Extreme Hafnium Values in Archean Banded Iron Formations: Evidence for Sedimentary Lu/Hf Fractionation at 3.2 Ga or Diagenesis?

NASA Astrophysics Data System (ADS)

Foster, I. S.; Agranier, A.; Heubeck, C. E.; Köhler, I.; Homann, M.; Tripati, A. K.; Nonnotte, P.; Ponzevera, E.; Lalonde, S.

2017-12-01

The emergence of continental crust above sea level in the early Precambrian would have created the first terrestrial habitats, and initiated atmosphere-driven weathering of the continents, yet the history of continental emergence is largely unknown[1]. Precambrian chemical sediments, specifically Banded Iron Formation (BIF), appear to have sampled the Hf-Nd isotope composition of ancient seawater, and may preserve a historical record of the emergence of the continental landmass[2] via Lu/Hf fractionation induced by subaerial differential weathering[3,4]. However, paired Hf-Nd isotope data are available for only one BIF to date, indicating appreciable emerged continental landmass ca. 2.7 Ga[2]. Our work extends this record back into the Eo- and Meso-Archean using samples of 3.8 Ga BIF from Isua, Greenland, and 3.2 Ga BIF from the Moodies Group, S. Africa. Isua samples appear to have been altered by amphibolite-grade metamorphism, however Moodies Group samples appear primary, having experienced significantly lower metamorphic grades. Moodies samples appear to retain their primary seawater signatures, however, their range of ˜Hf(i) values, from -54.6 to +40.7, is among the most extreme ever reported. Such extreme values may be indicative of one of several possibilities: unusual and intense sedimentary Lu/Hf fractionation during the Mesoarchean relative to today, sampling of a continuum of compositions from two sources with distinct Hf-compositions, or the result of early diagenetic processes occurring soon after the deposition of the Moodies Group BIF. These results suggest that interpretation of ˜Hf and ˜Nd data from BIF is not as straightforward as previously suggested[2], and positive ˜Hf values are not necessarily indicative of emerged continental crust. [1] Flament et al. (2013), Precambrian Research, 229, 177-188. [2] Veihmann et al. (2014), Geology, 42, 115-118. [3] Bayon et al. (2006), Geology, 34, 433-436. [4] Vervoort et al. (2011), Geochimica et Cosmochimica Acta, 75, 5903-5926.

Ni Isotope Signatures in Banded Iron Formations Before, During, and After the Great Oxidation Event

NASA Astrophysics Data System (ADS)

Wasylenki, L.; Wang, S.

2016-12-01

We have measured the Ni isotope compositions of banded iron formations (BIF) in an effort to test the hypothesis of Konhauser et al. that a decrease in supply of Ni to the Late Archean oceans may have triggered a decline in methanogen productivity [1,2]. These microorganisms are critically dependent on Ni, and their decline may have triggered a drop in atmospheric CH4 that allowed the first sustained rise of free O2 in the atmosphere at 2.4 Ga. While simultaneously considering other processes that may have controlled the Ni isotope composition of Precambrian seawater, and in turn the BIF, we are looking for a shift in δ60/58Ni over time that correlates with the decrease in BIF Ni/Fe ratios documented previously [1,2] and that possibly reflects a dramatic change in methanogen Ni status over that same time interval (2.7-2.4 Ga). Our preliminary results indicate that the ocean's Ni isotope composition has varied considerably over geologic time. Using results from an accompanying experimental study of Ni fractionation during incorporation into ferric oxyhydroxides/oxides, we can reconstruct the Ni isotope compositions of seawater from which BIF precipitated. We observe that Precambrian seawater was generally considerably enriched in light isotopes of Ni relative to modern seawater. So far we observe the widest range of δ60/58Ni values in those BIF samples aged 2.7-2.4 Ga, implying significant changes in the controls on δ60/58Ni in the Late Archean Eon and possibly much greater sensitivity of the biogeochemical cycle of Ni to perturbations in Ni sources, such as oxidative weathering of sulfides, input/output fluxes, or biological uptake. [1] Konhauser et al. (2009) Nature 458,750; [2] Konhauser et al. (2015) Astrobiology 15,804.

Field evidence of Eros-scale asteroids and impact-forcing of Precambrian geodynamic episodes, Kaapvaal (South Africa) and Pilbara (Western Australia) Cratons

NASA Astrophysics Data System (ADS)

Glikson, Andrew Y.

2008-03-01

The role of asteroid and comet impacts as triggers of mantle-crust processes poses one of the fundamental questions in Earth science. I present direct field evidence for close associations between impact ejecta/fallout units, major unconformities and lithostratigraphic boundaries in Archaean and early Proterozoic terrains, including abrupt changes in the composition of volcanic and sedimentary assemblages across stratigraphic impact boundaries, with implications for the nature and composition of their provenance terrains. As originally observed by D.R. Lowe and G.R. Byerly, in the Barberton Greenstone Belt, eastern Kaapvaal Craton, South Africa, 3.26-3.24 Ga asteroid mega-impact units are closely associated with the abrupt break between an underlying simatic mafic-ultramafic volcanic crust and an overlying association of turbidites, banded iron formations, felsic tuff and conglomerates of continental affinities. Contemporaneous stratigraphic relationships are identified in the Pilbara Craton, Western Australia. Evidence for enrichment of seawater in ferrous iron in the wake of major asteroid impacts reflects emergence of new source terrains, likely dominated by mafic compositions, attributed to impact-triggered oceanic volcanic activity. Relationships between impact and volcanic activity are supported by the onset of major mafic dyke systems associated with ~ 2.48 Ga and possibly the 2.56 Ga mega-impact events.

A strong protective action of guttiferone-A, a naturally occurring prenylated benzophenone, against iron-induced neuronal cell damage.

PubMed

Figueredo, Yanier Núñez; García-Pupo, Laura; Cuesta Rubio, Osmany; Delgado Hernández, René; Naal, Zeki; Curti, Carlos; Pardo Andreu, Gilberto L

2011-01-01

Guttiferone-A (GA) is a natural occurring polyisoprenylated benzophenone with several reported pharmacological actions. We have assessed the protective action of GA on iron-induced neuronal cell damage by employing the PC12 cell line and primary culture of rat cortical neurons (PCRCN). A strong protection by GA, assessed by the 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carbox-anilide (XTT) assay, was revealed, with IC(50) values <1 µM. GA also inhibited Fe(3+)-ascorbate reduction, iron-induced oxidative degradation of 2-deoxiribose, and iron-induced lipid peroxidation in rat brain homogenate, as well as stimulated oxygen consumption by Fe(2+) autoxidation. Absorption spectra and cyclic voltammograms of GA-Fe(2+)/Fe(3+) complexes suggest the formation of a transient charge transfer complex between Fe(2+) and GA, accelerating Fe(2+) oxidation. The more stable Fe(3+) complex with GA would be unable to participate in Fenton-Haber Weiss-type reactions and the propagation phase of lipid peroxidation. The results show a potential of GA against neuronal diseases associated with iron-induced oxidative stress.

Influence of GaAs surface termination on GaSb/GaAs quantum dot structure and band offsets

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zech, E. S.; Chang, A. S.; Martin, A. J.

2013-08-19

We have investigated the influence of GaAs surface termination on the nanoscale structure and band offsets of GaSb/GaAs quantum dots (QDs) grown by molecular-beam epitaxy. Transmission electron microscopy reveals both coherent and semi-coherent clusters, as well as misfit dislocations, independent of surface termination. Cross-sectional scanning tunneling microscopy and spectroscopy reveal clustered GaSb QDs with type I band offsets at the GaSb/GaAs interfaces. We discuss the relative influences of strain and QD clustering on the band offsets at GaSb/GaAs interfaces.

Oral iron treatment has a positive effect on iron metabolism in elite soccer players.

PubMed

Villanueva, Jesús; Soria, Marisol; González-Haro, Carlos; Ezquerra, Laura; Nieto, José L; Escanero, Jesús F

2011-09-01

The purpose of this study was to assess the effects of oral iron supplementation on hematological and iron metabolism in elite soccer players. Thirty-five members of the Real Zaragoza SAD soccer team took part in this study: group A (GA, n = 24; Spanish Premier League) took an oral iron supplement of 80 mg day(-1) for 3 weeks, and group B (GB, n = 11; Spanish Third Division League) did not receive any supplementation. In GA, the parameters were measured before and after giving the iron supplements, while in GB, measurements were only made at the time of collecting the second set of data from GA. After supplementation, GA showed an increase in serum iron (SI) (P < 0.05), serum ferritin (Ftn) (P < 0.01), and transferrin saturation (Sat) (P < 0.01) with respect to the basal values. In addition, GA showed higher values of hematocrit (P < 0.01), mean corpuscular volume (P < 0.01), Ftn (P < 0.01), and Sat (P < 0.01) than GB. No significant differences were found in any other parameters. More specifically, a higher percentage of players had Ftn levels above upper limits in GA vs. GB (P < 0.05), and GB had a higher incidence of Ftn below lower limits with respect to subjects in GA (P < 0.01). Further, after treatment, 58.3% of GA had >800 mg of SI, while all players in GB presented levels below the lower limits. In conclusion, iron supplementation with 80 mg·day(-1) for 3 weeks, before the start of the soccer season, can be recommended for elite soccer players.

Understanding band alignments in semiconductor heterostructures: Composition dependence and type-I-type-II transition of natural band offsets in nonpolar zinc-blende AlxGa1 -xN /AlyGa1 -yN composites

NASA Astrophysics Data System (ADS)

Landmann, M.; Rauls, E.; Schmidt, W. G.

2017-04-01

The composition dependence of the natural band alignment at nonpolar AlxGa1 -xN /AlyGa1 -yN heterojunctions is investigated via hybrid functional based density functional theory. Accurate band-gap data are provided using Heyd-Scuseria-Ernzerhof (HSE) type hybrid functionals with a composition dependent exact-exchange contribution. The unstrained band alignment between zincblende (zb) AlxGa1 -xN semiconductor alloys is studied within the entire ternary composition range utilizing the Branch-point technique to align the energy levels related to the bulklike direct Γv→Γc and indirect, pseudodirect, respectively, Γv→Xc type transitions in zb-AlxGa1 -xN . While the zb-GaN/AlxGa1 -xN band edges consistently show a type-I alignment, the relative position of fundamental band edges changes to a type-II alignment in the Al-rich composition ranges of zb-AlxGa1 -xN /AlN and zb-AlxGa1 -xN /AlyGa1 -yN systems. The presence of a direct-indirect band-gap transition at xc=0.63 in zb-AlxGa1 -xN semiconductor alloys gives rise to a notably different composition dependence of band discontinuities in the direct and indirect energy-gap ranges. Below the critical direct-indirect Al/Ga-crossover concentration, the band offsets show a close to linear dependence on the alloy composition. In contrast, notable bowing characteristics of all band discontinuities are observed above the critical crossover composition.

Paleoproterozoic snowball earth: extreme climatic and geochemical global change and its biological consequences

NASA Technical Reports Server (NTRS)

Kirschvink, J. L.; Gaidos, E. J.; Bertani, L. E.; Beukes, N. J.; Gutzmer, J.; Maepa, L. N.; Steinberger, R. E.

2000-01-01

Geological, geophysical, and geochemical data support a theory that Earth experienced several intervals of intense, global glaciation ("snowball Earth" conditions) during Precambrian time. This snowball model predicts that postglacial, greenhouse-induced warming would lead to the deposition of banded iron formations and cap carbonates. Although global glaciation would have drastically curtailed biological productivity, melting of the oceanic ice would also have induced a cyanobacterial bloom, leading to an oxygen spike in the euphotic zone and to the oxidative precipitation of iron and manganese. A Paleoproterozoic snowball Earth at 2.4 Giga-annum before present (Ga) immediately precedes the Kalahari Manganese Field in southern Africa, suggesting that this rapid and massive change in global climate was responsible for its deposition. As large quantities of O(2) are needed to precipitate this Mn, photosystem II and oxygen radical protection mechanisms must have evolved before 2.4 Ga. This geochemical event may have triggered a compensatory evolutionary branching in the Fe/Mn superoxide dismutase enzyme, providing a Paleoproterozoic calibration point for studies of molecular evolution.

Gallium-Protoporphyrin IX Inhibits Pseudomonas aeruginosa Growth by Targeting Cytochromes.

PubMed

Hijazi, Sarah; Visca, Paolo; Frangipani, Emanuela

2017-01-01

Pseudomonas aeruginosa is a challenging pathogen due to both innate and acquired resistance to antibiotics. It is capable of causing a variety of infections, including chronic lung infection in cystic fibrosis (CF) patients. Given the importance of iron in bacterial physiology and pathogenicity, iron-uptake and metabolism have become attractive targets for the development of new antibacterial compounds. P. aeruginosa can acquire iron from a variety of sources to fulfill its nutritional requirements both in the environment and in the infected host. The adaptation of P. aeruginosa to heme iron acquisition in the CF lung makes heme utilization pathways a promising target for the development of new anti- Pseudomonas drugs. Gallium [Ga(III)] is an iron mimetic metal which inhibits P. aeruginosa growth by interfering with iron-dependent metabolism. The Ga(III) complex of the heme precursor protoporphyrin IX (GaPPIX) showed enhanced antibacterial activity against several bacterial species, although no inhibitory effect has been reported on P. aeruginosa . Here, we demonstrate that GaPPIX is indeed capable of inhibiting the growth of clinical P. aeruginosa strains under iron-deplete conditions, as those encountered by bacteria during infection, and that GaPPIX inhibition is reversed by iron. Using P. aeruginosa PAO1 as model organism, we show that GaPPIX enters cells through both the heme-uptake systems has and phu , primarily via the PhuR receptor which plays a crucial role in P. aeruginosa adaptation to the CF lung. We also demonstrate that intracellular GaPPIX inhibits the aerobic growth of P. aeruginosa by targeting cytochromes, thus interfering with cellular respiration.

Gallium-Protoporphyrin IX Inhibits Pseudomonas aeruginosa Growth by Targeting Cytochromes

PubMed Central

Hijazi, Sarah; Visca, Paolo; Frangipani, Emanuela

2017-01-01

Pseudomonas aeruginosa is a challenging pathogen due to both innate and acquired resistance to antibiotics. It is capable of causing a variety of infections, including chronic lung infection in cystic fibrosis (CF) patients. Given the importance of iron in bacterial physiology and pathogenicity, iron-uptake and metabolism have become attractive targets for the development of new antibacterial compounds. P. aeruginosa can acquire iron from a variety of sources to fulfill its nutritional requirements both in the environment and in the infected host. The adaptation of P. aeruginosa to heme iron acquisition in the CF lung makes heme utilization pathways a promising target for the development of new anti-Pseudomonas drugs. Gallium [Ga(III)] is an iron mimetic metal which inhibits P. aeruginosa growth by interfering with iron-dependent metabolism. The Ga(III) complex of the heme precursor protoporphyrin IX (GaPPIX) showed enhanced antibacterial activity against several bacterial species, although no inhibitory effect has been reported on P. aeruginosa. Here, we demonstrate that GaPPIX is indeed capable of inhibiting the growth of clinical P. aeruginosa strains under iron-deplete conditions, as those encountered by bacteria during infection, and that GaPPIX inhibition is reversed by iron. Using P. aeruginosa PAO1 as model organism, we show that GaPPIX enters cells through both the heme-uptake systems has and phu, primarily via the PhuR receptor which plays a crucial role in P. aeruginosa adaptation to the CF lung. We also demonstrate that intracellular GaPPIX inhibits the aerobic growth of P. aeruginosa by targeting cytochromes, thus interfering with cellular respiration. PMID:28184354

Angular dependent XPS study of surface band bending on Ga-polar n-GaN

NASA Astrophysics Data System (ADS)

Huang, Rong; Liu, Tong; Zhao, Yanfei; Zhu, Yafeng; Huang, Zengli; Li, Fangsen; Liu, Jianping; Zhang, Liqun; Zhang, Shuming; Dingsun, An; Yang, Hui

2018-05-01

Surface band bending and composition of Ga-polar n-GaN with different surface treatments were characterized by using angular dependent X-ray photoelectron spectroscopy. Upward surface band bending of varying degree was observed distinctly upon to the treatment methods. Besides the nitrogen vacancies, we found that surface states of oxygen-containing absorbates (O-H component) also contribute to the surface band bending, which lead the Fermi level pined at a level further closer to the conduction band edge on n-GaN surface. The n-GaN surface with lower surface band bending exhibits better linear electrical properties for Ti/GaN Ohmic contacts. Moreover, the density of positively charged surface states could be derived from the values of surface band bending.

Defect-related photoluminescence in Mg-doped GaN nanostructures

NASA Astrophysics Data System (ADS)

Reshchikov, M. A.; Shahedipour-Sandvik, F.; Messer, B. J.; Jindal, V.; Tripathi, N.; Tungare, M.

2009-12-01

Thin film of GaN:Mg, pyramidal GaN:Mg on GaN, sapphire and AlN substrates were grown in a MOCVD system under same growth conditions and at the same time. In samples with Mg-doped GaN pyramids on GaN:Si template a strong ultraviolet (UVL) band with few phonon replicas dominated at low temperature and was attributed to transitions from shallow donors to shallow Mg acceptor. In samples grown on sapphire and AlN substrates the UVL band appeared as a structureless band with the maximum at about 3.25 eV. There is a possibility that the structureless UVL band and the UVL band with phonon structure have different origin. In addition to the UVL band, the blue luminescence (BL) band peaking at 2.9 eV was observed in samples representing GaN:Mg pyramids on GaN:Si substrate. It is preliminary attributed to transitions from shallow donors to Zn acceptor in GaN:Si substrate.

Updating the Geologic Barcodes for South China: Discovery of Late Archean Banded Iron Formations in the Yangtze Craton.

PubMed

Ye, Hui; Wu, Chang-Zhi; Yang, Tao; Santosh, M; Yao, Xi-Zhu; Gao, Bing-Fei; Wang, Xiao-Lei; Li, Weiqiang

2017-11-08

Banded iron formations (BIFs) in Archean cratons provide important "geologic barcodes" for the global correlation of Precambrian sedimentary records. Here we report the first finding of late Archean BIFs from the Yangtze Craton, one of largest Precambrian blocks in East Asia with an evolutionary history of over 3.3 Ga. The Yingshan iron deposit at the northeastern margin of the Yangtze Craton, displays typical features of BIF, including: (i) alternating Si-rich and Fe-rich bands at sub-mm to meter scales; (ii) high SiO 2  + Fe 2 O 3total contents (average 90.6 wt.%) and Fe/Ti ratios (average 489); (iii) relative enrichment of heavy rare earth elements and positive Eu anomalies (average 1.42); (iv) and sedimentary Fe isotope compositions (δ 56 Fe IRMM-014 as low as -0.36‰). The depositional age of the BIF is constrained at ~2464 ± 24 Ma based on U-Pb dating of zircon grains from a migmatite sample of a volcanic protolith that conformably overlied the Yingshan BIF. The BIF was intruded by Neoproterozoic (805.9 ± 4.7 Ma) granitoids that are unique in the Yangtze Craton but absent in the North China Craton to the north. The discovery of the Yingshan BIF provides new constraints for the tectonic evolution of the Yangtze Craton and has important implications in the reconstruction of Pre-Nuna/Columbia supercontinent configurations.

Type I band alignment in GaAs{sub 81}Sb{sub 19}/GaAs core-shell nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, T.; Key Laboratory of Advanced Display and System Application, Shanghai University, 149 Yanchang Road, Shanghai 200072; Wei, M. J.

2015-09-14

The composition and band gap of the shell that formed during the growth of axial GaAs/GaAs{sub 81}Sb{sub 19}/ GaAs heterostructure nanowires have been investigated by transmission electron microscopy combined with energy dispersion spectroscopy, scanning tunneling spectroscopy, and density functional theory calculations. On the GaAs{sub 81}Sb{sub 19} intermediate segment, the shell is found to be free of Sb (pure GaAs shell) and transparent to the tunneling electrons, despite the (110) biaxial strain that affects its band gap. As a result, a direct measurement of the core band gap allows the quantitative determination of the band offset between the GaAs{sub 81}Sb{sub 19}more » core and the GaAs shell and identifies it as a type I band alignment.« less

Hybrid density functional study of band alignment in ZnO-GaN and ZnO-(Ga(1-x)Zn(x))(N(1-x)O(x))-GaN heterostructures.

PubMed

Wang, Zhenhai; Zhao, Mingwen; Wang, Xiaopeng; Xi, Yan; He, Xiujie; Liu, Xiangdong; Yan, Shishen

2012-12-05

The band alignment in ZnO-GaN and related heterostructures is crucial for uses in solar harvesting technology. Here, we report our density functional calculations of the band alignment and optical properties of ZnO-GaN and ZnO-(Ga(1-x)Zn(x))(N(1-x)O(x))-GaN heterostructures using a Heyd-Scuseria-Ernzerhof (HSE) hybrid functional. We found that the conventional GGA functionals underestimate not only the band gap but also the band offset of these heterostructures. Using the hybrid functional calculations, we show that the (Ga(1-x)Zn(x))(N(1-x)O(x)) solid solution has a direct band gap of about 2.608 eV, in good agreement with the experimental data. More importantly, this solid solution forms type-II band alignment with the host materials. A GaN-(Ga(1-x)Zn(x))(N(1-x)O(x))-ZnO core-shell solar cell model is presented to improve the visible light absorption ability and carrier collection efficiency.

Theoretical studies on band structure and optical gain of GaInAsN/GaAs /GaAs cylindrical quantum dot

NASA Astrophysics Data System (ADS)

Mal, Indranil; Samajdar, Dip Prakash; John Peter, A.

2018-07-01

Electronic band structure, effective masses, band offsets and optical gain of Ga0.661In0.339N0.0554As0.9446/GaAs quantum dot systems are investigated using 10 band k·p Hamiltonian for various nitrogen and indium concentrations. The calculations include the effects of strain generated due to the lattice mismatch and the effective band gap of GaInAsN/GaAs heterostructures. The variation of conduction band, light hole and heavy hole band offsets with indium and nitrogen compositions in the alloy are obtained. The band structure of Ga0.661In0.339N0.0554As0.9446/GaAs quantum dot is found in the crystal directions Δ (100) and Λ (111) using 10 band k·p Hamiltonian. The optical gain of the cylindrical quantum dot structures as functions of surface carrier concentration and the dot radius is investigated. Our results show that the tensile strain of 1.34% generates a band gap of 0.59 eV and the compressive strain of 2.2% produces a band gap of 1.28 eV and the introduction of N atoms has no effect on the spin orbit split off band. The variation of optical gain with the dot size and the carrier concentration indicates that the optical gain increases with the decrease in the radius of the quantum dot. The results may be useful for the potential applications in optical devices.

Mesoarchean Banded Iron Formation sequences in Dixon Island-Cleaverville Formation, Pilbara Australia: Oxygenic signal from DXCL project

NASA Astrophysics Data System (ADS)

Kiyokawa, S.; Ito, T.; Ikehara, M.; Yamaguchi, K. E.; Naraoka, H.; Onoue, T.; Horie, K.; Sakamoto, R.; Aihara, Y.; Miki, T.

2013-12-01

The 3.2-3.1 Ga Dixon island-Cleaverville formations are well-preserved Banded Iron Formation (BIF) within hydrothermal oceanic sequence at oceanic island arc setting (Kiyokawa et al., 2002, 2006, 2012). The stratigraphy of the Dixon Island (3195+15Ma) -Cleaverville (3108+13Ma) formations shows the well preserved environmental condition at the Mesoarchean ocean floor. The stratigraphy of these formations are formed about volcano-sedimentary sequences with hydrothermal chert, black shale and banded iron formation to the top. Based on the scientific drilling of DXCL project at 2007 and 2011, detail lithology between BIF sequence was clearly understood. Four drilling holes had been done at coastal sites; the Dixon Island Formation is DX site (100m) and the Cleaverville Formation is CL2 (40m), CL1 (60m) and CL3 (200m) sites and from stratigraphic bottom to top. Coarsening and thickening upward black shale-BIF sequences are well preserved of the stratigraphy form the core samples. The Dixon Island Formation consists komatiite-rhyolite sequences with many hydrothermal veins and very fine laminated cherty rocks above them. The Cleaverville Formation contains black shale, fragments-bearing pyroclastic beds, white chert, greenish shale and BIF. The CL3 core, which drilled through BIF, shows siderite-chert beds above black shale identified before magnetite lamination bed. U-Pb SHRIMP data of the tuff in lower Dixon Island Formation is 3195+15 Ma and the pyroclastic sequence below the Cleaverville BIF is 3108+13 Ma. Sedimentation rate of these sequence is 2-8 cm/ 1000year. The hole section of the organic carbon rich black shales below BIF are similar amount of organic content and 13C isotope (around -30per mill). There are very weak sulfur MIF signal (less 0.2%) in these black shale sequence. Our result show that thick organic rich sediments may be triggered to form iron rich siderite and magnetite iron beds. The stratigraphy in this sequence quite resemble to other Iron formation (eg. Hamersley BIF). So we investigate that the Cleaverville iron formation, which is one of the best well known Mesoarchean iron formation, was already started cyanobacteria oxygen production system to used pre-syn iron sedimentation at anoxic oceanic condition.

Composition dependence of band alignments in GaxIn1-xAsySb1-y heterojunctions lattice matched to GaSb and InAs

NASA Astrophysics Data System (ADS)

Shim, Kyurhee

2013-11-01

A theoretical model utilizing a universal tight binding method and a correlated function expansion technique is presented to calculate the valence band maximum (VBM) and the conduction band minimum (CBM) of the binary (GaAs, InAS, GaSb, and InSb) and quaternary alloy GaxIn1-xAsySb1-y systems. By organizing the relative positions of the VBM and CBM between semiconductors, the band alignments and band types in the heterojunctions are determined. A straddling (type-I) band alignment in InAs/GaAs, InSb/GaAs, and GaSb/InSb, staggered (type-II) band alignment in GaSb/GaAs, and broken (type-III) band alignment in InSb/InAs and InAs/GaSb are found respectively. In addition, the compositional variations of VBM, CBM, valence band offset, conduction band offset, and band type for the alloy GaxIn1-xAsySb1-y lattice matched on GaSb and InAs are obtained as increasing the composition x. A pronounced upward bowing for the VBM and a very slight upward bowing (almost linear) for CBM are found, respectively. By controlling the compositions (x, y), band type transitions occur. The GaxIn1-xAsySb1-y heterojunctions lattice matched to GaSb changes their band types from type-III at x ˜0→ to type-II at x = 0.07, and → to type-I at x = 0.38. In contrast, the GaxIn1-xAsySb1-y heterojunctions lattice matched to InAs changes their band types from type-II x ˜0→ to type-III at x = 0.32. Reasonable agreement is obtained between our theoretical results and existing experimental data.

8-band and 14-band kp modeling of electronic band structure and material gain in Ga(In)AsBi quantum wells grown on GaAs and InP substrates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gladysiewicz, M.; Wartak, M. S.; Department of Physics and Computer Science, Wilfrid Laurier University, Waterloo, Ontario N2L 3C5

The electronic band structure and material gain have been calculated for GaAsBi/GaAs quantum wells (QWs) with various bismuth concentrations (Bi ≤ 15%) within the 8-band and 14-band kp models. The 14-band kp model was obtained by extending the standard 8-band kp Hamiltonian by the valence band anticrossing (VBAC) Hamiltonian, which is widely used to describe Bi-related changes in the electronic band structure of dilute bismides. It has been shown that in the range of low carrier concentrations n < 5 × 10{sup 18 }cm{sup −3}, material gain spectra calculated within 8- and 14-band kp Hamiltonians are similar. It means that the 8-band kp model can be usedmore » to calculate material gain in dilute bismides QWs. Therefore, it can be applied to analyze QWs containing new dilute bismides for which the VBAC parameters are unknown. Thus, the energy gap and electron effective mass for Bi-containing materials are used instead of VBAC parameters. The electronic band structure and material gain have been calculated for 8 nm wide GaInAsBi QWs on GaAs and InP substrates with various compositions. In these QWs, Bi concentration was varied from 0% to 5% and indium concentration was tuned in order to keep the same compressive strain (ε = 2%) in QW region. For GaInAsBi/GaAs QW with 5% Bi, gain peak was determined to be at about 1.5 μm. It means that it can be possible to achieve emission at telecommunication windows (i.e., 1.3 μm and 1.55 μm) for GaAs-based lasers containing GaInAsBi/GaAs QWs. For GaInAsBi/Ga{sub 0.47}In{sub 0.53}As/InP QWs with 5% Bi, gain peak is predicted to be at about 4.0 μm, i.e., at the wavelengths that are not available in current InP-based lasers.« less

Band-Bending of Ga-Polar GaN Interfaced with Al2O3 through Ultraviolet/Ozone Treatment.

PubMed

Kim, Kwangeun; Ryu, Jae Ha; Kim, Jisoo; Cho, Sang June; Liu, Dong; Park, Jeongpil; Lee, In-Kyu; Moody, Baxter; Zhou, Weidong; Albrecht, John; Ma, Zhenqiang

2017-05-24

Understanding the band bending at the interface of GaN/dielectric under different surface treatment conditions is critically important for device design, device performance, and device reliability. The effects of ultraviolet/ozone (UV/O 3 ) treatment of the GaN surface on the energy band bending of atomic-layer-deposition (ALD) Al 2 O 3 coated Ga-polar GaN were studied. The UV/O 3 treatment and post-ALD anneal can be used to effectively vary the band bending, the valence band offset, conduction band offset, and the interface dipole at the Al 2 O 3 /GaN interfaces. The UV/O 3 treatment increases the surface energy of the Ga-polar GaN, improves the uniformity of Al 2 O 3 deposition, and changes the amount of trapped charges in the ALD layer. The positively charged surface states formed by the UV/O 3 treatment-induced surface factors externally screen the effect of polarization charges in the GaN, in effect, determining the eventual energy band bending at the Al 2 O 3 /GaN interfaces. An optimal UV/O 3 treatment condition also exists for realizing the "best" interface conditions. The study of UV/O 3 treatment effect on the band alignments at the dielectric/III-nitride interfaces will be valuable for applications of transistors, light-emitting diodes, and photovoltaics.

Different actions of deferoxamine and iron on Ga-67 abscess detection in rats

DOE Office of Scientific and Technical Information (OSTI.GOV)

Oberhensli, R.D.; Mueller, R.M.; Fridrich, R.

1984-06-01

The contrast-enhancing properties of iron (Fe) and deferoxamine (DFO) in abscess imaging with Ga-67 citrate were compared in rats bearing turpentine-induced abscesses. Iron administration shifted Ga-67 from plasma into tissues such as muscle and fat. As a result, the abscess-to-plasma ratio increased whereas the abscess-to-muscle ratio decreased. DFO enhanced the abscess-to-muscle and abscess-to-plasma ratios by increasing urinary Ga-67 excretion. The authors conclude that abscess imaging with Ga-67 citrate may be improved by administration of (a) Fe for detection of abscesses masked by blood activity, or (b) DFO for detection of abscesses surrounded by muscle tissue.

Formation of high electrical-resistivity thin surface layer on carbonyl-iron powder (CIP) and thermal stability of nanocrystalline structure and vortex magnetic structure of CIP

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sugimura, K.; Miyajima, Y.; Sonehara, M.

2016-05-15

This study focuses on the carbonyl-iron powder (CIP) used in the metal composite bulk magnetic core for high-efficient/light-weight SiC/GaN power device MHz switching dc-dc converter, where the fine CIP with a mean diameter of 1.1 μm is used to suppress the MHz band eddy current inside the CIP body. When applying the CIP to composite core together with the resin matrix, high electrical resistivity layer must be formed on the CIP-surface in order to suppress the overlapped eddy current between adjacent CIPs. In this study, tens nm thick silica (SiO{sub 2}) was successfully deposited on the CIP-surface by using hydrolysismore » of TEOS (Si(OC{sub 2}H{sub 5}){sub 4}). Also tens nm thick oxidized layer of the CIP-surface was successfully formed by using CIP annealing in dry air. The SiC/GaN power device can operate at ambient temperature over 200 degree-C, and the composite magnetic core is required to operate at such ambient temperature. The as-made CIP had small coercivity below 800 A/m (10 Oe) due to its nanocrystalline-structure and had a single vortex magnetic structure. From the experimental results, both nanocrystalline and single vortex magnetic structure were maintained after heat-exposure of 250 degree-C, and the powder coercivity after same heat-exposure was nearly same as that of the as-made CIP. Therefore, the CIP with thermally stable nanocrystalline-structure and vortex magnetic state was considered to be heat-resistant magnetic powder used in the iron-based composite core for SiC/GaN power electronics.« less

Formation of high electrical-resistivity thin surface layer on carbonyl-iron powder (CIP) and thermal stability of nanocrystalline structure and vortex magnetic structure of CIP

NASA Astrophysics Data System (ADS)

Sugimura, K.; Miyajima, Y.; Sonehara, M.; Sato, T.; Hayashi, F.; Zettsu, N.; Teshima, K.; Mizusaki, H.

2016-05-01

This study focuses on the carbonyl-iron powder (CIP) used in the metal composite bulk magnetic core for high-efficient/light-weight SiC/GaN power device MHz switching dc-dc converter, where the fine CIP with a mean diameter of 1.1 μm is used to suppress the MHz band eddy current inside the CIP body. When applying the CIP to composite core together with the resin matrix, high electrical resistivity layer must be formed on the CIP-surface in order to suppress the overlapped eddy current between adjacent CIPs. In this study, tens nm thick silica (SiO2) was successfully deposited on the CIP-surface by using hydrolysis of TEOS (Si(OC2H5)4). Also tens nm thick oxidized layer of the CIP-surface was successfully formed by using CIP annealing in dry air. The SiC/GaN power device can operate at ambient temperature over 200 degree-C, and the composite magnetic core is required to operate at such ambient temperature. The as-made CIP had small coercivity below 800 A/m (10 Oe) due to its nanocrystalline-structure and had a single vortex magnetic structure. From the experimental results, both nanocrystalline and single vortex magnetic structure were maintained after heat-exposure of 250 degree-C, and the powder coercivity after same heat-exposure was nearly same as that of the as-made CIP. Therefore, the CIP with thermally stable nanocrystalline-structure and vortex magnetic state was considered to be heat-resistant magnetic powder used in the iron-based composite core for SiC/GaN power electronics.

Valence band offset of β-Ga2O3/wurtzite GaN heterostructure measured by X-ray photoelectron spectroscopy.

PubMed

Wei, Wei; Qin, Zhixin; Fan, Shunfei; Li, Zhiwei; Shi, Kai; Zhu, Qinsheng; Zhang, Guoyi

2012-10-10

A sample of the β-Ga2O3/wurtzite GaN heterostructure has been grown by dry thermal oxidation of GaN on a sapphire substrate. X-ray diffraction measurements show that the β-Ga2O3 layer was formed epitaxially on GaN. The valence band offset of the β-Ga2O3/wurtzite GaN heterostructure is measured by X-ray photoelectron spectroscopy. It is demonstrated that the valence band of the β-Ga2O3/GaN structure is 1.40 ± 0.08 eV.

Valence band offset of β-Ga2O3/wurtzite GaN heterostructure measured by X-ray photoelectron spectroscopy

PubMed Central

2012-01-01

A sample of the β-Ga2O3/wurtzite GaN heterostructure has been grown by dry thermal oxidation of GaN on a sapphire substrate. X-ray diffraction measurements show that the β-Ga2O3 layer was formed epitaxially on GaN. The valence band offset of the β-Ga2O3/wurtzite GaN heterostructure is measured by X-ray photoelectron spectroscopy. It is demonstrated that the valence band of the β-Ga2O3/GaN structure is 1.40 ± 0.08 eV. PMID:23046910

Lasing in optimized two-dimensional iron-nail-shaped rod photonic crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kwon, Soon-Yong; Moon, Seul-Ki; Yang, Jin-Kyu, E-mail: jinkyuyang@kongju.ac.kr

2016-03-15

We demonstrated lasing at the Γ-point band-edge (BE) modes in optimized two-dimensional iron-nail-shaped rod photonic crystals by optical pulse pumping at room temperature. As the radius of the rod increased quadratically toward the edge of the pattern, the quality factor of the Γ-point BE mode increased up to three times, and the modal volume decreased to 56% compared with the values of the original Γ-point BE mode because of the reduction of the optical loss in the horizontal direction. Single-mode lasing from an optimized iron-nail-shaped rod array with an InGaAsP multiple quantum well embedded in the nail heads was observedmore » at a low threshold pump power of 160 μW. Real-image-based numerical simulations showed that the lasing actions originated from the optimized Γ-point BE mode and agreed well with the measurement results, including the lasing polarization, wavelength, and near-field image.« less

Mixed nickel-gallium tellurides Ni{sub 3−x}GaTe{sub 2} as a matrix for incorporating magnetic cations: A Ni{sub 3−x}Fe{sub x}GaTe{sub 2} series

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kuznetsov, Alexey N., E-mail: alexei@inorg.chem.msu.ru; N.S. Kurnakov Institute of General and Inorganic Chemistry, RAS, Leninsky pr. 31, GSP-1, 119991 Moscow; Stroganova, Ekaterina A.

Using a high-temperature ampoule technique, a series of mixed nickel-iron-gallium metal-rich tellurides with layered structures, Ni{sub 3-x}Fe{sub x}GaTe{sub 2}, were prepared and characterized based on X-ray powder diffraction, energy-dispersive spectroscopy, and {sup 57}Fe Mössbauer spectroscopy data. These compounds may be regarded as a result of partial substitution of nickel by iron in the recently reported ternary Ni{sub 3-x}GaTe{sub 2} series, which are based on NiAs/Ni{sub 2}In type of structure. The compositional boundary for the substitution was found to be at x~1. According to the Mössbauer spectroscopy data, the substitution is not statistical, and iron atoms with the increase in xmore » tend to preferentially occupy those nickel positions that are partially vacant in the initial ternary compound. Magnetic measurements data for the Ni{sub 3-x}Fe{sub x}GaTe{sub 2} series show dramatic change in behavior from temperature-independent paramagnetic properties of the initial matrix to a low-temperature (~75 K) ferromagnetic ordering in the Ni{sub 2}FeGaTe{sub 2}. - Graphical abstract: Ordered substitution of nickel by iron in the Ni{sub 3−x}GaTe{sub 2} series leading to ferromagnetic ordering. - Highlights: • A series of Ni{sub 3−x}Fe{sub x}GaTe{sub 2} compounds were synthesized. • They adopt the NiAs/Ni{sub 2}In type of structure with ordered iron distribution. • The distribution of iron was studied using {sup 57}Fe Mössbauer spectroscopy. • An increase in iron content leads to the strong ferromagnetic coupling.« less

Pyoverdine and proteases affect the response of Pseudomonas aeruginosa to gallium in human serum.

PubMed

Bonchi, Carlo; Frangipani, Emanuela; Imperi, Francesco; Visca, Paolo

2015-09-01

Gallium is an iron mimetic which has recently been repurposed as an antibacterial agent due to its capability to disrupt bacterial iron metabolism. In this study, the antibacterial activity of gallium nitrate [Ga(NO3)3] was investigated in complement-free human serum (HS) on 55 Pseudomonas aeruginosa clinical isolates from cystic fibrosis and non-cystic fibrosis patients. The susceptibility of P. aeruginosa to Ga(NO3)3 in HS was dependent on the bacterial ability to acquire iron from serum binding proteins (i.e., transferrin). The extent of serum protein degradation correlated well with P. aeruginosa growth in HS, while pyoverdine production did not. However, pyoverdine-deficient P. aeruginosa strains were unable to grow in HS and overcome iron restriction, albeit capable of releasing proteases. Predigestion of HS with proteinase K promoted the growth of all strains, irrespective of their ability to produce proteases and/or pyoverdine. The MICs of Ga(NO3)3 were higher in HS than in an iron-poor Casamino Acids medium, where proteolysis does not affect iron availability. Coherently, strains displaying high proteolytic activity were less susceptible to Ga(NO3)3 in HS. Our data support a model in which both pyoverdine and proteases affect the response of P. aeruginosa to Ga(NO3)3 in HS. The relatively high Ga(NO3)3 concentration required to inhibit the growth of highly proteolytic P. aeruginosa isolates in HS poses a limitation to the potential of Ga(NO3)3 in the treatment of P. aeruginosa bloodstream infections. Copyright © 2015, American Society for Microbiology. All Rights Reserved.

Pyoverdine and Proteases Affect the Response of Pseudomonas aeruginosa to Gallium in Human Serum

PubMed Central

Bonchi, Carlo; Frangipani, Emanuela; Imperi, Francesco

2015-01-01

Gallium is an iron mimetic which has recently been repurposed as an antibacterial agent due to its capability to disrupt bacterial iron metabolism. In this study, the antibacterial activity of gallium nitrate [Ga(NO3)3] was investigated in complement-free human serum (HS) on 55 Pseudomonas aeruginosa clinical isolates from cystic fibrosis and non-cystic fibrosis patients. The susceptibility of P. aeruginosa to Ga(NO3)3 in HS was dependent on the bacterial ability to acquire iron from serum binding proteins (i.e., transferrin). The extent of serum protein degradation correlated well with P. aeruginosa growth in HS, while pyoverdine production did not. However, pyoverdine-deficient P. aeruginosa strains were unable to grow in HS and overcome iron restriction, albeit capable of releasing proteases. Predigestion of HS with proteinase K promoted the growth of all strains, irrespective of their ability to produce proteases and/or pyoverdine. The MICs of Ga(NO3)3 were higher in HS than in an iron-poor Casamino Acids medium, where proteolysis does not affect iron availability. Coherently, strains displaying high proteolytic activity were less susceptible to Ga(NO3)3 in HS. Our data support a model in which both pyoverdine and proteases affect the response of P. aeruginosa to Ga(NO3)3 in HS. The relatively high Ga(NO3)3 concentration required to inhibit the growth of highly proteolytic P. aeruginosa isolates in HS poses a limitation to the potential of Ga(NO3)3 in the treatment of P. aeruginosa bloodstream infections. PMID:26149986

The melting of subducted banded iron formations

NASA Astrophysics Data System (ADS)

Kang, Nathan; Schmidt, Max W.

2017-10-01

Banded iron formations (BIF) were common shelf and ocean basin sediments 3.5-1.8 Ga ago. To understand the fate of these dense rocks upon subduction, the melting relations of carbonated BIF were determined in Fe-Ca-(Mg)-Si-C-O2 at 950-1400 °C, 6 and 10 GPa, oxidizing (fO2 = hematite-magnetite, HM) and moderately reducing (fO2 ∼CO2-graphite/diamond, CCO) conditions. Solidus temperatures under oxidizing conditions are 950-1025 °C with H2O, and 1050-1150 °C anhydrous, but 250-175 °C higher at graphite saturation (values at 6-10 GPa). The combination of Fe3+ and carbonate leads to a strong melting depression. Solidus curves are steep with 17-20 °C/GPa. Near-solidus melts are ferro-carbonatites with ∼22 wt.% FeOtot, ∼48 wt% CO2 and 1-5 wt.% SiO2 at fO2 ∼ HM and ∼49 wt.% FeOtot, ∼20 wt% CO2 and 19-25 wt.% SiO2 at fO2 ∼ CCO . At elevated subduction geotherms, as likely for the Archean, C-bearing BIF could melt out all carbonate around 6 GPa. Fe-rich carbonatites would rise but stagnate gravitationally near the slab/mantle interface until they react with the mantle through Fe-Mg exchange and partial reduction. The latter would precipitate diamond and yield Fe- and C-rich mantle domains, yet, Fe-Mg is expected to diffusively re-equilibrate over Ga time scales. We propose that the oldest subduction derived diamonds stem from BIF derived melts.

Electronic structure and optical properties of noncentrosymmetric LiGaSe2: Experimental measurements and DFT band structure calculations

NASA Astrophysics Data System (ADS)

Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Ananchenko, L. N.; Isaenko, L. I.; Yelisseyev, A. P.; Khyzhun, O. Y.

2017-04-01

We report on measurements of X-ray photoelectron (XP) spectra for pristine and Ar+ ion-irradiated surfaces of LiGaSe2 single crystal grown by Bridgman-Stockbarger method. Electronic structure of the LiGaSe2 compound is studied from a theoretical and experimental viewpoint. In particular, total and partial densities of states of LiGaSe2 are investigated by density functional theory (DFT) calculations employing the augmented plane wave + local orbitals (APW + lo) method and they are verified by data of X-ray spectroscopy measurements. The DFT calculations indicate that the main contributors to the valence band of LiGaSe2 are the Se 4p states, which contribute mainly at the top and in the upper portion of the valence band, with also essential contributions of these states in the lower portion of the band. Other substantial contributions to the valence band of LiGaSe2 emerge from the Ga 4s and Ga 4p states contributing mainly at the lower ant upper portions of the valence band, respectively. With respect to the conduction band, the calculations indicate that its bottom is composed mainly from contributions of the unoccupied Ga s and Se p states. The present calculations are confirmed experimentally when comparing the XP valence-band spectrum of the LiGaS2 single crystal on a common energy scale with the X-ray emission bands representing the energy distribution of the Ga 4p and Se 4p states. Measurements of the fundamental absorption edges at room temperature reveal that bandgap value, Eg, of LiGaSe2 is equal to 3.47 eV and the Eg value increases up to 3.66 eV when decreasing temperature to 80 K. The main optical characteristics of the LiGaSe2 compound are clarified by the DFT calculations.

Hybrid functional study of band structures of GaAs1-xNx and GaSb1-xNx alloys

NASA Astrophysics Data System (ADS)

Virkkala, Ville; Havu, Ville; Tuomisto, Filip; Puska, Martti J.

2012-02-01

Band structures of GaAs1-xNx and GaSb1-xNx alloys are studied in the framework of the density functional theory within the hybrid functional scheme (HSE06). We find that the scheme gives a clear improvement over the traditional (semi)local functionals in describing, in a qualitative agreement with experiments, the bowing of electron energy band gap in GaAs1-xNx alloys. In the case of GaSb1-xNx alloys, the hybrid functional used makes the study of band structures possible ab initio without any empirical parameter fitting. We explain the trends in the band gap reductions in the two materials that result mainly from the positions of the nitrogen-induced states with respect to the bottoms of the bulk conduction bands.

Gibberellins regulate iron deficiency-response by influencing iron transport and translocation in rice seedlings (Oryza sativa)

PubMed Central

Wang, Baolan; Wei, Haifang; Xue, Zhen

2017-01-01

Background and aims Gibberellins (GAs) are a class of plant hormones with diverse functions. However, there has been little information on the role of GAs in response to plant nutrient deficiency. Methods To evaluate the roles of GAs in regulation of Fe homeostasis, the effects of GA on Fe accumulation and Fe translocation in rice seedlings were investigated using wild-type, a rice mutant (eui1) displaying enhnaced endogenous GA concentrations due to a defect in GA deactivation, and transgenic rice plants overexpressing OsEUI. Key Results Exposure to Fe-deficient medium significantly reduced biomass of rice plants. Both exogenous application of GA and an endogenous increase of bioactive GA enhanced Fe-deficiency response by exaggerating foliar chlorosis and reducing growth. Iron deficiency significantly suppressed production of GA1 and GA4, the biologically active GAs in rice. Exogenous application of GA significantly decreased leaf Fe concentration regardless of Fe supply. Iron concentration in shoot of eui1 mutants was lower than that of WT plants under both Fe-sufficient and Fe-deficient conditions. Paclobutrazol, an inhibitor of GA biosynthesis, alleviated Fe-deficiency responses, and overexpression of EUI significantly increased Fe concentration in shoots and roots. Furthermore, both exogenous application of GA and endogenous increase in GA resulting from EUI mutation inhibited Fe translocation within shoots by suppressing OsYSL2 expression, which is involved in Fe transport and translocation. Conclusions The novel findings provide compelling evidence to support the involvement of GA in mediation of Fe homeostasis in strategy II rice plants by negatively regulating Fe transport and translocation. PMID:28065924

Magnesium acceptor in gallium nitride. I. Photoluminescence from Mg-doped GaN

NASA Astrophysics Data System (ADS)

Reshchikov, M. A.; Ghimire, P.; Demchenko, D. O.

2018-05-01

Defect-related photoluminescence (PL) is analyzed in detail for n -type, p -type, and semi-insulating Mg-doped GaN grown by different techniques. The ultraviolet luminescence (UVL) band is the dominant PL band in conductive n -type and p -type GaN:Mg samples grown by hydride vapor phase epitaxy (HVPE) and molecular beam epitaxy. The UVL band in undoped and Mg-doped GaN samples is attributed to the shallow M gGa acceptor with the ionization energy of 223 meV. In semi-insulating GaN:Mg samples, very large shifts of the UVL band (up to 0.6 eV) are observed with variation of temperature or excitation intensity. The shifts are attributed to diagonal transitions, likely due to potential fluctuations or near-surface band bending. The blue luminescence (B LMg ) band is observed only in GaN:Mg samples grown by HVPE or metalorganic chemical vapor deposition when the concentration of Mg exceeds 1019c m-3 . The B LMg band is attributed to electron transitions from an unknown deep donor to the shallow M gGa acceptor. Basic properties of the observed PL are explained with a phenomenological model.

Energy band structure and electrical properties of Ga-oxide/GaN interface formed by remote oxygen plasma

NASA Astrophysics Data System (ADS)

Yamamoto, Taishi; Taoka, Noriyuki; Ohta, Akio; Truyen, Nguyen Xuan; Yamada, Hisashi; Takahashi, Tokio; Ikeda, Mitsuhisa; Makihara, Katsunori; Nakatsuka, Osamu; Shimizu, Mitsuaki; Miyazaki, Seiichi

2018-06-01

The energy band structure of a Ga-oxide/GaN structure formed by remote oxygen plasma exposure and the electrical interface properties of the GaN metal–oxide–semiconductor (MOS) capacitors with the SiO2/Ga-oxide/GaN structures with postdeposition annealing (PDA) at various temperatures have been investigated. Reflection high-energy electron diffraction and X-ray photoelectron spectroscopy clarified that the formed Ga-oxide layer is neither a single nor polycrystalline phase with high crystallinity. We found that the energy band offsets at the conduction band minimum and at the valence band maximum between the Ga-oxide layer and the GaN surface were 0.4 and 1.2 ± 0.2 eV, respectively. Furthermore, capacitance–voltage (C–V) characteristics revealed that the interface trap density (D it) is lower than the evaluation limit of Terman method without depending on the PDA temperatures, and that the SiO2/Ga-oxide stack can work as a protection layer to maintain the low D it, avoiding the significant decomposition of GaN at the high PDA temperature of 800 °C.

Compositional dependence of the band gap in Ga(NAsP) quantum well heterostructures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jandieri, K., E-mail: kakhaber.jandieri@physik.uni-marburg.de; Ludewig, P.; Wegele, T.

We present experimental and theoretical studies of the composition dependence of the direct band gap energy in Ga(NAsP)/GaP quantum well heterostructures grown on either (001) GaP- or Si-substrates. The theoretical description takes into account the band anti-crossing model for the conduction band as well as the modification of the valence subband structure due to the strain resulting from the pseudomorphic epitaxial growth on the respective substrate. The composition dependence of the direct band gap of Ga(NAsP) is obtained for a wide range of nitrogen and phosphorus contents relevant for laser applications on Si-substrate.

Modulation of transferrin receptor mRNA by transferrin-gallium in human myeloid HL60 and lymphoid CCRF-CEM leukaemic cells.

PubMed Central

Ul-Haq, R; Chitambar, C R

1993-01-01

Gallium binds to the iron transport protein transferrin (Tf), is incorporated into cells through transferrin receptors (TfR) and inhibits iron-dependent DNA synthesis. Since cellular TfR expression is tightly regulated by the availability of iron, we investigated the effects of transferrin-gallium (Tf-Ga) on TfR mRNA levels in myeloid HL60 and lymphoid CCRF-CEM cells. In HL60 cells, Tf-Ga increased TfR mRNA levels in a dose-dependent fashion. This increase in TfR mRNA was blocked by Tf-Fe and by cycloheximide. Analysis of the rate of mRNA decay in the presence of actinomycin D revealed that the half-life of TfR mRNA was increased in HL60 cells incubated with Tf-Ga. The rate of transcription of TfR mRNA was not increased by Tf-Ga. In contrast with HL60 cells, CCRF-CEM cells displayed a decrease in the level of TfR mRNA after incubation with Tf-Ga. Tf-Ga inhibited iron uptake in both HL60 and CCRF-CEM cells but increased the level of TfR mRNA only in HL60 cells, suggesting that the Tf-Ga induction of TfR mRNA was not solely due to inhibition of cellular iron uptake. At growth-inhibitory concentrations, Tf-Ga increased the TfR mRNA level in HL60 cells but decreased it in CCRF-CEM cells. Our studies suggest that in HL60 cells, gallium regulates TfR expression at the post-transcriptional level by mechanisms which require de novo protein synthesis and involve interaction with iron. The divergent effects of Tf-Ga on TfR mRNA in myeloid HL60 and lymphoid CCRF-CEM cells suggest that differences exist in the regulation of TfR expression between these two cell types. Images Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 PMID:8379943

Band offsets of non-polar A-plane GaN/AlN and AlN/GaN heterostructures measured by X-ray photoemission spectroscopy.

PubMed

Sang, Ling; Zhu, Qin Sheng; Yang, Shao Yan; Liu, Gui Peng; Li, Hui Jie; Wei, Hong Yuan; Jiao, Chun Mei; Liu, Shu Man; Wang, Zhan Guo; Zhou, Xiao Wei; Mao, Wei; Hao, Yue; Shen, Bo

2014-01-01

The band offsets of non-polar A-plane GaN/AlN and AlN/GaN heterojunctions are measured by X-ray photoemission spectroscopy. A large forward-backward asymmetry is observed in the non-polar GaN/AlN and AlN/GaN heterojunctions. The valence-band offsets in the non-polar A-plane GaN/AlN and AlN/GaN heterojunctions are determined to be 1.33 ± 0.16 and 0.73 ± 0.16 eV, respectively. The large valence-band offset difference of 0.6 eV between the non-polar GaN/AlN and AlN/GaN heterojunctions is considered to be due to piezoelectric strain effect in the non-polar heterojunction overlayers.

Photoluminescence and Band Alignment of Strained GaAsSb/GaAs QW Structures Grown by MBE on GaAs

PubMed Central

Sadofyev, Yuri G.; Samal, Nigamananda

2010-01-01

An in-depth optimization of growth conditions and investigation of optical properties including discussions on band alignment of GaAsSb/GaAs quantum well (QW) on GaAs by molecular beam epitaxy (MBE) are reported. Optimal MBE growth temperature of GaAsSb QW is found to be 470 ± 10 °C. GaAsSb/GaAs QW with Sb content ~0.36 has a weak type-II band alignment with valence band offset ratio QV ~1.06. A full width at half maximum (FWHM) of ~60 meV in room temperature (RT) photoluminescence (PL) indicates fluctuation in electrostatic potential to be less than 20 meV. Samples grown under optimal conditions do not exhibit any blue shift of peak in RT PL spectra under varying excitation.

Electrical and optical properties of Si-doped Ga2O3

NASA Astrophysics Data System (ADS)

Li, Yin; Yang, Chuanghua; Wu, Liyuan; Zhang, Ru

2017-05-01

The charge densities, band structure, density of states, dielectric functions of Si-doped β-Ga2O3 have been investigated based on the density functional theory (DFT) within the hybrid functional HSE06. The heavy doping makes conduction band split out more bands and further influences the band structure. It decreases the band gap and changes from a direct gap to an indirect gap. After doping, the top of the valence bands is mainly composed by the O-2p states, Si-3p states and Ga-4p states and the bottom of the conduction bands is almost formed by the Si-3s, Si-3p and Ga-4s orbits. The anisotropic optical properties have been investigated by means of the complex dielectric function. After the heavy Si doping, the position of absorption band edges did not change much. The slope of the absorption curve descends and indicates that the absorption became more slow for Si-doped β-Ga2O3 than undoped one due to the indirect gap of Si-doped β-Ga2O3.

Fine structure of the red luminescence band in undoped GaN

DOE Office of Scientific and Technical Information (OSTI.GOV)

Reshchikov, M. A., E-mail: mreshchi@vcu.edu; Usikov, A.; Saint-Petersburg National Research University of Information Technologies, Mechanics and Optics, 49 Kronverkskiy Ave., 197101 Saint Petersburg

2014-01-20

Many point defects in GaN responsible for broad photoluminescence (PL) bands remain unidentified. Their presence in thick GaN layers grown by hydride vapor phase epitaxy (HVPE) detrimentally affects the material quality and may hinder the use of GaN in high-power electronic devices. One of the main PL bands in HVPE-grown GaN is the red luminescence (RL) band with a maximum at 1.8 eV. We observed the fine structure of this band with a zero-phonon line (ZPL) at 2.36 eV, which may help to identify the related defect. The shift of the ZPL with excitation intensity and the temperature-related transformation of the RLmore » band fine structure indicate that the RL band is caused by transitions from a shallow donor (at low temperature) or from the conduction band (above 50 K) to an unknown deep acceptor having an energy level 1.130 eV above the valence band.« less

Crystallographic and optical properties and band diagrams of CuGaS2 and CuGa5S8 phases in Cu-poor Cu2S-Ga2S3 pseudo-binary system

NASA Astrophysics Data System (ADS)

Maeda, Tsuyoshi; Yu, Ying; Chen, Qing; Ueda, Kenta; Wada, Takahiro

2017-04-01

We synthesized Cu-poor Cu-Ga-S samples such, as CuGaS2 and CuGa5S8 with the composition of (1 - x)Cu2S-(x)Ga2S3 with 0.5 ≤ x ≤ 1.0, by a mechanochemical process and sequential heating. The crystal structure changes from tetragonal chalcopyrite-type CuGaS2 (0.5 ≤ x ≤ 0.55) to tetragonal stannite-type CuGa5S8 (x = 0.8). For samples with 0.60 ≤ x ≤ 0.75, the diffraction peaks were identified to be those of a mixed phase of the chalcopyrite- and stannite-type structures. The band-gap energies of Cu-poor Cu-Ga-S samples increase in a stepwise manner with increasing x. The band-gap energy of CuGa5S8 (x = 0.8) with the tetragonal stannite-type structure is approximately 2.66 eV, which is wider than that of chalcopyrite-type CuGaS2 (2.45 eV). The energy levels of valence band maxima (VBMs) were estimated from the ionization energies measured by photoemission yield spectroscopy (PYS). The energy levels of the VBM and conduction band minimum (CBM) of the Cu-poor Cu-Ga-S samples decrease significantly with increasing x (decreasing Cu/Ga ratio). The energy level of the VBM of CuGaS2 (-5.8 eV) is considerably deeper than those of CuInSe2 (-5.2 eV) and CuInS2 (-5.5 eV). The VBM of stannite-type CuGa5S8 with x = 0.8 (-6.4 eV) is much deeper than that of chalcopyrite-type CuGaS2 (-5.8 eV) and stannite-type CuIn3Se5 (-5.6 eV). In order to understand the band structures of chalcopyrite-type CuGaS2 and stannite-type CuGa5S8, we performed first-principles calculations using the Heyd-Scuseria-Ernzerhof (HSE06), nonlocal screened hybrid density functional method. The theoretical band-gap energy of stannite-type CuGa5S8 (2.2 eV) is wider than that of chalcopyrite-type CuGaS2 (2.0 eV). Both the theoretical and experimental band gaps of stannite-type CuGa5S8 are about 0.2 eV wider than those of chalcopyrite-type CuGaS2.

Enhanced electronic and magnetic properties by functionalization of monolayer GaS via substitutional doping and adsorption.

PubMed

Ur Rahman, Altaf; Rahman, Gul; Kratzer, Peter

2018-05-16

The structural, electronic, and magnetic properties of two-dimensional (2D) GaS are investigated using density functional theory (DFT). After confirming that the pristine 2D GaS is a non-magnetic, indirect band gap semiconductor, we consider N and F as substitutional dopants or adsorbed atoms. Except for N substituting for Ga (N Ga ), all considered cases are found to possess a magnetic moment. Fluorine, both in its atomic and molecular form, undergoes a highly exothermic reaction with GaS. Its site preference (F S or F Ga ) as substitutional dopant depends on Ga-rich or S-rich conditions. Both for F Ga and F adsorption at the Ga site, a strong F-Ga bond is formed, resulting in broken bonds within the GaS monolayer. As a result, F Ga induces p-type conductivity in GaS, whereas F S induces a dispersive, partly occupied impurity band about 0.5 e below the conduction band edge of GaS. Substitutional doping with N at both the S and the Ga site is exothermic when using N atoms, whereas only the more favourable site under the prevailing conditions can be accessed by the less reactive N 2 molecules. While N Ga induces a deep level occupied by one electron at 0.5 eV above the valence band, non-magnetic N S impurities in sufficiently high concentrations modify the band structure such that a direct transition between N-induced states becomes possible. This effect can be exploited to render monolayer GaS a direct-band gap semiconductor for optoelectronic applications. Moreover, functionalization by N or F adsorption on GaS leads to in-gap states with characteristic transition energies that can be used to tune light absorption and emission. These results suggest that GaS is a good candidate for design and construction of 2D optoelectronic and spintronics devices.

Electronic and optical properties of GaSb:N from first principles

NASA Astrophysics Data System (ADS)

Jadaun, Priyamvada; Nair, Hari; Lordi, Vincenzo; Bank, Seth; Banerjee, Sanjay

2014-03-01

We present an ab-initio study of dilute nitride III-Vs, focusing on dilute nitride GaSb (GaSb:N). GaSb:N displays promise towards realization of optoelectronic devices accessing the mid-infrared wavelength regime. Theoretical and experimental results on its electronic and optical properties are however few. To address this, we present a first principles, density functional theory study using the hybrid HSE06 exchange-correlation functional of GaSb doped with 1.6% nitrogen. We conduct a comparative study on GaAs:N, also with 1.6% nitrogen mole fraction, and find that GaSb:N has a smaller band gap and displays more band gap bowing than GaAs:N. In addition we examine the orbital character of the bands, finding the lowest conduction band to be quasi-delocalized, with a large N-3s contribution. At high concentrations, the N atoms interact via the host matrix, forming a dispersive band of their own which governs optoelectronic properties and dominates band gap bowing. While this band drives the optical and electronic properties of GaSb:N, its physics is not captured by traditional models for dilute-nitrides. We thus propose that a complete theory of dilute-nitrides should incorporate orbital character examination, especially at high N concentrations. Texas Advanced Computing Center (TACC), U.S. Department of Energy, Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344.

Gibberellins regulate iron deficiency-response by influencing iron transport and translocation in rice seedlings (Oryza sativa).

PubMed

Wang, Baolan; Wei, Haifang; Xue, Zhen; Zhang, Wen-Hao

2017-04-01

Gibberellins (GAs) are a class of plant hormones with diverse functions. However, there has been little information on the role of GAs in response to plant nutrient deficiency. To evaluate the roles of GAs in regulation of Fe homeostasis, the effects of GA on Fe accumulation and Fe translocation in rice seedlings were investigated using wild-type, a rice mutant ( eui1 ) displaying enhnaced endogenous GA concentrations due to a defect in GA deactivation, and transgenic rice plants overexpressing OsEUI . Exposure to Fe-deficient medium significantly reduced biomass of rice plants. Both exogenous application of GA and an endogenous increase of bioactive GA enhanced Fe-deficiency response by exaggerating foliar chlorosis and reducing growth. Iron deficiency significantly suppressed production of GA 1 and GA 4 , the biologically active GAs in rice. Exogenous application of GA significantly decreased leaf Fe concentration regardless of Fe supply. Iron concentration in shoot of eui1 mutants was lower than that of WT plants under both Fe-sufficient and Fe-deficient conditions. Paclobutrazol, an inhibitor of GA biosynthesis, alleviated Fe-deficiency responses, and overexpression of EUI significantly increased Fe concentration in shoots and roots. Furthermore, both exogenous application of GA and endogenous increase in GA resulting from EUI mutation inhibited Fe translocation within shoots by suppressing OsYSL2 expression, which is involved in Fe transport and translocation. The novel findings provide compelling evidence to support the involvement of GA in mediation of Fe homeostasis in strategy II rice plants by negatively regulating Fe transport and translocation. © The Author 2017. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oup.com

Optical characteristics of p-type GaAs-based semiconductors towards applications in photoemission infrared detectors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lao, Y. F.; Perera, A. G. U., E-mail: uperera@gsu.edu; Center for Nano-Optics

2016-03-14

Free-carrier effects in a p-type semiconductor including the intra-valence-band and inter-valence-band optical transitions are primarily responsible for its optical characteristics in infrared. Attention has been paid to the inter-valence-band transitions for the development of internal photoemission (IPE) mid-wave infrared (MWIR) photodetectors. The hole transition from the heavy-hole (HH) band to the spin-orbit split-off (SO) band has demonstrated potential applications for 3–5 μm detection without the need of cooling. However, the forbidden SO-HH transition at the Γ point (corresponding to a transition energy Δ{sub 0}, which is the split-off gap between the HH and SO bands) creates a sharp drop around 3.6 μmmore » in the spectral response of p-type GaAs/AlGaAs detectors. Here, we report a study on the optical characteristics of p-type GaAs-based semiconductors, including compressively strained InGaAs and GaAsSb, and a dilute magnetic semiconductor, GaMnAs. A model-independent fitting algorithm was used to derive the dielectric function from experimental reflection and transmission spectra. Results show that distinct absorption dip at Δ{sub 0} is observable in p-type InGaAs and GaAsSb, while GaMnAs displays enhanced absorption without degradation around Δ{sub 0}. This implies the promise of using GaMnAs to develop MWIR IPE detectors. Discussions on the optical characteristics correlating with the valence-band structure and free-hole effects are presented.« less

Chemical Classification of Iron Meteorites: XII. New Members of the Magmatic Groups

NASA Astrophysics Data System (ADS)

Wasson, John T.; Choi, Byeon-Gak; Jerde, Eric A.; Ulff-Møller, Finn

1998-02-01

Data are reported for thirty iron meteorites that are members of the magmatic groups, for three main group pallasites, one anomalous mesosiderite, and for three ungrouped irons and an ungrouped pallasite that are similar to IIIAB irons in their Ni, Ga, and Ge contents. The set includes four observed falls (11% of iron falls) Ban Rong Du, Chisenga, Nyaung and Sterlitamak, and Zaisho, one of two known pallasite falls. Two of the ungrouped irons (Ban Rong Du and Mount Howe 88403) and the ungrouped pallasite Yamato 8451, although having Ni, Ga, and Ge contents in the same general range as IIIAB, have very different contents of Co and exhibit significant differences for several other elements; they are clearly not related to IIIAB or to its little sister, group IIIE. A fourth ungrouped iron, Tres Castillos, chiefly differs from IIIAB in terms of its low Ga and high Ge contents; its Ga/Ge ratio is 35% higher than that of any other IIIAB iron. We report data on four new IIAB irons, all falling within established fields; the Bilibino iron is somewhat unusual, having a low Ir content (0.12 μg/g) and a structure altered by reheating. The IVA irons are also typical. One, Albion, may be a mislabeled specimen of Gibeon; another, Page City, exhibits large cracks (up to 3 cm). The Chaunskij anomalous mesosiderite has exceptionally high Ni and very low Ir concentrations. Two of three new main group pallasites are anomalous; Pecora Escarpment 91004 has an Ir content above the normal range, and Zaisho has an exceptionally high Fa content in the olivine.

High temperature and low pressure chemical vapor deposition of silicon nitride on AlGaN: Band offsets and passivation studies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Reddy, Pramod; Washiyama, Shun; Kaess, Felix

2016-04-14

In this work, we employed X-ray photoelectron spectroscopy to determine the band offsets and interface Fermi level at the heterojunction formed by stoichiometric silicon nitride deposited on Al{sub x}Ga{sub 1-x}N (of varying Al composition “x”) via low pressure chemical vapor deposition. Silicon nitride is found to form a type II staggered band alignment with AlGaN for all Al compositions (0 ≤ x ≤ 1) and present an electron barrier into AlGaN even at higher Al compositions, where E{sub g}(AlGaN) > E{sub g}(Si{sub 3}N{sub 4}). Further, no band bending is observed in AlGaN for x ≤ 0.6 and a reduced band bending (by ∼1 eV in comparison to that atmore » free surface) is observed for x > 0.6. The Fermi level in silicon nitride is found to be at 3 eV with respect to its valence band, which is likely due to silicon (≡Si{sup 0/−1}) dangling bonds. The presence of band bending for x > 0.6 is seen as a likely consequence of Fermi level alignment at Si{sub 3}N{sub 4}/AlGaN hetero-interface and not due to interface states. Photoelectron spectroscopy results are corroborated by current-voltage-temperature and capacitance-voltage measurements. A shift in the interface Fermi level (before band bending at equilibrium) from the conduction band in Si{sub 3}N{sub 4}/n-GaN to the valence band in Si{sub 3}N{sub 4}/p-GaN is observed, which strongly indicates a reduction in mid-gap interface states. Hence, stoichiometric silicon nitride is found to be a feasible passivation and dielectric insulation material for AlGaN at any composition.« less

In-situ, Gate Bias Dependent Study of Neutron Irradiation Effects on AlGaN/GaN HFETs

DTIC Science & Technology

2010-03-01

band gap and high breakdown field, AlGaN devices can operate at very high temperature and operating frequency. AlGaN/GaN based structures, have been...stable under ambient conditions [3]. GaN has a wide, direct band gap of 3.4 eV. It is therefore suitable for high temperature devices. Its high...also be grown with a wurtzite crystal structure and has a band - gap of 6.1 eV. Aluminum, due to having smaller atoms than gallium, forms a smaller

Tunnel optical radiation in In{sub x}Ga{sub 1−x}N

DOE Office of Scientific and Technical Information (OSTI.GOV)

Alexandrov, Dimiter; Skerget, Shawn

2014-02-21

An investigation of tunnel optical radiation in epitaxial layers of n-type In{sub x}Ga{sub 1−x}N grown on p-type GaN by novel plasma based migration enhanced epitaxy is presented. Experimental results of electro-luminescence spectra for In{sub x}Ga{sub 1−x}N/p−GaN hetero-junctions were obtained and they show two well expressed optical bands - one in range 500-540 nm and other in range 550-610 nm. An interesting detail is that each band begins and ends by sharp drops of the radiation, which nearly approach zero. A theoretical investigation of the unusual behavior of these spectra was done using LCAO electron band structure calculations. The optical rangesmore » of these bands show that the radiation occurs in the In{sub x}Ga{sub 1−x}N region. In fact, substitutions of In atoms in Ga sites creates defects in the structure of In{sub x}Ga{sub 1−x}N and the corresponding LCAO matrix elements are found on this basis. The LCAO electron band structures are calculated considering the interactions between nearest-neighbor orbitals. Electron energy pockets are found in both the conduction and the valence bands at the Γ point of the electron band structures. Also it is found that these pockets are separated by distances, for which there is overlapping between the electron wave functions describing localized states belonging to the pockets, and as a result tunnel optical radiation can take place. This type of electron transition - between such a pocket in the conduction band and a pocket in the valence band - occurs in In{sub x}Ga{sub 1−x}N, causing the above described optical bands. This conclusion concurs with the fact that the shapes of these bands change with change of the applied voltage.« less

The localized effect of the Bi level on the valence band in the dilute bismuth GaBixAs1-x alloy

NASA Astrophysics Data System (ADS)

Zhao, Chuan-Zhen; Zhu, Min-Min; Wang, Jun; Wang, Sha-Sha; Lu, Ke-Qing

2018-05-01

The research on the temperature dependence of the band gap energy of the dilute bismuth GaBixAs1-x alloy has been done. It is found that its temperature insensitiveness is due to the enhanced localized character of the valence band state and the small decrease of the temperature coefficient for the conduction band minimum (CBM). The enhanced localized character of the valence band state is the main factor. In order to describe the localized effect of the Bi levels on the valence band, the localized energy is introduced into the Varshni's equation. It is found that the effect of the localized Bi level on the valence band becomes strong with increasing Bi content. In addition, it is found that the pressure dependence of the band gap energy of GaBixAs1-x does not seem to be influenced by the localized Bi levels. It is due to two factors. One is that the pressure dependence of the band gap energy is mainly determined by the D CBM of GaBixAs1-x. The D CBM of GaBixAs1-x is not influenced by the localized Bi levels. The other is that the small variation of the pressure coefficient for the D valence band maximum (VBM) state of GaBixAs1-x can be cancelled by the variation of the pressure coefficient for the D CBM state of GaBixAs1-x.

Verification of band offsets and electron effective masses in GaAsN/GaAs quantum wells: Spectroscopic experiment versus 10-band k·p modeling

NASA Astrophysics Data System (ADS)

Ryczko, K.; Sek, G.; Sitarek, P.; Mika, A.; Misiewicz, J.; Langer, F.; Höfling, S.; Forchel, A.; Kamp, M.

2013-06-01

Optical transitions in GaAs1-xNx/GaAs quantum wells (QWs) have been probed by two complementary techniques, modulation spectroscopy in a form of photoreflectance and surface photovoltage spectroscopy. Transition energies in QWs of various widths and N contents have been compared with the results of band structure calculations based on the 10-band k.p Hamiltonian. Due to the observation of higher order transitions in the measured spectra, the band gap discontinuities at the GaAsN/GaAs interface and the electron effective masses could be determined, both treated as semi-free parameters to get the best matching between the theoretical and experimental energies. We have obtained the chemical conduction band offset values of 86% for x = 1.2% and 83% for x = 2.2%, respectively. For these determined band offsets, the electron effective masses equal to about 0.09 mo in QWs with 1.2% N and 0.15 mo for the case of larger N content of 2.2%.

Radiative recombination in GaN/InGaN heterojunction bipolar transistors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kao, Tsung-Ting; Lee, Yi-Che; Kim, Hee-Jin

2015-12-14

We report an electroluminescence (EL) study on npn GaN/InGaN heterojunction bipolar transistors (HBTs). Three radiative recombination paths are resolved in the HBTs, corresponding to the band-to-band transition (3.3 eV), conduction-band-to-acceptor-level transition (3.15 eV), and yellow luminescence (YL) with the emission peak at 2.2 eV. We further study possible light emission paths by operating the HBTs under different biasing conditions. The band-to-band and the conduction-band-to-acceptor-level transitions mostly arise from the intrinsic base region, while a defect-related YL band could likely originate from the quasi-neutral base region of a GaN/InGaN HBT. The I{sub B}-dependent EL intensities for these three recombination paths are discussed. The resultsmore » also show the radiative emission under the forward-active transistor mode operation is more effective than that using a diode-based emitter due to the enhanced excess electron concentration in the base region as increasing the collector current increases.« less

Determining the band alignment of TbAs:GaAs and TbAs:In 0.53Ga 0.47As

DOE PAGES

Bomberger, Cory C.; Vanderhoef, Laura R.; Rahman, Abdur; ...

