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Sample records for al cr ga

  1. Optical properties of heusler alloys Co2FeSi, Co2FeAl, Co2CrAl, and Co2CrGa

    NASA Astrophysics Data System (ADS)

    Shreder, E. I.; Svyazhin, A. D.; Belozerova, K. A.

    2013-11-01

    The results of an investigation of optical properties and the calculations of the electronic structure of Co2FeSi, Co2FeAl, Co2CrAl, and Co2CrGa Heusler alloys are presented. The main focus of our attention is the study of the spectral dependence of the real part (ɛ1) and imaginary part (ɛ2) of the dielectric constant in the range of wavelengths λ = 0.3-13 μm using the ellipsometric method. An anomalous behavior of the optical conductivity σ(ω) has been found in the infrared range in the Co2CrAl and Co2CrGa alloys, which differs substantially from that in the Co2FeSi and Co2FeAl alloys. The results obtained are discussed based on the calculations of the electronic structure.

  2. YZ (Y = V, Cr; Z = Al, Ga) under pressure: a DFT study

    NASA Astrophysics Data System (ADS)

    Seema, K.; Kumar, Ranjan

    2014-09-01

    The structural, electronic and magnetic properties of Co-based Heusler compounds Co2YZ (Y = V, Cr; Z = Al, Ga) under pressure are studied using first principles density functional theory. The calculations are performed within generalized gradient approximation. The total magnetic moment decreases slightly on compression. Under application of external pressure, the valence band and conduction band are shifted downward which leads to the modification of electronic structure. There exists an indirect band gap along Г- X for all the alloys studied. Co2CrAl shows half-metallic nature up to 85 GPa. After this pressure transition from true half-metallic behavior to nearly half-metallic behavior is observed and at 90 GPa it shows metallic behavior. Co2CrGa shows nearly half-metallic behavior at ambient pressure, but true half-metallic behavior is observed as pressure is increased to 100 GPa. For Co2VGa, true half-metallic to nearly half-metallic transition is observed at 40 GPa and around 100 GPa, Co2VGa shows metallic behavior. For Co2VAl, true half-metallic behavior is not observed at ambient as well as higher pressures. The half metal-to-metal transition in Co2VAl and Co2CrAl is accompanied by quenching of magnetic moment.

  3. Cr4 + :Gd3Sc2Ga3O12 passive Q-switch for the Cr3 + :LiCaAlF6 laser

    NASA Astrophysics Data System (ADS)

    Kuo, Yen-Kuang; Yang, Yang; Birnbaum, Milton

    1994-05-01

    A Cr4+:Gd3Sc2Ga3O12 (Cr4+:GSGG) broad-band saturable absorber has been demonstrated to be an excellent passive Q-switch for the flashlamp-pumped tunable Cr3+:LiCaAlF6 (Cr:LiCAF) laser at room temperature. A single Q-switched laser output pulse of 11 mJ in energy and 37 ns in duration at 778 nm was obtained in a nonoptimized laser.

  4. Electronic structure, magnetism and stability of Co2CrX (X =Al, Ga, In) ab initio study

    NASA Astrophysics Data System (ADS)

    Dahmane, F.; Mesri, D.; Tadjer, A.; Khenata, R.; Benalia, S.; Djoudi, L.; Doumi, B.; Boumia, L.; Aourag, H.

    2016-01-01

    The structural, electronic as well as the magnetic properties of the Co2CrX (X =Al, Ga and In) full-Heusler alloy have been studied using first-principles calculations performed in the framework of density functional theory (DFT) within the generalized gradient approximation (GGA). It was taken into account both possible L21 structures (i.e. Hg2CuTi- and Cu2MnAl-type). Basically, for all compounds, the Cu2MnAl-type structure is energetically more stable than Hg2CuTi-type structure at the equilibrium volume. The electronic structure calculations for Co2CrAl reveal that half-metallic (HM) character in Cu2MnAl-type structure, Co2CrGa show nearly HM behavior and Co2CrIn has a metallic character. The predicted total magnetic moment is 3μB for Co2CrX (X =Al, Ga) which is in good convergence with the Slater-Pauling (SP) rule.

  5. Waste polyethylene terephthalate (PET) materials as sustainable precursors for the synthesis of nanoporous MOFs, MIL-47, MIL-53(Cr, Al, Ga) and MIL-101(Cr).

    PubMed

    Lo, Sheng-Han; Senthil Raja, Duraisamy; Chen, Chia-Wei; Kang, Yu-Hao; Chen, Jiun-Jen; Lin, Chia-Her

    2016-06-21

    In our novel green approach, the waste polyethylene terephthalate (PET) bottle material has effectively been used as the starting precursor instead of terephthalic acid for the synthesis of five terephthalate based nanoporous trivalent metal-organic frameworks (MOFs) namely MIL-47, MIL-53(Cr), MIL-53(Al), MIL-53(Ga), and MIL-101(Cr). The optimum reaction parameters to achieve the green synthesis were studied. These MOFs were structurally identified by using powder X-ray diffraction (PXRD) measurements. Scanning electron microscopy (SEM) images confirm the particle nature and size of the synthesized MOFs. Nitrogen gas sorption measurements have been done for some of the MOFs to check their porous properties. All the characterization techniques strongly supported that the synthesized MOFs using PET are similar to their literature reports. The gas adsorption studies for the synthesized MIL-53(Cr) and MIL-101(Cr) showed their significant gas uptake capability towards CO2 and H2 gases. Further, the synthesized MIL-47 and MIL-101(Cr) have been tested for their catalytic ability in chemical fixation of CO2 gas through the conversion of CO2 and epoxides to the corresponding cyclic carbonates which shows promising results to use them as catalysts. PMID:27198203

  6. Near-infrared luminescence and color tunable chromophores based on Cr(3+)-doped mullite-type Bi2(Ga,Al)4O9 solid solutions.

    PubMed

    Liu, Chengyin; Xia, Zhiguo; Chen, Mingyue; Molokeev, Maxim S; Liu, Quanlin

    2015-02-16

    Cr(3+)-activated mullite-type Bi2Ga(4-x)Al(x)O9 (x = 0, 1, 2, 3, and 4) solid solutions were prepared by the solid state reaction, and their spectroscopic properties were investigated in conjunction with the structural evolution. Under excitation at 610 nm, Bi2[Ga(4-y)Al(y)]3.97O9:0.03Cr(3+) (y = 0, 1, 2, 3, and 4) phosphors exhibited broad-band near-infrared (NIR) emission peaking at ∼710 nm in the range 650-850 nm, and the optimum Cr(3+) concentrations and concentration quenching mechanism were determined. Except for the interesting NIR emission, the body color changed from white (at x = 0) to green (at x = 0.08) for Bi2Ga(4-x)O9:xCr(3+), and from light yellow (at x = 0) to deep brown (at x = 0.08) for Bi2Al(4-x)O9:xCr(3+), respectively. Moreover, as a result of variable Al/Ga ratio, the observed body color for Bi2[Ga(4-y)Al(y)]3.97O9:0.03Cr(3+) (y = 0, 1, 2, 3, and 4) varied from deep brown to green. The relationship between the observed colors and their diffuse reflectance spectra were also studied for the understanding of the different absorption bands. The results indicated that Cr(3+)-doped Bi2Ga(4-x)Al(x)O9 solid solutions appeared as the bifunctional materials with NIR phosphors and color-tunable pigments. PMID:25622046

  7. A critical evaluation of GGA + U modeling for atomic, electronic and magnetic structure of Cr2AlC, Cr2GaC and Cr2GeC

    NASA Astrophysics Data System (ADS)

    Dahlqvist, M.; Alling, B.; Rosen, J.

    2015-03-01

    In this work we critically evaluate methods for treating electron correlation effects in multicomponent carbides using a GGA + U framework, addressing doubts from previous works on the usability of density functional theory in the design of magnetic MAX phases. We have studied the influence of the Hubbard U-parameter, applied to Cr 3d orbitals, on the calculated lattice parameters, magnetic moments, magnetic order, bulk modulus and electronic density of states of Cr2AlC, Cr2GaC and Cr2GeC. By considering non-, ferro-, and five different antiferromagnetic spin configurations, we show the importance of including a broad range of magnetic orders in the search for MAX phases with finite magnetic moments in the ground state. We show that when electron correlation is treated on the level of the generalized gradient approximation (U = 0 eV), the magnetic ground state of Cr2AC (A = Al, Ga, Ge) is in-plane antiferromagnetic with finite Cr local moments, and calculated lattice parameters and bulk modulus close to experimentally reported values. By comparing GGA and GGA + U results with experimental data we find that using a U-value larger than 1 eV results in structural parameters deviating strongly from experimentally observed values. Comparisons are also done with hybrid functional calculations (HSE06) resulting in an exchange splitting larger than what is obtained for a U-value of 2 eV. Our results suggest caution and that investigations need to involve several different magnetic orders before lack of magnetism in calculations are blamed on the exchange-correlation approximations in this class of magnetic MAX phases.

  8. Optical properties and storage capabilities in AB2O4:Cr3+ (A=Zn, Mg, B=Ga, Al)

    NASA Astrophysics Data System (ADS)

    Sharma, S. K.; Bessiere, A.; Gourier, D.; Sraiki, G.; Viana, B.; Dereń, P. J.; Rudnicka, D.; Watras, A.; Basavaraju, N.; Priolkar, K. R.; Maldiney, T.; Scherman, D.; Richard, C.

    2014-03-01

    Red emitting long-lasting phosphorescence (LLP) material, are useful biomarker for small animal in vivo imaging. We report here our investigations on the optical features of chromium doped AB2O4 spinels (A=Zn, Mg and B=Ga, Al…) suitable for such applications. It is possible to tune the emission wavelengths of Cr3+ by a crystal field variation to be well centered in the biological window and it is also possible to adjust the traps depth in order to better control the release of the traps. These traps are therefore stable at room temperature and could be emptied by thermal or near infrared source making this material a potential new photostimulated/optically compound. Photoluminescence (PL) and thermally stimulated luminescence (TSL) studies are reported.

  9. Effect of Cr Addition on Wetting Behavior Between Cu and High-Temperature Zn-25Sn-0.15Al-0.1Ga- xCr Pb-Free Solder

    NASA Astrophysics Data System (ADS)

    Liu, Chin-Wei; Lin, Kwang-Lung

    2014-12-01

    In this study the effect of Cr content (0.02 wt.% to 0.2 wt.%) on the wetting interaction between high-temperature Zn-25Sn-0.15Al-0.1Ga- xCr Pb-free solders and Cu has been investigated using the wetting balance method. Differential scanning calorimetry (DSC) investigation showed that Cr addition reduces the liquidus temperature slightly while raising the solidus temperature. Flux-assisted wetting experiments were carried out at an immersion rate of 15 mm/s at 435°C. The results show that the shortest wetting time of around 0.7 s was achieved when 0.1 wt.% Cr was added, but the wetting force tends to decrease with the Cr content. Cr addition enhances the formation of interfacial Cu5Zn8 intermetallic compound (IMC) during air cooling, but the opposite effect was observed for water cooling. The results suggest that Cr addition depresses the formation of the Cu5Zn8 layer in liquid/solid reaction, but enhances the formation of the Cu5Zn8 layer in solid/solid reaction.

  10. Persistent luminescence of AB2O4:Cr3+ (A = Zn, Mg, B = Ga, Al) spinels: New biomarkers for in vivo imaging

    NASA Astrophysics Data System (ADS)

    Sharma, S. K.; Gourier, D.; Viana, B.; Maldiney, T.; Teston, E.; Scherman, D.; Richard, C.

    2014-09-01

    Recently red emitting long-lasting phosphorescence (LLP) materials have been demonstrated to be useful biomarkers for small animal in vivo imaging. We report here our investigations on the optical properties of chromium doped AB2O4 spinels (with A = Zn, Mg and B = Ga, Al) suitable for such applications. It is possible to tune the absorption wavelengths of Cr3+ by a crystal field variation and also slightly vary the emission to be better centered in the biological window and to adjust the trap depth in order to better control the release of the charges. These traps are therefore stable at room temperature and could be emptied by thermal or near infrared source, which makes these materials potential new optically photo-storage compounds.

  11. The importance of inversion disorder in the visible light induced persistent luminescence in Cr³⁺ doped AB₂O₄ (A = Zn or Mg and B = Ga or Al).

    PubMed

    Basavaraju, Neelima; Priolkar, Kaustubh R; Gourier, Didier; Sharma, Suchinder K; Bessière, Aurélie; Viana, Bruno

    2015-01-21

    Cr(3+) doped spinel compounds AB2O4 with A = Zn, Mg and B = Ga, Al exhibit a long, near infrared persistent luminescence when excited with UV or X-rays. In addition, the persistent luminescence of ZnGa2O4, and to a lesser extent MgGa2O4, can also be induced by visible light excitation via (4)A2→(4)T2 transition of Cr(3+), which makes these compounds suitable as biomarkers for in vivo optical imaging of small animals. We correlate this peculiar optical property with the presence of antisite defects, which are present in ZnGa2O4 and MgGa2O4. By using X-ray absorption fine structure (XAFS) spectroscopy, associated with electron paramagnetic resonance (EPR) and optical emission spectroscopy, it is shown that an increase in antisite defects concentration results in a decrease in the Cr-O bond length and the octahedral crystal field energy. A part of the defects occurs in the close environment of Cr(3+) ions, as shown by the increasing strain broadening of EPR and XAFS peaks observed upon increasing antisite disorder. It appears that ZnAl2O4, which exhibits the largest crystal field splitting of Cr(3+) and the smallest antisite disorder, does not show considerable persistent luminescence upon visible light excitation as compared to ZnGa2O4 and MgGa2O4. These results highlight the importance of Cr(3+) ions with neighboring antisite defects in the mechanism of persistent luminescence exhibited by Cr(3+) doped AB2O4 spinel compounds. PMID:25462833

  12. Investigation of structural, electronic and magnetic properties of 1:1:1:1 stoichiometric quaternary Heusler alloys YCoCrZ (Z=Si, Ge, Ga, Al): An ab-initio study

    NASA Astrophysics Data System (ADS)

    Rasool, M. Nasir; Mehmood, Salman; Sattar, M. Atif; Khan, Muhammad Azhar; Hussain, Altaf

    2015-12-01

    Full potential linearized augmented plane wave method (FPLAPW) has been employed to probe the structural, electronic and magnetic properties of equiatomic yttrium based quaternary Heusler alloys YCoCrZ (Z=Si, Ge, Ga, Al). These calculations have been carried out via ab -initio simulations based on density functional theory (DFT) approach coded by Wien2K. The generalized gradient approximation of Perdew-Burke-Ernzerhof 96 scheme is engaged for calculations in all alloys under investigation. Equilibrium lattice constants are studied by structural optimization performed by computing total energies versus volumes. Structural optimization demonstrates that Y(3/4,3/4,3/4)Co(0,0,0)Cr(1/2,1/2,1/2)Z(1/4,1/4,1/4) (Type-1) configuration is the most stable one. The calculated electronic and magnetic properties based on type-1, indicate that YCoCrZ alloys are half-metallic ferromagnetic. The calculation of spin polarization is also made and further their total magnetic moments follow the Slater Pauling rule of Mtot=NVE-18 conceding the integer value i.e. 4.00μB and 3.00μB for YCoCrSi, Ge and YCoCrGa, Al respectively. The results of density of states (DOS) revealed that yttrium based quaternary Heusler alloys exhibit excellent band gaps i.e. 0.70, 0.65, 0.46 and 0.35 eV for YCoCrSi, Ge, Ga and Al respectively. The formation of band gaps owing to hybridization effect is also described. The half-metallic gaps of these compounds comprising the order YCoCrGa>YCoCrSi>YCoCrAl>YCoCrGe by size, is also manipulated. The incredible spin gapless semiconductor (SGS) type character of YCoCrGa and YCoCrAl having bantam DOS in spin up version is also discoursed. The optimised results of these compounds signpost that these are suitable candidates for spintronics applications.

  13. The structural, electronic, magnetic and mechanical properties of quaternary Heusler alloys ZrTiCrZ (Z  =  Al, Ga, In, Si, Ge, Sn): a first-principles study

    NASA Astrophysics Data System (ADS)

    Yan, Peng-Li; Zhang, Jian-Min; Zhou, Bo; Xu, Ke-Wei

    2016-06-01

    The structural, electronic, magnetic and mechanical properties of the quaternary Heusler alloys ZrTiCrZ (Z  =  Al, Ga, In, Si, Ge, Sn) have been investigated firstly by using the first-principles calculations. The preferred configurations of the ZrTiCrZ alloys are all Y-type (I). At their equilibrium lattice constants, the ZrTiCrZ alloys are half-metallic (HM) ferrimagnets for Z  =  Al, Ga and In, while spin-gapless semiconductor (SGS) antiferromagnets (AFM) for Z  =  Si, Ge and Sn. The total magnetic moments {μt} of the ZrTiCrZ alloys are  ‑1 {μ\\text{B}}/\\text{f}\\text{.u}\\text{.} for Z  =  Al, Ga and In, while 0 {μ\\text{B}}/\\text{f}\\text{.u}\\text{.} for Z  =  Si, Ge and Sn, both linearly scaled with the total number of valence electrons {{Z}\\text{t}} by Slater–Pauling rule {μ\\text{t}}={{Z}\\text{t}}-18 . The elastic constants {{C}11} , {{C}12} and {{C}44} of the single crystal and the related elastic moduli G , B , E , \\upsilon and A of the polycrystalline aggregates are also calculated and used to study the mechanical stability of these alloys. Although the Curie temperatures {{T}\\text{C}} of the ZrTiCrZ alloys are overestimated by using the mean field approximation (MFA), they can be better estimated by including the exchange interactions. Finally, the HM stabilities as well as the total and atomic magnetic moments of the ZrTiCrZ alloys (Z  =  Al, Ga, In) under either hydrostatic strain or tetragonal strain are also discussed.

  14. Vibrational analysis of the elpasolites Cs{sub 2}NaAlF{sub 6} and Cs{sub 2}NaGaF{sub 6} doped with Cr{sup 3+} ions by fluorescence spectroscopy.

    SciTech Connect

    Bordallo, H. N.; Sosman, L. P.; Tavares, A. D., Jr.; da Fonseca, R. J. M.

    1999-09-07

    Interest in 3d transition metal impurities in ionic crystals has increased due to their important role in the laser activity of these materials. Moreover, recent advances in tunable solid-state lasers and high-power semiconductor laser diode arrays have generated a strong interest in investigating new compounds that emit in the visible and near-infrared spectral regions. In particular, many optical studies have been devoted to Cr{sup 3+}-doped fluoride crystals as a consequence of the high quality of some Cr{sup 3+}-based laser materials. In the present investigation, the low temperature emission spectra of Cr{sup 3+} ions in the hexagonal elpasolites Cs{sub 2}NaAlF{sub 6} and Cs{sub 2}NaGaF{sub 6} have been measured. Each compound has two crystallographically inequivalent octahedral sites for the Al{sup 3+} and Ga{sup 3+} ions that can be occupied by Cr{sup 3+} ions. For both materials, the luminescence spectrum presents two zero-phonon lines accompanied by a well-defined vibrational structure. The different peaks of the emission broadband are described in terms of phonons of the lattice and normal modes of the octahedral complex [CrF{sub 6}]{sup 3{minus}}. A detailed analysis of the vibrational structure observed leads to the conclusion that the {sup 2}E and {sup 4}T{sub 2} excited states of the [CrF{sub 6}]{sup 3{minus}} ions are displaced along the e{sub g}, a{sub 1g} and probably the t{sub 2g} coordinates.

  15. Ohmic contact formation between metal and AlGaN/GaN heterostructure via graphene insertion

    NASA Astrophysics Data System (ADS)

    Sung Park, Pil; Reddy, Kongara M.; Nath, Digbijoy N.; Yang, Zhichao; Padture, Nitin P.; Rajan, Siddharth

    2013-04-01

    A simple method for the creation of Ohmic contact to 2D electron gas in AlGaN/GaN high electron-mobility transistors using Cr/graphene layer is demonstrated. A weak temperature dependence of this Ohmic contact observed in the range 77 to 300 K precludes thermionic emission or trap-assisted hopping as possible carrier-transport mechanisms. It is suggested that the Cr/graphene combination acts akin to a doped n-type semiconductor in contact with AlGaN/GaN heterostructure, and promotes carrier transport along percolating Al-lean paths through the AlGaN layer. This use of graphene offers a simple method for making Ohmic contacts to AlGaN/GaN heterostructures, circumventing complex additional processing steps involving high temperatures. These results could have important implications for the fabrication and manufacturing of AlGaN/GaN-based microelectronic and optoelectronic devices/sensors of the future.

  16. LiCaAl/sub 6/:Cr/sup 3+/

    SciTech Connect

    Payne, S.A.; Chase, L.L.; Newkirk, H.W.; Smith, L.K.; Krupke, W.F. )

    1988-11-01

    The authors report the discovery of a new laser, LiCaAIF/sub 6/:Cr/sup 3/ (Cr/sup 3+/ :LiCAF). The intrinsic (extrapolated maximum) slope efficiency was found to be 67 percent. For comparison, they also measured the intrinsic slope efficiencies of BeAl/sub 2/O/sub 4/:Cr/sup 3+/ (alexandrite), Na/sub 3/Ga/sub 2/Li/sub 3/F/sub 12/:Cr/sup 3+/, and ScBO/sub 3/:Cr/sup 3+/, and obtained values of 65,28, and 26 percent, respectively. The tuning range of LiCaAIF/sub 6/:Cr/sup 3+/ was determined to be at least 720-840 nm. The conventional spectroscopic properties, such as the absorption, emission, and emission lifetimes as a function of temperature, are reported as well.

  17. A photovoltaic effect in the metal-high-resistive GaAs:Cr contact

    NASA Astrophysics Data System (ADS)

    Budnitskii, D. L.; Novikov, V. A.; Prudaev, I. A.; Тоlbanov, О. P.; Yaskevich, Т. М.

    2012-12-01

    The results of studies of photovoltaic effect in the contacts of a number of metals with high-resistive GaAs:Cr are reported. High-resistive (HR) GaAs was obtained by diffusion of chromium in n-GaAs. V, Cr, and Al were used as metals. In was employed in order to produce ohmic contacts. Photovoltage was excited by red light (hν = 1.85 eV), and the excitation intensity amounted to 1.5ṡ1021 сm-2ṡs-1. Photovoltage was measured in the presence of asymmetric pairs of contacts to HR-GaAs: V-In, Cr-In, and Al-In. It is shown that V, Cr, and Al form barriers for electrons in the contact with high-resistive GaAs:Cr. The photovoltage of the contacts is determined by the inversion of conductivity type of the near-surface GaAs layer under the metal contact. The hole concentration in the inversion layer can be as high as ≈1015 сm-3. An In contact to high-resistive GaAs:Cr is an ohmic injecting contact for electrons with the barrier height for holes ≈0.9 eV.

  18. Magnetometory of AlGaN/GaN heterostructure wafers

    NASA Astrophysics Data System (ADS)

    Tsubaki, K.; Maeda, N.; Saitoh, T.; Kobayashi, N.

    2005-06-01

    AlGaN/GaN heterostructure wafers are becoming a key technology for next generation cellar-phone telecommunication system because of their potential for high-performance microwave applications. Therefore, the electronic properties of a 2DEG in AlGaN/GaN heterostructures have recently been discussed. In this paper, we performed the extraordinary Hall effect measurement and the SQUID magnetometory of AlGaN/GaN heterostructure wafer at low temperature. The AlGaN/GaN heterostructures were grown by low-pressure metal-organic chemical vapour phase epitaxy on (0001) SiC substrate using AlN buffers. The electron mobility and electron concentration at 4.2 K are 9,540cm2/V s and 6.6 × 1012cm-2, respectively. In the extraordinary Hall effect measurement of AlGaN/GaN heterostructures, the hysteresis of Hall resistance appeared below 4.5 K and disappeared above 4.5 K. On the other hand, the hysteresis of magnetometric data obtained by SQUID magnetometory appears near zero magnetic field when the temperature is lower than 4.5 K. At the temperature larger than 4.5 K, the hysteresis of magnetometric data disappears. And the slopes of magnetometric data with respect to magnetic field become lower as obeying Currie-Weiss law and the Curie temperature TC is 4.5 K. Agreement of TC measured by the extraordinary Hall effect and the SQUID magnetometory implies the ferromagnetism at the AlGaN/GaN heterojunction. However, the conformation of the ferromagnetism of AlGaN/GaN heterostructure is still difficult and the detailed physical mechanism is still unclear.

  19. High breakdown voltage in AlGaN/GaN HEMTs using AlGaN/GaN/AlGaN quantum-well electron-blocking layers

    PubMed Central

    2014-01-01

    In this paper, we numerically study an enhancement of breakdown voltage in AlGaN/GaN high-electron-mobility transistors (HEMTs) by using the AlGaN/GaN/AlGaN quantum-well (QW) electron-blocking layer (EBL) structure. This concept is based on the superior confinement of two-dimensional electron gases (2-DEGs) provided by the QW EBL, resulting in a significant improvement of breakdown voltage and a remarkable suppression of spilling electrons. The electron mobility of 2-DEG is hence enhanced as well. The dependence of thickness and composition of QW EBL on the device breakdown is also evaluated and discussed. PMID:25206318

  20. 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.

  1. Comparison of trap characteristics between AlGaN/GaN and AlGaN/InGaN/GaN heterostructure by frequency dependent conductance measurement

    SciTech Connect

    Chakraborty, Apurba; Biswas, Dhrubes

    2015-02-23

    Frequency dependent conductance measurement is carried out to observe the trapping effect in AlGaN/InGaN/GaN double heterostructure and compared that with conventional AlGaN/GaN single heterostructure. It is found that the AlGaN/InGaN/GaN diode structure does not show any trapping effect, whereas single heterostructure AlGaN/GaN diode suffers from two kinds of trap energy states in near depletion to higher negative voltage bias region. This conductance behaviour of AlGaN/InGaN/GaN heterostructure is owing to more Fermi energy level shift from trap energy states at AlGaN/InGaN junction compare to single AlGaN/GaN heterostructure and eliminates the trapping effects. Analysis yielded interface trap energy state in AlGaN/GaN is to be with time constant of (33.8–76.5) μs and trap density of (2.38–0.656) × 10{sup 12 }eV{sup −1} cm{sup −2} in −3.2 to −4.8 V bias region, whereas for AlGaN/InGaN/GaN structure no interface energy states are found and the extracted surface trap energy concentrations and time constants are (5.87–4.39) ×10{sup 10} eV{sup −1} cm{sup −2} and (17.8–11.3) μs, respectively, in bias range of −0.8–0.0 V.

  2. Molecular beam epitaxy growth and magnetic properties of Cr-Co-Ga Heusler alloy films

    SciTech Connect

    Feng, Wuwei Wang, Weihua; Zhao, Chenglong; Van Quang, Nguyen; Cho, Sunglae; Dung, Dang Duc

    2015-11-15

    We have re-investigated growth and magnetic properties of Cr{sub 2}CoGa films using molecular beam epitaxy technique. Phase separation and precipitate formation were observed experimentally again in agreement with observation of multiple phases separation in sputtered Cr{sub 2}CoGa films by M. Meinert et al. However, significant phase separation could be suppressed by proper control of growth conditions. We showed that Cr{sub 2}CoGa Heusler phase, rather than Co{sub 2}CrGa phase, constitutes the majority of the sample grown on GaAs(001) at 450 {sup o}C. The measured small spin moment of Cr{sub 2}CoGa is in agreement with predicted HM-FCF nature; however, its Curie temperature is not as high as expected from the theoretical prediction probably due to the off-stoichiometry of Cr{sub 2}CoGa and the existence of the disorders and phase separation.

  3. Ti{sub 2}GaC, Ti{sub 4}GaC{sub 3} and Cr{sub 2}GaC-Synthesis, crystal growth and structure analysis of Ga-containing MAX-phases M{sub n+1}GaC{sub n} with M=Ti, Cr and n=1, 3

    SciTech Connect

    Etzkorn, Johannes; Ade, Martin; Kotzott, Dominik; Kleczek, Monique; Hillebrecht, Harald

    2009-05-15

    Single crystals of Ga-containing MAX-phases Ti{sub 2}GaC, Ti{sub 4}GaC{sub 3}, and Cr{sub 2}GaC were grown from a metallic melt generated by an excess of Ga. This technique allows the crystal growth at different temperatures to control the product distribution. Compounds developed were Ti{sub 2}GaC and TiC at 1500 deg. C, and Ti{sub 2}GaC and Ti{sub 4}GaC{sub 3} at 1300 deg. C. Crystal structures were refined from single crystal data. Ti{sub 2}GaC and Cr{sub 2}GaC were previously known, and belong to the Cr{sub 2}AlC type as well as the solid solutions V{sub 2}Ga{sub 1-x}Al{sub x}C and Cr{sub 2}Ga{sub 1-x}Al{sub x}C. Ti{sub 4}GaC{sub 3} is one of the few 413-phases (P6{sub 3}/mmc, a=3.0690(4) A, c=23.440(5) A) and the first Ga-containing representative. The crystal structures of MAX-phases are intergrowths of layers of an intermetallic MGa in a hexagonal stacking sequence with carbidic layers (MC){sub n} of the NaCl type. The thickness of the layer depends from the value of n. The results of the structure refinements also demonstrate that also the structural details follow this description. - Graphical abstract: Single crystals of Ga-containing MAX-Phases (TiC){sub n}(TiGa) (n=1, 3) were grown from a metallic melt including and characterised by X-ray diffraction. Ti{sub 4}GaC{sub 3} is one of the few 413-phase and the first containing Ga.

  4. Stimulated emission in AlGaN/AlGaN quantum wells with different Al content

    NASA Astrophysics Data System (ADS)

    Mickevičius, J.; Jurkevičius, J.; Kazlauskas, K.; Žukauskas, A.; Tamulaitis, G.; Shur, M. S.; Shatalov, M.; Yang, J.; Gaska, R.

    2012-02-01

    Stimulated emission (SE) is studied in AlGaN/AlGaN multiple quantum wells (MQWs) with different Al content grown on sapphire substrate. The spectra of spontaneous and stimulated emission and their transformations with increasing temperature as well as stimulated emission thresholds were measured in the temperature range from 8 to 300 K. Phonon-assisted band broadening in low-Al-content MQWs and double-scaled potential profile in high-Al-content MQWs were observed in the samples and linked with carrier localization conditions. The temperature dependence of the stimulated emission threshold was similar in the samples where the stimulated transitions occur between extended states and in the samples where the transitions occur in localized states. The stimulated emission threshold depends predominantly on the density of nonradiative recombination centers.

  5. Cr Atom Alignment in Cr-Delta-Doped GaN

    NASA Astrophysics Data System (ADS)

    Kimura, S.; Emura, S.; Ofuchi, H.; Nakata, Y.; Zhou, Y. K.; Choi, S. W.; Yamauchi, Y.; Hasegawa, S.; Asahi, H.

    2007-02-01

    Structural properties and Cr atom alignments in Cr-delta doped GaN grown by molecular beam epitaxy are studied with transmission electron microscopy and X-ray absorption fine structure (XAFS) measurements. It is found that the environment around Cr atoms in delta-doped samples is dramatically changed under various growth conditions. The XAFS analysis of these synthesized layers suggests that new Cr-related complexes are grown.

  6. Scattering induced by Al segregation in AlGaN/GaN heterostructures

    SciTech Connect

    Liu, Xiwen; Lu, Yanwu; Ji, Dong

    2015-08-17

    The effect of Al segregation near dislocations on the mobility of two-dimensional electron gas in AlGaN/GaN heterostructure-based high-electron-mobility transistors was investigated. Exponentially varied composition fluctuation was effective in describing Al segregation near dislocations when calculating scattering behavior. Mobility, which was limited by Al segregation surrounding dislocation lines, was calculated to be in the order of 10{sup 3} cm{sup 2}/Vs to 10{sup 6} cm{sup 2}/Vs. Results indicated that the mobility in AlGaN/GaN heterojunction was enhanced upon the reduction of dislocation density at low temperature. This study contributes to generating higher electron mobility in AlGaN/GaN heterojunctions.

  7. Analysis of InGaN light-emitting diodes with GaN-AlGaN and AlGaN-GaN composition-graded barriers

    SciTech Connect

    Yang, Yujue; Wang, Junxi; Li, Jinmin; Zeng, Yiping

    2014-06-21

    The effects of InGaN-based light-emitting diodes (LEDs) with Al composition increasing and decreasing GaN-AlGaN barriers along the growth direction are studied numerically. Simulation results suggest that the LEDs with GaN-AlGaN composition-decreased barriers show more significant enhancement of light-output power and internal quantum efficiency than LEDs with composition-increasing GaN-AlGaN barriers when compared with the conventional LED with GaN barriers, due to the improvement in hole injection efficiency and electron blocking capability. Moreover, the optical performance is further improved by replacing GaN-AlGaN barriers with AlGaN-GaN barriers of the same Al composition-decreasing range, which are mainly attributed to the modified band diagrams. In addition, the major causes of the different efficiency droop behaviors for all the designed structures are explained by the electron leakage current and the different increase rates of hole concentration with injection current.

  8. AlGaAs-GaAs cascade solar cell

    NASA Technical Reports Server (NTRS)

    Lamorte, M. F.; Abbott, D. H.

    1980-01-01

    Computer modeling studies are reported for a monolithic, two junction, cascade solar cell using the AlGaAs GaAs materials combination. An optimum design was obtained through a serial optimization procedure by which conversion efficiency is maximized for operation at 300 K, AM 0, and unity solar concentration. Under these conditions the upper limit on efficiency was shown to be in excess of 29 percent, provided surface recombination velocity did not exceed 10,000 cm/sec.

  9. Bulk modulus and specific heat of B-site doped (La0.3Pr0.7)0.65Ca0.35Mn1-xBxO3 (B=Fe, Cr, Ru, Al, Ga)

    NASA Astrophysics Data System (ADS)

    Srivastava, Archana; Thakur, Rasna; Gaur, N. K.

    2014-04-01

    Specific heat (Cp) thermal expansion (α) and Bulk modulus (BT) of lightly doped Rare Earth manganites (La0.3Pr0.7)0.65Ca0.35Mn1-xBxO3 (B3+ = Fe3+,Cr3+,Ga3+,Al3+,Ru4+); (0.3

  10. Structural and optical properties of Cr-doped semi-insulating GaN epilayers

    SciTech Connect

    Mei, F.; Wu, K. M.; Pan, Y.; Han, T.; Liu, C.; Gerlach, J. W.; Rauschenbach, B.

    2008-09-15

    The properties of Cr-doped GaN epilayers grown by rf-plasma-assisted molecular beam epitaxy were studied. The deep acceptor nature of Cr was used to grow semi-insulating GaN epilayers on sapphire substrates for electronic device applications. The room-temperature (RT) sheet resistivity of the epilayers reached 10{sup 10} {omega}/square. The activation energy of dark conductivity was about 0.48 eV. Step-graded Al{sub x}Ga{sub 1-x}N/GaN (x=0.3-0.2) superlattices (SLs) were designed to filter dislocations. Transmission electron microscopy images showed that the SLs can dramatically reduce dislocation density. Al{sub 0.35}Ga{sub 0.65}N/GaN heterostructure grown on Cr-doped semi-insulating GaN epilayer exhibited a RT mobility of 960 cm{sup 2}/V s and sheet carrier density of 2.1x10{sup 13} cm{sup -2}.

  11. Structure, magnetism, and electron-transport properties of Mn2CrGa-based nanomaterials

    NASA Astrophysics Data System (ADS)

    Zhang, Wenyong; Kharel, Parashu; Skomski, Ralph; Valloppilly, Shah; Li, Xingzhong; Sellmyer, David J.

    2016-05-01

    Mn2CrGa in the disordered cubic structure has been synthesized using rapid quenching and subsequent annealing. The cubic phase transforms to a stable tetragonal phase when a fraction of Cr or Ga is replaced by Pt or Al, respectively. All samples are ferrimagnetic with high Curie temperatures (Tc); Mn2CrGa exhibits the highest Tc of about 813 K. The tetragonal samples have appreciable values of magnetocrystalline anisotropy energy, which leads to an increase in coercivity (Hc) that approaches about 10 kOe in the Pt-doped sample. The Hc linearly increases with a decrease of temperature, concomitant with the anisotropy change with temperature. All samples are metallic and show negative magnetoresistance with room-temperature resistivities on the order of 1 mΩcm. The magnetic properties including high Tc and low magnetic moment suggest that these tetragonal materials have potential for spin-transfer-torque-based devices.

  12. High performance AlGaN/GaN HEMTs with AlN/SiNx passivation

    NASA Astrophysics Data System (ADS)

    Xin, Tan; Yuanjie, Lü; Guodong, Gu; Li, Wang; Shaobo, Dun; Xubo, Song; Hongyu, Guo; Jiayun, Yin; Shujun, Cai; Zhihong, Feng

    2015-07-01

    AlGaN/GaN high electron-mobility transistors (HEMTs) with 5 nm AlN passivation by plasma enhanced atomic layer deposition (PEALD) were fabricated, covered by 50 nm SiNx which was grown by plasma enhanced chemical vapor deposition (PECVD). With PEALD AlN passivation, current collapse was suppressed more effectively and the devices show better subthreshold characteristics. Moreover, the insertion of AlN increased the RF transconductance, which lead to a higher cut-off frequency. Temperature dependence of DC characteristics demonstrated that the degradations of drain current and maximum transconductance at elevated temperatures for the AlN/SiNx passivated devices were much smaller compared with the devices with SiNx passivation, indicating that PEALD AlN passivation can improve the high temperature operation of the AlGaN/GaN HEMTs. Project supported by the National Natural Science Foundation of China (No. 60890192).

  13. Reverse bias leakage current mechanism of AlGaN/InGaN/GaN heterostructure

    NASA Astrophysics Data System (ADS)

    Chakraborty, Apurba; Ghosh, Saptarsi; Mukhopadhyay, Partha; Jana, Sanjay K.; Dinara, Syed Mukulika; Bag, Ankush; Mahata, Mihir K.; Kumar, Rahul; Das, Subhashis; Das, Palash; Biswas, Dhrubes

    2016-03-01

    The reverse bias leakage current mechanism of AlGaN/InGaN/GaN heterostructure is investigated by current-voltage measurement in temperature range from 298 K to 423 K. The Higher electric field across the AlGaN barrier layer of AlGaN/InGaN/GaN double heterostructure due to higher polarization charge is found to be responsible for strong Fowler-Nordheim (FN) tunnelling in the electric field higher than 3.66 MV/cm. For electric field less than 3.56 MV/cm, the reverse bias leakage current is also found to follow the trap assisted Frenkel-Poole (FP) emission in low negative bias region. Analysis of reverse FP emission yielded the barrier height of trap energy level of 0.34 eV with respect to Fermi level. [Figure not available: see fulltext.

  14. Surface half-metallicity of half-Heusler compound FeCrSe and interface half-metallicity of FeCrSe/GaP

    NASA Astrophysics Data System (ADS)

    Khalaf Al-zyadi, Jabbar M.; Jolan, Mudhahir H.; Yao, Kai-Lun

    2016-04-01

    Recent studies showed that half-Heusler FeCrSe exhibits half-metallic ferromagnetism (Huang et al. [20]). In this paper, we investigate extensively the electronic, magnetic, and half-metallic properties of the half-Heusler alloy FeCrSe (111) and (001) surfaces and the interface with GaP (111) substrate by using the first-principles calculations within the density functional theory. The atomic density of states demonstrates that the half-me tallicity verified in the bulk FeCrSe is maintained at the CrSe-terminated (001) and Se-terminated (111) surfaces, but lost at both Cr- and Fe-terminated (111) surfaces and the Fe-terminated (001) surface. Alternatively, for the interface of FeCrSe/GaP (111), the bulk half-metallicity is destroyed at Se-P configuration while Se-Ga interface and subinterface show nearly 100% spin polarization. Moreover, the calculated interfacial adhesion energies exhibit that Se-Ga shape is more stable than the Se-P one. The calculated magnetic moments of Se, Ga at the Se-Ga (111) interface and P at the Se-P (111) interface increase with respect to the corresponding bulk values while the atomic magnetic moment of Se atom at the Se-P (111) interface decreases. We also notice that the magnetic moments of subinterface Fe at both Se-Ga and Se-P (111) interfaces decrease compared to the bulk values.

  15. Introducing the AlGaN/GaN/InAlGaN/GaN Dh-Hemt Structure and it is Functional Analysis

    NASA Astrophysics Data System (ADS)

    Shahrjerdi, Roshanak Manzari; Faez, Rahim; Madadi, Robab

    2011-12-01

    In this article we present a new Al0.3Ga0.7N/GaN/InxAlyGa1-x-yN/GaN double Heterojunction HEMT (DH-HEMT) structure. In this structure a thin layer of InxAlyGa1-x-yN was inserted within the GaN channel. Compared with the baseline AlGaN/GaN HEMTs, The InAlGaN layer provides a unique piezoelectric polarization field which results in a higher potential barrier at the backside of the two-dimensional electron gas channel, effectively improving the carrier confinement and then reducing the buffer leakage. Also the DH-HEMT shows higher carriers density and conductivity. This device is obtained, replacing the InAlGaN for InGaN in Al0.3Ga0.7N/GaN/In0.1Ga0.9N/GaN DH-HEMT structure, for increasing the breakdown voltage. Because in this structure the InGaN alloy, which has the role of minor channel in the structure, has a small critical electric field (˜1.75 Mv/cm) rather than the (2 Mv/cm) GaN alloy (the channel in the conventional AlGaN/GaN HEMTs). So in order to increase the breakdown voltage in this structure it has been tried to find a substitute for the InGaN, so that it can keep the proper specifications of InGaN and have a bigger critical electric field (Ecrit) rather than GaN and InGaN. Lately the InAlGaN alloy, possessing the goal specification, has been noticeable. In order to achieve the best specifications, different amounts of In, Al are considered in the InAlGaN structure so that, the Ecrit have a bigger magnitude 2 Mv/cm (Ecrit in GaN alloy) and the polarization charge shows a proper magnitude as well. In AlGaN/GaN/In0.15Al0.2GaN/GaN DH-HEMT structure rather than the AlGaN/GaN/InGaN/GaN DH-HEMT has small decrease in carrier density and conductivity, but the breakdown voltage has increased, Hence there is no remarkable breakdown voltage rather than the conventional AlGaN/GaN HEMTs, also at the end we have improved structure of AlGaN/GaN/In0.15Al0.08GaN/GaN DH-HEMT that carrier density and conductivity and breakdown voltage was improvement rather than InGaN DH-HEMT.

  16. Ferromagnetic Fe2CrAl Nanowires

    NASA Astrophysics Data System (ADS)

    Dulal, Rajendra; Dahal, Bishnu; Pegg, Ian L.; Philip, John

    Heusler alloy Fe2CrAl (FCA) nanowires were grown on silicon substrates. Nanowires have diameters in the range 50 to 200 nm and lengths up to 100 µm. They exhibit cubic L21 and A2 type structure with a space group, Pm m. Magnetic characterization reveals that they display ferromagnetic behavior and has a Curie temperature above 400 K. Magnetic behavior of FCA nanowires is different from the reported bulk behavior. Bulk FCA with L21 structure has a Curie temperature around 274 K. National Science Foundation under ECCS-0845501 and NSF-MRI, DMR-0922997.

  17. Enhancement of Red Persistent Luminescence in Cr3+-Doped ZnGa2O4 Phosphors by Bi2O3 Codoping

    NASA Astrophysics Data System (ADS)

    Zhuang, Yixi; Ueda, Jumpei; Tanabe, Setsuhisa

    2013-05-01

    Bi2O3 was proved to be an effective codopant to enhance red persistent luminescence in Cr3+-doped ZnGa2O4 spinel. The Cr-Bi-codoped ZnGa2O4 phosphors showed about 10 times higher persistent luminescence intensity than the Cr-singly-doped phosphors. The radiance (in mW sr-1m-2) of persistent luminescence in ZnGa2O4:Cr,Bi phosphors was comparable to that in commercialized SrAl2O4:Eu,Dy phosphors. Increases of Cr3+ absorption and photoluminescence were also observed in the Cr-Br-codoped ZnGa2O4 sample. The obtained results suggest that Bi2O3 may play a critical role in stabilizing Cr3+ in ZnGa2O4 spinel.

  18. Multicharacterization approach for studying InAl(Ga)N/Al(Ga)N/GaN heterostructures for high electron mobility transistors

    SciTech Connect

    Naresh-Kumar, G. Trager-Cowan, C.; Vilalta-Clemente, A.; Morales, M.; Ruterana, P.; Pandey, S.; Cavallini, A.; Cavalcoli, D.; Skuridina, D.; Vogt, P.; Kneissl, M.; Behmenburg, H.; Giesen, C.; Heuken, M.; Gamarra, P.; Di Forte-Poisson, M. A.; Patriarche, G.; Vickridge, I.

    2014-12-15

    We report on our multi–pronged approach to understand the structural and electrical properties of an InAl(Ga)N(33nm barrier)/Al(Ga)N(1nm interlayer)/GaN(3μm)/ AlN(100nm)/Al{sub 2}O{sub 3} high electron mobility transistor (HEMT) heterostructure grown by metal organic vapor phase epitaxy (MOVPE). In particular we reveal and discuss the role of unintentional Ga incorporation in the barrier and also in the interlayer. The observation of unintentional Ga incorporation by using energy dispersive X–ray spectroscopy analysis in a scanning transmission electron microscope is supported with results obtained for samples with a range of AlN interlayer thicknesses grown under both the showerhead as well as the horizontal type MOVPE reactors. Poisson–Schrödinger simulations show that for high Ga incorporation in the Al(Ga)N interlayer, an additional triangular well with very small depth may be exhibited in parallel to the main 2–DEG channel. The presence of this additional channel may cause parasitic conduction and severe issues in device characteristics and processing. Producing a HEMT structure with InAlGaN as the barrier and AlGaN as the interlayer with appropriate alloy composition may be a possible route to optimization, as it might be difficult to avoid Ga incorporation while continuously depositing the layers using the MOVPE growth method. Our present work shows the necessity of a multicharacterization approach to correlate structural and electrical properties to understand device structures and their performance.

  19. Study of nuclear quadrupole interactions and quadrupole Raman processes of 69Ga and 71Ga in a β-Ga 2O 3:Cr 3+ single crystal

    NASA Astrophysics Data System (ADS)

    Yeom, Tae Ho; Lim, Ae Ran

    2009-10-01

    Nuclear magnetic resonance (NMR) data and the spin-lattice relaxation times, T1, of 69Ga and 71Ga nuclei in a β-Ga 2O 3:Cr 3+ single crystal were obtained using FT NMR spectrometry. Four sets of NMR spectra for 69Ga ( I = 3/2) and 71Ga ( I = 3/2) were obtained in the crystallographic planes. The 69Ga and 71Ga nuclei each had two chemically inequivalent Ga I and Ga II centers. Each of the 69Ga and 71Ga isotopes yielded two different central NMR resonance lines originating from Ga I and Ga II sites. The nuclear quadrupole coupling constants and asymmetry parameters of 69Ga I, 69Ga II, 71Ga I, and 71Ga II centers in a β-Ga 2O 3:Cr 3+ crystal were obtained. Analysis of the EFG tensor principal axes (PAs) for Ga nuclei and the ZFS tensor PAs for the Cr 3+ ion confirmed that the Cr 3+ paramagnetic impurity ion substitutes for the Ga 3+ ion in the oxygen octahedron. In addition, the temperature dependencies of the 69Ga and 71Ga relaxation rates were consistent with Raman processes, as T1-1 ∝ T2. Even though the Cr 3+ impurities are paramagnetic, the relaxations were dominated by electric quadrupole interactions of the nuclear spins in the temperature range investigated.

  20. Asymmetric quantum-well structures for AlGaN/GaN/AlGaN resonant tunneling diodes

    NASA Astrophysics Data System (ADS)

    Yang, Lin'an; Li, Yue; Wang, Ying; Xu, Shengrui; Hao, Yue

    2016-04-01

    Asymmetric quantum-well (QW) structures including the asymmetric potential-barrier and the asymmetric potential-well are proposed for AlGaN/GaN/AlGaN resonant tunneling diodes (RTDs). Theoretical investigation gives that an appropriate decrease in Al composition and thickness for emitter barrier as well as an appropriate increase of both for collector barrier can evidently improve the negative-differential-resistance characteristic of RTD. Numerical simulation shows that RTD with a 1.5-nm-thick GaN well sandwiched by a 1.3-nm-thick Al0.15Ga0.85N emitter barrier and a 1.7-nm-thick Al0.25Ga0.75N collector barrier can yield the I-V characteristic having the peak current (Ip) and the peak-to-valley current ratio (PVCR) of 0.39 A and 3.6, respectively, about double that of RTD with a 1.5-nm-thick Al0.2Ga0.8N for both barriers. It is also found that an introduction of InGaN sub-QW into the diode can change the tunneling mode and achieve higher transmission coefficient of electron. The simulation demonstrates that RTD with a 2.8-nm-thick In0.03Ga0.97N sub-well in front of a 2.0-nm-thick GaN main-well can exhibit the I-V characteristic having Ip and PVCR of 0.07 A and 11.6, about 7 times and double the value of RTD without sub-QW, respectively. The purpose of improving the structure of GaN-based QW is to solve apparent contradiction between the device structure and the device manufacturability of new generation RTDs for sub-millimeter and terahertz applications.

  1. Creep and Toughness of Cryomilled NiAl Containing Cr

    NASA Technical Reports Server (NTRS)

    Whittenberger, J. Daniel; Aikin, Beverly; Salem, Jon

    2000-01-01

    NiAl-AlN + Cr composites were produced by blending cryomilled NiAl powder with approx. 10 vol % Cr flakes. In comparison to the as-consolidated matrices, hot isostatically pressed Cr-modified materials did not demonstrate any significant improvement in toughness. Hot extruded NiAl-AlN+10.5Cr, however, possessed a toughness twice that determine for the base NiAl-AlN alloy. Measurement of the 1200 to 1400 K plastic flow properties revealed that the strength of the composites was completely controlled by the properties of the NiAl-AlN matrices. This behavior could be successfully modeled by the Rule-of-Mixtures, where load is shed from the weak Cr to the strong matrix.

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

    SciTech Connect

    Kleinerman, Nadezhda M. Serikov, Vadim V. Vershinin, Aleksandr V. Mushnikov, Nikolai V. Stashkova, Liudmila A.

    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 enter the nearest surroundings of iron atoms, thus forming binary Fe-Ga regions (or phases)

  3. Formation of perovskite solid solutions and lithium-ion conductivity in the compositions, Li 2 xSr 1-2 xM III0.5- xTa 0.5+ xO 3 (M = Cr, Fe, Co, Al, Ga, In, Y)

    NASA Astrophysics Data System (ADS)

    Watanabe, Hiroyuki; Kuwano, Jun

    Formation of solid solutions with perovskite structure and ionic conductivity have been investigated in the systems Li 2 xSr 1-2 xM III0.5- xTa 0.5+ xO 3 (M = Cr, Fe, Co, Al, Ga, In, Y). Perovskite solid solutions formed in ranges of x ≤ 0.25 for M = Fe, Cr, x < 0.22 for Ga and x < ˜ 0.16 for M = Co, In. No single-phase samples of perovskite solid solution were prepared for M = Al, Y. The solid solutions Li 0.5Sr 0.5M III0.25Ta 0.75O 3 (M = Fe, Cr; x = 0.25) had high bulk conductivities of 1.0 × 10 -4 S cm -1 and 6.0 × 10 -5 S cm -1 at room temperature, respectively. The former value was the highest of those reported to date for lithium electrolytes based on tantalates. They were simple cubic perovskites (space group: Pm3 m; a = 396.39 pm for Fe, a = 395.03 pm for Cr), indicating that the Li and Sr ions, and the M (Fe or Cr) and Ta ions are randomly distributed over the A-sites and B-sites, respectively. Frame emission analysis for Li in both sintered pellets revealed that the loss of the Li content took place during sintering. The high conductivity is probably attributed to an A-site deficient perovskite phase resulting from the loss. The perovskite solid solutions in the other systems exhibited conductivities as low as 10 -7-10 -8 S cm -1. The Fe- and Cr-containing solid solutions are the first of tantalate-based perovskites with high Li-ion conductivity.

  4. Breakdown voltage enhancement of AlGaN/GaN high electron mobility transistors by polyimide/chromium composite thin film passivation

    NASA Astrophysics Data System (ADS)

    Futong, Chu; Chao, Chen; Xingzhao, Liu

    2014-03-01

    A novel AlGaN/GaN high electric mobility transistor (HEMT) with polyimide (PI)/chromium (Cr) as the passivation layer is proposed for enhancing breakdown voltage and its DC performance is also investigated. The Cr nanoparticles firstly introduced in PI thin films by the co-evaporation can be used to increase the permittivity of PI film. The high-permittivity PI/Cr passivation acting as field plate can suppress the fringing electric field peak at the drain-side edge of the gate electrode. This mechanism is demonstrated in accord with measured results. The experimental results show that in comparison with the AlGaN/GaN HEMTs without passivation, the breakdown voltage of HEMTs with the PI/Cr composite thin films can be significantly improved, from 122 to 248 V.

  5. Near-infrared multi-wavelengths long persistent luminescence of Nd3+ ion through persistent energy transfer in Ce3+, Cr3+ co-doped Y3Al2Ga3O12 for the first and second bio-imaging windows

    NASA Astrophysics Data System (ADS)

    Xu, Jian; Tanabe, Setsuhisa; Sontakke, Atul D.; Ueda, Jumpei

    2015-08-01

    We developed a persistent phosphor of Y3Al2Ga3O12 doped with Nd3+, Ce3+, Cr3+ ions (YAGG:Nd-Ce-Cr) exhibiting long (>10 h) persistent luminescence at multi-wavelengths of around 880, 1064, and 1335 nm due to f-f transitions of Nd3+ and at 505 nm due to Ce3+:5d1→4f transition. The intense near-infrared (NIR) persistent luminescence bands from Nd3+ match well with the first (650-950 nm) and second (1000-1400 nm) bio-imaging windows. The NIR persistent radiance of the YAGG:Nd-Ce-Cr phosphor (0.33 × 10-1 mW/Sr/m2) at 60 min after ceasing blue light illumination was over 2 times higher than that of the widely used ZnGa2O4:Cr3+ red persistent phosphor (0.15 × 10-1 mW/Sr/m2).

  6. Ultrafast optical studies on GaAs/AlGaAs/GaMnAs quantum wells

    NASA Astrophysics Data System (ADS)

    Schulz, R.; Korn, T.; Wurstbauer, U.; Schuh, D.; Wegscheider, W.; Schüller, C.

    2010-01-01

    GaMnAs is a highly interesting material system for future spintronic devices. We present a study of nonmagnetic GaAs quantum wells (QW) embedded in AlGaAs barriers, close to a ferromagnetic GaMnAs layer. The samples were grown on semi-insulating GaAs(001) and contain two QWs, where one QW is close to the GaMnAs layer and the other one is farther away (120 nm), and serves as a reference. We studied the influence of the barrier material, e.g. a short-period AlAs/GaAs superlattice. The photoluminescence (PL) of the upper QWs, close to the GaMnAs layer, show a significant broadening. Additionally, time-resolved Faraday rotation (TRFR) reveals that the spin lifetime in the upper QW is up to 30 times longer than that in the lower QW. We attribute these observations to backdiffusion of Mn into the QW during and after growth. Both, the PL and the TRFR, are highly sensitive to small quantities (below 0.05 %) of Mn and allow us to study the efficiency of barrier layers in suppressing Mn diffusion.

  7. Optical gain characteristics in Al-rich AlGaN/AlN quantum wells

    SciTech Connect

    Oto, Takao; Banal, Ryan G.; Funato, Mitsuru; Kawakami, Yoichi

    2014-05-05

    The optical gain characteristics of Al-rich AlGaN/AlN quantum wells (QWs) were assessed by the variable stripe length method at room temperature. An Al{sub 0.79}Ga{sub 0.21}N/AlN QW with a well width of 5 nm had a large optical gain of 140 cm{sup −1}. Increasing the excitation length induced a redshift due to the gain consumption and the consequent saturation of the amplified spontaneous emission. Moreover, a change in the dominant gain polarization with Al composition, which was attributed to switching of the valence band ordering of strained AlGaN/AlN QWs at Al compositions of ∼0.8, was experimentally demonstrated.

  8. AlGaN/InGaN Photocathode Development

    SciTech Connect

    Buckley, J. H.; Leopold, D. J.

    2008-12-24

    An increase in quantum efficiency in photodetectors could result in a proportional reduction in the area of atmospheric Cherenkov telescopes and an even larger reduction in cost. We report on the development of high quantum efficiency, high gain, UV/blue photon-counting detectors based on AlGaN/InGaN photocathode heterostructures grown by molecular beam epitaxy. This research could eventually result in nearly ideal light detectors with a number of distinct advantages over existing technologies for numerous applications in high-energy physics and particle astrophysics. Potential advantages include much lower noise detection, better stability and radiation resistance than other cathode structures, high VUV sensitivity and very low radioactive background levels for deep underground experiments, and high detection efficiency for the detection of individual VUV-visible photons. We are also developing photocathodes with intrinsic gain, initially improving the detection efficiency of hybrid semiconductor-vacuum tube devices and eventually leading to a new type of all-solid-state photomultiplier device.

  9. Preparation of Al-Cr-Fe Coatings by Heat Treatment of Electrodeposited Cr/Al Composite Coatings

    NASA Astrophysics Data System (ADS)

    Zhang, Min; Chen, Chang'an; Zhang, Guikai; Rao, Yongchu; Ling, Guoping

    Al-Cr-Fe coatings have been widely used in the surface engineering field of materials, due to their excellent corrosion resistance to water vapor and fused salt deposits. In this study, a new two-step approach was developed to prepare Al-Cr-Fe coatings on surfaces of SUS430 stainless steels. First, the Cr/Al composite coatings were prepared by electrodepositing Cr from aqueous solution then electrodepositing Al from AlCl3-1-ethyl-3-methyl-imidazolium chloride (AlCl3-EMIC) ionic liquid on SUS430 stainless steel substrate. In the second, heat treatment of the Cr/Al composite coatings was carried out to acquire Al-Cr-Fe coatings. Effects of the thickness of Cr/Al composite coatings, the time and temperature of heat treatment on composition and phase structure of alloy layers were studied by using scanning electron microscope (SEM), backscattered electron (BSE), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The structure transformation process and formation mechanism of Al-Cr-Fe coatings were discussed.

  10. Positron annihilation in AlN and GaN

    NASA Astrophysics Data System (ADS)

    Arutyunov, N. Yu.; Emtsev, V. V.; Mikhailin, A. V.; Davidov, V. Yu.

    2001-12-01

    The measurements of one-dimensional angular correlation of the annihilation radiation (1D-ACAR) have been carried out for AlN and GaN as well as for some related materials (Al, Ga, GaP, GaAs, GaSb) which have been used as samples of references the analysis of results. The numeral values of characteristic length of radius of spherical volume to be occupied by annihilating electron ( rs‧) have differed significantly from the corresponding values ( rs) calculated by the conventional independent-particle-model (IPM) for ideal Fermi-gas: rs‧ (AlN)≃1.28 rs, where rs (AlN)≃1.61 a.u., and rs‧ (GaN)≃1.66 rs, where rs (GaN)≃1.64 a.u. The electron-positron “ion radii” reconstructed by the high-momentum components (HMC) of 1D-ACAR for Al 3+, Ga 3+ cores as well as numeral rs‧ values provide some reasons to believe that Ga- and Al-vacancies and their impurity complexes are effective centers of the positron localization in AlN and GaN; it is assumed that these complexes include V Ga, V Al, and N atom (V Ga-N Ga in GaN and V Al-N Al in AlN) where the nitrogen atom is likely to be in the configuration of substitution (anti-site), N +Ga and N +Al, respectively.

  11. Investigation of amber light-emitting diodes based on InGaN/AlN/AlGaN quantum wells

    NASA Astrophysics Data System (ADS)

    Iida, Daisuke; Lu, Shen; Hirahara, Sota; Niwa, Kazumasa; Kamiyama, Satoshi; Ohkawa, Kazuhiro

    2016-05-01

    We investigated InGaN-based amber light-emitting diodes (LEDs) with AlN/(Al)GaN barrier layers grown by metalorganic vapor-phase epitaxy. Tensilely strained AlN/Al0.03Ga0.97N barriers improved the crystalline quality of compressively strained InGaN quantum wells. We found that strain compensation among wells and barriers improves the external quantum efficiency of high-In-content InGaN-based amber LEDs. The amber LEDs with AlN/Al0.03Ga0.97N barriers have shown an electroluminescence (EL) intensity approximately 2.5-fold that of LEDs with the AlN/GaN barriers at 20 mA.

  12. Reactive codoping of GaAlInP compound semiconductors

    DOEpatents

    Hanna, Mark Cooper; Reedy, Robert

    2008-02-12

    A GaAlInP compound semiconductor and a method of producing a GaAlInP compound semiconductor are provided. The apparatus and method comprises a GaAs crystal substrate in a metal organic vapor deposition reactor. Al, Ga, In vapors are prepared by thermally decomposing organometallic compounds. P vapors are prepared by thermally decomposing phospine gas, group II vapors are prepared by thermally decomposing an organometallic group IIA or IIB compound. Group VIB vapors are prepared by thermally decomposing a gaseous compound of group VIB. The Al, Ga, In, P, group II, and group VIB vapors grow a GaAlInP crystal doped with group IIA or IIB and group VIB elements on the substrate wherein the group IIA or IIB and a group VIB vapors produced a codoped GaAlInP compound semiconductor with a group IIA or IIB element serving as a p-type dopant having low group II atomic diffusion.

  13. Surface morphology of Cr:Ga2Se3 heteroepitaxy on Si(001)

    NASA Astrophysics Data System (ADS)

    Yitamben, E. N.; Lovejoy, T. C.; Paul, D. F.; Callaghan, J. B.; Ohuchi, F. S.; Olmstead, M. A.

    2009-08-01

    Addition of the transition-metal dopant Cr to Ga2Se3 during heteroepitaxial growth on Si(001), a system of interest as a prototype silicon-compatible, dilute magnetic semiconductor, has been studied with scanning-tunneling microscopy and scanning Auger microscopy as a function of Cr concentration and the presence or absence of an undoped buffer or capping layer. Chromium incorporates into laminar Ga2Se3 films up to a solubility limit of several atomic percent, after which Cr-rich islands nucleate. At low Cr concentrations, the vacancy-ordered nanoridge structure characteristic of pure Ga2Se3 remains but nanoridge aspect ratios decrease with Cr concentration; this is likely associated with Cr removing intrinsic vacancies. At higher Cr concentrations, faceted, Cr-rich islands nucleate, often surrounded by trenches, and the terrace morphology no longer resembles pure Ga2Se3 . Growth of Cr-doped Ga2Se3 directly on Si(001):As is qualitatively similar to growth on a pure Ga2Se3 buffer layer; however, the island structure changes dramatically upon coverage of a highly doped layer with undoped Ga2Se3 . Addition of Cr stabilizes cubic overlayer growth under Se-poor growth conditions beyond that of pure Ga2Se3 ; no growth of the hexagonal layered structure characteristic of bulk GaSe was observed.

  14. Hot Corrosion Performance of AlO-CrO/NiCoCrAlYTa and AlO/NiCoCrAlYTa Coatings Deposited by Atmospheric Plasma Spraying

    NASA Astrophysics Data System (ADS)

    Tao, Chong; Wang, Lei; Cheng, Nailiang; Hu, Hengfa; Liu, Yang; Song, Xiu

    2016-04-01

    AlO-CrO/NiCoCrAlYTa and AlO/NiCoCrAlYTa coatings were deposited on 316L stainless steel substrate using atmospheric plasma spraying, respectively, in order to improve the oxidation and corrosion resistance. The hot corrosion performance of the coatings at 700 and 900 °C were studied, and the detailed microstructures and phase composition of the coatings were analyzed using x-ray diffraction, scanning electron microscope with energy dispersive spectrometer, and transmission electron microscope. The results show that both coatings are structurally featured by slatted layers, consisting of amorphous phase, Cr2O3, Ni3Al, and Al2O3. The hot corrosion resistance of AlO-CrO/NiCoCrAlYTa coating is better than that of AlO/NiCoCrAlYTa coating. This improvement is attributed to lower porosity and more compact Cr2O3 in AlO-CrO/NiCoCrAlYTa coating which performs better than Al2O3 in blocking further inward progress of corrosion and oxidization.

  15. Coordination and activation of Al-H and Ga-H bonds.

    PubMed

    Abdalla, Joseph A B; Riddlestone, Ian M; Turner, Joshua; Kaufman, Paul A; Tirfoin, Remi; Phillips, Nicholas; Aldridge, Simon

    2014-12-22

    The modes of interaction of donor-stabilized Group 13 hydrides (E=Al, Ga) were investigated towards 14- and 16-electron transition-metal fragments. More electron-rich N-heterocyclic carbene-stabilized alanes/gallanes of the type NHC⋅EH3 (E=Al or Ga) exclusively generate κ(2) complexes of the type [M(CO)4 (κ(2)-H3 E⋅NHC)] with [M(CO)4 (COD)] (M=Cr, Mo), including the first κ(2) σ-gallane complexes. β-Diketiminato ('nacnac')-stabilized systems, {HC(MeCNDipp)2 }EH2 , show more diverse reactivity towards Group 6 carbonyl reagents. For {HC(MeCNDipp)2 }AlH2, both κ(1) and κ(2) complexes were isolated, while [Cr(CO)4 (κ(2)-H2 Ga{(NDippCMe)2 CH})] is the only simple κ(2) adduct of the nacnac-stabilized gallane which can be trapped, albeit as a co-crystallite with the (dehydrogenated) gallylene system [Cr(CO)5 (Ga{(NDippCMe)2 CH})]. Reaction of [Co2 (CO)8] with {HC(MeCDippN)2 }AlH2 generates [(OC)3 Co(μ-H)2 Al{(NdippCme)2 CH}][Co(CO)4] (12), which while retaining direct AlH interactions, features a hitherto unprecedented degree of bond activation in a σ-alane complex. PMID:25358970

  16. Radiation Hard AlGaN Detectors and Imager

    SciTech Connect

    2012-05-01

    Radiation hardness of AlGaN photodiodes was tested using a 65 MeV proton beam with a total proton fluence of 3x10{sup 12} protons/cm{sup 2}. AlGaN Deep UV Photodiode have extremely high radiation hardness. These new devices have mission critical applications in high energy density physics (HEDP) and space explorations. These new devices satisfy radiation hardness requirements by NIF. NSTec is developing next generation AlGaN optoelectronics and imagers.

  17. Effect of buffer structures on AlGaN/GaN high electron mobility transistor reliability

    SciTech Connect

    Liu, L.; Xi, Y. Y.; Ren, F.; Pearton, S. J.; Laboutin, O.; Cao, Yu; Johnson, Wayne J.; Kravchenko, Ivan I

    2012-01-01

    AlGaN/GaN high electron mobility transistors (HEMTs) with three different types of buffer layers, including a GaN/AlGaN composite layer, or 1 or 2 lm GaN thick layers, were fabricated and their reliability compared. The HEMTs with the thick GaN buffer layer showed the lowest critical voltage (Vcri) during off-state drain step-stress, but this was increased by around 50% and 100% for devices with the composite AlGaN/GaN buffer layers or thinner GaN buffers, respectively. The Voff - state for HEMTs with thin GaN and composite buffers were 100 V, however, this degraded to 50 60V for devices with thick GaN buffers due to the difference in peak electric field near the gate edge. A similar trend was observed in the isolation breakdown voltage measurements, with the highest Viso achieved based on thin GaN or composite buffer designs (600 700 V), while a much smaller Viso of 200V was measured on HEMTs with the thick GaN buffer layers. These results demonstrate the strong influence of buffer structure and defect density on AlGaN/GaN HEMT performance and reliability.

  18. AlGaAs/InGaAsN/GaAs PnP double heterojunction bipolar transistor

    SciTech Connect

    Chang, P.C.; Baca, A.G.; Li, N.Y.; Sharps, P.R.; Hou, H.Q.; Laroche, J.R.; Ren, F.

    2000-01-04

    The authors demonstrated a functional PnP double heterojunction bipolar transistor (DHBT) using AlGaAs, InGaAsN, and GaAs. The band alignment between InGaAsN and GaAs has a large {triangle}E{sub c} and negligible {triangle}E{sub v}, this unique characteristic is very suitable for PnP DHBT applications. The metalorganic vapor phase epitaxy (MOCVD) grown Al{sub 0.3}Ga{sub 0.7}As/In{sub 0.03}Ga{sub 0.97}As{sub 0.99}N{sub 0.01}/GaAs PnP DHBT is lattice matched to GaAs and has a peak current gain of 25. Because of the smaller bandgap (E{sub g}=1.20eV) of In{sub 0.03}Ga{sub 0.97}As{sub 0.99}N{sub 0.01} used for the base layer, this device has a low V{sub ON} of 0.79 V, which is 0.25 V lower than in a comparable Pnp AlGaAs/GaAs HBT. And because GaAs is used for the collector, its BV{sub CEO} is 12 V, consistent with BV{sub CEO} of AlGaAs/GaAs HBTs.

