Science.gov

Sample records for earth implanted gan

  1. The structure of crystallographic damage in GaN formed during rare earth ion implantation with and without an ultrathin AlN capping layer

    NASA Astrophysics Data System (ADS)

    Gloux, F.; Ruterana, P.; Wojtowicz, T.; Lorenz, K.; Alves, E.

    2006-10-01

    The crystallographic nature of the damage created in GaN implanted by rare earth ions at 300 keV and room temperature has been investigated by transmission electron microscopy versus the fluence, from 7×10 13 to 2×10 16 at/cm 2, using Er, Eu or Tm ions. The density of point defect clusters was seen to increase with the fluence. From about 3×10 15 at/cm 2, a highly disordered 'nanocrystalline layer' (NL) appears on the GaN surface. Its structure exhibits a mixture of voids and misoriented nanocrystallites. Basal stacking faults (BSFs) of I 1, E and I 2 types have been noticed from the lowest fluence, they are I 1 in the majority. Their density increases and saturates when the NL is observed. Many prismatic stacking faults (PSFs) with Drum atomic configuration have been identified. The I 1 BSFs are shown to propagate easily through GaN by folding from basal to prismatic planes thanks to the PSFs. When implanting through a 10 nm AlN cap, the NL threshold goes up to about 3×10 16 at/cm 2. The AlN cap plays a protective role against the dissociation of the GaN up to the highest fluences. The flat surface after implantation and the absence of SFs in the AlN cap indicate its high resistance to the damage formation.

  2. Characterisation of Cs ion implanted GaN by DLTS

    NASA Astrophysics Data System (ADS)

    Ngoepe, P. N. M.; Meyer, W. E.; Auret, F. D.; Omotoso, E.; Hlatshwayo, T. T.; Diale, M.

    2018-04-01

    Deep level transient spectroscopy (DLTS) was used to characterise Cs implanted GaN grown by hydride vapour phase epitaxy (HVPE). This implantation was done at room temperature using energy of 360 keV to a fluence of 10-11 cm-2. A defect with activation energy of 0.19 eV below the conduction band and an apparent capture cross section of 1.1 × 10-15 cm2 was induced. This defect has previously been observed after rare earth element (Eu, Er and Pr) implantation. It has also been reported after electron, proton and He ion implantation.

  3. Novel activation process for Mg-implanted GaN

    NASA Astrophysics Data System (ADS)

    Hashimoto, Shin; Nakamura, Takao; Honda, Yoshio; Amano, Hiroshi

    2014-02-01

    A novel activation process for Mg-implanted GaN was demonstrated. As opposed to the conventional thermal annealing process, an H2/NH3 alternating supply annealing process achieved better optical activation, stronger near-ultraviolet luminescence and weaker yellow luminescence in the photoluminescence spectroscopy. After this process, small hexagonal hillocks were observed on the surface, which indicated that crystal regrowth was induced by this process, consisting of decomposition of GaN by H2 supplies and re-crystallization by NH3 supplies. It was revealed that the implanted Mg could easily be located at the activation site by means of crystal regrowth by this process.

  4. Photoluminescence of Zn-implanted GaN

    NASA Technical Reports Server (NTRS)

    Pankove, J. I.; Hutchby, J. A.

    1974-01-01

    The photoluminescence spectrum of Zn-implanted GaN peaks at 2.87 eV at room temperature. The emission efficiency decreases linearly with the logarithm of the Zn concentration in the range from 1 x 10 to the 18th to 20 x 10 to the 18th Zn/cu cm.

  5. Thermal stability of implanted dopants in GaN

    NASA Astrophysics Data System (ADS)

    Wilson, R. G.; Pearton, S. J.; Abernathy, C. R.; Zavada, J. M.

    1995-04-01

    Results are reported of measurements of depth profiles and stability against redistribution with annealing up to 800 or 900 °C, for implanted Be, C, Mg, Si, S, Zn, Ge, and Se as dopants in GaN. The results confirm the high-temperature stability of dopants in this material up to temperatures that vary from 600 to 900 °C. S redistributes for temperatures above 600 °C, and Zn and Se, for temperatures above 800 °C. All of the other elements are stable to 900 °C. These results indicate that direct implantation of dopants rather than masked diffusion will probably be necessary to define selective area doping of III-V nitride device structures based on these results for GaN.

  6. Photoluminescence of ion-implanted GaN

    NASA Technical Reports Server (NTRS)

    Pankove, J. I.; Hutchby, J. A.

    1976-01-01

    Thirty-five elements were implanted in GaN. Their photoluminescence spectra were measured and compared to those of an unimplanted control sample. Most impurities emit a peak at about 2.15 eV. Mg, Zn, Cd, Ca, As, Hg, and Ag have more characteristic emissions. Zn provides the most efficient recombination center. A set of midgap states is generated during the damage-annealing treatment.

  7. Characterization of an Mg-implanted GaN p-i-n Diode

    DTIC Science & Technology

    2016-03-31

    unintentionally doped GaN layer was grown by metal organic chemical vapor deposition (MOCVD) on a n+ Ga -face c-oriented GaN substrate. The as-grown MOCVD film...their proper lattice sites. In the case of Mg implanted GaN , the Mg must replace Ga to result in p-type material. In many other semiconductor...Characterization of an Mg-implanted GaN p-i-n Diode Travis J. Anderson, Jordan D. Greenlee, Boris N. Feigelson, Karl D. Hobart, and Francis J

  8. Effects of hydrogen implantation into GaN

    NASA Astrophysics Data System (ADS)

    Pearton, S. J.; Abernathy, C. R.; Wilson, R. G.; Zavada, J. M.; Song, C. Y.; Weinstein, M. G.; Stavola, M.; Han, J.; Shul, R. J.

    1999-01-01

    Proton implantation in GaN is found to reduce the free carrier density through two mechanisms - first, by creating electron and hole traps at around EC - 0.8 eV and EV + 0.9 eV that lead to compensation in both n- and p-type material, and second, by leading to formation of (AH)° complexes, where A is any acceptor (Mg, Ca, Zn, Be, Cd). The former mechanism is useful in creating high resistivity regions for device isolation, whereas the latter produces unintentional acceptor passivation that is detrimental to device performance. The strong affinity of hydrogen for acceptors leads to markedly different redistribution behavior for implanted H + in n- and p-GaN due to the chemical reaction to form neutral complexes in the latter. The acceptors may be reactivated by simple annealing at ⩾600°C, or by electron injection at 25-150°C that produces debonding of the (AH)° centers. Implanted hydrogen is also strongly attracted to regions of strain in heterostructure samples during annealing, leading to pile-up at epi-epi and epi-substrate interfaces. IR spectroscopy shows that implanted hydrogen also decorates VGa defects in undoped and n-GaN.

  9. Radiation Damage Formation And Annealing In Mg-Implanted GaN

    SciT

    Whelan, Sean; Kelly, Michael J.; Yan, John

    2005-06-30

    We have implanted GaN with Mg ions over an energy range of 200keV to 1MeV at substrate temperatures of -150 (cold) and +300 deg. C (hot). The radiation damage formation in GaN was increased for cold implants when compared to samples implanted at elevated temperatures. The increase in damage formation is due to a reduction in the dynamic defect annealing during ion irradiation. The dopant stopping in the solid also depends upon the implant temperature. For a fixed implant energy and dose, Mg ions have a shorter range in GaN for cold implants when compared to hot implants which ismore » caused by the increase in scattering centres (disorder)« less

  10. Electronic Characteristics of Rare Earth Doped GaN Schottky Diodes

    DTIC Science & Technology

    2013-03-21

    REPORT TYPE Master’s Thesis 3. DATES COVERED (From – To) 04 Sep 2011 - Mar 2013 4. TITLE AND SUBTITLE ELECTRONIC CHARACTERISTICS OF RARE EARTH ...ELECTRONIC CHARACTERISTICS OF RARE EARTH DOPED GaN SCHOTTKY DIODES THESIS Aaron B. Blanning...United States. AFIT-ENP-13-M-03 Electronic Characteristics of Rare Earth Doped GaN Schottky Diodes THESIS Presented to the Faculty

  11. Evaluation of lattice displacement in Mg - Implanted GaN by Rutherford backscattering spectroscopy

    NASA Astrophysics Data System (ADS)

    Nishikata, N.; Kushida, K.; Nishimura, T.; Mishima, T.; Kuriyama, K.; Nakamura, T.

    2017-10-01

    Evaluation of lattice displacement in Mg-ion implanted GaN is studied by combining elastic recoil detection analysis (ERDA), Rutherford backscattering spectroscopy (RBS) and Photoluminescence (PL) measurements. Mg-ion implantation into GaN single crystal wafer is performed with energies of 30 keV (ion fluence; 3.5 × 1014 cm-2) and 60 keV (6.5 × 1014 cm-2) at room temperature. The ERDA measurements using the 1.5 MeV helium beam can evaluate hydrogen from the surface to ∼300 nm. The hydrogen concentration for un-implanted and as-implanted GaN is 3.1 × 1014 cm-2 and 6.1 × 1014 cm-2 at around 265 nm in depth. χmin (the ratio of aligned and random yields) near the surface of the 〈0 0 0 1〉 direction for Ga is 1.61% for un-implanted and 2.51% for Mg-ion implanted samples. On the other hand, the value of χmin for N is 10.08% for un-implanted and 11.20% for Mg-ion implanted samples. The displacement concentration of Ga and N estimated from these χmin values is 4.01 × 1020 cm-3 and 5.46 × 1020 cm-3, respectively. This suggests that Ga vacancy (VGa), N vacancy (VN), Ga interstitial (Gai), and N interstitial (Ni) is introduced in Mg-ion implanted GaN. A strong emission at around 400 nm in as-implanted GaN is related to a VN donor and some acceptor pairs. It is suggested that the origin of the very high resistivity after the Mg-ion implantation is attributed to the carrier compensation effect due to the deep level of Ni as a non-radiative center.

  12. Multicycle rapid thermal annealing technique and its application for the electrical activation of Mg implanted in GaN

    NASA Astrophysics Data System (ADS)

    Feigelson, B. N.; Anderson, T. J.; Abraham, M.; Freitas, J. A.; Hite, J. K.; Eddy, C. R.; Kub, F. J.

    2012-07-01

    No reliable results were reported up-to-date on electrical activation of Mg implanted GaN without co-doping with other ions. The main reason of the poor ion-implanted activation in GaN is lack of the adequate GaN annealing technique. We have developed a new approach, Multicycle Rapid Thermal Annealing to overcome this limitation and enable longer annealing times at high temperature. We have applied this new technique to Mg-implanted GaN, and demonstrated p-type conductivity.

  13. Microwave annealing of Mg-implanted and in situ Be-doped GaN

    NASA Astrophysics Data System (ADS)

    Aluri, Geetha S.; Gowda, Madhu; Mahadik, Nadeemullah A.; Sundaresan, Siddarth G.; Rao, Mulpuri V.; Schreifels, John A.; Freitas, J. A.; Qadri, S. B.; Tian, Y.-L.

    2010-10-01

    An ultrafast microwave annealing method, different from conventional thermal annealing, is used to activate Mg-implants in GaN layer. The x-ray diffraction measurements indicated complete disappearance of the defect sublattice peak, introduced by the implantation process for single-energy Mg-implantation, when the annealing was performed at ≥1400 °C for 15 s. An increase in the intensity of Mg-acceptor related luminescence peak (at 3.26 eV) in the photoluminescence spectra confirms the Mg-acceptor activation in single-energy Mg-implanted GaN. In case of multiple-energy implantation, the implant generated defects persisted even after 1500 °C/15 s annealing, resulting in no net Mg-acceptor activation of the Mg-implant. The Mg-implant is relatively thermally stable and the sample surface roughness is 6 nm after 1500 °C/15 s annealing, using a 600 nm thick AlN cap. In situ Be-doped GaN films, after 1300 °C/5 s annealing have shown Be out-diffusion into the AlN layer and also in-diffusion toward the GaN/SiC interface. The in-diffusion and out-diffusion of the Be increased with increasing annealing temperature. In fact, after 1500 °C/5 s annealing, only a small fraction of in situ doped Be remained in the GaN layer, revealing the inadequateness of using Be-implantation for forming p-type doped layers in the GaN.

  14. Structures and optical properties of \\text{H}_{2}^{+} -implanted GaN epi-layers

    NASA Astrophysics Data System (ADS)

    Li, B. S.; Wang, Z. G.

    2015-06-01

    The implantation damage build-up and optical properties of GaN epitaxial films under \\text{H}2+ ion implantation have been investigated by a combination of Rutherford backscattering in channeling geometry, Raman spectroscopy, UV-visible spectroscopy and transmission electron microscopy. GaN epitaxial films were implanted with 134 keV \\text{H}2+ ions to doses ranging from 3.75   ×   1016 to 1.75   ×   1017 \\text{H}2+  cm-2 at room temperature or the same dose of 1.5   ×   1017 \\text{H}2+  cm-2 at room temperature, 573 and 723 K. The dependence of lattice disorder induced by \\text{H}2+ -implantation on the ion dose can be divided into a three-step damage process. A strong influence of the H concentration on the defect accumulation is discussed. The decrease in relative Ga disorder induced by \\text{H}2+ -implantation is linear with increasing implantation temperature. The absorption coefficient of GaN epitaxial films increases with increasing ion dose, leading to the decrease in Raman scattering spectra of Ga-N vibration. With increasing implantation doses up to 5   ×   1016 \\text{H}2+  cm-2, nanoscale hydrogen bubbles are observed in the H deposition peak region. Interstitial-type dislocation loops are observed in the damaged layer located near the damage peak region, and the geometry of the dislocation loops produced by H implantation is analyzed. The surface layer is almost free of lattice disorder induced by \\text{H}2+ -implantation.

  15. P-type doping of GaN(000\\bar{1}) by magnesium ion implantation

    NASA Astrophysics Data System (ADS)

    Narita, Tetsuo; Kachi, Tetsu; Kataoka, Keita; Uesugi, Tsutomu

    2017-01-01

    Magnesium ion implantation has been performed on a GaN(000\\bar{1}) substrate, whose surface has a high thermal stability, thus allowing postimplantation annealing without the use of a protective layer. The current-voltage characteristics of p-n diodes fabricated on GaN(000\\bar{1}) showed distinct rectification at a turn-on voltage of about 3 V, although the leakage current varied widely among the diodes. Coimplantation with magnesium and hydrogen ions effectively suppressed the leakage currents and device-to-device variations. In addition, an electroluminescence band was observed at wavelengths shorter than 450 nm for these diodes. These results provide strong evidence that implanted magnesium ions create acceptors in GaN(000\\bar{1}).

  16. Structural and optical properties of vanadium ion-implanted GaN

    NASA Astrophysics Data System (ADS)

    Macková, A.; Malinský, P.; Jagerová, A.; Sofer, Z.; Klímová, K.; Sedmidubský, D.; Mikulics, M.; Lorinčík, J.; Veselá, D.; Böttger, R.; Akhmadaliev, S.

    2017-09-01

    The field of advanced electronic and optical devices searches for a new generation of transistors and lasers. The practical development of these novel devices depends on the availability of materials with the appropriate magnetic and optical properties, which is strongly connected to the internal morphology and the structural properties of the prepared doped structures. In this contribution, we present the characterisation of V ion-doped GaN epitaxial layers. GaN layers, oriented along the (0 0 0 1) crystallographic direction, grown by low-pressure metal-organic vapour-phase epitaxy (MOVPE) on c-plane sapphire substrates were implanted with 400 keV V+ ions at fluences of 5 × 1015 and 5 × 1016 cm-2. Elemental depth profiling was accomplished by Rutherford Backscattering Spectrometry (RBS) and Secondary Ion Mass Spectrometry (SIMS) to obtain precise information about the dopant distribution. Structural investigations are needed to understand the influence of defect distribution on the crystal-matrix recovery and the desired structural and optical properties. The structural properties of the ion-implanted layers were characterised by RBS-channelling and Raman spectroscopy to get a comprehensive insight into the structural modification of implanted GaN and to study the influence of subsequent annealing on the crystalline matrix reconstruction. Photoluminescence measurement was carried out to check the optical properties of the prepared structures.

  17. Effects of Mn Ion Implantation on XPS Spectroscopy of GaN Thin Films

    NASA Astrophysics Data System (ADS)

    Majid, Abdul; Ahmad, Naeem; Rizwan, Muhammad; Khan, Salah Ud-Din; Ali, Fekri Abdulraqeb Ahmed; Zhu, Jianjun

    2018-02-01

    Gallium nitride (GaN) thin film was deposited onto a sapphire substrate and then implanted with 250 keV Mn ions at two different doses of 2 × 1016 ions/cm2 and 5 × 1016 ions/cm2. The as-grown and post-implantation-thermally-annealed samples were studied in detail using x-ray photoelectron spectroscopy (XPS). The XPS peaks of Ga 3 d, Ga 2 p, N 1 s, Mn 2 p and C 1 s were recorded in addition to a full survey of the samples. The doublet peaks of Ga 2 p for pure GaN were observed blue-shifted when compared with elemental Ga, and appeared further shifted to higher energies for the implanted samples. These observations point to changes in the bonds and the chemical environment of the host as a result of ion implantation. The results revealed broadening of the N 1 s peak after implantation, which is interpreted in terms of the presence of N-Mn bonds in addition to N-Ga bonds. The XPS spectra of Mn 2 p recorded for ion-implanted samples indicated splitting of Mn 2 p 1/2 and Mn 2 p 3/2 peaks higher than that for metallic Mn, which helps rule out the possibility of clustering and points to substitutional doping of Mn. These observations provide a framework that sheds light on the local environment of the material for understanding the mechanism of magnetic exchange interactions in Mn:GaN based diluted magnetic semiconductors.

  18. Effects of Phosphorus Implantation on the Activation of Magnesium Doped in GaN

    NASA Astrophysics Data System (ADS)

    Liu, Kuan-Ting; Chang, Shoou-Jinn; Wu, Sean

    2009-08-01

    The effects of phosphorus implantation on the activation of magnesium doped in GaN at different dopant concentration ratios have been systematically investigated. Hall effect measurements show that P implantation improves the hole concentration, and that this improvement is dependent on P/Mg dopant concentration ratio and annealing conditions. This phenomenon is attributable to the reduction in self-compensation that results from the formation of deep donors and the enhanced Mg atom activation, which is in reasonable agreement with the optical properties observed by photoluminescence measurements. In addition, a new photoluminescence peak resulting from P-related transitions is also observed, evidently owing to the recombination of electrons from the shallow native donors with holes previously captured by isoelectronic P traps.

  19. Formation of definite GaN p-n junction by Mg-ion implantation to n--GaN epitaxial layers grown on a high-quality free-standing GaN substrate

    NASA Astrophysics Data System (ADS)

    Oikawa, Takuya; Saijo, Yusuke; Kato, Shigeki; Mishima, Tomoyoshi; Nakamura, Tohru

    2015-12-01

    P-type conversion of n--GaN by Mg-ion implantation was successfully performed using high quality GaN epitaxial layers grown on free-standing low-dislocation-density GaN substrates. These samples showed low-temperature PL spectra quite similar to those observed from Mg-doped MOVPE-grown p-type GaN, consisting of Mg related donor-acceptor pair (DAP) and acceptor bound exciton (ABE) emission. P-n diodes fabricated by the Mg-ion implantation showed clear rectifying I-V characteristics and UV and blue light emissions were observed at forward biased conditions for the first time.

  20. Impact of Mg-ion implantation with various fluence ranges on optical properties of n-type GaN

    NASA Astrophysics Data System (ADS)

    Tsuge, Hirofumi; Ikeda, Kiyoji; Kato, Shigeki; Nishimura, Tomoaki; Nakamura, Tohru; Kuriyama, Kazuo; Mishima, Tomoyoshi

    2017-10-01

    Optical characteristics of Mg-ion implanted GaN layers with various fluence ranges were evaluated. Mg ion implantation was performed twice at energies of 30 and 60 keV on n-GaN layers. The first implantation at 30 keV was performed with three different fluence ranges of 1.0 × 1014, 1.0 × 1015 and 5.0 × 1015 cm-2. The second implantation at an energy of 60 keV was performed with a fluence of 6.5 × 1013 cm-2. After implantation, samples were annealed at 1250 °C for 1 min under N2 atmosphere. Photoluminescence (PL) spectrum of the GaN layer with the Mg ion implantation at the fluence range of 1.0 × 1014 cm-2 at 30 keV was similar to the one of Mg-doped p-GaN layers grown by MOVPE (Metal-Organic Vapor Phase Epitaxy) on free-standing GaN substrates and those at the fluence ranges over 1.0 × 1015 cm-2 were largely degraded.

  1. Lattice disorder produced in GaN by He-ion implantation

    NASA Astrophysics Data System (ADS)

    Han, Yi; Peng, Jinxin; Li, Bingsheng; Wang, Zhiguang; Wei, Kongfang; Shen, Tielong; Sun, Jianrong; Zhang, Limin; Yao, Cunfeng; Gao, Ning; Gao, Xing; Pang, Lilong; Zhu, Yabin; Chang, Hailong; Cui, Minghuan; Luo, Peng; Sheng, Yanbin; Zhang, Hongpeng; Zhang, Li; Fang, Xuesong; Zhao, Sixiang; Jin, Jin; Huang, Yuxuan; Liu, Chao; Tai, Pengfei; Wang, Dong; He, Wenhao

    2017-09-01

    The lattice disorders induced by He-ion implantation in GaN epitaxial films to fluences of 2 × 1016, 5 × 1016 and 1 × 1017 cm-2 at room temperature (RT) have been investigated by a combination of Raman spectroscopy, high-resolution X-ray diffraction (HRXRD), nano-indentation, and transmission electron microscopy (TEM). The experimental results present that Raman intensity decreases with increasing fluence. Raman frequency "red shift" occurs after He-ion implantation. Strain increases with increasing fluence. The hardness of the highly damaged layer increases monotonically with increasing fluence. Microstructural results demonstrate that the width of the damage band and the number density of observed dislocation loops increases with increasing fluence. High-resolution TEM images exhibit that He-ion implantation lead to the formation of planar defects and most of the lattice defects are of interstitial-type basal loops. The relationships of Raman intensity, lattice strain, swelling and hardness with He-implantation-induced lattice disorders are discussed.

  2. Multicycle rapid thermal annealing optimization of Mg-implanted GaN: Evolution of surface, optical, and structural properties

    SciT

    Greenlee, Jordan D., E-mail: jordan.greenlee.ctr@nrl.navy.mil; Feigelson, Boris N.; Anderson, Travis J.

    2014-08-14

    The first step of a multi-cycle rapid thermal annealing process was systematically studied. The surface, structure, and optical properties of Mg implanted GaN thin films annealed at temperatures ranging from 900 to 1200 °C were investigated by Raman spectroscopy, photoluminescence, UV-visible spectroscopy, atomic force microscopy, and Nomarski microscopy. The GaN thin films are capped with two layers of in-situ metal organic chemical vapor deposition -grown AlN and annealed in 24 bar of N{sub 2} overpressure to avoid GaN decomposition. The crystal quality of the GaN improves with increasing annealing temperature as confirmed by UV-visible spectroscopy and the full widths at halfmore » maximums of the E{sub 2} and A{sub 1} (LO) Raman modes. The crystal quality of films annealed above 1100 °C exceeds the quality of the as-grown films. At 1200 °C, Mg is optically activated, which is determined by photoluminescence measurements. However, at 1200 °C, the GaN begins to decompose as evidenced by pit formation on the surface of the samples. Therefore, it was determined that the optimal temperature for the first step in a multi-cycle rapid thermal anneal process should be conducted at 1150 °C due to crystal quality and surface morphology considerations.« less

  3. Multicycle rapid thermal annealing optimization of Mg-implanted GaN: Evolution of surface, optical, and structural properties

    NASA Astrophysics Data System (ADS)

    Greenlee, Jordan D.; Feigelson, Boris N.; Anderson, Travis J.; Tadjer, Marko J.; Hite, Jennifer K.; Mastro, Michael A.; Eddy, Charles R.; Hobart, Karl D.; Kub, Francis J.

    2014-08-01

    The first step of a multi-cycle rapid thermal annealing process was systematically studied. The surface, structure, and optical properties of Mg implanted GaN thin films annealed at temperatures ranging from 900 to 1200 °C were investigated by Raman spectroscopy, photoluminescence, UV-visible spectroscopy, atomic force microscopy, and Nomarski microscopy. The GaN thin films are capped with two layers of in-situ metal organic chemical vapor deposition -grown AlN and annealed in 24 bar of N2 overpressure to avoid GaN decomposition. The crystal quality of the GaN improves with increasing annealing temperature as confirmed by UV-visible spectroscopy and the full widths at half maximums of the E2 and A1 (LO) Raman modes. The crystal quality of films annealed above 1100 °C exceeds the quality of the as-grown films. At 1200 °C, Mg is optically activated, which is determined by photoluminescence measurements. However, at 1200 °C, the GaN begins to decompose as evidenced by pit formation on the surface of the samples. Therefore, it was determined that the optimal temperature for the first step in a multi-cycle rapid thermal anneal process should be conducted at 1150 °C due to crystal quality and surface morphology considerations.

  4. Quantitative secondary ion mass spectrometric analysis of secondary ion polarity in GaN films implanted with oxygen

    NASA Astrophysics Data System (ADS)

    Hashiguchi, Minako; Sakaguchi, Isao; Adachi, Yutaka; Ohashi, Naoki

    2016-10-01

    Quantitative analyses of N and O ions in GaN thin films implanted with oxygen ions (16O+) were conducted by secondary ion mass spectrometry (SIMS). Positive (CsM+) and negative secondary ions extracted by Cs+ primary ion bombardment were analyzed for oxygen quantitative analysis. The oxygen depth profiles were obtained using two types of primary ion beams: a Gaussian-type beam and a broad spot beam. The oxygen peak concentrations in GaN samples were from 3.2 × 1019 to 7.0 × 1021 atoms/cm3. The depth profiles show equivalent depth resolutions in the two analyses. The intensity of negative oxygen ions was approximately two orders of magnitude higher than that of positive ions. In contrast, the O/N intensity ratio measured using CsM+ molecular ions was close to the calculated atomic density ratio, indicating that the SIMS depth profiling using CsM+ ions is much more effective for the measurements of O and N ions in heavy O-implanted GaN than that using negative ions.

  5. Injection Laser Using Rare Earth Doped GaN Thin Films for Visible and Infrared Applications

    DTIC Science & Technology

    2010-05-01

    system is used to measure the conductivity of GaN thin films in Nanolab. Undoped GaN thin films are usually n-type conductive with the electron ...being investigated for potential applications in optical communication and displays, due to the wide and direct energy bandgap of GaN resulting in low...enhancement on Eu3+ PL intensity. The electrical properties of GaN:RE thin films were changed from high resistive to

  6. Electrically active point defects in Mg implanted n-type GaN grown by metal-organic chemical vapor deposition

    NASA Astrophysics Data System (ADS)

    Alfieri, G.; Sundaramoorthy, V. K.; Micheletto, R.

    2018-05-01

    Magnesium (Mg) is the p-type doping of choice for GaN, and selective area doping by ion implantation is a routine technique employed during device processing. While electrically active defects have been thoroughly studied in as-grown GaN, not much is known about defects generated by ion implantation. This is especially true for the case of Mg. In this study, we carried out an electrical characterization investigation of point defects generated by Mg implantation in GaN. We have found at least nine electrically active levels in the 0.2-1.2 eV energy range, below the conduction band. The isochronal annealing behavior of these levels showed that most of them are thermally stable up to 1000 °C. The nature of the detected defects is then discussed in the light of the results found in the literature.

  7. Depth Profiles of Mg, Si, and Zn Implants in GaN by Trace Element Accelerator Mass Spectrometry

    NASA Astrophysics Data System (ADS)

    Ravi Prasad, G. V.; Pelicon, P.; Mitchell, L. J.; McDaniel, F. D.

    2003-08-01

    GaN is one of the most promising electronic materials for applications requiring high-power, high frequencies, or high-temperatures as well as opto-electronics in the blue to ultraviolet spectral region. We have recently measured depth profiles of Mg, Si, and Zn implants in GaN substrates by the TEAMS particle counting method for both matrix and trace elements, using a gas ionization chamber. Trace Element Accelerator Mass Spectrometry (TEAMS) is a combination of Secondary Ion Mass Spectrometry (SIMS) and Accelerator Mass Spectrometry (AMS) to measure trace elements at ppb levels. Negative ions from a SIMS like source are injected into a tandem accelerator. Molecular interferences inherent with the SIMS method are eliminated in the TEAMS method. Negative ion currents are extremely low with GaN as neither gallium nor nitrogen readily forms negative ions making the depth profile measurements more difficult. The energies of the measured ions are in the range of 4-8 MeV. A careful selection of mass/charge ratios of the detected ions combined with energy-loss behavior of the ions in the ionization chamber eliminated molecular interferences.

  8. Detection of deep-level defects and reduced carrier concentration in Mg-ion-implanted GaN before high-temperature annealing

    NASA Astrophysics Data System (ADS)

    Akazawa, Masamichi; Yokota, Naoshige; Uetake, Kei

    2018-02-01

    We report experimental results for the detection of deep-level defects in GaN after Mg ion implantation before high-temperature annealing. The n-type GaN samples were grown on GaN free-standing substrates by metalorganic vapor phase epitaxy. Mg ions were implanted at 50 keV with a small dosage of 1.5×1011 cm-2, which did not change the conduction type of the n-GaN. By depositing Al2O3 and a Ni/Au electrode onto the implanted n-GaN, metal-oxide-semiconductor (MOS) diodes were fabricated and tested. The measured capacitance-voltage (C-V) characteristics showed a particular behavior with a plateau region and a region with an anomalously steep slope. Fitting to the experimental C-V curves by simulation showed the existence of deep-level defects and a reduction of the carrier concentration near the GaN surface. By annealing at 800oC, the density of the deep-level defects was reduced and the carrier concentration partially recovered.

  9. Electronic structure of ytterbium-implanted GaN at ambient and high pressure: experimental and crystal field studies.

    PubMed

    Kaminska, A; Ma, C-G; Brik, M G; Kozanecki, A; Boćkowski, M; Alves, E; Suchocki, A

    2012-03-07

    The results of high-pressure low-temperature optical measurements in a diamond-anvil cell of bulk gallium nitride crystals implanted with ytterbium are reported in combination with crystal field calculations of the Yb(3+) energy levels. Crystal field analysis of splitting of the (2)F(7/2) and (2)F(5/2) states has been performed, with the aim of assigning all features of the experimental luminescence spectra. A thorough analysis of the pressure behavior of the Yb(3+) luminescence lines in GaN allowed the determination of the ambient-pressure positions and pressure dependence of the Yb(3+) energy levels in the trigonal crystal field as well as the pressure-induced changes of the spin-orbit coupling coefficient.

  10. 1.54 micron Emission from Erbium implanted GaN for Photonic Applications

    NASA Technical Reports Server (NTRS)

    Thaik, Myo; Hommerich, U.; Schwartz, R. N.; Wilson, R. G.; Zavada, J. M.

    1998-01-01

    The development of efficient and compact light sources operating at 1.54 micron is of enormous importance for the advancement of new optical communication systems. Erbium (1%) doped fiber amplifiers (EDFA's) or semiconductor lasers are currently being employed as near infrared light sources. Both devices, however, have inherent limitations due to their mode of operation. EDFA's employ an elaborate optical pumping scheme, whereas diode lasers have a strongly temperature dependent lasing wavelength. Novel light emitters based on erbium doped III-V semiconductors could overcome these limitations. Er doped semiconductors combine the convenience of electrical excitation with the excellent luminescence properties of Er(3+) ions. Electrically pumped, compact, and temperature stable optoelectronic devices are envisioned from this new class of luminescent materials. In this paper we discuss the potential of Er doped GaN for optoelectronic applications based on temperature dependent photoluminescence excitation studies.

  11. Defect formation in MeV H+ implanted GaN and 4H-SiC investigated by cross-sectional Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Huang, Kai; Jia, Qi; You, Tiangui; Zhang, Shibin; Lin, Jiajie; Zhang, Runchun; Zhou, Min; Yu, Wenjie; Zhang, Bo; Ou, Xin; Wang, Xi

    2017-09-01

    Cross-sectional Raman spectroscopy is used to characterize the defect formation and the defect recovery in MeV H+ implanted bulk GaN and 4H-SiC in the high energy MeV ion-cut process. The Raman intensity decreases but the forbidden modes are activated at the damage region, and the intensity decrease is proportional to the damage level. The Raman spectrum is quite sensitive to detect the damage recovery after annealing. The main peak intensity increases and the forbidden mode disappears in both annealed GaN and 4H-SiC samples. The Raman spectra of GaN samples annealed at different temperatures suggest that higher annealing temperature is more efficient for damage recovery. While, the Raman spectra of SiC indicate that higher implantation temperature results in heavier lattice damage and other polytype clusters might be generated by high annealing temperature in the annealed SiC samples. The cross-sectional Raman spectroscopy is a straightforward method to characterize lattice damage and damage recovery in high energy ion-cut process. It can serve as a fast supplementary measurement technique to Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA) and transmission electron microscope (TEM) for the defect characterizations.