2015-09-10

Here, we propose and systematically justify a band structure for TbAs nanoparticles in GaAs and In 0.53Ga 0.47As host matrices. Moreover, fluence-dependent optical-pump terahertz-probe measurements suggest the TbAs nanoparticles have a band gap and provide information on the carrier dynamics, which are determined by the band alignment. Spectrophotometry measurements provide the energy of optical transitions in the nanocomposite systems and reveal a large blue shift in the absorption energy when the host matrix is changed from In 0.53Ga 0.47As to GaAs. Finally, Hall data provides the approximate Fermi level in each system. From this data, we deduce that the TbAs:GaAsmore » system forms a type I (straddling) heterojunction and the TbAs:In 0.53Ga 0.47As system forms a type II (staggered) heterojunction.« less

Band alignment of SiO2/(AlxGa1-x)2O3 (0 ≤ x ≤ 0.49) determined by X-ray photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Feng, Zhaoqing; Feng, Qian; Zhang, Jincheng; Li, Xiang; Li, Fuguo; Huang, Lu; Chen, Hong-Yan; Lu, Hong-Liang; Hao, Yue

2018-03-01

In this work, we report the investigation of the band alignment of SiO2/(AlxGa1-x)2O3 (0 ≤ x ≤ 0.49) utilizing the high resolution X-ray photoelectron spectroscopy (XPS) measurements. The single crystallinity and orientation of β-(AlxGa1-x)2O3 films grown on sapphire by pulsed laser deposition were studied with the high resolution X-ray diffraction. The Ga 2p3/2 and Si 2p core-level spectra as well as valence band spectra were used in the analysis of band alignment. As the mole fraction x of Al increases from 0 to 0.49, the bandgap and conduction band offset values of SiO2/(AlxGa1-x)2O3 increases from 4.9 to 5.6 eV and from 1.5 to 2.1 eV, respectively, while that of valence band offset decreases from 2.2 to 0.9 eV. From the results obtained, the energy band diagram of the studied SiO2/(AlxGa1-x)2O3 (0 ≤ x ≤ 0.49) interfaces is found to be of type I. Energy band lineups of SiO2/(AlxGa1-x)2O3 were thus determined which can be used as for Ga2O3 based power device technology.

Temperature-dependent internal photoemission probe for band parameters

NASA Astrophysics Data System (ADS)

Lao, Yan-Feng; Perera, A. G. Unil

2012-11-01

The temperature-dependent characteristic of band offsets at the heterojunction interface was studied by an internal photoemission (IPE) method. In contrast to the traditional Fowler method independent of the temperature (T), this method takes into account carrier thermalization and carrier/dopant-induced band-renormalization and band-tailing effects, and thus measures the band-offset parameter at different temperatures. Despite intensive studies in the past few decades, the T dependence of this key band parameter is still not well understood. Re-examining a p-type doped GaAs emitter/undoped AlxGa1-xAs barrier heterojunction system disclosed its previously ignored T dependency in the valence-band offset, with a variation up to ˜-10-4 eV/K in order to accommodate the difference in the T-dependent band gaps between GaAs and AlGaAs. Through determining the Fermi energy level (Ef), IPE is able to distinguish the impurity (IB) and valence bands (VB) of extrinsic semiconductors. One important example is to determine Ef of dilute magnetic semiconductors such as GaMnAs, and to understand whether it is in the IB or VB.

E+ Transition in GaAs1-xNx and GaAs1-xBix Due to Isoelectronic-Impurity-Induced Perturbation of the Conduction Band

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fluegel, B.; Mascarenhas, A.; Ptak, A. J.

2007-01-01

An above-band-gap transition E{sub +} is experimentally observed in the dilute GaAs{sub 1-x}Bi{sub x} alloy. Precise measurements at very low dilutions are made of the above-band-gap transition E{sub +} that is observed in GaAs{sub 1-x}N{sub x}, making it possible to compare the behavior of the different isoelectronic traps Bi and N in the common host GaAs with respect to their perturbation to the host electronic structure. We suggest that the origin of the E{sub +} level observed in GaAs is not the isolated isoelectronic impurity level N{sub x}, as is presumed in the band-anticrossing model, but rather the isoelectronic-impurity-induced perturbationmore » of the conduction band L{sub 6}{sup c}.« less

Iron and intrinsic deep level states in Ga2O3

NASA Astrophysics Data System (ADS)

Ingebrigtsen, M. E.; Varley, J. B.; Kuznetsov, A. Yu.; Svensson, B. G.; Alfieri, G.; Mihaila, A.; Badstübner, U.; Vines, L.

2018-01-01

Using a combination of deep level transient spectroscopy, secondary ion mass spectrometry, proton irradiation, and hybrid functional calculations, we identify two similar deep levels that are associated with Fe impurities and intrinsic defects in bulk crystals and molecular beam epitaxy and hydride vapor phase epitaxi-grown epilayers of β-Ga2O3. First, our results indicate that FeGa, and not an intrinsic defect, acts as the deep acceptor responsible for the often dominating E2 level at ˜0.78 eV below the conduction band minimum. Second, by provoking additional intrinsic defect generation via proton irradiation, we identified the emergence of a new level, labeled as E2*, having the ionization energy very close to that of E2, but exhibiting an order of magnitude larger capture cross section. Importantly, the properties of E2* are found to be consistent with its intrinsic origin. As such, contradictory opinions of a long standing literature debate on either extrinsic or intrinsic origin of the deep acceptor in question converge accounting for possible contributions from E2 and E2* in different experimental conditions.

Electronic structure and optical properties of defect chalcopyrite HgGa2Se4

NASA Astrophysics Data System (ADS)

Gabrelian, B. V.; Lavrentyev, A. A.; Vu, Tuan V.; Parasyuk, O. V.; Khyzhun, O. Y.

2018-01-01

We report on studies from an experimental and theoretical viewpoint of the electronic structure of mercury digallium selenide, HgGa2Se4, a very promising optoelectronic material. In particular, the method of X-ray photoelectron spectroscopy (XPS) was used to evaluate binding energies of the constituent element core electrons and the shape of the valence band for pristine and Ar+-ion bombarded surfaces of HgGa2Se4 single crystal. First principles band-structure calculations were performed in the present work using the augmented plane wave + local orbitals (APW+lo). These calculations indicate that the Se 4p states are the main contributors at the top and in the upper portion of the valence band with slightly smaller contributions of the Ga 4p states in the upper portion of the band as well. Further, the central portion of the valence band is determined mainly by contributions of the Ga 4s states, and the Hg 5d states are the principal contributors to the bottom of the valence band. These theoretical data are in fair agreement when matching on a common energy scale of the X-ray emission bands giving information on the energy distribution of the Se 4p and Ga 4p states and the XPS valence-band spectrum of the HgGa2Se4 crystal. The principal optical constants are elucidated from the DFT calculations.

Theoretical prediction of a self-forming gallium oxide layer at an n-type GaN/SiO2 interface

NASA Astrophysics Data System (ADS)

Chokawa, Kenta; Narita, Tetsuo; Kikuta, Daigo; Kachi, Tetsu; Shiozaki, Koji; Shiraishi, Kenji

2018-03-01

We examine the energy band diagram at the n-type GaN (n-GaN)/SiO2 interface and show that electron transfer from n-GaN to SiO2 leads to the formation of negatively charged oxygen vacancies in the SiO2, resulting in the self-formation of an n-GaN/Ga2O3/SiO2 structure. On the other hand, it is difficult to automatically form Ga2O3 at a p-type GaN (p-GaN)/SiO2 interface. This electron-transfer-induced self-formation of Ga2O3 causes an interface dipole, which leads to band bending, resulting in an increase in the conduction band offset between GaN and SiO2. Accordingly, by using this self-forming phenomenon, GaN MOSFETs with lower leakage current can be realized.

Electronic Structures of Free-Standing Nanowires made from Indirect Bandgap Semiconductor Gallium Phosphide

PubMed Central

Liao, Gaohua; Luo, Ning; Chen, Ke-Qiu; Xu, H. Q.

2016-01-01

We present a theoretical study of the electronic structures of freestanding nanowires made from gallium phosphide (GaP)—a III-V semiconductor with an indirect bulk bandgap. We consider [001]-oriented GaP nanowires with square and rectangular cross sections, and [111]-oriented GaP nanowires with hexagonal cross sections. Based on tight binding models, both the band structures and wave functions of the nanowires are calculated. For the [001]-oriented GaP nanowires, the bands show anti-crossing structures, while the bands of the [111]-oriented nanowires display crossing structures. Two minima are observed in the conduction bands, while the maximum of the valence bands is always at the Γ-point. Using double group theory, we analyze the symmetry properties of the lowest conduction band states and highest valence band states of GaP nanowires with different sizes and directions. The band state wave functions of the lowest conduction bands and the highest valence bands of the nanowires are evaluated by spatial probability distributions. For practical use, we fit the confinement energies of the electrons and holes in the nanowires to obtain an empirical formula. PMID:27307081

Fine structure of Fe-Co-Ga and Fe-Cr-Ga alloys with low Ga content

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kleinerman, Nadezhda M., E-mail: kleinerman@imp.uran.ru; Serikov, Vadim V., E-mail: kleinerman@imp.uran.ru; Vershinin, Aleksandr V., E-mail: kleinerman@imp.uran.ru

2014-10-27

Investigation of Ga influence on the structure of Fe-Cr and Fe-Co alloys was performed with the use of {sup 57}Fe Mössbauer spectroscopy and X-ray diffraction methods. In the alloys of the Fe-Cr system, doping with Ga handicaps the decomposition of solid solutions, observed in the binary alloys, and increases its stability. In the alloys with Co, Ga also favors the uniformity of solid solutions. The analysis of Mössbauer experiments gives some grounds to conclude that if, owing to liquation, clusterization, or initial stages of phase separation, there exist regions enriched in iron, some amount of Ga atoms prefer to entermore » the nearest surroundings of iron atoms, thus forming binary Fe-Ga regions (or phases)« less

Effects of Exogenous Gibberellic Acid3 on Iron and Manganese Plaque Amounts and Iron and Manganese Uptake in Rice

PubMed Central

Guo, Yue; Zhu, Changhua; Gan, Lijun; Ng, Denny; Xia, Kai

2015-01-01

Gibberellins (GA) regulate various components of plant development. Iron and Mn plaque result from oxiding and hydroxiding Fe and Mn, respectively, on the roots of aquatic plant species such as rice (Oryza sativa L.). In this study, we found that exogenous gibberellic acid3 (GA3) spray decreased Fe plaque, but increased Mn plaque, with applications of Kimura B nutrient solution. Similar effects from GA3, leading to reduced Fe plaque and increased Mn plaque, were also found by scanning electron microscopy and energy dispersive X-ray spectrometric microanalysis. Reduced Fe plaque was observed after applying GA3 to the groups containing added Fe2+ (17 and 42 mg•L-1) and an increasing trend was detected in Mn plaques of the Mn2+ (34 and 84 mg•L-1) added treatments. In contrast, an inhibitor of GA3, uniconazole, reversed the effects of GA3. The uptake of Fe or Mn in rice plants was enhanced after GA3 application and Fe or Mn plaque production. Strong synergetic effects of GA3 application on Fe plaque production were detected. However, no synergetic effects on Mn plaque production were detected. PMID:25710173

High-temperature luminescence in an n-GaSb/n-InGaAsSb/p-AlGaAsSb light-emitting heterostructure with a high potential barrier

DOE Office of Scientific and Technical Information (OSTI.GOV)

Petukhov, A. A., E-mail: andrey-rus29@rambler.ru; Zhurtanov, B. E.; Kalinina, K. V.

2013-09-15

The electroluminescent properties of an n-GaSb/n-InGaAsSb/p-AlGaAsSb heterostructure with a high potential barrier in the conduction band (large conduction-band offset) at the n-GaSb/n-InGaAsSb type-II heterointerface ({Delta}E{sub c} = 0.79 eV) are studied. Two bands with peaks at 0.28 and 0.64 eV at 300 K, associated with radiative recombination in n-InGaAsSb and n-GaSb, respectively, are observed in the electroluminescence (EL) spectrum. In the entire temperature range under study, T = 290-480 K, additional electron-hole pairs are formed in the n-InGaAsSb active region by impact ionization with hot electrons heated as a result of the conduction-band offset. These pairs contribute to radiative recombination,more » which leads to a nonlinear increase in the EL intensity and output optical power with increasing pump current. A superlinear increase in the emission power of the long-wavelength band is observed upon heating in the temperature range T = 290-345 K, and a linear increase is observed at T > 345 K. This work for the first time reports an increase in the emission power of a light-emitting diode structure with increasing temperature. It is shown that this rise is caused by a decrease in the threshold energy of the impact ionization due to narrowing of the band gap of the active region.« less

Theoretical study of nitride short period superlattices

NASA Astrophysics Data System (ADS)

Gorczyca, I.; Suski, T.; Christensen, N. E.; Svane, A.

2018-02-01

Discussion of band gap behavior based on first principles calculations of electronic band structures for various short period nitride superlattices is presented. Binary superlattices, as InN/GaN and GaN/AlN as well as superlattices containing alloys, as InGaN/GaN, GaN/AlGaN, and GaN/InAlN are considered. Taking into account different crystallographic directions of growth (polar, semipolar and nonpolar) and different strain conditions (free-standing and pseudomorphic) all the factors influencing the band gap engineering are analyzed. Dependence on internal strain and lattice geometry is considered, but the main attention is devoted to the influence of the internal electric field and the hybridization of well and barrier wave functions. The contributions of these two important factors to band gap behavior are illustrated and estimated quantitatively. It appears that there are two interesting ranges of layer thicknesses; in one (few atomic monolayers in barriers and wells) the influence of the wave function hybridization is dominant, whereas in the other (layers thicker than roughly five to six monolayers) dependence of electric field on the band gaps is more important. The band gap behavior in superlattices is compared with the band gap dependence on composition in the corresponding ternary and quaternary alloys. It is shown that for superlattices it is possible to exceed by far the range of band gap values, which can be realized in ternary alloys. The calculated values of the band gaps are compared with the photoluminescence emission energies, when the corresponding data are available. Finally, similarities and differences between nitride and oxide polar superlattices are pointed out by comparison of wurtzite GaN/AlN and ZnO/MgO.

Abnormal broadening of the optical transitions in (Ga,As)N/GaAs quantum wells

NASA Astrophysics Data System (ADS)

Turcotte, S.; Beaudry, J.-N.; Masut, R. A.; Desjardins, P.; Bentoumi, G.; Leonelli, R.

2012-01-01

We have measured the near band-gap absorption of structurally well characterized GaAs1-xNx quantum wells grown on GaAs(001) with x<0.014. The spectra were reproduced by a model that includes electron-hole correlations. We find that the width of the excitonic and band-to-band optical transitions are more than twice larger than what is found in conventional III-V alloy heterostructures. This confirms the presence of strong nitrogen-configuration induced band-gap fluctuations reported previously by Bentoumi [Phys. Rev. BPRBMDO1098-012110.1103/PhysRevB.70.035315 70, 035315 (2004)] for bulk dilute GaAsN alloys.

Enhanced electronic and magnetic properties by functionalization of monolayer GaS via substitutional doping and adsorption

NASA Astrophysics Data System (ADS)

Rahman, Altaf Ur; Rahman, Gul; Kratzer, Peter

2018-05-01

The structural, electronic, and magnetic properties of two-dimensional (2D) GaS are investigated using density functional theory (DFT). After confirming that the pristine 2D GaS is a non-magnetic, indirect band gap semiconductor, we consider N and F as substitutional dopants or adsorbed atoms. Except for N substituting for Ga (NGa), all considered cases are found to possess a magnetic moment. Fluorine, both in its atomic and molecular form, undergoes a highly exothermic reaction with GaS. Its site preference (FS or FGa) as substitutional dopant depends on Ga-rich or S-rich conditions. Both for FGa and F adsorption at the Ga site, a strong F–Ga bond is formed, resulting in broken bonds within the GaS monolayer. As a result, FGa induces p-type conductivity in GaS, whereas FS induces a dispersive, partly occupied impurity band about 0.5 e below the conduction band edge of GaS. Substitutional doping with N at both the S and the Ga site is exothermic when using N atoms, whereas only the more favourable site under the prevailing conditions can be accessed by the less reactive N2 molecules. While NGa induces a deep level occupied by one electron at 0.5 eV above the valence band, non-magnetic NS impurities in sufficiently high concentrations modify the band structure such that a direct transition between N-induced states becomes possible. This effect can be exploited to render monolayer GaS a direct-band gap semiconductor for optoelectronic applications. Moreover, functionalization by N or F adsorption on GaS leads to in-gap states with characteristic transition energies that can be used to tune light absorption and emission. These results suggest that GaS is a good candidate for design and construction of 2D optoelectronic and spintronics devices.

Band offsets in ITO/Ga2O3 heterostructures

NASA Astrophysics Data System (ADS)

Carey, Patrick H.; Ren, F.; Hays, David C.; Gila, B. P.; Pearton, S. J.; Jang, Soohwan; Kuramata, Akito

2017-11-01

The valence band offsets in rf-sputtered Indium Tin Oxide (ITO)/single crystal β-Ga2O3 (ITO/Ga2O3) heterostructures were measured with X-Ray Photoelectron Spectroscopy using the Kraut method. The bandgaps of the component materials in the heterostructure were determined by Reflection Electron Energy Loss Spectroscopy as 4.6 eV for Ga2O3 and 3.5 eV for ITO. The valence band offset was determined to be -0.78 ± 0.30 eV, while the conduction band offset was determined to be -0.32 ± 0.13 eV. The ITO/Ga2O3 system has a nested gap (type I) alignment. The use of a thin layer of ITO between a metal and the Ga2O3 is an attractive approach for reducing contact resistance on Ga2O3-based power electronic devices and solar-blind photodetectors.

Epitaxial growth of MgO/Ga2O3 heterostructure and its band alignment studied by X-ray photoemission spectroscopy

NASA Astrophysics Data System (ADS)

Matsuo, Norihiro; Doko, Naoki; Yasukawa, Yukiko; Saito, Hidekazu; Yuasa, Shinji

2018-07-01

We have grown an epitaxial MgO/Ga2O3 heterostructure on a MgO(001) substrate by molecular beam epitaxy. Crystallographic studies revealed the out-of-plane and in-plane crystal orientations between the MgO overlayer and the Ga2O3 layer, which were MgO(001) ∥ β-Ga2O3(001) and MgO[100] ∥ β-Ga2O3 [02\\bar{1}], respectively. The valence band offset at the MgO/β-Ga2O3 interface was determined to be 0.19 eV (type-II band alignment) by X-ray photoelectron spectroscopy, resulting in a large conduction band offset of 2.7–3.2 eV. These results indicate that MgO is a promising potential barrier for electrons in an epitaxial MgO/Ga2O3 multilayered structure.

Photocatalysis using a Wide Range of the Visible Light Spectrum: Hydrogen Evolution from Doped AgGaS2.

PubMed

Yamato, Kohei; Iwase, Akihide; Kudo, Akihiko

2015-09-07

Doping of nickel into AgGaS2 yields a new absorption band, at a wavelength longer than the intrinsic absorption band of the AgGaS2 host. The doped nickel forms an electron donor level in a forbidden band of AgGaS2 . The nickel-doped AgGaS2 with rhodium co-catalyst shows photocatalytic activity for sacrificial H2 evolution under the light of up to 760 nm due to the transition from the electron donor level consisting of Ni(2+) to the conduction band of AgGaS2 . Apparent quantum yields for the sacrificial H2 evolution at 540-620 nm are about 1 %. Moreover, the nickel-doped AgGa0.75 In0.25 S2 also responds to near-IR light, up to 900 nm. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Photoreflectance from GaAs and GaAs/GaAs interfaces

NASA Astrophysics Data System (ADS)

Sydor, Michael; Angelo, James; Wilson, Jerome J.; Mitchel, W. C.; Yen, M. Y.

1989-10-01

Photoreflectance from semi-insulating GaAs, and GaAs/GaAs interfaces, is discussed in terms of its behavior with temperature, doping, epilayer thickness, and laser intensity. Semi-insulating substrates show an exciton-related band-edge signal below 200 K and an impurity-related photoreflectance above 400 K. At intermediate temperatures the band-edge signal from thin GaAs epilayers contains a contribution from the epilayer-substrate interface. The interface effect depends on the epilayer's thickness, doping, and carrier mobility. The effect broadens the band-edge photoreflectance by 5-10 meV, and artifically lowers the estimates for the critical-point energy, ECP, obtained through the customary third-derivative functional fit to the data.

The effects of the chemical composition and strain on the electronic properties of GaSb/InAs core-shell nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ning, Feng; Wang, Dan; Tang, Li-Ming, E-mail: lmtang@hnu.edu.cn

2014-09-07

The effects of the chemical composition and strain on the electronic properties of [111] zinc-blende (ZB) and [0001] wurtzite (WZ) GaSb/InAs core-shell nanowires (NWs) with different core diameters and shell thicknesses are studied using first-principles methods. The band structures of the [111] ZB GaSb/InAs core-shell NWs underwent a noticeable type-I/II band alignment transition, associated with a direct-to-indirect band gap transition under a compressive uniaxial strain. The band structures of the [0001] WZ GaSb/InAs core-shell NWs preserved the direct band gap under either compressive or tensile uniaxial strains. In addition, the band gaps and the effective masses of the carriers couldmore » be tuned by their composition. For the core-shell NWs with a fixed GaSb-core size, the band gaps decreased linearly with an increasing InAs-shell thickness, caused by the significant downshift of the conduction bands. For the [111] ZB GaSb/InAs core-shell NWs, the calculated effective masses indicated that the transport properties could be changed from hole-dominated conduction to electron-dominated conduction by changing the InAs-shell thickness.« less

Gallium Maltolate Disrupts Tumor Iron Metabolism and Retards the Growth of Glioblastoma by Inhibiting Mitochondrial Function and Ribonucleotide Reductase.

PubMed

Chitambar, Christopher R; Al-Gizawiy, Mona M; Alhajala, Hisham S; Pechman, Kimberly R; Wereley, Janine P; Wujek, Robert; Clark, Paul A; Kuo, John S; Antholine, William E; Schmainda, Kathleen M

2018-06-01

Gallium, a metal with antineoplastic activity, binds transferrin (Tf) and enters tumor cells via Tf receptor1 (TfR1); it disrupts iron homeostasis leading to cell death. We hypothesized that TfR1 on brain microvascular endothelial cells (BMEC) would facilitate Tf-Ga transport into the brain enabling it to target TfR-bearing glioblastoma. We show that U-87 MG and D54 glioblastoma cell lines and multiple glioblastoma stem cell (GSC) lines express TfRs, and that their growth is inhibited by gallium maltolate (GaM) in vitro After 24 hours of incubation with GaM, cells displayed a loss of mitochondrial reserve capacity followed by a dose-dependent decrease in oxygen consumption and a decrease in the activity of the iron-dependent M2 subunit of ribonucleotide reductase (RRM2). IHC staining of rat and human tumor-bearing brains showed that glioblastoma, but not normal glial cells, expressed TfR1 and RRM2, and that glioblastoma expressed greater levels of H- and L-ferritin than normal brain. In an orthotopic U-87 MG glioblastoma xenograft rat model, GaM retarded the growth of brain tumors relative to untreated control ( P = 0.0159) and reduced tumor mitotic figures ( P = 0.045). Tumors in GaM-treated animals displayed an upregulation of TfR1 expression relative to control animals, thus indicating that gallium produced tumor iron deprivation. GaM also inhibited iron uptake and upregulated TfR1 expression in U-87 MG and D54 cells in vitro We conclude that GaM enters the brain via TfR1 on BMECs and targets iron metabolism in glioblastoma in vivo, thus inhibiting tumor growth. Further development of novel gallium compounds for brain tumor treatment is warranted. Mol Cancer Ther; 17(6); 1240-50. ©2018 AACR . ©2018 American Association for Cancer Research.

Optical properties of InGaN grown by MOCVD on sapphire and on bulk GaN

NASA Astrophysics Data System (ADS)

Osinski, Marek; Eliseev, Petr G.; Lee, Jinhyun; Smagley, Vladimir A.; Sugahara, Tamoya; Sakai, Shiro

1999-11-01

Experimental data on photoluminescence of various bulk and quantum-well epitaxial InGaN/GaN structures grown by MOCVD are interpreted in terms of a band-tail model of inhomogeneously broadened radiative recombination. The anomalous temperature-induced blue spectral is shown to result from band-tail recombination under non-degenerate conditions. Significant differences are observed between epilayers grown on sapphire substrates and on GaN substrates prepared by the sublimination method, with no apparent evidence of band tails in homoepitaxial structures, indicating their higher crystalline quality.

Electron and hole photoemission detection for band offset determination of tunnel field-effect transistor heterojunctions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Wei; Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871; Zhang, Qin

2014-11-24

We report experimental methods to ascertain a complete energy band alignment of a broken-gap tunnel field-effect transistor based on an InAs/GaSb hetero-junction. By using graphene as an optically transparent electrode, both the electron and hole barrier heights at the InAs/GaSb interface can be quantified. For a Al{sub 2}O{sub 3}/InAs/GaSb layer structure, the barrier height from the top of the InAs and GaSb valence bands to the bottom of the Al{sub 2}O{sub 3} conduction band is inferred from electron emission whereas hole emissions reveal the barrier height from the top of the Al{sub 2}O{sub 3} valence band to the bottom ofmore » the InAs and GaSb conduction bands. Subsequently, the offset parameter at the broken gap InAs/GaSb interface is extracted and thus can be used to facilitate the development of predicted models of electron quantum tunneling efficiency and transistor performance.« less

Electron band bending of polar, semipolar and non-polar GaN surfaces

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bartoš, I.; Romanyuk, O., E-mail: romanyuk@fzu.cz; Houdkova, J.

2016-03-14

The magnitudes of the surface band bending have been determined by X-ray photoelectron spectroscopy for polar, semipolar, and non-polar surfaces of wurtzite GaN crystals. All surfaces have been prepared from crystalline GaN samples grown by the hydride-vapour phase epitaxy and separated from sapphire substrates. The Ga 3d core level peak shifts have been used for band bending determination. Small band bending magnitudes and also relatively small difference between the band bendings of the surfaces with opposite polarity have been found. These results point to the presence of electron surface states of different amounts and types on surfaces of different polaritymore » and confirm the important role of the electron surface states in compensation of the bound surface polarity charges in wurtzite GaN crystals.« less

Band alignment at β-(AlxGa1-x)2O3/β-Ga2O3 (100) interface fabricated by pulsed-laser deposition

NASA Astrophysics Data System (ADS)

Wakabayashi, Ryo; Hattori, Mai; Yoshimatsu, Kohei; Horiba, Koji; Kumigashira, Hiroshi; Ohtomo, Akira

2018-06-01

High-quality β-(AlxGa1-x)2O3 (x = 0-0.37) films were epitaxially grown on β-Ga2O3 (100) substrates by oxygen-radical-assisted pulsed-laser deposition with repeating alternate ablation of single crystals of β-Ga2O3 and α-Al2O3. The bandgap was tuned from 4.55 ± 0.01 eV (x = 0) to 5.20 ± 0.02 eV (x = 0.37), where bowing behavior was observed. The band alignment at the β-(AlxGa1-x)2O3/β-Ga2O3 interfaces was found to be type-I with conduction- and valence-band offsets of 0.52 ± 0.08 eV (0.37 ± 0.08 eV) and 0.13 ± 0.07 eV (0.02 ± 0.07 eV) for x = 0.37 (0.27), respectively. The large conduction-band offsets are ascribed to the dominant contribution of the cation-site substitution to the conduction band.

A Zn isotope perspective on the rise of continents.

PubMed

Pons, M-L; Fujii, T; Rosing, M; Quitté, G; Télouk, P; Albarède, F

2013-05-01

Zinc isotope abundances are fairly constant in igneous rocks and shales and are left unfractionated by hydrothermal processes at pH < 5.5. For that reason, Zn isotopes in sediments can be used to trace the changing chemistry of the hydrosphere. Here, we report Zn isotope compositions in Fe oxides from banded iron formations (BIFs) and iron formations of different ages. Zinc from early Archean samples is isotopically indistinguishable from the igneous average (δ(66) Zn ~0.3‰). At 2.9-2.7 Ga, δ(66) Zn becomes isotopically light (δ(66) Zn < 0‰) and then bounces back to values >1‰ during the ~2.35 Ga Great Oxygenation Event. By 1.8 Ga, BIF δ(66) Zn has settled to the modern value of FeMn nodules and encrustations (~0.9‰). The Zn cycle is largely controlled by two different mechanisms: Zn makes strong complexes with phosphates, and phosphates in turn are strongly adsorbed by Fe hydroxides. We therefore review the evidence that the surface geochemical cycles of Zn and P are closely related. The Zn isotope record echoes Sr isotope evidence, suggesting that erosion starts with the very large continental masses appearing at ~2.7 Ga. The lack of Zn fractionation in pre-2.9 Ga BIFs is argued to reflect the paucity of permanent subaerial continental exposure and consequently the insignificant phosphate input to the oceans and the small output of biochemical sediments. We link the early decline of δ(66) Zn between 3.0 and 2.7 Ga with the low solubility of phosphate in alkaline groundwater. The development of photosynthetic activity at the surface of the newly exposed continents increased the oxygen level in the atmosphere, which in turn triggered acid drainage and stepped up P dissolution and liberation of heavy Zn into the runoff. Zinc isotopes provide a new perspective on the rise of continents, the volume of carbonates on continents, changing weathering conditions, and compositions of the ocean through time. © 2013 Blackwell Publishing Ltd.

Valence-band offsets of CoTiSb/In0.53Ga0.47As and CoTiSb/In0.52Al0.48As heterojunctions

NASA Astrophysics Data System (ADS)

Harrington, S. D.; Sharan, A.; Rice, A. D.; Logan, J. A.; McFadden, A. P.; Pendharkar, M.; Pennachio, D. J.; Wilson, N. S.; Gui, Z.; Janotti, A.; Palmstrøm, C. J.

2017-08-01

The valence-band offsets, ΔEv, between semiconducting half-Heusler compound CoTiSb and lattice-matched III-V In0.53Ga0.47As and In0.52Al0.48As heterojunction interfaces have been measured using X-ray photoemission spectroscopy (XPS). These interfaces were formed using molecular beam epitaxy and transferred in situ for XPS measurements. Valence-band offsets of 0.30 eV and 0.58 eV were measured for CoTiSb/In0.53Ga0.47As and CoTiSb/In0.52Al0.48As, respectively. By combining these measurements with previously reported XPS ΔEv (In0.53Ga0.47As/In0.52Al0.48As) data, the results suggest that band offset transitivity is satisfied. In addition, the film growth order of the interface between CoTiSb and In0.53Ga0.47As is explored and does not seem to affect the band offsets. Finally, the band alignments of CoTiSb with GaAs, AlAs, and InAs are calculated using the density function theory with the HSE06 hybrid functional and applied to predict the band alignment of CoTiSb with In0.53Ga0.47As and In0.52Al0.48As. Good agreement is found between the calculated valence-band offsets and those determined from XPS.

Band crossing in isovalent semiconductor alloys with large size mismatch

NASA Astrophysics Data System (ADS)

Deng, Hui-Xiong; Wei, Su-Huai

2012-02-01

Mixing isovalent compounds AC with BC to form alloys A1-xBxC has been an effective way in band structure engineering to enhance the availability of material properties. In most cases, the mixed isovalent atoms A and B, such as Al and Ga in Al1-xGaxAs or As and Sb in GaAs1-xSbx are similar in their atomic sizes and chemical potentials; therefore, the physical properties of A1-xBxC change smoothly from AC to BC. However, in some cases when the chemical and size differences between the isovalent atoms A and B are large, adding a small amount of B to AC or vice versa can lead to a discontinuous change in the electronic band structure. These large size- and chemicalmismatched (LSCM) systems often show unusual and abrupt changes in the alloys' material properties, which provide great potential in material design for novel device applications. In this report, based on first-principles band-structure calculations we show that for LSCM GaAs1-xNx and GaAs1-xBix alloys at the impurity limit the N (Bi)-induced impurity level is above (below) the conduction-(valence-) band edge of GaAs. These trends reverse at high concentration, i.e., the conduction-band edge of GaAs1-xNx becomes an N-derived state and the valence-band edge of GaAs1-xBix becomes a Bi-derived state, as expected from their band characters. We show that this band crossing phenomenon cannot be described by the popular BAC model but can be naturally explained by a simple band broadening picture.

Evaluation of band alignment of α-Ga2O3/α-(Al x Ga1‑ x )2O3 heterostructures by X-ray photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Uchida, Takayuki; Jinno, Riena; Takemoto, Shu; Kaneko, Kentaro; Fujita, Shizuo

2018-04-01

The band alignment at an α-Ga2O3/α-(Al x Ga1‑ x )2O3 heterointerface, with different Al compositions (x), grown on a c-plane sapphire substrate was evaluated by X-ray photoelectron spectroscopy. The experimental results show that the heterointerface has the type-I band discontinuity with the valence band offsets of 0.090, 0.12, and 0.14 eV, and the conduction band offsets of 0.34, 0.79, and 1.87 eV, for x values of 0.1, 0.4, and 0.8, respectively. The small band offset for the valence band is attributed to the fact that the valence band of oxides is constituted by the localized O 2p level, which is dominated by the nature of oxygen atoms. The type-I band discontinuity is desirable for a variety of heterostructure devices.

Identification of point defects in HVPE-grown GaN by steady-state and time-resolved photoluminescence

NASA Astrophysics Data System (ADS)

Reshchikov, M. A.; Demchenko, D. O.; Usikov, A.; Helava, H.; Makarov, Yu.

2015-03-01

We have investigated point defects in GaN grown by HVPE by using steady-state and time-resolved photoluminescence (PL). Among the most common PL bands in this material are the red luminescence band with a maximum at 1.8 eV and a zero-phonon line (ZPL) at 2.36 eV (attributed to an unknown acceptor having an energy level 1.130 eV above the valence band), the blue luminescence band with a maximum at 2.9 eV (attributed to ZnGa), and the ultraviolet luminescence band with the main peak at 3.27 eV (related to an unknown shallow acceptor). In GaN with the highest quality, the dominant defect-related PL band at high excitation intensity is the green luminescence band with a maximum at about 2.4 eV. We attribute this band to transitions of electrons from the conduction band to the 0/+ level of the isolated CN defect. The yellow luminescence (YL) band, related to transitions via the -/0 level of the same defect, has a maximum at 2.1 eV. Another yellow luminescence band, which has similar shape but peaks at about 2.2 eV, is observed in less pure GaN samples and is attributed to the CNON complex. In semi-insulating GaN, the GL2 band with a maximum at 2.35 eV (attributed to VN) and the BL2 band with a maximum at 3.0 eV and the ZPL at 3.33 eV (attributed to a defect complex involving hydrogen) are observed. We also conclude that the gallium vacancy-related defects act as centers of nonradiative recombination.

Cu isotopes in marine black shales record the Great Oxidation Event

PubMed Central

Rodríguez, Nathalie P.; Partin, Camille A.; Andersson, Per; Weiss, Dominik J.; El Albani, Abderrazak; Rodushkin, Ilia; Konhauser, Kurt O.

2016-01-01

The oxygenation of the atmosphere ∼2.45–2.32 billion years ago (Ga) is one of the most significant geological events to have affected Earth’s redox history. Our understanding of the timing and processes surrounding this key transition is largely dependent on the development of redox-sensitive proxies, many of which remain unexplored. Here we report a shift from negative to positive copper isotopic compositions (δ65CuERM-AE633) in organic carbon-rich shales spanning the period 2.66–2.08 Ga. We suggest that, before 2.3 Ga, a muted oxidative supply of weathering-derived copper enriched in 65Cu, along with the preferential removal of 65Cu by iron oxides, left seawater and marine biomass depleted in 65Cu but enriched in 63Cu. As banded iron formation deposition waned and continentally sourced Cu became more important, biomass sampled a dissolved Cu reservoir that was progressively less fractionated relative to the continental pool. This evolution toward heavy δ65Cu values coincides with a shift to negative sedimentary δ56Fe values and increased marine sulfate after the Great Oxidation Event (GOE), and is traceable through Phanerozoic shales to modern marine settings, where marine dissolved and sedimentary δ65Cu values are universally positive. Our finding of an important shift in sedimentary Cu isotope compositions across the GOE provides new insights into the Precambrian marine cycling of this critical micronutrient, and demonstrates the proxy potential for sedimentary Cu isotope compositions in the study of biogeochemical cycles and oceanic redox balance in the past. PMID:27091980

Multi-shell spherical GaAs /AlxGa1-x As quantum dot shells-size distribution as a mechanism to generate intermediate band energy levels

NASA Astrophysics Data System (ADS)

Rodríguez-Magdaleno, K. A.; Pérez-Álvarez, R.; Martínez-Orozco, J. C.; Pernas-Salomón, R.

2017-04-01

In this work the generation of an intermediate band of energy levels from multi-shell spherical GaAs /AlxGa1-x As quantum dot shells-size distribution is reported. Within the effective mass approximation the electronic structure of a GaAs spherical quantum-dot surrounded by one, two and three shells is studied in detail using a numerically stable transfer matrix method. We found that a shells-size distribution characterized by continuously wider GaAs domains is a suitable mechanism to generate the intermediate band whose width is also dependent on the Aluminium concentration x. Our results suggest that this effective mechanism can be used for the design of wider intermediate band than reported in other quantum systems with possible solar cells enhanced performance.

Investigation on the interfacial chemical state and band alignment for the sputtering-deposited CaF2/p-GaN heterojunction by angle-resolved X-ray photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Zhang, Kexiong; Liao, Meiyong; Sumiya, Masatomo; Koide, Yasuo; Sang, Liwen

2016-11-01

The interfacial chemical state and the band alignment of the sputtering-deposited CaF2/p-GaN hetero-structure were investigated by angle-resolved X-ray photoelectron spectroscopy. The dependence of Ga 3p core-level positions on the collection angles proves that the downward band bending of p-GaN is reduced from 1.51 to 0.85 eV after the deposition of CaF2, which may be due to the reduction of Mg-Ga-O-related interface states by the oxygen-free deposition of CaF2. The band gap of sputtering-deposited CaF2 is estimated to be about 7.97 eV with a potential gradient of 0.48 eV obtained by the variation of the Ca 2p3/2 position on different collection angles. By taking into account the p-GaN surface band bending and potential gradient in the CaF2 layer, large valence and conduction band offsets of 2.66 ± 0.20 and 1.92 ± 0.20 eV between CaF2 and p-GaN are obtained. These results indicate that CaF2 is a promising gate dielectric layer on the p-GaN for the application of metal-insulator-semiconductor devices.

Electronic structure and optical properties of noncentrosymmetric LiGaGe2Se6, a promising nonlinear optical material

NASA Astrophysics Data System (ADS)

Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Ananchenko, L. N.; Isaenko, L. I.; Yelisseyev, A.; Krinitsin, P. G.; Khyzhun, O. Y.

2016-11-01

X-ray photoelectron core-level and valence-band spectra are measured for pristine and Ar+ ion-bombarded surfaces of LiGaGe2Se6 single crystal grown by Bridgman-Stockbarger technique. Further, electronic structure of LiGaGe2Se6 is elucidated from both theoretical and experimental viewpoints. Density functional theory (DFT) calculations are made using the augmented plane wave +local orbitals (APW+lo) method to study total and partial densities of states in the LiGaGe2Se6 compound. The present calculations indicate that the principal contributors to the valence band are the Se 4p states: they contribute mainly at the top and in the central portion of the valence band of LiGaGe2Se6, with also their significant contributions in its lower portion. The Ge 4s and Ge 4p states are among other significant contributors to the valence band of LiGaGe2Se6, contributing mainly at the bottom and in the central portion, respectively. In addition, the calculations indicate that the bottom of the conduction band is composed mainly from the unoccupied Ge s and Se p states. The present DFT calculations are supported experimentally by comparison on a common energy scale of the X-ray emission bands representing the energy distribution of the 4p states associated with Ga, Ge and Se and the XPS valence-band spectrum of the LiGaGe2Se6 single crystal. The main optical characteristics of the LiGaGe2Se6 compound are elucidated by the first-principles calculations.

Spectrally resolved localized states in GaAs 1– xBi x

DOE PAGES

Christian, Theresa M.; Alberi, Kirstin; Beaton, Daniel A.; ...

2017-02-01

In this study, the role of localized states and their influence on the broader band structure remains a crucial question in understanding the band structure evolution in GaAs 1-xBi x. Here in this work, we present clear spectroscopic observations of recombination at several localized states in GaAs 1-xBi x. Sharp and recognizable photoluminescence features appear in multiple samples and redshift as a function of GaBi fraction between x = 0.16% and 0.4% at a linearized rate of 34 meV per % Bi, weaker than the redshift associated with band-to-band recombination. Interpreting these results in terms of radiative recombination between localizedmore » holes and free electrons sheds light on the relative movement of the conduction band minimum and the characteristics of localized bismuth-related trap states in GaAs 1-xBi x alloys.« less

Luminescence from defects in GaN

NASA Astrophysics Data System (ADS)

Reshchikov, M. A.; Morkoç, H.

2006-04-01

We briefly review the luminescence properties of defects in GaN and focus on the most interesting defects. In particular, the blue luminescence band peaking at about 3 eV is assigned to different defects and even different types of transitions in undoped, Zn-, C-, and Mg-doped GaN. Another omnipresent luminescence band, the yellow luminescence band may have different origin in nearly dislocation-free freestanding GaN templates, undoped thin layers, and carbon-doped GaN. The Y4 and Y7 lines are caused by recombination at unidentified point defects captured by threading edge dislocations.

Antibonding Holes Induce Good Thermoelectric Properties of p-type Ca5Ga2As6

NASA Astrophysics Data System (ADS)

Yu, Qingxiu; Wang, Yuan Xu; Shao, Hehong

2017-07-01

The arrangement of anionic tetrahedra in Zintl compounds plays a key role in determining their thermoelectric properties. We manifest this idea by investigating the crystal structure, electronic structure, and thermoelectric properties of the Zintl compounds Ca3GaAs3 and Ca5Ga2As6. By comparing various properties of Ca3GaAs3 and Ca5Ga2As6, we found that with decreasing calcium content from Ca3GaAs3 to Ca5Ga2As6, the two adjacent covalent chains formed by GaAs4 tetrahedra are connected by As-As bonds. In Ca5Ga2As6, the appearance of such As-As bonds not only supports the charge balance but also provides two nearly degenerate bands at the top of its valence bands. These two bands determine the thermoelectric behavior of p-type Ca5Ga2As6. The calculated band-decomposed charge density shows that the two bands have a π* antibonding feature of the As pz orbital. Our calculations also reveal that the formation or non-formation of As-As bonds plays an important role in the difference in the thermoelectric properties between Ca3GaAs3 and Ca5Ga2As6. The optimal carrier concentration for achieving the highest thermoelectric performance was explored by calculating the trends in their thermoelectric properties with the carrier concentration. Our work may stimulate further experimental and theoretical work to increase understanding of Zintl chemistry and improve the thermoelectric performance of Zintl compounds.

Origin of band gap bowing in dilute GaAs1-xNx and GaP1-xNx alloys: A real-space view

NASA Astrophysics Data System (ADS)

Virkkala, Ville; Havu, Ville; Tuomisto, Filip; Puska, Martti J.

2013-07-01

The origin of the band gap bowing in dilute nitrogen doped gallium based III-V semiconductors is largely debated. In this paper we show the dilute GaAs1-xNx and GaP1-xNx as representative examples that the nitrogen-induced states close to the conduction band minimum propagate along the zigzag chains on the {110} planes. Thereby states originating from different N atoms interact with each other resulting in broadening of the nitrogen-induced states which narrows the band gap. Our modeling based on ab initio theoretical calculations explains the experimentally observed N concentration dependent band gap narrowing both qualitatively and quantitatively.

Influence of Bi-related impurity states on the bandgap and spin-orbit splitting energy of dilute III-V-Bi alloys: InP1-xBix, InAs1-xBix, InSb1-xBix and GaSb1-xBix

NASA Astrophysics Data System (ADS)

Samajdar, D. P.; Dhar, S.

2016-01-01

Valence Band Anticrossing (VBAC) Model is used to calculate the changes in band structure of Bi containing alloys such as InP1-xBix, InAs1-xBix, InSb1-xBix and GaSb1-xBix due to the incorporation of dilute concentrations of bismuth. The coupling parameter CBi which gives the magnitude of interaction of Bi impurity states with the LH, HH and SO sub bands in VBAC depends on the increase in the HH/LH related energy level EHH/LH+, location of the Bi related impurity level EBi and valence band offset ΔEVBM between the endpoint compounds in the corresponding III-V-Bi. The reduction in band gap as well as the enhancement of the spin-orbit splitting energy is well explained using this model and the calculated results are compared with the results of Virtual Crystal Approximation (VCA) and Density Functional Theory (DFT) calculations, as well as with the available experimental data and are found to have good agreement. The incorporation of Bi mainly perturbs the valence band due to the interaction of the Bi impurity states with the HH, LH and SO bands. The lowering of the conduction band minimum (CBM) due to VCA is added with the upward movement of the HH/LH bands to get the total reduction in band gap for the bismides. The valence band shifts of 31.9, 32.5, 20.8 and 12.4 meV/at%Bi for InP1-xBix, InAs1-xBix, InSb1-xBix and GaSb1-xBix respectively constitute 65, 76, 59 and 31% of the total band gap reduction and the rest is the contribution of the conduction band shift. The spin-orbit splitting energy also shows significant increase with the maximum change in InPBi and the minimum in InSbBi. The same is true for Ga containing bismides if we make a comparison with the available values for GaAsBi and GaPBi with that of GaSbBi. It has also been observed that the increase in splitting energy is greater in case of the bismides such as InAsBi, InPBi and GaAsBi than the bismides such as InSbBi and GaSbBi with the parent substrates having higher values of splitting energy. This may be due to the proximity of the Bi related impurity level EBi with the SO bands of InAs, InP and GaAs.

Direct Band Gap Wurtzite Gallium Phosphide Nanowires

PubMed Central

2013-01-01

The main challenge for light-emitting diodes is to increase the efficiency in the green part of the spectrum. Gallium phosphide (GaP) with the normal cubic crystal structure has an indirect band gap, which severely limits the green emission efficiency. Band structure calculations have predicted a direct band gap for wurtzite GaP. Here, we report the fabrication of GaP nanowires with pure hexagonal crystal structure and demonstrate the direct nature of the band gap. We observe strong photoluminescence at a wavelength of 594 nm with short lifetime, typical for a direct band gap. Furthermore, by incorporation of aluminum or arsenic in the GaP nanowires, the emitted wavelength is tuned across an important range of the visible light spectrum (555–690 nm). This approach of crystal structure engineering enables new pathways to tailor materials properties enhancing the functionality. PMID:23464761

Conduction Band-Edge Non-Parabolicity Effects on Impurity States in (In,Ga)N/GaN Cylindrical QWWs

NASA Astrophysics Data System (ADS)

Haddou El, Ghazi; Anouar, Jorio

2014-02-01

In this paper, the conduction band-edge non-parabolicity (NP) and the circular cross-section radius effects on hydrogenic shallow-donor impurity ground-state binding energy in zinc-blende (ZB) InGaN/GaN cylindrical QWWs are reported. The finite potential barrier between (In,Ga)N well and GaN environment is considered. Two models of the conduction band-edge non-parabolicity are taking into account. The variational approach is used within the framework of single band effective-mass approximation with one-parametric 1S-hydrogenic trial wave-function. It is found that NP effect is more pronounced in the wire of radius equal to effective Bohr radius than in large and narrow wires. Moreover, the binding energy peak shifts to narrow wire under NP effect. A good agreement is shown compared to the findings results.

Cyclotron resonance of dirac fermions in InAs/GaSb/InAs quantum wells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Krishtopenko, S. S.; Ikonnikov, A. V., E-mail: antikon@ipmras.ru; Maremyanin, K. V.

2017-01-15

The band structure of three-layer symmetric InAs/GaSb/InAs quantum wells confined between AlSb barriers is analyzed theoretically. It is shown that, depending on the thicknesses of the InAs and GaSb layers, a normal band structure, a gapless state with a Dirac cone at the center of the Brillouin zone, or inverted band structure (two-dimensional topological insulator) can be realized in this system. Measurements of the cyclotron resonance in structures with gapless band spectra carried out for different electron concentrations confirm the existence of massless Dirac fermions in InAs/GaSb/InAs quantum wells.

Valence-band offsets of CoTiSb/In 0.53Ga 0.47As and CoTiSb/In 0.52Al 0.48As heterojunctions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Harrington, S. D.; Sharan, A.; Rice, A. D.

2017-08-11

The valence-band offsets, ΔE v, between semiconducting half-Heusler compound CoTiSb and lattice-matched III-V In 0.53Ga 0.47As and In 0.52Al 0.48As heterojunction interfaces have been measured using X-ray photoemission spectroscopy (XPS). These interfaces were formed using molecular beam epitaxy and transferred in situ for XPS measurements. Valence-band offsets of 0.30 eV and 0.58 eV were measured for CoTiSb/In 0.53Ga 0.47As and CoTiSb/In 0.52Al 0.48As, respectively. By combining these measurements with previously reported XPS ΔE v (In 0.53Ga 0.47As/In 0.52Al 0.48As) data, the results suggest that band offset transitivity is satisfied. In addition, the film growth order of the interface between CoTiSbmore » and In 0.53Ga 0.47As is explored and does not seem to affect the band offsets. Finally, the band alignments of CoTiSb with GaAs, AlAs, and InAs are calculated using the density function theory with the HSE06 hybrid functional and applied to predict the band alignment of CoTiSb with In 0.53Ga 0.47As and In 0.52Al 0.48As. As a result, good agreement is found between the calculated valence-band offsets and those determined from XPS.« less

Probing the magnetic field dependence of the light hole transition in GaAs/AlGaAs quantum wells using optically pumped NMR

NASA Astrophysics Data System (ADS)

Willmering, Matthew M.; Sesti, Erika L.; Hayes, Sophia E.; Wood, Ryan M.; Bowers, Clifford R.; Thapa, Sunil K.; Stanton, Christopher J.; Reyes, Arneil P.; Kuhns, Philip; McGill, Stephen

2018-02-01

Optically pumped NMR (OPNMR) of the NMR-active Ga/7169 species has been shown to be a unique method to probe electronic energy bands in GaAs, with sensitivity to the light hole-to-conduction band transition. This transition is often obscured in other optical measurements such as magnetoabsorption. Using OPNMR, we exploit the hyperfine interaction between conduction band electrons (and their spin states) and nuclear spins, which are detected through phase-sensitive radio-frequency (NMR) spectroscopy. Measurements were made over a range of external magnetic fields (B0) in two different labs with separate experimental setups to obtain the magnetic field dependence of the light hole-to-conduction band transition energy. In addition, k .p theory was used to interpret the experimental results, mapping out this specific transition's magnetic field dependence in an AlGaAs/GaAs quantum well. The combination of theory and experiment point to a mixing of valence bands at a field of approximately B0=4.7 T, swapping the dominant character of the absorption transition and, thus, explaining the magnetic field dependence. Lastly, the experimental dependence of the light hole-to-conduction band transition energy on B0 is found to be less steep compared to the calculated trend, indicating that inclusion of additional effects may be necessary to accurately model the spin-split band structure. The additional insight gained by Ga/7169 OPNMR about the light hole states will facilitate future testing of more complex band structure models.

Effect of varying Ga content in ZnO:GaN solid solution synthesized by solution combustion technique for photocatalytic applications

NASA Astrophysics Data System (ADS)

Menon, Sumithra Sivadas; Janani, R.; Baskar, K.; Gupta, Bhavana; Singh, Shubra

2017-05-01

ZnO:GaN (oxy)nitride solid solution has been established as the most efficient non-oxide photocatalyst for water splitting under visible irradiation with one step photoexcitation and also boasts a band gap tunability from 2.8 eV to 2.5 eV[1]. The solid solution of GaN in ZnO is formed by the intersubstitution of few of Zn/O ions by Ga/N ions, and this results in the introduction of new defect levels above the valence band which narrows the effective band gap enabling activity under visible region of spectra. In this work, we report the synthesis of ZnO:GaN solid solution by a solution combustion technique where metal nitrates and urea are used as precursors. The Zn/Ga ratio was varied from 16 to 1 in the precursors. The as synthesized samples were characterized as phase pure by X-ray diffraction, where the wurtzite structure was retained up to Zn/Ga ratio of 5. The Diffuse reflectance spectroscopy studies revealed that as the Ga content in the solid solution increases there is a reduction in band gap, from 2.9 eV to 2.4 eV. The reduced band gap of the samples facilitates its photocatalytic activity under visible region of the spectra as evaluated by photoelectrochemical measurements.

MOVPE of GaSb/InGaAsSb Multilayers and Fabrication of Dual Band Photodetectors

NASA Technical Reports Server (NTRS)

Xiao, Ye-Gao; Bhat, Ishwara; Refaat, Tamer F.; Abedin, M. Nurul; Shao, Qing-Hui

2005-01-01

Metalorganic vapor phase epitaxy (MOVPE) of GaSb/InGaAsSb multilayer thin films and fabrication of bias-selectable dual band photodetectors are reported. For the dual band photodetectors the short wavelength detector, or the upper p- GaSb/n-GaSb junction photodiode, is placed optically ahead of the long wavelength one, or the lower photodiode. The latter is based on latticed-matched In0.13Ga0.87As0.11Sb0.89 with bandgap near 0.6 eV. Specifically, high quality multilayer thin films are grown sequentially from top to bottom as p+-GaSb/p-GaSb/n-GaSb/n-InGaAsSb/p-InGaAsSb/p-GaSb on undoped p-type GaSb substrate, and as n-GaSb/p-GaSb/p-InGaAsSb/n-InGaAsSb/n-GaSb on Te-doped n-type GaSb substrate respectively. The multilayer thin films are characterized by optical microscope, atomic force microscope (AFM), electron microprobe analyses etc. The photodiode mesa steps are patterned by photolithography with wet chemical etching and the front metallization is carried out by e-beam evaporation with Pd/Ge/Au/Ti/Au to give ohmic contact on both n- and p-type Sb based layer surfaces. Dark I-V measurements show typical diode behavior for both the upper and lower photodiodes. The photoresponsivity measurements indicate that both the upper and lower photodiodes can sense the infrared illumination corresponding to their cutoff wavelengths respectively, comparable with the simulation results. More work is underway to bring the long wavelength band to the medium infrared wavelength region near 4 micrometers.

Interband emission energy in a dilute nitride quaternary semiconductor quantum dot for longer wavelength applications

NASA Astrophysics Data System (ADS)

Mageshwari, P. Uma; Peter, A. John; Lee, Chang Woo; Duque, C. A.

2016-07-01

Excitonic properties are studied in a strained Ga1-xInxNyAs1-y/GaAs cylindrical quantum dot. The optimum condition for the desired band alignment for emitting wavelength 1.55 μm is investigated using band anticrossing model and the model solid theory. The band gap and the band discontinuities of a Ga1-xInxNyAs1-y/GaAs quantum dot on GaAs are computed with the geometrical confinement effect. The binding energy of the exciton, the oscillator strength and its radiative life time for the optimum condition are found taking into account the spatial confinement effect. The effects of geometrical confinement and the nitrogen incorporation on the interband emission energy are brought out. The result shows that the desired band alignment for emitting wavelength 1.55 μm is achieved for the inclusion of alloy contents, y=0.0554% and x=0.339% in Ga1-xInxNyAs1-y/GaAs quantum dot. And the incorporation of nitrogen and indium shows the red-shift and the geometrical confinement shows the blue-shift. And it can be applied for fibre optical communication networks.

Electronic Band Structure Tuning of Highly-Mismatched-Alloys for Energy Conversion Applications

NASA Astrophysics Data System (ADS)

Ting, Min

Highly-mismatched alloys: ZnO1-xTe x and GaN1-xSb x are discussed within the context of finding the suitable material for a cost-effective Si-based tandem solar cell (SBTSC). SBTSC is an attractive concept for breaking through the energy conversion efficiency theoretical limit of a single junction solar cell. Combining with a material of 1.8 eV band gap, SBTSC can theoretically achieve energy conversion efficiency > 45%. ZnO and GaN are wide band gap semiconductors. Alloying Te in ZnO and alloying Sb in GaN result in large band gap reduction to < 2 eV from 3.3 eV and 3.4 eV respectively. The band gap reduction is majorly achieved by the upward shift of valence band (VB). Incorporating Te in ZnO modifies the VB of ZnO through the valence-band anticrossing (VBAC) interaction between localized Te states and ZnO VB delocalized states, which forms a Te-derived VB at 1 eV above the host VB. Similar band structure modification is resulted from alloying Sb in GaN. Zn1-xTex and GaN 1-xSbx thin films are synthesized across the whole composition range by pulsed laser deposition (PLD) and low temperature molecular beam epitaxy (LT-MBE) respectively. The electronic band edges of these alloys are measured by synchrotron X-ray absorption, emission, and the X-ray photoelectron spectroscopies. Modeling the optical absorption coefficient with the band anticrossing (BAC) model revealed that the Te and Sb defect levels to be at 0.99 eV and 1.2 eV above the VB of ZnO and GaN respectively. Electrically, Zn1-xTex is readily n-type conductive and GaN1-xSbx is strongly p-type conductive. A heterojunction device of p-type GaN 0.93Sb0.07 with n-type ZnO0.77Te0.93 upper cell (band gap at 1.8 eV) on Si bottom cell is proposed as a promising SBTSC device.

Luminescence and luminescence quenching in Gd3(Ga,Al)5O12 scintillators doped with Ce3+.

PubMed

Ogiegło, Joanna M; Katelnikovas, Arturas; Zych, Aleksander; Jüstel, Thomas; Meijerink, Andries; Ronda, Cees R

2013-03-28

The optical properties of gadolinium gallium aluminum garnet, Gd3(Ga,Al)5O12, doped with Ce(3+) are investigated as a function of the Ga/Al ratio, aimed at an improved understanding of the energy flow and luminescence quenching in these materials. A decrease of both the crystal field strength and band gap with increasing content of Ga(3+) is observed and explained by the geometrical influence of Ga(3+) on the crystal field splitting of the 5d level in line with theoretical work of Muñoz-García et al. ( uñoz-García, A. B.; Seijo, L. Phys. Rev. B 2010, 82, 184118 ). Thermal quenching results in shorter decay times as well as reduced emission intensities for all samples in the temperature range from 100 to 500 K. An activation energy for emission quenching is calculated from the data. The band gap of the host is measured upon Ga substitution and the decrease in band gap is related to Ga(3+) substitution into tetrahedral sites after all octahedral sites are occupied in the garnet material. Based on the change in band gap and crystal field splitting, band diagrams can be constructed explaining the low thermal quenching temperatures in the samples with high Ga content. The highest luminescence intensity is found for Gd3(Ga,Al)5O12 with 40% of Al(3+) replaced by Ga(3+).

(abstract) Transmission Electron Microscopy of Al(sub x)Ga(sub 1-x)N/SiC Multilayer Structures Grown on Sapphire Substrates

NASA Technical Reports Server (NTRS)

Pike, W. T.; George, T.; Khan, M. A.; Kuznia, J. N.

1994-01-01

The potential of wide-band-gap III-V nitrides as ultraviolet sensors and light emitters has prompted an increasing amount of work recently, including the fabrication of the first UV sensors from as-deposited single crystal GaN. We have used high resolution transmission electron microscopy (TEM) to study the microstructure of two novel developments of wide-band-gap III-V nitrides: the growth of ultra-short period GaN/AlN superlattices; and the incorporation of SiC layers into Al(sub x)Ga(sub 1-x)N structures. By varying the relative periods in a GaN/AlN superlattice, the band gap of the composite can be tailored to lie between the elemental values of 365 nm for GaN and 200 nm for AlN. The group IV semiconductor, SiC, has a wide band-gap and has a close lattice match (less than 3 %) to Al(sub x)Ga(sub 1-x)N for growth on the basal plane. Demonstration of epitaxial growth for Al(sub x)Ga(sub 1-x)N/SiC multilayers would introduce a wide band-gap analog to the already existing family of III-V and Si(sub 1-x)Ge(sub x) heteroepitaxial growth systems. Although good quality growth of GaN on SiC substrates has been demonstrated, Al(sub x)Ga(sub 1-x)N/SiC multilayer structures have never been grown and the interfacial structure is unknown.

Contactless electroreflectance study of the Fermi level pinning on GaSb surface in n-type and p-type GaSb Van Hoof structures

NASA Astrophysics Data System (ADS)

Kudrawiec, R.; Nair, H. P.; Latkowska, M.; Misiewicz, J.; Bank, S. R.; Walukiewicz, W.

2012-12-01

Contactless electroreflectance (CER) has been applied to study the Fermi-level position on GaSb surface in n-type and p-type GaSb Van Hoof structures. CER resonances, followed by strong Franz-Keldysh oscillation of various periods, were clearly observed for two series of structures. This period was much wider (i.e., the built-in electric field was much larger) for n-type structures, indicating that the GaSb surface Fermi level pinning position is closer to the valence-band than the conduction-band. From analysis of the built-in electric fields in undoped GaSb layers, it was concluded that on GaSb surface the Fermi-level is located ˜0.2 eV above the valence band.

Mechanism of radiative recombination in acceptor-doped bulk GaN crystals

NASA Astrophysics Data System (ADS)

Godlewski, M.; Suski, T.; Grzegory, I.; Porowski, S.; Bergman, J. P.; Chen, W. M.; Monemar, B.

1999-12-01

Optical and electrical properties of acceptor-doped bulk GaN crystals are discussed. Though introducing Zn and Ca to bulk GaN does not significantly change electron concentration, it results in the appearance of a blue photoluminescence band accompanying the relatively strong yellow band usually present. Highly resistive GaN : Mg crystals are obtained when high amount of Mg is introduced to the Ga melt during high-pressure synthesis. Change of electrical properties of Mg-doped bulk crystals is accompanied by the appearance of a strong blue emission of GaN similar to that in Ca- and Zn-doped crystals. Optically detected magnetic resonance investigations indicate a multi-band character of this blue emission and suggest possible mechanism of compensation in acceptor-doped bulk GaN.

Performance analysis of high efficiency InxGa1-xN/GaN intermediate band quantum dot solar cells

NASA Astrophysics Data System (ADS)

Chowdhury, Injamam Ul Islam; Sarker, Jith; Shifat, A. S. M. Zadid; Shuvro, Rezoan A.; Mitul, Abu Farzan

2018-06-01

In this subsistent fifth generation era, InxGa1-xN/GaN based materials have played an imperious role and become promising contestant in the modernistic fabrication technology because of some of their noteworthy attributes. On our way of illustrating the performance, the structure of InxGa1-xN/GaN quantum dot (QD) intermediate band solar cell (IBSC) is investigated by solving the Schrödinger equation in light of the Kronig-Penney model. In comparison with p-n homojunction and heterojunction solar cells, InxGa1-xN/GaN IBQD solar cell manifests larger power conversion efficiency (PCE). PCE strongly depends on position and width of the intermediate bands (IB). Position of IBs can be controlled by tuning the size of QDs and the Indium content of InxGa1-xN whereas, width of IB can be controlled by tuning the interdot distance. PCE can also be controlled by tuning the position of fermi energy bands as well as changing the doping concentration. In this work, maximum conversion efficiency is found approximately 63.2% for a certain QD size, interdot distance, Indium content and doping concentration.

Photovoltaic Properties of Selenized CuGa/In Films with Varied Compositions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Muzzillo, Christopher P.; Mansfield, Lorelle M.; Ramanathan, Kannan

2016-11-21

Thin CuGa/In films with varied compositions were deposited by co-evaporation and then selenized in situ with evaporated selenium. The selenized Cu(In, Ga)Se2 absorbers were used to fabricate 390 solar cells. Cu/(Ga+In) and Ga/(Ga+In) (Cu/III and Ga/III) were independently varied, and photovoltaic performance was optimal at Cu/III of 77-92% for all Ga/III compositions studied (Ga/III ~ 30, 50, and 70%). The best absorbers at each Ga/III composition were characterized with time-resolved photoluminescence, scanning electron microscopy, and secondary ion mass spectrometry, and devices were studied with temperature-dependent current density-voltage, light and electrical biased quantum efficiency, and capacitance-voltage. The best cells with Ga/IIImore » ~ 30, 50, and 70% had efficiencies of 14.5, 14.4, and 12.2% and maximum power temperature coefficients of -0.496, -0.452, and -0.413%/degrees C, respectively. This resulted in the Ga/III ~ 50% champion having the highest efficiency at temperatures greater than 40 degrees C, making it the optimal composition for practical purposes. This optimum is understood as a result of the absorber's band gap grading- where minimum band gap dominates short-circuit current density, maximum space charge region band gap dominates open-circuit voltage, and average absorber band gap dominates maximum power temperature coefficient.« less

Shallow Carrier Trap Levels in GaAsN Investigated by Photoluminescence

NASA Astrophysics Data System (ADS)

Inagaki, Makoto; Suzuki, Hidetoshi; Suzuki, Akio; Mutaguchi, Kazumasa; Fukuyama, Atsuhiko; Kojima, Nobuaki; Ohshita, Yoshio; Yamagichi, Masafumi

2011-04-01

Shallow carrier trap levels in GaAs1-xNx (0.0010≤x≤0.0038) were investigated by photoluminescence (PL) and photoreflectance (PR) ranging from 4.2 to 300 K. The band gap energies of the GaAsN were clearly determined in the whole temperature range by the PR fitting analysis. It is clarified by peak decomposing that there were three emission peaks in the near-band-edge PL spectra of GaAsN. One of them was originated from band-to-band transition. The energies of two emission peaks were located at approximately 6 and 17 meV below the band edge. The existence of these peaks is evidence of carrier localization at the near-band-edge. The intensity ratio of the peak at the low energy side to other peaks increases with increasing N composition. This behavior is similar to the degradation of electrical properties.

Hydroxypyridinone Chelators: From Iron Scavenging to Radiopharmaceuticals for PET Imaging with Gallium-68

PubMed Central

Cusnir, Ruslan; Imberti, Cinzia; Hider, Robert C.; Blower, Philip J.; Ma, Michelle T.

2017-01-01

Derivatives of 3,4-hydroxypyridinones have been extensively studied for in vivo Fe3+ sequestration. Deferiprone, a 1,2-dimethyl-3,4-hydroxypyridinone, is now routinely used for clinical treatment of iron overload disease. Hexadentate tris(3,4-hydroxypyridinone) ligands (THP) complex Fe3+ at very low iron concentrations, and their high affinities for oxophilic trivalent metal ions have led to their development for new applications as bifunctional chelators for the positron emitting radiometal, 68Ga3+, which is clinically used for molecular imaging in positron emission tomography (PET). THP-peptide bioconjugates rapidly and quantitatively complex 68Ga3+ at ambient temperature, neutral pH and micromolar concentrations of ligand, making them amenable to kit-based radiosynthesis of 68Ga PET radiopharmaceuticals. 68Ga-labelled THP-peptides accumulate at target tissue in vivo, and are excreted largely via a renal pathway, providing high quality PET images. PMID:28075350

Hydroxypyridinone Chelators: From Iron Scavenging to Radiopharmaceuticals for PET Imaging with Gallium-68.

PubMed

Cusnir, Ruslan; Imberti, Cinzia; Hider, Robert C; Blower, Philip J; Ma, Michelle T

2017-01-08

Derivatives of 3,4-hydroxypyridinones have been extensively studied for in vivo Fe 3+ sequestration. Deferiprone, a 1,2-dimethyl-3,4-hydroxypyridinone, is now routinely used for clinical treatment of iron overload disease. Hexadentate tris(3,4-hydroxypyridinone) ligands (THP) complex Fe 3+ at very low iron concentrations, and their high affinities for oxophilic trivalent metal ions have led to their development for new applications as bifunctional chelators for the positron emitting radiometal, 68 Ga 3+ , which is clinically used for molecular imaging in positron emission tomography (PET). THP-peptide bioconjugates rapidly and quantitatively complex 68 Ga 3+ at ambient temperature, neutral pH and micromolar concentrations of ligand, making them amenable to kit-based radiosynthesis of 68 Ga PET radiopharmaceuticals. 68 Ga-labelled THP-peptides accumulate at target tissue in vivo, and are excreted largely via a renal pathway, providing high quality PET images.

Many-body design of highly strained GaInNAs electroabsorption modulators on GaInAs ternary substrates

NASA Astrophysics Data System (ADS)

Fujisawa, Takeshi; Arai, Masakazu; Kano, Fumiyoshi

2010-05-01

Electroabsorption in highly strained GaInAs and GaInNAs quantum wells (QWs) grown on GaInAs or quasi-GaInAs substrates is investigated by using microscopic many-body theory. The effects of various parameters, such as strain, barrier height, substrate composition, and temperature are thoroughly examined. It is shown that the value of the absorption coefficient strongly depends on the depth of the QWs under large bias electric field due to the small overlap integral of wave functions between the conduction and valence bands. The use of GaInNAs QWs makes the strain in the well layer very small. Further, the effective quantum-well depth is increased in GaInNAs QWs due to the anticrossing interaction between the conduction and N-resonant bands, making it possible to obtain larger absorption coefficient under large bias electric fields without using wide-band gap materials for barriers.

In vitro and in vivo antimicrobial activities of gallium nitrate against multidrug-resistant Acinetobacter baumannii.

PubMed

Antunes, Luísa C S; Imperi, Francesco; Minandri, Fabrizia; Visca, Paolo

2012-11-01

Multidrug-resistant Acinetobacter baumannii poses a tremendous challenge to traditional antibiotic therapy. Due to the crucial role of iron in bacterial physiology and pathogenicity, we investigated iron metabolism as a possible target for anti-A. baumannii chemotherapy using gallium as an iron mimetic. Due to chemical similarity, gallium competes with iron for binding to several redox enzymes, thereby interfering with a number of essential biological reactions. We found that Ga(NO(3))(3), the active component of an FDA-approved drug (Ganite), inhibits the growth of a collection of 58 A. baumannii strains in both chemically defined medium and human serum, at concentrations ranging from 2 to 80 μM and from 4 to 64 μM, respectively. Ga(NO(3))(3) delayed the entry of A. baumannii into the exponential phase and drastically reduced bacterial growth rates. Ga(NO(3))(3) activity was strongly dependent on iron availability in the culture medium, though the mechanism of growth inhibition was independent of dysregulation of gene expression controlled by the ferric uptake regulator Fur. Ga(NO(3))(3) also protected Galleria mellonella larvae from lethal A. baumannii infection, with survival rates of ≥75%. At therapeutic concentrations for humans (28 μM plasma levels), Ga(NO(3))(3) inhibited the growth in human serum of 76% of the multidrug-resistant A. baumannii isolates tested by ≥90%, raising expectations on the therapeutic potential of gallium for the treatment of A. baumannii bloodstream infections. Ga(NO(3))(3) also showed strong synergism with colistin, suggesting that a colistin-gallium combination holds promise as a last-resort therapy for infections caused by pan-resistant A. baumannii.

In Vitro and In Vivo Antimicrobial Activities of Gallium Nitrate against Multidrug-Resistant Acinetobacter baumannii

PubMed Central

Antunes, Luísa C. S.; Imperi, Francesco; Minandri, Fabrizia

2012-01-01

Multidrug-resistant Acinetobacter baumannii poses a tremendous challenge to traditional antibiotic therapy. Due to the crucial role of iron in bacterial physiology and pathogenicity, we investigated iron metabolism as a possible target for anti-A. baumannii chemotherapy using gallium as an iron mimetic. Due to chemical similarity, gallium competes with iron for binding to several redox enzymes, thereby interfering with a number of essential biological reactions. We found that Ga(NO3)3, the active component of an FDA-approved drug (Ganite), inhibits the growth of a collection of 58 A. baumannii strains in both chemically defined medium and human serum, at concentrations ranging from 2 to 80 μM and from 4 to 64 μM, respectively. Ga(NO3)3 delayed the entry of A. baumannii into the exponential phase and drastically reduced bacterial growth rates. Ga(NO3)3 activity was strongly dependent on iron availability in the culture medium, though the mechanism of growth inhibition was independent of dysregulation of gene expression controlled by the ferric uptake regulator Fur. Ga(NO3)3 also protected Galleria mellonella larvae from lethal A. baumannii infection, with survival rates of ≥75%. At therapeutic concentrations for humans (28 μM plasma levels), Ga(NO3)3 inhibited the growth in human serum of 76% of the multidrug-resistant A. baumannii isolates tested by ≥90%, raising expectations on the therapeutic potential of gallium for the treatment of A. baumannii bloodstream infections. Ga(NO3)3 also showed strong synergism with colistin, suggesting that a colistin-gallium combination holds promise as a last-resort therapy for infections caused by pan-resistant A. baumannii. PMID:22964249

Calculation of Vertical and Horizontal Mobilities in InAs/GaSb Superlattices (Postprint)

DTIC Science & Technology

2011-10-13

width 2a and GaSb having width 2b, with the period = 2a + 2b. For energies near the band gap edges, the carrier wave function can be approximated by a...online) Electron energy bands along the growth direction for three combinations of InAs/ GaSb layer widths. For typical carrier densities, at low...Fermi energies , parallel masses, and band gaps from the 8×8 EFA model. Sheet carrier Calculated Measured Calculated InAs GaSb concentration per period

First principles investigation of GaNbO{sub 4} as a photocatalytic material

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sharma, Neelam, E-mail: sneelam@issc.unipune.ac.in; Verma, Mukta; Shah, Vaishali

We have performed first principles density functional total energy calculations on pure and doped GaNbO{sub 4} to investigate its applicability as a photo catalyst. Pure GaNbO{sub 4} is an indirect, wide band gap semiconductor similar to the widely investigated TiO{sub 2} which is known to be a photo catalyst in UV light [K. Yang et. al. Chem. Mater. 20, 6528 (2008)]. S atom doping of TiO{sub 2} reduces the band gap [F. Tian et. al. J. Phys. Chem. B 110, 17866 (2006)], and increases its efficiency in the visible light range. It has been experimentally reported that S doping ofmore » GaNbO{sub 4} at the O site, decreases its photo catalytic efficiency. Our band structure calculations show that both pure and doped GaNbO{sub 4} have indirect band gaps and S atom doping reduces the band gap in agreement with experiments. The decrease in the band gap is due to the lowering of the conduction band minimum towards the Fermi level. An unequal reduction in the band gap was observed at the four inequivalent O sites chosen for S doping. This suggests that the photo catalytic activity varies with the dopant site.« less

Effect of temperature on In_{{\\varvec{x}}} Ga_{1-{{\\varvec{x}}}} As/GaAs quantum dots

NASA Astrophysics Data System (ADS)

Borji, Mahdi Ahmadi; Reyahi, Ali; Rajaei, Esfandiar; Ghahremani, Mohsen

2017-08-01

In this paper, the strain, band-edge, and energy levels of pyramidal In_x Ga_{1-x} As/GaAs quantum dots are investigated by 1-band effective mass approach. It is shown that while temperature has no remarkable effect on the strain tensor, the band gap lowers and the radiation wavelength elongates by increasing temperature. Also, band gap and energy do not linearly decrease by temperature rise. Our results appear to agree with former researches. This can be used in designing laser devices and sensors when applied in different working temperatures. Furthermore, when the device works for a long time, self-heating occurs which changes the characteristics of the output.