  19. Ti-Cr-Al-O Thin Film Resistors

    SciTech Connect

    Jankowski, A F; Hayes, J P

    2002-03-21

    Thin films of Ti-Cr-Al-O are produced for use as an electrical resistor material. The films are rf sputter deposited from ceramic targets using a reactive working gas mixture of Ar and O{sub 2}. Vertical resistivity values from 10{sup 4} to 10{sup 10} Ohm-cm are measured for Ti-Cr-Al-O films. The film resistivity can be design selected through control of the target composition and the deposition parameters. The Ti-Cr-Al-O thin film resistor is found to be thermally stable unlike other metal-oxide films.

  20. Oxidation of Fe-Cr-Al and Fe-Cr-Al-Y Single Crystals

    NASA Astrophysics Data System (ADS)

    Grabke, H. J.; Siegers, M.; Tolpygo, V. K.

    1995-03-01

    Single crystal samples of the alloy Fe-20%Cr-5%Al with and without Y-doping were used to study the "reactive element" (RE) effect, which causes improved oxidation behaviour and formation of a protective Al2O3 layer on this alloy. The oxidation was followed by AES at 10-7 mbar O2 up to about 1000 °C. Most observations were peculiar for this low pO2 environment, but yttrium clearly favors the formation of Al-oxide and stabilizes it also under these conditions, probably by favoring its nucleation. The oxides formed are surface compounds of about monolayer thickness, not clearly related to bulk oxides. Furthermore, the morphologies of oxide scales were investigated by SEM, after oxidation at 1000°C for 100 h at 133 mbar O2. On Fe-Cr-Al the scale is strongly convoluted and tends to spalling, whereas the presence of Y leads to flat scales which are well adherent. This difference is explained by a change in growth mechanism. The tendency for separation of oxide and metal was highest for the samples with low energy metal surface, i.e. (100) and (110), the scale was better adherent on the (111) oriented surface and on the polycrystalline specimen, since in the latter cases the overall energy for scale/metal separation is higher. All observations, from the low and from the high pO2 experiments, are discussed in relation to the approximately ten mechanisms proposed in the literature for explanation of the RE effects.

  1. Piezoelectric Franz-Keldysh effect in a GaN/InGaN/AlGaN multilayer structure

    NASA Astrophysics Data System (ADS)

    Hou, Yong T.; Teo, Kie L.; Li, Ming Fu; Uchida, Kazuo; Tokunaga, Hiroki; Akutsu, Nakao; Matsumoto, Koh

    1999-11-01

    Contactless electroreflectance (CER) of a GaN/InGaN/AlGaN multilayer structure grown on sapphire has been measured in the temperature range of 15K and 450K. Except for the GaN exciton structures, well-defined Franz-Keldysh Oscillations are observed above the AlGaN band gap. An electomodulational model based on complex Airy functions is used to analyze the FKOs line shape. The temperature dependence of transition energies is obtained both for GaN and AlGaN. The magnitude of the built in electric field in AlGaN layer is also determined. The temperature dependence of the electric field is found to be consistent with the variation of thermal strain in the epilayer. It is demonstrated that the built-in electric field can be identified to be due to the piezoelectric effect.

  2. Traps and defects in pre- and post-proton irradiated AlGaN-GaN high electron mobility transistors and AlGaN Schottky diodes

    NASA Astrophysics Data System (ADS)

    Sin, Yongkun; Foran, Brendan; Presser, Nathan; LaLumondiere, Stephen; Lotshaw, William; Moss, Steven C.

    2013-03-01

    High electron mobility transistors (HEMTs) based on AlGaN-GaN hetero-structures are promising for both commercial and military applications that require high voltage, high power, and high efficiency operation. Study of reliability and radiation effects of AlGaN-GaN HEMTs is necessary before solid state power amplifiers based on GaN HEMT technology are successfully deployed in satellite communication systems. Several AlGaN HEMT manufacturers have recently reported encouraging reliability data, but long-term reliability of these devices in the space environment still remains a major concern because a large number of traps and defects are present both in the bulk as well as at the surface leading to undesirable characteristics. This study is to investigate the effects of the AlGaN-GaN HEMTs and AlGaN Schottky diodes irradiated with protons.

  3. Hot-electron real-space transfer and longitudinal transport in dual AlGaN/AlN/{AlGaN/GaN} channels

    NASA Astrophysics Data System (ADS)

    Šermukšnis, E.; Liberis, J.; Matulionis, A.; Avrutin, V.; Ferreyra, R.; Özgür, Ü.; Morkoç, H.

    2015-03-01

    Real-space transfer of hot electrons is studied in dual-channel GaN-based heterostructure operated at or near plasmon-optical phonon resonance in order to attain a high electron drift velocity at high current densities. For this study, pulsed electric field is applied in the channel plane of a nominally undoped Al0.3Ga0.7N/AlN/{Al0.15Ga0.85N/GaN} structure with a composite channel of Al0.15Ga0.85N/GaN, where the electrons with a sheet density of 1.4 × 1013 cm-2, estimated from the Hall effect measurements, are confined. The equilibrium electrons are situated predominantly in the Al0.15Ga0.85N layer as confirmed by capacitance-voltage experiment and Schrödinger-Poisson modelling. The main peak of the electron density per unit volume decreases as more electrons occupy the GaN layer at high electric fields. The associated decrease in the plasma frequency induces the plasmon-assisted decay of non-equilibrium optical phonons (hot phonons) confirmed by the decrease in the measured hot-phonon lifetime from 0.95 ps at low electric fields down below 200 fs at fields of E \\gt 4 kV cm-1 as the plasmon-optical phonon resonance is approached. The onset of real-space transfer is resolved from microwave noise measurements: this source of noise dominates for E \\gt 8 kV cm-1. In this range of fields, the longitudinal current exceeds the values measured for a mono channel reference Al0.3Ga0.7N/AlN/GaN structure. The results are explained in terms of the ultrafast decay of hot phonons and reduced alloy scattering caused by the real-space transfer in the composite channel.

  4. Mechanical Characterization of CrN/CrAlN Multilayer Coatings Deposited by Magnetron Sputtering System

    NASA Astrophysics Data System (ADS)

    Kaouther, Khlifi; Hafedh, Dhiflaoui; Lassaad, Zoghlami; Ahmed, Ben Cheikh Larbi

    2015-10-01

    Chromium-based coatings are deposited on a 100Cr6 (AISI 52100) substrate by a physical vapor deposition magnetron sputtering system. The coatings have different structures, such as a CrN monolayer and CrAlN multilayer. The structural and morphological compositions of the coatings were evaluated using glow discharge optical emission spectroscopy, atomic force microscopy, and cross-sectional scanning electron microscopy. Nano-indentation tests were performed to investigate the mechanical properties. Domes and craters are shown to be uniformly distributed over the entire surfaces of the two coatings. Additionally, the CrN/CrAlN multilayer coating exhibits a rough surface, attractive mechanical properties, a high compressive stress, and a high plastic and elastic deformation resistance. The improvement of the mechanical properties of the CrN/CrAlN coating is mainly attributed to a reduction in the crystallite size. We found that this reduction was related to three factors: (1) the compositional change resulting from the substitution of aluminum for chromium, which can produce a decrease in the interatomic distance; (2) the structure of CrN/CrAlN, which was characterized by grain size refinement; and (3) the high number of interfaces, which explains the widely accepted concept of dislocation blocking by the layer interfaces.

  5. TI--CR--AL--O thin film resistors

    DOEpatents

    Jankowski, Alan F.; Schmid, Anthony P.

    2000-01-01

    Thin films of Ti--Cr--Al--O are used as a resistor material. The films are rf sputter deposited from ceramic targets using a reactive working gas mixture of Ar and O.sub.2. Resistivity values from 10.sup.4 to 10.sup.10 Ohm-cm have been measured for Ti--Cr--Al--O film <1 .mu.m thick. The film resistivity can be discretely selected through control of the target composition and the deposition parameters. The application of Ti--Cr--Al--O as a thin film resistor has been found to be thermodynamically stable, unlike other metal-oxide films. The Ti--Cr--Al--O film can be used as a vertical or lateral resistor, for example, as a layer beneath a field emission cathode in a flat panel display; or used to control surface emissivity, for example, as a coating on an insulating material such as vertical wall supports in flat panel displays.

  6. Multiple quantum well AlGaAs nanowires.

    PubMed

    Chen, Chen; Braidy, Nadi; Couteau, Christophe; Fradin, Cécile; Weihs, Gregor; LaPierre, Ray

    2008-02-01

    This letter reports on the growth, structure, and luminescent properties of individual multiple quantum well (MQW) AlGaAs nanowires (NWs). The composition modulations (MQWs) are obtained by alternating the elemental flux of Al and Ga during the molecular beam epitaxy growth of the AlGaAs wire on GaAs (111)B substrates. Transmission electron microscopy and energy dispersive X-ray spectroscopy performed on individual NWs are consistent with a configuration composed of conical segments stacked along the NW axis. Microphotoluminescence measurements and confocal microscopy showed enhanced light emission from the MQW NWs as compared to nonsegmented NWs due to carrier confinement and sidewall passivation. PMID:18184023

  7. Fretting Wear of Ti-48Al-2Cr-2Nb

    NASA Technical Reports Server (NTRS)

    Miyoshi, Kazuhisa; Lerch, Bradley A.; Draper, Susan L.

    2001-01-01

    An investigation was conducted to examine the wear behavior of gamma titanium aluminide (Ti-48Al-2Cr-2Nb in atomic percent) in contact with a typical nickel-base superalloy under repeated microscopic vibratory motion in air at temperatures from 296-823 K. The surface damage observed on the interacting surfaces of both Ti-48Al-2Cr-2Nb and superalloy consisted of fracture pits, oxides, metallic debris, scratches, craters, plastic deformation, and cracks. The Ti-48Al-2Cr-2Nb transferred to the superalloy at all fretting conditions and caused scuffing or galling. The increasing rate of oxidation at elevated temperatures led to a drop in Ti-48Al-2Cr-2Nb wear at 473 K. Mild oxidative wear was observed at 473 K. However, fretting wear increased as the temperature was increased from 473-823 K. At 723 and 823 K, oxide disruption generated cracks, loose wear debris, and pits on the Ti-48Al-2Cr-2Nb wear surface. Ti-48Al-2Cr-2Nb wear generally decreased with increasing fretting frequency. Both increasing slip amplitude and increasing load tended to produce more metallic wear debris, causing severe abrasive wear in the contacting metals. Keywords

  8. AlGaAs/GaAs/AlGaAs quantum wells as a sensitive tool for the MOVPE reactor environment

    NASA Astrophysics Data System (ADS)

    Dimastrodonato, V.; Mereni, L. O.; Young, R. J.; Pelucchi, E.

    2010-10-01

    We present in this work a simple quantum well (QW) structure consisting of GaAs wells with AlGaAs barriers as a probe for measuring the performance of arsine purifiers within a metalorganic vapour phase epitaxy system. Comparisons between two different commercially available purifiers are based on the analysis of low-temperature photoluminescence emission spectra from thick QWs, grown on GaAs substrates misoriented slightly from (1 0 0). Neutral excitons emitted from these structures show extremely narrow linewidths, comparable with those that can be obtained by molecular beam epitaxy in an ultra-high vacuum environment, suggesting that purifications well below the 1 ppb level are needed to achieve high quality quantum well growth.

  9. Multibarrier heterostructure GaAs/AlAs switch

    NASA Astrophysics Data System (ADS)

    Reklaitis, A.

    1999-02-01

    A multibarrier heterostructure GaAs/AlAs current switching diode has been proposed and investigated by Monte Carlo particle simulations. The switching phenomenon is based on electron tunneling and thermoemission from the GaAs wells to the AlAs barriers, electron drift across the thin AlAs barriers followed by a subsequent impact ionization in the undoped GaAs layers. The calculated switching voltage is close to 100 V for the diode involving four AlAs barriers at 300 K lattice temperature. The estimated switching time is in the order of 10 ps.

  10. Electroreflectance and photoluminescence measurement of passivation by hydrogenation in GaAs/AlGaAs structures

    NASA Astrophysics Data System (ADS)

    Yang, D.; Garland, J. W.; Raccah, P. M.; Coluzza, C.; Frankl, P.; Capizzi, M.; Chambers, F.; Devane, G.

    1991-04-01

    We have studied by electrolyte electroreflectance and photoluminescence a GaAs/AlGaAs resonant tunneling structure (RTS) with a highly n-doped GaAs cap, before and after hydrogenation. We measured the amount of passivation of shallow donor states and of deep traps in the cap and found the approximate pinning levels and interace charges of the RTS.

  11. Magnetometory Measurement of AlGaN/GaN 2DEG

    NASA Astrophysics Data System (ADS)

    Tsubaki, K.; Maeda, N.; Saitoh, T.; Kobayashi, N.

    2004-03-01

    AlGaN/GaN heterostructure devices have been attracting much attention because of their potential for high-performance microwave applications. Therefore, the electronic properties of a 2DEG in AlGaN/GaN heterostructures have recently been discussed. In this paper, we performed the magnetometory measurement of AlGaN/GaN 2DEG at low temperature. The AlGaN/GaN heterostructures were grown by low-pressure metal-organic chemical vapour phase epitaxy on (0001) SiC substrate using AlN buffers. The electron mobility and electron concentration at 4.2 K are 9,540 cm^2/Vs and 6.6 × 10^12 cm-2, respectively. When the temperature is lower than 4.5 K the hysteresis of magnetometric data is observed near zero magnetic field. At the temperature larger than 4.5 K, the hysteresis of magnetometric data disappears and the slope of magnetometric data with respect to magnetic field becomes lower as obeying Currie-Weiss law. In general the hysteresis and Currie-Weiss law behavior in magnetometric data imply the possibility of the ferromagnetism, but the conformation of the ferromagnetism of AlGaN/GaN heterostructure is still difficult and the detailed physical mechanism is still unclear.

  12. On the AlGaInP-bulk and AlGaInP/GaAs-superlattice confinement effects for heterostructure-emitter bipolar transistors

    SciTech Connect

    Tsai, Jung-Hui

    2015-02-09

    The confinement effect and electrical characteristics of heterostructure-emitter bipolar transistors with an AlGaInP bulk-confinement layer and an AlGaInP/GaAs superlattice-confinement layer are first demonstrated and compared by experimentally results. In the two devices, the relatively large valence band discontinuity at AlGaInP/GaAs heterojunction provides excellent confinement effect for holes to enhance current gain. As to the AlGaInP/GaAs superlattice-confinement device, part of thermionic-emission electrons will be trapped in the GaAs quantum wells of the superlattice. This will result in lower collector current and current gain as compared with the bulk-confinement device. Nevertheless, the superlattice-confinement device exhibits a larger current-gain cutoff frequency, which can be attributed that the tunneling behavior is included in the carrier transportation and transporting time across the emitter region could be substantially reduced.

  13. Low density GaAs /AlGaAs quantum dots grown by modified droplet epitaxy

    NASA Astrophysics Data System (ADS)

    Mantovani, V.; Sanguinetti, S.; Guzzi, M.; Grilli, E.; Gurioli, M.; Watanabe, K.; Koguchi, N.

    2004-10-01

    Low temperature photoluminescence spectroscopy is used to analyze the effects of the Ga coverage and of the postgrowth thermal annealing on the electronic properties of low density (≈1×109cm-2) self-assembled GaAs /AlGaAs quantum dots (QDs) grown by modified droplet epitaxy (MDE). We demonstrate that with the MDE method it is possible to obtain low density and high efficiency QD samples with high photoluminescence efficiency. Large modifications of the photoluminescence band, which depend on Ga coverage and thermal annealing, are found and quantitatively interpreted by means of a simple model based on the Al-Ga interdiffusion.

  14. High-efficiency double-heterostructure AlGaAs/GaAs solar cells

    NASA Technical Reports Server (NTRS)

    Gale, R. P.; Fan, John C. C.; Turner, G. W.; Chapman, R. L.

    1985-01-01

    Double-heterostructure solar cells have been fabricated from wafers prepared by using organometallic chemical vapor deposition to grow a p GaAs absorbing layer sandwiched between p(+) and n(+) AlGaAs layers. The best cell, which incorporates an abrupt AlGaAs/GaAs shallow heterojunction, exhibits a global AM1 one-sun conversion efficiency of 23 percent. The rate at which the open-circuit voltage decreases with increasing temperature is lower for the double-heterostructure cells than for GaAs shallow-homojunction cells.

  15. Leakage mechanism in GaN and AlGaN Schottky interfaces

    NASA Astrophysics Data System (ADS)

    Hashizume, Tamotsu; Kotani, Junji; Hasegawa, Hideki

    2004-06-01

    Based on detailed temperature-dependent current-voltage (I-V-T) measurements the mechanism of leakage currents through GaN and AlGaN Schottky interfaces is discussed. The experiments were compared to calculations based on thin surface barrier model in which the effects of surface defects were taken into account. Our simulation method reproduced the experimental I-V-T characteristics of the GaN and AlGaN Schottky diodes, and gave excellent fitting results to the reported Schottky I-V curves in GaN for both forward and reverse biases at different temperatures. The present results indicate that the barrier thinning caused by unintentional surface-defect donors enhances the tunneling transport processes, leading to large leakage currents through GaN and AlGaN Schottky interfaces.

  16. Electrical properties of GaN-based heterostructures adopting InAlN/AlGaN bilayer barriers

    NASA Astrophysics Data System (ADS)

    Xu, Z. Y.; Xu, F. J.; Huang, C. C.; Wang, J. M.; Zhang, X.; Yang, Z. J.; Wang, X. Q.; Shen, B.

    2016-08-01

    Electrical properties of GaN-based heterostructures adopting InAlN/AlGaN bilayer barriers are investigated by Hall-effect and current-voltage measurements. It is found that this structure possesses both merits of high two-dimensional electron gas (2DEG) density and low gate leakage current density, while maintaining high 2DEG mobility. Furthermore, temperature dependence of the 2DEG density in this structure is verified to follow a combined tendency of InAlN/GaN (increase) and AlGaN/GaN (decrease) heterostructures with increasing temperature from 90 K to 400 K, which is mainly caused by superposition of the effects from carrier thermal activation induced by extrinsic factors in InAlN layer and the reduced conduction-band discontinuity.

  17. AlGaAs/InGaAsN/GaAs PnP double heterojunction bipolar transistor

    SciTech Connect

    Chang, P.C.; Baca, A.G.; Li, N.Y.; Sharps, P.R.; Hou, H.Q.; Laroche, J.R.; Ren, F.

    2000-01-11

    The authors have demonstrated a functional MOCVD-grown AlGaAs/InGaAsN/GaAsPnP DHBT that is lattice matched to GaAs and has a peak current gain ({beta}) of 25. Because of the smaller bandgap (E{sub g}=1.20eV)of In{sub 0.03}Ga{sub 0.97}As{sub 0.99}N{sub 0.01} used for the base layer, this device has a low V{sub ON} of 0.79 V, 0.25 V lower than in a comparable Pnp AlGaAs/GaAs HBT. The BV{sub CEO} is 12 V, consistent with its GaAs collector thickness and doping level.

  18. Electron mobility in very low density GaN/AlGaN/GaN heterostructures

    SciTech Connect

    Manfra, M.J.; Baldwin, K.W.; Sergent, A.M.; Molnar, R.J.; Caissie, J.

    2004-09-06

    We report on the transport properties of a tunable two-dimensional electron gas (2DEG) confined at the lower interface of a GaN/Al{sub 0.06}Ga{sub 0.94}N/GaN heterostructure grown by plasma-assisted molecular beam epitaxy on semi-insulating GaN templates prepared by hydride vapor phase epitaxy. Using an insulated gate Hall bar structure, the electron density is continuously tuned from {approx}2x10{sup 12} down to 1.5x10{sup 11} cm{sup -2}. At T=300 mK, the 2DEG displays a maximum mobility of 80 000 cm{sup 2}/V s at a sheet density of 1.75x10{sup 12} cm{sup -2}. At low densities, the mobility exhibits a power law dependence on density -{mu}{approx}n{sub e}{sup {alpha}}, with {alpha}{approx}1.0, over the range of 2x10{sup 11}-1x10{sup 12} cm{sup -2}. In this density regime, the mobility is no longer limited by alloy scattering and long-range Coulomb scattering dominates. We discuss the dominant scattering mechanisms that presently limit low temperature mobility at electron densities below 1x10{sup 12} cm{sup -2}.

  19. Characterization of interface reaction of Ti/Al-based ohmic contacts on AlGaN/GaN epitaxial layers on GaN substrate

    NASA Astrophysics Data System (ADS)

    Zadeh, Daryoush H.; Tanabe, Shinichi; Watanabe, Noriyuki; Matsuzaki, Hideaki

    2016-05-01

    The ohmic properties of Ti/Al/Mo/Au contacts on a high-quality AlGaN/GaN heterostructure epitaxially grown on a GaN substrate were investigated. Systematic structural and electrical analyses of the metal/AlGaN interface after annealing in N2 at 700 and 900 °C were conducted. After annealing at 900 °C, a new Al-rich interlayer with nitrogen vacancies was formed at the metal/AlGaN interface. Ohmic contacts with a low specific contact resistance (ρc) of 5.1 × 10‑6 Ω cm2 and a dominant field emission carrier transport mechanism were achieved. The fabrication of recessed-AlGaN-structured ohmic contact with ρc as low as 2.4 × 10‑5 Ω cm2 at a low annealing temperature of 650 °C, was also successfully demonstrated. This result indicates that a process methodology can be provided for fabricating low-resistivity ohmic contacts with a low thermal budget on a high-quality AlGaN/GaN structure, which is based on an appropriate control of the metal/AlGaN interface and AlGaN thickness rather than relying on the existence of threading dislocations.

  20. Multi-bands photoconductive response in AlGaN/GaN multiple quantum wells

    SciTech Connect

    Chen, G.; Rong, X.; Xu, F. J.; Tang, N.; Wang, X. Q. Shen, B.; Fu, K.; Zhang, B. S.; Hashimoto, H.; Yoshikawa, A.; Ge, W. K.

    2014-04-28

    Based on the optical transitions among the quantum-confined electronic states in the conduction band, we have fabricated multi-bands AlGaN/GaN quantum well infrared photodetectors. Crack-free AlGaN/GaN multiple quantum wells (MQWs) with atomically sharp interfaces have been achieved by inserting an AlN interlayer, which releases most of the tensile strain in the MQWs grown on the GaN underlayer. With significant reduction of dark current by using thick AlGaN barriers, photoconductive responses are demonstrated due to intersubband transition in multiple regions with center wavelengths of 1.3, 2.3, and 4 μm, which shows potential applications on near infrared detection.

  1. Synthesis, morphology and optical properties of GaN and AlGaN semiconductor nanostructures

    SciTech Connect

    Kuppulingam, B. Singh, Shubra Baskar, K.

    2014-04-24

    Hexagonal Gallium Nitride (GaN) and Aluminum Gallium Nitride (AlGaN) nanoparticles were synthesized by sol-gel method using Ethylene Diamine Tetra Acetic acid (EDTA) complex route. Powder X-ray diffraction (PXRD) analysis confirms the hexagonal wurtzite structure of GaN and Al{sub 0.25}Ga{sub 0.75}N nanoparticles. Surface morphology and elemental analysis were carried out by Scanning Electron Microscope (SEM) and Energy Dispersive X-ray spectroscopy (EDX). The room temperature Photoluminescence (PL) study shows the near band edge emission for GaN at 3.35 eV and at 3.59 eV for AlGaN nanoparticles. The Aluminum (Al) composition of 20% has been obtained from PL emission around 345 nm.

  2. 2.4 Micrometer Cutoff Wavelength AlGaAsSb/InGaAsSb Phototransistors

    NASA Technical Reports Server (NTRS)

    Sulima, O. V.; Swaminathan, K.; Refaat, T. F.; Faleev, N. N.; Semenov, A. N.; Solov'ev, V. A.; Ivanov, S. V.; Abedin, M. N.; Singh, U. N.; Prather, D.

    2006-01-01

    We report the first AlGaAsSb/InGaAsSb phototransistors with a cutoff wavelength (50% of peak responsivity) of 2.4 micrometers operating in a broad range of temperatures. These devices are also the first AlGaAsSb/InGaAsSb heterojunction phototransistors (HPT) grown by molecular beam epitaxy (MBE). This work is a continuation of a preceding study, which was carried out using LPE (liquid phase epitaxy)-grown AlGaAsSb/InGaAsSb/GaSb heterostructures. Although the LPE-related work resulted in the fabrication of an HPT with excellent parameters [1-4], the room temperature cutoff wavelength of these devices (approximately 2.15 micrometers) was determined by fundamental limitations implied by the close-to-equilibrium growth from Al-In-Ga-As-Sb melts. As the MBE technique is free from the above limitations, AlGaAsSb/InGaAsSb/GaSb heterostructures for HPT with a narrower bandgap of the InGaAsSb base and collector - and hence sensitivity at longer wavelengths (lambda) - were grown in this work. Moreover, MBE - compared to LPE - provides better control over doping levels, composition and width of the AlGaAsSb and InGaAsSb layers, compositional and doping profiles, especially with regard to abrupt heterojunctions. The new MBE-grown HPT exhibited both high responsivity R (up to 2334 A/W for lambda=2.05 micrometers at -20 deg C.) and specific detectivity D* (up to 2.1 x 10(exp 11) cmHz(exp 1/2)/W for lambda=2.05 micrometers at -20 deg C).

  3. Medium energy proton radiation damage to (AlGa)As-GaAs solar cells

    NASA Technical Reports Server (NTRS)

    Loo, R. Y.; Kamath, G. S.; Knechtli, R. C.

    1982-01-01

    The performance of (AlGa)As-GaAs solar cells irradiated by medium energy 2, 5, and 10 MeV protons was evaluated. The Si cells without coverglass and a number of GaAs solar cells with 12 mil coverglass were irradiated simultaneously with bare GaAs cells. The cell degradation is directly related to the penetration of depth of protons with GaAs. The influence of periodic and continuous thermal annealing on the GaAs solar cells was investigated.

  4. Influence of FeCrAl Content on Microstructure and Bonding Strength of Plasma-Sprayed FeCrAl/Al2O3 Coatings

    NASA Astrophysics Data System (ADS)

    Zhou, Liang; Luo, Fa; Zhou, Wancheng; Zhu, Dongmei

    2016-02-01

    Low-power plasma-sprayed FeCrAl/Al2O3 composite coatings with 1.5 mm thickness have been fabricated for radar absorption applications. The effects of FeCrAl content on the coating properties were studied. The FeCrAl presents in the form of a few thin lamellae and numerous particles, demonstrating relatively even distribution in all the coatings. Results show that the micro-hardness and porosity decrease with the increase in FeCrAl content. With FeCrAl content increasing from 28 to 47 wt.%, the bonding strength of the coatings with 1.5 mm thickness increases from 10.5 to 27 MPa, and the failure modes are composed of cohesive and adhesive failure, which are ascribed to the coating microstructure and the residual stress, respectively.

  5. Simplified 2DEG carrier concentration model for composite barrier AlGaN/GaN HEMT

    SciTech Connect

    Das, Palash Biswas, Dhrubes

    2014-04-24

    The self consistent solution of Schrodinger and Poisson equations is used along with the total charge depletion model and applied with a novel approach of composite AlGaN barrier based HEMT heterostructure. The solution leaded to a completely new analytical model for Fermi energy level vs. 2DEG carrier concentration. This was eventually used to demonstrate a new analytical model for the temperature dependent 2DEG carrier concentration in AlGaN/GaN HEMT.

  6. InGaAsN/AlGaAs Pnp Heterojunction Bipolar Transistor

    SciTech Connect

    BACA,ALBERT G.; CHANG,PING-CHIH; HOU,H.Q.; LAROCHE,J.R.; LI,N.Y.; REN,F.; SHARPS,P.R.

    1999-11-03

    The authors have demonstrated a functional Pnp heterojunction bipolar transistor (HBT) using InGaAsN. The metalorganic vapor phase epitaxy (MOCVD) grown Al{sub 0.3}Ga{sub 0.7}As/In{sub 0.03}Ga{sub 0.97}As{sub 0.99}N{sub 0.01} HBT takes advantage of the narrower bandgap energy (E{sub g} = 1.25eV) of In{sub 0.03}Ga{sub 0.97}As{sub 0.99}N{sub 0.01}, which is lattice matched to GaAs. Compared with the Al{sub 0.3}Ga{sub 0.7}As/GaAs material system, the Al{sub 0.3}Ga{sub 0.7}As/In{sub 0.03}Ga{sub 0.97}As{sub 0.99}N{sub 0.01} material system has a larger conduction band offset, while the valence band offset remains comparable. This characteristic band alignment is very suitable for Pnp HBT applications. The device's peak current gain is 23 and it has a turn on voltage of 0.77V, which is 0.25V lower than in a comparable Pnp Al{sub 0.3}Ga{sub 0.7}As/GaAs HBT.

  7. An extrinsic fmax > 100 GHz InAlN/GaN HEMT with AlGaN back barrier

    NASA Astrophysics Data System (ADS)

    Bo, Liu; Zhihong, Feng; Shaobo, Dun; Xiongwen, Zhang; Guodong, Gu; Yuangang, Wang; Peng, Xu; Zezhao, He; Shujun, Cai

    2013-04-01

    We report the DC and RF performance of InAlN/GaN high-electron mobility transistors with AlGaN back barrier grown on SiC substrates. These presented results confirm the high performance that is reachable by InAlN-based technology. The InAlN/GaN HEMT sample showed a high 2DEG mobility of 1550 cm2/(V·s) at a 2DEG density of 1.7 × 1013 cm-2. DC and RF measurements were performed on the unpassivated device with 0.2 μm “T“ gate. The maximum drain current density at VGS = 2 V is close to 1.05 A/mm in a reproducible way. The reduction in gate leakage current helps to increase the frequency performance of AlGaN back barrier devices. The power gain cut-off frequency of a transistor with an AlGaN back barrier is 105 GHz, which is much higher than that of the device without an AlGaN back barrier at the same gate length. These results indicate InAlN/GaN HEMT is a promising candidate for millimeter-wave application.

  8. Terahertz GaAs/AlAs quantum-cascade lasers

    NASA Astrophysics Data System (ADS)

    Schrottke, L.; Lü, X.; Rozas, G.; Biermann, K.; Grahn, H. T.

    2016-03-01

    We have realized GaAs/AlAs quantum-cascade lasers operating at 4.75 THz exhibiting more than three times higher wall plug efficiencies than GaAs/Al0.25Ga0.75As lasers with an almost identical design. At the same time, the threshold current density at 10 K is reduced from about 350 A/cm2 for the GaAs/Al0.25Ga0.75As laser to about 120 A/cm2 for the GaAs/AlAs laser. Substituting AlAs for Al0.25Ga0.75As barriers leads to a larger energy separation between the subbands reducing the probability for leakage currents through parasitic states and for reabsorption of the laser light. The higher barriers allow for a shift of the quasi-continuum of states to much higher energies. The use of a binary barrier material may also reduce detrimental effects due to the expected composition fluctuations in ternary alloys.

  9. Mid-infrared Photoconductive Response in AlGaN/GaN Step Quantum Wells.

    PubMed

    Rong, X; Wang, X Q; Chen, G; Zheng, X T; Wang, P; Xu, F J; Qin, Z X; Tang, N; Chen, Y H; Sang, L W; Sumiya, M; Ge, W K; Shen, B

    2015-01-01

    AlGaN/GaN quantum structure is an excellent candidate for high speed infrared detectors based on intersubband transitions. However, fabrication of AlGaN/GaN quantum well infrared detectors suffers from polarization-induced internal electric field, which greatly limits the carrier vertical transport. In this article, a step quantum well is proposed to attempt solving this problem, in which a novel spacer barrier layer is used to balance the internal electric field. As a result, a nearly flat band potential profile is obtained in the step barrier layers of the AlGaN/GaN step quantum wells and a bound-to-quasi-continuum (B-to-QC) type intersubband prototype device with detectable photocurrent at atmosphere window (3-5 μm) is achieved in such nitride semiconductors. PMID:26395756

  10. Prostate specific antigen detection using AlGaN /GaN high electron mobility transistors

    NASA Astrophysics Data System (ADS)

    Kang, B. S.; Wang, H. T.; Lele, T. P.; Tseng, Y.; Ren, F.; Pearton, S. J.; Johnson, J. W.; Rajagopal, P.; Roberts, J. C.; Piner, E. L.; Linthicum, K. J.

    2007-09-01

    Antibody-functionalized Au-gated AlGaN /GaN high electron mobility transistors (HEMTs) were used to detect prostate specific antigen (PSA). The PSA antibody was anchored to the gate area through the formation of carboxylate succinimdyl ester bonds with immobilized thioglycolic acid. The AlGaN /GaN HEMT drain-source current showed a rapid response of less than 5s when target PSA in a buffer at clinical concentrations was added to the antibody-immobilized surface. The authors could detect a wide range of concentrations from 10pg/mlto1μg/ml. The lowest detectable concentration was two orders of magnitude lower than the cutoff value of PSA measurements for clinical detection of prostate cancer. These results clearly demonstrate the promise of portable electronic biological sensors based on AlGaN /GaN HEMTs for PSA screening.

  11. Mid-infrared Photoconductive Response in AlGaN/GaN Step Quantum Wells

    PubMed Central

    Rong, X.; Wang, X. Q.; Chen, G.; Zheng, X. T.; Wang, P.; Xu, F. J.; Qin, Z. X.; Tang, N.; Chen, Y. H.; Sang, L. W.; Sumiya, M.; Ge, W. K.; Shen, B.

    2015-01-01

    AlGaN/GaN quantum structure is an excellent candidate for high speed infrared detectors based on intersubband transitions. However, fabrication of AlGaN/GaN quantum well infrared detectors suffers from polarization-induced internal electric field, which greatly limits the carrier vertical transport. In this article, a step quantum well is proposed to attempt solving this problem, in which a novel spacer barrier layer is used to balance the internal electric field. As a result, a nearly flat band potential profile is obtained in the step barrier layers of the AlGaN/GaN step quantum wells and a bound-to-quasi-continuum (B-to-QC) type intersubband prototype device with detectable photocurrent at atmosphere window (3–5 μm) is achieved in such nitride semiconductors. PMID:26395756

  12. Internal quantum efficiency in yellow-amber light emitting AlGaN-InGaN-GaN heterostructures

    SciTech Connect

    Ngo, Thi Huong; Gil, Bernard; Valvin, Pierre; Damilano, Benjamin; Lekhal, Kaddour; De Mierry, Philippe

    2015-09-21

    We determine the internal quantum efficiency of strain-balanced AlGaN-InGaN-GaN hetero-structures designed for yellow-amber light emission, by using a recent model based on the kinetics of the photoluminescence decay initiated by Iwata et al. [J. Appl. Phys. 117, 075701 (2015)]. Our results indicate that low temperature internal quantum efficiencies sit in the 50% range and we measure that adding an AlGaN layer increases the internal quantum efficiency from 50% up to 57% with respect to the GaN-InGaN case. More dramatic, it almost doubles from 2.5% up to 4.3% at room temperature.

  13. AlGaN/GaN heterostructure prepared on a Si (110) substrate via pulsed sputtering

    SciTech Connect

    Watanabe, T.; Ohta, J.; Kondo, T.; Ohashi, M.; Ueno, K.; Kobayashi, A.; Fujioka, H.

    2014-05-05

    GaN films were grown on Si (110) substrates using a low-temperature growth technique based on pulsed sputtering. Reduction of the growth temperature suppressed the strain in the GaN films, leading to an increase in the critical thickness for crack formation. In addition, an AlGaN/GaN heterostructure with a flat heterointerface was prepared using this technique. Furthermore, the existence of a two dimensional electron gas at the heterointerface with a mobility of 1360 cm{sup 2}/Vs and a sheet carrier density of 1.3 × 10{sup 13} cm{sup −2} was confirmed. Finally, the use of the AlGaN/GaN heterostructure in a high electron mobility transistor was demonstrated. These results indicate that low-temperature growth via pulsed sputtering is quite promising for the fabrication of GaN-based electronic devices.

  14. Composite droplets: evolution of InGa and AlGa alloys on GaAs(100).

    PubMed

    Sablon, K A; Wang, Zh M; Salamo, G J

    2008-03-26

    We present a comparative study for the evolution of utilizing indium gallium (InGa) and aluminum gallium (AlGa) alloys fabricated on GaAs(100) by means of simultaneous and sequential droplet formation. The composite alloys reported using the sequential approach lack the ability to precisely determine the final alloy composition as well as consistency in the density of the droplets. Further, the composition of the InGa alloy is not uniform, as seen by the size distribution using an atomic force microscope (AFM). Although this approach may be acceptable for materials with similar surface kinetics, as in the case of AlGa, it is not acceptable for InGa. This investigation reveals that the simultaneous approach for fabricating composite alloys is the optimum approach for producing InGa alloys with better control on composition for plasmonic applications such as plasmonic waveguides. PMID:21817741

  15. Laser diode bars based on strain-compensated AlGaPAs/GaAs heterostructures

    SciTech Connect

    Marmalyuk, Aleksandr A; Ladugin, M A; Yarotskaya, I V; Panarin, V A; Mikaelyan, G T

    2012-01-31

    Traditional (in the AlGaAs/GaAs system) and phosphorus-compensated (in the AlGaAs/AlGaPAs/GaAs system) laser heterostructures emitting at a wavelength of 850 nm are grown by MOVPE and studied. Laser diode bars are fabricated and their output characteristics are studied. The method used to grow heterolayers allowed us to control (minimise) mechanical stresses in the AlGaPAs/GaAs laser heterostructure, which made it possible to keep its curvature at the level of the initial curvature of the substrate. It is shown that the use of a compensated AlGaPAs/GaAs heterostructure improves the linear distribution of emitting elements in the near field of laser diode arrays and allows the power - current characteristic to retain its slope at high pump currents owing to a uniform contact of all emitting elements with the heat sink. The radius of curvature of the grown compensated heterostructures turns out to be smaller than that of traditional heterostructures.

  16. Improved mobility of AlGaN channel heterojunction material using an AlGaN/GaN composite buffer layer

    NASA Astrophysics Data System (ADS)

    Wen, Hui-Juan; Zhang, Jin-Cheng; Lu, Xiao-Li; Wang, Zhi-Zhe; Ha, Wei; Ge, Sha-Sha; Cao, Rong-Tao; Hao, Yue

    2014-03-01

    The quality of an AlGaN channel heterojunction on a sapphire substrate is massively improved by using an AlGaN/GaN composite buffer layer. We demonstrate an Al0.4Ga0.5N/Al0.18Ga0.82N heterojunction with a state-of-the-art mobility of 815 cm2/(V·s) and a sheet resistance of 890 Ω/□ under room temperature. The crystalline quality and the electrical properties of the AlGaN heterojunction material are analyzed by atomic force microscopy, high-resolution X-ray diffraction, and van der Pauw Hall and capacitance—voltage (C—V) measurements. The results indicate that the improved electrical properties should derive from the reduced surface roughness and low dislocation density.

  17. Groove-type channel enhancement-mode AlGaN/GaN MIS HEMT with combined polar and nonpolar AlGaN/GaN heterostructures

    NASA Astrophysics Data System (ADS)

    Duan, Xiao-Ling; Zhang, Jin-Cheng; Xiao, Ming; Zhao, Yi; Ning, Jing; Hao, Yue

    2016-08-01

    A novel groove-type channel enhancement-mode AlGaN/GaN MIS high electron mobility transistor (GTCE-HEMT) with a combined polar and nonpolar AlGaN/GaN heterostucture is presented. The device simulation shows a threshold voltage of 1.24 V, peak transconductance of 182 mS/mm, and subthreshold slope of 85 mV/dec, which are obtained by adjusting the device parameters. Interestingly, it is possible to control the threshold voltage accurately without precisely controlling the etching depth in fabrication by adopting this structure. Besides, the breakdown voltage (V B) is significantly increased by 78% in comparison with the value of the conventional MIS-HEMT. Moreover, the fabrication process of the novel device is entirely compatible with that of the conventional depletion-mode (D-mode) polar AlGaN/GaN HEMT. It presents a promising way to realize the switch application and the E/D-mode logic circuits. Project supported by the National Science and Technology Major Project, China (Grant No. 2013ZX02308-002) and the National Natural Science Foundation of China (Grant Nos. 11435010, 61474086, and 61404099).

  18. Hybrid UV Imager Containing Face-Up AlGaN/GaN Photodiodes

    NASA Technical Reports Server (NTRS)

    Zheng, Xinyu; Pain, Bedabrata

    2005-01-01

    A proposed hybrid ultraviolet (UV) image sensor would comprise a planar membrane array of face-up AlGaN/GaN photodiodes integrated with a complementary metal oxide/semiconductor (CMOS) readout-circuit chip. Each pixel in the hybrid image sensor would contain a UV photodiode on the AlGaN/GaN membrane, metal oxide/semiconductor field-effect transistor (MOSFET) readout circuitry on the CMOS chip underneath the photodiode, and a metal via connection between the photodiode and the readout circuitry (see figure). The proposed sensor design would offer all the advantages of comparable prior CMOS active-pixel sensors and AlGaN UV detectors while overcoming some of the limitations of prior (AlGaN/sapphire)/CMOS hybrid image sensors that have been designed and fabricated according to the methodology of flip-chip integration. AlGaN is a nearly ideal UV-detector material because its bandgap is wide and adjustable and it offers the potential to attain extremely low dark current. Integration of AlGaN with CMOS is necessary because at present there are no practical means of realizing readout circuitry in the AlGaN/GaN material system, whereas the means of realizing readout circuitry in CMOS are well established. In one variant of the flip-chip approach to integration, an AlGaN chip on a sapphire substrate is inverted (flipped) and then bump-bonded to a CMOS readout circuit chip; this variant results in poor quantum efficiency. In another variant of the flip-chip approach, an AlGaN chip on a crystalline AlN substrate would be bonded to a CMOS readout circuit chip; this variant is expected to result in narrow spectral response, which would be undesirable in many applications. Two other major disadvantages of flip-chip integration are large pixel size (a consequence of the need to devote sufficient area to each bump bond) and severe restriction on the photodetector structure. The membrane array of AlGaN/GaN photodiodes and the CMOS readout circuit for the proposed image sensor would

  19. CBE growth of AlGaAs/GaAs heterostructures and their device applications

    NASA Astrophysics Data System (ADS)

    Houng, Yu-Min

    1990-10-01

    We demonstrate that the chemical beam epitaxial (CBE) technique is suitable for growing high quality AlGaAs/GaAs heterostructures for high-speed and photonic device applications. Substantial improvements in electrical and optical properties of Al xGa 1- xAs were achieved using triisobutylaluminum (TIBAl) instead of triethylaluminum source. Highly uniform Al xGa 1- xAs:Si and Al xGa 1- xAs:C films with very low surface defect density were grown. The electrical and optical properties of these materials are comparable to those of high quality organometallic vapor phase epitaxial (OMVPE) Al xGa 1- xAs. CBE grown 0.25 °m gate length modulation-doped field effect transistors (MODFET) have been fabricated. MODFETs having fT greater than 38 GHz and a 1.7 dB noise figure with 10 dB associated gain at 18 GHz are reported. Device quality AlGaAs/GaAs heterojunction bipolar transistor (HBT), electro-optic modulator and light emitter array structures grown by CBE technique were also demonstrated.

  20. Variable angle spectroscopic ellipsometry - Application to GaAs-AlGaAs multilayer homogeneity characterization

    NASA Technical Reports Server (NTRS)

    Alterovitz, Samuel A.; Snyder, Paul G.; Merkel, Kenneth G.; Woollam, John A.; Radulescu, David C.

    1988-01-01

    Variable angle spectroscopic ellipsometry has been applied to a GaAs-AlGaAs multilayer structure to obtain a three-dimensional characterization, using repetitive measurements at several spots on the same sample. The reproducibility of the layer thickness measurements is of order 10 A, while the lateral dimension is limited by beam diameter, presently of order 1 mm. Thus, the three-dimensional result mainly gives the sample homogeneity. In the present case three spots were used to scan the homogeneity over 1 in of a wafer which had molecular-beam epitaxially grown layers. The thickness of the AlGaAs, GaAs, and oxide layers and the Al concentration varied by 1 percent or less from edge to edge. This result was confirmed by two methods of data analysis. No evidence of an interfacial layer was observed on top of the AlGaAs.

  1. Waveguide Optimization for Semipolar (In,Al,Ga)N Lasers

    NASA Astrophysics Data System (ADS)

    Rass, Jens; Ploch, Simon; Wernicke, Tim; Frentrup, Martin; Weyers, Markus; Kneissl, Michael

    2013-08-01

    In this work the optical waveguiding in semipolar InGaN-based laser diodes is analyzed. Different designs of the separate confinement heterostructure with AlGaN or GaN cladding layers and GaN or InGaN waveguide layers are studied. The influence of waveguide material, thickness and composition on the optical confinement factor Γ, the accumulated strain energy E and the refractive index contrast is calculated. Measurements of the threshold and the far field intensity distributions of lasers with differing waveguide design confirm the predictions from model calculations. The optimum waveguide for a 410 nm single quantum well laser is found to consist of a symmetric In0.04Ga0.96N waveguide of 2×85 nm thickness with GaN cladding layers.

  2. Electrochemical etching of p-n-GaN/AlGaN photoelectrodes

    NASA Astrophysics Data System (ADS)

    Usikov, A. S.; Helava, H.; Nikiforov, A.; Puzyk, M. V.; Papchenko, B. P.; Kovaleva, Yu. V.; Makarov, Yu. N.

    2016-05-01

    Specific features of etching of GaN/AlGaN p-n structures in a KOH-based electrolyte have been studied. It was found that the corrosion process first passes across p layers through vertical channels associated with threading structural defects. Then, the corrosion process occurs in the lateral direction along n layers of the structure, with local hollows and voids thereby formed. The lateral etching is due to the presence of positive piezoelectric charges at boundaries of n-AlGaN and n-GaN layers and positively charged ionized donors in the space-charge region of the p-n junction.

  3. Magnetic coupling in ferromagnetic semiconductor (Ga,Mn)As/(Al,Ga,Mn)As bilayers

    NASA Astrophysics Data System (ADS)

    Wang, M.; Wadley, P.; Campion, R. P.; Rushforth, A. W.; Edmonds, K. W.; Gallagher, B. L.; Charlton, T. R.; Kinane, C. J.; Langridge, S.

    2015-08-01

    We report on a study of ferromagnetic semiconductor (Ga,Mn)As/(Al,Ga,Mn)As bilayers using magnetometry and polarized neutron reflectivity (PNR). From depth-resolved characterization of the magnetic structure obtained by PNR, we concluded that the (Ga,Mn)As and (Al,Ga,Mn)As layers have in-plane and perpendicular-to-plane magnetic easy axes, respectively, with weak interlayer coupling. Therefore, the layer magnetizations align perpendicular to each other under low magnetic fields and parallel at high fields.

  4. Ga/1-x/Al/x/As LED structures grown on GaP substrates.

    NASA Technical Reports Server (NTRS)

    Woodall, J. M.; Potemski, R. M.; Blum, S. E.; Lynch, R.

    1972-01-01

    Ga(1-x)Al(x)As light-emitting diode structures have been grown on GaP substrates by the liquid-phase-epitaxial method. In spite of the large differences in lattice constants and thermal-expansion coefficients, room-temperature efficiencies up to 5.5% in air have been observed for a peak emission of 8500 A. Using undoped GaP substrates, which are transparent to the infrared and red portions of the spectrum, thin structures of Ga(1-x)Al(x)As with large external efficiencies can now be made.

  5. Magnetic coupling in ferromagnetic semiconductor (Ga,Mn)As/(Al,Ga,Mn)As bilayers

    SciTech Connect

    Wang, M.; Wadley, P.; Campion, R. P.; Rushforth, A. W.; Edmonds, K. W.; Gallagher, B. L.; Charlton, T. R.; Kinane, C. J.; Langridge, S.

    2015-08-07

    We report on a study of ferromagnetic semiconductor (Ga,Mn)As/(Al,Ga,Mn)As bilayers using magnetometry and polarized neutron reflectivity (PNR). From depth-resolved characterization of the magnetic structure obtained by PNR, we concluded that the (Ga,Mn)As and (Al,Ga,Mn)As layers have in-plane and perpendicular-to-plane magnetic easy axes, respectively, with weak interlayer coupling. Therefore, the layer magnetizations align perpendicular to each other under low magnetic fields and parallel at high fields.

  6. Si-doped GaAs/AlGaAs TJS laser by MBE

    SciTech Connect

    Mitsunaga, K.; Fujiwara, K.; Nunoshita, M.; Nakayama, T.

    1984-04-01

    The effect of high temperature annealing on the properties of silicon-doped GaAs/AlGaAs double heterostructure (DH) grown by molecular beam expitaxy (MBE)= and its application to the fabrication of transverse junction stripe (TJS) lasers are reported. In spite of the amphoteric nature of Si, it was found that the high temperature annealing gave little influence on the electrical and optical quality of the n-type DH wafer. The TJS laser using Si-doped GaAs/AlGaAs wafer has been oscillated cw at room temperature and exhibited low threshold current of 30 mA and high quantum efficiency of 60%.

  7. Persistent photoconductivity study in AlGaN superlattice

    NASA Astrophysics Data System (ADS)

    Wang, Li-wei; Xu, Jin-tong; Wang, Nili; Xu, Peng-xiao; Li, Xiangyang

    2014-11-01

    The transport properties of GaN and its alloys are attracting increasing interest due to the potential application of these materials for solar blind photodetectors and high mobility transistors. Because of the large band gap, the applications of AlxGa1-xN are extensive, such as for visible-blind ultraviolet detectors, laser diodes, and short-wave light emitting diodes (LEDs). However, the persistent photoconductivity (PPC) of GaN based photoconductive devices affects its applications. In order to study the origin of PPC, we designed solar blind ultraviolet photoconductive detector, which consists of n - Al0.65Ga0.35N top contact layer (100nm), n-Al0.42Ga0.58N/i-Al0.65Ga0.35N superlattice layers (200nm), i- Al0.65Ga0.35N layer (600nm), AlN buffer layer and double polished sapphire substrate. Moreover, there are photoconductive devices with different photosensitive areas. Investigations of electric-field effects and thermal effects on PPC in n-Al0.42Ga0.58N/i-Al0.65Ga0.35N superlattice are presented. We have observed that, by applying a high-voltage pulse, the course of PPC was effectively accelerated: With the same pulse width and different voltage, in the appropriate range, the higher of the voltage, the course of PPC was more effectively accelerated; with the same voltage and different pulse width, in the appropriate range, the wider of the pulse width, the course of PPC was more effectively accelerated. And PPC effect strongly depends on the temperature. The decay time of the PPC depend on the temperature and become longer with a decreasing temperature.

  8. Improved degradation resistance of (AlGa)As lasers

    NASA Technical Reports Server (NTRS)

    Kressel, H.; Ladany, J.

    1980-01-01

    Simultaneous doping with Ge and Zn improves degradation resistance of short-wavelength (AlGa)As lasers. Method opens up prospects for greatly increased reliability in lasers and LED's operating at 7,500 angstroms or below.

  9. Nanostructure and strain properties of core-shell GaAs/AlGaAs nanowires

    NASA Astrophysics Data System (ADS)

    Kehagias, Th; Florini, N.; Kioseoglou, J.; Pavloudis, Th; Komninou, Ph; Walther, T.; Moratis, K.; Hatzopoulos, Z.; Pelekanos, N. T.

    2015-11-01

    GaAs/AlGaAs core-shell nanowires (NWs) were grown on Si(111) by Ga-assisted molecular beam epitaxy via the vapor-liquid-solid mechanism. High-resolution and scanning transmission electron microscopy observations showed that NWs were predominantly zinc-blende single crystals of hexagonal shape, grown along the [111] direction. GaAs core NWs emerged from the Si surface and subsequently, the NW growth front advanced by a continuous sequence of (111) rotational twins, while the AlGaAs shell lattice was perfectly aligned with the core lattice. Occasionally, single or multiple stacking faults induced wurtzite structure NW pockets. The AlGaAs shell occupied at least half of the NW’s projected diameter, while the average Al content of the shell, estimated by energy dispersive x-ray analysis, was x = 0.35. Furthermore, molecular dynamics simulations of hexagonal cross-section NW slices, under a new parametrization of the Tersoff interatomic potential for AlAs, showed increased atom relaxation at the hexagon vertices of the shell. This, in conjunction with the compressively strained Al0.35Ga0.65As shell close to the GaAs core, can trigger a kinetic surface mechanism that could drive Al adatoms to accumulate at the relaxed sites of the shell, namely along the diagonals of the shell’s hexagon. Moreover, the absence of long-range stresses in the GaAs/Al0.35Ga0.65As core-shell system may account for a highly stable heterostructure. The latter was consolidated by temperature-dependent photoluminescence spectroscopy.

  10. Impact ionization in N-polar AlGaN/GaN high electron mobility transistors

    SciTech Connect

    Killat, N. E-mail: Martin.Kuball@bristol.ac.uk; Uren, M. J.; Kuball, M. E-mail: Martin.Kuball@bristol.ac.uk; Keller, S.; Kolluri, S.; Mishra, U. K.

    2014-08-11

    The existence of impact ionization as one of the open questions for GaN device reliability was studied in N-polar AlGaN/GaN high electron mobility transistors. Electroluminescence (EL) imaging and spectroscopy from underneath the device gate contact revealed the presence of hot electrons in excess of the GaN bandgap energy even at moderate on-state bias conditions, enabling impact ionization with hole currents up to several hundreds of pA/mm. The detection of high energy luminescence from hot electrons demonstrates that EL analysis is a highly sensitive tool to study degradation mechanisms in GaN devices.

  11. New Al0.25Ga0.75N/GaN high electron mobility transistor with partial etched AlGaN layer

    NASA Astrophysics Data System (ADS)

    Yuan, Song; Duan, Baoxing; Yuan, Xiaoning; Cao, Zhen; Guo, Haijun; Yang, Yintang

    2016-05-01

    In this letter, a new Al0.25Ga0.75N/GaN high electron mobility transistor (HEMT) with the AlGaN layer is partial etched is reported for the first time. The two-dimensional electron gas (2DEG) density in the HEMTs is changed by partially etching the AlGaN layer. A new electric field peak is introduced along the interface between the AlGaN layer and the GaN buffer by the electric field modulation effect. The high electric field near the gate in the proposed Al0.25Ga0.75N/GaN HEMT is effectively decreased, which makes the surface electric field more uniform. Compared with the conventional structure, the breakdown voltage can be improved by 58% for the proposed Al0.25Ga0.75N/GaN HEMT and the current collapse can be reduced resulting from the more uniform surface electric field.

  12. Monolithic AlGaAs second-harmonic nanoantennas.

    PubMed

    Gili, V F; Carletti, L; Locatelli, A; Rocco, D; Finazzi, M; Ghirardini, L; Favero, I; Gomez, C; Lemaître, A; Celebrano, M; De Angelis, C; Leo, G

    2016-07-11

    We demonstrate monolithic aluminum gallium arsenide (AlGaAs) optical nanoantennas. Using a selective oxidation technique, we fabricated epitaxial semiconductor nanocylinders on an aluminum oxide substrate. Second harmonic generation from AlGaAs nanocylinders of 400 nm height and varying radius pumped with femtosecond pulses delivered at 1554-nm wavelength has been measured, revealing a peak conversion efficiency exceeding 10-5 for nanocylinders with an optimized geometry. PMID:27410864

  13. Graded-bandgap AlGaAs solar cells for AlGaAs/Ge cascade cells

    NASA Technical Reports Server (NTRS)

    Timmons, M. L.; Venkatasubramanian, R.; Colpitts, T. S.; Hills, J. S.; Hutchby, J. A.; Iles, P. A.; Chu, C. L.

    1991-01-01

    Some p/n graded-bandgap Al(x)Ga(1-x)As solar cells were fabricated and show AMO conversion efficiencies in excess of 15 percent without antireflection (AR) coatings. The emitters of these cells are graded between 0.008 is less than or equal to x is less than or equal to 0.02 during growth of 0.25 to 0.30 micron thick layers. The keys to achieving this performance were careful selection of organometallic sources and scrubbing oxygen and water vapor from the AsH3 source. Source selection and growth were optimized using time-resolved photoluminescence. Preliminary radiation-resistance measurements show AlGaAs cells degraded less than GaAs cells at high 1 MeV electron fluences, and AlGaAs cells grown on GaAs and Ge substrates degrade comparably.

  14. Effect of high density H 2 plasmas on InGaP/GaAs and AlGaAs/GaAs HEMTs

    NASA Astrophysics Data System (ADS)

    Ren, F.; Kopf, R. F.; Kuo, J. M.; Lothian, J. R.; Lee, J. W.; Pearton, S. J.; Shul, R. J.; Constantine, C.; Johnson, D.

    1998-05-01

    InGaP/GaAs and AlGaAs/GaAs high electron mobility transistors have been exposed to inductively coupled plasma or electron cyclotron resonance H 2 plasmas as a function of pressure, source power and rf chuck power. The transconductance, gate ideality factor and saturated drain-source current are all degraded by the plasma treatment. Two mechanisms are identified: passivation of Si dopants in the InGaP or AlGaAs donor layers by H 0 and lattice disorder created by H + and H 2+ ion bombardment. HEMTs are found to be more susceptible to plasma-induced degradation than heterojunction bipolar transistors.

  15. Room temperature green to red electroluminescence from (Al,Ga)As/GaP QDs and QWs

    NASA Astrophysics Data System (ADS)

    Golz, Christian; Dadgostar, Shabnam; Masselink, W. Ted; Hatami, Fariba

    2016-03-01

    We present the growth, fabrication, and characterization of light-emitting diodes based on (Al,Ga)As quantum wells and dots embedded in a p-n GaP structure. Samples were grown on Sulphur-doped GaP (001) substrate using gas-source molecular beam epitaxy. The structures include either GaAs quantum structures with nominal coverage between 1.2 and 3.6 monolayers or Al0.3Ga0.7As quantum wells. For structures with GaAs layer thicker than 1.5 monolayers the 3.6% lattice mismatch in the materials system results in formation of quantum dots via Stranski-Krastanow growth mode with areal density of about 8×1010 cm-2. The atomic-force and transmission-electron microscopy show that with increasing coverage of GaAs from 1.8 to 3.6 monolayers the average lateral size and height of dots change in the range of 17-34 nm and 0.9-2 nm, respectively. The diode structures emit light from the red to the green spectral range up to room temperature. The GaAs/GaP QDs show electroluminescence between 1.8 eV and 2 eV, whereas the Al0.3Ga0.7As quantum wells emit light between 2 eV and 2.2 eV.

  16. Rapid thermal anneal in InP, GaAs and GaAs/GaAlAs

    NASA Astrophysics Data System (ADS)

    Descouts, B.; Duhamel, N.; Godefroy, S.; Krauz, P.

    Ion implantation in semiconductors provides a doping technique with several advantages over more conventional doping methods and is now extensively used for device applications, e.g. field effect transistors (MESFET GaAs, MIS (InP), GaAs/GaAlAs heterojunction bipolar transistors (HBT). Because of the lattice disorder produced by the implantation, the dopant must be made electrically active by a postimplant anneal. As the device performances are very dependent on its electrical characteristics, the anneal is a very important stage of the process. Rapid anneal is known to provide less exodiffusion and less induffusion of impurities compared to conventional furnace anneal, so this technique has been used in this work to activate an n-type dopant (Si) in InP and a p-type dopant (Mg) in GaAs and GaAs/GaAIAs. These two ions have been chosen to realize implanted MIS InP and the base contacts for GaAs/GaAlAs HBTs. The experimental conditions to obtain the maximum electrical activity in these two cases will be detailed. For example, although we have not been able to obtain a flat profile in Mg + implanted GaAs/GaAlAs heterostructure by conventional thermal anneal, rapid thermal anneal gives a flat hole profile over a depth of 0.5 μm with a concentration of 1 x 10 19 cm -3.

  17. Engineering of electric field distribution in GaN(cap)/AlGaN/GaN heterostructures: theoretical and experimental studies

    NASA Astrophysics Data System (ADS)

    Gladysiewicz, M.; Janicki, L.; Misiewicz, J.; Sobanska, M.; Klosek, K.; Zytkiewicz, Z. R.; Kudrawiec, R.

    2016-09-01

    Polarization engineering of GaN-based heterostructures opens a way to develop advanced transistor heterostructures, although measurement of the electric field in such heterostructures is not a simple task. In this work, contactless electroreflectance (CER) spectroscopy has been applied to measure the electric field in GaN-based heterostructures. For a set of GaN(d  =  0, 5, 15, and 30 nm)/AlGaN(20 nm)/GaN(buffer) heterostructures a decrease of electric field in the GaN(cap) layer from 0.66 MV cm‑1 to 0.27 MV cm‑1 and an increase of the electric field in the AlGaN layer from 0.57 MV cm‑1 to 0.99 MV cm‑1 have been observed with the increase in the GaN(cap) thickness from 5–30 nm. For a set of GaN(20 nm)/AlGaN(d  =  10, 20, 30, and 40 nm)/GaN(buffer) heterostructures a decrease of the electric field in the AlGaN layer from 1.77 MV cm‑1 to 0.64 MV cm‑1 and an increase of the electric field in the GaN layer from 0.57 MV cm‑1 to 0.99 MV cm‑1 were observed with the increase in the AlGaN thickness from 10–40 nm. To determine the distribution of the electric field in these heterostructures the Schrödinger and Poisson equations are solved in a self-consistent manner and matched with experimental data. It is shown that the built-in electric field in the GaN(cap) and AlGaN layers obtained from measurements does not reach values of electric field resulting only from polarization effects. The measured electric fields are smaller due to a screening of polarization effects by free carriers, which are inhomogeneously distributed across the heterostructure and accumulate at interfaces. The results clearly demonstrate that CER measurements supported by theoretical calculations are able to determine the electric field distribution in GaN-based heterostructures quantitatively, which is very important for polarization engineering in this material system.

  18. Efficient AlGaAs shallow-homojunction solar cells

    NASA Technical Reports Server (NTRS)

    Gale, R. P.; Fan, J. C. C.; Turner, G. W.; Chapman, R. L.; Pantano, J. V.

    1984-01-01

    Shallow-homojunction n+/p/p+ solar cells with one-sun, AM1 conversion efficiencies as high as 12.9 percent have been fabricated in Al0.2Ga0.8As epitaxial layers grown by organometallic chemical vapor deposition on single-crystal GaAs substrates. For these cells, which have n+ layers thinned by anodic oxidation to about 500 A, the quantum efficiencies in the short-wavelength portion of the spectrum are as high as the best reported for AlGaAs cells with high band-gap window layers.

  19. Investigation of the wet-etching mechanism of Ga-polar AlGaN/GaN micro-pillars

    NASA Astrophysics Data System (ADS)

    Kang, Junjie; Li, Zhi; Liu, Zhiqiang; Li, Hongjian; Zhao, Yongbing; Tian, Yingdong; Ma, Ping; Yi, Xiaoyan; Wang, Guohong

    2014-01-01

    The wet-etching mechanism of Ga-polar AlGaN/GaN micro-pillars is analyzed in this work. By using solutions of KOH in ethylene glycol (KE), we discover the difference in wet-etching processes with and without an AlGaN layer. In the Ga-polar micro-pillar GaN-based nano-wire, a micro-pillar with {1-100} facet and a pyramid with a {11-212} facet are realized by accurately controlling the wet-etching rate and time, respectively. Schematic drawings are established to explain the dissimilarity of these two structures. Our research attempts to fabricate lager scale nano-wires and quasi-polarity substrates.

  20. Nanoscale investigation of AlGaN/GaN-on-Si high electron mobility transistors

    NASA Astrophysics Data System (ADS)

    Fontserè, A.; Pérez-Tomás, A.; Placidi, M.; Llobet, J.; Baron, N.; Chenot, S.; Cordier, Y.; Moreno, J. C.; Jennings, M. R.; Gammon, P. M.; Fisher, C. A.; Iglesias, V.; Porti, M.; Bayerl, A.; Lanza, M.; Nafría, M.

    2012-10-01

    AlGaN/GaN HEMTs are devices which are strongly influenced by surface properties such as donor states, roughness or any kind of inhomogeneity. The electron gas is only a few nanometers away from the surface and the transistor forward and reverse currents are considerably affected by any variation of surface property within the atomic scale. Consequently, we have used the technique known as conductive AFM (CAFM) to perform electrical characterization at the nanoscale. The AlGaN/GaN HEMT ohmic (drain and source) and Schottky (gate) contacts were investigated by the CAFM technique. The estimated area of these highly conductive pillars (each of them of approximately 20-50 nm radius) represents around 5% of the total contact area. Analogously, the reverse leakage of the gate Schottky contact at the nanoscale seems to correlate somehow with the topography of the narrow AlGaN barrier regions producing larger currents.

  1. Characterization of multilayer GaAs/AlGaAs transistor structures by variable angle spectroscopic ellipsometry

    NASA Technical Reports Server (NTRS)

    Merkel, Kenneth G.; Snyder, Paul G.; Woollam, John A.; Alterovitz, Samuel; Rai, A. K.

    1989-01-01

    Variable angle of incidence spectroscopic ellipsometry (VASE) has been implemented as a means of determining layer thickness, alloy composition, and growth quality of GaAs/AlGaAs samples composed of relatively thick layers as well as superlattices. The structures studied in this work contained GaAs/AlGaAs multilayers with a superlattice 'barrier' and were grown for later formation of modulation-doped field effect transistors (MODFETs). Sample modeling was performed by treating the superlattice as a bulk AlGaAs layer of unknown composition. Extremely good data fits were realized when five layer thicknesses and two alloy ratios were allowed to vary in a regression analysis. Room temperature excitonic effects associated with the e-hh(1), e-lh(1) and e-hh(2) transitions were observed in the VASE data.

  2. Voltage sensitivity of a point-contact GaAs/AlGaAs heterojunction microwave detector

    NASA Astrophysics Data System (ADS)

    Sužiedėlis, A.; Ašmontas, S.; Kundrotas, J.; Nargelienė, V.; Gradauskas, J.

    2012-03-01

    The detection properties of a microwave detector containing a point-contact GaAs/AlGaAs heterojunction were investigated in the X to Ka frequency range. The detector had a small-area GaAs/AlGaAs heterojunction connected in series with small- and large-area n-n+ homojunctions. Both theoretically and experimentally, the frequency dependence of the voltage sensitivity of the microwave detector was established, which we relate with the intervalley electromotive force arising in Al0.3Ga0.7As. The intervalley electromotive force is expressed using phenomenological electron transport theory in a many-valley semiconductor, and an intervalley relaxation time of 20 ps is estimated using the frequency dependence of the detector voltage sensitivity.

  3. Growth of high quality and uniformity AlGaN/GaN heterostructures on Si substrates using a single AlGaN layer with low Al composition.

    PubMed

    Cheng, Jianpeng; Yang, Xuelin; Sang, Ling; Guo, Lei; Zhang, Jie; Wang, Jiaming; He, Chenguang; Zhang, Lisheng; Wang, Maojun; Xu, Fujun; Tang, Ning; Qin, Zhixin; Wang, Xinqiang; Shen, Bo

    2016-01-01

    By employing a single AlGaN layer with low Al composition, high quality and uniformity AlGaN/GaN heterostructures have been successfully grown on Si substrates by metal-organic chemical vapor deposition (MOCVD). The heterostructures exhibit a high electron mobility of 2150 cm(2)/Vs with an electron density of 9.3 × 10(12) cm(-2). The sheet resistance is 313 ± 4 Ω/◻ with ±1.3% variation. The high uniformity is attributed to the reduced wafer bow resulting from the balance of the compressive stress induced and consumed during the growth, and the thermal tensile stress induced during the cooling down process. By a combination of theoretical calculations and in situ wafer curvature measurements, we find that the compressive stress consumed by the dislocation relaxation (~1.2 GPa) is comparable to the value of the thermal tensile stress (~1.4 GPa) and we should pay more attention to it during growth of GaN on Si substrates. Our results demonstrate a promising approach to simplifying the growth processes of GaN-on-Si to reduce the wafer bow and lower the cost while maintaining high material quality. PMID:26960730

  4. Growth of high quality and uniformity AlGaN/GaN heterostructures on Si substrates using a single AlGaN layer with low Al composition

    NASA Astrophysics Data System (ADS)

    Cheng, Jianpeng; Yang, Xuelin; Sang, Ling; Guo, Lei; Zhang, Jie; Wang, Jiaming; He, Chenguang; Zhang, Lisheng; Wang, Maojun; Xu, Fujun; Tang, Ning; Qin, Zhixin; Wang, Xinqiang; Shen, Bo

    2016-03-01

    By employing a single AlGaN layer with low Al composition, high quality and uniformity AlGaN/GaN heterostructures have been successfully grown on Si substrates by metal-organic chemical vapor deposition (MOCVD). The heterostructures exhibit a high electron mobility of 2150 cm2/Vs with an electron density of 9.3 × 1012 cm‑2. The sheet resistance is 313 ± 4 Ω/◻ with ±1.3% variation. The high uniformity is attributed to the reduced wafer bow resulting from the balance of the compressive stress induced and consumed during the growth, and the thermal tensile stress induced during the cooling down process. By a combination of theoretical calculations and in situ wafer curvature measurements, we find that the compressive stress consumed by the dislocation relaxation (~1.2 GPa) is comparable to the value of the thermal tensile stress (~1.4 GPa) and we should pay more attention to it during growth of GaN on Si substrates. Our results demonstrate a promising approach to simplifying the growth processes of GaN-on-Si to reduce the wafer bow and lower the cost while maintaining high material quality.