  12. High carrier activation of Mg ion-implanted GaN by conventional rapid thermal annealing

    NASA Astrophysics Data System (ADS)

    Niwa, Takaki; Fujii, Takahiro; Oka, Tohru

    2017-09-01

    A high activation ratio of Mg ion implantation by conventional rapid thermal annealing (RTA) was demonstrated. To obtain the high activation ratio of Mg ion implantation, the dependence of hole concentration on Mg dose was investigated. A maximum hole concentration and a high activation ratio of 2.3% were obtained at a Mg dose of 2.3 × 1014 cm-2 between 9.2 × 1013 and 2.3 × 1015 cm-2. The ratio is, to the best of our knowledge, the highest ever obtained by conventional RTA.

  13. Nitrogen vacancies as a common element of the green luminescence and nonradiative recombination centers in Mg-implanted GaN layers formed on a GaN substrate

    NASA Astrophysics Data System (ADS)

    Kojima, Kazunobu; Takashima, Shinya; Edo, Masaharu; Ueno, Katsunori; Shimizu, Mitsuaki; Takahashi, Tokio; Ishibashi, Shoji; Uedono, Akira; Chichibu, Shigefusa F.

    2017-06-01

    The photoluminescences of ion-implanted (I/I) and epitaxial Mg-doped GaN (GaN:Mg) are compared. The intensities and lifetimes of the near-band-edge and ultraviolet luminescences associated with a MgGa acceptor of I/I GaN:Mg were significantly lower and shorter than those of the epilayers, respectively. Simultaneously, the green luminescence (GL) became dominant. These emissions were quenched far below room temperature. The results indicate the generation of point defects common to GL and nonradiative recombination centers (NRCs) by I/I. Taking the results of positron annihilation measurement into account, N vacancies are the prime candidate to emit GL and create NRCs with Ga vacancies, (VGa) m (VN) n , as well as to inhibit p-type conductivity.

  14. Rare Earth Doped GaN Laser Structures Using Metal Modulated Epitaxy

    DTIC Science & Technology

    2015-03-30

    from Eu-doped GaN,” Appl. Phys. Lett., vol. 75, pp. 1189–1191, 1999. 24. D. S . Lee and A. J. Steckl, “Room-temperature-grown rare- earth -doped GaN...luminescent thin films,” Appl. Phys. Lett., vol. 79, pp. 1962–1964,2001. 25. D. S . Lee and A. J. Steckl, “Lateral color integration on rare- earth doped... s . 0.259nm/ s =1.14E13cm-2/ s =1 ML/ s .Our plasma source was optimized to work at 1.5 sccm and 230 W RF power and it provides a growth rate of 0.8 ML/ s

  15. Tunable Solid-State Quantum Memory Using Rare-Earth-Ion-Doped Crystal, Nd(3+):GaN

    DTIC Science & Technology

    2017-04-01

    by plasma-assisted molecular beam epitaxy in a modular Gen II reactor using liquid gallium, solid Nd, and a nitrogen plasma. The photoluminescence (PL...provide a tunable memory. To vary the applied field, we designed and grew a series of Nd-doped GaN p-i-n structures, strain- balanced superlattice...27 Fig. 23 Electric field vs. GaN well/ AlxGa(1-x)N barrier thickness for strain- balanced superlattice (SBSL) structures with

  16. Production yield of rare-earth ions implanted into an optical crystal

    SciT

    Kornher, Thomas, E-mail: t.kornher@physik.uni-stuttgart.de; Xia, Kangwei; Kolesov, Roman

    2016-02-01

    Rare-earth (RE) ions doped into desired locations of optical crystals might enable a range of novel integrated photonic devices for quantum applications. With this aim, we have investigated the production yield of cerium and praseodymium by means of ion implantation. As a measure, the collected fluorescence intensity from both implanted samples and single centers was used. With a tailored annealing procedure for cerium, a yield up to 53% was estimated. Praseodymium yield amounts up to 91%. Such high implantation yield indicates a feasibility of creation of nanopatterned rare-earth doping and suggests strong potential of RE species for on-chip photonic devices.

  17. Cathodoluminescence of rare earth implanted Ga2O3 and GeO2 nanostructures.

    PubMed

    Nogales, E; Hidalgo, P; Lorenz, K; Méndez, B; Piqueras, J; Alves, E

    2011-07-15

    Rare earth (RE) doped gallium oxide and germanium oxide micro- and nanostructures, mostly nanowires, have been obtained and their morphological and optical properties have been characterized. Undoped oxide micro- and nanostructures were grown by a thermal evaporation method and were subsequently doped with gadolinium or europium ions by ion implantation. No significant changes in the morphologies of the nanostructures were observed after ion implantation and thermal annealing. The luminescence emission properties have been studied with cathodoluminescence (CL) in a scanning electron microscope (SEM). Both β-Ga(2)O(3) and GeO(2) structures implanted with Eu show the characteristic red luminescence peak centered at around 610 nm, due to the (5)D(0)-(7)F(2) Eu(3+) intraionic transition. Sharpening of the luminescence peaks after thermal annealing is observed in Eu implanted β-Ga(2)O(3), which is assigned to the lattice recovery. Gd(3+) as-implanted samples do not show rare earth related luminescence. After annealing, optical activation of Gd(3+) is obtained in both matrices and a sharp ultraviolet peak centered at around 315 nm, associated with the Gd(3+) (6)P(7/2)-(8)S(7/2) intraionic transition, is observed. The influence of the Gd ion implantation and the annealing temperature on the gallium oxide broad intrinsic defect band has been analyzed.

  18. Isolation and characterization of Burkholderia fungorum Gan-35 with the outstanding ammonia nitrogen-degrading ability from the tailings of rare-earth-element mines in southern Jiangxi, China.

    PubMed

    Feng, Ai-Juan; Xiao, Xi; Ye, Cong-Cong; Xu, Xiao-Ming; Zhu, Qing; Yuan, Jian-Ping; Hong, Yue-Hui; Wang, Jiang-Hai

    2017-12-01

    The exploitation of rare-earth-element (REE) mines has resulted in severe ammonia nitrogen pollution and induced hazards to environments and human health. Screening microorganisms with the ammonia nitrogen-degrading ability provides a basis for bioremediation of ammonia nitrogen-polluted environments. In this study, a bacterium with the outstanding ammonia nitrogen-degrading capability was isolated from the tailings of REE mines in southern Jiangxi Province, China. This strain was identified as Burkholderia fungorum Gan-35 according to phenotypic and phylogenetic analyses. The optimal conditions for ammonia-nitrogen degradation by strain Gan-35 were determined as follows: pH value, 7.5; inoculum dose, 10%; incubation time, 44 h; temperature, 30 °C; and C/N ratio, 15:1. Strain Gan-35 degraded 68.6% of ammonia nitrogen under the optimized conditions. Nepeta cataria grew obviously better in the ammonia nitrogen-polluted soil with strain Gan-35 than that without inoculation, and the decrease in ammonia-nitrogen contents of the former was also more obvious than the latter. Besides, strain Gan-35 exhibited the tolerance to high salinities. In summary, strain Gan-35 harbors the ability of both ammonia-nitrogen degradation at high concentrations and promoting plant growth. This work has reported a Burkholderia strain with the ammonia nitrogen-degrading capability for the first time and is also the first study on the isolation of a bacterium with the ammonia nitrogen-degrading ability from the tailings of REE mines. The results are useful for developing an effective method for microbial remediation of the ammonia nitrogen-polluted tailings of REE mines.

  19. Intramaxillary and intermaxillary absolute anchorage with an endosseous dental implant and rare-earth magnets.

    PubMed

    Uribe, Flavio; Nanda, Ravindra

    2009-07-01

    This case report describes the management of a partially edentulous adult with a severe deepbite malocclusion and a collapsed buccal segment because of supraeruption of the maxillary second and third molars. The treatment involved placement of an endosseous dental implant to achieve absolute intermaxillary and intramaxillary anchorage. Rare-earth magnets embedded in acrylic bite-blocks were used to intrude the supraerupted maxillary molars on the right side. After use as an anchorage device, the implant was restored and brought into occlusion. Good occlusal and esthetic results were achieved with this cost-effective treatment.

  20. P-type doping of GaN

    SciT

    Wong, Raechelle Kimberly

    2000-04-01

    After implantation of As, As + Be, and As + Ga into GaN and annealing for short durations at temperatures as high as 1500 C, the GaN films remained highly resistive. It was apparent from c-RBS studies that although implantation damage did not create an amorphous layer in the GaN film, annealing at 1500 C did not provide enough energy to completely recover the radiation damage. Disorder recovered significantly after annealing at temperatures up to 1500 C, but not completely. From SIMS analysis, oxygen contamination in the AIN capping layer causes oxygen diffusion into the GaN film above 1400 C.more » The sapphire substrate (A1203) also decomposed and oxygen penetrated into the backside of the GaN layer above 1400 C. To prevent donor-like oxygen impurities from the capping layer and the substrate from contaminating the GaN film and compensating acceptors, post-implantation annealing should be done at temperatures below 1500 C. Oxygen in the cap could be reduced by growing the AIN cap on the GaN layer after the GaN growth run or by depositing the AIN layer in a ultra high vacuum (UHV) system post-growth to minimize residual oxygen and water contamination. With longer annealing times at 1400 C or at higher temperatures with a higher quality AIN, the implantation drainage may fully recover.« less

  1. Structure guided GANs

    NASA Astrophysics Data System (ADS)

    Cao, Feidao; Zhao, Huaici; Liu, Pengfei

    2017-11-01

    Generative adversarial networks (GANs) has achieved success in many fields. However, there are some samples generated by many GAN-based works, whose structure is ambiguous. In this work, we propose Structure Guided GANs that introduce structural similar into GANs to overcome the problem. In order to achieve our goal, we introduce an encoder and a decoder into a generator to design a new generator and take real samples as part of the input of a generator. And we modify the loss function of the generator accordingly. By comparison with WGAN, experimental results show that our proposed method overcomes largely sample structure ambiguous and can generate higher quality samples.

  2. Magnetic Resonance Imaging Distortion and Targeting Errors from Strong Rare Earth Metal Magnetic Dental Implant Requiring Revision.

    PubMed

    Seong-Cheol, Park; Chong Sik, Lee; Seok Min, Kim; Eu Jene, Choi; Do Hee, Lee; Jung Kyo, Lee

    2016-12-22

    Recently, the use of magnetic dental implants has been re-popularized with the introduction of strong rare earth metal, for example, neodymium, magnets. Unrecognized magnetic dental implants can cause critical magnetic resonance image distortions. We report a case involving surgical failure caused by a magnetic dental implant. A 62-year-old man underwent deep brain stimulation for medically insufficiently controlled Parkinson's disease. Stereotactic magnetic resonance imaging performed for the first deep brain stimulation showed that the overdenture was removed. However, a dental implant remained and contained a neodymium magnet, which was unrecognized at the time of imaging; the magnet caused localized non-linear distortions that were the largest around the dental magnets. In the magnetic field, the subthalamic area was distorted by a 4.6 mm right shift and counter clockwise rotation. However, distortions were visually subtle in the operation field and small for distant stereotactic markers, with approximately 1-2 mm distortions. The surgeon considered the distortion to be normal asymmetry or variation. Stereotactic marker distortion was calculated to be in the acceptable range in the surgical planning software. Targeting errors, approximately 5 mm on the right side and 2 mm on the left side, occurred postoperatively. Both leads were revised after the removal of dental magnets. Dental magnets may cause surgical failures and should be checked and removed before stereotactic surgery. Our findings should be considered when reviewing surgical precautions and making distortion-detection algorithm improvements.

  3. Investigation on thermodynamics of ion-slicing of GaN and heterogeneously integrating high-quality GaN films on CMOS compatible Si(100) substrates.

    PubMed

    Huang, Kai; Jia, Qi; You, Tiangui; Zhang, Runchun; Lin, Jiajie; Zhang, Shibin; Zhou, Min; Zhang, Bo; Yu, Wenjie; Ou, Xin; Wang, Xi

    2017-11-08

    Die-to-wafer heterogeneous integration of single-crystalline GaN film with CMOS compatible Si(100) substrate using the ion-cutting technique has been demonstrated. The thermodynamics of GaN surface blistering is in-situ investigated via a thermal-stage optical microscopy, which indicates that the large activation energy (2.5 eV) and low H ions utilization ratio (~6%) might result in the extremely high H fluence required for the ion-slicing of GaN. The crystalline quality, surface topography and the microstructure of the GaN films are characterized in detail. The full width at half maximum (FWHM) for GaN (002) X-ray rocking curves is as low as 163 arcsec, corresponding to a density of threading dislocation of 5 × 10 7  cm -2 . Different evolution of the implantation-induced damage was observed and a relationship between the damage evolution and implantation-induced damage is demonstrated. This work would be beneficial to understand the mechanism of ion-slicing of GaN and to provide a platform for the hybrid integration of GaN devices with standard Si CMOS process.

  4. Evolution of the protolunar disk: Dynamics, cooling timescale and implantation of volatiles onto the Earth

    NASA Astrophysics Data System (ADS)

    Charnoz, Sébastien; Michaut, Chloé

    2015-11-01

    It is thought that the Moon accreted from the protolunar disk that was assembled after the last giant impact on Earth. Due to its high temperature, the protolunar disk may act as a thermochemical reactor in which the material is processed before being incorporated into the Moon. Outstanding issues like devolatilisation and istotopic evolution are tied to the disk evolution, however its lifetime, dynamics and thermodynamics are unknown. Here, we numerically explore the long term viscous evolution of the protolunar disk using a one dimensional model where the different phases (vapor and condensed) are vertically stratified. Viscous heating, radiative cooling, phase transitions and gravitational instability are accounted for whereas Moon's accretion is not considered for the moment. The viscosity of the gas, liquid and solid phases dictates the disk evolution. We find that (1) the vapor condenses into liquid in ∼10 years, (2) a large fraction of the disk mass flows inward forming a hot and compact liquid disk between 1 and 1.7 Earth's radii, a region where the liquid is gravitationally stable and can accumulate, (3) the disk finally solidifies in 103 to 105 years. Viscous heating is never balanced by radiative cooling. If the vapor phase is abnormally viscous, due to magneto-rotational instability for instance, most of the disk volatile components are transported to Earth leaving a disk enriched in refractory elements. This opens a way to form a volatile-depleted Moon and would suggest that the missing Moon's volatiles are buried today into the Earth. The disk cooling timescale may be long enough to allow for planet/disk isotopic equilibration. However large uncertainties on the disk physics remain because of the complexity of its multi-phased structure.

  5. Defects in N/Ge coimplanted GaN studied by positron annihilation

    NASA Astrophysics Data System (ADS)

    Nakano, Yoshitaka; Kachi, Tetsu

    2002-01-01

    We have applied positron annihilation spectroscopy to study the depth distributions and species of defects in N-, Ge-, and N/Ge-implanted GaN at dosages of 1×1015 cm-2. For all the implanted samples, Ga vacancies introduced by ion-implantation are found to diffuse into much deeper regions of the GaN layers during the implantation and to change into some other vacancy-type defects by the annealing at 1300 °C. In particular, markedly different defects turn out to be newly created in the electrically activated regions for both the Ge- and N/Ge-implanted samples after annealing, indicating that these new defects are probably associated with the presence of the implanted Ge dopant atoms.

  6. Cubic crystalline erbium oxide growth on GaN(0001) by atomic layer deposition

    NASA Astrophysics Data System (ADS)

    Chen, Pei-Yu; Posadas, Agham B.; Kwon, Sunah; Wang, Qingxiao; Kim, Moon J.; Demkov, Alexander A.; Ekerdt, John G.

    2017-12-01

    Growth of crystalline Er2O3, a rare earth sesquioxide, on GaN(0001) is described. Ex situ HCl and NH4OH solutions and an in situ N2 plasma are used to remove impurities on the GaN surface and result in a Ga/N stoichiometry of 1.02. Using atomic layer deposition with erbium tris(isopropylcyclopentadienyl) [Er(iPrCp)3] and water, crystalline cubic Er2O3 (C-Er2O3) is grown on GaN at 250 °C. The orientation relationships between the C-Er2O3 film and the GaN substrate are C-Er2O3(222) ǁ GaN(0001), C-Er2O3⟨-440⟩ ǁ GaN ⟨11-20⟩, and C-Er2O3⟨-211⟩ ǁ GaN ⟨1-100⟩. Scanning transmission electron microscopy and electron energy loss spectroscopy are used to examine the microstructure of C-Er2O3 and its interface with GaN. With post-deposition annealing at 600 °C, a thicker interfacial layer is observed, and two transition layers, crystalline GaNwOz and crystalline GaErxOy, are found between GaN and C-Er2O3. The tensile strain in the C-Er2O3 film is studied with x-ray diffraction by changes in both out-of-plane and in-plane d-spacing. Fully relaxed C-Er2O3 films on GaN are obtained when the film thickness is around 13 nm. Additionally, a valence band offset of 0.7 eV and a conduction band offset of 1.2 eV are obtained using x-ray photoelectron spectroscopy.

  7. Earth

    2012-01-30

    Behold one of the more detailed images of the Earth yet created. This Blue Marble Earth montage shown above -- created from photographs taken by the Visible/Infrared Imager Radiometer Suite (VIIRS) instrument on board the new Suomi NPP satellite -- shows many stunning details of our home planet. The Suomi NPP satellite was launched last October and renamed last week after Verner Suomi, commonly deemed the father of satellite meteorology. The composite was created from the data collected during four orbits of the robotic satellite taken earlier this month and digitally projected onto the globe. Many features of North America and the Western Hemisphere are particularly visible on a high resolution version of the image. http://photojournal.jpl.nasa.gov/catalog/PIA18033

  8. GaN for LED applications

    NASA Technical Reports Server (NTRS)

    Pankove, J. I.

    1973-01-01

    In order to improve the synthesis of GaN the effect of various growth and doping parameters has been studied. Although Be, Li, Mg, and Dy can be used to overcompensate native donors, the most interesting acceptor element is Zn. The emission spectrum and the luminescence efficiency depend on the growth temperature (below 800 C), on the partial pressure of the doping impurity, and on the duration of growth. Blue-green electroluminescence with a power efficiency of 0.1 percent and a brightness of 850 fL (at 0.6 mA and 22.5 V) was obtained. Some diodes allow the color of the emitted light to change by reversing the polarity of the bias. Continuous operation of a diode over a period of 5 months showed no evidence of degradation. The luminescence properties of ion-implanted GaN were studied. Delay effects were found in the electroluminescence of diodes, although, with a dc bias, a 70-MHz modulation was possible.

  9. Specific peptide for functionalization of GaN

    NASA Astrophysics Data System (ADS)

    Estephan, E.; Larroque, C.; Cloitre, T.; Cuisinier, F. J. G.; Gergely, C.

    2008-04-01

    Nanobiotechnology aims to exploit biomolecular recognition and self-assembly capabilities for integrating advanced materials into medicine and biology. However frequent problems are encountered at the interface of substrate-biological molecule, as the direct physical adsorption of biological molecules is dependent of unpredictable non-specific interactions with the surface, often causing their denaturation. Therefore, a proper functionalization of the substrate should avoid a loss of biological activity. In this work we address the functionalization of the semiconductor GaN (0001) for biosensing applications. The basic interest of using III-V class semiconductors is their good light emitting properties and a fair chemical stability that allows various applications of these materials. The technology chosen to elaborate GaN-specific peptides is the combinatorial phage-display method, a biological screening procedure based on affinity selection. An M13 bacteriophage library has been used to screen 10 10 different peptides against the GaN (0001) surface to finally isolate one specific peptide. The preferential attachment of the biotinylated selected peptide onto the GaN (0001), in close proximity to a surface of different chemical and structural composition has been demonstrated by fluorescence microscopy. Further physicochemical studies have been initiated to evaluate the semiconductor-peptide interface and understand the details in the specific recognition of peptides for semiconductor substrates. Fourier Transform Infrared spectroscopy in Attenuated Total Reflection mode (FTIR-ATR) has been employed to prove the presence of peptides on the surface. Our Atomic Force Microscopy (AFM) studies on the morphology of the GaN surface after functionalization revealed a total surface coverage by a very thin, homogeneous peptide layer. Due to its good biocompatibility, functionalized GaN devices might evolve in a new class of implantable biosensors for medical applications.

  10. Luminescence properties of defects in GaN

    NASA Astrophysics Data System (ADS)

    Reshchikov, Michael A.; Morkoç, Hadis

    2005-03-01

    other impurities, such as C, Si, H, O, Be, Mn, Cd, etc., on the luminescence properties of GaN are also reviewed. Further, atypical luminescence lines which are tentatively attributed to the surface and structural defects are discussed. The effect of surfaces and surface preparation, particularly wet and dry etching, exposure to UV light in vacuum or controlled gas ambient, annealing, and ion implantation on the characteristics of the defect-related emissions is described.

  11. Roma Gans: Still Writing at 95.

    ERIC Educational Resources Information Center

    Sullivan, Joanna

    1991-01-01

    Recounts discussions with reading educator Roma Gans over a 25-year period. Presents Gans' views about reading, teachers, her family, and her years at Teachers College, Columbia. Notes that Gans has seen the teaching of reading come full circle since her first teaching assignment in 1919. (RS)

  12. First-principles Study of Hydrogen depassivation of Mg acceptor by Be in GaN

    NASA Astrophysics Data System (ADS)

    Zhang, Qiming; Wang, Xiao; Wang, Chihsiang

    2010-03-01

    The process of hydrogen depassivation of the acceptor by can convert the as-grown high-resistivity -doped into a - conducting material. A first-principles study on the process will be presented. The formation energies of various complex of impurities and point defects have been calculated and compared. The diffusion barriers of the hydrogen atom in the doped GaN have been obtained by the Nudge-Elastic-Band method. The results explain successfully the experimental observation that the hole concentration has been significantly enhanced in a Be-implanted Mg-doped GaN.

  13. Luminescence from defects in GaN

    NASA Astrophysics Data System (ADS)

    Reshchikov, M. A.; Morkoç, H.

    2006-04-01

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

  14. Electrical contact of wurtzite GaN mircrodisks on p-type GaN template

    NASA Astrophysics Data System (ADS)

    Tsai, Cheng-Da; Lo, Ikai; Wang, Ying-Chieh; Hsu, Yu-Chi; Shih, Cheng-Hung; Pang, Wen-Yuan; You, Shuo-Ting; Hu, Chia-Hsuan; Chou, Mitch M. C.; Yang, Chen-Chi; Lin, Yu-Chiao

    2015-03-01

    We developed a back processing to fabricate a secure electrical contact of wurtzite GaN microdisk on a transparent p-type GaN template with the orientation, [10-10]disk // [10-10]template. GaN microdisks were grown on LiAlO2 substrate by using plasma-assisted molecular beam epitaxy. In the further study, we analyzed the TEM specimen of a sample with annealed GaN microdisk/p-typed GaN template by selection area diffraction (SAD) to confirm the alignment of the microdisks with the template at the interface. From the I-V measurements performed on the samples, we obtained a threshold voltage of ~ 5.9 V for the current passing through the GaN microdisks with a resistance of ~ 45 K Ω. The electrical contact can be applied to the nanometer-scaled GaN light-emitting diode.

  15. TOPICAL REVIEW: The doping process and dopant characteristics of GaN

    NASA Astrophysics Data System (ADS)

    Sheu, J. K.; Chi, G. C.

    2002-06-01

    The characteristic effects of doping with impurities such as Si, Ge, Se, O, Mg, Be, and Zn on the electrical and optical properties of GaN-based materials are reviewed. In addition, the roles of unintentionally introduced impurities, such as C, H, and O, and grown-in defects, such as vacancy and antisite point defects, are also discussed. The doping process during epitaxial growth of GaN, AlGaN, InGaN, and their superlattice structures is described. Doping using the diffusion process and ion implantation techniques is also discussed. A p-n junction formed by Si implantation into p-type GaN is successfully fabricated. The results on crystal structure, electrical resistivity, carrier mobility, and optical spectra obtained by means of x-rays, low-temperature Hall measurements, and photoluminescence are also discussed.

  16. Bandgap engineering of GaN nanowires

    SciT

    Ming, Bang-Ming; Yan, Hui; Wang, Ru-Zhi, E-mail: wrz@bjut.edu.cn, E-mail: yamcy@csrc.ac.cn

    2016-05-15

    Bandgap engineering has been a powerful technique for manipulating the electronic and optical properties of semiconductors. In this work, a systematic investigation of the electronic properties of [0001] GaN nanowires was carried out using the density functional based tight-binding method (DFTB). We studied the effects of geometric structure and uniaxial strain on the electronic properties of GaN nanowires with diameters ranging from 0.8 to 10 nm. Our results show that the band gap of GaN nanowires depends linearly on both the surface to volume ratio (S/V) and tensile strain. The band gap of GaN nanowires increases linearly with S/V, whilemore » it decreases linearly with increasing tensile strain. These linear relationships provide an effect way in designing GaN nanowires for their applications in novel nano-devices.« less

  17. Vertical GaN power diodes with a bilayer edge termination

    DOE PAGES

    Dickerson, Jeramy R.; Allerman, Andrew A.; Bryant, Benjamin N.; ...

    2015-12-07

    Vertical GaN power diodes with a bilayer edge termination (ET) are demonstrated. The GaN p-n junction is formed on a low threading dislocation defect density (10 4 - 10 5 cm -2) GaN substrate, and has a 15-μm-thick n-type drift layer with a free carrier concentration of 5 × 10 15 cm -3. The ET structure is formed by N implantation into the p+-GaN epilayer just outside the p-type contact to create compensating defects. The implant defect profile may be approximated by a bilayer structure consisting of a fully compensated layer near the surface, followed by a 90% compensated (p)more » layer near the n-type drift region. These devices exhibit avalanche breakdown as high as 2.6 kV at room temperature. In addition simulations show that the ET created by implantation is an effective way to laterally distribute the electric field over a large area. This increases the voltage at which impact ionization occurs and leads to the observed higher breakdown voltages.« less

  18. Lattice Location of Mg in GaN: A Fresh Look at Doping Limitations

    NASA Astrophysics Data System (ADS)

    Wahl, U.; Amorim, L. M.; Augustyns, V.; Costa, A.; David-Bosne, E.; Lima, T. A. L.; Lippertz, G.; Correia, J. G.; da Silva, M. R.; Kappers, M. J.; Temst, K.; Vantomme, A.; Pereira, L. M. C.

    2017-03-01

    Radioactive 27Mg (t1 /2=9.5 min ) was implanted into GaN of different doping types at CERN's ISOLDE facility and its lattice site determined via β- emission channeling. Following implantations between room temperature and 800 °C , the majority of 27Mg occupies the substitutional Ga sites; however, below 350 °C significant fractions were also found on interstitial positions ˜0.6 Å from ideal octahedral sites. The interstitial fraction of Mg was correlated with the GaN doping character, being highest (up to 31%) in samples doped p type with 2 ×1019 cm-3 stable Mg during epilayer growth, and lowest in Si-doped n -GaN, thus giving direct evidence for the amphoteric character of Mg. Implanting above 350 °C converts interstitial 27Mg to substitutional Ga sites, which allows estimating the activation energy for migration of interstitial Mg as between 1.3 and 2.0 eV.

  19. White emission from non-planar InGaN/GaN MQW LEDs grown on GaN template with truncated hexagonal pyramids.

    PubMed

    Lee, Ming-Lun; Yeh, Yu-Hsiang; Tu, Shang-Ju; Chen, P C; Lai, Wei-Chih; Sheu, Jinn-Kong

    2015-04-06

    Non-planar InGaN/GaN multiple quantum well (MQW) structures are grown on a GaN template with truncated hexagonal pyramids (THPs) featuring c-plane and r-plane surfaces. The THP array is formed by the regrowth of the GaN layer on a selective-area Si-implanted GaN template. Transmission electron microscopy shows that the InGaN/GaN epitaxial layers regrown on the THPs exhibit different growth rates and indium compositions of the InGaN layer between the c-plane and r-plane surfaces. Consequently, InGaN/GaN MQW light-emitting diodes grown on the GaN THP array emit multiple wavelengths approaching near white light.

  20. The origins and properties of intrinsic nonradiative recombination centers in wide bandgap GaN and AlGaN

    NASA Astrophysics Data System (ADS)

    Chichibu, S. F.; Uedono, A.; Kojima, K.; Ikeda, H.; Fujito, K.; Takashima, S.; Edo, M.; Ueno, K.; Ishibashi, S.

    2018-04-01

    The nonradiative lifetime (τNR) of the near-band-edge emission in various quality GaN samples is compared with the results of positron annihilation measurement, in order to identify the origin and to determine the capture-cross-section of the major intrinsic nonradiative recombination centers (NRCs). The room-temperature τNR of various n-type GaN samples increased with decreasing the concentration of divacancies composed of a Ga vacancy (VGa) and a N vacancy (VN), namely, VGaVN. The τNR value also increased with increasing the diffusion length of positrons, which is almost proportional to the inverse third root of the gross concentration of all point defects. The results indicate that major intrinsic NRC in n-type GaN is VGaVN. From the relationship between its concentration and τNR, its hole capture-cross-section is estimated to be about 7 × 10-14 cm2. Different from the case of 4H-SiC, the major NRCs in p-type and n-type GaN are different: the major NRCs in Mg-doped p-type GaN epilayers are assigned to multiple vacancies containing a VGa and two (or three) VNs, namely, VGa(VN)n (n = 2 or 3). The ion-implanted Mg-doped GaN films are found to contain larger size vacancy complexes such as (VGa)3(VN)3. In analogy with GaN, major NRCs in Al0.6Ga0.4N alloys are assigned to vacancy complexes containing an Al vacancy or a VGa.

  1. Mechanical properties of nanoporous GaN and its application for separation and transfer of GaN thin films.

    PubMed

    Huang, Shanjin; Zhang, Yu; Leung, Benjamin; Yuan, Ge; Wang, Gang; Jiang, Hao; Fan, Yingmin; Sun, Qian; Wang, Jianfeng; Xu, Ke; Han, Jung

    2013-11-13

    Nanoporous (NP) gallium nitride (GaN) as a new class of GaN material has many interesting properties that the conventional GaN material does not have. In this paper, we focus on the mechanical properties of NP GaN, and the detailed physical mechanism of porous GaN in the application of liftoff. A decrease in elastic modulus and hardness was identified in NP GaN compared to the conventional GaN film. The promising application of NP GaN as release layers in the mechanical liftoff of GaN thin films and devices was systematically studied. A phase diagram was generated to correlate the initial NP GaN profiles with the as-overgrown morphologies of the NP structures. The fracture toughness of the NP GaN release layer was studied in terms of the voided-space-ratio. It is shown that the transformed morphologies and fracture toughness of the NP GaN layer after overgrowth strongly depends on the initial porosity of NP GaN templates. The mechanical separation and transfer of a GaN film over a 2 in. wafer was demonstrated, which proves that this technique is useful in practical applications.

  2. Nanoair-bridged lateral overgrowth of GaN on ordered nanoporous GaN template

    NASA Astrophysics Data System (ADS)

    Wang, Y. D.; Zang, K. Y.; Chua, S. J.; Tripathy, S.; Chen, P.; Fonstad, C. G.

    2005-12-01

    We report the growth of high-quality GaN epilayers on an ordered nanoporous GaN template by metalorganic chemical vapor deposition. The nanopores in GaN template were created by inductively coupled plasma etching using anodic aluminum oxide film as an etch mask. The average pore diameter and interpore distance is about 65 and 110nm, respectively. Subsequent overgrowth of GaN first begins at the GaN crystallite surface between the pores, and then air-bridge-mediated lateral overgrowth leads to the formation of the continuous layer. Microphotoluminescence and micro-Raman measurements show improved optical properties and significant strain relaxation in the overgrown layer when compared to GaN layer of same thickness simultaneously grown on sapphire without any template. Similar to conventional epitaxial lateral overgrown GaN, such overgrown GaN on a nanopatterned surface would also serve as a template for the growth of ultraviolet-visible light-emitting III-nitride devices.

  3. Ion implantation damage, annealing and dopant activation in epitaxial gallium nitride

    NASA Astrophysics Data System (ADS)

    Suvkhanov, Agajan

    2001-07-01

    Successful n- and p-doping of GaN is an extremely important technological problem. More recently, ion implantation has been used to achieve both n- and p-type GaN. The ion implantation process is accompanied by the presence of radiation defects as the result of the ion-solid interactions. The temperatures (above 1000°C) required for recovery of the implantation induced damage and dopant activation strongly affect the GaN's surface integrity due to the significant nitrogen vapor pressure. Preservation of the surface integrity of GaN during high temperature post-implantation annealing is one of the key issues in the fabrication of GaN-based light-emitting devices. The radiation damage build-up in the implanted GaN layers has been investigated as a function of ion dose and the substrate's temperature. Results of measurements of structural damage by the Rutherford backscattering/Channeling (RBS/C) and the spectroscopic ellipsometry (SE) techniques have demonstrated the complex nature of the damage build-up. Analysis of GaN implanted at high temperature has demonstrated the presence of competing processes of layer-by-layer damage build-up and defect annihilation. Using a capping layer and annealing in a sealed quartz capsule filled with dry nitrogen can preserve the integrity of the GaN's surface. In this work the ion-implanted GaN samples were capped with 40 run MOCVD (Metal Organic Chemical Vapor Deposition) grown AlN film prior to annealing. The results of this work showed the advantage of high-temperature annealing of implanted GaN in a quartz capsule with nitrogen ambient, as compared with annealing in argon and nitrogen gas flow. Partial to complete decomposition of the AlN cap and underlying GaN has been observed by RBS/C and SEM (Scanning electron microscopy) for the samples annealed in flowing argon, as well as for the samples processed in flowing nitrogen. Encapsulation with nitrogen overpressure prevented the decomposition of the AlN capping film and the GaN

  4. Effect of hydrogen on Ca and Mg acceptors in GaN

    SciT

    Lee, J.W.; Pearton, S.J.; Zolper, J.C.