Elimination of surface band bending on N-polar InN with thin GaN capping

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kuzmík, J., E-mail: Jan.Kuzmik@savba.sk; Haščík, Š.; Kučera, M.

2015-11-09

0.5–1 μm thick InN (0001) films grown by molecular-beam epitaxy with N- or In-polarity are investigated for the presence of native oxide, surface energy band bending, and effects introduced by 2 to 4 monolayers of GaN capping. Ex situ angle-resolved x-ray photo-electron spectroscopy is used to construct near-surface (GaN)/InN energy profiles, which is combined with deconvolution of In3d signal to trace the presence of InN native oxide for different types of polarity and capping. Downwards surface energy band bending was observed on bare samples with native oxide, regardless of the polarity. It was found that the In-polar InN surface is mostmore » readily oxidized, however, with only slightly less band bending if compared with the N-polar sample. On the other hand, InN surface oxidation was effectively mitigated by GaN capping. Still, as confirmed by ultra-violet photo-electron spectroscopy and by energy band diagram calculations, thin GaN cap layer may provide negative piezoelectric polarization charge at the GaN/InN hetero-interface of the N-polar sample, in addition to the passivation effect. These effects raised the band diagram up by about 0.65 eV, reaching a flat-band profile.« less

Infrared and infrared emission spectroscopy of gallium oxide alpha-GaO(OH) nanostructures.

PubMed

Yang, Jing Jeanne; Zhao, Yanyan; Frost, Ray L

2009-10-01

Infrared spectroscopy has been used to study nano- to micro-sized gallium oxyhydroxide alpha-GaO(OH), prepared using a low temperature hydrothermal route. Rod-like alpha-GaO(OH) crystals with average length of approximately 2.5 microm and width of 1.5 microm were prepared when the initial molar ratio of Ga to OH was 1:3. beta-Ga(2)O(3) nano and micro-rods were prepared through the calcination of alpha-GaO(OH). The initial morphology of alpha-GaO(OH) is retained in the beta-Ga(2)O(3) nanorods. The combination of infrared and infrared emission spectroscopy complimented with dynamic thermal analysis were used to characterise the alpha-GaO(OH) nanotubes and the formation of beta-Ga(2)O(3) nanorods. Bands at around 2903 and 2836 cm(-1) are assigned to the -OH stretching vibration of alpha-GaO(OH) nanorods. Infrared bands at around 952 and 1026 cm(-1) are assigned to the Ga-OH deformation modes of alpha-GaO(OH). A significant number of bands are observed in the 620-725 cm(-1) region and are assigned to GaO stretching vibrations.

Device Performance and Reliability Improvements of AlGaBN/GaN/Si MOSFET

DTIC Science & Technology

2016-02-04

Metal insulator semiconductor AlGaN /GaN high electron mobility transistors (MISHEMTs) are promising for power device applications due to a lower leakage...current than the conventional Schottky AlGaN/GaN HEMTs.1–3 Among a large number of insulator materials, an Al2O3 dielectric layer, deposited by...atomic layer deposition (ALD), is often employed as the gate insulator because of a large band gap (and the resultant high conduction band offset on

X-ray spectra and electronic structure of the Ca3Ga2Ge3О12 compound

NASA Astrophysics Data System (ADS)

Shcherba, I. D.; Kostyk, L. V.; Noga, H.; Bekenov, L. V.; Uskokovich, D.; Jatsyk, B. M.

2017-09-01

The band structure of Ca3Ga2Ge3О12 with the garnet structure has been determined for the first time by X-ray emission and photoelectron spectroscopy. It has been established that the bottom of the valence band is formed by Ge d states, which are not dominant in the chemical bonding. Strong hybridization of oxygen 2s states with 4p states of Ga and Ge revealed by the presence of an extra structure in the X-ray emission spectra has been found. The middle of the valence band has been demonstrated to be occupied by d states of Ga, while Ga and Ge 4рstates with a considerable admixture of oxygen 2p states form the top of the valence band.

Valence band offsets of Sc x Ga1-x N/AlN and Sc x Ga1-x N/GaN heterojunctions

NASA Astrophysics Data System (ADS)

Tsui, H. C. L.; Goff, L. E.; Palgrave, R. G.; Beere, H. E.; Farrer, I.; Ritchie, D. A.; Moram, M. A.

2016-07-01

The valence band offsets of Sc x Ga1-x N/AlN heterojunctions were measured by x-ray photoelectron spectroscopy (XPS) and were found to increase from 0.42 eV to 0.95 eV as the Sc content x increased from 0 to 0.15. The increase in valence band offset with increasing x is attributed to the corresponding increase in spontaneous polarization of the wurtzite structure. The Sc x Ga1-x N/AlN heterojunction is type I, similar to other III-nitride-based heterojunctions. The data also indicate that a type II staggered heterojunction, which can enhance spatial charge separation, could be formed if Sc x Ga1-x N is grown on GaN.

Thermal quenching of the yellow luminescence in GaN

NASA Astrophysics Data System (ADS)

Reshchikov, M. A.; Albarakati, N. M.; Monavarian, M.; Avrutin, V.; Morkoç, H.

2018-04-01

We observed varying thermal quenching behavior of the yellow luminescence band near 2.2 eV in different GaN samples. In spite of the different behavior, the yellow band in all the samples is caused by the same defect—the YL1 center. In conductive n-type GaN, the YL1 band quenches with exponential law, and the Arrhenius plot reveals an ionization energy of ˜0.9 eV for the YL1 center. In semi-insulating GaN, an abrupt and tunable quenching of the YL1 band is observed, where the apparent activation energy in the Arrhenius plot is not related to the ionization energy of the defect. In this case, the ionization energy can be found by analyzing the shift of the characteristic temperature of PL quenching with excitation intensity. We conclude that only one defect, namely, the YL1 center, is responsible for the yellow band in undoped and doped GaN samples grown by different techniques.

Role of biaxial strain and microscopic ordering for structural and electronic properties of InxGa1 -xN

NASA Astrophysics Data System (ADS)

Cui, Ying; Lee, Sangheon; Freysoldt, Christoph; Neugebauer, Jörg

2015-08-01

The structural and electronic properties of InxGa1 -xN alloys are studied as a function of c -plane biaxial strain and In ordering by density functional theory with the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional. A nonlinear variation of the c lattice parameter with In content is observed in biaxial strain and should be taken into account when deducing In content from interplanar distances. From compressive to tensile strain, the character of the top valence-band state changes, leading to a nonlinear variation of the band gap in InxGa1 -xN . Interestingly, the well-known bowing of the InxGa1 -xN band gap is largely removed for alloys grown strictly coherently on GaN, while the actual values for band gaps at x <0.33 are hardly affected by strain. Ordering plays a minor role for lattice constants but may induce changes of the band gap up to 0.15 eV.

Gallic acid prevents nonsteroidal anti-inflammatory drug-induced gastropathy in rat by blocking oxidative stress and apoptosis.

PubMed

Pal, Chinmay; Bindu, Samik; Dey, Sumanta; Alam, Athar; Goyal, Manish; Iqbal, Mohd Shameel; Maity, Pallab; Adhikari, Susanta S; Bandyopadhyay, Uday

2010-07-15

Nonsteroidal anti-inflammatory drug (NSAID)-induced oxidative stress plays a critical role in gastric mucosal cell apoptosis and gastropathy. NSAIDs induce the generation of hydroxyl radical ((*)OH) through the release of free iron, which plays an important role in developing gastropathy. Thus, molecules having both iron-chelating and antiapoptotic properties will be beneficial in preventing NSAID-induced gastropathy. Gallic acid (GA), a polyphenolic natural product, has the capacity to chelate free iron. Here, we report that GA significantly prevents, as well as heals, NSAID-induced gastropathy. In vivo, GA blocks NSAID-mediated mitochondrial oxidative stress by preventing mitochondrial protein carbonyl formation, lipid peroxidation, and thiol depletion. In vitro, GA scavenges free radicals and blocks (*)OH-mediated oxidative damage. GA also attenuates gastric mucosal cell apoptosis in vivo as well as in vitro in cultured gastric mucosal cells as evident from the TUNEL assay. GA prevents NSAID-induced activation of caspase-9, a marker for the mitochondrial pathway of apoptosis, and restores NSAID-mediated collapse of the mitochondrial transmembrane potential and dehydrogenase activity. Thus, the inhibition of mitochondrial oxidative stress by GA is associated with the inhibition of NSAID-induced mitochondrial dysfunction and activation of apoptosis in gastric mucosal cells, which are responsible for gastric injury or gastropathy. Copyright 2010 Elsevier Inc. All rights reserved.

Intersubband absorption in GaN nanowire heterostructures at mid-infrared wavelengths.

PubMed

Ajay, Akhil; Blasco, Rodrigo; Polaczynski, Jakub; Spies, Maria; den Hertog, Martien; Monroy, Eva

2018-06-27

In this paper, we study intersubband characteristics of GaN/AlN and GaN/Al0.4Ga0.6N heterostructures in GaN nanowires structurally designed to absorb in the mid-infrared wavelength region. Increasing the GaN well width from 1.5 to 5.7 nm leads to a red shift of the intersubband absorption from 1.4 to 3.4 µm. The red shift in larger quantum wells is amplified by the fact that one of the GaN/AlN heterointerfaces (corresponding to the growth of GaN on AlN) is not sharp but rather a graded alloy extending around 1.5-2 nm. Using AlGaN instead of AlN for the same barrier dimensions, we observe the effects of reduced polarization, which blue shifts the band-to-band transitions and red shifts the intersubband transitions. In heavily doped GaN/AlGaN nanowires, a broad absorption band is observed in the 4.5-6.4 µm spectral region. © 2018 IOP Publishing Ltd.

Nanostructuring-induced modification of optical properties of p-GaAs (1 0 0)

NASA Astrophysics Data System (ADS)

Naddaf, M.; Saloum, S.

2009-10-01

A pulsed anodic etching method has been utilized for nanostructuring of p-type GaAs (1 0 0) surface, using HCl-based solution as electrolyte. The resulting porous GaAs layer is characterized by atomic force microscopy (AFM), room temperature photoluminescence (PL), Raman spectroscopy and optical reflectance measurements. AFM imaging reveals that the porous GaAs layer is consisted of a pillar-like of few nm in width distributed between more-reduced size nanostructures. In addition to the “infrared” PL band of un-etched GaAs, a strong “green” PL band is observed in the etched sample. The broad visible PL band of a high-energy (3.82 eV) excitation is found to compose of two PL band attributed to excitons confinement in two different sizes distribution of GaAs nanocrystals. The quantum confinement effects in GaAs nanocrystallites is also evidenced from Raman spectroscopy through the pronounced appearance of the transverse optical (TO) phonon line in the spectra of the porous sample. Porosity-induced a significant reduction of the specular reflection, in the spectral range (400-800 nm), is also demonstrated.

Origins of Fermi-level pinning on GaN and InN polar and nonpolar surfaces

NASA Astrophysics Data System (ADS)

Segev, D.; Van de Walle, C. G.

2006-10-01

Using band structure and total energy methods, we study the atomic and electronic structures of the polar (+c and - c plane) and nonpolar (a and m plane) surfaces of GaN and InN. We identify two distinct microscopic origins for Fermi-level pinning on GaN and InN, depending on surface stoichiometry and surface polarity. At moderate Ga/N ratios unoccupied gallium dangling bonds pin the Fermi level on n-type GaN at 0.5 0.7 eV below the conduction-band minimum. Under highly Ga-rich conditions metallic Ga adlayers lead to Fermi-level pinning at 1.8 eV above the valence-band maximum. We also explain the source of the intrinsic electron accumulation that has been universally observed on polar InN surfaces. It is caused by In-In bonds leading to occupied surface states above the conduction-band minimum. We predict that such a charge accumulation will be absent on the nonpolar surfaces of InN, when prepared under specific conditions.

Theoretical investigation of GaAsBi/GaAsN tunneling field-effect transistors with type-II staggered tunneling junction

NASA Astrophysics Data System (ADS)

Wang, Yibo; Liu, Yan; Han, Genquan; Wang, Hongjuan; Zhang, Chunfu; Zhang, Jincheng; Hao, Yue

2017-06-01

We investigate GaAsBi/GaAsN system for the design of type-II staggered hetero tunneling field-effect transistor (hetero-TFET). Strain-symmetrized GaAsBi/GaAsN with effective lattice match to GaAs exhibits a type-II band lineup, and the effective bandgap EG,eff at interface is significantly reduced with the incorporation of Bi and N elements. The band-to-band tunneling (BTBT) rate and drive current of GaAsBi/GaAsN hetero-TFETs are boosted due to the utilizing of the type-II staggered tunneling junction with the reduced EG,eff. Numerical simulation shows that the drive current and subthreshold swing (SS) characteristics of GaAsBi/GaAsN hetero-TFETs are remarkably improved by increasing Bi and N compositions. The dilute content GaAs0.85Bi0.15/GaAs0.92N0.08 staggered hetero-nTFET achieves 7.8 and 550 times higher ION compared to InAs and In0.53Ga0.47As homo-TFETs, respectively, at the supply voltage of 0.3 V. GaAsBi/GaAsN heterostructure is a potential candidate for high performance TFET.

Ballistic-electron-emission spectroscopy of Al{sub x}Ga{sub 1{minus}x}As/GaAs heterostructures: Conduction-band offsets, transport mechanisms, and band-structure effects

DOE Office of Scientific and Technical Information (OSTI.GOV)

OShea, J.J.; Brazel, E.G.; Rubin, M.E.

1997-07-01

We report an extensive investigation of semiconductor band-structure effects in single-barrier Al{sub x}Ga{sub 1{minus}x}As/GaAs heterostructures using ballistic-electron-emission spectroscopy (BEES). The transport mechanisms in these single-barrier structures were studied systematically as a function of temperature and Al composition over the full compositional range (0{le}x{le}1). The initial ({Gamma}) BEES thresholds for Al{sub x}Ga{sub 1{minus}x}As single barriers with 0{le}x{le}0.42 were extracted using a model which includes the complete transmission probability of the metal-semiconductor interface and the semiconductor heterostructure. Band offsets measured by BEES are in good agreement with previous measurements by other techniques which demonstrates the accuracy of this technique. BEES measurements atmore » 77 K give the same band-offset values as at room temperature. When a reverse bias is applied to the heterostructures, the BEES thresholds shift to lower voltages in good agreement with the expected bias-induced band-bending. In the indirect band-gap regime ({ital x}{gt}0.45), spectra show a weak ballistic-electron-emission microscopy current contribution due to intervalley scattering through Al{sub x}Ga{sub 1{minus}x}As {ital X} valley states. Low-temperature spectra show a marked reduction in this intervalley current component, indicating that intervalley phonon scattering at the GaAs/Al{sub x}Ga{sub 1{minus}x}As interface produces a significant fraction of this{ital X} valley current. A comparison of the BEES thresholds with the expected composition dependence of the Al{sub x}Ga{sub 1{minus}x}As {Gamma}, {ital L}, and {ital X} points yields good agreement over the entire composition range. {copyright} {ital 1997} {ital The American Physical Society}« less

Spatially resolved In and As distributions in InGaAs/GaP and InGaAs/GaAs quantum dot systems.

PubMed

Shen, J; Song, Y; Lee, M L; Cha, J J

2014-11-21

InGaAs quantum dots (QDs) on GaP are promising for monolithic integration of optoelectronics with Si technology. To understand and improve the optical properties of InGaAs/GaP QD systems, detailed measurements of the QD atomic structure as well as the spatial distributions of each element at high resolution are crucial. This is because the QD band structure, band alignment, and optical properties are determined by the atomic structure and elemental composition. Here, we directly measure the inhomogeneous distributions of In and As in InGaAs QDs grown on GaAs and GaP substrates at the nanoscale using energy dispersive x-ray spectral mapping in a scanning transmission electron microscope. We find that the In distribution is broader on GaP than on GaAs, and as a result, the QDs appear to be In-poor using a GaP matrix. Our findings challenge some of the assumptions made for the concentrations and distributions of In within InGaAs/GaAs or InGaAs/GaP QD systems and provide detailed structural and elemental information to modify the current band structure understanding. In particular, the findings of In deficiency and inhomogeneous distribution in InGaAs/GaP QD systems help to explain photoluminescence spectral differences between InGaAs/GaAs and InGaAs/GaP QD systems.

Characterization of Dual-Band Infrared Detectors for Application to Remote Sensing

NASA Technical Reports Server (NTRS)

Abedin, M. Nurul; Refaat, Tamer F.; Xiao, Yegao; Bhat, Ishwara

2005-01-01

NASA Langley Research Center (LaRC), in partnership with the Rensselaer Polytechnic Institute (RPI), developed photovoltaic infrared (IR) detectors suitable at two different wavelengths using Sb-based material systems. Using lattice-matched InGaAsSb grown on GaSb substrates, dual wavelength detectors operating at 1.7 and 2.5 micron wavelengths can be realized. P-N junction diodes are fabricated on both GaSb and InGaAsSb materials. The photodiode on GaSb detects wavelengths at 1.7 micron and the InGaAsSb detector detects wavelengths at 2.2 micron or longer depending on the composition. The films for these devices are grown by metal-organic vapor phase epitaxy (MOVPE). The cross section of the independently accessed back-to-back photodiode dual band detector consists of a p-type substrate on which n-on-p GaInAsSb junction is grown, followed by a p-on-n GaSb junction. There are three ohmic contacts in this structure, one to the p-GaSb top layer, one to the n-GaSb/n-GaInAsSb layer and one to the p-type GaSb substrate. The common terminal is the contact to the n-GaSb/n-GaInAsSb layer. The contact to the n-GaSb/p-GaInAsSb region of the photodiode in the dual band is electrically connected and is accessed at the edge of the photodiode. NASA LaRC acquired the fabricated dual band detector from RPI and characterized the detector at its Detector Characterization Laboratory. Characterization results, such as responsivity, noise, quantum efficiency, and detectivity will be presented.

Theoretical constraints on oxygen and carbon dioxide concentrations in the Precambrian atmosphere

NASA Technical Reports Server (NTRS)

Kasting, J. F.

1987-01-01

Simple (one-dimensional) climate models suggest that carbon dioxide concentrations during the Archean must have been at least 100-1000 times the present level to keep the Earth's surface temperature above freezing in the face of decreased solar luminosity. Such models provide only lower bounds on CO2, so it is possible that CO2 levels were substantially higher than this and that the Archean climate was much warmer than today. Periods of extensive glaciation during the early and late Proterozoic, on the other hand, indicate that the climate at these times was relatively cool. To be consistent with climate models CO2 partial pressures must have declined from approximately 0.03 to 0.3 bar around 2.5 Ga ago to between 10(-3) and 10(-2) bar at 0.8 Ga ago. This steep decrease in carbon dioxide concentrations may be inconsistent with paleosol data, which implies that pCO2 did not change appreciably during that time. Oxygen was essentially absent from the Earth's atmosphere and oceans prior to the emergence of a photosynthetic source, probably during the late Archean. During the early Proterozoic the atmosphere and surface ocean were apparently oxidizing, while the deep ocean remained reducing. An upper limit of 6 x 10(-3) bar for pO2 at this time can be derived by balancing the burial rate of organic carbon with the rate of oxidation of ferrous iron in the deep ocean. The establishment of oxidizing conditions in the deep ocean, marked by the disappearance of banded iron formations approximately 1.7 Ga ago, permitted atmospheric oxygen to climb to its present level. O2 concentrations may have remained substantially lower than today, however, until well into the Phanerozoic.

Band engineered epitaxial 3D GaN-InGaN core-shell rod arrays as an advanced photoanode for visible-light-driven water splitting.

PubMed

Caccamo, Lorenzo; Hartmann, Jana; Fàbrega, Cristian; Estradé, Sonia; Lilienkamp, Gerhard; Prades, Joan Daniel; Hoffmann, Martin W G; Ledig, Johannes; Wagner, Alexander; Wang, Xue; Lopez-Conesa, Lluis; Peiró, Francesca; Rebled, José Manuel; Wehmann, Hergo-Heinrich; Daum, Winfried; Shen, Hao; Waag, Andreas

2014-02-26

3D single-crystalline, well-aligned GaN-InGaN rod arrays are fabricated by selective area growth (SAG) metal-organic vapor phase epitaxy (MOVPE) for visible-light water splitting. Epitaxial InGaN layer grows successfully on 3D GaN rods to minimize defects within the GaN-InGaN heterojunctions. The indium concentration (In ∼ 0.30 ± 0.04) is rather homogeneous in InGaN shells along the radial and longitudinal directions. The growing strategy allows us to tune the band gap of the InGaN layer in order to match the visible absorption with the solar spectrum as well as to align the semiconductor bands close to the water redox potentials to achieve high efficiency. The relation between structure, surface, and photoelectrochemical property of GaN-InGaN is explored by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), current-voltage, and open circuit potential (OCP) measurements. The epitaxial GaN-InGaN interface, pseudomorphic InGaN thin films, homogeneous and suitable indium concentration and defined surface orientation are properties demanded for systematic study and efficient photoanodes based on III-nitride heterojunctions.

Optical characterization of type-I to type-II band alignment transition in GaAs/Al x Ga1-x As quantum rings grown by droplet epitaxy

NASA Astrophysics Data System (ADS)

Su, Linlin; Wang, Ying; Guo, Qinglin; Li, Xiaowei; Wang, Shufang; Fu, Guangsheng; Mazur, Yuriy I.; E Ware, Morgan; Salamo, Gregory J.; Liang, Baolai; Huffaker, Diana L.

2017-08-01

Optical properties of GaAs/Al x Ga1-x As quantum rings (QRs) grown on GaAs (1 0 0) by droplet epitaxy have been investigated as a function of the Al-composition in the Al x Ga1-x As barrier. A transition from type-I to type-II band alignment is observed for the QRs via photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. While x  ⩽  0.45, the QR PL spectra show a blue-shift and an increasing intensity with increasing Al-composition, revealing the enhancement of quantum confinement in the QRs with type-I band alignment. While x  ⩾  0.60, the characteristic large blue-shift with excitation intensity and the much longer lifetime indicate the realization of a type-II band alignment. Due to the height fluctuation of QR structures grown by droplet epitaxy mode, it is not the large blue-shift of emission energy, but the long lifetime that becomes the more important feature to identify the type-II band alignment.

Experimental study of the effect of local atomic ordering on the energy band gap of melt grown InGaAsN alloys

NASA Astrophysics Data System (ADS)

Milanova, M.; Donchev, V.; Kostov, K. L.; Alonso-Álvarez, D.; Valcheva, E.; Kirilov, K.; Asenova, I.; Ivanov, I. G.; Georgiev, S.; Ekins-Daukes, N.

2017-08-01

We present a study of melt grown dilute nitride InGaAsN layers by x-ray photoelectron spectroscopy (XPS), Raman and photoluminescence (PL) spectroscopy. The purpose of the study is to determine the degree of atomic ordering in the quaternary alloy during the epitaxial growth at near thermodynamic equilibrium conditions and its influence on band gap formation. Despite the low In concentration (˜3%) the XPS data show a strong preference toward In-N bonding configuration in the InGaAsN samples. Raman spectra reveal that most of the N atoms are bonded to In instead of Ga atoms and the formation of N-centred In3Ga1 clusters. PL measurements reveal smaller optical band gap bowing as compared to the theoretical predictions for random alloy and localised tail states near the conduction band minimum.

Valence-band structure of the ferromagnetic semiconductor GaMnAs studied by spin-dependent resonant tunneling spectroscopy.

PubMed

Ohya, Shinobu; Muneta, Iriya; Hai, Pham Nam; Tanaka, Masaaki

2010-04-23

The valence-band structure and the Fermi level (E(F)) position of ferromagnetic-semiconductor GaMnAs are quantitatively investigated by electrically detecting the resonant tunneling levels of a GaMnAs quantum well (QW) in double-barrier heterostructures. The resonant level from the heavy-hole first state is clearly observed in the metallic GaMnAs QW, indicating that holes have a high coherency and that E(F) exists in the band gap. Clear enhancement of tunnel magnetoresistance induced by resonant tunneling is demonstrated in these double-barrier heterostructures.

Determination of band offsets at GaN/single-layer MoS{sub 2} heterojunction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tangi, Malleswararao; Mishra, Pawan; Ng, Tien Khee

2016-07-18

We report the band alignment parameters of the GaN/single-layer (SL) MoS{sub 2} heterostructure where the GaN thin layer is grown by molecular beam epitaxy on CVD deposited SL-MoS{sub 2}/c-sapphire. We confirm that the MoS{sub 2} is an SL by measuring the separation and position of room temperature micro-Raman E{sup 1}{sub 2g} and A{sup 1}{sub g} modes, absorbance, and micro-photoluminescence bandgap studies. This is in good agreement with HRTEM cross-sectional analysis. The determination of band offset parameters at the GaN/SL-MoS{sub 2} heterojunction is carried out by high-resolution X-ray photoelectron spectroscopy accompanying with electronic bandgap values of SL-MoS{sub 2} and GaN. Themore » valence band and conduction band offset values are, respectively, measured to be 1.86 ± 0.08 and 0.56 ± 0.1 eV with type II band alignment. The determination of these unprecedented band offset parameters opens up a way to integrate 3D group III nitride materials with 2D transition metal dichalcogenide layers for designing and modeling of their heterojunction based electronic and photonic devices.« less

Synthesis, structural, electronic and linear electro-optical features of new quaternary Ag2Ga2SiS6 compound

NASA Astrophysics Data System (ADS)

Piasecki, M.; Myronchuk, G. L.; Parasyuk, O. V.; Khyzhun, O. Y.; Fedorchuk, A. O.; Pavlyuk, V. V.; Kozer, V. R.; Sachanyuk, V. P.; El-Naggar, A. M.; Albassam, A. A.; Jedryka, J.; Kityk, I. V.

2017-02-01

For the first time phase equilibria and phase diagram of the AgGaS2-SiS2 system were successfully explored by differential thermal and X-ray phase analysis methods. Crystal structure of low-temperature (LT) modification of Ag2Ga2SiS6 (LT- Ag2Ga2SiS6) was studied by X-ray powder method and it belongs to tetragonal space group I-42d, with unit cell parameters a=5.7164(4) Å, c=9.8023(7) Å, V=320.32(7) Å3. Additional details regarding the crystal structure exploration are available at the web page Fachinformationszentrum Karlsruhe. X-ray photoelectron core-level and valence-band spectra were measured for pristine LT- Ag2Ga2SiS6 crystal surface. In addition, the X-ray photoelectron valence-band spectrum of LT-Ag2Ga2SiS6 was matched on a common energy scale with the X-ray emission S Kβ1,3 and Ga Kβ2 bands, which give information on the energy distribution of the S 3p and Ga 4p states, respectively. The presented X-ray spectroscopy results indicate that the valence S p and Ga p atomic states contribute mainly to the upper and central parts of the valence band of LT-Ag2Ga2SiS6, respectively, with a less significant contribution also to other valence-band regions. Band gap energy was estimated by measuring the quantum energy in the spectral range of the fundamental absorption. We have found that energy gap Eg is equal to 2.35 eV at 300 K. LT-Ag2Ga2SiS6 is a photosensitive material and reveals two spectral maxima on the curve of spectral photoconductivity spectra at λmax1 =590 nm and λmax2 =860 nm. Additionally, linear electro-optical effect of LT-Ag2Ga2SiS6 for the wavelengths of a cw He-Ne laser at 1150 nm was explored.

Growth of β-Ga2O3 and GaN nanowires on GaN for photoelectrochemical hydrogen generation.

PubMed

Hwang, Jih-Shang; Liu, Tai-Yan; Chattopadhyay, Surjit; Hsu, Geng-Ming; Basilio, Antonio M; Chen, Han-Wei; Hsu, Yu-Kuei; Tu, Wen-Hsun; Lin, Yan-Gu; Chen, Kuei-Hsien; Li, Chien-Cheng; Wang, Sheng-Bo; Chen, Hsin-Yi; Chen, Li-Chyong

2013-02-08

Enhanced photoelectrochemical (PEC) performances of Ga(2)O(3) and GaN nanowires (NWs) grown in situ from GaN were demonstrated. The PEC conversion efficiencies of Ga(2)O(3) and GaN NWs have been shown to be 0.906% and 1.09% respectively, in contrast to their 0.581% GaN thin film counterpart under similar experimental conditions. A low crystallinity buffer layer between the grown NWs and the substrate was found to be detrimental to the PEC performance, but the layer can be avoided at suitable growth conditions. A band bending at the surface of the GaN NWs generates an electric field that drives the photogenerated electrons and holes away from each other, preventing recombination, and was found to be responsible for the enhanced PEC performance. The enhanced PEC efficiency of the Ga(2)O(3) NWs is aided by the optical absorption through a defect band centered 3.3 eV above the valence band of Ga(2)O(3). These findings are believed to have opened up possibilities for enabling visible absorption, either by tailoring ion doping into wide bandgap Ga(2)O(3) NWs, or by incorporation of indium to form InGaN NWs.

Internal Rb-Sr Age and Initial Sr-87/Sr-86 of a Silicate Inclusion from the Campo Del Cielo Iron Meteorite

NASA Technical Reports Server (NTRS)

Liu, Y.; Nyquist, L.; Wiesmann, H.; Shih, C.; Schwandt, C.; Takeda, H.

2003-01-01

The largest group of iron meteorites, IAB, is distinguished by the presence of diverse silicate inclusions. In principle, Rb-Sr and Sm-Nd radiometric dating of these silicate inclusions by internal isochron techniques can determine both the times of melting and parent/daughter ratios in the precursor materials via initial Sr-87/Sr-86 and Nd-143/Nd-144 ratios. The Sr-87/Sr-86 and Nd-143/Nd-144 ratios could distinguish chondritic precursors from already differentiated silicates. We reported Rb-Sr and Sm-Nd internal ischron ages of 4.52+/-0.03 Ga and 4.50+/-0.04 Ga, respectively, for plagioclase-diopside-rich material in the Caddo County IAB iron meteorite. These results are essentially identical to literature values of its Ar-Ar age of 4.520+/-0.005 Ga and its Sm-Nd age of 4.53+/-0.02 Ga. The purpose of this study is to evaluate the formation and evolution of silicate inclusions in IAB iron meteorites by determination of their initial Sr-87/Sr-86 ratios combined with higher-resolution chronology and mineralogical and geochemical studies.

Band Structure Engineering and Thermoelectric Properties of Charge-Compensated Filled Skutterudites

PubMed Central

Shi, Xiaoya; Yang, Jiong; Wu, Lijun; Salvador, James R.; Zhang, Cheng; Villaire, William L.; Haddad, Daad; Yang, Jihui; Zhu, Yimei; Li, Qiang

2015-01-01

Thermoelectric properties of semiconductors are intimately related to their electronic band structure, which can be engineered via chemical doping. Dopant Ga in the cage-structured skutterudite Co4Sb12 substitutes Sb sites while occupying the void sites. Combining quantitative scanning transmission electron microscopy and first-principles calculations, we show that Ga dual-site occupancy breaks the symmetry of the Sb-Sb network, splits the deep triply-degenerate conduction bands, and drives them downward to the band edge. The charge-compensating nature of the dual occupancy Ga increases overall filling fraction limit. By imparting this unique band structure feature, and judiciously doping the materials by increasing the Yb content, we promote the Fermi level to a point where carriers are in energetic proximity to these features. Increased participation of these heavier bands in electronic transport leads to increased thermopower and effective mass. Further, the localized distortion from Ga/Sb substitution enhances the phonon scattering to reduce the thermal conductivity effectively. PMID:26456013

Band structure engineering and thermoelectric properties of charge-compensated filled skutterudites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shi, Xiaoya; Yang, Jiong; Wu, Lijun

2015-10-12

Thermoelectric properties of semiconductors are intimately related to their electronic band structure, which can be engineered via chemical doping. Dopant Ga in the cage-structured skutterudite Co 4Sb 12 substitutes Sb sites while occupying the void sites. Combining quantitative scanning transmission electron microscopy and first-principles calculations, we show that Ga dual-site occupancy breaks the symmetry of the Sb-Sb network, splits the deep triply-degenerate conduction bands, and drives them downward to the band edge. The charge-compensating nature of the dual occupancy Ga increases overall filling fraction limit. By imparting this unique band structure feature, and judiciously doping the materials by increasing themore » Yb content, we promote the Fermi level to a point where carriers are in energetic proximity to these features. Increased participation of these heavier bands in electronic transport leads to increased thermopower and effective mass. Further, the localized distortion from Ga/Sb substitution enhances the phonon scattering to reduce the thermal conductivity effectively.« less

Band Structure Engineering and Thermoelectric Properties of Charge-Compensated Filled Skutterudites

NASA Astrophysics Data System (ADS)

Shi, Xiaoya; Yang, Jiong; Wu, Lijun; Salvador, James R.; Zhang, Cheng; Villaire, William L.; Haddad, Daad; Yang, Jihui; Zhu, Yimei; Li, Qiang

2015-10-01

Thermoelectric properties of semiconductors are intimately related to their electronic band structure, which can be engineered via chemical doping. Dopant Ga in the cage-structured skutterudite Co4Sb12 substitutes Sb sites while occupying the void sites. Combining quantitative scanning transmission electron microscopy and first-principles calculations, we show that Ga dual-site occupancy breaks the symmetry of the Sb-Sb network, splits the deep triply-degenerate conduction bands, and drives them downward to the band edge. The charge-compensating nature of the dual occupancy Ga increases overall filling fraction limit. By imparting this unique band structure feature, and judiciously doping the materials by increasing the Yb content, we promote the Fermi level to a point where carriers are in energetic proximity to these features. Increased participation of these heavier bands in electronic transport leads to increased thermopower and effective mass. Further, the localized distortion from Ga/Sb substitution enhances the phonon scattering to reduce the thermal conductivity effectively.

Band gap tuning and optical absorption in type-II InAs/GaSb mid infrared short period superlattices: 14 bands K Dot-Operator p study

DOE Office of Scientific and Technical Information (OSTI.GOV)

AbuEl-Rub, Khaled M.

2012-09-06

The MBE growth of short-period InAs/GaSb type-II superlattice structures, varied around 20.5 A InAs/24 A GaSb were [J. Applied physics, 96, 2580 (2004)] carried out by Haugan et al. These SLs were designed to produce devices with an optimum mid-infrared photoresponse and a sharpest photoresponse cutoff. We have used a realistic and reliable 14-band k.p formalism description of the superlattice electronic band structure to calculate the absorption coefficient in such short-period InAs/GaSb type-II superlattices. The parameters for this formalism are known from fitting to independent experiments for the bulk materials. The band-gap energies are obtained without any fitting parameters, andmore » are in good agreement with experimental data.« less

First-principles study of direct and narrow band gap semiconducting β -CuGaO 2

DOE PAGES

Nguyen, Manh Cuong; Zhao, Xin; Wang, Cai-Zhuang; ...

2015-04-16

Semiconducting oxides have attracted much attention due to their great stability in air or water and the abundance of oxygen. Recent success in synthesizing a metastable phase of CuGaO 2 with direct narrow band gap opens up new applications of semiconducting oxides as absorber layer for photovoltaics. Using first-principles density functional theory calculations, we investigate the thermodynamic and mechanical stabilities as well as the structural and electronic properties of the β-CuGaO 2 phase. Our calculations show that the β-CuGaO 2 structure is dynamically and mechanically stable. The energy band gap is confirmed to be direct at the Γ point ofmore » Brillouin zone. In conclusion, the optical absorption occurs right at the band gap edge and the density of states near the valance band maximum is large, inducing an intense absorption of light as observed in experiment.« less

Influence of functionalized nanoparticles on conformational stability of type I collagen for possible biomedical applications.

PubMed

Kandamchira, Aswathy; Selvam, Sangeetha; Marimuthu, Nidhin; Janardhanan, Sreeram Kalarical; Fathima, Nishter Nishad

2013-12-01

Collagen-nanoparticle interactions are vital for many biomedical applications including drug delivery and tissue engineering applications. Iron oxide nanoparticles synthesized using starch template according to our earlier reported procedures were functionalized by treating them with Gum Arabic (GA), a biocompatible polysaccharide, so as to enhance the interaction between nanoparticle surfaces and collagen. Viscosity, circular dichroism (CD) and Fourier transform infrared spectroscopy (FTIR) techniques have been used to study the collagen-nanoparticle interactions. The relative viscosity for collagen-nanoparticle conjugate was found to increase with increase in concentration of the nanoparticle within the concentration range investigated, which is due to the aggregation of protein onto the surface of nanoparticle. The CD spectra for the collagen-nanoparticle at different concentration ratios do not have much variation in the Rpn values (ratio of positive peak intensity over negative peak intensity) after functionalization with GA. The variation of molar ellipticity values for collagen-nanoparticle is due to the glycoprotein present in GA. The collagen triple helical structure is maintained after interaction with nanoparticles. The FTIR spectra of native collagen, Coll-Fs (nanoparticle without functionalization) and Coll-FsG (nanoparticle functionalized with GA) show clearly the amide I, II, III bands, with respect to collagen. The ability of polysaccharide stabilized/functionalized nanoparticles to maintain the collagen properties would help in its biomedical applications. © 2013.

Electronic structure and optical properties of LiGa0.5In0.5Se2 single crystal, a nonlinear optical mid-IR material

NASA Astrophysics Data System (ADS)

Lavrentyev, A. A.; Gabrelian, B. V.; Vu, Tuan V.; Isaenko, L. I.; Yelisseyev, A. P.; Khyzhun, O. Y.

2018-06-01

Measurements of X-ray photoelectron core-level and valence-band spectra for pristine and irradiated with Ar+ ions surfaces of LiGa0.5In0.5Se2 single crystal, novel nonlinear optical mid-IR selenide grown by a modified vertical Bridgman-Stockbarger technique, are reported. Electronic structure of LiGa0.5In0.5Se2 is elucidated from theoretical and experimental points of view. Notably, total and partial densities of states (DOSs) of the LiGa0.5In0.5Se2 compound are calculated based on density functional theory (DFT) using the augmented plane wave + local orbitals (APW + lo) method. In accordance with the DFT calculations, the principal contributors to the valence band are the Se 4p states, making the main input at the top and in the upper part of the band, while its bottom is dominated by contributions of the valence s states associated with Ga and In atoms. The theoretical total DOS curve peculiarities are found to be in excellent agreement with the shape of the X-ray photoelectron valence-band spectrum of the LiGa0.5In0.5Se2 single crystal. The bottom of the conduction band of LiGa0.5In0.5Se2 is formed mainly by contributions of the unoccupied Ga 4s and In 5s states in almost equal proportion, with somewhat smaller contributions of the unoccupied Se 4p states as well. Our calculations indicate that the LiGa0.5In0.5Se2 compound is a direct gap semiconductor. The principal optical constants of LiGa0.5In0.5Se2 are calculated in the present work.

Impurity-Band Model for GaP1-xNx

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fluegel, B.; Zhang, Y.; Geisz, J. F.

2005-11-01

Low-temperature absorption studies on free-standing GaP1-xNx films provide direct experimental evidence that the host conduction-band minimum (CBM) near X1C does not plunge downward with increased nitrogen doping, contrary to what has been suggested recently; rather, it remains stationary for x up to 0.1%. This fact, combined with the results of earlier studies of the CBM at ..GAMMA.. and conduction-band edge near L, confirms that the giant bandgap lowering observed in GaP1-xNx results from a CBM that evolves purely from nitrogen impurity bands.

Exchange-mediated anisotropy of (ga,mn)as valence-band probed by resonant tunneling spectroscopy.

PubMed

Elsen, M; Jaffrès, H; Mattana, R; Tran, M; George, J-M; Miard, A; Lemaître, A

2007-09-21

We report on experiments and theory of resonant tunneling anisotropic magnetoresistance (TAMR) in AlAs/GaAs/AlAs quantum wells (QW) contacted by a (Ga,Mn)As ferromagnetic electrode. Such resonance effects manifest themselves by bias-dependent oscillations of the TAMR signal correlated to the successive positions of heavy (HH) and light (LH) quantized hole energy levels in GaAs QW. We have modeled the experimental data by calculating the spin-dependent resonant tunneling transmission in the frame of the 6 x 6 valence-band k.p theory. The calculations emphasize the opposite contributions of the (Ga,Mn)As HH and LH subbands near the Gamma point, unraveling the anatomy of the diluted magnetic semiconductor valence band.

Theoretical studies of optical gain tuning by hydrostatic pressure in GaInNAs/GaAs quantum wells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gladysiewicz, M.; Wartak, M. S.; Department of Physics and Computer Science, Wilfrid Laurier University, Waterloo, Ontario N2L 3C5

In order to describe theoretically the tuning of the optical gain by hydrostatic pressure in GaInNAs/GaAs quantum wells (QWs), the optical gain calculations within kp approach were developed and applied for N-containing and N-free QWs. The electronic band structure and the optical gain for GaInNAs/GaAs QW were calculated within the 10-band kp model which takes into account the interaction of electron levels in the QW with the nitrogen resonant level in GaInNAs. It has been shown that this interaction increases with the hydrostatic pressure and as a result the optical gain for GaInNAs/GaAs QW decreases by about 40% and 80%more » for transverse electric and transverse magnetic modes, respectively, for the hydrostatic pressure change from 0 to 40 kilobars. Such an effect is not observed for N-free QWs where the dispersion of electron and hole energies remains unchanged with the hydrostatic pressure. This is due to the fact that the conduction and valence band potentials in GaInAs/GaAs QW scale linearly with the hydrostatic pressure.« less

Oxygen deficiency and Sn doping of amorphous Ga{sub 2}O{sub 3}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Heinemann, M. D.; Unold, T.; Berry, J.

2016-01-11

The potential of effectively n-type doping Ga{sub 2}O{sub 3} considering its large band gap has made it an attractive target for integration into transistors and solar cells. As a result amorphous GaO{sub x} is now attracting interest as an electron transport layer in solar cells despite little information on its opto-electrical properties. Here we present the opto-electronic properties, including optical band gap, electron affinity, and charge carrier density, for amorphous GaO{sub x} thin films deposited by pulsed laser deposition. These properties are strongly dependent on the deposition temperature during the deposition process. The deposition temperature has no significant influence onmore » the general structural properties but produces significant changes in the oxygen stoichiometry of the films. The density of the oxygen vacancies is found to be related to the optical band gap of the GaO{sub x} layer. It is proposed that the oxygen deficiency leads to defect band below the conduction band minimum that increases the electron affinity. These properties facilitate the use of amorphous GaO{sub x} as an electron transport layer in Cu(In,Ga)Se{sub 2} and in Cu{sub 2}O solar cells. Further it is shown that at low deposition temperatures, extrinsic doping with Sn is effective at low Sn concentrations.« less

Molecular Beam Epitaxial Growth of Iron Nitrides on Zinc-Blende Gallium Nitride(001)

NASA Astrophysics Data System (ADS)

Pak, Jeongihm; Lin, Wenzhi; Chinchore, Abhijit; Wang, Kangkang; Smith, Arthur R.

2008-03-01

Iron nitrides are attractive materials for their high magnetic moments, corrosion, and oxidation resistance. We present the successful epitaxial growth of iron nitride on zinc-blende gallium nitride (c-GaN) in order to develop a novel magnetic transition metal nitride/semiconductor system. First, GaN is grown on magnesium oxide (MgO) substrates having (001) orientation using rf N2-plasma molecular beam epitaxy. Then we grow FeN at substrate temperature of ˜ 210 ^oC up to a thickness of ˜ 10.5 nm. In-situ reflection high-energy electron diffraction (RHEED) is used to monitor the surface during growth. Initial results suggest that the epitaxial relationship is FeN[001] || GaN[001] and FeN[100] || GaN[100]. Work in progress is to investigate the surface using in-situ scanning tunneling microscopy (STM) to reveal the surface structure at atomic scale, as well as to explore more Fe-rich magnetic phases.

Thermoreflectance characterization of beta-Ga2O3 thin-film nanostrips.

PubMed

Ho, Ching-Hwa; Tseng, Chiao-Yeh; Tien, Li-Chia

2010-08-02

Nanostructure of beta-Ga(2)O(3) is wide-band-gap material with white-light-emission function because of its abundance in gap states. In this study, the gap states and near-band-edge transitions in beta-Ga(2)O(3) nanostrips have been characterized using temperature-dependent thermoreflectance (TR) measurements in the temperature range between 30 and 320 K. Photoluminescence (PL) measurements were carried to identify the gap-state transitions in the beta-Ga(2)O(3) nanostrips. Experimental analysis of the TR spectra revealed that the direct gap (E(0)) of beta-Ga(2)O(3) is 4.656 eV at 300 K. There are a lot of gap-state and near-band-edge (GSNBE) transitions denoted as E(D3), E(W1), E(W2), E(W3), E(D2), EDBex, E(DB), E(D1), E(0), and E(0)' can be detected in the TR and PL spectra at 30 K. Transition origins for the GSNBE features in the beta-Ga(2)O(3) nanostrips are respectively evaluated. Temperature dependences of transition energies of the GSNBE transitions in the beta-Ga(2)O(3) nanostrips are analyzed. The probable band scheme for the GSNBE transitions in the beta-Ga(2)O(3) nanostrips is constructed.

Infrared-sensitive photocathode

DOEpatents

Mariella, Jr., Raymond P.; Cooper, Gregory A.

1995-01-01

A single-crystal, multi-layer device incorporating an IR absorbing layer that is compositionally different from the Ga.sub.x Al.sub.1-x Sb layer which acts as the electron emitter. Many different IR absorbing layers can be envisioned for use in this embodiment, limited only by the ability to grow quality material on a chosen substrate. A non-exclusive list of possible IR absorbing layers would include GaSb, InAs and InAs/Ga.sub.w In.sub.y Al.sub.1-y-w Sb superlattices. The absorption of the IR photon excites an electron into the conduction band of the IR absorber. An externally applied electric field then transports electrons from the conduction band of the absorber into the conduction band of the Ga.sub.x Al.sub.1-x Sb, from which they are ejected into vacuum. Because the band alignments of Ga.sub.x Al.sub.1-x Sb can be made the same as that of GaAs, emitting efficiencies comparable to GaAs photocathodes are obtainable. The present invention provides a photocathode that is responsive to wavelengths within the range of 0.9 .mu.m to at least 10 .mu.m.

Effects of strain and thickness on the electronic and optical behaviors of two-dimensional hexagonal gallium nitride

NASA Astrophysics Data System (ADS)

Behzad, Somayeh

2017-06-01

The full potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory has been used to study effects of strain and thickness on the electronic and optical properties of two-dimensional GaN. The band gap of monolayer and bilayer GaN under compressive in-plane strain change from indirect to direct with bond length shortening. Also, the semiconductor to semimetal transition occurs for monolayer and bilayer GaN under in-plane tensile strain with bond length elongation. It is found that the tensile and compressive strains cause the red and blue shifts in the optical spectra, respectively, for both monolayer and bilayer GaN. Applying the perpendicular strain on the bilayer GaN by decreasing the inter layer distance leads to the shift of valence band maximum towards the Γ point in the band structure and shift of peak positions and variation of peak intensities in ε2(ω) spectrum. The results show that the n-layer GaN has an indirect band gap for n < 16. The results suggest that monolayer and multilayer GaN are good candidates for application in optoelectronics and flexible electronics.

Infrared-sensitive photocathode

DOEpatents

Mariella, R.P. Jr.; Cooper, G.A.

1995-04-04

A single-crystal, multi-layer device is described incorporating an IR absorbing layer that is compositionally different from the Ga{sub x}Al{sub 1{minus}x}Sb layer which acts as the electron emitter. Many different IR absorbing layers can be envisioned for use in this embodiment, limited only by the ability to grow quality material on a chosen substrate. A non-exclusive list of possible IR absorbing layers would include GaSb, InAs and InAs/Ga{sub w}In{sub y}Al{sub 1{minus}y{minus}w}Sb superlattices. The absorption of the IR photon excites an electron into the conduction band of the IR absorber. An externally applied electric field then transports electrons from the conduction band of the absorber into the conduction band of the Ga{sub x}Al{sub 1{minus}x}Sb, from which they are ejected into vacuum. Because the band alignments of Ga{sub x}Al{sub 1{minus}x}Sb can be made the same as that of GaAs, emitting efficiencies comparable to GaAs photocathodes are obtainable. The present invention provides a photocathode that is responsive to wavelengths within the range of 0.9 {mu}m to at least 10 {mu}m. 9 figures.

The electronic structure of lithium metagallate.

PubMed

Johnson, N W; McLeod, J A; Moewes, A

2011-11-09

Herein we present a study of the electronic structure of lithium metagallate (LiGaO(2)), a material of interest in the field of optoelectronics. We use soft x-ray spectroscopy to probe the electronic structure of both the valence and conduction bands and compare our measurements to ab initio density functional theory calculations. We use several different exchange-correlation functionals, but find that no single theoretical approach used herein accurately quantifies both the band gap and the Ga 3d(10) states in LiGaO(2). We derive a band gap of 5.6 eV, and characterize electron hybridization in both the valence and conduction bands. Our study of the x-ray spectra may prove useful in analysing spectra from more complicated LiGaO(2) heterostructures. © 2011 IOP Publishing Ltd

Synchrotron radiation based cross-sectional scanning photoelectron microscopy and spectroscopy of n-ZnO:Al/p-GaN:Mg heterojunction

NASA Astrophysics Data System (ADS)

Lee, Kai-Hsuan; Chang, Ping-Chuan; Chen, Tse-Pu; Chang, Sheng-Po; Shiu, Hung-Wei; Chang, Lo-Yueh; Chen, Chia-Hao; Chang, Shoou-Jinn

2013-02-01

Al-doped ZnO (AZO) deposited by radio frequency co-sputtering is formed on epitaxial Mg-doped GaN template at room temperature to achieve n-AZO/p-GaN heterojunction. Alignment of AZO and GaN bands is investigated using synchrotron radiation based cross-sectional scanning photoelectron microscopy and spectroscopy on the nonpolar side-facet of a vertically c-axis aligned heterostructure. It shows type-II band configuration with valence band offset of 1.63 ± 0.1 eV and conduction band offset of 1.61 ± 0.1 eV, respectively. Rectification behavior is clearly observed, with a ratio of forward-to-reverse current up to six orders of magnitude when the bias is applied across the p-n junction.

Synchrotron radiation based cross-sectional scanning photoelectron microscopy and spectroscopy of n-ZnO:Al/p-GaN:Mg heterojunction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Kai-Hsuan; Chen, Chia-Hao; Chang, Ping-Chuan

2013-02-18

Al-doped ZnO (AZO) deposited by radio frequency co-sputtering is formed on epitaxial Mg-doped GaN template at room temperature to achieve n-AZO/p-GaN heterojunction. Alignment of AZO and GaN bands is investigated using synchrotron radiation based cross-sectional scanning photoelectron microscopy and spectroscopy on the nonpolar side-facet of a vertically c-axis aligned heterostructure. It shows type-II band configuration with valence band offset of 1.63 {+-} 0.1 eV and conduction band offset of 1.61 {+-} 0.1 eV, respectively. Rectification behavior is clearly observed, with a ratio of forward-to-reverse current up to six orders of magnitude when the bias is applied across the p-n junction.

Quantum-confinement effects on conduction band structure of rectangular cross-sectional GaAs nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tanaka, H., E-mail: tanaka@semicon.kuee.kyoto-u.ac.jp; Morioka, N.; Mori, S.

2014-02-07

The conduction band structure and electron effective mass of GaAs nanowires with various cross-sectional shapes and orientations were calculated by two methods, a tight-binding method and an effective mass equation taking the bulk full-band structure into account. The effective mass of nanowires increases as the cross-sectional size decreases, and this increase in effective mass depends on the orientations and substrate faces of nanowires. Among [001], [110], and [111]-oriented rectangular cross-sectional GaAs nanowires, [110]-oriented nanowires with wider width along the [001] direction showed the lightest effective mass. This dependence originates from the anisotropy of the Γ valley of bulk GaAs. Themore » relationship between effective mass and bulk band structure is discussed.« less

Iron minerals within specific microfossil morphospecies of the 1.88 Ga Gunflint Formation.

PubMed

Lepot, Kevin; Addad, Ahmed; Knoll, Andrew H; Wang, Jian; Troadec, David; Béché, Armand; Javaux, Emmanuelle J

2017-03-23

Problematic microfossils dominate the palaeontological record between the Great Oxidation Event 2.4 billion years ago (Ga) and the last Palaeoproterozoic iron formations, deposited 500-600 million years later. These fossils are often associated with iron-rich sedimentary rocks, but their affinities, metabolism, and, hence, their contributions to Earth surface oxidation and Fe deposition remain unknown. Here we show that specific microfossil populations of the 1.88 Ga Gunflint Iron Formation contain Fe-silicate and Fe-carbonate nanocrystal concentrations in cell interiors. Fe minerals are absent in/on all organically preserved cell walls. These features are consistent with in vivo intracellular Fe biomineralization, with subsequent in situ recrystallization, but contrast with known patterns of post-mortem Fe mineralization. The Gunflint populations that display relatively large cells (thick-walled spheres, filament-forming rods) and intra-microfossil Fe minerals are consistent with oxygenic photosynthesizers but not with other Fe-mineralizing microorganisms studied so far. Fe biomineralization may have protected oxygenic photosynthesizers against Fe 2+ toxicity during the Palaeoproterozoic.

Band alignment of atomic layer deposited SiO2 and HfSiO4 with (\\bar{2}01) β-Ga2O3

NASA Astrophysics Data System (ADS)

Carey, Patrick H., IV; Ren, Fan; Hays, David C.; Gila, Brent P.; Pearton, Stephen J.; Jang, Soohwan; Kuramata, Akito

2017-07-01

The valence band offset at both SiO2/β-Ga2O3 and HfSiO4/β-Ga2O3 heterointerfaces was measured using X-ray photoelectron spectroscopy. Both dielectrics were deposited by atomic layer deposition (ALD) onto single-crystal β-Ga2O3. The bandgaps of the materials were determined by reflection electron energy loss spectroscopy as 4.6 eV for Ga2O3, 8.7 eV for Al2O3 and 7.0 eV for HfSiO4. The valence band offset was determined to be 1.23 ± 0.20 eV (straddling gap, type I alignment) for ALD SiO2 on β-Ga2O3 and 0.02 ± 0.003 eV (also type I alignment) for HfSiO4. The respective conduction band offsets were 2.87 ± 0.70 eV for ALD SiO2 and 2.38 ± 0.50 eV for HfSiO4, respectively.

Electron tunneling spectroscopy study of electrically active traps in AlGaN/GaN high electron mobility transistors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Jie, E-mail: jie.yang@yale.edu; Cui, Sharon; Ma, T. P.

2013-11-25

We investigate the energy levels of electron traps in AlGaN/GaN high electron mobility transistors by the use of electron tunneling spectroscopy. Detailed analysis of a typical spectrum, obtained in a wide gate bias range and with both bias polarities, suggests the existence of electron traps both in the bulk of AlGaN and at the AlGaN/GaN interface. The energy levels of the electron traps have been determined to lie within a 0.5 eV band below the conduction band minimum of AlGaN, and there is strong evidence suggesting that these traps contribute to Frenkel-Poole conduction through the AlGaN barrier.

First principles investigation of SiC/AlGaN(0001) band offset

NASA Astrophysics Data System (ADS)

Kojima, E.; Endo, K.; Shirakawa, H.; Chokawa, K.; Araidai, M.; Ebihara, Y.; Kanemura, T.; Onda, S.; Shiraishi, K.

2017-06-01

We are attempting to develop a new type of vertical MOSFET with SiC/AlGaN heterojunction. Toward the realization of the vertical MOSFET, the control of conduction-band offset is one of the crucial subjects. We investigated the conduction-band offset of 4H-SiC/AlxGa1-xN interface by the first-principles electronic structure calculations. We found that the offset of the interface with 40% Al content becomes almost zero. Therefore, 4H-SiC/Al0.4Ga0.6N interface is one of the most promising candidates for the vertical MOSFET in future power conversion devices.

Multi-junction, monolithic solar cell using low-band-gap materials lattice matched to GaAs or Ge

DOEpatents

Olson, Jerry M.; Kurtz, Sarah R.; Friedman, Daniel J.

2001-01-01

A multi-junction, monolithic, photovoltaic solar cell device is provided for converting solar radiation to photocurrent and photovoltage with improved efficiency. The solar cell device comprises a plurality of semiconductor cells, i.e., active p/n junctions, connected in tandem and deposited on a substrate fabricated from GaAs or Ge. To increase efficiency, each semiconductor cell is fabricated from a crystalline material with a lattice constant substantially equivalent to the lattice constant of the substrate material. Additionally, the semiconductor cells are selected with appropriate band gaps to efficiently create photovoltage from a larger portion of the solar spectrum. In this regard, one semiconductor cell in each embodiment of the solar cell device has a band gap between that of Ge and GaAs. To achieve desired band gaps and lattice constants, the semiconductor cells may be fabricated from a number of materials including Ge, GaInP, GaAs, GaInAsP, GaInAsN, GaAsGe, BGaInAs, (GaAs)Ge, CuInSSe, CuAsSSe, and GaInAsNP. To further increase efficiency, the thickness of each semiconductor cell is controlled to match the photocurrent generated in each cell. To facilitate photocurrent flow, a plurality of tunnel junctions of low-resistivity material are included between each adjacent semiconductor cell. The conductivity or direction of photocurrent in the solar cell device may be selected by controlling the specific p-type or n-type characteristics for each active junction.

Electronic structure modeling of InAs/GaSb superlattices with hybrid density functional theory

NASA Astrophysics Data System (ADS)

Garwood, T.; Modine, N. A.; Krishna, S.

2017-03-01

The application of first-principles calculations holds promise for greatly improving our understanding of semiconductor superlattices. Developing a procedure to accurately predict band gaps using hybrid density functional theory lays the groundwork for future studies investigating more nuanced properties of these structures. Our approach allows a priori prediction of the properties of SLS structures using only the band gaps of the constituent materials. Furthermore, it should enable direct investigation of the effects of interface structure, e.g., intermixing or ordering at the interface, on SLS properties. In this paper, we present band gap data for various InAs/GaSb type-II superlattice structures calculated using the generalized Kohn-Sham formulation of density functional theory. A PBE0-type hybrid functional was used, and the portion of the exact exchange was tuned to fit the band gaps of the binary compounds InAs and GaSb with the best agreement to bulk experimental values obtained with 18% of the exact exchange. The heterostructures considered in this study are 6 monolayer (ML) InAs/6 ML GaSb, 8 ML InAs/8 ML GaSb and 10 ML InAs/10 ML GaSb with deviations from the experimental band gaps ranging from 3% to 11%.

Electronic structure modeling of InAs/GaSb superlattices with hybrid density functional theory

DOE Office of Scientific and Technical Information (OSTI.GOV)

Garwood, Tristan; Modine, Normand A.; Krishna, S.

2016-12-18

The application of first-principles calculations holds promise for greatly improving our understanding of semiconductor superlattices. By developing a procedure to accurately predict band gaps using hybrid density functional theory, it lays the groundwork for future studies investigating more nuanced properties of these structures. Our approach allows a priori prediction of the properties of SLS structures using only the band gaps of the constituent materials. Furthermore, it should enable direct investigation of the effects of interface structure, e.g., intermixing or ordering at the interface, on SLS properties. In this paper, we present band gap data for various InAs/GaSb type-II superlattice structuresmore » calculated using the generalized Kohn-Sham formulation of density functional theory. A PBE0-type hybrid functional was used, and the portion of the exact exchange was tuned to fit the band gaps of the binary compounds InAs and GaSb with the best agreement to bulk experimental values obtained with 18% of the exact exchange. The heterostructures considered in this study are 6 monolayer (ML) InAs/6 ML GaSb, 8 ML InAs/8 ML GaSb and 10 ML InAs/10 ML GaSb with deviations from the experimental band gaps ranging from 3% to 11%.« less

Comparison of as-grown and annealed GaN/InGaN : Mg samples

NASA Astrophysics Data System (ADS)

Deng, Qingwen; Wang, Xiaoliang; Xiao, Hongling; Wang, Cuimei; Yin, Haibo; Chen, Hong; Lin, Defeng; Jiang, Lijuan; Feng, Chun; Li, Jinmin; Wang, Zhanguo; Hou, Xun

2011-08-01

Mg-doped InGaN was grown on unintentionally doped GaN layer, and Mg and defect behaviours in both GaN and InGaN : Mg were investigated through photoluminescence measurement at 7 K. Mg acceptor was found in unintentionally doped GaN after thermal annealing in N2 ambient, and Mg activation energy was estimated to be 200 meV and 110 meV for GaN and InGaN, respectively. Particularly, the ultraviolet band (3.0-3.2 eV) in the GaN layer was infrequently observed in the unannealed sample but quenched in the annealed sample; this band may be associated with oxygen-substituted nitrogen defects. Moreover, the measurement errors of photoluminescence and x-ray diffraction originated from strain were taken into account.

X-ray photoelectron spectroscopy investigations of band offsets in Ga0.02Zn0.98O/ZnO heterojunction for UV photodetectors

NASA Astrophysics Data System (ADS)

Singh, Karmvir; Rawal, Ishpal; Punia, Rajesh; Dhar, Rakesh

2017-10-01

Here, we report the valence and conduction band offset measurements in pure ZnO and the Ga0.02Zn0.98O/ZnO heterojunction by X-Ray photoelectron spectroscopy studies for UV photodetector applications. For detailed investigations on the band offsets and UV photodetection behavior of Ga0.02Zn0.98O/ZnO heterostructures, thin films of pristine ZnO, Ga-doped ZnO (Ga0.02Zn0.98O), and heterostructures of Ga-doped ZnO with ZnO (Ga0.02Zn0.98O/ZnO) were deposited using a pulsed laser deposition technique. The deposited thin films were characterized by X-ray diffraction, atomic force microscopy, and UV-Vis spectroscopy. X-ray photoelectron spectroscopy studies were carried out on all the thin films for the investigation of valence and conduction band offsets. The valence band was found to be shifted by 0.28 eV, while the conduction band has a shifting of -0.272 eV in the Ga0.02Zn0.98O/ZnO heterojunction as compared to pristine ZnO thin films. All the three samples were analyzed for photoconduction behavior under UVA light of the intensity of 3.3 mW/cm2, and it was observed that the photoresponse of pristine ZnO (19.75%) was found to increase with 2 wt. % doping of Ga (22.62%) and heterostructured thin films (29.10%). The mechanism of UV photodetection in the deposited samples has been discussed in detail, and the interaction of chemisorbed oxygen on the ZnO surface with holes generated by UV light exposure has been the observed mechanism for the change in electrical conductivity responsible for UV photoresponse on the present deposited ZnO films.

Surface and bulk electronic structures of unintentionally and Mg-doped In0.7Ga0.3N epilayer by hard X-ray photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Imura, Masataka; Tsuda, Shunsuke; Takeda, Hiroyuki; Nagata, Takahiro; Banal, Ryan G.; Yoshikawa, Hideki; Yang, AnLi; Yamashita, Yoshiyuki; Kobayashi, Keisuke; Koide, Yasuo; Yamaguchi, Tomohiro; Kaneko, Masamitsu; Uematsu, Nao; Wang, Ke; Araki, Tsutomu; Nanishi, Yasushi

2018-03-01

The surface and bulk electronic structures of In0.7Ga0.3N epilayers are investigated by angle-resolved hard X-ray photoelectron spectroscopy (HX-PES) combined with soft X-PES. The unintentionally and Mg-doped In0.7Ga0.3N (u-In0.7Ga0.3N and In0.7Ga0.3N:Mg, respectively) epilayers are grown by radio-frequency plasma-assisted molecular beam epitaxy. Here three samples with different Mg concentrations ([Mg] = 0, 7 × 1019, and 4 × 1020 cm-3) are chosen for comparison. It is found that a large downward energy band bending exists in all samples due to the formation of a surface electron accumulation (SEA) layer. For u-In0.7Ga0.3N epilayer, band bending as large as 0.8 ± 0.05 eV occurs from bulk to surface. Judged from the valence band spectral edge and numerical analysis of energy band with a surface quantum well, the valence band maximum (VBM) with respect to Fermi energy (EF) level in the bulk is determined to be 1.22 ± 0.05 eV. In contrast, for In0.7Ga0.3N:Mg epilayers, the band bending increases and the VBM only in the bulk tends to shift toward the EF level owing to the Mg acceptor doping. Hence, the energy band is considered to exhibit a downward bending structure due to the coexistence of the n+ SEA layer and Mg-doped p layer formed in the bulk. When [Mg] changes from 7 × 1019 to 4 × 1020 cm-3, the peak split occurs in HX-PES spectra under the bulk sensitive condition. This result indicates that the energy band forms an anomalous downward bending structure with a singular point due to the generation of a thin depleted region at the n+ p interface. For In0.7Ga0.3N:Mg epilayers, the VBM in the bulk is assumed to be slightly lower than EF level within 0.1 eV.

Rectifying behavior in the GaN/graded-AlxGa1‑xN/GaN double heterojunction structure

NASA Astrophysics Data System (ADS)

Wang, Caiwei; Jiang, Yang; Ma, Ziguang; Zuo, Peng; Yan, Shen; Die, Junhui; Wang, Lu; Jia, Haiqiang; Wang, Wenxin; Chen, Hong

2018-05-01

Rectifying characteristics induced by the polarization fields are achieved in the GaN/graded-AlxGa1‑xN/GaN double heterojunction structure (DHS). By grading AlxGa1‑xN from x  =  0.4(0.3) to 0.1, the DHS displays a better conductivity for smaller reverse bias than for forward bias voltages (reverse rectifying behavior) which is opposite to p–n junction rectifying characteristics. The mechanism of reverse rectifying behavior is illustrated via calculating the energy band structures of the samples. The band gap narrowing caused by decreasing Al composition could compensate the for the band tilt due to the polarization effect in AlxGa1‑xN barriers, thus lowering the barrier height for electron transport from top to bottom. The reverse rectifying behavior could be enhanced by increasing the Al content and the thickness of the multi-layer graded AlxGa1‑xN barriers. This work gives a better understanding of the mechanism of carrier transport in a DHS and makes it possible to realize novel GaN-based heterojunction transistors.

Change in optimum genetic algorithm solution with changing band discontinuities and band widths of electrically conducting copolymers

NASA Astrophysics Data System (ADS)

Kaur, Avneet; Bakhshi, A. K.

2010-04-01

The interest in copolymers stems from the fact that they present interesting electronic and optical properties leading to a variety of technological applications. In order to get a suitable copolymer for a specific application, genetic algorithm (GA) along with negative factor counting (NFC) method has recently been used. In this paper, we study the effect of change in the ratio of conduction band discontinuity to valence band discontinuity (Δ Ec/Δ Ev) on the optimum solution obtained from GA for model binary copolymers. The effect of varying bandwidths on the optimum GA solution is also investigated. The obtained results show that the optimum solution changes with varying parameters like band discontinuity and band width of constituent homopolymers. As the ratio Δ Ec/Δ Ev increases, band gap of optimum solution decreases. With increasing band widths of constituent homopolymers, the optimum solution tends to be dependent on the component with higher band gap.

First principles calculations of the electronic structure and magnetic properties of Y(Fe,M)9.2 and Y(Fe,M)9.2C (M= Si, Ga, Zr)

NASA Astrophysics Data System (ADS)

Tian, Guang; Zha, Liang; Yang, Wenyun; Qiao, Guanyi; Wang, Changsheng; Yang, Yingchang; Yang, Jinbo

2018-06-01

The preferential site substitution of the Fe by Si, Ga and Zr in the Y(Fe,M)9.2 and Y(Fe,M)9.2C compounds, and the doping effects on the magnetic properties have been studied by the first-principles calculations. It is found that the doping of the Si or Zr can improve the thermodynamic stability of the 1:9 phase, while the substitution of the Fe by Ga makes it unstable. Si atom tends to enter the 3g crystal site and Zr prefers to occupy the 2e site when Y(Fe,M)9.2 and their carbides are synthesized. Although the substitution of the Fe by Si and Zr will reduce the total magnetic moments of the YFe9.2 and their carbides, the volumetric and the d-band narrowing effects caused by the doping can still modify the electron density distributions of the Fe near the Fermi level, improving the magnetic ordering temperature of the non-carbonated compound YFe9.2. The calculated magnetic ordering temperatures of Y(Fe,M)9.2C decrease with the increasing content of the doping elements M due to the stronger hybridization of the d bands in the carbides. For the rare-earth(RE) iron based intermetallics REFe9.2 with the TbCu7-type structure, it is suggested that Zr is able to stabilize the phase and enhance the magnetic ordering temperature, indicating the possible further application in the field of permanent magnets, which has not been reported before.

Effect of quantum-well thickness on the optical polarization of AlGaN-based ultraviolet light-emitting diodes

NASA Astrophysics Data System (ADS)

Liu, Cheng; Zhang, Jing

2018-02-01

Optical polarization from AlGaN quantum well (QW) is crucial for realizing high-efficiency deep-ultraviolet (UV) light-emitting diodes (LEDs) because it determines the light emission patterns and light extraction mechanism of the devices. As the Al-content of AlGaN QW increases, the valence bands order changes and consequently the light polarization switches from transverse-electric (TE) to transverse-magnetic (TM) owing to the different sign and the value of the crystal field splitting energy between AlN (-169meV) and GaN (10meV). Several groups have reported that the ordering of the bands and the TE/TM crossover Al-content could be influenced by the strain state and the quantum confinement from the AlGaN QW system. In this work, we investigate the influence of QW thickness on the optical polarization switching point from AlGaN QW with AlN barriers by using 6-band k•p model. The result presents a decreasing trend of the critical Al-content where the topmost valence band switches from heave hole (HH) to crystal field spilt-off (CH) with increasing QW thicknesses due to the internal electric field and the strain state from the AlGaN QW. Instead, the TE- and TM-polarized spontaneous emission rates switching Al-content rises first and falls later because of joint consequence of the band mixing effect and the Quantum Confined Stark Effect. The reported optical polarization from AlGaN QW emitters in the UV spectral range is assessed in this work and the tendency of the polarization switching point shows great consistency with the theoretical results, which deepens the understanding of the physics from AlGaN QW UV LEDs.

Visibility of an iron-containing fiducial marker in magnetic resonance imaging for high-precision external beam prostate radiotherapy.

PubMed

Tanaka, Osamu; Komeda, Hisao; Hirose, Shigeki; Taniguchi, Takuya; Ono, Kousei; Matsuo, Masayuki

2017-11-29

Visualization of fiducial gold markers is critical for registration on computed tomography (CT) and magnetic resonance imaging (MRI) for imaging-guided radiotherapy. Although larger markers provide better visualization on MRI, they tend to generate artifacts on CT. MRI is strongly influenced by the presence of metals, such as iron, in the body. Here we compared efficacies of a 0.5% iron-containing gold marker (GM) and a traditional non-iron-containing marker. Twenty-seven patients underwent CT/MRI fusion-based intensity-modulated radiotherapy. Markers were placed by urologists under local anesthesia. Gold Anchor (GA; diameter: 0.28 mm; length: 10 mm), an iron-containing marker, was placed on the right side of the prostate using a 22-G needle and VISICOIL (VIS; diameter: 0.35 mm; length: 10 mm), a non-iron-containing marker, was placed on the left side using a 19-G needle. T2*-weighted images MRI sequences were obtained. Two radiation oncologists and a radiation technologist evaluated and assigned scores for visual quality on a five-point scale (1, poor; 5, best visibility). Artifact generation on CT was slightly greater with GA than with VIS. The mean marker visualization scores on MRI of all three observers were significantly superior for GA than for VIS (3.5 vs 3.2, 3.9 vs 3.2, and 4.0 vs 2.9). The actual size of the spherical GA was about 2 mm in diameter, but the signal void on MRI was approximately 5 mm. Although both markers were well visualized and can be recommended clinically, the results suggest that GA has some subtle advantages for quantitative visualization that could prove useful in certain situations of stereotactic body radiotherapy and intensity-modulated radiotherapy. © 2017 John Wiley & Sons Australia, Ltd.

Electron affinity and surface states of GaN m -plane facets: Implication for electronic self-passivation

NASA Astrophysics Data System (ADS)

Portz, V.; Schnedler, M.; Eisele, H.; Dunin-Borkowski, R. E.; Ebert, Ph.

2018-03-01

The electron affinity and surface states are of utmost importance for designing the potential landscape within (heterojunction) nanowires and hence for tuning conductivity and carrier lifetimes. Therefore, we determined for stoichiometric nonpolar GaN (10 1 ¯0 ) m -plane facets, i.e., the dominating sidewalls of GaN nanowires, the electron affinity to 4.06 ±0.07 eV and the energy of the empty Ga-derived surface state in the band gap to 0.99 ±0.08 eV below the conduction band minimum using scanning tunneling spectroscopy. These values imply that the potential landscape within GaN nanowires is defined by a surface state-induced Fermi-level pinning, creating an upward band bending at the sidewall facets, which provides an electronic passivation.

Ferromagnetism and the electronic band structure in (Ga,Mn)(Bi,As) epitaxial layers

NASA Astrophysics Data System (ADS)

Yastrubchak, O.; Sadowski, J.; Gluba, L.; Domagala, J. Z.; Rawski, M.; Żuk, J.; Kulik, M.; Andrearczyk, T.; Wosinski, T.

2014-08-01

Impact of Bi incorporation into (Ga,Mn)As layers on their electronic- and band-structures as well as their magnetic and structural properties has been studied. Homogenous (Ga,Mn)(Bi,As) layers of high structural perfection have been grown by the low-temperature molecular-beam epitaxy technique. Post-growth annealing treatment of the layers results in an improvement of their structural and magnetic properties and an increase in the hole concentration in the layers. The modulation photoreflectance spectroscopy results are consistent with the valence-band model of hole-mediated ferromagnetism in the layers. This material combines the properties of (Ga,Mn)As and Ga(Bi,As) ternary compounds and offers the possibility of tuning its electrical and magnetic properties by controlling the alloy composition.