  5. Growth of high quality and uniformity AlGaN/GaN heterostructures on Si substrates using a single AlGaN layer with low Al composition

    PubMed Central

    Cheng, Jianpeng; Yang, Xuelin; Sang, Ling; Guo, Lei; Zhang, Jie; Wang, Jiaming; He, Chenguang; Zhang, Lisheng; Wang, Maojun; Xu, Fujun; Tang, Ning; Qin, Zhixin; Wang, Xinqiang; Shen, Bo

    2016-01-01

    By employing a single AlGaN layer with low Al composition, high quality and uniformity AlGaN/GaN heterostructures have been successfully grown on Si substrates by metal-organic chemical vapor deposition (MOCVD). The heterostructures exhibit a high electron mobility of 2150 cm2/Vs with an electron density of 9.3 × 1012 cm−2. The sheet resistance is 313 ± 4 Ω/◻ with ±1.3% variation. The high uniformity is attributed to the reduced wafer bow resulting from the balance of the compressive stress induced and consumed during the growth, and the thermal tensile stress induced during the cooling down process. By a combination of theoretical calculations and in situ wafer curvature measurements, we find that the compressive stress consumed by the dislocation relaxation (~1.2 GPa) is comparable to the value of the thermal tensile stress (~1.4 GPa) and we should pay more attention to it during growth of GaN on Si substrates. Our results demonstrate a promising approach to simplifying the growth processes of GaN-on-Si to reduce the wafer bow and lower the cost while maintaining high material quality. PMID:26960730

  6. Cation distribution and optical properties of Cr-doped MgGa2O4 nanocrystals

    NASA Astrophysics Data System (ADS)

    Duan, Xiulan; Liu, Jian; Wang, Xinqiang; Jiang, Huaidong

    2014-11-01

    The distribution of cations in the spinel-type MgCr2yGa2-2yO4 (y = 0-0.6) nanocrystals and their optical properties as a function of annealing temperature and chromium content were investigated by using X-ray photoelectron spectroscopy (XPS) in combination with absorption spectroscopy. The cations in MgCr2yGa2-2yO4 nanocrystals are disorderly distributed with mixing of divalent and trivalent cations occupying the tetrahedral and octahedral sites. With the increase of annealing temperature, the inversion parameter (the fraction of Mg2+ ions in octahedral sites) decreases, which has the same varying tendency with the proportion of tetrahedral Ga3+ or Cr3+ ions. The inversion parameter increases with increasing Cr3+ concentration. The absorption spectra indicate that Cr3+ ions are located in the octahedral sites as well as in the tetrahedral sites. The fraction of tetrahedral Cr3+ decreases with Cr-enrichment. The optical absorption properties of Cr-doped MgGa2O4 nanocrystals may be tuned by varying the preparation temperature or Cr concentration.

  7. Analytical modeling of AlGaN/AlN/GaN heterostructures including effects of distributed surface donor states

    SciTech Connect

    Goyal, Nitin; Fjeldly, Tor A.

    2014-07-14

    In this paper, a physics based analytical model is presented for calculation of the two-dimensional electron gas density and the bare surface barrier height of AlGaN/AlN/GaN material stacks. The presented model is based on the concept of distributed surface donor states and the self-consistent solution of Poisson equation at the different material interfaces. The model shows good agreement with the reported experimental data and can be used for the design and characterization of advanced GaN devices for power and radio frequency applications.

  8. Temperature dependencies of current-voltage characteristics of GaAs:Cr

    NASA Astrophysics Data System (ADS)

    Kolesnikova, I.; Lozinskaya, A.; Mihaylov, T.; Novikov, V.; Shemeryankina, A.; Sherbakov, I.; Tolbanov, O.; Tyazhev, A.; Zarubin, A.

    2016-03-01

    A detailed analysis of temperature dependencies of I-V-characteristics of high resistive chromium compensated gallium arsenide (HR GaAs:Cr) sensors is presented. Samples had Cr/Ni contacts made using electron-beam deposition that formed Schottky barrier contact to GaAs. Thus the structure of the samples was Ni/Cr - HR GaAs:Cr - Cr/Ni. The I-V curves were investigated in the temperature range from 23°C to 70°C. Current-voltage characteristics quite well obey the thermionic emission model in all ranges of temperature. The barrier height of the Schottky barrier was calculated from experimental data. The results of the investigation are discussed in this paper.

  9. The substructure and luminescence of low-temperature AlGaAs/GaAs(100) heterostructures

    SciTech Connect

    Seredin, P. V. Glotov, A. V.; Domashevskaya, E. P.; Arsentyev, I. N. Vinokurov, D. A.; Tarasov, I. S.; Zhurbina, I. A.

    2010-02-15

    The substructure and luminescence of low-temperature epitaxial AlGaAs alloys are studied by Raman spectroscopy and photoluminescence spectroscopy. It is shown that the experimental data obtained in the study are consistent with the results of the previous structural and optical study. The assumption that, at high concentrations of carbon acceptors, the acceptor atoms concentrate at lattice defects of the AlGaAs crystal alloys to form carbon nanocrystals is confirmed.

  10. AlGaAs/GaAs photovoltaic converters for high power narrowband radiation

    SciTech Connect

    Khvostikov, Vladimir; Kalyuzhnyy, Nikolay; Mintairov, Sergey; Potapovich, Nataliia; Shvarts, Maxim; Sorokina, Svetlana; Andreev, Viacheslav; Luque, Antonio

    2014-09-26

    AlGaAs/GaAs-based laser power PV converters intended for operation with high-power (up to 100 W/cm{sup 2}) radiation were fabricated by LPE and MOCVD techniques. Monochromatic (λ = 809 nm) conversion efficiency up to 60% was measured at cells with back surface field and low (x = 0.2) Al concentration 'window'. Modules with a voltage of 4 V and the efficiency of 56% were designed and fabricated.

  11. Parallel magnetic field-induced conductance fluctuations in GaAs/AlGaAs ballistic quantum dots.

    NASA Astrophysics Data System (ADS)

    Faniel, S.; Gustin, C.; Melinte, S.; Hackens, B.; Bayot, V.; Shayegan, M.

    2004-03-01

    We present magnetotransport measurements in ballistic quantum dots under a parallel magnetic field. The dots were fabricated on two different GaAs/AlGaAs quantum wells with thicknesses of 15 and and 45 nm and with one and two subbands occupied, respectively. The samples were patterned using e-beam lithography and wet etching. A Cr/Au electrostatic top gate was used in order to tune the width of the dot openings. The measurements were performed down to 300 mK with the magnetic field applied strictly parallel to the plane of the two-dimensional electron gas. For both dots, we observe universal conductance fluctuations and, in the case of the wide quantum well, a reduction of their amplitude at large magnetic field. We discuss these conductance fluctuations in terms of orbital effect(V.I. Fal'ko and T. Jungwirth, Phys Rev B 65), 081306 (2002) and magnetic subband depopulation.

  12. Phonon Knudsen flow in GaAs/AlAs superlattices

    SciTech Connect

    Hyldgaard, P.; Mahan, G.D. |

    1995-09-01

    The measured in-plane thermal conductivity, {delta}{sub SL} of GaAs/AlAs superlattices with even moderate layer thicknesses are significantly smaller than the weighted average, {delta}{sub l} = 67 W/Km, of the bulk GaAs and AlAs conductivities. One expects a suppression of the thermal conductivity to that of an actual Al{sub 0.5}Ga{sub 0.5}As alloy when the thickness of the GaAs and AlAs layers approaches that of a single monolayer. However, the observed superlattice thermal conductivity remains suppressed even at layer thickness {approx_gt} 10 nm. The low thermal conductivities, and very high mobilities, make n-doped GaAs/AlAs superlattices attractive possibilities for thermoelectric devices. With Molecular-Beam-Epitaxial grown GaAs/AlAs superlattices one can expect the individual GaAs and AlAs layers to be extremely clean. Defect and/or alloy scattering is limited to be near the heterostructure interfaces. The authors estimate the room-temperature phonon mean-free-path to be 42 (22) nm for the longitudinal (transverse) mode and thus comparable to or smaller than the layer thicknesses. Thus they expect an important phonon scattering at the interfaces. They study this phonon scattering at the superlattice interfaces assuming a Knudsen flow characterized by diffusive scattering. The solid curve in the figure shows the Knudsen-flow theory estimated for the superlattice thermal conductivity which shows a significant reduction when the layer thickness is shorter than the estimated phonon mean free paths.

  13. InGaAsN/AlGaAs P-n-p heterojunction bipolar transistor

    SciTech Connect

    Chang, P. C.; Baca, A. G.; Li, N. Y.; Sharps, P. R.; Hou, H. Q.; Laroche, J. R.; Ren, F.

    2000-05-08

    We have demonstrated a functional P-n-p heterojunction bipolar transistor (HBT) using InGaAsN. The metalorganic-vapor-phase-epitaxy-grown Al{sub 0.3}Ga{sub 0.7}As/In{sub 0.03}Ga{sub 0.97}As{sub 0.99}N{sub 0.01} HBT takes advantage of the narrower band gap energy (E{sub g}=1.2 eV) of In{sub 0.03}Ga{sub 0.97}As{sub 0.99}N{sub 0.01}, which is lattice matched to GaAs. Compared with the Al{sub 0.3}Ga{sub 0.7}As/GaAs material system, the Al{sub 0.3}Ga{sub 0.7}As/In{sub 0.03}Ga{sub 0.97}As{sub 0.99}N{sub 0.01} material system has a larger conduction-band offset, while the valence-band offset remains comparable. This characteristic band alignment is very suitable for P-n-p HBT applications. The device's peak current gain is 23, and it has a turn-on voltage of 0.77 V, which is 0.25 V lower than in a comparable P-n-p Al{sub 0.3}Ga{sub 0.7}As/GaAs HBT. (c) 2000 American Institute of Physics.

  14. Comparative Study on Intersubband Absorption in AlGaN/GaN and AlInN/GaN Heterostructures Grown on Low-Defect Substrates

    NASA Astrophysics Data System (ADS)

    Edmunds, Colin; Tang, Liang; Shao, Jiayi; Li, Donghui; Gardner, Geoff; Manfra, Michael; Malis, Oana; Grier, Andrew; Ikonic, Zoran; Harrison, Paul; Zakharov, Dimitri

    2013-03-01

    Intersubband (ISB) devices utilizing III-nitrides have attracted attention for near- and far-infrared optoelectronic applications. However, the lattice mismatch between GaN and commonly used substrates results in a high defect density that hinders the vertical transport required for these devices. Furthermore, most devices in the literature utilize AlGaN/GaN heterostructures for which there is no lattice-matched alloy composition. Due to this lattice mismatch, AlGaN is not ideal for the development of complex devices such as quantum cascade lasers that often require active-region thicknesses on the order of microns for efficient operation. Fortunately, exact lattice matching occurs in AlInN/GaN heterostructures at roughly 18% In composition. To investigate the challenges of lattice-matched nitrides, we presents a comparative study of ISB absorption in high-quality AlGaN/GaN and near lattice-matched AlInN/GaN heterostructures grown by molecular-beam epitaxy on low-defect free-standing GaN substrates. Experimental measurements of transition energy, integrated absorbance and linewidth were compared to theoretical predictions that included many-body effects, interface roughness and calculations of the transition lifetime.

  15. Hydrogen permeation characteristics of some Fe-Cr-Al alloys

    NASA Astrophysics Data System (ADS)

    Van Deventer, E. H.; Maroni, V. A.

    1983-01-01

    Hydrogen permeation data are reported for two Fe-Cr-Al alloys, Type-405 SS (Cr 14-A1 0.2) and a member of the Fecralloy family of alloys (Cr 16-A1 5). The hydrogen permeability of each alloy (in a partially oxidized condition) was measured over a period of several weeks at randomly selected temperatures (between 150 and 850°C) and upstream H 2 pressures (between 2 and 1.5 × 10 4 Pa). The permeabilities showed considerable scatter with both time and temperature and were 10 2 to 10 3 times lower than those of pure iron, even in strongly reducing environments. The exponent, n, for the relationship between upstream H 2 pressure, P, and permeability, φ, ( φ ~ Pn) was closer to 0.7 than to the expected 0.5, indicating a process limited by surface effects (e.g., surface oxide films) as opposed to bulk material effects. Comparison of these results with prior permeation measurements on other Fe-Cr-Al alloys, on Fe-Cr alloys, and on pure iron shows that the presence of a few weight percent aluminum offers the best prospects for achieving low tritium permeabilities with martensitic and ferritic steels used in fusion-reactor first wall and blanket applications.

  16. Grain-size control in Ti-48Al-2Cr-2Nb with yttrium additions

    NASA Astrophysics Data System (ADS)

    Trivedi, P. B.; Patankar, S. N.; (Sam) Froes, F. H.; Baburaj, E. G.; Genç, A.; Ovecoglu, L.

    2002-08-01

    A gas-atomized (GA) prealloyed powder of the Ti-48Al-2Cr-2Nb intermetallic and 1.6 wt pct Y were mechanically alloyed (MA) and hot isostatically pressed (hipped) to produce a fully dense nanocrystalline material. Mechanical alloying of the as-blended powder for 16 hours resulted in the formation of a disordered fcc phase. Hipping of the alloy powder produced a single-phase nanocrystalline TiAl intermetallic, containing a distribution of 20 to 35-nm-sized Al2Y4O9 particles. The formation of oxide particles occurred by the chemical combination of Al and Y with oxygen, which entered as a contaminant during milling. Oxide particles increased the hardness of the intermetallic compound and minimized grain growth even at 0.8 T m , where T m is the melting point of the compound.

  17. A crystalline oxide passivation for Al{sub 2}O{sub 3}/AlGaN/GaN

    SciTech Connect

    Qin, Xiaoye; Dong, Hong; Kim, Jiyoung; Wallace, Robert M.

    2014-10-06

    In situ X-ray photoelectron spectroscopy and low energy electron diffraction are performed to study the formation of a crystalline oxide on the AlGaN surface. The oxidation of the AlGaN surface is prepared by annealing and remote N{sub 2} + O{sub 2} plasma pretreatments resulting in a stable crystalline oxide. The impact of the oxide on the interface state density is studied by capacitance voltage (C-V) measurements. It is found that a remote plasma exposure at 550 °C shows the smallest frequency dispersion. Crystalline oxide formation may provide a novel passivation method for high quality AlGaN/GaN devices.

  18. Growth of Cr2CoGa and inverse Heusler thin films using Molecular Beam Epitaxy

    NASA Astrophysics Data System (ADS)

    Jamer, Michelle; Decapua, Matthew; Player, Gabriel; Heiman, Don

    Theoretical calculations have predicted the existence of inverse Heusler compounds that exhibit zero-moment magnetization while retaining their half-metallicity. These unique compounds have been labeled spin gapless semiconductors (SGS), where the density of states (DOS) can behave as a half-metal or gapless semiconductor. There is a special interest for zero-moment SGS compounds since traditional antiferromagnets cannot be spin-polarized. Such compounds are experimentally attractive for future spintronic devices due to their large magnetic transition temperature (400-800 K). This work focuses on zero-moment inverse Heusler compounds including Cr2CoGa and Mn3Al. Thin films have been grown using MBE and their magnetic, structural, and electrical properties of these compounds have been characterized by various techniques, including XMCD and magnetometry. The atomic moments are found to be large, but significant cancellations lead to small average moments. Supported by NSF Grant ECCS-1402738.

  19. Theoretical investigation of spin-filtering in CrAs/GaAs heterostructures

    SciTech Connect

    Stickler, B. A.; Ertler, C.; Pötz, W.; Chioncel, L.

    2013-12-14

    The electronic structure of bulk zinc-blende GaAs, zinc-blende and tetragonal CrAs, and CrAs/GaAs supercells, computed within linear muffin-tin orbital (LMTO) local spin-density functional theory, is used to extract the band alignment for the [1,0,0] GaAs/CrAs interface in dependence of the spin orientation. With the lateral lattice constant fixed to the experimental bulk GaAs value, a local energy minimum is found for a tetragonal CrAs unit cell with a longitudinal ([1,0,0]) lattice constant reduced by ≈2%. Due to the identified spin-dependent band alignment, half-metallicity of CrAs no longer is a key requirement for spin-filtering. Based on these findings, we study the spin-dependent tunneling current in [1,0,0] GaAs/CrAs/GaAs heterostructures within the non-equilibrium Green's function approach for an effective tight-binding Hamiltonian derived from the LMTO electronic structure. Results indicate that these heterostructures are promising candidates for efficient room-temperature all-semiconductor spin-filtering devices.

  20. Persistent luminescence and photocatalytic properties of Ga2O3:Cr3+,Zn2+ phosphors

    NASA Astrophysics Data System (ADS)

    Wang, Yinhai; Xu, Ke; Li, Darong; Zhao, Hui; Hu, Zhengfa

    2014-09-01

    Ga2O3:Cr3+0.01, Zn2+0.005 and Ga2O3:Cr3+0.01 phosphors were synthesized by the high temperature solid state reaction. The emission and excitation spectra, persistent luminescence decay curve, thermoluminescence curve and photocatalytic properties were measured. The emission and excitation spectra indicate that the Zn2+ doping has no effect on photoluminescent properties of Ga2O3:Cr3+0.01. The red persistent luminescence of Ga2O3:Cr3+0.01, Zn2+0.005 could be observed for 20 min by naked eye. The thermoluminescence (TL) curve shows that a suitable trap was introduced into the Ga2O3:Cr3+ host by Zn2+ doping. The photocatalytic activity toward rhodamineB (RhB) was obtained to evaluate photocatalytic properties by Zn2+ doping. The results indicate that the Zn2+ doping could improve the photocatalytic properties of Ga2O3:Cr3+0.01 phosphors. It was considered that the new traps play an important role for trapping the electrons or holes to decrease the combination of the electrons and holes producing by the light irradiation and increase the photocatalytic activity.

  1. On the optimization of asymmetric barrier layers in InAlGaAs/AlGaAs laser heterostructures on GaAs substrates

    SciTech Connect

    Zhukov, A. E.; Asryan, L. V.; Semenova, E. S.; Zubov, F. I.; Kryzhanovskaya, N. V.; Maximov, M. V.

    2015-07-15

    Band offsets at the heterointerface are calculated for various combinations of InAlGaAs/AlGaAs heteropairs that can be synthesized on GaAs substrates in the layer-by-layer pseudomorphic growth mode. Patterns which make it possible to obtain an asymmetric barrier layer providing the almost obstruction-free transport of holes and the highest possible barrier height for electrons are found. The optimal compositions of both compounds (In{sup 0.232}Al{sup 0.594}Ga{sup 0.174}As/Al{sup 0.355}Ga{sup 0.645}As) at which the flux of electrons across the barrier is at a minimum are determined with consideration for the critical thickness of the indium-containing quaternary solid solution.

  2. Electron and hole accumulations at GaN/AlInN/GaN interfaces and conductive n-type AlInN/GaN distributed Bragg reflectors

    NASA Astrophysics Data System (ADS)

    Yoshida, Shotaro; Ikeyama, Kazuki; Yasuda, Toshiki; Furuta, Takashi; Takeuchi, Tetsuya; Iwaya, Motoaki; Kamiyama, Satoshi; Akasaki, Isamu

    2016-05-01

    We investigated electron and hole accumulations at GaN/AlInN/GaN interfaces by Hall effect measurement. The InN mole fraction and temperature dependences on the sheet carrier densities at the interfaces reveal that electrons and holes were induced by large positive and negative polarization charges to satisfy the charge neutrality conditions, respectively. On the basis of the above results, we then designed and demonstrated a low-resistity 10-pair Si-doped n-type AlInN/GaN distributed Bragg reflector (DBR) by using high Si doped and graded layers at the GaN/AlInN interfaces. The low-resistity n-type AlInN/GaN DBR will reduce the resistance and the internal loss in blue vertical-cavity surface emitting lasers.

  3. Emergence of half-metallic ferromagnetism in Ga1- x Cr x As

    NASA Astrophysics Data System (ADS)

    Rani, Anita; Kumar, Ranjan

    2016-08-01

    We have studied the structural, electronic and half-metallic ferromagnetic properties of Ga1- x Cr x As compounds at dopant concentrations x = 0.25, 0.125 and 0.0625. First principle calculations based on density functional theories as implemented in SIESTA code using LDA + U (local density approximation + U) as exchange correlation potential have been used to study the properties of these compounds. Here, U is the Hubbard's parameter. The calculated results predict that Cr-doped GaAs diluted magnetic semiconductors exhibit half-metallic properties at different concentrations, in which Cr atoms form deep levels in forbidden energy gap. The results also predict that with increase of fraction of Cr atoms, half-metallic energy band gap of Ga1- x Cr x As decreases. Total magnetic moment of these compounds is due to Cr states, and also p-d hybridization between Ga-p and Cr-d induces small magnetic moment on nonmagnetic atoms (Ga and As) for all concentrations.

  4. Some TEM observations of Al2O3 scales formed on NiCrAl alloys

    NASA Technical Reports Server (NTRS)

    Smialek, J.; Gibala, R.

    1979-01-01

    The microstructural development of Al2O3 scales on NiCrAl alloys has been examined by transmission electron microscopy. Voids have been observed within grains in scales formed on a pure NiCrAl alloy. Both voids and oxide grains grew measurably with oxidation time at 1100 C. The size and amount of porosity decreased towards the oxide-metal growth interface. It was postulated that the voids resulted from an excess number of oxygen vacancies near the oxide-metal interface. Short-circuit diffusion paths were discussed in reference to current growth stress models for oxide scales. Transient oxidations of pure, Y-doped, and Zr-doped NiCrAl was also examined. Oriented alpha-(Al,Cr)2O3 and Ni(Al,Cr)2O4 scales often coexisted in layered structures on all three alloys. Close-packed oxygen planes and directions in the corundum and spinel layers were parallel. The close relationships between oxide layers provided a gradual transition from initial transient scales to steady state Al2O3 growth.

  5. Some TEM observations of Al2O3 scales formed on NiCrAl alloys

    NASA Technical Reports Server (NTRS)

    Smialek, J.; Gibala, R.

    1979-01-01

    The microstructural development of Al2O3 scales on NiCrAl alloys has been examined by transmission electron microscopy. Voids were observed within grains in scales formed on a pure NiCrAl alloy. Both voids and oxide grains grew measurably with oxidation time at 1100 C. The size and amount of porosity decreased towards the oxide-metal growth interface. The voids resulted from an excess number of oxygen vacancies near the oxidemetal interface. Short-circuit diffusion paths were discussed in reference to current growth stress models for oxide scales. Transient oxidation of pure, Y-doped, and Zr-doped NiCrAl was also examined. Oriented alpha-(Al, Cr)2O3 and Ni(Al, Cr)2O4 scales often coexisted in layered structures on all three alloys. Close-packed oxygen planes and directions in the corundum and spinel layers were parallel. The close relationship between oxide layers provided a gradual transition from initial transient scales to steady state Al2O3 growth.

  6. Si diffusion and intermixing in AlGaAs/GaAs structures using buried impurity sources

    SciTech Connect

    Beernink, K.J.; Thornton, R.L.; Anderson, G.B.; Emanuel, M.A.

    1995-05-08

    Si diffusion and impurity-induced layer intermixing from a buried impurity source have been studied by transmission electron microscopy and secondary ion mass spectroscopy of isolated, Si-doped GaAs layers in an undoped Al{sub 0.4}Ga{sub 0.6}As/GaAs superlattice and by photoluminescence measurements on Si-doped GaAs quantum wells with undoped Al{sub 0.4}Ga{sub 0.6}As barriers. In annealed samples, the Si profile suggests a Si diffusion process involving multiply ionized column III vacancies. The width of the resulting Si profile and the spatial extent and completeness of intermixing strongly depend on the initial Si concentration in the doped layer.

  7. A V-grooved AlGaAs/GaAs passivated pn junction

    NASA Technical Reports Server (NTRS)

    Bailey, S. G.; Leon, R. P.; Arrison, A.

    1987-01-01

    A passivated, V-grooved GaAs solar cell offers important advantages in terms of improved optical coupling, higher short-circuit current, and increased tolerance to particle radiation when compared to the planar cell configuration. An AlGaAs epilayer has been deposited on a p-type GaAs epilayer grown on an n-type V-grooved GaAs surface using MOCVD. A wet chemical etching process was used to produce a V-pattern with a 7.0-micron periodicity. Reflectivity measurements substantiate the expected decrease in solar reflectance. Scanning electron microscopy techniques were used to confirm the presence of the AlGaAs layer and verify the existence of a pn junction.

  8. A V-grooved AlGaAs/GaAs passivated PN junction

    NASA Technical Reports Server (NTRS)

    Bailey, Sheila G.; Leon, Rosa P.; Arrison, Anne

    1987-01-01

    A passivated, V-grooved GaAs solar cell offers important advantages in terms of improved optical coupling, higher short circuit current, and increased tolerance to particle radiation when compared to the planar cell configuration. An AlGaAs epilayer has been deposited on a p-type GaAs epilayer grown on an n-type V-grooved GaAs surface using MOCVD. A wet chemical etching process was used to produce a V-pattern with a 7.0 micron periodicity. Reflectivity measurements substantiate the expected decrease in solar reflectance. Scanning electron microscopy techniques were used to confirm the presence of the AlGaAs layer and verify the existence of a pn junction.

  9. V-grooved AlGaAs/GaAs passivated PN junction

    SciTech Connect

    Bailey, S.G.; Leon, R.P.; Arrison, A.

    1987-05-01

    A passivated, V-grooved GaAs solar cell offers important advantages in terms of improved optical coupling, higher short circuit current, and increased tolerance to particle radiation when compared to the planar cell configuration. An AlGaAs epilayer has been deposited on a p-type GaAs epilayer grown on an n-type V-grooved GaAs surface using MOCVD. A wet chemical etching process was used to produce a V-pattern with a 7.0 micron periodicity. Reflectivity measurements substantiate the expected decrease in solar reflectance. Scanning electron microscopy techniques were used to confirm the presence of the AlGaAs layer and verify the existence of a pn junction.

  10. Two-dimensional electron gas in AlGaN/GaN heterostructures

    SciTech Connect

    Li, J.Z.; Lin, J.Y.; Jiang, H.X.; Khan, M.A.; Chen, Q.

    1997-07-01

    The formation of a two-dimensional electron gas (2DEG) system by an AlGaN/GaN heterostructure has been further confirmed by measuring its electrical properties. The effect of persistent photoconductivity (PPC) has been observed and its unique features have been utilized to study the properties of 2DEG formed by the AlGaN/GaN heterointerface. Sharp electronic transitions from the first to the second subbands in the 2DEG channel have been observed by monitoring the 2DEG carrier mobility as a function of carrier concentration through the use of PPC. These results are expected to have significant implications on field-effect transistor and high electron mobility transistor applications based on the GaN system. {copyright} {ital 1997 American Vacuum Society.}

  11. Fabrication and characterization of graphene/AlGaN/GaN ultraviolet Schottky photodetector

    NASA Astrophysics Data System (ADS)

    Kumar, M.; Jeong, H.; Polat, K.; Okyay, A. K.; Lee, D.

    2016-07-01

    We report on the fabrication and characterization of a Schottky ultraviolet graphene/AlGaN/GaN photodetector (PD). The fabricated device clearly exhibits rectification behaviour, indicating that the Schottky barrier is formed between the AlGaN and the mechanically transferred graphene. The Schottky parameters are evaluated using an equivalent circuit with two diodes connected back-to-back in series. The PD shows a low dark current of 4.77  ×  10‑12 A at a bias voltage of  ‑2.5 V. The room temperature current–voltage (I–V) measurements of the graphene/AlGaN/GaN Schottky PD exhibit a large photo-to-dark contrast ratio of more than four orders of magnitude. Furthermore, the device shows peak responsivity at a wavelength of 350 nm, corresponding to GaN band edge and a small hump at 300 nm associated to the AlGaN band edge. In addition, we examine the behaviour of Schottky PDs with responsivities of 0.56 and 0.079 A W‑1 at 300 and 350 nm, respectively, at room temperature.

  12. Electron transport through cubic InGaN/AlGaN resonant tunneling diodes

    NASA Astrophysics Data System (ADS)

    Yahyaoui, N.; Sfina, N.; Nasrallah, S. Abdi-Ben; Lazzari, J.-L.; Said, M.

    2014-12-01

    We theoretically study the electron transport through a resonant tunneling diode (RTD) based on strained AlxGa1-xN/In0.1Ga0.9N/AlxGa1-xN quantum wells embedded in relaxed n- Al0.15Ga0.85N/strained In0.1Ga0.9N emitter and collector. The aluminum composition in both injector and collector contacts is taken relatively weak; this does not preclude achieving a wide band offset at the border of the pre-confinement wells. The epilayers are assumed with a cubic crystal structure to reduce spontaneous and piezoelectric polarization effects. The resonant tunneling and the thermally activated transfer through the barriers are the two mechanisms of transport taken into account in the calculations based on the Schrödinger, Poisson and kinetic equations resolved self-consistently. Using the transfer matrix formalism, we have analyzed the influence of the double barrier height on the resonant current. With an Al composition in the barriers varying between 30% and 50%, we have found that resonant tunneling dominates over the transport mediated by the thermally activated charge transfer for low applied voltages. It is also found that the designed n-type InGaN/AlGaN RTD with 30% of Al composition in the barriers is a potential candidate for achieving a resonant tunneling diode.

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

    SciTech Connect

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

    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, 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.

  14. MOCVD growth of AlGaN UV LEDs

    SciTech Connect

    Han, J.; Crawford, M.H.

    1998-09-01

    Issues related to the MOCVD growth of AlGaN, specifically the gas-phase parasitic reactions among TMG, TMA, and NH{sub 3}, are studied using an in-situ optical reflectometer. It is observed that the presence of the well-known gas phase adduct (TMA: NH{sub 3}) could seriously hinder the incorporation behavior of TMGa. Relatively low reactor pressures (30--50 Torr) are employed to grow an AlGaN/GaN SCH QW p-n diode structure. The UV emission at 360 nm (FWHM {approximately} 10 nm) represents the first report of LED operation from an indium-free GaN QW diode.

  15. High-efficiency AlGaAs-GaAs solar cells with internal Bragg reflector

    SciTech Connect

    Andreev, V.M.; Komin, V.V.; Kochnev, I.V.; Lantratov, V.M.; Shvarts, M.Z.

    1994-12-31

    The work presents an investigation of solar cells (SCs) based on AlGaAs/GaAs heterostructures with internal Bragg reflectors grown by low-pressure MOCVD on n-GaAs substrates in a horizontal resistively heated reactor. The typical structure consists of: Bragg reflector (BR) having 12 periods, n-GaAs base layer with the thickness of 1,500--2,000 nm, 400--500 nm thick p-GaAs emitter, 70 nm thick p-AlGaAs passivating window and top p-GaAs contact layers. The BR with the reflectance maximum centered at the wavelength of 860 nm consists of twelve pairs of AlAs/GaAs layers. The resulting BR thicknesses is 71.6 nm for A.As and 59 nm for GaAs. In this case the peak of reflectance spectrum is in the area of 830--900 nm where the reflectance is close to unit. This multi-layer quasi-dielectric stack selectively reflect weakly absorbed photons with energies near the GaAs band gap for a second pass through the photoactive region increasing the photocurrent. The employment of the BR allows to increase the external quantum efficiency in the long wavelength range of the spectrum and to fabricate simultaneously a thinner n-GaAs base layer. The use of the internal BR, Ta{sub 2}O{sub 5} for antireflecting coating and prismatic cover allowed them to obtain efficiency of 23.4% (17.7 suns, AM0, 25 C) and 27.2% (23.4 suns, Am 1.5).

  16. Carbon incorporation in AlGaAs grown by CBE

    NASA Astrophysics Data System (ADS)

    Lee, B. J.; Houng, Y. M.; Miller, J. N.; Turner, J. E.

    1990-10-01

    The incorporation of carbon into unintentionally doped Al xGa 1- xAs epilayers grown by chemical beam epitaxy (CBE) using arsine and various combinations of group III sources was investigated. Growth of unintentionally doped Al xGa 1- xAs using triethylgallium (TEGa)+triisobutylaluminum (TIBAl) resulted in lower hole and carbon concentrations than those grown from TEGa+triethylaluminum (TEAl). The carbon concentration in AlGaAs epilayers increased with decreasing growth temperature below 560°C and increased with increasing growth temperature above 560°C. This "U-shaped" dependence of carbon concentration on growth temperature exhibited its minimum value at ˜ 560°C for both the TEGa+TEAl and TEGa+TIBAl systems. The alkyl-Al compounds are thought to be the controlling species for the carbon incorporation in the low temperature regime, while the AlCH 3 formed through β-methyl elimination is responsible for the carbon incorporation in the high temperature regime. Based on this study, we are able to grow high quality AlGaAs epilayers with reduced carbon contamination by using TIBAl instead of TEAl at the growth temperature of 560°C with a V/III ratio of 20. AlGaAs/GaAs modulation-doped structures grown from TEGa+TIBAl show a two-dimensional electron gas mobility as high as 88,600 cm 2/V·s at 77 K, which is a 40% improvement over that grown from TEGa+TEAl, with a sheet carrier concentration of 6x10 11 cm -2 and a spacer layer thickness of 150 Å.

  17. Identification of photoluminescence bands in AlGaAs/InGaAs/GaAs PHEMT heterostructures with donor-acceptor-doped barriers

    SciTech Connect

    Gulyaev, D. V. Zhuravlev, K. S.; Bakarov, A. K.; Toropov, A. I.

    2015-02-15

    The photoluminescence of AlGaAs/InGaAs/GaAs pseudomorphic high-electron mobility transistor heterostructures with donor-acceptor-doped AlGaAs barriers is studied. It is found that the introduction of additional p{sup +}-doped AlGaAs layers into the design brings about the appearance of new bands in the photoluminescence spectra. These bands are identified as resulting from transitions (i) in donor-acceptor pairs in doped AlGaAs layers and (ii) between the conduction subband and acceptor levels in the undoped InGaAs quantum well.

  18. Pure AlN layers in metal-polar AlGaN/AlN/GaN and AlN/GaN heterostructures grown by low-temperature ammonia-based molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Kaun, Stephen W.; Mazumder, Baishakhi; Fireman, Micha N.; Kyle, Erin C. H.; Mishra, Umesh K.; Speck, James S.

    2015-05-01

    When grown at a high temperature (820 °C) by ammonia-based molecular beam epitaxy (NH3-MBE), the AlN layers of metal-polar AlGaN/AlN/GaN heterostructures had a high GaN mole fraction (∼0.15), as identified by atom probe tomography in a previous study (Mazumder et al 2013 Appl. Phys. Lett. 102 111603). In the study presented here, growth at low temperature (<740 °C) by NH3-MBE yielded metal-polar AlN layers that were essentially pure at the alloy level. The improved purity of the AlN layers grown at low temperature was correlated to a dramatic increase in the sheet density of the two-dimensional electron gas (2DEG) at the AlN/GaN heterointerface. Through application of an In surfactant, metal-polar AlN(3.5 nm)/GaN and AlGaN/AlN(2.5 nm)/GaN heterostructures grown at low temperature yielded low 2DEG sheet resistances of 177 and 285 Ω/□, respectively.

  19. Sheet resistance under Ohmic contacts to AlGaN/GaN heterostructures

    SciTech Connect

    Hajłasz, M.; Donkers, J. J. T. M.; Sque, S. J.; Heil, S. B. S.; Gravesteijn, D. J.; Rietveld, F. J. R.; Schmitz, J.

    2014-06-16

    For the determination of specific contact resistance in semiconductor devices, it is usually assumed that the sheet resistance under the contact is identical to that between the contacts. This generally does not hold for contacts to AlGaN/GaN structures, where an effective doping under the contact is thought to come from reactions between the contact metals and the AlGaN/GaN. As a consequence, conventional extraction of the specific contact resistance and transfer length leads to erroneous results. In this Letter, the sheet resistance under gold-free Ti/Al-based Ohmic contacts to AlGaN/GaN heterostructures on Si substrates has been investigated by means of electrical measurements, transmission electron microscopy, and technology computer-aided design simulations. It was found to be significantly lower than that outside of the contact area; temperature-dependent electrical characterization showed that it exhibits semiconductor-like behavior. The increase in conduction is attributed to n-type activity of nitrogen vacancies in the AlGaN. They are thought to form during rapid thermal annealing of the metal stack when Ti extracts nitrogen from the underlying semiconductor. The high n-type doping in the region between the metal and the 2-dimensional electron gas pulls the conduction band towards the Fermi level and enhances horizontal electron transport in the AlGaN. Using this improved understanding of the properties of the material underneath the contact, accurate values of transfer length and specific contact resistance have been extracted.

  20. Sheet resistance under Ohmic contacts to AlGaN/GaN heterostructures

    NASA Astrophysics Data System (ADS)

    Hajłasz, M.; Donkers, J. J. T. M.; Sque, S. J.; Heil, S. B. S.; Gravesteijn, D. J.; Rietveld, F. J. R.; Schmitz, J.

    2014-06-01

    For the determination of specific contact resistance in semiconductor devices, it is usually assumed that the sheet resistance under the contact is identical to that between the contacts. This generally does not hold for contacts to AlGaN/GaN structures, where an effective doping under the contact is thought to come from reactions between the contact metals and the AlGaN/GaN. As a consequence, conventional extraction of the specific contact resistance and transfer length leads to erroneous results. In this Letter, the sheet resistance under gold-free Ti/Al-based Ohmic contacts to AlGaN/GaN heterostructures on Si substrates has been investigated by means of electrical measurements, transmission electron microscopy, and technology computer-aided design simulations. It was found to be significantly lower than that outside of the contact area; temperature-dependent electrical characterization showed that it exhibits semiconductor-like behavior. The increase in conduction is attributed to n-type activity of nitrogen vacancies in the AlGaN. They are thought to form during rapid thermal annealing of the metal stack when Ti extracts nitrogen from the underlying semiconductor. The high n-type doping in the region between the metal and the 2-dimensional electron gas pulls the conduction band towards the Fermi level and enhances horizontal electron transport in the AlGaN. Using this improved understanding of the properties of the material underneath the contact, accurate values of transfer length and specific contact resistance have been extracted.

  1. Mismatch relaxation by stacking fault formation of AlN islands in AlGaN/GaN structures on m-plane GaN substrates

    SciTech Connect

    Smalc-Koziorowska, Julita; Sawicka, Marta; Skierbiszewski, Czeslaw; Grzegory, Izabella

    2011-08-08

    We study the mismatch relaxation of 2-5 nm thin elongated AlN islands formed during growth of AlGaN on bulk m-plane GaN by molecular beam epitaxy. The relaxation of these m-plane AlN layers is anisotropic and occurs through the introduction of stacking faults in [0001] planes during island coalescence, while no relaxation is observed along the perpendicular [1120] direction. This anisotropy in the mismatch relaxation and the formation of stacking faults in the AlN islands are explained by the growth mode of the AlN platelets and their coalescence along the [0001] direction.

  2. Trivalent cation-controlled phase space of new U(IV) fluorides, Na3MU6F30 (M = Al(3+), Ga(3+), Ti(3+), V(3+), Cr(3+), Fe(3+)): mild hydrothermal synthesis including an in situ reduction step, structures, optical, and magnetic properties.

    PubMed

    Yeon, Jeongho; Smith, Mark D; Morrison, Gregory; zur Loye, Hans-Conrad

    2015-02-16

    A series of new, complex U(IV) fluorides, namely, Na3MU6F30 (M = Al(3+), Ga(3+), Ti(3+), V(3+), Cr(3+), and Fe(3+)), containing trivalent transition- and main-group metal cations were synthesized via an in situ reduction step of U(VI) to U(IV). Single crystals of the series were grown in high yield under mild hydrothermal conditions and were characterized by single-crystal X-ray diffraction. The reported compounds crystallize in the trigonal space group P3̅c1 and exhibit complex crystal structures with a three-dimensional (3-D) framework composed of corner- and edge-shared UF9 polyhedra. The arrangement of U2F16 dimers forms two types of hexagonal channels, where MF6 polyhedra and sodium atoms are located. The most interesting structural feature is the presence of the 3-D framework that can accommodate various transition-metal ions in low oxidation states, indicating that the framework acts as an excellent host. Trivalent transition metal ions, even reduced Ti(3+) and V(3+), were stabilized by both the rigid framework and by our synthetic conditions. Utilizing ionic radii of transition metal ions, a phase boundary was investigated, suggesting that there exists a size limit for the M site in the crystal structure. The valence state of uranium was studied by U 4f X-ray photoelectron spectroscopy, which confirmed the presence of U(4+). Temperature-dependent magnetic susceptibility measurements yielded effective magnetic moments of 3.50 and 3.35 μB for Na3MU6F30 (M = Al(3+) and Ga(3+)), respectively. For the other compounds, combined effective magnetic moments of 8.93, 9.09, 9.18, and 10.39 μB were obtained for Ti, V, Cr, and Fe members, respectively. In all cases, large negative Weiss constants were observed, which are indicative of the existence of a spin gap in U(4+). Field-dependent magnetic property measurements at 2 K for Na3FeU6F30 demonstrated that U(4+) attains a nonmagnetic singlet ground state at low temperature. Optical and thermal properties were

  3. Adherent Al2O3 scales formed on undoped NiCrAl alloys

    NASA Technical Reports Server (NTRS)

    Smialek, James L.

    1987-01-01

    Changes in the spalling behavior of Al2O3 scales formed on an undoped NiCrAl alloy are described. Two samples of Ni-15Cr-13Al (wt pct), one a control and the other sanded, were subjected to 25 oxidation cycles. It is observed that adherent scales formed on the sanded sample; however, the control sample had speckled, spalled scales. The data reveal that the adherent scales are caused by repeated removal of surface layers after each oxidation cycle. It is determined that interfacial segregation of sulfur influences spallation and sulfur removal increases bonding. The effect of moisture on scale adhesions is investigated.

  4. Photoemission characteristics of graded band-gap AlGaAs/GaAs wire photocathode

    NASA Astrophysics Data System (ADS)

    Ding, Xiaojun; Ge, Xiaowan; Zou, Jijun; Zhang, Yijun; Peng, Xincun; Deng, Wenjuan; Chen, Zhaoping; Zhao, Wenjun; Chang, Benkang

    2016-05-01

    A photoemission model of graded band-gap AlGaAs/GaAs wire NEA photocathode is developed based on the numerical solution of coupled Poisson and continuity equations. The emission current density and integral sensitivity of graded band-gap AlGaAs/GaAs wire photocathode as a function of incident light wavelength, Al composition range, and wire length, are simulated according to the model. The simulation results show that, compared with the GaAs (Al composition 0) wire photocathode, the peak integral sensitivities for the photocathodes with wire width of 1 μm and linearly graded Al composition ranges of 0 to 0.1, 0.2, 0.3, and 0.4 increase by 29.5%, 38.5%, 42.1%, and 43.8%, respectively. The optimum wire lengths are 4.7, 5.9, 7.1, and 8.4 μm for the wire photocathodes with Al composition ranges of 0 to 0.1, 0.2, 0.3, and 0.4, respectively.

  5. Observation of large Zeeman splitting in GaGdN/AlGaN ferromagnetic semiconductor double quantum well superlattices

    NASA Astrophysics Data System (ADS)

    Zhou, YiKai; Almokhtar, Mohamed; Kubo, Hitoshi; Mori, Nobuya; Emura, Shuichi; Hasegawa, Shigehiko; Asahi, Hajime

    2012-07-01

    Symmetric GaGdN/AlGaN (Gd concentration: 2%) and GaN/AlGaN double quantum well superlattices (DQW-SLs) were grown by radio-frequency plasma-assisted molecular-beam epitaxy on GaN (0001) templates. Atomic steps were observed on all the sample surfaces by atomic force microscope. X-ray diffraction θ/2θ scan curves exhibited well-defined satellite structures. Room temperature ferromagnetism was confirmed for the GaGdN/AlGaN DQW-SL samples by using alternating gradient magnetometer. Strong photoluminescence was observed from both GaGdN and GaN QWs at higher energy side of GaN excitonic peak. Magneto-photoluminescence spectra for GaGdN/AlGaN DQW-SL samples showed a large magnetic field dependence of the excitonic energy by applying a magnetic field up to 7 T. The observed strong redshift of excitonic PL indicated an enhancement of Zeeman splitting of the free carrier energy levels in magnetic GaGdN/AlGaN DQW-SL. Enhanced g-factor was estimated to be about 60 for GaGdN/AlGaN DQW-SL sample with QW thickness of 1 nm.

  6. Electrical spin injection using GaCrN in a GaN based spin light emitting diode

    SciTech Connect

    Banerjee, D.; Ganguly, S.; Saha, D.; Adari, R.; Sankaranarayan, S.; Kumar, A.; Aldhaheri, R. W.; Hussain, M. A.; Balamesh, A. S.

    2013-12-09

    We have demonstrated electrical spin-injection from GaCrN dilute magnetic semiconductor (DMS) in a GaN-based spin light emitting diode (spin-LED). The remanent in-plane magnetization of the thin-film semiconducting ferromagnet has been used for introducing the spin polarized electrons into the non-magnetic InGaN quantum well. The output circular polarization obtained from the spin-LED closely follows the normalized in-plane magnetization curve of the DMS. A saturation circular polarization of ∼2.5% is obtained at 200 K.

  7. Temporal evolution of ion energy distribution functions and ion charge states of Cr and Cr-Al pulsed arc plasmas

    SciTech Connect

    Tanaka, Koichi; Anders, André

    2015-11-15

    To study the temporal evolution of ion energy distribution functions, charge-state-resolved ion energy distribution functions of pulsed arc plasmas from Cr and Cr-Al cathodes were recorded with high time resolution by using direct data acquisition from a combined energy and mass analyzer. The authors find increases in intensities of singly charged ions, which is evidence that charge exchange reactions took place in both Cr and Cr-Al systems. In Cr-Al plasmas, the distributions of high-charge-state ions exhibit high energy tails 50 μs after discharge ignition, but no such tails were observed at 500 μs. The energy ratios of ions of different charge states at the beginning of the pulse, when less neutral atoms were in the space in front of the cathode, suggest that ions are accelerated by an electric field. The situation is not so clear after 50 μs due to particle collisions. The initial mean ion charge state of Cr was about the same in Cr and in Cr-Al plasmas, but it decreased more rapidly in Cr-Al plasmas compared to the decay in Cr plasma. The faster decay of the mean ion charge state and ion energy caused by the addition of Al into a pure Cr cathode suggests that the mean ion charge state is determined not only by ionization processes at the cathode spot but also by inelastic collision between different elements.

  8. Phonon-assisted stimulated emission in thin (<55 A) AlGaAs-GaAs-AlGaAs single quantum wells

    SciTech Connect

    Lo, Y.C.; Kolbas, R.M.

    1988-12-05

    Since the initial report of phonon-assisted stimulated emission in AlGaAs-GaAs quantum wells, several laboratories have performed similar experiments which have produced a wide range of experimental data and interpretations. We present photopumped laser data (77 K) from three different AlGaAs-GaAs-AlGaAs single quantum well heterostructures grown by molecular beam epitaxy. Closely spaced end-to-end laser modes are observed one phonon energy (approx. =36 meV) below the n = 1' electron to light hole confined-particle state which we attribute to phonon-assisted stimulated emission. Also, the data are inconsistent with impurity related luminescence or optical absorption losses which have been suggested as alternative explanations. These results are important because these are the thinnest quantum wells to exhibit phonon-assisted stimulated emission and the first independent observation and confirmation of this important phenomenon.

  9. Phase Separation kinetics in an Fe-Cr-Al alloy

    SciTech Connect

    Capdevila, C.; Miller, Michael K; Chao, J.

    2012-01-01

    The {alpha}-{alpha}{prime} phase separation kinetics in a commercial Fe-20 wt.% Cr-6 wt.% Al oxide dispersion-strengthened PM 2000{trademark} steel have been characterized with the complementary techniques atom probe tomography and thermoelectric power measurements during isothermal aging at 673, 708, and 748 K for times up to 3600 h. A progressive decrease in the Al content of the Cr-rich {alpha}{prime} phase was observed at 708 and 748 K with increasing time, but no partitioning was observed at 673 K. The variation in the volume fraction of the {alpha}{prime} phase well inside the coarsening regime, along with the Avrami exponent 1.2 and activation energy 264 kJ mol{sup -1}, obtained after fitting the experimental results to an Austin-Rickett type equation, indicates that phase separation in PM 2000{trademark} is a transient coarsening process with overlapping nucleation, growth, and coarsening stages.

  10. Polarization field engineering of GaN/AlN/AlGaN superlattices for enhanced thermoelectric properties

    SciTech Connect

    Sztein, Alexander; Bowers, John E.; DenBaars, Steven P.; Nakamura, Shuji

    2014-01-27

    A novel polarization field engineering based strategy to simultaneously achieve high electrical conductivity and low thermal conductivity in thermoelectric materials is demonstrated. Polarization based electric fields are used to confine electrons into two-dimensional electron gases in GaN/AlN/Al{sub 0.2}Ga{sub 0.8}N superlattices, resulting in improved electron mobilities as high as 1176 cm{sup 2}/Vs and in-plane thermal conductivity as low as 8.9 W/mK. The resulting room temperature ZT values reach 0.08, a factor of four higher than InGaN and twelve higher than GaN, demonstrating the potential benefits of this polarization based engineering strategy for improving the ZT and efficiencies of thermoelectric materials.

  11. Comparative study of intersubband absorption in AlGaN/GaN and AlInN/GaN superlattices: Impact of material inhomogeneities

    NASA Astrophysics Data System (ADS)

    Edmunds, C.; Tang, L.; Cervantes, M.; Shirazi-HD, M.; Shao, J.; Grier, A.; Valavanis, A.; Cooper, J. D.; Li, D.; Gardner, G.; Zakharov, D. N.; Ikonić, Z.; Indjin, D.; Harrison, P.; Manfra, M. J.; Malis, O.

    2013-12-01

    We report a systematic and quantitative study of near-infrared intersubband absorption in strained AlGaN/GaN and lattice-matched AlInN/GaN superlattices grown by plasma-assisted molecular-beam epitaxy as a function of Si-doping profile with and without δ doping. For AlGaN/GaN, we obtained good theoretical agreement with experimental measurements of transition energy, integrated absorbance and linewidth by considering many-body effects, interface roughness, and calculations of the transition lifetime that include dephasing. For the AlInN/GaN system, experimental measurements of the integrated absorbance due to the superlattice transitions produced values more than one order of magnitude lower than AlGaN/GaN heterostructures at similar doping levels. Furthermore, observed transition energies were roughly 150 meV higher than expected. The weak absorption and high transition energies measured in these structures is attributed to columnar alloy inhomogeneity in the AlInN barriers observed in high-angle annular dark-field scanning transmission electron microscopy. We simulated the effect of these inhomogeneities using three-dimensional band-structure calculations. The inhomogeneities were modeled as AlInN nanorods with radially varying In composition embedded in the barrier material of the superlattice. We show that inclusion of the nanorods leads to the depletion of the quantum wells (QWs) due to localization of charge carriers in high-In-containing regions. The higher energy of the intersubband transitions was attributed to the relatively uniform regions of the QWs surrounded by high Al (95%) composition barriers. The calculated transition energy assuming Al0.95In0.05N barriers was in good agreement with experimental results.

  12. AlP/GaP distributed Bragg reflectors

    SciTech Connect

    Emberger, Valentin; Hatami, Fariba; Ted Masselink, W.; Peters, Sven

    2013-07-15

    Distributed Bragg reflectors with high reflectivity bands centered at wavelengths from 530 to 690 nm (green to red) based on AlP/GaP quarter-wave stacks are prepared on (001)GaP using gas-source molecular-beam epitaxy. Additionally, the complex refractive index of AlP is measured using spectroscopic ellipsometry within the range of 330-850 nm in order to facilitate an accurate reflector design. Structures consisting of 15 quarter-wave stacks reach a peak reflectance between 95% and 98%, depending on the spectral position of the maximum.

  13. MOVPE growth of (AlGaIn)P/(GaIn)P heterostructures using TBP

    NASA Astrophysics Data System (ADS)

    El-Zein, N.; Greiling, A.; Koch, J.; Stolz, W.; Reinhard, S.; McDermott, B. T.

    2004-12-01

    (AlInGa)P red laser diodes have become increasingly attractive as light sources for various applications. High-quality AlInGaP and InGaP layers are of key importance for the performance of these lasers. In this report we explore the metalorganic vapor phase epitaxy (MOVPE) growth of red lasers using liquid precursors, i.e. replacing the highly toxic gaseous hydrides with liquid MO-V sources. This report will show material data (X-ray, PL, SIMS, etc.) on In(Al)GaP layers grown using TBP. The material performance of AlInGaP and InGaP layers grown using TBP and having different compositions will be compared to similar layers grown using PH 3. Growth parameters (temperature, V/III ratio) that will be presented will outline the advantages of MO-V growth such as lower growth temperatures and lower V/III ratios, as well as potential benefits on the maintenance of the reactor. Finally, we will show preliminary data on the full red laser epitaxial stack using the optimized growth conditions for each layer.

  14. Strain-compensated AlGaN /GaN/InGaN cladding layers in homoepitaxial nitride devices

    NASA Astrophysics Data System (ADS)

    Czernecki, R.; Krukowski, S.; Targowski, G.; Prystawko, P.; Sarzynski, M.; Krysko, M.; Kamler, G.; Grzegory, I.; Leszczynski, M.; Porowski, S.

    2007-12-01

    One of the most important problems in III-nitride violet laser diode technology is the lattice mismatch between the AlGaN cladding layers and the rest of the epitaxial structure. For efficiently working devices, it is necessary to have both a high Al content and thick claddings. This leads, however, to severe sample bowing and even cracking of the upper layer. In this work, we propose a cladding structure of strain-compensated AlGaN /GaN/InGaN superlattice grown by metal-organic vapor phase epitaxy on bulk GaN substrates. Various thicknesses and compositions of the layers were employed. We measured the radius of bowing, lattice mismatches, aluminum and indium contents, and densities of threading dislocations. The proposed cladding structures suppress bowing and cracking, which are the two parasitic effects commonly experienced in laser diodes with bulk AlGaN claddings. The suppression of cracking and bowing is shown to occur due to modified strain energy distribution of the superlattices structure.

  15. Theoretical and experimental studies of electric field distribution in N-polar GaN/AlGaN/GaN heterostructures

    SciTech Connect

    Gladysiewicz, M. Janicki, L.; Kudrawiec, R.; Siekacz, M.; Cywinski, G.

    2015-12-28

    Electric field distribution in N-polar GaN(channel)/AlGaN/GaN(buffer) heterostructures was studied theoretically by solving Schrodinger and Poisson equations in a self-consistent manner for various boundary conditions and comparing results of these calculations with experimental data, i.e., measurements of electric field in GaN(channel) and AlGaN layers by electromodulation spectroscopy. A very good agreement between theoretical calculations and experimental data has been found for the Fermi-level located at ∼0.3 eV below the conduction band at N-polar GaN surface. With this surface boundary condition, the electric field distribution and two dimensional electron gas concentration are determined for GaN(channel)/AlGaN/GaN(buffer) heterostructures of various thicknesses of GaN(channel) and AlGaN layers.

  16. Enhanced light output power of InGaN-based amber LEDs by strain-compensating AlN/AlGaN barriers

    NASA Astrophysics Data System (ADS)

    Iida, Daisuke; Lu, Shen; Hirahara, Sota; Niwa, Kazumasa; Kamiyama, Satoshi; Ohkawa, Kazuhiro

    2016-08-01

    We investigated the effect of a combined AlN/Al0.03Ga0.97N barrier on InGaN-based amber light-emitting diodes (LEDs) grown by metalorganic vapor-phase epitaxy. InGaN-based multiple quantum wells with a combined AlN/Al0.03Ga0.97N barrier showed intense photoluminescence with a narrow full-width at half-maximum. The amber LEDs with a combined AlN/Al0.03Ga0.97N barrier achieved a light output power enhanced approximately 2.5-fold at 20 mA compared to that of the LED with a combined AlN/GaN barrier, owing to the reduction of defects in InGaN active layers. Thus, the efficiency of high-In-content InGaN-based LEDs can be improved in the spectrum range of amber.

  17. Reliability comparison of GaAlAs/GaAs and aluminum-free high-power laser diodes

    NASA Astrophysics Data System (ADS)

    Pendse, D. R.; Chin, Aland K.; Dabkowski, Ferdynand P.; Clausen, Edward M., Jr.

    1998-08-01

    Aluminum-free laser diodes are InGaAsP/GaAs devices whose epitaxial layers do not contain aluminum. Studies comparing the GaAslAs/GaAs and InGaAsP/GaAs high power laser diodes allegedly indicate that aluminum-free lasers are more reliable due to a reduction of dark-line defects, sudden failures, and gradual degradation. The improved reliability of aluminum-free lasers is presumed to result from the elimination of oxidation of the aluminum-containing epitaxial layers of the laser facets. In this presentation, the performance and reliability of GaAlAs/GaAs and InGaAsP/GaAs high power laser diodes will be reviewed and compared. The present data shows that high reliable GaAlAs/GaAs lasers can be produced with good manufacturing practices.

  18. Two-Phase (TiAl+TiCrAl) Coating Alloys for Titanium Aluminides

    NASA Technical Reports Server (NTRS)

    Brady, Michael P. (Inventor); Smialek, James L. (Inventor); Brindley, William J. (Inventor)

    1998-01-01

    A coating for protecting titanium aluminide alloys, including the TiAl gamma + Ti3Al (alpha(sub 2)) class, from oxidative attack and interstitial embrittlement at temperatures up to at least 1000 C. is disclosed. This protective coating consists essentially of titanium, aluminum. and chromium in the following approximate atomic ratio: Ti(41.5-34.5)Al(49-53)Cr(9.5-12.5)

  19. Estimation of excess energies and activity coefficients for the penternary Ni-Cr-Co-Al-Mo system and its subsystems

    NASA Astrophysics Data System (ADS)

    Dogan, A.; Arslan, H.; Dogan, T.

    2015-06-01

    Using different prediction methods, such as the General Solution Model of Kohler and Muggianu, the excess energy and activities of molybdenum for the sections of the phase diagram for the penternary Ni-Cr-Co-Al-Mo system with mole ratios xNi/ xMo = 1, xCr/ xMo = 1, xCo/ xMo = 1, and xAl/ xMo = r = 0.5 and 1, were thermodynamically investigated at a temperature of 2000 K, whereas the excess energy and activities of Bi for the section corresponding to the ternary Bi-Ga-Sb system with mole ratio xGa/ xSb = 1/9 were thermodynamically investigated at a temperature of 1073 K. In the case of r = 0.5 and 1 in the alloys Ni-Cr-Co-Al-Mo, a positive deviation in the activity coefficient was revealed, as molybdenum content increased. Moreover, in the calculations performed in Chou's GSM model, the obtained values for excess Gibbs energies are negative in the whole concentration range of bismuth at 1073 K and exhibit the minimum of about -2.2 kJ/mol at the mole ratio xGa/ xSb = 1/9 in the alloy Bi-Ga-Sb.

  20. Structure and wear behavior of AlCrSiN-based coatings

    NASA Astrophysics Data System (ADS)

    Chen, Yun; Du, Hao; Chen, Ming; Yang, Jun; Xiong, Ji; Zhao, Haibo

    2016-05-01

    AlCrN, AlCrSiCN, AlCrSiN/MoN, and AlCrSiN/NbN coatings have been deposited on high-polished WC-Co cemented carbide substrate and tools by mid-frequency magnetron sputtering in Ar/N2 mixtures. Al0.6Cr0.4, Al0.6Cr0.3Si0.1, and C/Mo/Nb targets were used during the deposition. The microstructure and mechanical properties of as-deposited coatings were investigated. Investigations of the wear behaviors of coated tools were also performed. The results showed that cubic structure was formed in the coatings. Broader CrAlN (1 1 1) and (2 0 0) peaks without SiNx peak were formed in the AlCrSiN/MexN coatings, which showed a nanocomposited structure. Meanwhile, according to SEM micrographs, AlCrN exhibited a columnar structure, while, AlCrSiCN, AlCrSiN/MoN, and AlCrSiN/NbN coatings showed nanocrystalline morphology. The nano-multilayered coatings performed higher hardness, H/E, and H3/E2 ratios compared with AlCrN coating. Through the Rockwell adhesion test, all the coatings exhibited adhesion strength quality HF1. After turning Inconel 718 under dry condition, the nano-multilyered coatings showed better wear resistance than AlCrN coating. Due to the molybdenum and niobium in the coating, AlCrSiN/MoN and AlCrSiN/NbN coatings showed the best wear resistance.

  1. Optical charge transfer for the dope in GaAs>

    SciTech Connect

    Vakulenko, O.V.; Skirda, A.S.; Skryshevskii, V.A.

    1984-05-01

    It is concluded that the chromium dope is amphoteric in behavior on the basis of the spectra, kinetics, and lux-ampere characteristics of the absorption and photoconductivity induced by 1.15-um IR laser radiation in high-resistance specimens of GaAs>. It is assumed that the additional IR illumination produces optical charge transfer in the chromium in accordance with Cr/sup 3 +/3d/sup 3/ + h..nu.. ..-->.. Cr/sup 2 +/3d/sup 4/ + Cr/sup 4 +/3d/sup 2/. The photoneutralization of the Cr/sup 4 +/3d/sup 2/ centers is responsible for additional optical-absorption and photoconductivity bands appearing in the long-wave region. The kinetic equations for these centers are solved, which describes the experimental results satisfactorily. It is suggested that chromium may compensate not only shallow donors in GaAs but also shallow acceptors.

  2. Comparison of AlCrN and AlCrTiSiN coatings deposited on the surface of plasma nitrocarburized high carbon steels

    NASA Astrophysics Data System (ADS)

    Chen, Wanglin; Zheng, Jie; Lin, Yue; Kwon, Sikchol; Zhang, Shihong

    2015-03-01

    The AlCrN and AlCrTiSiN coatings were produced on the surface of plasma nitrocarburized T10 steels by multi-arc ion plating. The comparison of the microstructures and mechanical properties of the duplex coatings were investigated by means of X-ray diffraction, optical microscope, scanning electron microscope and transmission electron microscope, in association with mechanical property measurement. The results show that the AlCrN coatings with columnar grown are mainly composed of nanocrytalline fcc-(Cr,Al)N phases with {111} preferred orientation, whereas the superlattice and nanocomposite AlCrTiSiN coatings with planar growth mainly consist of nanocrystalline fcc-(Cr,Al)N phases with {100} perfected orientation, hcp-AlN and Si3N4 amorphous phases. The AlCrTiSiN duplex coating with the compound layer reveals higher hardness, adhesion strength, load capacity and lower friction coefficient when compared with the other duplex coatings, which is due to its superlattice and nanocomposite structure. Additionally, these improved properties are related to the appearance of the γ‧-phase which plays the nucleation sites for the coating nitrides and provides a strong supporting effect for the AlCrN and AlCrTiSiN coatings. The main wear mechanism of the duplex coatings without compound layer is spalling and chipping wear as well as tribooxidation wear, whereas the main wear mechanism of the duplex coatings with compound layer is tribooxidation wear.

  3. A highly selective, chlorofluorocarbon-free GaAs on AlGaAs etch

    SciTech Connect

    Smith, L.E. . Solid State Technology Center)

    1993-07-01

    A highly selective reactive ion etching process using SiCl[sub 4], CF[sub 4], O[sub 2], and He is reported. The selectivity of the etch, which is adjustable, ranges from 308:1 to 428:1 for GaAs to Al[sub 0.11]Ga[sub 0.89]As. This variability in selectivity is achieved by adjusting the helium flow rate. One very attractive feature of this etch is that it uses no chlorofluorocarbons and therefore complies with future bans on these substances imposed at both federal and corporate levels. The etch is demonstrated on a GaAs field effect transistor structure with an underlying Al[sub 0.11]Ga[sub 0.89]As stop-etch layer. The etch can be used for both anisotropic and isotropic applications.

  4. Electric field driven plasmon dispersion in AlGaN/GaN high electron mobility transistors

    NASA Astrophysics Data System (ADS)

    Tan, Ren-Bing; Qin, Hua; Zhang, Xiao-Yu; Xu, Wen

    2013-11-01

    We present a theoretical study on the electric field driven plasmon dispersion of the two-dimensional electron gas (2DEG) in AlGaN/GaN high electron mobility transistors (HEMTs). By introducing a drifted Fermi—Dirac distribution, we calculate the transport properties of the 2DEG in the AlGaN/GaN interface by employing the balance-equation approach based on the Boltzmann equation. Then, the nonequilibrium Fermi—Dirac function is obtained by applying the calculated electron drift velocity and electron temperature. Under random phase approximation (RPA), the electric field driven plasmon dispersion is investigated. The calculated results indicate that the plasmon frequency is dominated by both the electric field E and the angle between wavevector q and electric field E. Importantly, the plasmon frequency could be tuned by the applied source—drain bias voltage besides the gate voltage (change of the electron density).

  5. Composition profiling of GaAs/AlGaAs quantum dots grown by droplet epitaxy

    SciTech Connect

    Bocquel, J.; Koenraad, P. M.; Giddings, A. D.; Prosa, T. J.; Larson, D. J.; Mano, T.

    2014-10-13

    Droplet epitaxy (DE) is a growth method which can create III-V quantum dots (QDs) whose optoelectronic properties can be accurately controlled through the crystallisation conditions. In this work, GaAs/AlGaAs DE-QDs have been analyzed with the complimentary techniques of cross-sectional scanning tunneling microscopy and atom probe tomography. Structural details and a quantitative chemical analysis of QDs of different sizes are obtained. Most QDs were found to be pure GaAs, while a small proportion exhibited high intermixing caused by a local etching process. Large QDs with a high aspect ratio were observed to have an Al-rich crown above the GaAs QD. This structure is attributed to differences in mobility of the cations during the capping phase of the DE growth.

  6. Height stabilization of GaSb/GaAs quantum dots by Al-rich capping

    SciTech Connect

    Smakman, E. P. Koenraad, P. M.; DeJarld, M.; Martin, A. J.; Millunchick, J.; Luengo-Kovac, M.; Sih, V.

    2014-09-01

    GaSb quantum dots (QDs) in a GaAs matrix are investigated with cross-sectional scanning tunneling microscopy (X-STM) and photoluminescence (PL). We observe that Al-rich capping materials prevent destabilization of the nanostructures during the capping stage of the molecular beam epitaxy (MBE) growth process and thus preserves the QD height. However, the strain induced by the absence of destabilization causes many structural defects to appear around the preserved QDs. These defects originate from misfit dislocations near the GaSb/GaAs interface and extend into the capping layer as stacking faults. The lack of a red shift in the QD PL suggests that the preserved dots do not contribute to the emission spectra. We suggest that a better control over the emission wavelength and an increase of the PL intensity is attainable by growing smaller QDs with an Al-rich overgrowth.

  7. Fabrication and characterization of an undoped GaAs/AlGaAs quantum dot device

    SciTech Connect

    Li, Hai-Ou; Cao, Gang; Xiao, Ming You, Jie; Wei, Da; Tu, Tao; Guo, Guang-Can; Guo, Guo-Ping; Jiang, Hong-Wen

    2014-11-07

    We demonstrate the development of a double quantum dot with an integrated charge sensor fabricated in undoped GaAs/AlGaAs heterostructures using a double top-gated design. Based on the evaluation of the integrated charge sensor, the double quantum dot can be tuned to a few-electron region. Additionally, the inter-dot coupling of the double quantum dot can be tuned to a large extent according to the voltage on the middle gate. The quantum dot is shown to be tunable from a single dot to a well-isolated double dot. To assess the stability of such design, the potential fluctuation induced by 1/f noise was measured. Based on the findings herein, the quantum dot design developed in the undoped GaAs/AlGaAs semiconductor shows potential for the future exploitation of nano-devices.

  8. Schottky barrier diode embedded AlGaN/GaN switching transistor

    NASA Astrophysics Data System (ADS)

    Park, Bong-Ryeol; Lee, Jung-Yeon; Lee, Jae-Gil; Lee, Dong-Myung; Kim, Moon-Kyung; Cha, Ho-Young

    2013-12-01

    We developed a Schottky barrier diode (SBD) embedded AlGaN/GaN switching transistor to allow negative current flow during off-state condition. An SBD was embedded in a recessed normally-off AlGaN/GaN-on-Si metal-oxide-semiconductor heterostructure field-effect transistor (MOSHFET). The fabricated device exhibited normally-off characteristics with a gate threshold voltage of 2.8 V, a diode turn-on voltage of 1.2 V, and a breakdown voltage of 849 V for the anode-to-drain distance of 8 µm. An on-resistance of 2.66 mΩcm2 was achieved at a gate voltage of 16 V in the forward transistor mode. Eliminating the need for an external diode, the SBD embedded switching transistor has advantages of significant reduction in parasitic inductance and chip area.