    The influence of minority carrier injection on the reactivation of hydrogen passivated Mg in GaN at 175 C has been investigated in p-n junction diodes. The dissociation of the neutral MgH complexes is greatly enhanced in the presence of minority carrier and the reactivation process follows second order kinetics. Conventional annealing under zero-bias conditions does not produce Mg-H dissociation until temperatures {ge} 450 C. These results provide an explanation for the e-beam induced reactivation of Mg acceptors in hydrogenated GaN. Exposure to a hydrogen plasma at 250 C of p-type GaN (Ca) prepared by either Ca{sup +} or Ca{sup +}more » plus P{sup +} coimplantation leads to a reduction in sheet carrier density of approximately an order of magnitude (1.6 {times} 10{sup 12} cm{sup {minus}2} to 1.8 {times} 10{sup 11} cm{sup {minus}2}), and an accompanying increase in hole mobility (6 cm{sup 2}/Vs to 18 cm{sup 2}/Vs). The passivation process can be reversed by post-hydrogenation annealing at 400--500 C under a N{sub 2} ambient. This reactivation of the acceptors is characteristic of the formation of neutral (Ca-H) complexes in the GaN. The thermal stability of the passivation is similar to that of Mg-H complexes in material prepared in the same manner (implantation) with similar initial doping levels. Hydrogen passivation of acceptor dopants in GaN appears to be a ubiquitous phenomenon, as it is in other p-type semiconductors.« less

  5. The Formation and Characterization of GaN Hexagonal Pyramids

    NASA Astrophysics Data System (ADS)

    Zhang, Shi-Ying; Xiu, Xiang-Qian; Lin, Zeng-Qin; Hua, Xue-Mei; Xie, Zi-Li; Zhang, Rong; Zheng, You-Dou

    2013-05-01

    GaN with hexagonal pyramids is fabricated using the photo-assisted electroless chemical etching method. Defective areas of the GaN substrate are selectively etched in a mixed solution of KOH and K2S2O8 under ultraviolet illumination, producing submicron-sized pyramids. Hexagonal pyramids on the etched GaN with well-defined {101¯1¯} facets and very sharp tips are formed. High-resolution x-ray diffraction shows that etched GaN with pyramids has a higher crystal quality, and micro-Raman spectra reveal a tensile stress relaxation in GaN with pyramids compared with normal GaN. The cathodoluminescence intensity of GaN after etching is significantly increased by three times, which is attributed to the reduction in the internal reflection, high-quality GaN with pyramids and the Bragg effect.

  6. Utilization of native oxygen in Eu(RE)-doped GaN for enabling device compatibility in optoelectronic applications

    DOE PAGES

    Mitchell, Brandon; Timmerman, D.; Poplawsky, Jonathan D.; ...

    2016-01-04

    The detrimental influence of oxygen on the performance and reliability of V/III nitride based devices is well known. However, the influence of oxygen on the nature of the incorporation of other co-dopants, such as rare earth ions, has been largely overlooked in GaN. Here, we report the first comprehensive study of the critical role that oxygen has on Eu in GaN, as well as atomic scale observation of diffusion and local concentration of both atoms in the crystal lattice. We find that oxygen plays an integral role in the location, stability, and local defect structure around the Eu ions thatmore » were doped into the GaN host. Although the availability of oxygen is essential for these properties, it renders the material incompatible with GaN-based devices. However, the utilization of the normally occurring oxygen in GaN is promoted through structural manipulation, reducing its concentration by 2 orders of magnitude, while maintaining both the material quality and the favorable optical properties of the Eu ions. Furthermore, these findings open the way for full integration of RE dopants for optoelectronic functionalities in the existing GaN platform.« less

  7. Lattice Location of Mg in GaN: A Fresh Look at Doping Limitations.

    PubMed

    Wahl, U; Amorim, L M; Augustyns, V; Costa, A; David-Bosne, E; Lima, T A L; Lippertz, G; Correia, J G; da Silva, M R; Kappers, M J; Temst, K; Vantomme, A; Pereira, L M C

    2017-03-03

    Radioactive ^{27}Mg (t_{1/2}=9.5  min) was implanted into GaN of different doping types at CERN's ISOLDE facility and its lattice site determined via β^{-} emission channeling. Following implantations between room temperature and 800 °C, the majority of ^{27}Mg occupies the substitutional Ga sites; however, below 350 °C significant fractions were also found on interstitial positions ∼0.6  Å from ideal octahedral sites. The interstitial fraction of Mg was correlated with the GaN doping character, being highest (up to 31%) in samples doped p type with 2×10^{19}  cm^{-3} stable Mg during epilayer growth, and lowest in Si-doped n-GaN, thus giving direct evidence for the amphoteric character of Mg. Implanting above 350 °C converts interstitial ^{27}Mg to substitutional Ga sites, which allows estimating the activation energy for migration of interstitial Mg as between 1.3 and 2.0 eV.

  8. Piezotronic Effect in Polarity-Controlled GaN Nanowires.

    PubMed

    Zhao, Zhenfu; Pu, Xiong; Han, Changbao; Du, Chunhua; Li, Linxuan; Jiang, Chunyan; Hu, Weiguo; Wang, Zhong Lin

    2015-08-25

    Using high-quality and polarity-controlled GaN nanowires (NWs), we studied the piezotronic effect in crystal orientation defined wurtzite structures. By applying a normal compressive force on c-plane GaN NWs with an atomic force microscopy tip, the Schottky barrier between the Pt tip and GaN can be effectively tuned by the piezotronic effect. In contrast, the normal compressive force cannot change the electron transport characteristics in m-plane GaN NWs whose piezoelectric polarization axis is turned in the transverse direction. This observation provided solid evidence for clarifying the difference between the piezotronic effect and the piezoresistive effect. We further demonstrated a high sensitivity of the m-plane GaN piezotronic transistor to collect the transverse force. The integration of c-plane GaN and m-plane GaN indicates an overall response to an external force in any direction.

  9. GaN Micromechanical Resonators with Meshed Metal Bottom Electrode.

    PubMed

    Ansari, Azadeh; Liu, Che-Yu; Lin, Chien-Chung; Kuo, Hao-Chung; Ku, Pei-Cheng; Rais-Zadeh, Mina

    2015-03-17

    This work describes a novel architecture to realize high-performance gallium nitride (GaN) bulk acoustic wave (BAW) resonators. The method is based on the growth of a thick GaN layer on a metal electrode grid. The fabrication process starts with the growth of a thin GaN buffer layer on a Si (111) substrate. The GaN buffer layer is patterned and trenches are made and refilled with sputtered tungsten (W)/silicon dioxide (SiO₂) forming passivated metal electrode grids. GaN is then regrown, nucleating from the exposed GaN seed layer and coalescing to form a thick GaN device layer. A metal electrode can be deposited and patterned on top of the GaN layer. This method enables vertical piezoelectric actuation of the GaN layer using its largest piezoelectric coefficient ( d 33 ) for thickness-mode resonance. Having a bottom electrode also results in a higher coupling coefficient, useful for the implementation of acoustic filters. Growth of GaN on Si enables releasing the device from the frontside using isotropic xenon difluoride (XeF₂) etch and therefore eliminating the need for backside lithography and etching.

  10. GaN based nanorods for solid state lighting

    NASA Astrophysics Data System (ADS)

    Li, Shunfeng; Waag, Andreas

    2012-04-01

    In recent years, GaN nanorods are emerging as a very promising novel route toward devices for nano-optoelectronics and nano-photonics. In particular, core-shell light emitting devices are thought to be a breakthrough development in solid state lighting, nanorod based LEDs have many potential advantages as compared to their 2 D thin film counterparts. In this paper, we review the recent developments of GaN nanorod growth, characterization, and related device applications based on GaN nanorods. The initial work on GaN nanorod growth focused on catalyst-assisted and catalyst-free statistical growth. The growth condition and growth mechanisms were extensively investigated and discussed. Doping of GaN nanorods, especially p-doping, was found to significantly influence the morphology of GaN nanorods. The large surface of 3 D GaN nanorods induces new optical and electrical properties, which normally can be neglected in layered structures. Recently, more controlled selective area growth of GaN nanorods was realized using patterned substrates both by metalorganic chemical vapor deposition (MOCVD) and by molecular beam epitaxy (MBE). Advanced structures, for example, photonic crystals and DBRs are meanwhile integrated in GaN nanorod structures. Based on the work of growth and characterization of GaN nanorods, GaN nanoLEDs were reported by several groups with different growth and processing methods. Core/shell nanoLED structures were also demonstrated, which could be potentially useful for future high efficient LED structures. In this paper, we will discuss recent developments in GaN nanorod technology, focusing on the potential advantages, but also discussing problems and open questions, which may impose obstacles during the future development of a GaN nanorod based LED technology.

  11. Cochlear Implants.

    ERIC Educational Resources Information Center

    Clark, Catherine; Scott, Larry

    This brochure explains what a cochlear implant is, lists the types of individuals with deafness who may be helped by a cochlear implant, describes the process of evaluating people for cochlear implants, discusses the surgical process for implanting the aid, traces the path of sound through the cochlear implant to the brain, notes the costs of…

  12. GaN membrane MSM ultraviolet photodetectors

    NASA Astrophysics Data System (ADS)

    Muller, A.; Konstantinidis, G.; Kostopoulos, A.; Dragoman, M.; Neculoiu, D.; Androulidaki, M.; Kayambaki, M.; Vasilache, D.; Buiculescu, C.; Petrini, I.

    2006-12-01

    GaN exhibits unique physical properties, which make this material very attractive for wide range of applications and among them ultraviolet detection. For the first time a MSM type UV photodetector structure was manufactured on a 2.2 μm. thick GaN membrane obtained using micromachining techniques. The low unintentionally doped GaN layer structure was grown by MOCVD on high resistivity (ρ>10kΩcm) <111> oriented silicon wafers, 500μm thick. The epitaxially grown layers include a thin AlN layer in order to reduce the stress in the GaN layer and avoid cracking. Conventional contact lithography, e-gun Ni/Au (10nm /200nm) evaporation and lift-off techniques were used to define the interdigitated Schottky metalization on the top of the wafer. Ten digits with a width of 1μm and a length of 100μm were defined for each electrode. The distance between the digits was also 1μm. After the backside lapping of the wafer to a thickness of approximately 150μm, a 400nm thick Al layer was patterned and deposited on the backside, to be used as mask for the selective reactive ion etching of silicon. The backside mask, for the membrane formation, was patterned using double side alignment techniques and silicon was etched down to the 2.2μm thin GaN layer using SF 6 plasma. A very low dark current (30ρA at 3V) was obtained. Optical responsivity measurements were performed at 1.5V. A maximum responsivity of 18mA/W was obtained at a wavelength of 370nm. This value is very good and can be further improved using transparent contacts for the interdigitated structure.

  13. High-fluence ion implantation in silicon carbide for fabrication of a compliant substrate

    NASA Astrophysics Data System (ADS)

    Lioubtchenko, Mikhail

    GaN and related nitrides are promising materials for applications as UV/blue light emitters and in high-power, high-temperature electonic devices. Unfortunately, the vast potential of these materials cannot be realized effectively due to a large density of threading dislocations, arising from large lattice mismatch between GaN and utilized substrates. Therefore, a new approach to the heteroepitaxial growth is desirable, and a compliant substrate might help to remedy the situation. A modified model for the compliant substrate consisting of the compliant membrane glued to a thick handling substrate by a soft layer was proposed. We have chosen 6H-SiC as a starting substrate and ion implantation as a means of creating a buried layer. High fluence ion implantation of different species in 6H-SiC was performed at elevated temperatures and damage removal/accumulation was studied. It was found that temperatures around 1600°C are necessary to successfully recrystallize the radiation damage for Ti, Ga, Si and C implantations, but no damage removal was monitored for In implantation. In order to minimize the damage produced during ion implantation, it was decided to employ a multistep process in which each implantation step was followed by annealing. This approach was realized for 125 keV Ti++ and 300 keV Ga+ implantations up to a total dose of 1.8 x 1017 cm--2. Ti-implanted substrates were shown to retain good quality in the top layer, whereas Ga implantation preserves the quality of the near-surface region only at lower doses. The implanted species concentration was monitored after each step using Rutherford Backscattering (RBS). GaN films were grown on the prepared substrates and a control SiC sample by MOCVD. TEM and photoluminescence measurements have demonstrated that the quality of GaN films improves upon growth on compliant substrates.

  14. Later Leaders in Education: Roma Gans--Teacher of Teachers.

    ERIC Educational Resources Information Center

    Almy, Millie

    1990-01-01

    Retired teacher Roma Gans is described in terms of her early life and education, early teaching experience, teaching experience at Teachers College, publishing experience, citizenship, and retirement. (DG)

  15. Acceptor Ionization Energies in GaN*

    NASA Astrophysics Data System (ADS)

    Wang, Hao; Ban Chen, An

    2001-03-01

    The k.p Hamiltonian and a model potential are used to deduce the acceptor ionization energies in GaN from a systematic study of the chemical trend in GaAs, GaP, and InP. The acceptors studied include Be, Mg, Ca, Zn, and Cd on the cation sites and C, Si, and Ge on the anion sites. Our calculated acceptor ionization energies are estimated to be accurate to better than ten percent across the board. The ionization energies of C and Be (152 and 187 meV respectively) in wurtzite GaN are found to be lower than that of Mg (224 meV). The C was found to behave like the hydrogenic acceptor in all systems and it has the smallest ionization energy among all the acceptors studied.

  16. Dislocation filtering in GaN nanostructures.

    PubMed

    Colby, Robert; Liang, Zhiwen; Wildeson, Isaac H; Ewoldt, David A; Sands, Timothy D; García, R Edwin; Stach, Eric A

    2010-05-12

    Dislocation filtering in GaN by selective area growth through a nanoporous template is examined both by transmission electron microscopy and numerical modeling. These nanorods grow epitaxially from the (0001)-oriented GaN underlayer through the approximately 100 nm thick template and naturally terminate with hexagonal pyramid-shaped caps. It is demonstrated that for a certain window of geometric parameters a threading dislocation growing within a GaN nanorod is likely to be excluded by the strong image forces of the nearby free surfaces. Approximately 3000 nanorods were examined in cross-section, including growth through 50 and 80 nm diameter pores. The very few threading dislocations not filtered by the template turn toward a free surface within the nanorod, exiting less than 50 nm past the base of the template. The potential active region for light-emitting diode devices based on these nanorods would have been entirely free of threading dislocations for all samples examined. A greater than 2 orders of magnitude reduction in threading dislocation density can be surmised from a data set of this size. A finite element-based implementation of the eigenstrain model was employed to corroborate the experimentally observed data and examine a larger range of potential nanorod geometries, providing a simple map of the different regimes of dislocation filtering for this class of GaN nanorods. These results indicate that nanostructured semiconductor materials are effective at eliminating deleterious extended defects, as necessary to enhance the optoelectronic performance and device lifetimes compared to conventional planar heterostructures.

  17. Change in equilibrium position of misfit dislocations at the GaN/sapphire interface by Si-ion implantation into sapphire—I. Microstructural characterization

    SciT

    Lee, Sung Bo, E-mail: bolee@snu.ac.kr; Han, Heung Nam, E-mail: hnhan@snu.ac.kr; Lee, Dong Nyung

    Much research has been done to reduce dislocation densities for the growth of GaN on sapphire, but has paid little attention to the elastic behavior at the GaN/sapphire interface. In this study, we have examined effects of the addition of Si to a sapphire substrate on its elastic property and on the growth of GaN deposit. Si atoms are added to a c-plane sapphire substrate by ion implantation. The ion implantation results in scratches on the surface, and concomitantly, inhomogeneous distribution of Si. The scratch regions contain a higher concentration of Si than other regions of the sapphire substrate surface,more » high-temperature GaN being poorly grown there. However, high-temperature GaN is normally grown in the other regions. The GaN overlayer in the normally-grown regions is observed to have a lower TD density than the deposit on the bare sapphire substrate (with no Si accommodated). As compared with the film on an untreated, bare sapphire, the cathodoluminescence defect density decreases by 60 % for the GaN layer normally deposited on the Si-ion implanted sapphire. As confirmed by a strain mapping technique by transmission electron microscopy (geometric phase analysis), the addition of Si in the normally deposited regions forms a surface layer in the sapphire elastically more compliant than the GaN overlayer. The results suggest that the layer can largely absorb the misfit strain at the interface, which produces the overlayer with a lower defect density. Our results highlight a direct correlation between threading-dislocation density in GaN deposits and the elastic behavior at the GaN/sapphire interface, opening up a new pathway to reduce threading-dislocation density in GaN deposits.« less

  18. Resistivity control of unintentionally doped GaN films

    NASA Astrophysics Data System (ADS)

    Grzegorczyk, A. P.; Macht, L.; Hageman, P. R.; Rudzinski, M.; Larsen, P. K.

    2005-05-01

    GaN epilayers were grown on sapphire substrates via low temperature GaN and AlN nucleation layers (NL) by metalorganic chemical vapor phase epitaxy (MOCVD). The morphology of the individual NLs strongly depends on the carrier gas used during the growth and recrystallization and this is the key factor for control of the resistivity of the GaN layer grown on it. The GaN nucleation layer grown in presence of N2 has a higher density of islands with a statistically smaller diameter than the samples grown in H2 atmosphere. The NL grown in N2 enables the growth GaN with a sheet resistivity higher than 3×104 cm as opposed to a 0.5 cm value obtained for the NL grown in H2. Introduction of an additional intermediate (IL) low temperature (GaN or AlN) nucleation layer changes the GaN epilayer resistivity to about 50 cm, regardless of the carrier gas used during the growth of the IL. Defect selective etching demonstrated that control of the type and density of the dislocations in GaN enables the growth of highly resistive layers without any intentional acceptor doping (Mg, Zn). It will be demonstrated that by changing the ratio of edge type to screw dislocations the resistivity of the layer can be changed by a few orders of magnitude.

  19. High optical quality GaN nanopillar arrays

    NASA Astrophysics Data System (ADS)

    Wang, Y. D.; Chua, S. J.; Tripathy, S.; Sander, M. S.; Chen, P.; Fonstad, C. G.

    2005-02-01

    GaN nanopillar arrays have been fabricated by inductively coupled plasma etching of GaN films using anodic aluminum oxide film as an etch mask. The average diameter and length of these pillars are 60-65nm and 350-400nm, respectively. Ultraviolet microphotoluminescence measurements indicate high photoluminescence intensity and stress relaxation in these GaN nanopillars as compared to the starting epitaxial GaN films. Evidence of good crystalline quality is also observed by micro-Raman measurements, wherein a redshift of the E2high mode from GaN nanopillars suggests partial relaxation of the compressive strain. In addition, breakdown of the polarization selection rules led to the appearance of symmetry-forbidden and quasipolar modes.

  20. Stacking fault effects in Mg-doped GaN

    NASA Astrophysics Data System (ADS)

    Schmidt, T. M.; Miwa, R. H.; Orellana, W.; Chacham, H.

    2002-01-01

    First-principles total energy calculations are performed to investigate the interaction of a stacking fault with a p-type impurity in both zinc-blende and wurtzite GaN. For both structures we find that, in the presence of a stacking fault, the impurity level is a more localized state in the band gap. In zinc-blende GaN, the minimum energy position of the substitutional Mg atom is at the plane of the stacking fault. In contrast, in wurtzite GaN the substitutional Mg atom at the plane of the stacking fault is a local minimum and the global minimum is the substitutional Mg far from the fault. This behavior can be understood as a packing effect which induces a distinct strain relief process, since the local structure of the stacking fault in zinc-blende GaN is similar to fault-free wurtzite GaN and vice-versa.

  1. Atomic force microscopy studies of homoepitaxial GaN layers grown on GaN template by laser MBE

    SciT

    Choudhary, B. S.; Rajasthan Technical University, Rawatbhata Road, Kota 324010; Singh, A.

    We have grown homoepitaxial GaN films on metal organic chemical vapor deposition (MOCVD) grown 3.5 µm thick GaN on sapphire (0001) substrate (GaN template) using an ultra-high vacuum (UHV) laser assisted molecular beam epitaxy (LMBE) system. The GaN films were grown by laser ablating a polycrystalline solid GaN target in the presence of active r.f. nitrogen plasma. The influence of laser repetition rates (10-30 Hz) on the surface morphology of homoepitaxial GaN layers have been studied using atomic force microscopy. It was found that GaN layer grown at 10 Hz shows a smooth surface with uniform grain size compared to the rough surfacemore » with irregular shape grains obtained at 30 Hz. The variation of surface roughness of the homoepitaxial GaN layer with and without wet chemical etching has been also studied and it was observed that the roughness of the film decreased after wet etching due to the curved structure/rough surface.« less

  2. Preparation of freestanding GaN wafer by hydride vapor phase epitaxy on porous silicon

    NASA Astrophysics Data System (ADS)

    Wu, Xian; Li, Peng; Liang, Renrong; Xiao, Lei; Xu, Jun; Wang, Jing

    2018-05-01

    A freestanding GaN wafer was prepared on porous Si (111) substrate using hydride vapor phase epitaxy (HVPE). To avoid undesirable effects of the porous surface on the crystallinity of the GaN, a GaN seed layer was first grown on the Si (111) bare wafer. A pattern with many apertures was fabricated in the GaN seed layer using lithography and etching processes. A porous layer was formed in the Si substrate immediately adjacent to the GaN seed layer by an anodic etching process. A 500-μm-thick GaN film was then grown on the patterned GaN seed layer using HVPE. The GaN film was separated from the Si substrate through the formation of cracks in the porous layer caused by thermal mismatch stress during the cooling stage of the HVPE. Finally, the GaN film was polished to obtain a freestanding GaN wafer.

  3. Energetics of Mg incorporation at GaN(0001) and GaN(0001¯) surfaces

    NASA Astrophysics Data System (ADS)

    Sun, Qiang; Selloni, Annabella; Myers, T. H.; Doolittle, W. Alan

    2006-04-01

    By using density functional calculations in the generalized gradient approximation, we investigate the energetics of Mg adsorption and incorporation at GaN(0001) and GaN(0001¯) surfaces under various Ga and Mg coverage conditions as well as in presence of light or electron beam-induced electronic excitation. We find significant differences in Mg incorporation between Ga- and N-polar surfaces. Mg incorporation is easier at the Ga-polar surface, but high Mg coverages are found to cause important distortions which locally change the polarity from Ga to N polar. At the N-rich and moderately Ga-rich GaN(0001) surface, 0.25 ML of Mg substituting Ga in the top bilayer strongly reduce the surface diffusion barriers of Ga and N adatoms, in agreement with the surfactant effect observed in experiments. As the Mg coverage exceeds 0.5 ML, partial incorporation in the subsurface region (second bilayer) becomes favorable. A surface structure with 0.5 ML of incorporated Mg in the top bilayer and 0.25 ML in the second bilayer is found to be stable over a wide range of Ga chemical potential. At the Ga bilayer-terminated GaN(0001) surface, corresponding to Ga-rich conditions, configurations where Mg is incorporated in the interface region between the metallic Ga bilayer and the underlying GaN bilayer appear to be favored. At the N-polar surface, Mg is not incorporated under N-rich or moderately Ga-rich conditions, whereas incorporation in the adlayer may take place under Ga-rich conditions. In the presence of light or electron beam induced excitation, energy differences between Mg incorporated at the surface and in deeper layers are reduced so that the tendency toward surface segregation is also reduced.

  4. Cochlear Implants

    MedlinePlus

    ... newsroom@entnet.org . A cochlear implant is an electronic device that restores partial hearing to individuals with ... An internal component that consists of a small electronic device that is surgically implanted under the skin ...

  5. Goserelin Implant

    MedlinePlus

    Goserelin implant is used in combination with radiation therapy and other medications to treat localized prostate cancer ... and other symptoms) and to help with the treatment of abnormal bleeding of the uterus. Goserelin implant ...

  6. Probing the cellular damage in bacteria induced by GaN nanoparticles using confocal laser Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Sahoo, Prasana; Murthy, P. Sriyutha; Dhara, S.; Venugopalan, V. P.; Das, A.; Tyagi, A. K.

    2013-08-01

    Understanding the mechanism of nanoparticle (NP) induced toxicity in microbes is of potential importance to a variety of disciplines including disease diagnostics, biomedical implants, and environmental analysis. In this context, toxicity to bacterial cells and inhibition of biofilm formation by GaN NPs and their functional derivatives have been investigated against gram positive and gram negative bacterial species down to single cellular level. High levels of inhibition of biofilm formation (>80 %) was observed on treatments with GaN NPs at sub-micro molar concentrations. These results were substantiated with morphological features investigated with field emission scanning electron microscope, and the observed changes in vibrational modes of microbial cells using Raman spectroscopy. Raman spectra provided molecular interpretation of cell damage by registering signatures of molecular vibrations of individual living microbial cells and mapping the interplay of proteins at the cell membrane. As compared to the untreated cells, Raman spectra of NP-treated cells showed an increase in the intensities of characteristic protein bands, which confirmed membrane damage and subsequent release of cellular contents outside the cells. Raman spectral mapping at single cellular level can facilitate understanding of the mechanistic aspect of toxicity of GaN NPs. The effect may be correlated to passive diffusion causing mechanical damage to the membrane or ingress of Ga3+ (ionic radius 0.076 nm) which can potentially interfere with bacterial metabolism, as it resembles Fe2+ (ionic radius 0.077 nm), which is essential for energy metabolism.

  7. Determination of carrier diffusion length in GaN

    NASA Astrophysics Data System (ADS)

    Hafiz, Shopan; Zhang, Fan; Monavarian, Morteza; Avrutin, Vitaliy; Morkoç, Hadis; Özgür, Ümit; Metzner, Sebastian; Bertram, Frank; Christen, Jürgen; Gil, Bernard

    2015-01-01

    Diffusion lengths of photo-excited carriers along the c-direction were determined from photoluminescence (PL) and cross-sectional cathodoluminescence (CL) measurements in p- and n-type GaN epitaxial layers grown on c-plane sapphire by metal-organic chemical vapor deposition. The investigated samples incorporate a 6 nm thick In0.15Ga0.85N active layer capped with either 500 nm p-GaN or 1500 nm n-GaN. The top GaN layers were etched in steps and PL from the InGaN active region and the underlying layers was monitored as a function of the top GaN thickness upon photo-generation near the surface region by above bandgap excitation. Taking into consideration the absorption in the top GaN layer as well as active and underlying layers, the diffusion lengths at 295 K and at 15 K were measured to be 93 ± 7 nm and 70 ± 7 nm for Mg-doped p-type GaN and 432 ± 30 nm and 316 ± 30 nm for unintentionally doped n-type GaN, respectively, at photogenerated carrier densities of 4.2 × 1018 cm-3 using PL spectroscopy. CL measurements of the unintentionally doped n-type GaN layer at much lower carrier densities of 1017 cm-3 revealed a longer diffusion length of 525 ± 11 nm at 6 K.

  8. UMA/GAN network architecture analysis

    NASA Astrophysics Data System (ADS)

    Yang, Liang; Li, Wensheng; Deng, Chunjian; Lv, Yi

    2009-07-01

    This paper is to critically analyze the architecture of UMA which is one of Fix Mobile Convergence (FMC) solutions, and also included by the third generation partnership project(3GPP). In UMA/GAN network architecture, UMA Network Controller (UNC) is the key equipment which connects with cellular core network and mobile station (MS). UMA network could be easily integrated into the existing cellular networks without influencing mobile core network, and could provides high-quality mobile services with preferentially priced indoor voice and data usage. This helps to improve subscriber's experience. On the other hand, UMA/GAN architecture helps to integrate other radio technique into cellular network which includes WiFi, Bluetooth, and WiMax and so on. This offers the traditional mobile operators an opportunity to integrate WiMax technique into cellular network. In the end of this article, we also give an analysis of potential influence on the cellular core networks ,which is pulled by UMA network.

  9. Photoemission stability of negative electron affinity GaN photocathode

    NASA Astrophysics Data System (ADS)

    Zhang, Junju; Wang, Xiaohui; Yang, Wenzheng; Tang, Weidong; Fu, Xiaoqian; Li, Biao; Chang, Benkang

    2012-11-01

    The stability for reflection-mode GaN photocathode has been investigated by monitoring the photocurrent and the spectral response at room temperature. We watch that the photocurrent of the cathode decays with time in the vacuum system, and compare the spectral response curves after activation and after degradation. The photocurrent decay mechanism for reflection-mode NEA GaN photocathode was studied by the surface model ?GaN (Mg) :Cs ?:O-Cs. The reduction of the effective dipole quantity, which is caused by harmful gases, is the key factor of the photocurrent reduction.

  10. GaN grown on nano-patterned sapphire substrates

    NASA Astrophysics Data System (ADS)

    Jing, Kong; Meixin, Feng; Jin, Cai; Hui, Wang; Huaibing, Wang; Hui, Yang

    2015-04-01

    High-quality gallium nitride (GaN) film was grown on nano-patterned sapphire substrates (NPSS) and investigated using XRD and SEM. It was found that the optimum thickness of the GaN buffer layer on the NPSS is 15 nm, which is thinner than that on micro-patterned sapphire substrates (MPSS). An interesting phenomenon was observed for GaN film grown on NPSS:GaN mainly grows on the trench regions and little grows on the sidewalls of the patterns at the initial growth stage, which is dramatically different from GaN grown on MPSS. In addition, the electrical and optical properties of LEDs grown on NPSS were characterized. Project supported by the Suzhou Nanojoin Photonics Co., Ltd and the High-Tech Achievements Transformation of Jiangsu Province, China (No.BA2012010).

  11. GaN microcavities: Giant Rabi splitting and optical anisotropy

    NASA Astrophysics Data System (ADS)

    Kavokin, Alexey; Gil, Bernard

    1998-06-01

    Numerical simulation of light reflection from a λ/2 GaN microcavity with Ga0.8Al0.2N/Ga0.5Al0.5N Bragg mirrors grown on the A surface of Al2O3 revealed a Rabi splitting of the order of 50 meV and remarkable optical anisotropy. These effects are originated from the giant exciton oscillator strength in GaN and a pronounced uniaxial strain in the structure.

  12. Characterization of Deep and Shallow Levels in GaN

    NASA Astrophysics Data System (ADS)

    Wessels, Bruce

    1997-03-01

    The role of native defects and impurities in compensating n-type GaN was investigated. From the observed dependence of carrier concentration on dopant partial pressure the compensating acceptor in n-type material is attributed to the triply charged gallium vacancy. This is consistent with recent calculations on defect stability using density functional theory. The interaction of hydrogen and point defects in GaN was also investigated using FTIR. The role of these defects in compensation will be discussed.

  13. ARM MJO Investigation Experiment on Gan Island (AMIE-Gan) Science Plan

    SciT

    Long, CL; Del Genio, A; Deng, M

    2011-04-11

    The overarching campaign, which includes the ARM Mobile Facility 2 (AMF2) deployment in conjunction with the Dynamics of the Madden-Julian Oscillation (DYNAMO) and the Cooperative Indian Ocean experiment on intraseasonal variability in the Year 2011 (CINDY2011) campaigns, is designed to test several current hypotheses regarding the mechanisms responsible for Madden-Julian Oscillation (MJO) initiation and propagation in the Indian Ocean area. The synergy between the proposed AMF2 deployment with DYNAMO/CINDY2011, and the corresponding funded experiment on Manus, combine for an overarching ARM MJO Investigation Experiment (AMIE) with two components: AMF2 on Gan Island in the Indian Ocean (AMIE-Gan), where the MJOmore » initiates and starts its eastward propagation; and the ARM Manus site (AMIE-Manus), which is in the general area where the MJO usually starts to weaken in climate models. AMIE-Gan will provide measurements of particular interest to Atmospheric System Research (ASR) researchers relevant to improving the representation of MJO initiation in climate models. The framework of DYNAMO/CINDY2011 includes two proposed island-based sites and two ship-based locations forming a square pattern with sonde profiles and scanning precipitation and cloud radars at both island and ship sites. These data will be used to produce a Variational Analysis data set coinciding with the one produced for AMIE-Manus. The synergy between AMIE-Manus and AMIE-Gan will allow studies of the initiation, propagation, and evolution of the convective cloud population within the framework of the MJO. As with AMIE-Manus, AMIE-Gan/DYNAMO also includes a significant modeling component geared toward improving the representation of MJO initiation and propagation in climate and forecast models. This campaign involves the deployment of the second, marine-capable, AMF; all of the included measurement systems; and especially the scanning and vertically pointing radars. The campaign will include

  14. Synthesis of p-type GaN nanowires.

    PubMed

    Kim, Sung Wook; Park, Youn Ho; Kim, Ilsoo; Park, Tae-Eon; Kwon, Byoung Wook; Choi, Won Kook; Choi, Heon-Jin

    2013-09-21

    GaN has been utilized in optoelectronics for two decades. However, p-type doping still remains crucial for realization of high performance GaN optoelectronics. Though Mg has been used as a p-dopant, its efficiency is low due to the formation of Mg-H complexes and/or structural defects in the course of doping. As a potential alternative p-type dopant, Cu has been recognized as an acceptor impurity for GaN. Herein, we report the fabrication of Cu-doped GaN nanowires (Cu:GaN NWs) and their p-type characteristics. The NWs were grown vertically via a vapor-liquid-solid (VLS) mechanism using a Au/Ni catalyst. Electrical characterization using a nanowire-field effect transistor (NW-FET) showed that the NWs exhibited n-type characteristics. However, with further annealing, the NWs showed p-type characteristics. A homo-junction structure (consisting of annealed Cu:GaN NW/n-type GaN thin film) exhibited p-n junction characteristics. A hybrid organic light emitting diode (OLED) employing the annealed Cu:GaN NWs as a hole injection layer (HIL) also demonstrated current injected luminescence. These results suggest that Cu can be used as a p-type dopant for GaN NWs.