A comparative study of thermoelectric properties of CuGaTe2 by using PBE and MBJ potentials

NASA Astrophysics Data System (ADS)

Sharma, Sonu; Singh, Birender; Kumar, Pradeep

2018-04-01

We have investigated the electronic and thermoelectric properties of CuGaTe2 by combining the first principle calculations with Boltzmann transport theory. The electronic properties show that CuGaTe2 is a direct band semiconductor with large band gap at r-point. The band gaps are computed by using PBE and mBJ potentials and value obtained with mBJ is much closer to the experimental value. Partial density of states plots show that the band gap is formed by the hybridization between 3d states of Cu atom, 4s and 4p states of Ga atom and 5p states of Te atom. Very large value (˜300 µVK-1) of Seebeck coefficient is obtained for this compound. Figure-of-merit calculated by using transport coefficients is also found to be very large for the entire temperature range and CuGaTe2 is a good thermoelectric material.

Direct Band Gap Gallium Antimony Phosphide (GaSbxP1−x) Alloys

PubMed Central

Russell, H. B.; Andriotis, A. N.; Menon, M.; Jasinski, J. B.; Martinez-Garcia, A.; Sunkara, M. K.

2016-01-01

Here, we report direct band gap transition for Gallium Phosphide (GaP) when alloyed with just 1–2 at% antimony (Sb) utilizing both density functional theory based computations and experiments. First principles density functional theory calculations of GaSbxP1−x alloys in a 216 atom supercell configuration indicate that an indirect to direct band gap transition occurs at x = 0.0092 or higher Sb incorporation into GaSbxP1−x. Furthermore, these calculations indicate band edge straddling of the hydrogen evolution and oxygen evolution reactions for compositions ranging from x = 0.0092 Sb up to at least x = 0.065 Sb making it a candidate for use in a Schottky type photoelectrochemical water splitting device. GaSbxP1−x nanowires were synthesized by reactive transport utilizing a microwave plasma discharge with average compositions ranging from x = 0.06 to x = 0.12 Sb and direct band gaps between 2.21 eV and 1.33 eV. Photoelectrochemical experiments show that the material is photoactive with p-type conductivity. This study brings attention to a relatively uninvestigated, tunable band gap semiconductor system with tremendous potential in many fields. PMID:26860470

Valence-band-edge shift due to doping in p + GaAs

NASA Astrophysics Data System (ADS)

Silberman, J. A.; de Lyon, T. J.; Woodall, J. M.

1991-05-01

Accurate knowledge of the shifts in valence- and conduction-band edges due to heavy doping effects is crucial in modeling GaAs device structures that utilize heavily doped layers. X-ray photoemission spectroscopy was used to deduce the shift in the valence-band-edge induced by carbon (p type) doping to a carrier density of 1×1020 cm-3 based on a determination of the bulk binding energy of the Ga and As core levels in this material. Analysis of the data indicates that the shift of the valence-band maximum into the gap and the penetration of the Fermi level into the valence bands exactly compensate at this degenerate carrier concentration, to give ΔEv =0.12±0.05 eV.

Time-Resolved IR-Absorption Spectroscopy of Hot-Electron Dynamics in Satellite and Upper Conduction Bands in GaP

NASA Technical Reports Server (NTRS)

Cavicchia, M. A.; Alfano, R. R.

1995-01-01

The relaxation dynamics of hot electrons in the X6 and X7 satellite and upper conduction bands in GaP was directly measured by femtosecond UV-pump-IR-probe absorption spectroscopy. From a fit to the induced IR-absorption spectra the dominant scattering mechanism giving rise to the absorption at early delay times was determined to be intervalley scattering of electrons out of the X7 upper conduction-band valley. For long delay times the dominant scattering mechanism is electron-hole scattering. Electron transport dynamics of the upper conduction band of GaP has been time resolved.

Discrimination of iron ore deposits of granulite terrain of Southern Peninsular India using ASTER data

NASA Astrophysics Data System (ADS)

Rajendran, Sankaran; Thirunavukkarasu, A.; Balamurugan, G.; Shankar, K.

2011-04-01

This work describes a new image processing technique for discriminating iron ores (magnetite quartzite deposits) and associated lithology in high-grade granulite region of Salem, Southern Peninsular India using visible, near-infrared and short wave infrared reflectance data of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). Image spectra show that the magnetite quartzite and associated lithology of garnetiferrous pyroxene granulite, hornblende biotite gneiss, amphibolite, dunite, and pegmatite have absorption features around spectral bands 1, 3, 5, and 7. ASTER band ratios ((1 + 3)/2, (3 + 5)/4, (5 + 7)/6) in RGB are constructed by summing the bands representing the shoulders of absorption features as a numerator, and the band located nearest the absorption feature as a denominator to map iron ores and band ratios ((2 + 4)/3, (5 + 7)/6, (7 + 9)/8) in RGB for associated lithology. The results show that ASTER band ratios ((1 + 3)/2, (3 + 5)/4, (5 + 7)/6) in a Red-Green-Blue (RGB) color combination identifies the iron ores much better than previously published ASTER band ratios analysis. A Principal Component Analysis (PCA) is applied to reduce redundant information in highly correlated bands. PCA (3, 2, and 1 for iron ores and 5, 4, 2 for granulite rock) in RGB enabled the discrimination between the iron ores and garnetiferrous pyroxene granulite rock. Thus, this image processing technique is very much suitable for discriminating the different types of rocks of granulite region. As outcome of the present work, the geology map of Salem region is provided based on the interpretation of ASTER image results and field verification work. It is recommended that the proposed methods have great potential for mapping of iron ores and associated lithology of granulite region with similar rock units of granulite regions of Southern Peninsular India. This work also demonstrates the ability of ASTER's to provide information on iron ores, which is valuable for mineral prospecting and exploration activities.

Rhenium-osmium isotope systematics in meteorites. I - Magmatic iron meteorite groups IIAB and IIIAB

NASA Technical Reports Server (NTRS)

Morgan, John W.; Walker, Richard J.; Grossman, Jeffery N.

1992-01-01

Resonance ionization mass spectrometry is used to determine the Re and Os abundances by isotope dilution (ID) and to measure Os-187/Os-186 ratios from 19 iron meteorites. Abundances range from 1.4 to 4800 ppb Re, and from 13 to 65,000 ppb Os, and generally agree well with previous ID and neutron activation results. The Re and Os data suggest that abundance trends in these iron groups may be entirely explained by fractional crystallization. Whole-rock isochrons for the IIAB and IIIAB groups are statistically indistinguishable. Pooled data yield an initial Os-187/Os-186 of 0.794 +/- 0.010 Ga. Given the errors in the slope and half life, this age does not differ significantly from the canonical chondrite age of 4.56 Ga, but could be as young as 4.46 Ga.

Valence and conduction band offsets of β-Ga2O3/AlN heterojunction

NASA Astrophysics Data System (ADS)

Sun, Haiding; Torres Castanedo, C. G.; Liu, Kaikai; Li, Kuang-Hui; Guo, Wenzhe; Lin, Ronghui; Liu, Xinwei; Li, Jingtao; Li, Xiaohang

2017-10-01

Both β-Ga2O3 and wurtzite AlN have wide bandgaps of 4.5-4.9 and 6.1 eV, respectively. We calculated the in-plane lattice mismatch between the (-201) plane of β-Ga2O3 and the (0002) plane of AlN, which was found to be 2.4%. This is the smallest mismatch between β-Ga2O3 and binary III-nitrides which is beneficial for the formation of a high quality β-Ga2O3/AlN heterojunction. However, the valence and conduction band offsets (VBO and CBO) at the β-Ga2O3/AlN heterojunction have not yet been identified. In this study, a very thin (less than 2 nm) β-Ga2O3 layer was deposited on an AlN/sapphire template to form the heterojunction by pulsed laser deposition. High-resolution X-ray photoelectron spectroscopy revealed the core-level (CL) binding energies of Ga 3d and Al 2p with respect to the valence band maximum in individual β-Ga2O3 and AlN layers, respectively. The separation between Ga 3d and Al 2p CLs at the β-Ga2O3/AlN interface was also measured. Eventually, the VBO was found to be -0.55 ± 0.05 eV. Consequently, a staggered-gap (type II) heterojunction with a CBO of -1.75 ± 0.05 eV was determined. The identification of the band alignment of the β-Ga2O3/AlN heterojunction could facilitate the design of optical and electronic devices based on these and related alloys.

Study of GaN nanorods converted from β-Ga2O3

NASA Astrophysics Data System (ADS)

Li, Yuewen; Xiong, Zening; Zhang, Dongdong; Xiu, Xiangqian; Liu, Duo; Wang, Shuang; Hua, Xuemei; Xie, Zili; Tao, Tao; Liu, Bin; Chen, Peng; Zhang, Rong; Zheng, Youdou

2018-05-01

We report here high-quality β-Ga2O3 nanorods (NRs) grown on sapphire substrates by hydrothermal method. Ammoniating the β-Ga2O3 NRs results in strain-free wurtzite gallium nitride (GaN) NRs. It was shown by XRD and Raman spectroscopy that β-Ga2O3 was partially converted to GaN/β-Ga2O3 at 1000 °C and then completely converted to GaN NRs at 1050 °C, as confirmed by high-resolution transmission electron microscopy (HRTEM). There is no band-edge emission of β-Ga2O3 in the cathodoluminescence spectrum, and only a deep-level broad emission observed at 3.68-3.73 eV. The band edge emission (3.39 eV) of GaN NRs converted from β-Ga2O3 can also be observed.

Band alignments at Ga2O3 heterojunction interfaces with Si and Ge

NASA Astrophysics Data System (ADS)

Gibbon, J. T.; Jones, L.; Roberts, J. W.; Althobaiti, M.; Chalker, P. R.; Mitrovic, Ivona Z.; Dhanak, V. R.

2018-06-01

Amorphous Ga2O3 thin films were deposited on p-type (111) and (100) surfaces of silicon and (100) germanium by atomic layer deposition (ALD). X-ray photoelectron spectroscopy (XPS) was used to investigate the band alignments at the interfaces using the Kraut Method. The valence band offsets were determined to be 3.49± 0.08 eV and 3.47± 0.08 eV with Si(111) and Si(100) respectively and 3.51eV± 0.08 eV with Ge(100). Inverse photoemission spectroscopy (IPES) was used to investigate the conduction band of a thick Ga2O3 film and the band gap of the film was determined to be 4.63±0.14 eV. The conduction band offsets were found to be 0.03 eV and 0.05eV with Si(111) and Si(100) respectively, and 0.45eV with Ge(100). The results indicate that the heterojunctions of Ga2O3 with Si(100), Si(111) and Ge(100) are all type I heterojunctions.

A model for the energy band gap of GaSbxAs1-x and InSbxAs1-x in the whole composition range

NASA Astrophysics Data System (ADS)

Zhao, Chuan-Zhen; Ren, He-Yu; Wei, Tong; Wang, Sha-Sha; Wang, Jun

2018-04-01

The band gap evolutions of GaSbxAs1-x and InSbxAs1-x in the whole composition range are investigated. It is found that the band gap evolutions of GaSbxAs1-x and InSbxAs1-x are determined by two factors. One is the impurity-host interaction in the As-rich and Sb-rich composition ranges. The other is the intraband coupling within the conduction band and separately within the valence band in the moderate composition range. Based on the band gap evolutions of GaSbxAs1-x and InSbxAs1-x, a model is established. In addition, it is found that the impurity-host interaction is determined by not only the mismatches in size and electronegativity between the introduced atoms in the host material and the anions of the host material, but also the difference in electronegativity between the introduced atoms in the host material and the cations of the host material.

Local band gap measurements by VEELS of thin film solar cells.

PubMed

Keller, Debora; Buecheler, Stephan; Reinhard, Patrick; Pianezzi, Fabian; Pohl, Darius; Surrey, Alexander; Rellinghaus, Bernd; Erni, Rolf; Tiwari, Ayodhya N

2014-08-01

This work presents a systematic study that evaluates the feasibility and reliability of local band gap measurements of Cu(In,Ga)Se2 thin films by valence electron energy-loss spectroscopy (VEELS). The compositional gradients across the Cu(In,Ga)Se2 layer cause variations in the band gap energy, which are experimentally determined using a monochromated scanning transmission electron microscope (STEM). The results reveal the expected band gap variation across the Cu(In,Ga)Se2 layer and therefore confirm the feasibility of local band gap measurements of Cu(In,Ga)Se2 by VEELS. The precision and accuracy of the results are discussed based on the analysis of individual error sources, which leads to the conclusion that the precision of our measurements is most limited by the acquisition reproducibility, if the signal-to-noise ratio of the spectrum is high enough. Furthermore, we simulate the impact of radiation losses on the measured band gap value and propose a thickness-dependent correction. In future work, localized band gap variations will be measured on a more localized length scale to investigate, e.g., the influence of chemical inhomogeneities and dopant accumulations at grain boundaries.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shiogai, J.; Institute of Materials Research, Tohoku University, Sendai 980-8577, Miyagi; Ciorga, M., E-mail: mariusz.ciorga@ur.de

We investigate the angular dependence of the tunneling anisotropic magnetoresistance in (Ga,Mn)As/n-GaAs spin Esaki diodes in the regime where the tunneling process is dominated by the excess current through midgap states in (Ga,Mn)As. We compare it to similar measurements performed in the regime of band-to-band tunneling. Whereas the latter show biaxial symmetry typical for magnetic anisotropy observed in (Ga,Mn)As samples, the former is dominated by uniaxial anisotropy along the 〈110〉 axes.

Prediction of Quantum Anomalous Hall Insulator in half-fluorinated GaBi Honeycomb

PubMed Central

Chen, Sung-Ping; Huang, Zhi-Quan; Crisostomo, Christian P.; Hsu, Chia-Hsiu; Chuang, Feng-Chuan; Lin, Hsin; Bansil, Arun

2016-01-01

Using first-principles electronic structure calculations, we predict half-fluorinated GaBi honeycomb under tensile strain to harbor a quantum anomalous Hall (QAH) insulator phase. We show that this QAH phase is driven by a single inversion in the band structure at the Γ point. Moreover, we have computed the electronic spectrum of a half-fluorinated GaBi nanoribbon with zigzag edges, which shows that only one edge band crosses the Fermi level within the band gap. Our results suggest that half-fluorination of the GaBi honeycomb under tensile strain could provide a new platform for developing novel spintronics devices based on the QAH effect. PMID:27507248

Modeling of the absorption properties of Ga1-xInxAs1-yNy/GaAs quantum well structures for photodetection applications

NASA Astrophysics Data System (ADS)

Aissat, A.; Bestam, R.; Alshehri, B.; Vilcot, J. P.

2015-06-01

This work reports on theoretical studies on the GaInNAs material properties (bandgap, lattice mismatch, absorption coefficient) as grown on GaAs substrate. The Band Anti-Crossing (BAC) kṡp 8 × 8 model has been used to determine the influence of indium and nitrogen concentrations on the position of conduction and valence bands. The incorporation of nitrogen at a level lower than 5% causes the split of the conduction band. For indium and nitrogen concentrations of 38% and 3.5%, respectively, the strained bandgap energy is 0.70 eV and the absorption coefficient of indium and nitrogen-rich compounds increases significantly.

Prediction of Quantum Anomalous Hall Insulator in half-fluorinated GaBi Honeycomb

DOE PAGES

Chen, Sung-Ping; Huang, Zhi-Quan; Crisostomo, Christian P.; ...

2016-08-10

Using first-principles electronic structure calculations, we predict half-fluorinated GaBi honeycomb under tensile strain to harbor a quantum anomalous Hall (QAH) insulator phase. We show that this QAH phase is driven by a single inversion in the band structure at the Γ point. Moreover, we have computed the electronic spectrum of a half-fluorinated GaBi nanoribbon with zigzag edges, which shows that only one edge band crosses the Fermi level within the band gap. In conclusion, our results suggest that half-fluorination of the GaBi honeycomb under tensile strain could provide a new platform for developing novel spintronics devices based on the QAHmore » effect.« less

Isotopic Signature of the Ancient Biosphere

NASA Technical Reports Server (NTRS)

DesMarais, D. J.; Chang, Sherwood (Technical Monitor)

1997-01-01

The age distribution of 261 field localities, sampled for their well-preserved Archean and Proterozoic sedimentary rocks, revealed a 500-700 Ma episodicity. Assuming that the numbers of sites are a proxy for mass of sediments, the record of well-preserved sediments is more abundant in the intervals 3.5-3.3, 2.8-2.5, 2.1-1.8, 1.5-1.3, and 1.0-0.54 Ga than in the intervening intervals. It is proposed that the crustal inventory of photosynthetic organic carbon was modulated by the volume of sedimentation in sites favorable for the burial and long-term preservation of organic carbon. Tectonic processes controlled this sediment volume. Episodic increases in the organic inventory led to stepwise increases in oxidized reservoirs (e.g., O2, SO4(2-), Fe(3+). The interval 2.9-2.5 Ga recorded a large rise in seawater Sr-87/Sr-86, the oldest-known extensive banded iron formations, and the first evidence (C-13-depleted kerogens) of O2 use by methylotrophic bacteria. The interval 2.2-1.8 Ga has both carbon isotopic evidence for a stepwise increase in the organic reservoir and also paleosol evidence for an O2 increase. The interval 1.1-0.6 Ga shows isotopic evidence for another organic carbon increase. The interval 1.5-1.3 Ga revealed no such increases as yet, perhaps because incomplete rifting of the mid-Proterozoic supercontinent was associated with extensive sedimentation in oxidized continental basins, producing redbeds, coarse clastics, etc. Such sedimentation did not promote the burial of reduced carbon.

Heteroepitaxial growth of ɛ-(AlxGa1-x)2O3 alloy films on c-plane AlN templates by mist chemical vapor deposition

NASA Astrophysics Data System (ADS)

Tahara, Daisuke; Nishinaka, Hiroyuki; Morimoto, Shota; Yoshimoto, Masahiro

2018-04-01

In this study, ɛ-(AlxGa1-x)2O3 alloy films were grown on c-plane AlN templates by mist chemical vapor deposition. The Al content of two samples was determined by Rutherford backscattering analysis. The lattice constant of the ɛ-(AlxGa1-x)2O3 alloy films followed Vegard's law, and the Al contents of other samples were determined to be as high as x = 0.395 by Vegard's law. The direct bandgap was obtained in the range of 5.0-5.9 eV by transmittance measurements. The valence-band offset between ɛ-(Al0.395Ga0.605)2O3 and ɛ-Ga2O3 was analyzed to be 0.2 eV, and the conduction-band offset was calculated to be 0.7 eV by X-ray photoelectron spectroscopy. The ɛ-(AlxGa1-x)2O3/ɛ-Ga2O3 interface band discontinuity was type I. Our experimental results will be important for the actual application of ɛ-(AlxGa1-x)2O3/ɛ-Ga2O3 heterojunction devices.

Growth and photoluminescence study of several single crystal segments relevant to monolithic semiconductor cascade solar cells

NASA Astrophysics Data System (ADS)

Sillmon, Roger S.; Schreiner, Anton F.; Timmons, Michael

1983-09-01

Several representative single crystal stacked layers of III-V compound and alloy semiconductors were grown which are spatial regions relevant to a monolithic cascade solar cell, including the substrate, n-GaAs(Si), which was pre-growth heat treated in H 2(g) prior to its use. These structures were then studied by cryogenic laser excited photoluminescence (PL), and the substrate portion was explored in a depth profiling mode. Within the forbidden band gap region up to seven recombinations were observed and identified for undoped GaAs layers or the GaAs(Si) substrate, and several other PL recombinations were observed for undoped Al xGa 1- xAs and Al yGa 1- ySb zAs 1- z layers. In addition to the valence and conduction bands, these optical bands are also associa ted with the presence of C Ga, Si Ga, Si As, Cu Ga, V As, V Ga and vacancy-impurity complexes involving several of these defect types even in the absence of intentional doping. The findings also relate to problems of self-compensation and type inversion, so that the need for growth modifications is indicated.

Entirely relaxed lattice-mismatched GaSb/GaAs/Si(001) heterostructure grown via metalorganic chemical vapor deposition

NASA Astrophysics Data System (ADS)

Ha, Minh Thien Huu; Hoang Huynh, Sa; Binh Do, Huy; Nguyen, Tuan Anh; Luc, Quang Ho; Lee, Ching Ting; Chang, Edward Yi

2018-05-01

A GaSb epilayer is grown on a GaAs/Si(001) epitaxial substrate via metalorganic chemical vapor deposition. High-resolution transmission electron microscopy micrographs and high-resolution X-ray reciprocal space mapping indicate an entirely relaxed interfacial misfit (IMF) array GaSb epilayer. The valence-band offset and conduction-band offset of the Al2O3/GaSb/GaAs/Si structure are estimated to be 2.39 and 3.65 eV, respectively. The fabricated Al2O3/p-GaSb/GaAs/Si MOS capacitors exhibited good capacitance–voltage characteristics with a small accumulation frequency dispersion of approximately 1.05% per decade. These results imply that the GaSb epilayer grown on the GaAs/Si platform in the IMF mode can be used for future complementary metal–oxide semiconductor applications.

Ultra-small iron-gallic acid coordination polymer nanoparticles for chelator-free labeling of 64Cu and multimodal imaging-guided photothermal therapy.

PubMed

Jin, Qiutong; Zhu, Wenjun; Jiang, Dawei; Zhang, Rui; Kutyreff, Christopher J; Engle, Jonathan W; Huang, Peng; Cai, Weibo; Liu, Zhuang; Cheng, Liang

2017-08-31

Cancer nanotechnology has become the hot topic nowadays. While various kinds of nanomaterials have been widely explored for innovative cancer imaging and therapy applications, safe multifunctional nano-agents without long-term retention and toxicity are still demanded. Herein, iron-gallic acid coordination nanoparticles (Fe-GA CPNs) with ultra-small sizes are successfully synthesized by a simple method for multimodal imaging-guided cancer therapy. After surface modification with polyethylene glycol (PEG), the synthesized Fe-GA-PEG CPNs show high stability in various physiological solutions. Taking advantage of high near-infrared (NIR) absorbance as well as the T 1 -MR contrasting ability of Fe-GA-PEG CPNs, in vivo photoacoustic tomography (PAT) and magnetic resonance (MR) bimodal imaging are carried out, revealing the efficient passive tumor targeting of these ultra-small CPNs after intravenous (i.v.) injection. Interestingly, such Fe-GA-PEG CPNs could be labeled with the 64 Cu isotope via a chelator-free method for in vivo PET imaging, which also illustrates the high tumor uptake of Fe-GA CPNs. We further utilize Fe-GA-PEG CPNs for in vivo photothermal therapy and achieve highly effective tumor destruction after i.v. injection of Fe-GA-PEG CPNs and the following NIR laser irradiation of the tumors, without observing any apparent toxicity of such CPNs to the treated animals. Our work highlights the promise of ultra-small iron coordination nanoparticles for imaging-guided cancer therapy.

Deferiprone and Gallium-Protoporphyrin Have the Capacity to Potentiate the Activity of Antibiotics in Staphylococcus aureus Small Colony Variants

PubMed Central

Richter, Katharina; Thomas, Nicky; Zhang, Guimin; Prestidge, Clive A.; Coenye, Tom; Wormald, Peter-John; Vreugde, Sarah

2017-01-01

Small colony variants (SCVs) of bacteria like Staphylococcus aureus are characterized by a reduced colony size and are linked to increased antibiotic tolerance and resistance. Their altered expression of virulence factors, slow growing properties and their ability to form biofilms make the eradication of SCVs challenging. In the context of biofilm-related infectious diseases involving S. aureus SCVs, a therapy targeting bacterial iron metabolism was evaluated. The combination of the iron-chelator deferiprone (Def) and the heme-analog gallium-protoporphyrin (GaPP), in solution and incorporated in a surgical wound gel, was tested for activity against planktonic and sessile SCVs. To this end, the activity of Def-GaPP was assessed against planktonic S. aureus SCVs, as well as against in vitro and in vivo biofilms in the colony biofilm model, an artificial wound model and a Caenorhabditis elegans infection model. While Def alone failed to show substantial antibacterial activity, GaPP and the combination of Def-GaPP demonstrated concentration- and strain-dependent antibacterial properties. Specifically, the Def-GaPP combination significantly reduced the bacterial load in an artificial wound model and increased the survival of S. aureus SCV infected C. elegans. When Def-GaPP were combined with gentamicin or ciprofloxacin, the triple combinations exceeded the antibiofilm activity of the individual compounds in the colony biofilm model. In targeting bacterial iron metabolism, Def-GaPP showed significant activity against planktonic and sessile SCVs. Moreover, Def-GaPP could potentiate the activity of gentamicin and ciprofloxacin. Delivered in a wound healing gel, Def-GaPP showed promise as a new topical strategy against infections with S. aureus SCVs. PMID:28690982

Local ordering and magnetism in Ga{sub 0.9}Fe{sub 3.1}N

DOE Office of Scientific and Technical Information (OSTI.GOV)

Burghaus, Jens; Sougrati, Moulay T., E-mail: moulay-tahar.sougrati@univ-montp2.fr; Moechel, Anne

Prior investigations of the ternary nitride series Ga{sub 1-x}Fe{sub 3+x}N (0{<=}x{<=}1) have indicated a transition from ferromagnetic {gamma}'-Fe{sub 4}N to antiferromagnetic 'GaFe{sub 3}N'. The ternary nitride 'GaFe{sub 3}N' has been magnetically and spectroscopically reinvestigated in order to explore the weakening of the ferromagnetic interactions through the gradual incorporation of gallium into {gamma}'-Fe{sub 4}N. A hysteretic loop at RT reveals the presence of a minority phase of only 0.1-0.2 at%, in accord with the sound two-step synthesis. The composition of the gallium-richest phase 'GaFe{sub 3}N' was clarified by Prompt Gamma-ray Activation Analysis and leads to the berthollide formula Ga{sub 0.91(1)}Fe{sub 3.09(10)}N{submore » 1.05(7)}. Magnetic measurements indicate a transition around 8 K, further supported by Moessbauer spectral data. The weakening of the ferromagnetic coupling through an increasing gallium concentration is explained by a simple Stoner argument. In Ga{sub 0.9}Fe{sub 3.1}N, the presence of iron on the gallium site affects the magnetism by the formation of 13-atom iron clusters. - Graphical Abstract: The crystal structure of GaFe{sub 3}N with green nitrogen atoms in the very center, red iron atoms at the face centers, and gray gallium atoms at the corner positions. Highlights: > Almost phase-pure synthesis of Ga{sub 0.9}Fe{sub 3.1}N. > Prompt gamma-ray activation analysis yields precise composition. > Magnetic ordering of the facial Fe atoms at the lowest temperatures. > Moessbauer spectroscopy suggests percolation or RKKY-type interaction. > Fe{sub 13} clusters due to berthollide character.« less

Deferiprone and Gallium-Protoporphyrin Have the Capacity to Potentiate the Activity of Antibiotics in Staphylococcus aureus Small Colony Variants.

PubMed

Richter, Katharina; Thomas, Nicky; Zhang, Guimin; Prestidge, Clive A; Coenye, Tom; Wormald, Peter-John; Vreugde, Sarah

2017-01-01

Small colony variants (SCVs) of bacteria like Staphylococcus aureus are characterized by a reduced colony size and are linked to increased antibiotic tolerance and resistance. Their altered expression of virulence factors, slow growing properties and their ability to form biofilms make the eradication of SCVs challenging. In the context of biofilm-related infectious diseases involving S. aureus SCVs, a therapy targeting bacterial iron metabolism was evaluated. The combination of the iron-chelator deferiprone (Def) and the heme-analog gallium-protoporphyrin (GaPP), in solution and incorporated in a surgical wound gel, was tested for activity against planktonic and sessile SCVs. To this end, the activity of Def-GaPP was assessed against planktonic S. aureus SCVs, as well as against in vitro and in vivo biofilms in the colony biofilm model, an artificial wound model and a Caenorhabditis elegans infection model. While Def alone failed to show substantial antibacterial activity, GaPP and the combination of Def-GaPP demonstrated concentration- and strain-dependent antibacterial properties. Specifically, the Def-GaPP combination significantly reduced the bacterial load in an artificial wound model and increased the survival of S. aureus SCV infected C. elegans . When Def-GaPP were combined with gentamicin or ciprofloxacin, the triple combinations exceeded the antibiofilm activity of the individual compounds in the colony biofilm model. In targeting bacterial iron metabolism, Def-GaPP showed significant activity against planktonic and sessile SCVs. Moreover, Def-GaPP could potentiate the activity of gentamicin and ciprofloxacin. Delivered in a wound healing gel, Def-GaPP showed promise as a new topical strategy against infections with S. aureus SCVs.

Holmium Doped Solid State Laser Resonantly Pumped and Q-Switched by Novel GaSb-Based Photonic Devices

DTIC Science & Technology

2011-08-31

dominant role of inter valence band absorption [7]. Details of the conduction band structure of the particular 0 20 40 60 80 100 0 10 20 30 CW 30s...here the n-cladding composition resulted into material with three valleys in conduction band to have almost the same energy minimum so no inter...emitting GaSb -based diode lasers was improved by utilization of the waveguide structure with asymmetric claddings. The AlGaAsSb p-cladding contained

First-principles study of structural, electronic, and optical properties of surface defects in GaAs(001) - β2(2x4)

NASA Astrophysics Data System (ADS)

Bacuyag, Dhonny; Escaño, Mary Clare Sison; David, Melanie; Tani, Masahiko

2018-06-01

We performed first-principles calculations based on density functional theory (DFT) to investigate the role of point defects in the structural, electronic, and optical properties of the GaAs(001)- β2(2x4). In terms of structural properties, AsGa is the most stable defect structure, consistent with experiments. With respect to the electronic structure, band structures revealed the existence of sub-band and midgap states for all defects. The induced sub-bands and midgap states originated from the redistributions of charges towards these defects and neighboring atoms. The presence of these point defects introduced deep energy levels characteristic of EB3 (0.97 eV), EL4 (0.52 eV), and EL2 (0.82 eV) for AsGa, GaAs, GaV, respectively. The optical properties are found to be strongly related to these induced gap states. The calculated onset values in the absorption spectra, corresponding to the energy gaps, confirmed the absorption below the known bulk band gap of 1.43 eV. These support the possible two-step photoabsorption mediated by midgap states as observed in experiments.

Electronic Structure of I-M8Ga16Sn30 (M = Ba, Sr, Yb) by First-Principles Calculation

NASA Astrophysics Data System (ADS)

Wang, Jin-song; Liu, Hong-xia; Deng, Shuping; Li, De-cong; Shen, Lan-xian; Cheng, Feng; Deng, Shu-kang

2017-05-01

Sn-based clathrates possess excellent thermoelectric properties ascribed to their higher Seebeck coefficient and lower thermal conductivity. Guest atoms significantly modulate the thermoelectric properties of Sn-based calculates because of their diverse atomic radius and interactions with framework atoms. Thus, we explored the electronic structure of I-M8Ga16Sn30 (M = Ba, Sr, Yb) by first-principles calculation. Results revealed significant differences between Yb8Ga16Sn30 and M8Ga16Sn30 (M = Ba, Sr,). In particular, the Yb-filled compound substitution possesses lowest formation energy and the off-center distance of the Yb atom is the largest compared with the other structures. I-M8Ga16Sn30 (M = Ba, Sr, Yb) is an indirect band gap semiconductor, and the enhanced hybridization effect between the guest and framework atoms' orbits exists because the Yb f orbit results in a decrease in band gap. Ba- and Sr-filled clathrates have similar valence bands but slightly different conduction bands; however, Yb8Ga16Sn30 possess the spiculate density of states near the Fermi level that reveals excellent thermoelectric properties.

Effect of Sb in thick InGaAsSbN layers grown by liquid phase epitaxy

NASA Astrophysics Data System (ADS)

Donchev, V.; Milanova, M.; Asenova, I.; Shtinkov, N.; Alonso-Álvarez, D.; Mellor, A.; Karmakov, Y.; Georgiev, S.; Ekins-Daukes, N.

2018-02-01

Dilute nitride InGaAsSbN layers grown by low-temperature liquid phase epitaxy are studied in comparison with quaternary InGaAsN layers grown at the same growth conditions to understand the effect of Sb in the alloy. The lattice mismatch to the GaAs substrate is found to be slightly larger for the InGaAsSbN layers, which is explained by the large atomic radius of Sb. A reduction of the band gap energy with respect to InGaAsN is demonstrated by means of photoluminescence (PL), surface photovoltage (SPV) spectroscopy and tight-binding calculations. The band-gap energies determined from PL and ellipsometry measurements are in good agreement, while the SPV spectroscopy and the tight-binding calculations provide lower values. Possible reasons for these discrepancies are discussed. The PL spectra reveal localized electronic states in the band gap near the conduction band edge, which is confirmed by SPV spectroscopy. The analysis of the power dependence of the integrated PL has allowed determining the dominant radiative recombination mechanisms in the layers. The values of the refraction index in a wide spectral region are found to be higher for the Sb containing layers.

Morphological and chemical evidence of stromatolitic deposits in the 2.75 Ga Carajás banded iron formation, Brazil

USGS Publications Warehouse

Ribeiro da Luz, Beatriz; Crowley, James K.

2012-01-01

We describe evidence of biogenicity in the morphology and carbon content of well-preserved, Neoarchean samples of banded iron formation (BIF) from Carajás, Brazil. Silica-rich BIF layers contain translucent ellipsoidal or trapezoidal structures (∼5–10 μm diameter) composed of silica, hematite, and kerogen, which are arranged in larger ring-like forms (rosettes). Stable carbon isotope analysis yields a δ13C value of −24.5‰ indicating that the contained carbon is likely biogenic. Raman and SEM analyses, as well as wavelength-dispersive X-ray elemental maps, show kerogen inside the rosette forms. Within the iron-rich BIF layers, tubular structures (0.5–5 μm) were observed between hematite granules and blades. Kerogen and kaolinite are present in these structures. Both the rosettes and the tubular structures resemble morphologies that are characteristic of some bacterial species.We hypothesize that the Carajás BIFs originated as biomats formed by one or more species that over time produced large stromatolitic structures. The rosettes and the tubular structures, associated with chert-rich and iron-rich BIF layers, respectively, may represent two different species, or perhaps, two phases of a bacterium life cycle. For example, some modern myxobacteria exhibit similar morphologies in their resting and vegetative stages.Fe(III) precipitation may have occurred by contact of Fe(II) with bacterial slime, leading to oxidation by chemical reactions with exposed polysaccharide hydroxyl and carboxyl groups. The Fe(III) would then have been available for use as a source of energy in a dissimilatory iron reduction type of metabolism. Organic carbon input presumably came from primary producers (not necessarily aerobic) within the local water column, perhaps in shallow-water communities. Alternatively, the carbon may have originated by Fischer–Tropsch synthesis at ocean hydrothermal vents. The observed lateral continuity of BIF layers may perhaps be explained by chemical signaling by the bacteria of favorable or unfavorable environmental conditions, leading to nearly synchronous cell morphogenesis from a vegetative to resting phase and vice versa.

Photoluminescence as a tool for characterizing point defects in semiconductors

NASA Astrophysics Data System (ADS)

Reshchikov, Michael

2012-02-01

Photoluminescence is one of the most powerful tools used to study optically-active point defects in semiconductors, especially in wide-bandgap materials. Gallium nitride (GaN) and zinc oxide (ZnO) have attracted considerable attention in the last two decades due to their prospects in optoelectronics applications, including blue and ultraviolet light-emitting devices. However, in spite of many years of extensive studies and a great number of publications on photoluminescence from GaN and ZnO, only a few defect-related luminescence bands are reliably identified. Among them are the Zn-related blue band in GaN, Cu-related green band and Li-related orange band in ZnO. Numerous suggestions for the identification of other luminescence bands, such as the yellow band in GaN, or green and yellow bands in ZnO, do not stand up under scrutiny. In these conditions, it is important to classify the defect-related luminescence bands and find their unique characteristics. In this presentation, we will review the origin of the major luminescence bands in GaN and ZnO. Through simulations of the temperature and excitation intensity dependences of photoluminescence and by employing phenomenological models we are able to obtain important characteristics of point defects such as carrier capture cross-sections for defects, concentrations of defects, and their charge states. These models are also used to find the absolute internal quantum efficiency of photoluminescence and obtain information about nonradiative defects. Results from photoluminescence measurements will be compared with results of the first-principle calculations, as well as with the experimental data obtained by other techniques such as positron annihilation spectroscopy, deep-level transient spectroscopy, and secondary ion mass spectrometry.

Play the heavy: An effective mass study for α-Fe{sub 2}O{sub 3} and corundum oxides

DOE Office of Scientific and Technical Information (OSTI.GOV)

Neufeld, Ofer; Caspary Toroker, Maytal, E-mail: maytalc@tx.technion.ac.il

2016-04-28

Iron(III) oxide (α-Fe{sub 2}O{sub 3}) is a known water splitting catalyst commonly used in photoelectrochemical cells. These cells are severely impaired by poor conductivity in α-Fe{sub 2}O{sub 3}, and resolving the conductivity issue is therefore crucial. One of the most intrinsic properties of matter, which governs conductivity, is the carrier effective masses. In this work, we investigate the carrier effective masses in α-Fe{sub 2}O{sub 3} and other corundum oxides, including Al{sub 2}O{sub 3}, Cr{sub 2}O{sub 3}, Ga{sub 2}O{sub 3}, and In{sub 2}O{sub 3} with different theoretical constructs: density functional theory (DFT), DFT+U, hybrid DFT, and G{sub 0}W{sub 0}. We findmore » DFT sufficiently describes the carrier masses and a quasi-particle theory is only required for accuracies better than 30% for the conduction band effective mass. Additionally, we compare the density of states (DOS) and band effective mass approximations and conclude the DOS effective mass provides poor results whenever the band structure is anisotropic. We find that the charge carriers in Fe{sub 2}O{sub 3} “play the heavy” since they have large effective masses that reduce conductivity and device efficiency. Finally, we conclude that the less heavy electron effective masses of other corundum oxides studied relative to Fe{sub 2}O{sub 3} could contribute to efficiency improvements in Fe{sub 2}O{sub 3} upon Al{sub 2}O{sub 3}, Ga{sub 2}O{sub 3}, and In{sub 2}O{sub 3} coverage.« less

X-band inverse class-F GaN internally-matched power amplifier

NASA Astrophysics Data System (ADS)

Zhao, Bo-Chao; Lu, Yang; Han, Wen-Zhe; Zheng, Jia-Xin; Zhang, Heng-Shuang; Ma, Pei-jun; Ma, Xiao-Hua; Hao, Yue

2016-09-01

An X-band inverse class-F power amplifier is realized by a 1-mm AlGaN/GaN high electron mobility transistor (HEMT). The intrinsic and parasitic components inside the transistor, especially output capacitor Cds, influence the harmonic impedance heavily at the X-band, so compensation design is used for meeting the harmonic condition of inverse class-F on the current source plane. Experiment results show that, in the continuous-wave mode, the power amplifier achieves 61.7% power added efficiency (PAE), which is 16.3% higher than the class-AB power amplifier realized by the same kind of HEMT. To the best of our knowledge, this is the first inverse class-F GaN internally-matched power amplifier, and the PAE is quite high at the X-band. Project supported by the National High Technology Research and Development Program of China (Grant No. 2015AA016801).

InGaP Heterojunction Barrier Solar Cells

NASA Technical Reports Server (NTRS)

Welser, Roger E. (Inventor)

2014-01-01

A new solar cell structure called a heterojunction barrier solar cell is described. As with previously reported quantum-well and quantum-dot solar cell structures, a layer of narrow band-gap material, such as GaAs or indium-rich InGaP, is inserted into the depletion region of a wide band-gap PN junction. Rather than being thin, however, the layer of narrow band-gap material is about 400-430 nm wide and forms a single, ultrawide well in the depletion region. Thin (e.g., 20-50 nm), wide band-gap InGaP barrier layers in the depletion region reduce the diode dark current. Engineering the electric field and barrier profile of the absorber layer, barrier layer, and p-type layer of the PN junction maximizes photogenerated carrier escape. This new twist on nanostructured solar cell design allows the separate optimization of current and voltage to maximize conversion efficiency.

Type-II InP quantum dots in wide-bandgap InGaP host for intermediate-band solar cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tayagaki, Takeshi, E-mail: tayagaki-t@aist.go.jp; Sugaya, Takeyoshi

2016-04-11

We demonstrate type-II quantum dots (QDs) with long carrier lifetimes in a wide-bandgap host as a promising candidate for intermediate-band solar cells. Type-II InP QDs are fabricated in a wide-bandgap InGaP host using molecular beam epitaxy. Time-resolved photoluminescence measurements reveal an extremely long carrier lifetime (i.e., greater than 30 ns). In addition, from temperature-dependent PL spectra, we find that the type-II InP QDs form a negligible valence band offset and conduction band offset of ΔE{sub c} ≈ 0.35 eV in the InGaP host. Such a type-II confinement potential for InP/InGaP QDs has a significant advantage for realizing efficient two-step photon absorption and suppressed carriermore » capture in QDs via Auger relaxation.« less

Compositional bowing of band energies and their deformation potentials in strained InGaAs ternary alloys: A first-principles study

NASA Astrophysics Data System (ADS)

Khomyakov, Petr A.; Luisier, Mathieu; Schenk, Andreas

2015-08-01

Using first-principles calculations, we show that the conduction and valence band energies and their deformation potentials exhibit a non-negligible compositional bowing in strained ternary semiconductor alloys such as InGaAs. The electronic structure of these compounds has been calculated within the framework of local density approximation and hybrid functional approach for large cubic supercells and special quasi-random structures, which represent two kinds of model structures for random alloys. We find that the predicted bowing effect for the band energy deformation potentials is rather insensitive to the choice of the functional and alloy structural model. The direction of bowing is determined by In cations that give a stronger contribution to the formation of the InxGa1-xAs valence band states with x ≳ 0.5, compared to Ga cations.

Negative differential velocity in ultradilute GaAs1-xNx alloys

NASA Astrophysics Data System (ADS)

Vogiatzis, N.; Rorison, J. M.

2011-04-01

We present theoretical results on steady state characteristics in bulk GaAs1-xNx alloys (x ≤ 0.2) using the single electron Monte-Carlo method. Two approaches have been used; the first assumes a GaAs band with a strong nitrogen scattering resonance and the second uses the band anti-crossing model, in which the localized N level interacts with the GaAs band strongly perturbing the conduction band. In the first model we observe two negative differential velocity peaks, the lower one associated with nitrogen scattering while the higher one with polar optical phonon emission accounting for the nonparabolicity effect. In the second model one negative differential velocity peak is observed associated with polar optical phonon emission. Good agreement with experimental low field mobility is obtained from the first model. We also comment on the results from both Models when the intervalley Г → L transfer is accounted for.

Band-to-Band Tunneling Transistors: Scalability and Circuit Performance

DTIC Science & Technology

2013-05-01

to this point. The inability to create GaN ingots as cost effective substrates (or Silicon Carbide ingots coupled with GaN deposition) means that...was vastly different than standard Silicon CMOS (e.g. HEMTs and GaN channel devices were included, but not III-V-channel MOS or Germanium-channel MOS...the same wafer, wafer bonding has been used by Chung et al. to attach GaN to Silicon wafers, where a p-type Si device can be used [15]. Since

Yb5Ga2Sb6: a mixed valent and narrow-band gap material in the RE5M2X6 family.

PubMed

Subbarao, Udumula; Sarkar, Sumanta; Gudelli, Vijay Kumar; Kanchana, V; Vaitheeswaran, G; Peter, Sebastian C

2013-12-02

A new compound Yb5Ga2Sb6 was synthesized by the metal flux technique as well as high frequency induction heating. Yb5Ga2Sb6 crystallizes in the orthorhombic space group Pbam (no. 55), in the Ba5Al2Bi6 structure type, with a unit cell of a = 7.2769(2) Å, b = 22.9102(5) Å, c = 4.3984(14) Å, and Z = 2. Yb5Ga2Sb6 has an anisotropic structure with infinite anionic double chains (Ga2Sb6)(10-) cross-linked by Yb(2+) and Yb(3+) ions. Each single chain is made of corner-sharing GaSb4 tetrahedra. Two such chains are bridged by Sb2 groups to form double chains of 1/∞ [Ga2Sb6(10-)]. The compound satisfies the classical Zintl-Klemm concept and is a narrow band gap semiconductor with an energy gap of around 0.36 eV calculated from the electrical resistivity data corroborating with the experimental absorption studies in the IR region (0.3 eV). Magnetic measurements suggest Yb atoms in Yb5Ga2Sb6 exist in the mixed valent state. Temperature dependent magnetic susceptibility data follows the Curie-Weiss behavior above 100 K and no magnetic ordering was observed down to 2 K. Experiments are accompanied by all electron full-potential linear augmented plane wave (FP-LAPW) calculations based on density functional theory to calculate the electronic structure and density of states. The calculated band structure shows a weak overlap of valence band and conduction band resulting in a pseudo gap in the density of states revealing semimetallic character.

Intrinsic transmission magnetic circular dichroism spectra of GaMnAs

NASA Astrophysics Data System (ADS)

Terada, Hiroshi; Ohya, Shinobu; Tanaka, Masaaki

2018-03-01

Transmission magnetic circular dichroism (MCD) spectroscopy has been widely used to reveal the spin-dependent band structure of ferromagnetic semiconductors. In these previous studies, some band pictures have been proposed from the spectral shapes observed in transmission MCD; however, extrinsic signals originating from optical interference have not been appropriately considered. In this study, we calculate the MCD spectra taking into account the optical interference of the layered structure of samples and show that the spectral shape of MCD is strongly influenced by optical interference. To correctly understand the transmission MCD, we also calculate the intrinsic MCD spectra of GaMnAs that are not influenced by the optical interference. The spectral shape of the intrinsic MCD can be explained by the characteristic band structure of GaMnAs, that is, the spin-polarized valence band and the impurity band existing above the valence band top.

InGaAsBi alloys on InP for efficient near- and mid-infrared light emitting devices

NASA Astrophysics Data System (ADS)

Jin, Shirong; John Sweeney, Stephen

2013-12-01

We present the band parameters such as band gap, spin-orbit splitting energy, band offsets and strain of InGaAsBi on InP based on recent experimental data. It is shown that InGaAsBi is promising for near- and mid-infrared photonic devices operating from 0.3-0.8 eV (1.5-4 μm) on conventional InP substrates. We also show how bismuth may be used to form alloys whereby the spin-orbit splitting energy (ΔSO) is large and controllable and can, for example, be made larger than the band gap (Eg) thereby providing a means of suppressing non-radiative hot-hole producing Auger recombination and inter-valence band absorption both involving the spin-orbit band. This is expected to improve the high-temperature performance and thermal stability of light emitting devices.

Influence of internal electric fields on band gaps in short period GaN/GaAlN and InGaN/GaN polar superlattices

NASA Astrophysics Data System (ADS)

Gorczyca, I.; Skrobas, K.; Suski, T.; Christensen, N. E.; Svane, A.

2015-08-01

The electronic structures of short period mGaN/nGayAl1-yN and mInyGa1-yN/nGaN superlattices grown along the wurtzite c axis have been calculated for different alloy compositions y and various small numbers m of well- and n of barrier-monolayers. The general trends in gap behavior can, to a large extent, be related to the strength of the internal electric field, E, in the GaN and InGaN quantum wells. In the GaN/GaAlN superlattices, E reaches 4 MV/cm, while in the InGaN/GaN superlattices, values as high as E ≈ 6.5 MV/cm are found. The strong electric fields are caused by spontaneous and piezoelectric polarizations, the latter contribution dominating in InGaN/GaN superlattices. The influence of different arrangements of In atoms (indium clustering) on the band gap values in InGaN/GaN superlattices is examined.

Theoretical prediction of the band offsets at the ZnO/anatase TiO2 and GaN/ZnO heterojunctions using the self-consistent ab initio DFT/GGA-1/2 method.

PubMed

Fang, D Q; Zhang, S L

2016-01-07

The band offsets of the ZnO/anatase TiO2 and GaN/ZnO heterojunctions are calculated using the density functional theory/generalized gradient approximation (DFT/GGA)-1/2 method, which takes into account the self-energy corrections and can give an approximate description to the quasiparticle characteristics of the electronic structure of semiconductors. We present the results of the ionization potential (IP)-based and interfacial offset-based band alignments. In the interfacial offset-based band alignment, to get the natural band offset, we use the surface calculations to estimate the change of reference level due to the interfacial strain. Based on the interface models and GGA-1/2 calculations, we find that the valence band maximum and conduction band minimum of ZnO, respectively, lie 0.64 eV and 0.57 eV above those of anatase TiO2, while lie 0.84 eV and 1.09 eV below those of GaN, which agree well with the experimental data. However, a large discrepancy exists between the IP-based band offset and the calculated natural band offset, the mechanism of which is discussed. Our results clarify band alignment of the ZnO/anatase TiO2 heterojunction and show good agreement with the GW calculations for the GaN/ZnO heterojunction.

Iron minerals within specific microfossil morphospecies of the 1.88 Ga Gunflint Formation

PubMed Central

Lepot, Kevin; Addad, Ahmed; Knoll, Andrew H.; Wang, Jian; Troadec, David; Béché, Armand; Javaux, Emmanuelle J.

2017-01-01

Problematic microfossils dominate the palaeontological record between the Great Oxidation Event 2.4 billion years ago (Ga) and the last Palaeoproterozoic iron formations, deposited 500–600 million years later. These fossils are often associated with iron-rich sedimentary rocks, but their affinities, metabolism, and, hence, their contributions to Earth surface oxidation and Fe deposition remain unknown. Here we show that specific microfossil populations of the 1.88 Ga Gunflint Iron Formation contain Fe-silicate and Fe-carbonate nanocrystal concentrations in cell interiors. Fe minerals are absent in/on all organically preserved cell walls. These features are consistent with in vivo intracellular Fe biomineralization, with subsequent in situ recrystallization, but contrast with known patterns of post-mortem Fe mineralization. The Gunflint populations that display relatively large cells (thick-walled spheres, filament-forming rods) and intra-microfossil Fe minerals are consistent with oxygenic photosynthesizers but not with other Fe-mineralizing microorganisms studied so far. Fe biomineralization may have protected oxygenic photosynthesizers against Fe2+ toxicity during the Palaeoproterozoic. PMID:28332570

Nitrogen-related intermediate band in P-rich GaN xP yAs 1-x-y alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zelazna, K.; Gladysiewicz, M.; Polak, M. P.

The electronic band structure of phosphorus-rich GaN xP yAs 1-x-y alloys (x ~ 0.025 and y ≥ 0.6) is studied experimentally using optical absorption, photomodulated transmission, contactless electroreflectance, and photoluminescence. Here, it is shown that incorporation of a few percent of N atoms has a drastic effect on the electronic structure of the alloys. The change of the electronic band structure is very well described by the band anticrossing (BAC) model in which localized nitrogen states interact with the extended states of the conduction band of GaAsP host. The BAC interaction results in the formation of a narrow intermediate bandmore » (E - band in BAC model) with the minimum at the Γ point of the Brillouin zone resulting in a change of the nature of the fundamental band gap from indirect to direct. The splitting of the conduction band by the BAC interaction is further confirmed by a direct observation of the optical transitions to the E + band using contactless electroreflectance spectroscopy.« less

Nitrogen-related intermediate band in P-rich GaN xP yAs 1-x-y alloys

DOE PAGES

Zelazna, K.; Gladysiewicz, M.; Polak, M. P.; ...

2017-11-16

The electronic band structure of phosphorus-rich GaN xP yAs 1-x-y alloys (x ~ 0.025 and y ≥ 0.6) is studied experimentally using optical absorption, photomodulated transmission, contactless electroreflectance, and photoluminescence. Here, it is shown that incorporation of a few percent of N atoms has a drastic effect on the electronic structure of the alloys. The change of the electronic band structure is very well described by the band anticrossing (BAC) model in which localized nitrogen states interact with the extended states of the conduction band of GaAsP host. The BAC interaction results in the formation of a narrow intermediate bandmore » (E - band in BAC model) with the minimum at the Γ point of the Brillouin zone resulting in a change of the nature of the fundamental band gap from indirect to direct. The splitting of the conduction band by the BAC interaction is further confirmed by a direct observation of the optical transitions to the E + band using contactless electroreflectance spectroscopy.« less

Polarization Enhanced Charge Transfer: Dual-Band GaN-Based Plasmonic Photodetector.

PubMed

Jia, Ran; Zhao, Dongfang; Gao, Naikun; Liu, Duo

2017-01-13

Here, we report a dual-band plasmonic photodetector based on Ga-polar gallium nitride (GaN) for highly sensitive detection of UV and green light. We discover that decoration of Au nanoparticles (NPs) drastically increases the photoelectric responsivities by more than 50 times in comparition to the blank GaN photodetector. The observed behaviors are attributed to polarization enhanced charge transfer of optically excited hot electrons from Au NPs to GaN driven by the strong spontaneous polarization field of Ga-polar GaN. Moreover, defect ionization promoted by localized surface plasmon resonances (LSPRs) is also discussed. This novel type of photodetector may shed light on the design and fabrication of photoelectric devices based on polar semiconductors and microstructural defects.

Nonradiative recombination centers in GaAs:N δ-doped superlattice revealed by two-wavelength-excited photoluminescence

NASA Astrophysics Data System (ADS)

Dulal Haque, Md.; Kamata, Norihiko; Fukuda, Takeshi; Honda, Zentaro; Yagi, Shuhei; Yaguchi, Hiroyuki; Okada, Yoshitaka

2018-04-01

We use two-wavelength-excited photoluminescence (PL) to investigate nonradiative recombination (NRR) centers in GaAs:N δ-doped superlattice (SL) structures grown by molecular beam epitaxy. The change in photoluminescence (PL) intensity due to the superposition of below-gap excitation at energies of 0.75, 0.80, 0.92, and 0.95 eV and above-gap excitation at energies of 1.69 or 1.45 eV into the GaAs conduction band and the E- band implies the presence of NRR centers inside the GaAs:N δ-doped SL and/or GaAs layers. The change in PL intensity as a function of the photon number density of below-gap excitation is examined for both bands, which enables us to determine the distribution of NRR centers inside the GaAs:N δ-doped SL and GaAs layers. We propose recombination models to explain the experimental results. Defect-related parameters that give a qualitative insight into the samples are investigated systematically by fitting the rate equations to the experimental data.

Dislocation blocking by AlGaN hot electron injecting layer in the epitaxial growth of GaN terahertz Gunn diode

NASA Astrophysics Data System (ADS)

Li, Liang; Yang, Lin'an; Zhang, Jincheng; Hao, Yue

2013-09-01

This paper reports an efficient method to improve the crystal quality of GaN Gunn diode with AlGaN hot electron injecting layer (HEI). An evident reduction of screw dislocation and edge dislocation densities is achieved by the strain management and the enhanced lateral growth in high temperature grown AlGaN HEI layer. Compared with the top hot electron injecting layer (THEI) structure, the bottom hot electron injecting layer (BHEI) structure enhances the crystal quality of transit region due to the growth sequence modulation of HEI layer. A high Hall mobility of 2934 cm2/Vs at 77 K, a nearly flat downtrend of Hall mobility at the temperature ranging from 300 to 573 K, a low intensity of ratio of yellow luminescence band to band edge emission, a narrow band edge emission line-width, and a smooth surface morphology are observed for the BHEI structural epitaxy of Gunn diode, which indicates that AlGaN BHEI structure is a promising candidate for fabrication of GaN Gunn diodes in terahertz regime.

Interface states and internal photoemission in p-type GaAs metal-oxide-semiconductor surfaces

NASA Technical Reports Server (NTRS)

Kashkarov, P. K.; Kazior, T. E.; Lagowski, J.; Gatos, H. C.

1983-01-01

An interface photodischarge study of p-type GaAs metal-oxide-semiconductor (MOS) structures revealed the presence of deep interface states and shallow donors and acceptors which were previously observed in n-type GaAs MOS through sub-band-gap photoionization transitions. For higher photon energies, internal photoemission was observed, i.e., injection of electrons to the conduction band of the oxide from either the metal (Au) or from the GaAs valence band; the threshold energies were found to be 3.25 and 3.7 + or - 0.1 eV, respectively. The measured photoemission current exhibited a thermal activation energy of about 0.06 eV, which is consistent with a hopping mechanism of electron transport in the oxide.

Charge carrier relaxation in InGaAs-GaAs quantum wire modulation-doped heterostructures

NASA Astrophysics Data System (ADS)

Kondratenko, S. V.; Iliash, S. A.; Mazur, Yu I.; Kunets, V. P.; Benamara, M.; Salamo, G. J.

2017-09-01

The time dependencies of the carrier relaxation in modulation-doped InGaAs-GaAs low-dimensional structures with quantum wires have been studied as functions of temperature and light excitation levels. The photoconductivity (PC) relaxation follows a stretched exponent with decay constant, which depends on the morphology of InGaAs epitaxial layers, presence of deep traps, and energy disorder due to inhomogeneous distribution of size and composition. A hopping model, where electron tunnels between bands of localized states, gives appropriate interpretation for temperature-independent PC decay across the temperature range 150-290 K. At low temperatures (T < 150 K), multiple trapping-retrapping via 1D states of InGaAs quantum wires (QWRs), sub-bands of two-dimensional electron gas of modulation-doped n-GaAs spacers, as well as defect states in the GaAs environment are the dominant relaxation mechanism. The PC and photoluminescence transients for samples with different morphologies of the InGaAs nanostructures are compared. The relaxation rates are found to be largely dependent on energy disorder due to inhomogeneous distribution of strain, nanostructure size and composition, and piezoelectric fields in and around nanostructures, which have a strong impact on efficiency of carrier exchange between bands of the InGaAs QWRs, GaAs spacers, or wetting layers; presence of local electric fields; and deep traps.

The effect of Ga vacancies on the defect and magnetic properties of Mn-doped GaN

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kang, Joongoo; Chang, K. J.; Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea and Korea Institute for Advanced Study, Seoul 130-722

2007-10-15

We perform first-principles theoretical calculations to investigate the effect of the presence of Ga vacancy on the defect and magnetic properties of Mn-doped GaN. When a Ga vacancy (V{sub Ga}) is introduced to the Mn ions occupying the Ga lattice sites, a charge transfer occurs from the Mn d band to the acceptor levels of V{sub Ga}, and strong Mn-N bonds are formed between the Mn ion and the N atoms in the neighborhood of V{sub Ga}. The charge transfer and chemical bonding effects significantly affect the defect and magnetic properties of Mn-doped GaN. In a Mn-V{sub Ga} complex, whichmore » consists of a Ga vacancy and one Mn ion, the dangling bond orbital of the N atom involved in the Mn-N bond is electrically deactivated, and the remaining dangling bond orbitals of V{sub Ga} lead to the shallowness of the defect level. When a Ga vacancy forms a complex with two Mn ions located at a distance of about 6 A, which corresponds to the percolation length in determining the Curie temperature in diluted Mn-doped GaN, the Mn d band is broadened and the density of states at the Fermi level is reduced due to two strong Mn-N bonds. Although the broadening and depopulation of the Mn d band weaken the ferromagnetic stability between the Mn ions, the ferromagnetism is still maintained because of the lack of antiferromagnetic superexchange interactions at the percolation length.« less

Interface Engineering of Monolayer MoS2/GaN Hybrid Heterostructure: Modified Band Alignment for Photocatalytic Water Splitting Application by Nitridation Treatment.

PubMed

Zhang, Zhaofu; Qian, Qingkai; Li, Baikui; Chen, Kevin J

2018-05-23

Interface engineering is a key strategy to deal with the two-dimensional (2D)/three-dimensional (3D) hybrid heterostructure, since the properties of this atomic-layer-thick 2D material can easily be impacted by the substrate environment. In this work, the structural, electronic, and optical properties of the 2D/3D heterostructure of monolayer MoS 2 on wurtzite GaN surface without and with nitridation interfacial layer are systematically investigated by first-principles calculation and experimental analysis. The nitridation interfacial layer can be introduced into the 2D/3D heterostructure by remote N 2 plasma treatment to GaN sample surface prior to stacking monolayer MoS 2 on top. The calculation results reveal that the 2D/3D integrated heterostructure is energetically favorable with a negative formation energy. Both interfaces demonstrate indirect band gap, which is a benefit for longer lifetime of the photoexcited carriers. Meanwhile, the conduction band edge and valence band edge of the MoS 2 side increases after nitridation treatment. The modification to band alignment is then verified by X-ray photoelectron spectroscopy measurement on MoS 2 /GaN heterostructures constructed by a modified wet-transfer technique, which indicates that the MoS 2 /GaN heterostructure without nitridation shows a type-II alignment with a conduction band offset (CBO) of only 0.07 eV. However, by the deployment of interface nitridation, the band edges of MoS 2 move upward for ∼0.5 eV as a result of the nitridized substrate property. The significantly increased CBO could lead to better electron accumulation capability at the GaN side. The nitridized 2D/3D heterostructure with effective interface treatment exhibits a clean band gap and substantial optical absorption ability and could be potentially used as practical photocatalyst for hydrogen generation by water splitting using solar energy.

Tunable electronic properties of silicene/GaP heterobilayer: Effects of electric field or biaxial tensile strain

NASA Astrophysics Data System (ADS)

Zhang, Peng; Yang, Xibin; Wu, Wei; Tian, Lifen; Cui, Heping; Zheng, Kai; Jiang, Junke; Chen, Xianping; Ye, Huaiyu

2018-05-01

We systematically investigate the electronic properties the two-dimensional (2D) silicene/GaP heterobilayer by using density functional theory calculations. We find the silicene and GaP monolayer are bounded to each other via orbital hybridization, and the charge redistribution occurring at the silicene/GaP interface leads to the opening of a direct energy band gap of about 0.997 eV in silicene. Importantly, by applying external electric field, the band structure of silicene/GaP heterostructure can be effectively modulated, and a semiconductor-metal transition even emerges. These intriguing properties make the silicene/GaP heterobilayer a promising 2D material for future electronics and strain sensors.

Effect of stacking faults on the photoluminescence spectrum of zincblende GaN

NASA Astrophysics Data System (ADS)

Church, S. A.; Hammersley, S.; Mitchell, P. W.; Kappers, M. J.; Lee, L. Y.; Massabuau, F.; Sahonta, S. L.; Frentrup, M.; Shaw, L. J.; Wallis, D. J.; Humphreys, C. J.; Oliver, R. A.; Binks, D. J.; Dawson, P.

2018-05-01

The photoluminescence spectra of a zincblende GaN epilayer grown via metal-organic chemical vapour deposition upon 3C-SiC/Si (001) substrates were investigated. Of particular interest was a broad emission band centered at 3.4 eV, with a FWHM of 200 meV, which extends above the bandgap of both zincblende and wurtzite GaN. Photoluminescence excitation measurements show that this band is associated with an absorption edge centered at 3.6 eV. Photoluminescence time decays for the band are monoexponential, with lifetimes that reduce from 0.67 ns to 0.15 ns as the recombination energy increases. TEM measurements show no evidence of wurtzite GaN inclusions which are typically used to explain emission in this energy range. However, dense stacking fault bunches are present in the epilayers. A model for the band alignment at the stacking faults was developed to explain this emission band, showing how both electrons and holes can be confined adjacent to stacking faults. Different stacking fault separations can change the carrier confinement energies sufficiently to explain the width of the emission band, and change the carrier wavefunction overlap to account for the variation in decay time.

InN/InGaN complementary heterojunction-enhanced tunneling field-effect transistor with enhanced subthreshold swing and tunneling current

NASA Astrophysics Data System (ADS)

Peng, Yue; Han, Genquan; Wang, Hongjuan; Zhang, Chunfu; Liu, Yan; Wang, Yibo; Zhao, Shenglei; Zhang, Jincheng; Hao, Yue

2016-05-01

InN/In0.75Ga0.25N complementary heterojunction-enhanced tunneling field-effect transistors (HE-TFETs) were characterized using the numerical simulation. InN/In0.75Ga0.25N HE-TFET has an InN/In0.75Ga0.25N heterojunction located in the channel region with a distance of LT-H from the source/channel tunneling junction. We demonstrate that, for both n- and p-channel devices, HE-TFETs have a delay of onset voltage VONSET, a steeper subthreshold swing (SS), and an enhanced on-state current ION in comparison with the homo-TFETs. InN/In0.75Ga0.25N n- and p-channel HE-TFETs with a gate length LG of 25 nm and a LT-H of 5 nm achieve a 7 and 9 times ION improvement in comparison with the homo devices, respectively, at a supply voltage of 0.3 V. The performance enhancement in HE-TFETs is attributed to the modulating effect of heterojunction on band-to-band tunneling (BTBT). Because InN/In0.75Ga0.25N heterointerface shows the similar band offsets at conduction and valence bands, the InN/In0.75Ga0.25N heterojunction exhibits the improved effect on BTBT for both n- and p-channel devices. This makes InN/In0.75Ga0.25N heterojunction a promising structure for high performance complementary TFETs.

Carbonates of the Gunflint Banded Iron Formation as Analogs of Martian Carbonates

NASA Technical Reports Server (NTRS)

Pun, A.; Papike, J. J.; Shearer, C. K.

2001-01-01

Terrestrial iron formations preserve remnants of life on Earth and may serve as analogs for identifying evidence of biologic activity in martian rocks. We report on the petrography, mineralogy and trace-element abundances of carbonates of the Gunflint banded iron formation. Additional information is contained in the original extended abstract.

GaN nanophosphors for white-light applications

NASA Astrophysics Data System (ADS)

Kumar, Mirgender; Singh, V. P.; Dubey, Sarvesh; Suh, Youngsuk; Park, Si-Hyun

2018-01-01

GaN nanoparticles (NPs) were synthesized by carbothermal reduction combined with nitridation, using Ga2O3 powder and graphitic carbon nitride (g-C3N4) as precursors. Characterization of the NPs was performed by X-ray diffraction, scanning electron microscopy, and room-temperature photoluminescence measurements. X-ray photoelectron spectroscopy was also performed to detect the chemical states of the different species. A universal yellow luminescence (YL) band was observed from complexes of Ga vacancies with O anti-sites and of O anti-sites with C. Further increments in the C content were observed with continued growth and induced an additional blue luminescence (BL) band. Tuning of the YL and BL bands resulted in white-light emission under certain experimental conditions, thus offering a new way of employing GaN nanophosphors for solid-state white lighting. Calculations of the correlated color temperature and color-quality scale parameters confirmed the utility of the experimental process for different applications.

Band lineup of lattice mismatched InSe/GaSe quantum well structures prepared by van der Waals epitaxy: Absence of interfacial dipoles

NASA Astrophysics Data System (ADS)

Lang, O.; Klein, A.; Pettenkofer, C.; Jaegermann, W.; Chevy, A.

1996-10-01

Epitaxial growth of the strongly lattice mismatched (6.5%) layered chalcogenides InSe and GaSe on each other is obtained with the concept of van der Waals epitaxy as proven by low-energy electron diffraction and scanning tunnel microscope. InSe/GaSe/InSe and GaSe/InSe/GaSe quantum well structures were prepared by molecular beam epitaxy and their interface properties were characterized by soft x-ray photoelectron spectroscopy. Valence and conduction band offsets are determined to be 0.1 and 0.9 eV, respectively, and do not depend on deposition sequence (commutativity). As determined from the measured work functions the interface dipole is 0.05 eV; the band lineup between the two materials is correctly predicted by the Anderson model (electron affinity rule).

Orientation independence of heterojunction-band offsets at GaAs-AlAs heterointerfaces characterized by x-ray photoemission spectroscopy

NASA Astrophysics Data System (ADS)

Hirakawa, K.; Hashimoto, Y.; Ikoma, T.

1990-12-01

We systematically studied the orientation and the growth sequence dependence of the valence-band offset ΔEv at the lattice-matched common anion GaAs-AlAs interfaces. High quality GaAs-AlAs heterojunctions were carefully grown on GaAs substrates with three major orientations, namely, (100), (110), and (111)B. The core level energy distance ΔECL between Ga 3d and Al 2p levels was measured by in situ x-ray photoemission spectroscopy. ΔECL is found to be independent of the substrate orientation and the growth sequence, which clearly indicates the face independence of ΔEv. This result suggests that the band lineup at lattice-matched isovalent semiconductor heterojunctions is determined by the bulk properties of the constituent materials. ΔEv is determined to be 0.44 ± 0.05 eV.

Comparative study on Ga1-xZnxN1-yOy oxynitride synthesized by different techniques for application in photocatalytic hydrogen production

NASA Astrophysics Data System (ADS)

Menon, Sumithra Sivadas; Baskar, K.; Singh, Shubra

2017-06-01

Hydrogen evolution by overall water splitting has emerged as a potential method for green energy generation due to the introduction of highly efficient photocatalysts active under visible region of spectra. In the present work, we focus on a comparative study of the properties of Ga1-xZnxN1-yOy oxynitride samples synthesized by two techniques and their effect on the sample properties. The samples were prepared by both traditional nitridation technique and solution combustion method. Room temperature photoluminescence studies revealed the introduction of additional energy levels above the valence band which in turns broadens the valence band and subsequently reduces the band gap. The band gap narrowing was further confirmed using diffuse reflectance spectroscopy and Valence band X-ray photoelectron spectroscopy (VB-XPS). It was also realized from VB XPS that the reduction of band gap in both the samples was due to upshift of valence band without affecting the conduction band. The presence of disorder activated modes in the samples was examined using temperature dependent Raman spectroscopy. In this work we corroborate the theoretical prediction reported by Al-Jassim et. al that the bandgap narrowing mechanism in ZnO rich solid solution and GaN rich solid solution is asymmetric and a significant bandgap reduction could be observed for ZnO rich solid solution than GaN rich.

Dual Band Deep Ultraviolet AlGaN Photodetectors

NASA Technical Reports Server (NTRS)

Aslam, S.; Miko, L.; Stahle, C.; Franz, D.; Pugel, D.; Guan, B.; Zhang, J. P.; Gaska, R.

2007-01-01

We report on the design, fabrication and characterization of a back-illuminated voltage bias selectable dual-band AlGaN UV photodetector. The photodetector can separate UVA and W-B band radiation by bias switching a two terminal n-p-n homojunction structure that is fabricated in the same pixel. When a forward bias is applied between the top and bottom electrodes, the detector can sense UV-A and reject W-B band radiation. Alternatively, under reverse bias, the photodetector can sense UV-B and reject UV-A band radiation.

X-ray photoelectron spectrum and electronic properties of a noncentrosymmetric chalcopyrite compound HgGa(2)S(4): LDA, GGA, and EV-GGA.

PubMed

Reshak, Ali Hussain; Khenata, R; Kityk, I V; Plucinski, K J; Auluck, S

2009-04-30

An all electron full potential linearized augmented plane wave method has been applied for a theoretical study of the band structure, density of states, and electron charge density of a noncentrosymmetric chalcopyrite compound HgGa(2)S(4) using three different approximations for the exchange correlation potential. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Gamma resulting in a direct energy gap of about 2.0, 2.2, and 2.8 eV for local density approximation (LDA), generalized gradient approximation (GGA), and Engel-Vosko (EVGGA) compared to the experimental value of 2.84 eV. We notice that EVGGA shows excellent agreement with the experimental data. This agreement is attributed to the fact that the Engel-Vosko GGA formalism optimizes the corresponding potential for band structure calculations. We make a detailed comparison of the density of states deduced from the X-ray photoelectron spectra with our calculations. We find that there is a strong covalent bond between the Hg and S atoms and Ga and S atoms. The Hg-Hg, Ga-Ga, and S-S bonds are found to be weaker than the Hg-S and Ga-S bonds showing that a covalent bond exists between Hg and S atoms and Ga and S atoms.

Polarized edge emission from GaN-based light-emitting diodes sandwiched by dielectric/metal hybrid reflectors

NASA Astrophysics Data System (ADS)

Yan, L. J.; Sheu, J. K.; Huang, F. W.; Lee, M. L.

2010-12-01

Edge-emitting c-plane GaN/sapphire-based light-emitting diodes (LEDs) sandwiched by two dielectric/metal hybrid reflectors on both sapphire and GaN surfaces were studied to determine their light emission polarization. The hybrid reflectors comprised dielectric multiple thin films and a metal layer. The metal layers of Au or Ag used in this study were designed to enhance the polarization ratio from S-polarization (transverse electric wave, TE) to P-polarization (transverse magnetic wave, TM). The two sets of optimized dielectric multi thin films served as matching layers for wide-angle incident light on both sapphire and GaN surfaces. To determine which reflector scheme would achieve a higher polarization ratio, simulations of the reflectance at the hybrid reflectors on sapphire (or GaN) interface were performed before the fabrication of experimental LEDs. Compared with conventional c-plane InGaN/GaN/sapphire LEDs without dielectric/metal hybrid reflectors, the experimental LEDs exhibited higher polarization ratio (ITE-max/ITM-max) with r=2.174 (˜3.37 dB) at a wavelength of 460 nm. In contrast, the original polarized light (without dielectric/metal hybrid reflectors) was partially contributed (r=1.398) by C-HH or C-LH (C band to the heavy-hole sub-band or C band to the crystal-field split-off sub-band) transitions along the a-plane or m-plane direction.

Polarity determination of polar and semipolar (112¯2) InN and GaN layers by valence band photoemission spectroscopy

NASA Astrophysics Data System (ADS)

Skuridina, D.; Dinh, D. V.; Lacroix, B.; Ruterana, P.; Hoffmann, M.; Sitar, Z.; Pristovsek, M.; Kneissl, M.; Vogt, P.

2013-11-01

We demonstrate that the polarity of polar (0001), (0001¯) and semipolar (112¯2) InN and GaN thin layers can be determined by valence band X-ray photoemission spectroscopy (XPS). The polarity of the layers has been confirmed by wet etching and convergent beam electron diffraction. Unlike these two techniques, XPS is a non-destructive method and unaffected by surface oxidation or roughness. Different intensities of the valence band states in spectra recorded by using AlKα X-ray radiation are observed for N-polar and group-III-polar layers. The highest intensity of the valence band state at ≈3.5 eV for InN and ≈5.2 eV for GaN correlates with the group-III polarity, while the highest intensity at ≈6.7 eV for InN and ≈9.5 eV for GaN correlates with the N-polarity. The difference between the peaks for the group-III- and N-polar orientations was found to be statistically significant at the 0.05 significance level. The polarity of semipolar (112¯2) InN and GaN layers can be determined by recording valence band photoelectrons emitted along the [000 ± 1] direction.