  9. Local stress-induced effects on AlGaAs/AlOx oxidation front shape

    SciTech Connect

    Chouchane, F.; Almuneau, G. Arnoult, A.; Lacoste, G.; Fontaine, C.; Cherkashin, N.

    2014-07-28

    The lateral oxidation of thick AlGaAs layers (>500 nm) is studied. An uncommon shape of the oxide tip is evidenced and attributed to the embedded stress distribution, inherent to the oxidation reaction. Experimental and numerical studies of the internal strain in oxidized Al{sub x}Ga{sub 1−x}As/GaAs structures were carried out by dark-field electron holography and finite element methods. A mapping of the strain distribution around the AlGaAs/oxide interface demonstrates the main role of internal stress on the shaping of the oxide front. These results demonstrate the high relevance of strain in oxide-confined III-V devices, in particular, with over-500-nm thick AlOx confinement layers.

  10. Optical and electron paramagnetic resonance studies of Cr doped Ga{sub 2}O{sub 3} nanoparticles

    SciTech Connect

    Popa, A. Toloman, D.; Stan, M.; Silipas, T. D.; Biris, A. R.

    2015-12-23

    In the present work we report the experimental results obtained on Ga{sub 2}O{sub 3} nanoparticles doped with Cr ions. X-ray diffraction analysis confirms the substitution of Ga ions with Cr ions. A secondary phase of Cr{sub 2}O{sub 5} oxides was evidence at high doping Cr concentration by Raman spectroscopy. Different valence state of Cr ions was highlighted by UV-VIS spectroscopy. EPR spectroscopy data show the presence of different environments for Cr ions, depending on the Cr addition.

  11. Fabrication of dry etched and subsequently passivated laser facets in GaAs/AlGaAs

    SciTech Connect

    Deichsel, Eckard; Franz, Gerhard

    2004-09-01

    The aging behavior of edge emitting laser diodes based on GaAs/AlGaAs is investigated by comparing devices with facets that are alternatively cleaved or dry etched and consecutively treated with H{sub 2}S. In this work we demonstrate that an in situ exposure to H{sub 2}S gas is not sufficient to prevent ageing but an additional plasma treatment is rather required to obtain comparable ageing results to lasers with cleaved facets.

  12. Performance of AlGaN/GaN high electron mobility transistors at nanoscale gate lengths.

    PubMed

    Johnson, J W; Ren, F; Pearton, S J; Baca, A G; Han, J; Dabiran, A M; Chow, P P

    2002-01-01

    The DC and RF performance of AlGaN/GaN high electron mobility transistors with nanoscale gate lengths is presented. The layer structures were grown by either metal organic chemical vapor deposition or rf plasma-assisted molecular beam epitaxy. Excellent scaling properties were observed as a function of both gate length and width and confirm that these devices are well suited to both high speed switching and power microwave applications. PMID:12908259

  13. Radio-frequency reflectometry on an undoped AlGaAs/GaAs single electron transistor

    SciTech Connect

    MacLeod, S. J.; See, A. M.; Keane, Z. K.; Scriven, P.; Micolich, A. P.; Hamilton, A. R.; Aagesen, M.; Lindelof, P. E.

    2014-01-06

    Radio frequency reflectometry is demonstrated in a sub-micron undoped AlGaAs/GaAs device. Undoped single electron transistors (SETs) are attractive candidates to study single electron phenomena, due to their charge stability and robust electronic properties after thermal cycling. However, these devices require a large top-gate, which is unsuitable for the fast and sensitive radio frequency reflectometry technique. Here, we demonstrate that rf reflectometry is possible in an undoped SET.

  14. Photoelectric properties of an array of axial GaAs/AlGaAs nanowires

    NASA Astrophysics Data System (ADS)

    Grigor'ev, R. V.; Shtrom, I. V.; Grigor'eva, N. R.; Novikov, B. V.; Soshnikov, I. P.; Samsonenko, Yu. B.; Khrebtov, A. I.; Buravleuv, A. D.; Cirlin, G. E.

    2015-05-01

    The results of studies of photoelectric properties of an array of axial n-type GaAs/Al x Ga1 - x As ( x ≈ 0.3) nanowires grown using molecular beam epitaxy on a p-type silicon substrate are presented. The ability to separate charges efficiently in a wide spectral range (from 450 to 1100 nm) is demonstrated. Such properties are important for designing active elements of photodetectors and solar cells.

  15. Reduction of exciton mass by uniaxial stress in GaAs/AlGaAs quantum wells

    NASA Astrophysics Data System (ADS)

    Loginov, D. K.; Grigoryev, P. S.; Efimov, Yu. P.; Eliseev, S. A.; Lovtcius, V. A.; Petrov, V. V.; Ubyivovk, E. V.; Ignatiev, I. V.

    2016-08-01

    It is experimentally shown that the pressure applied along the twofold symmetry axis of a heterostructure with a wide GaAs/AlGaAs quantum well leads to considerable modification of the polariton reflectance spectra. This effect is treated as the stress-induced decrease of the heavy-hole exciton mass. Theoretical modeling of the effect supports this assumption. The 5\\%-decrease of the exciton mass is obtained at pressure P=0.23 GPa.

  16. Electron mass tunneling along the growth direction of (Al,Ga)As/GaAs semiconductor superlattices

    NASA Astrophysics Data System (ADS)

    Duffield, T.; Bhat, R.; Koza, M.; Derosa, F.; Hwang, D. M.

    1986-06-01

    The effective mass of electrons tunneling through thin barriers in (Al,Ga)As/GaAs superlattices is measured by performing cyclotron-resonance experiments with the magnetic field oriented perpendicular to the growth direction. The effective mass is shown to increase with the barrier height at a rate that is in rough agreement with a model that includes the change in band mass in the barrier.

  17. Bulk modulus and specific heat of B-site doped (La{sub 0.3}Pr{sub 0.7}){sub 0.65}Ca{sub 0.35}Mn{sub 1−x}B{sub x}O{sub 3} (B=Fe, Cr, Ru, Al, Ga)

    SciTech Connect

    Srivastava, Archana; Thakur, Rasna; Gaur, N. K.

    2014-04-24

    Specific heat (C{sub p}) thermal expansion (α) and Bulk modulus (B{sub T}) of lightly doped Rare Earth manganites (La{sub 0.3}Pr{sub 0.7}){sub 0.65}Ca{sub 0.35}Mn{sub 1−x}B{sub x}O{sub 3} (B{sup 3+} = Fe{sup 3+},Cr{sup 3+},Ga{sup 3+},Al{sup 3+},Ru4+); (0.3

  18. Experimental evidence of Cr magnetic moments at low temperature in Cr2A(A=Al, Ge)C.

    PubMed

    Jaouen, M; Bugnet, M; Jaouen, N; Ohresser, P; Mauchamp, V; Cabioc'h, T; Rogalev, A

    2014-04-30

    From x-ray magnetic circular dichroism experiments performed at low temperature on Cr2AlC and Cr2GeC thin films, it is evidenced that Cr atoms carry a net magnetic moment in these ternary phases. It is shown that the Cr magnetization of the Al-based compound nearly vanished at 100 K in agreement with what has been recently observed on bulk. X-ray linear dichroism measurements performed at various angles of incidence and temperatures clearly demonstrate the existence of a charge ordering along the c axis of the structure of Cr2AlC. All these experimental observations support, in part, theoretical calculations claiming that Cr dd correlations have to be considered to correctly describe the structure and properties of these Cr-based ternary phases. PMID:24721758

  19. Fast detection of a protozoan pathogen, Perkinsus marinus, using AlGaN/GaN high electron mobility transistors

    NASA Astrophysics Data System (ADS)

    Wang, Yu-Lin; Chu, B. H.; Chen, K. H.; Chang, C. Y.; Lele, T. P.; Papadi, G.; Coleman, J. K.; Sheppard, B. J.; Dungen, C. F.; Pearton, S. J.; Johnson, J. W.; Rajagopal, P.; Roberts, J. C.; Piner, E. L.; Linthicum, K. J.; Ren, F.

    2009-06-01

    Antibody-functionalized, Au-gated AlGaN/GaN high electron mobility transistors (HEMTs) were used to detect Perkinsus marinus. The antibody was anchored to the gate area through immobilized thioglycolic acid. The AlGaN/GaN HEMT drain-source current showed a rapid response of less than 5 s when the infected solution was added to the antibody-immobilized surface. The sensor can be recycled with a phosphate buffered saline wash. These results clearly demonstrate the promise of field-deployable electronic biological sensors based on AlGaN/GaN HEMTs for Perkinsus marinus detection.

  20. HgNO3 sensitivity of AlGaN/GaN field effect transistors functionalized with phytochelating peptides

    NASA Astrophysics Data System (ADS)

    Rohrbaugh, Nathaniel; Hernandez-Balderrama, Luis; Kaess, Felix; Kirste, Ronny; Collazo, Ramon; Ivanisevic, Albena

    2016-06-01

    This study examined the conductance sensitivity of AlGaN/GaN field effect transistors in response to varying Hg/HNO3 solutions. FET surfaces were covalently functionalized with phytochelatin-5 peptides in order to detect Hg in solution. Results showed a resilience of peptide-AlGaN/GaN bonds in the presence of strong HNO3 aliquots, with significant degradation in FET ID signal. However, devices showed strong and varied response to Hg concentrations of 1, 10, 100, and 1000 ppm. The gathered statistically significant results indicate that peptide terminated AlGaN/GaN devices are capable of differentiating between Hg solutions and demonstrate device sensitivity.

  1. Effective mass from microwave photoresistance measurements in GaAs/AlGaAs quantum wells

    NASA Astrophysics Data System (ADS)

    Hatke, A. T.; Zudov, M. A.; Watson, J. D.; Manfra, M. J.; Pfeiffer, L. N.; West, K. W.

    2013-08-01

    We have performed microwave photoresistance measurements in high mobility GaAs/AlGaAs quantum wells and investigated the value of the effective mass. The effective mass, obtained from the period of microwave-induced resistance oscillations (MIRO), was found to be about 12 % lower than the band mass in GaAs, m*b. In contrast, the measured magneto-plasmon dispersion (MPR) revealed an effective mass which is close to m*b, in accord with previous studies. These findings suggest that, in contrast to MPR, the MIRO dispersion contains corrections due to electron-electron interaction effects.

  2. Evidence for effective mass reduction in GaAs/AlGaAs quantum wells

    NASA Astrophysics Data System (ADS)

    Hatke, A. T.; Zudov, M. A.; Watson, J. D.; Manfra, M. J.; Pfeiffer, L. N.; West, K. W.

    2013-04-01

    We have performed microwave photoresistance measurements in high mobility GaAs/AlGaAs quantum wells and investigated the value of the effective mass. Surprisingly, the effective mass, obtained from the period of microwave-induced resistance oscillations, is found to be about 12% lower than the band mass in GaAs, mb. This finding provides strong evidence for electron-electron interactions which can be probed by microwave photoresistance in very high Landau levels. In contrast, the measured magnetoplasmon dispersion revealed an effective mass which is close to mb, in accord with previous studies.

  3. Few-hole double quantum dot in an undoped GaAs/AlGaAs heterostructure

    SciTech Connect

    Tracy, L. A.; Hargett, T. W.; Reno, J. L.

    2014-03-24

    We demonstrate a hole double quantum dot in an undoped GaAs/AlGaAs heterostructure. The interdot coupling can be tuned over a wide range, from formation of a large single dot to two well-isolated quantum dots. Using charge sensing, we show the ability to completely empty the dot of holes and control the charge occupation in the few-hole regime. The device should allow for control of individual hole spins in single and double quantum dots in GaAs.

  4. Assembly of phosphonic acids on GaN and AlGaN

    NASA Astrophysics Data System (ADS)

    Simpkins, B. S.; Hong, S.; Stine, R.; Mäkinen, A. J.; Theodore, N. D.; Mastro, M. A.; Eddy, C. R., Jr.; Pehrsson, P. E.

    2010-01-01

    Self-assembled monolayers of octadecylphosphonic acid and 16-phosphonohexadecanoic acid (PHDA) were formed on the semiconductor substrates gallium nitride (GaN) and aluminium gallium nitride (AlGaN). The presence of the molecular layers was verified through x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. Structural information was acquired with infrared spectroscopy which verified the bonding orientation of the carboxyl-containing PHDA. The impact of the molecular layers on the channel conductivity and the surface electronic structure of an AlGaN/GaN heterostructure was measured. Our results indicate that pinning of the surface Fermi level prohibits modification of the channel conductivity by the layer. However, a surface dipole of ~0.8 eV is present and associated with both phosphonic acid layers. These results are of direct relevance to field-effect-based biochemical sensors and metal-semiconductor contact formation for this system and provide a fundamental basis for further applications of GaN and AlGaN technology in the fields of biosensing and microelectronics.

  5. High efficient radiation stable AlGaAs/GaAs solar cells with internal Bragg reflector

    NASA Technical Reports Server (NTRS)

    Andreev, V. M.; Kalinovsky, V. S.; Komin, V. V.; Kochnev, I. V.; Lantratov, V. M.; Shvarts, M. Z.

    1995-01-01

    An investigation of solar cells based on AlGaAs/GaAs heterostructures with an internal Bragg reflector as the back-surface reflector is presented. The Bragg reflector is grown by low pressure metalorganic chemical vapor deposition on n-GaAs substrates in a horizontal resistively heated reactor. The Bragg reflector with its maximum reflectance centered at a wavelength of 860 nm consists of 12 pairs of AlAs/GaAs layers. The resulting Bragg reflector has a thickness of 0.072 micrometers for AlAs and 0.059 micrometers for GaAs. The multi-layered quasi-dielectric stack selectively reflects weakly absorbed photons with energies near to the GaAs band gap for a second pass through the photoactive region, thus increasing the photocurrent. The use of the Bragg reflector allows the external quantum efficiency to be increased in the long wavelength of the spectrum. The use of the Bragg reflector and an antireflective coating and prismatic cover allowed an efficiency of 23.4 percent to be obtained.

  6. Studies of electroluminescence in pAlGaAs-pGaAs-nGaAs heterophotocells with distributed parameters

    NASA Astrophysics Data System (ADS)

    Aripov, Kh. K.; Koroleva, N. S.; Larionov, V. R.; Nuller, T. A.; Rumiantsev, V. D.

    1983-02-01

    Electroluminescence techniques for studying defects and internal resistive losses are illustrated using pAlGaAs-pGaAs-nGaAs heterophotocells which convert unconcentrated sunlight into electricity. A laboratory technique for testing heterophotocells is proposed which involves measuring the photocurrent when the cell is illuminated by simulated low-intensity sunlight (in order to find the absolute photosensitivity of the p-n junction) and observing the spatial electroluminescence intensity distribution at the normal working current while simultaneously measuring the forward voltage over the cell.

  7. Al/sub 2/O/sub 3/ adherence on CoCrAl alloys

    SciTech Connect

    Kingsley, L.M.

    1980-04-01

    Adherence of protective oxides on NiCrAl and CoCrAl superalloys has been promoted by a dispersion of a highly oxygen reactive element or its oxide being produced within the protection system. Two aspects of this subject are investigated here: the use of Al/sub 2/O/sub 3/ as both the dispersion and protective oxide; and the production of an HfO/sub 2/ dispersion while simultaneously aluminizing the alloy. It was found that an Al/sub 2/O/sub 3/ dispersion will act to promote the adherence of an external scale of Al/sub 2/O/sub 3/ to a degree comparable to previously tested dispersions and an HfO/sub 2/ dispersion comparable to that produced by a Rhines pack treatment is produced during aluminization.

  8. Growth and Fabrication of GaN/AlGaN Heterojunction Bipolar Transistor

    SciTech Connect

    Abernathy, C.R.; Baca, A.G.; Cao, X.A.; Cho, H.; Dang, G.T.; Donovan, S.M.; Han, J.; Jung, K.B.; Pearton, S.J.; Ren, F.; Shul, R.J.; Willison, C.G.; Wilson, R.G.; Zhang, A.P.; Zhang, L

    1999-03-16

    A GaN/AlGaN heterojunction bipolar transistor structure with Mg doping in the base and Si Doping in the emitter and collector regions was grown by Metal Organic Chemical Vapor Deposition in c-axis Al(2)O(3). Secondary Ion Mass Spectrometry measurements showed no increase in the O concentration (2-3x10(18) cm(-3)) in the AlGaN emitter and fairly low levels of C (~4-5x10(17) cm (-3)) throughout the structure. Due to the non-ohmic behavior of the base contact at room temperature, the current gain of large area (~90 um diameter) devices was <3. Increasing the device operating temperature led to higher ionization fractions of the mg acceptors in the base, and current gains of ~10 were obtained at 300 degree C.

  9. Rare-earth chromium gallides RE4CrGa12 (RE=Tb-Tm)

    NASA Astrophysics Data System (ADS)

    Slater, Brianna R.; Bie, Haiying; Stoyko, Stanislav S.; Bauer, Eric D.; Thompson, Joe D.; Mar, Arthur

    2012-12-01

    The ternary rare-earth-metal chromium gallides RE4CrGa12 (RE=Tb-Tm) have been prepared by reactions of the elements at 1000 °C in the presence of excess gallium used as a self-flux. Their structures are derived by inserting Cr atoms into a quarter of the empty Ga6 octahedral clusters found in the parent binary gallides REGa3 (AuCu3-type), although single-crystal X-ray diffraction studies suggest that complex superstructures may be adopted. An ideal ordered Y4PdGa12-type structure was successfully refined for a crystal of Dy4CrGa12 (Pearson symbol cI34, space group Im3barm, Z=2, a=8.572(1) Å). Magnetic measurements on single-crystal samples reveal ferromagnetic or possibly ferrimagnetic ordering for the Tb, Dy, and Er members (TC=22, 15, and 2.8 K, respectively) and antiferromagnetic ordering for the Ho member (TN=7.5 K). Band structure calculations on a hypothetical “Y4CrGa12” model suggest that the Cr atoms carry no local magnetic moment.

  10. Comparison of AlGaInP-VECSEL gain structures

    NASA Astrophysics Data System (ADS)

    Baumgärtner, Stefan; Kahle, Hermann; Bek, Roman; Schwarzbäck, Thomas; Jetter, Michael; Michler, Peter

    2015-03-01

    We present a comparison of epitaxial designs for barrier pumped vertical external-cavity surface-emitting lasers in the red spectral range. The VECSEL chips are grown by metal-organic vapor-phase epitaxy as GaInP/AlGaInP multi-quantum well structures with 20 and 21 compressively strained quantum wells, respectively. The QWs are placed in various packages in a separate confinement heterostructure with quaternary AlGaInP-barrier and cladding layers. Beneath the active region there is a distributed Bragg reflector consisting of 55 λ / 4 pairs of Al0.5Ga0.5As/AlAs. We compare three different QW distributions: one design includes 20 QWs arranged in 10 pairs, the second one contains 20 QWs arranged in 10 pairs with tensile strained barriers to compensate the compressive strain of the QWs, the third one has an exponential distribution of the QWs in tensile strained barriers. Laser parameters such as wavelength, differential efficiency, optical output power and absorption of the pump laser were measured for the different designs. By using the exponential distribution of QWs we could improve the differential efficiency by 42% and the output power by 20% compared to the not strain compensated 10×2 QW design. Furthermore we could improve the absorption efficiency by 60 % at nearly the same laser wavelength.

  11. Luminescence Properties of GaN:Tb, GaN/AlGaN:Eu Superlattice, and AlN:Tb and Eu.

    NASA Astrophysics Data System (ADS)

    Lozykowski, H. J.; Jadwisienczak, W. M.; Brown, I. G.

    2001-03-01

    We report on recent progress in the investigation of cathodoluminescence (CL) of GaN doped with Tb, the visible photoluminescence (PL) and CL of GaN and Al_0.14Ga_0.86N /GaN superlattice doped with Eu, and the CL from AlN doped with Eu and Tb. The CL of GaN:Tb shows sharp emission lines corresponding to Tb^3+ ions transitions resolved in the spectral range from 350 nm to 750 nm, and observed over the temperature range of 7 - 330 K. The luminescence exhibits transitions which originate in the ^5D3 and ^5D4 levels and terminate in the ^7F manifolds. The depth resolved CL spectra analysis show a luminescence surface dead layer thickness of ~20 nm. The decay times for ^5D_3-->^7F5 (423.4nm) and ^5D_4-->^7F5 (551.6nm) transitions at 7 K are ~0.7 and ~1.8 ms, with little change with temperature. The visible PL and CL of GaN and Al_0.14Ga_0.86N/GaN superlattice doped with Eu ions, show sharp characteristic emission lines corresponding to Eu^3+ intra-4f^6-shell transitions. The luminescence shows dominant ^5D_0-->^7F_1,2,3 and weaker ^5D_0-->^7F_4,5,6 and ^5D_1-->^7F1 transitions. The intensity of Eu emission from Al_0.14Ga_0.86N/GaN superlattice annealed in N2 is ~58% stronger than from Eu in the GaN epilayer. Strong CL was observed from AlN thin single crystal films doped with Eu^3+ and Tb^3+ ions. The space group symmetry of the wurtzite AlN is C-P6_3mc and the Al cation occupies the site of point group symmetry C_3v. We assume that those implanted RE ions in AlN occupy relaxed substitutional Al-sites with hexagonal C_3v crystal symmetry. Emission lines corresponding to Eu^3+ and Tb^3+ intra-4f^n-shell transitions are resolved in the spectral range from 300 to 900 nm. The CL kinetics for several transitions of Eu^3+ (^5D_0), Tb^3+ (^5D_3,4) were analyzed. (Electronic mail: lozykows/@bobcat.ent.ohiou.edu)

  12. A study of electrically active traps in AlGaN/GaN high electron mobility transistor

    NASA Astrophysics Data System (ADS)

    Yang, Jie; Cui, Sharon; Ma, T. P.; Hung, Ting-Hsiang; Nath, Digbijoy; Krishnamoorthy, Sriram; Rajan, Siddharth

    2013-10-01

    We have studied electron conduction mechanisms and the associated roles of the electrically active traps in the AlGaN layer of an AlGaN/GaN high electron mobility transistor structure. By fitting the temperature dependent I-V (Current-Voltage) curves to the Frenkel-Poole theory, we have identified two discrete trap energy levels. Multiple traces of I-V measurements and constant-current injection experiment all confirm that the main role of the traps in the AlGaN layer is to enhance the current flowing through the AlGaN barrier by trap-assisted electron conduction without causing electron trapping.

  13. Importance of doping and frustration in itinerant Fe-doped Cr2Al

    DOE PAGESBeta

    Susner, M. A.; Parker, D. S.; Sefat, A. S.

    2015-05-12

    We performed an experimental and theoretical study comparing the effects of Fe-doping of Cr2Al, an antiferromagnet with a N el temperature of 670 K, with known results on Fe-doping of antiferromagnetic bcc Cr. (Cr1-xFex)2Al materials are found to exhibit a rapid suppression of antiferromagnetic order with the presence of Fe, decreasing TN to 170 K for x=0.10. Antiferromagnetic behavior disappears entirely at x≈0.125 after which point increasing paramagnetic behavior is exhibited. Moreover, this is unlike the effects of Fe doping of bcc antiferromagnetic Cr, in which TN gradually decreases followed by the appearance of a ferromagnetic state. Theoretical calculations explainmore » that the Cr2Al-Fe suppression of magnetic order originates from two effects: the first is band narrowing caused by doping of additional electrons from Fe substitution that weakens itinerant magnetism; the second is magnetic frustration of the Cr itinerant moments in Fe-substituted Cr2Al. In pure-phase Cr2Al, the Cr moments have an antiparallel alignment; however, these are destroyed through Fe substitution and the preference of Fe for parallel alignment with Cr. This is unlike bulk Fe-doped Cr alloys in which the Fe anti-aligns with the Cr atoms, and speaks to the importance of the Al atoms in the magnetic structure of Cr2Al and Fe-doped Cr2Al.« less

  14. Interstitial precipitation in Fe-Cr-Al alloys

    NASA Astrophysics Data System (ADS)

    Spear, W. S.; Polonis, D. H.

    1994-06-01

    Two separate stages of precipitation have been identified during the aging of ternary Fel8Cr3Al and Fel8Cr5Al alloys at temperatures in the vicinity of 475 °C. The first stage involves the formation of interstitial precipitates resulting from C and N impurities; the second and slower stage is the formation of the Cr-rich α' phase. Transmission electron microscopy (TEM) results show that carbonitride precipitation occurs preferentially at dislocations, stacking faults, and grain boundaries, and also uniformly through the matrix. Aging for times in excess of 400 hours at 475 °C promotes coarsening of the heterogeneous precipitates and dissolution of the uniformly distributed matrix particles. A resistometric analysis shows that the kinetics of the initial stages of precipitation can be described by a (time)2/3 relation. This kinetic behavior is explained in terms of stress-assisted diffusion in the highly stressed matrix resulting from coherency strains accompanying carbonitride precipitation. Experimental values of the activation energy for the first stage reaction correlate closely with those reported for the interstitial diffusion of C and N in alpha iron.

  15. A model-base comparison - GaAs/GaAlAs HBT versus silicon bipolar

    NASA Astrophysics Data System (ADS)

    Kurata, M.; Katoh, R.; Yoshida, J.; Akagi, J.

    1986-10-01

    A pure model-base comparison is made between the GaAs/GaAlAs heterojunction bipolar transistor and the silicon bipolar transistor for the high-speed switching performance under ring oscillator operation. Full utilization is made of the earlier developed (Kurata et al., 1984 and 1985) modeling tools, which include a 'physical' one-dimensional transistor model, a hybrid model to represent a realistic device structure, and a circuit simulator to allow direct access to the physical model. Delay time versus power characteristics, as well as dynamic carrier profiles are demonstrated, with discussion about limiting factors for the switching speed.

  16. Degradation mechanisms of Ti/Al/Ni/Au-based Ohmic contacts on AlGaN/GaN HEMTs

    DOE PAGESBeta

    Hwang, Ya-Hsi; Ahn, Shihyun; Dong, Chen; Zhu, Weidi; Kim, Byung-Jae; Le, Lingcong; Ren, Fan; Lind, Aaron G.; Dahl, James; Jones, Kevin S.; et al

    2015-04-27

    We investigated the degradation mechanism of Ti/Al/Ni/Au-based Ohmic metallization on AlGaN/GaN high electron mobility transistors upon exposure to buffer oxide etchant (BOE). The major effect of BOE on the Ohmic metal was an increase of sheet resistance from 2.89 to 3.69 Ω/ₜafter 3 min BOE treatment. The alloyed Ohmic metallization consisted 3–5 μm Ni-Al alloy islands surrounded by Au-Al alloy-rings. The morphology of both the islands and ring areas became flatter after BOE etching. Lastly, we used energy dispersive x-ray analysis and Auger electron microscopy to analyze the compositions and metal distributions in the metal alloys prior to and aftermore » BOE exposure.« less

  17. Degradation mechanisms of Ti/Al/Ni/Au-based Ohmic contacts on AlGaN/GaN HEMTs

    SciTech Connect

    Hwang, Ya-Hsi; Ahn, Shihyun; Dong, Chen; Zhu, Weidi; Kim, Byung-Jae; Le, Lingcong; Ren, Fan; Lind, Aaron G.; Dahl, James; Jones, Kevin S.; Pearton, Stephen J.; Kravchenko, Ivan I.; Zhang, Ming-Lan

    2015-04-27

    We investigated the degradation mechanism of Ti/Al/Ni/Au-based Ohmic metallization on AlGaN/GaN high electron mobility transistors upon exposure to buffer oxide etchant (BOE). The major effect of BOE on the Ohmic metal was an increase of sheet resistance from 2.89 to 3.69 Ω/ₜafter 3 min BOE treatment. The alloyed Ohmic metallization consisted 3–5 μm Ni-Al alloy islands surrounded by Au-Al alloy-rings. The morphology of both the islands and ring areas became flatter after BOE etching. Lastly, we used energy dispersive x-ray analysis and Auger electron microscopy to analyze the compositions and metal distributions in the metal alloys prior to and after BOE exposure.

  18. The influences of AlN/GaN superlattices buffer on the characteristics of AlGaN/GaN-on-Si (1 1 1) template

    NASA Astrophysics Data System (ADS)

    Ni, Yiqiang; He, Zhiyuan; Zhou, Deqiu; Yao, Yao; Yang, Fan; Zhou, Guilin; Shen, Zhen; Zhong, Jian; Zhen, Yue; Zhang, Baijun; Liu, Yang

    2015-07-01

    The influence of AlN/GaN superlattices (SL) buffer on the characteristics of AlGaN/GaN-on-Si (1 1 1) template was studied in detail. There existed an optimized Relative AlN Thickness (RAT) in the superlattices buffer which can not only further filtering the edge- and screw-type dislocations to the upper epilayer and lead to a good crystal quality with narrowest (0 0 0 2) and (1 0 -1 2) full width of half maximum (FWHMs), 439″ and 843″, but also improve the surface roughness to enhance the Two dimensional electron gas (2DEG) mobility and superior electrical properties were achieved. Moreover, an optimized RAT in SL can induce a proper compressive stress to the subsequently grown GaN epilayer and protect it from crack during the cooling step, which can also lead to a better wafer bending.

  19. SEMICONDUCTOR MATERIALS: AlGaSb/GaSb quantum wells grown on an optimized AlSb nucleation layer

    NASA Astrophysics Data System (ADS)

    Hanchao, Gao; Cai, Wen; Wenxin, Wang; Zhongwei, Jiang; Haitao, Tian; Tao, He; Hui, Li; Hong, Chen

    2010-05-01

    Five-period AlGaSb/GaSb multiple quantum wells (MQW) are grown on a GaSb buffer. Through optimizing the AlSb nucleation layer, the low threading dislocation density of the MQW is found to be (2.50 ± 0.91) × 108 cm-2 in 1-μm GaSb buffer, as determined by plan-view transmission election microscopy (TEM) images. High resolution TEM clearly shows the presence of 90° misfit dislocations with an average spacing of 5.4 nm at the AlSb/GaAs interface, which effectively relieve most of the strain energy. In the temperature range from T = 26 K to 300 K, photoluminescence of the MQW is dominated by the ground state electron to ground state heavy hole (e1-hh1) transition, while a high energy shoulder clearly seen at T > 76 K can be attributed to the ground state electron to ground state light hole (e1-lh1) transition.

  20. Normal and excess nitrogen uptake by iron-based Fe-Cr-Al alloys: the role of the Cr/Al atomic ratio

    NASA Astrophysics Data System (ADS)

    Jung, K. S.; Schacherl, R. E.; Bischoff, E.; Mittemeijer, E. J.

    2011-06-01

    Upon nitriding ferritic iron-based Fe-Cr-Al alloys, containing a total of 1.50 at. % (Cr + Al) alloying elements with varying Cr/Al atomic ratio (0.21-2.00), excess nitrogen uptake occurred, i.e. more nitrogen was incorporated in the specimens than compatible with only inner nitride formation and equilibrium nitrogen solubility of the unstrained ferrite matrix. The amount of excess nitrogen increased with decreasing Cr/Al atomic ratio. The microstructure of the nitrided zone was investigated by X-ray diffraction, electron probe microanalysis, transmission electron microscopy and electron energy loss spectroscopy. Metastable, fine platelet-type, mixed Cr1- x Al x N nitride precipitates developed in the nitrided zone for all of the investigated specimens. The degree of coherency of the nitride precipitates with the surrounding ferrite matrix is discussed in view of the anisotropy of the misfit. Analysis of nitrogen-absorption isotherms, recorded after subsequent pre- and de-nitriding treatments, allowed quantitative differentiation of different types of nitrogen taken up. The amounts of the different types of excess nitrogen as function of the Cr/Al atomic ratio are discussed in terms of the nitride/matrix misfit and the different chemical affinities of Cr and Al for N. The strikingly different nitriding behaviors of Fe-Cr-Al and Fe-Cr-Ti alloys could be explained on this basis.

  1. Electrical defects in AlGaN and InAlN

    NASA Astrophysics Data System (ADS)

    Johnstone, D.; Leach, Jacob H.; Kovalskii, Vladimir A.; Fan, Qian; Xie, Jingqiao; Morkoç, Hadis

    2009-02-01

    Compound semiconductors based on GaN have multiple functional applications. Useful compositions include GaN, and ternary and quaternary compositions of (AlGaIn)N. Defects arising from lattice mismatch, point defects, or impurities may act as electrical trapping centers and degrade device efficiency. Current-voltage, capacitance-voltage, thermal admittance spectroscopy (TAS), and deep level transient spectroscopy (DLTS) measurements are applied to characterize the defects in Al0.40Ga0.80N and In0.18Al0.82N in this report. Broad peaks with a shoulder at high temperature dominate the DLTS spectra in each of the materials. An acceptor trap associated with a dislocation appears at 340 K in AlGaN. The defect has an energy of 0.2 eV and capture cross section of 10-21 cm2. A second trap at 0.35 eV, 10-14 cm2 appears in the TAS measurements in addition to the trap at 0.2 eV. Defects in InAlN are dominated by a peak near 150 K. Two traps appear in the TAS measurements. Both traps in the InAlN are acceptors, based on a lack of field dependent emission rates using double pulse DLTS (DDLTS). The two energy levels in InAlN appear to be coupled, with only one state occupied at a time.

  2. Vertical Transport in GaN/AlGaN Resonant Tunneling Diodes and Superlattices

    NASA Astrophysics Data System (ADS)

    Warde, Elias; Sakr, Salam; Tchernycheva, Maria; Julien, Francois Henry

    2012-05-01

    Using the transfer matrix formalism, we have theoretically studied the vertical ballistic transport in GaN/AlGaN resonant tunneling diodes (RTDs) and superlattices with a small number of periods. We have calculated the transmission probability versus the longitudinal electron energy ( T- E) and the current density-voltage ( J- V) characteristics. Calculations of both T- E and J- V characteristics have been performed for different Al contents in the barriers. The asymmetry effects due to the internal electric field in the barriers are discussed. Applied to the RTD structure, our calculations demonstrate: (i) the increase of the peak-to-valley ratio of the negative differential resistance (NDR) with increasing Al content in the barriers, (ii) the dependence of the J- V resonance values on the current direction, and (iii) the asymmetry of the NDR with respect to the current direction due to the huge internal electric field in the structure. In the case of multiple quantum well structure (MQWS), the calculation results confirm the same trends as in the RTD case when the Al content is varied. In spite of the fact that it is more difficult to analyze the results in the case of MQWS, the obtained calculations demonstrate the applicability of the used model and of the numerical method to study GaN/AlGaN devices based on quantum well (QW) heterostructures. Furthermore, a design of an optimized 7QW structure operating symmetrically whatever the direction of the applied voltage is presented.

  3. Carbon Nanostructures Grown on Fe-Cr-Al Alloy

    NASA Astrophysics Data System (ADS)

    Čaplovičová, Mária; Čaplovič, Ľubomír; Búc, Dalibor; Vinduška, Peter; Janík, Ján

    2010-11-01

    The morphology and nanostructure of carbon nanotubes (CNTs), synthesized directly on Fe-Cr-Al-based alloy substrate using an alcohol catalytic chemical vapour deposition method (ACCVD), were examined by transmission electron microscopy (TEM). The grown CNTs were entangled with chain-like, bamboo-like, and necklace-like morphologies. The CNT morphology was affected by the elemental composition of catalysts and local instability of deposition process. Straight and bended CNTs with bamboo-like nanostructure grew mainly on γ-Fe and Fe3C particles. The synthesis of necklace-like nanostructures was influenced by silicon oxide, and growth of chain-like nanostructures was supported by a catalysts consisting of Fe, Si, oxygen and trace of Cr. Most of nanotubes grew according to base growth mechanism.

  4. Enhanced thermoelectric transport in modulation-doped GaN/AlGaN core/shell nanowires

    SciTech Connect

    Song, Erdong; Li, Qiming; Swartzentruber, Brian; Pan, Wei; Wang, George T.; Martinez, Julio A.

    2015-11-25

    The thermoelectric properties of unintentionally n-doped core GaN/AlGaN core/shell N-face nanowires are reported. We found that the temperature dependence of the electrical conductivity is consistent with thermally activated carriers with two distinctive donor energies. The Seebeck coefficient of GaN/AlGaN nanowires is more than twice as large as that for the GaN nanowires alone. However, an outer layer of GaN deposited onto the GaN/AlGaN core/shell nanowires decreases the Seebeck coefficient at room temperature, while the temperature dependence of the electrical conductivity remains the same. We attribute these observations to the formation of an electron gas channel within the heavily-doped GaN core of the GaN/AlGaN nanowires. The room-temperature thermoelectric power factor for the GaN/AlGaN nanowires can be four times higher than the GaN nanowires. As a result, selective doping in bandgap engineered core/shell nanowires is proposed for enhancing the thermoelectric power.

  5. Enhanced thermoelectric transport in modulation-doped GaN/AlGaN core/shell nanowires

    DOE PAGESBeta

    Song, Erdong; Li, Qiming; Swartzentruber, Brian; Pan, Wei; Wang, George T.; Martinez, Julio A.

    2015-11-25

    The thermoelectric properties of unintentionally n-doped core GaN/AlGaN core/shell N-face nanowires are reported. We found that the temperature dependence of the electrical conductivity is consistent with thermally activated carriers with two distinctive donor energies. The Seebeck coefficient of GaN/AlGaN nanowires is more than twice as large as that for the GaN nanowires alone. However, an outer layer of GaN deposited onto the GaN/AlGaN core/shell nanowires decreases the Seebeck coefficient at room temperature, while the temperature dependence of the electrical conductivity remains the same. We attribute these observations to the formation of an electron gas channel within the heavily-doped GaN coremore » of the GaN/AlGaN nanowires. The room-temperature thermoelectric power factor for the GaN/AlGaN nanowires can be four times higher than the GaN nanowires. As a result, selective doping in bandgap engineered core/shell nanowires is proposed for enhancing the thermoelectric power.« less

  6. Local stress-induced effects on AlGaAs/AlOx oxidation front shape

    NASA Astrophysics Data System (ADS)

    Chouchane, F.; Almuneau, G.; Cherkashin, N.; Arnoult, A.; Lacoste, G.; Fontaine, C.

    2014-07-01

    The lateral oxidation of thick AlGaAs layers (>500 nm) is studied. An uncommon shape of the oxide tip is evidenced and attributed to the embedded stress distribution, inherent to the oxidation reaction. Experimental and numerical studies of the internal strain in oxidized AlxGa1-xAs/GaAs structures were carried out by dark-field electron holography and finite element methods. A mapping of the strain distribution around the AlGaAs/oxide interface demonstrates the main role of internal stress on the shaping of the oxide front. These results demonstrate the high relevance of strain in oxide-confined III-V devices, in particular, with over-500-nm thick AlOx confinement layers.

  7. Response of timepix detector with GaAs:Cr and Si sensor to heavy ions

    NASA Astrophysics Data System (ADS)

    Abu Al Azm, S. M.; Chelkov, G.; Kozhevnikov, D.; Guskov, A.; Lapkin, A.; Leyva Fabelo, A.; Smolyanskiy, P.; Zhemchugov, A.

    2016-05-01

    The response of the Timepix detector to neon ions with kinetic energy 77 and 158.4 MeV has been studied at the cyclotron U-400M of the JINR Flerov Laboratory of Nuclear Reaction. Sensors produced from gallium arsenide compensated by chromium and from silicon are used for these measurements. While in Timepix detector with Si sensor the well-known so-called "volcano effect" observed, in Timepix detector with GaAs:Cr sensor such effect was completely absent. In the work the behavior of the Timepix detector with GaAs:Cr sensor under irradiation with heavy ions is described in comparison with the detector based on Si sensor. Also the possible reason for absence of "volcano" effect in GaAs:Cr detector is proposed.

  8. Performance improvement of GaN-based near-UV LEDs with InGaN/AlGaN superlattices strain relief layer and AlGaN barrier

    NASA Astrophysics Data System (ADS)

    Jia, Chuanyu; Yu, Tongjun; Feng, Xiaohui; Wang, Kun; Zhang, Guoyi

    2016-09-01

    The carrier confinement effect and piezoelectric field-induced quantum-confined stark effect of different GaN-based near-UV LED samples from 395 nm to 410 nm emission peak wavelength were investigated theoretically and experimentally. It is found that near-UV LEDs with InGaN/AlGaN multiple quantum wells (MQWs) active region have higher output power than those with InGaN/GaN MQWs for better carrier confinement effect. However, as emission peak wavelength is longer than 406 nm, the output power of the near-UV LEDs with AlGaN barrier is lower than that of the LEDs with GaN barrier due to more serious spatial separation of electrons and holes induced by the increase of piezoelectric field. The N-doped InGaN/AlGaN superlattices (SLs) were adopted as a strain relief layer (SRL) between n-GaN and MQWs in order to suppress the polarization field. It is demonstrated the output power of near-UV LEDs is increased obviously by using SLs SRL and AlGaN barrier for the discussed emission wavelength range. Besides, the forward voltage of near-UV LEDs with InGaN/AlGaN SLs SRL is lower than that of near-UV LEDs without SRL.

  9. Thermal expansion in Cr:LiSrGaF{sub 6}

    SciTech Connect

    Grzechnik, Andrzej . E-mail: andrzej@wm.lc.ehu.es; Azcona, Zunbeltz Izaola; Bereciartua, Pablo; Friese, Karen; Doyle, Stephen

    2005-11-03

    High-temperature behaviour of LiSrGaF{sub 6} doped with 1.5 at.% of Cr{sup 3+} was studied with high-resolution synchrotron angle-dispersive X-ray powder diffraction in the temperature range 298-539 K. No phase transitions were detected. The origin of negative thermal expansion along the c axis is discussed based on the temperature dependencies of structural parameters and octahedral distortions obtained with the Rietveld method. The SrF{sub 6} slab contracts with increasing temperatures because of the diminishing F-Sr-F octahedral angles without any changes in the F-F octahedral edges not only around strontium but also around lithium and gallium. At the same time, the angular distortions of the SrF{sub 6} octahedra are largely diminished. Such a behaviour is discussed in comparison with the thermal expansion of LiCaAlF{sub 6} and LiSrAlF{sub 6}.

  10. Novel model of a AlGaN/GaN high electron mobility transistor based on an artificial neural network

    NASA Astrophysics Data System (ADS)

    Cheng, Zhi-Qun; Hu, Sha; Liu, Jun; Zhang, Qi-Jun

    2011-03-01

    In this paper we present a novel approach to modeling AlGaN/GaN high electron mobility transistor (HEMT) with an artificial neural network (ANN). The AlGaN/GaN HEMT device structure and its fabrication process are described. The circuit-based Neuro-space mapping (neuro-SM) technique is studied in detail. The EEHEMT model is implemented according to the measurement results of the designed device, which serves as a coarse model. An ANN is proposed to model AlGaN/GaN HEMT based on the coarse model. Its optimization is performed. The simulation results from the model are compared with the measurement results. It is shown that the simulation results obtained from the ANN model of AlGaN/GaN HEMT are more accurate than those obtained from the EEHEMT model. Project supported by the National Natural Science Foundation of China (Grant No. 60776052).

  11. Recent advancements in monolithic AlGaAs/GaAs solar cells for space applications

    NASA Technical Reports Server (NTRS)

    Wickham, K. R.; Chung, B.-C.; Klausmeier-Brown, M.; Kuryla, M. S.; Ristow, M. Ladle; Virshup, G. F.; Werthen, J. G.

    1991-01-01

    High efficiency, two terminal, multijunction AlGaAs/GaAs solar cells were reproducibly made with areas of 0.5 sq cm. The multiple layers in the cells were grown by Organo Metallic Vapor Phase Epitaxy (OMVPE) on GaAs substrates in the n-p configuration. The upper AlGaAs cell has a bandgap of 1.93 eV and is connected in series to the lower GaAs cell (1.4 eV) via a metal interconnect deposited during post-growth processing. A prismatic coverglass is installed on top of the cell to reduce obscuration caused by the gridlines. The best 0.5 sq cm cell has a two terminal efficiency of 23.0 pct. at 1 sun, air mass zero (AM0) and 25 C. To date, over 300 of these cells were grown and processed for a manufacturing demonstration. Yield and efficiency data for this demonstration are presented. As a first step toward the goal of a 30 pct. efficient cell, a mechanical stack of the 0.5 sq cm cells described above, and InGaAsP (0.95 eV) solar cells was made. The best two terminal measurement to date yields an efficiency of 25.2 pct. AM0. This is the highest reported efficiency of any two terminal, 1 sun space solar cell.

  12. Electron and proton degradation in /AlGa/As-GaAs solar cells

    NASA Technical Reports Server (NTRS)

    Loo, R.; Knechtli, R. C.; Kamath, G. S.; Goldhammer, L.; Anspaugh, B.

    1978-01-01

    Results on radiation damage in (AlGa)As-GaAs solar cells by 1 MeV electron fluences up to 10 to the 16th electrons/sq cm and by 15, 20, 30 and 40 MeV proton fluences up to 5 times 10 to the 11th protons/sq cm are presented. The damage is compared with data on state-of-the-art silicon cells which were irradiated along with the gallium arsenide cells. The theoretical expectation that the junction depth has to be kept relatively shallow, to minimize radiation damage has been verified experimentally. The damage to the GaAs cells as a function of irradiation, is correlated with the change in their spectral response and dark I-V characteristics. The effect of thermal annealing on the (AlGa)As-GaAs solar cells was also investigated. This data is used to predict further avenues of optimization of the GaAs cells.

  13. Finite element analysis of AlGaN/GaN micro-diaphragms with diamond coating

    NASA Astrophysics Data System (ADS)

    Dzuba, J.; Vanko, G.; Vojs, M.; Rýger, I.; Ižák, T.; Jirásek, V.; Kutiš, V.; Lalinský, T.

    2015-05-01

    In this work, we present a pressure sensor based on diamond coated AlGaN/GaN diaphragm with integrated high electron mobility transistor (HEMT). The influence of the diamond film thickness (in the range of 1 μm to 50 μm) on the properties of the AlGaN/GaN diaphragm is studied by finite element simulation method (FEM). The effect of thermal buckling as well as the induced piezoelectric charge of HEMTs as a function of pressure and temperature is investigated. It was found out that diamond coated sensor better prevents the effect known as thermal buckling of the diaphragm at elevated temperature. Thermal buckling of diaphragms with 1, 5, 10 μm diamond coating occurs at temperature 40, 73 and 142 °C, respectively. Compared with original GaN diaphragm, diamond expanded the operational temperature range of the pressure sensor. Moreover, compared with the operational range of pressure sensor based on pure GaN diaphragm (up to 30 kPa), diamond coated modified MEMS sensors withstand relatively higher pressures (2.2 MPa). The maximum load on the diaphragm increased two times by adding only 1 μm of diamond coating.

  14. AlGaN/GaN current aperture vertical electron transistors with regrown channels

    NASA Astrophysics Data System (ADS)

    Ben-Yaacov, Ilan; Seck, Yee-Kwang; Mishra, Umesh K.; DenBaars, Steven P.

    2004-02-01

    AlGaN/GaN current aperture vertical electron transistors with regrown aperture and source regions have been fabricated and tested. A 2 μm thick GaN:Si drain region followed by a 0.4 μm GaN:Mg current-blocking layer were grown by metalorganic chemical vapor deposition on a c-plane sapphire substrate. Channel apertures were etched, and a maskless regrowth was performed to grow unintentionally doped GaN inside the aperture as well as above the insulating layer, and to add an AlGaN cap layer. Cl2 reactive ion etching was used to pattern the device mesa, and source, drain, and gate metals were then deposited. Devices were achieved with a maximum source-drain current of 750 mA/mm, an extrinsic transconductance of 120 mS/mm, and a 2-terminal gate breakdown of 65 V while exhibiting almost no DC-RF dispersion for 80 μs pulsed I-V curves. The suppression of DC-RF dispersion was shown to result from the absence of the large electric fields at the surface on the drain-side edge of the gate that are present in high electron mobility transistors. Parasitic leakage currents, which were present in all devices, have been studied in detail. Three leakage paths have been identified, and methods to eliminate them are discussed.

  15. Nonpolar AlGaN/GaN HFETs with a normally off operation

    NASA Astrophysics Data System (ADS)

    Ishida, M.; Kuroda, M.; Ueda, T.; Tanaka, T.

    2012-02-01

    Nonpolar AlGaN/GaN heterojunction field-effect transistors (HFETs) with a normally off operation have been demonstrated. The nonpolar (1-120) a-plane's epitaxial layers are grown on (1-102) r-plane sapphire substrates by metal organic chemical vapour deposition. We have found that a thicker AlN buffer layer achieves a GaN layer with a narrower full-width at half-maximum of the x-ray rocking curve and higher electron mobility. We have fabricated AlGaN/GaN HFETs with different gate directions. It is found that the drain current strongly depends on the gate directions, and higher drain current flows to the (0001) direction that is parallel to the hair-lined morphology. To realize a complete normally off operation, we have fabricated a-plane metal-insulator-semiconductor HFETs (MIS-HFETs) with a 2 nm-thick SiN as an insulator. The fabricated MIS-HFET exhibits a threshold voltage of +1.3 V with a high drain current of 112 mA mm-1. The presented MIS-HFETs will be desirable in next-generation power switching applications.

  16. AlGaN/GaN high-electron mobility transistor-based sensors for environmental and bio-applications

    NASA Astrophysics Data System (ADS)

    Chu, B. H.; Wang, Y. L.; Chen, K. H.; Chang, C. Y.; Lo, C. F.; Pearton, S. J.; Papadi, G.; Coleman, J. K.; Sheppard, B. J.; Dungen, C. F.; Kroll, Kevin; Denslow, Nancy; Dabiran, A.; Chow, P. P.; Johnson, J. W.; Pine, E. L.; Linthicum, K. J.; Ren, F.

    2010-04-01

    A promising sensing technology utilizing AlGaN/GaN high electron mobility transistors (HEMTs) has been developed to analyze a wide variety of environmental and biological gases and liquids. The conducting 2DEG channel of GaN/AlGaN HEMTs is very close to the surface and extremely sensitive to adsorption of analytes. Examples of detecting mercury ions, perkinsus, lactic acid, carbon dioxide, and vitellogenin are discussed in this paper.

  17. Short circuit current changes in electron irradiated GaAlAs/GaAs solar cells

    NASA Technical Reports Server (NTRS)

    Walker, G. H.; Conway, E. J.

    1978-01-01

    Heteroface p-GaAlAs/p-GaAs/n-GaAs solar cells with junction depths of 0.8, 1.5, and 4 microns were irradiated with 1 MeV electrons. The short-circuit current for the 4 micron junction depth cells is significantly reduced by the electron irradiation. Reduction of the junction depth to 1.5 microns improves the electron radiation resistance of the cells while further reduction of the junction depth to 0.8 microns improves the stability of the cells even more. Primary degradation is in the blue region of the spectrum. Considerable recovery of lost response is obtained by annealing the cells at 200 C. Computer modeling shows that the degradation is caused primarily by a reduction in the minority carrier diffusion length in the p-GaAs.

  18. Simulation of GaN/AlGaN heterojunction bipolar transistors: part I npn structures

    NASA Astrophysics Data System (ADS)

    Cao, X. A.; Van Hove, J. M.; Klaassen, J. J.; Polley, C. J.; Wowchak, A. M.; Chow, P. P.; King, D. J.; Zhang, A. P.; Dang, G.; Monier, C.; Pearton, S. J.; Ren, F.

    2000-07-01

    A drift-diffusion model was employed to calculate the dc performance of GaN/AlGaN heterojunction bipolar transistors (HBTs). The dc current gain was found to vary from ˜7 to 60 for collector currents of 10 -12-10 -2 A in 100 μm contact diameter devices with 2000 Å thick p-GaN base layers (P=2×10 17 cm-3) . The effects of base grading, base thickness, minority carrier lifetime and mobility in the base, base contact resistance and device operating temperature (25-300°C) were examined. The HBTs were found to have a significantly better gain at low collector current densities than the GaN bipolar junction transistors, due to the valence band offset.

  19. Ultrafast Photodetection in the Quantum Wells of Single AlGaAs/GaAs-Based Nanowires

    NASA Astrophysics Data System (ADS)

    Erhard, N.; Zenger, S.; Morkötter, S.; Rudolph, D.; Weiss, M.; Krenner, H. J.; Karl, H.; Abstreiter, G.; Finley, J. J.; Koblmüller, G.; Holleitner, A. W.

    2015-10-01

    We investigate the ultrafast optoelectronic properties of single Al0.3Ga0.7As/GaAs-core-shell-nanowires. The nanowires contain GaAs-based quantum wells. For a resonant excitation of the quantum wells, we find a picosecond photocurrent which is consistent with an ultrafast lateral expansion of the photogenerated charge carriers. This Dember-effect does not occur for an excitation of the GaAs-based core of the nanowires. Instead, the core exhibits an ultrafast displacement current and a photo-thermoelectric current at the metal Schottky contacts. Our results uncover the optoelectronic dynamics in semiconductor core-shell nanowires comprising quantum wells, and they demonstrate the possibility to use the low-dimensional quantum well states therein for ultrafast photoswitches and photodetectors.

  20. Ultrafast Photodetection in the Quantum Wells of Single AlGaAs/GaAs-Based Nanowires.

    PubMed

    Erhard, N; Zenger, S; Morkötter, S; Rudolph, D; Weiss, M; Krenner, H J; Karl, H; Abstreiter, G; Finley, J J; Koblmüller, G; Holleitner, A W

    2015-10-14

    We investigate the ultrafast optoelectronic properties of single Al0.3Ga0.7As/GaAs core-shell nanowires. The nanowires contain GaAs-based quantum wells. For a resonant excitation of the quantum wells, we find a picosecond photocurrent which is consistent with an ultrafast lateral expansion of the photogenerated charge carriers. This Dember-effect does not occur for an excitation of the GaAs-based core of the nanowires. Instead, the core exhibits an ultrafast displacement current and a photothermoelectric current at the metal Schottky contacts. Our results uncover the optoelectronic dynamics in semiconductor core-shell nanowires comprising quantum wells, and they demonstrate the possibility to use the low-dimensional quantum well states therein for ultrafast photoswitches and photodetectors. PMID:26356189

  1. Room-Temperature Transport of Indirect Excitons in (Al ,Ga )N /GaN Quantum Wells

    NASA Astrophysics Data System (ADS)

    Fedichkin, F.; Guillet, T.; Valvin, P.; Jouault, B.; Brimont, C.; Bretagnon, T.; Lahourcade, L.; Grandjean, N.; Lefebvre, P.; Vladimirova, M.

    2016-07-01

    We report on the exciton propagation in polar (Al ,Ga )N /GaN quantum wells over several micrometers and up to room temperature. The key ingredient to achieve this result is the crystalline quality of GaN quantum wells grown on GaN substrate that limits nonradiative recombination. From the comparison of the spatial and temporal dynamics of photoluminescence, we conclude that the propagation of excitons under continuous-wave excitation is assisted by efficient screening of the in-plane disorder. Modeling within drift-diffusion formalism corroborates this conclusion and suggests that exciton propagation is still limited by the exciton scattering on defects rather than by exciton-exciton scattering so that improving interface quality can boost exciton transport further. Our results pave the way towards room-temperature excitonic devices based on gate-controlled exciton transport in wide-band-gap polar heterostructures.

  2. Enhanced sheet carrier densities in polarization controlled AlInN/AlN/GaN/InGaN field-effect transistor on Si (111)

    SciTech Connect

    Hennig, J. Dadgar, A.; Witte, H.; Bläsing, J.; Lesnik, A.; Strittmatter, A.; Krost, A.

    2015-07-15

    We report on GaN based field-effect transistor (FET) structures exhibiting sheet carrier densities of n = 2.9 10{sup 13} cm{sup −2} for high-power transistor applications. By grading the indium-content of InGaN layers grown prior to a conventional GaN/AlN/AlInN FET structure control of the channel width at the GaN/AlN interface is obtained. The composition of the InGaN layer was graded from nominally x{sub In} = 30 % to pure GaN just below the AlN/AlInN interface. Simulations reveal the impact of the additional InGaN layer on the potential well width which controls the sheet carrier density within the channel region of the devices. Benchmarking the In{sub x}Ga{sub 1−x}N/GaN/AlN/Al{sub 0.87}In{sub 0.13}N based FETs against GaN/AlN/AlInN FET reference structures we found increased maximum current densities of I{sub SD} = 1300 mA/mm (560 mA/mm). In addition, the InGaN layer helps to achieve broader transconductance profiles as well as reduced leakage currents.

  3. Photovoltaic detection of infrared light in a GaAs/AlGaAs superlattice

    NASA Astrophysics Data System (ADS)

    Kastalsky, A.; Duffield, T.; Allen, S. J.; Harbison, J.

    1988-04-01

    The observation of photovoltaic infrared photodetection in an n-type GaAs/AlGaAs superlattice within the optical range 3.6-6.2 microns is reported. A built-in graded AlGaAs barrier provides charge polarization in the sample by allowing the optically excited electrons in the second miniband of the superlattice to diffuse over this barrier. The optical polarization of the infrared signal is consistent with the selection rules applicable to the superlattice. The infrared photoresponse results from first-to-second miniband as well as shallow donor-to-second miniband photoexcitations within the superlattice. Donor-to-second miniband photoexcitation is dominant at low temperatures.

  4. Atomic layer epitaxy of AlAs and AlGaAs

    NASA Astrophysics Data System (ADS)

    Meguro, T.; Iwai, S.; Aoyagi, Y.; Ozaki, K.; Yamamoto, Y.; Suzuki, T.; Okano, Y.; Hirata, A.

    1990-01-01

    Atomic layer epitaxy (ALE) of AlAs and AlGaAs with metalorganic vapor-phase epitaxy (MOVPE) under Ar-ion laser irradiation has been successfully realized in a triethylaluminum (TEA)/AsH 3 system for the first time. Comparison with the growth characteristics of MOVPE with alternative feeding modes of TMA/AsH 3 and TEA/AsH 3 is discussed. Application to laser-ALE of AlGaAs using a triethylgallium (TEG)/TEA/AsH 3 system is also discussed.

  5. Zener Relaxation Peak in an Fe-Cr-Al Alloy

    NASA Astrophysics Data System (ADS)

    Zhou, Zheng-Cun; Cheng, He-Fa; Gong, Chen-Li; Wei, Jian-Ning; Han, Fu-Sheng

    2002-11-01

    We have studied the temperature spectra of internal friction and relative dynamic modulus of the Fe-(25 wt%)Cr-(5 wt%)Al alloy with different grain sizes. It is found that a peak appears in the internal friction versus temperature plot at about 550°C. The peak is of a stable relaxation and is reversible, which occurs not only during heating but also during cooling. Its activation energy is 2.5 (+/- 0.15) eV in terms of the Arrhenius relation. In addition, the peak is not obvious in specimens with a smaller grain size. It is suggested that the peak originates from Zener relaxation.

  6. Optimisation of a carbon doped buffer layer for AlGaN/GaN HEMT devices

    NASA Astrophysics Data System (ADS)

    Gamarra, Piero; Lacam, Cedric; Tordjman, Maurice; Splettstösser, Jörg; Schauwecker, Bernd; di Forte-Poisson, Marie-Antoinette

    2015-03-01

    This work reports on the optimisation of carbon doping GaN buffer layer (BL) for AlGaN/GaN HEMT (high electron mobility transistor) structures, grown by low pressure metal-organic vapour phase epitaxy (LP-MOVPE) on 3 in. SiC semi-insulating substrates. The incorporation of carbon impurities in GaN is studied as a function of the growth conditions, without using an external carbon source. We observed that the C incorporation can be effectively controlled over more than one order of magnitude by tuning the reactor pressure and the growth temperature, without degradation of the crystalline properties of the GaN layers. HEMT structures with a specific barrier design were grown with different carbon dopings in the GaN BL and processed into transistors to evaluate the impact of the BL doping on the device performances. A significant improvement of the HEMT drain leakage current and of the breakdown voltage was obtained by increasing the carbon incorporation in the GaN BL. The RF performances of the devices show a trade-off between leakage currents and trapping phenomena which are enhanced by the use of carbon doping, limiting the delivered output power. An output power as high as 6.5 W/mm with a Power Added Efficiency of 70% has been achieved at 2 GHz by the HEMT structures with the lowest carbon doping in the BL.

  7. Transport imaging for contact-free measurements of minority carrier diffusion in GaN, GaN/AlGaN, and GaN/InGaN core-shell nanowires

    SciTech Connect

    Baird, Lee; Ong, C. P.; Cole, R. Adam; Haegel, N. M.; Talin, A. Alec; Li, Qiming; Wang, George T.

    2011-03-28

    Minority carrier diffusion lengths Ld are calculated for GaN, GaN/AlGaN, and GaN/InGaN core-shell nanowires using a procedure based on imaging of recombination luminescence. The result of shell material on conveyance properties is recorded. An AlGaN shell produces Ld values in surplus of 1μm and a relative insensitivity to wire diameter. An InGaN shell reduces effective diffusion length, while a dependence of Ld on diameter is observed for unshielded nanowires.

  8. Vertical electron transistor (VET) in GaAs with a heterojunction (AlGaAs-GaAs) cathode

    NASA Astrophysics Data System (ADS)

    Mishra, U.; Maki, P. A.; Wendt, J. R.; Schaff, W.; Kohn, E.; Eastman, L. F.

    1984-02-01

    The successful fabrication of submicrometer channel length (0.75 micron) and gate length (0.15 micron) vertical electron transistors with AlGaAs cathodes is reported. Lack of electron velocity enhancement has been proposed to be due to high operating channel temperatures, and low temperature measurements were hindered by carrier freeze-out.

  9. Bulk moduli of Cr[subscript 2]GaC and Ti[subscript 2]GaN up to 50 GPa

    SciTech Connect

    Manoun, B.; Kulkarni, S.; Pathak, N.; Saxena, S.K.; Amini, S.; Barsoum, M.W.

    2010-10-22

    Using a synchrotron radiation source and a diamond anvil cell, we measured the pressure dependencies of the lattice parameters of two Ga-containing polycrystalline MAX phases: Ti{sub 2}GaN and Cr{sub 2}GaC. The bulk moduli were calculated to be 189 {+-} 4 and 188 {+-} 5 GPa, respectively. Up to a pressure of about 50 GPa, no phase transformations were observed. Ti{sub 2}GaN compresses more easily along the c-axis, while Cr{sub 2}GaC compresses equally in both directions.

  10. High-pressure laser spectroscopy of Cr3+:Gd3Sc2Ga3O12 and Cr3+:Gd3Ga5O12

    NASA Astrophysics Data System (ADS)

    Hömmerich, U.; Bray, K. L.

    1995-05-01

    We report on the effect of high pressure on the room-temperature emission spectra and lifetimes of Cr3+:GSGG (Gd3Sc2Ga3O12) and Cr3+:GGG (Gd3Ga5O12). In both systems we observed a dramatic change of the overall emission band shape upon increasing pressure, from a nearly structureless broadband (4T2-->4A2) to a highly structured narrow band (2E-->4A2). From the peak energy of the broadband emission, we estimated the pressure-induced blueshift of the 4T2-->4A2 transition to be 10 (+/-2) cm-1/kbar. High-resolution measurements in the R-line region (~700 nm) revealed that the 2E-->4A2 transition hardly shifts at low pressures (<40 kbar), whereas at higher pressures (>60 kbar) a nearly linear redshift of 0.65 (+/-0.05) cm-1/kbar is observed. Besides pressure-induced spectral changes, an enormous increase in the emission lifetime with increasing pressure was found for both systems. In the case of Cr3+:GSGG, the lifetime changed from 110 μs at ambient pressure to 4.4 ms at 125 kbar. For Cr3+:GGG, the lifetime increased from 168 μs to 7.3 ms for the same pressure range. The pressure-induced spectral and lifetime changes are described by a single configurational coordinate model that considers the effect of pressure on the thermal and spin-orbit coupling of the 2E and 4T2 states. A previously reported pressure-induced R-line-shift reversal in Cr3+:GSGG and the effect of high pressure on the lifetime in Cr3+:YAG are also discussed within the same framework.

  11. Growth and characterization of TiAlN/CrAlN superlattices prepared by reactive direct current magnetron sputtering

    SciTech Connect

    Barshilia, Harish C.; Deepthi, B.; Rajam, K. S.; Bhatti, Kanwal Preet; Chaudhary, Sujeet

    2009-01-15

    TiAlN and CrAlN coatings were prepared using a reactive direct current magnetron sputtering system from TiAl and CrAl targets. Structural characterization of the coatings using x-ray diffraction (XRD) revealed the B1 NaCl structure of TiAlN and CrAlN coatings with a prominent reflection along the (111) plane. The XPS data confirmed the bonding structures of TiAlN and CrAlN single layer coatings. Subsequently, nanolayered multilayer coatings of TiAlN/CrAlN were deposited on silicon and mild steel (MS) substrates at different modulation wavelengths ({lambda}) with a total thickness of approximately 1.0 {mu}m. The modulation wavelengths were calculated from the x-ray reflectivity data using modified Bragg's law. TiAlN/CrAlN multilayer coatings were textured along (111) for {lambda}<200 A and the XRD patterns showed the formation of superlattice structure for coatings deposited at {lambda}=102 A. The x-ray reflectivity data showed reflections of fifth and seventh orders for multilayer coatings deposited at {lambda}=102 and 138 A, respectively, indicating the formation of sharp interfaces between TiAlN and CrAlN layers. The cross-sectional scanning electron microscopy image of TiAlN/CrAlN multilayer coatings indicated a noncolumnar and dense microstructure. A maximum hardness of 39 GPa was observed for TiAlN/CrAlN multilayer coatings deposited at {lambda}=93 A, which was higher than the rule-of-mixture value (30 GPa) for TiAlN and CrAlN. Study of thermal stability of the coatings in air using micro-Raman spectroscopy indicated that the TiAlN/CrAlN multilayer coatings were stable up to 900 deg. C in air. TiAlN/CrAlN multilayer coatings also exhibited improved corrosion resistance when compared to the MS substrate.

  12. Performance characteristics of InGaAs/GaAs and GaAs/InGaAlAs coherently strained superlattice photodiodes

    NASA Technical Reports Server (NTRS)

    Das, Utpal; Zebda, Yousef; Bhattacharya, Pallab; Chin, Albert

    1987-01-01

    The properties of In(0.24)Ga(0.76)As/GaAs and GaAs/In(0.05)Ga(0.58)Al(0.37)As superlattice photodiodes grown by molecular beam epitaxy have been investigated. From the temporal response characteristics, deconvolved rise times about 60-100 ps are obtained. The measured responsivities of the photodiodes with dark currents of 5-10 nA at 10 V are about 0.4 A/W, which correspond to peak external quantum efficiencies of about 60 percent. These results indicate that very high performance photodiodes can be realized with strained layers.

  13. Development and High Temperature Property Evaluation of Ni-Co-Cr-Al Composite Electroforms

    NASA Astrophysics Data System (ADS)

    Srivastava, Meenu; Siju; Balaraju, J. N.; Ravisankar, B.

    2015-05-01

    Ni-Co-Cr-Al composite electroforms were developed with cobalt content of 10 and 40 wt.%. Cr and Al nano-particles were suspended in sulphamate electrolyte and co-deposited in the Ni-Co matrices. The surface morphology was investigated using field emission scanning electron microscope and the composition analyzed by energy-dispersive x-ray analysis. The oxidation resistance of the electroforms was studied from 600 to 1000 °C. The weight gain of Ni-10 wt.%Co-Cr-Al was less (better oxidation resistance) compared to Ni-Cr-Al and Ni-40 wt.%Co-Cr-Al. The x-ray diffraction studies revealed that the oxidation product formed on the surface of Ni-Cr-Al and Ni-10 wt.%Co-Cr-Al consisted of NiO and Al2O3, while Ni-40 wt.%Co-Cr-Al comprised oxides such as NiCo2O4, CrO3, CoO, NiO, and Al2O3. The hot corrosion behavior was investigated in 75%Na2SO4 + 25%NaCl environment at 800 °C. It was found that the hot corrosion resistance of the composite coating improved with increase in cobalt content. The probable composition suitable for high-temperature applications was found to be Ni-10 wt.%Co-Cr-Al.

  14. Electronic structure of Ga1- x Al x As nanostructures grown on the GaAs surface by ion implantation

    NASA Astrophysics Data System (ADS)

    Donaev, S. B.; Umirzakov, B. E.; Tashmukhamedova, D. A.

    2015-10-01

    The surface morphology and electronic properties of nanocrystalline phases and 2-7-nm-thick Ga1- x Al x As films grown on the GaAs(111) surface by Al+ ion implantation with subsequent (laser + thermal) annealing are studied. It is found that bandgap E g of the Ga0.5Al0.5As nanocrystalline surface phase 25-30 nm in size equals 2.8-2.9 eV.