  15. Effect of photocatalytic oxidation technology on GaN CMP

    NASA Astrophysics Data System (ADS)

    Wang, Jie; Wang, Tongqing; Pan, Guoshun; Lu, Xinchun

    2016-01-01

    GaN is so hard and so chemically inert that it is difficult to obtain a high material removal rate (MRR) in the chemical mechanical polishing (CMP) process. This paper discusses the application of photocatalytic oxidation technology in GaN planarization. Three N-type semiconductor particles (TiO2, SnO2, and Fe2O3) are used as catalysts and added to the H2O2-SiO2-based slurry. By optical excitation, highly reactive photoinduced holes are produced on the surface of the particles, which can oxidize OH- and H2O absorbed on the surface of the catalysts; therefore, more OH* will be generated. As a result, GaN MRRs in an H2O2-SiO2-based polishing system combined with catalysts are improved significantly, especially when using TiO2, the MRR of which is 122 nm/h. The X-ray photoelectron spectroscopy (XPS) analysis shows the variation trend of chemical composition on the GaN surface after polishing, revealing the planarization process. Besides, the effect of pH on photocatalytic oxidation combined with TiO2 is analyzed deeply. Furthermore, the physical model of GaN CMP combined with photocatalytic oxidation technology is proposed to describe the removal mechanism of GaN.

  16. Polarity Control of Heteroepitaxial GaN Nanowires on Diamond.

    PubMed

    Hetzl, Martin; Kraut, Max; Hoffmann, Theresa; Stutzmann, Martin

    2017-06-14

    Group III-nitride materials such as GaN nanowires are characterized by a spontaneous polarization within the crystal. The sign of the resulting sheet charge at the top and bottom facet of a GaN nanowire is determined by the orientation of the wurtzite bilayer of the different atomic species, called N and Ga polarity. We investigate the polarity distribution of heteroepitaxial GaN nanowires on different substrates and demonstrate polarity control of GaN nanowires on diamond. Kelvin Probe Force Microscopy is used to determine the polarity of individual selective area-grown and self-assembled nanowires over a large scale. At standard growth conditions, mixed polarity occurs for selective GaN nanowires on various substrates, namely on silicon, on sapphire and on diamond. To obtain control over the growth orientation on diamond, the substrate surface is modified by nitrogen and oxygen plasma exposure prior to growth, and the growth parameters are adjusted simultaneously. We find that the surface chemistry and the substrate temperature are the decisive factors for obtaining control of up to 93% for both polarity types, whereas the growth mode, namely selective area or self-assembled growth, does not influence the polarity distribution significantly. The experimental results are discussed by a model based on the interfacial bonds between the GaN nanowires, the termination layer, and the substrate.

  17. High nitrogen pressure solution growth of GaN

    NASA Astrophysics Data System (ADS)

    Bockowski, Michal

    2014-10-01

    Results of GaN growth from gallium solution under high nitrogen pressure are presented. Basic of the high nitrogen pressure solution (HNPS) growth method is described. A new approach of seeded growth, multi-feed seed (MFS) configuration, is demonstrated. The use of two kinds of seeds: free-standing hydride vapor phase epitaxy GaN (HVPE-GaN) obtained from metal organic chemical vapor deposition (MOCVD)-GaN/sapphire templates and free-standing HVPE-GaN obtained from the ammonothermally grown GaN crystals, is shown. Depending on the seeds’ structural quality, the differences in the structural properties of pressure grown material are demonstrated and analyzed. The role and influence of impurities, like oxygen and magnesium, on GaN crystals grown from gallium solution in the MFS configuration is presented. The properties of differently doped GaN crystals are discussed. An application of the pressure grown GaN crystals as substrates for electronic and optoelectronic devices is reported.

  18. Autonomic nervous system involvement in the giant axonal neuropathy (GAN) KO mouse: implications for human disease.

    PubMed

    Armao, Diane; Bailey, Rachel M; Bouldin, Thomas W; Kim, Yongbaek; Gray, Steven J

    2016-08-01

    Giant axonal neuropathy (GAN) is an inherited severe sensorimotor neuropathy. The aim of this research was to investigate the neuropathologic features and clinical autonomic nervous system (ANS) phenotype in two GAN knockout (KO) mouse models. Little is known about ANS involvement in GAN in humans, but autonomic signs and symptoms are commonly reported in early childhood. Routine histology and immunohistochemistry was performed on GAN KO mouse specimens taken at various ages. Enteric dysfunction was assessed by quantifying the frequency, weight, and water content of defecation in GAN KO mice. Histological examination of the enteric, parasympathetic and sympathetic ANS of GAN KO mice revealed pronounced and widespread neuronal perikaryal intermediate filament inclusions. These neuronal inclusions served as an easily identifiable, early marker of GAN in young GAN KO mice. Functional studies identified an age-dependent alteration in fecal weight and defecation frequency in GAN KO mice. For the first time in the GAN KO mouse model, we described the early, pronounced and widespread neuropathologic features involving the ANS. In addition, we provided evidence for a clinical autonomic phenotype in GAN KO mice, reflected in abnormal gastrointestinal function. These findings in GAN KO mice suggest that consideration should be given to ANS involvement in human GAN, especially when considering treatments and patient care.

  19. Role of the ganSPQAB Operon in Degradation of Galactan by Bacillus subtilis.

    PubMed

    Watzlawick, Hildegard; Morabbi Heravi, Kambiz; Altenbuchner, Josef

    2016-10-15

    Bacillus subtilis possesses different enzymes for the utilization of plant cell wall polysaccharides. This includes a gene cluster containing galactan degradation genes (ganA and ganB), two transporter component genes (ganQ and ganP), and the sugar-binding lipoprotein-encoding gene ganS (previously known as cycB). These genes form an operon that is regulated by GanR. The degradation of galactan by B. subtilis begins with the activity of extracellular GanB. GanB is an endo-β-1,4-galactanase and is a member of glycoside hydrolase (GH) family 53. This enzyme was active on high-molecular-weight arabinose-free galactan and mainly produced galactotetraose as well as galactotriose and galactobiose. These galacto-oligosaccharides may enter the cell via the GanQP transmembrane proteins of the galactan ABC transporter. The specificity of the galactan ABC transporter depends on the sugar-binding lipoprotein, GanS. Purified GanS was shown to bind galactotetraose and galactotriose using thermal shift assay. The energy for this transport is provided by MsmX, an ATP-binding protein. The transported galacto-oligosaccharides are further degraded by GanA. GanA is a β-galactosidase that belongs to GH family 42. The GanA enzyme was able to hydrolyze short-chain β-1,4-galacto-oligosaccharides as well as synthetic β-galactopyranosides into galactose. Thermal shift assay as well as electrophoretic mobility shift assay demonstrated that galactobiose is the inducer of the galactan operon regulated by GanR. DNase I footprinting revealed that the GanR protein binds to an operator overlapping the -35 box of the σ(A)-type promoter of Pgan, which is located upstream of ganS IMPORTANCE: Bacillus subtilis is a Gram-positive soil bacterium that utilizes different types of carbohydrates, such as pectin, as carbon sources. So far, most of the pectin degradation systems and enzymes have been thoroughly studied in B. subtilis Nevertheless, the B. subtilis utilization system of galactan, which is

  20. Breast Implants

    MedlinePlus

    ... sale in the United States: saline-filled and silicone gel-filled. Both types have a silicone outer shell. They vary in size, shell thickness, ... implant them. Provide information on saline-filled and silicone gel-filled breast implants, including data supporting a ...

  1. Viability and proliferation of endothelial cells upon exposure to GaN nanoparticles.

    PubMed

    Braniste, Tudor; Tiginyanu, Ion; Horvath, Tibor; Raevschi, Simion; Cebotari, Serghei; Lux, Marco; Haverich, Axel; Hilfiker, Andres

    2016-01-01

    Nanotechnology is a rapidly growing and promising field of interest in medicine; however, nanoparticle-cell interactions are not yet fully understood. The goal of this work was to examine the interaction between endothelial cells and gallium nitride (GaN) semiconductor nanoparticles. Cellular viability, adhesion, proliferation, and uptake of nanoparticles by endothelial cells were investigated. The effect of free GaN nanoparticles versus the effect of growing endothelial cells on GaN functionalized surfaces was examined. To functionalize surfaces with GaN, GaN nanoparticles were synthesized on a sacrificial layer of zinc oxide (ZnO) nanoparticles using hydride vapor phase epitaxy. The uptake of GaN nanoparticles by porcine endothelial cells was strongly dependent upon whether they were fixed to the substrate surface or free floating in the medium. The endothelial cells grown on surfaces functionalized with GaN nanoparticles demonstrated excellent adhesion and proliferation, suggesting good biocompatibility of the nanostructured GaN.

  2. Gallium hydride vapor phase epitaxy of GaN nanowires

    PubMed Central

    2011-01-01

    Straight GaN nanowires (NWs) with diameters of 50 nm, lengths up to 10 μm and a hexagonal wurtzite crystal structure have been grown at 900°C on 0.5 nm Au/Si(001) via the reaction of Ga with NH3 and N2:H2, where the H2 content was varied between 10 and 100%. The growth of high-quality GaN NWs depends critically on the thickness of Au and Ga vapor pressure while no deposition occurs on plain Si(001). Increasing the H2 content leads to an increase in the growth rate, a reduction in the areal density of the GaN NWs and a suppression of the underlying amorphous (α)-like GaN layer which occurs without H2. The increase in growth rate with H2 content is a direct consequence of the reaction of Ga with H2 which leads to the formation of Ga hydride that reacts efficiently with NH3 at the top of the GaN NWs. Moreover, the reduction in the areal density of the GaN NWs and suppression of the α-like GaN layer is attributed to the reaction of H2 with Ga in the immediate vicinity of the Au NPs. Finally, the incorporation of H2 leads to a significant improvement in the near band edge photoluminescence through a suppression of the non-radiative recombination via surface states which become passivated not only via H2, but also via a reduction of O2-related defects. PMID:21711801

  3. Gallium hydride vapor phase epitaxy of GaN nanowires.

    PubMed

    Zervos, Matthew; Othonos, Andreas

    2011-03-28

    Straight GaN nanowires (NWs) with diameters of 50 nm, lengths up to 10 μm and a hexagonal wurtzite crystal structure have been grown at 900°C on 0.5 nm Au/Si(001) via the reaction of Ga with NH3 and N2:H2, where the H2 content was varied between 10 and 100%. The growth of high-quality GaN NWs depends critically on the thickness of Au and Ga vapor pressure while no deposition occurs on plain Si(001). Increasing the H2 content leads to an increase in the growth rate, a reduction in the areal density of the GaN NWs and a suppression of the underlying amorphous (α)-like GaN layer which occurs without H2. The increase in growth rate with H2 content is a direct consequence of the reaction of Ga with H2 which leads to the formation of Ga hydride that reacts efficiently with NH3 at the top of the GaN NWs. Moreover, the reduction in the areal density of the GaN NWs and suppression of the α-like GaN layer is attributed to the reaction of H2 with Ga in the immediate vicinity of the Au NPs. Finally, the incorporation of H2 leads to a significant improvement in the near band edge photoluminescence through a suppression of the non-radiative recombination via surface states which become passivated not only via H2, but also via a reduction of O2-related defects.

  4. Native defects in GaN: a hybrid functional study

    NASA Astrophysics Data System (ADS)

    Diallo, Ibrahima Castillo; Demchenko, Denis

    Intrinsic defects play an important role in the performance of GaN-based devices. We present hybrid density functional calculations of the electronic and possible optical properties of interstitial N (Ni-Ni) , N antisite (NGa) , interstitial Ga (Gai) , Ga antisite (GaN) , Ga vacancy (VGa) , N vacancy (VN) and Ga-N divacancies (VGaVN) in GaN. Our results show that the vacancies display relatively low formation energies in certain samples, whereas antisites and interstitials are energetically less favorable. However, interstitials can be created by electron irradiation. For instance, in 2.5 MeV electron-irradiated GaN samples, a strong correlation between the frequently observed photoluminescence (PL) band centered around 0.85 eV accompanied with a rich phonon sideband of ~0.88 eV and the theoretical optical behavior of interstitial Ga is discussed. N vacancies are found to likely contribute to the experimentally obtained green luminescence band (GL2) peaking at 2.24 eV in high-resistivity undoped and Mg-doped GaN. National Science Foundation (DMR-1410125) and the Thomas F. and Kate Miller Jeffress Memorial Trust.

  5. Energy bands and acceptor binding energies of GaN

    NASA Astrophysics Data System (ADS)

    Xia, Jian-Bai; Cheah, K. W.; Wang, Xiao-Liang; Sun, Dian-Zhao; Kong, Mei-Ying

    1999-04-01

    The energy bands of zinc-blende and wurtzite GaN are calculated with the empirical pseudopotential method, and the pseudopotential parameters for Ga and N atoms are given. The calculated energy bands are in agreement with those obtained by the ab initio method. The effective-mass theory for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN for both structures are given. The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor are 24 and 142 meV for the zinc-blende structure, 20 and 131, and 97 meV for the wurtzite structure, respectively, which are consistent with recent experimental results. It is proposed that there are two kinds of acceptor in wurtzite GaN. One kind is the general acceptor such as C, which substitutes N, which satisfies the effective-mass theory. The other kind of acceptor includes Mg, Zn, Cd, etc., the binding energy of these acceptors is deviated from that given by the effective-mass theory. In this report, wurtzite GaN is grown by the molecular-beam epitaxy method, and the photoluminescence spectra were measured. Three main peaks are assigned to the donor-acceptor transitions from two kinds of acceptors. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material.

  6. The 2018 GaN power electronics roadmap

    NASA Astrophysics Data System (ADS)

    Amano, H.; Baines, Y.; Beam, E.; Borga, Matteo; Bouchet, T.; Chalker, Paul R.; Charles, M.; Chen, Kevin J.; Chowdhury, Nadim; Chu, Rongming; De Santi, Carlo; Merlyne De Souza, Maria; Decoutere, Stefaan; Di Cioccio, L.; Eckardt, Bernd; Egawa, Takashi; Fay, P.; Freedsman, Joseph J.; Guido, L.; Häberlen, Oliver; Haynes, Geoff; Heckel, Thomas; Hemakumara, Dilini; Houston, Peter; Hu, Jie; Hua, Mengyuan; Huang, Qingyun; Huang, Alex; Jiang, Sheng; Kawai, H.; Kinzer, Dan; Kuball, Martin; Kumar, Ashwani; Boon Lee, Kean; Li, Xu; Marcon, Denis; März, Martin; McCarthy, R.; Meneghesso, Gaudenzio; Meneghini, Matteo; Morvan, E.; Nakajima, A.; Narayanan, E. M. S.; Oliver, Stephen; Palacios, Tomás; Piedra, Daniel; Plissonnier, M.; Reddy, R.; Sun, Min; Thayne, Iain; Torres, A.; Trivellin, Nicola; Unni, V.; Uren, Michael J.; Van Hove, Marleen; Wallis, David J.; Wang, J.; Xie, J.; Yagi, S.; Yang, Shu; Youtsey, C.; Yu, Ruiyang; Zanoni, Enrico; Zeltner, Stefan; Zhang, Yuhao

    2018-04-01

    Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

  7. Activation and evaluation of GaN photocathodes

    NASA Astrophysics Data System (ADS)

    Qian, Yunsheng; Chang, Benkang; Qiao, Jiangliang; Zhang, Yijun; Fu, Rongguo; Qiu, Yafeng

    2009-09-01

    Gallium Nitride (GaN) photocathodes are potentially attractive as UV detective materials and electron sources. Based on the activation and evaluation system for GaAs photocathode, which consists of ultra-high vacuum (UHV) activation chamber, multi-information measurement system, X-ray photoelectron spectroscopy (XPS), and ultraviolet ray photoelectron spectroscopy (UPS), the control and measurement system for the activation of UV photocathodes was developed. The developed system, which consists of Xenon lamp, monochromator with scanner, signal-processing module, power control unit of Cs and O source, A/D adapter, digital I/O card, computer and software, can control the activation of GaN photocathodes and measure on-line the spectral response curves of GaN photocathodes. GaN materials on sapphire substrate were grown by Metal-Organic Chemical Vapor Deposition (MOCVD) with p-type Mg doping. The GaN materials were activated by Cs-O. The spectral response and quantum efficiency (QE) were measured and calculated. The experiment results are discussed.

  8. Implant ethics

    PubMed Central

    Hansson, S

    2005-01-01

    Implant ethics is defined here as the study of ethical aspects of the lasting introduction of technological devices into the human body. Whereas technological implants relieve us of some of the ethical problems connected with transplantation, other difficulties arise that are in need of careful analysis. A systematic approach to implant ethics is proposed. The major specific problems are identified as those concerning end of life issues (turning off devices), enhancement of human capabilities beyond normal levels, mental changes and personal identity, and cultural effects. PMID:16131553

  9. Alignment control and atomically-scaled heteroepitaxial interface study of GaN nanowires.

    PubMed

    Liu, Qingyun; Liu, Baodan; Yang, Wenjin; Yang, Bing; Zhang, Xinglai; Labbé, Christophe; Portier, Xavier; An, Vladimir; Jiang, Xin

    2017-04-20

    Well-aligned GaN nanowires are promising candidates for building high-performance optoelectronic nanodevices. In this work, we demonstrate the epitaxial growth of well-aligned GaN nanowires on a [0001]-oriented sapphire substrate in a simple catalyst-assisted chemical vapor deposition process and their alignment control. It is found that the ammonia flux plays a key role in dominating the initial nucleation of GaN nanocrystals and their orientation. Typically, significant improvement of the GaN nanowire alignment can be realized at a low NH 3 flow rate. X-ray diffraction and cross-sectional scanning electron microscopy studies further verified the preferential orientation of GaN nanowires along the [0001] direction. The growth mechanism of GaN nanowire arrays is also well studied based on cross-sectional high-resolution transmission electron microscopy (HRTEM) characterization and it is observed that GaN nanowires have good epitaxial growth on the sapphire substrate following the crystallographic relationship between (0001) GaN ∥(0001) sapphire and (101[combining macron]0) GaN ∥(112[combining macron]0) sapphire . Most importantly, periodic misfit dislocations are also experimentally observed in the interface region due to the large lattice mismatch between the GaN nanowire and the sapphire substrate, and the formation of such dislocations will favor the release of structural strain in GaN nanowires. HRTEM analysis also finds the existence of "type I" stacking faults and voids inside the GaN nanowires. Optical investigation suggests that the GaN nanowire arrays have strong emission in the UV range, suggesting their crystalline nature and chemical purity. The achievement of aligned GaN nanowires will further promote the wide applications of GaN nanostructures toward diverse high-performance optoelectronic nanodevices including nano-LEDs, photovoltaic cells, photodetectors etc.

  10. Structural defects in bulk GaN

    NASA Astrophysics Data System (ADS)

    Liliental-Weber, Z.; dos Reis, R.; Mancuso, M.; Song, C. Y.; Grzegory, I.; Porowski, S.; Bockowski, M.

    2014-10-01

    Transmission Electron Microscopy (TEM) studies of undoped and Mg doped GaN layers grown on the HVPE substrates by High Nitrogen Pressure Solution (HNPS) with the multi-feed-seed (MFS) configuration are shown. The propagation of dislocations from the HVPE substrate to the layer is observed. Due to the interaction between these dislocations in the thick layers much lower density of these defects is observed in the upper part of the HNPS layers. Amorphous Ga precipitates with attached voids pointing toward the growth direction are observed in the undoped layer. This is similar to the presence of Ga precipitates in high-pressure platelets, however the shape of these precipitates is different. The Mg doped layers do not show Ga precipitates, but MgO rectangular precipitates are formed, decorating the dislocations. Results of TEM studies of HVPE layers grown on Ammonothermal substrates are also presented. These layers have superior crystal quality in comparison to the HNPS layers, as far as density of dislocation is concern. Occasionally some small inclusions can be found, but their chemical composition was not yet determined. It is expected that growth of the HNPS layers on these substrate will lead to large layer thickness obtained in a short time and with high crystal perfection needed in devices.

  11. Penile Implants

    MedlinePlus

    ... ED that's situational, the result of a relationship conflict or potentially reversible An infection, such as a ... your partner in the discussion with your doctor. Types of penile implants There are two main types ...

  12. Cochlear Implants

    MedlinePlus

    ... learn or relearn the sense of hearing. Not everyone performs at the same level with this device. The decision to receive an implant should involve discussions with medical specialists, including an experienced ...

  13. Carmustine Implant

    MedlinePlus

    ... works by slowing or stopping the growth of cancer cells in your body. ... are pregnant, plan to become pregnant, or are breast-feeding. If you become pregnant while receiving carmustine implant, call your doctor. Carmustine may harm the fetus.

  14. Codoping characteristics of Zn with Mg in GaN

    NASA Astrophysics Data System (ADS)

    Kim, K. S.; Han, M. S.; Yang, G. M.; Youn, C. J.; Lee, H. J.; Cho, H. K.; Lee, J. Y.

    2000-08-01

    The doping characteristics of Mg-Zn codoped GaN films grown by metalorganic chemical vapor deposition are investigated. By means of the concept of Mg-Zn codoping technique, we have grown p-GaN showing a low electrical resistivity (0.72 Ω cm) and a high hole concentration (8.5×1017cm-3) without structural degradation of the film. It is thought that the codoping of Zn atoms with Mg raises the Mg activation ratio by reducing the hydrogen solubility in p-GaN. In addition, the measured specific contact resistance of Mg-Zn codoped GaN film is 5.0×10-4 Ω cm2, which is one order of magnitude lower than that of Mg doped only GaN film (1.9×10-3 Ω cm2).

  15. High-Sensitivity GaN Microchemical Sensors

    NASA Technical Reports Server (NTRS)

    Son, Kyung-ah; Yang, Baohua; Liao, Anna; Moon, Jeongsun; Prokopuk, Nicholas

    2009-01-01

    Systematic studies have been performed on the sensitivity of GaN HEMT (high electron mobility transistor) sensors using various gate electrode designs and operational parameters. The results here show that a higher sensitivity can be achieved with a larger W/L ratio (W = gate width, L = gate length) at a given D (D = source-drain distance), and multi-finger gate electrodes offer a higher sensitivity than a one-finger gate electrode. In terms of operating conditions, sensor sensitivity is strongly dependent on transconductance of the sensor. The highest sensitivity can be achieved at the gate voltage where the slope of the transconductance curve is the largest. This work provides critical information about how the gate electrode of a GaN HEMT, which has been identified as the most sensitive among GaN microsensors, needs to be designed, and what operation parameters should be used for high sensitivity detection.

  16. Curvature and bow of bulk GaN substrates

    SciT

    Foronda, Humberto M.; Young, Erin C.; Robertson, Christian A.

    2016-07-21

    We investigate the bow of free standing (0001) oriented hydride vapor phase epitaxy grown GaN substrates and demonstrate that their curvature is consistent with a compressive to tensile stress gradient (bottom to top) present in the substrates. The origin of the stress gradient and the curvature is attributed to the correlated inclination of edge threading dislocation (TD) lines away from the [0001] direction. A model is proposed and a relation is derived for bulk GaN substrate curvature dependence on the inclination angle and the density of TDs. The model is used to analyze the curvature for commercially available GaN substratesmore » as determined by high resolution x-ray diffraction. The results show a close correlation between the experimentally determined parameters and those predicted from theoretical model.« less

  17. The controlled growth of GaN nanowires.

    PubMed

    Hersee, Stephen D; Sun, Xinyu; Wang, Xin

    2006-08-01

    This paper reports a scalable process for the growth of high-quality GaN nanowires and uniform nanowire arrays in which the position and diameter of each nanowire is precisely controlled. The approach is based on conventional metalorganic chemical vapor deposition using regular precursors and requires no additional metal catalyst. The location, orientation, and diameter of each GaN nanowire are controlled using a thin, selective growth mask that is patterned by interferometric lithography. It was found that use of a pulsed MOCVD process allowed the nanowire diameter to remain constant after the nanowires had emerged from the selective growth mask. Vertical GaN nanowire growth rates in excess of 2 mum/h were measured, while remarkably the diameter of each nanowire remained constant over the entire (micrometer) length of the nanowires. The paper reports transmission electron microscopy and photoluminescence data.

  18. Transient atomic behavior and surface kinetics of GaN

    NASA Astrophysics Data System (ADS)

    Moseley, Michael; Billingsley, Daniel; Henderson, Walter; Trybus, Elaissa; Doolittle, W. Alan

    2009-07-01

    An in-depth model for the transient behavior of metal atoms adsorbed on the surface of GaN is developed. This model is developed by qualitatively analyzing transient reflection high energy electron diffraction (RHEED) signals, which were recorded for a variety of growth conditions of GaN grown by molecular-beam epitaxy (MBE) using metal-modulated epitaxy (MME). Details such as the initial desorption of a nitrogen adlayer and the formation of the Ga monolayer, bilayer, and droplets are monitored using RHEED and related to Ga flux and shutter cycles. The suggested model increases the understanding of the surface kinetics of GaN, provides an indirect method of monitoring the kinetic evolution of these surfaces, and introduces a novel method of in situ growth rate determination.

  19. Vacancy-hydrogen complexes in ammonothermal GaN

    NASA Astrophysics Data System (ADS)

    Tuomisto, F.; Kuittinen, T.; Zając, M.; Doradziński, R.; Wasik, D.

    2014-10-01

    We have applied positron annihilation spectroscopy to study in-grown vacancy defects in bulk GaN crystals grown by the ammonothermal method. We observe a high concentration of Ga vacancy related defects in n-type samples with varying free electron and oxygen content. The positron lifetimes found in these samples suggest that the Ga vacancies are complexed with hydrogen impurities. The number of hydrogen atoms in each vacancy decreases with increasing free electron concentration and oxygen and hydrogen content. The local vibrational modes observed in infrared absorption support this conclusion. Growth of high-quality ammonothermal GaN single crystals with varying electron concentrations. Identification of defect complexes containing a Ga vacancy and 1 or more hydrogen atoms, and possibly O. These vacancy complexes provide a likely explanation for electrical compensation in ammonothermal GaN.

  20. Temperature dependent growth of GaN nanowires using CVD technique

    SciT

    Kumar, Mukesh, E-mail: mukeshjihrnp@gmail.com; Singh, R.; Kumar, Vikram

    2016-05-23

    Growth of GaN nanowires have been carried out on sapphire substrates with Au as a catalyst using chemical vapour deposition technique. GaN nanowires growth have been studied with the experimental parameter as growth temperature. Diameter of grown GaN nanowires are in the range of 50 nm to 100 nm while the nanowire length depends on growth temperature. Morphology of the GaN nanowires have been studied by scanning electron microscopy. Crystalline nature has been observed by XRD patterns. Optical properties of grown GaN nanowires have been investigated by photoluminescence spectra.

  1. Conductivity based on selective etch for GaN devices and applications thereof

    DOEpatents

    Zhang, Yu; Sun, Qian; Han, Jung

    2015-12-08

    This invention relates to methods of generating NP gallium nitride (GaN) across large areas (>1 cm.sup.2) with controlled pore diameters, pore density, and porosity. Also disclosed are methods of generating novel optoelectronic devices based on porous GaN. Additionally a layer transfer scheme to separate and create free-standing crystalline GaN thin layers is disclosed that enables a new device manufacturing paradigm involving substrate recycling. Other disclosed embodiments of this invention relate to fabrication of GaN based nanocrystals and the use of NP GaN electrodes for electrolysis, water splitting, or photosynthetic process applications.

  2. A DFT study on NEA GaN photocathode with an ultrathin n-type Si-doped GaN cap layer

    NASA Astrophysics Data System (ADS)

    Xia, Sihao; Liu, Lei; Kong, Yike; Diao, Yu

    2016-10-01

    Due to the drawbacks of conventional negative electron affinity (NEA) GaN photocathodes activated by Cs or Cs/O, a new-type NEA GaN photocathodes with heterojunction surface dispense with Cs activation are proposed. This structure can be obtained through the coverage of an ultrathin n-type Si-doped GaN cap layer on the p-type Mg-doped GaN emission layer. The influences of the cap layer on the photocathode are calculated using DFT. This study indicates that the n-type cap layer can promote the photoemission characteristics of GaN photocathode and demonstrates the probability of the preparation of a NEA GaN photocathode with an n-type cap layer.

  3. Electrical properties of polycrystalline GaN films functionalized with cysteine and stabilization of GaN nanoparticles in aqueous media.

    PubMed

    Arízaga, Gregorio Guadalupe Carbajal; Oviedo, Mariana J; López, Oscar Edel Contreras

    2012-10-01

    GaN was synthesized onto sapphire substrates by chemical vapor deposition, reacting gallium, ammonium chloride and ammonia. The polycrystalline films were immersed in glycine, aspartic acid and cysteine solutions. Cysteine chemisorbed onto GaN films produced detectable changes in conductivity, mobility and Hall coefficient indicating that GaN is capable of detecting and reacting with thiolate groups, which was confirmed by X-ray photoelectron spectroscopy. The Cys-GaN film solution was adjusted to pH 10, upon which the GaN nanoparticles were transferred to the aqueous phase forming a suspension stable for seven days. The alkaline colloid was then further adjusted down to pH 3 retaining stability for three days. The GaN colloid obtained represents a suitable medium to study GaN properties for biological applications. Copyright © 2012 Elsevier B.V. All rights reserved.

  4. Mg doping of GaN by molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Lieten, R. R.; Motsnyi, V.; Zhang, L.; Cheng, K.; Leys, M.; Degroote, S.; Buchowicz, G.; Dubon, O.; Borghs, G.

    2011-04-01

    We present a systematic study on the influence of growth conditions on the incorporation and activation of Mg in GaN layers grown by plasma-assisted molecular beam epitaxy. We show that high quality p-type GaN layers can be obtained on GaN-on-silicon templates. The Mg incorporation and the electrical properties have been investigated as a function of growth temperature, Ga : N flux ratio and Mg : Ga flux ratio. It was found that the incorporation of Mg and the electrical properties are highly sensitive to the Ga : N flux ratio. The highest hole mobility and lowest resistivity were achieved for slightly Ga-rich conditions. In addition to an optimal Ga : N ratio, an optimum Mg : Ga flux ratio was also observed at around 1%. We observed a clear Mg flux window for p-type doping of GaN : 0.31% < Mg : Ga < 5.0%. A lowest resistivity of 0.98 Ω cm was obtained for optimized growth conditions. The p-type GaN layer then showed a hole concentration of 4.3 × 1017 cm-3 and a mobility of 15 cm2 V-1 s-1. Temperature-dependent Hall effect measurements indicate an acceptor depth in these samples of 100 meV for a hole concentration of 5.5 × 1017 cm-3. The corresponding Mg concentration is 5 × 1019 cm-3, indicating approximately 1% activation at room temperature. In addition to continuous growth of Mg-doped GaN layers we also investigated different modulated growth procedures. We show that a modulated growth procedure has only limited influence on Mg doping at a growth temperature of 800 °C or higher. This result is thus in contrast to previously reported GaN : Mg doping at much lower growth temperatures of 500 °C.

  5. Controlled growth of ordered nanopore arrays in GaN.

    PubMed

    Wildeson, Isaac H; Ewoldt, David A; Colby, Robert; Stach, Eric A; Sands, Timothy D

    2011-02-09

    High-quality, ordered nanopores in semiconductors are attractive for numerous biological, electrical, and optical applications. Here, GaN nanorods with continuous pores running axially through their centers were grown by organometallic vapor phase epitaxy. The porous nanorods nucleate on an underlying (0001)-oriented GaN film through openings in a SiN(x) template that are milled by a focused ion beam, allowing direct placement of porous nanorods. Nanopores with diameters ranging from 20-155 nm were synthesized with crystalline sidewalls.