Impact of Wide-Ranging Nanoscale Chemistry on Band Structure at Cu(In, Ga)Se 2 Grain Boundaries

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stokes, Adam; Al-Jassim, Mowafak; Diercks, David

The relative chemistry from grain interiors to grain boundaries help explain why grain boundaries may be beneficial, detrimental or benign towards device performance. 3D Nanoscale chemical analysis extracted from atom probe tomography (APT) (10’s of parts-per-million chemical sensitivity and sub-nanometer spatial resolution) of twenty grain boundaries in a high-efficiency Cu(In, Ga)Se 2 solar cell shows the matrix and alkali concentrations are wide-ranging. The concentration profiles are then related to band structure which provide a unique insight into grain boundary electrical performance. Fluctuating Cu, In and Ga concentrations result in a wide distribution of potential barriers at the valence band maximummore » (VBM) (-10 to -160 meV) and the conduction band minimum (CBM) (-20 to -70 meV). Furthermore, Na and K segregation is not correlated to hampering donors, (In, Ga) Cu and V Se, contrary to what has been previously reported. In addition, Na and K are predicted to be n-type dopants at grain boundaries. An overall band structure at grain boundaries is presented.« less

Impact of Wide-Ranging Nanoscale Chemistry on Band Structure at Cu(In, Ga)Se 2 Grain Boundaries

DOE PAGES

Stokes, Adam; Al-Jassim, Mowafak; Diercks, David; ...

2017-10-26

The relative chemistry from grain interiors to grain boundaries help explain why grain boundaries may be beneficial, detrimental or benign towards device performance. 3D Nanoscale chemical analysis extracted from atom probe tomography (APT) (10’s of parts-per-million chemical sensitivity and sub-nanometer spatial resolution) of twenty grain boundaries in a high-efficiency Cu(In, Ga)Se 2 solar cell shows the matrix and alkali concentrations are wide-ranging. The concentration profiles are then related to band structure which provide a unique insight into grain boundary electrical performance. Fluctuating Cu, In and Ga concentrations result in a wide distribution of potential barriers at the valence band maximummore » (VBM) (-10 to -160 meV) and the conduction band minimum (CBM) (-20 to -70 meV). Furthermore, Na and K segregation is not correlated to hampering donors, (In, Ga) Cu and V Se, contrary to what has been previously reported. In addition, Na and K are predicted to be n-type dopants at grain boundaries. An overall band structure at grain boundaries is presented.« less

Growth and characterization of dilute nitride GaN{sub x}P{sub 1−x} nanowires and GaN{sub x}P{sub 1−x}/GaN{sub y}P{sub 1−y} core/shell nanowires on Si (111) by gas source molecular beam epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sukrittanon, S.; Kuang, Y. J.; Dobrovolsky, A.

2014-08-18

We have demonstrated self-catalyzed GaN{sub x}P{sub 1−x} and GaN{sub x}P{sub 1−x}/GaN{sub y}P{sub 1−y} core/shell nanowire growth by gas-source molecular beam epitaxy. The growth window for GaN{sub x}P{sub 1−x} nanowires was observed to be comparable to that of GaP nanowires (∼585 °C to ∼615 °C). Transmission electron microscopy showed a mixture of cubic zincblende phase and hexagonal wurtzite phase along the [111] growth direction in GaN{sub x}P{sub 1−x} nanowires. A temperature-dependent photoluminescence (PL) study performed on GaN{sub x}P{sub 1−x}/GaN{sub y}P{sub 1−y} core/shell nanowires exhibited an S-shape dependence of the PL peaks. This suggests that at low temperature, the emission stems from N-related localizedmore » states below the conduction band edge in the shell, while at high temperature, the emission stems from band-to-band transition in the shell as well as recombination in the GaN{sub x}P{sub 1−x} core.« less

Point-defect energies in the nitrides of aluminum, gallium, and indium

NASA Astrophysics Data System (ADS)

Tansley, T. L.; Egan, R. J.

1992-05-01

Experimental data on the nature and energetic location of levels associated with native point defects in the group-III metal nitrides are critically reviewed and compared with theoretical estimates. All three show strong evidence of the existence of a triplet of donorlike states associated with the nitrogen vacancy. Ground states are at about 150, 400, and 900 meV from the conduction-band edge in InN, GaN, and AlN, respectively, with their charged derivatives lying closer to the band edge. These values agree with both modified-hydrogenic and deep-level calculations, surprisingly well in view of the inherent approximations in each in this depth range. The InN donor ground state is both optically active and usually occupied, showing a distinctive absorption band which is very well described by quantum-defect analysis. Variation of threshold with electron concentration shows a Moss-Burstein shift commensurate with that observed in band-to-band absorption. In both GaN and AlN, levels have been identified at about 1/4EG and about 3/4EG, which correlate well with predictions for the antisite defects NM and MN, respectively, while similar behavior in InN is at odds with theory. The metal-vacancy defect appears to generate a level somewhat below midgap in AlN and close to the valence-band edge in GaN, but has not been located experimentally in InN, where it is predicted to lie very close to the valence-band edge. A tentative scheme for the participation of two of the native defects in GaN, namely VN and NGa, in the four broad emission bands found in Zn-compensated and undoped GaN is offered.

Recombination by band-to-defect tunneling near semiconductor heterojunctions: A theoretical model

DOE PAGES

Wampler, William R.; Samuel M. Myers; Modine, Normand A.

2016-10-04

Carrier transport and recombination are modeled for a heterojunction diode containing defect traps. Here, particular attention is given to the role of band-to-trap tunneling and how it is affected by band offsets at the junction. Tunneled states are characterized by numerical solution of the Schrodinger equation, and the interaction with traps is treated assuming capture and emission by the multi-phonon mechanism. It is shown that tunneling can increase carrier recombination at defects by orders magnitude in the presence of large band offsets. This explains why InGaP/GaAs/GaAs Npn HBTs with displacement damage from energetic particle irradiation have higher carrier recombination inmore » the emitter-base depletion region.« less

Recombination by band-to-defect tunneling near semiconductor heterojunctions: A theoretical model

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wampler, William R.; Samuel M. Myers; Modine, Normand A.

Carrier transport and recombination are modeled for a heterojunction diode containing defect traps. Here, particular attention is given to the role of band-to-trap tunneling and how it is affected by band offsets at the junction. Tunneled states are characterized by numerical solution of the Schrodinger equation, and the interaction with traps is treated assuming capture and emission by the multi-phonon mechanism. It is shown that tunneling can increase carrier recombination at defects by orders magnitude in the presence of large band offsets. This explains why InGaP/GaAs/GaAs Npn HBTs with displacement damage from energetic particle irradiation have higher carrier recombination inmore » the emitter-base depletion region.« less

First principles study of size and external electric field effects on the atomic and electronic properties of gallium nitride nanostructures

NASA Astrophysics Data System (ADS)

Yilmaz, Hulusi

A comprehensive density functional theory study of atomic and the electronic properties of wurtzite gallium nitride (GaN) nanostructures with different sizes and shapes is presented and the effect of external electric field on these properties is examined. We show that the atomic and electronic properties of [101¯0] facet single-crystal GaN nanotubes (quasi-1D), nanowires (1D) and nanolayers (2D) are mainly determined by the surface to volume ratio. The shape dependent quantum confinement and strain effects on the atomic and electronic properties of these GaN nanostructures are found to be negligible. Based on this similarity between the atomic and electronic properties of the small size GaN nanostructures, we calculated the atomic and electronic properties of the practical size (28.1 A wall thickness) single-crystal GaN nanotubes through computational much economical GaN nanoslabs (nanolayers). Our results show that, regardless of diameter, hydrogen saturated single-crystal GaN tubes with the wall thickness of 28.1 A are energetically stable and they have a noticeably larger band gap with respect to the band gap of bulk GaN. The band gap of unsaturated single-crystal GaN tubes, on the other hand, is always smaller than the band gap of the wurtzite bulk GaN. In a separate study, we show that a transverse electric field induces a homojunction across the diameter of initially semiconducting GaN single-crystal nanotubes and nanowires. The homojunction arises due to the decreased energy of the electronic states in the higher potential region with respect to the energy of those states in the lower potential region under the transverse electric field. Calculations on single-crystal GaN nanotubes and nanowires of different diameter and wall thickness show that the threshold electric field required for the semiconductor-homojunction induction increases with increasing wall thickness and decreases significantly with increasing diameter.

Wide Bandgap Extrinsic Photoconductive Switches

NASA Astrophysics Data System (ADS)

Sullivan, James Stephen

Wide Bandgap Extrinsic Photoconductive Switches Semi-insulating Gallium Nitride, 4H and 6H Silicon Carbide are attractive materials for compact, high voltage, extrinsic, photoconductive switches due to their wide bandgap, high dark resistance, high critical electric field strength and high electron saturation velocity. These wide bandgap semiconductors are made semi-insulating by the addition of vanadium (4H and 6H-SiC) and iron (2H-GaN) impurities that form deep acceptors. These deep acceptors trap electrons donated from shallow donor impurities. The electrons can be optically excited from these deep acceptor levels into the conduction band to transition the wide bandgap semiconductor materials from a semi-insulating to a conducting state. Extrinsic photoconductive switches with opposing electrodes have been constructed using vanadium compensated 6H-SiC and iron compensated 2H-GaN. These extrinsic photoconductive switches were tested at high voltage and high power to determine if they could be successfully used as the closing switch in compact medical accelerators. The successful development of a vanadium compensated, 6H-SiC extrinsic photoconductive switch for use as a closing switch for compact accelerator applications was realized by improvements made to the vanadium, nitrogen and boron impurity densities. The changes made to the impurity densities were based on the physical intuition outlined and simple rate equation models. The final 6H-SiC impurity 'recipe' calls for vanadium, nitrogen and boron densities of 2.5 e17 cm-3, 1.25e17 cm-3 and ≤ 1e16 cm-3, respectively. This recipe was originally developed to maximize the quantum efficiency of the vanadium compensated 6H-SiC, while maintaining a thermally stable semi-insulating material. The rate equation models indicate that, besides increasing the quantum efficiency, the impurity recipe should be expected to also increase the carrier recombination time. Three generations of 6H-SiC materials were tested. The third generation vanadium compensated 6H-SiC has average impurity densities close to the recipe values. Extrinsic photoconductive switches constructed from the third generation vanadium compensated, 6H-SiC, 1 mm thick, 1 cm2, substrates have achieved high power operation at 16 kV with pulsed currents exceeding 1400 Amperes and a minimum on resistance of 1 ohm. The extrinsic photoconductive switch performance of the third generation 6H-SiC material was improved by a factor of up to 50 for excitation at the 532 nm wavelength compared to the initial 6H-SiC material. Switches based on this material have been incorporated into a prototype compact proton medical accelerator being developed by the Compact Particle Acceleration Corporation (CPAC). The vanadium compensated, 6H-SiC, extrinsic photoconductive switch operates differently when excited by 1064, or 532 nm, wavelength light. The 6H-SiC extrinsic photoconductive switch is a unipolar device when excited with 1064 nm light. The carriers are electrons excited from filled vanadium acceptor levels and other electron traps located within 1.17 eV of the conduction band. The switch is bipolar at 532 nm since the carriers consist of holes, as well as electrons. The holes are primarily generated by the excitation of valence band electrons into empty trap/acceptor levels and by two-photon absorption. Carrier generation by two-photon absorption becomes more important at high applied optical intensity at 532 nm and contributes to the supralinear behavior of switch conductance as a function of optical power. The 6H-SiC switch material is trap dominated at low nitrogen to vanadium ratios. The trap dominated vanadium compensated 6H-SiC exhibits low quantum efficiency when excited with 1064 and 532 nm light and has a carrier recombination time of ˜ 150 - 300 ps. The vanadium compensated 6H-SiC transitions to an impurity dominated material as the ratio of nitrogen to vanadium is increased to 0.5. The increased nitrogen doping produces a material with much higher quantum efficiency and carrier recombination time of 0.9 to 1.0 ns. The iron compensated 2H-GaN did not perform well as an extrinsic photoconductive switch. The density of carriers generated at 1064 nm was, low indicating that there were very few electrons trapped in the iron acceptor level located at 0.5 - 0.6 eV below the conduction band. Carrier generation at 532 nm was dominated by two photon absorption resulting in the switch conductance increasing as the square of applied optical intensity. A minimum switch resistance of 0.8 ohms was calculated for the 400 nm thick, 1.2 by 1.2 cm, 2H-GaN switch for an applied optical intensity of 41.25 MW/cm2. An optical intensity of ˜ 70 MW/cm2 at 532 nm would be required to achieve a 0.8 ohm on resistance for a 1 mm thick, 1 cm2, 2H-GaN switch.

GaN: From three- to two-dimensional single-layer crystal and its multilayer van der Waals solids

NASA Astrophysics Data System (ADS)

Onen, A.; Kecik, D.; Durgun, E.; Ciraci, S.

2016-02-01

Three-dimensional (3D) GaN is a III-V compound semiconductor with potential optoelectronic applications. In this paper, starting from 3D GaN in wurtzite and zinc-blende structures, we investigated the mechanical, electronic, and optical properties of the 2D single-layer honeycomb structure of GaN (g -GaN ) and its bilayer, trilayer, and multilayer van der Waals solids using density-functional theory. Based on high-temperature ab initio molecular-dynamics calculations, we first showed that g -GaN can remain stable at high temperature. Then we performed a comparative study to reveal how the physical properties vary with dimensionality. While 3D GaN is a direct-band-gap semiconductor, g -GaN in two dimensions has a relatively wider indirect band gap. Moreover, 2D g -GaN displays a higher Poisson ratio and slightly less charge transfer from cation to anion. In two dimensions, the optical-absorption spectra of 3D crystalline phases are modified dramatically, and their absorption onset energy is blueshifted. We also showed that the physical properties predicted for freestanding g -GaN are preserved when g -GaN is grown on metallic as well as semiconducting substrates. In particular, 3D layered blue phosphorus, being nearly lattice-matched to g -GaN , is found to be an excellent substrate for growing g -GaN . Bilayer, trilayer, and van der Waals crystals can be constructed by a special stacking sequence of g -GaN , and they can display electronic and optical properties that can be controlled by the number of g -GaN layers. In particular, their fundamental band gap decreases and changes from indirect to direct with an increasing number of g -GaN layers.

Influence of internal electric fields on band gaps in short period GaN/GaAlN and InGaN/GaN polar superlattices

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gorczyca, I., E-mail: iza@unipress.waw.pl; Skrobas, K.; Suski, T.

2015-08-21

The electronic structures of short period mGaN/nGa{sub y}Al{sub 1−y}N and mIn{sub y}Ga{sub 1-y}N/nGaN superlattices grown along the wurtzite c axis have been calculated for different alloy compositions y and various small numbers m of well- and n of barrier-monolayers. The general trends in gap behavior can, to a large extent, be related to the strength of the internal electric field, E, in the GaN and InGaN quantum wells. In the GaN/GaAlN superlattices, E reaches 4 MV/cm, while in the InGaN/GaN superlattices, values as high as E ≈ 6.5 MV/cm are found. The strong electric fields are caused by spontaneous and piezoelectric polarizations,more » the latter contribution dominating in InGaN/GaN superlattices. The influence of different arrangements of In atoms (indium clustering) on the band gap values in InGaN/GaN superlattices is examined.« less

Isolation and characterization of gallium resistant Pseudomonas aeruginosa mutants.

PubMed

García-Contreras, Rodolfo; Lira-Silva, Elizabeth; Jasso-Chávez, Ricardo; Hernández-González, Ismael L; Maeda, Toshinari; Hashimoto, Takahiro; Boogerd, Fred C; Sheng, Lili; Wood, Thomas K; Moreno-Sánchez, Rafael

2013-12-01

Pseudomonas aeruginosa PA14 cells resistant to the novel antimicrobial gallium nitrate (Ga) were developed using transposon mutagenesis and by selecting spontaneous mutants. The mutants showing the highest growth in the presence of Ga were selected for further characterization. These mutants showed 4- to 12-fold higher Ga minimal inhibitory growth concentrations and a greater than 8-fold increase in the minimum biofilm eliminating Ga concentration. Both types of mutants produced Ga resistant biofilms whereas the formation of wild-type biofilms was strongly inhibited by Ga. The gene interrupted in the transposon mutant was hitA, which encodes a periplasmic iron binding protein that delivers Fe³⁺ to the HitB iron permease; complementation of the mutant with the hitA gene restored the Ga sensitivity. This hitA mutant showed a 14-fold decrease in Ga internalization versus the wild-type strain, indicating that the HitAB system is also involved in the Ga uptake. Ga uptake in the spontaneous mutant was also lower, although no mutations were found in the hitAB genes. Instead, this mutant harbored 64 non-silent mutations in several genes including those of the phenazine pyocyanin biosynthesis. The spontaneous mutant produced 2-fold higher pyocyanin basal levels than the wild-type; the addition of this phenazine to wild-type cultures protected them from the Ga bacteriostatic effect. The present data indicate that mutations affecting Ga transport and probably pyocyanin biosynthesis enable cells to develop resistance to Ga. Copyright © 2013 Elsevier GmbH. All rights reserved.

Electronic structure of β-Ga2O3 single crystals investigated by hard X-ray photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Li, Guo-Ling; Zhang, Fabi; Cui, Yi-Tao; Oji, Hiroshi; Son, Jin-Young; Guo, Qixin

2015-07-01

By combination of hard X-ray photoelectron spectroscopy (HAXPES) and first-principles band structure calculations, the electronic states of β-Ga2O3 were investigated to deepen the understanding of bulk information for this compound. The valence band spectra of HAXPES presented the main contribution from Ga 4sp, which are well represented by photoionization cross section weighted partial density of states. The experimental data complemented with the theoretical study yield a realistic picture of the electronic structure for β-Ga2O3.

Structuring β-Ga2O3 photonic crystal photocatalyst for efficient degradation of organic pollutants.

PubMed

Li, Xiaofang; Zhen, Xiuzheng; Meng, Sugang; Xian, Jiangjun; Shao, Yu; Fu, Xianzhi; Li, Danzhen

2013-09-03

Coupling photocatalysts with photonic crystals structure is based on the unique property of photonic crystals in confining, controlling, and manipulating the incident photons. This combination enhances the light absorption in photocatalysts and thus greatly improves their photocatalytic performance. In this study, Ga2O3 photonic crystals with well-arranged skeleton structures were prepared via a dip-coating infiltration method. The positions of the electronic band absorption for Ga2O3 photonic crystals could be made to locate on the red edge, on the blue edge, and away from the edge of their photonic band gaps by changing the pore sizes of the samples, respectively. Particularly, the electronic band absorption of the Ga2O3 photonic crystal with a pore size of 135 nm was enhanced more than other samples by making it locate on the red edge of its photonic band gap, which was confirmed by the higher instantaneous photocurrent and photocatalytic activity for the degradation of various organic pollutants under ultraviolet light irradiation. Furthermore, the degradation mechanism over Ga2O3 photonic crystals was discussed. The design of Ga2O3 photonic crystals presents a prospective application of photonic crystals in photocatalysis to address light harvesting and quantum efficiency problems through manipulating photons or constructing photonic crystal structure as groundwork.

Evolution of optical properties and band structure from amorphous to crystalline Ga2O3 films

NASA Astrophysics Data System (ADS)

Zhang, Fabi; Li, Haiou; Cui, Yi-Tao; Li, Guo-Ling; Guo, Qixin

2018-04-01

The optical properties and band structure evolution from amorphous to crystalline Ga2O3 films was investigated in this work. Amorphous and crystalline Ga2O3 films were obtained by changing the growth substrate temperatures of pulsed laser deposition and the crystallinity increase with the rising of substrate temperature. The bandgap value and ultraviolet emission intensity of the films increase with the rising of crystallinity as observed by means of spectrophotometer and cathodoluminescence spectroscopy. Abrupt bandgap value and CL emission variations were observed when amorphous to crystalline transition took place. X-ray photoelectron spectroscopy core level spectra reveal that more oxygen vacancies and disorders exist in amorphous Ga2O3 film grown at lower substrate temperature. The valence band spectra of hard X-ray photoelectron spectroscopy present the main contribution from Ga 4sp for crystalline film deposited at substrate temperature of 500 oC, while extra subgap states has been observed in amorphous film deposited at 300 oC. The oxygen vacancy and the extra subgap density of states are suggested to be the parts of origin of bandgap and CL spectra variations. The experimental data above yields a realistic picture of optical properties and band structure variation for the amorphous to crystalline transition of Ga2O3 films.

Energy-band alignment of (HfO2)x(Al2O3)1-x gate dielectrics deposited by atomic layer deposition on β-Ga2O3 (-201)

NASA Astrophysics Data System (ADS)

Yuan, Lei; Zhang, Hongpeng; Jia, Renxu; Guo, Lixin; Zhang, Yimen; Zhang, Yuming

2018-03-01

Energy band alignments between series band of Al-rich high-k materials (HfO2)x(Al2O3)1-x and β-Ga2O3 are investigated using X-Ray Photoelectron Spectroscopy (XPS). The results exhibit sufficient conduction band offsets (1.42-1.53 eV) in (HfO2)x(Al2O3)1-x/β-Ga2O3. In addition, it is also obtained that the value of Eg, △Ec, and △Ev for (HfO2)x(Al2O3)1-x/β-Ga2O3 change linearly with x, which can be expressed by 6.98-1.27x, 1.65-0.56x, and 0.48-0.70x, respectively. The higher dielectric constant and higher effective breakdown electric field of (HfO2)x(Al2O3)1-x compared with Al2O3, coupled with sufficient barrier height and lower gate leakage makes it a potential dielectric for high voltage β-Ga2O3 power MOSFET, and also provokes interest in further investigation of HfAlO/β-Ga2O3 interface properties.

Electronic structures of [001]- and [111]-oriented InSb and GaSb free-standing nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liao, Gaohua; Department of Applied Physics and Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha 410082; Luo, Ning

We report on a theoretical study of the electronic structures of InSb and GaSb nanowires oriented along the [001] and [111] crystallographic directions. The nanowires are described by atomistic, tight-binding models, including spin-orbit interaction. The band structures and the wave functions of the nanowires are calculated by means of a Lanczos iteration algorithm. For the [001]-oriented InSb and GaSb nanowires, the systems with both square and rectangular cross sections are considered. Here, it is found that all the energy bands are doubly degenerate. Although the lowest conduction bands in these nanowires show good parabolic dispersions, the top valence bands showmore » rich and complex structures. In particular, the topmost valence bands of the nanowires with a square cross section show a double maximum structure. In the nanowires with a rectangular cross section, this double maximum structure is suppressed, and the top valence bands gradually develop into parabolic bands as the aspect ratio of the cross section is increased. For the [111]-oriented InSb and GaSb nanowires, the systems with hexagonal cross sections are considered. It is found that all the bands at the Γ-point are again doubly degenerate. However, some of them will split into non-degenerate bands when the wave vector moves away from the Γ-point. Although the lowest conduction bands again show good parabolic dispersions, the topmost valence bands do not show the double maximum structure. Instead, they show a single maximum structure with its maximum at a wave vector slightly away from the Γ-point. The wave functions of the band states near the band gaps of the [001]- and [111]-oriented InSb and GaSb nanowires are also calculated and are presented in terms of probability distributions in the cross sections. It is found that although the probability distributions of the band states in the [001]-oriented nanowires with a rectangular cross section could be qualitatively described by one-band effective mass theory, the probability distributions of the band states in the [001]-oriented nanowires with a square cross section and the [111]-oriented nanowires with a hexagonal cross section show characteristic patterns with symmetries closely related to the irreducible representations of the relevant double point groups and, in general, go beyond the prediction of a simple one-band effective mass theory. We also investigate the effects of quantum confinement on the band structures of the [001]- and [111]-oriented InSb and GaSb nanowires and present an empirical formula for the description of quantization energies of the band edge states in the nanowires, which could be used to estimate the enhancement of the band gaps of the nanowires as a result of quantum confinement. The size dependencies of the electron and hole effective masses in these nanowires are also investigated and discussed.« less

Electronic structures of [001]- and [111]-oriented InSb and GaSb free-standing nanowires

NASA Astrophysics Data System (ADS)

Liao, Gaohua; Luo, Ning; Yang, Zhihu; Chen, Keqiu; Xu, H. Q.

2015-09-01

We report on a theoretical study of the electronic structures of InSb and GaSb nanowires oriented along the [001] and [111] crystallographic directions. The nanowires are described by atomistic, tight-binding models, including spin-orbit interaction. The band structures and the wave functions of the nanowires are calculated by means of a Lanczos iteration algorithm. For the [001]-oriented InSb and GaSb nanowires, the systems with both square and rectangular cross sections are considered. Here, it is found that all the energy bands are doubly degenerate. Although the lowest conduction bands in these nanowires show good parabolic dispersions, the top valence bands show rich and complex structures. In particular, the topmost valence bands of the nanowires with a square cross section show a double maximum structure. In the nanowires with a rectangular cross section, this double maximum structure is suppressed, and the top valence bands gradually develop into parabolic bands as the aspect ratio of the cross section is increased. For the [111]-oriented InSb and GaSb nanowires, the systems with hexagonal cross sections are considered. It is found that all the bands at the Γ-point are again doubly degenerate. However, some of them will split into non-degenerate bands when the wave vector moves away from the Γ-point. Although the lowest conduction bands again show good parabolic dispersions, the topmost valence bands do not show the double maximum structure. Instead, they show a single maximum structure with its maximum at a wave vector slightly away from the Γ-point. The wave functions of the band states near the band gaps of the [001]- and [111]-oriented InSb and GaSb nanowires are also calculated and are presented in terms of probability distributions in the cross sections. It is found that although the probability distributions of the band states in the [001]-oriented nanowires with a rectangular cross section could be qualitatively described by one-band effective mass theory, the probability distributions of the band states in the [001]-oriented nanowires with a square cross section and the [111]-oriented nanowires with a hexagonal cross section show characteristic patterns with symmetries closely related to the irreducible representations of the relevant double point groups and, in general, go beyond the prediction of a simple one-band effective mass theory. We also investigate the effects of quantum confinement on the band structures of the [001]- and [111]-oriented InSb and GaSb nanowires and present an empirical formula for the description of quantization energies of the band edge states in the nanowires, which could be used to estimate the enhancement of the band gaps of the nanowires as a result of quantum confinement. The size dependencies of the electron and hole effective masses in these nanowires are also investigated and discussed.

Mechanical, Thermodynamic and Electronic Properties of Wurtzite and Zinc-Blende GaN Crystals.

PubMed

Qin, Hongbo; Luan, Xinghe; Feng, Chuang; Yang, Daoguo; Zhang, Guoqi

2017-12-12

For the limitation of experimental methods in crystal characterization, in this study, the mechanical, thermodynamic and electronic properties of wurtzite and zinc-blende GaN crystals were investigated by first-principles calculations based on density functional theory. Firstly, bulk moduli, shear moduli, elastic moduli and Poisson's ratios of the two GaN polycrystals were calculated using Voigt and Hill approximations, and the results show wurtzite GaN has larger shear and elastic moduli and exhibits more obvious brittleness. Moreover, both wurtzite and zinc-blende GaN monocrystals present obvious mechanical anisotropic behavior. For wurtzite GaN monocrystal, the maximum and minimum elastic moduli are located at orientations [001] and <111>, respectively, while they are in the orientations <111> and <100> for zinc-blende GaN monocrystal, respectively. Compared to the elastic modulus, the shear moduli of the two GaN monocrystals have completely opposite direction dependences. However, different from elastic and shear moduli, the bulk moduli of the two monocrystals are nearly isotropic, especially for the zinc-blende GaN. Besides, in the wurtzite GaN, Poisson's ratios at the planes containing [001] axis are anisotropic, and the maximum value is 0.31 which is located at the directions vertical to [001] axis. For zinc-blende GaN, Poisson's ratios at planes (100) and (111) are isotropic, while the Poisson's ratio at plane (110) exhibits dramatically anisotropic phenomenon. Additionally, the calculated Debye temperatures of wurtzite and zinc-blende GaN are 641.8 and 620.2 K, respectively. At 300 K, the calculated heat capacities of wurtzite and zinc-blende are 33.6 and 33.5 J mol -1 K -1 , respectively. Finally, the band gap is located at the G point for the two crystals, and the band gaps of wurtzite and zinc-blende GaN are 3.62 eV and 3.06 eV, respectively. At the G point, the lowest energy of conduction band in the wurtzite GaN is larger, resulting in a wider band gap. Densities of states in the orbital hybridization between Ga and N atoms of wurtzite GaN are much higher, indicating more electrons participate in forming Ga-N ionic bonds in the wurtzite GaN.

Synthesis, structure, and electronic structure of CsAgGa{sub 2}Se{sub 4}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mei Dajiang; Yin Wenlong; Feng Kai

2012-02-15

The new metal chalcogenide CsAgGa{sub 2}Se{sub 4} has been synthesized by means of the reactive flux method. It crystallizes in the space group P2{sub 1}/c of the monoclinic system with cell dimensions of a=11.225(2) A, b=7.9443(16) A, c=21.303(4) A, {beta}=103.10(3), V=1850.3(6), and Z=8. The structure contains two-dimensional {sub {infinity}}{sup 2}[AgGa{sub 2}Se{sub 4}]{sup -} layers separated by Cs{sup +} cations. The {sub {infinity}}{sup 2}[AgGa{sub 2}Se{sub 4}]{sup -} superlayer possesses a novel chain-sublayer-chain structure: a {sub {infinity}}{sup 2}[Ag{sub 2}GaSe{sub 6}]{sup 7-} sublayer, composed of {sub {infinity}}{sup 1}[AgGaSe{sub 4}]{sup 4-} chains that are further connected by Ag{sup +} ions, is sandwiched by parallelmore » {sub {infinity}}{sup 1}[Ga{sub 3}Se{sub 8}]{sup 7-} chains to generate the {sub {infinity}}{sup 2}[AgGa{sub 2}Se{sub 4}]{sup -} superlayer. From a band structure calculation, the orbitals of all atoms have contributions to the bottoms of conduction bands, but the band gap is mainly determined by the 4s, 4p orbitals of Ga and Se. - Graphical Abstract: CsAgGa{sub 2}Se{sub 4} contains two-dimensional {sub {infinity}}{sup 2}[AgGa{sub 2}Se{sub 4}]{sup -} layers with a novel chain-sublayer-chain structure. Highlights: Black-Right-Pointing-Pointer New chalcogenide CsAgGa{sub 2}Se{sub 4} has been synthesized. Black-Right-Pointing-Pointer It possesses a new structure type with {sub {infinity}}{sup 2}[AgGa{sub 2}Se{sub 4}]{sup -} layers separated by Cs{sup +} cations. Black-Right-Pointing-Pointer {sub {infinity}}{sup 2}[AgGa{sub 2}Se{sub 4}]{sup -} consists of a {sub {infinity}}{sup 2}[Ag{sub 2}GaSe{sub 6}]{sup 7-} sublayer sandwiched by {sub {infinity}}{sup 1}[Ga{sub 3}Se{sub 8}]{sup 7-} chains. Black-Right-Pointing-Pointer Band gap of CsAgGa{sub 2}Se{sub 4} is mainly determined by the 4s, 4p orbitals of Ga and Se.« less

Mechanical, Thermodynamic and Electronic Properties of Wurtzite and Zinc-Blende GaN Crystals

PubMed Central

Luan, Xinghe; Feng, Chuang; Yang, Daoguo; Zhang, Guoqi

2017-01-01

For the limitation of experimental methods in crystal characterization, in this study, the mechanical, thermodynamic and electronic properties of wurtzite and zinc-blende GaN crystals were investigated by first-principles calculations based on density functional theory. Firstly, bulk moduli, shear moduli, elastic moduli and Poisson’s ratios of the two GaN polycrystals were calculated using Voigt and Hill approximations, and the results show wurtzite GaN has larger shear and elastic moduli and exhibits more obvious brittleness. Moreover, both wurtzite and zinc-blende GaN monocrystals present obvious mechanical anisotropic behavior. For wurtzite GaN monocrystal, the maximum and minimum elastic moduli are located at orientations [001] and <111>, respectively, while they are in the orientations <111> and <100> for zinc-blende GaN monocrystal, respectively. Compared to the elastic modulus, the shear moduli of the two GaN monocrystals have completely opposite direction dependences. However, different from elastic and shear moduli, the bulk moduli of the two monocrystals are nearly isotropic, especially for the zinc-blende GaN. Besides, in the wurtzite GaN, Poisson’s ratios at the planes containing [001] axis are anisotropic, and the maximum value is 0.31 which is located at the directions vertical to [001] axis. For zinc-blende GaN, Poisson’s ratios at planes (100) and (111) are isotropic, while the Poisson’s ratio at plane (110) exhibits dramatically anisotropic phenomenon. Additionally, the calculated Debye temperatures of wurtzite and zinc-blende GaN are 641.8 and 620.2 K, respectively. At 300 K, the calculated heat capacities of wurtzite and zinc-blende are 33.6 and 33.5 J mol−1 K−1, respectively. Finally, the band gap is located at the G point for the two crystals, and the band gaps of wurtzite and zinc-blende GaN are 3.62 eV and 3.06 eV, respectively. At the G point, the lowest energy of conduction band in the wurtzite GaN is larger, resulting in a wider band gap. Densities of states in the orbital hybridization between Ga and N atoms of wurtzite GaN are much higher, indicating more electrons participate in forming Ga-N ionic bonds in the wurtzite GaN. PMID:29231902

Tailoring the electronic structure of β-Ga2O3 by non-metal doping from hybrid density functional theory calculations.

PubMed

Guo, Weiyan; Guo, Yating; Dong, Hao; Zhou, Xin

2015-02-28

A systematic study using density functional theory has been performed for β-Ga2O3 doped with non-metal elements X (X = C, N, F, Si, P, S, Cl, Se, Br, and I) to evaluate the effect of doping on the band edges and photocatalytic activity of β-Ga2O3. The utilization of a more reliable hybrid density functional, as prescribed by Heyd, Scuseria and Ernzerhof, is found to be effective in predicting the band gap of β-Ga2O3 (4.5 eV), in agreement with the experimental result (4.59 eV). Based on the relaxed structures of X-doped systems, the defect formation energies and the plots of density of states have been calculated to analyze the band edges, the band gap states and the preferred doping sites. Our results show that the doping is energetically favored under Ga-rich growth conditions with respect to O-rich growth conditions. It is easier to replace the threefold coordinated O atom with non-metal elements compared to the fourfold coordinated O atom. X-doped systems (X = C, Si, P) show no change in the band gap, with the presence of discrete midgap states, which have adverse effect on the photocatalytic properties. The photocatalytic redox ability can be improved to a certain extent by doping with N, S, Cl, Se, Br, and I. The band alignments for Se-doped and I-doped β-Ga2O3 are well positioned for the feasibility of both photo-oxidation and photo-reduction of water, which are promising photocatalysts for water splitting in the visible region.

A high efficiency C-band internally-matched harmonic tuning GaN power amplifier

NASA Astrophysics Data System (ADS)

Lu, Y.; Zhao, B. C.; Zheng, J. X.; Zhang, H. S.; Zheng, X. F.; Ma, X. H.; Hao, Y.; Ma, P. J.

2016-09-01

In this paper, a high efficiency C-band gallium nitride (GaN) internally-matched power amplifier (PA) is presented. This amplifier consists of 2-chips of self-developed GaN high-electron mobility transistors (HEMTs) with 16 mm total gate width on SiC substrate. New harmonic manipulation circuits are induced both in the input and output matching networks for high efficiency matching at fundamental and 2nd-harmonic frequency, respectively. The developed amplifier has achieved 72.1% power added efficiency (PAE) with 107.4 W output power at 5 GHz. To the best of our knowledge, this amplifier exhibits the highest PAE in C-band GaN HEMT amplifiers with over 100 W output power. Additionally, 1000 hours' aging test reveals high reliability for practical applications.

Visible-light absorption and large band-gap bowing of GaN 1-xSb x from first principles

DOE PAGES

Sheetz, R. Michael; Richter, Ernst; Andriotis, Antonis N.; ...

2011-08-01

Applicability of the Ga(Sb x)N 1-x alloys for practical realization of photoelectrochemical water splitting is investigated using first-principles density functional theory incorporating the local density approximation and generalized gradient approximation plus the Hubbard U parameter formalism. Our calculations reveal that a relatively small concentration of Sb impurities is sufficient to achieve a significant narrowing of the band gap, enabling absorption of visible light. Theoretical results predict that Ga(Sb x)N 1-x alloys with 2-eV band gaps straddle the potential window at moderate to low pH values, thus indicating that dilute Ga(Sb x)N 1-x alloys could be potential candidates for splitting watermore » under visible light irradiation.« less

The potential of desferrioxamine-gallium as an anti-Pseudomonas therapeutic agent

PubMed Central

Banin, Ehud; Lozinski, Alina; Brady, Keith M.; Berenshtein, Eduard; Butterfield, Phillip W.; Moshe, Maya; Chevion, Mordechai; Greenberg, Everett Peter; Banin, Eyal

2008-01-01

The opportunistic pathogen Pseudomonas aeruginosa causes infections that are difficult to treat by antibiotic therapy. This bacterium can cause biofilm infections where it shows tolerance to antibiotics. Here we report the novel use of a metallo-complex, desferrioxamine-gallium (DFO-Ga) that targets P. aeruginosa iron metabolism. This complex kills free-living bacteria and blocks biofilm formation. A combination of DFO-Ga and the anti-Pseudomonas antibiotic gentamicin caused massive killing of P. aeruginosa cells in mature biofilms. In a P. aeruginosa rabbit corneal infection, topical administration of DFO-Ga together with gentamicin decreased both infiltrate and final scar size by about 50% compared to topical application of gentamicin alone. The use of DFO-Ga as a Trojan horse delivery system that interferes with iron metabolism shows promise as a treatment for P. aeruginosa infections. PMID:18931304

Variation of crystal structure and optical properties of wurtzite-type oxide semiconductor alloys of β-Cu(Ga,Al)O2

NASA Astrophysics Data System (ADS)

Nagatani, Hiraku; Mizuno, Yuki; Suzuki, Issei; Kita, Masao; Ohashi, Naoki; Omata, Takahisa

2017-06-01

Band-gap engineering of β-CuGaO2 was demonstrated by the alloying of gallium with aluminum, that is, Cu(Ga1-xAlx)O2. The ternary wurtzite β-NaFeO2-type alloys were obtained in the range 0 ≤ x ≤ 0.7, and γ-LiAlO2-type phase appeared in the range 0.7 ≤ x ≤ 1. The energy band gap of wurtzite β-CuGaO2 was controlled in the range between 1.47 and 2.09 eV. A direct band gap for x < 0.6 and indirect band gap for x ≥ 0.6 were proposed based on the structural distortion in the β-NaFeO2-type phase and density functional theory (DFT) calculation of β-CuAlO2. The DFT calculation also indicated that the γ-LiAlO2-type phases appeared in 0.7 ≤ x ≤ 1 are also indirect-gap semiconductors.

Determination of the electron-capture coefficients and the concentration of free electrons in GaN from time-resolved photoluminescence

PubMed Central

Reshchikov, M. A.; McNamara, J. D.; Toporkov, M.; Avrutin, V.; Morkoç, H.; Usikov, A.; Helava, H.; Makarov, Yu.

2016-01-01

Point defects in high-purity GaN layers grown by hydride vapor phase epitaxy are studied by steady-state and time-resolved photoluminescence (PL). The electron-capture coefficients for defects responsible for the dominant defect-related PL bands in this material are found. The capture coefficients for all the defects, except for the green luminescence (GL1) band, are independent of temperature. The electron-capture coefficient for the GL1 band significantly changes with temperature because the GL1 band is caused by an internal transition in the related defect, involving an excited state acting as a giant trap for electrons. By using the determined electron-capture coefficients, the concentration of free electrons can be found at different temperatures by a contactless method. A new classification system is suggested for defect-related PL bands in undoped GaN. PMID:27901025

Ab initio simulation study of defect assisted Zener tunneling in GaAs diode

NASA Astrophysics Data System (ADS)

Lu, Juan; Fan, Zhi-Qiang; Gong, Jian; Jiang, Xiang-Wei

2017-06-01

The band to band tunneling of defective GaAs nano-junction is studied by using the non-equilibrium Green's function formalism with density functional theory. Aiming at performance improvement, two types of defect-induced transport behaviors are reported in this work. By examining the partial density of states of the system, we find the substitutional defect OAs that locates in the middle of tunneling region will introduce band-gap states, which can be used as stepping stones to increase the tunneling current nearly 3 times higher at large bias voltage (Vb≥0.3V). Another type of defects SeAs and VGa (Ga vacancy) create donor and acceptor states at the edge of conduction band (CB) and valence band (VB)respectively, which can change the band bending of the junction as well as increase the tunneling field obtaining a 1.5 times higher ON current. This provides an effective defect engineering approach for next generation TFET device design.

Creep rupture behavior of Stirling engine materials

NASA Technical Reports Server (NTRS)

Titran, R. H.; Scheuerman, C. M.; Stephens, J. R.

1985-01-01

The automotive Stirling engine, being investigated jointly by the Department of Energy and NASA Lewis as an alternate to the internal combustion engine, uses high-pressure hydrogen as the working fluid. The long-term effects of hydrogen on the high temperature strength properties of materials is relatively unknown. This is especially true for the newly developed low-cost iron base alloy NASAUT 4G-A1. This iron-base alloy when tested in air has creep-rupture strengths in the directionally solidified condition comparable to the cobalt base alloy HS-31. The equiaxed (investment cast) NASAUT 4G-A1 has superior creep-rupture to the equiaxed iron-base alloy XF-818 both in air and 15 MPa hydrogen.

Preparation and morphology, magnetic properties of yttrium iron garnet nanodot arrays on Gd3Ga5O12 substrate

NASA Astrophysics Data System (ADS)

Xu, Zhiwang; Zheng, Hui; Han, Mangui

2017-07-01

In this work, yttrium iron garnet nanodot array has been deposited on Gd3Ga5O12 substrate by pulsed laser deposition through an ultrathin alumina mask. The morphology and magnetic properties of YIG nanodot array have been investigated. Scanning electron microscopy displays the prepared nanodot array has a sharp distribution in diameter centered at 330 nm with standard deviation of 20 nm. X-ray diffraction θ-2θ and pole figure analysis show the yttrium iron garnet nanodot array has oriented growth. Moreover, typical hysteresis loops and ferromagnetic resonance spectra display larger coercivity and multi-resonance peaks which are ascribed to this unique structure.

Photo-assisted Kelvin probe force microscopy investigation of three dimensional GaN structures with various crystal facets, doping types, and wavelengths of illumination

NASA Astrophysics Data System (ADS)

Ali Deeb, Manal; Ledig, Johannes; Wei, Jiandong; Wang, Xue; Wehmann, Hergo-Heinrich; Waag, Andreas

2017-08-01

Three dimensional GaN structures with different crystal facets and doping types have been investigated employing the surface photo-voltage (SPV) method to monitor illumination-induced surface charge behavior using Kelvin probe force microscopy. Various photon energies near and below the GaN bandgap were used to modify the generation of electron-hole pairs and their motion under the influence of the electric field near the GaN surface. Fast and slow processes for Ga-polar c-planes on both Si-doped n-type as well as Mg-doped p-type GaN truncated pyramid micro-structures were found and their origin is discussed. The immediate positive (for n-type) and negative (for p-type) SPV response dominates at band-to-band and near-bandgap excitation, while only the slow process is present at sub-bandgap excitation. The SPV behavior for the semi-polar facets of the p-type GaN truncated pyramids has a similar characteristic to that on its c-plane, which indicates that it has a comparable band bending and no strong influence of the polarity-induced charges is detectable. The SPV behavior of the non-polar m-facets of the Si-doped n-type part of a transferred GaN column is similar to that of a clean c-plane GaN surface during illumination. However, the SPV is smaller in magnitude, which is attributed to intrinsic surface states of m-plane surfaces and their influence on the band bending. The SPV behavior of the non-polar m-facet of the slightly Mg-doped part of this GaN column is found to behave differently. Compared to c- and r-facets of p-type surfaces of GaN-light-emitting diode micro-structures, the m-plane is more chemically stable.

Electronic band structure and optical gain of GaN{sub x}Bi{sub y}As{sub 1−x−y}/GaAs pyramidal quantum dots

DOE Office of Scientific and Technical Information (OSTI.GOV)

Song, Zhi-Gang; State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083; Bose, Sumanta

The electronic band structure and optical gain of GaN{sub x}Bi{sub y}As{sub 1−x−y}/GaAs pyramidal quantum dots (QDs) are investigated using the 16-band k ⋅ p model with constant strain. The optical gain is calculated taking both homogeneous and inhomogeneous broadenings into consideration. The effective band gap falls as we increase the composition of nitrogen (N) and bismuth (Bi) and with an appropriate choice of composition we can tune the emission wavelength to span within 1.3 μm–1.55 μm, for device application in fiber technology. The extent of this red shift is more profound in QDs compared with bulk material due to quantum confinement. Othermore » factors affecting the emission characteristics include virtual crystal, strain profile, band anticrossing (BAC), and valence band anticrossing (VBAC). The strain profile has a profound impact on the electronic structure, specially the valence band of QDs, which can be determined using the composition distribution of wave functions. All these factors eventually affect the optical gain spectrum. With an increase in QD size, we observe a red shift in the emission energy and emergence of secondary peaks owing to transitions or greater energy compared with the fundamental transition.« less

Ku and K band GaN High Power Amplifier MMICs

DTIC Science & Technology

2017-03-20

end Ku-band HPA operates from 13 to 14.5 GHz and delivers 48 Watts of output power with 43% PAE. A high-end Ku-band HPA operates from 15.5 to 18 GHz and...delivers 25 Watts of output power with 45% PAE. A K-band HPA operates from 19.5 to 22 GHz and delivers 18 Watts of output power with 29% PAE...15.5 and 18 GHz. The circuit is a three-stage reactively-matched amplifier. A photograph of a fabricated high-end Ku-band GaN HPA is shown as an

On a direct connection of the transition metal impurity levels to the band edge discontinuities in semiconductor heterojunctions

NASA Astrophysics Data System (ADS)

Langer, Jerzy M.; Heinrich, Helmut

1985-11-01

Our recent proposal of using the transition metal impurity levels to predict the isovalent heterojunction (HJ) band-edge discontinuities is further discussed. It is shown that for Ga 1-xAl xAs/GaAs heterojunctions most of the recent discontinuity data follow within experimental error the prediction of the ΔE cb: ΔE vb= 0.64:0.36 discontinuity ratio derived from the Fe 2+ level position in Ga 1-xAl xAs compound. Predictions of valence-band discontinuities for the other III-V and II-VI HJ systems are also given.

Determination of the energy structure of recombination centers in heavily doped AlxGa1-xN:Si epitaxial layers with x > 0.5

NASA Astrophysics Data System (ADS)

Osinnykh, I. V.; Malin, T. V.; Zhuravlev, K. S.

2018-03-01

The photoluminescence properties of the intensive defect-related emission in heavily doped Al x Ga l-x N:Si layers with x > 0.5 have been investigated by photoluminescence (PL) spectroscopy. The PL band in AlN was attributed to donor-acceptor (DA) transitions. At the lowest Al content, the impurity band merges with the conduction band and DA transitions are replaced by electron-acceptor transitions involving the same acceptor. The energy structure of recombination centers was obtained using the model of configuration coordinates for Al0.67Ga0.33N.

A new approach to high-efficiency multi-band-gap solar cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Barnham, K.W.J.; Duggan, G.

1990-04-01

The advantages of using multi-quantum-well or superlattice systems as the absorbers in concentrator solar cells are discussed. By adjusting the quantum-well width, an effective band-gap variation that covers the high-efficiency region of the solar spectrum can be obtained. Higher efficiencies should result from the ability to optimize separately current and voltage generating factors. Suitable structures to ensure good carrier separation and collection and to obtain higher open-circuit voltages are presented using the (AlGa)As/GaAs/(InGa)As system. Efficiencies above existing single-band-gap limits should be achievable, with upper limits in excess of 40%.

High Sensitivity Long-Wavelength Infrared QWIP Focal Plane Array Based Instrument for Remote Sensing of Icy Satellites

NASA Technical Reports Server (NTRS)

Gunapala, S.; Bandara, S.; Ivanov, A.

2003-01-01

GaAs based Quantum Well Infrared Photodetector (QWIP) technology has shown remarkable success in advancing low cost, highly uniform, high-operability, large format multi-color focal plane arrays. QWIPs afford greater flexibility than the usual extrinsically doped semiconductor IR detectors. The wavelength of the peak response and cutoff can be continuously tailored over a range wide enough to enable light detection at any wavelength range between 6 and 20 micron. The spectral band-width of these detectors can be tuned from narrow (Deltalambda/lambda is approximately 10%) to wide (Deltalambda/lambda is approximately 40%) allowing various applications. Furthermore, QWIPs offer low cost per pixel and highly uniform large format focal plane arrays due to mature GaAs/AlGaAs growth and processing technologies. The other advantages of GaAs/AlGaAs based QWIPS are higher yield, lower l/f noise and radiation hardness (1.5 Mrad). In this presentation, we will discuss our recent demonstrations of 640x512 pixel narrow-band, broad-band, multi-band focal plane arrays, and the current status of the development of 1024x1024 pixel long-wavelength infrared QWIP focal plane arrays.

Polarization-induced interfacial coupling modulations in BaTiO3/GaN heterojunction devices

NASA Astrophysics Data System (ADS)

Bhat, Thirumaleshwara N.; Pandey, B. K.; Krupanidhi, S. B.

2017-07-01

We report on the ferroelectric polarization-induced switchable interfacial coupling modulations in BaTiO3/GaN heterojunction transport behaviour. The ferroelectric barium titanate, BaTiO3 (BTO) was integrated with polar semiconductor gallium nitride (GaN). BTO with a tetragonal structure was deposited on a wurtzite (0 0 0 1) epitaxial GaN/c-Al2O3 substrate by pulsed laser deposition, which was further confirmed by x-ray diffraction and Raman spectroscopy. BTO/GaN heterojunctions with resistive switching behaviour exhibited modulations in transport characteristics due to the interfacial coupling. The ferroelectric nature and interfacial coupling effect of this heterojunction was confirmed with the help of piezo-response force microscopy. A valence band offset of 0.82 eV and conduction band offset of 0.62 eV were obtained for BTO/GaN heterojunctions by x-ray photo-electron spectroscopy. This interfacial coupling phenomenon was analysed and its effect on the carrier conduction in the heterojunction was investigated by band alignment studies.

Tunable terahertz reflection spectrum based on band gaps of GaP materials excited by ultrasonic

NASA Astrophysics Data System (ADS)

Cui, H.; Zhang, X. B.; Wang, X. F.; Wang, G. Q.

2018-02-01

Tunable terahertz (THz) reflection spectrum, ranged from 0.2 to 8 THz, in band gaps of gallium phosphide (GaP) materials excited by ultrasonic is investigated in the present paper, in which tunable ultrasonic and terahertz wave collinear transmission in the same direction is postulated. Numerical simulation results show that, under the acousto-optic interaction, band gaps of transverse optical phonon polariton dispersion curves are turned on, this leads to a dis-propagation of polariton in GaP bulk. On the other side, GaP material has less absorption to THz wave according to experimental studies, as indicates that THz wave could be reflected by the band gaps spontaneously. The band gaps width and acousto-optic coupling strength are proportional with ultrasonic frequency and its intensity in ultrasonic frequency range of 0-250 MHz, in which low-frequency branch of transverse optical phonon polariton dispersion curves demonstrate periodicity and folding as well as. With the increase of ultrasonic frequency, frequency of band gap is blue-shifted, and total reflectivity decreased with -1-order and -2-order reflectivity decrease. The band gaps converge to the restrahlen band infinitely with frequency of ultrasonic exceeding over 250 MHz, total reflectivity of which is attenuated. As is show above, reflection of THz wave can be accommodated by regulating the frequency and its intensity of ultrasonic frequency. Relevant technology may be available in tunable THz frequency selection and filtering.

Self-consistent vertical transport calculations in AlxGa1-xN/GaN based resonant tunneling diode

NASA Astrophysics Data System (ADS)

Rached, A.; Bhouri, A.; Sakr, S.; Lazzari, J.-L.; Belmabrouk, H.

2016-03-01

The formation of two-dimensional electron gases (2DEGs) at AlxGa1-xN/GaN hexagonal double-barriers (DB) resonant tunneling diodes (RTD) is investigated by numerical self-consistent (SC) solutions of the coupled Schrödinger and Poisson equations. Spontaneous and piezoelectric effects across the material interfaces are rigorously taken into account. Conduction band profiles, band edges and corresponding envelope functions are calculated in the AlxGa1-xN/GaN structures and likened to those where no polarization effects are included. The combined effect of the polarization-induced bound charge and conduction band offsets between the hexagonal AlGaN and GaN results in the formation of 2DEGs on one side of the DB and a depletion region on the other side. Using the transfer matrix formalism, the vertical transport (J-V characteristics) in AlGaN/GaN RTDs is calculated with a fully SC calculation in the ballistic regime. Compared to standard calculations where the voltage drop along the structure is supposed to be linear, the SC method leads to strong quantitative changes in the J-V characteristics showing that the applied electric field varies significantly in the active region of the structure. The influences of the aluminum composition and the GaN(AlGaN) thickness layers on the evolution of the current characteristics are also self-consistently investigated and discussed. We show that the electrical characteristics are very sensitive to the potential barrier due to the interplay between the potential symmetry and the barrier height and width. More interestingly, we demonstrate that the figures of merit namely the peak-to-valley ratio (PVR) of GaN/AlGaN RTDs can be optimized by increasing the quantum well width.

Emission properties of Ga2O3 nano-flakes: effect of excitation density.

PubMed

Pozina, G; Forsberg, M; Kaliteevski, M A; Hemmingsson, C

2017-02-08

In the quest of developing high performance electronic and optical devices and more cost effective fabrication processes of monoclinic β-Ga 2 O 3 , new growth techniques and fundamental electronic and optical properties of defects have to be explored. By heating of dissolved metallic Ga in HCl in a NH 3 and N 2 atmosphere, nano-flake films of monoclinic β-phase Ga 2 O 3 were grown as confirmed by XRD. From optical measurements, we observe two strong emissions. A red band peaking at ~2.0 eV and a UV band at ~3.8 eV. The band at ~2.0 eV is attributed to donor-acceptor pair recombination where the donor and acceptor level is suggested to be related to V O and nitrogen, respectively. By studying the dependence of the intensity of the UV band at 3.8 eV versus excitation density, a model is suggested. In the model, it is assumed that local potential fluctuations forming minima (maxima), where the carriers would be localized with a summarized band offset for conduction and valence band of 1 eV. The origin of the fluctuations is tentatively suggested to be related to micro-inclusions of different phases in the film.

Emission properties of Ga2O3 nano-flakes: effect of excitation density

PubMed Central

Pozina, G.; Forsberg, M.; Kaliteevski, M. A.; Hemmingsson, C.

2017-01-01

In the quest of developing high performance electronic and optical devices and more cost effective fabrication processes of monoclinic β-Ga2O3, new growth techniques and fundamental electronic and optical properties of defects have to be explored. By heating of dissolved metallic Ga in HCl in a NH3 and N2 atmosphere, nano-flake films of monoclinic β-phase Ga2O3 were grown as confirmed by XRD. From optical measurements, we observe two strong emissions. A red band peaking at ~2.0 eV and a UV band at ~3.8 eV. The band at ~2.0 eV is attributed to donor-acceptor pair recombination where the donor and acceptor level is suggested to be related to VO and nitrogen, respectively. By studying the dependence of the intensity of the UV band at 3.8 eV versus excitation density, a model is suggested. In the model, it is assumed that local potential fluctuations forming minima (maxima), where the carriers would be localized with a summarized band offset for conduction and valence band of 1 eV. The origin of the fluctuations is tentatively suggested to be related to micro-inclusions of different phases in the film. PMID:28176841

Structural changes during annealing of GaInAsN

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kurtz, Sarah; Webb, J.; Gedvilas, L.

2001-02-05

The alloy GaInAsN has great potential as a lower-band-gap material lattice matched to GaAs, but there is little understanding of what causes its poor optoelectronic properties and why these improve with annealing. This study provides information about the structural changes that occur when GaInAsN is annealed. The Fourier transform infrared spectra exhibit two primary features: a triplet at {approx}470 cm-1 (Ga--N stretch) and two or three bands at {approx}3100 cm-1 (N--H stretch). The change in the Ga--N stretch absorption can be explained if the nitrogen environment is converted from NGa{sub 4} to NInGa{sub 3} after annealing. The N--H stretch ismore » also changed after annealing, implying a second, and unrelated, structural change.« less

Efficient Band-to-Trap Tunneling Model Including Heterojunction Band Offset

DOE PAGES

Gao, Xujiao; Huang, Andy; Kerr, Bert

2017-10-25

In this paper, we present an efficient band-to-trap tunneling model based on the Schenk approach, in which an analytic density-of-states (DOS) model is developed based on the open boundary scattering method. The new model explicitly includes the effect of heterojunction band offset, in addition to the well-known field effect. Its analytic form enables straightforward implementation into TCAD device simulators. It is applicable to all one-dimensional potentials, which can be approximated to a good degree such that the approximated potentials lead to piecewise analytic wave functions with open boundary conditions. The model allows for simulating both the electric-field-enhanced and band-offset-enhanced carriermore » recombination due to the band-to-trap tunneling near the heterojunction in a heterojunction bipolar transistor (HBT). Simulation results of an InGaP/GaAs/GaAs NPN HBT show that the proposed model predicts significantly increased base currents, due to the hole-to-trap tunneling enhanced by the emitter-base junction band offset. Finally, the results compare favorably with experimental observation.« less

Evidence of type-II band alignment in III-nitride semiconductors: experimental and theoretical investigation for In 0.17 Al 0.83 N/GaN heterostructures.

PubMed

Wang, Jiaming; Xu, Fujun; Zhang, Xia; An, Wei; Li, Xin-Zheng; Song, Jie; Ge, Weikun; Tian, Guangshan; Lu, Jing; Wang, Xinqiang; Tang, Ning; Yang, Zhijian; Li, Wei; Wang, Weiying; Jin, Peng; Chen, Yonghai; Shen, Bo

2014-10-06

Type-II band alignment structure is coveted in the design of photovoltaic devices and detectors, since it is beneficial for the transport of photogenerated carriers. Regrettably, for group-III-nitride wide bandgap semiconductors, all existing devices are limited to type-I heterostructures, owing to the unavailable of type-II ones. This seriously restricts the designing flexibility for optoelectronic devices and consequently the relevant performance of this material system. Here we show a brandnew type-II band alignment of the lattice-matched In 0.17 Al 0.83 N/GaN heterostructure from the perspective of both experimental observations and first-principle theoretical calculations. The band discontinuity is dominated by the conduction band offset ΔEC, with a small contribution from the valence band offset ΔEV which equals 0.1 eV (with E(AlInN(VBM) being above E(GaN)(VBM)). Our work may open up new prospects to realize high-performance III-Nitrides optoelectronic devices based on type-II energy band engineering.

Efficient Band-to-Trap Tunneling Model Including Heterojunction Band Offset

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gao, Xujiao; Huang, Andy; Kerr, Bert

In this paper, we present an efficient band-to-trap tunneling model based on the Schenk approach, in which an analytic density-of-states (DOS) model is developed based on the open boundary scattering method. The new model explicitly includes the effect of heterojunction band offset, in addition to the well-known field effect. Its analytic form enables straightforward implementation into TCAD device simulators. It is applicable to all one-dimensional potentials, which can be approximated to a good degree such that the approximated potentials lead to piecewise analytic wave functions with open boundary conditions. The model allows for simulating both the electric-field-enhanced and band-offset-enhanced carriermore » recombination due to the band-to-trap tunneling near the heterojunction in a heterojunction bipolar transistor (HBT). Simulation results of an InGaP/GaAs/GaAs NPN HBT show that the proposed model predicts significantly increased base currents, due to the hole-to-trap tunneling enhanced by the emitter-base junction band offset. Finally, the results compare favorably with experimental observation.« less

Modified InGaN/GaN quantum wells with dual-wavelength green-yellow emission

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fang, Z. L., E-mail: zhilaifang@hotmail.com; Li, Q. F.; Shen, X. Y.

2014-01-28

Energy band engineering by indium pretreatment of the bottom GaN barriers and control of the growth temperature profile for the InGaN active layers were employed to improve the green-yellow emitting InGaN/GaN quantum well (QW). The modified InGaN/GaN QWs were investigated by various characterization techniques and demonstrated to be of good interface abruptness and well-defined indium concentration profile, composed of 0.52 nm In{sub 0.35}Ga{sub 0.65}N “wetting layer,” 1.56 nm In{sub 0.35-0.22}Ga{sub 0.65-0.78}N graded layers, and 1.56 nm In{sub 0.22}Ga{sub 0.78}N layer along the growth direction. Broad-band dual-wavelength green-yellow emission at about 497 and 568 nm was observed and attributed to the major contribution of enhancedmore » interband transitions from the first and second quantized electron states “e1” and “e2” to the first quantized hole state “h1.” With the modified QW structure, electron overflow loss would be suppressed by filling of the excited electron state with electrons at high carrier injection density and reduction in polarization-induced band bending. APSYS simulation shows efficiency and droop improvements due to the enhanced overlapping of electron and hole wave functions inside the modified InGaN active layers, and the enhanced interband transitions involving the excited electron state.« less

Application of ASTER and Landsat 8 imagery data and mathematical evaluation method in detecting iron minerals contamination in the Chadormalu iron mine area, central Iran

NASA Astrophysics Data System (ADS)

Moghtaderi, Arsia; Moore, Farid; Ranjbar, Hojjatollah

2017-01-01

Satellite images are widely used to map geological and environmental features at different map scales. The ability of visible to near-infrared (VNIR) scanner systems to map gossans, rich in iron and associated with weathered sulfide occurrences, as well as to characterize regoliths, is perhaps one of the most important current applications of this technology. Initial results of this study show that advanced space-borne thermal emission and reflection (ASTER), VNIR, and short-wave infrared radiometer scanner systems can be used successfully to map iron ores. By applying internal average relative reflectance, false color composite, minimum noise fraction transform, and mathematical evaluation method (MEM) techniques, iron contaminations were successfully detected in the Chadormalu iron mine area of central Iran. An attempt was also made to discriminate between the geogenic and anthropogenic iron contaminations in the vicinity of the Chadormalu iron deposit. This research compares ASTER and Landsat 8 data images and the MEM with the band ratio method in a full scope view scale and demonstrates ASTER image data capability in detecting iron contaminations in the Chadormalu area. This indicates that ASTER bands 3, 2, and 1 have a higher spatial (15 m) resolution compared with sensors used in previous works. In addition, the capability of the MEM in detecting Fe-contaminants, unlike the color judgments of the band ratio method, can discriminate between iron pollution in an alluvial plain and the Fe-contents of the host and country rocks in the study area. This study proved that Landsat 8 data illustrate exaggeration both in the MEM and band ratio final results (outputs) and cannot display iron contamination in detail.

Third Working Meeting on Gallium Arsenide Solar Cells

NASA Technical Reports Server (NTRS)

Walker, G. H. (Compiler)

1976-01-01

Research results are reported for GaAs Schottky barrier solar cells, GaAlAs/GaAs heteroface solar cells, and GaAlAs graded band gap solar cells. Related materials studies are presented. A systems study for GaAs and Si solar concentrator systems is given.

Ab initio density functional theory study on the atomic and electronic structure of GaP/Si(001) heterointerfaces

NASA Astrophysics Data System (ADS)

Romanyuk, O.; Supplie, O.; Susi, T.; May, M. M.; Hannappel, T.

2016-10-01

The atomic and electronic band structures of GaP/Si(001) heterointerfaces were investigated by ab initio density functional theory calculations. Relative total energies of abrupt interfaces and mixed interfaces with Si substitutional sites within a few GaP layers were derived. It was found that Si diffusion into GaP layers above the first interface layer is energetically unfavorable. An interface with Si/Ga substitution sites in the first layer above the Si substrate is energetically the most stable one in thermodynamic equilibrium. The electronic band structure of the epitaxial GaP/Si(001) heterostructure terminated by the (2 ×2 ) surface reconstruction consists of surface and interface electronic states in the common band gap of two semiconductors. The dispersion of the states is anisotropic and differs for the abrupt Si-Ga, Si-P, and mixed interfaces. Ga 2 p , P 2 p , and Si 2 p core-level binding-energy shifts were computed for the abrupt and the lowest-energy heterointerface structures. Negative and positive core-level shifts due to heterovalent bonds at the interface are predicted for the abrupt Si-Ga and Si-P interfaces, respectively. The distinct features in the heterointerface electronic structure and in the core-level shifts open new perspectives in the experimental characterization of buried polar-on-nonpolar semiconductor heterointerfaces.

Sonochemical Assisted Solvothermal Synthesis of Gallium Oxynitride Nanosheets and their Solar-Driven Photoelectrochemical Water-Splitting Applications.

PubMed

Iqbal, Naseer; Khan, Ibrahim; Yamani, Zain H; Qurashi, Ahsanullhaq

2016-08-26

Gallium oxynitride (GaON) nanosheets for photoelectrochemical (PEC) analysis are synthesized via direct solvothermal approach. Their FE-SEM revealed nanosheets morphology of GaON prepared at a reaction time of 24 hours at 180 °C. The elemental composition and mapping of Ga, O and N are carried out through electron dispersive X-ray spectroscopy (EDX). The cubic structure of GaON nanosheets is elucidated by X-ray diffraction (XRD)analysis. The X-ray Photoelectron Spectroscopy (XPS) further confirms Ga, O and N in their respective ratios and states. The optical properties of GaON nanosheets are evaluated via UV-Visible, Photoluminescence (PL) and Raman spectroscopy's. The band gap energy of ~1.9 eV is calculated from both absorption and diffused reflectance spectroscopy's which showed stronger p-d repulsions in the Ga (3d) and N (2p) orbitals. This effect and chemical nitridation caused upward shift of valence band and band gap reduction. The GaON nanosheets are investigated for PEC studies in a standard three electrode system under 1 Sun irradiation in 0.5 M Na2SO4. The photocurrent generation, oxidation and reduction reactions during the measurements are observed by Chronoampereometry, linear sweep Voltametry (LSV) and Cyclic Voltametry (CV) respectively. Henceforward, these GaON nanosheets can be used as potential photocatalyts for solar water splitting.

Sonochemical Assisted Solvothermal Synthesis of Gallium Oxynitride Nanosheets and their Solar-Driven Photoelectrochemical Water-Splitting Applications

PubMed Central

Iqbal, Naseer; Khan, Ibrahim; Yamani, Zain H.; Qurashi, Ahsanullhaq

2016-01-01

Gallium oxynitride (GaON) nanosheets for photoelectrochemical (PEC) analysis are synthesized via direct solvothermal approach. Their FE-SEM revealed nanosheets morphology of GaON prepared at a reaction time of 24 hours at 180 °C. The elemental composition and mapping of Ga, O and N are carried out through electron dispersive X-ray spectroscopy (EDX). The cubic structure of GaON nanosheets is elucidated by X-ray diffraction (XRD)analysis. The X-ray Photoelectron Spectroscopy (XPS) further confirms Ga, O and N in their respective ratios and states. The optical properties of GaON nanosheets are evaluated via UV-Visible, Photoluminescence (PL) and Raman spectroscopy’s. The band gap energy of ~1.9 eV is calculated from both absorption and diffused reflectance spectroscopy’s which showed stronger p-d repulsions in the Ga (3d) and N (2p) orbitals. This effect and chemical nitridation caused upward shift of valence band and band gap reduction. The GaON nanosheets are investigated for PEC studies in a standard three electrode system under 1 Sun irradiation in 0.5 M Na2SO4. The photocurrent generation, oxidation and reduction reactions during the measurements are observed by Chronoampereometry, linear sweep Voltametry (LSV) and Cyclic Voltametry (CV) respectively. Henceforward, these GaON nanosheets can be used as potential photocatalyts for solar water splitting. PMID:27561646

Two-way feedback between biology and deep Earth processes

NASA Astrophysics Data System (ADS)

Sleep, N. H.; Pope, E.; Bird, D.

2012-12-01

The presence of the metamorphic products of banded iron formation and black shale indicate that the Earth teemed with life by the time of the earliest preserved rocks, ca. 3.85 Ga. Iron and sulfur-based anoxygenic photosynthesis with full carbon cycles was present by this time. The pH of the ocean was ~8. The lack of older rock record cloaks pre-biotic evolution and the origin of life. Nascent and early life obtained energy from chemical disequilibria in rocks rather than sunlight. Appraising putative rock pre-biological environments is difficult in that life has modified the composition of the atmosphere, the hydrosphere, and sedimentary rocks. It has greatly affected the composition of crystalline crustal rocks and measurably modified the mantle. Conversely, hard crustal rocks and the mantle likely sequester a very ancient record of last resort. Theory provides additional insight. The Earth's surface and interior cooled following the moon-forming impact. The oceans passed through conditions favored by thermophile organisms before becoming clement. Ocean pH was ~6 and bars of CO2 existed in the atmosphere. Subduction removed the CO2 into the mantle before the time of rock record. Serpentinite likely existed in land, tidal, and marine environments as it does today. Seafloor spreading and arc volcanism likely drove hydrothermal circulation. The late heavy bombardment occurred after ca. 4.1 Ga; low heat flow environments and hence habitable subsurface refugia existed. It is conceivable that one or a few ocean-boiling impacts left thermophile survivors in their wake. Overall, the molecular biology of extant life likely conserves features that relate to its earliest abodes.

The Electronic Structure of the Cs/ n-GaN(0001) Nano-Interface

NASA Astrophysics Data System (ADS)

Benemanskaya, G. V.; Lapushkin, M. N.; Marchenko, D. E.; Timoshnev, S. N.

2018-03-01

Electronic structures of the n-GaN(0001) surface and Cs/ n-GaN(0001) interface with submonolayer Cs coverages were studied for the first time in situ by the photoelectron spectroscopy (PES) method. The spectra of photoemission from the valence band, surface electron states, and core levels (Ga 3 d, Cs 4 d, Cs 5 p) under synchrotron excitation were measured in a range of photon energies within 50-150 eV. Evolution of the spectrum of surface states near the valence-band maximum was revealed by PES during the adsorption of Cs atoms. A metallic character of the Cs/ n-GaN(0001) nano-interface is demonstrated.

Opening the band gap of graphene through silicon doping for the improved performance of graphene/GaAs heterojunction solar cells

NASA Astrophysics Data System (ADS)

Zhang, S. J.; Lin, S. S.; Li, X. Q.; Liu, X. Y.; Wu, H. A.; Xu, W. L.; Wang, P.; Wu, Z. Q.; Zhong, H. K.; Xu, Z. J.

2015-12-01

Graphene has attracted increasing interest due to its remarkable properties. However, the zero band gap of monolayered graphene limits it's further electronic and optoelectronic applications. Herein, we have synthesized monolayered silicon-doped graphene (SiG) with large surface area using a chemical vapor deposition method. Raman and X-ray photoelectron spectroscopy measurements demonstrate that the silicon atoms are doped into graphene lattice at a doping level of 2.7-4.5 at%. Electrical measurements based on a field effect transistor indicate that the band gap of graphene has been opened via silicon doping without a clear degradation in carrier mobility, and the work function of SiG, deduced from ultraviolet photoelectron spectroscopy, was 0.13-0.25 eV larger than that of graphene. Moreover, when compared with the graphene/GaAs heterostructure, SiG/GaAs exhibits an enhanced performance. The performance of 3.4% silicon doped SiG/GaAs solar cell has been improved by 33.7% on average, which was attributed to the increased barrier height and improved interface quality. Our results suggest that silicon doping can effectively engineer the band gap of monolayered graphene and SiG has great potential in optoelectronic device applications.Graphene has attracted increasing interest due to its remarkable properties. However, the zero band gap of monolayered graphene limits it's further electronic and optoelectronic applications. Herein, we have synthesized monolayered silicon-doped graphene (SiG) with large surface area using a chemical vapor deposition method. Raman and X-ray photoelectron spectroscopy measurements demonstrate that the silicon atoms are doped into graphene lattice at a doping level of 2.7-4.5 at%. Electrical measurements based on a field effect transistor indicate that the band gap of graphene has been opened via silicon doping without a clear degradation in carrier mobility, and the work function of SiG, deduced from ultraviolet photoelectron spectroscopy, was 0.13-0.25 eV larger than that of graphene. Moreover, when compared with the graphene/GaAs heterostructure, SiG/GaAs exhibits an enhanced performance. The performance of 3.4% silicon doped SiG/GaAs solar cell has been improved by 33.7% on average, which was attributed to the increased barrier height and improved interface quality. Our results suggest that silicon doping can effectively engineer the band gap of monolayered graphene and SiG has great potential in optoelectronic device applications. Electronic supplementary information (ESI) available: Synthetic process of the SiG sheet; UPS spectra of SiG and graphene; J-V curves for the SiG/GaAs and graphene/GaAs solar cells under dark conditions and AM1.5 illumination at 100 mW cm-2, respectively; Statistic PCE of SiG/GaAs solar cells with different Si doping levels; EQE of SiG/GaAs and graphene/GaAs solar cells; a comparison of the parameters between the SiG and graphene/GaAs solar cells. See DOI: 10.1039/c5nr06345k

Structural and electronic properties of GaAs and GaP semiconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rani, Anita; Kumar, Ranjan

2015-05-15

The Structural and Electronic properties of Zinc Blende phase of GaAs and GaP compounds are studied using self consistent SIESTA-code, pseudopotentials and Density Functional Theory (DFT) in Local Density Approximation (LDA). The Lattice Constant, Equillibrium Volume, Cohesive Energy per pair, Compressibility and Band Gap are calculated. The band gaps calcultated with DFT using LDA is smaller than the experimental values. The P-V data fitted to third order Birch Murnaghan equation of state provide the Bulk Modulus and its pressure derivatives. Our Structural and Electronic properties estimations are in agreement with available experimental and theoretical data.

Defect interactions in GaAs single crystals

NASA Technical Reports Server (NTRS)

Gatos, H. C.; Lagowski, J.

1984-01-01

The two-sublattice structural configuration of GaAs and deviations from stoichiometry render the generation and interaction of electrically active point defects (and point defect complexes) critically important for device applications and very complex. Of the defect-induced energy levels, those lying deep into the energy band are very effective lifetime ""killers". The level 0.82 eV below the condition band, commonly referred to as EL2, is a major deep level, particularly in melt-grown GaAs. This level is associated with an antisite defect complex (AsGa - VAS). Possible mechanisms of its formation and its annihilation were further developed.