  15. AlGaAs/InGaAlP tunnel junctions for multijunction solar cells

    SciTech Connect

    SHARPS,P.R.; LI,N.Y.; HILLS,J.S.; HOU,H.; CHANG,PING-CHIH; BACA,ALBERT G.

    2000-05-16

    Optimization of GaInP{sub 2}/GaAs dual and GaInP{sub 2}/GaAs/Ge triple junction cells, and development of future generation monolithic multi-junction cells will involve the development of suitable high bandgap tunnel junctions. There are three criteria that a tunnel junction must meet. First, the resistance of the junction must be kept low enough so that the series resistance of the overall device is not increased. For AMO, 1 sun operation, the tunnel junction resistance should be below 5 x 10{sup {minus}2} {Omega}-cm. Secondly, the peak current density for the tunnel junction must also be larger than the J{sub sc} of the cell so that the tunnel junction I-V curve does not have a deleterious effect on the I-V curve of the multi-junction device. Finally, the tunnel junction must be optically transparent, i.e., there must be a minimum of optical absorption of photons that will be collected by the underlying subcells. The paper reports the investigation of four high bandgap tunnel junctions grown by metal-organic chemical vapor deposition.

  16. Host Atom Diffusion in Ternary Fe-Cr-Al Alloys

    NASA Astrophysics Data System (ADS)

    Rohrberg, Diana; Spitzer, Karl-Heinz; Dörrer, Lars; Kulińska, Anna J.; Borchardt, Günter; Fraczkiewicz, Anna; Markus, Torsten; Jacobs, Michael H. G.; Schmid-Fetzer, Rainer

    2014-01-01

    In the Fe-rich corner of the Fe-Cr-Al ternary phase diagram, both interdiffusion experiments [1048 K to 1573 K (775 °C to 1300 °C)] and 58Fe tracer diffusion experiments [873 K to 1123 K (600 °C to 850 °C)] were performed along the Fe50Cr50-Fe50Al50 section. For the evaluation of the interdiffusion data, a theoretical model was used which directly yields the individual self-diffusion coefficients of the three constituents and the shift of the original interface of the diffusion couple through inverse modeling. The driving chemical potential gradients were derived using a phenomenological Gibbs energy function which was based on thoroughly assessed thermodynamic data. From the comparison of the individual self-diffusivities of Fe as obtained from interdiffusion profiles and independent 58Fe tracer diffusivities, the influence of the B2-A2 order-disorder transition becomes obvious, resulting in a slightly higher activation enthalpy for the bcc-B2 phase and a significantly lower activation entropy for this phase.

  17. Laterally oxidized GaInP/AlGaInP quantum well visible laser diodes

    NASA Astrophysics Data System (ADS)

    Floyd, P. D.; Sun, D.; Treat, D. W.

    1997-09-01

    High efficiency, low threshold visible AlGaInP/GaInP laser diodes using a buried AlAs native oxide for carrier and optical confinement are described. The lasers incorporate a thin AlAs layer in the upper cladding region, which when laterally wet oxidized, forms a narrow aperture. The lasers exhibited modest performance under continuous wave (cw) operation. Low temperature (400 °C) post-fabrication annealing was shown to dramatically improve the device characteristics. The lasers operate with room temperature cw threshold currents of 20 mA with external differential quantum efficiencies of 27% per facet (0.25 W/A per facet) for an uncoated 625-μm-long, 3.5-μm-wide device.

  18. Device characteristics of the PnP AlGaAs/InGaAsN/GaAs double heterojunction bipolar transistor

    SciTech Connect

    CHANG,PING-CHIH; LI,N.Y.; LAROCHE,J.R.; BACA,ALBERT G.; HOU,H.Q.; REN,F.

    2000-02-09

    The authors have demonstrated a functional PnP double heterojunction bipolar transistor (DHBT) using AlGaAs, InGaAsN, and GaAs. The band alignment between InGaAsN and GaAs has a large {triangle}E{sub C} and a negligible {triangle}E{sub V}, and this unique characteristic is very suitable for PnP DHBT applications. The metalorganic vapor phase epitaxy (MOCVD) grown Al{sub 0.3}Ga{sub 0.7}As/In{sub 0.03}Ga{sub 0.97}As{sub 0.99}N{sub 0.01}/GaAs PnP DHBT is lattice matched to GaAs and has a peak current gain of 25. Because of the smaller bandgap (Eg = 1.20 eV) of In{sub 0.03}Ga{sub 0.97}As{sub 0.99}N{sub 0.01} used for the base layer, this device has a low V{sub ON} of 0.79 V, which is 0.25 V lower than in a comparable Pnp AlGaAs/GaAs HBT. And because GaAs is used for the collector, its BV{sub CEO} is 12 V, consistent with BV{sub CEO} of AlGaAs/GaAs HBTs of comparable collector thickness and doping level.

  19. High-quality eutectic-metal-bonded AlGaAs-GaAs thin films on Si substrates

    NASA Astrophysics Data System (ADS)

    Venkatasubramanian, R.; Timmons, M. L.; Humphreys, T. P.; Keyes, B. M.; Ahrenkiel, R. K.

    1992-02-01

    Device quality GaAs-AlGaAs thin films have been obtained on Si substrates, using a novel approach called eutectic-metal-bonding (EMB). This involves the lattice-matched growth of GaAs-AlGaAs thin films on Ge substrates, followed by bonding onto a Si wafer. The Ge substrates are selectively removed by a CF4/O2 plasma etch, leaving high-quality GaAs-AlGaAs thin films on Si substrates. A minority-carrier lifetime of 103 ns has been obtained in a EMB GaAs-AlGaAs double heterostructure on Si, which is nearly forty times higher than the state-of-the-art lifetime for heteroepitaxial GaAs on Si, and represents the largest reported minority-carrier lifetime for a freestanding GaAs thin film. In addition, a negligible residual elastic strain in the EMB GaAs-AlGaAs films has been determined from Raman spectroscopy measurements.

  20. AlGaN/GaN field effect transistors for power electronics—Effect of finite GaN layer thickness on thermal characteristics

    SciTech Connect

    Hodges, C. Anaya Calvo, J.; Kuball, M.; Stoffels, S.; Marcon, D.

    2013-11-11

    AlGaN/GaN heterostructure field effect transistors with a 150 nm thick GaN channel within stacked Al{sub x}Ga{sub 1−x}N layers were investigated using Raman thermography. By fitting a thermal simulation to the measured temperatures, the thermal conductivity of the GaN channel was determined to be 60 W m{sup −1} K{sup −1}, over 50% less than typical GaN epilayers, causing an increased peak channel temperature. This agrees with a nanoscale model. A low thermal conductivity AlGaN buffer means the GaN spreads heat; its properties are important for device thermal characteristics. When designing power devices with thin GaN layers, as well as electrical considerations, the reduced channel thermal conductivity must be considered.

  1. Effect of stress on the Al composition evolution in AlGaN grown using metal organic vapor phase epitaxy

    NASA Astrophysics Data System (ADS)

    He, Chenguang; Qin, Zhixin; Xu, Fujun; Zhang, Lisheng; Wang, Jiaming; Hou, Mengjun; Zhang, Shan; Wang, Xinqiang; Ge, Weikun; Shen, Bo

    2016-05-01

    Two series of AlGaN samples with different stresses were designed to investigate the effect of stress on the Al composition. X-ray diffraction reciprocal space mapping (XRD RSM) demonstrated that the AlGaN epilayers with different stresses have large Al composition differences despite the same growth conditions. The largest Al composition difference reached up to 21.3%, which was also confirmed using secondary ion mass spectroscopy (SIMS). This result is attributed to a large stress discrepancy in the AlGaN epilayers. Finally, the dependences of the solid-phase Al composition on the gas-phase Al composition under different stresses were systematically analyzed.

  2. Hot corrosion of Co-Cr, Co-Cr-Al, and Ni-Cr alloys in the temperature range of 700-750 deg C

    NASA Technical Reports Server (NTRS)

    Chiang, K. T.; Meier, G. H.

    1980-01-01

    The effect of SO3 pressure in the gas phase on the Na2SO4 induced hot corrosion of Co-Cr, Ni-Cr, and Co-Cr-Al alloys was studied in the temperature range 700 to 750 C. The degradation of the Co-Cr and Ni-Cr alloys was found to be associated with the formation of liquid mixed sulfates (CoSO4-Na2SO4 or NiSO4-Na2SO4) which provided a selective dissolution of the Co or Ni and a subsequent sulfidation oxidation mode of attack which prevented the maintenance of a protective Cr2O3 film. A clear mechanism was not developed for the degradation of Co-Cr-Al alloys. A pitting corrosion morphology was induced by a number of different mechanisms.

  3. Effect of Doping Profile and Concentration on the Near-Infrared Optical Properties of AlGaN/GaN and AlInN/GaN Heterostructures

    NASA Astrophysics Data System (ADS)

    Cervantes, Mayra; Edmunds, Colin; Li, Donghui; Tang, Liang; Shao, Jiayi; Gardner, Geoff; Manfra, Michael; Malis, Oana

    2012-02-01

    Intersubband (ISB) devices utilizing III-nitrides have recently attracted attention for near- and far- infrared optoelectronic applications. In order to achieve efficient ISB transitions, large doping densities are typically required (>1E18 cm-3). The large impurity density has significant effects on the band structure and material quality, effects that are reflected in important device parameters such as transition energies and linewidths. To determine the optimal doping concentration and profile for III-N intersubband devices, we carried out a systematic study of optical and structural properties of strained AlGaN/GaN and lattice-matched AlInN/GaN heterostructures grown by MBE on quasi-bulk GaN substrates. The lattice-matched AlInN/GaN system is targeted because it allows growth of thick strain-free materials. However, it also presents some considerable growth challenges due to the vastly different optimal growth conditions for Al and In containing nitrides. The transition energy and line profile were determined by direct and photoinduced absorption measurements, while the material quality was assessed using TEM and high resolution x-ray diffraction. The FWHM of the ISB transition at 1.9 μm was found to vary up to 60% with the position of delta doping in the quantum well.

  4. Lateral and Vertical Transistors Using the AlGaN/GaN Heterostructure

    SciTech Connect

    Chowdhury, S; Mishra, UK

    2013-10-01

    Power conversion losses are endemic in all areas of electricity consumption, including motion control, lighting, air conditioning, and information technology. Si, the workhorse of the industry, has reached its material limits. Increasingly, the lateral AlGaN/GaN HEMT based on gallium nitride (GaN-on-Si) is becoming the device of choice for medium power electronics as it enables high-power conversion efficiency and reduced form factor at attractive pricing for wide market penetration. The reduced form factor enabled by high-efficiency operation at high frequency further enables significant system price reduction because of savings in bulky extensive passive elements and heat sink costs. The high-power market, however, still remains unaddressed by lateral GaN devices. The current and voltage demand for high power conversion application makes the chip area in a lateral topology so large that it becomes more difficult to manufacture. Vertical GaN devices would play a big role alongside of silicon carbide (SiC) to address the high power conversion needs. In this paper, the development, performance, and status of lateral and vertical GaN devices are discussed.

  5. Photoreflectance and photoluminescence study of defect passivation by hydrogen in GaAlAs/GaAs/GaAs heterostructures

    NASA Astrophysics Data System (ADS)

    Capizzi, Mario; Coluzza, Carlo; Frankl, P.; Frova, Andrea; Yin, X.; Pollak, Fred H.; Sacks, Robert N.

    1990-08-01

    Using both photoreflectance (PR) at 80 and 300 K and photoluminescence (PL) at 77 K, we have investigated the passivation by H-gun treatment of the surface, interface and volume defects in Ga83Al017As/GaAs/GaAs MBE-grown heterostructure on LEC substrate. Both amplitude of PR and phase delay angle between laser excitation and response were measured. After H-treatment, substantial changes were observed in all properties. Room temperature PR is indicative of the ready disappearance of surface and interface defects at early stages of hydrogenation. Deep trap passivation in the bulk gives rise to increased PL emission in both GaA1As and GaAs, the effect being large only for the latter. A concomitant low in the phase delay angle is detected. The optimum occurs when the total dose of H ions reaching the surface is 1017 cm2. When the largest doses of H are attained, a high density of new bulk defects develops in GaAs, which virtually wipe-out luminescence and reinstate a large phase delay.

  6. Prediction of semiconducting behavior in minority spin of Co2CrZ (Z = Ga, Ge, As): LSDA

    NASA Astrophysics Data System (ADS)

    Rai, D. P.; Thapa, R. K.

    2012-12-01

    Volume optimization was performed to obtain the theoretical lattice constants by using the generalized gradient approximation (GGA). The electronic and magnetic properties of Heusler alloys Co2CrZ (Z = Ga, Ge, As) were investigated by using local spin density approximation (LSDA). Amongst the systems under investigation, Co2CrGe and Co2CrGa give 100% spin polarization at the Fermi level (EF). Co2CrGe and Co2CrGa are the most stable half-metallic ferromagnets (HMFs); their EF lie exactly at the gap of 0.24 eV and 0.38 eV, respectively, in the spin-down channel. Even though Co2CrAs gives a distinct and bigger gap as compared to Co2CrGa and Co2CrGe, its EF is not located at the middle of the gap in the spin-down channel. We have also found that the total magnetic moments increase as the Z goes from Ga to As. The calculated density of states and band structures show the HMF character for Co2CrGe and Co2CrGa.

  7. State-of-the-art performance of GaAlAs/GaAs avalanche photodiodes

    NASA Technical Reports Server (NTRS)

    Law, H. D.; Nakano, K.; Tomasetta, L. R.

    1979-01-01

    Ga(0.15)Al(0.85)As/GaAs avalanche photodiodes have been successfully fabricated. The performance of these detectors is characterized by a rise time of less than 35 ps, an external quantum efficiency with an antireflection coating of 95% at 0.53 microns, and a microwave optical gain of 42 dB. The dark current density is in the low range (10 to the minus A/sq cm) at one-half the breakdown voltages, and rises to 0.0001 A/sq cm at 42 dB optical gain.

  8. A field induced guide-antiguide modulator of GaAs-AlGaAs

    NASA Technical Reports Server (NTRS)

    Huang, T. C.; Chung, Y.; Young, D. B.; Dagli, N.; Coldren, L. A.

    1991-01-01

    A guide-antiguide modulator of GaAs-AlGaAs using the electric-field-induced waveguide concept was demonstrated. The device was formed with a central waveguide electrode sandwiched between two antiguide electrodes on the surface of a p-i-n multiple quantum well (MQW). Switching between lateral guiding and antiguiding was accomplished by reverse biasing either the central electrode or the adjacent electrodes to increase the index beneath these respective regions. The on-off ratio was measured to be 20:1 with a propagation loss of the on-state of about 5 dB/mm.

  9. Simplified gas sensor model based on AlGaN/GaN heterostructure Schottky diode

    SciTech Connect

    Das, Subhashis Majumdar, S.; Kumar, R.; Bag, A.; Chakraborty, A.; Biswas, D.

    2015-08-28

    Physics based modeling of AlGaN/GaN heterostructure Schottky diode gas sensor has been investigated for high sensitivity and linearity of the device. Here the surface and heterointerface properties are greatly exploited. The dependence of two dimensional electron gas (2DEG) upon the surface charges is mainly utilized. The simulation of Schottky diode has been done in Technology Computer Aided Design (TCAD) tool and I-V curves are generated, from the I-V curves 76% response has been recorded in presence of 500 ppm gas at a biasing voltage of 0.95 Volt.

  10. High-efficiency AlGaAs-GaAs Cassegrainian concentrator cells

    NASA Technical Reports Server (NTRS)

    Werthen, J. G.; Hamaker, H. C.; Virshup, G. F.; Lewis, C. R.; Ford, C. W.

    1985-01-01

    AlGaAs-GaAs heteroface space concentrator solar cells have been fabricated by metalorganic chemical vapor deposition. AMO efficiencies as high as 21.1% have been observed both for p-n and np structures under concentration (90 to 100X) at 25 C. Both cell structures are characterized by high quantum efficiencies and their performances are close to those predicted by a realistic computer model. In agreement with the computer model, the n-p cell exhibits a higher short-circuit current density.

  11. Simplified gas sensor model based on AlGaN/GaN heterostructure Schottky diode

    NASA Astrophysics Data System (ADS)

    Das, Subhashis; Majumdar, S.; Kumar, R.; Chakraborty, A.; Bag, A.; Biswas, D.

    2015-08-01

    Physics based modeling of AlGaN/GaN heterostructure Schottky diode gas sensor has been investigated for high sensitivity and linearity of the device. Here the surface and heterointerface properties are greatly exploited. The dependence of two dimensional electron gas (2DEG) upon the surface charges is mainly utilized. The simulation of Schottky diode has been done in Technology Computer Aided Design (TCAD) tool and I-V curves are generated, from the I-V curves 76% response has been recorded in presence of 500 ppm gas at a biasing voltage of 0.95 Volt.

  12. Near-Infrared Absorption in Lattice-Matched AlInN/GaN and Strained AlGaN/GaN Heterostructures Grown by MBE on Low-Defect GaN Substrates

    NASA Astrophysics Data System (ADS)

    Edmunds, C.; Tang, L.; Li, D.; Cervantes, M.; Gardner, G.; Paskova, T.; Manfra, M. J.; Malis, O.

    2012-05-01

    We have investigated near-infrared absorption and photocurrent in lattice-matched AlInN/GaN and strained AlGaN/GaN heterostructures grown by molecular-beam epitaxy (MBE) on low-defect GaN substrates for infrared device applications. The AlGaN/GaN heterostructures were grown under Ga-rich conditions at 745°C. Material characterization via atomic force microscopy and high-resolution x-ray diffraction indicates that the AlGaN/GaN heterostructures have smooth and well-defined interfaces. A minimum full-width at half-maximum of 92 meV was obtained for the width of the intersubband absorption peak at 675 meV of a 13.7 Å GaN/27.5 Å Al0.47Ga0.53N superlattice. The variation of the intersubband absorption energy across a 1 cm × 1 cm wafer was ±1%. An AlGaN/GaN-based electromodulated absorption device and a quantum well infrared detector were also fabricated. Using electromodulated absorption spectroscopy, the full-width at half-maximum of the absorption peak was reduced by 33% compared with the direct absorption measurement. This demonstrates the suitability of the electromodulated absorption technique for determining the intrinsic width of intersubband transitions. The detector displayed a peak responsivity of 195 μA/W at 614 meV (2.02 μm) without bias. Optimal MBE growth conditions for lattice-matched AlInN on low-defect GaN substrates were also studied as a function of total metal flux and growth temperature. A maximum growth rate of 3.8 nm/min was achieved while maintaining a high level of material quality. Intersubband absorption in AlInN/GaN superlattices was observed at 430 meV with full-width at half-maximum of 142 meV. Theoretical calculations of the intersubband absorption energies were found to be in agreement with the experimental results for both AlGaN/GaN and AlInN/GaN heterostructures.

  13. Al-augite and Cr-diopside ultramafic xenoliths in basaltic rocks from western United States

    USGS Publications Warehouse

    Wilshire, H.G.; Shervais, J.W.

    1975-01-01

    Ultramafic xenoliths in basalts from the western United States are divided into Al-augite and Cr-diopside groups. The Al-augite group is characterized by Al, Ti-rich augites, comparatively Fe-rich olivine and orthopyroxene, and Al-rich spinel, the Cr-diopside group by Cr-rich clinopyroxene and spinel and by Mg-rich olivine and pyroxenes. Both groups have a wide range of subtypes, but the Al-augite group is dominated by augite-rich varieties, and the Cr-diopside group by olivine-rich lherzolites. ?? 1975.

  14. Process development for Ni-Cr-ThO2 and Ni-Cr-Al-ThO2 sheet

    NASA Technical Reports Server (NTRS)

    Cook, R. C.; Norris, L. F.

    1973-01-01

    A process was developed for the production of thin gauge Ni-Cr-ThO2 sheet. The process was based on the elevated temperature deposition of chromium onto a wrought Ni-2%ThO2 sheet and subsequent high temperature diffusion heat treatments to minimize chromium concentration gradients within the sheet. The mechanical properties of the alloy were found to be critically dependent on those of the Ni-2%ThO2 sheet. A similar process for the production of a Ni-Cr-Al-ThO2 alloy having improved oxidation resistance was investigated but the non-reproducible deposition of aluminum from duplex Cr/Al packs precluded successful scale-up. The mechanical properties of the Ni-Cr-Al-ThO2 alloys were generally equivalent to the best Ni-Cr-ThO2 alloy produced in the programme.

  15. A sulfur segregation study of PWA 1480, NiCrAl, and NiAl alloys

    NASA Technical Reports Server (NTRS)

    Jayne, D. T.; Smialek, J. L.

    1993-01-01

    Some nickel based superalloys show reduced oxidation resistance from the lack of an adherent oxide layer during high temperature cyclic oxidation. The segregation of sulfur to the oxide-metal interface is believed to effect oxide adhesion, since low sulfur alloys exhibit enhanced adhesion. X ray Photoelectron Spectroscopy (XPS) was combined with an in situ sample heater to measure sulfur segregation in NiCrAl, PWA 1480, and NiAl alloys. The polished samples with a 1.5 to 2.5 nm (native) oxide were heated from 650 to 1100 C with hold times up to 6 hr. The sulfur concentration was plotted as a function of temperature versus time at temperature. One NiCrAl sulfur study was performed on the same casting used by Browning to establish a base line between previous Auger Electron Spectroscopy (AES) results and the XPS results of this study. Sulfur surface segregation was similar for PWA 1480 and NiCrAl and reached a maximum of 30 at% at 800 to 850 C. Above 900 C the sulfur surface concentration decreased to about 3 at% at 1100 C. These results are contrasted to the minimal segregation observed for low sulfur hydrogen annealed materials which exhibit improved scale adhesion.

  16. Carrier Localization and Decay in Wide-band-gap AlGaN/AlGaN Quantum Wells

    NASA Astrophysics Data System (ADS)

    Tamulaitis, G.; Mickevičius, J.; Kazlauskas, K.; Žukauskas, A.; Kuokštis, E.; Shur, M. S.; Yang, J.; Gaska, R.

    2010-01-01

    Carrier dynamics in wide-band-gap AlGaN/AlGaN multiple quantum wells (MQWs) has been investigated by studying a set of MQWs with well widths varying from 1.65 to 5.0 nm. The structures were grown by metal-organic chemical vapor deposition and investigated using photoluminescence spectroscopy under transient and quasi-steady-state conditions and light-induced transient grating technique. It is demonstrated that the fast initial PL intensity decay is caused by recovery of built-in electric field, while the carrier lifetime is strongly influenced by carrier localization. The scale of the localization was estimated by fitting the results of Monte Carlo simulation of exciton hopping with experimental PL linewidth dependence on temperature.

  17. Investigation of InGaP/(In)AlGaAs/GaAs triple-junction top cells for smart stacked multijunction solar cells grown using molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Sugaya, Takeyoshi; Mochizuki, Toru; Makita, Kikuo; Oshima, Ryuji; Matsubara, Koji; Okano, Yoshinobu; Niki, Shigeru

    2015-08-01

    We report high-quality InGaP/(In)AlGaAs/GaAs triple-junction solar cells fabricated using solid-source molecular beam epitaxy (MBE) for the first time. The triple-junction cells can be used as top cells for smart stacked multijunction solar cells. A growth temperature of 480 °C was found to be suitable for an (In)AlGaAs second cell to obtain high-quality tunnel junctions. The properties of AlGaAs solar cells were better than those of InAlGaAs solar cells when a second cell was grown at 480 °C. The high-quality InGaP/AlGaAs/GaAs solar cell had an impressive open-circuit voltage of 3.1 V. This result indicates that high-performance InGaP/AlGaAs/GaAs triple-junction solar cells can be fabricated using solid-source MBE.

  18. Nanoscale Cellular Structures at Phase Boundaries of Ni-Cr-Al-Ti and Ni-Cr-Mo-Al-Ti Superalloys

    NASA Astrophysics Data System (ADS)

    Wang, Cong; Dunand, David C.

    2015-06-01

    The microstructural evolution of Ni-20 pct Cr wires was studied during pack cementation where Al and Ti, with and without prior cementation with Mo, are deposited to the surface of the Ni-Cr wires and subsequently homogenized in their volumes. Mo deposition promotes the formation of Kirkendall pores and subsequent co-deposition of Al and Ti creates a triple-layered diffusional coating on the wire surface. Subsequent homogenization drives the alloying element to distribute evenly in the wires which upon further heat treatment exhibit the γ + γ' superalloy structure. Unexpectedly, formation of cellular structures is observed at some of the boundaries between primary γ' grains and γ matrix grains. Based on additional features ( i.e., ordered but not perfectly periodic structure, confinement at γ + γ' phase boundaries as a cellular film with ~100 nm width, as well as lack of topologically close-packed phases), and considering that similar, but much larger, microstructures were reported in commercial superalloys, it is concluded that the present cellular structure solidified as a thin film, composed of eutectic γ + γ' and from which the γ' phase was subsequently etched, which was created by incipient melting of a region near the phase boundary with high solute segregation.

  19. Structural and magnetic impact of Cr+-implantation into GaN thin film

    NASA Astrophysics Data System (ADS)

    Husnain, G.; Shu-De, Yao; Ahmad, Ishaq; Rafique, H. M.

    2012-06-01

    Thin films of GaN with thickness of 2 μm were synthesized on sapphire. Cr+ ions were implanted into GaN with150 keV energy at a fluence of 3 × 1015 cm-2. The annealing of the samples was carried out for a short time using rapid thermal annealing (RTA). Structural properties of the implanted samples were undertaken by XRD and Rutherford backscattering. The annealed samples demonstrated lattice recovery and damages caused by implantation. The structural properties were also studied by High-resolution X-ray Diffraction (HRXRD). Magnetic measurements of the samples were performed by Alternating Gradient Magnetometer (AGM) at room temperature and by SQUID in the range of 5-380 K. The SQUID results showed ferromagnetic behavior at T = 5 K and above 380 K for Cr+-implanted GaN.

  20. Integrating AlGaN/GaN high electron mobility transistor with Si: A comparative study of integration schemes

    NASA Astrophysics Data System (ADS)

    Mohan, Nagaboopathy; Singh, Manikant; Soman, Rohith; Raghavan, Srinivasan

    2015-10-01

    AlGaN/GaN high electron mobility transistor stacks deposited on a single growth platform are used to compare the most common transition, AlN to GaN, schemes used for integrating GaN with Si. The efficiency of these transitions based on linearly graded, step graded, interlayer, and superlattice schemes on dislocation density reduction, stress management, surface roughness, and eventually mobility of the 2D-gas are evaluated. In a 500 nm GaN probe layer deposited, all of these transitions result in total transmission electron microscopy measured dislocations densities of 1 to 3 × 109/cm2 and <1 nm surface roughness. The 2-D electron gas channels formed at an AlGaN-1 nm AlN/GaN interface deposited on this GaN probe layer all have mobilities of 1600-1900 cm2/V s at a carrier concentration of 0.7-0.9 × 1013/cm2. Compressive stress and changes in composition in GaN rich regions of the AlN-GaN transition are the most effective at reducing dislocation density. Amongst all the transitions studied the step graded transition is the one that helps to implement this feature of GaN integration in the simplest and most consistent manner.

  1. Integrating AlGaN/GaN high electron mobility transistor with Si: A comparative study of integration schemes

    SciTech Connect

    Mohan, Nagaboopathy; Raghavan, Srinivasan; Manikant,; Soman, Rohith

    2015-10-07

    AlGaN/GaN high electron mobility transistor stacks deposited on a single growth platform are used to compare the most common transition, AlN to GaN, schemes used for integrating GaN with Si. The efficiency of these transitions based on linearly graded, step graded, interlayer, and superlattice schemes on dislocation density reduction, stress management, surface roughness, and eventually mobility of the 2D-gas are evaluated. In a 500 nm GaN probe layer deposited, all of these transitions result in total transmission electron microscopy measured dislocations densities of 1 to 3 × 10{sup 9}/cm{sup 2} and <1 nm surface roughness. The 2-D electron gas channels formed at an AlGaN-1 nm AlN/GaN interface deposited on this GaN probe layer all have mobilities of 1600–1900 cm{sup 2}/V s at a carrier concentration of 0.7–0.9 × 10{sup 13}/cm{sup 2}. Compressive stress and changes in composition in GaN rich regions of the AlN-GaN transition are the most effective at reducing dislocation density. Amongst all the transitions studied the step graded transition is the one that helps to implement this feature of GaN integration in the simplest and most consistent manner.

  2. Liquid phase sensors based on chemically functionalized GaAs/AlGaAs heterostructures

    NASA Astrophysics Data System (ADS)

    Luber, S. M.; Adlkofer, K.; Rant, U.; Ulman, A.; Gölzhäuser, A.; Grunze, M.; Schuh, D.; Tanaka, M.; Tornow, M.; Abstreiter, G.

    2004-03-01

    We report on surface-near two-dimensional electron gases in GaAs/AlGaAs heterostructures for application in potential chemical or biochemical sensors in liquid environment. GaAs cap layers of the heterostructures were coated with self-assembled monolayers of 4‧-substituted 4-mercaptobiphenyls (MBP), showing stable device performance in physiological (aqueous) buffers. Deposition of MBP with different 4‧-substituents led to systematic changes in the lateral resistance, which can be correlated to the electrical potential drop across the established surface dipole layers. Furthermore, the lateral resistance showed a clear dependency on organic solvents with different polarities, suggesting its high sensitivity to the polarity of physisorbed molecules.

  3. Terahertz intersubband photodetectors based on semi-polar GaN/AlGaN heterostructures

    NASA Astrophysics Data System (ADS)

    Durmaz, Habibe; Nothern, Denis; Brummer, Gordie; Moustakas, Theodore D.; Paiella, Roberto

    2016-05-01

    Terahertz intersubband photodetectors are developed based on GaN/AlGaN quantum wells grown on a free-standing semi-polar ( 20 2 ¯ 1 ¯ ) GaN substrate. These quantum wells are nearly free of the polarization-induced internal electric fields that severely complicate the design of nitride intersubband devices on traditional c-plane substrates. As a result, a promising bound-to-quasi-bound THz photodetector design can be implemented. Pronounced photocurrent peaks at the design frequency near 10 THz are measured, covering frequencies that are fundamentally inaccessible to existing arsenide intersubband devices due to reststrahlen absorption. This materials system provides a favorable platform to utilize the intrinsic advantages of nitride semiconductors for THz optoelectronics.

  4. Photon degradation of AlGaAs/GaAs solar cells

    NASA Technical Reports Server (NTRS)

    Anspaugh, Bruce; Kachare, Ram; Iles, Peter

    1988-01-01

    The behavior of GaAs solar cells after photon illumination for a prolonged exposure time is presented. More than 55 OMCVD AlGaAs/GaAs solar cells were exposed for over 400 h to AM0 photons at 28 C in three separate, well-controlled runs. Significant degradation of solar cell efficiency was observed in two out of three runs. Although noticeable losses in the open-circuit voltage, fill factor, and maximum power were observed, no change in the short-circuit current was found. In one of the runs, no change was seen in either the test cells or the control cells. Each cell in this run was protected with a coverglass. The cells in this run had thicker buffer layers and thinner window layers than the cells in the other two runs.

  5. Photoluminescence of GaAs/AlGaAs quantum ring arrays

    SciTech Connect

    Sibirmovskii, Yu. D. Vasil’evskii, I. S.; Vinichenko, A. N.; Eremin, I. S.; Zhigunov, D. M.; Kargin, N. I.; Kolentsova, O. S.; Martyuk, P. A.; Strikhanov, M. N.

    2015-05-15

    Samples of epitaxial structures with GaAs/AlGaAs quantum rings different in morphology are grown by droplet epitaxy. The photoluminescence spectra of the samples are recorded at temperatures of 20–90 and 300 K. Intense peaks defined by quantum confinement of the charge-carrier energy in the quantum rings are observed in the optical region. The peaks are identified by estimating the energy of the ground state of electrons and holes in GaAs quantum rings and by recording the spectra of the samples after removing the layers with the quantum rings by etching. The average dimensions of the quantum rings are determined by atomic force microscopy and scanning electron microscopy. Some inferences about the factors that influence the emission spectrum and intensity of the epitaxial structures with quantum rings are drawn.

  6. AlGaAs-GaAs solar cell development - Electrical analysis and prospects for space application

    NASA Astrophysics Data System (ADS)

    Therez, F.; Alcubilla, R.; Chikouche, A.

    1986-08-01

    AlGaAs-GaAs solar cells fabricated using an improved liquid phase epitaxy process involving saturated melts show improved epitaxial layer uniformity, and the 2kT direct current in the GaAs diodes has been analyzed to determine its dependence on surface and perimeter values. Photovoltaic samples demonstrated a short-circuit current density of 24.7 mA/sq cm, a fill factor of 0.81, and an open circuit voltage of 970 mV. An efficiency of 20.2 percent was found under 400 mW/sq cm incident light power and AM 1.5 spectral conditions, while the AM 0 spectrum gave an efficiency of 17.2 percent. Short-circuit current degradations have been obtained in terms of junction depths.

  7. Observation of 8600 K electron temperature in AlGaN/GaN high electron mobility transistors on Si substrate

    NASA Astrophysics Data System (ADS)

    Narita, Tomotaka; Fujimoto, Yuichi; Wakejima, Akio; Egawa, Takashi

    2016-03-01

    The electron temperature (T e) in AlGaN/GaN high electron mobility transistors (HEMTs) on Si was studied by spectroscopic measurements of its electroluminescence (EL). The EL spectrum has been followed by the Maxwell-Boltzmann distribution and no signal at equivalent energy as a band-gap of GaN has been observed. These experimental results imply that the EL is dominated by an intra-band transition. The highest T e of 8600 K in AlGaN/GaN HEMTs was extracted at the drain voltage of 60 V. The experimental results are in agreement with results previously predicted by a Monte Carlo simulation.

  8. Chemical beam epitaxy growth of AlGaAs/GaAs tunnel junctions using trimethyl aluminium for multijunction solar cells

    SciTech Connect

    Paquette, B.; DeVita, M.; Turala, A.; Kolhatkar, G.; Boucherif, A.; Jaouad, A.; Aimez, V.; Arès, R.; Wilkins, M.; Wheeldon, J. F.; Walker, A. W.; Hinzer, K.; Fafard, S.

    2013-09-27

    AlGaAs/GaAs tunnel junctions for use in high concentration multijunction solar cells were designed and grown by chemical beam epitaxy (CBE) using trimethyl aluminium (TMA) as the p-dopant source for the AlGaAs active layer. Controlled hole concentration up to 4⋅10{sup 20} cm{sup −3} was achieved through variation in growth parameters. Fabricated tunnel junctions have a peak tunneling current up to 6140 A/cm{sup 2}. These are suitable for high concentration use and outperform GaAs/GaAs tunnel junctions.

  9. Recent advances of high voltage AlGaN/GaN power HFETs

    NASA Astrophysics Data System (ADS)

    Uemoto, Yasuhiro; Ueda, Tetsuzo; Tanaka, Tsuyoshi; Ueda, Daisuke

    2009-02-01

    We review our recent advances of GaN-based high voltage power transistors. These are promising owing to low on-state resistance and high breakdown voltage taking advantages of superior material properties. However, there still remain a couple of technical issues to be solved for the GaN devices to replace the existing Si-based power devices. The most critical issue is to achieve normally-off operation which is strongly desired for the safety operation, however, it has been very difficult because of the built-in polarization electric field. Our new device called GIT (Gate Injection Transistor) utilizing conductivity modulation successfully achieves the normally-off operation keeping low on-state resistance. The fabricated GIT on a Si substrate exhibits threshold voltage of +1.0V. The obtained on-state resistance and off-state breakdown voltage were 2.6mΩ•cm2 and 800V, respectively. Remaining technical issue is to further increase the breakdown voltage. So far, the reported highest off-state breakdown voltage of AlGaN/GaN HFETs has been 1900V. Overcoming these issues by a novel device structure, we have demonstrated the world highest breakdown voltages of 10400V using thick poly-crystalline AlN as a passivation film and Via-holes through sapphire which enable very efficient layout of the lateral HFET array avoiding any undesired breakdown of passivation films. Since conventional wet or dry etching cannot be used for chemically stable sapphire, high power pulsed laser is used to form the via-holes. The presented GaN power devices demonstrate that GaN is advantageous for high voltage power switching applications replacing currently used Si-based power MOSFETs and IGBTs.

  10. NiAl(110)/Cr(110) interface: A density functional theory study

    NASA Astrophysics Data System (ADS)

    Liu, W.; Li, J. C.; Zheng, W. T.; Jiang, Q.

    2006-05-01

    The optimal geometries, thermodynamic properties, and electronic structures of NiAl(110)/Cr(110) interface are studied using a first-principle density functional plane-wave ultrasoft pseudopotential method. Surface energies of different NiAl surfaces are compared with those obtained based on the classical broken-bond rule. Simulation results indicate that the structure of Ni and Al placed in the hollow sites of Cr atoms at the interface is more thermodynamically stable, and the NiCr bonding is dominated by 3d electrons of Ni and Cr. It is found that NiAl(110)/Cr(110) alloying could lower brittleness of NiAl compounds. With simulated values of adhesion work and interface energy for NiAl(110)/Cr(110) system, its mechanical and thermodynamic properties are also discussed.

  11. Improved high-temperature characteristics of a symmetrically graded AlGaAs/InxGa1-xAs/AlGaAs pHEMT

    NASA Astrophysics Data System (ADS)

    Huang, Jun-Chin; Hsu, Wei-Chou; Lee, Ching-Sung; Chang, Wei-Chen; Huang, Dong-Hai

    2006-12-01

    This work investigates the superior high-temperature and high-linearity characteristics of a double δ-doped AlGaAs/InxGa1-xAs/AlGaAs pseudomorphic high electron mobility transistor (pHEMT) with a symmetrically linearly graded InxGa1-xAs channel and a wide energy gap AlGaAs barrier. Distinguished high-temperature device characteristics are presented, including an extrinsic transconductance (gm,max) of 182 (223) mS mm-1, a drain-source saturation current density (IDSS) of 428 (524) mA mm-1, an output conductance of 0.334 (0.352) mS mm-1, a gate-voltage swing (GVS) of 1.45 (1.5) V, a voltage gain (Av) of 505 (658) and a reverse breakdown voltage (BVGD) of -24.1 (-31.2) V at 500 (300) K, respectively, with gate dimensions of 0.65 × 200 µm2. In addition, the device demonstrates a superior stable thermal threshold coefficient (∂Vth/∂T) of -0.55 mV K-1, a thermal GVS coefficient (∂GVS/∂T) of -0.25 mV K-1 and a wide gate-bias range of 1.25 V for a unity-gain cut-off frequency (ft) of over 20 GHz. Consequently, the proposed device shows good potential for high-temperature and high-linearity circuit applications.

  12. Manipulation of emission energy in GaAs/AlGaAs core-shell nanowires with radial heterostructure

    SciTech Connect

    Barbosa, B. G.; Arakaki, H.; Souza, C. A. de; Pusep, Yu. A.

    2014-03-21

    Photoluminescence was studied in GaAs/AlGaAs nanowires (NWs) with different radial heterostructures. We demonstrated that manipulation of the emission energy may be achieved by appropriate choice of the shell structure. The emission at highest energy is generated in the NWs with tunneling thin AlGaAs inner shell and thin GaAs outer shell due to recombination of the photoexcited electrons confined in the outer shell with the holes in the core. Lower energy emission was shown to occur in the NWs with thick outer shell grown in the form of a short-period GaAs/AlGaAs multiple quantum well structure. In this case, the tunneling probability through the multiple quantum wells controls the energy emitted by the NWs. The doping of core results in dominated low energy emission from the GaAs core.

  13. Rapid fabrication and packaging of AlGaN/GaN high-temperature ultraviolet photodetectors using direct wire bonding

    NASA Astrophysics Data System (ADS)

    So, Hongyun; Senesky, Debbie G.

    2016-07-01

    Cost-effective fabrication and rapid packaging of AlGaN/GaN ultraviolet (UV) photodetectors was demonstrated using direct wire bonding between aluminum wires and a GaN surface. The fabricated photodetectors showed stable dark current levels through the highly conductive 2D electron gas (2DEG), which was electrically connected to aluminum bonding wires. At room temperature, the current passing through the 2DEG rapidly increased upon exposure to UV light because of the generated electrons excited in the AlGaN/GaN layers. In addition, the devices showed consistent and reliable operation at high temperatures up to 100 °C with mechanically stable bonding wires (pull strength of 3–5.2 gram-force), supporting the use of direct wire bonding techniques to fabricate simple AlGaN/GaN sensors for UV detection within harsh environments, such as downhole and space exploration applications.

  14. Ga, Ca, and 3d transition element (Cr through Zn) partitioning among spinel-lherzolite phases from the Lanzo massif, Italy: Analytical results and crystal chemistry

    SciTech Connect

    Wogelius, R.A.; Fraser, D.G.

    1994-06-01

    Ultramafic rocks exposed in Lanzo massif, Italy is a record of mantle geochemistry, melting, sub-solidus re-equilibration. Plagioclase(+ spinel)-lherzolite samples were analyzed by Scanning Proton Microscopy, other techniques. Previous work postulated partial melting events and a two-stage sub-solidus cooling history; this paper notes Ga enrichment on spinel-clinopyroxene grain boundaries, high Ga and transition element content of spinel, and pyroxene zonation in Ca and Al. Trace element levels in olivine and orthopyroxene are also presented. Zoning trends are interpreted as due to diffusion during cooling. Olivine-clinopyroxene Cr and Ca exchange as well as clinopyroxene and spinel zonation trends indicate that the massif experienced at least two sub-solidus cooling episodes, one at 20 kbar to 1000 C and one at 8 kbar <750C. Ga levels in cores of Lanzo high-Cr spinels are high (82-66 ppM) relative to other mantle spinels (66-40 ppM), indicating enrichment. Ga content of ultramafic spinels apparently increases with Cr content; this may be due to: increased Ga solubility stemming from crystal chemical effects and/or higher Ga activities in associated silicate melts. Thus, during melting, high-Cr residual spinel may tend to buffer solid-phase Ga level. These spinels are not only rich in Ga and Cr (max 26.37 el. wt %), but also in Fe (max 21.07 el. wt %), Mn (max 3400 ppM), and Zn (max 2430 ppM). These enrichments are again due to melt extraction and partitioning into spinel structure. Low Ni (min 1050 ppM) levels are due to unsuccessful competition of Ni with Cr for octahedral structural sites caused by crystal field. Comparisons of change in partitioning vs Cr content among several 3d transition elements for spinels from Lanzo, other localities allow us to separate crystal field effects from bulk chemical effects and to show that in typical assemblages, inversion of olivine-spinel partition coefficient for Ni from <1 to >1 should occur at 11% el. wt. Cr in spinel.

  15. Gate metal dependent electrical characteristics of AlGaN/GaN HEMTs

    SciTech Connect

    Koo, Sang-Mo Kang, Min-Seok

    2014-10-15

    Highlights: • We investigated transfer characteristics of AlGaN/GaN high electron mobility transistors. • We demonstrate the effect of the barrier height of Schottky gate metals. • The conduction mechanisms examine by comparing the experimental results with numerical simulations. • 2-DEG concentration depends on the barrier height of Schottky gate metals. - Abstract: We investigated transfer characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) and the effect of the barrier height of Schottky gate metals. It is found that the threshold voltage of the HEMT structures with the Ni Schottky contact shows a positive shift compared to that of the Ti Schottky contacts (ΔV{sub th} = 2.9 V). The maximum saturation current of the HEMT structures with the Ti Schottky contact (∼1.4 × 10{sup 7} A/cm{sup 2}) is found to be ∼2.5 times higher than that of the Ni Schottky contact (2.9 × 10{sup 7} A/cm{sup 2}). The conduction mechanisms have been examined by comparing the experimental results with numerical simulations, which confirm that the increased barrier height is mainly attributed to the reduction of 2-DEG concentration.

  16. High mobility AlGaN/GaN devices for β--dosimetry

    NASA Astrophysics Data System (ADS)

    Schmid, Martin; Howgate, John; Ruehm, Werner; Thalhammer, Stefan

    2016-05-01

    There is a high demand in modern medical applications for dosimetry sensors with a small footprint allowing for unobtrusive or high spatial resolution detectors. To this end we characterize the sensoric response of radiation resistant high mobility AlGaN/GaN semiconductor devices when exposed to β--emitters. The samples were operated as a floating gate transistor, without a field effect gate electrode, thus excluding any spurious effects from β--particle interactions with a metallic surface covering. We demonstrate that the source-drain current is modulated in dependence on the kinetic energy of the incident β--particles. Here, the signal is shown to have a linear dependence on the absorbed energy calculated from Monte Carlo simulations. Additionally, a stable and reproducible sensor performance as a β--dose monitor is shown for individual radioisotopes. Our experimental findings and the characteristics of the AlGaN/GaN high mobility layered devices indicate their potential for future applications where small sensor size is necessary, like for instance brachytherapy.

  17. Effect of Proton Irradiation on 2DEG in AlGaN/GaN Heterostructures

    NASA Astrophysics Data System (ADS)

    Abderrahmane, A.; Koide, S.; Tahara, T.; Sato, S.; Ohshima, T.; Okada, H.; Sandhu, A.

    2013-04-01

    Low temperature Hall effect measurements were carried on AlGaN/GaN micro-Hall effect sensors before and after irradiation with 380 keV and fluence of 1014 protons/cm2 protons. The sheet electron density after irradiation did not show significant changes but there was a dramatic decrease in the electron mobility of the heterostructures. Prior to irradiation, the observation of well-defined Landau plateaus in the Hall resistance and Shubnikov-de Haas oscillations (SdH) at 4.5 T was indicative of the high quality the heterojunction confining the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface of micro-Hall effect sensors. In contrast, the Landau plateaus disappeared after irradiation and the threshold magnetic field required for the observation of the SdH increased, which was accompanied by a decrease of the electron mobility. Temperature dependent magnetoresistance measurements were used to deduce the effective mass and the quantum scattering time before irradiation. A negative magnetoresistance was observed at low magnetic fields which is related to weak localization and parabolic negative magnetoresistance attributed to electron-electron interaction in both samples.

  18. Electrical detection of biomaterials using AlGaN/GaN high electron mobility transistors

    NASA Astrophysics Data System (ADS)

    Kang, B. S.; Wang, H. T.; Ren, F.; Pearton, S. J.

    2008-08-01

    Chemical sensors can be used to analyze a wide variety of environmental and biological gases and liquids and may need to be able to selectively detect a target analyte. Different methods, including gas chromatography, chemiluminescence, selected ion flow tube, and mass spectroscopy, have been used to measure biomarkers. These methods show variable results in terms of sensitivity for some applications and may not meet the requirements for a handheld biosensor. A promising sensing technology utilizes AlGaN/GaN high electron mobility transistors (HEMTs). HEMT structures have been developed for use in microwave power amplifiers due to their high two dimensional electron gas (2DEG) mobility and saturation velocity. The conducting 2DEG channel of AlGaN/GaN HEMTs is very close to the surface and extremely sensitive to adsorption of analytes. HEMT sensors can be used for detecting gases, ions, pH values, proteins, and DNA. In this paper we review recent progress on functionalizing the surface of HEMTs for specific detection of glucose, kidney marker injury molecules, prostate cancer, and other common substances of interest in the biomedical field.

  19. Temperature-dependent hall measurement of AlGaN/GaN heterostructures on Si substrates

    NASA Astrophysics Data System (ADS)

    Lee, In Hak; Kim, Yong Hyun; Chang, Young Jun; Shin, Jong Hoon; Jang, T.; Jang, Seung Yup

    2015-01-01

    For AlGaN/GaN high electron mobility transistor (HEMT) devices grown on Si substrates, we studied the influence of the surface defect density on the Hall mobility and the carrier concentration. We established a Hall measurement system over wide temperature range (8 ~ 800 K) by combining a closed-cycle refrigerator and a heating chamber. The two-dimensional electron gas (2DEG) in the epitaxially-grown AlGaN/GaN HEMT structures showed high mobilities, i.e., > 1500 cm2/Vs at 300 K and > 7000 cm2/Vs at 8 K. As the surface defect density increased, the mobility values at 8 K decreased due to scattering from charged impurities at low temperatures. However, at high temperatures where optical phonons are a major source of scattering, the mobilities do not show a significant dependence on the surface defect density. In addition, the carrier density showed an unexpected decrease at temperatures above 300 K, which is ascribed to a change in the misfit strain upon heating.

  20. Study of surface leakage current of AlGaN/GaN high electron mobility transistors

    SciTech Connect

    Chen, YongHe; Zhang, Kai; Cao, MengYi; Zhao, ShengLei; Zhang, JinCheng; Hao, Yue; Ma, XiaoHua

    2014-04-14

    Temperature-dependent surface current measurements were performed to analyze the mechanism of surface conductance of AlGaN/GaN channel high-electron-mobility transistors by utilizing process-optimized double gate structures. Different temperatures and electric field dependence have been found in surface current measurements. At low electric field, the mechanism of surface conductance is considered to be two-dimensional variable range hopping. At elevated electric field, the Frenkel–Poole trap assisted emission governs the main surface electrons transportation. The extracted energy barrier height of electrons emitting from trapped state near Fermi energy level into a threading dislocations-related continuum state is 0.38 eV. SiN passivation reduces the surface leakage current by two order of magnitude and nearly 4 orders of magnitude at low and high electric fields, respectively. SiN also suppresses the Frenkel–Poole conductance at high temperature by improving the surface states of AlGaN/GaN. A surface treatment process has been introduced to further suppress the surface leakage current at high temperature and high field, which results in a decrease in surface current of almost 3 orders of magnitude at 476 K.

  1. Auger effect in yellow light emitters based on InGaN–AlGaN–GaN quantum wells

    NASA Astrophysics Data System (ADS)

    Huong Ngo, Thi; Gil, Bernard; Valvin, Pierre; Damilano, Benjamin; Lekhal, Kaddour; De Mierry, Philippe

    2016-05-01

    The Auger effect and its impact on the internal quantum efficiency (IQE) of yellow light emitters based on silicon-doped InGaN–AlGaN–GaN quantum wells are investigated by power dependence measurement and using an ABC model. Photoluminescence intensity recorded as a function of excitation power density follows a linear dependence up to a threshold P T that depends on the design of the sample. Above this threshold, the variation of the intensity becomes sublinear, which is characteristic of the onset of Auger recombination processes. After extracting the evolution of IQE with pump power from the experimental data, we use a modified ABC modeling that includes the residual n-type doping to estimate the contribution of different recombination channels. We find that the Auger effect dominates in the high-excitation regime. In addition, we find that intercalating an AlGaN-strain-compensating layer reduces not only the coefficient of nonradiative recombination rates but also reduces the onset of Auger recombination.

  2. High power AlGaAs-GaAs visible diode lasers

    SciTech Connect

    Tihanyi, P.L.; Jain, F.C. . Dept. of Electrical Engineering); Robinson, M.J.; Dixon, J.E. ); Williams, J.E.; Meehan, K.; O'Neill, M.S.; Heath, L.S.; Beyea, D.M. . Microelectronics and Materials Center)

    1994-07-01

    A high-power room-temperature AlGaAs graded index separately confined heterostructure (GRINSCH) laser emitting in the visible spectral regime ([approx equal]715 nm) is reported for the first time. The device is gain-guided and consists of 12 stripes, each 5 [mu]m in width with a centerline separation of 9 [mu]m. This high-power visible laser has been successfully fabricated using a GaAlAs active layer. The epitaxial layer was grown with significantly lower levels of oxygen compared to those grown using standard metalorganic sources. Threshold currents of 310 mA at 10 C were routinely measured on uncoated devices. The uncoated device had a catastrophic optical damage limit of 540 mW and has a slope efficiency as high as 0.48. No degradation in device performance was observed during a 50-hour 150-mW burn-in.

  3. Ballistic transport in GaN/AlGaN resonant tunneling diodes

    NASA Astrophysics Data System (ADS)

    Sakr, S.; Warde, E.; Tchernycheva, M.; Julien, F. H.

    2011-01-01

    In this work we theoretically study the vertical transport in GaN/AlGaN resonant tunneling diodes in the ballistic regime. Heterostructures based on III-nitride compounds are characterized by a large conduction band discontinuity and a presence of an internal electric field, both of which have important effects on the electronic transport. Using the transfer matrix formalism, we investigate the effect of the energetic barrier height on the resonant current. Our calculations show an increase in the peak to valley ratio (PVR) with increasing Al content of the barriers which arises from the large decrease in the valley current. Furthermore, we show that the current resonances are different for positive and negative applied voltages. We also demonstrate that, due to the asymmetry of the conduction band profile, only one current direction leads to a significant PVR. Finally, we present an approach to achieve large PVR in both current directions by reducing the asymmetry induced by the internal field.

  4. Excitonic recombination dynamics in non-polar GaN/AlGaN quantum wells

    SciTech Connect

    Rosales, D.; Gil, B.; Bretagnon, T.; Guizal, B.; Zhang, F.; Okur, S.; Monavarian, M.; Izyumskaya, N.; Avrutin, V.; Özgür, Ü.; Morkoç, H.; Leach, J. H.

    2014-02-21

    The optical properties of GaN/Al{sub 0.15}Ga{sub 0.85}N multiple quantum wells are examined in 8 K–300 K temperature range. Both polarized CW and time resolved temperature-dependent photoluminescence experiment are performed so that we can deduce the relative contributions of the non-radiative and radiative recombination processes. From the calculation of the proportion of the excitonic population having wave vector in the light cone, we can deduce the variation of the radiative decay time with temperature. We find part of the excitonic population to be localized in concert with the report of Corfdir et al. (Jpn. J. Appl. Phys., Part 2 52, 08JC01 (2013)) in case of a-plane quantum wells.

  5. Comparative study on interface and bulk charges in AlGaN/GaN metal-insulator-semiconductor heterostructures with Al2O3, AlN, and Al2O3/AlN laminated dielectrics

    NASA Astrophysics Data System (ADS)

    Zhu, Jie-Jie; Ma, Xiao-Hua; Chen, Wei-Wei; Hou, Bin; Xie, Yong; Hao, Yue

    2016-05-01

    In this paper, the interface and bulk charges in AlGaN/GaN metal-insulator-semiconductor (MIS) heterostructures with AlN, Al2O3, and Al2O3/AlN laminated dielectrics were studied. In situ plasma pretreatment resulted negligible interface trap states and voltage hysteresis. The fixed charge density at Al2O3/AlN (or Al2O3/barrier) interface was estimated to be 1.66 × 1013 cm-2 by using flat-band voltage shift, and the oxide bulk charge concentration was 2.86 × 1017 cm-3. The interface charge density at other interfaces were at the order of 1011 cm-2. Simulation results using the above charge density/concentration indicated that Al2O3/AlN interface fixed charges dominated the dielectric-related voltage shift in AlGaN/GaN MIS heterostructures, which caused a large voltage shift of -3 V with 10 nm Al2O3 thickness, while the flat-band voltage variety resulting from other types of charges was within 0.1 V.

  6. Resolution characteristics of graded doping and graded composition reflection-mode AlGaAs/GaAs photocathodes

    NASA Astrophysics Data System (ADS)

    Deng, Wenjuan; Zhang, Daoli; Zou, Jijun; Peng, Xincun; Wang, Weilu; Zhang, Yijun; Chang, Benkang

    2015-11-01

    The resolution model of graded doping and graded composition reflection-mode AlGaAs/GaAs photocathode is solved numerically from the two-dimensional continuity equations. According to the model, the theoretical modulation transfer functions (MTFs) of different structure reflection-mode photocathodes were calculated, and the effects of doping concentration, Al composition, AlGaAs and GaAs layer thickness on the resolution of cathodes were analyzed. The simulation results show that both graded composition and graded doping structures can increase the resolution of photocathode, and the effect of graded composition structure is more pronounced. The resolution improvement is attributed to the built-in electric field induced by a graded composition or doping structure. The simulation results also show that the MTFs of cathodes are affected by the AlGaAs and GaAs layer thickness.

  7. Realization of defect-free epitaxial core-shell GaAs/AlGaAs nanowire heterostructures

    SciTech Connect

    Tambe, Michael J.; Lim, Sung Keun; Smith, Matthew J.; Gradecak, Silvija; Allard, Lawrence F.

    2008-10-13

    We report the controlled growth of vertically aligned GaAs/AlGaAs core-shell nanowires. By optimizing the shell deposition temperature and catalyst density we maintain high temperature stability and achieve defect-free epitaxial AlGaAs shell deposition with high aluminum incorporation. Energy dispersive x-ray analysis determines the shell composition to be Al{sub 0.9}Ga{sub 0.1}As and measures the uniformity of the shell thickness. Lattice-resolved high-angle annular dark-field scanning transmission electron microscopy images confirm the core-shell interface to be defect-free, epitaxial, and atomically sharp. The ability to realize GaAs/AlGaAs core-shell nanowires with precise control over the morphology and composition is essential to the development of nanowire-based high mobility electronics.

  8. Temperature dependence of mid-infrared intersubband absorption in AlGaN/GaN multiple quantum wells

    NASA Astrophysics Data System (ADS)

    Kotani, Teruhisa; Arita, Munetaka; Hoshino, Katsuyuki; Arakawa, Yasuhiko

    2016-02-01

    The temperature dependence of the mid-infrared intersubband (ISB) absorption in non-polar (m-plane) and polar (c-plane) AlGaN/GaN quantum wells (QWs) is studied. The ISB absorption shifts to higher energy as the temperature is reduced from 300 K to below 10 K. Both m-plane and c-plane QWs show a small energy shift (1.6-2.6 meV) compared to AlGaAs/GaAs (3.5-5.2 meV) and AlSb/InAs (6.2 and 12 meV) QWs. Theoretical calculations considering the temperature induced material constant changes show good agreement with the experimental results. These results suggest that ISB transition energies in AlGaN/GaN QWs are more stable against temperature change mainly because of the heavy effective masses and small nonparabolicities.

  9. Strained InGaSb/AlGa(As)Sb Quantum Wells for p-Channel Transistors

    NASA Astrophysics Data System (ADS)

    Bennett, Brian R.; Podpirka, Adrian A.; Boos, J. B.; Kumar, Satvika L.

    2016-06-01

    Quantum wells of InGaSb clad by AlGa(As)Sb were grown by molecular beam epitaxy. Well and barrier compositions were chosen to yield biaxial compressive strain and enhanced hole mobility in the InGaSb. Wells with thickness of 7.5 nm exhibited room-temperature mobilities of 1000 cm2/V s to 1100 cm2/V s, with the surface-layer material influencing two-dimensional hole densities. The introduction of As into the barrier material allows a wider range of p-channel well/barrier combinations and lattice constants. These could be compatible with n-channel InGaAs wells for complementary field-effect transistor circuits which utilize a common buffer layer. InGaSb wells with thicknesses of 20 nm to 30 nm and compressive strains of 1.0% to 1.5% exhibited hole mobilities of 700 cm2/V s to 900 cm2/V s.

  10. Low energy proton radiation damage to (AlGa)As-GaAs solar cells

    NASA Technical Reports Server (NTRS)

    Loo, R.; Kamath, S.; Knechtli, R. C.

    1979-01-01

    Twenty-seven 2 times 2 sq cm (AlGa)As-GaAs solar cells were fabricated and subjected to 50 keV, 100 keV, and 290 keV of proton irradiation along with eighteen high efficiency silicon solar cells. The results of the study further corroborate the advantages for space missions offered by GaAs cells over state of the art silicon cells. Thus, even though the GaAs cells showed greater degradation when irradiated by protons with energy less than 5 MeV, the solar cells were normally protected from these protons by the glass covers used in space arrays. The GaAs cells also offered superior end of life power capability compared with silicon. The change in the open circuit voltage, short circuit current, spectral response, and dark 1-5 characteristics after irradiation at each proton energy and fluence were found to be consistent with the explanation of the effect of the protons. Also dark 1-5 characteristics showed that a new recombination center dominates the current transport mechanism after irradiation.

  11. Gain measurements and average power capabilities of Cr(3+):LiSrAlF6

    NASA Astrophysics Data System (ADS)

    Hanson, F.; Bendall, C.; Poirier, P.

    1993-09-01

    Gain for flash-lamp-pumped Cr:LiSrAlF6 and Cr:Li(Sr(1-x),Ca(x))AlF6 from 790 to 1000 nm is reported. For Cr:LiSrAlF6 thermal fracture occurred at heat loadings of 4.5 W/cm and about 6 W/sq cm in the rod and slab geometry, respectively. Excited-state upconversion losses were evident in the slabs at high pump energy.

  12. Degradation of blue AlGaN/InGaN/GaN LEDs subjected to high current pulses

    SciTech Connect

    Barton, D.L.; Zeller, J.; Phillips, B.S.; Chiu, P.C.; Askar, S.; Lee, D.S.; Osinski, M.; Malloy, K.J.

    1994-12-31

    Short-wavelength, visible-light emitting optoelectronic devices are needed for a wide range of commercial applications, including high-density optical data storage, full-color displays, and underwater communications. In 1994, high-brightness blue LEDs based on gallium nitride and related compounds (InGaN/AlGaN) were introduced by Nichia Chemical Industries. The Nichia diodes are 100 times brighter than the previously available SiC blue LEDs. Group-III nitrides combine a wide, direct bandgap with refractory properties and high physical strength. So far, no studies of degradation of GaN based LEDs have been reported. The authors study, reported in this paper, focuses on the performance of GaN LEDs under high electrical stress conditions. Their observations indicate that, in spite of a high defect density, which normally would have been fatal to other III-V devices, defects in group-III nitrides are not mobile even under high electrical stress. Defect tubes, however, can offer a preferential path for contact metals to electromigrate towards the p-n junction, eventually resulting in a short. The proposed mechanism of GaN diode degradation raises concern for prospects of reliable lasers in the group-III nitrides grown on sapphire.

  13. Spin and phase relaxation dynamics in GaN and GaN/AlGaN quantum wells (Presentation Recording)

    NASA Astrophysics Data System (ADS)

    Gallart, Mathieu; Ziegler, Marc; Hönerlage, Bernd H.; Gilliot, Pierre; Feltin, Eric; Carlin, Jean-François; Butté, Raphaël.; Grandjean, Nicolas

    2015-09-01

    By performing time-resolved optical non-degenerate pump-probe experiments, we study the relaxation dynamics of spin-polarized excitons in wurtzite epitaxial GaN and in nitride nanostructures. Those materials are indeed promising candidates for spintronic applications because of their weak spin-orbit coupling and large exciton binding energy (~ 17 meV and ~ 26meV in bulk GaN, respectively). In epilayers, we show that the high density of dislocations increases dramatically the spin relaxation of electrons and holes through the defect assisted Elliott-Yafet mechanism. That makes the exciton dephasing time very short. In high quality GaN/AlGaN quantum wells, both the exciton-spin lifetime S and the exciton dephasing-time T2 were determined via pump-probe spectroscopy using polarized laser pulses and time-resolved four wave-mixing experiments. The evolution of both quantities with temperature shows that spin relaxation occurs in the motional narrowing regime up to 80 K. Above this threshold, the thermal energy becomes large enough for excitons to escape from the QW. Such measurements demonstrate that GaN-based heterostructures can reach a very high degree of control that was previously mostly restricted to conventional III-V semiconductors and more specifically to the arsenide family.

  14. Metalorganic chemical vapor deposition growth of high-mobility AlGaN/AlN/GaN heterostructures on GaN templates and native GaN substrates

    SciTech Connect

    Chen, Jr-Tai Hsu, Chih-Wei; Forsberg, Urban; Janzén, Erik

    2015-02-28

    Severe surface decomposition of semi-insulating (SI) GaN templates occurred in high-temperature H{sub 2} atmosphere prior to epitaxial growth in a metalorganic chemical vapor deposition system. A two-step heating process with a surface stabilization technique was developed to preserve the GaN template surface. Utilizing the optimized heating process, a high two-dimensional electron gas mobility ∼2000 cm{sup 2}/V·s was obtained in a thin AlGaN/AlN/GaN heterostructure with an only 100-nm-thick GaN spacer layer homoepitaxially grown on the GaN template. This technique was also demonstrated viable for native GaN substrates to stabilize the surface facilitating two-dimensional growth of GaN layers. Very high residual silicon and oxygen concentrations were found up to ∼1 × 10{sup 20 }cm{sup −3} at the interface between the GaN epilayer and the native GaN substrate. Capacitance-voltage measurements confirmed that the residual carbon doping controlled by growth conditions of the GaN epilayer can be used to successfully compensate the donor-like impurities. State-of-the-art structural properties of a high-mobility AlGaN/AlN/GaN heterostructure was then realized on a 1 × 1 cm{sup 2} SI native GaN substrate; the full width at half maximum of the X-ray rocking curves of the GaN (002) and (102) peaks are only 21 and 14 arc sec, respectively. The surface morphology of the heterostructure shows uniform parallel bilayer steps, and no morphological defects were noticeable over the entire epi-wafer.

  15. Metalorganic chemical vapor deposition growth of high-mobility AlGaN/AlN/GaN heterostructures on GaN templates and native GaN substrates

    NASA Astrophysics Data System (ADS)

    Chen-Tai, Jr.; Hsu, Chih-Wei; Forsberg, Urban; Janzén, Erik

    2015-02-01

    Severe surface decomposition of semi-insulating (SI) GaN templates occurred in high-temperature H2 atmosphere prior to epitaxial growth in a metalorganic chemical vapor deposition system. A two-step heating process with a surface stabilization technique was developed to preserve the GaN template surface. Utilizing the optimized heating process, a high two-dimensional electron gas mobility ˜2000 cm2/V.s was obtained in a thin AlGaN/AlN/GaN heterostructure with an only 100-nm-thick GaN spacer layer homoepitaxially grown on the GaN template. This technique was also demonstrated viable for native GaN substrates to stabilize the surface facilitating two-dimensional growth of GaN layers. Very high residual silicon and oxygen concentrations were found up to ˜1 × 1020 cm-3 at the interface between the GaN epilayer and the native GaN substrate. Capacitance-voltage measurements confirmed that the residual carbon doping controlled by growth conditions of the GaN epilayer can be used to successfully compensate the donor-like impurities. State-of-the-art structural properties of a high-mobility AlGaN/AlN/GaN heterostructure was then realized on a 1 × 1 cm2 SI native GaN substrate; the full width at half maximum of the X-ray rocking curves of the GaN (002) and (102) peaks are only 21 and 14 arc sec, respectively. The surface morphology of the heterostructure shows uniform parallel bilayer steps, and no morphological defects were noticeable over the entire epi-wafer.

  16. Properties of atomic layer deposited dielectrics for AlGaN/GaN device passivation

    NASA Astrophysics Data System (ADS)

    Ramanan, Narayanan; Lee, Bongmook; Kirkpatrick, Casey; Suri, Rahul; Misra, Veena

    2013-07-01

    In order to minimize ac-dc dispersion, reduce gate leakage and maximize ac transconductance, there is a critical need to identify optimal interfaces, low-k passivation dielectrics and high-k gate dielectrics. In this paper, an investigation of different atomic layer deposited (ALD) passivation dielectrics on AlGaN/GaN-based hetero-junction field effect transistors (HFETs) was performed. Angle-resolved x-ray photoelectron spectroscopy revealed that HCl/HF and NH4OH cleans resulted in a reduction of native oxide and carbon levels at the GaN surface. The role of high temperature anneals, following the ALD, on the effectiveness of passivation was also explored. Gate-lag measurements on HFETs passivated with a thin ALD high-k Al2O3 or HfAlO layer capped with a thick plasma enhanced chemical vapor deposited (PECVD) low-k SiO2 layer, annealed at 600-700 °C, were found to be as good as or even better than those with conventional PECVD silicon nitride passivation. Further, it was observed that different passivation dielectric stacks required different anneal temperatures for improved gate-lag behavior compared to the as-deposited case.

  17. AlGaN/GaN-based HEMTs for electrical stimulation of neuronal cell cultures

    NASA Astrophysics Data System (ADS)

    Witte, H.; Warnke, C.; Voigt, T.; de Lima, A.; Ivanov, I.; Vidakovic-Koch, T. R.; Sundmacher, K.; Krost, A.

    2011-09-01

    Unipolar source-drain voltage pulses of GaN/AlGaN-high electron mobility transistors (HEMTs) were used for stimulation of cultured neuronal networks obtained from embryonic rat cerebral cortex. The HEMT sensor was grown by metal organic vapour phase epitaxy on a 2 inch sapphire substrate consisting of 10 single HEMTs concentrically arranged around the wafer centre. Electrolytic reactions between the HEMT sensor surface and the culture medium were not detected using cyclic voltammetry. During voltage pulses and resulting neuronal excitation, capacitances were recharged giving indications of the contributions of the AlGaN and AlOx isolation layers between the two-dimensional electron gas channel and the neuron culture. The resulting threshold current for stimulation of neuron activity strongly depended on the culture and HEMT position on the sensor surface under consideration which was caused by different impedances of each neuron culture and position within the culture. The differences of culture impedances could be explained by variations of composition, thickness and conductivity of the culture areas.