  6. Surface cleaning for negative electron affinity GaN photocathode

    NASA Astrophysics Data System (ADS)

    Qiao, Jianliang; Yin, Yingpeng; Gao, Youtang; Niu, Jun; Qian, Yunsheng; Chang, Benkang

    2012-10-01

    In the preparation process for negative electron affinity (NEA) GaN photocathode, the surface cleanness is very important to activation, it influences the sensitivity and stability of NEA GaN photocathode. The traditional corrosion methods based on oxidizing and dissolving can't remove oxygen (O) and carbon (C) on GaN surface effectively. How to get an ideal atom clean surface is still an important question at present. The cleaning techniques for GaN photocathode was studied by using NEA photocathode activation system and XPS surface analysis system. The experiment sample is p-type GaN doped with Mg, doped concentration is 1.37×1017 cm-3, the transfer rate is 3.08 cm2/V-S, and the thickness of activation layer is 0.51 μm, the substrate is 300 μm thick sapphire. The sample was dealed with chemical cleaning depuration at first. And to get the atom clean surface, the vacuum heat cleaning process was needed. The methods of chemical cleaning and the vacuum heating cleaning were given in detail. According to the X-ray photoelectron spectroscopy of GaN surface after chemical cleaning and the vacuum degree curve of the activation chamber during the heat cleaning, the cleaning effect and the cleaning mechanism were discussed. After the effective chemical cleaning and the heating of 700 Centigrade degree about 20 minutes in ultrahigh vacuum system, the oxides and carbon contaminants on cathode surface can be removed effectively, and the ideal atom clean surface can be obtained. The purpose of heating depuration process is that not only to get the atom clean GaN surface, but also to guarantee the contents of Ga, N on GaN surface stabilize and to keep the system ultra-high vacuum degree. Because of the volatilization of oxide and carbon impurity on the cathode surface, the vacuum degree curve drops with the rising of temperature on the whole.

  7. GaN epitaxial layers grown on multilayer graphene by MOCVD

    NASA Astrophysics Data System (ADS)

    Li, Tianbao; Liu, Chenyang; Zhang, Zhe; Yu, Bin; Dong, Hailiang; Jia, Wei; Jia, Zhigang; Yu, Chunyan; Gan, Lin; Xu, Bingshe

    2018-04-01

    In this study, GaN epitaxial layers were successfully deposited on a multilayer graphene (MLG) by using metal-organic chemical vapor deposition (MOCVD). Highly crystalline orientations of the GaN films were confirmed through electron backscatter diffraction (EBSD). An epitaxial relationship between GaN films and MLG is unambiguously established by transmission electron microscope (TEM) analysis. The Raman spectra was used to analyze the internal stress of GaN films, and the spectrum shows residual tensile stress in the GaN films. Moreover, the results of the TEM analysis and Raman spectra indicate that the high quality of the MLG substrate is maintained even after the growth of the GaN film. This high-quality MLG makes it possible to easily remove epitaxial layers from the supporting substrate by micro-mechanical exfoliation technology. This work can aid in the development of transferable devices using GaN films.

  8. The origin of the residual conductivity of GaN films on ferroelectric materials

    NASA Astrophysics Data System (ADS)

    Lee, Kyoung-Keun; Cai, Zhuhua; Ziemer, Katherine; Doolittle, William Alan

    2009-08-01

    In this paper, the origin of the conductivity of GaN films grown on ferroelectric materials was investigated using XPS, AES, and XRD analysis tools. Depth profiles confirmed the existence of impurities in the GaN film originating from the substrates. Bonding energy analysis from XPS and AES verified that oxygen impurities from the substrates were the dominant origin of the conductivity of the GaN film. Furthermore, Ga-rich GaN films have a greater chance of enhancing diffusion of lithium oxide from the substrates, resulting in more substrate phase separation and a wider inter-mixed region confirmed by XRD. Therefore, the direct GaN film growth on ferroelectric materials causes impurity diffusion from the substrates, resulting in highly conductive GaN films. Future work needs to develop non-conductive buffer layers for impurity suppression in order to obtain highly resistive GaN films.

  9. Breast reconstruction - implants

    MedlinePlus

    Breast implants surgery; Mastectomy - breast reconstruction with implants; Breast cancer - breast reconstruction with implants ... it harder to find a tumor if your breast cancer comes back. Getting breast implants does not take ...

  10. Applicability of Rayleigh–Gans Scattering to Spherical Particles

    SciT

    Kerker, M.; Farone, W. A.; Matijevic, E.

    1963-01-01

    Exact computations of scattering functions for spheres are compared with those obtained from the Rayleigh-Gans theory of scattering of electromagnetic radiation. The range of validity for spheres as a guide for non-homogeneous particles and other geometries. This study is limited to non-absorbing particles with real indices of refraction. (C.E.S.)

  11. Basic ammonothermal GaN growth in molybdenum capsules

    NASA Astrophysics Data System (ADS)

    Pimputkar, S.; Speck, J. S.; Nakamura, S.

    2016-12-01

    Single crystal, bulk gallium nitride (GaN) crystals were grown using the basic ammonothermal method in a high purity growth environment created using a non-hermetically sealed molybdenum (Mo) capsule and compared to growths performed in a similarly designed silver (Ag) capsule and capsule-free René 41 autoclave. Secondary ion mass spectrometry (SIMS) analysis revealed transition metal free (<1×1017 cm-3) GaN crystals. Anomalously low oxygen concentrations ((2-6)×1018 cm-3) were measured in a {0001} seeded crystal boule grown using a Mo capsule, despite higher source material oxygen concentrations ((1-5)×1019 cm-3) suggesting that molybdenum (or molybdenum nitrides) may act to getter oxygen under certain conditions. Total system pressure profiles from growth runs in a Mo capsule system were comparable to those without a capsule, with pressures peaking within 2 days and slowly decaying due to hydrogen diffusional losses. Measured Mo capsule GaN growth rates were comparable to un-optimized growth rates in capsule-free systems and appreciably slower than in Ag-capsule systems. Crystal quality replicated that of the GaN seed crystals for all capsule conditions, with high quality growth occurring on the (0001) Ga-face. Optical absorption and impurity concentration characterization suggests reduced concentrations of hydrogenated gallium vacancies (VGa-Hx).

  12. Thermal quenching of the yellow luminescence in GaN

    NASA Astrophysics Data System (ADS)

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

    2018-04-01

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

  13. GaN and ZnO nanostructures

    NASA Astrophysics Data System (ADS)

    Fündling, Sönke; Sökmen, Ünsal; Behrends, Arne; Al-Suleiman, Mohamed Aid Mansur; Merzsch, Stephan; Li, Shunfeng; Bakin, Andrey; Wehmann, Hergo-Heinrich; Waag, Andreas; Lähnemann, Jonas; Jahn, Uwe; Trampert, Achim; Riechert, Henning

    2010-07-01

    GaN and ZnO are both wide band gap semiconductors with interesting properties concerning optoelectronic and sensor device applications. Due to the lack or the high costs of native substrates, alternatives like sapphire, silicon, or silicon carbide are taken, but the resulting lattice and thermal mismatches lead to increased defect densities which reduce the material quality. In contrast, nanostructures with high aspect ratio have lower defect densities as compared to layers. In this work, we give an overview on our results achieved on both ZnO as well as GaN based nanorods. ZnO nanostructures were grown by a wet chemical approach as well as by VPT on different substrates - even on flexible polymers. To compare the growth results we analyzed the structures by XRD and PL and show possible device applications. The GaN nano- and microstructures were grown by metal organic vapor phase epitaxy either in a self- organized process or by selective area growth for a better control of shape and material composition. Finally we take a look onto possible device applications, presenting our attempts, e.g., to build LEDs based on GaN nanostructures.

  14. Ultradeep electron cyclotron resonance plasma etching of GaN

    DOE PAGES

    Harrison, Sara E.; Voss, Lars F.; Torres, Andrea M.; ...

    2017-07-25

    Here, ultradeep (≥5 μm) electron cyclotron resonance plasma etching of GaN micropillars was investigated. Parametric studies on the influence of the applied radio-frequency power, chlorine content in a Cl 2/Ar etch plasma, and operating pressure on the etch depth, GaN-to-SiO 2 selectivity, and surface morphology were performed. Etch depths of >10 μm were achieved over a wide range of parameters. Etch rates and sidewall roughness were found to be most sensitive to variations in RF power and % Cl 2 in the etch plasma. Selectivities of >20:1 GaN:SiO 2 were achieved under several chemically driven etch conditions where a maximummore » selectivity of ~39:1 was obtained using a 100% Cl 2 plasma. The etch profile and (0001) surface morphology were significantly influenced by operating pressure and the chlorine content in the plasma. Optimized etch conditions yielded >10 μm tall micropillars with nanometer-scale sidewall roughness, high GaN:SiO 2 selectivity, and nearly vertical etch profiles. These results provide a promising route for the fabrication of ultradeep GaN microstructures for use in electronic and optoelectronic device applications. In addition, dry etch induced preferential crystallographic etching in GaN microstructures is also demonstrated, which may be of great interest for applications requiring access to non- or semipolar GaN surfaces.« less

  15. Properties of Epitaxial GaN on Refractory Metal Substrates

    DTIC Science & Technology

    2007-02-28

    Rowland Aymont Technology, Inc., Scotia, New York 12302 Jihyun Kim College of Engineering, Korea University, Seoul 136-701, South Korea Mohammad Fatemi...M. Li, D. Wang, C. Ahyi, C.-C. Tin, J. Williams, and M. Park , Appl. Phys. Lett. 88, 113509 2001. FIG. 4. I-V characteristic of the GaN film on 111

  16. Refractive index of erbium doped GaN thin films

    SciT

    Alajlouni, S.; Sun, Z. Y.; Li, J.

    2014-08-25

    GaN is an excellent host for erbium (Er) to provide optical emission in the technologically important as well as eye-safe 1540 nm wavelength window. Er doped GaN (GaN:Er) epilayers were synthesized on c-plane sapphire substrates using metal organic chemical vapor deposition. By employing a pulsed growth scheme, the crystalline quality of GaN:Er epilayers was significantly improved over those obtained by conventional growth method of continuous flow of reaction precursors. X-ray diffraction rocking curve linewidths of less than 300 arc sec were achieved for the GaN (0002) diffraction peak, which is comparable to the typical results of undoped high quality GaN epilayers andmore » represents a major improvement over previously reported results for GaN:Er. Spectroscopic ellipsometry was used to determine the refractive index of the GaN:Er epilayers in the 1540 nm wavelength window and a linear dependence on Er concentration was found. The observed refractive index increase with Er incorporation and the improved crystalline quality of the GaN:Er epilayers indicate that low loss GaN:Er optical waveguiding structures are feasible.« less

  17. Use of hydrogen etching to remove existing dislocations in GaN epitaxial layers

    NASA Astrophysics Data System (ADS)

    Yeh, Yen-Hsien; Chu, Chung-Ming; Wu, Yin-Hao; Hsu, Ying-Chia; Yu, Tzu-Yi; Lee, Wei-I.

    2015-08-01

    In this paper, based on the anisotropic nature of hydrogen (H2) etching on GaN, we describe a new approach to the removal of threading dislocations in GaN layers. The top surfaces of c-plane (Ga-face) and a-plane GaNs are considered stable in H2; therefore, H2 etches only crystal imperfections such as dislocation and basal plane stacking fault (BSF) sites. We used H2 to etch undoped c-plane GaN, n-type c-plane GaN, a-plane GaN, and an InGaN/GaN multiple quantum well structure. Several examinations were performed, indicating deep cavities on the c-plane GaN samples after H2 etching; furthermore, gorge-like grooves were observed on the a-plane GaN samples. The deep cavities on the c-plane GaN were considered the etched dislocation sites, and the gorge-like grooves on the a-plane GaN were considered the etched BSF sites. Photoluminescence measurements were performed and the results indicated that the H2-etched samples demonstrate superior optoelectronic properties, probably because of the elimination of dislocations.

  18. Morphological evolution and characterization of GaN pyramid arrays fabricated by photo-assisted chemical etching

    NASA Astrophysics Data System (ADS)

    Zhang, Shiying; Xiu, Xiangqian; Xu, Qingjun; Li, Yuewen; Hua, Xuemei; Chen, Peng; Xie, Zili; Liu, Bin; Zhou, Yugang; Han, Ping; Zhang, Rong; Zheng, Youdou

    2016-12-01

    GaN pyramid arrays have been successfully synthesized by selective photo-assisted chemical etching in a K2S2O8/KOH solution. A detailed analysis of time evolution of surface morphology has been conducted, which describes an etching process of GaN pyramids. Room temperature cathodoluminescence images indicate that these pyramids are composed of crystalline GaN surrounding dislocations, which is caused by the greater recombination rate of electrons and holes at dislocation than that of crystalline GaN. The Raman results show a stress relaxation in GaN pyramids compared with unetched GaN. The optical property of both unetched GaN and GaN pyramids has been studied by photoluminescence. The formation mechanism and feature of GaN pyramids are also rationally explained.

  19. Substitutional and interstitial oxygen in wurtzite GaN

    NASA Astrophysics Data System (ADS)

    Wright, A. F.

    2005-11-01

    Density-functional theory was used to compute energy-minimum configurations and formation energies of substitutional and interstitial oxygen (O) in wurtzite GaN. The results indicate that O substituted at a N site (ON) acts as a single donor with the ionized state (ON+1) being the most stable O state in p-type GaN. In n-type GaN, interstitial O (OI) is predicted to be a double acceptor and O substituted at a Ga site (OGa) is predicted to be a triple acceptor. The formation energies of these two species are comparable to that of ON in n-type GaN and, as such, they should form and compensate the ON donors. The extent of compensation was estimated for both Ga-rich and N-rich conditions with a total O concentration of 1017cm-3. Ga-rich conditions yielded negligible compensation and an ON concentration in excess of 9.9×1016cm-3. N-rich conditions yielded a 25% lower ON concentration, due to the increased stability of OI and OGa relative to ON, and moderate compensation. These findings are consistent with experimental results indicating that O acts as a donor in GaN(O). Complexes of ON with the Mg acceptor and OI with the Si donor were examined. Binding energies for charge-conserving reactions were ⩾0.5eV, indicating that these complexes can exist in equilibrium at room temperature. Complexes of ON with the Ga vacancy in n-type GaN were also examined and their binding energies were 1.2 and 1.4eV, indicating that appreciable concentrations can exist in equilibrium even at elevated temperatures.

  20. Ground Albedo Neutron Sensing (GANS) for Measurement of Integral Soil Water Content at the Small Catchment Scale

    NASA Astrophysics Data System (ADS)

    Rivera Villarreyes, C.; Baroni, G.; Oswald, S. E.

    2012-12-01

    to precipitation events through summer and autumn, but soil water content estimations were affected by water stored in snow and partly biomass. Thus, when calibration parameters were transferred to different crops (e.g. from sunflower to rye), the changes in biomass water will have to be considered. Finally, these results imply that GANS measurements can be a reliable ground-truthing possibility as well as additional constraint for hydrological models. References (1) Rivera Villarreyes, C.A., Baroni, G., and Oswald, S.E. (2011): Integral quantification of seasonal soil moisture changes in farmland by cosmic-ray neutrons, Hydrol. Earth Syst. Sci., 15, 3843-3859. (2) Rivera Villarreyes, C.A., Baroni, G., and Oswald, S.E. (2012): Evaluation of the Ground Albedo Neutron Sensing (GANS) method for soil moisture estimations in different crop fields (in preparation for Hydrological Processes). (3) Zreda, M., Shuttleworth, W.J., Zeng, X., Zweck, C., Desilets, D., Franz, T., Rosolem, R., and Ferre, T.P.A. (2012): COSMOS: The COsmic-ray Soil Moisture Observing System. Hydrol. Earth Syst. Sci. Discuss., 9, 4505-4551.

  1. Total photoelectron yield spectroscopy of energy distribution of electronic states density at GaN surface and SiO2/GaN interface

    NASA Astrophysics Data System (ADS)

    Ohta, Akio; Truyen, Nguyen Xuan; Fujimura, Nobuyuki; Ikeda, Mitsuhisa; Makihara, Katsunori; Miyazaki, Seiichi

    2018-06-01

    The energy distribution of the electronic state density of wet-cleaned epitaxial GaN surfaces and SiO2/GaN structures has been studied by total photoelectron yield spectroscopy (PYS). By X-ray photoelectron spectroscopy (XPS) analysis, the energy band diagram for a wet-cleaned epitaxial GaN surface such as the energy level of the valence band top and electron affinity has been determined to obtain a better understanding of the measured PYS signals. The electronic state density of GaN surface with different carrier concentrations in the energy region corresponding to the GaN bandgap has been evaluated. Also, the interface defect state density of SiO2/GaN structures was also estimated by not only PYS analysis but also capacitance–voltage (C–V) characteristics. We have demonstrated that PYS analysis enables the evaluation of defect state density filled with electrons at the SiO2/GaN interface in the energy region corresponding to the GaN midgap, which is difficult to estimate by C–V measurement of MOS capacitors.

  2. Large electron capture-cross-section of the major nonradiative recombination centers in Mg-doped GaN epilayers grown on a GaN substrate

    NASA Astrophysics Data System (ADS)

    Chichibu, S. F.; Shima, K.; Kojima, K.; Takashima, S.; Edo, M.; Ueno, K.; Ishibashi, S.; Uedono, A.

    2018-05-01

    Complementary time-resolved photoluminescence and positron annihilation measurements were carried out at room temperature on Mg-doped p-type GaN homoepitaxial films for identifying the origin and estimating the electron capture-cross-section ( σ n ) of the major nonradiative recombination centers (NRCs). To eliminate any influence by threading dislocations, free-standing GaN substrates were used. In Mg-doped p-type GaN, defect complexes composed of a Ga-vacancy (VGa) and multiple N-vacancies (VNs), namely, VGa(VN)2 [or even VGa(VN)3], are identified as the major intrinsic NRCs. Different from the case of 4H-SiC, atomic structures of intrinsic NRCs in p-type and n-type GaN are different: VGaVN divacancies are the major NRCs in n-type GaN. The σ n value approximately the middle of 10-13 cm2 is obtained for VGa(VN)n, which is larger than the hole capture-cross-section (σp = 7 × 10-14 cm2) of VGaVN in n-type GaN. Combined with larger thermal velocity of an electron, minority carrier lifetime in Mg-doped GaN becomes much shorter than that of n-type GaN.

  3. Earth Observation

    2014-06-01

    ISS040-E-006327 (1 June 2014) --- A portion of International Space Station solar array panels and Earth?s horizon are featured in this image photographed by an Expedition 40 crew member on the space station.

  4. Optical properties of P ion implanted ZnO

    NASA Astrophysics Data System (ADS)

    Pong, Bao-Jen; Chou, Bo-Wei; Pan, Ching-Jen; Tsao, Fu-Chun; Chi, Gou-Chung

    2006-02-01

    Red and green emissions are observed from P ion implanted ZnO. Red emission at ~680 nm (1.82 eV) is associated with the donor-acceptor pair (DAP) transition, where the corresponding donor and acceptor are interstitial zinc (Zn i) and interstitial oxygen (O i), respectively. Green emission at ~ 516 nm (2.40 eV) is associated with the transition between the conduction band and antisite oxygen (O Zn). Green emission at ~516nm (2.403 eV) was observed for ZnO annealed at 800 oC under ambient oxygen, whereas, it was not visible when it was annealed in ambient nitrogen. Hence, the green emission is most likely not related to oxygen vacancies on ZnO sample, which might be related to the cleanliness of ZnO surface, a detailed study is in progress. The observed micro-strain is larger for N ion implanted ZnO than that for P ion implanted ZnO. It is attributed to the larger straggle of N ion implanted ZnO than that of P ion implanted ZnO. Similar phenomenon is also observed in Be and Mg ion implanted GaN.

  5. Enhanced light extraction efficiency of GaN-based light-emittng diodes by nitrogen implanted current blocking layer

    SciT

    Kim, Yong Deok; Oh, Seung Kyu; Park, Min Joo

    Highlights: • A nitrogen implanted current-blocking layer was successfully demonstrated. • Light-extraction efficiency and radiant intensity was increased by more than 20%. • Ion implantation was successfully implemented in GaN based light-emitting diodes. - Abstract: GaN-based light emitting diodes (LEDs) with a nitrogen implanted current-blocking layer (CBL) were successfully demonstrated for improving the light extraction efficiency (LEE) and radiant intensity. The LEE and radiant intensity of the LEDs with a shallow implanted CBL with nitrogen was greatly increased by more than 20% compared to that of a conventional LED without the CBL due to an increase in the effective currentmore » path, which reduces light absorption at the thick p-pad electrode. Meanwhile, deep implanted CBL with a nitrogen resulted in deterioration of the LEE and radiant intensity because of formation of crystal damage, followed by absorption of the light generated at the multi-quantum well(MQW). These results clearly suggest that ion implantation method, which is widely applied in the fabrication of Si based devices, can be successfully implemented in the fabrication of GaN based LEDs by optimization of implanted depth.« less

  6. Chemical lift-off of (11-22) semipolar GaN using periodic triangular cavities

    NASA Astrophysics Data System (ADS)

    Jeon, Dae-Woo; Lee, Seung-Jae; Jeong, Tak; Baek, Jong Hyeob; Park, Jae-Woo; Jang, Lee-Woon; Kim, Myoung; Lee, In-Hwan; Ju, Jin-Woo

    2012-01-01

    Chemical lift-off of (11-22) semipolar GaN using triangular cavities was investigated. The (11-22) semipolar GaN was grown using epitaxial lateral overgrowth by metal-organic chemical vapor deposition on m-plane sapphire, in such a way as to keep N terminated surface of c-plane GaN exposed in the cavities. After regrowing 300 μm thick (11-22) semipolar GaN by hydride vapor phase epitaxy for a free-standing (11-22) semipolar GaN substrate, the triangular cavities of the templates were chemically etched in molten KOH. The (000-2) plane in the triangular cavities can be etched in the [0002] direction with the high lateral etching rate of 196 μm/min. The resulting free-standing (11-22) semipolar GaN substrate was confirmed to be strain-free by the Raman analysis.

  7. Graphene oxide assisted synthesis of GaN nanostructures for reducing cell adhesion.

    PubMed

    Yang, Rong; Zhang, Ying; Li, Jingying; Han, Qiusen; Zhang, Wei; Lu, Chao; Yang, Yanlian; Dong, Hongwei; Wang, Chen

    2013-11-21

    We report a general approach for the synthesis of large-scale gallium nitride (GaN) nanostructures by the graphene oxide (GO) assisted chemical vapor deposition (CVD) method. A modulation effect of GaN nanostructures on cell adhesion has been observed. The morphology of the GaN surface can be controlled by GO concentrations. This approach, which is based on the predictable choice of the ratio of GO to catalysts, can be readily extended to the synthesis of other materials with controllable nanostructures. Cell studies show that GaN nanostructures reduced cell adhesion significantly compared to GaN flat surfaces. The cell-repelling property is related to the nanostructure and surface wettability. These observations of the modulation effect on cell behaviors suggest new opportunities for novel GaN nanomaterial-based biomedical devices. We believe that potential applications will emerge in the biomedical and biotechnological fields.

  8. Lattice distortions in GaN on sapphire using the CBED-HOLZ technique.

    PubMed

    Sridhara Rao, D V; McLaughlin, K; Kappers, M J; Humphreys, C J

    2009-09-01

    The convergent beam electron diffraction (CBED) methodology was developed to investigate the lattice distortions in wurtzite gallium nitride (GaN) from a single zone-axis pattern. The methodology enabled quantitative measurements of lattice distortions (alpha, beta, gamma and c) in transmission electron microscope (TEM) specimens of a GaN film grown on (0,0,0,1) sapphire by metal-organic vapour-phase epitaxy. The CBED patterns were obtained at different distances from the GaN/sapphire interface. The results show that GaN is triclinic above the interface with an increased lattice parameter c. At 0.85 microm from the interface, alpha=90 degrees , beta=8905 degrees and gamma=11966 degrees . The GaN lattice relaxes steadily back to hexagonal further away from the sapphire substrate. The GaN distortions are mainly confined to the initial stages of growth involving the growth and the coalescence of 3D GaN islands.

  9. Growth of GaN micro/nanolaser arrays by chemical vapor deposition.

    PubMed

    Liu, Haitao; Zhang, Hanlu; Dong, Lin; Zhang, Yingjiu; Pan, Caofeng

    2016-09-02

    Optically pumped ultraviolet lasing at room temperature based on GaN microwire arrays with Fabry-Perot cavities is demonstrated. GaN microwires have been grown perpendicularly on c-GaN/sapphire substrates through simple catalyst-free chemical vapor deposition. The GaN microwires are [0001] oriented single-crystal structures with hexagonal cross sections, each with a diameter of ∼1 μm and a length of ∼15 μm. A possible growth mechanism of the vertical GaN microwire arrays is proposed. Furthermore, we report room-temperature lasing in optically pumped GaN microwire arrays based on the Fabry-Perot cavity. Photoluminescence spectra exhibit lasing typically at 372 nm with an excitation threshold of 410 kW cm(-2). The result indicates that these aligned GaN microwire arrays may offer promising prospects for ultraviolet-emitting micro/nanodevices.

  10. Ohmic contacts to n-GaN formed by ion-implanted Si into p-GaN

    NASA Astrophysics Data System (ADS)

    Bao, Xichang; Xu, Jintong; Zhang, Wenjing; Wang, Ling; Chu, Kaihui; Li, Chao; Li, Xiangyang

    2009-07-01

    In this paper, we report the ohmic contact to n-GaN fabricated by implanting silicon into Mg-doped GaN using an alloy of Ti/Al/Ti/Au metallization. The used materials were grown on (001) sapphire substrates by metal-organic chemical-vapor deposition (MOCVD). The layer structure was comprised of a GaN buffer layer and followed by a 2 μm thickness Mg-doped GaN (Na=5×1017cm-3) and then double silicon implantation was performed in order to convert p-type GaN into n-type GaN films. The as-implanted samples were then thermal annealed at 1150 °C for 5 min in N2 ambient. The carrier concentration and Hall mobility were 3.13×1018 cm3 and 112 cm2/ (VÂ.s) measured by Hall method. Multilayer electrode of Ti (50 nm)/Al (50 nm)/Ti (30 nm)/Au (30 nm) was deposited on n-GaN using an electron-beam evaporation and contacts were formed by a N2 annealing technique ranging from 600 to 900 °C. After annealing lower than 700 °C, the contacts exhibited a rectifying behavior and became ohmic contact only after high temperature processes (>=700 °C). Specific contact resistance was as low as 9.58×10-4 ΩÂ.cm2 after annealing at 800 °C for 60 seconds. While annealing temperature is higher than 800 °C, the specific contact resistance becomes worse. This phenomenon is caused by the surface morphology degradation.

  11. Growth of Single Crystals and Fabrication of GaN and AlN Wafers

    DTIC Science & Technology

    2006-03-01

    Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Synthesis and Decomposition of Ammonia ", 4, Elsevier Scientific Publishing Company...Solid Surfaces and Heterogeneous Catalysis, Synthesis and Decomposition of Ammonia ", 4, Elsevier Scientific Publishing Company, Amsterdam (1982). 119...GaN(s), (2) Ga(g) + _ N2(g) = GaN(s) 93 APPENDIX C: AMMONIA DECOMPOSITION Despite the apparent simplicity of the GaN synthesis from elemental Ga and

  12. Epitaxy of GaN in high aspect ratio nanoscale holes over silicon substrate

    NASA Astrophysics Data System (ADS)

    Wang, Kejia; Wang, Anqi; Ji, Qingbin; Hu, Xiaodong; Xie, Yahong; Sun, Ying; Cheng, Zhiyuan

    2017-12-01

    Dislocation filtering in gallium nitride (GaN) by epitaxial growth through patterned nanoscale holes is studied. GaN grown from extremely high aspect ratio holes by metalorganic chemical vapor deposition is examined by transmission electron microscopy and high-resolution transmission electron microscopy. This selective area epitaxial growth method with a reduced epitaxy area and an increased depth to width ratio of holes leads to effective filtering of dislocations within the hole and improves the quality of GaN significantly.

  13. Short Implants: New Horizon in Implant Dentistry.

    PubMed

    Jain, Neha; Gulati, Manisha; Garg, Meenu; Pathak, Chetan

    2016-09-01

    The choice of implant length is an essential factor in deciding the survival rates of these implants and the overall success of the prosthesis. Placing an implant in the posterior part of the maxilla and mandible has always been very critical due to poor bone quality and quantity. Long implants can be placed in association with complex surgical procedures such as sinus lift and bone augmentation. These techniques are associated with higher cost, increased treatment time and greater morbidity. Hence, there is need for a less invasive treatment option in areas of poor bone quantity and quality. Data related to survival rates of short implants, their design and prosthetic considerations has been compiled and structured in this manuscript with emphasis on the indications, advantages of short implants and critical biomechanical factors to be taken into consideration when choosing to place them. Studies have shown that comparable success rates can be achieved with short implants as those with long implants by decreasing the lateral forces to the prosthesis, eliminating cantilevers, increasing implant surface area and improving implant to abutment connection. Short implants can be considered as an effective treatment alternative in resorbed ridges. Short implants can be considered as a viable treatment option in atrophic ridge cases in order to avoid complex surgical procedures required to place long implants. With improvement in the implant surface geometry and surface texture, there is an increase in the bone implant contact area which provides a good primary stability during osseo-integration.

  14. Short Implants: New Horizon in Implant Dentistry

    PubMed Central

    Gulati, Manisha; Garg, Meenu; Pathak, Chetan

    2016-01-01

    The choice of implant length is an essential factor in deciding the survival rates of these implants and the overall success of the prosthesis. Placing an implant in the posterior part of the maxilla and mandible has always been very critical due to poor bone quality and quantity. Long implants can be placed in association with complex surgical procedures such as sinus lift and bone augmentation. These techniques are associated with higher cost, increased treatment time and greater morbidity. Hence, there is need for a less invasive treatment option in areas of poor bone quantity and quality. Data related to survival rates of short implants, their design and prosthetic considerations has been compiled and structured in this manuscript with emphasis on the indications, advantages of short implants and critical biomechanical factors to be taken into consideration when choosing to place them. Studies have shown that comparable success rates can be achieved with short implants as those with long implants by decreasing the lateral forces to the prosthesis, eliminating cantilevers, increasing implant surface area and improving implant to abutment connection. Short implants can be considered as an effective treatment alternative in resorbed ridges. Short implants can be considered as a viable treatment option in atrophic ridge cases in order to avoid complex surgical procedures required to place long implants. With improvement in the implant surface geometry and surface texture, there is an increase in the bone implant contact area which provides a good primary stability during osseo-integration. PMID:27790598

  15. Optical properties of Mg doped p-type GaN nanowires

    NASA Astrophysics Data System (ADS)

    Patsha, Avinash; Pandian, Ramanathaswamy; Dhara, S.; Tyagi, A. K.

    2015-06-01

    Mg doped p-type GaN nanowires are grown using chemical vapor deposition technique in vapor-liquid-solid (VLS) process. Morphological and structural studies confirm the VLS growth process of nanowires and wurtzite phase of GaN. We report the optical properties of Mg doped p-type GaN nanowires. Low temperature photoluminescence studies on as-grown and post-growth annealed samples reveal the successful incorporation of Mg dopants. The as-grwon and annealed samples show passivation and activation of Mg dopants, respectively, in GaN nanowires.

  16. Dislocation-induced nanoparticle decoration on a GaN nanowire.

    PubMed

    Yang, Bing; Yuan, Fang; Liu, Qingyun; Huang, Nan; Qiu, Jianhang; Staedler, Thorsten; Liu, Baodan; Jiang, Xin

    2015-02-04

    GaN nanowires with homoepitaxial decorated GaN nanoparticles on their surface along the radial direction have been synthesized by means of a chemical vapor deposition method. The growth of GaN nanowires is catalyzed by Au particles via the vapor-liquid-solid (VLS) mechanism. Screw dislocations are generated along the radial direction of the nanowires under slight Zn doping. In contrast to the metal-catalyst-assisted VLS growth, GaN nanoparticles are found to prefer to nucleate and grow at these dislocation sites. High-resolution transmission electron microscopy (HRTEM) analysis demonstrates that the GaN nanoparticles possess two types of epitaxial orientation with respect to the corresponding GaN nanowire: (I) [1̅21̅0]np//[1̅21̅0]nw, (0001)np//(0001)nw; (II) [1̅21̅3]np//[12̅10]nw, (101̅0)np//(101̅0)nw. An increased Ga signal in the energy-dispersive spectroscopy (EDS) profile lines of the nanowires suggests GaN nanoparticle growth at the edge surface of the wires. All the crystallographic results confirm the importance of the dislocations with respect to the homoepitaxial growth of the GaN nanoparticles. Here, screw dislocations situated on the (0001) plane provide the self-step source to enable nucleation of the GaN nanoparticles.

  17. N-Face GaN Electronics for Heteroepitaxial and Bonded Structures

    DTIC Science & Technology

    2015-08-27

    GaN ! ?" InGaAs’Channel’ InAlAs’ !!!!!S! !!!!!!D! !!!!G! Ga (In)N’Dri2 ’Region! Wafer* Bonded! Junc2on! !!!!!S...Gate InGaAs InAlAs (In, Ga )N Source GaN on Sapphire Aperture CBL WBI InGaN n-InGaAs InAlAs n+ GaN S D WBI...about. Polarization effects at the interface may need to be considered. For Ga -polar InGaN- GaN homojunctions,

  18. Structural defects in GaN revealed by Transmission Electron Microscopy

    SciT

    Liliental-Weber, Zuzanna

    This paper reviews the various types of structural defects observed by Transmission Electron Microscopy in GaN heteroepitaxial layers grown on foreign substrates and homoepitaxial layers grown on bulk GaN substrates. The structural perfection of these layers is compared to the platelet self-standing crystals grown by High Nitrogen Pressure Solution. Defects in undoped and Mg doped GaN are discussed. Lastly, some models explaining the formation of inversion domains in heavily Mg doped layers that are possible defects responsible for the difficulties of p-doping in GaN are also reviewed.