Bound-to-bound midinfrared intersubband absorption in carbon-doped GaAs /AlGaAs quantum wells

NASA Astrophysics Data System (ADS)

Malis, Oana; Pfeiffer, Loren N.; West, Kenneth W.; Sergent, A. Michael; Gmachl, Claire

2005-08-01

Bound-to-bound intersubband absorption in the valence band of modulation-doped GaAs quantum wells with digitally alloyed AlGaAs barriers was studied in the midinfrared wavelength range. A high-purity solid carbon source was used for the p-type doping. Strong narrow absorption peaks due to heavy-to-heavy hole transitions are observed with out-of-plane polarized light, and weaker broader features with in-plane polarized light. The heavy-to-heavy hole transition energy spans the spectral range between 206 to 126 meV as the quantum well width is increased from 25 to 45 Å. The experimental results are found to be in agreement with calculations of a six-band k •p model taking into account the full band structure of the digital alloy.

0.15 {mu}m InGaAs/AlGaAs/GaAs HEMT production process for high performance and high yield v-band power MMICs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lai, R.; Biedenbender, M.; Lee, J.

1995-12-31

The authors present a unique high yield, high performance 0.15 {mu}m HEMT production process which supports fabrication of MMW power MMICs up to 70 GHz. This process has been transferred successfully from an R&D process to TRW`s GaAs production line. This paper reports the on-wafer test results of more than 1300 V-band MMIC PA circuits measured over 24 wafers. The best 2-stage V-band power MMICs have demonstrated state-of-the-art performance with 9 dB power gain, 20% PAE and 330 mW output power. An excellent RF yield of 60% was achieved with an 8 dB power gain and 250 mW output powermore » specification.« less

Electronic structure and thermoelectric performance of Zintl compound Sr{sub 3}GaSb{sub 3}: A first-principles study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Feng Shi, Qing; Li Yan, Yu; Xu Wang, Yuan, E-mail: wangyx@henu.edu.cn

2014-01-06

By using first-principles method and Boltzmann theory, we simulated the thermoelectric transport properties of p-type and n-type Sr{sub 3}GaSb{sub 3}. It is found that the thermoelectric figure-of merit (ZT) of n-type Sr{sub 3}GaSb{sub 3} is probably better than that of p-type, mainly due to its large band degeneracy. Moreover, a high ZT value of 1.74 at 850 K can be achieved for n-type Sr{sub 3}GaSb{sub 3} along the yy direction, corresponding to the carrier concentration 3.5 × 10{sup 20} e cm{sup −3}. We propose that the high ZT value of experimentally synthesized p-type Sr{sub 3}GaSb{sub 3} is originated from appearing of the larger numbermore » of band valley on the top of valence bands.« less

Direct evidence of recombination between electrons in InGaN quantum discs and holes in p-type GaN.

PubMed

Sun, Xiaoxiao; Wang, Xinqiang; Wang, Ping; Wang, Tao; Sheng, Bowen; Zheng, Xiantong; Li, Mo; Zhang, Jian; Yang, Xuelin; Xu, Fujun; Ge, Weikun; Shen, Bo

2017-11-27

Intense emission from an InGaN quantum disc (QDisc) embedded in a GaN nanowire p-n junction is directly resolved by performing cathodoluminescence spectroscopy. The luminescence observed from the p-type GaN region is exclusively dominated by the emission at 380 nm, which has been usually reported as the emission from Mg induced impurity bands. Here, we confirm that the robust emission from 380 nm is actually not due to the Mg induced impurity bands, but rather due to being the recombination between electrons in the QDisc and holes in the p-type GaN. This identification helps to get a better understanding of the confused luminescence from nanowires with thin QDiscs embedded for fabricating electrically driven single photon emitters.

GaAsP on GaP top solar cells

NASA Technical Reports Server (NTRS)

Mcneely, J. B.; Negley, G. H.; Barnett, A. M.

1985-01-01

GaAsP on GaP top solar cells as an attachment to silicon bottom solar cells are being developed. The GaAsP on GaP system offers several advantages for this top solar cell. The most important is that the gallium phosphide substrate provides a rugged, transparent mechanical substrate which does not have to be removed or thinned during processing. Additional advantages are that: (1) gallium phosphide is more oxidation resistant than the III-V aluminum compounds, (2) a range of energy band gaps higher than 1.75 eV is readily available for system efficiency optimization, (3) reliable ohmic contact technology is available from the light-emitting diode industry, and (4) the system readily lends itself to graded band gap structures for additional increases in efficiency.

Selective principal component regression analysis of fluorescence hyperspectral image to assess aflatoxin contamination in corn

USDA-ARS?s Scientific Manuscript database

Selective principal component regression analysis (SPCR) uses a subset of the original image bands for principal component transformation and regression. For optimal band selection before the transformation, this paper used genetic algorithms (GA). In this case, the GA process used the regression co...

Thermal photovoltaic cells

NASA Astrophysics Data System (ADS)

Sharps, P. R.; Timmons, M. L.; Venkatasubramanian, R.; Hills, J. S.; O'Quinn, B.; Hutchby, J. A.; Iles, P. A.; Chu, C. L.

1995-01-01

Most current emphasis is on GaInAs alloys or GaSb for thermal photovoltaic converters operating in a band gap range between about 0.50 to 0.75 eV. In this paper the growth and fabrication of GaInAs devices with nominal band gaps of 0.6 eV are described. Yield statistics are presented for the growth of a large number of devices, and I-V data are presented. Alternative cell structures are also described, and manufacturing issues are discussed.

Strain and electric field induced metallization in the GaX (X = N, P, As & Sb) monolayer

NASA Astrophysics Data System (ADS)

Bahuguna, Bhagwati Prasad; Saini, L. K.; Sharma, Rajesh O.; Tiwari, Brajesh

2018-05-01

We investigate the strain and electric field dependent electronic properties of two dimensional Ga-based group III-V monolayer from the first-principles approach within density functional theory. The energy bandgap of GaX monolayer increases upto the certain value of compressive strain and then decreases. On the other hand, the energy bandgap of GaX monolayer is monotonically decreased with increasing tensile strain and become metallic at the higher value. Furthermore, the perpendicular electric field decreases the energy band gap of unstrained GaX monolayer and shows semiconductor to metal transition. These results suggest that the nature of energy bands and value of energy bandgap in GaX monolayer can be tuned by the biaxial mechanical strain or perpendicular electrical field. Additionally, we have also studied the optical response of unstrained GaX monolayer in term of optical conductivity. These findings may provide valuable information to develop the Ga-based optoelectronic devices and further the understanding of the GaX monolayer.

Mesoarchean black shale -iron sedimentary sequences in Cleaverville Formation, Pilbara Australia: drilling preliminary result of DXCL2

NASA Astrophysics Data System (ADS)

Kiyokawa, S.; Ito, T.; Ikehara, M.; Yamaguchi, K. E.; Onoue, T.; Horie, K.; Sakamoto, R.; Teraji, S.; Aihara, Y.

2012-12-01

The 3.2-3.1 Ga Dixon island-Cleaverville formations are well-preserved hydrothermal oceanic sequence at oceanic island arc setting (Kiyokawa et al., 2002, 2006, 2012). The Dixon Island (3195+15 Ma) - Cleaverville (3108+13 Ma) formations formed volcano-sedimentary sequences with hydrothermal chert, black shale and banded iron formation to the top. Based on the scientific drilling as DXCL1 at 2007 and DXCL2 at 2011, lithology was clearly understood. Four drilling holes had been done at coastal sites; the Dixon Island Formation is DX site (100m) and the Cleaverville Formation is CL2 (40m), CL1 (60m) and CL3 (200m) sites and from stratigraphic bottom to top. These sequences formed coarsening and thickening upward black shale-BIF sequences. The Dixon Island Formation consists komatiite-rhyolite sequences with many hydrothermal veins and very fine laminated cherty rocks above them. The Cleaverville Formation contains black shale, fragments-bearing pyroclastic beds, white chert, greenish shale and BIF. Especially, CL3 core, which drilled through the Iron formation, shows siderite-chert beds above black shale identified before magnetite lamination bed. The magnetite bed formed very thin laminated bed with siderite lamination. This magnetite bed was covered by black shale beds again. New U-Pb SHRIMP data of the pyroclastic in black shale is 3109Ma. Estimated 2-8 cm/1000year sedimentation rate are identified in these sequences. Our preliminary result show that siderite and chert layers formed before magnetite iron sedimentation. The lower-upper sequence of organic carbon rich black shales are similar amount of organic content and 13C isotope (around -30per mill). So we investigate that the Archean iron formation, especially Cleaverville iron formation, was highly related by hydrothermal input and started pre-syn iron sedimentation at anoxic oceanic condition.

Role of surface energy on the morphology and optical properties of GaP micro & nano structures grown on polar and non-polar substrates

NASA Astrophysics Data System (ADS)

Roychowdhury, R.; Kumar, Shailendra; Wadikar, A.; Mukherjee, C.; Rajiv, K.; Sharma, T. K.; Dixit, V. K.

2017-10-01

Role of surface energy on the morphology, crystalline quality, electronic structure and optical properties of GaP layer grown on Si (001), Si (111), Ge (111) and GaAs (001) is investigated. GaP layers are grown on four different substrates under identical growth kinetics by metal organic vapour phase epitaxy. The atomic force microscopy images show that GaP layer completely covers the surface of GaAs substrate. On the other hand, the surfaces of Si (001), Si (111), Ge (111) substrates are partially covered with crystallographically morphed GaP island type micro and nano-structures. Origin of these crystallographically morphed GaP island is explained by the theoretical calculation of surface energy of the layer and corresponding substrates respectively. The nature of GaP island type micro and nano-structures and layers are single crystalline with existence of rotational twins on Si and Ge (111) substrates which is confirmed by the phi, omega and omega/2theta scans of high resolution x-ray diffraction. The electronic valence band offsets between the GaP and substrates have been determined from the valence band spectra of ultraviolet photoelectron spectroscopy. The valence electron plasmon of GaP are investigated by studying the energy values of Ga (3d) core level along with loss peaks in the energy dependent photoelectron spectra. The peak observed within the range of 3-6 eV from the Ga (3d) core level in the photoelectron spectra are associated to inter band transitions as their energy values are estimated from the pseudo dielectric function by the spectroscopic ellipsometry.

Growth and characterization of metamorphic InAs/GaSb tunnel heterojunction on GaAs by molecular beam epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Jheng-Sin; Clavel, Michael B.; Hudait, Mantu K., E-mail: mantu.hudait@vt.edu

The structural, morphological, optical, and electrical transport characteristics of a metamorphic, broken-gap InAs/GaSb p-i-n tunnel diode structure, grown by molecular beam epitaxy on GaAs, were demonstrated. Precise shutter sequences were implemented for the strain-balanced InAs/GaSb active layer growth on GaAs, as corroborated by high-resolution X-ray analysis. Cross-sectional transmission electron microscopy and detailed micrograph analysis demonstrated strain relaxation primarily via the formation of 90° Lomer misfit dislocations (MDs) exhibiting a 5.6 nm spacing and intermittent 60° MDs at the GaSb/GaAs heterointerface, which was further supported by a minimal lattice tilt of 180 arc sec observed during X-ray analysis. Selective area diffraction and Fastmore » Fourier Transform patterns confirmed the full relaxation of the GaSb buffer layer and quasi-ideal, strain-balanced InAs/GaSb heteroepitaxy. Temperature-dependent photoluminescence measurements demonstrated the optical band gap of the GaSb layer. Strong optical signal at room temperature from this structure supports a high-quality material synthesis. Current–voltage characteristics of fabricated InAs/GaSb p-i-n tunnel diodes measured at 77 K and 290 K demonstrated two bias-dependent transport mechanisms. The Shockley–Read–Hall generation–recombination mechanism at low bias and band-to-band tunneling transport at high bias confirmed the p-i-n tunnel diode operation. This elucidated the importance of defect control in metamorphic InAs/GaSb tunnel diodes for the implementation of low-voltage and high-performance tunnel field effect transistor applications.« less

Luminescent N-polar (In,Ga)N/GaN quantum wells achieved by plasma-assisted molecular beam epitaxy at temperatures exceeding 700 °C

NASA Astrophysics Data System (ADS)

Chèze, C.; Feix, F.; Lähnemann, J.; Flissikowski, T.; Kryśko, M.; Wolny, P.; Turski, H.; Skierbiszewski, C.; Brandt, O.

2018-01-01

Previously, we found that N-polar (In,Ga)N/GaN quantum wells prepared on freestanding GaN substrates by plasma-assisted molecular beam epitaxy at conventional growth temperatures of about 650 °C do not exhibit any detectable luminescence even at 10 K. In the present work, we investigate (In,Ga)N/GaN quantum wells grown on Ga- and N-polar GaN substrates at a constant temperature of 730 °C . This exceptionally high temperature results in a vanishing In incorporation for the Ga-polar sample. In contrast, quantum wells with an In content of 20% and abrupt interfaces are formed on N-polar GaN. Moreover, these quantum wells exhibit a spatially uniform green luminescence band up to room temperature, but the intensity of this band is observed to strongly quench with temperature. Temperature-dependent photoluminescence transients show that this thermal quenching is related to a high density of nonradiative Shockley-Read-Hall centers with large capture coefficients for electrons and holes.

Native defects in GaN: a hybrid functional study

NASA Astrophysics Data System (ADS)

Diallo, Ibrahima Castillo; Demchenko, Denis

Intrinsic defects play an important role in the performance of GaN-based devices. We present hybrid density functional calculations of the electronic and possible optical properties of interstitial N (Ni-Ni) , N antisite (NGa) , interstitial Ga (Gai) , Ga antisite (GaN) , Ga vacancy (VGa) , N vacancy (VN) and Ga-N divacancies (VGaVN) in GaN. Our results show that the vacancies display relatively low formation energies in certain samples, whereas antisites and interstitials are energetically less favorable. However, interstitials can be created by electron irradiation. For instance, in 2.5 MeV electron-irradiated GaN samples, a strong correlation between the frequently observed photoluminescence (PL) band centered around 0.85 eV accompanied with a rich phonon sideband of ~0.88 eV and the theoretical optical behavior of interstitial Ga is discussed. N vacancies are found to likely contribute to the experimentally obtained green luminescence band (GL2) peaking at 2.24 eV in high-resistivity undoped and Mg-doped GaN. National Science Foundation (DMR-1410125) and the Thomas F. and Kate Miller Jeffress Memorial Trust.

Electronic coupling in iron oxide-modified TiO2 leads to a reduced band gap and charge separation for visible light active photocatalysis.

PubMed

Nolan, Michael

2011-10-28

In recent experiments Tada et al. have shown that TiO(2) surfaces modified with iron oxide display visible light photocatalytic activity. This paper presents first principles simulations of iron oxide clusters adsorbed at the rutile TiO(2) (110) surface to elucidate the origin of the visible light photocatalytic activity of iron oxide modified TiO(2). Small iron oxide clusters adsorb at rutile (110) surface and their presence shifts the valence band so that the band gap of the composite is narrowed towards the visible, thus confirming the origin of the visible light activity of this composite material. The presence of iron oxide at the TiO(2) surface leads to charge separation, which is the origin of enhanced photocatalytic efficiency, consistent with experimental photoluminesence and photocurrent data. Surface modification of a metal oxide is thus an interesting route in the development of visible light photocatalytic materials. This journal is © the Owner Societies 2011

Growth, strain relaxation properties and high-κ dielectric integration of mixed-anion GaAs{sub 1-y}Sb{sub y} metamorphic materials

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, Y.; Clavel, M.; Goley, P.

Mixed-anion, GaAs{sub 1-y}Sb{sub y} metamorphic materials with a wide range of antimony (Sb) compositions extending from 15% to 62%, were grown by solid source molecular beam epitaxy (MBE) on GaAs substrates. The impact of different growth parameters on the Sb composition in GaAs{sub 1-y}Sb{sub y} materials was systemically investigated. The Sb composition was well-controlled by carefully optimizing the As/Ga ratio, the Sb/Ga ratio, and the substrate temperature during the MBE growth process. High-resolution x-ray diffraction demonstrated a quasi-complete strain relaxation within each composition of GaAs{sub 1-y}Sb{sub y}. Atomic force microscopy exhibited smooth surface morphologies across the wide range of Sbmore » compositions in the GaAs{sub 1-y}Sb{sub y} structures. Selected high-κ dielectric materials, Al{sub 2}O{sub 3}, HfO{sub 2}, and Ta{sub 2}O{sub 5} were deposited using atomic layer deposition on the GaAs{sub 0.38}Sb{sub 0.62} material, and their respective band alignment properties were investigated by x-ray photoelectron spectroscopy (XPS). Detailed XPS analysis revealed a valence band offset of >2 eV for all three dielectric materials on GaAs{sub 0.38}Sb{sub 0.62}, indicating the potential of utilizing these dielectrics on GaAs{sub 0.38}Sb{sub 0.62} for p-type metal-oxide-semiconductor (MOS) applications. Moreover, both Al{sub 2}O{sub 3} and HfO{sub 2} showed a conduction band offset of >2 eV on GaAs{sub 0.38}Sb{sub 0.62}, suggesting these two dielectrics can also be used for n-type MOS applications. The well-controlled Sb composition in several GaAs{sub 1-y}Sb{sub y} material systems and the detailed band alignment analysis of multiple high-κ dielectric materials on a fixed Sb composition, GaAs{sub 0.38}Sb{sub 0.62}, provides a pathway to utilize GaAs{sub 1-y}Sb{sub y} materials in future microelectronic and optoelectronic applications.« less

Sensitivity of gap symmetry to an incipient band: Application to iron based superconductors

NASA Astrophysics Data System (ADS)

Mishra, Vivek; Scalapino, Douglas; Maier, Thomas

Observation of high temperature superconductivity in iron-based superconductors with a submerged hole band has attracted wide interest. A spin fluctuation mediated pairing mechanism has been proposed as a possible explanation for the high transition temperatures observed in these systems. Here we discuss the importance of the submerged band in the context of the gap symmetry. We show that the incipient band can lead to an attractive pairing interaction and thus have significant effects on the pairing symmetry. We propose a framework to include the effect of the incipient band in the standard multi-orbital spin-fluctuation theories which are widely used for studying various iron-based superconductors. Research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy.

Gallium(iii) and iron(iii) complexes of quinolone antimicrobials.

PubMed

Mjos, Katja Dralle; Cawthray, Jacqueline F; Polishchuk, Elena; Abrams, Michael J; Orvig, Chris

2016-08-16

Iron is an essential nutrient for many microbes. According to the "Trojan Horse Hypothesis", biological systems have difficulties distinguishing between Fe(3+) and Ga(3+), which constitutes the antimicrobial efficacy of the gallium(iii) ion. Nine novel tris(quinolono)gallium(iii) complexes and their corresponding iron(iii) analogs have been synthesized and fully characterized. Quinolone antimicrobial agents from three drug generations were used in this study: ciprofloxacin, enoxacin, fleroxacin, levofloxacin, lomefloxacin, nalidixic acid, norfloxacin, oxolinic acid, and pipemidic acid. The antimicrobial efficacy of the tris(quinolono)gallium(iii) complexes was studied against E. faecalis and S. aureus (both Gram-positive), as well as E. coli, K. pneumonia, and P. aeruginosa (all Gram-negative) in direct comparison to the tris(quinolono)iron(iii) complexes and the corresponding free quinolone ligands at various concentrations. For the tris(quinolono)gallium(iii) complexes, no combinational antimicrobial effects between Ga(3+) and the quinolone antimicrobial agents were observed.

Influence of hydropower dams on the composition of the suspended and riverbank sediments in the Danube.

PubMed

Klaver, Gerard; van Os, Bertil; Negrel, Philippe; Petelet-Giraud, Emmanuelle

2007-08-01

Large hydropower dams have major impacts on flow regime, sediment transport and the characteristics of water and sediment in downstream rivers. The Gabcikovo and Iron Gate dams divide the studied Danube transect (rkm 1895-795) into three parts. In the Gabcikovo Reservoir (length of 40km) only a part of the incoming suspended sediments were deposited. Contrary to this, in the much larger Iron Gate backwater zone and reservoir (length of 310km) all riverine suspended sediments were deposited within the reservoir. Subsequently, suspended sediments were transported by tributaries into the Iron Gate backwater zone. Here they were modified by fractional sedimentation before they transgressed downstream via the dams. Compared with undammed Danube sections, Iron Gate reservoir sediment and suspended matter showed higher clay contents and different K/Ga and Metal/Ga ratios. These findings emphasize the importance of reservoir-river sediment-fractionation.

Rare-earth gate oxides for GaAs MOSFET application

NASA Astrophysics Data System (ADS)

Kwon, Kwang-Ho; Yang, Jun-Kyu; Park, Hyung-Ho; Kim, Jongdae; Roh, Tae Moon

2006-08-01

Rare-earth oxide films for gate dielectric on n-GaAs have been investigated. The oxide films were e-beam evaporated on S-passivated GaAs, considering interfacial chemical bonding state and energy band structure. Rare-earth oxides such as Gd 2O 3, (Gd xLa 1- x) 2O 3, and Gd-silicate were employed due to high resistivity and no chemical reaction with GaAs. Structural and bonding properties were characterized by X-ray photoemission, absorption, and diffraction. The electrical characteristics of metal-oxide-semiconductor (MOS) diodes were correlated with material properties and energy band structures to guarantee the feasibility for MOS field effect transistor (FET) application. Gd 2O 3 films were grown epitaxially on S-passivated GaAs (0 0 1) at 400 °C. The passivation induced a lowering of crystallization temperature with an epitaxial relationship of Gd 2O 3 (4 4 0) and GaAs (0 0 1). A better lattice matching relation between Gd 2O 3 and GaAs substrate was accomplished by the substitution of Gd with La, which has larger ionic radius. The in-plane relationship of (Gd xLa 1- x) 2O 3 (4 4 0) with GaAs (0 0 1) was found and the epitaxial films showed an improved crystalline quality. Amorphous Gd-silicate film was synthesized by the incorporation of SiO 2 into Gd 2O 3. These amorphous Gd-silicate films excluded defect traps or current flow path due to grain boundaries and showed a relatively larger energy band gap dependent on the contents of SiO 2. Energy band parameters such as Δ EC, Δ EV, and Eg were effectively controlled by the film composition.

Soft X-ray photoemission study of Co2(Cr1-xFex)Ga Heusler compounds

NASA Astrophysics Data System (ADS)

Tsunekawa, Masanori; Hattori, Yoshiro; Sekiyama, Akira; Fujiwara, Hidenori; Suga, Shigemasa; Muro, Takayuki; Kanomata, Takeshi; Imada, Shin

2015-08-01

We have performed soft X-ray photoemission spectroscopy (SXPES) and X-ray absorption spectroscopy (XAS) of the Co-based Heusler compounds Co2(Cr1-xFex)Ga (x = 0.0, 0.4, and 1.0) in order to study their electronic structures. Band-structure calculation was carried out and compared with the experimental results. SXPES spectra show hν-dependence, revealing the contributions of the Co, Cr, and Fe 3d electronic states in the valence band. The band width observed by the SXPES seems to be narrower than that predicted by the band-structure calculation. XAS spectra depend strongly on the the value of x in Co2(Cr1-xFex)Ga. The electron correlation effects are found to be stronger as x changes from 0.0 to 1.0.

Doping effects in InN/GaN short-period quantum well structures-Theoretical studies based on density functional methods

NASA Astrophysics Data System (ADS)

Strak, Pawel; Kempisty, Pawel; Sakowski, Konrad; Krukowski, Stanislaw

2014-09-01

Density functional theory studies were conducted to determine an influence of the carrier concentration on the optical and electronic properties of InN/GaN superlattice system. The oscillator strength values, energy gaps and the band profiles were obtained. The band profiles were found to be strongly affected for technically possible heavy n-type doping while for p-type doping the carrier influence, both screening and band shift, is negligible. Blue shift of the transition energy between conduction band minima and valence band maxima was observed for high concentrations of both type carriers.

Synthesis, structural, electronic and linear electro-optical features of new quaternary Ag{sub 2}Ga{sub 2}SiS{sub 6} compound

DOE Office of Scientific and Technical Information (OSTI.GOV)

Piasecki, M., E-mail: m.piasecki@ajd.czest.pl; Myronchuk, G.L.; Parasyuk, O.V.

For the first time phase equilibria and phase diagram of the AgGaS{sub 2}–SiS{sub 2} system were successfully explored by differential thermal and X-ray phase analysis methods. Crystal structure of low-temperature (LT) modification of Ag{sub 2}Ga{sub 2}SiS{sub 6} (LРў- Ag{sub 2}Ga{sub 2}SiS{sub 6}) was studied by X-ray powder method and it belongs to tetragonal space group I-42d, with unit cell parameters a=5.7164(4) Å, c=9.8023(7) Å, V=320.32(7) Å{sup 3}. Additional details regarding the crystal structure exploration are available at the web page Fachinformationszentrum Karlsruhe. X-ray photoelectron core-level and valence-band spectra were measured for pristine LРў- Ag{sub 2}Ga{sub 2}SiS{sub 6} crystal surface. Inmore » addition, the X-ray photoelectron valence-band spectrum of LРў-Ag{sub 2}Ga{sub 2}SiS{sub 6} was matched on a common energy scale with the X-ray emission S Kβ{sub 1,3} and Ga Kβ{sub 2} bands, which give information on the energy distribution of the S 3p and Ga 4p states, respectively. The presented X-ray spectroscopy results indicate that the valence S p and Ga p atomic states contribute mainly to the upper and central parts of the valence band of LРў-Ag{sub 2}Ga{sub 2}SiS{sub 6}, respectively, with a less significant contribution also to other valence-band regions. Band gap energy was estimated by measuring the quantum energy in the spectral range of the fundamental absorption. We have found that energy gap Eg is equal to 2.35 eV at 300 K. LT-Ag{sub 2}Ga{sub 2}SiS{sub 6} is a photosensitive material and reveals two spectral maxima on the curve of spectral photoconductivity spectra at λ{sub max1} =590 nm and λ{sub max2} =860 nm. Additionally, linear electro-optical effect of LT-Ag{sub 2}Ga{sub 2}SiS{sub 6} for the wavelengths of a cw He-Ne laser at 1150 nm was explored. - Graphical abstract: Manuscript present the technology of growth and investigation of properties a new quaternary compound Ag{sub 2}Ga{sub 2}SiS{sub 6} including the detailed study of the phase equilibria in the AgGaS{sub 2}–SiS{sub 2} system to determine the physico-chemical conditions of its formation. We examined crystal structure of the compound by X-ray powder method, its electronic structure by XPS method. Finally we have found these crystals like promising material for nonlinear optical, electrooptical and piezoelectric applications. - Highlights: • Phase diagram of the AgGaS{sub 2}–SiS{sub 2} system was successfully described at first time. • Crystal Ag{sub 2}Ga{sub 2}SiS{sub 6} was grown and its structure was determined. • Electronic structure and chemical bonding of the Ag{sub 2}Ga{sub 2}SiS{sub 6} crystal were examined by. • X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES) methods. • Optical and photoelectrical properties were investigated.« less

Bias Selectable Dual Band AlGaN Ultra-violet Detectors

NASA Technical Reports Server (NTRS)

Yan, Feng; Miko, Laddawan; Franz, David; Guan, Bing; Stahle, Carl M.

2007-01-01

Bias selectable dual band AlGaN ultra-violet (UV) detectors, which can separate UV-A and UV-B using one detector in the same pixel by bias switching, have been designed, fabricated and characterized. A two-terminal n-p-n photo-transistor-like structure was used. When a forward bias is applied between the top electrode and the bottom electrode, the detectors can successfully detect W-A and reject UV-B. Under reverse bias, they can detect UV-B and reject UV-A. The proof of concept design shows that it is feasible to fabricate high performance dual-band UV detectors based on the current AlGaN material growth and fabrication technologies.

Multi-stacked GaSb/GaAs type-II quantum nanostructures for application to intermediate band solar cells

NASA Astrophysics Data System (ADS)

Shoji, Yasushi; Tamaki, Ryo; Okada, Yoshitaka

2017-06-01

We have investigated the performance of 10-layer stacked GaSb/GaAs quantum dot (QD) and quantum ring (QR) solar cells (SCs) having a type-II band alignment. For both SCs, the external quantum efficiency (EQE) increased in the longer wavelength region beyond GaAs bandedge wavelength of λ > 870 nm due to an additive contribution from GaSb/GaAs QD or QR layers inserted in the intrinsic region of p-i-n SC structure. The EQE of GaSb/GaAs QRSC was higher than that of QDSC at room temperature and the photoluminescence intensity from GaSb/GaAs QRs was stronger compared with GaSb/GaAs QDs. These results indicate that crystal quality of GaSb/GaAs QRs is superior to that of GaSb/GaAs QDs. Furthermore, a photocurrent production due to two-step photo-absorption via GaSb/GaAs QD states or QR states, ΔEQE was measured at low temperature and the ratio of two-step absorption to total carrier extraction defined as ΔEQE / (ΔEQE + EQE), was higher for GaSb/GaAs QRSC than that of QDSC. The ratio of GaSb/GaAs QRSC exceeds 80% over the wavelength region of λ = 950 - 1250 nm. This suggests that two-step absorption process is more dominant for carrier extraction from GaSb/GaAs QR structure.

Gallium Compounds Exhibit Potential as New Therapeutic Agents against Mycobacterium abscessus

PubMed Central

Abdalla, Maher Y.; Switzer, Barbara L.; Goss, Christopher H.; Aitken, Moira L.; Singh, Pradeep K.

2015-01-01

The rapidly growing nontuberculous mycobacterial species Mycobacterium abscessus has recently emerged as an important pathogen in patients with cystic fibrosis (CF). Treatment options are limited because of the organism's innate resistance to standard antituberculous antibiotics, as well as other currently available antibiotics. New antibiotic approaches to the treatment of M. abscessus are urgently needed. The goal of the present study was to assess the growth-inhibitory activity of different Ga compounds against an American Type Culture Collection (ATCC) strain and clinical isolates of M. abscessus obtained from CF and other patients. In our results, using Ga(NO3)3 and all of the other Ga compounds tested inhibited the growth of ATCC 19977 and clinical isolates of M. abscessus. Inhibition was mediated by disrupting iron uptake, as the addition of exogenous iron (Fe) restored basal growth. There were modest differences in inhibition among the isolates for the same Ga chelates, and for most Ga chelates there was only a slight difference in potency from Ga(NO3)3. In contrast, Ga-protoporphyrin completely and significantly inhibited the ATCC strain and clinical isolates of M. abscessus at much lower concentrations than Ga(NO3)3. In in vitro broth culture, Ga-protoporphyrin was more potent than Ga(NO3)3. When M. abscessus growth inside the human macrophage THP-1 cell line was assessed, Ga-protoporphyrin was >20 times more active than Ga(NO3)3. The present work suggests that Ga exhibits potent growth-inhibitory capacity against the ATCC strain, as well as against antibiotic-resistant clinical isolates of M. abscessus, including the highly antibiotic-resistant strain MC2638. Ga-based therapy offers the potential for further development as a novel therapy against M. abscessus. PMID:26033732

Experimental visualization of covalent bonds and structural disorder in a gallium zinc oxynitride photocatalyst (Ga(1-x)Znx)(N(1-x)Ox): origin of visible light absorption.

PubMed

Yashima, Masatomo; Yamada, Hiroki; Maeda, Kazuhiko; Domen, Kazunari

2010-04-14

We present the experimental visualization of covalent bonding, positional disorders and split anion sites in visible-light responsive photocatalyst (Ga(0.885)Zn(0.115))(N(0.885)O(0.115)). ZnO alloying into GaN reduces the band gap, leading to the visible-light response. DFT calculations indicated no significant difference in band gap between structural models with and without split sites.

Effect of Variation of Silicon Nitride Passivation Layer on Electron Irradiated Aluminum Gallium Nitride/Gallium Nitride HEMT Structures

DTIC Science & Technology

2014-06-19

the AlGaN is unintentionally doped . Figure 2.3. AlGaN/GaN band diagram showing polarization charges. The band diagram in Figure 2.3 shows...intentionally doped as are MESFETS, and the channel gets its electrons from the unintentional doping . There is less Coulomb scattering in the...temperature measurements are often used to provide spatial PL maps of doping and trap densities. Laser excitation (quasi-monochromatic) is

Structural defects and recombination behavior of excited carriers in Cu(In,Ga)Se2 solar cells

NASA Astrophysics Data System (ADS)

Yang, J.; Du, H. W.; Li, Y.; Gao, M.; Wan, Y. Z.; Xu, F.; Ma, Z. Q.

2016-08-01

The carriers' behavior in neutral region (NTR) and space charged region (SCR) of Cu(In,Ga)Se2 thin film based solar cells has been investigated by temperature dependent photoluminescence (PL-T), electroluminescence (EL-T) and current-voltage (IV-T) from 10 to 300 K. PL-T spectra show that three kinds of defects, namely VSe, InCu and (InCu+VCu), are localized within the band gap of NTR and SCR of CIGS layer, corresponding to the energy levels of EC-0.08, EC-0.20 and EC-0.25 eV, respectively. The InCu and (InCu+VCu) deep level defects are non-radiative recombination centers at room temperature. The IV-T and EL-T analysis reveals that the injection modes of electrons from ZnO conduction band into Cu(In,Ga)Se2 layer are tunneling, thermally-excited tunneling and thermionic emission under 10-40, 60-160, and 180-300 K, respectively. At 10-160 K, the electrons tunnel into (InCu+VCu) and Vse defect levels in band gap of SCR and the drifting is involved in the emission bands at 0.96 and 1.07 eV, which is the direct evidence for a tunneling assisted recombination. At 180-300 K, the electrons are directly injected into the Cu(In,Ga)Se2 conduction band, and the emission of 1.13 eV are ascribed to the transitions from the conduction band to the valence band.

Total photoelectron yield spectroscopy of energy distribution of electronic states density at GaN surface and SiO2/GaN interface

NASA Astrophysics Data System (ADS)

Ohta, Akio; Truyen, Nguyen Xuan; Fujimura, Nobuyuki; Ikeda, Mitsuhisa; Makihara, Katsunori; Miyazaki, Seiichi

2018-06-01

The energy distribution of the electronic state density of wet-cleaned epitaxial GaN surfaces and SiO2/GaN structures has been studied by total photoelectron yield spectroscopy (PYS). By X-ray photoelectron spectroscopy (XPS) analysis, the energy band diagram for a wet-cleaned epitaxial GaN surface such as the energy level of the valence band top and electron affinity has been determined to obtain a better understanding of the measured PYS signals. The electronic state density of GaN surface with different carrier concentrations in the energy region corresponding to the GaN bandgap has been evaluated. Also, the interface defect state density of SiO2/GaN structures was also estimated by not only PYS analysis but also capacitance–voltage (C–V) characteristics. We have demonstrated that PYS analysis enables the evaluation of defect state density filled with electrons at the SiO2/GaN interface in the energy region corresponding to the GaN midgap, which is difficult to estimate by C–V measurement of MOS capacitors.

Controlled synthesis of AlN/GaN multiple quantum well nanowire structures and their optical properties.

PubMed

Qian, Fang; Brewster, Megan; Lim, Sung K; Ling, Yichuan; Greene, Christopher; Laboutin, Oleg; Johnson, Jerry W; Gradečak, Silvija; Cao, Yu; Li, Yat

2012-06-13

We report the controlled synthesis of AlN/GaN multi-quantum well (MQW) radial nanowire heterostructures by metal-organic chemical vapor deposition. The structure consists of a single-crystal GaN nanowire core and an epitaxially grown (AlN/GaN)(m) (m = 3, 13) MQW shell. Optical excitation of individual MQW nanowires yielded strong, blue-shifted photoluminescence in the range 340-360 nm, with respect to the GaN near band-edge emission at 368.8 nm. Cathodoluminescence analysis on the cross-sectional MQW nanowire samples showed that the blue-shifted ultraviolet luminescence originated from the GaN quantum wells, while the defect-associated yellow luminescence was emitted from the GaN core. Computational simulation provided a quantitative analysis of the mini-band energies in the AlN/GaN superlattices and suggested the observed blue-shifted emission corresponds to the interband transitions between the second subbands of GaN, as a result of quantum confinement and strain effect in these AlN/GaN MQW nanowire structures.

Electronic structure of LiGaS 2

NASA Astrophysics Data System (ADS)

Atuchin, V. V.; Isaenko, L. I.; Kesler, V. G.; Lobanov, S.; Huang, H.; Lin, Z. S.

2009-04-01

X-ray photoelectron spectroscopy (XPS) measurement has been performed to determine the valence band structure of LiGaS 2 crystals. The experimental measurement is compared with the electronic structure obtained from the density functional calculations. It is found that the Ga 3d states in the XPS spectrum are much higher than the calculated results. In order to eliminate this discrepancy, the LDA+ U method is employed and reasonable agreement is achieved. Further calculations show that the difference of the linear and nonlinear optical coefficients between LDA and LDA+ U calculations is negligibly small, indicating that the Ga 3d states are actually independent of the excited properties of LiGaS 2 crystals since they are located at a very deep position in the valence bands.

Growth and Characterization of GaN As(P) for High Efficiency Solar Cells: Final Subcontract Report, 29 July 1999--28 September 2003

DOE Office of Scientific and Technical Information (OSTI.GOV)

Narayanamurti, V.

2004-06-01

This report describes the characterization of GaAs1-xNx and GaP1-xNx alloys using scanning tunneling microscopy (STM) and ballistic electron emission microscopy (BEEM). One objective was to understand the origins of the giant bandgap bowing of these compound semiconductor alloys as a function of nitrogen concentration. The STM and BEEM have been used to characterize GaNxAs1-x and GaNxP1-x as a function of composition. The reduction in bandgap has been measured. Detailed studies of the band structure as a function of N composition has led to a basic understanding of the materials system. The major results of this work include: (i) determination ofmore » relative contributions of the G- and L-like bands of GaNxAs1-x to the BEEM spectra; (ii) determination of the composition dependence of the Au/ G- and L-like bands of GaNxAs1-x Schottky barrier height; (iii) development of a model to describe the BEEM results at nonepitaxial metal/semiconductor interfaces; and (iv) ballistic electron emission spectroscopy studies of GaNxP1-x samples that demonstrated possible splitting in the degeneracy of the X valley due to the nitrogen. The data were qualitatively described by the recent perturbed host states model of Kent and Zunger.« less

Optical gain in GaAsBi/GaAs quantum well diode lasers

PubMed Central

Marko, Igor P.; Broderick, Christopher A.; Jin, Shirong; Ludewig, Peter; Stolz, Wolfgang; Volz, Kerstin; Rorison, Judy M.; O’Reilly, Eoin P.; Sweeney, Stephen J.

2016-01-01

Electrically pumped GaAsBi/GaAs quantum well lasers are a promising new class of near-infrared devices where, by use of the unusual band structure properties of GaAsBi alloys, it is possible to suppress the dominant energy-consuming Auger recombination and inter-valence band absorption loss mechanisms, which greatly impact upon the device performance. Suppression of these loss mechanisms promises to lead to highly efficient, uncooled operation of telecommunications lasers, making GaAsBi system a strong candidate for the development of next-generation semiconductor lasers. In this report we present the first experimentally measured optical gain, absorption and spontaneous emission spectra for GaAsBi-based quantum well laser structures. We determine internal optical losses of 10–15 cm−1 and a peak modal gain of 24 cm−1, corresponding to a material gain of approximately 1500 cm−1 at a current density of 2 kA cm−2. To complement the experimental studies, a theoretical analysis of the spontaneous emission and optical gain spectra is presented, using a model based upon a 12-band k.p Hamiltonian for GaAsBi alloys. The results of our theoretical calculations are in excellent quantitative agreement with the experimental data, and together provide a powerful predictive capability for use in the design and optimisation of high efficiency lasers in the infrared. PMID:27363930

Ultrawide Band Gap β-Ga2O3 Nanomechanical Resonators with Spatially Visualized Multimode Motion.

PubMed

Zheng, Xu-Qian; Lee, Jaesung; Rafique, Subrina; Han, Lu; Zorman, Christian A; Zhao, Hongping; Feng, Philip X-L

2017-12-13

Beta gallium oxide (β-Ga 2 O 3 ) is an emerging ultrawide band gap (4.5 eV-4.9 eV) semiconductor with attractive properties for future power electronics, optoelectronics, and sensors for detecting gases and ultraviolet radiation. β-Ga 2 O 3 thin films made by various methods are being actively studied toward such devices. Here, we report on the experimental demonstration of single-crystal β-Ga 2 O 3 nanomechanical resonators using β-Ga 2 O 3 nanoflakes grown via low-pressure chemical vapor deposition (LPCVD). By investigating β-Ga 2 O 3 circular drumhead structures, we demonstrate multimode nanoresonators up to the sixth mode in high and very high frequency (HF/VHF) bands, and also realize spatial mapping and visualization of the multimode motion. These measurements reveal a Young's modulus of E Y = 261 GPa and anisotropic biaxial built-in tension of 37.5 MPa and 107.5 MPa. We find that thermal annealing can considerably improve the resonance characteristics, including ∼40% upshift in frequency and ∼90% enhancement in quality (Q) factor. This study lays a foundation for future exploration and development of mechanically coupled and tunable β-Ga 2 O 3 electronic, optoelectronic, and physical sensing devices.

Temperature influence on luminescent coupling efficiency in concentrator MJ SCs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shvarts, Maxim, E-mail: shvarts@scell.ioffe.ru; Emelyanov, Viktor; Mintairov, Mikhail

2015-09-28

In the work, presented are the results of investigation of temperature dependencies of the luminescent coupling effectiveness in lattice-matched (LM) GaInP/GaAs/Ge and metamorphic (MM) GaInP/GaInAs/Ge solar cells. The “ordinary” luminescent coupling effectiveness rise has been observed with temperature decrease for GaAs-Ge, GaInP-GaInAs and GaInAs-Ge pairs of subcells, and its limiting values have been defined. A “reverse” behavior of the luminescent coupling effectiveness for the GaInP-GaAs pair has been found, determined emittance potential drop of wideband GaInP p-n junction. It is shown that the established “unusual” behavior of the LC efficiency may be determined by the presence of thermalized centers ofmore » non-radiative recombination of charge carriers for the GaInP subcell in GaInP/GaAs/Ge LM structure. Estimation of characteristic parameters for the nonradiative recombination processes in wideband GaInP p-n junction has been carried out, and values for the energy of the nonradiative center thermalization (E{sub nrad2} =79.42meV) and for the activation energy of nonradiative band-to-band recombination (E{sub A}=33.4meV) have been obtained.« less

Theoretical investigation of structural, mechanical and electronic properties of GaAs1-xNx alloys under ambient and high pressure

NASA Astrophysics Data System (ADS)

Li, Jian; Han, Xiuxun; Dong, Chen; Fan, Changzeng

2017-12-01

Using first-principles total energy calculations, we have studied the structural, mechanical and electronic properties of GaAs1-xNx ternary semiconductor alloys with the zinc-blende crystal structure over the whole nitrogen concentration range (with x from 0 to 1) within density functional theory (DFT) framework. To obtain the ideal band gap, we employ the semi-empirical approach called local density approximation plus the multi-orbital mean-field Hubbard model (LDA+U). The calculated results illustrate the varying lattice constants and band gap in GaAs1-xNx alloys as functions of the nitrogen concentration x. According to the pressure dependence of the lattice constants and volume, the higher N concentration alloy exhibits the better anti-compressibility. In addition, an increasing band gap is predicted under 20 GPa pressure for GaAs1-xNx alloys.

Investigation of Localized States in GaAsSb Epilayers Grown by Molecular Beam Epitaxy

PubMed Central

Gao, Xian; Wei, Zhipeng; Zhao, Fenghuan; Yang, Yahui; Chen, Rui; Fang, Xuan; Tang, Jilong; Fang, Dan; Wang, Dengkui; Li, Ruixue; Ge, Xiaotian; Ma, Xiaohui; Wang, Xiaohua

2016-01-01

We report the carrier dynamics in GaAsSb ternary alloy grown by molecular beam epitaxy through comprehensive spectroscopic characterization over a wide temperature range. A detailed analysis of the experimental data reveals a complex carrier relaxation process involving both localized and delocalized states. At low temperature, the localized degree shows linear relationship with the increase of Sb component. The existence of localized states is also confirmed by the temperature dependence of peak position and band width of the emission. At temperature higher than 60 K, emissions related to localized states are quenched while the band to band transition dominates the whole spectrum. This study indicates that the localized states are related to the Sb component in the GaAsSb alloy, while it leads to the poor crystal quality of the material, and the application of GaAsSb alloy would be limited by this deterioration. PMID:27381641

Intrinsic optical confinement for ultrathin InAsN quantum well superlattices

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sakri, A.; Robert, C.; Pedesseau, L.

We study energy-band engineering with InAsN monolayer in GaAs/GaP quantum well structure. A tight-binding calculation indicates that both type I alignment along with direct band-gap behavior can be obtained. We show that the optical transitions are less sensitive to the position of the probe.

Hidden edge Dirac point and robust quantum edge transport in InAs/GaSb quantum wells

NASA Astrophysics Data System (ADS)

Li, Chang-An; Zhang, Song-Bo; Shen, Shun-Qing

2018-01-01

The robustness of quantum edge transport in InAs/GaSb quantum wells in the presence of magnetic fields raises an issue on the fate of topological phases of matter under time-reversal symmetry breaking. A peculiar band structure evolution in InAs/GaSb quantum wells is revealed: the electron subbands cross the heavy hole subbands but anticross the light hole subbands. The topologically protected band crossing point (Dirac point) of the helical edge states is pulled to be close to and even buried in the bulk valence bands when the system is in a deeply inverted regime, which is attributed to the existence of the light hole subbands. A sizable Zeeman energy gap verified by the effective g factors of edge states opens at the Dirac point by an in-plane or perpendicular magnetic field; however, it can also be hidden in the bulk valance bands. This provides a plausible explanation for the recent observation on the robustness of quantum edge transport in InAs/GaSb quantum wells subjected to strong magnetic fields.

Two-band analysis of hole mobility and Hall factor for heavily carbon-doped p-type GaAs

NASA Astrophysics Data System (ADS)

Kim, B. W.; Majerfeld, A.

1996-02-01

We solve a pair of Boltzmann transport equations based on an interacting two-isotropic-band model in a general way first to get transport parameters corresponding to the relaxation time. We present a simple method to calculate effective relaxation times, separately for each band, which compensate for the inherent deficiencies in using the relaxation time concept for polar optical-phonon scattering. Formulas for calculating momentum relaxation times in the two-band model are presented for all the major scattering mechanisms of p-type GaAs for simple, practical mobility calculations. In the newly proposed theoretical framework, first-principles calculations for the Hall mobility and Hall factor of p-type GaAs at room temperature are carried out with no adjustable parameters in order to obtain direct comparisons between the theory and recently available experimental results. In the calculations, the light-hole-band nonparabolicity is taken into account on the average by the use of energy-dependent effective mass obtained from the kṡp method and valence-band anisotropy is taken partly into account by the use the Wiley's overlap function.. The calculated Hall mobilities show a good agreement with our experimental data for carbon-doped p-GaAs samples in the range of degenerate hole densities. The calculated Hall factors show rH=1.25-1.75 over hole densities of 2×1017-1×1020 cm-3.

Excitation mechanisms of Er optical centers in GaN epilayers

DOE Office of Scientific and Technical Information (OSTI.GOV)

George, D. K.; Hawkins, M. D.; McLaren, M.

2015-10-26

We report direct evidence of two mechanisms responsible for the excitation of optically active Er{sup 3+} ions in GaN epilayers grown by metal-organic chemical vapor deposition. These mechanisms, resonant excitation via the higher-lying inner 4f shell transitions and band-to-band excitation of the semiconductor host, lead to narrow emission lines from isolated and the defect-related Er optical centers. However, these centers have different photoluminescence spectra, local defect environments, decay dynamics, and excitation cross sections. The photoluminescence at 1.54 μm from the isolated Er optical center which can be excited by either mechanism has the same decay dynamics, but possesses a much highermore » excitation cross-section under band-to-band excitation. In contrast, the photoluminescence at 1.54 μm from the defect-related Er optical center can only be observed through band-to-band excitation but has the largest excitation cross-section. These results explain the difficulty in achieving gain in Er doped GaN and indicate approaches for realization of optical amplification, and possibly lasing, at room temperature.« less

Electronic structure and p-type doping of ZnSnN2

NASA Astrophysics Data System (ADS)

Wang, Tianshi; Janotti, Anderson; Ni, Chaoying

ZnSnN2 is a promising solar-cell absorber material composed of earth abundant elements. Little is known about doping, defects, and how the valence and conduction bands in this material align with the bands in other semiconductors. Using density functional theory with the the Heyd-Scuseria-Ernzerhof hybrid functional (HSE06), we investigate the electronic structure of ZnSnN2, its band alignment to other semiconductors, such as GaN and ZnO, the possibility of p-type doping, and the possible causes of the observed unintentional n-type conductivity. We find that the position of the valence-band maximum of ZnSnN2 is 0.55 eV higher than that of GaN, yet the conduction-band minimum is close to that in ZnO. As possible p-type dopants, we explore Li, Na, and K substituting on the Zn site. Finally, we discuss the cause of unintentional n-type conductivity by analyzing the position of the conduction-band minimum with respect to that of GaN and ZnO.

Bandgap engineering of GaN nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ming, Bang-Ming; Yan, Hui; Wang, Ru-Zhi, E-mail: wrz@bjut.edu.cn, E-mail: yamcy@csrc.ac.cn

2016-05-15

Bandgap engineering has been a powerful technique for manipulating the electronic and optical properties of semiconductors. In this work, a systematic investigation of the electronic properties of [0001] GaN nanowires was carried out using the density functional based tight-binding method (DFTB). We studied the effects of geometric structure and uniaxial strain on the electronic properties of GaN nanowires with diameters ranging from 0.8 to 10 nm. Our results show that the band gap of GaN nanowires depends linearly on both the surface to volume ratio (S/V) and tensile strain. The band gap of GaN nanowires increases linearly with S/V, whilemore » it decreases linearly with increasing tensile strain. These linear relationships provide an effect way in designing GaN nanowires for their applications in novel nano-devices.« less

Band gap engineering of N-alloyed Ga{sub 2}O{sub 3} thin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Song, Dongyu; Li, Bingsheng, E-mail: libingsheng@hit.edu.cn, E-mail: ashen@ccny.cuny.edu; Sui, Yu

2016-06-15

The authors report the tuning of band gap of GaON ternary alloy in a wide range of 2.75 eV. The samples were prepared by a two-step nitridation method. First, the samples were deposited on 2-inch fused silica substrates by megnetron sputtering with NH{sub 3} and Ar gas for 60 minutes. Then they were annealed in NH{sub 3} ambience at different temperatures. The optical band gap energies are calculated from transmittance measurements. With the increase of nitridation temperature, the band gap gradually decreases from 4.8 eV to 2.05 eV. X-ray diffraction results indicate that as-deposited amorphous samples can crystallize into monoclinicmore » and hexagonal structures after they were annealed in oxygen or ammonia ambience, respectively. The narrowing of the band gap is attributed to the enhanced repulsion of N2p -Ga3d orbits and formation of hexagonal structure.« less

Zero-phonon line and fine structure of the yellow luminescence band in GaN

NASA Astrophysics Data System (ADS)

Reshchikov, M. A.; McNamara, J. D.; Zhang, F.; Monavarian, M.; Usikov, A.; Helava, H.; Makarov, Yu.; Morkoç, H.

2016-07-01

The yellow luminescence band was studied in undoped and Si-doped GaN samples by steady-state and time-resolved photoluminescence. At low temperature (18 K), the zero-phonon line (ZPL) for the yellow band is observed at 2.57 eV and attributed to electron transitions from a shallow donor to a deep-level defect. At higher temperatures, the ZPL at 2.59 eV emerges, which is attributed to electron transitions from the conduction band to the same defect. In addition to the ZPL, a set of phonon replicas is observed, which is caused by the emission of phonons with energies of 39.5 meV and 91.5 meV. The defect is called the YL1 center. The possible identity of the YL1 center is discussed. The results indicate that the same defect is responsible for the strong YL1 band in undoped and Si-doped GaN samples.

Band offsets and growth mode of molecular beam epitaxy grown MgO (111) on GaN (0002) by x-ray photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Craft, H. S.; Collazo, R.; Losego, M. D.; Mita, S.; Sitar, Z.; Maria, J.-P.

2007-10-01

MgO is a proposed dielectric for use as a tunneling barrier in devices integrating GaN and ferroelectric oxides. In this study, we present data regarding the growth mode and band offsets of MgO grown epitaxially on GaN (0002) surfaces using molecular beam epitaxy. Using in situ x-ray photoelectron spectroscopy (XPS) and molecular beam epitaxy, we determine, from sequential growth experiments, that the growth of MgO proceeds via the Volmer-Weber (three-dimensional) mode, and full coalescence of the film does not occur until approximately 12nm of MgO has been deposited. The observation of a three-dimensional growth mode is in agreement with previously published data. For the valence band offset, we find a value of 1.2±0.2eV, which corresponds to a 3.2eV conduction band offset. XPS measurements suggest a chemically abrupt interface and no effect on band lineup due to the slow coalescence behavior.

Fabrication of bi-layer graphene and theoretical simulation for its possible application in thin film solar cell.

PubMed

Behura, Sanjay K; Mahala, Pramila; Nayak, Sasmita; Yang, Qiaoqin; Mukhopadhyay, Indrajit; Janil, Omkar

2014-04-01

High quality graphene film is fabricated using mechanical exfoliation of highly-oriented pyrolytic graphite. The graphene films on glass substrates are characterized using field-emission scanning electron microscopy, atomic force microscopy, Raman spectroscopy, UV-vis spectroscopy and Fourier transform infrared spectroscopy. A very high intensity ratio of 2D to G-band (to approximately 1.67) and narrow 2D-band full-width at half maximum (to approximately 40 cm(-1)) correspond to the bi-layer graphene formation. The bi-layer graphene/p-GaN/n-InGaN/n-GaN/GaN/sAl2O3 system is studied theoretically using TCAD Silvaco software, in which the properties of exfoliated bi-layer graphene are used as transparent and conductive film, and the device exhibits an efficiency of 15.24% compared to 13.63% for ITO/p-GaN/n-InGaN/n-GaN/GaN/Al2O3 system.

First principles calculations of La2O3/GaAs interface properties under biaxial strain and hydrostatic pressure

NASA Astrophysics Data System (ADS)

Shi, Li-Bin; Li, Ming-Biao; Xiu, Xiao-Ming; Liu, Xu-Yang; Zhang, Kai-Cheng; Li, Chun-Ran; Dong, Hai-Kuan

2017-04-01

La2O3 is a potential dielectric material with high permittivity (high-κ) for metal-oxide-semiconductor (MOS) devices. However, band offsets and oxide defects should still be concerned. Smaller band offsets and carrier traps increase leakage current, and degenerate performance of the devices. In this paper, the interface behaviors of La2O3/GaAs under biaxial strain and hydrostatic pressure are investigated, which is performed by first principles calculations based on density functional theory (DFT). Strain engineering is attempted to improve performance of the metal/La2O3/GaAs devices. First of all, we creatively realize band alignment of La2O3/GaAs interface under biaxial strain and hydrostatic pressure. The proper biaxial tensile strain can effectively increase valence band offsets (VBO) and conduction band offsets (CBO), which can be used to suppress leakage current. However, the VBO will decrease with the increase of hydrostatic pressure, indicating that performance of the devices is degenerated. Then, a direct tunneling leakage current model is used to investigate current and voltage characteristics of the metal/La2O3/GaAs. The impact of biaxial strain and hydrostatic pressure on leakage current is discussed. At last, formation energies and transition levels of oxygen interstitial (Oi) and oxygen vacancy (VO) in La2O3 are assessed. We investigate how they will affect performance of the devices.

Photoemission study of CaF2- and SrF2-GaAs(110) interfaces formed at room temperature

NASA Astrophysics Data System (ADS)

Mao, D.; Young, K.; Kahn, A.; Zanoni, R.; McKinley, J.; Margaritondo, G.

1989-06-01

Interfaces formed by evaporating CaF2 or SrF2 on room-temperature GaAs(110) are studied with synchrotron-radiation photoemission spectroscopy. The fluoride films grow uniformly on the GaAs surface. The deposition of CaF2 and SrF2 induces a large initial band bending on p-type GaAs (~0.9 eV) and a small initial band bending on n-type GaAs (~0.25 eV). The valence band is dominated by the F 2p peak which shifts toward high binding energies by ~1.5 eV after the deposition of >=16 Å fluoride. This shift reflects an increase in the valence-band offset between the two materials as the film forms. The final band offsets are estimated at 7.7 and 8.0 eV for CaF2 and SrF2, respectively, and are in qualitative agreement with those expected from the fluoride-Si data. Core-level measurements indicate that no reaction or decomposition of the MF2 molecule takes place at the interface. The F 2s core-level line shape and the increase in the binding-energy separation of F 2s and Ca 3p with increasing coverage suggest the presence of an interface F component. Contrary to the CaF2/Si case, no measurable Ca-substrate bonding effect is observed. The dissociative effect of uv irradiation on the CaF2 film is also investigated.

Microwave power amplifiers based on AlGaN/GaN transistors with a two-dimensional electron gas

NASA Astrophysics Data System (ADS)

Vendik, O. G.; Vendik, I. B.; Tural'chuk, P. A.; Parnes, Ya. M.; Parnes, M. D.

2016-11-01

A technique for synthesis of microwave power amplifiers based on transistors with a AlGaN/GaN heterojunction is discussed. Special focus is on the development of a technique for synthesis of transformation circuits of the power amplifier to increase efficiency with a retained high output power. The use of independent matching at the harmonic frequencies and fundamental frequency makes it possible to control the attainable efficiency in a wide frequency band along with the total suppression of harmonics beyond the operational band. Microwave power amplifiers for operation at 4 and 9 GHz have been developed and experimentally investigated.

Undoped p-type GaN1-xSbx alloys: Effects of annealing

NASA Astrophysics Data System (ADS)

Segercrantz, N.; Baumgartner, Y.; Ting, M.; Yu, K. M.; Mao, S. S.; Sarney, W. L.; Svensson, S. P.; Walukiewicz, W.

2016-12-01

We report p-type behavior for undoped GaN1-xSbx alloys with x ≥ 0.06 grown by molecular beam epitaxy at low temperatures (≤400 °C). Rapid thermal annealing of the GaN1-xSbx films at temperatures >400 °C is shown to generate hole concentrations greater than 1019 cm-3, an order of magnitude higher than typical p-type GaN achieved by Mg doping. The p-type conductivity is attributed to a large upward shift of the valence band edge resulting from the band anticrossing interaction between localized Sb levels and extended states of the host matrix.

Electronic properties of core-shell nanowire resonant tunneling diodes

PubMed Central

2014-01-01

The electronic sub-band structure of InAs/InP/InAs/InP/InAs core-shell nanowire resonant tunneling diodes has been investigated in the effective mass approximation by varying the core radius and the thickness of the InP barriers and InAs shells. A top-hat, double-barrier potential profile and optimal energy configuration are obtained for core radii and surface shells >10 nm, InAs middle shells <10 nm, and 5 nm InP barriers. In this case, two sub-bands exist above the Fermi level in the InAs middle shell which belongs to the m = 0 and m = 1 ladder of states that have similar wave functions and energies. On the other hand, the lowest m = 0 sub-band in the core falls below the Fermi level but the m = 1 states do not contribute to the current transport since they reside energetically well above the Fermi level. We compare the case of GaAs/AlGaAs/GaAs/AlGaAs/GaAs which may conduct current with smaller applied voltages due to the larger effective mass of electrons in GaAs and discuss the need for doping. PMID:25288912

Effect of arsenic on the optical properties of GaSb-based type II quantum wells with quaternary GaInAsSb layers

NASA Astrophysics Data System (ADS)

Janiak, F.; Motyka, M.; Sek, G.; Dyksik, M.; Ryczko, K.; Misiewicz, J.; Weih, R.; Höfling, S.; Kamp, M.; Patriarche, G.

2013-12-01

Optical properties of molecular beam epitaxially grown type II "W" shaped GaSb/AlSb/InAs/GaIn(As)Sb/InAs/AlSb/GaSb quantum wells (QWs) designed for the active region of interband cascade lasers have been investigated. Temperature dependence of Fourier-transformed photoluminescence and photoreflectance was employed to probe the effects of addition of arsenic into the original ternary valence band well of GaInSb. It is revealed that adding arsenic provides an additional degree of freedom in terms of band alignment and strain tailoring and allows enhancing the oscillator strength of the active type II transition. On the other hand, however, arsenic incorporation apparently also affects the structural and optical material quality via generating carrier trapping states at the interfaces, which can deteriorate the radiative efficiency. These have been evidenced in several spectroscopic features and are also confirmed by cross-sectional transmission electron microscopy images. While arsenic incorporation into type II QWs is a powerful heterostructure engineering tool for optoelectronic devices, a compromise has to be found between ideal band structure properties and high quality morphological properties.

Electronic properties of core-shell nanowire resonant tunneling diodes.

PubMed

Zervos, Matthew

2014-01-01

The electronic sub-band structure of InAs/InP/InAs/InP/InAs core-shell nanowire resonant tunneling diodes has been investigated in the effective mass approximation by varying the core radius and the thickness of the InP barriers and InAs shells. A top-hat, double-barrier potential profile and optimal energy configuration are obtained for core radii and surface shells >10 nm, InAs middle shells <10 nm, and 5 nm InP barriers. In this case, two sub-bands exist above the Fermi level in the InAs middle shell which belongs to the m = 0 and m = 1 ladder of states that have similar wave functions and energies. On the other hand, the lowest m = 0 sub-band in the core falls below the Fermi level but the m = 1 states do not contribute to the current transport since they reside energetically well above the Fermi level. We compare the case of GaAs/AlGaAs/GaAs/AlGaAs/GaAs which may conduct current with smaller applied voltages due to the larger effective mass of electrons in GaAs and discuss the need for doping.

Microstructure and conductance-slope of InAs/GaSb tunnel diodes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Iutzi, Ryan M., E-mail: iutzi@mit.edu; Fitzgerald, Eugene A.

2014-06-21

InAs/GaSb and similar materials systems have generated great interest as a heterojunction for tunnel field effect transistors (TFETs) due to favorable band alignment. However, little is currently understood about how such TFETs are affected by materials defects and nonidealities. We present measurements of the conductance slope for various InAs/GaSb heterojunctions via two-terminal electrical measurements, which removes three-terminal parasitics and enables direct study on the effect of microstructure on tunnelling. Using this, we can predict how subthreshold swings in TFETs can depend on microstructure. We also demonstrate growth and electrical characterization for structures grown by metalorganic chemical vapor deposition (MOCVD)—a generallymore » more scalable process compared with molecular beam epitaxy (MBE). We determine that misfit dislocations and point defects near the interface can lead to energy states in the band-gap and local band bending that result in trap-assisted leakage routes and nonuniform band alignment across the junction area that lower the steepness of the conductance slope. Despite the small lattice mismatch, misfit dislocations still form in InAs on GaSb due to relaxation as a result of large strain from intermixed compositions. This can be circumvented by growing GaSb on InAs, straining the GaSb underlayer, or lowering the InAs growth temperature in the region of the interface. The conductance slope can also be improved by annealing the samples at higher temperatures, which we believe acts to annihilate point defects and average out major fluctuations in band alignment across the interface. Using a combination of these techniques, we can greatly improve the steepness of the conductance slope which could result in steeper subthreshold swings in TFETs in the future.« less

Heterostructures with diffused interfaces: Luminescent technique for ascertainment of band alignment type

NASA Astrophysics Data System (ADS)

Abramkin, D. S.; Gutakovskii, A. K.; Shamirzaev, T. S.

2018-03-01

The experimental ascertainment of band alignment type for semiconductor heterostructures with diffused interfaces is discussed. A method based on the analysis of the spectral shift of photoluminescence (PL) band with excitation density (Pex) that takes into account state filling and band bending effects on the PL band shift is developed. It is shown that the shift of PL band maximum position is proportional to ℏωmax ˜ (Ue + Uh).ln(Pex) + b.Pex1/3, where Ue (Uh) are electron (hole) Urbach energy tail, and parameter b characterizes the effect of band bending or is equal to zero for heterostructures with type-II or type-I band alignment, respectively. The method was approved with InAs/AlAs, GaAs/AlAs, GaSb/AlAs, and AlSb/AlAs heterostructures containing quantum wells.

Assessing Biogenecity of Stromatolites: Return to the Facies

NASA Astrophysics Data System (ADS)

Shapiro, R. S.; Jameson, S.; Rutter, A.; McCarthy, K.; Planavsky, N. J.; Severson, M.

2013-12-01

The discovery of richly microfossiliferous cherty stromatolites near Schreiber and Kakabeka Falls, Ontario, in the 1.9 Ga Gunflint Iron Formation, firmly established the field of pre-Cambrian paleontology. In the half-century since this discovery, paradigm shifts in the ecology of the microfossils as well as the utility of stromatolites as biological markers has caused a re-evaluation of our understanding of the pre-Cambrian fossil record. This research summarizes facies evaluation of the two stromatolite marker beds in the Gunflint-correlative Biwabik Iron Formation of Minnesota. The centimeter-scale microstratigraphy of cores drilled through the central and eastern Mesabi Iron Range was coupled with field descriptions of outcrops and mines in both the Biwabik and Gunflint iron formations. Eight lithologic facies associated with the stromatolites are identified: A) Pebble conglomerate clasts ranging in size of 0.5-3 cm, syneresis cracks, and septarian nodules with medium to coarse grain matrix; B) siltstone with subparallel sub-mm to 5 cm magnetitic and non-magnetic bands; C) stromatolitic boundstone comprising stratiform, pseudocolumnar, domal, undulatory, flat-laminated, dendritic, columnar, and mico-digitate forms and oncoids 0.5 to 2 cm diameter; D) grainstone with medium to coarse siliceous and carbonate ooids and peloids; E) massive green crystalline beds with bands of magnetite, quartz, calcite, disseminated pyrite and localized ankerite; F) autobreccicated fabric of 0.3 to 10 mm clasts; G) medium to coarse sandstone; H) quartzite. Correlation between 11 cores near Hoyt Lakes and 9 cores through the basal stromatolite layer at the MinnTac Mine near Virginia revealed that stromatolites formed both on conglomerate and medium quartz sandstone. Multiple forms of stromatolite may occur in a vertical succession (flat-laminated to undulatory to psuedocolumnar to columnar) or a core may be dominated by one type, typically columnar-stratiform. Where stromatolites do not occur, the interval is autobrecciated with many clasts containing syneresis cracks. Flow-banded iron oxides in these layers are not stromatolitic but likely formed from fluid flow at the boundary between well- and poorly-cemented lithofacies. Mesostructure is similar between collection sites that are essentially unaltered, low-grade diagenetic, and amphibolite-grade metamorphic. Furthermore, mesostructure is consistent in most stromatolites throughout both layers in the Biwabik as well as the Gunflint. These new results provide a much broader context then previous studies and demonstrate that the upper and lower stromatolite horizons in the Biwabik and Gunflint iron ranges are formed in similar facies. Complexity in stromatolite forms occurs on the sub-meter scale, as would be expected for a nearshore, microbially mediated system. However, there appears to be a gradient in the degree of microbial control of stromatolite morphology, with both ';biogenic' and ';abiogenic' forms present. More broadly, this work highlights the importance of field and regional context when deciphering the significance of microbialites.

Characterization of Cu(In,Ga)Se 2 (CIGS) films with varying gallium ratios

DOE PAGES

Claypoole, Jesse; Peace, Bernadette; Sun, Neville; ...

2015-09-05

Cu(In 1–x,Ga x)Se 2 (CIGS) absorber layers were deposited on molybdenum (Mo) coated soda-lime glass substrates with varying Ga content (described as Ga/(In + Ga) ratios) with respect to depth. As the responsible mechanisms for the limitation of the performance of the CIGS solar cells with high Ga contents are not well understood, the goal of this work was to investigate different properties of CIGS absorber films with Ga/(In + Ga) ratios varied between 0.29 and 0.41 (as determined by X-ray florescence spectroscopy (XRF)) in order to better understand the role that the Ga content has on film quality. Themore » Ga grading in the CIGS layer has the effect causing a higher band gap toward the surface and Mo contact while the band gap in the middle of the CIGS layer is lower. Also, a wider and larger Ga/(In + Ga) grading dip located deeper in the CIGS absorber layers tend to produce larger grains in the regions of the films that have lower Ga/(In + Ga) ratios. Moreover, it was found that surface roughness decreases from 51.2 nm to 41.0 nm with increasing Ga/(In + Ga) ratios. Furthermore, the surface roughness generally decreases if the Ga grading occurs deeper in the absorber layer.« less

Rhenium-osmium isotope systematics in meteorites I: Magmatic iron meteorite groups IIAB and IIIAB

USGS Publications Warehouse

Morgan, J.W.; Walker, R.J.; Grossman, J.N.

1992-01-01

Using resonance ionization mass spectrometry (RIMS), Re and Os abundances were determined by isotope dilution (ID) and 187Os 186Os ratios measured in nineteen iron meteorites: eight from group IIAB, ten from group IIIAB, and Treysa (IIIB anomalous). Abundances range from 1.4 to 4800 ppb Re, and from 13 to 65000 ppb Os, and generally agree well with previous ID and neutron activation (NAA) results. The Re and Os data suggest that abundance trends in these iron groups may be entirely explained by fractional crystallization. Addition of late-formed metal to produce ReOs variation in the B subgroups is not essential but cannot be excluded. Whole-rock isochrons for the IIAB and IIIAB groups are statistically indistinguishable. Pooled data yield an initial 187Os 186Os of 0.794 ?? 0.010, with a slope of (7.92 ?? 0.20) ?? 10-2 corresponding to a magmatic iron meteorite age of 4.65 ?? 0.11 Ga (using a decay constant of 1.64 ?? 10-11 a-1). Given the errors in the slope and half life, this age does not differ significantly from the canonical chondrite age of 4.56 Ga, but could be as young as 4.46 Ga. ?? 1992.

Transition-metal impurities in semiconductors and heterojunction band lineups

NASA Astrophysics Data System (ADS)

Langer, Jerzy M.; Delerue, C.; Lannoo, M.; Heinrich, Helmut

1988-10-01

The validity of a recent proposal that transition-metal impurity levels in semiconductors may serve as a reference in band alignment in semiconductor heterojunctions is positively verified by using the most recent data on band offsets in the following lattice-matched heterojunctions: Ga1-xAlxAs/GaAs, In1-xGaxAsyP1-y/InP, In1-xGaxP/GaAs, and Cd1-xHgxTe/CdTe. The alignment procedure is justified theoretically by showing that transition-metal energy levels are effectively pinned to the average dangling-bond energy level, which serves as the reference level for the heterojunction band alignment. Experimental and theoretical arguments showing that an increasingly popular notion on transition-metal energy-level pinning to the vacuum level is unjustified and must be abandoned in favor of the internal-reference rule proposed recently [J. M. Langer and H. Heinrich, Phys. Rev. Lett. 55, 1414 (1985)] are presented.

Effect of low and staggered gap quantum wells inserted in GaAs tunnel junctions

NASA Astrophysics Data System (ADS)

Louarn, K.; Claveau, Y.; Marigo-Lombart, L.; Fontaine, C.; Arnoult, A.; Piquemal, F.; Bounouh, A.; Cavassilas, N.; Almuneau, G.

2018-04-01

In this article, we investigate the impact of the insertion of either a type I InGaAs or a type II InGaAs/GaAsSb quantum well on the performances of MBE-grown GaAs tunnel junctions (TJs). The devices are designed and simulated using a quantum transport model based on the non-equilibrium Green’s function formalism and a 6-band k.p Hamiltonian. We experimentally observe significant improvements of the peak tunneling current density on both heterostructures with a 460-fold increase for a moderately doped GaAs TJ when the InGaAs QW is inserted at the junction interface, and a 3-fold improvement on a highly doped GaAs TJ integrating a type II InGaAs/GaAsSb QW. Thus, the simple insertion of staggered band lineup heterostructures enables us to reach a tunneling current well above the kA cm‑2 range, equivalent to the best achieved results for Si-doped GaAs TJs, implying very interesting potential for TJ-based components, such as multi-junction solar cells, vertical cavity surface emitting lasers and tunnel-field effect transistors.

Investigation on Surface Polarization of Al2O3-capped GaN/AlGaN/GaN Heterostructure by Angle-Resolved X-ray Photoelectron Spectroscopy.

PubMed

Duan, Tian Li; Pan, Ji Sheng; Wang, Ning; Cheng, Kai; Yu, Hong Yu

2017-08-17

The surface polarization of Ga-face gallium nitride (GaN) (2 nm)/AlGaN (22 nm)/GaN channel (150 nm)/buffer/Si with Al 2 O 3 capping layer is investigated by angle-resolved X-ray photoelectron spectroscopy (ARXPS). It is found that the energy band varies from upward bending to downward bending in the interface region, which is believed to be corresponding to the polarization variation. An interfacial layer is formed between top GaN and Al 2 O 3 due to the occurrence of Ga-N bond break and Ga-O bond forming during Al 2 O 3 deposition via the atomic layer deposition (ALD). This interfacial layer is believed to eliminate the GaN polarization, thus reducing the polarization-induced negative charges. Furthermore, this interfacial layer plays a key role for the introduction of the positive charges which lead the energy band downward. Finally, a N 2 annealing at 400 °C is observed to enhance the interfacial layer growth thus increasing the density of positive charges.

Band bending at the heterointerface of GaAs/InAs core/shell nanowires monitored by synchrotron X-ray photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Khanbabaee, B.; Bussone, G.; Knutsson, J. V.; Geijselaers, I.; Pryor, C. E.; Rieger, T.; Demarina, N.; Grützmacher, D.; Lepsa, M. I.; Timm, R.; Pietsch, U.

2016-10-01

Unique electronic properties of semiconductor heterostructured nanowires make them useful for future nano-electronic devices. Here, we present a study of the band bending effect at the heterointerface of GaAs/InAs core/shell nanowires by means of synchrotron based X-ray photoelectron spectroscopy. Different Ga, In, and As core-levels of the nanowire constituents have been monitored prior to and after cleaning from native oxides. The cleaning process mainly affected the As-oxides and was accompanied by an energy shift of the core-level spectra towards lower binding energy, suggesting that the As-oxides turn the nanowire surfaces to n-type. After cleaning, both As and Ga core-levels revealed an energy shift of about -0.3 eV for core/shell compared to core reference nanowires. With respect to depth dependence and in agreement with calculated strain distribution and electron quantum confinement, the observed energy shift is interpreted by band bending of core-levels at the heterointerface between the GaAs nanowire core and the InAs shell.