  18. Third element effect in the surface zone of Fe-Cr-Al alloys

    NASA Astrophysics Data System (ADS)

    Airiskallio, E.; Nurmi, E.; Heinonen, M. H.; Väyrynen, I. J.; Kokko, K.; Ropo, M.; Punkkinen, M. P. J.; Pitkänen, H.; Alatalo, M.; Kollár, J.; Johansson, B.; Vitos, L.

    2010-01-01

    The third element effect to improve the high temperature corrosion resistance of the low-Al Fe-Cr-Al alloys is suggested to involve a mechanism that boosts the recovering of the Al concentration to the required level in the Al-depleted zone beneath the oxide layer. We propose that the key factor in this mechanism is the coexistent Cr depletion that helps to maintain a sufficient Al content in the depleted zone. Several previous experiments related to our study support that conditions for such a mechanism to be functional prevail in real oxidation processes of Fe-Cr-Al alloys.

  19. Low resistance as-deposited Cr /Au contacts on p-type GaN

    NASA Astrophysics Data System (ADS)

    Kalaitzakis, F. G.; Pelekanos, N. T.; Prystawko, P.; Leszczynski, M.; Konstantinidis, G.

    2007-12-01

    The influence of several predeposition surface treatments and different contact metals to the electrical properties of metal/p-GaN contacts was studied. A low resistance as-deposited Cr /Au Ohmic contact was achieved, using boiling aqua regia as surface treatment. The Ohmic resistance of Cr /Au contacts with 50μm interspacing was found to be 50Ω, while the specific contact resistivity value was measured 2.6×10-3Ωcm2. Direct comparison with the standard oxidized Ni /Au contacts confirmed the superior characteristics of the Cr /Au contact scheme. Violet emission was readily obtained when the as-deposited Cr /Au contacts were used as the p electrode of a light emitting diode emitting at 385nm.

  20. Characterization of AlInN/AlN/GaN Heterostructures with Different AlN Buffer Thickness

    NASA Astrophysics Data System (ADS)

    Çörekçi, S.; Dugan, S.; Öztürk, M. K.; Çetin, S. Ş.; Çakmak, M.; Özçelik, S.; Özbay, E.

    2016-05-01

    Two AlInN/AlN/GaN heterostructures with 280-nm- and 400-nm-thick AlN buffer grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) have been investigated by x-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) and Hall-effect measurements. The symmetric (0002) plane with respect to the asymmetric (10bar{1} 2) plane in the 280-nm-thick AlN buffer has a higher crystal quality, as opposed to the 400-nm-thick buffer. The thinner buffer improves the crystallinity of both (0002) and (10bar{1} 2) planes in the GaN layers, it also provides a sizeable reduction in dislocation density of GaN. Furthermore, the lower buffer thickness leads to a good quality surface with an rms roughness of 0.30 nm and a dark spot density of 4.0 × 108 cm-2. The optical and transport properties of the AlInN/AlN/GaN structure with the relatively thin buffer are compatible with the enhancement in its structural quality, as verified by XRD and AFM results.

  1. Characterization of AlInN/AlN/GaN Heterostructures with Different AlN Buffer Thickness

    NASA Astrophysics Data System (ADS)

    Çörekçi, S.; Dugan, S.; Öztürk, M. K.; Çetin, S. Ş.; Çakmak, M.; Özçelik, S.; Özbay, E.

    2016-07-01

    Two AlInN/AlN/GaN heterostructures with 280-nm- and 400-nm-thick AlN buffer grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) have been investigated by x-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) and Hall-effect measurements. The symmetric (0002) plane with respect to the asymmetric (10bar{1}2) plane in the 280-nm-thick AlN buffer has a higher crystal quality, as opposed to the 400-nm-thick buffer. The thinner buffer improves the crystallinity of both (0002) and (10bar{1}2) planes in the GaN layers, it also provides a sizeable reduction in dislocation density of GaN. Furthermore, the lower buffer thickness leads to a good quality surface with an rms roughness of 0.30 nm and a dark spot density of 4.0 × 108 cm-2. The optical and transport properties of the AlInN/AlN/GaN structure with the relatively thin buffer are compatible with the enhancement in its structural quality, as verified by XRD and AFM results.

  2. N-face GaN/AlN/GaN/InAlN and GaN/AlN/AlGaN/GaN/InAlN high-electron-mobility transistor structures grown by plasma-assisted molecular beam epitaxy on vicinal substrates

    NASA Astrophysics Data System (ADS)

    Ahmadi, Elaheh; Wu, Feng; Li, Haoran; Kaun, Stephen W.; Tahhan, Maher; Hestroffer, Karine; Keller, Stacia; Speck, James S.; Mishra, Umesh K.

    2015-05-01

    Since on-axis GaN-on-sapphire substrates with low threading dislocation density are not available in the N-face orientation, we explored the growth of InAlN on vicinal (4° miscut along GaN ≤ft< 10\\bar{1}0 \\right> ) GaN-on-sapphire substrates. The microstructure of In0.18Al0.82N layers grown by plasma-assisted molecular beam epitaxy at different temperatures was studied using scanning transmission electron microscopy (STEM). The cross-sectional and plan-view STEM images revealed lateral variations in the InAlN composition along ≤ft< 10\\bar{1}0 \\right> (perpendicular to the step edges). Also, step bunching was observed in InAlN layers thicker than 10 nm. N-face high-electron-mobility transistor structures with lattice-matched InAlN backbarriers were then grown on these vicinal substrates with different InAlN thicknesses. Transmission line measurements showed that step bunching and lateral variation of InAlN composition degraded the two-dimensional electron gas (2DEG) mobility in the directions parallel and perpendicular to the steps. A 2DEG charge density of 1.1 × 1013 cm-2 and mobility of 1850 cm2 V-1 s-1 were achieved on a GaN/AlN/InAlN/GaN structure with 7.5 nm thick In0.18Al0.82N. By designing a double backbarrier (In0.18Al0.82N(7.5 nm)/Al0.57Ga0.43N(7 nm)), a 2DEG charge density of 2 × 1013 cm-2 and mobility of 1360 cm2 V-1 s-1 were attained, which resulted in a sheet resistance of 230 Ω/□.

  3. Performance improvements of AlGaN/GaN HEMTs by strain modification and unintentional carbon incorporation

    NASA Astrophysics Data System (ADS)

    Luong, Tien-Tung; Tran, Binh Tinh; Ho, Yen-Teng; Ha, Minh-Thien-Huu; Hsiao, Yu-Lin; Liu, Shih-Chien; Chiu, Yu-Sheng; Chang, Edward-Yi

    2015-03-01

    An advanced AlGaN/GaN HEMT structure, grown on a sapphire substrate by MOCVD utilizing a high temperature (HT) AlN interlayer (IL) and a multilayer high-low-high temperature (HLH) AlN buffer layer, demonstrates a superior performance both in breakdown voltage (>200 V) and maximum drain current (IDSS = 667 mA/mm). The HT AlN IL produces an additional compressive strain into the above GaN layer. Accordingly, an AlGaN barrier, grown on the more compressive GaN, introduces less tensile strain leading to an improvement in surface morphology (RMS = 0.19 nm in 2 × 2 μm2), a remarkable increase in 2DEG mobility by 46% ( μ s = 1900 cm2/Vs) and a decrease in densities of defects acting as paths for the leakage current through the AlGaN barrier. A high semi-insulating buffer is achieved by eliminating leakage paths both through the buffer layer and the buffer-substrate interfacial layer. These result from an increase in unintentional carbon introduced by AlN layers, especially by a low temperature AlN layer; which are grown under low pressure (50 Torr). Lastly, the decrease in AlGaN barrier tensile strain and low leakage current in the advanced HEMTs structure using an HT AlN IL and an HLH AlN buffer are promising for an improvement in AlGaN/GaN HEMTs' reliability. [Figure not available: see fulltext.

  4. Alloy inhomogeneity and carrier localization in AlGaN sections and AlGaN/AlN nanodisks in nanowires with 240–350 nm emission

    SciTech Connect

    Himwas, C.; Hertog, M. den; Dang, Le Si; Songmuang, R.; Monroy, E.

    2014-12-15

    We present structural and optical studies of AlGaN sections and AlGaN/AlN nanodisks (NDs) in nanowires grown by plasma-assisted molecular beam epitaxy. The Al-Ga intermixing at Al(Ga)N/GaN interfaces and the chemical inhomogeneity in AlGaN NDs evidenced by scanning transmission electron microscopy are attributed to the strain relaxation process. This interpretation is supported by the three-dimensional strain distribution calculated by minimizing the elastic energy in the structure. The alloy inhomogeneity increases with the Al content, leading to enhanced carrier localization signatures in the luminescence characteristics, i.e., red shift of the emission, s-shaped temperature dependence, and linewidth broadening. Despite these effects, the emission energy of AlGaN/AlN NDs can be tuned in the 240–350 nm range with internal quantum efficiencies around 30%.

  5. Influence of InGaN sub-quantum-well on performance of InAlN/GaN/InAlN resonant tunneling diodes

    NASA Astrophysics Data System (ADS)

    Chen, Haoran; Yang, Lin'an; Hao, Yue

    2014-08-01

    The resonant tunneling mechanism of the GaN based resonant tunneling diode (RTD) with an InGaN sub-quantum-well has been investigated by means of numerical simulation. At resonant-state, Electrons in the InGaN/InAlN/GaN/InAlN RTD tunnel from the emitter region through the aligned discrete energy levels in the InGaN sub-quantum-well and GaN main-quantum-well into the collector region. The implantation of the InGaN sub-quantum-well alters the dominant transport mechanism, increase the transmission coefficient and give rise to the peak current and peak-to-valley current ratio. We also demonstrate that the most pronounced negative-differential-resistance characteristic can be achieved by choosing appropriately the In composition of InxGa1-xN at around x = 0.06.

  6. Influence of InGaN sub-quantum-well on performance of InAlN/GaN/InAlN resonant tunneling diodes

    SciTech Connect

    Chen, Haoran; Yang, Lin'an Hao, Yue

    2014-08-21

    The resonant tunneling mechanism of the GaN based resonant tunneling diode (RTD) with an InGaN sub-quantum-well has been investigated by means of numerical simulation. At resonant-state, Electrons in the InGaN/InAlN/GaN/InAlN RTD tunnel from the emitter region through the aligned discrete energy levels in the InGaN sub-quantum-well and GaN main-quantum-well into the collector region. The implantation of the InGaN sub-quantum-well alters the dominant transport mechanism, increase the transmission coefficient and give rise to the peak current and peak-to-valley current ratio. We also demonstrate that the most pronounced negative-differential-resistance characteristic can be achieved by choosing appropriately the In composition of In{sub x}Ga{sub 1−x}N at around x = 0.06.

  7. Metal-interconnection-free integration of InGaN/GaN light emitting diodes with AlGaN/GaN high electron mobility transistors

    NASA Astrophysics Data System (ADS)

    Liu, Chao; Cai, Yuefei; Liu, Zhaojun; Ma, Jun; Lau, Kei May

    2015-05-01

    We report a metal-interconnection-free integration scheme for InGaN/GaN light emitting diodes (LEDs) and AlGaN/GaN high electron mobility transistors (HEMTs) by combining selective epi removal (SER) and selective epitaxial growth (SEG) techniques. SER of HEMT epi was carried out first to expose the bottom unintentionally doped GaN buffer and the sidewall GaN channel. A LED structure was regrown in the SER region with the bottom n-type GaN layer (n-electrode of the LED) connected to the HEMTs laterally, enabling monolithic integration of the HEMTs and LEDs (HEMT-LED) without metal-interconnection. In addition to saving substrate real estate, minimal interface resistance between the regrown n-type GaN and the HEMT channel is a significant improvement over metal-interconnection. Furthermore, excellent off-state leakage characteristics of the driving transistor can also be guaranteed in such an integration scheme.

  8. Metal-interconnection-free integration of InGaN/GaN light emitting diodes with AlGaN/GaN high electron mobility transistors

    SciTech Connect

    Liu, Chao; Cai, Yuefei; Liu, Zhaojun; Ma, Jun; Lau, Kei May

    2015-05-04

    We report a metal-interconnection-free integration scheme for InGaN/GaN light emitting diodes (LEDs) and AlGaN/GaN high electron mobility transistors (HEMTs) by combining selective epi removal (SER) and selective epitaxial growth (SEG) techniques. SER of HEMT epi was carried out first to expose the bottom unintentionally doped GaN buffer and the sidewall GaN channel. A LED structure was regrown in the SER region with the bottom n-type GaN layer (n-electrode of the LED) connected to the HEMTs laterally, enabling monolithic integration of the HEMTs and LEDs (HEMT-LED) without metal-interconnection. In addition to saving substrate real estate, minimal interface resistance between the regrown n-type GaN and the HEMT channel is a significant improvement over metal-interconnection. Furthermore, excellent off-state leakage characteristics of the driving transistor can also be guaranteed in such an integration scheme.

  9. 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.

  10. Polarized XAFS study of Al K-edge for m-plane AlGaN films

    NASA Astrophysics Data System (ADS)

    Miyanaga, T.; Azuhata, T.; Nakajima, K.; Nagoya, H.; Hazu, K.; Chichibu, S. F.

    2014-04-01

    Local structures around Al atoms in high-quality m-plane AlxGa1-xN films (x=0.32 and 0.58) deposited on m-plane GaN substrates by the NH3 source molecular beam epitaxy method were investigated by Al K-edge X-ray absorption fine structure (XAFS) for the first time. XAFS spectra were measured using a linearly-polarized X-ray source from synchrotron radiation for three different directions; along the c-, a-, and m-axes. The interatomic distances along the a-axis are close to Ga-Ga distance in GaN, indicating that the local structures are strongly affected by GaN substrates. The localization of Al atoms was observed for the Al0.32Ga0.68N film.

  11. Spectroscopic characteristics of LiGaSiO4:Cr nano-glass-ceramics and glassy precursors

    NASA Astrophysics Data System (ADS)

    Subbotin, K. A.; Veber, A. A.; Nikolaev, D. A.; Senin, V. G.; Smirnov, V. A.; Osipova, Yu. N.; Zharikov, E. V.; Shcherbakov, I. A.

    2013-09-01

    We have performed a complex of spectroscopic investigations (extinction, luminescence, and luminescence excitation spectra and luminescence decay kinetics) of LiGaSiO4:Cr nano-glass-ceramics and Cr-Li-Ga-Si-O glassy precursors. It has been shown that 94-96% of chromium in precursors synthesized in air are in the trivalent form, while remaining chromium is in the form of Cr6+ and Cr4+. The luminescence of the latter form in precursors at 300 K is strongly quenched. In precursors synthesized in an inert atmosphere, only trivalent chromium occurs. We have calculated the absorption cross sections of Cr3+ and Cr6+ ions in Cr-Li-Ga-Si-O glassy precursors. It has been shown that, in the case of synthesis of these precursors, a considerable amount of chromium is lost as a result of evaporation and fails to enter the specimens. Upon partial controlled crystallization of the precursors (formation of LiGaSiO4:Cr nano-glass-ceramics), the oxidation state of chromium that passed to crystallites from the glass phase becomes tetravalent. In this case, two types of luminescence centers arise, which correspond to Cr4+ ions localized in two different polymorphic modifications of LiGaSiO4. Spectral characteristics of these two centers (bandshape, peak position, and spectral range) are almost identical, whereas luminescence lifetimes are radically different.

  12. Persistent photoconductivity in AlGaN/GaN heterojunction channels caused by the ionization of deep levels in the AlGaN barrier layer

    SciTech Connect

    Murayama, H.; Akiyama, Y.; Niwa, R.; Sakashita, H.; Sakaki, H.; Kachi, T.; Sugimoto, M.

    2013-12-04

    Time-dependent responses of drain current (I{sub d}) in an AlGaN/GaN HEMT under UV (3.3 eV) and red (2.0 eV) light illumination have been studied at 300 K and 250 K. UV illumination enhances I{sub d} by about 10 %, indicating that the density of two-dimensional electrons is raised by about 10{sup 12} cm{sup −2}. When UV light is turned off at 300 K, a part of increased I{sub d} decays quickly but the other part of increment is persistent, showing a slow decay. At 250 K, the majority of increment remains persistent. It is found that such a persistent increase of I{sub d} at 250 K can be partially erased by the illumination of red light. These photo-responses are explained by a simple band-bending model in which deep levels in the AlGaN barrier get positively charged by the UV light, resulting in a parabolic band bending in the AlGaN layer, while some potion of those deep levels are neutralized by the red light.

  13. Interface structural defects and photoluminescence properties of epitaxial GaN and AlGaN/GaN layers grown on sapphire

    SciTech Connect

    Klad'ko, V. P.; Chornen'kii, S. V.; Naumov, A. V. Komarov, A. V.; Tacano, M.; Sveshnikov, Yu. N.; Vitusevich, S. A.; Belyaev, A. E.

    2006-09-15

    Overall characterization of the GaN and AlGaN/GaN epitaxial layers by X-ray diffractometry and optical spectral analysis is carried out. The layers are grown by metalloorganic gas-phase epitaxy on (0001)-oriented single crystal sapphire wafers. The components of strains and the density of dislocations are determined. The effects of strains and dislocations on the photoluminescence intensity and spectra are studied. The results allow better understanding of the nature and mechanisms of the formation of defects in the epitaxial AlGaN/GaN heterostructures.

  14. The generation of misfit dislocations in facet-controlled growth of AlGaN /GaN films

    NASA Astrophysics Data System (ADS)

    Cherns, D.; Sahonta, S.-L.; Liu, R.; Ponce, F. A.; Amano, H.; Akasaki, I.

    2004-11-01

    The relaxation of tensile stresses in AlGaN layers grown on GaN /(0001)sapphire by facet-controlled epitaxial lateral overgrowth is reported. It is shown that a-type misfit dislocations are introduced at inclined {112¯2} AlGaN /GaN interfaces, with strong evidence for a half-loop nucleation and glide mechanism driven by shear stresses present on the (0001) slip plane. In addition to relieving misfit stresses, these dislocations introduce grain rotations of up to 10-2rad across the AlGaN /GaN boundaries, leading to tilt boundaries at the meeting front between laterally growing wings and between regions growing in the lateral and [0001] directions. The effects of these processes on the defect density in subsequent layers are examined.

  15. Current Collapse and Deep Levels of AlGaN/GaN Heterostructures monitored by LFN Measurements

    NASA Astrophysics Data System (ADS)

    Tacano, M.; Tanuma, N.; Yagi, S.; Okumura, H.; Matsui, T.; Sikula, J.

    2009-04-01

    The correlation between the current collapse in the IV characteristics of AlGaN/GaN HFETs and the traps monitored through the unpassivated and SiN-passivation processes of the AlGaN/GaN heterostructures by the low frequency noise measurements is reported: the noise level of E1(47 meV) trap decreased by 10 dBA/√Hz by the SiN passivation process together with the current collapse recovery in IV curves, while E2(131 meV) and E3(235 meV) levels became apparent after SiN passivation, indicating the remarkable suppression of the E1 trap by the passivation. The commercially available AlGaAs/GaAs LED heads for the page and FAX printers found several deep levels introduced during the contact formation processes, which could not be assigned by the DLTS measurements.

  16. Conduction electrons in acceptor-doped GaAs/GaAlAs heterostructures: a review

    NASA Astrophysics Data System (ADS)

    Zawadzki, Wlodek; Raymond, Andre; Kubisa, Maciej

    2016-05-01

    We review magneto-optical and magneto-transport effects in GaAs/GaAlAs heterostructures doped in GaAlAs barriers with donors, providing two-dimensional (2D) electron gas (2DEG) in GaAs quantum wells (QWS), and additionally doped with smaller amounts of acceptors (mostly Be atoms) in the vicinity of 2DEG. One may also deal with residual acceptors (mostly C atoms). The behavior of such systems in the presence of a magnetic field differs appreciably from those doped in the vicinity of 2DEG with donors. Three subjects related to the acceptor-doped heterostructures are considered. First is the problem of bound states of conduction electrons confined to the vicinity of negatively charged acceptors by the joint effect of a QW and an external magnetic field parallel to the growth direction. A variational theory of such states is presented, demonstrating that an electron turning around a repulsive center has discrete energies above the corresponding Landau levels. Experimental evidence for the discrete electron energies comes from the work on interband photo-magneto-luminescence, intraband cyclotron resonance and quantum magneto-transport (the Quantum Hall and Shubnikov-de Haas effects). An electron rain-down effect at weak electric fields and a boil-off effect at strong electric fields are introduced. It is demonstrated, both theoretically and experimentally, that a negatively charged acceptor can localize more than one electron. The second subject describes experiment and theory of asymmetric quantized Hall and Shubnikov-de Haas plateaus in acceptor-doped GaAs/GaAlAs heterostructures. It is shown that the main features of the plateau asymmetry can be attributed to asymmetric density of Landau states in the presence of acceptors. However, at high magnetic fields, the rain-down effect is also at work. The third subject deals with the so-called disorder modes (DMs) in the cyclotron resonance of conduction electrons. The DMs originate from random distributions of negatively

  17. GaAs/AlGaAs resonant tunneling diodes with a GaInNAs absorption layer for telecommunication light sensing

    NASA Astrophysics Data System (ADS)

    Hartmann, F.; Langer, F.; Bisping, D.; Musterer, A.; Höfling, S.; Kamp, M.; Forchel, A.; Worschech, L.

    2012-04-01

    Al0.6Ga0.4As/GaAs/Al0.6Ga0.4As double-barrier resonant-tunneling diodes (RTD) were grown by molecular beam epitaxy with a nearby, lattice-matched Ga0.89In0.11N0.04As0.96 absorption layer. RTD mesas with ring contacts and an aperture for optical excitation of charge carriers were fabricated on the epitaxial layers. Electrical and optical properties of the RTDs were investigated for different thicknesses of a thin GaAs spacer layer incorporated between the AlGaAs tunnel barrier adjacent to the GaInNAs absorption layer. Illumination of the RTDs with laser light of 1.3 μm wavelength leads to a pronounced photo-effect with a sensitivities of around 103 A/W.

  18. Strain-compensated (Ga,In)N/(Al,Ga)N/GaN multiple quantum wells for improved yellow/amber light emission

    SciTech Connect

    Lekhal, K.; Damilano, B. De Mierry, P.; Vennéguès, P.; Ngo, H. T.; Rosales, D.; Gil, B.; Hussain, S.

    2015-04-06

    Yellow/amber (570–600 nm) emitting In{sub x}Ga{sub 1−x}N/Al{sub y}Ga{sub 1−y}N/GaN multiple quantum wells (QWs) have been grown by metal organic chemical vapor deposition on GaN-on- sapphire templates. When the (Al,Ga)N thickness of the barrier increases, the room temperature photoluminescence is red-shifted while its yield increases. This is attributed to an increase of the QW internal electric field and an improvement of the material quality due to the compensation of the compressive strain of the In{sub x}Ga{sub 1−x}N QWs by the Al{sub y}Ga{sub 1−y}N layers, respectively.

  19. Ternary Borides Cr2AlB2, Cr3AlB4, and Cr4AlB6: The First Members of the Series (CrB2)nCrAl with n = 1, 2, 3 and a Unifying Concept for Ternary Borides as MAB-Phases.

    PubMed

    Ade, Martin; Hillebrecht, Harald

    2015-07-01

    Single crystals of the ternary borides Cr2AlB2, Cr3AlB4, Cr4AlB6, MoAlB, WAlB, Mn2AlB2, and Fe2AlB2 were grown from the elements with an excess of Al. Structures were refined by X-ray methods on the basis of single crystal data. All compounds crystallize in orthorhombic space groups. In each case boron atoms show the typical trigonal prisms BM6. The BM6-units are linked by common rectangular faces forming B-B-bonds. Thus, zigzag chains of boron atoms are obtained for MoAlB, WAlB, and M2AlB2 (M = Cr, Mn, Fe); chains of hexagons for Cr3AlB4; and double chains of hexagons for Cr4AlB6. The same subunits are known for the binary borides CrB, Cr3B4, Cr2B3, and β-WB, too. The boride partial structures are separated by single layers of Al-atoms in the case of the chromium compounds and double layers for WAlB, i.e., W2Al2B2. All crystal structures can be described using a unified building set principle with quadratic 4(4)-nets of metal atoms. The different compositions and crystal structures are obtained by different numbers of metal layers in the corresponding parts according to the formula (MB)2Aly(MB2)x. This principle is an extension of a scheme which was developed for the boridecarbides of niobium. Furthermore, there is a close similarity to the group of ternary carbides MAl(MC)n, so-called MAX-phases. Therefore, they might be named as "MAB-phases". The pronounced two-dimensionality and the mixture of strong covalent and metallic interactions make MAB-phases to promising candidates for interesting material properties. All compositions were confirmed by EDX measurements. Additionally, microhardness measurements were performed. PMID:26069993

  20. AlGaAs/GaAs transverse junction stripe lasers with distributed feedback

    NASA Technical Reports Server (NTRS)

    Hafich, M. J.; Skogman, R. A.; Petersen, P. E.; Kawanishi, H.

    1981-01-01

    Transverse junction stripe (TJS) lasers with periodic feedback were fabricated in two geometries. An interferometric and wet chemical etching technique was used to create a feedback grating across the entire pumping region for the distributed feedback (DFB) TJS laser and to create the separate distributed Bragg reflectors/DBR) for the TJS/DBR laser. The TJS/DFB laser was a double heterostructure device grown by liquid phase epitaxy (LPE) and had a third order grating etched in the top ALO.2GaO.8As layer. The grating was buried by growing an ALO.35GaO.65As layer on the grating by metal organic chemical vapor deposition (MO-CVD). The TJS/DBR laser was also fabricated in an LPE double heterostructure. The top AlGaAs layer was thinned to 0.1 micron over more than half of the laser so that the grating would be close to the GaAs active layer and optical field. Single mode operation in both configurations was obtained. The thermal shift of the laser wavelength in both cases was less than 1 Angstrom/deg K, compared to the 3 Angstrom/deg K shift of the spontaneous emission peak.

  1. GaAs nanowires and GaAs/AlGaAs core/shell nanowires synthesized by MOCVD

    NASA Astrophysics Data System (ADS)

    Peters, Brian; Minutillo, Nicholas; Carlin, John; Yang, Fengyuan

    2011-03-01

    Nanowires made by the ``bottom-up'' approach can be used in a variety of electrical and optoelectronic devices as well as in the study of low dimensional transport physics. We have grown GaAs nanowires using Au catalysts in a closed couple showerhead MOCVD system. A number of growth parameters, including the substrate temperature, growth rate, and Arsine/TMGa ratio, are explored to identify optimal conditions for growth of GaAs nanowires with large aspect ratio and minimal tapering. Higher substrate temperatures result in larger tapering and lower temperature leads to ``kinks.'' Meanwhile, large V/III source ratio gives large tapering as well. We have found that our optimal conditions are at a substrate temperature of 420°C and V/III ratio of ~ 25 , which gives a tapering of less than 1 nm increase in diameter per micron in length. In addition, GaAs/AlGaAs core/shell structured nanowires were also grown to minimize the surface states. Characterizations by SEM and photoluminescence will be presented. This work is supported by Department of Energy (DE-SC0001304).

  2. In-situ monitoring of GaSb, GaInAsSb, and AlGaAsSb

    SciTech Connect

    Vineis, C.J. |; Wang, C.A.; Jensen, K.F.; Breiland, W.G.

    1998-06-01

    Suitability of silicon photodiode detector arrays for monitoring the spectral reflectance during epitaxial growths of GaSb, AlGaAsSb, and GaInAsSb, which have cutoff wavelengths of 1.7, 1.2, and 2.3 {micro}m, respectively, is demonstrated. These alloys were grown lattice matched to GaSb in a vertical rotating-disk reactor, which was modified to accommodate near normal reflectance without affecting epilayer uniformity. By using a virtual interface model, the growth rate and complex refractive index at the growth temperature are extracted for these alloys over the 600 to 950 nm spectral range. Excellent agreement is obtained between the extracted growth rate and that determined by ex-situ measurement. Optical constants are compared to theoretical predictions based on an existing dielectric function model for these materials. Furthermore, quantitative analysis of the entire reflectance spectrum yields valuable information on the approximate thickness of overlayers on the pregrowth substrate.

  3. Highly coherent long cavity GaAs/AlGaAs single-quantum-well lasers

    SciTech Connect

    Larsson, A. ); Andrekson, P.A.; Jonsson, B.; Lindstrom, C. )

    1989-09-01

    The authors report on measurements of the spectral properties of ridge waveguide graded index separate confinement heterostructure single-quantum-well GaAs/AlGaAs lasers. Long cavity lasers (800{mu}m) exhibit remarkably pure single-longitudinal-mode spectra under continuous operation in spite of the short cavity mode spacing. At an output power of 5 mW, the sidemode suppression exceeds 24 dB and the linewidth is 1.5 MHz. This is among the narrowest linewidths reported for solitary AlGaAs lasers. The linewidth-power product is 6.4 MHz mW. Measurements of the linewidth-power product as a function of cavity length L gives an L/sup -2/ dependence in agreement with theory for lasers with small internal loss. No significant deviation from this dependence was observed for lasers short enough to operate at the second quantized state. The results are also used to deduce the linewidth enhancement factor {alpha} at the gain peak wavelength and its dependence on the excitation level. The sublinear gain-carrier density relation in the single quantum well results in an increase in a with increasing carrier density (decreasing cavity length) in contrast to conventional double heterostructure lasers and multiple-quantum-well lasers. In addition, a decrease in {alpha} was observed for lasers operating at the second quantized state due to recovery of the differential gain.

  4. Optical constants of GaAs-AlGaAs superlattices and multiple quantum wells

    NASA Technical Reports Server (NTRS)

    Kahen, K. B.; Leburton, J. P.

    1986-01-01

    The optical properties of GaAs-Al sub x Ga sub 1-xAs superlattices are calculated as a function of the frequency and superlattice structure. The comutations are performed using a partition method which combines the vectors k.p method with the pseudopotential technique. The influence of the super-structure on the electronic properties of the systems is accounted for by appropriate quantization conditions. The anisotropy and structure dependence of the dielectric constant result mainly from the contribution of the gamma region while the contributions of the other regions of the Brillouin zone are rather insensitive to the superlattice structure. The superlattice index of refraction values are shown to attain maxima at the various quantized transition energies, where for certain structures, the difference between the refractive indices of the superlattices and its corresponding Al sub x Ga sub 1-xAs alloy can be as large as 2%. In general results are in good agreement with the experimental data.

  5. Linear and nonlinear optical properties of GaAs/Al{sub x}Ga{sub 1−x}As/GaAs/Al{sub y}Ga{sub 1−y}As multi-shell spherical quantum dot

    SciTech Connect

    Emre Kavruk, Ahmet E-mail: aekavruk@gmail.com; Koc, Fatih; Sahin, Mehmet E-mail: mehsahin@gmail.com

    2013-11-14

    In this work, the optical properties of GaAs/Al{sub x}Ga{sub 1−x}As/GaAs/Al{sub y}Ga{sub 1−y}As multi-shell quantum dot heterostructure have been studied as a function of Al doping concentrations for cases with and without a hydrogenic donor atom. It has been observed that the absorption coefficient strength and/or resonant absorption wavelength can be adjusted by changing the Al content of inner-barrier and/or outer-barrier regions. Besides, it has been shown that the donor atom has an important effect on the control of the electronic and optical properties of the structure. The results have been presented as a function of the Al contents of the inner-barrier x and outer-barrier y regions and probable physical reasons have been discussed.

  6. Solid state phase equilibria and intermetallic compounds of the Al-Cr-Ho system

    SciTech Connect

    Pang, Mingjun; Zhan, Yongzhong; Du, Yong

    2013-02-15

    The solid state phase equilibria of the Al-Cr-Ho ternary system at 500 Degree-Sign C were experimentally investigated. The phase relations at 500 Degree-Sign C are governed by 14 three-phase regions, 29 two-phase regions and 15 single-phase regions. The existences of 10 binary compounds and 2 ternary phases have been confirmed. Al{sub 11}Cr{sub 2}, Al{sub 11}Cr{sub 4} and Al{sub 17}Ho{sub 2} were not found at 500 Degree-Sign C. Crystal structures of Al{sub 9}Cr{sub 4} and Al{sub 8}Cr{sub 4}Ho were determined by the Rietveld X-ray powder data refinement. Al{sub 9}Cr{sub 4} was found to exhibit cubic structure with space group I4-bar 3m (no. 217) and lattice parameters a=0.9107(5) nm. Al{sub 8}Cr{sub 4}Ho crystallizes in ThMn{sub 12} structure type with space group I4/mmm (no. 139) and lattice parameters a=0.8909(4) nm, c=0.5120(5) nm. It is concluded that the obtained Al{sub 4}Cr phase in this work should be {mu}-Al{sub 4}Cr by comparing with XRD pattern of the hexagonal {mu}-Al{sub 4}Mn compound. - Graphical abstract: The solid state phase equilibria of the Al-Cr-Ho ternary system at 500 Degree-Sign C. Highlights: Black-Right-Pointing-Pointer Al-Cr-Ho system has been investigated. Black-Right-Pointing-Pointer Al{sub 9}Cr{sub 4} has cubic structure with space group I4-bar 3m. Black-Right-Pointing-Pointer Al{sub 8}Cr{sub 4}Ho crystallizes in ThMn{sub 12} type with space group I4/mmm. Black-Right-Pointing-Pointer Al{sub 4}Cr phase is {mu}-type at 500 Degree-Sign C.

  7. Probing temperature gradients within the GaN buffer layer of AlGaN/GaN high electron mobility transistors with Raman thermography

    SciTech Connect

    Hodges, C. Pomeroy, J.; Kuball, M.

    2014-02-14

    We demonstrate the ability of confocal Raman thermography using a spatial filter and azimuthal polarization to probe vertical temperature gradients within the GaN buffer layer of operating AlGaN/GaN high electron mobility transistors. Temperature gradients in the GaN layer are measured by using offset focal planes to minimize the contribution from different regions of the GaN buffer. The measured temperature gradient is in good agreement with a thermal simulation treating the GaN thermal conductivity as homogeneous throughout the layer and including a low thermal conductivity nucleation layer to model the heat flow between the buffer and substrate.

  8. Microstructural stability of Fe-Cr-Al alloys at 450-550 °C

    NASA Astrophysics Data System (ADS)

    Ejenstam, Jesper; Thuvander, Mattias; Olsson, Pär; Rave, Fernando; Szakalos, Peter

    2015-02-01

    Iron-Chromium-Aluminium (Fe-Cr-Al) alloys have been widely investigated as candidate materials for various nuclear applications. Albeit the excellent corrosion resistance, conventional Fe-Cr-Al alloys suffer from α-α‧ phase separation and embrittlement when subjected to temperatures up to 500 °C, due to their high Cr-content. Low-Cr Fe-Cr-Al alloys are anticipated to be embrittlement resistant and provide adequate oxidation properties, yet long-term aging experiments and simulations are lacking in literature. In this study, Fe-10Cr-(4-8)Al alloys and a Fe-21Cr-5Al were thermally aged in the temperature interval of 450-550 °C for times up to 10,000 h, and the microstructures were evaluated mainly using atom probe tomography. In addition, a Kinetic Monte Carlo (KMC) model of the Fe-Cr-Al system was developed. No phase separation was observed in the Fe-10Cr-(4-8)Al alloys, and the developed KMC model yielded results in good agreement with the experimental data.

  9. Microstructure/Oxidation/Microhardness Correlations in Gamma-Based and Tau-Based Al-Ti-Cr Alloys

    NASA Technical Reports Server (NTRS)

    Brady, Michael P.; Smialek, J. L.; Humphrey, D. L.

    1994-01-01

    The relationships between alloy microstructure and air oxidation kinetics and alloy microstructure and microhardness in the Al-Ti-Cr system for exposures at 800 C and 1000 C were investigated. The relevant phases were identified as tau (Ll2), gamma (LIO), r-Al2Ti, TiCrAl (laves), and Cr2AI. Protective alumina formation was associated with tau, Al-rich TiCrAl, and gamma/TiCrAl mixtures. Brittleness was associated with the TiCrAl phase and tau decomposition to A12Ti + Cr2AI. It was concluded that two-phase gamma + TiCrAl alloys offer the greatest potential for oxidation resistance and room temperature ductility in the Al-Ti-Cr system.

  10. High electron mobility AlGaN/AlN/GaN HEMT structure with a nano-scale AlN interlayer

    NASA Astrophysics Data System (ADS)

    Huang, Shih-Chun; Chen, Wen-Ray; Hsu, Yu-Ting; Lin, Jia-Ching; Chang, Kuo-Jen; Lin, Wen-Jen

    2012-10-01

    Epitaxies of AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures with different thickness of nano-scale AlN interlayers have been realized by metalorganic chemical vapor deposition (MOCVD) technology. After epitaxy, high resolution X-ray diffraction (HRXRD), temperature-dependent Hall Effect and atomic force microscopy (AFM) measurements were used to characterize the properties of these samples. First, it was found that the Al composition of AlGaN layer increases from 21.6 to 34.2% with increasing the thickness of AlN interlayer from 0 to 5 nm under the same AlGaN growth conditions. This result may due to the influences of compressive stress and Al incorporation induced by the AlN interlayer. Then, we also found that the room-temperature (RT) electron mobility stays higher than 1500 cm2/Vs in the samples within AlN interlayer thickness range of 1.5 nm, on the other hand, the low-temperature (80K) electron mobility drops dramatically from 8180 to 5720 cm2/Vs in the samples with AlN interlayer thickness increasing from 1 to 1.5 nm. Furthermore, it was found that the two-dimensional electron gas (2DEG) density increases from 1.15×1013 to 1.58×1013 cm-2 beyond the AlN interlayer thickness of 1 nm. It was also found that the temperature independent 2DEG densities are observed in the samples with AlN interlayer thickness of 0.5 and 1 nm. The degenerated characteristics of the samples with AlN thickness thicker than 1.5 nm show the degraded crystalline quality which matched the observation of surface defects and small cracks formations from their AFM images. Finally, the 2DEG mobilities of the proposed structures can be achieved as high as 1705 and 8180 cm2/Vs at RT and 80K, respectively.

  11. Variable angle of incidence spectroscopic ellipsometry Application to GaAs-Al(x)Ga(1-x)As multiple heterostructures

    NASA Technical Reports Server (NTRS)

    Snyder, Paul G.; Rost, Martin C.; Bu-Abbud, George H.; Woollam, John A.; Alterovitz, Samuel A.

    1986-01-01

    The sensitivity of spectroscopic ellipsometry data to multilayer model parameters is shown to be a strong function of the angle of incidence. A quantitative study of sensitivity versus angle of incidence is performed for a GaAs-Al(x)Ga(1-x)As-GaAs substrate structure, showing that maximum sensitivity to layer thicknesses and AlGaAs composition occurs near the wavelength-dependent principal angle. These results are verified by experimental measurements on two molecular-beam epitaxy grown samples.

  12. Enhancement of Rashba interaction in GaAs/AlGaAs quantum wells due to the incorporation of bismuth

    SciTech Connect

    Simmons, R. A.; Jin, S. R.; Sweeney, S. J.; Clowes, S. K.

    2015-10-05

    This paper reports on the predicted increase in the Rashba interaction due to the incorporation of Bi in GaAs/AlGaAs heterostructures. Band structure parameters obtained from the band anti-crossing theory have been used in combination with self-consistent Schrödinger-Poisson calculations and k.p models to determine the electron spin-splitting caused by structural inversion asymmetry and increased spin-orbit interaction. A near linear seven fold increase in the strength of the Rashba interaction is predicted for a 10% concentration of Bi in a GaAsBi/AlGaAs quantum well heterostructure.

  13. The beam properties of high-power InGaAs/AlGaAs quantum well lasers

    NASA Astrophysics Data System (ADS)

    Wu, Xiang; Lu, Zukang; Wang, You; Takiguchi, Yoshihiro; Kan, Hirofumi

    2003-11-01

    The vertical beam quality factor of the fundamental TE propagating mode for InGaAs/AlGaAs SCH DQW lasers emitting at 940 nm is investigated by using the transfer matrix method and the non-paraxial vectorial moment theory for non-paraxial beams. An experimental approach is given for the measurement of the equivalent vertical beam quality factor of an InGaAs/AlGaAs SCH DQW laser. It has been shown that the vertical beam quality factor Mx2 is always larger than unity, whether the thickness of the active region of LDs is much smaller than the emission wavelength or not.

  14. Surface donor states distribution post SiN passivation of AlGaN/GaN heterostructures

    SciTech Connect

    Goyal, Nitin; Fjeldly, Tor A.

    2014-07-21

    In this paper, we present a physics based analytical model to describe the effect of SiN passivation on two-dimensional electron gas density and surface barrier height in AlGaN/GaN heterostructures. The model is based on an extraction technique to calculate surface donor density and surface donor level at the SiN/AlGaN interface. The model is in good agreement with the experimental results and promises to become a useful tool in advanced design and characterization of GaN based heterostructures.

  15. Electrical properties of ferromagnetic Ni{sub 2}MnGa and Co{sub 2}CrGa Heusler alloys

    SciTech Connect

    Kourov, N. I. Marchenkov, V. V.; Pushin, V. G.; Belozerova, K. A.

    2013-07-15

    The electrical properties of ferromagnetic Ni{sub 2}MnGa and Co{sub 2}CrGa Heusler alloys are measured in the temperature range 4-900 K. The effect of the energy gap near the Fermi level in the electronic spectrum on the behavior of electrical resistivity and absolute differential thermopower is discussed.

  16. Theoretical model of the polarization Coulomb field scattering in strained AlGaN/AlN/GaN heterostructure field-effect transistors

    SciTech Connect

    Luan, Chongbiao; Lin, Zhaojun Zhao, Jingtao; Wang, Yutang; Lv, Yuanjie; Chen, Hong; Wang, Zhanguo

    2014-07-28

    The theoretical model of the polarization Coulomb field scattering (PCF) caused by the polarization charge density variation at the AlGaN/AlN interface in strained AlGaN/AlN/GaN heterostructure field-effect transistors has been developed. And the theoretical values for the electron drift mobility, which were calculated using the Matthiessen's rule that includes PCF, piezoelectric scattering, polar optical-phonon scattering, and interface roughness scattering, are in good agreement with our experimental values. Therefore, the theoretical model for PCF has been confirmed.

  17. High-temperature molecular beam epitaxial growth of AlGaN/GaN on GaN templates with reduced interface impurity levels

    SciTech Connect

    Koblmueller, G.; Chu, R. M.; Raman, A.; Mishra, U. K.; Speck, J. S.

    2010-02-15

    We present combined in situ thermal cleaning and intentional doping strategies near the substrate regrowth interface to produce high-quality AlGaN/GaN high electron mobility transistors on semi-insulating (0001) GaN templates with low interfacial impurity concentrations and low buffer leakage. By exposing the GaN templates to an optimized thermal dissociation step in the plasma-assisted molecular beam epitaxy environment, oxygen, carbon, and, to lesser extent, Si impurities were effectively removed from the regrowth interface under preservation of good interface quality. Residual Si was further compensated by C-doped GaN via CBr{sub 4} to yield highly resistive GaN buffer layers. Improved N-rich growth conditions at high growth temperatures were then utilized for subsequent growth of the AlGaN/GaN device structure, yielding smooth surface morphologies and low residual oxygen concentration with large insensitivity to the (Al+Ga)N flux ratio. Room temperature electron mobilities of the two-dimensional electron gas at the AlGaN/GaN interface exceeded >1750 cm{sup 2}/V s and the dc drain current reached {approx}1.1 A/mm at a +1 V bias, demonstrating the effectiveness of the applied methods.

  18. Small-signal model parameter extraction for AlGaN/GaN HEMT

    NASA Astrophysics Data System (ADS)

    Le, Yu; Yingkui, Zheng; Sheng, Zhang; Lei, Pang; Ke, Wei; Xiaohua, Ma

    2016-03-01

    A new 22-element small signal equivalent circuit model for the AlGaN/GaN high electron mobility transistor (HEMT) is presented. Compared with the traditional equivalent circuit model, the gate forward and breakdown conductions (G gsf and G gdf) are introduced into the new model to characterize the gate leakage current. Additionally, for the new gate-connected field plate and the source-connected field plate of the device, an improved method for extracting the parasitic capacitances is proposed, which can be applied to the small-signal extraction for an asymmetric device. To verify the model, S-parameters are obtained from the modeling and measurements. The good agreement between the measured and the simulated results indicate that this model is accurate, stable and comparatively clear in physical significance.

  19. The electron g factor in AlGaN/GaN quantum wells

    NASA Astrophysics Data System (ADS)

    Li, Ming; Feng, Zhi-Bo.; Fan, Libo; Zhao, Yilong; Han, Hongpei; Feng, Tuanhui

    2016-04-01

    Considering the Rashba and Zeeman effects, the effective Hamiltonian for electrons in AlGaN/GaN quantum wells (QWs) with the magnetic field is obtained, and the effective transverse and longitudinal g-factor (g⊥,//), are derived. The small anisotropy of the g factor in bulk wurtzite materials is clearly shown, while the anisotropy in QWs induced by the quantum confined effect is evident. Moreover, the average g factor (g*) depends greatly on the position of the origin along the growth axis (c axis). With increasing well thickness, both g⊥ and g// increase, and the g-factor anisotropy first decreases and then increases slowly. Results show the g-factor and its anisotropy in III-nitride QWs can be modulated by the well thickness, and they are greatly affected by the internal electric field and the quantum confined effect.

  20. A dimer PT -symmetric model simulated in GaAs/AlGaAs quantum wells

    NASA Astrophysics Data System (ADS)

    Meng, Li-Chen; Zhang, Wen-Jing; Liu, Jibing; Xie, Xiao-Tao

    2016-05-01

    We perform the possibility to generate a dimer PT -symmetric model based on a double lambda four-level system in GaAs/AlGaAs quantum wells with biexcitonic transitions. By presenting the detuning management and modulating the Rabi frequencies of the two strong coupling laser fields, we show that the PT -symmetric model can be realized by the spatial evolution of the weak probe laser and four-wave mixing (FWM)-generated field along the propagation direction. The two weak fields in our model may be used to simulate two laser propagating in two PT -symmetric parallel waveguides. The diffraction effect also can be studied in some conditions. Our scheme offers two advantages: the complex refractive index is controlled by the strong coupling fields; the symmetry energy exchange between a dimer PT -symmetric structure is guaranteed by the four-wave mixing process. The present investigation may provide research opportunities in optical experiments.

  1. Room temperature spin transport in undoped (110) GaAs/AlGaAs quantum wells

    SciTech Connect

    Yokota, Nobuhide Aoshima, Yohei; Ikeda, Kazuhiro; Kawaguchi, Hitoshi

    2014-02-17

    We are reporting on our first observation of a micrometer-order electron spin transport in a (110) GaAs/AlGaAs multiple quantum well (QW) at room temperature using a space- and time-resolved Kerr rotation technique. A 37-μm transport was observed within an electron spin lifetime of 1.2 ns at room temperature when using an in-plane electric field of 1.75 kV/cm. The spatio-temporal profiles of electron spins were well reproduced by the spin drift-diffusion equations coupled with the Poisson equation, supporting the validity of the measurement. The results suggest that (110) QWs are useful as a spin transport layer for semiconductor spintronic devices operating at room temperature.

  2. AlGaAs-InGaAs SHEP's for space applications

    NASA Technical Reports Server (NTRS)

    Educato, J. L.; Wagner, M.; Leburton, J. P.

    1987-01-01

    The authors propose a novel class of cascade photovoltaics which are intended to overcome two problems with current cascade solar cell designs: poor upper subcell performance and excessive joule losses in the intercell ohmic contact (IOC). The devices feature multijunction upper subcells which improve spectral response by reducing the mean collection distance for photogenerated carriers. They circumvent the need for a monolithic IOC by using npn and pnp three-terminal cells arranged in a complementary configuration. Emphasis is on the AlGaAs-InGaAs system, which allows optimal partitioning of the solar spectrum to attain maximum possible efficiency for a two-gap cascade design. Computer simulations of these cells show that one-sun beginning-of-life efficiencies in excess of 27 percent are possible.

  3. Phonon-pumped terahertz gain in n-type GaAs/AlGaAs superlattices

    NASA Astrophysics Data System (ADS)

    Sun, Gregory; Soref, Richard A.

    2001-05-01

    Local population inversion and far-IR gain are proposed and theoretically analyzed for an unbiased n-doped GaAs/Al0.15Ga0.85As superlattice pumped solely by phonons. The lasing transition occurs at the Brillouin zone boundary of the superlattice wave vector kz between the two conduction minibands CB1 and CB2 of the opposite curvature in kz space. The proposed waveguided structure is contacted above and below by heat sinks at 300 K and 77 K, respectively. Atop the superlattice, a heat buffer layer confines longitudinal optical phonons for enhanced optical-phonon pumping of CB1 electrons. A gain of 345 cm-1 at 4.5 THz is predicted for a doping density of 2.8×1016cm-3.

  4. Dynamic properties of excitons in ZnO/AlGaN/GaN hybrid nanostructures

    PubMed Central

    Forsberg, Mathias; Hemmingsson, Carl; Amano, Hiroshi; Pozina, Galia

    2015-01-01

    Hybrid samples based on ZnO colloidal nanocrystals (NCs) deposited on AlGaN/GaN quantum well (QW) structures with different top barrier thickness d = 3, 6 and 9 nm are studied by time-resolved photoluminescence. Thermal behavior of the QW exciton lifetime in the hybrids and in the bare QW structures has been compared and it has been found that the QW exciton recombination rate increases in the hybrid having d = 3 nm and decreases in the hybrid with d = 6 nm, while no change has been observed for the structure with d = 9 nm. It is suggested that non-radiative resonance energy transfer from the QW excitons to the ZnO NCs and a variation of the surface potential can both influence the QW exciton lifetime in the hybrids. PMID:25601650

  5. SEMICONDUCTOR DEVICES: A high-performance enhancement-mode AlGaN/GaN HEMT

    NASA Astrophysics Data System (ADS)

    Zhihong, Feng; Shengyin, Xie; Rui, Zhou; Jiayun, Yin; Wei, Zhou; Shujun, Cai

    2010-08-01

    An enhancement-mode AlGaN/GaN HEMT with a threshold voltage of 0.35 V was fabricated by fluorine plasma treatment. The enhancement-mode device demonstrates high-performance DC characteristics with a saturation current density of 667 mA/mm at a gate bias of 4 V and a peak transconductance of 201 mS/mm at a gate bias of 0.8 V. The current-gain cut-off frequency and the maximum oscillation frequency of the enhancement-mode device with a gate length of 1 μm are 10.3 GHz and 12.5 GHz, respectively, which is comparable with the depletion-mode device. A numerical simulation supported by SIMS results was employed to give a reasonable explanation that the fluorine ions act as an acceptor trap center in the barrier layer.

  6. Small signal modeling of AlGaN/GaN HEMTs with consideration of CPW capacitances

    NASA Astrophysics Data System (ADS)

    Jiangfeng, Du; Peng, Xu; Kang, Wang; Chenggong, Yin; Yang, Liu; Zhihong, Feng; Shaobo, Dun; Qi, Yu

    2015-03-01

    Given the coplanar waveguide (CPW) effect on AlGaN/GaN high electron mobility transistors at a high frequency, the traditional equivalent circuit model cannot accurately describe the electrical characteristics of the device. The admittance of CPW capacitances is large when the frequency is higher than 40 GHz; its impact on the device cannot be ignored. In this study, a small-signal equivalent circuit model considering CPW capacitance is provided. To verify the model, S-parameters are obtained from the modeling and measurements. A good agreement is observed between the simulation and measurement results, indicating the reliability of the model. Project supported by the National Natural Science Foundation of China (Nos. 61376078, 61274086) and the Fundamental Research Funds for the Central Universities of China (No. ZYGX2012J041).

  7. AlGaAs/GaAs laser diode bars (λ = 808 nm) with improved thermal stability

    SciTech Connect

    Marmalyuk, A A; Ladugin, M A; Andreev, A Yu; Telegin, K Yu; Yarotskaya, I V; Meshkov, A S; Konyaev, V P; Sapozhnikov, S M; Lebedeva, E I; Simakov, V A

    2013-10-31

    Two series of AlGaAs/GaAs laser heterostructures have been grown by metal-organic vapour phase epitaxy, and 808-nm laser diode bars fabricated from the heterostructures have been investigated. The heterostructures differed in waveguide thickness and quantum well depth. It is shown that increasing the barrier height for charge carriers in the active region has an advantageous effect on the output parameters of the laser sources in the case of the heterostructures with a narrow symmetric waveguide: the slope of their power – current characteristics increased from 0.9 to 1.05 W A{sup -1}. Thus, the configuration with a narrow waveguide and deep quantum well is better suited for high-power laser diode bars under hindered heat removal conditions. (lasers)

  8. Electrically biased GaAs/AlGaAs heterostructures for enhanced detection of bacteria

    NASA Astrophysics Data System (ADS)

    Aziziyan, Mohammad R.; Hassen, Walid M.; Dubowski, Jan J.

    2016-03-01

    We have examined the influence of electrical bias on immobilization of bacteria on the surface of GaAs/AlGaAs heterostructures, functionalized with an alkanethiol based architecture. A mixture of biotinylated polyethylene glycol (PEG) thiol and hexadecanethiol was applied to attach neutravidin and antibodies targeting specific immobilization of Legionella pneumophila. An electrochemical setup was designed to bias biofunctionalized samples with the potential measured versus silver/silver chloride reference electrode in a three electrode configuration system. The immobilization efficiency has been examined with fluorescence microscopy after tagging captured bacteria with fluorescein labeled antibodies. We demonstrate more than 2 times enhanced capture of Legionella pneumophila, suggesting the potential of electrically biased biochips to deliver enhanced sensitivity in detecting these bacteria.

  9. Microwave characterization and modeling of GaAs/AlGaAs heterojunction bipolar transistors

    NASA Technical Reports Server (NTRS)

    Simons, Rainee N.; Romanofsky, Robert R.

    1987-01-01

    The characterization and modeling of a microwave GaAs/AlGaAs heterojunction Bipolar Transistor (HBT) are discussed. The de-embedded scattering parameters are used to derive a small signal lumped element equivalent circuit model using EEsof's Touchstone software package. Each element in the equivalent circuit model is shown to have its origin within the device. The model shows good agreement between the measured and modeled scattering parameters over a wide range of bias currents. Further, the MAG (maximum available power gain) and the h sub 21 (current gain) calculated from the measured data and those predicted by the model are also in good agreement. Consequently, the model should also be capable of predicting the f sub max and the f sub T of other HBTs.

  10. Proton implantation for the isolation of AlGaAs/GaAs quantum cascade lasers

    NASA Astrophysics Data System (ADS)

    Szerling, A.; Kosiel, K.; Kozubal, M.; Myśliwiec, M.; Jakieła, R.; Kuc, M.; Czyszanowski, T.; Kruszka, R.; Pągowska, K.; Karbownik, P.; Barcz, A.; Kamińska, E.; Piotrowska, A.

    2016-07-01

    The novel fabrication scheme of the mid-infrared (∼9.5 μm) Al0.45Ga0.55As/GaAs plasmon-enhanced-waveguide quantum cascade laser (QCL) is reported. The electric isolation was made exclusively by 6.5 μm-deep proton implantation. The applied implantation allowed us to suppress the current spreading and at the same time enabled the laser radiation confinement without any mesa formation. A galvanic gold layer at least 3.5 μm thick covering the top ohmic contact was used as a mask for implantation. This mask was not removed after the implantation, but it served for heat spreading from the laser. A considerable reduction in the necessary technological steps was obtained with the presented novel fabrication scheme, in comparison with the standard mesa-etching-based method.

  11. Self-aligned Si-Zn diffusion into GaAs and AlGaAs

    SciTech Connect

    Zou, W.X.; Corzine, S.; Vawter, G.A.; Merz, J.L.; Coldren, L.A.; Hu, E.L.

    1988-08-15

    A practical technology for self-aligned Si-Zn diffusion into GaAs and AlGaAs has been developed. It is found that the use of a Si film alone for self-aligned Si-Zn diffusion is subject to serious problems of morphology degradation and doping contamination during the process of the Si diffusion. A procedure combining the use of a SiO/sub 2/ film as an encapsulant with a sputtered Si film as source for Si diffusion and mask for Zn diffusion is investigated in detail. Optimum thicknesses of the Si and SiO/sub 2/ films are determined to be 180 and 550 A, respectively.

  12. Large linear magnetoresistance in a GaAs/AlGaAs heterostructure

    SciTech Connect

    Aamir, Mohammed Ali Goswami, Srijit Ghosh, Arindam; Baenninger, Matthias; Farrer, Ian; Ritchie, David A.; Tripathi, Vikram; Pepper, Michael

    2013-12-04

    We report non-saturating linear magnetoresistance (MR) in a two-dimensional electron system (2DES) at a GaAs/AlGaAs heterointerface in the strongly insulating regime. We achieve this by driving the gate voltage below the pinch-off point of the device and operating it in the non-equilibrium regime with high source-drain bias. Remarkably, the magnitude of MR is as large as 500% per Tesla with respect to resistance at zero magnetic field, thus dwarfing most non-magnetic materials which exhibit this linearity. Its primary advantage over most other materials is that both linearity and the enormous magnitude are retained over a broad temperature range (0.3 K to 10 K), thus making it an attractive candidate for cryogenic sensor applications.

  13. Al Incorporation at All Growth Stages of Al x Ga1- x N Epilayers Using SiN Treatment

    NASA Astrophysics Data System (ADS)

    Benzarti, Z.; Halidou, I.; Touré, A.; El Jani, B.

    2016-02-01

    Al compositional distribution of Al x Ga1- x N epilayers grown on SiN-treated sapphire substrate by atmospheric pressure metalorganic vapor phase epitaxy is investigated. The growth process was interrupted at various stages allowing a systematic study of Al x Ga1- x N epilayers during the smoothing process. A transition from three-dimensional (3D) to two-dimensional (2D) growth mode is revealed by in situ laser reflectometry (λ = 632.8 nm) as well as by atomic force microscopic images. Then, ion mass spectrometry analysis was performed to obtain the solid Al composition ( x) profile as well as by photoluminescence measurements. Moreover, the in situ reflectivity signal is simulated; thereby Al x Ga1- x N growth rate is derived and compared with that of GaN layer in order to study the effect of the aluminum incorporation on the growth mechanism. It is worth emphasising that the growth mode of Al x Ga1- x N layers is dictated by SiN treatment, which influences the Al compositional distribution. Electron mobility and refractive index against the thickness of Al x Ga1- x N layers have similar trends, which confirm a competitive mechanism between growth mode and Al incorporation. Therefore, the correlation between the Al composition and morphological, optical, and electrical properties of Al x Ga1- x N layers is established.

  14. Interaction of Cr3+ with valence and conduction bands in the long persistent phosphor ZnGa2O4:Cr3+, studied by ENDOR spectroscopy

    NASA Astrophysics Data System (ADS)

    Binet, Laurent; Sharma, Suchinder K.; Gourier, Didier

    2016-09-01

    Cr3+-doped zinc gallate ZnGa2O4 is a red-near infrared (IR) long persistent phosphor that can be excited by orange-red light, in the transparency window of living tissues. With this property, persistent luminescence nanoparticles were recently used for in vivo optical imaging of tumors in mice. In order to understand the origin of the excitability of persistent luminescence by visible light in this material, a Q-band ENDOR investigation of 71/69Ga and 53Cr nuclei was performed in ZnGa2O4:Cr3+ to get information on the interaction of Cr3+ with valence and conduction bands. The positive electron spin density at Ga nuclei revealed a dominant interaction of the 4A2 ground state of Cr3+ with the valence band, and a weaker interaction with the conduction band. The latter may occur only in the excited 2E and 4T2 states of Cr3+. It is proposed that when these two interactions are present, pairs of electrons and holes can be generated from excited Cr3+ in distorted sites undergoing local electric field produced by neighboring defects with opposite charges.

  15. Interaction of Cr(3+) with valence and conduction bands in the long persistent phosphor ZnGa2O4:Cr(3+), studied by ENDOR spectroscopy.

    PubMed

    Binet, Laurent; Sharma, Suchinder K; Gourier, Didier

    2016-09-28

    Cr(3+)-doped zinc gallate ZnGa2O4 is a red-near infrared (IR) long persistent phosphor that can be excited by orange-red light, in the transparency window of living tissues. With this property, persistent luminescence nanoparticles were recently used for in vivo optical imaging of tumors in mice. In order to understand the origin of the excitability of persistent luminescence by visible light in this material, a Q-band ENDOR investigation of (71/69)Ga and (53)Cr nuclei was performed in ZnGa2O4:Cr(3+) to get information on the interaction of Cr(3+) with valence and conduction bands. The positive electron spin density at Ga nuclei revealed a dominant interaction of the (4)A2 ground state of Cr(3+) with the valence band, and a weaker interaction with the conduction band. The latter may occur only in the excited (2)E and (4)T2 states of Cr(3+). It is proposed that when these two interactions are present, pairs of electrons and holes can be generated from excited Cr(3+) in distorted sites undergoing local electric field produced by neighboring defects with opposite charges. PMID:27460505

  16. Contribution of alloy clustering to limiting the two-dimensional electron gas mobility in AlGaN/GaN and InAlN/GaN heterostructures: Theory and experiment

    SciTech Connect

    Ahmadi, Elaheh; Mishra, Umesh K.; Chalabi, Hamidreza; Kaun, Stephen W.; Shivaraman, Ravi; Speck, James S.

    2014-10-07

    The influence of alloy clustering on fluctuations in the ground state energy of the two-dimensional electron gas (2DEG) in AlGaN/GaN and InAlN/GaN heterostructures is studied. We show that because of these fluctuations, alloy clustering degrades the mobility even when the 2DEG wavefunction does not penetrate the alloy barrier unlike alloy disorder scattering. A comparison between the results obtained for AlGaN/GaN and InAlN/GaN heterostructures shows that alloy clustering limits the 2DEG mobility to a greater degree in InAlN/GaN heterostructures. Our study also reveals that the inclusion of an AlN interlayer increases the limiting mobility from alloy clustering. Moreover, Atom probe tomography is used to demonstrate the random nature of the fluctuations in the alloy composition.

  17. InGaAsSb/AlGaAsSb Heterojunction Phototransistors for Infrared Applications

    NASA Technical Reports Server (NTRS)

    Refaat, Tamer F.; Abedin, M. N.; Sulima, Oleg V.; Ismail, Syed; Singh, Upendra N.

    2006-01-01

    High quality infrared (IR) quantum detectors are important for several applications, such as atmospheric remote sensing, chemical detection and absorption spectroscopy. Although several IR detectors are commercially available, with different materials and structures, they provide limited performance regarding the signal-to-noise ratio and the corresponding minimum detectable signal. InGaAsSb/AlGaAsSb heterojunction based phototransistors show strong potential for developing IR sensors with improved performance. In this paper, the performance of a novel npn InGaAsSb/AlGaAsSb heterojunction phototransistor is presented. This performance study is based on experimental characterization of the device dark current, noise and spectral response. Detectivity of 1.7x10(exp 9) cmHz(exp 1/2)/W at 2 microns was obtained at 100 C temperature and 2 V bias voltage. This corresponds to a responsivity of 94.7 A/W and an internal gain of 156 with about 37.7% quantum efficiency. Reducing the temperature to -30 C allows to increase the bias to 3V and enhance the detectivity to 8.7x10(exp 10) cmHz(exp 1/2)/W at the same wavelength, which corresponds to a responsivity of 386.5 A/W and an internal gain of 288.2 with about 83.3% quantum efficiency. The device impulse response and linearity, including the corresponding dynamic range, also are presented. Impulse response analysis indicated a settling time of about 1.1 s at 2V and 100 C, while linearity measurements indicated a constant responsivity in the radiation intensity range of 1.6x10(exp -7) W/sq cm and 31.6 mW/sq cm.

  18. Synthesis and characterization of Sr(Al,Cr) 12O 19 solid solution

    NASA Astrophysics Data System (ADS)

    Sandiumenge, Felip; Galí, Salvador

    1989-09-01

    Magnetoplumbite-type crystalline powders with SrAl 12- xCr xO 19 composition were synthesized in the range 0 ≤ x ≤ 4.4 by two different solid state reactions in air. The limited substitution of Al by trivalent Cr is accompanied by the appearance of a sesquioxide Al 2- yCr yO 3 solid solution and a new cubic phase with composition 3Al 2O 3 · SrCrO 4. These results are compared with those obtained in the synthesis of Sr(Fe,Cr) 12O 19 and Sr(Al,Fe) 12O 19 solid solutions. Finally, the cation distribution of Al and Cr in the system Sr(Al,Cr) 12O 19(Al,Cr) 2O 3 was obtained from X-ray diffraction profile analysis. The cationic distribution suggests that the solid solution range depends more on the crystal chemistry of the substitution than on the starting compounds used in the synthesis.

  19. Improved performance of AlGaN-based deep ultraviolet light-emitting diodes with n-AlGaN underlayers

    NASA Astrophysics Data System (ADS)

    Li, Lei; Tsutsumi, Tatsuya; Miyachi, Yuta; Miyoshi, Makoto; Egawa, Takashi

    2015-12-01

    We demonstrate notable performance improvement of 270 nm deep ultraviolet light-emitting diodes (DUV-LEDs) by inserting an n-AlGaN underlayer (UL), which was directly beneath an AlGaN multiple quantum well (MQW) active region. This DUV-LED exhibited significantly improved forward and reverse bias current-voltage characteristics, and spectral properties. As a result, the turn-on voltage was decreased from 7 to 4.6 V by introducing the n-AlGaN UL, and the maximum light output power and external quantum efficiency of DUV-LEDs with the n-AlGaN UL, obtained via on-wafer measurement, were increased by factors of 3.6 and 2.2, respectively. Further enhancements can be realized by using high-quality AlN/sapphire templates together with the n-AlGaN UL. These improvements were considered to be attributed to the introduction of the n-AlGaN UL which improved electron injection and reduced the quantum confined stark effect in the AlGaN MQWs.

  20. Comparative studies on the thermal stability and corrosion resistance of CrN, CrSiN, and CrSiN/AlN coatings

    SciTech Connect

    Kim, Gwang Seok; Kim, Sung Min; Lee, Sang Yul; Lee, Bo Young

    2009-07-15

    In this work, three kinds of Cr-based nitride coatings such as monolithic CrN, CrSiN coatings, and multilayered CrSiN/AlN coating with bilayer period of 3.0 nm were deposited on both Si (100) wafer and AISI H13 steel substrates by unbalanced magnetron sputtering. Thermal stability of these coatings was evaluated by annealing the coatings at temperatures between 600 and 1000 degree sign C for 30 min in air. In addition, the corrosion behaviors of these coatings were investigated by potentiodynamic polarization tests in a deaerated 3.5 wt. % NaCl solution at 40 degree sign C. Results from annealing test show the monolithic CrN and CrSiN coatings were completely oxidized after annealed at 800 and 900 degree sign C, and their cross sectional images and atomic force microscopy showed a loose and very porous morphology due to the oxidation. Also, the hardness values of the monolithic CrN and CrSiN coatings were decreased significantly from 22 and 27 GPa to 8 and 14 GPa, respectively. However, the multilayered CrSiN/AlN coating still exhibited a dense microstructure without visible change after annealed at 1000 degree sign C, and moreover, the relatively high hardness of 25 GPa was maintained. The superior thermal stability of the CrSiN/AlN multilayer coating could be attributed to the formation of the dense and stable oxidation barrier consisted of the Al{sub 2}O{sub 3}, Cr{sub 2}O{sub 3}, and amorphous SiO{sub 2} phases near the surface region, which retard the diffusion of oxygen into the coating. In the potentiodynamic polarization test results, it was found that the significantly improved corrosion resistance of the multilayered CrSiN/AlN coating was observed in comparison with those from the monolithic CrN and CrSiN coatings, and its corrosion current density (i{sub corr}) and protective efficiency were measured to be approximately 4.21 {mu}A/cm{sup 2} and 95%, respectively.

  1. An inverted AlGaAs/GaAs patterned-Ge tunnel junction cascade concentrator solar cell

    SciTech Connect

    Venkatasubramanian, R. )

    1993-01-01

    This report describes work to develop inverted-grown Al[sub 0.34]Ga[sub 0.66]As/GaAs cascades. Several significant developments are reported on as follows: (1) The AM1.5 1-sun total-area efficiency of the top Al[sub 0.34]Ga[sub 0.66]As cell for the cascade was improved from 11.3% to 13.2% (NREL measurement [total-area]). (2) The cycled'' organometallic vapor phase epitaxy growth (OMVPE) was studied in detail utilizing a combination of characterization techniques including Hall-data, photoluminescence, and secondary ion mass spectroscopy. (3) A technique called eutectic-metal-bonding (EMB) was developed by strain-free mounting of thin GaAs-AlGaAs films (based on lattice-matched growth on Ge substrates and selective plasma etching of Ge substrates) onto Si carrier substrates. Minority-carrier lifetime in an EMB GaAs double-heterostructure was measured as high as 103 nsec, the highest lifetime report for a freestanding GaAs thin film. (4) A thin-film, inverted-grown GaAs cell with a 1-sun AM1.5 active-area efficiency of 20.3% was obtained. This cell was eutectic-metal-bonded onto Si. (5) A thin-film inverted-grown, Al[sub 0.34]Ga[sub 0.66]As/GaAs cascade with AM1.5 efficiency of 19.9% and 21% at 1-sun and 7-suns, respectively, was obtained. This represents an important milestone in the development of an AlGaAs/GaAs cascade by OMVPE utilizing a tunnel interconnect and demonstrates a proof-of-concept for the inverted-growth approach.