  19. Structural defects in GaN revealed by Transmission Electron Microscopy

    DOE PAGES

    Liliental-Weber, Zuzanna

    2014-09-08

    This paper reviews the various types of structural defects observed by Transmission Electron Microscopy in GaN heteroepitaxial layers grown on foreign substrates and homoepitaxial layers grown on bulk GaN substrates. The structural perfection of these layers is compared to the platelet self-standing crystals grown by High Nitrogen Pressure Solution. Defects in undoped and Mg doped GaN are discussed. Lastly, some models explaining the formation of inversion domains in heavily Mg doped layers that are possible defects responsible for the difficulties of p-doping in GaN are also reviewed.

  20. Characterizations of GaN film growth by ECR plasma chemical vapor deposition

    NASA Astrophysics Data System (ADS)

    Fu, Silie; Chen, Junfang; Zhang, Hongbin; Guo, Chaofen; Li, Wei; Zhao, Wenfen

    2009-06-01

    The electron cyclotron resonance plasma-enhanced metalorganic chemical vapor deposition technology (ECR-MOPECVD) is adopted to grow GaN films on (0 0 0 1) α-Al2O3 substrate. The gas sources are pure N2 and trimethylgallium (TMG). Optical emission spectroscopy (OES) and thermodynamic analysis of GaN growth are applied to understand the GaN growth process. The OES of ECR plasma shows that TMG is significantly dissociated in ECR plasma. Reactants N and Ga in the plasma, obtained easily under the self-heating condition, are essential for the GaN growth. They contribute to the realization of GaN film growth at a relatively low temperature. The thermodynamic study shows that the driving force for the GaN growth is high when N2:TMG>1. Furthermore, higher N2:TMG flow ratio makes the GaN growth easier. Finally, X-ray diffraction, photoluminescence, and atomic force microscope are applied to investigate crystal quality, morphology, and roughness of the GaN films. The results demonstrate that the ECR-MOPECVD technology is favorable for depositing GaN films at low temperatures.

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

    PubMed

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

    2017-12-12

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

  2. Direct growth of freestanding GaN on C-face SiC by HVPE.

    PubMed

    Tian, Yuan; Shao, Yongliang; Wu, Yongzhong; Hao, Xiaopeng; Zhang, Lei; Dai, Yuanbin; Huo, Qin

    2015-06-02

    In this work, high quality GaN crystal was successfully grown on C-face 6H-SiC by HVPE using a two steps growth process. Due to the small interaction stress between the GaN and the SiC substrate, the GaN was self-separated from the SiC substrate even with a small thickness of about 100 μm. Moreover, the SiC substrate was excellent without damage after the whole process so that it can be repeatedly used in the GaN growth. Hot phosphoric acid etching (at 240 °C for 30 min) was employed to identify the polarity of the GaN layer. According to the etching results, the obtained layer was Ga-polar GaN. High-resolution X-ray diffraction (HRXRD) and electron backscatter diffraction (EBSD) were done to characterize the quality of the freestanding GaN. The Raman measurements showed that the freestanding GaN film grown on the C-face 6H-SiC was stress-free. The optical properties of the freestanding GaN layer were determined by photoluminescence (PL) spectra.

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

    PubMed Central

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

    2017-01-01

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

  4. Synthesis and Raman scattering of GaN nanorings, nanoribbons and nanowires

    NASA Astrophysics Data System (ADS)

    Li, Z. J.; Chen, X. L.; Li, H. J.; Tu, Q. Y.; Yang, Z.; Xu, Y. P.; Hu, B. Q.

    Low-dimensional GaN materials, including nanorings, nanoribbons and smooth nanowires have been synthesized by reacting gallium and ammonia using Ag particles as a catalyst on the substrate of MgO single crystals. They were characterized by field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). EDX, XRD indicated that the low-dimensional nanomaterials were wurtzite GaN. New features are found in Raman scatterings for these low-dimensional GaN materials, which are different from the previous observations of GaN materials.

  5. Discover Earth

    NASA Technical Reports Server (NTRS)

    Steele, Colleen

    1998-01-01

    Discover Earth is a NASA-sponsored project for teachers of grades 5-12, designed to: (1) enhance understanding of the Earth as an integrated system; (2) enhance the interdisciplinary approach to science instruction; and (3) provide classroom materials that focus on those goals. Discover Earth is conducted by the Institute for Global Environmental Strategies in collaboration with Dr. Eric Barron, Director, Earth System Science Center, The Pennsylvania State University; and Dr. Robert Hudson, Chair, the Department of Meteorology, University of Maryland at College Park. The enclosed materials: (1) represent only part of the Discover Earth materials; (2) were developed by classroom teachers who are participating in the Discover Earth project; (3) utilize an investigative approach and on-line data; and (4) can be effectively adjusted to classrooms with greater/without technology access. The Discover Earth classroom materials focus on the Earth system and key issues of global climate change including topics such as the greenhouse effect, clouds and Earth's radiation balance, surface hydrology and land cover, and volcanoes and climate change. All the materials developed to date are available on line at (http://www.strategies.org) You are encouraged to submit comments and recommendations about these materials to the Discover Earth project manager, contact information is listed below. You are welcome to duplicate all these materials.

  6. Calculations of acceptor ionization energies in GaN

    NASA Astrophysics Data System (ADS)

    Wang, H.; Chen, A.-B.

    2001-03-01

    The k.p Hamiltonian and a model potential are used to deduce the acceptor ionization energies in GaN from a systematic study of the chemical trend in GaAs, GaP, and InP. The acceptors studied include Be, Mg, Ca, Zn, and Cd on the cation sites and C, Si, and Ge on the anion sites. Our calculated acceptor ionization energies are estimated to be accurate to better than 10% across the board. The ionization energies of C and Be (152 and 187 meV, respectively) in wurtzite GaN are found to be lower than that of Mg (224 meV). The C was found to behave like the hydrogenic acceptor in all systems and it has the smallest ionization energy among all the acceptors studied.

  7. Fermi level dependence of hydrogen diffusivity in GaN

    NASA Astrophysics Data System (ADS)

    Polyakov, A. Y.; Smirnov, N. B.; Pearton, S. J.; Ren, F.; Theys, B.; Jomard, F.; Teukam, Z.; Dmitriev, V. A.; Nikolaev, A. E.; Usikov, A. S.; Nikitina, I. P.

    2001-09-01

    Hydrogen diffusion studies were performed in GaN samples with different Fermi level positions. It is shown that, at 350 °C, hydrogen diffusion is quite fast in heavily Mg doped p-type material with the Fermi level close to Ev+0.15 eV, considerably slower in high-resistivity p-GaN(Zn) with the Fermi level Ev+0.9 eV, while for conducting and semi-insulating n-GaN samples with the Fermi level in the upper half of the band gap no measurable hydrogen diffusion could be detected. For these latter samples it is shown that higher diffusion temperature of 500 °C and longer times (50 h) are necessary to incorporate hydrogen to appreciable depth. These findings are in line with previously published theoretical predictions of the dependence of hydrogen interstitials formation in GaN on the Fermi level position.

  8. Size dictated thermal conductivity of GaN

    DOE PAGES

    Thomas Edwin Beechem; McDonald, Anthony E.; Fuller, Elliot James; ...

    2016-04-01

    The thermal conductivity on n- and p-type doped gallium nitride (GaN) epilayers having thickness of 3-4 μm was investigated using time domain thermoreflectance (TDTR). Despite possessing carrier concentrations ranging across 3 decades (10 15 – 10 18 cm –3), n-type layers exhibit a nearly constant thermal conductivity of 180 W/mK. The thermal conductivity of p-type epilayers, in contrast, reduces from 160 to 110 W/mK with increased doping. These trends–and their overall reduction relative to bulk–are explained leveraging established scattering models where it is shown that size effects play a primary role in limiting thermal conductivity for layers even tens ofmore » microns thick. GaN device layers, even of pristine quality, will therefore exhibit thermal conductivities less than the bulk value of 240 W/mK owing to their finite thickness.« less

  9. Anharmonic phonon decay in cubic GaN

    NASA Astrophysics Data System (ADS)

    Cuscó, R.; Domènech-Amador, N.; Novikov, S.; Foxon, C. T.; Artús, L.

    2015-08-01

    We present a Raman-scattering study of optical phonons in zinc-blende (cubic) GaN for temperatures ranging from 80 to 750 K. The experiments were performed on high-quality, cubic GaN films grown by molecular-beam epitaxy on GaAs (001) substrates. The observed temperature dependence of the optical phonon frequencies and linewidths is analyzed in the framework of anharmonic decay theory, and possible decay channels are discussed in the light of density-functional-theory calculations. The longitudinal-optical (LO) mode relaxation is found to occur via asymmetric decay into acoustic phonons, with an appreciable contribution of higher-order processes. The transverse-optical mode linewidth shows a weak temperature dependence and its frequency downshift is primarily determined by the lattice thermal expansion. The LO phonon lifetime is derived from the observed Raman linewidth and an excellent agreement with previous theoretical predictions is found.

  10. Recovery of GaN surface after reactive ion etching

    NASA Astrophysics Data System (ADS)

    Fan, Qian; Chevtchenko, S.; Ni, Xianfeng; Cho, Sang-Jun; Morko, Hadis

    2006-02-01

    Surface properties of GaN subjected to reactive ion etching and the impact on device performance have been investigated by surface potential, optical and electrical measurements. Different etching conditions were studied and essentially high power levels and low chamber pressures resulted in higher etch rates accompanying with the roughening of the surface morphology. Surface potential for the as-grown c-plane GaN was found to be in the range of 0.5~0.7 V using Scanning Kevin Probe Microscopy. However, after reactive ion etching at a power level of 300 W, it decreased to 0.1~0.2 V. A nearly linear reduction was observed on c-plane GaN with increasing power. The nonpolar a-plane GaN samples also showed large surface band bending before and after etching. Additionally, the intensity of the near band-edge photoluminescence decreased and the free carrier density increased after etching. These results suggest that the changes in the surface potential may originate from the formation of possible nitrogen vacancies and other surface oriented defects and adsorbates. To recover the etched surface, N II plasma, rapid thermal annealing, and etching in wet KOH were performed. For each of these methods, the surface potential was found to increase by 0.1~0.3 V, also the reverse leakage current in Schottky diodes fabricated on treated samples was reduced considerably compared with as-etched samples, which implies a partial-to-complete recovery from the plasma-induced damage.

  11. Thermal functionalization of GaN surfaces with 1-alkenes.

    PubMed

    Schwarz, Stefan U; Cimalla, Volker; Eichapfel, Georg; Himmerlich, Marcel; Krischok, Stefan; Ambacher, Oliver

    2013-05-28

    A thermally induced functionalization process for gallium nitride surfaces with 1-alkenes is introduced. The resulting functionalization layers are characterized with atomic force microscopy and X-ray photoelectron spectroscopy and compared to reference samples without and with a photochemically generated functionalization layer. The resulting layers show very promising characteristics as functionalization for GaN based biosensors. On the basis of the experimental results, important characteristics of the functionalization layers are estimated and a possible chemical reaction scheme is proposed.

  12. First-Principles Study of Defects in GaN

    DTIC Science & Technology

    2009-07-29

    This means both Mg and Be are not suitable p-type dopants in AlN. c) We have calculated the Ga Frenkel pairs (interstitial Ga and gallium vacancy... gallium vacancy complexes) in GaN. We studied both the stability of the pair at different separations and the barriers for the pair to form/disintegrate...high in energy than vacancy defects, especially for covalent materials. However, in ionic materials the charged interstitial defects can have low

  13. Gallium Nitride (GaN) High Power Electronics (FY11)

    DTIC Science & Technology

    2012-01-01

    GaN films grown by metal-organic chemical vapor deposition (MOCVD) and ~1010 in films grown by molecular beam epitaxy (MBE) when they are deposited...inductively coupled plasma I-V current-voltage L-HVPE low doped HVPE MBE molecular beam epitaxy MOCVD metal-organic chemical vapor deposition...figure of merit HEMT high electron mobility transistor H-HVPE high doped HVPE HPE high power electronics HVPE hydride vapor phase epitaxy ICP

  14. Growth and characterizations of various GaN nanostructures on C-plane sapphire using laser MBE

    NASA Astrophysics Data System (ADS)

    Ch., Ramesh; Tyagi, P.; Maurya, K. K.; Kumar, M. Senthil; Kushvaha, S. S.

    2017-05-01

    We have grown various GaN nanostructures such as three-dimensional islands, nanowalls and nanocolumns on c-plane sapphire substrates using laser assisted molecular beam epitaxy (LMBE) system. The shape of the GaN nanostructures was controlled by using different nucleation surfaces such as bare and nitridated sapphire with GaN or AlN buffer layers. The structural and surface morphological properties of grown GaN nanostructures were characterized by ex-situ high resolution x-ray diffraction, Raman spectroscopy and field emission scanning electron microscopy. The symmetric x-ray rocking curve along GaN (0002) plane shows that the GaN grown on pre-nitridated sapphire with GaN or AlN buffer layer possesses good crystalline quality compared to sapphire without nitridation. The Raman spectroscopy measurements revealed the wurtzite phase for all the GaN nanostructures grown on c-sapphire.

  15. Radiation sensors based on GaN microwires

    NASA Astrophysics Data System (ADS)

    Verheij, D.; Peres, M.; Cardoso, S.; Alves, L. C.; Alves, E.; Durand, C.; Eymery, J.; Lorenz, K.

    2018-05-01

    GaN microwires were shown to possess promising characteristics as building blocks for radiation resistant particle detectors. They were grown by metal organic vapour phase epitaxy with diameters between 1 and 2 μm and lengths around 20 μm. Devices were fabricated by depositing gold contacts at the extremities of the wires using photolithography. The response of these single wire radiation sensors was then studied under irradiation with 2 MeV protons. Severe degradation of the majority of devices only sets in for fluences above protons cm‑2 revealing good radiation resistance. During proton irradiation, a clear albeit small current gain was observed with a corresponding decay time below 1 s. Photoconductivity measurements upon irradiation with UV light were carried out before and after the proton irradiation. Despite a relatively low gain, attributed to significant dark currents caused by a high dopant concentration, fast response times of a few seconds were achieved comparable to state-of-the-art GaN nanowire photodetectors. Irradiation and subsequent annealing resulted in an overall improvement of the devices regarding their response to UV radiation. The photocurrent gain increased compared to the values that were obtained prior to the irradiation, without compromising the decay times. The results indicate the possibility of using GaN microwires not only as UV detectors, but also as particle detectors.

  16. Dopant Adsorption and Incorporation at Irradiated GaN Surfaces

    NASA Astrophysics Data System (ADS)

    Sun, Qiang; Selloni, Annabella; Myers, Thomas; Doolittle, W. Alan

    2006-03-01

    Mg and O are two of the common dopants in GaN, but, in spite of extensive investigation, the atomic scale understanding of their adsorption and incorporation is still incomplete. In particular, high-energy electron irradiation, such as occurring during RHEED, has been reported to have an important effect on the incorporation of these impurities, but no study has addressed the detailed mechanisms of this effect yet. Here we use DFT calculations to study the adsorption and incorporation of Mg and O at the Ga- and N-polar GaN surfaces under various Ga, Mg and O coverage conditions as well as in presence of light or electron beam-induced electronic excitation. We find that the adsorption and incorporation of the two impurities have opposite surface polarity dependence: substitutional Mg prefers to incorporate at the GaN(0001) surface, while O prefers to adsorb and incorporate at the N-polar surface. In addition, our results indicate that in presence of light irradiation the tendency of Mg to surface-segregate is reduced. The O adsorption energy on the N-polar surface is also significantly reduced, consistent with the experimental observation of a much smaller concentration of oxygen in the irradiated samples.

  17. Si Complies with GaN to Overcome Thermal Mismatches for the Heteroepitaxy of Thick GaN on Si.

    PubMed

    Tanaka, Atsunori; Choi, Woojin; Chen, Renjie; Dayeh, Shadi A

    2017-10-01

    Heteroepitaxial growth of lattice mismatched materials has advanced through the epitaxy of thin coherently strained layers, the strain sharing in virtual and nanoscale substrates, and the growth of thick films with intermediate strain-relaxed buffer layers. However, the thermal mismatch is not completely resolved in highly mismatched systems such as in GaN-on-Si. Here, geometrical effects and surface faceting to dilate thermal stresses at the surface of selectively grown epitaxial GaN layers on Si are exploited. The growth of thick (19 µm), crack-free, and pure GaN layers on Si with the lowest threading dislocation density of 1.1 × 10 7 cm -2 achieved to date in GaN-on-Si is demonstrated. With these advances, the first vertical GaN metal-insulator-semiconductor field-effect transistors on Si substrates with low leakage currents and high on/off ratios paving the way for a cost-effective high power device paradigm on an Si CMOS platform are demonstrated. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  18. Earth Wisdom.

    ERIC Educational Resources Information Center

    Van Matre, Steve

    1985-01-01

    In our human-centered ignorance and arrogance we are rapidly destroying the earth. We must start helping people understand the big picture of ecological concepts. What these concepts mean for our own lives and how we must begin to change our lifestyles in order to live more harmoniously with the earth. (JHZ)

  19. Earth Science

    1976-01-01

    The LAGEOS I (Laser Geodynamics Satellite) was developed and launched by the Marshall Space Flight Center on May 4, 1976 from Vandenberg Air Force Base, California . The two-foot diameter satellite orbited the Earth from pole to pole and measured the movements of the Earth's surface.

  20. Viability and proliferation of endothelial cells upon exposure to GaN nanoparticles

    PubMed Central

    Braniste, Tudor; Tiginyanu, Ion; Horvath, Tibor; Raevschi, Simion; Cebotari, Serghei; Lux, Marco; Haverich, Axel

    2016-01-01

    Summary Nanotechnology is a rapidly growing and promising field of interest in medicine; however, nanoparticle–cell interactions are not yet fully understood. The goal of this work was to examine the interaction between endothelial cells and gallium nitride (GaN) semiconductor nanoparticles. Cellular viability, adhesion, proliferation, and uptake of nanoparticles by endothelial cells were investigated. The effect of free GaN nanoparticles versus the effect of growing endothelial cells on GaN functionalized surfaces was examined. To functionalize surfaces with GaN, GaN nanoparticles were synthesized on a sacrificial layer of zinc oxide (ZnO) nanoparticles using hydride vapor phase epitaxy. The uptake of GaN nanoparticles by porcine endothelial cells was strongly dependent upon whether they were fixed to the substrate surface or free floating in the medium. The endothelial cells grown on surfaces functionalized with GaN nanoparticles demonstrated excellent adhesion and proliferation, suggesting good biocompatibility of the nanostructured GaN. PMID:27826507

  1. The investigation of stress in freestanding GaN crystals grown from Si substrates by HVPE.

    PubMed

    Lee, Moonsang; Mikulik, Dmitry; Yang, Mino; Park, Sungsoo

    2017-08-17

    We investigate the stress evolution of 400 µm-thick freestanding GaN crystals grown from Si substrates by hydride vapour phase epitaxy (HVPE) and the in situ removal of Si substrates. The stress generated in growing GaN can be tuned by varying the thickness of the MOCVD AlGaN/AlN buffer layers. Micro Raman analysis shows the presence of slight tensile stress in the freestanding GaN crystals and no stress accumulation in HVPE GaN layers during the growth. Additionally, it is demonstrated that the residual tensile stress in HVPE GaN is caused only by elastic stress arising from the crystal quality difference between Ga- and N-face GaN. TEM analysis revealed that the dislocations in freestanding GaN crystals have high inclination angles that are attributed to the stress relaxation of the crystals. We believe that the understanding and characterization on the structural properties of the freestanding GaN crystals will help us to use these crystals for high-performance opto-electronic devices.

  2. Electronic and Optical Properties of Two-Dimensional GaN from First-Principles.

    PubMed

    Sanders, Nocona; Bayerl, Dylan; Shi, Guangsha; Mengle, Kelsey A; Kioupakis, Emmanouil

    2017-12-13

    Gallium nitride (GaN) is an important commercial semiconductor for solid-state lighting applications. Atomically thin GaN, a recently synthesized two-dimensional material, is of particular interest because the extreme quantum confinement enables additional control of its light-emitting properties. We performed first-principles calculations based on density functional and many-body perturbation theory to investigate the electronic, optical, and excitonic properties of monolayer and bilayer two-dimensional (2D) GaN as a function of strain. Our results demonstrate that light emission from monolayer 2D GaN is blueshifted into the deep ultraviolet range, which is promising for sterilization and water-purification applications. Light emission from bilayer 2D GaN occurs at a similar wavelength to its bulk counterpart due to the cancellation of the effect of quantum confinement on the optical gap by the quantum-confined Stark shift. Polarized light emission at room temperature is possible via uniaxial in-plane strain, which is desirable for energy-efficient display applications. We compare the electronic and optical properties of freestanding two-dimensional GaN to atomically thin GaN wells embedded within AlN barriers in order to understand how the functional properties are influenced by the presence of barriers. Our results provide microscopic understanding of the electronic and optical characteristics of GaN at the few-layer regime.

  3. Advanced processing of gallium nitride and gallium nitride-based devices: Ultra-high temperature annealing and implantation incorporation

    NASA Astrophysics Data System (ADS)

    Yu, Haijiang

    This dissertation is focused on three fields: ultra-high temperature annealing of GaN, activation of implanted GaN and the implantation incorporation into AlGaN/GaN HEMT processing, with an aim to increase the performance, manufacturability and reliability of AlGaN/GaN HEMTs. First, the ultra high temperature (around 1500°C) annealing of MOCVD grown GaN on sapphire has been studied, and a thermally induced threading dislocation (TD) motion and reaction are reported. Using a rapid thermal annealing (RTA) approach capable of heating 2 inch wafers to around 1500°C with 100 bar N2 over-pressure, evidence of dislocation motion was first observed in transmission electron microscopy (TEM) micrographs of both planar and patterned GaN films protected by an AIN capping layer. An associated decrease in x-ray rocking curve (XRC) full-width-half-maximum (FWHM) was also observed for both the symmetric and asymmetric scans. After annealing, the AIN capping layer remained intact, and optical measurements showed no degradation of the opto-electronic properties of the films. Then activation annealing of Si implants in MOCVD grown GaN has been studied for use in ohmic contacts. Si was implanted in semi-insulating GaN at 100 keV with doses from 5 x 1014 cm-2 to 1.5 x 1016 cm-2. Rapid thermal annealing at 1500°C with 100 bar N2 over-pressure was used for dopant activation, resulting in a minimum sheet resistance of 13.9 O/square for a dose of 7 x 1015 cm-2. Secondary ion mass spectroscopy measurements showed a post-activation broadening of the dopant concentration peak by 20 nm (at half the maximum), while X-Ray triple axis o-2theta scans indicated nearly complete implant damage recovery. Transfer length method measurements of the resistance of Ti/Al/Ni/Au contacts to activated GaN:Si (5 x 1015 cm-2 at 100 keV) indicated lowest contact resistances of 0.07 Omm and 0.02 Omm for as-deposited and subsequently annealed contacts, respectively. Finally, the incorporation of Si implantation

  4. Characterization of Pb-Doped GaN Thin Films Grown by Thermionic Vacuum Arc

    NASA Astrophysics Data System (ADS)

    Özen, Soner; Pat, Suat; Korkmaz, Şadan

    2018-03-01

    Undoped and lead (Pb)-doped gallium nitride (GaN) thin films have been deposited by a thermionic vacuum arc (TVA) method. Glass and polyethylene terephthalate were selected as optically transparent substrates. The structural, optical, morphological, and electrical properties of the deposited thin films were investigated. These physical properties were interpreted by comparison with related analysis methods. The crystalline structure of the deposited GaN thin films was hexagonal wurtzite. The optical bandgap energy of the GaN and Pb-doped GaN thin films was found to be 3.45 eV and 3.47 eV, respectively. The surface properties of the deposited thin films were imaged using atomic force microscopy and field-emission scanning electron microscopy, revealing a nanostructured, homogeneous, and granular surface structure. These results confirm that the TVA method is an alternative layer deposition system for Pb-doped GaN thin films.

  5. Mechanism of radiative recombination in acceptor-doped bulk GaN crystals

    NASA Astrophysics Data System (ADS)

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

    1999-12-01

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

  6. Study on GaN nanostructures: Growth and the suppression of the yellow emission

    NASA Astrophysics Data System (ADS)

    Wang, Ting; Chen, Fei; Ji, Xiaohong; Zhang, Qinyuan

    2018-07-01

    GaN nanostructures were synthesized via a simple chemical vapor deposition using Ga2O3 and NH3 as precursors. Structural and morphological properties were systematically characterized by field emission scanning electron microscopy, X-ray diffractometer, transmission electron microscopy, and Raman spectroscopy. The configuration of GaN nanostructures was found to be strongly dependent on the growth temperature and the NH3 flow rate. Photoluminescence analysis revealed that all the fabricated GaN NSs exhibited a strong ultra-violet emission (∼364 nm), and the yellow emission of GaN nanorods can be suppressed at appropriate III/V ratio. The suppression of the yellow emission was attributed to the low density of surface or the VGa defect. The work demonstrates that the GaN nanostructures have potential applications in the optoelectronic and nanoelectronic devices.

  7. Spin injection in epitaxial MnGa(111)/GaN(0001) heterostructures

    NASA Astrophysics Data System (ADS)

    Zube, Christian; Malindretos, Joerg; Watschke, Lars; Zamani, Reza R.; Disterheft, David; Ulbrich, Rainer G.; Rizzi, Angela; Iza, Michael; Keller, Stacia; DenBaars, Steven P.

    2018-01-01

    Ferromagnetic MnGa(111) layers were grown on GaN(0001) by molecular beam epitaxy. MnGa/GaN Schottky diodes with a doping level of around n = 7 × 1018 cm-3 were fabricated to achieve single step tunneling across the metal/semiconductor junction. Below the GaN layer, a thin InGaN quantum well served as optical spin detector ("spin-LED"). For electron spin injection from MnGa into GaN and subsequent spin transport through a 45 nm (70 nm) thick GaN layer, we observe a circular polarization of 0.3% (0.2%) in the electroluminescence at 80 K. Interface mixing, spin polarization losses during electrical transport in the GaN layer, and spin relaxation in the InGaN quantum well are discussed in relation with the low value of the optically detected spin polarization.

  8. Prospects for the application of GaN power devices in hybrid electric vehicle drive systems

    NASA Astrophysics Data System (ADS)

    Su, Ming; Chen, Chingchi; Rajan, Siddharth

    2013-07-01

    GaN, a wide bandgap semiconductor successfully implemented in optical and high-speed electronic devices, has gained momentum in recent years for power electronics applications. Along with rapid progress in material and device processing technologies, high-voltage transistors over 600 V have been reported by a number of teams worldwide. These advances make GaN highly attractive for the growing market of electrified vehicles, which currently employ bipolar silicon devices in the 600-1200 V class for the traction inverter. However, to capture this billion-dollar power market, GaN has to compete with existing IGBT products and deliver higher performance at comparable or lower cost. This paper reviews key achievements made by the GaN semiconductor industry, requirements of the automotive electric drive system and remaining challenges for GaN power devices to fit in the inverter application of hybrid vehicles.

  9. The optimal thickness of a transmission-mode GaN photocathode

    NASA Astrophysics Data System (ADS)

    Wang, Xiao-Hui; Shi, Feng; Guo, Hui; Hu, Cang-Lu; Cheng, Hong-Chang; Chang, Ben-Kang; Ren, Ling; Du, Yu-Jie; Zhang, Jun-Ju

    2012-08-01

    A 150-nm-thick GaN photocathode with a Mg doping concentration of 1.6 × 1017 cm-3 is activated by Cs/O in an ultrahigh vacuum chamber, and a quantum efficiency (QE) curve of the negative electron affinity transmission-mode (t-mode) of the GaN photocathode is obtained. The maximum QE reaches 13.0% at 290 nm. According to the t-mode QE equation solved from the diffusion equation, the QE curve is fitted. From the fitting results, the electron escape probability is 0.32, the back-interface recombination velocity is 5 × 104 cm·s-1, and the electron diffusion length is 116 nm. Based on these parameters, the influence of GaN thickness on t-mode QE is simulated. The simulation shows that the optimal thickness of GaN is 90 nm, which is better than the 150-nm GaN.

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

    NASA Astrophysics Data System (ADS)

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

    2016-02-01

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

  11. Ancient Earth, Alien Earths Event

    2014-08-20

    Panelists pose for a group photo at the “Ancient Earth, Alien Earths” Event at NASA Headquarters in Washington, DC Wednesday, August 20, 2014. The event was sponsored by NASA, the National Science Foundation (NSF), and the Smithsonian Institution and highlighted how research on early Earth could help guide our search for habitable planets orbiting other stars. Photo Credit: (NASA/Aubrey Gemignani)

  12. Bilayer Implants

    PubMed Central

    Schagemann, Jan C.; Rudert, Nicola; Taylor, Michelle E.; Sim, Sotcheadt; Quenneville, Eric; Garon, Martin; Klinger, Mathias; Buschmann, Michael D.; Mittelstaedt, Hagen

    2016-01-01

    Objective To compare the regenerative capacity of 2 distinct bilayer implants for the restoration of osteochondral defects in a preliminary sheep model. Methods Critical sized osteochondral defects were treated with a novel biomimetic poly-ε-caprolactone (PCL) implant (Treatment No. 2; n = 6) or a combination of Chondro-Gide and Orthoss (Treatment No. 1; n = 6). At 19 months postoperation, repair tissue (n = 5 each) was analyzed for histology and biochemistry. Electromechanical mappings (Arthro-BST) were performed ex vivo. Results Histological scores, electromechanical quantitative parameter values, dsDNA and sGAG contents measured at the repair sites were statistically lower than those obtained from the contralateral surfaces. Electromechanical mappings and higher dsDNA and sGAG/weight levels indicated better regeneration for Treatment No. 1. However, these differences were not significant. For both treatments, Arthro-BST revealed early signs of degeneration of the cartilage surrounding the repair site. The International Cartilage Repair Society II histological scores of the repair tissue were significantly higher for Treatment No. 1 (10.3 ± 0.38 SE) compared to Treatment No. 2 (8.7 ± 0.45 SE). The parameters cell morphology and vascularization scored highest whereas tidemark formation scored the lowest. Conclusion There was cell infiltration and regeneration of bone and cartilage. However, repair was incomplete and fibrocartilaginous. There were no significant differences in the quality of regeneration between the treatments except in some histological scoring categories. The results from Arthro-BST measurements were comparable to traditional invasive/destructive methods of measuring quality of cartilage repair. PMID:27688843

  13. Earth Observation

    2013-08-20

    Earth observation taken during day pass by an Expedition 36 crew member on board the International Space Station (ISS). Per Twitter message: Looking southwest over northern Africa. Libya, Algeria, Niger.

  14. Earth Observation

    2014-09-01

    Earth Observation taken during a night pass by the Expedition 40 crew aboard the International Space Station (ISS). Folder lists this as: New Zealand Aurora night pass. On crewmember's Flickr page - Look straight down into an aurora.

  15. Earth Observation

    2014-06-07

    ISS040-E-008174 (7 June 2014) --- Layers of Earth's atmosphere, brightly colored as the sun rises, are featured in this image photographed by an Expedition 40 crew member on the International Space Station.

  16. Earth Observation

    2014-06-02

    ISS040-E-006817 (2 June 2014) --- Intersecting the thin line of Earth's atmosphere, International Space Station solar array wings are featured in this image photographed by an Expedition 40 crew member on the International Space Station.

  17. Earth Science

    1992-07-18

    Workers at Launch Complex 17 Pad A, Kennedy Space Center (KSC) encapsulate the Geomagnetic Tail (GEOTAIL) spacecraft (upper) and attached payload Assist Module-D upper stage (lower) in the protective payload fairing. GEOTAIL project was designed to study the effects of Earth's magnetic field. The solar wind draws the Earth's magnetic field into a long tail on the night side of the Earth and stores energy in the stretched field lines of the magnetotail. During active periods, the tail couples with the near-Earth magnetosphere, sometimes releasing energy stored in the tail and activating auroras in the polar ionosphere. GEOTAIL measures the flow of energy and its transformation in the magnetotail and will help clarify the mechanisms that control the imput, transport, storage, release, and conversion of mass, momentum, and energy in the magnetotail.

  18. Discover Earth

    NASA Technical Reports Server (NTRS)

    1997-01-01

    Discover Earth is a NASA-funded project for teachers of grades 5-12 who want to expand their knowledge of the Earth system, and prepare to become master teachers who promote Earth system science in their own schools, counties, and throughout their state. Participants from the following states are invited to apply: Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, and Washington, DC. Teachers selected for the project participate in a two-week summer workshop conducted at the University of Maryland, College Park; develop classroom-ready materials during the workshop for broad dissemination; conduct a minimum of two peer training activities during the coming school year; and participate in other enrichment/education opportunities as available and desired. Discover Earth is a team effort that utilizes expertise from a range of contributors, and balances science content with hands-on classroom applications.