Holmium Doped Solid State Laser Resonantly Pumped and Q-Switched by Novel GaSb-Based Photonic Devices

DTIC Science & Technology

2011-08-31

increased overlap with p-cladding, presumably due to dominant role of inter valence band absorption [7]. Details of the conduction band structure of the...absorption to total loss. In the specific structures used here the n-cladding composition resulted into material with three valleys in conduction band to...materials. The beam properties of the high power 2 μm emitting GaSb -based diode lasers was improved by utilization of the waveguide structure with

Electrical, Optical and Structural Studies of INAS/INGASB VLWIR Superlattices

DTIC Science & Technology

2013-01-01

period measured by x-ray diffraction and the optical band gap energy determined by the photoresponse spectra. Sample InAs (Å) GaSb (Å) In (%) IF (Å...8x8 EFA. 22 Temperature-dependent lattice constants, band gap energies , and other physical data for InAs and GaSb are taken from Vurgaftman et al...gallium antimonide to achieve energy band gaps less than 50 meV with a superlattice period on the order of 68 Å. Similar to the work reported on

Luminescence emission from Al0.3Ga0.7N/GaN multi quantum disc core/shell nanowire: Numerical approach

NASA Astrophysics Data System (ADS)

Namvari, E.; Shojaei, S.; Asgari, A.

2017-09-01

In this work, a numerical approach to investigate the room temperature luminescence emission from core/shell nanowire is presented where GaN quantum discs (QDiscs), periodically distributed in AlxGa1-xN nanowire, is considered as core and AlxGa1-xN as shell. Thin disc shaped (Ring shaped) n-doped region has been placed at the GaN/ AlxGa1-xN (AlxGa1-xN /air) interface in AlxGa1-xN region in axial (radial) directions. To obtain energy levels and related wavefunctions, self-consistent procedure has been employed to solve Schrodinger-Poisson equations with considering the spontaneous and piezoelectric polarization. Then luminescence spectrum is studied in details to recognize the parameters influent in luminescence. The results show that the amount of doping, size of QDiscs and theirs numbers have remarkable effects on the band to band luminescence emission. Our numerical calculations gives some insights into the luminescence emission of core/shell nanowire and exhibits a useful tool to analyze findings in experiments.

Optical band gap of thermally deposited Ge-S-Ga thin films

NASA Astrophysics Data System (ADS)

Rana, Anjli; Heera, Pawan; Singh, Bhanu Pratap; Sharma, Raman

2018-05-01

Thin films of Ge20S80-xGax glassy alloy, obtained from melt quenching technique, were deposited on the glass substrate by thermal evaporation technique under a high vacuum conditions (˜ 10-5 Torr). Absorption spectrum fitting method (ASF) is employed to obtain the optical band gap from absorption spectra. This method requires only the measurement of the absorption spectrum of the sample. The width of the band tail was also determined. Optical band gap computed from absorption spectra is found to decrease with an increase in Ga content. The evaluated optical band gap (Eg) is in well agreement with the theoretically predicted Eg and obtained from transmission spectra.

Greens Function-Based Defect Identification in InAs-InA1-xSbx Strained Layer Superlattices (Postprint)

DTIC Science & Technology

2017-06-15

the GaSb valance band edge, in agreement with values deduced recently from lifetime measurements and analysis [Aytac et al . Phys. Rev. Appl., 5...meV below the GaSb valance band edge, in agreement with values deduced recently from lifetime mea- surements and analysis [Aytac et al . Phys. Rev

A Topical Hydrogel with Deferiprone and Gallium-Protoporphyrin Targets Bacterial Iron Metabolism and Has Antibiofilm Activity.

PubMed

Richter, Katharina; Thomas, Nicky; Claeys, Jolien; McGuane, Jonathan; Prestidge, Clive A; Coenye, Tom; Wormald, Peter-John; Vreugde, Sarah

2017-06-01

Many infectious diseases are associated with multidrug-resistant (MDR) bacteria residing in biofilms that require high antibiotic concentrations. While oral drug delivery is frequently ineffective, topical treatments have the potential to deliver higher drug concentrations to the infection site while reducing systemic side effects. This study determined the antibiofilm activity of a surgical wound gel loaded with the iron chelator deferiprone (Def) and the heme analogue gallium-protoporphyrin (GaPP), alone and in combination with ciprofloxacin. Activity against MDR Staphylococcus aureus , Staphylococcus epidermidis , Pseudomonas aeruginosa , and Acinetobacter johnsonii biofilms was assessed in the colony biofilm and artificial wound model by enumeration of CFU and correlative light/electron microscopy. While Staphylococcus biofilms were equally susceptible to GaPP and Def-GaPP gels (log 10 reduction of 3.8 and 3.7, respectively), the Def-GaPP combination was crucial for significant activity against P. aeruginosa biofilms (log 10 reduction of 1.3 for GaPP and 3.3 for Def-GaPP). When Def-GaPP gel was combined with ciprofloxacin, the efficacy exceeded the activity of the individual compounds. Def-GaPP delivered in a surgical wound gel showed significant antibiofilm activity against different MDR strains and could enhance the gel's wound-healing properties. Moreover, Def-GaPP indicated a potentiation of ciprofloxacin. This antibiofilm strategy has potential for clinical utilization as a therapy for topical biofilm-related infections. Copyright © 2017 American Society for Microbiology.

A Topical Hydrogel with Deferiprone and Gallium-Protoporphyrin Targets Bacterial Iron Metabolism and Has Antibiofilm Activity

PubMed Central

Claeys, Jolien; McGuane, Jonathan; Prestidge, Clive A.; Wormald, Peter-John

2017-01-01

ABSTRACT Many infectious diseases are associated with multidrug-resistant (MDR) bacteria residing in biofilms that require high antibiotic concentrations. While oral drug delivery is frequently ineffective, topical treatments have the potential to deliver higher drug concentrations to the infection site while reducing systemic side effects. This study determined the antibiofilm activity of a surgical wound gel loaded with the iron chelator deferiprone (Def) and the heme analogue gallium-protoporphyrin (GaPP), alone and in combination with ciprofloxacin. Activity against MDR Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa, and Acinetobacter johnsonii biofilms was assessed in the colony biofilm and artificial wound model by enumeration of CFU and correlative light/electron microscopy. While Staphylococcus biofilms were equally susceptible to GaPP and Def-GaPP gels (log10 reduction of 3.8 and 3.7, respectively), the Def-GaPP combination was crucial for significant activity against P. aeruginosa biofilms (log10 reduction of 1.3 for GaPP and 3.3 for Def-GaPP). When Def-GaPP gel was combined with ciprofloxacin, the efficacy exceeded the activity of the individual compounds. Def-GaPP delivered in a surgical wound gel showed significant antibiofilm activity against different MDR strains and could enhance the gel's wound-healing properties. Moreover, Def-GaPP indicated a potentiation of ciprofloxacin. This antibiofilm strategy has potential for clinical utilization as a therapy for topical biofilm-related infections. PMID:28396543

Gallium Disrupts Iron Metabolism of Mycobacteria Residing within Human Macrophages

PubMed Central

Olakanmi, Oyebode; Britigan, Bradley E.; Schlesinger, Larry S.

2000-01-01

Mycobacterium tuberculosis and M. avium complex (MAC) enter and multiply within monocytes and macrophages in phagosomes. In vitro growth studies using standard culture media indicate that siderophore-mediated iron (Fe) acquisition plays a critical role in the growth and metabolism of both M. tuberculosis and MAC. However, the applicability of such studies to conditions within the macrophage phagosome is unclear, due in part to the absence of experimental means to inhibit such a process. Based on the ability of gallium (Ga3+) to concentrate within mononuclear phagocytes and on evidence that Ga disrupts cellular Fe-dependent metabolic pathways by substituting for Fe3+ and failing to undergo redox cycling, we hypothesized that Ga could disrupt Fe acquisition and Fe-dependent metabolic pathways of mycobacteria. We find that Ga(NO3)3 and Ga-transferrin produce an Fe-reversible concentration-dependent growth inhibition of M. tuberculosis strains and MAC grown extracellularly and within human macrophages. Ga is bactericidal for M. tuberculosis growing extracellularly and within macrophages. Finally, we provide evidence that exogenously added Fe is acquired by intraphagosomal M. tuberculosis and that Ga inhibits this Fe acquisition. Thus, Ga(NO3)3 disruption of mycobacterial Fe metabolism may serve as an experimental means to study the mechanism of Fe acquisition by intracellular mycobacteria and the role of Fe in intracellular survival. Furthermore, given the inability of biological systems to discriminate between Ga and Fe, this approach could have broad applicability to the study of Fe metabolism of other intracellular pathogens. PMID:10992462

Triggered high-purity telecom-wavelength single-photon generation from p-shell-driven InGaAs/GaAs quantum dot.

PubMed

Dusanowski, Ł; Holewa, P; Maryński, A; Musiał, A; Heuser, T; Srocka, N; Quandt, D; Strittmatter, A; Rodt, S; Misiewicz, J; Reitzenstein, S; Sęk, G

2017-12-11

We report on the experimental demonstration of triggered single-photon emission at the telecom O-band from In(Ga)As/GaAs quantum dots (QDs) grown by metal-organic vapor-phase epitaxy. Micro-photoluminescence excitation experiments allowed us to identify the p-shell excitonic states in agreement with high excitation photoluminescence on the ensemble of QDs. Hereby we drive an O-band-emitting GaAs-based QD into the p-shell states to get a triggered single photon source of high purity. Applying pulsed p-shell resonant excitation results in strong suppression of multiphoton events evidenced by the as measured value of the second-order correlation function at zero delay of 0.03 (and ~0.005 after background correction).

Swift heavy ion-beam induced amorphization and recrystallization of yttrium iron garnet.

PubMed

Costantini, Jean-Marc; Miro, Sandrine; Beuneu, François; Toulemonde, Marcel

2015-12-16

Pure and (Ca and Si)-substituted yttrium iron garnet (Y3Fe5O12 or YIG) epitaxial layers and amorphous films on gadolinium gallium garnet (Gd3Ga5O12, or GGG) single crystal substrates were irradiated by 50 MeV (32)Si and 50 MeV (or 60 MeV) (63)Cu ions for electronic stopping powers larger than the threshold value (~4 MeV μm(-1)) for amorphous track formation in YIG crystals. Conductivity data of crystalline samples in a broad ion fluence range (10(11)-10(16) cm(-2)) are modeled with a set of rate equations corresponding to the amorphization and recrystallization induced in ion tracks by electronic excitations. The data for amorphous layers confirm that a recrystallization process takes place above ~10(14) cm(-2). Cross sections for both processes deduced from this analysis are discussed in comparison to previous determinations with reference to the inelastic thermal-spike model of track formation. Micro-Raman spectroscopy was also used to follow the related structural modifications. Raman spectra show the progressive vanishing and randomization of crystal phonon modes in relation to the ion-induced damage. For crystalline samples irradiated at high fluences (⩾10(14) cm(-2)), only two prominent broad bands remain like for amorphous films, thereby reflecting the phonon density of states of the disordered solid, regardless of samples and irradiation conditions. The main band peaked at ~660 cm(-1) is assigned to vibration modes of randomized bonds in tetrahedral (FeO4) units.

Demonstration of high-responsivity epitaxial β-Ga2O3/GaN metal–heterojunction-metal broadband UV-A/UV-C detector

NASA Astrophysics Data System (ADS)

Kalra, Anisha; Vura, Sandeep; Rathkanthiwar, Shashwat; Muralidharan, Rangarajan; Raghavan, Srinivasan; Nath, Digbijoy N.

2018-06-01

We demonstrate epitaxial β-Ga2O3/GaN-based vertical metal–heterojunction-metal (MHM) broadband UV-A/UV-C photodetectors with high responsivity (3.7 A/W) at 256 and 365 nm, UV-to-visible rejection >103, and a photo-to-dark current ratio of ∼100. A small (large) conduction (valence) band offset at the heterojunction of pulsed laser deposition (PLD)-grown β-Ga2O3 on metal organic chemical vapor deposition (MOCVD)-grown GaN-on-silicon with epitaxial registry, as confirmed by X-ray diffraction (XRD) azimuthal scanning, is exploited to realize detectors with an asymmetric photoresponse and is explained with one-dimensional (1D) band diagram simulations. The demonstrated novel vertical MHM detectors on silicon are fully scalable and promising for enabling focal plane arrays for broadband ultraviolet sensing.

Continuous-wave lasing from InP/InGaAs nanoridges at telecommunication wavelengths

NASA Astrophysics Data System (ADS)

Han, Yu; Li, Qiang; Zhu, Si; Ng, Kar Wei; Lau, Kei May

2017-11-01

We report continuous-wave lasing from InP/InGaAs nanoridges grown on a patterned (001) Si substrate by aspect ratio trapping. Multi-InGaAs ridge quantum wells inside InP nanoridges are designed as active gain materials for emission in the 1500 nm band. The good crystalline quality and optical property of the InGaAs quantum wells are attested by transmission electron microscopy and microphotoluminescence measurements. After transfer of the InP/InGaAs nanoridges onto a SiO2/Si substrate, amplified Fabry-Perot resonant modes at room temperature and multi-mode lasing behavior in the 1400 nm band under continuous-wave optical pumping at 4.5 K are observed. This result thus marks an important step towards integrating InP/InGaAs nanolasers directly grown on microelectronic standard (001) Si substrates.

Initial experience of using an iron-containing fiducial marker for radiotherapy of prostate cancer: Advantages in the visualization of markers in Computed Tomography and Magnetic Resonance Imaging

NASA Astrophysics Data System (ADS)

Tanaka, Osamu; Iida, Takayoshi; Komeda, Hisao; Tamaki, Masayoshi; Seike, Kensaku; Kato, Daiki; Yokoyama, Takamasa; Hirose, Shigeki; Kawaguchi, Daisuke

2016-12-01

Visualization of markers is critical for imaging modalities such as computed tomography (CT) and magnetic resonance imaging (MRI). However, the size of the marker varies according to the imaging technique. While a large-sized marker is more useful for visualization in MRI, it results in artifacts on CT and causes substantial pain on administration. In contrast, a small-sized marker reduces the artifacts on CT but hampers MRI detection. Herein, we report a new ironcontaining marker and compare its utility with that of non-iron-containing markers. Five patients underwent CT/MRI fusion-based intensity-modulated radiotherapy, and the markers were placed by urologists. A Gold Anchor™ (GA; diameter, 0.28 mm; length, 10 mm) was placed using a 22G needle on the right side of the prostate. A VISICOIL™ (VIS; diameter, 0.35 mm; length, 10 mm) was placed using a 19G needle on the left side. MRI was performed using T2*-weighted imaging. Three observers evaluated and scored the visual qualities of the acquired images. The mean score of visualization was almost identical between the GA and VIS in radiography and cone-beam CT (Novalis Tx). The artifacts in planning CT were slightly larger using the GA than using the VIS. The visualization of the marker on MRI using the GA was superior to that using the VIS. In conclusion, the visualization quality of radiography, conebeam CT, and planning CT was roughly equal between the GA and VIS. However, the GA was more strongly visualized than was the VIS on MRI due to iron containing.

Effect of U on the electronic properties of neodymium gallate (NdGaO3): theoretical and experimental studies.

PubMed

Reshak, Ali Hussain; Piasecki, M; Auluck, S; Kityk, I V; Khenata, R; Andriyevsky, B; Cobet, C; Esser, N; Majchrowski, A; Swirkowicz, M; Diduszko, R; Szyrski, W

2009-11-19

We have performed a density functional calculation for the centrosymmetric neodymium gallate using a full-potential linear augmented plane wave method with the LDA and LDA+U exchange correlation. In particular, we explored the influence of U on the band dispersion and optical transitions. Our calculations show that U = 0.55 Ry gives the best agreement with our ellipsometry data taken in the VUV spectral range with a synchrotron source. Our LDA+U (U = 0.55) calculation shows that the valence band maximum (VBM) is located at T and the conduction band minimum (CBM) is located at the center of the Brillouin zone, resulting in a wide indirect energy band gap of about 3.8 eV in excellent agreement with our experiment. The partial density of states show that the upper valence band originates predominantly from Nd-f and O-p states, with a small admixture of Nd-s/p and Ga-p B-p states, while the lower conduction band prevailingly originates from the Nd-f and Nd-d terms with a small contribution of O-p-Ga-s/p states. The Nd-f states in the upper valence band and lower conduction band have a significant influence on the energy band gap dispersion which is illustrated by our calculations. The calculated frequency dependent optical properties show a small positive uniaxial anisotropy.

Alteration mineral mapping for iron prospecting using ETM+ data, Tonkolili iron field, northern Sierra Leone

NASA Astrophysics Data System (ADS)

Mansaray, Lamin R.; Liu, Lei; Zhou, Jun; Ma, Zhimin

2013-10-01

The Tonkolili iron field in northern Sierra Leone has the largest known iron ore deposit in Africa. It occurs in a greenstone belt in an Achaean granitic basement. This study focused mainly on mapping areas with iron-oxide and hydroxyl bearing minerals, and identifying potential areas for haematite mineralization and banded iron formations (BIFs) in Tonkolili. The predominant mineral assemblage at the surface (laterite duricrust) of this iron field is haematitegoethite- limonite ±magnetite. The mineralization occurs in quartzitic banded ironstones, layered amphibolites, granites, schists and hornblendites. In this study, Crosta techniques were applied on Enhanced Thematic Mapper (ETM+) data to enhance areas with alteration minerals and target potential areas of haematite and BIF units in the Tonkolili iron field. Synthetic analysis shows that alteration zones mapped herein are consistent with the already discovered magnetite BIFs in Tonkolili. Based on the overlaps of the simplified geological map and the remote sensing-based alteration mineral maps obtained in this study, three new haematite prospects were inferred within, and one new haematite prospect was inferred outside the tenement boundary of the Tonkolili exploration license. As the primary iron mineral in Tonkolili is magnetite, the study concludes that, these haematite prospects could also be underlain by magnetite BIFs. This study also concludes that, the application of Crosta techniques on ETM+ data is effective not only in mapping iron-oxide and hydroxyl alterations but can also provide a basis for inferring areas of potential iron resources in Algoma-type banded iron formations (BIFs), such as those in the Tonkolili field.

Investigation of the interface characteristics of Y2O3/GaAs under biaxial strain, triaxial strain, and non-strain conditions

NASA Astrophysics Data System (ADS)

Shi, Li-Bin; Liu, Xu-Yang; Dong, Hai-Kuan

2016-09-01

We investigate the interface behaviors of Y2O3/GaAs under biaxial strain, triaxial strain, and non-strain conditions. This study is performed by first principles calculations based on density functional theory (DFT). First of all, the biaxial strain is realized by changing the lattice constants in ab plane. Averaged electrostatic potential (AEP) is aligned by establishing Y2O3 and GaAs (110) surfaces. The band offsets of Y2O3/GaAs interface under biaxial strain are investigated by generalized gradient approximation and Heyd-Scuseria-Ernzerhof (HSE) functionals. The interface under biaxial strain is suitable for the design of metal oxide semiconductor (MOS) devices because the valence band offsets (VBO) and conduction band offsets (CBO) are larger than 1 eV. Second, the triaxial strain is applied to Y2O3/GaAs interface by synchronously changing the lattice constants in a, b, and c axis. The band gaps of Y2O3 and GaAs under triaxial strain are investigated by HSE functional. We compare the VBO and CBO under triaxial strain with those under biaxial strain. Third, in the absence of lattice strain, the formation energies, charge state switching levels, and migration barriers of native defects in Y2O3 are assessed. We investigate how they will affect the MOS device performance. It is found that VO+2 and Oi-2 play a very dangerous role in MOS devices. Finally, a direct tunneling leakage current model is established. The model is used to analyze current and voltage characteristics of the metal/Y2O3/GaAs.

Tunneling calculations for GaAs-Al(x)Ga(1-x) as graded band-gap sawtooth superlattices. Thesis

NASA Technical Reports Server (NTRS)

Forrest, Kathrine A.; Meijer, Paul H. E.

1991-01-01

Quantum mechanical tunneling calculations for sawtooth (linearly graded band-gap) and step-barrier AlGaAs superlattices were performed by means of a transfer matrix method, within the effective mass approximation. The transmission coefficient and tunneling current versus applied voltage were computed for several representative structures. Particular consideration was given to effective mass variations. The tunneling properties of step and sawtooth superlattices show some qualitative similarities. Both structures exhibit resonant tunneling, however, because they deform differently under applied fields, the J-V curves differ.

Mineralogy of approximately 1-10 Micrometer Iron Spheres Within 3.4 Ga Rocks (Towers Formation, Warrawoona Group, Northwestern Australia)

NASA Technical Reports Server (NTRS)

Morris, P. A.; Wentworth, Susan J.; Thomas-Keprta, Kathie L.; Allen, Carlton C.; Schwandt, Craig S.; McKay, David S.; Westall, Frances; Bell, Mary Sue; Gibson, Everett K.

2000-01-01

Iron-bearing spherules in Archean Warrawoona rocks are composed of hematite and goethite. They are clearly syngenetic with the rock but their origin, whether biological or abiogenic, is not yet known.

Nitrogen-Induced Perturbation of the Valence Band States in GaP1-xNx Alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dudiy, S. V.; Zunger, A.; Felici, M.

2006-01-01

The effects of diluted nitrogen impurities on the valence- and conduction-band states of GaP{sub 1-x}N{sub x} have been predicted and measured experimentally. The calculation uses state-of-the-art atomistic modeling: we use large supercells with screened pseudopotentials and consider several random realizations of the nitrogen configurations. These calculations agree with photoluminescence excitation (PLE) measurements performed for nitrogen concentrations x up to 0.035 and photon energies up to 1 eV above the GaP optical-absorption edge, as well as with published ellipsometry data. In particular, a predicted nitrogen-induced buildup of the L character near the valence- and conduction-band edges accounts for the surprising broad-absorptionmore » plateau observed in PLE between the X{sub 1c} and the {Lambda}{sub 1c} critical points of GaP. Moreover, theory accounts quantitatively for the downward bowing of the indirect conduction-band edge and for the upward bowing of the direct transition with increasing nitrogen concentration. We review some of the controversies in the literature regarding the shifts in the conduction band with composition, and conclude that measured results at ultralow N concentration cannot be used to judge behavior at a higher concentration. In particular, we find that at the high concentrations of nitrogen studied here ({approx}1%) the conduction-band edge (CBE) is a hybridized state made from the original GaP X{sub 1c} band-edge state plus all cluster states. In this limit, the CBE plunges down in energy as the N concentration increases, in quantitative agreement with the measurements reported here. However, at ultralow nitrogen concentrations (<0.1%), the CBE is the nearly unperturbed host X{sub 1c}, which does not sense the nitrogen cluster levels. Thus, this state does not move energetically as nitrogen is added and stays pinned in energy, in agreement with experimental results.« less

Simulation and optimization performance of GaAs/GaAs0.5Sb0.5/GaSb mechanically stacked tandem solar cells

NASA Astrophysics Data System (ADS)

Tayubi, Y. R.; Suhandi, A.; Samsudin, A.; Arifin, P.; Supriyatman

2018-05-01

Different approaches have been made in order to reach higher solar cells efficiencies. Concepts for multilayer solar cells have been developed. This can be realised if multiple individual single junction solar cells with different suitably chosen band gaps are connected in series in multi-junction solar cells. In our work, we have simulated and optimized solar cells based on the system mechanically stacked using computer simulation and predict their maximum performance. The structures of solar cells are based on the single junction GaAs, GaAs0.5Sb0.5 and GaSb cells. We have simulated each cell individually and extracted their optimal parameters (layer thickness, carrier concentration, the recombination velocity, etc), also, we calculated the efficiency of each cells optimized by separation of the solar spectrum in bands where the cell is sensible for the absorption. The optimal values of conversion efficiency have obtained for the three individual solar cells and the GaAs/GaAs0.5Sb0.5/GaSb tandem solar cells, that are: η = 19,76% for GaAs solar cell, η = 8,42% for GaAs0,5Sb0,5 solar cell, η = 4, 84% for GaSb solar cell and η = 33,02% for GaAs/GaAs0.5Sb0.5/GaSb tandem solar cell.

Recovery of magnetite from low grade banded magnetite quartzite (BMQ) ore

NASA Astrophysics Data System (ADS)

Tripathy, Alok; Bagchi, Subhankar; Rao, Danda Srinivas; Nayak, Bijaya Ketana; Rout, Prashanta Kumar; Biswal, Surendra Kumar

2018-04-01

There has been a steady increase of iron ore demand in the last few decades. This growing demand could be countered by use of low grade iron ore after beneficiation. Banded iron formations (BIF) are one of the resources of such low grade iron ores. Banded magnetite quartzite (BMQ) is one such BIF and a source of iron phase mineral in the form of magnetite. In the present study a low grade BMQ ore containing around 25.47% Fe was beneficiated for recovery of magnetite. XRD study shows that quartz, magnetite, hematite, and goethite are the major minerals phases present in the low grade BMQ sample. Unit operations such as crushing, scrubbing, grinding, and magnetic separations were used for recovering magnetite. Based on the large scale beneficiation studies the process flowsheet has been developed for enrichment of magnetite. It was found that with the help of developed process flowsheet it is possible to enrich Fe value up to 65.14% in the concentrate with a yield of 24.59%.

Extraction of dielectric and magnetic properties of carbonyl iron powder composites at high frequencies

NASA Astrophysics Data System (ADS)

Zivkovic, I.; Murk, A.

2012-06-01

In this paper, we examine carbonyl iron composites in silicone rubber and epoxy matrices. Transmission measurements were performed at W (70 to 110 GHz) and Ka (26 to 40 GHz) bands and effective permittivity and permeability of composites with 10% volume fraction of carbonyl iron powder (CIP) were extracted at these frequencies. To extract permittivity and permeability of carbonyl iron powder in W and Ka bands, we use Looyenga formula. We extract permittivity and permeability of CIP from both silicone rubber and epoxy based composites and good agreement is achieved.

Iron Deficiency Induces a Partial Inhibition of the Photosynthetic Electron Transport and a High Sensitivity to Light in the Diatom Phaeodactylum tricornutum.

PubMed

Roncel, Mercedes; González-Rodríguez, Antonio A; Naranjo, Belén; Bernal-Bayard, Pilar; Lindahl, Anna M; Hervás, Manuel; Navarro, José A; Ortega, José M

2016-01-01

Iron limitation is the major factor controlling phytoplankton growth in vast regions of the contemporary oceans. In this study, a combination of thermoluminescence (TL), chlorophyll fluorescence, and P700 absorbance measurements have been used to elucidate the effects of iron deficiency in the photosynthetic electron transport of the marine diatom P. tricornutum. TL was used to determine the effects of iron deficiency on photosystem II (PSII) activity. Excitation of iron-replete P. tricornutum cells with single turn-over flashes induced the appearance of TL glow curves with two components with different peaks of temperature and contributions to the total signal intensity: the B band (23°C, 63%), and the AG band (40°C, 37%). Iron limitation did not significantly alter these bands, but induced a decrease of the total TL signal. Far red excitation did not increase the amount of the AG band in iron-limited cells, as observed for iron-replete cells. The effect of iron deficiency on the photosystem I (PSI) activity was also examined by measuring the changes in P700 redox state during illumination. The electron donation to PSI was substantially reduced in iron-deficient cells. This could be related with the important decline on cytochrome c 6 content observed in these cells. Iron deficiency also induced a marked increase in light sensitivity in P. tricornutum cells. A drastic increase in the level of peroxidation of chloroplast lipids was detected in iron-deficient cells even when grown under standard conditions at low light intensity. Illumination with a light intensity of 300 μE m(-2) s(-1) during different time periods caused a dramatic disappearance in TL signal in cells grown under low iron concentration, this treatment not affecting to the signal in iron-replete cells. The results of this work suggest that iron deficiency induces partial blocking of the electron transfer between PSII and PSI, due to a lower concentration of the electron donor cytochrome c 6. This decreased electron transfer may induce the over-reduction of the plastoquinone pool and consequently the appearance of acceptor side photoinhibition in PSII even at low light intensities. The functionality of chlororespiratory electron transfer pathway under iron restricted conditions is also discussed.

Iron Deficiency Induces a Partial Inhibition of the Photosynthetic Electron Transport and a High Sensitivity to Light in the Diatom Phaeodactylum tricornutum

PubMed Central

Roncel, Mercedes; González-Rodríguez, Antonio A.; Naranjo, Belén; Bernal-Bayard, Pilar; Lindahl, Anna M.; Hervás, Manuel; Navarro, José A.; Ortega, José M.

2016-01-01

Iron limitation is the major factor controlling phytoplankton growth in vast regions of the contemporary oceans. In this study, a combination of thermoluminescence (TL), chlorophyll fluorescence, and P700 absorbance measurements have been used to elucidate the effects of iron deficiency in the photosynthetic electron transport of the marine diatom P. tricornutum. TL was used to determine the effects of iron deficiency on photosystem II (PSII) activity. Excitation of iron-replete P. tricornutum cells with single turn-over flashes induced the appearance of TL glow curves with two components with different peaks of temperature and contributions to the total signal intensity: the B band (23°C, 63%), and the AG band (40°C, 37%). Iron limitation did not significantly alter these bands, but induced a decrease of the total TL signal. Far red excitation did not increase the amount of the AG band in iron-limited cells, as observed for iron-replete cells. The effect of iron deficiency on the photosystem I (PSI) activity was also examined by measuring the changes in P700 redox state during illumination. The electron donation to PSI was substantially reduced in iron-deficient cells. This could be related with the important decline on cytochrome c6 content observed in these cells. Iron deficiency also induced a marked increase in light sensitivity in P. tricornutum cells. A drastic increase in the level of peroxidation of chloroplast lipids was detected in iron-deficient cells even when grown under standard conditions at low light intensity. Illumination with a light intensity of 300 μE m-2 s-1 during different time periods caused a dramatic disappearance in TL signal in cells grown under low iron concentration, this treatment not affecting to the signal in iron-replete cells. The results of this work suggest that iron deficiency induces partial blocking of the electron transfer between PSII and PSI, due to a lower concentration of the electron donor cytochrome c6. This decreased electron transfer may induce the over-reduction of the plastoquinone pool and consequently the appearance of acceptor side photoinhibition in PSII even at low light intensities. The functionality of chlororespiratory electron transfer pathway under iron restricted conditions is also discussed. PMID:27536301

Atomic layer epitaxy of GaN over sapphire using switched metalorganic chemical vapor deposition

NASA Astrophysics Data System (ADS)

Khan, M. A.; Skogman, R. A.; van Hove, J. M.; Olson, D. T.; Kuznia, J. N.

1992-03-01

In this letter the first switched atomic layer epitaxy (SALE) of single crystal GaN over basal plane sapphire substrates is reported. A low pressure metalorganic chemical vapor deposition (LPMOCVD) system was used for the epilayer depositions. In contrast to conventional LPMOCVD requiring temperatures higher than 700 C, the SALE process resulted in single crystal insulating GaN layers at growth temperatures ranging from 900 to 450 C. The band-edge transmission and the photoluminescence of the films from the SALE process were comparable to the best LPMOCVD films. As best as is known this is the first report of insulating GaN films which show excellent band-edge photoluminescence.

Atomic layer epitaxy of GaN over sapphire using switched metalorganic chemical vapor deposition

NASA Astrophysics Data System (ADS)

Asif Khan, M.; Skogman, R. A.; Van Hove, J. M.; Olson, D. T.; Kuznia, J. N.

1992-03-01

In this letter we report the first switched atomic layer epitaxy (SALE) of single crystal GaN over basal plane sapphire substrates. A low pressure metalorganic chemical vapor deposition (LPMOCVD) system was used for the epilayer depositions. In contrast to conventional LPMOCVD requiring temperatures higher than 700 °C, the SALE process resulted in single crystal insulating GaN layers at growth temperatures ranging from 900 to 450 °C. The band-edge transmission and the photoluminescence of the films from the SALE process were comparable to the best LPMOCVD films. To the best of our knowledge this is the first report of insulating GaN films which show excellent band-edge photoluminescence.

Gallium Disrupts Iron Uptake by Intracellular and Extracellular Francisella Strains and Exhibits Therapeutic Efficacy in a Murine Pulmonary Infection Model ▿

PubMed Central

Olakanmi, Oyebode; Gunn, John S.; Su, Shengchang; Soni, Shilpa; Hassett, Daniel J.; Britigan, Bradley E.

2010-01-01

Francisella tularensis requires iron (Fe) for growth, but the biologic sources of Fe for this organism are largely unknown. We found that Francisella sp. growing in broth culture or within human macrophages can acquire Fe from the two major host Fe-binding proteins, lactoferrin (Lf) and transferrin (Tf). Fe acquisition is a potential target for novel therapies. Gallium (Ga) is a transition metal that interferes with cellular Fe metabolism by competing with Fe for uptake/utilization. Growth of either F. tularensis live vaccine strain (LVS) or Francisella novicida was inhibited by ≥2 μM Ga chelated to Tf or Lf, with GaLf being somewhat more potent. Francisella spp. express two Fe-containing antioxidant enzymes, catalase (KatG) and Fe cofactored superoxide dismutase (FeSOD). Growth of LVS with 10 μM GaTf or GaLf led to a dramatic decrease in bacterial catalase activity and in FeSOD activity that was associated with an increased susceptibility to H2O2. Ga also protected mice from intranasal challenge with F. novicida. Whereas 100% of the F. novicida-infected mice died by day 9, 75% of the mice receiving Ga continued to survive to at least day 15. Thus, a single intranasal dose of Ga followed by daily intraperitoneal Ga at a dose tolerated by the animals resulted in prolonged survival. These data support the potential utility of Ga as a therapy for F. tularensis infection of the lung. PMID:19917753

Pseudogap and anharmonic phonon behavior in Ba{sub 8}Ga{sub 16}Ge{sub 30}: An NMR study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sirusi, Ali A., E-mail: alisirusi@tamu.edu; Ross, Joseph H., E-mail: jhross@tamu.edu; Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843

2016-08-07

We have performed {sup 69}Ga, {sup 71}Ga, and {sup 137}Ba NMR on Ba{sub 8}Ga{sub 16}Ge{sub 30}, a clathrate semiconductor which has been of considerable interest due to its large figure of merit for thermoelectric applications. In measurements from 4 K to 450 K, we used measurements on the two Ga nuclei to separate the magnetic and electric quadrupole hyperfine contributions and thereby gain information about the metallic and phonon behavior. The results show the presence of a pseudogap in the Ga electronic states within the conduction band, superposed upon a large Ba contribution to the conduction band. Meanwhile the phononmore » contributions to the Ga relaxation rates are large and increase more rapidly with temperature than typical semiconductors. These results provide evidence for enhanced anharmonicity of the propagative phonon modes over a wide range, providing experimental evidence for enhanced phonon-phonon scattering as a mechanism for the reduced thermal conductivity.« less

Role of Ga particulates on the structure and optical properties of Y3(Al,Ga)5O12:Tb thin films prepared by PLD

NASA Astrophysics Data System (ADS)

Yousif, A.; Duvenhage, M. M.; Ntwaeaborwa, O. M.; Swart, H. C.

2018-04-01

Y3(Al,Ga)5O12:Tb thin films (70 nm) have been prepared by pulsed laser deposition on a Si (100) substrate at the substrate temperature of 300 °C. The effect of annealing time on the structural, morphological and luminescence properties of Y3(Al,Ga)5O12:Tb thin films at 800 °C were studied. The crystal structure of the samples was studied by X- ray diffraction (XRD) and showed shifts in the peak positions to lower diffraction angles for the annealed film compared to the XRD peak positions of the commercial Y3(Al,Ga)5O12:Tb powder. A new excitation band different from the original Y3(Al,Ga)5O12:Tb powder was also observed for the annealed films. The shift in the XRD pattern and the new excitation band for the annealed film suggested that the films were enriched with Ga after annealing.

The Calculation Study of Electronic Properties of Doped RE (Eu, Er and Tm)-GaN using Density Functional Theory

NASA Astrophysics Data System (ADS)

Zaharo, Aflah; Purqon, Acep

2017-07-01

The calculation of the structure and electronic properties of Rare Earth (RE) at the wurtzite Gallium Nitride (GaN) based on DFT has completed. GGA approximation used for exchange correlation and Ultra soft pseudo potential too. The stability structure of GaN is seen that difference lattice parameter 11% lower than another calculation and experiment result. It is shown the stability structure GaN have direct band gap energy on Gamma point hexagonal lattice Brillouin zone. The width Eg is 2.6 eV. When one atom Ga is substituted with one atom RE, the bond length is change 12 % longest. An in good agreement with theoretical doping RE concentration increases, the edge of energy level shifted towards to make the band gap narrow which is allow the optical transitions and help to improve the optical performance of GaN. The RE doped GaN is potentially applicable for various color of LED with lower energy consumption and potentially energy saving application

Type-II GaSb/GaAs quantum-dot intermediate band with extended optical absorption range for efficient solar cells

NASA Astrophysics Data System (ADS)

Boustanji, Hela; Jaziri, Sihem

2018-02-01

GaSb/GaAs type-II quantum-dot solar cells (QD SCs) have attracted attention as highly efficient intermediate band SCs due to their infrared absorption. Type-II QDs exhibited a staggered confinement potential, where only holes are strongly confined within the dots. Long wavelength light absorption of the QDSCs is enhanced through the improved carriers number in the IB. The absorption of dots depends on their shape, material quality, and composition. Therefore, the optical properties of the GaSbGaAs QDs before and after thermal treatment are studied. Our intraband studies have shown an extended absorption into the long wavelength region 1.77 μ {m}. The annealed QDs have shown significantly more infrared response of 7.2 μ {m} compared to as-grown sample. The photon absorption and hole extraction depend strongly on the thermal annealing process. In this context, emission of holes from localized states in GaSb QDs has been studied using conductance-voltage ( G- V ) characteristics.

Formation and photoluminescence of GaAs1-xNx dilute nitride achieved by N-implantation and flash lamp annealing

NASA Astrophysics Data System (ADS)

Gao, Kun; Prucnal, S.; Skorupa, W.; Helm, M.; Zhou, Shengqiang

2014-07-01

In this paper, we present the fabrication of dilute nitride semiconductor GaAs1-xNx by nitrogen-ion-implantation and flash lamp annealing (FLA). N was implanted into the GaAs wafers with atomic concentration of about ximp1 = 0.38% and ximp2 = 0.76%. The GaAs1-xNx layer is regrown on GaAs during FLA treatment in a solid phase epitaxy process. Room temperature near band-edge photoluminescence (PL) has been observed from the FLA treated GaAs1-xNx samples. According to the redshift of the near band-edge PL peak, up to 80% and 44% of the implanted N atoms have been incorporated into the lattice by FLA for ximp1 = 0.38% and ximp2 = 0.76%, respectively. Our investigation shows that ion implantation followed by ultrashort flash lamp treatment, which allows for large scale production, exhibits a promising prospect on bandgap engineering of GaAs based semiconductors.

Novel BTlGaN semiconducting materials for infrared opto-electronic devices

NASA Astrophysics Data System (ADS)

Assali, Abdenacer; Bouslama, M'hamed

2017-03-01

BTlGaN quaternary alloys are proposed as new semiconductor materials for infrared opto-electronic applications. The structural and opto-electronic properties of zinc blende BxTlyGa1-x-yN alloys lattice matched to GaN with (0 ⩽ x and y ⩽ 0.187) are studied using density functional theory (DFT) within full-potential linearized augmented plane wave (FP-LAPW) method. The calculated structural parameters such as lattice constant a0 and bulk modulus B0 are found to be in good agreement with experimental data using the new form of generalized gradient approximation (GGA-WC). The band gaps of the compounds are also found very close to the experimental results using the recently developed Tran-Blaha-modified Becke-Johnson (TB-mBJ) exchange potential. A quaternary BxTlyGa1-x-yN is expected to be lattice matched to the GaN substrate with concentrations x = 0.125 and y = 0.187 allows to produce high interface layers quality. It has been found that B incorporation into BTlGaN does not significantly affect the band gap, while the addition of dilute Tl content leads to induce a strong reduction of the band gap, which in turn increases the emission wavelengths to the infrared region. The refractivity, reflectivity and absorption coefficient of these alloys were investigated. BTlGaN/GaN is an interesting new material to be used as active layer/barriers in quantum wells suitable for realizing advanced Laser Diodes and Light-Emitting Diodes as new sources of light emitting in the infrared spectrum region.

Energy bands and acceptor binding energies of GaN

NASA Astrophysics Data System (ADS)

Xia, Jian-Bai; Cheah, K. W.; Wang, Xiao-Liang; Sun, Dian-Zhao; Kong, Mei-Ying

1999-04-01

The energy bands of zinc-blende and wurtzite GaN are calculated with the empirical pseudopotential method, and the pseudopotential parameters for Ga and N atoms are given. The calculated energy bands are in agreement with those obtained by the ab initio method. The effective-mass theory for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN for both structures are given. The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor are 24 and 142 meV for the zinc-blende structure, 20 and 131, and 97 meV for the wurtzite structure, respectively, which are consistent with recent experimental results. It is proposed that there are two kinds of acceptor in wurtzite GaN. One kind is the general acceptor such as C, which substitutes N, which satisfies the effective-mass theory. The other kind of acceptor includes Mg, Zn, Cd, etc., the binding energy of these acceptors is deviated from that given by the effective-mass theory. In this report, wurtzite GaN is grown by the molecular-beam epitaxy method, and the photoluminescence spectra were measured. Three main peaks are assigned to the donor-acceptor transitions from two kinds of acceptors. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material.

Landau levels and shallow donor states in GaAs/AlGaAs multiple quantum wells at megagauss magnetic fields

NASA Astrophysics Data System (ADS)

Zybert, M.; Marchewka, M.; Sheregii, E. M.; Rickel, D. G.; Betts, J. B.; Balakirev, F. F.; Gordon, M.; Stier, A. V.; Mielke, C. H.; Pfeffer, P.; Zawadzki, W.

2017-03-01

Landau levels and shallow donor states in multiple GaAs/AlGaAs quantum wells (MQWs) are investigated by means of the cyclotron resonance at megagauss magnetic fields. Measurements of magneto-optical transitions were performed in pulsed fields up to 140 T and temperatures from 6-300 K. The 14 ×14 P.p band model for GaAs is used to interpret free-electron transitions in a magnetic field. Temperature behavior of the observed resonant structure indicates, in addition to the free-electron Landau states, contributions of magnetodonor states in the GaAs wells and possibly in the AlGaAs barriers. The magnetodonor energies are calculated using a variational procedure suitable for high magnetic fields and accounting for conduction band nonparabolicity in GaAs. It is shown that the above states, including their spin splitting, allow one to interpret the observed magneto-optical transitions in MQWs in the middle infrared region. Our experimental and theoretical results at very high magnetic fields are consistent with the picture used previously for GaAs/AlGaAs MQWs at lower magnetic fields.

Mutations in the HFE, TFR2, and SLC40A1 genes in patients with hemochromatosis.

PubMed

Del-Castillo-Rueda, Alejandro; Moreno-Carralero, María-Isabel; Cuadrado-Grande, Nuria; Alvarez-Sala-Walther, Luis-Antonio; Enríquez-de-Salamanca, Rafael; Méndez, Manuel; Morán-Jiménez, María-Josefa

2012-10-15

Hereditary hemochromatosis causes iron overload and is associated with a variety of genetic and phenotypic conditions. Early diagnosis is important so that effective treatment can be administered and the risk of tissue damage avoided. Most patients are homozygous for the c.845G>A (p.C282Y) mutation in the HFE gene; however, rare forms of genetic iron overload must be diagnosed using a specific genetic analysis. We studied the genotype of 5 patients who had hyperferritinemia and an iron overload phenotype, but not classic mutations in the HFE gene. Two patients were undergoing phlebotomy and had no iron overload, 1 with metabolic syndrome and no phlebotomy had mild iron overload, and 2 patients had severe iron overload despite phlebotomy. The patients' first-degree relatives also underwent the analysis. We found 5 not previously published mutations: c.-408_-406delCAA in HFE, c.1118G>A (p.G373D), c.1473G>A (p.E491E) and c.2085G>C (p.S695S) in TFR2; and c.-428_-427GG>TT in SLC40A1. Moreover, we found 3 previously published mutations: c.221C>T (p.R71X) in HFE; c.1127C>A (p.A376D) in TFR2; and c.539T>C (p.I180T) in SLC40A1. Four patients were double heterozygous or compound heterozygous for the mutations mentioned above, and the patient with metabolic syndrome was heterozygous for a mutation in the TFR2 gene. Our findings show that hereditary hemochromatosis is clinically and genetically heterogeneous and that acquired factors may modify or determine the phenotype. Copyright © 2012. Published by Elsevier B.V.

Wurtzite/zinc-blende electronic-band alignment in basal-plane stacking faults in semi-polar GaN

NASA Astrophysics Data System (ADS)

Monavarian, Morteza; Hafiz, Shopan; Izyumskaya, Natalia; Das, Saikat; Özgür, Ümit; Morkoç, Hadis; Avrutin, Vitaliy

2016-02-01

Heteroepitaxial semipolar and nonpolar GaN layers often suffer from high densities of extended defects including basal plane stacking faults (BSFs). BSFs which are considered as inclusions of cubic zinc-blende phase in wurtzite matrix act as quantum wells strongly affecting device performance. Band alignment in BSFs has been discussed as type of band alignment at the wurtzite/zinc blende interface governs the response in differential transmission; fast decay after the pulse followed by slow recovery due to spatial splitting of electrons and heavy holes for type- II band alignment in contrast to decay with no recovery in case of type I band alignment. Based on the results, band alignment is demonstrated to be of type II in zinc-blende segments in wurtzite matrix as in BSFs.

Emission dynamics of hybrid plasmonic gold/organic GaN nanorods

NASA Astrophysics Data System (ADS)

Mohammadi, F.; Schmitzer, H.; Kunert, G.; Hommel, D.; Ge, J.; Duscher, G.; Langbein, W.; Wagner, H. P.

2017-12-01

We studied the emission of bare and aluminum quinoline (Alq3)/gold coated wurtzite GaN nanorods by temperature- and intensity-dependent time-integrated and time-resolved photoluminescence (PL). The GaN nanorods of ˜1.5 μm length and ˜250 nm diameter were grown by plasma-assisted molecular beam epitaxy. Gold/Alq3 coated GaN nanorods were synthesized by organic molecular beam deposition. The near band-edge and donor-acceptor pair luminescence was investigated in bare GaN nanorods and compared with multilevel model calculations providing the dynamical parameters for electron-hole pairs, excitons, impurity bound excitons, donors and acceptors. Subsequently, the influence of a 10 nm gold coating without and with an Alq3 spacer layer was studied and the experimental results were analyzed with the multilevel model. Without a spacer layer, a significant PL quenching and lifetime reduction of the near band-edge emission is found. The behavior is attributed to surface band-bending and Förster energy transfer from excitons to surface plasmons in the gold layer. Inserting a 5 nm Alq3 spacer layer reduces the PL quenching and lifetime reduction which is consistent with a reduced band-bending and Förster energy transfer. Increasing the spacer layer to 30 nm results in lifetimes which are similar to uncoated structures, showing a significantly decreased influence of the gold coating on the excitonic dynamics.

Emission dynamics of hybrid plasmonic gold/organic GaN nanorods.

PubMed

Mohammadi, F; Schmitzer, H; Kunert, G; Hommel, D; Ge, J; Duscher, G; Langbein, W; Wagner, H P

2017-12-15

We studied the emission of bare and aluminum quinoline (Alq 3 )/gold coated wurtzite GaN nanorods by temperature- and intensity-dependent time-integrated and time-resolved photoluminescence (PL). The GaN nanorods of ∼1.5 μm length and ∼250 nm diameter were grown by plasma-assisted molecular beam epitaxy. Gold/Alq 3 coated GaN nanorods were synthesized by organic molecular beam deposition. The near band-edge and donor-acceptor pair luminescence was investigated in bare GaN nanorods and compared with multilevel model calculations providing the dynamical parameters for electron-hole pairs, excitons, impurity bound excitons, donors and acceptors. Subsequently, the influence of a 10 nm gold coating without and with an Alq 3 spacer layer was studied and the experimental results were analyzed with the multilevel model. Without a spacer layer, a significant PL quenching and lifetime reduction of the near band-edge emission is found. The behavior is attributed to surface band-bending and Förster energy transfer from excitons to surface plasmons in the gold layer. Inserting a 5 nm Alq 3 spacer layer reduces the PL quenching and lifetime reduction which is consistent with a reduced band-bending and Förster energy transfer. Increasing the spacer layer to 30 nm results in lifetimes which are similar to uncoated structures, showing a significantly decreased influence of the gold coating on the excitonic dynamics.

Temperature dependent empirical pseudopotential theory for self-assembled quantum dots.

PubMed

Wang, Jianping; Gong, Ming; Guo, Guang-Can; He, Lixin

2012-11-28

We develop a temperature dependent empirical pseudopotential theory to study the electronic and optical properties of self-assembled quantum dots (QDs) at finite temperature. The theory takes the effects of both lattice expansion and lattice vibration into account. We apply the theory to InAs/GaAs QDs. For the unstrained InAs/GaAs heterostructure, the conduction band offset increases whereas the valence band offset decreases with increasing temperature, and there is a type-I to type-II transition at approximately 135 K. Yet, for InAs/GaAs QDs, the holes are still localized in the QDs even at room temperature, because the large lattice mismatch between InAs and GaAs greatly enhances the valence band offset. The single-particle energy levels in the QDs show a strong temperature dependence due to the change of confinement potentials. Because of the changes of the band offsets, the electron wavefunctions confined in QDs increase by about 1-5%, whereas the hole wavefunctions decrease by about 30-40% when the temperature increases from 0 to 300 K. The calculated recombination energies of excitons, biexcitons and charged excitons show red shifts with increasing temperature which are in excellent agreement with available experimental data.

Sub-band-gap absorption in Ga2O3

NASA Astrophysics Data System (ADS)

Peelaers, Hartwin; Van de Walle, Chris G.

2017-10-01

β-Ga2O3 is a transparent conducting oxide that, due to its large bandgap of 4.8 eV, exhibits transparency into the UV. However, the free carriers that enable the conductivity can absorb light. We study the effect of free carriers on the properties of Ga2O3 using hybrid density functional theory. The presence of free carriers leads to sub-band-gap absorption and a Burstein-Moss shift in the onset of absorption. We find that for a concentration of 1020 carriers, the Fermi level is located 0.23 eV above the conduction-band minimum. This leads to an increase in the electron effective mass from 0.27-0.28 me to 0.35-0.37 me and a sub-band-gap absorption band with a peak value of 0.6 × 103 cm-1 at 3.37 eV for light polarized along the x or z direction. Both across-the-gap and free-carrier absorption depend strongly on the polarization of the incoming light. We also provide parametrizations of the conduction-band shape and the effective mass as a function of the Fermi level.

Tuning the Electronic, Optical, and Magnetic Properties of Monolayer GaSe with a Vertical Electric Field

NASA Astrophysics Data System (ADS)

Ke, Congming; Wu, Yaping; Guo, Guang-Yu; Lin, Wei; Wu, Zhiming; Zhou, Changjie; Kang, Junyong

2018-04-01

Inspired by two-dimensional material with their unique physical properties and innovative device applications, here we report a design framework on monolayer GaSe, an important member of the two-dimensional material family, in an effort to tune the electronic, optical, and magnetic properties through a vertical electric field. A transition from indirect to direct band gap in monolayer GaSe is found with an electric field of 0.09 V /Å . The giant Stark effect results in a reduction of the band gap with a Stark coefficient of 3.54 Å. Optical and dielectric properties of monolayer GaSe are dependent on the vertical electric field. A large regulation range for polarization E ∥c ^ is found for the static dielectric constant. The optical anisotropy with the dipole transition from E ∥c ^ to E ⊥c ^ is achieved. Induced by the spin-orbit coupling, spin-splitting energy at the valence band maximum increases linearly with the electric field. The effective mass of holes is highly susceptible to the vertical electric field. Switchable spin-polarization features in spin texture of monolayer GaSe are predicted. The tunable electronic, optical, and magnetic properties of monolayer GaSe hold great promise for applications in both the optoelectronic and spintronic devices.

Effects of Thickness of a Low-Temperature Buffer and Impurity Incorporation on the Characteristics of Nitrogen-polar GaN.

PubMed

Yang, Fann-Wei; Chen, Yu-Yu; Feng, Shih-Wei; Sun, Qian; Han, Jung

2016-12-01

In this study, effects of the thickness of a low temperature (LT) buffer and impurity incorporation on the characteristics of Nitrogen (N)-polar GaN are investigated. By using either a nitridation or thermal annealing step before the deposition of a LT buffer, three N-polar GaN samples with different thicknesses of LT buffer and different impurity incorporations are prepared. It is found that the sample with the thinnest LT buffer and a nitridation step proves to be the best in terms of a fewer impurity incorporations, strong PL intensity, fast mobility, small biaxial strain, and smooth surface. As the temperature increases at ~10 K, the apparent donor-acceptor-pair band is responsible for the decreasing integral intensity of the band-to-band emission peak. In addition, the thermal annealing of the sapphire substrates may cause more impurity incorporation around the HT-GaN/LT-GaN/sapphire interfacial regions, which in turn may result in a lower carrier mobility, larger biaxial strain, larger bandgap shift, and stronger yellow luminescence. By using a nitridation step, both a thinner LT buffer and less impurity incorporation are beneficial to obtaining a high quality N-polar GaN.

Ultrafast Hot Carrier Dynamics in GaN and Its Impact on the Efficiency Droop.

PubMed

Jhalani, Vatsal A; Zhou, Jin-Jian; Bernardi, Marco

2017-08-09

GaN is a key material for lighting technology. Yet, the carrier transport and ultrafast dynamics that are central in GaN light-emitting devices are not completely understood. We present first-principles calculations of carrier dynamics in GaN, focusing on electron-phonon (e-ph) scattering and the cooling and nanoscale dynamics of hot carriers. We find that e-ph scattering is significantly faster for holes compared to electrons and that for hot carriers with an initial 0.5-1 eV excess energy, holes take a significantly shorter time (∼0.1 ps) to relax to the band edge compared to electrons, which take ∼1 ps. The asymmetry in the hot carrier dynamics is shown to originate from the valence band degeneracy, the heavier effective mass of holes compared to electrons, and the details of the coupling to different phonon modes in the valence and conduction bands. We show that the slow cooling of hot electrons and their long ballistic mean free paths (over 3 nm at room temperature) are a possible cause of efficiency droop in GaN light-emitting diodes. Taken together, our work sheds light on the ultrafast dynamics of hot carriers in GaN and the nanoscale origin of efficiency droop.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zybert, M.; Marchweka, M.; Sheregii, E. M.

Landau levels and shallow donor states in multiple GaAs/AlGaAs quantum wells (MQWs) are investigated by means of the cyclotron resonance at mega-gauss magnetic fields. Measurements of magneto-optical transitions were performed in pulsed fields up to 140 T and temperatures from 6 to 300 K. The 14 x 14 P.p band model for GaAs is used to interpret free-electron transitions in a magnetic field. Temperature behavior of the observed resonant structure indicates, in addition to the free-electron Landau states, contributions of magneto-donor states in the GaAs wells and possibly in the AlGaAs barriers. The magneto-donor energies are calculated using a variationalmore » procedure suitable for high magnetic fields and accounting for conduction band nonparabolicity in GaAs. It is shown that the above states, including their spin splitting, allow one to interpret the observed mengeto-optical transitions in MQWs in the middle infrared region. Our experimental and theoretical results at very high magnetic fields are consistent with the picture used previously for GaAs/AlGaAs MQWs at lower magnetic fields.« less

Spin Decoherence in III-V Quantum Wells and Superlattices

NASA Astrophysics Data System (ADS)

Lau, Wayne H.; Flatté, Michael E.

2001-03-01

Electron spin decoherence in zincblende type quantum wells (QW) and superlattices (SL) near room temperature is dominated by the precessional D'yakonov-Perel' (DP) mechanism. The effective precession is a direct result of the spin splitting of the conduction band due to bulk inversion asymmetry (BIA) of the constituent zincblende semiconductors and also to any native interface asymmetry (NIA) of the heterointerfaces. The effect of BIA is dominant in common atom (CA) systems such as GaAs/AlGaAs QWs. However, in no common atom (NCA) systems such as InAs/GaSb, the interface bonds are different in character from those in the bulk and are asymmetrically oriented (giving rise to NIA). To accurately describe the DP spin relaxation mechanism we employ a nonperturbative nanostructure model based on a fourteen-bulk-band basis, including both BIA and NIA. Quantitative agreement between these calculations and measurements is found for GaAs/AlGaAs, InGaAs/InP, and GaSb/AlSb QW's, as well as for an InAs/GaSb SL.

p-GaN/n-ZnO heterojunction nanowires: optoelectronic properties and the role of interface polarity.

PubMed

Schuster, Fabian; Laumer, Bernhard; Zamani, Reza R; Magén, Cesar; Morante, Joan Ramon; Arbiol, Jordi; Stutzmann, Martin

2014-05-27

In this work, simulations of the electronic band structure of a p-GaN/n-ZnO heterointerface are presented. In contrast to homojunctions, an additional energy barrier due to the type-II band alignment hinders the flow of majority charge carriers in this heterojunction. Spontaneous polarization and piezoelectricity are shown to additionally affect the band structure and the location of the recombination region. Proposed as potential UV-LEDs and laser diodes, p-GaN/n-ZnO heterojunction nanowires were fabricated by plasma-assisted molecular beam epitaxy (PAMBE). Atomic resolution annular bright field scanning transmission electron microscopy (STEM) studies reveal an abrupt and defect-free heterointerface with a polarity inversion from N-polar GaN to Zn-polar ZnO. Photoluminescence measurements show strong excitonic UV emission originating from the ZnO-side of the interface as well as stimulated emission in the case of optical pumping above a threshold of 55 kW/cm(2).

Temperature dependence of quantized states in strained-layer In0.21Ga0.79As/GaAs single quantum well

NASA Astrophysics Data System (ADS)

Chi, Wuh-Sheng; Huang, Ying-Sheng; Qiang, Hao; Pollak, Fred H.; Pettit, David G.; Woodall, Jerry M.

1994-02-01

The piezoreflectance (PzR) and photoreflectance (PR) measurements of a strained-layer (001) In0.21Ga0.79As/GaAs single quantum well as a function of temperature in the range of 20 to 300 K have been carried out. A careful analysis of the PzR and PR spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state to the nth heavy (light)-hole band state. The parameters that describe the temperature dependence of E(sub mnH(L)) are evaluated. The detailed study of the temperature variation of excitonic transition energies indicates that the main influence of temperature on quantized transitions is through the temperature dependence of the band gap of the constituent material in the well. The temperature dependence of the linewidth of the 11H exciton is evaluated and compared with that of the bulk material.

Temperature Dependence of Quantized States in Strained-Layer In0.21Ga0.79As/GaAs Single Quantum Well

NASA Astrophysics Data System (ADS)

Chi, Wuh-Sheng; Huang, Ying-Sheng; Qiang, Hao; Pollak, Fred; Pettit, David; Woodall, Jerry

1994-02-01

The piezoreflectance (PzR) and photoreflectance (PR) measurements of a strained-layer (001) In0.21Ga0.79As/GaAs single quantum well as a function of temperature in the range of 20 to 300 K have been carried out. A careful analysis of the PzR and PR spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state to the nth heavy (light)-hole band state. The parameters that describe the temperature dependence of E mnH(L) are evaluated. The detailed study of the temperature variation of excitonic transition energies indicates that the main influence of temperature on quantized transitions is through the temperature dependence of the band gap of the constituent material in the well. The temperature dependence of the linewidth of the 11H exciton is evaluated and compared with that of the bulk material.

Rashba effect and enriched spin-valley coupling in Ga X /M X2 (M = Mo, W; X = S, Se, Te) heterostructures

NASA Astrophysics Data System (ADS)

Zhang, Qingyun; Schwingenschlögl, Udo

2018-04-01

Using first-principles calculations, we investigate the electronic properties of the two-dimensional Ga X /MX 2 (M = Mo, W; X = S, Se, Te) heterostructures. Orbital hybridization between Ga X and MX 2 is found to result in Rashba splitting at the valence-band edge around the Γ point, which grows for increasing strength of the spin-orbit coupling in the p orbitals of the chalcogenide atoms. The location of the valence-band maximum in the Brillouin zone can be tuned by strain and application of an out-of-plane electric field. The coexistence of Rashba splitting (in-plane spin direction) and band splitting at the K and K' valleys (out-of-plane spin direction) makes Ga X /MX 2 heterostructures interesting for spintronics and valleytronics. They are promising candidates for two-dimensional spin-field-effect transistors and spin-valley Hall effect devices. Our findings shed light on the spin-valley coupling in van der Waals heterostructures.

Remarkable optical red shift and extremely high optical absorption coefficient of V-Ga co-doped TiO2

NASA Astrophysics Data System (ADS)

Deng, Quanrong; Han, Xiaoping; Gao, Yun; Shao, Guosheng

2012-07-01

A first attempt has been made to study the effect of codoping of transition metal and sp metal on the electronic structure and associated optical properties of TiO2, through V-Ga codoped thin films. V-Ga codoped rutile TiO2 films were fabricated on fused quartz substrates using pulsed laser ablation, followed by heat treatment at high temperatures. Gigantic redshift in the optical absorption edge was observed in V-Ga co-doped TiO2 materials, from UV to infrared region with high absorption coefficient. Through combined structural characterization and theoretical modeling, this is attributed to the p-d hybridization between the two metals. This leads to additional energy bands to overlap with the minimum of the conduction band, leading to remarkably narrowed band gap free of mid-gap states. The direct-gap of the co-doped phase is key to the remarkably high optical absorption coefficient of the coped titania.

Effects of post-deposition annealing ambient on band alignment of RF magnetron-sputtered Y2O3 film on gallium nitride

PubMed Central

2013-01-01

The effects of different post-deposition annealing ambients (oxygen, argon, forming gas (95% N2 + 5% H2), and nitrogen) on radio frequency magnetron-sputtered yttrium oxide (Y2O3) films on n-type gallium nitride (GaN) substrate were studied in this work. X-ray photoelectron spectroscopy was utilized to extract the bandgap of Y2O3 and interfacial layer as well as establishing the energy band alignment of Y2O3/interfacial layer/GaN structure. Three different structures of energy band alignment were obtained, and the change of band alignment influenced leakage current density-electrical breakdown field characteristics of the samples subjected to different post-deposition annealing ambients. Of these investigated samples, ability of the sample annealed in O2 ambient to withstand the highest electric breakdown field (approximately 6.6 MV/cm) at 10−6 A/cm2 was related to the largest conduction band offset of interfacial layer/GaN (3.77 eV) and barrier height (3.72 eV). PMID:23360596

Optical study of the band structure of wurtzite GaP nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Assali, S., E-mail: simone.assali@polymtl.ca; Greil, J.; Zardo, I.

2016-07-28

We investigated the optical properties of wurtzite (WZ) GaP nanowires by performing photoluminescence (PL) and time-resolved PL measurements in the temperature range from 4 K to 300 K, together with atom probe tomography to identify residual impurities in the nanowires. At low temperature, the WZ GaP luminescence shows donor-acceptor pair emission at 2.115 eV and 2.088 eV, and Burstein-Moss band-filling continuum between 2.180 and 2.253 eV, resulting in a direct band gap above 2.170 eV. Sharp exciton α-β-γ lines are observed at 2.140–2.164–2.252 eV, respectively, showing clear differences in lifetime, presence of phonon replicas, and temperature-dependence. The excitonic nature of those peaks is critically discussed, leading tomore » a direct band gap of ∼2.190 eV and to a resonant state associated with the γ-line ∼80 meV above the Γ{sub 8C} conduction band edge.« less

Application of the bounds-analysis approach to arsenic and gallium antisite defects in gallium arsenide

DOE PAGES

Wright, A. F.; Modine, N. A.

2015-01-23

The As antisite in GaAs (AsGa) has been the subject of numerous experimental and theoretical studies. Recent density-functional-theory (DFT) studies report results in good agreement with experimental data for the +2, +1, and 0 charge states of the stable EL2 structure, the 0 charge state of the metastable EL2* structure, and the activation energy to transform from EL2* to EL2 in the 0 charge state. However, these studies did not report results for EL2* in the -1 charge state. In this paper, we report new DFT results for the +2, +1, 0, and -1 charge states of AsGa, obtained usingmore » a semilocal exchange-correlation functional and interpreted using a bounds-analysis approach. In good agreement with experimental data, we find a -1/0 EL2* level 0.06 eV below the conduction-band edge and an activation energy of 0.05 eV to transform from EL2* to EL2 in the -1 charge state. While the Ga antisite in GaAs (GaAs) has not been studied as extensively as AsGa, experimental studies report three charge states (-2, -1, 0) and two levels (-2/-1, -1/0) close to the valence-band edge. Recent DFT studies report the same charge states, but the levels are found to be well-separated from the valence-band edge. To resolve this disagreement, we performed new DFT calculations for GaAs and interpreted them using a bounds analysis. The analysis identified the -1 and 0 charge states as hole states weakly bound to a highly-localized -2 charge state. Moreover, the -2/-1, -1/0 levels were found to be near the valence-band edge, in good agreement with the experimental data.« less

Analysis of InxGa1-xN/Si p-n heterojunction solar cells and the effects of spontaneous and piezoelectric polarization charges

NASA Astrophysics Data System (ADS)

Zheng, Yangdong; Mihara, Akihiro; Yamamoto, Akio

2013-10-01

The band structures, current-voltage characteristics under solar illumination, and photovoltaic (PV) properties of InxGa1-xN/Si p-n heterojunction solar cells (SCs), as well as the effects of spontaneous and piezoelectric polarization (Psp-PPZ) induced charges are investigated theoretically and numerically. We find that the energy peaks on the conduction and valence bands could exponentially reduce the diffusion currents and photocurrents, thus profoundly affect the PV properties of the SCs. Except for large values, the Psp-PPZ induced interface charges have little influence on the band structures and the PV properties. These results should be useful in analysis and design for multijunction tandem InxGa1-xN/Si SC devices.

Determination of the band gap of indium-rich InGaN by means of photoacoustic spectroscopy

NASA Astrophysics Data System (ADS)

Oliva, Robert; Zelewski, Szymon J.; Janicki, Łukasz; Gwóźdź, Katarzyna R.; Serafińczuk, Jarosław; Rudziński, Mariusz; Özbay, Ekmel; Kudrawiec, Robert

2018-03-01

Photoacoustic (PA) measurements have been performed on a series of In x Ga1-x N thin films grown with x > 50%. In order to illustrate the usefulness of this technique, these measurements have been compared with the results obtained by the following conventional techniques: photoluminescence, transmittance and contactless electroreflectance. Amongst all these techniques, only PA spectroscopy exhibited signal without the undesired Fabry-Perot interferences arising from the thin film and buffer layer. By accurately assessing the strain state and composition of our samples, we were able to study the compositional dependence of the band gap of our epilayers. Our results show that a bowing parameter of 1.43 eV successfully describes the compositional dependence of the band gap of InGaN.

Characterisation and Processing of Some Iron Ores of India

NASA Astrophysics Data System (ADS)

Krishna, S. J. G.; Patil, M. R.; Rudrappa, C.; Kumar, S. P.; Ravi, B. P.

2013-10-01

Lack of process characterization data of the ores based on the granulometry, texture, mineralogy, physical, chemical, properties, merits and limitations of process, market and local conditions may mislead the mineral processing entrepreneur. The proper implementation of process characterization and geotechnical map data will result in optimized sustainable utilization of resource by processing. A few case studies of process characterization of some Indian iron ores are dealt with. The tentative ascending order of process refractoriness of iron ores is massive hematite/magnetite < marine black iron oxide sands < laminated soft friable siliceous ore fines < massive banded magnetite quartzite < laminated soft friable clayey aluminous ore fines < massive banded hematite quartzite/jasper < massive clayey hydrated iron oxide ore < manganese bearing iron ores massive < Ti-V bearing magnetite magmatic ore < ferruginous cherty quartzite. Based on diagnostic process characterization, the ores have been classified and generic process have been adopted for some Indian iron ores.

Insertion of NiO electron blocking layer in fabrication of GaN-organic heterostructures

NASA Astrophysics Data System (ADS)

Li, Junmei; Guo, Wei; Jiang, Jie'an; Gao, Pingqi; Bo, Baoxue; Ye, Jichun

2018-03-01

We report the fabrication of a NiO thin film on top of an n-type GaN epitaxial layer. The electron-blocking capability of NiO in a hybrid organic/inorganic heterostructure consisting of n-GaN/NiO/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is discussed. Surface morphology, crystallography orientation, bandgap, and fermi level information of NiO films were investigated in detail. A rectifying property consistent with the proposed band diagram was observed in the current-voltage measurement. Theoretical analysis also demonstrated the effective electron blocking due to band alignment and a more balanced carrier distribution inside the GaN region with NiO inserted into the n-GaN/PEDOT:PSS heterostructure. This work provides a promising approach to the fabrication of high-efficiency hybrid optoelectronic devices.

Molecular fossils and the late rise of oxygenic photosynthesis

NASA Astrophysics Data System (ADS)

Brocks, J. J.

2012-04-01

Biomarkers are the molecular fossils of natural products such as lipids and pigments. They can yield a wealth of information about early microbial ecosystems and are particularly valuable when preserved in > 1 billion-year old (Ga) sedimentary rocks where conventional fossils are often lacking. Therefore, in 1999, the detection of traces of biomarkers in 2.5 to 2.7 Ga shales from Western Australia (Brocks et al. 1999, Summons et al. 1999) was celebrated as a breakthrough. The discovery, which was later confirmed by several independent studies, led to far reaching conclusions about the early evolution of oxygenic photosynthesis (Summons et al. 1999) and ancestral eukaryotes (Brocks et al. 1999). However, here we present new data based on the carbon isotopic composition of solidified hydrocarbons (Rasmussen et al. 2008) and the spatial distribution of liquid hydrocarbons within the original 2.5 and 2.7 Ga shales (Brocks 2011) that demonstrate that the molecules must have entered the rocks much later in Earth's history and therefore provide no information about the Archean (>2.5 Ga) biosphere or environment. The elimination of the Archean biomarker data has immense implications for our understanding of Earth's early biosphere. 2-Methylhopanes have been interpreted as evidence for the existence of cyanobacteria at 2.7 Ga, about ~300 million years before the atmosphere became mildly oxygenated in the Great Oxidation Event (GOE; between 2.45 and 2.32 Ga). Now, the oldest direct fossil evidence for cyanobacteria reverts back to 2.15 Ga, and the most ancient robust sign for oxygenic photosynthesis becomes the GOE itself. Moreover, the presence of steranes has been interpreted as evidence for the existence of ancestral eukaryotes at 2.7 Ga. However, without the steranes, the oldest fossil evidence for the domain falls into the range ~1.78-1.68 Ga. Recognition that the biomarkers from Archean rocks are not of Archean age renders permissive hypotheses about a late evolution of oxygenic photosynthesis, and an anoxygenic phototrophic origin of the vast deposits of Archean banded iron formation. Brocks et al. (1999) Science 285, 1033-1036. Brocks (2011) Geochim. Cosmochim. Acta, 75, 3196-3213. Rasmussen et al. (2008) Nature 455, 1101-1104. Summons et al. (1999) Nature 400, 554-557.

Efficient sorption and reduction of U(VI) on zero-valent iron-polyaniline-graphene aerogel ternary composite.

PubMed

Chen, Lili; Feng, Shaojie; Zhao, Donglin; Chen, Shaohua; Li, Feifei; Chen, Changlun

2017-03-15

In this work, zero-valent iron-polyaniline-graphene aerogel composite (Fe-PANI-GA) was prepared and applied in the removal of U(VI) from aqueous solutions by batch sorption experiments. The experimental results showed that the Fe-PANI-GA composite had an excellent removal capacity for the removal of U(VI) in acidic solutions. The results also showed that the maximum removal capacity of the Fe-PANI-GA toward U(VI) was 350.47mg/g at pH 5.5. The sorption kinetics data were well-described by pseudo-second-order. The sorption isotherms of U(VI) fitted well with Langmuir isotherm and exhibited better removal efficiency with the increase of temperature. The thermodynamic parameters (ΔG, ΔS, ΔH) indicated that the sorption of U(VI) on the Fe-PANI-GA was an endothermic and spontaneous process. Moreover, removal mechanisms were studied based on the results of XRD, FTIR and XPS. Both U(VI) sorption and partially reductive precipitation of U(VI) to U(IV) contributed to the removal of U(VI) on Fe-PANI-GA. Therefore, Fe-PANI-GA was an economic and effective material for the removal of uranium from nuclear waste in practical application. Copyright © 2016 Elsevier Inc. All rights reserved.

Effects of hydrogen treatment on ohmic contacts to p-type GaN films

NASA Astrophysics Data System (ADS)

Huang, Bohr-Ran; Chou, Chia-Hui; Ke, Wen-Cheng; Chou, Yi-Lun; Tsai, Chia-Lung; Wu, Meng-chyi

2011-06-01

This study investigated the effects of hydrogen (H 2) treatment on metal contacts to Mg-doped p-GaN films by Hall-effect measurement, current-voltage ( I- V) analyzer and X-ray photoemission spectra (XPS). The interfacial oxide layer on the p-GaN surface was found to be the main reason for causing the nonlinear I- V behavior of the untreated p-GaN films. The increased nitrogen vacancy (V N) density due to increased GaN decomposition rate at high-temperature hydrogen treatment is believed to form high density surface states on the surface of p-GaN films. Compared to untreated p-GaN films, the surface Fermi level determined by the Ga 2p core-level peak on 1000 °C H 2-treated p-GaN films lies about ˜2.1 eV closer to the conduction band edge (i.e., the surface inverted to n-type behavior). The reduction in barrier height due to the high surface state density pinned the surface Fermi level close to the conduction band edge, and allowed the electrons to easily flow over the barrier from the metal into the p-GaN films. Thus, a good ohmic contact was achieved on the p-GaN films by the surface inversion method.

High-efficiency V-band GaAs IMPATT diodes

NASA Technical Reports Server (NTRS)

Ma, Y. E.; Benko, E.; Trinh, T.; Erickson, L. P.; Mattord, T. J.