  2. Detection of prostate-specific antigen with biomolecule-gated AlGaN/GaN high electron mobility transistors

    NASA Astrophysics Data System (ADS)

    Li, Jia-dong; Cheng, Jun-jie; Miao, Bin; Wei, Xiao-wei; Xie, Jie; Zhang, Jin-cheng; Zhang, Zhi-qiang; Wu, Dong-min

    2014-07-01

    In order to improve the sensitivity of AlGaN/GaN high electron mobility transistor (HEMT) biosensors, a simple biomolecule-gated AlGaN/GaN HEMT structure was designed and successfully fabricated for prostate specific antigen (PSA) detection. UV/ozone was used to oxidize the GaN surface and then a 3-aminopropyl trimethoxysilane (APTES) self-assembled monolayer was bound to the sensing region. This monolayer serves as a binding layer for attachment of the prostate specific antibody (anti-PSA). The biomolecule-gated AlGaN/GaN HEMT sensor shows a rapid and sensitive response when the target prostate-specific antigen in buffer solution was added to the antibody-immobilized sensing area. The current change showed a logarithm relationship against the PSA concentration from 0.1 pg/ml to 0.993 ng/ml. The sensitivity of 0.215% is determined for 0.1 pg/ml PSA solution. The above experimental result of the biomolecule-gated AlGaN/GaN HEMT biosensor suggested that this biosensor might be a useful tool for prostate cancer screening.

  3. Effect of Al-mole fraction in Al{sub x}Ga{sub 1−x}N grown by MOCVD

    SciTech Connect

    Jayasakthi, M. Ramesh, R. Prabakaran, K. Loganathan, R. Kuppulingam, B. Balaji, M. Arivazhagan, P. Sankaranarayanan, S. Singh, Shubra Baskar, K.

    2014-04-24

    AlGaN/AlN layers were grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. The Al{sub x}Ga{sub 1−x}N layer composition was varied from 15% to 25%. The crystalline quality, thickness and aluminum (Al) composition of AlGaN were determined using high resolution X-ray diffraction (HRXRD). The growth rate decreases on increasing Al composition. Reciprocal space mapping (RSM) was used to estimate the strain and relaxation between AlGaN and AlN. The optical properties of AlGaN layers were investigated by room temperature Photoluminescence (PL). The AlGaN peak shifts towards lower wavelength with Al composition. The surface morphology of AlGaN was studied by atomic force microscopy (AFM). Root mean square (RMS) roughness values were found to be increased in AlGaN layers with composition.

  4. Influence of strain induced by AlN nucleation layer on the electrical properties of AlGaN/GaN heterostructures on Si(111) substrate

    NASA Astrophysics Data System (ADS)

    Christy, Dennis; Watanabe, Arata; Egawa, Takashi

    2014-10-01

    The crack-free metal-organic chemical vapor deposition (MOCVD) grown AlGaN/GaN heterostructures on Si substrate with modified growth conditions of AlN nucleation layer (NL) and its influence on the electrical and structural properties of conductive GaN layer are presented. From the Hall electrical measurements, a gradual decrease of two-dimensional electron gas (2DEG) concentration near heterointerface as the function of NL thickness is observed possibly due to the reduction in difference of piezoelectric polarization charge densities between AlGaN and GaN layers. It also indicates that the minimum tensile stress and a relatively less total dislocation density for high pressure grown NL can ensure a 20 % increment in mobility at room temperature irrespective of the interface roughness. The thickness and pressure variations in NL and the subsequent changes in growth mode of AlN contributing to the post growth residual tensile stress are investigated using X-ray diffraction and Raman scattering experiments, respectively. The post growth intrinsic residual stress in top layers of heterostructures arises from lattice mismatches, NL parameters and defect densities in GaN. Hence, efforts to reduce the intrinsic residual stress in current conducting GaN layer give an opportunity to further improve the electrical characteristics of AlGaN/GaN device structures on Si.

  5. Influence of strain induced by AlN nucleation layer on the electrical properties of AlGaN/GaN heterostructures on Si(111) substrate

    SciTech Connect

    Christy, Dennis; Watanabe, Arata; Egawa, Takashi

    2014-10-15

    The crack-free metal-organic chemical vapor deposition (MOCVD) grown AlGaN/GaN heterostructures on Si substrate with modified growth conditions of AlN nucleation layer (NL) and its influence on the electrical and structural properties of conductive GaN layer are presented. From the Hall electrical measurements, a gradual decrease of two-dimensional electron gas (2DEG) concentration near heterointerface as the function of NL thickness is observed possibly due to the reduction in difference of piezoelectric polarization charge densities between AlGaN and GaN layers. It also indicates that the minimum tensile stress and a relatively less total dislocation density for high pressure grown NL can ensure a 20 % increment in mobility at room temperature irrespective of the interface roughness. The thickness and pressure variations in NL and the subsequent changes in growth mode of AlN contributing to the post growth residual tensile stress are investigated using X-ray diffraction and Raman scattering experiments, respectively. The post growth intrinsic residual stress in top layers of heterostructures arises from lattice mismatches, NL parameters and defect densities in GaN. Hence, efforts to reduce the intrinsic residual stress in current conducting GaN layer give an opportunity to further improve the electrical characteristics of AlGaN/GaN device structures on Si.

  6. Narrow divergence, single quantum well, separate confinement, AlGaAs laser

    SciTech Connect

    Haw, T.E.; Williams, J.E.; Wober, M.A.

    1991-01-29

    This patent describes a improvement in a structure for a narrow divergence, single quantum well, separate confinement, laser. It comprises: an n-AlGaAs cladding epitaxial layer, a first AlGaAs waveguide epitaxial layer, a GaAs quantum well active epitaxial layer, a second AlGaAs waveguide epitaxial layer, a p-AlGaAs cladding epitaxial layer, and a GaAs cap epitaxial layer, all sequentially grown with respect to each other. The improvement comprises: the n-AlGaAs cladding layer dimensioned to a thickness which is greater than 2 microns and doped to a density less than 5 {times} 10{sup 18}/cm{sup 3}; the first AlGaAs waveguide layer dimensioned to a thickness in a range between 400 and 700 Angstroms; the GaAs quantum well layer dimensioned to a thickness in a range between 50 and 200 Angstroms; the second AlGaAs waveguide layer dimensioned to a thickness in a range between 400 and 700 Angstroms; and the p-AlGaAs cladding layer dimensioned to a thickness which is greater than 2.0 microns and doped to a density less than 5 {times} 10{sup 18}/cm{sup 3}.

  7. High temperature oxidation resistant coatings for the directionally solidified Ni-Nb-Cr-Al eutectic superalloy

    NASA Technical Reports Server (NTRS)

    Strangman, T. E.; Ulion, N. E.; Felten, E. J.

    1977-01-01

    Protective coatings required for the Ni-Nb-Cr-Al directionally solidified eutectic superalloy were developed and evaluated on the basis of oxidation resistance, diffusional stability, thermal fatigue, and creep resistance. NiCrAlY+Pt and NiCrAlY physical vapor-deposition coating systems exhibited the best combination of properties. Burner-rig testing indicated that the useful life of a 127-micron-thick NiCrAlY+Pt coating exceeds 1000 h at 1366 K. Eutectic-alloy creep lives at 1311 K and a stress of 151.7 MN/sq m were greater for NiCrAlY+Pt-coated specimens than for uncoated specimens by a factor of two.

  8. Plasma chemistries for dry etching GaN, AlN, InGaN and InAlN

    SciTech Connect

    Pearton, S.J.; Vartuli, C.B.; Lee, J.W.; Donovan, S.M.; MacKenzie, J.D.; Abernathy, C.R.; Shul, R.J.; McLane, G.F.; Ren, F.

    1996-04-01

    Etch rates up to 7,000 {angstrom}/min. for GaN are obtained in Cl{sub 2}/H{sub 2}/Ar or BCl{sub 3}/Ar ECR discharges at 1--3mTorr and moderate dc biases. Typical rates with HI/H{sub 2} are about a factor of three lower under the same conditions, while CH{sub 4}/H{sub 2} produces maximum rates of only {approximately}2,000 {angstrom}/min. The role of additives such as SF{sub 6}, N{sub 2}, H{sub 2} or Ar to the basic chlorine, bromine, iodine or methane-hydrogen plasma chemistries are discussed. Their effect can be either chemical (in forming volatile products with N) or physical (in breaking bonds or enhancing desorption of the etch products). The nitrides differ from conventional III-V`s in that bond-breaking to allow formation of the etch products is a critical factor. Threshold ion energies for the onset of etching of GaN, InGaN and InAlN are {ge} 75 eV.

  9. Monitoring and Controlling of Strain During MOCVD of AlGaN for UV Optoelectronics

    SciTech Connect

    Han, J.; Crawford, M.H.; Shul, R.J.; Hearne, S.J.; Chason, E.; Figiel, J.J.; Banas, M.

    1999-01-14

    The grown-in tensile strain, due to a lattice mismatch between AlGaN and GaN, is responsible for the observed cracking that seriously limits the feasibility of nitride-based ultraviolet (UV) emitters. We report in-situ monitoring of strain/stress during MOCVD of AlGaN based on a wafer-curvature measurement technique. The strain/stress measurement confirms the presence of tensile strain during growth of AlGaN pseudomorphically on a thick GaN layer. Further growth leads to the onset of stress relief through crack generation. We find that the growth of AlGaN directly on low-temperature (LT) GaN or AlN buffer layers results in a reduced and possibly controllable strain.

  10. Effects of GaN interlayer on the transport properties of lattice-matched AlInN/AlN/GaN heterostructures

    NASA Astrophysics Data System (ADS)

    Wu, F.; Gao, K. H.; Li, Z. Q.; Lin, T.; Zhou, W. Z.

    2015-04-01

    We study the effects of GaN interlayer on the transport properties of two-dimensional electron gases confined in lattice-matched AlInN/AlN/GaN heterostructures. It is found that the Hall mobility is evidently enhanced when an additional ultrathin GaN interlayer is introduced between AlInN and AlN layers. The enhancement of the Hall mobility is especially remarkable at low temperature. The high Hall mobility results in a low sheet resistance of 23 Ω / ◻ at 2 K. Meanwhile, Shubnikov-de Haas oscillations (SdH) are also remarkably enhanced due to the existence of GaN interlayer. The enhancement of the SdH oscillations is related to the larger quantum mobility μ q owing to the suppression of the interface roughness, alloy disorder, and ionized impurity scatterings by the GaN interlayer.

  11. Effects of GaN interlayer on the transport properties of lattice-matched AlInN/AlN/GaN heterostructures

    SciTech Connect

    Wu, F.; Gao, K. H. Li, Z. Q.; Lin, T.; Zhou, W. Z.

    2015-04-21

    We study the effects of GaN interlayer on the transport properties of two-dimensional electron gases confined in lattice-matched AlInN/AlN/GaN heterostructures. It is found that the Hall mobility is evidently enhanced when an additional ultrathin GaN interlayer is introduced between AlInN and AlN layers. The enhancement of the Hall mobility is especially remarkable at low temperature. The high Hall mobility results in a low sheet resistance of 23 Ω/◻ at 2 K. Meanwhile, Shubnikov-de Haas oscillations (SdH) are also remarkably enhanced due to the existence of GaN interlayer. The enhancement of the SdH oscillations is related to the larger quantum mobility μ{sub q} owing to the suppression of the interface roughness, alloy disorder, and ionized impurity scatterings by the GaN interlayer.

  12. Evaluation of Ti-48Al-2Cr-2Nb Under Fretting Conditions

    NASA Technical Reports Server (NTRS)

    Miyoshi, Kazuhisa; Lerch, Bradley A.; Draper, Susan L.; Raj, Sai V.

    2001-01-01

    The fretting behavior of Ti-48Al-2Cr-2Nb (y-TiAl) in contact with the nickel-base superalloy 718 was examined in air at temperatures from 296 to 823 K (23 to 550 C). The interfacial adhesive bonds between Ti-48Al-2Cr-2Nb and superalloy 718 were generally stronger than the cohesive bonds within Ti-48Al-2Cr-2Nb. The failed Ti-48Al-2Cr-2Nb debris subsequently transferred to the superalloy 718. In reference experiments conducted with Ti-6Al-4V against superalloy 718 under identical fretting conditions, the degree of transfer was greater for Ti-6A1-4V than for Ti-48Al-2Cr-2Nb. Wear of Ti-48Al-2Cr-2Nb generally decreased with increasing fretting frequency. The increasing rate of oxidation at elevated temperatures led to a drop in wear at 473 K. However, fretting wear increased as the temperature was increased from 473 to 823 K. At 723 and 823 K, oxide film disruption generated cracks, loose wear debris, and pits on the Ti-48Al-2Cr-2Nb wear surface. Both increasing slip amplitude and increasing load tended to produce more metallic wear debris, causing severe abrasive wear in the contacting metals.

  13. V-pit to truncated pyramid transition in AlGaN-based heterostructures

    NASA Astrophysics Data System (ADS)

    Mogilatenko, A.; Enslin, J.; Knauer, A.; Mehnke, F.; Bellmann, K.; Wernicke, T.; Weyers, M.; Kneissl, M.

    2015-11-01

    The formation of three-dimensional truncated pyramids after the deposition of AlN/GaN superlattices onto (0001) AlN/sapphire templates has been analysed by atomic force microscopy as well as transmission electron microscopy. V-pits in AlN layers and the formation of nano-mounds around the v-pit edges are suggested to be responsible for the pyramid formation. Keeping the individual AlN layer thickness at 2.5 nm in the 80xAlN/GaN superlattice, the transformation to the three-dimensional pyramids is observed when the individual GaN layer thickness exceeds 1.5 nm. A subsequent overgrowth of the pyramidal structures by AlGaN results in inhomogeneous Ga distribution in the layers and laterally inhomogeneous strain states. Nevertheless, compared to the growth on planar layers, the overgrowth of the truncated pyramids leads to a slight reduction in dislocation density from 1 · 1010 cm-2 (for GaN thickness of 1 nm in SL) to 7 · 109 cm-2 (for GaN thickness of 2 nm in SL). The non-planar growth front and thus the compositional inhomogeneity in AlGaN vanish gradually with increasing AlGaN thickness. As a result, homogeneous 4 μm thick Al0.5Ga0.5N buffer layers suitable for the fabrication of UV-B LED structures can be obtained.

  14. Novel attributes of AlGaN/AlN/GaN/SiC HEMTs with the multiple indented channel

    NASA Astrophysics Data System (ADS)

    Orouji, Ali A.; Ghaffari, Majid

    2015-11-01

    In this paper, a high performance AlGaN/AlN/GaN/SiC High Electron Mobility Transistor (HEMT) with the multiple indented channel (MIC-HEMT) is proposed. The main focus of the proposed structure is based on reduction of the space around the gate, stop of the spread of the depletion region around the source-drain, and decrement of the thickness of the channel between the gate and drain. Therefore, the breakdown voltage increases, meanwhile the elimination of the gate depletion layer extension to source/drain decreases the gate-source and gate-drain capacitances. The optimized results reveal that the breakdown voltage and the drain saturation current increase about 178% and 46% compared with a conventional HEMT (C-HEMT), respectively. Therefore, the maximum output power density is improved by factor 4.1 in comparison with conventional one. Also, the cut-off frequency of 25.2 GHz and the maximum oscillation frequency of 92.1 GHz for the MIC-HEMT are obtained compared to 13 GHz and 43 GHz for that of the C-HEMT and the minimum figure noise decreased consequently of reducing the gate-drain and gate-source capacitances by about 42% and 40%, respectively. The proposed MIC-HEMT shows a maximum stable gain (MSG) exceeding 24.1 dB at 3.1 GHz which the greatest gain is yet reported for HEMTs, showing the potential of this device for high power RF applications.

  15. FeAl underlayers for CoCrPt thin film longitudinal media

    SciTech Connect

    Lee, L.; Laughlin, D.E.; Lambeth, D.N.

    1997-04-01

    B2 ordered FeAl films with a small, uniform grain size have been produced by rf diode sputter deposition on glass substrates. CoCrPt films grown on FeAl underlayers were found to have the (10{bar 1}0) lamellar texture. The in-plane coercivities (H{sub c}) of the CoCrPt/FeAl films are comparable to those of the CoCrPt/Cr films and they can be further improved by inserting a thin Cr intermediate layer between the CoCrPt and the FeAl layers. By employing a MgO seed layer or a (002) textured Cr seed layer, (001) textured FeAl can be obtained. However, the (001) FeAl underlayer only induces a weak (11{bar 2}0) textured CoCrPt. Thus no improvement in H{sub c} over those produced on unseeded FeAl underlayers was observed. {copyright} {ital 1997 American Institute of Physics.}

  16. Structural disorder and magnetism in the spin-gapless semiconductor CoFeCrAl

    NASA Astrophysics Data System (ADS)

    Choudhary, Renu; Kharel, Parashu; Valloppilly, Shah R.; Jin, Yunlong; O'Connell, Andrew; Huh, Yung; Gilbert, Simeon; Kashyap, Arti; Sellmyer, D. J.; Skomski, Ralph

    2016-05-01

    Disordered CoFeCrAl and CoFeCrSi0.5Al0.5 alloys have been investigated experimentally and by first-principle calculations. The melt-spun and annealed samples all exhibit Heusler-type superlattice peaks, but the peak intensities indicate a substantial degree of B2-type chemical disorder. Si substitution reduces the degree of this disorder. Our theoretical analysis also considers several types of antisite disorder (Fe-Co, Fe-Cr, Co-Cr) in Y-ordered CoFeCrAl and partial substitution of Si for Al. The substitution transforms the spin-gapless semiconductor CoFeCrAl into a half-metallic ferrimagnet and increases the half-metallic band gap by 0.12 eV. Compared CoFeCrAl, the moment of CoFeCrSi0.5Al0.5 is predicted to increase from 2.01 μB to 2.50 μB per formula unit, in good agreement with experiment.

  17. Long range order and vacancy properties in Al-rich Fe{sub 3}Al and Fe{sub 3}Al(Cr) alloys

    SciTech Connect

    Kim, S.M.; Morris, D.G.

    1998-05-01

    Neutron powder diffraction measurements have been carried out in situ from room temperature to about 100 C in Fe28Al (28 at.% Al), Fe32.5Al (32.5 at.% Al) and Fe28Al15Cr (28 at.% Al, 5 at.% Cr) alloys. X-ray diffraction and TEM studies provided supporting information. The data were analyzed to obtain information about the temperature dependence of the DO{sub 3} and B2 long range order parameters, the location of the Cr atoms and their effect on the ordering energies, and on the vacancy formation and migration properties in Fe28Al and Fe32.5Al alloys. The location of the ternary alloying addition in DO{sub 3} and B2 ordered Al-rich Fe{sub 3}Al is shown to be consistent with considerations of interatomic bond energies.

  18. Persistent photoconductivity and carrier transport in AlGaN/GaN heterostructures treated by fluorine plasma

    SciTech Connect

    Li, B. K.; Ge, W. K.; Wang, J. N.; Chen, K. J.

    2008-02-25

    Fluorine plasma treatment technique can effectively incorporate fluorine atoms into the AlGaN barrier, depleting the two-dimensional electron gases (2DEGs) in the channel of AlGaN/GaN high electron mobility transistors and converting the device from depletion mode to enhancement mode. To reveal the underlying physical mechanisms, temperature dependent persistent photoconductivity (PPC) and Hall measurements are conducted in AlGaN/GaN heterostructures treated by CF{sub 4} plasma. Weakly temperature dependent 2DEG mobility and much more pronounced PPC effect are observed in the F-treated sample. An energy barrier of 624 meV for electrons recaptured by the F-related centers is extracted from the PPC decay behaviors.

  19. Corrosion and Discharge Behaviors of Al-Mg-Sn-Ga-In in Different Solutions

    NASA Astrophysics Data System (ADS)

    Xiong, Hanqing; Yin, Xiang; Yan, Yang; Dai, Yilong; Fan, Sufeng; Qiao, Xueyan; Yu, Kun

    2016-06-01

    Al-0.5 wt.%Mg-0.08 wt.%Sn-0.05 wt.%Ga-0.05 wt.%In and Al-0.5 wt.%Mg-0.08 wt.%Sn-0.05 wt.%Ga alloys were prepared by melting, casting and cold rolling. Corrosion and discharge behaviors of the two experimental alloys were investigated by electrochemical measurement, self-corrosion rate measurement, air battery testing, and scanning electron microscopy. The results showed that Al-Mg-Sn-Ga-In alloy exhibited higher electrochemical activity than Al-Mg-Sn-Ga alloy in 2 M NaCl solution, while it showed lower electrochemical activity than Al-Mg-Sn-Ga alloy in 4 M NaOH solution. By comparison with the air battery based on Al-Mg-Sn-Ga alloy, the battery with Al-Mg-Sn-Ga-In alloy cannot exhibit better discharge performance in 4 M NaOH electrolyte. However, the performance of the air battery based on Al-Mg-Sn-Ga-In alloy was greatly improved due to the In-rich inclusions and the uniform corroded morphology in 2 M NaCl electrolyte. Thus, Al-Mg-Sn-Ga-In alloy was a good anode material for Al-air battery in 2 M NaCl electrolyte.

  20. Efficient photo-enhancement of GaP and AlGaP growth in chemical beam epitaxy

    NASA Astrophysics Data System (ADS)

    Yoshimoto, M.; Ozasa, K.; Matsunami, H.

    1991-11-01

    The growth rate of GaP and AlGaP epitaxial layers was efficiently enhanced by N2-laser irradiation at low substrate temperatures in chemical beam epitaxy. The photo-enhancement efficiency, defined as the ratio of the number of deposited Ga atoms to the number of irradiating photons per unit area per unit time, is estimated to be 7×10-4. The decomposition of triethylaluminum was also enhanced by the irradiation, but less efficiently.

  1. Nitrogen-concentration control in GaNAs/AlGaAs quantum wells using nitrogen δ-doping technique

    SciTech Connect

    Mano, Takaaki; Jo, Masafumi; Kuroda, Takashi; Noda, Takeshi; Sugimoto, Yoshimasa; Sakuma, Yoshiki; Elborg, Martin; Sakoda, Kazuaki

    2014-05-15

    GaNAs/Al{sub 0.35}Ga{sub 0.65}As multiple quantum wells (MQWs) with nitrogen δ-doping were fabricated on GaAs (100) substrates by plasma-assisted molecular beam epitaxy. High controllability of nitrogen-concentrations in the MQWs was achieved by tuning nitrogen δ-doping time. The maximum nitrogen concentration in the MQWs was 2.8%. The MQWs exhibit intense, narrow photoluminescence emission.

  2. Microstructure evolution during annealing of TiAl/NiCoCrAl multilayer composite prepared by EB-PVD

    SciTech Connect

    Zhang, Rubing; Zhang, Deming; Chen, Guiqing; Wang, Yuesheng

    2014-07-01

    TiAl/NiCoCrAl laminate composite sheet with a thickness of 0.4–0.6 mm as well as a dimension of 150 mm × 100 mm was fabricated successfully by using electron beam physical vapor deposition (EB-PVD) method. The annealing treatment was processed at 1123 and 1323 K for 3 h in a high vacuum atmosphere, respectively. The phase composition and microstructure of TiAl/NiCoCrAl microlaminated sheet have been analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Based on the sheet characterization and results of the microstructure evolution during annealing treatment process, the diffusion mechanism of interfacial reaction in TiAl/NiCoCrAl microlaminate was investigated and discussed.

  3. Coaxial GaAs-AlGaAs core-multishell nanowire lasers with epitaxial gain control

    NASA Astrophysics Data System (ADS)

    Stettner, T.; Zimmermann, P.; Loitsch, B.; Döblinger, M.; Regler, A.; Mayer, B.; Winnerl, J.; Matich, S.; Riedl, H.; Kaniber, M.; Abstreiter, G.; Koblmüller, G.; Finley, J. J.

    2016-01-01

    We demonstrate the growth and single-mode lasing operation of GaAs-AlGaAs core-multishell nanowires (NW) with radial single and multiple GaAs quantum wells (QWs) as active gain media. When subject to optical pumping lasing emission with distinct s-shaped input-output characteristics, linewidth narrowing and emission energies associated with the confined QWs are observed. Comparing the low temperature performance of QW NW laser structures having 7 coaxial QWs with a nominally identical structure having only a single QW shows that the threshold power density reduces several-fold, down to values as low as ˜2.4 kW/cm2 for the multiple QW NW laser. This confirms that the individual radial QWs are electronically weakly coupled and that epitaxial design can be used to optimize the gain characteristics of the devices. Temperature-dependent investigations show that lasing prevails up to 300 K, opening promising new avenues for efficient III-V semiconductor NW lasers with embedded low-dimensional gain media.

  4. Irradiation-enhanced α' precipitation in model FeCrAl alloys

    DOE PAGESBeta

    Edmondson, Philip D.; Briggs, Samuel A.; Yamamoto, Yukinori; Howard, Richard H.; Sridharan, Kumar; Terrani, Kurt A.; Field, Kevin G.

    2016-02-17

    Model FeCrAl alloys with varying compositions (Fe(10–18)Cr(10–6)Al at.%) have been neutron irradiated at ~ 320 to damage levels of ~ 7 displacements per atom (dpa) to investigate the compositional influence on the formation of irradiation-induced Cr-rich α' precipitates using atom probe tomography. In all alloys, significant number densities of these precipitates were observed. Cluster compositions were investigated and it was found that the average cluster Cr content ranged between 51.1 and 62.5 at.% dependent on initial compositions. This is significantly lower than the Cr-content of α' in binary FeCr alloys. As a result, significant partitioning of the Al from themore » α' precipitates was also observed.« less

  5. Interface states in Al2O3/AlGaN/GaN metal-oxide-semiconductor structure by frequency dependent conductance technique

    NASA Astrophysics Data System (ADS)

    Liao, Xue-Yang; Zhang, Kai; Zeng, Chang; Zheng, Xue-Feng; En, Yun-Fei; Lai, Ping; Hao, Yue

    2014-05-01

    Frequency dependent conductance measurements are implemented to investigate the interface states in Al2O3/AlGaN/GaN metal-oxide-semiconductor (MOS) structures. Two types of device structures, namely, the recessed gate structure (RGS) and the normal gate structure (NGS), are studied in the experiment. Interface trap parameters including trap density Dit, trap time constant τit, and trap state energy ET in both devices have been determined. Furthermore, the obtained results demonstrate that the gate recess process can induce extra traps with shallower energy levels at the Al2O3/AlGaN interface due to the damage on the surface of the AlGaN barrier layer resulting from reactive ion etching (RIE).

  6. 1100 to 1500 K Slow Plastic Compressive Behavior of NiAl-xCr Single Crystals

    NASA Technical Reports Server (NTRS)

    Whittenberger, J. Daniel; Darolia, Ram

    2003-01-01

    The compressive properties of near <001> and <111> oriented NiAl-2Cr single crystals and near <011> oriented NiAl-6Cr samples have been measured between 1100 and 1500 K. The 2Cr addition produced significant solid solution strengthening in NiAl, and the <111> and <001> single crystals possessed similar strengths. The 6Cr crystals were not stronger than the 2Cr versions. At 1100 and 1200 K plastic flow in all three Cr-modified materials was highly dependent on stress with exponents > 10. The <011> oriented 6Cr alloy exhibited a stress exponent of about 8 at 1400 and 1500 K; whereas both <001> and <111> NiAl-2Cr crystals possessed stress exponents near 3 which is indicative of a viscous dislocation glide creep mechanism. While the Cottrell-Jaswon solute drag model predicted creep rates within a factor of 3 at 1500 K for <001>-oriented NiAl-2Cr; this mechanism greatly over predicted creep rates for other orientations and at 1400 K for <001> crystals.

  7. Identification of the Ga interstitial in Al(x)Ga(1-x)As by optically detected magnetic resonance

    NASA Technical Reports Server (NTRS)

    Kennedy, T. A.; Spencer, M. G.

    1986-01-01

    A new optically detected magnetic resonance spectrum in Al(x)Ga(1-x)As is reported and assigned to native Ga interstitials. Luminescence-quenching signals were observed over the energy region from 0.75 to 1.1 eV. The optically detected magnetic resonance is nearly isotropic, with spin-Hamiltonian parameters g = 2.025 + or - 0.006, central hyperfine splitting A(Ga-69) = 0.050 + or - 0.001/cm, and A(Ga-71) = 0.064 + or - 0.001/cm for H near the 001 line. The strong hyperfine coupling denotes an electronic state of A1 symmetry, which current theories predict for the Ga interstitial but not the Ga antisite. The slight anisotropy probably indicates that the Ga(i) is paired with a second, unknown defect.

  8. Short-wavelength, mid- and far-infrared intersubband absorption in nonpolar GaN/Al(Ga)N heterostructures

    NASA Astrophysics Data System (ADS)

    Lim, Caroline B.; Beeler, Mark; Ajay, Akhil; Lähnemann, Jonas; Bellet-Amalric, Edith; Bougerol, Catherine; Schörmann, Jörg; Eickhoff, Martin; Monroy, Eva

    2016-05-01

    This paper assesses nonpolar m-oriented GaN:Si/Al(Ga)N heterostructures grown on free-standing GaN for intersubband optoelectronics in the short-wavelength, mid- and far-infrared ranges. Characterization results are compared with reference c-plane samples and interpreted by correlation with self-consistent Schrödinger–Poisson calculations. In the near- and mid-infrared regions, we demonstrate m-GaN/Al(Ga)N multi-quantum-wells exhibiting room-temperature intersubband absorption tunable in the range of 1.5–5.8 µm (827–214 meV), the long wavelength limit being set by the second order of the Reststrahlen band in the GaN substrates. Extending the study to the far-infrared region, low-temperature intersubband transitions in the 1.5–9 THz range (6.3–37.4 meV) are observed in larger m-plane GaN/AlGaN multi-quantum-wells, covering most of the 7–10 THz band forbidden to GaAs-based technologies.

  9. Deep-level defects and turn-on capacitance recovery characteristics in AlGaN/GaN heterostructures

    NASA Astrophysics Data System (ADS)

    Nakano, Yoshitaka; Irokawa, Yoshihiro; Sumiya, Masatomo

    2015-06-01

    We report on turn-on capacitance recovery measurements as a simple short-time method of evaluating carrier-trapping phenomena in a two-dimensional electron gas (2DEG) in the bulk region of AlGaN/GaN heterostructures, employing their Schottky barrier diodes. Using this technique, we have investigated an in-depth relation between deep-level defects and 2DEG carrier trapping in an AlGaN/GaN heterostructure with a GaN buffer layer containing a high C concentration. Steady-state photo-capacitance spectroscopy measurements revealed three C-related deep-level defects located at ~2.07, ~2.80 and ~3.23 eV below the conduction band in the GaN buffer layer. Additionally, turn-on capacitance recovery measurements showed a large decrease in recovery time under white-light optical illuminations with long-pass filters between 370 and 390 nm. It is concluded that the ~3.23 eV level is mainly responsible for the 2DEG carrier-trapping phenomena in the GaN buffer layer of the AlGaN/GaN heterostructure.

  10. Two-photon passive electro-optic upconversion in a GaAs /AlGaAs heterostructure device

    NASA Astrophysics Data System (ADS)

    Zhao, Lai; Thompson, Pete; Faleev, N. N.; Prather, D. W.; Appelbaum, Ian

    2007-03-01

    A semiconductor heterostructure device that requires no external power source to upconvert two low-energy photons into one higher-energy photon is proposed. This passive device is fabricated in the AlGaAs /GaAs material system and it is used to demonstrate photon upconversion from 808to710nm at room temperature.

  11. Channel Temperature Estimates for Microwave AlGaN/GaN Power HEMTS on SiC and Sapphire

    NASA Technical Reports Server (NTRS)

    Freeman, Jon C.

    2003-01-01

    A simple technique to estimate the channel temperature of a generic AlGaN/GaN HEMTs on SiC or Sapphire, while incorporating temperature dependence of the thermal conductivity is presented. The procedure is validated b y comparing it's predictions with the experimentally measured temperatures in devices presented in three recently published articles.

  12. Oxygen-induced Al surface segregation in Al(x)Ga(1-x)As and the effect of Y overlayers on the oxidation of the Y/Al(x)Ga(1-x)As interface

    NASA Technical Reports Server (NTRS)

    Mesarwi, A.; Ignatiev, A.

    1992-01-01

    The oxidation of Al(x)Ga(1-x)As (x = 0.15, AlGaAs) was studied by AES and XPS at 350 C and different oxygen exposures (up to 5 x 10 exp 4 L). Also studied were the effects of yttrium overlayers (theta = 3 ML) on the oxidation of the AlGaAs surface. Substantial oxygen-induced Al surface segregation has been observed for both yttriated and nonyttriated AlGaAs surfaces which increased with increasing oxygen exposure. Also observed is a significant Y-enhanced oxidation of the AlGaAs surface. Oxidation of the yttriated AlGaAs surface was found to be a factor of 4 greater than that of the nonyttriated surface. Also, while oxidation of the nonyttriated AlGaAs yielded mainly Al2O(x) (x less than 3) and only little Ga2O3, the yttriated AlGaAs surface oxide layer was principally Ga2O3 and stoichiometric Al2O3. However, both the yttriated and nonyttriated surfaces were found to contain metallic As within the oxide layer.

  13. Diffusional transport during the cyclic oxidation of gamma + beta, Ni-Cr-Al(Y, Zr) alloys

    NASA Technical Reports Server (NTRS)

    Nesbitt, J. A.; Heckel, R. W.

    1988-01-01

    The cyclic oxidation behavior of several cast gamma + beta, Ni-Cr-Al(Y, Zr) alloys and one low-pressure plasma spraying gamma + beta, Ni-Co-Cr-Al(Y) alloy was studied. Cyclic oxidation was found to result in a decreasing Al concentration at the oxide-metal interface due to a high rate of Al consumption coupled with oxide scale cracking and spalling. Diffusion paths plotted on the ternary phase diagram showed higher Ni concentrations with increasing cyclic oxidation exposures. The alloy with the highest rate of Al consumption and the highest Al content underwent breakaway oxidation following 500 1-hr cycles at 1200 C.

  14. Novel high-current density GaN-based normally off transistor with tensile-strained quaternary InAlGaN barrier

    NASA Astrophysics Data System (ADS)

    Kajitani, Ryo; Tanaka, Kenichiro; Ogawa, Masahiro; Ishida, Hidetoshi; Ishida, Masahiro; Ueda, Tetsuzo

    2015-04-01

    A GaN-based normally off heterostructure field effect transistor (HFET) with high current density and with a quaternary InAlGaN barrier instead of an AlGaN barrier is investigated. It is difficult to obtain both high-current operation and normally off operation in the GaN-based HFET because of the need to control the polarization-induced charge. In order to obtain a normally off operation using quaternary InAlGaN barrier, the InAlGaN barrier is selectively removed and the p-AlGaN layer is formed as the gate electrode. The obtained threshold voltage of the InAlGaN-based HFET is +1.1 V. The maximum drain current reaches as high as 0.73 A/mm, which is almost twice that of a conventional AlGaN-based normally off gate injection transistor (GIT).

  15. Strain dependence on polarization properties of AlGaN and AlGaN-based ultraviolet lasers grown on AlN substrates

    SciTech Connect

    Bryan, Zachary Bryan, Isaac; Sitar, Zlatko; Collazo, Ramón; Mita, Seiji; Tweedie, James

    2015-06-08

    Since the band ordering in AlGaN has a profound effect on the performance of UVC light emitting diodes (LEDs) and even determines the feasibility of surface emitting lasers, the polarization properties of emitted light from c-oriented AlGaN and AlGaN-based laser structures were studied over the whole composition range, as well as various strain states, quantum confinements, and carrier densities. A quantitative relationship between the theoretical valence band separation, determined using k•p theory, and the experimentally measured degree of polarization is presented. Next to composition, strain was found to have the largest influence on the degree of polarization while all other factors were practically insignificant. The lowest crossover point from the transverse electric to transverse magnetic polarized emission of 245 nm was found for structures pseudomorphically grown on AlN substrates. This finding has significant implications toward the efficiency and feasibility of surface emitting devices below this wavelength.

  16. Self-assembled GaN quantum wires on GaN/AlN nanowire templates.

    PubMed

    Arbiol, Jordi; Magen, Cesar; Becker, Pascal; Jacopin, Gwénolé; Chernikov, Alexey; Schäfer, Sören; Furtmayr, Florian; Tchernycheva, Maria; Rigutti, Lorenzo; Teubert, Jörg; Chatterjee, Sangam; Morante, Joan R; Eickhoff, Martin

    2012-12-01

    We present a novel approach for self-assembled growth of GaN quantum wires (QWRs) exhibiting strong confinement in two spatial dimensions. The GaN QWRs are formed by selective nucleation on {112[combining macron]0} (a-plane) facets formed at the six intersections of {11[combining macron]00} (m-plane) sidewalls of AlN/GaN nanowires used as a template. Based on microscopy observations we have developed a 3D model explaining the growth mechanism of QWRs. We show that the QWR formation is governed by self-limited pseudomorphic growth on the side facets of the nanowires (NWs). Quantum confinement in the QWRs is confirmed by the observation of narrow photoluminescence lines originating from individual QWRs with emission energies up to 4.4 eV. Time-resolved photoluminescence studies reveal a short decay time (~120 ps) of the QWR emission. Capping of the QWRs with AlN allows enhancement of the photoluminescence, which is blue-shifted due to compressive strain. The emission energies from single QWRs are modelled assuming a triangular cross-section resulting from self-limited growth on a-plane facets. Comparison with the experimental results yields an average QWR diameter of about 2.7 nm in agreement with structural characterization. The presented results open a new route towards controlled realization of one-dimensional semiconductor quantum structures with a high potential both for fundamental studies and for applications in electronics and in UV light generation. PMID:23100169

  17. High growth rate of AlGaN for buffer structures for GaN on Si to increase throughput

    NASA Astrophysics Data System (ADS)

    Matsumoto, Koh; Ubukata, Akinori; Ikenaga, Kazutada; Naito, Kazuki; Yamamoto, Jun; Yano, Yoshiki; Tabuchi, Toshiya; Yamaguchi, Akira; Ban, Yuzaburo; Uchiyama, Kosuke

    2012-03-01

    Throughput requirement of the epitaxial process of GaN on Si is described. The impact of the growth rate of AlGaN for the buffer layer of GaN on Si is highlighted. In the attempt of growing GaN on Si, we have tested a production scale high flow speed MOVPE reactor (TAIYO NIPPON SANSO UR25k) for 6 inch X 7 wafers. Al0.58Ga0.42N was grown with the growth rate of 1.85μm/hr at 30 kPa. AlN was grown with the growth rate of 1.4μm/hr at 13kPa. AlN/GaN SLS (5nm/20nm) was also grown at the growth rate of 1.4μm/hr. An excellent uniformity of aluminum concentration of less than 0.5% was also obtained for Al0.58Ga0.42N. The challenge which we are facing to further increase of the throughput is summarized.

  18. High purity GaAs and Al xGa 1-xAs grown by metalorganic molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Furuhata, Naoki; Okamoto, Akihiko; Hoshino, Hitoshi

    1990-06-01

    High purity GaAs and Al xGa 1- xAs were grown by metalorganic molecular beam epitaxy (MOMBE) using metalorganic sources and arsenic (As 4). In GaAs growth using triethylgallium (TEG) and As 4, carrier concentrations depended on As 4 pressure and substrate temperatures. Below 1.5 x 10 -4 Torr As 4 pressure, the p-type carrier concentrations decreased with an increase in As 4 pressure. Above 1.5 x 10 -4 Torr As 4 pressure, the epilayer converted from p-type to n-type conductivity. Carrier concentrations decreased with a decrease in substrate temperatures. At 500°C and 1.5 x 10 -4 Torr As 4 pressure, the carrier concentration in the p-type GaAs epilayer exhibited 1.5 x 10 14 cm -3 with a room temperature mobility of 400 cm 2/V.s. Al xGa 1-xAs ( x=0.1-0. .2) was also grown using TEG, three different Al sources (triethylaluminum (TEA), trimethylaluminum (TMA) and dimethylaluminum hydride (DMAH) and As 4. Using TEG, TEA and As 4, the Al 0.15Ga 0.85As epilayer showed p-type conduction with carrier concentration of 1.2x10 15 cm -3 and mobility of 117 cm 2/V.s. This carrier concentration is the lowest value ever reported for Al xGa 1- xAs grown by MOMBE.

  19. Defect studies in ion irradiated AlGaN

    SciTech Connect

    Jagielski, Jacek; Thome, Lionel; Zhang, Yanwen; Wang, Chong M.; Turos, Andrzej; Nowicki, L.; Pagowska, K.; Jozwik, I.

    2010-06-01

    Defects created in Al0.4Ga0.6N crystals by 320 keV Ar ion irradiation were studied by using RBS/C and TEM techniques. One of the main aims of the work was to use a new version of McChasy, a Monte – Carlo simulation code of backscattering spectra, for the analysis of experimental results obtained for a dislocations-containing crystal. Transmission Electron Microscopy technique was used to get a better insight into dislocation and dislocation loop geometries in order to restrict the range of parameters used in simulations. RBS/C analysis was performed in a 1.5 MeV – 3 MeV energy range in order to check if MC simulations correctly reproduce backscattering spectra at different energies.

  20. The reliability of /AlGa/As CW laser diodes

    NASA Astrophysics Data System (ADS)

    Ettenberg, M.; Kressel, H.

    1980-02-01

    Major factors bearing on the reliability of (AlGa)As CW laser diodes are reviewed with attention given to the degradation modes of facet mirror damage, contact degradation, and internal damage. Detailed results are provided for the oxide-defined stripe-contact double-heterojunction lasers operated for more than 40,000 h with extrapolations indicating a median time to failure between 100,000 and 1,000,000. Facet damage and contact degradation appear to be under control, and internal damage remains the dominant failure mechanism. Most of the data deals with threshold current increase; however, shifts in far-field pattern and changes in laser modulation characteristics, including self-sustained oscillations, may affect laser performance in real systems.

  1. Metadynamics investigations of the AlN/GaN superlattice

    NASA Astrophysics Data System (ADS)

    Duan, Yifeng; Qin, Lixia; Liu, Hanyu

    2016-05-01

    Pressure-induced phase transitions of the AlN/GaN superlattice at ambient temperature are systematically investigated using density-functional–based metadynamics simulations. Accompanied with the hexagonal-to-tetragonal phase transition, the coordination number increases to six from four. The homogeneous deformation pathway is energetically favorable and driven by the dynamical instability. Furthermore, no stable intermediate structure of five-fold coordination appears during the evolutionary process of phase transition. The band gap of the hexagonal phase is always direct, and that of the tetragonal phase always indirect due to the strong p-d repulsion. As the metastep number increases, the band gap is enlarged to an ultraviolet-spectrum range.

  2. Spin dependent transport properties of Mn-Ga/MgO/Mn-Ga magnetic tunnel junctions with metal(Mg, Co, Cr) insertion layer

    SciTech Connect

    Liang, S. H.; Tao, L. L.; Liu, D. P. Han, X. F.; Lu, Y.

    2014-04-07

    We report a first principles theoretical investigation of spin polarized quantum transport in Mn{sub 2}Ga/MgO/Mn{sub 2}Ga and Mn{sub 3}Ga/MgO/Mn{sub 3}Ga magnetic tunneling junctions (MTJs) with the consideration of metal(Mg, Co, Cr) insertion layer effect. By changing the concentration of Mn, our calculation shows a considerable disparity in transport properties: A tunneling magnetoresistance (TMR) ratio of 852% was obtained for Mn{sub 2}Ga-based MTJs, however, only a 5% TMR ratio for Mn{sub 3}Ga-based MTJs. In addition, the influence of insertion layer has been considered in our calculation. We found the Co insertion layer can increase the TMR of Mn{sub 2}Ga-based MTJ to 904%; however, the Cr insertion layer can decrease the TMR by 668%; A negative TMR ratio can be obtained with Mg insertion layer. Our work gives a comprehensive understanding of the influence of different insertion layer in Mn-Ga based MTJs. It is proved that, due to the transmission can be modulated by the interfacial electronic structure of insertion, the magnetoresistance ratio of Mn{sub 2}Ga/MgO/Mn{sub 2}Ga MTJ can be improved by inserting Co layer.

  3. Tribological Properties of the Fe-Al-Cr Alloyed Layer by Double Glow Plasma Surface Metallurgy

    NASA Astrophysics Data System (ADS)

    Luo, Xixi; Yao, Zhengjun; Zhang, Pingze; Zhou, Keyin; Wang, Zhangzhong

    2016-07-01

    A Fe-Al-Cr alloyed layer was deposited onto the surface of Q235 low-carbon steel via double glow plasma surface metallurgy (DGPSM) to improve the steel's wear resistance. After the DGPSM treatment, the Fe-Al-Cr alloyed layer grown on the Q235 low-carbon steel was homogeneous and compact and had a thickness of 25 µm. The layer was found to be metallurgically adhered to the substrate. The frictional coefficient and specific wear rate of the sample with a Fe-Al-Cr alloyed layer (treated sample) were both lower than those of the bare substrate (untreated sample) at the measured temperatures (25, 250 and 450 °C). The results indicated that the substrate and the alloyed layer suffered oxidative wear and abrasive wear, respectively, and that the treated samples exhibited much better tribological properties than did the substrate. The formation of Fe2AlCr, Fe3Al(Cr), FeAl(Cr), Fe(Cr) sosoloid and Cr23C6 phases in the alloyed layer dramatically enhanced the wear resistance of the treated sample. In addition, the alloyed layer's oxidation film exhibited a self-healing capacity with lubrication action that also contributed to the improvement of the wear resistance at high temperature. In particular, at 450 °C, the specific wear rate of treated sample was 2.524 × 10-4 mm3/N m, which was only 45.2% of the untreated sample.

  4. Low-moment ferrimagnetic phase of the Heusler compound Cr2CoAl

    NASA Astrophysics Data System (ADS)

    Jamer, Michelle E.; Marshall, Luke G.; Sterbinsky, George E.; Lewis, Laura H.; Heiman, Don

    2015-11-01

    Synthesizing half-metallic fully compensated ferrimagnets that form in the inverse Heusler phase could lead to superior spintronic devices. These materials would have high spin polarization at room temperature with very little fringing magnetic fields. Previous theoretical studies indicated that Cr2CoAl should form in a stable inverse Heusler lattice due to its low activation energy. Here, stoichiometric Cr2CoAl samples were arc-melted and annealed at varying temperatures, followed by studies of their structural and magnetic properties. High-resolution synchrotron X-ray diffraction revealed a chemically ordered Heusler phase in addition to CoAl and Cr phases. Soft X-ray magnetic circular dichroism revealed that the Cr and Co magnetic moments are antiferromagnetically oriented leading to the observed low magnetic moment in Cr2CoAl.

  5. Comparison in mechanical and tribological properties of CrTiAlMoN and CrTiAlN nano-multilayer coatings deposited by magnetron sputtering

    NASA Astrophysics Data System (ADS)

    Wang, Tao; Zhang, Guojun; Jiang, Bailing

    2016-02-01

    CrTiAlN and CrTiAlMoN nano-multilayer coatings were deposited by closed field unbalanced magnetron sputtering. TiMoN and CrTiMoN nano-multilayer coatings with same Mo2N layer thickness were also prepared for comparison. The structure of these coatings is investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The mechanical and tribological properties were characterized and compared by nano-indentation and ball-on-disc test. It was found that these coatings were structured by fcc metal nitride phases (including CrN, TiN, AlN and Mo2N) and the preferred orientation changed from (1 1 1) to (2 0 0) with the increase of Mo content. The TEM results showed that the coatings exhibited typical columnar structure and nano-multilayer structure with modulation periods ranged from 3.2 nm to 7.6 nm. Among these coatings, CrTiAlMoN coatings presented the highest hardness, lowest coefficient of friction (COF) and wear rate. The hardness of these nano-multilayer coatings were determined by layer interfaces: TiN/Mo2N and AlN/Mo2N layer interface showed benefit on hardness enhancement while CrN/Mo2N layer interface led to a great hardness decrement. In comparison with the other as-deposited coatings, the low COF of CrTiAlMoN coatings was not only affected by Mo addition but also related to its oxidation behaviors.

  6. Characteristics of GaN/Si(1 1 1) epitaxy grown using Al 0.1Ga 0.9N/AlN composite nucleation layers having different thicknesses of AlN

    NASA Astrophysics Data System (ADS)

    Jang, Seong-Hwan; Lee, Seung-Jae; Seo, In-Seok; Ahn, Haeng-Keun; Lee, Oh-Yeon; Leem, Jae-Young; Lee, Cheul-Ro

    2002-06-01

    We have studied the effects of Al 0.1Ga 0.9N(150 nm)/AlN composite nucleation layers (CNLs) having different thicknesses of AlN ranging from 20 to 41 nm on the growth characteristics of GaN/Si(1 1 1) epitaxy. The surface morphology of the GaN epitaxial layers which were grown on Al 0.1Ga 0.9N(150 nm)/AlN CNLs showed that the number of thermal etch pits and cracks were abruptly decreased with the increase of AlN thickness from 20 to 35 nm. However, the morphology of GaN epitaxy which was grown on Al 0.1Ga 0.9N(150 nm)/AlN CNL having AlN of thickness 41 nm above 35 nm showed that the number of etch pits increased again. The GaN/Si(1 1 1) epitaxy grown by using Al 0.1Ga 0.9N(150 nm)/AlN(35 nm) CNL showed that the highest crystallinity having a FWHM of 1157 arcsec for the (0 0 0 2) diffraction. Photoluminescence (PL) spectrum at room temperature for GaN/Si(1 1 1) epitaxy grown using Al 0.1Ga 0.9N(150 nm)/AlN(35 nm) CNL showed sharp band edge emission at 364 nm, which do not have yellow luminescence related to various defects such as vacancy and dislocation. The PL spectra at room temperature for the GaN layers grown using other CNLs showed yellow luminescence at around 580 nm in addition to the band edge emission. Moreover, the FWHM of the main exitonic peak at 10 K for the GaN/Si(1 1 1) epitaxy, which was grown using Al 0.1Ga 0.9N(150 nm)/AlN(35 nm) CNL, has the lowest value of 12.81 meV. It is obvious that the Al 0.1Ga 0.9N(150 nm)/AlN CNL having suitable thickness of AlN plays an important role in improving the crystallinity and optical properties of GaN/Si(1 1 1) heteroepitaxy without showing any defects such as pits and cracks over the surface by reducing the mismatch of thermal expansion coefficient and lattice constant between GaN and Si(1 1 1) compared with other nucleation layer such as Al xGa 1- xN or AlN alone.

  7. An analysis of temperature dependent piezoelectric Franz-Keldysh effect in AlGaN

    NASA Astrophysics Data System (ADS)

    Hou, Y. T.; Teo, K. L.; Li, M. F.; Uchida, Kazuo; Tokunaga, Hiroki; Akutsu, Nakao; Matsumoto, Koh

    2000-02-01

    Strong Franz-Keldysh oscillations near the band gap of AlGaN are observed in the contactless electroreflectance (CER) studies of a GaN/InGaN/AlGaN multilayer structure. The line shape analysis of the CER spectra at different temperatures provides an accurate determination of the AlGaN band gap energies and the built-in electric fields. Using the existing data of the thermal expansion coefficients of GaN and sapphire, and the piezoelectric constants of AlGaN, the temperature dependence of the electric field is estimated and is in good agreement with the experimental results between 15 and 300 K. We attribute such electric field to the piezoelectric strain effect.

  8. Development of ODS FeCrAl alloys for accident-tolerant fuel cladding

    SciTech Connect

    Dryepondt, Sebastien N.; Hoelzer, David T.; Pint, Bruce A.; Unocic, Kinga A.

    2015-09-18

    FeCrAl alloys are prime candidates for accident-tolerant fuel cladding due to their excellent oxidation resistance up to 1400 C and good mechanical properties at intermediate temperature. Former commercial oxide dispersion strengthened (ODS) FeCrAl alloys such as PM2000 exhibit significantly better tensile strength than wrought FeCrAl alloys, which would alloy for the fabrication of a very thin (~250 m) ODS FeCrAl cladding and limit the neutronic penalty from the replacement of Zr-based alloys by Fe-based alloys. Several Fe-12-Cr-5Al ODS alloys where therefore fabricated by ball milling FeCrAl powders with Y2O3 and additional oxides such as TiO2 or ZrO2. The new Fe-12Cr-5Al ODS alloys showed excellent tensile strength up to 800 C but limited ductility. Good oxidation resistance in steam at 1200 and 1400 C was observed except for one ODS FeCrAl alloy containing Ti. Rolling trials were conducted at 300, 600 C and 800 C to simulate the fabrication of thin tube cladding and a plate thickness of ~0.6mm was reached before the formation of multiple edge cracks. Hardness measurements at different stages of the rolling process, before and after annealing for 1h at 1000 C, showed that a thinner plate thickness could likely be achieved by using a multi-step approach combining warm rolling and high temperature annealing. Finally, new Fe-10-12Cr-5.5-6Al-Z gas atomized powders have been purchased to fabricate the second generation of low-Cr ODS FeCrAl alloys. The main goals are to assess the effect of O, C, N and Zr contents on the ODS FeCrAl microstructure and mechanical properties, and to optimize the fabrication process to improve the ductility of the 2nd gen ODS FeCrAl while maintaining good mechanical strength and oxidation resistance.

  9. NMR study of the ternary carbides M2 AlC (M=Ti,V,Cr)

    NASA Astrophysics Data System (ADS)

    Lue, C. S.; Lin, J. Y.; Xie, B. X.

    2006-01-01

    We have performed a systematic study of the layered ternary carbides Ti2AlC , V2AlC , and Cr2AlC using Al27 NMR spectroscopy. The quadrupole splittings, Knight shifts, as well as spin-lattice relaxation times on each material have been identified. The sign of the isotropic Knight shift varies from positive for Ti2AlC and V2AlC to negative for Cr2AlC , attributed to the enhancement of hybridization with increasing valence electron count in the transition metal. Universally long relaxation times are found for these alloys. Results provide a measure of Al-s Fermi-level density of states Ns(EF) for Ti2AlC and V2AlC . In addition, the evidence that Ns(EF) correlates with the transition metal d -electron count has been explored in the present NMR investigation.

  10. Study of the effects of GaN buffer layer quality on the dc characteristics of AlGaN/GaN high electron mobility transistors

    DOE PAGESBeta

    Ahn, Shihyun; Zhu, Weidi; Dong, Chen; Le, Lingcong; Hwang, Ya-Hsi; Kim, Byung-Jae; Ren, Fan; Pearton, Stephen J.; Lind, Aaron G.; Jones, Kevin S.; et al

    2015-04-21

    Here we studied the effect of buffer layer quality on dc characteristics of AlGaN/GaN high electron mobility (HEMTs). AlGaN/GaN HEMT structures with 2 and 5 μm GaN buffer layers on sapphire substrates from two different vendors with the same Al concentration of AlGaN were used. The defect densities of HEMT structures with 2 and 5 μm GaN buffer layer were 7 × 109 and 5 × 108 cm₋2, respectively, as measured by transmission electron microscopy. There was little difference in drain saturation current or in transfer characteristics in HEMTs on these two types of buffer. However, there was no dispersionmore » observed on the nonpassivated HEMTs with 5 μm GaN buffer layer for gate-lag pulsed measurement at 100 kHz, which was in sharp contrast to the 71% drain current reduction for the HEMT with 2 μm GaN buffer layer.« less

  11. Study of the effects of GaN buffer layer quality on the dc characteristics of AlGaN/GaN high electron mobility transistors

    SciTech Connect

    Ahn, Shihyun; Zhu, Weidi; Dong, Chen; Le, Lingcong; Hwang, Ya-Hsi; Kim, Byung-Jae; Ren, Fan; Pearton, Stephen J.; Lind, Aaron G.; Jones, Kevin S.; Kravchenko, I. I.; Zhang, Ming-Lan

    2015-04-21

    Here we studied the effect of buffer layer quality on dc characteristics of AlGaN/GaN high electron mobility (HEMTs). AlGaN/GaN HEMT structures with 2 and 5 μm GaN buffer layers on sapphire substrates from two different vendors with the same Al concentration of AlGaN were used. The defect densities of HEMT structures with 2 and 5 μm GaN buffer layer were 7 × 109 and 5 × 108 cm₋2, respectively, as measured by transmission electron microscopy. There was little difference in drain saturation current or in transfer characteristics in HEMTs on these two types of buffer. However, there was no dispersion observed on the nonpassivated HEMTs with 5 μm GaN buffer layer for gate-lag pulsed measurement at 100 kHz, which was in sharp contrast to the 71% drain current reduction for the HEMT with 2 μm GaN buffer layer.

  12. The influence of Cr and Al pack cementation on low carbon steel to improve oxidation resistance

    NASA Astrophysics Data System (ADS)

    Prasetya, Didik; Sugiarti, Eni; Destyorini, Fredina; Thosin, Kemas Ahmad Zaini

    2012-06-01

    Pack chromizing and aluminizing has been widely used for many years to improve hot temperature oxidation and corrosion resistance of metals. The coating process involves packing the steel in a powder mixture which contain aluminum and chromium source, and inert filler (usually alumina), and halide activator NH4Cl. Al and Cr were deposited onto carbon steel by pack cementation process using elemental Al and Cr powder as Al and Cr source, whereas NiCo alloys codeposited by electrodeposition. The position of Al and Cr could be under or over Ni-Co alloys deposited. Pack cementation was heated on dry inert gas at temperature 800 °C about 5 hours and 20 minute for Cr and Al respectively. Al and Cr was successfully deposited. Laying down effect of Al and Cr onto carbon steel whether up and down toward NiCo alloys coating have affected to oxidation resistance. The pack aluminizing as top layer given best resitance to restrain excessive oxide scale, in contrast pack chromizing reveal bad oxidation resistance, moreover occured spallation on layer.

  13. A study of the impact of gate metals on the performance of AlGaN/AlN/GaN heterostructure field-effect transistors

    NASA Astrophysics Data System (ADS)

    Zhao, Jingtao; Lin, Zhaojun; Chen, Quanyou; Yang, Ming; Cui, Peng; Lv, Yuanjie; Feng, Zhihong

    2015-09-01

    The fabrication processes of heterostructure field-effect transistors (HFETs) often introduce stresses into the device structures. These stresses can significantly affect the electrical characteristics of the device. We investigated the additional strain induced by the gate metals (in this study, Au, Cu, Fe, Al, and Ni) in AlGaN/AlN/GaN HFETs using the measured capacitance-voltage and current-voltage characteristics. We found that the intensity of the additional strain of the AlGaN barrier layer is greatly affected by the types of gate metals. For those metals that mainly undergo a physical interaction with the AlGaN barrier layer, there is a negative correlation between the intensity of the additional strain and the Young's modulus of the gate metal. Therefore, the gate metal with a larger Young's modulus will be more favorable for weakening polarization Coulomb field scattering and increasing both the carrier mobility and the sheet carrier density in AlGaN/AlN/GaN HFETs.

  14. Kinetics of borided 31CrMoV9 and 34CrAlNi7 steels

    SciTech Connect

    Efe, Goezde Celebi; Ipek, Mediha; Ozbek, Ibrahim; Bindal, Cuma

    2008-01-15

    In this study, kinetics of borides formed on the surface of 31CrMoV9 and 34CrAlNi7 steels borided in solid medium consisting of Ekabor II at 850-900-950 deg. C for 2, 4, 6 and 8 h were investigated. Scanning electron microscopy and optical microscopy examinations showed that borides formed on the surface of borided steels have columnar morphology. The borides formed in the coating layer confirmed by X-ray diffraction analysis are FeB, Fe{sub 2}B, CrB, and Cr{sub 2}B. The hardnesses of boride layers are much higher than that of matrix. It was found that depending on process temperature and time the fracture toughness of boride layers ranged from 3.93 to 4.48 MPa m{sup 1/2} for 31CrMoV9 and from 3.87 to 4.40 MPa m{sup 1/2} for 34CrAlNi7 steel. Activation energy, growth rate and growth acceleration of boride layer calculated according to these kinetic studies revealed that lower activation energy results in the fast growth rate and high growth acceleration.

  15. Simultaneous observation of two dimensional electron gas and polarization in AlGaN/GaN heterostructure using scanning nonlinear dielectric microscopy

    NASA Astrophysics Data System (ADS)

    Hirose, Kotaro; Goto, Yasunori; Chinone, Norimichi; Cho, Yasuo

    2016-08-01

    The AlGaN/GaN heterostructure has two-dimensional electron gas (2DEG) and electric polarization. We observed both of them simultaneously in the cross section of the AlGaN/GaN heterostructure by scanning nonlinear dielectric microscopy (SNDM), which can visualize both carrier and polarization distributions. The AlGaN/GaN heterostructure was cross-sectioned at an angle of 20° from the [0001] axis and, hence, the perpendicular components of the polarizations of AlGaN and GaN were observed. Moreover, the 2DEG distribution was confirmed in a range of ∼10 nm at the AlGaN/GaN interface.

  16. Solid state reduction of chromium (VI) pollution for Al2O3-Cr metal ceramics application

    NASA Astrophysics Data System (ADS)

    Zhu, Hekai; Fang, Minghao; Huang, Zhaohui; Liu, Yangai; Tang, Hao; Min, Xin; Wu, Xiaowen

    2016-04-01

    Reduction of chromium (VI) from Na2CrO4 through aluminothermic reaction and fabrication of metal-ceramic materials from the reduction products have been investigated in this study. Na2CrO4 could be successfully reduced into micrometer-sized Cr particles in a flowing Ar atmosphere in presence of Al powder. The conversion ratio of Na2CrO4 to metallic Cr attained 96.16% efficiency. Al2O3-Cr metal-ceramic with different Cr content (5 wt%, 10 wt%, 15 wt%, 20 wt%) were further prepared from the reduction product Al2O3-Cr composite powder, and aluminum oxide nanopowder via pressure-less sintering. The phase composition, microstructure and mechanical properties of metal-ceramic composites were characterized to ensure the potential of the Al2O3-Cr composite powder to form ceramic materials. The highest relative density and bending strength can reach 93.4% and 205 MP, respectively. The results indicated that aluminothermic reduction of chromium (VI) for metal-ceramics application is a potential approach to remove chromium (VI) pollutant from the environment.

  17. Electronic, optical, structural, and elastic properties of MAX phases and (Cr2Hf)2Al3C3

    NASA Astrophysics Data System (ADS)

    Mo, Yuxiang

    The term "MAX phase" refers to a very interesting and important class of layered ternary transition-metal carbides and nitrides with a novel combination of both metal- and ceramic-like properties that have made these materials highly regarded candidates for numerous technological and engineering applications. In the present dissertation work, the electronic structure and optical conductivities of 20 MAX phases Ti3AC2 (A = Al, Si, Ge), Ti2AC (A = Al, Ga, In, Si, Ge, Sn, P, As, S), Ti2AlN, M2AlC (M = V, Nb, Cr), and Tan+1AlC n (n = 1 to 4) are studied using the first-principles orthogonalized linear combination of atomic orbitals (OLCAO) method. It is confirmed that the N(Ef) (total density of states at the Fermi level Ef) increases as the number of valence electrons of the composing elements increases. The local feature of total density of states (TDOS) near Ef is used to predict structural stability. The calculated effective charge on each atom shows that the M (transition-metal) atoms always lose charge to the X (C or N) atoms, whereas the A-group atoms mostly gain charge but some lose charge. Bond order values are obtained and critically analyzed for all types of interatomic bonds in the 20 MAX phases. Also included in this work is the exploration [using (Cr2Hf)2Al3C3 as an example] of the possibility of incorporating more types of elements into a MAX phase while maintaining the crystallinity, instead of creating solid solution phases. The crystal structure and elastic properties of (Cr2Hf)2Al 3C3 are studied using the Vienna ab initio Simulation Package. Unlike MAX phases with a hexagonal symmetry ( P63/mmc, #194), (Cr 2Hf)2Al3C3 crystallizes in the monoclinic space group of P21/m (#11). Its structure is found to be energetically much more favorable against the allotropic segregation and solid solution phases. Calculations using a stress versus strain approach and the VRH approximation for polycrystals also show that (Cr2Hf)2Al3C3 has outstanding elastic moduli.

  18. Magneto-Excitons in (411)A and (100)-Oriented GaAs/AlGaAs Multiple Quantum Well Structures

    SciTech Connect

    Bajaj, K.K.; Hiyamizu, S.; Jones, E.D.; Krivorotov, I.; Shimomura, S.; Shinohara, K.

    1999-01-20

    We report magneto-exciton spectroscopy studies of (411)A and (100)-oriented GaAs/Al{sub 0.3}Ga{sub 0.7}As multiquantum well structures. The samples consisted of seven GaAs quantum wells with widths varying between 0.6 and 12nm, were grown on (411)A and (100)-oriented GaAs substrates. The exciton diamagnetic energy shifts and linewidths were measured between 0 and 14T at 1.4K The dependence of the exciton diamagnetic shifts with magnetic field were calculated using a variational approach and good agreement with experiment for both substrate orientations was found.

  19. Cr3+-Doped Yb3Ga5O12 Nanophosphor: Synthesis, Optical, EPR, Studies

    NASA Astrophysics Data System (ADS)

    Singh, Vijay; Sivaramaiah, G.; Rao, J. L.; Singh, N.; Pathak, M. S.; Jirimali, H. D.; Singh, Pramod K.; Srivastava, Anoop K.; Dhoble, S. J.; Mohapatra, M.

    2016-08-01

    Gallium garnets of lanthanides are multifunctional materials especially known for their complicated structure and magnetic properties. In addition, with a suitable transition metal dopant ion, these matrices have been proved to be excellent materials for lasers. In particular, gallium garnet of ytterbium (Yb3Ga5O12) is known to possess excellent properties with regards to these applications. In this connection, Yb3Ga5O12 doped with Cr3+ nanophosphors were synthesized by a solution combustion route. The synthesized material was characterized by powder x-ray diffraction and scanning electron microscopy for phase purity and homogenous morphology. In order to ascertain the oxidation state of the doped ion, diffuse reflectance (DRF), photoluminescence (PL) and electron paramagnetic resonance (EPR) experiments were performed on the sample. The DRF and PL data suggested the stabilisation of the trivalent Cr ion in the matrix. The EPR spectra exhibited two resonance signals with effective g values at g ≈ 7.6 and 4. The EPR data corroborated the DRF and PL results, suggesting the stabilisation of Cr3+ in the matrix at octahedral-type geometries.

  20. Degradation and annealing effects caused by oxygen in AlGaN/GaN high electron mobility transistors

    NASA Astrophysics Data System (ADS)

    Jiang, R.; Shen, X.; Chen, J.; Duan, G. X.; Zhang, E. X.; Fleetwood, D. M.; Schrimpf, R. D.; Kaun, S. W.; Kyle, E. C. H.; Speck, J. S.; Pantelides, S. T.

    2016-07-01

    Hot-carrier degradation and room-temperature annealing effects are investigated in unpassivated ammonia-rich AlGaN/GaN high electron mobility transistors. Devices exhibit a fast recovery when annealed after hot carrier stress with all pins grounded. The recovered peak transconductance can exceed the original value, an effect that is not observed in control passivated samples. Density functional theory calculations suggest that dehydrogenation of pre-existing ON-H defects in AlGaN plays a significant role in the observed hot carrier degradation, and the resulting bare ON can naturally account for the "super-recovery" in the peak transconductance.

  1. Pack cementation Cr-Al coating of steels and Ge-doped silicide coating of Cr-Nb alloy

    SciTech Connect

    He, Y.R.; Zheng, M.H.; Rapp, R.A.

    1995-08-01

    Carbon steels or low-alloy steels used in utility boilers, heat exchangers, petrochemical plants and coal gasification systems are subjected to high temperature corrosion attack such as oxidation, sulfidation and hot corrosion. The pack cementation coating process has proven to be an economical and effective method to enhance the corrosion resistance by modifying the surface composition of steels. With the aid of a computer program, STEPSOL, pack cementation conditions to produce a ferrite Cr-Al diffusion coating on carbon-containing steels by using elemental Cr and Al powders have been calculated and experimentally verified. The cyclic oxidation kinetics for the Cr-Al coated steels are presented. Chromium silicide can maintain high oxidation resistance up to 1100{degrees}C by forming a SiO{sub 2} protective scale. Previous studies at Ohio State University have shown that the cyclic oxidation resistance of MOSi{sub 2} and TiSi{sub 2} can be further improved by Ge addition introduced during coating growth. The halide-activated pack cementation process was modified to produce a Ge-doped silicide diffusion coating in a single processing step for the ORNL-developed Cr-Nb advanced intermetallic alloy. The oxidation behavior of the silicide-coated Cr-Nb alloy was excellent: weight gain of about 1 mg/cm{sup 2} upon oxidation at 1100{degrees}C in air for 100 hours.