  19. Earth Observation

    2014-05-31

    Earth Observation taken during a day pass by the Expedition 40 crew aboard the International Space Station (ISS). Folder lists this as: CEO - Arena de Sao Paolo. View used for Twitter message: Cloudy skies over São Paulo Brazil

  20. Earth Observation

    2013-07-26

    Earth observation taken during day pass by an Expedition 36 crew member on board the International Space Station (ISS). Per Twitter message: Never tire of finding shapes in the clouds! These look very botanical to me. Simply perfect.

  1. Earth Observation

    2014-06-12

    Earth Observation taken during a day pass by the Expedition 40 crew aboard the International Space Station (ISS). Folder lists this as: Moon, Japan, Kamchatka with a wild cloud. Part of a solar array is also visible.

  2. Earth Science

    1990-10-24

    Solar Vector Magnetograph is used to predict solar flares, and other activities associated with sun spots. This research provides new understanding about weather on the Earth, and solar-related conditions in orbit.

  3. Earth Observation

    2013-08-03

    Earth observation taken during day pass by an Expedition 36 crew member on board the International Space Station (ISS). Per Twitter message: Perhaps a dandelion losing its seeds in the wind? Love clouds!

  4. Earth Observation

    2014-06-27

    Earth Observation taken during a day pass by the Expedition 40 crew aboard the International Space Station (ISS). Part of Space Station Remote Manipulator System (SSRMS) is visible. Folder lists this as: the Middle East, Israel.

  5. Immunological Response to Biodegradable Magnesium Implants

    NASA Astrophysics Data System (ADS)

    Pichler, Karin; Fischerauer, Stefan; Ferlic, Peter; Martinelli, Elisabeth; Brezinsek, Hans-Peter; Uggowitzer, Peter J.; Löffler, Jörg F.; Weinberg, Annelie-Martina

    2014-04-01

    The use of biodegradable magnesium implants in pediatric trauma surgery would render surgical interventions for implant removal after tissue healing unnecessary, thereby preventing stress to the children and reducing therapy costs. In this study, we report on the immunological response to biodegradable magnesium implants—as an important aspect in evaluating biocompatibility—tested in a growing rat model. The focus of this study was to investigate the response of the innate immune system to either fast or slow degrading magnesium pins, which were implanted into the femoral bones of 5-week-old rats. The main alloying element of the fast-degrading alloy (ZX50) was Zn, while it was Y in the slow-degrading implant (WZ21). Our results demonstrate that degrading magnesium implants beneficially influence the immune system, especially in the first postoperative weeks but also during tissue healing and early bone remodeling. However, rodents with WZ21 pins showed a slightly decreased phagocytic ability during bone remodeling when the degradation rate reached its maximum. This may be due to the high release rate of the rare earth-element yttrium, which is potentially toxic. From our results we conclude that magnesium implants have a beneficial effect on the innate immune system but that there are some concerns regarding the use of yttrium-alloyed magnesium implants, especially in pediatric patients.

  6. Earth Observations

    2010-06-16

    ISS024-E-006136 (16 June 2010) --- Polar mesospheric clouds, illuminated by an orbital sunrise, are featured in this image photographed by an Expedition 24 crew member on the International Space Station. Polar mesospheric, or noctilucent (?night shining?), clouds are observed from both Earth?s surface and in orbit by crew members aboard the space station. They are called night-shining clouds as they are usually seen at twilight. Following the setting of the sun below the horizon and darkening of Earth?s surface, these high clouds are still briefly illuminated by sunlight. Occasionally the ISS orbital track becomes nearly parallel to Earth?s day/night terminator for a time, allowing polar mesospheric clouds to be visible to the crew at times other than the usual twilight due to the space station altitude. This unusual photograph shows polar mesospheric clouds illuminated by the rising, rather than setting, sun at center right. Low clouds on the horizon appear yellow and orange, while higher clouds and aerosols are illuminated a brilliant white. Polar mesospheric clouds appear as light blue ribbons extending across the top of the image. These clouds typically occur at high latitudes of both the Northern and Southern Hemispheres, and at fairly high altitudes of 76?85 kilometers (near the boundary between the mesosphere and thermosphere atmospheric layers). The ISS was located over the Greek island of Kos in the Aegean Sea (near the southwestern coastline of Turkey) when the image was taken at approximately midnight local time. The orbital complex was tracking northeastward, nearly parallel to the terminator, making it possible to observe an apparent ?sunrise? located almost due north. A similar unusual alignment of the ISS orbit track, terminator position, and seasonal position of Earth?s orbit around the sun allowed for striking imagery of polar mesospheric clouds over the Southern Hemisphere earlier this year.

  7. Earth Rotation

    NASA Technical Reports Server (NTRS)

    Dickey, Jean O.

    1995-01-01

    The study of the Earth's rotation in space (encompassing Universal Time (UT1), length of day, polar motion, and the phenomena of precession and nutation) addresses the complex nature of Earth orientation changes, the mechanisms of excitation of these changes and their geophysical implications in a broad variety of areas. In the absence of internal sources of energy or interactions with astronomical objects, the Earth would move as a rigid body with its various parts (the crust, mantle, inner and outer cores, atmosphere and oceans) rotating together at a constant fixed rate. In reality, the world is considerably more complicated, as is schematically illustrated. The rotation rate of the Earth's crust is not constant, but exhibits complicated fluctuations in speed amounting to several parts in 10(exp 8) [corresponding to a variation of several milliseconds (ms) in the Length Of the Day (LOD) and about one part in 10(exp 6) in the orientation of the rotation axis relative to the solid Earth's axis of figure (polar motion). These changes occur over a broad spectrum of time scales, ranging from hours to centuries and longer, reflecting the fact that they are produced by a wide variety of geophysical and astronomical processes. Geodetic observations of Earth rotation changes thus provide insights into the geophysical processes illustrated, which are often difficult to obtain by other means. In addition, these measurements are required for engineering purposes. Theoretical studies of Earth rotation variations are based on the application of Euler's dynamical equations to the problem of finding the response of slightly deformable solid Earth to variety of surface and internal stresses.

  8. GaN Nanowire Devices: Fabrication and Characterization

    NASA Astrophysics Data System (ADS)

    Scott, Reum

    The development of microelectronics in the last 25 years has been characterized by an exponential increase of the bit density in integrated circuits (ICs) with time. Scaling solid-state devices improves cost, performance, and power; as such, it is of particular interest for companies, who gain a market advantage with the latest technology. As a result, the microelectronics industry has driven transistor feature size scaling from 10 μm to ~30 nm during the past 40 years. This trend has persisted for 40 years due to optimization, new processing techniques, device structures, and materials. But when noting processor speeds from the 1970's to 2009 and then again in 2010, the implication would be that the trend has ceased. To address the challenge of shrinking the integrated circuit (IC), current research is centered on identifying new materials and devices that can supplement and/or potentially supplant it. Bottom-up methods tailor nanoscale building blocks---atoms, molecules, quantum dots, and nanowires (NWs)---to be used to overcome these limitations. The Group IIIA nitrides (InN, AlN, and GaN) possess appealing properties such as a direct band gap spanning the whole solar spectrum, high saturation velocity, and high breakdown electric field. As a result nanostructures and nanodevices made from GaN and related nitrides are suitable candidates for efficient nanoscale UV/ visible light emitters, detectors, and gas sensors. To produce devices with such small structures new fabrication methods must be implemented. Devices composed of GaN nanowires were fabricated using photolithography and electron beam lithography. The IV characteristics of these devices were noted under different illuminations and the current tripled from 4.8*10-7 A to 1.59*10 -6 A under UV light which persisted for at least 5hrs.

  9. Chemical trends for acceptor impurities in GaN

    NASA Astrophysics Data System (ADS)

    Neugebauer, Jörg; Van de Walle, Chris G.

    1999-03-01

    We present a comprehensive investigation of acceptor impurities in GaN, based on first-principles total-energy calculations. Two main factors are identified that determine acceptor incorporation: the strength of chemical bonding between the acceptor and its neighbors (which can be assessed by comparison with existing compounds) and the atomic size match between the acceptor and the host atom for which it substitutes. None of the candidates (Li, Na, K, Be, Zn, and Ca) exhibits characteristics which surpass those of Mg in all respects. Only Be emerges as a potential alternative dopant, although it may suffer from compensation by Be interstitial donors.

  10. Transport mechanisms in Schottky diodes realized on GaN

    NASA Astrophysics Data System (ADS)

    Amor, Sarrah; Ahaitouf, Ali; Ahaitouf, Abdelaziz; Salvestrini, Jean Paul; Ougazzaden, Abdellah

    2017-03-01

    This work is focused on the conducted transport mechanisms involved on devices based in gallium nitride GaN and its alloys. With considering all conduction mechanisms of current, its possible to understanded these transport phenomena. Thanks to this methodology the current-voltage characteristics of structures with unusual behaviour are further understood and explain. Actually, the barrier height (SBH) is a complex problem since it depends on several parameters like the quality of the metal-semiconductor interface. This study is particularly interesting as solar cells are made on this material and their qualification is closely linked to their transport properties.

  11. Photoluminescence enhancement from GaN by beryllium doping

    NASA Astrophysics Data System (ADS)

    García-Gutiérrez, R.; Ramos-Carrazco, A.; Berman-Mendoza, D.; Hirata, G. A.; Contreras, O. E.; Barboza-Flores, M.

    2016-10-01

    High quality Be-doped (Be = 0.19 at.%) GaN powder has been grown by reacting high purity Ga diluted alloys (Be-Ga) with ultra high purity ammonia in a horizontal quartz tube reactor at 1200 °C. An initial low-temperature treatment to dissolve ammonia into the Ga melt produced GaN powders with 100% reaction efficiency. Doping was achieved by dissolving beryllium into the gallium metal. The powders synthesized by this method regularly consist of two particle size distributions: large hollow columns with lengths between 5 and 10 μm and small platelets in a range of diameters among 1 and 3 μm. The GaN:Be powders present a high quality polycrystalline profile with preferential growth on the [10 1 bar 1] plane, observed by means of X-ray diffraction. The three characteristics growth planes of the GaN crystalline phase were found by using high resolution TEM microscopy. The optical enhancing of the emission in the GaN powder is attributed to defects created with the beryllium doping. The room temperature photoluminescence emission spectra of GaN:Be powders, revealed the presence of beryllium on a shoulder peak at 3.39 eV and an unusual Y6 emission at 3.32eV related to surface donor-acceptor pairs. Also, a donor-acceptor-pair transition at 3.17 eV and a phonon replica transition at 3.1 eV were observed at low temperature (10 K). The well-known yellow luminescence band coming from defects was observed in both spectra at room and low temperature. Cathodoluminescence emission from GaN:Be powders presents two main peaks associated with an ultraviolet band emission and the yellow emission known from defects. To study the trapping levels related with the defects formed in the GaN:Be, thermoluminescence glow curves were obtained using UV and β radiation in the range of 50 and 150 °C.

  12. Stress related aspects of GaN technology physics

    NASA Astrophysics Data System (ADS)

    Suhir, Ephraim

    2015-03-01

    Simple, easy-to-use and physically meaningful analytical models have been developed for the assessment of the combined effect of the lattice and thermal mismatch on the induced stresses in an elongated bi-material assembly, as well as on the thermal mismatch on the thermal stresses in a tri-material assembly, in which the lattice mismatched stresses are eliminated in one way or another. This could be done, e.g., by using a polished or an etched substrate. The analysis is carried out in application to Gallium Nitride (GaN)-Silicon Carbide (SiC) and GaN-diamond (C) filmsubstrate assemblies. The calculated data are obtained, assuming that no annealing or other stress reduction means is applied. The data agree reasonably well with the reported (available) in-situ measurements. The most important conclusion from the computed data is that even if a reasonably good lattice match takes place (as, e.g., in the case of a GaN film fabricated on a SiC substrate, when the mismatch strain is only about 3%) and, in addition, the temperature change (from the fabrication/growth temperature to the operation temperature) is significant (as high as 1000 °C), the thermal stresses are still considerably lower than the lattice-mismatch stresses. Although there are structural and technological means for further reduction of the lattice-mismatch stresses (e.g., by high temperature annealing or by providing one or more buffering layers, or by using patterned or porous substrates), there is still a strong incentive to eliminate completely the lattice mismatch stresses. This seems to be indeed possible, if polished or otherwise flattened (e.g., chemically etched) substrates and sputter deposited GaN film is employed. In such a case only thermal stresses remain, but even these could be reduced, if necessary, by using compliant buffering layers, including layers of variable compliance, or by introducing variable compliance into the properly engineered substrate. In any event, it is expected

  13. Pressure-induced phase transition in GaN nanocrystals

    NASA Astrophysics Data System (ADS)

    Cui, Q.; Pan, Y.; Zhang, W.; Wang, X.; Zhang, J.; Cui, T.; Xie, Y.; Liu, J.; Zou, G.

    2002-11-01

    High-pressure in situ energy-dispersive x-ray diffraction experiments on GaN nanocrystals with 50 nm diameter have been carried out using a synchrotron x-ray source and a diamond-anvil cell up to about 79 GPa at room temperature. A pressure-induced first-order structural phase transition from the wurtzite-type structure to the rock-salt-type structure starts at about 48.8 GPa. The rock-salt-type phase persists to the highest pressure in our experimental range.

  14. Individual titanium zygomatic implant

    NASA Astrophysics Data System (ADS)

    Nekhoroshev, M. V.; Ryabov, K. N.; Avdeev, E. V.

    2018-03-01

    Custom individual implants for the reconstruction of craniofacial defects have gained importance due to better qualitative characteristics over their generic counterparts – plates, which should be bent according to patient needs. The Additive Manufacturing of individual implants allows reducing cost and improving quality of implants. In this paper, the authors describe design of zygomatic implant models based on computed tomography (CT) data. The fabrication of the implants will be carried out with 3D printing by selective laser melting machine SLM 280HL.

  15. Earth: Earth Science and Health

    NASA Technical Reports Server (NTRS)

    Maynard, Nancy G.

    2001-01-01

    A major new NASA initiative on environmental change and health has been established to promote the application of Earth science remote sensing data, information, observations, and technologies to issues of human health. NASA's Earth Sciences suite of Earth observing instruments are now providing improved observations science, data, and advanced technologies about the Earth's land, atmosphere, and oceans. These new space-based resources are being combined with other agency and university resources, data integration and fusion technologies, geographic information systems (GIS), and the spectrum of tools available from the public health community, making it possible to better understand how the environment and climate are linked to specific diseases, to improve outbreak prediction, and to minimize disease risk. This presentation is an overview of NASA's tools, capabilities, and research advances in this initiative.

  16. Ancient Earth, Alien Earths Event

    2014-08-20

    Panelists discuss how research on early Earth could help guide our search for habitable planets orbiting other stars at the “Ancient Earth, Alien Earths” Event at NASA Headquarters in Washington, DC Wednesday, August 20, 2014. The event was sponsored by NASA, the National Science Foundation (NSF), and the Smithsonian Institution and was moderated by Dr. David H. Grinspoon, Senior Scientist at the Planetary Science Institute. Photo Credit: (NASA/Aubrey Gemignani)

  17. Ancient Earth, Alien Earths Event

    2014-08-20

    Dr. David H. Grinspoon, Senior Scientist, Planetary Science Institute, moderates a panel at the “Ancient Earth, Alien Earths” Event at NASA Headquarters in Washington, DC Wednesday, August 20, 2014. The event was sponsored by NASA, the National Science Foundation (NSF), and the Smithsonian Institution and highlighted how research on early Earth could help guide our search for habitable planets orbiting other stars. Photo Credit: (NASA/Aubrey Gemignani)

  18. Ancient Earth, Alien Earths Event

    2014-08-20

    An audience member asks the panelists a question at the “Ancient Earth, Alien Earths” Event at NASA Headquarters in Washington, DC Wednesday, August 20, 2014. The event was sponsored by NASA, the National Science Foundation (NSF), and the Smithsonian Institution and was moderated by Dr. David H. Grinspoon, Senior Scientist at the Planetary Science Institute. Six scientists discussed how research on early Earth could help guide our search for habitable planets orbiting other stars. Photo Credit: (NASA/Aubrey Gemignani)

  19. Optical design of GaN nanowire arrays for photocatalytic applications

    NASA Astrophysics Data System (ADS)

    Winnerl, Julia; Hudeczek, Richard; Stutzmann, Martin

    2018-05-01

    GaN nanowire (NW) arrays are interesting candidates for photocatalytic applications due to their high surface-to-volume ratio and their waveguide character. The integration of GaN NW arrays on GaN-based light emitting diodes (LEDs), serving as a platform for electrically driven NW-based photocatalytic devices, enables an efficient coupling of the light from the planar LED to the GaN NWs. Here, we present a numerical study of the influence of the NW geometries, i.e., the NW diameter, length, and period, and the illumination wavelength on the transmission of GaN NW arrays on transparent substrates. A detailed numerical analysis reveals that the transmission characteristics for large periods are determined by the waveguide character of the single NW, whereas for dense GaN NW arrays inter-wire coupling and diffraction effects originating from the periodic arrangement of the GaN NWs dominate the transmission. The numerically simulated results are confirmed by experimental transmission measurements. We also investigate the influence of a dielectric NW shell and of the surrounding medium on the transmission characteristics of a GaN NW array.

  20. Theoretical study for heterojunction surface of NEA GaN photocathode dispensed with Cs activation

    NASA Astrophysics Data System (ADS)

    Xia, Sihao; Liu, Lei; Wang, Honggang; Wang, Meishan; Kong, Yike

    2016-09-01

    For the disadvantages of conventional negative electron affinity (NEA) GaN photocathodes activated by Cs or Cs/O, new-type NEA GaN photocathodes with heterojunction surface dispensed with Cs activation are investigated based on first-principle study with density functional theory. Through the growth of an ultrathin n-type GaN cap layer on p-type GaN emission layer, a p-n heterojunction is formed on the surface. According to the calculation results, it is found that Si atoms tend to replace Ga atoms to result in an n-type doped cap layer which contributes to the decreasing of work function. After the growth of n-type GaN cap layer, the atom structure near the p-type emission layer is changed while that away from the surface has no obvious variations. By analyzing the E-Mulliken charge distribution of emission surface with and without cap layer, it is found that the positive charge of Ga and Mg atoms in the emission layer decrease caused by the cap layer, while the negative charge of N atom increases. The conduction band moves downwards after the growth of cap layer. Si atom produces donor levels around the valence band maximum. The absorption coefficient of GaN emission layer decreases and the reflectivity increases caused by n-type GaN cap layer.

  1. Determination of carrier diffusion length in p- and n-type GaN

    NASA Astrophysics Data System (ADS)

    Hafiz, Shopan; Metzner, Sebastian; Zhang, Fan; Monavarian, Morteza; Avrutin, Vitaliy; Morkoç, Hadis; Karbaum, Christopher; Bertram, Frank; Christen, Jürgen; Gil, Bernard; Özgür, Ümit

    2014-03-01

    Diffusion lengths of photo-excited carriers along the c-direction were determined from photoluminescence (PL) measurements in p- and n-type GaN epitaxial layers grown on c-plane sapphire by metal-organic chemical vapor deposition. The investigated samples incorporate a 6 nm thick In0.15Ga0.85N active layer capped with either 500 nm p- GaN or 1300 nm n-GaN. The top GaN layers were etched in steps and PL from the InGaN active region and the underlying layers was monitored as a function of the top GaN thickness upon photogeneration near the surface region by above bandgap excitation. Taking into consideration the absorption in the active and underlying layers, the diffusion lengths at 295 K and at 15 K were measured to be about 92 ± 7 nm and 68 ± 7 nm for Mg-doped p-type GaN and 432 ± 30 nm and 316 ± 30 nm for unintentionally doped n-type GaN, respectively. Cross-sectional cathodoluminescence line-scan measurement was performed on a separate sample and the diffusion length in n-type GaN was measured to be 280 nm.

  2. Kinetic-limited etching of magnesium doping nitrogen polar GaN in potassium hydroxide solution

    NASA Astrophysics Data System (ADS)

    Jiang, Junyan; Zhang, Yuantao; Chi, Chen; Yang, Fan; Li, Pengchong; Zhao, Degang; Zhang, Baolin; Du, Guotong

    2016-01-01

    KOH based wet etchings were performed on both undoped and Mg-doped N-polar GaN films grown by metal-organic chemical vapor deposition. It is found that the etching rate for Mg-doped N-polar GaN gets slow obviously compared with undoped N-polar GaN. X-ray photoelectron spectroscopy analysis proved that Mg oxide formed on N-polar GaN surface is insoluble in KOH solution so that kinetic-limited etching occurs as the etching process goes on. The etching process model of Mg-doped N-polar GaN in KOH solution is tentatively purposed using a simplified ideal atomic configuration. Raman spectroscopy analysis reveals that Mg doping can induce tensile strain in N-polar GaN films. Meanwhile, p-type N-polar GaN film with a hole concentration of 2.4 ÿ 1017 cm⿿3 was obtained by optimizing bis-cyclopentadienyl magnesium flow rates.

  3. Impact of substrate off-angle on the m-plane GaN Schottky diodes

    NASA Astrophysics Data System (ADS)

    Yamada, Hisashi; Chonan, Hiroshi; Takahashi, Tokio; Shimizu, Mitsuaki

    2018-04-01

    We investigated the effects of the substrate off-angle on the m-plane GaN Schottky diodes. GaN epitaxial layers were grown by metal-organic chemical vapor deposition on m-plane GaN substrates having an off-angle of 0.1, 1.1, 1.7, or 5.1° toward [000\\bar{1}]. The surface of the GaN epitaxial layers on the 0.1°-off substrate consisted of pyramidal hillocks and contained oxygen (>1017 cm-3) and carbon (>1016 cm-3) impurities. The residual carbon and oxygen impurities decreased to <1016 cm-3 when the off-angle of the m-plane GaN substrate was increased. The leakage current of the 0.1°-off m-plane GaN Schottky diodes originated from the +c facet of the pyramidal hillocks. The leakage current was efficiently suppressed through the use of an off-angle that was observed to be greater than 1.1°. The off-angle of the m-plane GaN substrate is critical in obtaining high-performance Schottky diodes.

  4. Study on photoemission surface of varied doping GaN photocathode

    NASA Astrophysics Data System (ADS)

    Qiao, Jianliang; Du, Ruijuan; Ding, Huan; Gao, Youtang; Chang, Benkang

    2014-09-01

    For varied doping GaN photocathode, from bulk to surface the doping concentrations are distributed from high to low. The varied doping GaN photocathode may produce directional inside electric field within the material, so the higher quantum efficiency can be obtained. The photoemission surface of varied doping GaN photocathode is very important to the high quantum efficiency, but the forming process of the surface state after Cs activation or Cs/O activation has been not known completely. Encircling the photoemission mechanism of varied GaN photocathode, considering the experiment phenomena during the activation and the successful activation results, the varied GaN photocathode surface model [GaN(Mg):Cs]:O-Cs after activation with cesium and oxygen was given. According to GaN photocathode activation process and the change of electronic affinity, the comparatively ideal NEA property can be achieved by Cs or Cs/O activation, and higher quantum efficiency can be obtained. The results show: The effective NEA characteristic of GaN can be gotten only by Cs. [GaN(Mg):Cs] dipoles form the first dipole layer, the positive end is toward the vacuum side. In the activation processing with Cs/O, the second dipole layer is formed by O-Cs dipoles, A O-Cs dipole includes one oxygen atom and two Cs atoms, and the positive end is also toward the vacuum side thus the escape of electrons can be promoted.

  5. High-quality GaN epitaxially grown on Si substrate with serpentine channels

    NASA Astrophysics Data System (ADS)

    Wei, Tiantian; Zong, Hua; Jiang, Shengxiang; Yang, Yue; Liao, Hui; Xie, Yahong; Wang, Wenjie; Li, Junze; Tang, Jun; Hu, Xiaodong

    2018-06-01

    A novel serpentine-channeled mask was introduced to Si substrate for low-dislocation GaN epitaxial growth and the fully coalesced GaN film on the masked Si substrate was achieved for the first time. Compared with the epitaxial lateral overgrowth (ELOG) growth method, this innovative mask only requires one-step epitaxial growth of GaN which has only one high-dislocation region per mask opening. This new growth method can effectively reduce dislocation density, thus improving the quality of GaN significantly. High-quality GaN with low dislocation density ∼2.4 × 107 cm-2 was obtained, which accounted for about eighty percent of the GaN film in area. This innovative technique is promising for the growth of high-quality GaN templates and the subsequent fabrication of high-performance GaN-based devices like transistors, laser diodes (LDs), and light-emitting diodes (LEDs) on Si substrate.

  6. Estimating ice particle scattering properties using a modified Rayleigh-Gans approximation

    NASA Astrophysics Data System (ADS)

    Lu, Yinghui; Clothiaux, Eugene E.; Aydin, Kültegin; Verlinde, Johannes

    2014-09-01

    A modification to the Rayleigh-Gans approximation is made that includes self-interactions between different parts of an ice crystal, which both improves the accuracy of the Rayleigh-Gans approximation and extends its applicability to polarization-dependent parameters. This modified Rayleigh-Gans approximation is both efficient and reasonably accurate for particles with at least one dimension much smaller than the wavelength (e.g., dendrites at millimeter or longer wavelengths) or particles with sparse structures (e.g., low-density aggregates). Relative to the Generalized Multiparticle Mie method, backscattering reflectivities at horizontal transmit and receive polarization (HH) (ZHH) computed with this modified Rayleigh-Gans approach are about 3 dB more accurate than with the traditional Rayleigh-Gans approximation. For realistic particle size distributions and pristine ice crystals the modified Rayleigh-Gans approach agrees with the Generalized Multiparticle Mie method to within 0.5 dB for ZHH whereas for the polarimetric radar observables differential reflectivity (ZDR) and specific differential phase (KDP) agreement is generally within 0.7 dB and 13%, respectively. Compared to the A-DDA code, the modified Rayleigh-Gans approximation is several to tens of times faster if scattering properties for different incident angles and particle orientations are calculated. These accuracies and computational efficiencies are sufficient to make this modified Rayleigh-Gans approach a viable alternative to the Rayleigh-Gans approximation in some applications such as millimeter to centimeter wavelength radars and to other methods that assume simpler, less accurate shapes for ice crystals. This method should not be used on materials with dielectric properties much different from ice and on compact particles much larger than the wavelength.

  7. Rare Earth 4f Hybridization with the GaN Valence Band

    DTIC Science & Technology

    2012-01-01

    DOS of Ga17GdN18. and Auger-like electrons emitted in a super Coster– Kronig process [52] 4d104fN + hν → [4d94fN+1]∗ → 4d104fN−1 + e−, (2) where...similar, albeit not identical, response for GaN:Er compared to GaN:Gd at the photon energy characteristic for the Er 4d → 4f super Coster– Kronig resonance

  8. Hysteretic photochromic switching of Eu-Mg defects in GaN links the shallow transient and deep ground states of the Mg acceptor.

    PubMed

    Singh, A K; O'Donnell, K P; Edwards, P R; Lorenz, K; Kappers, M J; Boćkowski, M

    2017-02-03

    Although p-type activation of GaN by Mg underpins a mature commercial technology, the nature of the Mg acceptor in GaN is still controversial. Here, we use implanted Eu as a 'spectator ion' to probe the lattice location of Mg in doubly doped GaN(Mg):Eu. Photoluminescence spectroscopy of this material exemplifies hysteretic photochromic switching (HPS) between two configurations, Eu0 and Eu1(Mg), of the same Eu-Mg defect, with a hyperbolic time dependence on 'switchdown' from Eu0 to Eu1(Mg). The sample temperature and the incident light intensity at 355 nm tune the characteristic switching time over several orders of magnitude, from less than a second at 12.5 K, ~100 mW/cm 2 to (an estimated) several hours at 50 K, 1 mW/cm 2 . Linking the distinct Eu-Mg defect configurations with the shallow transient and deep ground states of the Mg acceptor in the Lany-Zunger model, we determine the energy barrier between the states to be 27.7(4) meV, in good agreement with the predictions of theory. The experimental results further suggest that at low temperatures holes in deep ground states are localized on N atoms axially bonded to Mg acceptors.

  9. Size dictated thermal conductivity of GaN

    NASA Astrophysics Data System (ADS)

    Beechem, Thomas E.; McDonald, Anthony E.; Fuller, Elliot J.; Talin, A. Alec; Rost, Christina M.; Maria, Jon-Paul; Gaskins, John T.; Hopkins, Patrick E.; Allerman, Andrew A.

    2016-09-01

    The thermal conductivity of n- and p-type doped gallium nitride (GaN) epilayers having thicknesses of 3-4 μm was investigated using time domain thermoreflectance. Despite possessing carrier concentrations ranging across 3 decades (1015-1018 cm-3), n-type layers exhibit a nearly constant thermal conductivity of 180 W/mK. The thermal conductivity of p-type epilayers, in contrast, reduces from 160 to 110 W/mK with increased doping. These trends—and their overall reduction relative to bulk—are explained leveraging established scattering models where it is shown that, while the decrease in p-type layers is partly due to the increased impurity levels evolving from its doping, size effects play a primary role in limiting the thermal conductivity of GaN layers tens of microns thick. Device layers, even of pristine quality, will therefore exhibit thermal conductivities less than the bulk value of 240 W/mK owing to their finite thickness.

  10. MARTA GANs: Unsupervised Representation Learning for Remote Sensing Image Classification

    NASA Astrophysics Data System (ADS)

    Lin, Daoyu; Fu, Kun; Wang, Yang; Xu, Guangluan; Sun, Xian

    2017-11-01

    With the development of deep learning, supervised learning has frequently been adopted to classify remotely sensed images using convolutional networks (CNNs). However, due to the limited amount of labeled data available, supervised learning is often difficult to carry out. Therefore, we proposed an unsupervised model called multiple-layer feature-matching generative adversarial networks (MARTA GANs) to learn a representation using only unlabeled data. MARTA GANs consists of both a generative model $G$ and a discriminative model $D$. We treat $D$ as a feature extractor. To fit the complex properties of remote sensing data, we use a fusion layer to merge the mid-level and global features. $G$ can produce numerous images that are similar to the training data; therefore, $D$ can learn better representations of remotely sensed images using the training data provided by $G$. The classification results on two widely used remote sensing image databases show that the proposed method significantly improves the classification performance compared with other state-of-the-art methods.

  11. Magneto-ballistic transport in GaN nanowires

    SciT

    Santoruvo, Giovanni, E-mail: giovanni.santoruvo@epfl.ch; Allain, Adrien; Ovchinnikov, Dmitry

    2016-09-05

    The ballistic filtering property of nanoscale crosses was used to investigate the effect of perpendicular magnetic fields on the ballistic transport of electrons on wide band-gap GaN heterostructures. The straight scattering-less trajectory of electrons was modified by a perpendicular magnetic field which produced a strong non-linear behavior in the measured output voltage of the ballistic filters and allowed the observation of semi-classical and quantum effects, such as quenching of the Hall resistance and manifestation of the last plateau, in excellent agreement with the theoretical predictions. A large measured phase coherence length of 190 nm allowed the observation of universal quantum fluctuationsmore » and weak localization of electrons due to quantum interference up to ∼25 K. This work also reveals the prospect of wide band-gap GaN semiconductors as a platform for basic transport and quantum studies, whose properties allow the investigation of ballistic transport and quantum phenomena at much larger voltages and temperatures than in other semiconductors.« less

  12. GaN nanophosphors for white-light applications

    NASA Astrophysics Data System (ADS)

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

    2018-01-01

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

  13. Properties of GaN grown on sapphire substrates

    NASA Technical Reports Server (NTRS)

    Crouch, R. K.; Debnam, W. J.; Fripp, A. L.

    1978-01-01

    Epitaxial growth of GaN on sapphire substrates using an open-tube growth furnace has been carried out to study the effects of substrate orientation and transfer gas upon the properties of the layers. It has been found that for the (0001) substrates, surface appearance was virtually independent of carrier gas and of doping levels. For the (1(-1)02) substrates surface faceting was greatly reduced when He was used as a transfer gas as opposed to H2. Faceting was also reduced when the GaN was doped with Zn, and the best surfaces for the (1(-1)02) substrates were obtained in a Zn-doped run using He as the transfer gas. The best sample in terms of electrical properties for the (1(-1)02) substrate had a mobility greater than 400 sq cm/V per sec and a carrier concentration of about 10 to the 17th per cu cm. This sample was undoped and used He as the transfer gas. The best (0001) sample was also grown undoped with He as the transfer gas and had a mobility of 300 sq cm/V per sec and a carrier concentration of 1 x 10 to the 18th per cu cm.