1984-01-01

Double-drift GaAs IMPATT diodes were designed for V-band frequency operations and fabricated using molecular-beam epitaxy. The diodes were fabricated in two configurations: (1) circular mesa diodes with silver-plated (integrated) heat sinks: (2) pill-type diodes bonded to diamond heat sinks. Both configurations utilized a miniature quartz-ring package. Output power greater than 1 W CW was achieved at V-band frequencies from diodes on diamond heat sinks. The best conversion efficiency was 13.3 percent at 55.5 GHz with 1 W output power.

X-ray absorption and reflection as probes of the GaN conduction bands: Theory and experiments

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lambrecht, W.R.L.; Rashkeev, S.N.; Segall, B.

1997-04-01

X-ray absorption measurements are a well-known probe of the unoccupied states in a material. The same information can be obtained by using glancing angle X-ray reflectivity. In spite of several existing band structure calculations of the group III nitrides and previous optical studies in UV range, a direct probe of their conduction band densities of states is of interest. The authors performed a joint experimental and theoretical investigation using both of these experimental techniques for wurtzite GaN.

Relationship between rabbit transferrin electrophoretic patterns and plasma iron concentrations.

PubMed

Zaragoza, P; Arana, A; Amorena, B

1987-01-01

Rabbit transferrin (Tf) was studied electrophoretically using 1141 blood samples from individuals belonging to seven populations (Spanish Common, Spanish Giant, Butterfly, Lyoné de Bourgogne, New Zealand White, Californian and New Zealand White X Californian hybrids). No Tf polymorphism was found by starch gel electrophoresis, but six patterns, differing in the presence and/or intensity of three bands ('a', anodic; 'b', intermediate; and 'c', cathodic) were observed by polyacrylamide gel electrophoresis. No genetic model could explain these patterns, since they reflect differences in plasma Tf iron content. The electrophoretic test allowed a direct observation of the relative in vivo levels of the different Tf molecular species; saturated (band 'a', Fe2Tf); semi-saturated (band 'b', Fe1Tf); and without iron (band 'c' Fe0Tf, apotransferrin). The degree of iron saturation of Tf varied among individuals and throughout the individual's life. Specifically, in pregnant females, Fe2Tf and Fe1Tf are generally observed, except in late pregnancy (from day 25 to parturition), when mainly apotransferrin is observed. Significantly, within 24 h post-partum, high levels of Fe2Tf are reached in the female's serum.

Experimental observation of incoherent-coherent crossover and orbital-dependent band renormalization in iron chalcogenide superconductors

DOE PAGES

Liu, Z. K.; Yi, M.; Zhang, Y.; ...

2015-12-22

The level of electronic correlation has been one of the key questions in understanding the nature of superconductivity. Among the iron-based superconductors, the iron chalcogenide family exhibits the strongest electron correlations. To gauge the correlation strength, we performed a systematic angle-resolved photoemission spectroscopy study on the iron chalcogenide series Fe 1+ySe xTe 1-x (0 < x < 0.59), a model system with the simplest structure. Our measurement reveals an incoherent-to-coherent crossover in the electronic structure as the selenium ratio increases and the system evolves from a weakly localized to a more itinerant state. Furthermore, we found that the effective massmore » of bands dominated by the d xy orbital character significantly decreases with increasing selenium ratio, as compared to the d xz/d yz orbital-dominated bands. The orbital-dependent change in the correlation level agrees with theoretical calculations on the band structure renormalization, and may help to understand the onset of superconductivity in Fe 1+ySe xTe 1-x.« less

Additional compound semiconductor nanowires for photonics

NASA Astrophysics Data System (ADS)

Ishikawa, F.

2016-02-01

GaAs related compound semiconductor heterostructures are one of the most developed materials for photonics. Those have realized various photonic devices with high efficiency, e. g., lasers, electro-optical modulators, and solar cells. To extend the functions of the materials system, diluted nitride and bismide has been paid attention over the past decade. They can largely decrease the band gap of the alloys, providing the greater tunability of band gap and strain status, eventually suppressing the non-radiative Auger recombinations. On the other hand, selective oxidation for AlGaAs is a vital technique for vertical surface emitting lasers. That enables precisely controlled oxides in the system, enabling the optical and electrical confinement, heat transfer, and mechanical robustness. We introduce the above functions into GaAs nanowires. GaAs/GaAsN core-shell nanowires showed clear redshift of the emitting wavelength toward infrared regime. Further, the introduction of N elongated the carrier lifetime at room temperature indicating the passivation of non-radiative surface recombinations. GaAs/GaAsBi nanowire shows the redshift with metamorphic surface morphology. Selective and whole oxidations of GaAs/AlGaAs core-shell nanowires produce semiconductor/oxide composite GaAs/AlGaOx and oxide GaOx/AlGaOx core-shell nanowires, respectively. Possibly sourced from nano-particle species, the oxide shell shows white luminescence. Those property should extend the functions of the nanowires for their application to photonics.

MANUFACTURING METHODS FOR PHASE SHIFTERS.

DTIC Science & Technology

MANUFACTURING), (*PHASE SHIFT CIRCUITS, FERRITES, GARNET , DIGITAL SYSTEMS, X BAND, C BAND, S BAND, RADAR EQUIPMENT, MAGNETIC MATERIALS, YTTRIUM COMPOUNDS, GADOLINIUM COMPOUNDS, ALUMINUM COMPOUNDS, IRON COMPOUNDS, OXIDES.

Increased p-type conductivity in GaN{sub x}Sb{sub 1−x}, experimental and theoretical aspects

DOE Office of Scientific and Technical Information (OSTI.GOV)

Segercrantz, N., E-mail: natalie.segercrantz@aalto.fi; Makkonen, I.; Slotte, J.

2015-08-28

The large increase in the p-type conductivity observed when nitrogen is added to GaSb has been studied using positron annihilation spectroscopy and ab initio calculations. Doppler broadening measurements have been conducted on samples of GaN{sub x}Sb{sub 1−x} layers grown by molecular beam epitaxy, and the results have been compared with calculated first-principle results corresponding to different defect structures. From the calculated data, binding energies for nitrogen-related defects have also been estimated. Based on the results, the increase in residual hole concentration is explained by an increase in the fraction of negative acceptor-type defects in the material. As the band gapmore » decreases with increasing N concentration, the ionization levels of the defects move closer to the valence band. Ga vacancy-type defects are found to act as positron trapping defects in the material, and the ratio of Ga vacancy-type defects to Ga antisites is found to be higher than that of the p-type bulk GaSb substrate. Beside Ga vacancies, the calculated results imply that complexes of a Ga vacancy and nitrogen could be present in the material.« less

In 0.35Ga 0.65P light-emitting diodes grown by gas-source MBE

NASA Astrophysics Data System (ADS)

Masselink, W. Ted; Zachau, Martin

1993-02-01

This paper describes the growth and optical characteristics of In yGa 1- yP with 0.3< y<0.5, and the LED operation of p-i-n structures in the same materials system. The InGaP is grown using gas-source molecular beam epitaxy (GSMBE). The non-lattice-matched In yGa 1- yP grown on GaAs using GSMBE has a specularly smooth surface morphology through the use of unique strained-layer superlattice (SLS) buffer. We have measured the luminescence, luminescence excitation, and Raman spectra of these undoped films and observe strong excitonic luminescence over the entire composition range investigated. The band gap derived from the luminescence excitation spectra corresponds to that of a fully relaxed InGaP film with no residual strain, which is confirmed by the Raman measurements. Light-emitting diodes with peak (300 K) emission centered at less than 590 nm have been fabricated from p-i-n junctions in In 0.35Ga 0.65P. This alloy is close to that with the largest direct band gap in the In yGa 1- y P system and has lattice mismatch from the GaAs substrate of 1%.

Magnetic susceptibility of the South African Agouron scientific drillcores quantifies iron and sulfur alteration relevant to geochemical oxygenation proxies

NASA Astrophysics Data System (ADS)

Raub, T. D.; Nayak, P. M.; Tikoo, S. M.; Johnson, J. E.; Peek, S.; Fischer, W. W.; Kirschvink, J. L.

2010-12-01

Various geochemical characteristics of sedimentary iron- and sulfur-bearing minerals motivate early- to late-oxygenation hypotheses from South African and Australian scientific drillcores. Most intervals of these drillcores appear to be remagnetized (in some cases multiple times); and ~2.0 Ga magnetic sulfide crystallization is particularly pervasive in carbonaceous siltstones of the ca. 2.7-2.2 Ga Griqualand margin of Kaapvaal craton. Robust interpretation of trace element abundances suggesting “whiffs” to “pervasive” levels of late Archean oxygen depends upon systematics of presumed depositional iron speciation; so multiple iron- and sulfur-mineral-altering events affecting existing drillcore records call straightforward interpretations into question. We report ca. 10,000 magnetic susceptibility measurements and associated detailed rock-magnetic results from all lithologies of Archean basinal and slope facies in drillcores GKP and GKF and relatively younger and shallower facies in Paleoproterozoic drillcores GEC and GTF. Specific carbonaceous siltstone and carbonate intervals are less-altered as revealed by coherent and relatively low magnetic susceptibilities: geochemical and biomarker interpretations based upon data from these intervals should be preferred to those from others. Magnetic susceptibility tracks subtle facies variation in drillcore GTF diamictite and suggests highly-structured Paleoproterozoic glacioeustatic sequence architecture consistent with assignment of Makganyene glaciation and its associated geochemical signature to a ca. 2.2 Ga “Snowball Earth” ice age.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Iyer, Abishek K.; Rudyk, Brent W.; Lin, Xinsong

The quaternary rare-earth chalcogenides RE{sub 3}CuGaS{sub 7} and RE{sub 3}CuGaSe{sub 7} (RE=La–Nd) have been prepared by reactions of the elements at 1050 °C and 900 °C, respectively. They crystallize in the noncentrosymmetric La{sub 3}CuSiS{sub 7}-type structure (hexagonal, space group P6{sub 3}, Z=2) in which the a-parameter is largely controlled by the RE component (a=10.0–10.3 Å for the sulfides and 10.3–10.6 Å for the selenides) whereas the c-parameter is essentially fixed by the choice of Ga and chalcogen atoms within tetrahedral units (c=6.1 Å for the sulfides and 6.4 Å for the selenides). They extend the series RE{sub 3}MGaCh{sub 7}, previouslymore » known for divalent metal atoms (M=Mn–Ni), differing in that the Cu atoms in RE{sub 3}CuGaCh{sub 7} occupy trigonal planar sites instead of octahedral sites. Among quaternary chalcogenides RE{sub 3}MM′Ch{sub 7}, the combination of monovalent (M=Cu) and trivalent (M′=Ga) metals is unusual because it appears to violate the condition of charge balance satisfied by most La{sub 3}CuSiS{sub 7}-type compounds. The possibility of divalent Cu atoms was ruled out by bond valence sum analysis, magnetic measurements, and X-ray photoelectron spectroscopy. The electron deficiency in RE{sub 3}CuGaCh{sub 7} is accommodated through S-based holes at the top of the valence band, as shown by band structure calculations on La{sub 3}CuGaS{sub 7}. An optical band gap of about 2.0 eV was found for La{sub 3}CuGaSe{sub 7}. - Graphical abstract: The chalcogenides RE{sub 3}CuGaCh{sub 7} contain monovalent Cu in trigonal planes and trivalent Ga in tetrahedra; they are electron-deficient representatives of La{sub 3}CuSiS{sub 7}-type compounds, which normally satisfy charge balance. - Highlights: • Quaternary chalcogenides RE{sub 3}CuGaCh{sub 7} (RE=La–Nd; Ch=S, Se) were prepared. • Bond valence sums, magnetism, and XPS data give evidence for monovalent Cu. • Crystal structures reveal high anisotropy of Cu displacement. • Electron deficiency is accommodated by S-based holes in valence band.« less

Electrical design of InAs-Sb/GaSb superlattices for optical detectors using full bandstructure sp3s* tight-binding method and Bloch boundary conditions

NASA Astrophysics Data System (ADS)

Mir, Raja N.; Frensley, William R.

2013-10-01

InAs-Sb/GaSb type-II strain compensated superlattices (SLS) are currently being used in mid-wave and long-wave infrared photodetectors. The electronic bandstructure of InSb and GaSb shows very strong anisotropy and non-parabolicity close to the Γ-point for the conduction band (CB) minimum and the valence band (VB) maximum. Particularly around the energy range of 45-80 meV from band-edge we observe strong non-parabolicity in the CB and light hole VB. The band-edge dispersion determines the electrical properties of a material. When the bulk materials are combined to form a superlattice we need a model of bandstructure which takes into account the full bandstructure details of the constituents and also the strong interaction between the conduction band of InAs and valence bands of GaSb. There can also be contact potentials near the interface between two dissimilar superlattices which will not be captured unless a full bandstructure calculation is done. In this study, we have done a calculation using second nearest neighbor tight binding model in order to accurately reproduce the effective masses. The calculation of mini-band structure is done by finding the wavefunctions within one SL period subject to Bloch boundary conditions ψ(L)=ψ(0)eikL. We demonstrate in this paper how a calculation of carrier concentration as a function of the position of the Fermi level (EF) within bandgap(Eg) should be done in order to take into account the full bandstructure of broken-bandgap material systems. This calculation is key for determining electron transport particularly when we have an interface between two dissimilar superlattices.

Structural defects and recombination behavior of excited carriers in Cu(In,Ga)Se{sub 2} solar cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, J.; Du, H. W.; Li, Y.

2016-08-15

The carriers’ behavior in neutral region (NTR) and space charged region (SCR) of Cu(In,Ga)Se{sub 2} thin film based solar cells has been investigated by temperature dependent photoluminescence (PL-T), electroluminescence (EL-T) and current-voltage (IV-T) from 10 to 300 K. PL-T spectra show that three kinds of defects, namely V{sub Se}, In{sub Cu} and (In{sub Cu}+V{sub Cu}), are localized within the band gap of NTR and SCR of CIGS layer, corresponding to the energy levels of E{sub C}-0.08, E{sub C}-0.20 and E{sub C}-0.25 eV, respectively. The In{sub Cu} and (In{sub Cu}+V{sub Cu}) deep level defects are non-radiative recombination centers at room temperature.more » The IV-T and EL-T analysis reveals that the injection modes of electrons from ZnO conduction band into Cu(In,Ga)Se{sub 2} layer are tunneling, thermally-excited tunneling and thermionic emission under 10-40, 60-160, and 180-300 K, respectively. At 10-160 K, the electrons tunnel into (In{sub Cu}+V{sub Cu}) and V{sub se} defect levels in band gap of SCR and the drifting is involved in the emission bands at 0.96 and 1.07 eV, which is the direct evidence for a tunneling assisted recombination. At 180-300 K, the electrons are directly injected into the Cu(In,Ga)Se{sub 2} conduction band, and the emission of 1.13 eV are ascribed to the transitions from the conduction band to the valence band.« less

Characterization and Applications of Micro- and Nano- Ferrites at Microwave and Millimeter Waves

NASA Astrophysics Data System (ADS)

Chao, Liu

Ferrite materials are one of the most widely used magnetic materials in microwave and millimeter wave applications such as radar, wireless communication. They provide unique properties for microwave and millimeter wave devices especially non-reciprocal devices. Some ferrite materials with strong magnetocrystalline anisotropy fields can extend these applications to tens of GHz range while reducing the size, weight and cost. This thesis focuses on characterization of such ferrite materials as micro- and nano-powder and the fabrication of the devices. The ferrite materials with strong magnetocrystalline anisotropy field are metal/non-metal substituted iron oxides oriented in low crystal symmetry. The ferrite materials characterized in this thesis include M-type hexagonal ferrites such as barium ferrite (BaFe12O19), strontium ferrite (SrFe12O19), epsilon phase iron oxide (epsilon-Fe 2O3), substituted epsilon phase iron oxide (epsilon-Ga xFe2-xO3, epsilon-AlxFe2-xO 3). These ferrites exhibit great anisotropic magnetic fields. A transmission-reflection based in-waveguide technique that employs a vector network analyzer was used to determine the scattering parameters for each sample in the microwave bands (8.2--40 GHz). From the S-parameters, complex dielectric permittivity and complex magnetic permeability are evaluated by an improved algorithm. The millimeter wave measurement is based on a free space quasi-optical spectrometer. Initially precise transmittance spectra over a broad millimeter wave frequency range from 40 GHz to 120 GHz are acquired. Later the transmittance spectra are converted into complex permittivity and permeability spectra. These ferrite powder materials are further characterized by x-ray diffraction (XRD) to understand the crystalline structure relating to the strength and the shift of the ferromagnetic resonance affected by the particle size. A Y-junction circulator working in the 60 GHz frequency band is designed based on characterized M-type barium micro- and nano-ferrite. A new fabrication process using ferrite composite is proposed to integrate the Y-junction circulator into the semiconductor substrate. Theoretical design of a high gain Traveling Wave Tube (TWT) amplifier using a metamaterial (MTM) structure and cold-test of the MTM structure are also included in this dissertation. An SWS working around 6 GHz below the X-band waveguide TE10 cutoff frequency is fabricated.

Semiconductor tunnel junction with enhancement layer

DOEpatents

Klem, John F.; Zolper, John C.

1997-01-01

The incorporation of a pseudomorphic GaAsSb layer in a runnel diode structure affords a new degree of freedom in designing runnel junctions for p-n junction device interconnects. Previously only doping levels could be varied to control the tunneling properties. This invention uses the valence band alignment band of the GaAsSb with respect to the surrounding materials to greatly relax the doping requirements for tunneling.

Band gap renormalization and Burstein-Moss effect in silicon- and germanium-doped wurtzite GaN up to 1020 cm-3

NASA Astrophysics Data System (ADS)

Feneberg, Martin; Osterburg, Sarah; Lange, Karsten; Lidig, Christian; Garke, Bernd; Goldhahn, Rüdiger; Richter, Eberhard; Netzel, Carsten; Neumann, Maciej D.; Esser, Norbert; Fritze, Stephanie; Witte, Hartmut; Bläsing, Jürgen; Dadgar, Armin; Krost, Alois

2014-08-01

The interplay between band gap renormalization and band filling (Burstein-Moss effect) in n-type wurtzite GaN is investigated. For a wide range of electron concentrations up to 1.6×1020cm-3 spectroscopic ellipsometry and photoluminescence were used to determine the dependence of the band gap energy and the Fermi edge on electron density. The band gap renormalization is the dominating effect up to an electron density of about 9×1018cm-3; at higher values the Burstein-Moss effect is stronger. Exciton screening, the Mott transition, and formation of Mahan excitons are discussed. A quantitative understanding of the near gap transition energies on electron density is obtained. Higher energy features in the dielectric functions up to 10eV are not influenced by band gap renormalization.

Electric field effects on the optical properties of buckled GaAs monolayer

NASA Astrophysics Data System (ADS)

Bahuguna, Bhagwati Prasad; Saini, L. K.; Sharma, Rajesh O.

2018-04-01

Buckled GaAs monolayer has a direct band gap semiconductor with energy gap of 1.31 eV in the absence of electric field. When we applied transverse electric field, the value of band gap decreases with increasing of electric field strength. In our previous work [1], it is observed that the buckled GaAs monolayer becomes metallic at 1.3 V/Å. In the present work, we investigate the optical properties such as photon energy-dependent dielectric functions, extinction coefficient, refractive index, absorption spectrum and reflectivity of buckled GaAs monolayer in the semiconducting phase i.e. absence of external electric field and metallic phase i.e. presence of external electric field using density functional theory.

Electronic structure and dynamics of thin Ge/GaAs(110) heterostructures

NASA Astrophysics Data System (ADS)

Haight, R.; Silberman, J. A.

1990-10-01

Using angle-resolved picosecond laser photoemission we have investigated both occupied and transiently excited empty states at the surface of Ge grown epitaxially on GaAs(110). We observe a normally unoccupied, Ge layer derived state whose separation from the valence-band maximum of the system is 700±50 meV at six monolayers Ge coverage. The evolution of the electronic structure is followed as a function of coverage and correlated with low-energy electron diffraction. The time dependence of the transiently occupied Ge signal is compared with that of the clean GaAs(110) surface and shows that electrons are prevented from diffusing into the GaAs bulk by the conduction-band offset of 330±40 meV.

Growth and properties of GaSbBi alloys

NASA Astrophysics Data System (ADS)

Rajpalke, M. K.; Linhart, W. M.; Birkett, M.; Yu, K. M.; Scanlon, D. O.; Buckeridge, J.; Jones, T. S.; Ashwin, M. J.; Veal, T. D.

2013-09-01

Molecular-beam epitaxy has been used to grow GaSb1-xBix alloys with x up to 0.05. The Bi content, lattice expansion, and film thickness were determined by Rutherford backscattering and x-ray diffraction, which also indicate high crystallinity and that >98% of the Bi atoms are substitutional. The observed Bi-induced lattice dilation is consistent with density functional theory calculations. Optical absorption measurements and valence band anticrossing modeling indicate that the room temperature band gap varies from 720 meV for GaSb to 540 meV for GaSb0.95Bi0.05, corresponding to a reduction of 36 meV/%Bi or 210 meV per 0.01 Å change in lattice constant.

Optical Control of Internal Electric Fields in Band Gap-Graded InGaN Nanowires

NASA Astrophysics Data System (ADS)

Erhard, N.; Sarwar, A. T. M. Golam; Yang, F.; McComb, D. W.; Myers, R. C.; Holleitner, A. W.

2015-01-01

InGaN nanowires are suitable building blocks for many future optoelectronic devices. We show that a linear grading of the indium content along the nanowire axis from GaN to InN introduces an internal electric field evoking a photocurrent. Consistent with quantitative band structure simulations we observe a sign change in the measured photocurrent as a function of photon flux. This negative differential photocurrent opens the path to a new type of nanowire-based photodetector. We demonstrate that the photocurrent response of the nanowires is as fast as 1.5 ps.

Gallium nitride based logpile photonic crystals.

PubMed

Subramania, Ganapathi; Li, Qiming; Lee, Yun-Ju; Figiel, Jeffrey J; Wang, George T; Fischer, Arthur J

2011-11-09

We demonstrate a nine-layer logpile three-dimensional photonic crystal (3DPC) composed of single crystalline gallium nitride (GaN) nanorods, ∼100 nm in size with lattice constants of 260, 280, and 300 nm with photonic band gap in the visible region. This unique GaN structure is created through a combined approach of a layer-by-layer template fabrication technique and selective metal organic chemical vapor deposition (MOCVD). These GaN 3DPC exhibit a stacking direction band gap characterized by strong optical reflectance between 380 and 500 nm. By introducing a "line-defect" cavity in the fifth (middle) layer of the 3DPC, a localized transmission mode with a quality factor of 25-30 is also observed within the photonic band gap. The realization of a group III nitride 3DPC with uniform features and a band gap at wavelengths in the visible region is an important step toward realizing complete control of the electromagnetic environment for group III nitride based optoelectronic devices.

Band structure calculations of CuAlO2, CuGaO2, CuInO2, and CuCrO2 by screened exchange

NASA Astrophysics Data System (ADS)

Gillen, Roland; Robertson, John

2011-07-01

We report density functional theory band structure calculations on the transparent conducting oxides CuAlO2, CuGaO2, CuInO2, and CuCrO2. The use of the hybrid functional screened-exchange local density approximation (sX-LDA) leads to considerably improved electronic properties compared to standard LDA and generalized gradient approximation (GGA) approaches. We show that the resulting electronic band gaps compare well with experimental values and previous quasiparticle calculations, and show the correct trends with respect to the atomic number of the cation (Al, Ga, In). The resulting energetic depths of Cu d and O p levels and the valence-band widths are considerable improvements compared to LDA and GGA and are in good agreement with available x-ray photoelectron spectroscopy data. Lastly, we show the calculated imaginary part of the dielectric function for all four systems.

Dominant transverse-electric polarized emission from 298 nm MBE-grown AlN-delta-GaN quantum well ultraviolet light-emitting diodes

NASA Astrophysics Data System (ADS)

Liu, Cheng; Ooi, Yu Kee; Islam, S. M.; Xing, Huili Grace; Jena, Debdeep; Zhang, Jing

2017-02-01

III-nitride based ultraviolet (UV) light emitting diodes (LEDs) are of considerable interest in replacing gas lasers and mercury lamps for numerous applications. Specifically, AlGaN quantum well (QW) based LEDs have been developed extensively but the external quantum efficiencies of which remain less than 10% for wavelengths <300 nm due to high dislocation density, difficult p-type doping and most importantly, the physics and band structure from the three degeneration valence subbands. One solution to address this issue at deep UV wavelengths is by the use of the AlGaN-delta-GaN QW where the insertion of the delta-GaN layer can ensure the dominant conduction band (C) - heavyhole (HH) transition, leading to large transverse-electric (TE) optical output. Here, we proposed and investigated the physics and polarization-dependent optical characterizations of AlN-delta- GaN QW UV LED at 300 nm. The LED structure is grown by Molecular Beam Epitaxy (MBE) where the delta-GaN layer is 3-4 monolayer (QW-like) sandwiched by 2.5-nm AlN sub-QW layers. The physics analysis shows that the use of AlN-delta-GaN QW ensures a larger separation between the top HH subband and lower-energy bands, and strongly localizes the electron and HH wave functions toward the QW center and hence resulting in 30-time enhancement in TEpolarized spontaneous emission rate, compared to that of a conventional Al0.35Ga0.65N QW. The polarization-dependent electroluminescence measurements confirm our theoretical analysis; a dominant TE-polarized emission was obtained at 298 nm with a minimum transverse-magnetic (TM) polarized emission, indicating the feasibility of high-efficiency TEpolarized UV emitters based on our proposed QW structure.

Growth and characterization of n-AlGaN 1-D structures with varying Al composition using u-GaN seeds

NASA Astrophysics Data System (ADS)

Kang, San; Chatterjee, Uddipta; Um, Dae-Young; Seo, In Seok; Lee, Cheul-Ro

2017-12-01

Like all the ternary alloys in III-nitride materials family, aluminum gallium nitride (AlGaN) has unique band gap tuning property which enables the alloy to be suitable for many opto-electronic applications. The direct band gap of AlGaN can be tuned from 3.4 to 6.2 eV by changing the composition. In this article, the growth of ternary n-AlGaN micro and nano structures on Si (1 1 1) substrate is demonstrated via 2-step growth method employing metal organic chemical vapor deposition. During the growth flow of Trimethygallium is varied to modulate the final Al/Ga ratio. After the growth, various morphological, crystalline and optical characterizations are carried out to probe in the properties of the grown structures. Recorded X-ray diffraction patterns reveal that the realized structures are wurtzite single crystalline n-AlGaN having a near homogeneous Al distribution and validated by energy dispersive X-ray spectroscopy. Low temperature cathodoluminescence spectra show band edge emission in deep UV region which enables the grown n-AlGaN structures to efficiently find opto-electronic applications in the aforementioned region. Finally, planar photoconductive devices are fabricated using the grown 1-D structures and photocurrent evolution is measured. Structure bearing highest Al content shows a manifold enhancement in photo activity compared to other grown samples. Absolute photoresponsivities of the grown samples are calculated to be 301.47, 116 and 38.13 mA/W which is in accord with the findings of low temperature cathodoluminescence investigation. Therefore, it can be concluded that the successful realization of n-AlGaN 1-D structures varying Al content facilitates the further developments of the field concerning nano- and opto-electronic devices.

Ultraviolet light-absorbing and emitting diodes consisting of a p-type transparent-semiconducting NiO film deposited on an n-type GaN homoepitaxial layer

NASA Astrophysics Data System (ADS)

Nakai, Hiroshi; Sugiyama, Mutsumi; Chichibu, Shigefusa F.

2017-05-01

Gallium nitride (GaN) and related (Al,Ga,In)N alloys provide practical benefits in the production of light-emitting diodes (LEDs) and laser diodes operating in ultraviolet (UV) to green wavelength regions. However, obtaining low resistivity p-type AlN or AlGaN of large bandgap energies (Eg) is a critical issue in fabricating UV and deep UV-LEDs. NiO is a promising candidate for useful p-type transparent-semiconducting films because its Eg is 4.0 eV and it can be doped into p-type conductivity of sufficiently low resistivity. By using these technologies, heterogeneous junction diodes consisting of a p-type transparent-semiconducting polycrystalline NiO film on an n-type single crystalline GaN epilayer on a low threading-dislocation density, free-standing GaN substrate were fabricated. The NiO film was deposited by using the conventional RF-sputtering method, and the GaN homoepitaxial layer was grown by metalorganic vapor phase epitaxy. They exhibited a significant photovoltaic effect under UV light and also exhibited an electroluminescence peak at 3.26 eV under forward-biased conditions. From the conduction and valence band (EV) discontinuities, the NiO/GaN heterointerface is assigned to form a staggered-type (TYPE-II) band alignment with the EV of NiO higher by 2.0 eV than that of GaN. A rectifying property that is consistent with the proposed band diagram was observed in the current-voltage characteristics. These results indicate that polycrystalline NiO functions as a hole-extracting and injecting layer of UV optoelectronic devices.

Band-gap tailoring and visible-light-driven photocatalytic performance of porous (GaN)1-x(ZnO)x solid solution.

PubMed

Wu, Aimin; Li, Jing; Liu, Baodan; Yang, Wenjin; Jiang, Yanan; Liu, Lusheng; Zhang, Xinglai; Xiong, Changmin; Jiang, Xin

2017-02-21

(GaN) 1-x (ZnO) x solid solution has attracted extensive attention due to its feasible band-gap tunability and excellent photocatalytic performance in overall water splitting. However, its potential application in the photodegradation of organic pollutants and environmental processing has rarely been reported. In this study, we developed a rapid synthesis process to fabricate porous (GaN) 1-x (ZnO) x solid solution with a tunable band gap in the range of 2.38-2.76 eV for phenol photodegradation. Under visible-light irradiation, (GaN) 0.75 (ZnO) 0.25 solid solution achieved the highest photocatalytic performance compared to other (GaN) 1-x (ZnO) x solid solutions with x = 0.45, 0.65 and 0.85 due to its higher redox capability and lower lattice deformation. Slight Ag decoration with a content of 1 wt% on the surface of the (GaN) 0.75 (ZnO) 0.25 solid solution leads to a significant enhancement in phenol degradation, with a reaction rate eight times faster than that of pristine (GaN) 0.75 (ZnO) 0.25 . Interestingly, phenol in aqueous solution (10 mg L -1 ) can also be completely degraded within 60 min, even under the direct exposure of sunlight irradiation. The photocurrent response indicates that the enhanced photocatalytic activity of (GaN) 0.75 (ZnO) 0.25 /Ag is directly induced by the improved transfer efficiency of the photogenerated electrons at the interface. The excellent phenol degradation performance of (GaN) 1-x (ZnO) x /Ag further broadens their promising photocatalytic utilization in environmental processing, besides in overall water splitting for hydrogen production.

Noncentrosymmetric rare-earth copper gallium chalcogenides RE3CuGaCh7 (RE=La-Nd; Ch=S, Se): An unexpected combination

NASA Astrophysics Data System (ADS)

Iyer, Abishek K.; Rudyk, Brent W.; Lin, Xinsong; Singh, Harpreet; Sharma, Arzoo Z.; Wiebe, Christopher R.; Mar, Arthur

2015-09-01

The quaternary rare-earth chalcogenides RE3CuGaS7 and RE3CuGaSe7 (RE=La-Nd) have been prepared by reactions of the elements at 1050 °C and 900 °C, respectively. They crystallize in the noncentrosymmetric La3CuSiS7-type structure (hexagonal, space group P63, Z=2) in which the a-parameter is largely controlled by the RE component (a=10.0-10.3 Å for the sulfides and 10.3-10.6 Å for the selenides) whereas the c-parameter is essentially fixed by the choice of Ga and chalcogen atoms within tetrahedral units (c=6.1 Å for the sulfides and 6.4 Å for the selenides). They extend the series RE3MGaCh7, previously known for divalent metal atoms (M=Mn-Ni), differing in that the Cu atoms in RE3CuGaCh7 occupy trigonal planar sites instead of octahedral sites. Among quaternary chalcogenides RE3MM‧Ch7, the combination of monovalent (M=Cu) and trivalent (M‧=Ga) metals is unusual because it appears to violate the condition of charge balance satisfied by most La3CuSiS7-type compounds. The possibility of divalent Cu atoms was ruled out by bond valence sum analysis, magnetic measurements, and X-ray photoelectron spectroscopy. The electron deficiency in RE3CuGaCh7 is accommodated through S-based holes at the top of the valence band, as shown by band structure calculations on La3CuGaS7. An optical band gap of about 2.0 eV was found for La3CuGaSe7.

Doping of Czochralski-grown bulk β-Ga2O3 single crystals with Cr, Ce and Al

NASA Astrophysics Data System (ADS)

Galazka, Zbigniew; Ganschow, Steffen; Fiedler, Andreas; Bertram, Rainer; Klimm, Detlef; Irmscher, Klaus; Schewski, Robert; Pietsch, Mike; Albrecht, Martin; Bickermann, Matthias

2018-03-01

We experimentally evaluated segregation of Cr, Ce and Al in bulk β-Ga2O3 single crystals grown by the Czochralski method, as well as the impact of these dopants on optical properties. The segregation of Cr and Ce and their incorporation into the β-Ga2O3 crystal structure strongly depends on O2 concentration in the growth atmosphere which has a noticeable impact on decomposition of Ga2O3 and Cr2O3, as well as on the charge state of Cr and Ce. Effective segregation coefficients for Cr are in the range of 3.1-1.5 at 7-24 vol% O2, while for Ce they are roughly below 0.01 at 1.5-34 vol% O2. The effective segregation coefficient for Al is 1.1 at 1.5-21 vol% O2. Both dopants Ce and Al have a thermodynamically stabilizing effect on β-Ga2O3 crystal growth by supressing decomposition. While Ce has no impact on the optical transmittance in the ultraviolet and visible regions, in Cr doped crystals we observe three absorption bands due to Cr3+ on octahedral Ga sites, one in the ultraviolet merging with the band edge absorption of β-Ga2O3 and two in the visible spectrum, for which we estimate the absorption cross sections. Al doping also does not induce dopant related absorption bands but clearly shifts the absorption edge as one expects for a solid-solution crystal Ga2(1-x)Al2xO3 still in the monoclinic phase. For the highest doping concentration (Ga1.9Al0.1O3) we estimate an increase of the energy gap by 0.11 eV.

Photoelectrochemical studies of InGaN/GaN MQW photoanodes

NASA Astrophysics Data System (ADS)

Butson, Joshua; Reddy Narangari, Parvathala; Krishna Karuturi, Siva; Yew, Rowena; Lysevych, Mykhaylo; Tan, Hark Hoe; Jagadish, Chennupati

2018-01-01

The research interest in photoelectrochemical (PEC) water splitting is ever growing due to its potential to contribute towards clean and portable energy. However, the lack of low energy band gap materials with high photocorrosion resistance is the primary setback inhibiting this technology from commercialisation. The ternary alloy InGaN shows promise to meet the photoelectrode material requirements due to its high chemical stability and band gap tunability. The band gap of InGaN can be modulated from the UV to IR regions by adjusting the In concentration so as to absorb the maximum portion of the solar spectrum. This paper reports on the influence of In concentration on the PEC properties of planar and nanopillar (NP) InGaN/GaN multi-quantum well (MQW) photoanodes, where NPs were fabricated using a top-down approach. Results show that changing the In concentration, while having a minor effect on the PEC performance of planar MQWs, has an enormous impact on the PEC performance of NP MQWs, with large variations in the photocurrent density observed. Planar photoanodes containing MQWs generate marginally lower photocurrents compared to photoanodes without MQWs when illuminated with sunlight. NP MQWs with 30% In generated the highest photocurrent density of 1.6 mA cm-2, 4 times greater than that of its planar counterpart and 1.8 times greater than that of the NP photoanode with no MQWs. The InGaN/GaN MQWs also slightly influenced the onset potential of both the planar and NP photoanodes. Micro-photoluminescence, diffuse reflectance spectroscopy and IPCE measurements are used to explain these results.

Low-Cost, Efficient, and Durable H2 Production by Photoelectrochemical Water Splitting with CuGa3Se5 Photocathodes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Muzzillo, Christopher; Klein, Walter E; Li, Zhen

Photoelectrochemical (PEC) water splitting is an elegant method of converting sunlight and water into H2 fuel. To be commercially advantageous, PEC devices must become cheaper, more efficient, and much more durable. This work examines low-cost polycrystalline chalcopyrite films, which are successful as photovoltaic absorbers, for application as PEC absorbers. In particular, Cu-Ga-Se films with wide band gaps can be employed as top cell photocathodes in tandem devices as a realistic route to high efficiencies. In this report, we demonstrate that decreasing Cu/Ga composition from 0.66 to 0.31 in Cu-Ga-Se films increased the band gap from 1.67 to 1.86 eV andmore » decreased saturated photocurrent density from 18 to 8 mA/cm2 as measured by chopped-light current-voltage (CLIV) measurements in a 0.5 M sulfuric acid electrolyte. Buffer and catalyst surface treatments were not applied to the Cu-Ga-Se films, and they exhibited promising stability, evidenced by unchanged CLIV after 9 months of storage in air. Finally, films with Cu/Ga = 0.36 (approximately stoichiometric CuGa3Se5) and 1.86 eV band gaps had exceptional durability and continuously split water for 17 days (~12 mA/cm2 at -1 V vs RHE). This is equivalent to ~17 200 C/cm2, which is a world record for any polycrystalline PEC absorber. These results indicate that CuGa3Se5 films are prime candidates for cheaply achieving efficient and durable PEC water splitting.« less

Low-Cost, Efficient, and Durable H2 Production by Photoelectrochemical Water Splitting with CuGa3Se5 Photocathodes.

PubMed

Muzzillo, Christopher P; Klein, W Ellis; Li, Zhen; DeAngelis, Alexander Daniel; Horsley, Kimberly; Zhu, Kai; Gaillard, Nicolas

2018-06-13

Photoelectrochemical (PEC) water splitting is an elegant method of converting sunlight and water into H 2 fuel. To be commercially advantageous, PEC devices must become cheaper, more efficient, and much more durable. This work examines low-cost polycrystalline chalcopyrite films, which are successful as photovoltaic absorbers, for application as PEC absorbers. In particular, Cu-Ga-Se films with wide band gaps can be employed as top cell photocathodes in tandem devices as a realistic route to high efficiencies. In this report, we demonstrate that decreasing Cu/Ga composition from 0.66 to 0.31 in Cu-Ga-Se films increased the band gap from 1.67 to 1.86 eV and decreased saturated photocurrent density from 18 to 8 mA/cm 2 as measured by chopped-light current-voltage (CLIV) measurements in a 0.5 M sulfuric acid electrolyte. Buffer and catalyst surface treatments were not applied to the Cu-Ga-Se films, and they exhibited promising stability, evidenced by unchanged CLIV after 9 months of storage in air. Finally, films with Cu/Ga = 0.36 (approximately stoichiometric CuGa 3 Se 5 ) and 1.86 eV band gaps had exceptional durability and continuously split water for 17 days (∼12 mA/cm 2 at -1 V vs RHE). This is equivalent to ∼17 200 C/cm 2 , which is a world record for any polycrystalline PEC absorber. These results indicate that CuGa 3 Se 5 films are prime candidates for cheaply achieving efficient and durable PEC water splitting.

Ultrafast Time-Resolved Photoluminescence Studies of Gallium-Arsenide

NASA Astrophysics Data System (ADS)

Johnson, Matthew Bruce

This thesis concerns the study of ultrafast phenomena in GaAs using time-resolved photoluminescence (PL). The thesis consists of five chapters. Chapter one is an introduction, which discusses the study of ultrafast phenomena in semiconductors. Chapter two is a description of the colliding-pulse mode-locked (CPM) ring dye laser, which is at the heart of the experimental apparatus used in this thesis. Chapter three presents a detailed experimental and theoretical investigation of photoluminescence excitation correlation spectroscopy (PECS), the novel technique which is used to time-resolve ultrafast PL phenomena. Chapters 4 and 5 discuss two applications of the PECS technique. In Chapter 4 the variation of PL intensity in In-alloyed GaAs substrate material is studied, while Chapter 5 discusses the variation of carrier lifetimes in ion-damaged GaAs used in photo-conductive circuit elements (PCEs). PECS is a pulse-probe technique that measures the cross correlation of photo-excited carrier populations. The theoretical model employed in this thesis is based upon the rate equation for a simple three-level system consisting of valence and conduction bands and a single trap level. In the limit of radiative band-to-band dominated recombination, no PECS signal should be observed; while in the capture -dominated recombination limit, the PECS signal from the band-to-band PL measures the cross correlation of the excited electron and hole populations and thus, the electron and hole lifetimes. PECS is experimentally investigated using a case study of PL in semi-insulating (SI) GaAs and In -alloyed GaAs. At 77 K, the PECS signal is characteristic of a capture-dominated system, yielding an electron-hole lifetime of about 200 ps. However, at 5 K the behavior is more complicated and shows saturation effects due to the C acceptor level, which is un-ionized at 5 K. As a first application, PECS is used to investigate the large band-to-band PL contrast observed near dislocations in In-alloyed GaAs. It is found that the PL intensity contrast between bright and dark areas correlates with the ratio of the lifetimes measured using PECS in these areas. Thus, the PL intensity contrast is due to the difference in the carrier lifetimes in the different regions. The carrier lifetimes in the bright and dark regions have different temperature dependences. (Abstract shortened with permission of author.).

Direct band gap measurement of Cu(In,Ga)(Se,S){sub 2} thin films using high-resolution reflection electron energy loss spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Heo, Sung; College of Information and Communication Engineering, Sungkyunkwan University, Cheoncheon-dong 300, Jangan-gu, Suwon 440-746; Lee, Hyung-Ik

2015-06-29

To investigate the band gap profile of Cu(In{sub 1−x},Ga{sub x})(Se{sub 1−y}S{sub y}){sub 2} of various compositions, we measured the band gap profile directly as a function of in-depth using high-resolution reflection energy loss spectroscopy (HR-REELS), which was compared with the band gap profile calculated based on the auger depth profile. The band gap profile is a double-graded band gap as a function of in-depth. The calculated band gap obtained from the auger depth profile seems to be larger than that by HR-REELS. Calculated band gaps are to measure the average band gap of the spatially different varying compositions with respectmore » to considering its void fraction. But, the results obtained using HR-REELS are to be affected by the low band gap (i.e., out of void) rather than large one (i.e., near void). Our findings suggest an analytical method to directly determine the band gap profile as function of in-depth.« less

Gallium-based anti-infectives: targeting microbial iron-uptake mechanisms.

PubMed

Kelson, Andrew B; Carnevali, Maia; Truong-Le, Vu

2013-10-01

Microbes have evolved elaborate iron-acquisition systems to sequester iron from the host environment using siderophores and heme uptake systems. Gallium(III) is structurally similar to iron(III), except that it cannot be reduced under physiological conditions, therefore gallium has the potential to serve as an iron analog, and thus an anti-microbial. Because Ga(III) can bind to virtually any complex that binds Fe(III), simple gallium salts as well as more complex siderophores and hemes are potential carriers to deliver Ga(III) to the microbes. These gallium complexes represent a new class of anti-infectives that is different in mechanism of action from conventional antibiotics. Simple gallium salts such as gallium nitrate, maltolate, and simple gallium siderophore complexes such as gallium citrate have shown good antibacterial activities. The most studied complex has been gallium citrate, which exhibits broad activity against many Gram negative bacteria at ∼1-5μg/ml MICs, strong biofilm activity, low drug resistance, and efficacy in vivo. Using the structural features of specific siderophore and heme made by pathogenic bacteria and fungi, researchers have begun to evaluate new gallium complexes to target key pathogens. This review will summarize potential iron-acquisition system targets and recent research on gallium-based anti-infectives. Copyright © 2013 Elsevier Ltd. All rights reserved.

The electronic and optical properties of quaternary GaAs1-x-y N x Bi y alloy lattice-matched to GaAs: a first-principles study.

PubMed

Ma, Xiaoyang; Li, Dechun; Zhao, Shengzhi; Li, Guiqiu; Yang, Kejian

2014-01-01

First-principles calculations based on density functional theory have been performed for the quaternary GaAs1-x-y N x Bi y alloy lattice-matched to GaAs. Using the state-of-the-art computational method with the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, electronic, and optical properties were obtained, including band structures, density of states (DOSs), dielectric function, absorption coefficient, refractive index, energy loss function, and reflectivity. It is found that the lattice constant of GaAs1-x-y N x Bi y alloy with y/x =1.718 can match to GaAs. With the incorporation of N and Bi into GaAs, the band gap of GaAs1-x-y N x Bi y becomes small and remains direct. The calculated optical properties indicate that GaAs1-x-y N x Bi y has higher optical efficiency as it has less energy loss than GaAs. In addition, it is also found that the electronic and optical properties of GaAs1-x-y N x Bi y alloy can be further controlled by tuning the N and Bi compositions in this alloy. These results suggest promising applications of GaAs1-x-y N x Bi y quaternary alloys in optoelectronic devices.

Photoluminescence emission from GaAs nanodisks in GaAs/AlGaAs nanopillar arrays fabricated by neutral beam etching

NASA Astrophysics Data System (ADS)

Ohori, Daisuke; Fukuyama, Atsuhiko; Sakai, Kentaro; Higo, Akio; Thomas, Cedric; Samukawa, Seiji; Ikari, Tetsuo

2017-05-01

GaAs quantum nanodisks (QNDs) in nanopillar (NP) arrays are considered to be an attractive candidate for photonic device applications. We report a damageless fabrication technique that can be used to produce large-area lattice-matched GaAs/AlGaAs heterostructure NP arrays through the use of a bio-template and neutral beam etching. We have successfully realized GaAs QNDs in NPs owing to nanoscale iron oxide masks included in poly(ethylene glycol)-decorated ferritin protein shells. We observed for first time the photoluminescence emission from as-etched GaAs QNDs and confirmed quantum confinement by quantum mechanical calculation. Our methodology is vital for high-efficiency pillar-based optoelectronic devices such as NP laser diodes.

Carrier and photon dynamics in a topological insulator Bi{sub 2}Te{sub 3}/GaN type II staggered heterostructure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chaturvedi, P.; Chouksey, S.; Banerjee, D.

2015-11-09

We have demonstrated a type-II band-aligned heterostructure between pulsed laser deposited topological insulator bismuth telluride and metal organic-chemical-vapour deposited GaN on a sapphire substrate. The heterostructure shows a large valence band-offset of 3.27 eV as determined from x-ray photoelectron spectroscopy, which is close to the bandgap of GaN (3.4 eV). Further investigation using x-ray diffraction, Raman spectroscopy, and energy-dispersive x-ray spectrum reveals the stoichiometric and material properties of bismuth telluride on GaN. Steady state photon emission from GaN is found to be modulated by the charge transfer process due to diffusion across the junction. The time constant involved with the charge transfermore » process is found to be 0.6 ns by transient absorption spectroscopy. The heterostructure can be used for designing devices with different functionalities and improving the performance of the existing devices on GaN.« less

Room temperature exciton-polariton resonant reflection and suppressed absorption in periodic systems of InGaN quantum wells

NASA Astrophysics Data System (ADS)

Bolshakov, A. S.; Chaldyshev, V. V.; Zavarin, E. E.; Sakharov, A. V.; Lundin, W. V.; Tsatsulnikov, A. F.; Yagovkina, M. A.

2017-04-01

We studied the optical properties of periodic InGaN/GaN multiple quantum well systems with different numbers of periods. A resonant increase in the optical reflection and simultaneous suppression of the optical absorption have been revealed experimentally at room temperature when the Bragg and exciton resonances were tuned to each other. Numerical modeling with a single set of parameters gave a quantitatively accurate fit of the experimental reflection and transmission spectra in a wide wavelength range and various angles of the light incidence. The model included both exciton resonance and non-resonant band-to-band transitions in the InGaN quantum wells, as well as Rayleigh light scattering in the GaN buffer layer. The analysis also involved x-ray diffraction and photoluminescence data. It allowed us to determine the key parameters of the structure. In particular, the radiative broadening of the InGaN QW excitons was evaluated as 0.20 ± 0.02 meV.

Improvement for the performance of solar-blind photodetector based on β-Ga2O3 thin films by doping Zn

NASA Astrophysics Data System (ADS)

Zhao, Xiaolong; Wu, Zhenping; Zhi, Yusong; An, Yuehua; Cui, Wei; Li, Linghong; Tang, Weihua

2017-03-01

Highly oriented (\\bar{2} 0 1 ) Ga2-x Zn x O3 thin films with different doping concentrations were grown on (0 0 0 1) sapphire substrates by laser molecular beam epitaxy technology. The expansion of lattice and the shrinkage of band gap with increasing doping level confirms the chemical substitution of Zn2+ ions into the Ga2O3 crystal lattice. The emission intensity of blue-violet emission bands enhanced with the increase of (ZnGa)‧ under 254 nm ultraviolet excitation, and the maximum was obtained at x  =  0.8. A metal-semiconductor-metal structured solar-blind photodetector based on Ga2-x Zn x O3 (x  =  0, 0.8) was made, the increasing responsivity and diminishing relaxation time constants for β-Ga2-x Zn x O3 (x  =  0.8) photodetector were observed with 254 nm ultraviolet illumination.

Method of manufacturing flexible metallic photonic band gap structures, and structures resulting therefrom

DOEpatents

Gupta, Sandhya; Tuttle, Gary L.; Sigalas, Mihail; McCalmont, Jonathan S.; Ho, Kai-Ming

2001-08-14

A method of manufacturing a flexible metallic photonic band gap structure operable in the infrared region, comprises the steps of spinning on a first layer of dielectric on a GaAs substrate, imidizing this first layer of dielectric, forming a first metal pattern on this first layer of dielectric, spinning on and imidizing a second layer of dielectric, and then removing the GaAs substrate. This method results in a flexible metallic photonic band gap structure operable with various filter characteristics in the infrared region. This method may be used to construct multi-layer flexible metallic photonic band gap structures. Metal grid defects and dielectric separation layer thicknesses are adjusted to control filter parameters.

Conduction- and Valence-Band Energies in Bulk InAs(1-x)Sb(x) and Type II InAs(1-x) Sb(x)/InAs Strained-Layer Superlattices

DTIC Science & Technology

2013-03-08

tions in the studied SLS structures . The fit of the dependence of the valence- band energy of unstrained InAs1!xSbx on the composition x with a... band . STRUCTURES Bulk InAsSb epilayers on metamorphic buffers and InAsSb/InAs strained-layer superlattices (SLS) were grown on GaSb substrates by solid...meV in InAs and Ev = 0 meV in InSb. For InAsSb with 22.5% Sb grown on GaSb , an unstrained valence- band energy of Ev = !457 meV was obtained. For the

On Optimizing an Archibald Rubber-Band Heat Engine.

ERIC Educational Resources Information Center

Mullen, J. G.; And Others

1978-01-01

Discusses the criteria and procedure for optimizing the performance of Archibald rubber-band heat engines by using the appropriate choice of dimensions, minimizing frictional torque, maximizing torque and balancing the rubber band system. (GA)

Polarized Infrared Reflectance Studies of Quaternary In0.04Al0.06Ga0.90N

NASA Astrophysics Data System (ADS)

Bakhori, S. K. Mohd; Lee, S. C.; Ahmad, M. A.; Ng, S. S.; Hassan, H. Abu

2010-07-01

Group III-nitride has re-gained considerable interest recently as wide direct band gap semiconductor materials for opto-electronic and high power devices. The quaternary InAlGaN have great flexibility in tailoring their band gap profile while maintaining their lattice-matching and structural integrity. In this study, we report for the first time the polarized infrared (IR) reflectance studies of quaternary In0.04Al0.06Ga0.90N by using Fourier transform infrared spectroscopy of Perkin-Elmer. The quaternary In0.04Al0.06Ga0.90N epilayers was grown on sapphire by molecular beam epitaxy. The polarized IR reflectance spectra obtained at incident angle of 15° were then compared with modeling spectrum of damped harmonic oscillator. Through this study, the transverse and longitudinal optical phonon modes of quaternary In0.04Al0.06Ga0.90N epilayers were obtained.

Redshift of the light emission from highly strained In0.3Ga0.7As/GaAs quantum wells by dipole δ doping

NASA Astrophysics Data System (ADS)

Fu, Y.; Wang, S.-M.; Wang, X.-D.; Larsson, A.

2005-08-01

We have studied theoretically the energy band structures and optical properties of highly strained dipole δ-doped In0.3Ga0.7As/GaAs single quantum wells. Including dopant diffusion effect, strain in the quantum well, spin-orbital interactions, and many-body effects, the self-consistent calculations of the eight-band k •p model and the Poisson equation show that the dipole δ doping induces an electric field across the In0.3Ga0.7As quantum well by the Stark effect so that both the interband transition energy and the wave-function overlap between the ground-state electrons and holes are reduced. Applying an external bias across the quantum well partially cancels the built-in electric field and reduces the wavelength redshift. The calculated material gain peak is close to the experimental lasing wavelength.

Suppression of electron overflow in 370-nm InGaN/AlGaN ultraviolet light emitting diodes with different insertion layer thicknesses

NASA Astrophysics Data System (ADS)

Wang, C. K.; Wang, Y. W.; Chiou, Y. Z.; Chang, S. H.; Jheng, J. S.; Chang, S. P.; Chang, S. J.

2017-06-01

In this study, the properties of 370-nm InGaN/AlGaN ultraviolet light emitting diodes (UV LEDs) with different thicknesses of un-doped Al0.3Ga0.7N insertion layer (IL) between the last quantum barrier and electron blocking layer (EBL) have been numerically simulated by Advance Physical Model of Semiconductor Devices (APSYS). The results show that the LEDs using the high Al composition IL can effectively improve the efficiency droop, light output power, and internal quantum efficiency (IQE) compared to the original structure. The improvements of the optical properties are mainly attributed to the energy band discontinuity and offset created by IL, which increase the potential barrier height of conduction band to suppress the electron overflow from the active region to the p-side layer.

Lack of quantum confinement in Ga2O3 nanolayers

NASA Astrophysics Data System (ADS)

Peelaers, Hartwin; Van de Walle, Chris G.

2017-08-01

β -Ga2Ox3 is a wide-band-gap semiconductor with promising applications in transparent electronics and in power devices. β -Ga2O3 has monoclinic crystal symmetry and does not display a layered structured characteristic of 2D materials in the bulk; nevertheless, monolayer-thin Ga2O3 layers can be created. We used first-principles techniques to investigate the structural and electronic properties of these nanolayers. Surprisingly, freestanding films do not exhibit any signs of quantum confinement and exhibit the same electronic structure as bulk material. A detailed examination reveals that this can be attributed to the presence of states that are strongly confined near the surface. When the Ga2O3 layers are embedded in a wider band-gap material such as Al2O3 , the expected effects of quantum confinement can be observed. The effective mass of electrons in all the nanolayers is small, indicating promising device applications.

AlGaAs diode pumped tunable chromium lasers

DOEpatents

Krupke, William F.; Payne, Stephen A.

1992-01-01

An all-solid-state laser system is disclosed wherein the laser is pumped in the longwave wing of the pump absorption band. By utilizing a laser material that will accept unusually high dopant concentrations without deleterious effects on the crystal lattice one is able to compensate for the decreased cross section in the wing of the absorption band, and the number of pump sources which can be used with such a material increases correspondingly. In a particular embodiment a chromium doped colquiriite-structure crystal such as Cr:LiSrAlF.sub.6 is the laser material. The invention avoids the problems associated with using AlGaInP diodes by doping the Cr:LiSrAlF.sub.6 heavily to enable efficient pumping in the longwave wing of the absorption band with more practical AlGaAs diodes.

Defect physics in intermediate-band materials: Insights from an optimized hybrid functional

NASA Astrophysics Data System (ADS)

Han, Miaomiao; Zeng, Zhi; Frauenheim, Thomas; Deák, Peter

2017-10-01

Despite the efforts to implement the idea of a deep level impurity intermediate band (IB) into bulk solar cell materials, a breakthrough in efficiency increase has not yet been achieved. Taking Sn-doped CuGaS2 as an example, we investigate the problem here from the perspective of defect physics, considering all possible charge states of the dopant and its interaction with native defects. Using an optimized hybrid functional, we find that SnGa has not only a donor-type (+/0), but also an acceptor-type (0 /- ) charge transition level. We estimate the probability of the optical transition of an electron from/to the neutral defect to/from the conduction-band edge to be about equal, therefore, the lifetimes of the excited carriers are probably quite short, limiting the enhancement of the photocurrent. In addition, we find that doping with SnGa leads to the spontaneous formation of the intrinsic acceptor CuGa defects which passivate the donor SnGa and pin the Fermi level to a position (1.4 eV above the valence-band edge) where both defects are ionized. As a result, the possibility of absorption in the middle of the visible range gets lost. These two recombination and passivation mechanisms appear to be quite likely the case for other donors and other similar host materials as well, explaining some of the experimental bottlenecks with IB solar cells based on deep level impurities.

Room-Temperature-Synthesized High-Mobility Transparent Amorphous CdO-Ga2O3 Alloys with Widely Tunable Electronic Bands.

PubMed

Liu, Chao Ping; Ho, Chun Yuen; Dos Reis, Roberto; Foo, Yishu; Guo, Peng Fei; Zapien, Juan Antonio; Walukiewicz, Wladek; Yu, Kin Man

2018-02-28

In this work, we have synthesized Cd 1-x Ga x O 1+δ alloy thin films at room temperature over the entire composition range by radio frequency magnetron sputtering. We found that alloy films with high Ga contents of x > 0.3 are amorphous. Amorphous Cd 1-x Ga x O 1+δ alloys in the composition range of 0.3 < x < 0.5 exhibit a high electron mobility of 10-20 cm 2 V -1 s -1 with a resistivity in the range of 10 -2 to high 10 -4 Ω cm range. The resistivity of the amorphous alloys can also be controlled over 5 orders of magnitude from 7 × 10 -4 to 77 Ω cm by controlling the oxygen stoichiometry. Over the entire composition range, these crystalline and amorphous alloys have a large tunable intrinsic band gap range of 2.2-4.8 eV as well as a conduction band minimum range of 5.8-4.5 eV below the vacuum level. Our results suggest that amorphous Cd 1-x Ga x O 1+δ alloy films with 0.3 < x < 0.4 have favorable optoelectronic properties as transparent conductors on flexible and/or organic substrates, whereas the band edges and electrical conductivity of films with 0.3 < x < 0.7 can be manipulated for transparent thin-film transistors as well as electron transport layers.

Photoluminescence spectroscopy and the effective mass theory of strained (In,Ga)As/GaAs heterostructures grown on (112)B GaAs substrates

NASA Technical Reports Server (NTRS)

Henderson, R. H.; Sun, D.; Towe, E.

1995-01-01

The photoluminescence characteristics of pseudomorphic In(0.19)Ga(0.81)As/GaAs quantum well structures grown on both the conventional (001) and the unconventional (112)B GaAs substrate are investigated. It is found that the emission spectra of the structures grown on the (112)B surface exhibit some spectral characteristics not observed on similar structures grown on the (001) surface. A spectral blue shift of the e yields hh1 transition with increasing optical pump intensity is observed for the quantum wells on the (112) surface. This shift is interpreted to be evidence of a strain-induced piezoelectric field. A second spectral feature located within the band gap of the In(0.19)Ga(0.81)As layer is also observed for the (112) structure; this feature is thought to be an impurity-related emission. The expected transition energies of the quantum well structures are calculated using the effective mass theory based on the 4 x 4 Luttinger valence band Hamiltonian, and related strain Hamiltonian.

Electronic structure of the dilute magnetic semiconductor G a1 -xM nxP from hard x-ray photoelectron spectroscopy and angle-resolved photoemission

NASA Astrophysics Data System (ADS)

Keqi, A.; Gehlmann, M.; Conti, G.; Nemšák, S.; Rattanachata, A.; Minár, J.; Plucinski, L.; Rault, J. E.; Rueff, J. P.; Scarpulla, M.; Hategan, M.; Pálsson, G. K.; Conlon, C.; Eiteneer, D.; Saw, A. Y.; Gray, A. X.; Kobayashi, K.; Ueda, S.; Dubon, O. D.; Schneider, C. M.; Fadley, C. S.

2018-04-01

We have investigated the electronic structure of the dilute magnetic semiconductor (DMS) G a0.98M n0.02P and compared it to that of an undoped GaP reference sample, using hard x-ray photoelectron spectroscopy (HXPS) and hard x-ray angle-resolved photoemission spectroscopy (HARPES) at energies of about 3 keV. We present experimental data, as well as theoretical calculations, to understand the role of the Mn dopant in the emergence of ferromagnetism in this material. Both core-level spectra and angle-resolved or angle-integrated valence spectra are discussed. In particular, the HARPES experimental data are compared to free-electron final-state model calculations and to more accurate one-step photoemission theory. The experimental results show differences between G a0.98M n0.02P and GaP in both angle-resolved and angle-integrated valence spectra. The G a0.98M n0.02P bands are broadened due to the presence of Mn impurities that disturb the long-range translational order of the host GaP crystal. Mn-induced changes of the electronic structure are observed over the entire valence band range, including the presence of a distinct impurity band close to the valence-band maximum of the DMS. These experimental results are in good agreement with the one-step photoemission calculations and a prior HARPES study of G a0.97M n0.03As and GaAs [Gray et al., Nat. Mater. 11, 957 (2012), 10.1038/nmat3450], demonstrating the strong similarity between these two materials. The Mn 2 p and 3 s core-level spectra also reveal an essentially identical state in doping both GaAs and GaP.

Studies on gallium nitride doped ferrite-polypyrrole nanocomposites

NASA Astrophysics Data System (ADS)

Indrakanti, Rajani; Brahmaji Rao, V.; Udaya Kiran, C.

2018-06-01

This communication reports the synthesis and characterisation of two novel Intrinsic conducting polymer nano composites (ICPN s) with the formulae Ga (2x+2) N Fe 2(49-x) O3—PPY synthesized using Impregnation technique. The Gallium nitride ferrite nano particles were synthesized for x = 1 and x = 5 using the above stichiometric equation earlier by Sol—Gel route. The chemical composition in the assembly of the ICPNs were Ga4NFe96O3-3%,10%,30% Polypyrrole, Ga12NFe88O3-3%,10%,30% Polypyrrole by weight. The Sci-Finder software failed to trace any earlier articles or reviews related to these ICNPs synthesised by us in the literature. X-ray Diffractometric (Structural), Morphological, EDAX SAED, IR spectroscopic characterizations were done on the synthesized nanocomposites. Structural studies reveal the semi-crystalline nature of composites. The average crystallite size of nano composites is decreased when compared with nano ferrites. SEM findings reveal that the shape for higher percentage of PPY is nano rods; for lower percentage it is globular. TEM reveals good dispersion and average particle size from histograms are calculated. The FT- IR bands of PPY and GaNFe2O3 are observed which show strong interaction between PPY- GaNFe2O3. Also there is a shift of bands in GaNFe2O3-PPY nano composites when compared to the bands of PPY.

Pressure Study of Photoluminescence in GaN/InGaN/ AlGaN Quantum Wells

NASA Astrophysics Data System (ADS)

Perlin, Piotr; Iota, V.; Weinstein, B. A.; Wisniewski, P.; Osinski, M.; Eliseev, P. G.

1997-03-01

We have studied the photoluminescence (PL) from two commercial high brightness single quantum well light emitting diodes (Nichia Chem. Industs.) with In_xGa_1-x N (x=0.45 and 0.2) as the active layers under hydrostatic pressures up to 7 GPa. These diodes are the best existing light emitters at short wavelengths, having the emission wavelengths of 430 nm and 530 nm depending on the content of indium in the 30 Åthick quantum wells. Although these devices show a remarkable quality and efficiency (luminosity as high as 12 cd), the mechanism of recombination remains obscure. We discovered that the pressure coefficient for each of the observed PL peaks is dramatically (2-3 times) lower than that of the energy gap of its InGaN active layer. These observations, in conjunction with the fact that the observed emission occurs below the energy gap of the quantum well material, and also considering the anomalous temperature behavior of the emission (peak energy increasing with temperature) suggest the involvement of localized states and exclude a simple band-to-band recombination picture. These localized states may be tentatively attributed to the presence of band tails in the gap which stem from composition fluctuations in the InGaN alloy. (figures)

Calculations and measurements of contact resistance of semi-transparent Ni/Pd contacts to p-GaN.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Crofton, John; Bogart, Katherine Huderle Andersen

2005-06-01

Calculations of specific contact resistance as a function of doping and barrier height were performed for p-type GaN. These calculations took into account two valence bands, each with different effective masses, and show that at low doping, the heavy hole band accounts for most of the conduction, whereas at heavier doping, the light hole band dominates conduction. These calculations also indicate the barrier height for typical contacts to p-GaN is between 0.75 eV and 1 eV. Specific contact resistance measurements were made for oxidized Ni/Au, Pd, and oxidized Ni/Pd ohmic contact metal schemes to p-GaN. The Ni/Pd contact had themore » lowest specific contact resistance, 6 x 10{sup -4} {Omega} cm{sup 2}. Auger sputter depth profile analysis showed some Ni diffused away from the GaN surface to the contact surface with the bulk of the Pd located in between two areas of Ni. Both Ni and Pd interdiffused with the GaN at the semiconductor surface. The majority of the oxygen observed was with the Ni as NiO. Angle-resolved-x-ray photoelectron spectroscopy (AR-XPS) analyses showed the formation of predominantly NiO and PdO species, with higher Ni and Pd oxides at the contact surface.« less

Direct charge carrier injection into Ga2O3 thin films using an In2O3 cathode buffer layer: their optical, electrical and surface state properties

NASA Astrophysics Data System (ADS)

Cui, W.; Zhao, X. L.; An, Y. H.; Guo, D. Y.; Qing, X. Y.; Wu, Z. P.; Li, P. G.; Li, L. H.; Cui, C.; Tang, W. H.

2017-04-01

Conductive Ga2O3 thin films with an In2O3 buffer layer have been prepared on c-plane sapphire substrates using a laser molecular beam epitaxy technique. The effects of the In2O3 buffer layer on the structure and optical, electrical and surface state properties of the Ga2O3 films have been studied. The change in conductivity of the thin films is attributed to different thicknesses of the In2O3 buffer layer, which determine the concentration of charge carriers injected into the upper Ga2O3 layer from the interface of the bilayer thin films. In addition, the increase in flat band voltage shift and capacitance values as the In2O3 buffer layer thickens are attributed to the increase in surface state density, which also contributes to the rapid shrinkage of the optical band gap of the Ga2O3. With transparency to visible light, high n-type conduction and the ability to tune the optical band gap and surface state density, we propose that Ga2O3/In2O3 bilayer thin film is an ideal n-type semiconductor for fabrication of transparent power devices, solar cell electrodes and gas sensors.

Semiconductor tunnel junction with enhancement layer

DOEpatents

Klem, J.F.; Zolper, J.C.

1997-10-21

The incorporation of a pseudomorphic GaAsSb layer in a runnel diode structure affords a new degree of freedom in designing runnel junctions for p-n junction device interconnects. Previously only doping levels could be varied to control the tunneling properties. This invention uses the valence band alignment band of the GaAsSb with respect to the surrounding materials to greatly relax the doping requirements for tunneling. 5 figs.

Opening the band gap of graphene through silicon doping for the improved performance of graphene/GaAs heterojunction solar cells.

PubMed

Zhang, S J; Lin, S S; Li, X Q; Liu, X Y; Wu, H A; Xu, W L; Wang, P; Wu, Z Q; Zhong, H K; Xu, Z J

2016-01-07

Graphene has attracted increasing interest due to its remarkable properties. However, the zero band gap of monolayered graphene limits it's further electronic and optoelectronic applications. Herein, we have synthesized monolayered silicon-doped graphene (SiG) with large surface area using a chemical vapor deposition method. Raman and X-ray photoelectron spectroscopy measurements demonstrate that the silicon atoms are doped into graphene lattice at a doping level of 2.7-4.5 at%. Electrical measurements based on a field effect transistor indicate that the band gap of graphene has been opened via silicon doping without a clear degradation in carrier mobility, and the work function of SiG, deduced from ultraviolet photoelectron spectroscopy, was 0.13-0.25 eV larger than that of graphene. Moreover, when compared with the graphene/GaAs heterostructure, SiG/GaAs exhibits an enhanced performance. The performance of 3.4% silicon doped SiG/GaAs solar cell has been improved by 33.7% on average, which was attributed to the increased barrier height and improved interface quality. Our results suggest that silicon doping can effectively engineer the band gap of monolayered graphene and SiG has great potential in optoelectronic device applications.

Electrical and band structural analyses of Ti1-x Al x O y films grown by atomic layer deposition on p-type GaAs

NASA Astrophysics Data System (ADS)

An, Youngseo; Mahata, Chandreswar; Lee, Changmin; Choi, Sungho; Byun, Young-Chul; Kang, Yu-Seon; Lee, Taeyoon; Kim, Jiyoung; Cho, Mann-Ho; Kim, Hyoungsub

2015-10-01

Amorphous Ti1-x Al x O y films in the Ti-oxide-rich regime (x  <  0.5) were deposited on p-type GaAs via atomic layer deposition with titanium isopropoxide, trimethylaluminum, and H2O precursor chemistry. The electrical properties and energy band alignments were examined for the resulting materials with their underlying substrates, and significant frequency dispersion was observed in the accumulation region of the Ti-oxide-rich Ti1-x Al x O y films. Although a further reduction in the frequency dispersion and leakage current (under gate electron injection) could be somewhat achieved through a greater addition of Al-oxide in the Ti1-x Al x O y film, the simultaneous decrease in the dielectric constant proved problematic in finding an optimal composition for application as a gate dielectric on GaAs. The spectroscopic band alignment measurements of the Ti-oxide-rich Ti1-x Al x O y films indicated that the band gaps had a rather slow increase with the addition of Al-oxide, which was primarily compensated for by an increase in the valance band offset, while a nearly-constant conduction band offset with a negative electron barrier height was maintained.

A simplified approach to the band gap correction of defect formation energies: Al, Ga, and In-doped ZnO

NASA Astrophysics Data System (ADS)

Saniz, R.; Xu, Y.; Matsubara, M.; Amini, M. N.; Dixit, H.; Lamoen, D.; Partoens, B.

2013-01-01

The calculation of defect levels in semiconductors within a density functional theory approach suffers greatly from the band gap problem. We propose a band gap correction scheme that is based on the separation of energy differences in electron addition and relaxation energies. We show that it can predict defect levels with a reasonable accuracy, particularly in the case of defects with conduction band character, and yet is simple and computationally economical. We apply this method to ZnO doped with group III elements (Al, Ga, In). As expected from experiment, the results indicate that Zn substitutional doping is preferred over interstitial doping in Al, Ga, and In-doped ZnO, under both zinc-rich and oxygen-rich conditions. Further, all three dopants act as shallow donors, with the +1 charge state having the most advantageous formation energy. Also, doping effects on the electronic structure of ZnO are sufficiently mild so as to affect little the fundamental band gap and lowest conduction bands dispersion, which secures their n-type transparent conducting behavior. A comparison with the extrapolation method based on LDA+U calculations and with the Heyd-Scuseria-Ernzerhof hybrid functional (HSE) shows the reliability of the proposed scheme in predicting the thermodynamic transition levels in shallow donor systems.

Quasiparticle band offset at the (001) interface and band gaps in ultrathin superlattices of GaAs-AlAs heterojunctions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, S.B.; Cohen, M.L.; Louie, S.G.

1990-05-15

A newly developed first-principles quasiparticle theory is used to calculate the band offset at the (001) interface and band gaps in 1{times}1 and 2{times}2 superlattices of GaAs-AlAs heterojunctions. We find a sizable many-body contribution to the valence-band offset which is dominated by the many-body corrections to bulk GaAs and AlAs quasiparticle energies. The resultant offset {Delta}{ital E}{sub {ital v}}=0.53{plus minus}0.05 eV is in good agreement with the recent experimental values of 0.50--0.56 eV. Our calculated direct band gaps for ultrathin superlattices are also in good agreement with experiment. The {ital X}{sub 1{ital c}}-derived state at point {bar {Gamma}}, is however,more » above the {Gamma}{sub 1{ital c}}-derived state for both the 1{times}1 and 2{times}2 lattices, contrary to results obtained under the virtual-crystal approximation (a limiting case for the Kronig-Penny model) and some previous local-density-approximation (corrected) calculations. The differences are explained in terms of atomic-scale localizations and many-body effects. Oscillator strengths and the effects of disorder on the spectra are discussed.« less

Density Functional Theory Calculation of the Band Alignment of (101̅0) In(x)Ga(1-x)N/Water Interfaces.

PubMed

Meng, Andrew C; Cheng, Jun; Sprik, Michiel

2016-03-03

Conduction band edge (CBE) and valence band edge (VBE) positions of InxGa1-xN photoelectrodes were computed using density functional theory methods. The band edges of fully solvated GaN and InN model systems were aligned with respect to the standard hydrogen electrode using a molecular dynamics hydrogen electrode scheme applied earlier to TiO2/water interfaces. Similar to the findings for TiO2, we found that the Purdew-Burke-Ernzerhof (PBE) functional gives a VBE potential which is too negative by 1 V. This cathodic bias is largely corrected by application of the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional containing a fraction of Hartree-Fock exchange. The effect of a change of composition was investigated using simplified model systems consisting of vacuum slabs covered on both sides by one monolayer of H2O. The CBE was found to vary linearly with In content. The VBE, in comparison, is much less sensitive to composition. The data show that the band edges straddle the hydrogen and oxygen evolution potentials for In fractions less than 47%. The band gap was found to exceed 2 eV for an In fraction less than 54%.

Theory of intermediate- and high-field mobility in dilute nitride alloys

NASA Astrophysics Data System (ADS)

Seifikar, Masoud; O'Reilly, Eoin P.; Fahy, Stephen

2011-10-01

We have solved the steady-state Boltzmann transport equation in bulk GaAs1-xNx. Two different models of the conduction band structure have been studied to investigate the behavior of electrons with increasing electric field in these alloys: (1) carriers in parabolic Γ and L bands are scattered by resonant nitrogen substitutional defect states, polar optic and acoustic phonons, and intervalley optical phonons; (2) carriers, constrained in the lower band of the band-anticrossing (BAC) model, are scattered by phonons and by nitrogen states. We consider scattering both by isolated N atoms and also by a full distribution of N states. We find that it is necessary to include the full distribution of levels in order to account for the small low-field mobility and the absence of a negative differential velocity regime observed experimentally with increasing x. Model 2 breaks down at intermediate and high field, due to the unphysical constraint of limiting carriers to the lower BAC band. For model 1, carrier scattering into the L bands is reduced at intermediate electric fields but is comparable at high fields to that observed in GaAs, with the calculated high-field mobility and carrier distribution then also being comparable to GaAs. Overall the results account well for a wide range of experimental data.