  2. Al-21Ti-23Cr high-temperature protective coating on TiAl intermetallic compounds by RF magnetron sputtering

    SciTech Connect

    Park, J.Y.; Lee, H.N.; Wee, D.M.; Park, S.W.; Oh, M.H.

    1997-12-31

    Ti-48Al specimens were coated with Al-21Ti-23Cr film at 200 W, 0.8 Pa and 573 K by RF magnetron sputtering. The oxidation behavior of the coated specimens was investigated through isothermal and cyclic oxidation tests, and the tensile deformation properties of the coated specimens were also investigated before and after oxidation. The isothermal and cyclic oxidation curves showed that the Al-21Ti-23Cr film was very effective in decreasing the oxidation rate of Ti-48Al. This excellent oxidation resistance is attributable to the formation of a protective Al{sub 2}O{sub 3} layer on the surface of the Al-21Ti-23Cr film. It was found from the results of the tensile test that the protective Al{sub 2}O{sub 3} layer on the surface of the Al-21Ti-23Cr film enabled the Ti-48Al to maintain its tensile properties in an oxidizing environment.

  3. Three-dimensional nanoscale study of Al segregation and quantum dot formation in GaAs/AlGaAs core-shell nanowires

    SciTech Connect

    Mancini, L.; Blum, I.; Vurpillot, F.; Rigutti, L.; Fontana, Y.; Conesa-Boj, S.; Francaviglia, L.; Russo-Averchi, E.; Heiss, M.; Morral, A. Fontcuberta i; Arbiol, J.

    2014-12-15

    GaAs/Al-GaAs core-shell nanowires fabricated by molecular beam epitaxy contain quantum confining structures susceptible of producing narrow photoluminescence (PL) and single photons. The nanoscale chemical mapping of these structures is analyzed in 3D by atom probe tomography (APT). The study allows us to confirm that Al atoms tend to segregate within the AlGaAs shells towards the vertices of the hexagons defining the nanowire cross section. We also find strong alloy fluctuations remaining AlGaAs shell, leading occasionally to the formation of quantum dots (QDs). The PL emission energies predicted in the framework of a 3D effective mass model for a QD analyzed by APT and the PL spectra measured on other nanowires from the same growth batch are consistent within the experimental uncertainties.

  4. Localized TiSi and TiN phases in Si/Ti/Al/Cu Ohmic contacts to AlGaN/GaN heterostructures

    NASA Astrophysics Data System (ADS)

    Yoon, Seonno; Song, Yunwon; Lee, Seung Min; Lee, Hi-Deok; Oh, Jungwoo

    2016-05-01

    Microstructural changes in Si/Ti/Al/Cu (10/40/60/50 nm) Ohmic contacts to AlGaN/GaN heterostructure were investigated for complementary metal-oxide semiconductor compatible processes. Si/Ti/Al/Cu metallization exhibited a low specific contact resistance of 3.6 × 10-6 Ω-cm2 and contact resistance of 0.46 Ω-mm when a Si interfacial layer was used. Without a designated barrier metal, TiSix alloys that formed in the metallic region effectively suppressed Cu diffusion. The shallow TiN junction in AlGaN/GaN was attributed to TiSix in the metallic regions. Microstructural changes were detected by systematic physical characterization.

  5. Ion damage and annealing of epitaxial gallium nitride and comparison with GaAs/AlGaAs materials

    SciTech Connect

    Tan, H.H.; Williams, J.S.; Yuan, C.; Pearton, S.J.

    1996-11-01

    Ion damage build up has been measured by ion channeling in good quality epitaxial GaN films on sapphire. GaN is found to be remarkably resistant to ion damage, with extremely efficient dynamic defect annihilation occurring at liquid nitrogen temperature during ion implantation. When disorder does accumulate at doses around 10{sup 16} cm{sup {minus}2} of 90 keV Si ions, the surface appears to be a strong sink for damage build up and possibly the nucleation of amorphous layers. Once ion disorder has been produced in GaN, it is extremely difficult to remove by annealing. GaN exhibits disordering and annealing behavior which is somewhat similar to that in high Al-content AlGaAs.

  6. Electrically conducting n-type AlGaN/GaN distributed Bragg reflectors grown by metalorganic chemical vapor deposition

    NASA Astrophysics Data System (ADS)

    Liu, Yuh-Shiuan; Haq, A. F. M. Saniul; Kao, Tsung-Ting; Mehta, Karan; Shen, Shyh-Chiang; Detchprohm, Theeradetch; Yoder, P. Douglas; Dupuis, Russell D.; Xie, Hongen; Ponce, Fernando A.

    2016-06-01

    We report an electrically conducting 40-pair silicon doped Al0.12Ga0.88N/GaN distributed Bragg reflector (DBR) grown by metalorganic chemical vapor deposition on a silicon doped n-type GaN template. Due to the relatively small lattice mismatch between AlGaN and GaN, strain managing layers are not required for crack-free n-DBR growth. The DBR demonstrates a peak reflectivity of 91.6% at 368 nm with stopband of 11 nm. In addition, the 40-pair n-DBR shows the vertical resistance of 5.5 Ω, which corresponds to bulk resistivity of 0.52 Ω cm, near the maximum measured current of 100 mA.

  7. High electron mobility recovery in AlGaN/GaN 2DEG channels regrown on etched surfaces

    NASA Astrophysics Data System (ADS)

    Chan, Silvia H.; Keller, Stacia; Tahhan, Maher; Li, Haoran; Romanczyk, Brian; DenBaars, Steven P.; Mishra, Umesh K.

    2016-06-01

    This paper reports high two-dimensional electron gas mobility attained from the regrowth of the AlGaN gating layer on ex situ GaN surfaces. To repair etch-damaged GaN surfaces, various pretreatments were conducted via metalorganic chemical vapor deposition, followed by a regrown AlGaN/GaN mobility test structure to evaluate the extent of recovery. The developed treatment process that was shown to significantly improve the electron mobility consisted of a N2 + NH3 pre-anneal plus an insertion of a 4 nm or thicker GaN interlayer prior to deposition of the AlGaN gating layer. Using the optimized process, a high electron mobility transistor (HEMT) device was fabricated which exhibited a high mobility of 1450 cm2 V‑1 s‑1 (R sh = 574 ohm/sq) and low dispersion characteristics. The additional inclusion of an in situ Al2O3 dielectric into the regrowth process for MOS-HEMTs still preserved the transport properties near etch-impacted areas.

  8. Thermodynamic analysis of chemical compatibility of several compounds with Fe-Cr-Al alloys

    NASA Technical Reports Server (NTRS)

    Misra, Ajay K.

    1993-01-01

    Chemical compatibility between Fe-19.8Cr-4.8Al (weight percent), which is the base composition for the commercial superalloy MA956, and several carbides, borides, nitrides, oxides, and silicides was analyzed from thermodynamic considerations. The effect of addition of minor alloying elements, such as Ti, Y, and Y2O3, to the Fe-Cr-Al alloy on chemical compatibility between the alloy and various compounds was also analyzed. Several chemically compatible compounds that can be potential reinforcement materials and/or interface coating materials for Fe-Cr-Al based composites were identified.

  9. Effect of AlGaN/GaN strained layer superlattice period on InGaN MQW laser diodes[Multiple Quantum Wells

    SciTech Connect

    Hansen, M.; Abare, A.C.; Kozodoy, P.; Katona, T.M.; Craven, M.D.; Speck, J.S.; Mishra, U.K.; Coldren, L.A.; DenBaars, S.P.

    2000-07-01

    AlGaN/GaN strained layer superlattices have been employed in the cladding layers of InGaN multi-quantum well laser diodes grown by metalorganic chemical vapor deposition (MOCVD). Superlattices have been investigated for strain relief of the cladding layer, as well as an enhanced hole concentration, which is more than ten times the value obtained for bulk AlGaN films. Laser diodes with strained layer superlattices as cladding layers were shown to have superior structural and electrical properties compared to laser diodes with bulk AlGaN cladding layers. As the period of the strained layer superlattices is decreased, the threshold voltage, as well as the threshold current density, is decreased. The resistance to vertical conduction through p-type superlattices with increasing superlattice period is not offset by the increase in hole concentration for increasing superlattice spacing, resulting in higher voltages.

  10. Spectroscopic properties of {Cr}^{3+} in the spinel solid solution {ZnAl}_{2-x}{Cr}x{O}_4

    NASA Astrophysics Data System (ADS)

    Verger, Louisiane; Dargaud, Olivier; Rousse, Gwenaelle; Rozsályi, Emese; Juhin, Amélie; Cabaret, Delphine; Cotte, Marine; Glatzel, Pieter; Cormier, Laurent

    2016-01-01

    The evolution of the structural environment of {Cr}{^{3+}} along the solid solution {ZnAl}_{2-x}{Cr}x{O}_4 has been investigated using a multi-analytical approach. X-ray diffraction confirms that the system follows Vegard's law. Diffuse reflectance spectra show a decrease of the crystal field parameter with the Cr content, usually related to the increase of the Cr-O bond length in a point charge model. This interpretation is discussed and compared to the data obtained by first-principle calculations based on density functional theory. X-ray absorption near edge structure spectra at the Cr K-edge show a pronounced evolution in the pre-edge with the Cr content, characterised by the appearance of a third feature. Calculations enable to assign the origin of this feature to Cr neighbours. The colour change from pink to brownish pink and eventually green along the solid solution has also been quantified by calculating the L*, a*, b* and x, y coefficients in the system defined by the International Commission on Illumination.

  11. Oxidation behavior of arc evaporated Al-Cr-Si-N thin films

    SciTech Connect

    Tritremmel, Christian; Daniel, Rostislav; Mitterer, Christian; Mayrhofer, Paul H.; Lechthaler, Markus; Polcik, Peter

    2012-11-15

    The impact of Al and Si on the oxidation behavior of Al-Cr-(Si)-N thin films synthesized by arc evaporation of powder metallurgically prepared Al{sub x}Cr{sub 1-x} targets with x = Al/(Al + Cr) of 0.5, 0.6, and 0.7 and (Al{sub 0.5}Cr{sub 0.5}){sub 1-z}Si{sub z} targets with Si contents of z = 0.05, 0.1, and 0.2 in N{sub 2} atmosphere was studied in detail by means of differential scanning calorimetry, thermogravimetric analysis (TGA), x-ray diffraction, and Raman spectroscopy. Dynamical measurements in synthetic air (up to 1440 Degree-Sign C) revealed the highest onset temperature of pronounced oxidation for nitride coatings prepared from the Al{sub 0.4}Cr{sub 0.4}Si{sub 0.2} target. Isothermal TGA at 1100, 1200, 1250, and 1300 Degree-Sign C highlight the pronounced improvement of the oxidation resistance of Al{sub x}Cr{sub 1-x}N coatings by the addition of Si. The results show that Si promotes the formation of a dense coating morphology as well as a dense oxide scale when exposed to air.

  12. Defects and annealing studies in 1-Me electron irradiated (AlGa)As-GaAs solar cells

    NASA Technical Reports Server (NTRS)

    Li, S. S.; Wang, W. L.; Loo, R. Y.; Rahilly, W. P.

    1982-01-01

    The deep-level defects and recombination mechanisms in the one-MeV electron irradiated (AlGa)As-GaAs solar cells under various irradiation and annealing conditions are discussed. Deep-level transient spectroscopy (DLTS) and capacitance-voltage (CV) techniques were used to determine the defect and recombination parameters such as energy levels and defect density, carrier capture cross sections and lifetimes for both electron and hole traps as well as hole diffusion lengths in these electron irradiated GaAs solar cells. GaAs solar cells used in this study were prepared by the infinite solution melt liquid phase epitaxial (LPE) technique at Hughes Research Lab., with (Al0.9Ga0.1)-As window layer, Be-diffused p-GaAs layer on Sn-doped n-GaAs or undoped n-GaAs active layer grown on n(+)-GaAs substrate. Mesa structure with area of 5.86x1000 sq cm was fabricated. Three different irradiation and annealing experiments were performed on these solar cells.

  13. Oxidation behavior of cubic phases formed by alloying Al3Ti with Cr and Mn

    NASA Technical Reports Server (NTRS)

    Parfitt, L. J.; Nic, J. P.; Mikkola, D. E.; Smialek, J. L.

    1991-01-01

    Gravimetric, SEM, and XRD data are presented which document the significant improvement obtainable in the oxidation resistance of Al3Ti-containing alloys through additions of Cr. The L1(2) Al(67)Cr(8)Ti25 alloy exhibited excellent cyclic oxidation resistance at 1473 K, with the primary oxide formed being the ideally protective alpha-Al2O3. The Al(67)Mn(8)Ti(25) alloy also tested for comparison exhibited poor cyclic oxidation resistance, with substantial occurrence of TiO2 in the protective scales. Catastrophic oxidation was also encountered in the quaternary alloy Al(67)Mn(8)Ti(22)V(3).

  14. Resolution characteristics of graded band-gap reflection-mode AlGaAs/GaAs photocathodes

    NASA Astrophysics Data System (ADS)

    Deng, Wenjuan; Zhang, Daoli; Zou, Jijun; Peng, Xincun; Wang, Weilu; Zhang, Yijun; Chang, Benkang

    2015-12-01

    The modulation transfer function (MTF) of graded band-gap AlGaAs/GaAs reflection-mode photocathodes was determined using two-dimensional Poisson and continuity equations through numerical method. Based on the MTF model, we calculated the theoretical MTF of graded and uniform band-gap reflection-mode photocathodes. We then analyzed the effects of Al composition, wavelength of incident photon, and thicknesses of AlGaAs and GaAs layer on the resolution. Calculation results show that graded band-gap structures can increase the resolution of reflection-mode photocathodes. When the spatial frequency is 800 lp/mm and wavelength is 600 nm, the resolution of graded band-gap photocathodes generally increases by 15.4-29.6%. The resolution improvement of graded band-gap photocathodes is attributed to the fact that the built-in electric field in graded band-gap photocathodes reduces the lateral diffusion distance of photoelectrons.

  15. Enhancement of optical polarization degree of AlGaN quantum wells by using staggered structure.

    PubMed

    Wang, Weiying; Lu, Huimin; Fu, Lei; He, Chenguang; Wang, Mingxing; Tang, Ning; Xu, Fujun; Yu, Tongjun; Ge, Weikun; Shen, Bo

    2016-08-01

    Staggered AlGaN quantum wells (QWs) are designed to enhance the transverse-electric (TE) polarized optical emission in deep ultraviolet (DUV) light- emitting diodes (LED). The optical polarization properties of the conventional and staggered AlGaN QWs are investigated by a theoretical model based on the k·p method as well as polarized photoluminescence (PL) measurements. Based on an analysis of the valence subbands and momentum matrix elements, it is found that AlGaN QWs with step-function-like Al content in QWs offers much stronger TE polarized emission in comparison to that from conventional AlGaN QWs. Experimental results show that the degree of the PL polarization at room temperature can be enhanced from 20.8% of conventional AlGaN QWs to 40.2% of staggered AlGaN QWs grown by MOCVD, which is in good agreement with the theoretical simulation. It suggests that polarization band engineering via staggered AlGaN QWs can be well applied in high efficiency AlGaN-based DUV LEDs. PMID:27505782

  16. Thermal wet oxidation of GaP and Al0.4Ga0.6P

    NASA Astrophysics Data System (ADS)

    Epple, J. H.; Chang, K. L.; Pickrell, G. W.; Cheng, K. Y.; Hsieh, K. C.

    2000-08-01

    Thermal wet oxidations of GaP and Al0.4Ga0.6P at 650 °C for various times have been performed. Comparisons are made on oxidation rates and post oxidation morphology. Transmission electron microscopy shows that when oxidizing GaP, polycrystalline monoclinic GaPO4ṡ2H2O forms without noticeable loss of phosphorus. Oxidation for 6 h or more leads to poor morphology resulting in cracks and detachment. A thickness expansion of about 2.5-3 times is noticed as a result of oxidation. In contrast, oxidized Al0.4Ga0.6P exhibits much better morphology without cracks or detachment from the substrate. The oxide has an almost amorphous-like microstructure. The oxidation process shows typical diffusion-limited reaction at long anneals. Preliminary work on the oxidation of AlP indicates that the reaction leads to formation of Al2O3 and possible volatile P2O5 diffusing out of the specimen. Thus, from the structural viewpoint, AlGaP forms a better oxide suitable for device needs.

  17. Effects of high-temperature AIN buffer on the microstructure of AlGaN/GaN HEMTs

    SciTech Connect

    Coerekci, S.; Oeztuerk, M. K.; Yu, Hongbo; Cakmak, M.; Oezcelik, S.; Oezbay, E.

    2013-06-15

    Effects on AlGaN/GaN high-electron-mobility transistor structure of a high-temperature AlN buffer on sapphire substrate have been studied by high-resolution x-ray diffraction and atomic force microscopy techniques. The buffer improves the microstructural quality of GaN epilayer and reduces approximately one order of magnitude the edge-type threading dislocation density. As expected, the buffer also leads an atomically flat surface with a low root-mean-square of 0.25 nm and a step termination density in the range of 10{sup 8} cm{sup -2}. Due to the high-temperature buffer layer, no change on the strain character of the GaN and AlGaN epitaxial layers has been observed. Both epilayers exhibit compressive strain in parallel to the growth direction and tensile strain in perpendicular to the growth direction. However, an high-temperature AlN buffer layer on sapphire substrate in the HEMT structure reduces the tensile stress in the AlGaN layer.

  18. A model for longitudinal optical phonons and electron-phonon coupling in GaAs/GaAlAs multilayer structure

    NASA Astrophysics Data System (ADS)

    Guillemot, C.; Clerot, F.

    A new model for long-wavelength longitudinal optical phonons in GaAsGaAlAs multi-layer structures is presented. Depending on the layer, the relative ionic displacements are written on the basis of GaAs or GaAs-type longitudinal optical phonons and treated in the framework of the Born-Huang model generalized to include isotropic dispersion effects in the Brillouin zone centre. For double heterostructures, a finite number of quantized confined modes is found. Interplay between the long range Coulomb interaction, which couples the vibrations of adjacent GaAs layers, and confinement effects, which prevent the displacements of adjacent GaAs layers to overlap, is evidenced in the case of superlattices. The strength of the electron-phonon coupling in double heterostructures stays within a factor of 2 of the electron-bulk phonon effective coupling strength for practical values of the parameters.

  19. In situ atomic layer deposition half cycle study of Al2O3 growth on AlGaN

    NASA Astrophysics Data System (ADS)

    Brennan, Barry; Qin, Xiaoye; Dong, Hong; Kim, Jiyoung; Wallace, Robert M.

    2012-11-01

    The atomic layer deposition (ALD) of Al2O3 on the native oxide and hydrofluoric acid treated Al0.25Ga0.75 N surface was studied using in situ X-ray photoelectron spectroscopy (XPS), after each individual "half cycle" of the ALD process. Initially, Al2O3, Ga2O3, and N-O states were detected on both surfaces at differing concentrations. During the course of the deposition process, the N-O bonds are seen to decrease to within XPS detection limits, as well as a small decrease in the Ga2O3 concentration. The Al2O3 growth rate initially is seen to be very low, indication of low reactivity between the trimethyl-aluminum molecule and the AlGaN surface.

  20. MAX phase - Alumina composites via elemental and exchange reactions in the Tin+1ACn systems (A=Al, Si, Ga, Ge, In and Sn)

    NASA Astrophysics Data System (ADS)

    Cuskelly, Dylan; Richards, Erin; Kisi, Erich

    2016-05-01

    Extension of the aluminothermal exchange reaction synthesis of Mn+1AXn phases to systems where the element 'A' is not the reducing agent was investigated in systems TiO2-A-Al-C for A=Al, Si, Ga, Ge, In and Sn as well as Cr2O3-Ga-Al-C. MAX phase-Al2O3 composites were made in all systems except those with A=Ga or In. The effectiveness of conversion to MAX phases was generally in the range 63-96% without optimisation of starting ratios. Optimisation in the Ti-Si-C system gave a MAX phase component with >98% Ti3SiC2.

  1. Luminescence Spectra of ZnAl 2 O 4 {:}Cr 3+ Spinel Nanopowders

    NASA Astrophysics Data System (ADS)

    Luc, H. H.; Nguyen, T. K.; Nguyen, V. M.; Suchocki, A.; Kamiñska, A.; Le, V. K.; Nguyen, V. H.; Luong, T. T.

    2002-12-01

    The synthetic ZnAl 2 O 4 spinels doped with Cr 3+ ions are prepared from ZnSO,dwi{4}>, Al 2 (SO 4 ) 3 , and Cr 2 (SO 4 ) 3 . The spinel single phase is detected from X-ray diffraction. Luminescence properties of Cr 3+ in ZnAl 2 O 4 were studied by low temperature luminescence and decay measurements. Four luminescence lines at 14570, 14520, 14460, and 14330 cm -1 were found to originate from structure distortion and the line at 14175 cm -1 - from chromium pairs. The broad emission band at about 13540 cm -1 is considered to arise from a new Cr 3+ center in ZnCr 2 O 4 .

  2. Cr Isotopes in Allende Ca-Al-rich Inclusions

    NASA Technical Reports Server (NTRS)

    Bogdanovski, O.; Papanastassiou, D. A.; Wasserburg, G. J.

    2002-01-01

    We have determined Cr isotope compositions in minerals from Allende CAI in order to address the initial 53Mn (half-life 3.7 Ma) abundance in the solar system. Additional information is contained in the original extended abstract.

  3. Investigation of gate-diode degradation in normally-off p-GaN/AlGaN/GaN high-electron-mobility transistors

    SciTech Connect

    Ťapajna, M. Kuzmík, J.; Hilt, O.; Bahat-Treidel, E.; Würfl, J.

    2015-11-09

    Gate diode conduction mechanisms were analyzed in normally-off p-GaN/AlGaN/GaN high-electron mobility transistors grown on Si wafers before and after forward bias stresses. Electrical characterization of the gate diodes indicates forward current to be limited by channel electrons injected through the AlGaN/p-GaN triangular barrier promoted by traps. On the other hand, reverse current was found to be consistent with carrier generation-recombination processes in the AlGaN layer. Soft breakdown observed after ∼10{sup 5 }s during forward bias stress at gate voltage of 7 V was attributed to formation of conductive channel in p-GaN/AlGaN gate stack via trap generation and percolation mechanism, likely due to coexistence of high electric field and high forward current density. Possible enhancement of localized conductive channels originating from spatial inhomogeneities is proposed to be responsible for the degradation.

  4. Generation and transportation mechanisms for two-dimensional hole gases in GaN/AlGaN/GaN double heterostructures

    SciTech Connect

    Nakajima, Akira Ogura, Masahiko; Makino, Toshiharu; Nishizawa, Shin-ichi; Ohashi, Hiromichi; Yamasaki, Satoshi; Liu, Pucheng; Kakushima, Kuniyuki; Iwai, Hiroshi

    2014-04-21

    The electrical properties of two-dimensional hole gases (2DHGs) in GaN/AlGaN/GaN double heterostructures were investigated. The layers were grown on sapphire substrates and a high-quality bulk GaN substrate. The coexistence of 2DHG and 2D electron gases on both sides of the AlGaN layer was confirmed by Hall effect measurements at 80–460 K. It was also verified that the 2DHGs were generated by negative polarization at the undoped GaN/AlGaN interface, which did not have a doped Mg acceptor. It was also demonstrated that the 2DHG density could be controlled by varying the AlGaN layer thickness and was inversely related to the 2DHG mobility. The measured relation indicated that the 2DHG mobility is mainly limited by phonon scatterings at around room temperature. As a result, the maximum 2DHG mobility of 16 cm{sup 2}/Vs at 300 K was achieved with a density of 1 × 10{sup 13} cm{sup −2}.

  5. Novel Phase Transitions in the Breathing Pyrochlore Lattice: Li7-NMR on LiInCr4O8 and LiGaCr4O8

    NASA Astrophysics Data System (ADS)

    Tanaka, Yu; Yoshida, Makoto; Takigawa, Masashi; Okamoto, Yoshihiko; Hiroi, Zenji

    2014-11-01

    We report Li7-NMR studies on LiInCr4O8 and LiGaCr4O8, in which Cr3 + ions with spin 3 /2 form a breathing pyrochlore lattice, a network of tetrahedra with alternating sizes. In LiInCr4O8 with large alternation, the nuclear relaxation rate 1 /T1 shows an activated temperature (T ) dependence down to 18 K, indicating a singlet ground state with a spin gap. This behavior, however, is disrupted by an antiferromagnetic transition at 13 K, which is preceded by another, most likely structural, transition at 16 K. In contrast, LiGaCr4O8 with a small alternation shows no spin gap but exhibits a first-order antiferromagnetic transition over a distributed T range 13-20 K. Nevertheless, 1 /T1 of the paramagnetic phase diverges toward 13 K, indicating proximity to a second-order transition. The results indicate that LiGaCr4O8 is located in the vicinity of a tricritical point in the phase diagram.

  6. AlGaN/GaN ohmic contact resistance variations across epitaxial suppliers

    NASA Astrophysics Data System (ADS)

    Gillespie, J.; Crespo, A.; Fitch, R.; Jessen, G.; Via, G.

    2005-04-01

    Six AlGaN/GaN epitaxy suppliers grew their standard films on SiC substrates as deliverables for Defense Advanced Research Projects Agency's (DARPA's) "Wide Bandgap Technology Initiative". An ohmic contact study was performed to determine an optimum process for each supplier and to study the impact of growth conditions on contact resistance. On each wafer four metal stacks were evaporated, with one being the epitaxial supplier's proprietary stack. The wafers were diced and alloyed using AFRL's standard conditions and the epitaxial supplier's condition. The optimal metal stack was found for each wafer and yielded contact resistances lower than 1 Ω mm. However, each wafer required a different metal stack in order to achieve the lowest contact resistance, which indicates that growth conditions can have a large impact on ohmic contact results. The molybdenum (Mo) based ohmic contacts showed little variation from sample to sample and produced marginally acceptable ohmic contact values across all material suppliers. The Mo based contacts show potential for optimization as a generic ohmic contact that is not dependent on the growth conditions of the material.

  7. Nanofabrication of Gate-defined GaAs/AlGaAs Lateral Quantum Dots

    PubMed Central

    Bureau-Oxton, Chloé; Camirand Lemyre, Julien; Pioro-Ladrière, Michel

    2013-01-01

    A quantum computer is a computer composed of quantum bits (qubits) that takes advantage of quantum effects, such as superposition of states and entanglement, to solve certain problems exponentially faster than with the best known algorithms on a classical computer. Gate-defined lateral quantum dots on GaAs/AlGaAs are one of many avenues explored for the implementation of a qubit. When properly fabricated, such a device is able to trap a small number of electrons in a certain region of space. The spin states of these electrons can then be used to implement the logical 0 and 1 of the quantum bit. Given the nanometer scale of these quantum dots, cleanroom facilities offering specialized equipment- such as scanning electron microscopes and e-beam evaporators- are required for their fabrication. Great care must be taken throughout the fabrication process to maintain cleanliness of the sample surface and to avoid damaging the fragile gates of the structure. This paper presents the detailed fabrication protocol of gate-defined lateral quantum dots from the wafer to a working device. Characterization methods and representative results are also briefly discussed. Although this paper concentrates on double quantum dots, the fabrication process remains the same for single or triple dots or even arrays of quantum dots. Moreover, the protocol can be adapted to fabricate lateral quantum dots on other substrates, such as Si/SiGe. PMID:24300661

  8. High Quantum Efficiency AlGaN/InGaN Photodetectors

    SciTech Connect

    Buckley, James H; Leopold, Daniel

    2009-11-24

    High efficiency photon counting detectors in use today for high energy particle detection applications have a significant spectral mismatch with typical sources and have a number of practical problems compared with conventional bialkali photomultiplier tubes. Numerous high energy physics experiments that employ scintillation light detectors or Cherenkov detectors would benefit greatly from photomultipliers with higher quantum efficiencies. The need for extending the sensitivity of photon detectors to the blue and UV wavebands comes from the fact that both Cherenkov light and some scintillators have an emission spectrum which is peaked at short wavelengths. This research involves the development of high quantum efficiency, high gain, UV/blue photon counting detectors based on AlGaN/InGaN photocathode heterostructures grown by molecular beam epitaxy (MBE). The work could eventually lead to nearly ideal light detectors with a number of distinct advantages over existing technologies for numerous applications in high-energy physics and particle astrophysics. Potential advantages include much lower noise detection, better stability and radiation resistance than other cathode structures, very low radioactive background levels for deep underground experiments and high detection efficiency of individual UV-visible photons. We are also working on the development of photocathodes with intrinsic gain, initially improving the detection efficiency of hybrid semiconductor-vacuum tube devices, and eventually leading to an all-solid-state photomultiplier device.

  9. Reliability of high-power AlGaAs/GaAs QW laser diodes

    NASA Astrophysics Data System (ADS)

    Dabkowski, Ferdynand P.; Pendse, D. R.; Barrett, Richard J.; Chin, Aland K.; Jollay, Richard A.; Clausen, Edward M., Jr.; Hughes, L. C.; Sanders, Neil B.

    1996-09-01

    High power laser diodes have been continuously gaining more practical applications. In the majority of these applications, device performance is a determining factor. However, device reliability determines whether a laser diode can be successfully introduced in a commercial product. We review some device reliability problems and their solutions found through customer experience while supplying packaged high power AlGaAs/GaAs quantum well laser diodes, utilized in medical, high resolution printers. The reliability problems were related to either photo-induced chemical reactions on the output facet leading to visible optical damage or the propensity of the material to rapidly develop dark line defects. To improve the reliability of high power laser diodes, we have performed numerous aging studies, followed by detailed failure mode analysis. Both hermetically packaged devices and devices exposed to air ambient were evaluated. The devices whose parameters deteriorated during aging were examined with optical microscopy, infrared microscopy, scanning electron microscopy, Auger spectroscopy, residual gas analysis and also electron beam induced current. We report the results of the failure mode analysis and suggest solutions to eliminate failures of high power laser diodes.

  10. With electroluminescence microcopy towards more reliable AlGaN/GaN transistors

    NASA Astrophysics Data System (ADS)

    Baeumler, Martina; Dammann, Michael; Wespel, Matthias; George, Roshna; Konstanzer, Helmer; Maroldt, Stephan; Polyakov, Vladimir M.; Müller, Stefan; Bronner, Wolfgang; Brückner, Peter; Benkhelifa, Fouad; Waltereit, Patrick; Quay, Rüdiger; Mikulla, Michael; Wagner, Joachim; Ambacher, Oliver; Graff, Andreas; Altmann, Frank; Simon-Najasek, Michél.; Lorenzini, Martino; Fagerlind, Martin; van der Wel, Paul J.; Roedle, Thomas

    2015-08-01

    Long-term stability and reliability of AlGaN/GaN high electron mobility transistors (HEMT) can be validated by various stress tests which allow studying the physical mechanisms responsible for degradation. As the electroluminescence (EL) intensity is related to the kinetic energy and density of the channel electrons accelerated in the electric field, both local current and electric field changes can result in an increase or decrease of the EL intensity. The electric field distribution in source drain direction peaks at the drain side edge of the gate foot and the field plates. It is strongly dependent on the gate design and the passivation /semiconductor interface trap density. Although the spatial resolution of the electroluminescence microscopy (ELM) set-up is limited to a few microns, the technique allows to monitor submicron changes in the device geometry and hence to identify elevated electric field peaks being responsible for the acceleration of the degradation process. ELM also benefits from the fact that submicron imperfections at the Schottky interface of the gate electrode result in strong local current variations. It has been used to selectively identify suitable positions for further failure analysis of focused ion beam prepared cross sections by scanning and transmission electron microscopy. Process induced imperfections as voids at the Schottky interface after stress have been localized.

  11. Temperature-dependent photoluminescence imaging of GaAs/AlGaAs heterostructure quantum well tubes

    NASA Astrophysics Data System (ADS)

    Shi, Teng; Jackson, Howard; Smith, Leigh; Yarrison-Rice, Jan; Jiang, Nian; Tan, Hoe; Gao, Qiang; Jagadish, Chennupati

    2014-03-01

    Two sets of GaAs/AlGaAs core-multi shell nanowire quantum well tubes (QWTs) grown by MOCVD, with QW widths of 2nm and 6nm are dispersed onto a 4mm diameter hemispherical solid immersion lens. We obtain high spatial resolution photoluminescence (PL) images of single nanowires (NWs) from 10 K up to 120 K. High spectral resolution PL spectra reveal several narrow emission lines on high energy side of the 2nm QWT at low temperatures. In the 6nm QW, such narrow emission lines are not observed. Spatially-resolved PL images show that these localized states are randomly distributed along the NW long axis. Temperature-dependent PL imaging indicates that the quantum dot emissions disappear at temperatures above 50K. The recombination lifetime for electrons and holes in the QWT for the 2nm and 6nm QWTs are 500ps and 800 ps, respectively. We observe the recombination lifetime increases slightly with increasing temperature. We acknowledge the NSF through DMR-1105362, 1105121 and ECCS-1100489, and the Australian Research Council.

  12. Hybrid architecture for shallow accumulation mode AlGaAs/GaAs heterostructures with epitaxial gates

    SciTech Connect

    MacLeod, S. J.; See, A. M.; Hamilton, A. R.; Farrer, I.; Ritchie, D. A.; Ritzmann, J.; Ludwig, A.; Wieck, A. D.

    2015-01-05

    Accumulation mode devices with epitaxially grown gates have excellent electrical stability due to the absence of dopant impurities and surface states. We overcome typical fabrication issues associated with epitaxially gated structures (e.g., gate leakage and high contact resistance) by using separate gates to control the electron densities in the Ohmic and Hall bar regions. This hybrid gate architecture opens up a way to make ultrastable nanoscale devices where the separation between the surface gates and the 2D electron gas is small. In this work, we demonstrate that the hybrid devices made from the same wafer have reproducible electrical characteristics, with identical mobility and density traces over a large range of 2D densities. In addition, thermal cycling does not influence the measured electrical characteristics. As a demonstration of concept, we have fabricated a hybrid single-electron transistor on a shallow (50 nm) AlGaAs/GaAs heterostructure that shows clear Coulomb blockade oscillations in the low temperature conductance.

  13. Detection of halide ions with AlGaN /GaN high electron mobility transistors

    NASA Astrophysics Data System (ADS)

    Kang, B. S.; Ren, F.; Kang, M. C.; Lofton, C.; Tan, Weihong; Pearton, S. J.; Dabiran, A.; Osinsky, A.; Chow, P. P.

    2005-04-01

    AlGaN /GaN high electron mobility transistors (HEMTs) both with and without a Au gate are found to exhibit significant changes in channel conductance upon exposing the gate region to various halide ions. The polar nature of the halide ions leads to a change of surface charge in the gate region on the HEMT, producing a change in the surface potential at the semiconductor/liquid interface. HEMTs with a Au-gate electrode not only doubled the sensitivity of changing the channel conductance as compared to gateless HEMT, but also showed the opposite conductance behavior. When anions adsorbed on the Au, they produced a counter charge for electrovalence. These anions drag some counter ions from the bulk solution or create an image positive charge on the metal for the required neutrality. The gateless HEMTs can be used as sensors for a range of chemicals through appropriate modification with covalently bonded halide functional groups on the Au surface. This creates many possibilities to functionalize the surface for a wide range of integrated biological, chemical, and fluid monitoring sensors.

  14. Enhanced optical property in quaternary GaInAsSb/AlGaAsSb quantum wells

    SciTech Connect

    Lin, Chien-Hung Lee, Chien-Ping

    2014-10-21

    High quality GaInAsSb/AlGaAsSb quantum wells (QWs) have been grown by molecular beam epitaxy using proper interface treatments. By controlling the group-V elements at interfaces, we obtained excellent optical quality QWs, which were free from undesired localized trap states, which may otherwise severely affect the exciton recombination. Strong and highly efficient exciton emissions up to room temperature with a wavelength of 2.2 μm were observed. A comprehensive investigation on the QW quality was carried out using temperature dependent and power dependent photoluminescence (PL) measurements. The PL emission intensity remains nearly constant at low temperatures and is free from the PL quenching from the defect induced localized states. The temperature dependent emission energy had a bulk-like behavior, indicating high quality well/barrier interfaces. Because of the uniformity of the QWs and smooth interfaces, the low temperature limit of inhomogeneous line width broadening is as small as 5 meV.

  15. Metalorganic chemical vapour deposition of junction isolated GaAlAs/GaAs LED structures

    NASA Astrophysics Data System (ADS)

    Bradley, R. R.; Ash, R. M.; Forbes, N. W.; Griffiths, R. J. M.; Jebb, D. P.

    1986-09-01

    A low-cost high-radiance Ga(1-x)Al(x)As/GaAs LED has been developed using p-n junction isolation to achieve current confinement in an inverted Burrus LED structure. Using this structure devices with a high internal quantum efficiency of 87 percent which launch up to 1.5 mW into 200-micron, 0.3 nA fiber at 150 mA and have rise and fall times of less than 5 ns, have been fabricated. The LED structure involved a two-stage growth process in which initially an n-type isolation layer was grown on a p-type substrate, followed by infill growth, through contact holes, of the inverted LED structure. This paper will describe in detail the growth and preparation of the optimized junction isolated LED structure, including growth rate and composition control, layer uniformity and deliberate interface grading. Exploration of the use of magnesium doping and triethylaluminum for precise control of active layer doping and composition will be discussed, and finally details of device characteristics and performance are presented.

  16. AlGaAs/GaAs quasi-bulk effect mixers: Analysis and experiments

    NASA Technical Reports Server (NTRS)

    Yngvesson, K. S.; Yang, J.-X.; Agahi, F.; Dai, D.; Musante, C.; Grammer, W.; Lau, K. M.

    1992-01-01

    The lowest noise temperature for any receiver in the 0.5 to 1 THz range has been achieved with the bulk InSb hot electron mixer, which unfortunately suffers from the problem of having a very narrow bandwidth (1-2 MHz). We have demonstrated a three order of magnitude improvement in the bandwidth of hot electron mixers, by using the two-dimensional electron gas (2DEG) medium at the hetero-interface between AlGaAs and GaAs. We have tested both inhouse MOCVD-grown material, and MBE materials, with similar results. The conversion loss (L(sub c)) at 94 GHz is presently 18 dB for a mixer operating at 20 K, and calculations indicate that L(sub c) can be decreased to about 10 dB in future devices. Calculated and measured curves of L(sub c), versus PLO and IDC, respectively, agree well. We argue that there are several different configurations of hot electron mixers, which will also show wide bandwidth, and that these devices are likely to become important as low-noise THz receivers in the future.

  17. Formation Mechanisms of Alloying Element Nitrides in Recrystallized and Deformed Ferritic Fe-Cr-Al Alloy

    NASA Astrophysics Data System (ADS)

    Akhlaghi, Maryam; Meka, Sai Ramudu; Jägle, Eric A.; Kurz, Silke J. B.; Bischoff, Ewald; Mittemeijer, Eric J.

    2016-07-01

    The effect of the initial microstructure (recrystallized or cold-rolled) on the nitride precipitation process upon gaseous nitriding of ternary Fe-4.3 at. pct Cr-8.1 at. pct Al alloy was investigated at 723 K (450 °C) employing X-ray diffraction (XRD) analyses, transmission electron microscopy (TEM), atom probe tomography (APT), and electron probe microanalysis (EPMA). In recrystallized Fe-Cr-Al specimens, one type of nitride develops: ternary, cubic, NaCl-type mixed Cr1-x Al x N. In cold-rolled Fe-Cr-Al specimens, precipitation of two types of nitrides occurs: ternary, cubic, NaCl-type mixed Cr1-x Al x N and binary, cubic, NaCl-type AlN. By theoretical analysis, it was shown that for the recrystallized specimens an energy barrier for the nucleation of mixed Cr1-x Al x N exists, whereas in the cold-rolled specimens no such energy barriers for the development of mixed Cr1-x Al x N and of binary, cubic AlN occur. The additional development of the cubic AlN in the cold-rolled microstructure could be ascribed to the preferred heterogeneous nucleation of cubic AlN on dislocations. The nitrogen concentration-depth profile of the cold-rolled specimen shows a stepped nature upon prolonged nitriding as a consequence of instantaneous nucleation of nitride upon arrival of nitrogen and nitride growth rate-limited by nitrogen transport through the thickening nitrided zone.

  18. Formation Mechanisms of Alloying Element Nitrides in Recrystallized and Deformed Ferritic Fe-Cr-Al Alloy

    NASA Astrophysics Data System (ADS)

    Akhlaghi, Maryam; Meka, Sai Ramudu; Jägle, Eric A.; Kurz, Silke J. B.; Bischoff, Ewald; Mittemeijer, Eric J.

    2016-09-01

    The effect of the initial microstructure (recrystallized or cold-rolled) on the nitride precipitation process upon gaseous nitriding of ternary Fe-4.3 at. pct Cr-8.1 at. pct Al alloy was investigated at 723 K (450 °C) employing X-ray diffraction (XRD) analyses, transmission electron microscopy (TEM), atom probe tomography (APT), and electron probe microanalysis (EPMA). In recrystallized Fe-Cr-Al specimens, one type of nitride develops: ternary, cubic, NaCl-type mixed Cr1- x Al x N. In cold-rolled Fe-Cr-Al specimens, precipitation of two types of nitrides occurs: ternary, cubic, NaCl-type mixed Cr1- x Al x N and binary, cubic, NaCl-type AlN. By theoretical analysis, it was shown that for the recrystallized specimens an energy barrier for the nucleation of mixed Cr1- x Al x N exists, whereas in the cold-rolled specimens no such energy barriers for the development of mixed Cr1- x Al x N and of binary, cubic AlN occur. The additional development of the cubic AlN in the cold-rolled microstructure could be ascribed to the preferred heterogeneous nucleation of cubic AlN on dislocations. The nitrogen concentration-depth profile of the cold-rolled specimen shows a stepped nature upon prolonged nitriding as a consequence of instantaneous nucleation of nitride upon arrival of nitrogen and nitride growth rate-limited by nitrogen transport through the thickening nitrided zone.

  19. Analysis of the AlGaN/GaN vertical bulk current on Si, sapphire, and free-standing GaN substrates

    SciTech Connect

    Perez-Tomas, A.; Fontsere, A.; Llobet, J.; Placidi, M.; Rennesson, S.; Chenot, S.; Moreno, J. C.; Cordier, Y.; Baron, N.

    2013-05-07

    The vertical bulk (drain-bulk) current (I{sub db}) properties of analogous AlGaN/GaN hetero-structures molecular beam epitaxially grown on silicon, sapphire, and free-standing GaN (FS-GaN) have been evaluated in this paper. The experimental I{sub db} (25-300 Degree-Sign C) have been well reproduced with physical models based on a combination of Poole-Frenkel (trap assisted) and hopping (resistive) conduction mechanisms. The thermal activation energies (E{sub a}), the (soft or destructive) vertical breakdown voltage (V{sub B}), and the effect of inverting the drain-bulk polarity have also been comparatively investigated. GaN-on-FS-GaN appears to adhere to the resistive mechanism (E{sub a} = 0.35 eV at T = 25-300 Degree-Sign C; V{sub B} = 840 V), GaN-on-sapphire follows the trap assisted mechanism (E{sub a} = 2.5 eV at T > 265 Degree-Sign C; V{sub B} > 1100 V), and the GaN-on-Si is well reproduced with a combination of the two mechanisms (E{sub a} = 0.35 eV at T > 150 Degree-Sign C; V{sub B} = 420 V). Finally, the relationship between the vertical bulk current and the lateral AlGaN/GaN transistor leakage current is explored.

  20. Persistent luminescence in ZnGa2O4:Cr: an outstanding biomarker for in-vivo imaging

    NASA Astrophysics Data System (ADS)

    Sharma, S. K.; Bessiere, A.; Gourier, D.; Binet, L.; Viana, B.; Basavaraju, N.; Priolkar, K.; Maldiney, T.; Scherman, D.; Richard, C.

    2014-03-01

    ZnGa2O4 (ZGO) is a normal spinel. When doped with Cr3+ ions, ZGO:Cr becomes a high brightness persistent luminescence material with an emission spectrum perfectly matching the transparency window of living tissues. It allows in vivo mouse imaging with a better signal to background ratio than classical fluorescent NIR probes. The most interesting characteristic of ZGO:Cr lies in the fact that its LLP can be excited with red light, well below its band gap energy and in the transparency window of living tissues. A mechanism based on the trapping of carriers localized around a special type of Cr3+ ions namely CrN2 can explain this singularity. The antisite defects of the structure are the main responsible traps in the persistent luminescence mechanism. When located around Cr3+ ions, they allow, via Cr3+ absorption, the storage of not only UV light but also all visible light from the excitation source.

  1. Coating MCPs with AlN and GaN

    NASA Technical Reports Server (NTRS)

    Bensaoula, Abdelhakim; Starikov, David; Boney, Chris

    2006-01-01

    A development effort underway at the time of reporting the information for this article is devoted to increasing the sensitivity of microchannel plates (MCPs) as detectors of photons and ions by coating the MCPs with nitrides of elements in period III of the periodic table. Conventional MCPs are relatively insensitive to slowly moving, large-mass ions for example, ions of biomolecules under analysis in mass spectrometers. The idea underlying this development is to coat an MCP to reduce its work function (decrease its electron affinity) in order to increase both (1) the emission of electrons in response to impingement of low-energy, large-mass ions and (2) the multiplying effect of secondary electron emission. Of particular interest as coating materials having appropriately low or even negative electron affinities are gallium nitride, aluminum nitride, and ternary alloys of general composition Al(x)Ga(1-x)N (where 0AlN and GaN both undoped and doped with Si were deposited on commercial MCPs by radio-frequency molecular-beam epitaxy (also known as plasma-assisted molecular-beam epitaxy) at temperatures <200 C. This deposition technique is particularly suitable because (1) MCPs cannot withstand the higher deposition-substrate temperatures used to decompose constituent compounds in some other deposition techniques and (2) in this technique, the constituent Al, Ga, and N

  2. AlGaInN laser diode technology and systems for defence and security applications

    NASA Astrophysics Data System (ADS)

    Najda, Stephen P.; Perlin, Piotr; Suski, Tadek; Marona, Lucja; Boćkowski, Mike; Leszczyński, Mike; Wisniewski, Przemek; Czernecki, Robert; Kucharski, Robert; Targowski, Grzegorz; Watson, Scott; Kelly, Antony E.

    2015-05-01

    The latest developments in AlGaInN laser diode technology are reviewed for defence and security applications such as underwater communications. The AlGaInN material system allows for laser diodes to be fabricated over a very wide range of wavelengths from u.v., ~380nm, to the visible ~530nm, by tuning the indium content of the laser GaInN quantum well. Thus AlGaInN laser diode technology is a key enabler for the development of new disruptive system level applications in displays, telecom, defence and other industries.

  3. Modeling on oxide dependent 2DEG sheet charge density and threshold voltage in AlGaN/GaN MOSHEMT

    NASA Astrophysics Data System (ADS)

    Panda, J.; Jena, K.; Swain, R.; Lenka, T. R.

    2016-04-01

    We have developed a physics based analytical model for the calculation of threshold voltage, two dimensional electron gas (2DEG) density and surface potential for AlGaN/GaN metal oxide semiconductor high electron mobility transistors (MOSHEMT). The developed model includes important parameters like polarization charge density at oxide/AlGaN and AlGaN/GaN interfaces, interfacial defect oxide charges and donor charges at the surface of the AlGaN barrier. The effects of two different gate oxides (Al2O3 and HfO2) are compared for the performance evaluation of the proposed MOSHEMT. The MOSHEMTs with Al2O3 dielectric have an advantage of significant increase in 2DEG up to 1.2 × 1013 cm‑2 with an increase in oxide thickness up to 10 nm as compared to HfO2 dielectric MOSHEMT. The surface potential for HfO2 based device decreases from 2 to ‑1.6 eV within 10 nm of oxide thickness whereas for the Al2O3 based device a sharp transition of surface potential occurs from 2.8 to ‑8.3 eV. The variation in oxide thickness and gate metal work function of the proposed MOSHEMT shifts the threshold voltage from negative to positive realizing the enhanced mode operation. Further to validate the model, the device is simulated in Silvaco Technology Computer Aided Design (TCAD) showing good agreement with the proposed model results. The accuracy of the developed calculations of the proposed model can be used to develop a complete physics based 2DEG sheet charge density and threshold voltage model for GaN MOSHEMT devices for performance analysis.

  4. Two-Step Electron Beam Excitation of Al2O3:Cr by Gallium Nitride Recombination Radiation

    NASA Astrophysics Data System (ADS)

    Zehe, A.; Ramírez, A.; Seifert, W.

    We report on a two-step excitation process of a GaN/A2O3:Cr specimen by an external electron beam of 30 keV. The epitaxial growth of GaN on the (0001) faces of ruby is described, as well as the experimental technique applied in the excitation and recording of the combined luminescence spectrum. It is known that the spinel and ruby are frequently used as substrate materials for the epitaxial deposition of GaN. At the same time is ruby one of the most useful layer materials with a red emission line λ = 694 nm. Due to its dielectric character, the pumping of ruby is performed usually subjecting it to the light of an intense flash lamp. Electron beam excitation, on the other hand, which could be more powerful, is impaired with electrical insulator material. This discrepancy can be removed by a two-step excitation, where at first the semiconducting GaN-layer, epitaxially grown on a (0001)Al2O3:Cr (ruby) crystal is excited to radiation emission by an external electron beam. The following internal absolption process, by the ruby crystal, of the GaN luminescence radiation provides for the characteristic 2E to 4A2-transition at 1.786 eV. We found a strong and sharp emission line of that photon energy, which additionally displays polarization, typical for the anisotropic uniaxial ruby crystal. Such an indirect electron-beam excitation of ruby allows to generate almost monochromatic red light, and might point into a direction of interesting practical applications.

  5. Multistage symmetry breaking in the breathing pyrochlore lattice Li(Ga ,In )Cr 4O8

    NASA Astrophysics Data System (ADS)

    Lee, S.; Do, S.-H.; Lee, W.-J.; Choi, Y. S.; Lee, M.; Choi, E. S.; Reyes, A. P.; Kuhns, P. L.; Ozarowski, A.; Choi, K.-Y.

    2016-05-01

    We present magnetic susceptibility, dielectric constant, high-frequency electron spin resonance, 7Li nuclear magnetic resonance, and zero-field muon spin relaxation measurements of LiACr4O8 (A =Ga , In), towards realizing a breathing pyrochlore lattice. Unlike the uniform pyrochlore ZnCr2O4 lattice, both the In and the Ga compounds feature two-stage symmetry breaking: a magnetostructural phase transition with subsequent antiferromagnetic ordering. We find a disparate symmetry breaking process between the In and the Ga compounds, having different degrees of bond alternation. Our data reveal that the Ga compound with moderate bond alternation shows the concomitant structural and magnetic transition at TS=15.2 K, followed by the magnetic ordering at Tm=12.9 K. In contrast, the In compound with strong bond alternation undergoes a thermal crossover at T*≈20.1 K from a tetramer singlet to a dimer singlet or a correlated paramagnet with a separate weak magnetostructural transition at TS=17.6 K and the second antiferromagnetic ordering at Tm=13.7 K. This suggests that the magnetic phases and correlations of the breathing pyrochlore lattice can be determined from the competition between bond alternation and spin-lattice coupling, thus stabilizing long-range magnetic ordering against a nonmagnetic singlet.

  6. Variation of structural and optical properties across an AlGaN/GaN HEMT structure directly imaged by cathodoluminescence microscopy

    NASA Astrophysics Data System (ADS)

    Bertram, F.; Christen, J.; Heikman, S.; Keller, S.; Denbaars, S. P.; Mishra, U. K.

    2003-11-01

    Non-planar selective area growth of GaN and Al0.1Ga0.9N was performed by metalorganic chemical vapor deposition, on selectively etched GaN samples masked with SiO2-ridges. We report on the characterization of this lateral structured AlGaN/GaN HEMT structure using spatially and spectrally resolved cathodoluminescence. A distinct change of emission energy of the AlGaN and GaN across the ridge was observed. The shift of AlGaN emission directly visualizes the strong lateral Al composition gradient across the ridge. The modulation of emission energy of the two dimensional electron gas (GaN emission) leads to the formation of a potential well with distinct barriers in the slope region and a pronounced lateral confinements inside of the well at the ridge edges. (

  7. Optimization of AlAs/AlGaAs quantum well heterostructures on on-axis and misoriented GaAs (111)B

    NASA Astrophysics Data System (ADS)

    Herzog, F.; Bichler, M.; Koblmüller, G.; Prabhu-Gaunkar, S.; Zhou, W.; Grayson, M.

    2012-05-01

    We report systematic growth optimization of high Al-content AlGaAs, AlAs, and associated modulation-doped quantum well (QW) heterostructures on on-axis and misoriented GaAs (111)B by molecular beam epitaxy. Growth temperatures TG > 690 °C and low As4 fluxes close to group III-rich growth significantly suppress twin defects in high-Al content AlGaAs on on-axis GaAs (111)B, as quantified by atomic force and transmission electron microscopy as well as x-ray diffraction. Mirror-smooth and defect-free AlAs with pronounced step-flow morphology was further achieved by growth on 2° misoriented GaAs (111)B toward [01¯1] and [21¯1¯] orientations. Successful fabrication of modulation-doped AlAs QW structures on these misoriented substrates yielded record electron mobilities (at 1.15 K) in excess of 13 000 cm2/Vs at sheet carrier densities of 5 × 1011 cm-2.

  8. Flat panel display using Ti-Cr-Al-O thin film

    DOEpatents

    Jankowski, Alan F.; Schmid, Anthony P.

    2002-01-01

    Thin films of Ti--Cr--Al--O are used as a resistor material. The films are rf sputter deposited from ceramic targets using a reactive working gas mixture of Ar and O.sub.2. Resistivity values from 10.sup.4 to 10.sup.10 Ohm-cm have been measured for Ti--Cr--Al--O film <1 .mu.m thick. The film resistivity can be discretely selected through control of the target composition and the deposition parameters. The application of Ti--Cr--Al--O as a thin film resistor has been found to be thermodynamically stable, unlike other metal-oxide films. The Ti--Cr--Al--O film can be used as a vertical or lateral resistor, for example, as a layer beneath a field emission cathode in a flat panel display; or used to control surface emissivity, for example, as a coating on an insulating material such as vertical wall supports in flat panel displays.

  9. Process for producing Ti-Cr-Al-O thin film resistors

    DOEpatents

    Jankowski, Alan F.; Schmid, Anthony P.

    2001-01-01

    Thin films of Ti-Cr-Al-O are used as a resistor material. The films are rf sputter deposited from ceramic targets using a reactive working gas mixture of Ar and O.sub.2. Resistivity values from 10.sup.4 to 10.sup.10 Ohm-cm have been measured for Ti-Cr-Al-O film <1 .mu.m thick. The film resistivity can be discretely selected through control of the target composition and the deposition parameters. The application of Ti-Cr-Al-O as a thin film resistor has been found to be thermodynamically stable, unlike other metal-oxide films. The Ti-Cr-Al-O film can be used as a vertical or lateral resistor, for example, as a layer beneath a field emission cathode in a flat panel display; or used to control surface emissivity, for example, as a coating on an insulating material such as vertical wall supports in flat panel displays.

  10. Performance of AlGaN/GaN Nanowire Omega-Shaped-Gate Fin-Shaped Field-Effect Transistor.

    PubMed

    Lee, Dong-Gi; Sindhuri, V; Jo, Young-Woo; Son, Dong-Hyeok; Kang, Hee-Sung; Lee, Jae-Hong; Lee, Jae-Hoon; Cristoloveanu, Sorin; Im, Ki-Sik; Lee, Jung-Hee

    2016-05-01

    The AlGaN/GaN nanowire omega-shaped-gate FinFET have been successfully fabricated demonstrating much improved performance compared to conventional AlGaN/GaN MISHFET. The AlGaN/GaN omega-shaped-gate FinFET exhibited the remarkable on-state performances, such as maximum drain current of 1.1 A/mm, low on-resistance, and low current collapse compared to that of the conventional device structure. In addition, the excellent off-state performances were measured: low off-state leakage current as low as -10(-10) mA, the theoretical SS value of -62 mV/dec, and high I(ON)/I(OFF) ratio (-10(9)). Improved dc performances were obtained for omega-shaped-gate structure due to the fully depletion of the active fin body and perfectly separation of the depleted fin from the underlying thick GaN buffer layer. Furthermore, the additional reason for the enhanced device performance of the proposed device is the improved gate controllability compared to the conventional MISHFET. The proposed nano-structure device is very promising candidate for the steep switching device applications. PMID:27483869

  11. NiAl-based Polyphase in situ Composites in the NiAl-Ta-X (X = Cr, Mo, or V) Systems

    NASA Technical Reports Server (NTRS)

    Johnson, D. R.; Oliver, B. F.; Noebe, R. D.; Whittenberger, J. D.

    1995-01-01

    Polyphase in situ composites were generated by directional solidification of ternary eutectics. This work was performed to discover if a balance of properties could be produced by combining the NiAl-Laves phase and the NiAl-refractory metal phase eutectics. The systems investigated were the Ni-Al-Ta-X (X = Cr, Mo, or V) alloys. Ternary eutectics were found in each of these systems and the eutectic composition, temperature, and morphology were determined. The ternary eutectic systems examined were the NiAl-NiAlTa-(Mo, Ta), NiAl-(Cr, Al) NiTa-Cr, and the NiAl-NiAlTa-V systems. Each eutectic consists of NiAl, a C14 Laves phase, and a refractory metal phase. Directional solidification was performed by containerless processing techniques in a levitation zone refiner to minimize alloy contamination. Room temperature fracture toughness of these materials was determined by a four-point bend test. Preliminary creep behavior was determined by compression tests at elevated temperatures, 1100-l400 K. Of the ternary eutectics, the one in the NiAl-Ta-Cr system was found to be the most promising. The fracture toughness of the NiAl-(Cr, Al)NiTa-Cr eutectic was intermediate between the values of the NiAl-NiAlTa eutectic and the NiAl-Cr eutectic. The creep strength of this ternary eutectic was similar to or greater than that of the NiAl-Cr eutectic.

  12. Wide bandgap engineering of (AlGa){sub 2}O{sub 3} films

    SciTech Connect

    Zhang, Fabi; Saito, Katsuhiko; Tanaka, Tooru; Nishio, Mitsuhiro; Guo, Qixin; Arita, Makoto

    2014-10-20

    Bandgap tunable (AlGa){sub 2}O{sub 3} films were deposited on sapphire substrates by pulsed laser deposition (PLD). The deposited films are of high transmittance as measured by spectrophotometer. The Al content in films is almost the same as that in targets. The measurement of bandgap energies by examining the onset of inelastic energy loss in core-level atomic spectra using X-ray photoelectron spectroscopy is proved to be valid for determining the bandgap of (AlGa){sub 2}O{sub 3} films as it is in good agreement with the bandgap values from transmittance spectra. The measured bandgap of (AlGa){sub 2}O{sub 3} films increases continuously with the Al content covering the whole Al content range from about 5 to 7 eV, indicating PLD is a promising growth technology for growing bandgap tunable (AlGa){sub 2}O{sub 3} films.

  13. X-ray characterization of GaN/AlGaN multiple quantum wells for ultraviolet laser diodes

    NASA Astrophysics Data System (ADS)

    Korakakis, D.; Ludwig, K. F.; Moustakas, T. D.

    1998-03-01

    GaN/Al0.20Ga0.80N (50 Å/50 Å) multiple quantum wells (MQW) with 15 periods were grown on (0001) sapphire substrates by molecular beam epitaxy and evaluated by x-ray diffraction. To simulate an ultraviolet laser diode structure, the substrate was coated first with n-GaN as the bottom contact layer and n-Al0.25Ga0.75N as the corresponding cladding layer. The crystal structure of this system was investigated by studying the reciprocal lattice map of off-axis diffraction peaks as well as the θ-2θ pattern around the (0002) reflection. The MQW was found to be coherent and has the a-lattice parameter of the underlying Al0.25Ga0.75N. The good agreement between experimental and theoretical data in the relative intensity of up to third-order satellite peaks supports that the interfaces of the MQW are abrupt, and thus, interdiffusion of Ga and Al atoms at the growth temperature was negligible.

  14. Abruptness improvement of the interfaces of AlGaN/GaN superlattice by cancelling asymmetric diffusion

    NASA Astrophysics Data System (ADS)

    Cai, Duanjun; Chen, Xiaohong; Lin, Na; Xu, Fuchun; Chen, Hangyang; Chen, Shanshan

    2012-02-01

    Interface abruptness has been an important issue in the construction of quantum wells as active layer in optoelectronic devices, which is extremely crucial in achieving stronger quantum confinement and consequently higher emission efficiency. The interfacial sharpness is highly associated with the crystal structure as well as the elemental transition. However, few studies have been done focusing on the elemental diffusion effect at the interface. In this work, the accurate determination was approached to the elemental inter-diffusion depth across the GaN/Al0.5Ga0.5N interfaces by using transmission electron microscopy, Auger electron microscopy, and X-ray diffraction. The GaN/Al0.5Ga0.5N superlattice was grown by metalorganic chemical vapor deposition (MOCVD) at high growth temperature (1070 ^oC). The results showed that the Al diffusion at the upper and lower interfaces of Al0.5Ga0.5N barrier appears an asymmetric behavior, which is 0.62 and 0.99 nm, respectively. Such will lead to the gradient interfacial region and asymmetric quantum well, affecting the carrier quantum confinement. To improve the abruptness of the interface and to modify the asymmetric diffusion, self-compensation pair technique was proposed and introduced to the growth of the lower Al0.5Ga0.5N/GaN interface, blocking the Al downward diffusion. Fist-principles simulations also showed that the structural relaxation at the strained heterointerface influences the electronic structure as well as elemental diffusion.

  15. Hydrogen permeation in FeCrAl alloys for LWR cladding application

    NASA Astrophysics Data System (ADS)

    Hu, Xunxiang; Terrani, Kurt A.; Wirth, Brian D.; Snead, Lance L.

    2015-06-01

    FeCrAl, an advanced oxidation-resistant iron-based alloy class, is a highly prevalent candidate as an accident-tolerant fuel cladding material. Compared with traditional zirconium alloy fuel cladding, increased tritium permeation through FeCrAl fuel cladding to the primary coolant is expected, raising potential safety concerns. In this study, the hydrogen permeability of several FeCrAl alloys was obtained using a static permeation test station, which was calibrated and validated using 304 stainless steel. The high hydrogen permeability of FeCrAl alloys leads to concerns with respect to potentially significant tritium release when used for fuel cladding in LWRs. The total tritium inventory inside the primary coolant of a light water reactor was quantified by applying a 1-dimensional steady state tritium diffusion model to demonstrate the dependence of tritium inventory on fuel cladding type. Furthermore, potential mitigation strategies for tritium release from FeCrAl fuel cladding were discussed and indicate the potential for application of an alumina layer on the inner clad surface to serve as a tritium barrier. More effort is required to develop a robust, economical mitigation strategy for tritium permeation in reactors using FeCrAl clad fuel assemblies.

  16. Hydrogen permeation in FeCrAl alloys for LWR cladding application

    DOE PAGESBeta

    Hu, Xunxiang; Terrani, Kurt A.; Wirth, Brian D.; Snead, Lance L.

    2015-03-19

    FeCrAl is an advanced oxidation-resistant iron-based alloy class, is a highly prevalent candidate as an accident-tolerant fuel cladding material. Compared with traditional zirconium alloy fuel cladding, increased tritium permeation through FeCrAl fuel cladding to the primary coolant is expected, raising potential safety concerns. In our study, the hydrogen permeability of several FeCrAl alloys was obtained using a static permeation test station, which was calibrated and validated using 304 stainless steel. The high hydrogen permeability of FeCrAl alloys leads to concerns with respect to potentially significant tritium release when used for fuel cladding in LWRs. Also, the total tritium inventory insidemore » the primary coolant of a light water reactor was quantified by applying a 1-dimensional steady state tritium diffusion model to demonstrate the dependence of tritium inventory on fuel cladding type. Furthermore, potential mitigation strategies for tritium release from FeCrAl fuel cladding were discussed and indicate the potential for application of an alumina layer on the inner clad surface to serve as a tritium barrier. More effort is required to develop a robust, economical mitigation strategy for tritium permeation in reactors using FeCrAl clad fuel assemblies.« less

  17. Hydrogen permeation in FeCrAl alloys for LWR cladding application

    SciTech Connect

    Hu, Xunxiang; Terrani, Kurt A.; Wirth, Brian D.; Snead, Lance L.

    2015-03-19

    FeCrAl is an advanced oxidation-resistant iron-based alloy class, is a highly prevalent candidate as an accident-tolerant fuel cladding material. Compared with traditional zirconium alloy fuel cladding, increased tritium permeation through FeCrAl fuel cladding to the primary coolant is expected, raising potential safety concerns. In our study, the hydrogen permeability of several FeCrAl alloys was obtained using a static permeation test station, which was calibrated and validated using 304 stainless steel. The high hydrogen permeability of FeCrAl alloys leads to concerns with respect to potentially significant tritium release when used for fuel cladding in LWRs. Also, the total tritium inventory inside the primary coolant of a light water reactor was quantified by applying a 1-dimensional steady state tritium diffusion model to demonstrate the dependence of tritium inventory on fuel cladding type. Furthermore, potential mitigation strategies for tritium release from FeCrAl fuel cladding were discussed and indicate the potential for application of an alumina layer on the inner clad surface to serve as a tritium barrier. More effort is required to develop a robust, economical mitigation strategy for tritium permeation in reactors using FeCrAl clad fuel assemblies.

  18. High-power single-element pseudomorphic InGaAs/GaAs/AlGaAs single quantum well lasers for pumping Er-doped fiber amplifiers

    NASA Technical Reports Server (NTRS)

    Larsson, A.; Forouhar, S.; Cody, J.; Lang, R. J.; Andrekson, P. A.

    1991-01-01

    A 980-nm-ridge waveguide pseudomorphic InGaAs/GaAs/AlGaAs single-quantum-well laser with a maximum single-ended output power of 240 mW from a facet-coated device is fabricated from a graded-index separate-confinement heterostructure grown by molecular-beam epitaxy. The laser oscillates in the fundamental spatial mode, allowing 22 percent coupling efficiency into a 1.55-micron single-mode optical fiber. Life testing at an output power of 30 mW per facet from uncoated devices reveals a superior reliability to GaAs/AlGaAs quantum-well lasers but also the need for protective facet coatings for long term reliability at power levels required for pumping Er-doped fiber amplifiers.

  19. AlGaAs phased array laser for optical communications

    NASA Technical Reports Server (NTRS)

    Carlson, N. W.

    1989-01-01

    Phased locked arrays of multiple AlGaAs diode laser emitters were investigated both in edge emitting and surface emitting configurations. CSP edge emitter structures, coupled by either evanescent waves or Y-guides, could not achieve the required powers (greater than or similar to 500 mW) while maintaining a diffraction limited, single lobed output beam. Indeed, although the diffraction limit was achieved in this type of device, it was at low powers and in the double lobed radiation pattern characteristic of out-of-phase coupling. Grating surface emitting (GSE) arrays were, therefore, investigated with more promising results. The incorporation of second order gratings in distribute Bragg reflector (DBR) structures allows surface emission, and can be configured to allow injection locking and lateral coupling to populate 2-D arrays that should be able to reach power levels commensurate with the needs of high performance, free space optical communications levels. Also, a new amplitude modulation scheme was developed for GSE array operation.

  20. An analysis of GaAlAs-GaAs solar cells - A study of the limiting mechanisms of heterojunction systems

    NASA Astrophysics Data System (ADS)

    Gazaleh, Y. A.

    1985-12-01

    Design and manufacturing techniques are presented for the production of optimized heterojunction GaAlAs-GaAs solar cells. The design goal is to obtain multispectral solar cells with enhanced efficiency and reliability at lower cost than now possible. Window layers with Al doping levels of 40 percent are shown to have efficiencies equivalent to layers with Al doping levels of 80-90 percent. Liquid phase epitaxial techniques are described for producing the cells. Supercooling methods are used to minimize the defect densities. Laboratory tests have yielded 0.04 sq cm cells with 20.2 percent efficiency under an illumination of 0.4 W/sq cm. In extending the study to bispectral cells, a cascade configuration is found promising for lowering production costs. Ternary and quaternary materials are determined to be the most suitable for obtaining the cascade cell, i.e., GaAlAsSb and GaInAsP substrates with GaAlAsSb window layers. A metal grid in the layer connection is demonstrated to give efficiencies of up to 25.4 percent, which are augmented to 27.3 percent with an AR coating. Finally, optimization of the metal contacts further enhances the cell efficiencies to 31 percent in AM0.