  14. Low voltage operation of GaN vertical nanowire MOSFET

    NASA Astrophysics Data System (ADS)

    Son, Dong-Hyeok; Jo, Young-Woo; Seo, Jae Hwa; Won, Chul-Ho; Im, Ki-Sik; Lee, Yong Soo; Jang, Hwan Soo; Kim, Dae-Hyun; Kang, In Man; Lee, Jung-Hee

    2018-07-01

    GaN gate-all-around (GAA) vertical nanowire MOSFET (VNWMOSFET) with channel length of 300 nm and diameter of 120 nm, the narrowest GaN-based vertical nanowire transistor ever achieved from the top-down approach, was fabricated by utilizing anisotropic side-wall wet etching in TMAH solution and photoresist etch-back process. The VNWMOSFET exhibited output characteristics with very low saturation drain voltage of less than 0.5 V, which is hardly observed from the wide bandgap-based devices. Simulation results indicated that the narrow diameter of the VNWMOSFET with relatively short channel length is responsible for the low voltage operation. The VNWMOSFET also demonstrated normally-off mode with threshold voltage (VTH) of 0.7 V, extremely low leakage current of ∼10-14 A, low drain-induced barrier lowering (DIBL) of 125 mV/V, and subthreshold swing (SS) of 66-122 mV/decade. The GaN GAA VNWMOSFET with narrow channel diameter investigated in this work would be promising for new low voltage logic application. He has been a Professor with the School of Electrical Engineering and Computer Science, Kyungpook National University, Daegu, Korea, since 1993

  15. GaN nanostructure design for optimal dislocation filtering

    NASA Astrophysics Data System (ADS)

    Liang, Zhiwen; Colby, Robert; Wildeson, Isaac H.; Ewoldt, David A.; Sands, Timothy D.; Stach, Eric A.; García, R. Edwin

    2010-10-01

    The effect of image forces in GaN pyramidal nanorod structures is investigated to develop dislocation-free light emitting diodes (LEDs). A model based on the eigenstrain method and nonlocal stress is developed to demonstrate that the pyramidal nanorod efficiently ejects dislocations out of the structure. Two possible regimes of filtering behavior are found: (1) cap-dominated and (2) base-dominated. The cap-dominated regime is shown to be the more effective filtering mechanism. Optimal ranges of fabrication parameters that favor a dislocation-free LED are predicted and corroborated by resorting to available experimental evidence. The filtering probability is summarized as a function of practical processing parameters: the nanorod radius and height. The results suggest an optimal nanorod geometry with a radius of ˜50b (26 nm) and a height of ˜125b (65 nm), in which b is the magnitude of the Burgers vector for the GaN system studied. A filtering probability of greater than 95% is predicted for the optimal geometry.

  16. Earth Science

    1994-03-08

    Workers at the Astrotech processing facility in Titusville prepared for a news media showing of the Geostationary Operational Environmental Satellite-1 (GOES-1). GOES-1 was the first in a new generation of weather satellites deployed above Earth. It was the first 3-axis, body-stabilized meteorological satellite to be used by the National Oceanic Atmospheric Administration (NOAA) and NASA. These features allowed GOES-1 to continuously monitor the Earth, rather than viewing it just five percent of the time as was the case with spin-stabilized meteorological satellites. GOES-1 also has independent imaging and sounding instruments which can operate simultaneously yet independently. As a result, observations provided by each instrument will not be interrupted. The imager produces visual and infrared images of the Earth's surface, oceans, cloud cover and severe storm development, while the prime sounding products include vertical temperature and moisture profiles, and layer mean moisture.

  17. Optical Probing of Low-Pressure Solution Grown GaN Crystal Properties

    DTIC Science & Technology

    2010-04-01

    observed in Mg and Si doped epitaxial films deposited by MBE and MOCVD on freestanding GaN HVPE substrates [23–25]. Considering the purity of the precursors...bands with similar energy positions here reported, a dominant deeper acceptor impurity has been assigned to Zn , a well known deep acceptor in GaN . Room...00-00-2010 to 00-00-2010 4. TITLE AND SUBTITLE Optical probing of low-pressure solution grown GaN crystal properties 5a. CONTRACT NUMBER 5b

  18. Impact of Group-II Acceptors on the Electrical and Optical Properties of GaN

    NASA Astrophysics Data System (ADS)

    Lyons, John L.; Janotti, Anderson; Van de Walle, Chris G.

    2013-08-01

    We explore the properties of group-II acceptors in GaN by performing hybrid density functional calculations. We find that MgGa gives rise to hole localization in zinc-blende GaN, similar to the behavior in the wurtzite phase. Alternative acceptor impurities, such as Zn and Be, also lead to localized holes in wurtzite GaN, and their ionization energies are larger than that of Mg. All these group-II acceptors also cause large lattice distortions in their neutral charge state, which in turn lead to deep and broad luminescence signals. We explore the consequences of these results for p-type doping.

  19. Interfacial Structure and Chemistry of GaN on Ge(111)

    NASA Astrophysics Data System (ADS)

    Zhang, Siyuan; Zhang, Yucheng; Cui, Ying; Freysoldt, Christoph; Neugebauer, Jörg; Lieten, Ruben R.; Barnard, Jonathan S.; Humphreys, Colin J.

    2013-12-01

    The interface of GaN grown on Ge(111) by plasma-assisted molecular beam epitaxy is resolved by aberration corrected scanning transmission electron microscopy. A novel interfacial structure with a 5∶4 closely spaced atomic bilayer is observed that explains why the interface is flat, crystalline, and free of GeNx. Density functional theory based total energy calculations show that the interface bilayer contains Ge and Ga atoms, with no N atoms. The 5∶4 bilayer at the interface has a lower energy than a direct stacking of GaN on Ge(111) and enables the 5∶4 lattice-matching growth of GaN.

  20. Photoassisted Kelvin probe force microscopy at GaN surfaces: The role of polarity

    NASA Astrophysics Data System (ADS)

    Wei, J. D.; Li, S. F.; Atamuratov, A.; Wehmann, H.-H.; Waag, A.

    2010-10-01

    The behavior of GaN surfaces during photoassisted Kelvin probe force microscopy is demonstrated to be strongly dependant on surface polarity. The surface photovoltage of GaN surfaces illuminated with above-band gap light is analyzed as a function of time and light intensity. Distinct differences between Ga-polar and N-polar surfaces could be identified, attributed to photoinduced chemisorption of oxygen during illumination. These differences can be used for a contactless, nondestructive, and easy-performable analysis of the polarity of GaN surfaces.

  1. Effect of precursors condition on the structural morphology of synthesized GaN

    NASA Astrophysics Data System (ADS)

    Muzammil, P.; Basha, S. Munawar; Muhammad, G. Shakil

    2018-05-01

    GaN nanostructures were synthesized using different mole concentration of precursor composing of gallium nitrate and PVP by sol-gel method. The structural analysis using X-ray diffraction shows the wurtzite form of GaN nanostructure, also it observed that the concentration of precursor play a vital role in structural quality as FWHM increase for higher concentration. From the SEM image it observed that for 0.25 and 0.5 M concentration the honey bee and nanorod structure were obtained. The micro-Raman analysis shows a strong E2H peak of GaN nanostructure.

  2. Earth Science

    1994-09-02

    This image depicts a full view of the Earth, taken by the Geostationary Operational Environment Satellite (GOES-8). The red and green charnels represent visible data, while the blue channel represents inverted 11 micron infrared data. The north and south poles were not actually observed by GOES-8. To produce this image, poles were taken from a GOES-7 image. Owned and operated by the National Oceanic and Atmospheric Administration (NOAA), GOES satellites provide the kind of continuous monitoring necessary for intensive data analysis. They circle the Earth in a geosynchronous orbit, which means they orbit the equatorial plane of the Earth at a speed matching the Earth's rotation. This allows them to hover continuously over one position on the surface. The geosynchronous plane is about 35,800 km (22,300 miles) above the Earth, high enough to allow the satellites a full-disc view of the Earth. Because they stay above a fixed spot on the surface, they provide a constant vigil for the atmospheric triggers for severe weather conditions such as tornadoes, flash floods, hail storms, and hurricanes. When these conditions develop, the GOES satellites are able to monitor storm development and track their movements. NASA manages the design and launch of the spacecraft. NASA launched the first GOES for NOAA in 1975 and followed it with another in 1977. Currently, the United States is operating GOES-8, positioned at 75 west longitude and the equator, and GOES-10, which is positioned at 135 west longitude and the equator. (GOES-9, which malfunctioned in 1998, is being stored in orbit as an emergency backup should either GOES-8 or GOES-10 fail. GOES-11 was launched on May 3, 2000 and GOES-12 on July 23, 2001. Both are being stored in orbit as a fully functioning replacement for GOES-8 or GOES-10 on failure.

  3. Earth Observation

    2010-08-23

    ISS024-E-016042 (23 Aug. 2010) --- This night time view captured by one of the Expedition 24 crew members aboard the International Space Station some 220 miles above Earth is looking southward from central Romania over the Aegean Sea toward Greece and it includes Thessaloniki (near center), the larger bright mass of Athens (left center), and the Macedonian capital of Skopje (lower right). Center point coordinates of the area pictured are 46.4 degrees north latitude and 25.5 degrees east longitude. The picture was taken in August and was physically brought back to Earth on a disk with the return of the Expedition 25 crew in November 2010.

  4. Earth Observation

    2014-07-19

    ISS040-E-070412 (19 July 2014) --- One of the Expedition 40 crew members aboard the Earth-orbiting International Space Station recorded this July 19 panorama featuring wildfires which are plaguing the Northwest and causing widespread destruction. (Note: south is at the top of the frame). The orbital outpost was flying 223 nautical miles above Earth at the time of the photo. Parts of Oregon and Washington are included in the scene. Mt. Jefferson, Three Sisters and Mt. St. Helens are all snow-capped and visible in the photo, and the Columbia River can also be delineated.

  5. Earth Observation

    2014-07-19

    ISS040-E-070424 (19 July 2014) --- One of the Expedition 40 crew members aboard the Earth-orbiting International Space Station recorded this July 19 image of wildfires which are plaguing the Northwest and causing widespread destruction. The orbital outpost was flying 223 nautical miles above Earth at the time of the photo. Lightning has been given as the cause of the Ochoco Complex fires in the Ochoco National Forest in central Oregon. The complex has gotten larger since this photo was taken.

  6. Earth observation

    2014-09-04

    ISS040-E-129950 (4 Sept. 2014) --- In this photograph. taken by one of the Expedition 40 crew members aboard the Earth-orbiting International Space Station, the orange spot located in the very center is the sun, which appears to be sitting on Earth's limb. At far right, a small bright spot is believed to be a reflection from somewhere in the camera system or something on the orbital outpost. When the photographed was exposed, the orbital outpost was flying at an altutude of 226 nautical miles above a point near French Polynesia in the Pacific Ocean.

  7. Earth Science

    2004-08-13

    This panoramic view of Hurricane Charley was photographed by the Expedition 9 crew aboard the International Space Station (ISS) on August 13, 2004, at a vantage point just north of Tampa, Florida. The small eye was not visible in this view, but the raised cloud tops near the center coincide roughly with the time that the storm began to rapidly strengthen. The category 2 hurricane was moving north-northwest at 18 mph packing winds of 105 mph. Crew Earth Observations record Earth surface changes over time, as well as more fleeting events such as storms, floods, fires, and volcanic eruptions.

  8. Earth Science

    2004-09-11

    This image hosts a look at the eye of Hurricane Ivan, one of the strongest hurricanes on record, as the storm topped the western Caribbean Sea on Saturday, September 11, 2004. The hurricane was photographed by astronaut Edward M. (Mike) Fincke from aboard the International Space Station (ISS) at an altitude of approximately 230 miles. At the time, the category 5 storm sustained winds in the eye of the wall that were reported at about 160 mph. Crew Earth Observations record Earth surface changes over time, as well as more fleeting events such as storms, floods, fires, and volcanic eruptions.

  9. Earth Science

    2004-09-15

    Except for a small portion of the International Space Station (ISS) in the foreground, Hurricane Ivan, one of the strongest hurricanes on record, fills this image over the northern Gulf of Mexico. As the downgraded category 4 storm approached landfall on the Alabama coast Wednesday afternoon on September 15, 2004, sustained winds in the eye of the wall were reported at about 135 mph. The hurricane was photographed by astronaut Edward M. (Mike) Fincke from aboard the ISS at an altitude of approximately 230 miles. Crew Earth Observations record Earth surface changes over time, as well as more fleeting events such as storms, floods, fires, and volcanic eruptions.

  10. Earth Science

    2004-09-15

    This image hosts a look into the eye of Hurricane Ivan, one of the strongest hurricanes on record, as the storm approached landfall on the central Gulf coast Wednesday afternoon on September 15, 2004. The hurricane was photographed by astronaut Edward M. (Mike) Fincke from aboard the International Space Station (ISS) at an altitude of approximately 230 miles. At the time, sustained winds in the eye of the wall were reported at about 135 mph as the downgraded category 4 storm approached the Alabama coast. Crew Earth Observations record Earth surface changes over time, as well as more fleeting events such as storms, floods, fires, and volcanic eruptions.

  11. Digital Earth - A sustainable Earth

    NASA Astrophysics Data System (ADS)

    Mahavir

    2014-02-01

    All life, particularly human, cannot be sustainable, unless complimented with shelter, poverty reduction, provision of basic infrastructure and services, equal opportunities and social justice. Yet, in the context of cities, it is believed that they can accommodate more and more people, endlessly, regardless to their carrying capacity and increasing ecological footprint. The 'inclusion', for bringing more and more people in the purview of development is often limited to social and economic inclusion rather than spatial and ecological inclusion. Economic investment decisions are also not always supported with spatial planning decisions. Most planning for a sustainable Earth, be at a level of rural settlement, city, region, national or Global, fail on the capacity and capability fronts. In India, for example, out of some 8,000 towns and cities, Master Plans exist for only about 1,800. A chapter on sustainability or environment is neither statutorily compulsory nor a norm for these Master Plans. Geospatial technologies including Remote Sensing, GIS, Indian National Spatial Data Infrastructure (NSDI), Indian National Urban Information Systems (NUIS), Indian Environmental Information System (ENVIS), and Indian National GIS (NGIS), etc. have potential to map, analyse, visualize and take sustainable developmental decisions based on participatory social, economic and social inclusion. Sustainable Earth, at all scales, is a logical and natural outcome of a digitally mapped, conceived and planned Earth. Digital Earth, in fact, itself offers a platform to dovetail the ecological, social and economic considerations in transforming it into a sustainable Earth.

  12. Earth Observation

    2014-08-10

    ISS040-E-091158 (10 Aug. 2014) --- One of the Expedition 40 crew members 225 nautical miles above Earth onboard the International Space Station used a 200mm lens to record this image of Hawke's Bay, New Zealand on Aug. 10, 2014. Napier and the bay area's most populous area are at bottom center of the frame.

  13. Earth Observation

    2013-06-13

    ISS036-E-007619 (13 June 2013) --- To a crew member aboard the International Space Station, the home planet is seen from many different angles and perspectives, as evdenced by this Expedition 36 image of Earth's atmophere partially obscured by one of the orbital outpost's solar panels.

  14. Think Earth.

    ERIC Educational Resources Information Center

    Niedermeyer, Fred; Ice, Kay

    1992-01-01

    Describes a series of environmental education instructional units for grades K-6 developed by the Think Earth Consortium that cover topics such as conservation, pollution control, and waste reduction. Provides testimony from one sixth-grade teacher that field tested the second-grade unit. (MDH)

  15. Earth Observation

    2014-09-01

    Earth Observation taken during a night pass by the Expedition 40 crew aboard the International Space Station (ISS). Folder lists this as: New Zealand Aurora night pass. Docked Soyuz and Progress spacecraft are visible. On crewmember's Flickr page - The Moon, about to dive into a glowing ocean of green᥿9.

  16. Earth Observation

    2013-07-21

    Earth observation taken during night pass by an Expedition 36 crew member on board the International Space Station (ISS). Per Twitter message this is labeled as : Tehran, Iran. Lights along the coast of the Caspian Sea visible through clouds. July 21.

  17. Earth Observation

    2013-05-19

    ISS036-E-002224 (21 May 2013) --- The sun is captured in a "starburst" mode over Earth's horizon by one of the Expedition 36 crew members as the orbital outpost was above a point in southwestern Minnesota on May 21, 2013.

  18. Earth Algebra.

    ERIC Educational Resources Information Center

    Schaufele, Christopher; Zumoff, Nancy

    Earth Algebra is an entry level college algebra course that incorporates the spirit of the National Council of Teachers of Mathematics (NCTM) Curriculum and Evaluation Standards for School Mathematics at the college level. The context of the course places mathematics at the center of one of the major current concerns of the world. Through…

  19. Earth Science

    1993-03-29

    Small Expendable Deployer System (SEDS) is a tethered date collecting satellite and is intended to demonstrate a versatile and economical way of delivering smaller payloads to higher orbits or downward toward Earth's atmosphere. 19th Navstar Global Positioning System Satellite mission joined with previously launched satellites used for navigational purposes and geodite studies. These satellites are used commercially as well as by the military.

  20. Earth Observation

    2014-06-14

    ISS040-E-011868 (14 June 2014) --- The dark waters of the Salton Sea stand out against neighboring cultivation and desert sands in the middle of the Southern California desert, as photographed by one of the Expedition 40 crew members aboard the Earth-orbiting International Space Station on June 14, 2014.

  1. Earth Observation

    2013-08-03

    Earth observation taken during day pass by an Expedition 36 crew member on board the International Space Station (ISS). Per Twitter message: From southernmost point of orbit over the South Pacific- all clouds seemed to be leading to the South Pole.

  2. Earth Sky

    1965-12-16

    S65-63282 (16 Dec. 1965) --- Area of Indian Ocean, just east of the island of Madagascar, as seen from the Gemini-6 spacecraft during its 15th revolution of Earth. Land mass at top of picture is the Malagasy Republic (Madagascar). Photo credit: NASA or National Aeronautics and Space Administration

  3. Rare earths

    Gambogi, J.

    2013-01-01

    Global mine production of rare earths was estimated to have declined slightly in 2012 relative to 2011 (Fig. 1). Production in China was estimated to have decreased to 95 from 105 kt (104,700 from 115,700 st) in 2011, while new mine production in the United States and Australia increased.

  4. Earth Observation

    2013-07-04

    ISS036-E-015354 (4 July 2013) --- A number of Quebec, Canada wildfires near the Manicouagan Reservoir (seen at lower left) were recorded as part of a series of photographs taken and downlinked to Earth on July 4 by the Expedition 36 crew members aboard the International Space Station.

  5. Earth Observation

    2013-07-04

    ISS036-E-015355 (4 July 2013) --- A number of Quebec, Canada wildfires near the Manicouagan Reservoir (seen at bottom center) were recorded in a series of photographs taken and downlinked to Earth on July 4 by the Expedition 36 crew members aboard the International Space Station.

  6. Earth Observation

    2013-07-03

    ISS036-E-015292 (3 July 2013) --- A number of Quebec, Canada wildfires southeast of James Bay were recorded as part of a series of photographs taken and downlinked to Earth on July 3-4 by the Expedition 36 crew members aboard the International Space Station. This image was recorded on July 3.

  7. Earth Observation

    2013-07-04

    ISS036-E-015342 (4 July 2013) --- A number of Quebec, Canada wildfires southeast of James Bay were recorded as part of a series of photographs taken and downlinked to Earth on July 4 by the Expedition 36 crew members aboard the International Space Station.

  8. Earth Observation

    2013-07-04

    ISS036-E-015335 (4 July 2013) --- A number of Quebec, Canada wildfires southeast of James Bay were recorded as part of a series of photographs taken and downlinked to Earth on July 4 by the Expedition 36 crew members aboard the International Space Station.

  9. Earth Observation

    2014-06-12

    Earth Observation taken during a day pass by the Expedition 40 crew aboard the International Space Station (ISS). Folder lists this as: Moon, Japan, Kamchatka with a wild cloud. Part of the U.S. Lab and PMM are also visible.

  10. Earth Observation

    2013-08-29

    ISS036-E-038117 (29 Aug. 2013) --- One of the Expedition 36 crew members aboard the Earth-orbiting International Space Station photographed massive smoke plumes from the California wildfires. When this image was exposed on Aug. 29, the orbital outpost was approximately 220 miles above a point located at 38.6 degrees north latitude and 123.2 degrees west longitude.

  11. Earth Observation

    2013-08-29

    ISS036-E-038114 (29 Aug. 2013) --- One of the Expedition 36 crew members aboard the Earth-orbiting International Space Station photographed massive smoke plumes from the California wildfires. When this image was exposed on Aug. 29, the orbital outpost was approximately 220 miles above a point located at 38.6 degrees north latitude and 123.3 degrees west longitude.

  12. Earth Observations

    2014-11-18

    ISS042E006751 (11/08/2014) --- Earth observation taken from the International Space Station of the coastline of the United Arab Emirates. The large wheel along the coast center left is "Jumeirah" Palm Island, with a conference center, hotels, recreation areas and a large marine zoo.

  13. Earth Moon

    1998-06-08

    NASA Galileo spacecraft took this image of Earth moon on December 7, 1992 on its way to explore the Jupiter system in 1995-97. The distinct bright ray crater at the bottom of the image is the Tycho impact basin. http://photojournal.jpl.nasa.gov/catalog/PIA00405

  14. Earth's horizon

    2005-07-30

    S114-E-6076 (30 July 2005) --- The blackness of space and Earth’s horizon form the backdrop for this view of the extended Space Shuttle Discovery’s remote manipulator system (RMS) robotic arm while docked to the International Space Station during the STS-114 mission.

  15. Peri-implantitis.

    PubMed

    Schwarz, Frank; Derks, Jan; Monje, Alberto; Wang, Hom-Lay

    2018-06-01

    This narrative review provides an evidence-based overview on peri-implantitis for the 2017 World Workshop on the Classification of Periodontal and Peri-Implant Diseases and Conditions. A literature review was conducted addressing the following topics: 1) definition of peri-implantitis; 2) conversion from peri-implant mucositis to peri-implantitis, 3) onset and pattern of disease progression, 4) characteristics of peri-implantitis, 5) risk factors/indicators for peri-implantitis, and 6) progressive crestal bone loss in the absence of soft tissue inflammation. 1)Peri-implantitis is a pathological condition occurring in tissues around dental implants, characterized by inflammation in the peri-implant connective tissue and progressive loss of supporting bone. 2)The histopathologic and clinical conditions leading to the conversion from peri-implant mucositis to peri-implantitis are not completely understood. 3)The onset of peri-implantitis may occur early during follow-up and the disease progresses in a non-linear and accelerating pattern. 4a)Peri-implantitis sites exhibit clinical signs of inflammation and increased probing depths compared to baseline measurements. 4b)At the histologic level, compared to periodontitis sites, peri-implantitis sites often have larger inflammatory lesions. 4c)Surgical entry at peri-implantitis sites often reveals a circumferential pattern of bone loss. 5a)There is strong evidence that there is an increased risk of developing peri-implantitis in patients who have a history of chronic periodontitis, poor plaque control skills, and no regular maintenance care after implant therapy. Data identifying "smoking" and "diabetes" as potential risk factors/indicators for peri-implantitis are inconclusive. 5b)There is some limited evidence linking peri-implantitis to other factors such as: post-restorative presence of submucosal cement, lack of peri-implant keratinized mucosa and positioning of implants that make it difficult to perform oral hygiene

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

    PubMed

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

    2013-02-08

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

  17. Ultralow threading dislocation density in GaN epilayer on near-strain-free GaN compliant buffer layer and its applications in hetero-epitaxial LEDs

    PubMed Central

    Shih, Huan-Yu; Shiojiri, Makoto; Chen, Ching-Hsiang; Yu, Sheng-Fu; Ko, Chung-Ting; Yang, Jer-Ren; Lin, Ray-Ming; Chen, Miin-Jang

    2015-01-01

    High threading dislocation (TD) density in GaN-based devices is a long unresolved problem because of the large lattice mismatch between GaN and the substrate, which causes a major obstacle for the further improvement of next-generation high-efficiency solid-state lighting and high-power electronics. Here, we report InGaN/GaN LEDs with ultralow TD density and improved efficiency on a sapphire substrate, on which a near strain-free GaN compliant buffer layer was grown by remote plasma atomic layer deposition. This “compliant” buffer layer is capable of relaxing strain due to the absorption of misfit dislocations in a region within ~10 nm from the interface, leading to a high-quality overlying GaN epilayer with an unusual TD density as low as 2.2 × 105 cm−2. In addition, this GaN compliant buffer layer exhibits excellent uniformity up to a 6” wafer, revealing a promising means to realize large-area GaN hetero-epitaxy for efficient LEDs and high-power transistors. PMID:26329829

  18. Ultralow threading dislocation density in GaN epilayer on near-strain-free GaN compliant buffer layer and its applications in hetero-epitaxial LEDs.

    PubMed

    Shih, Huan-Yu; Shiojiri, Makoto; Chen, Ching-Hsiang; Yu, Sheng-Fu; Ko, Chung-Ting; Yang, Jer-Ren; Lin, Ray-Ming; Chen, Miin-Jang

    2015-09-02

    High threading dislocation (TD) density in GaN-based devices is a long unresolved problem because of the large lattice mismatch between GaN and the substrate, which causes a major obstacle for the further improvement of next-generation high-efficiency solid-state lighting and high-power electronics. Here, we report InGaN/GaN LEDs with ultralow TD density and improved efficiency on a sapphire substrate, on which a near strain-free GaN compliant buffer layer was grown by remote plasma atomic layer deposition. This "compliant" buffer layer is capable of relaxing strain due to the absorption of misfit dislocations in a region within ~10 nm from the interface, leading to a high-quality overlying GaN epilayer with an unusual TD density as low as 2.2 × 10(5) cm(-2). In addition, this GaN compliant buffer layer exhibits excellent uniformity up to a 6" wafer, revealing a promising means to realize large-area GaN hetero-epitaxy for efficient LEDs and high-power transistors.

  19. Earth Observations

    2010-09-09

    ISS024-E-014071 (9 Sept. 2010) --- This striking panoramic view of the southwestern USA and Pacific Ocean is an oblique image photographed by an Expedition 24 crew member looking outwards at an angle from the International Space Station (ISS). While most unmanned orbital satellites view Earth from a nadir perspective?in other words, collecting data with a ?straight down? viewing geometry?crew members onboard the space station can acquire imagery at a wide range of viewing angles using handheld digital cameras. The ISS nadir point (the point on Earth?s surface directly below the spacecraft) was located in northwestern Arizona, approximately 260 kilometers to the east-southeast, when this image was taken. The image includes parts of the States of Arizona, Nevada, Utah, and California together with a small segment of the Baja California, Mexico coastline at center left. Several landmarks and physiographic features are readily visible. The Las Vegas, NV metropolitan area appears as a gray region adjacent to the Spring Mountains and Sheep Range (both covered by white clouds). The Grand Canyon, located on the Colorado Plateau in Arizona, is visible (lower left) to the east of Las Vegas with the blue waters of Lake Mead in between. The image also includes the Mojave Desert, stretching north from the Salton Sea (left) to the Sierra Nevada mountain range. The Sierra Nevada range is roughly 640 kilometers long (north-south) and forms the boundary between the Central Valley of California and the adjacent Basin and Range. The Basin and Range is so called due to the pattern of long linear valleys separated by parallel linear mountain ranges ? this landscape, formed by extension and thinning of Earth?s crust, is particularly visible at right.

  20. Ancient Earth, Alien Earths Event

    2014-08-20

    Dr. Shawn Domagal-Goldman, Research Space Scientist, NASA Goddard Space Flight Center, speaks on a panel at the “Ancient Earth, Alien Earths” Event at NASA Headquarters in Washington, DC Wednesday, August 20, 2014. The event was sponsored by NASA, the National Science Foundation (NSF), and the Smithsonian Institution and was moderated by Dr. David H. Grinspoon, Senior Scientist at the Planetary Science Institute. Six scientists discussed how research on early Earth could help guide our search for habitable planets orbiting other stars. Photo Credit: (NASA/Aubrey Gemignani)

  1. Ancient Earth, Alien Earths Event

    2014-08-20

    Dr. Phoebe Cohen, Professor of Geosciences, Williams College, speaks on a panel at the “Ancient Earth, Alien Earths” Event at NASA Headquarters in Washington, DC Wednesday, August 20, 2014. The event was sponsored by NASA, the National Science Foundation (NSF), and the Smithsonian Institution and was moderated by Dr. David H. Grinspoon, Senior Scientist at the Planetary Science Institute. Six scientists discussed how research on early Earth could help guide our search for habitable planets orbiting other stars. Photo Credit: (NASA/Aubrey Gemignani)

  2. Ancient Earth, Alien Earths Event

    2014-08-20

    Dr. Christopher House, Professor of Geosciences, Pennsylvania State University, speaks on a panel at the “Ancient Earth, Alien Earths” Event at NASA Headquarters in Washington, DC Wednesday, August 20, 2014. The event was sponsored by NASA, the National Science Foundation (NSF), and the Smithsonian Institution and was moderated by Dr. David H. Grinspoon, Senior Scientist at the Planetary Science Institute. Six scientists discussed how research on early Earth could help guide our search for habitable planets orbiting other stars. Photo Credit: (NASA/Aubrey Gemignani)

  3. Ancient Earth, Alien Earths Event

    2014-08-20

    Dr. Dawn Sumner, Professor of Geology, UC Davis, speaks on a panel at the “Ancient Earth, Alien Earths” Event at NASA Headquarters in Washington, DC Wednesday, August 20, 2014. The event was sponsored by NASA, the National Science Foundation (NSF), and the Smithsonian Institution and was moderated by Dr. David H. Grinspoon, Senior Scientist at the Planetary Science Institute. Six scientists discussed how research on early Earth could help guide our search for habitable planets orbiting other stars. Photo Credit: (NASA/Aubrey Gemignani)

  4. Ancient Earth, Alien Earths Event

    2014-08-20

    Dr. Timothy Lyons, Professor of Biogeochemistry, UC Riverside, speaks on a panel at the “Ancient Earth, Alien Earths” Event at NASA Headquarters in Washington, DC Wednesday, August 20, 2014. The event was sponsored by NASA, the National Science Foundation (NSF), and the Smithsonian Institution and was moderated by Dr. David H. Grinspoon, Senior Scientist at the Planetary Science Institute. Six scientists discussed how research on early Earth could help guide our search for habitable planets orbiting other stars. Photo Credit: (NASA/Aubrey Gemignani)

  5. Earth meandering

    NASA Astrophysics Data System (ADS)

    Asadiyan, H.; Zamani, A.

    2009-04-01

    In this paper we try to put away current Global Tectonic Model to look the tectonic evolution of the earth from new point of view. Our new dynamic model is based on study of river meandering (RM) which infer new concept as Earth meandering(EM). In a universal gravitational field if we consider a clockwise spiral galaxy model rotate above Ninety East Ridge (geotectonic axis GA), this system with applying torsion field (likes geomagnetic field) in side direction from Rocky Mt. (west geotectonic pole WGP) to Tibetan plateau TP (east geotectonic pole EGP),it seems that pulled mass from WGP and pushed it in EGP due to it's rolling dynamics. According to this idea we see in topographic map that North America and Green land like a tongue pulled from Pacific mouth toward TP. Actually this system rolled or meander the earth over itself fractaly from small scale to big scale and what we see in the river meandering and Earth meandering are two faces of one coin. River transport water and sediments from high elevation to lower elevation and also in EM, mass transport from high altitude-Rocky Mt. to lower altitude Himalaya Mt. along 'S' shape geodetic line-optimum path which connect points from high altitude to lower altitude as kind of Euler Elastica(EE). These curves are responsible for mass spreading (source) and mass concentration (sink). In this regard, tiltness of earth spin axis plays an important role, 'S' are part of sigmoidal shape which formed due to intersection of Earth rolling with the Earth glob and actual feature of transform fault and river meandering. Longitudinal profile in mature rivers as a part of 'S' curve also is a kind of EE. 'S' which bound the whole earth is named S-1(S order 1) and cube corresponding to this which represent Earth fracturing in global scale named C-1(cube order 1 or side vergence cube SVC), C-1 is a biggest cycle of spiral polygon, so it is not completely closed and it has separation about diameter of C-7. Inside SVC we introduce cone

  6. High breakdown single-crystal GaN p-n diodes by molecular beam epitaxy

    SciT

    Qi, Meng; Zhao, Yuning; Yan, Xiaodong

    2015-12-07

    Molecular beam epitaxy grown GaN p-n vertical diodes are demonstrated on single-crystal GaN substrates. A low leakage current <3 nA/cm{sup 2} is obtained with reverse bias voltage up to −20 V. With a 400 nm thick n-drift region, an on-resistance of 0.23 mΩ cm{sup 2} is achieved, with a breakdown voltage corresponding to a peak electric field of ∼3.1 MV/cm in GaN. Single-crystal GaN substrates with very low dislocation densities enable the low leakage current and the high breakdown field in the diodes, showing significant potential for MBE growth to attain near-intrinsic performance when the density of dislocations is low.

  7. Synthesis and excellent field emission properties of three-dimensional branched GaN nanowire homostructures

    NASA Astrophysics Data System (ADS)

    Li, Enling; Sun, Lihe; Cui, Zhen; Ma, Deming; Shi, Wei; Wang, Xiaolin

    2016-10-01

    Three-dimensional branched GaN nanowire homostructures have been synthesized on the Si substrate via a two-step approach by chemical vapor deposition. Structural characterization reveals that the single crystal GaN nanowire trunks have hexagonal wurtzite characteristics and grow along the [0001] direction, while the homoepitaxial single crystal branches grow in a radial direction from the six-sided surfaces of the trunks. The field emission measurements demonstrate that the branched GaN nanowire homostructures have excellent field emission properties, with low turn-on field at 2.35 V/μm, a high field enhancement factor of 2938, and long emission current stability. This indicates that the present branched GaN nanowire homostructures will become valuable for practical field emission applications.

  8. The GaN trench gate MOSFET with floating islands: High breakdown voltage and improved BFOM

    NASA Astrophysics Data System (ADS)

    Shen, Lingyan; Müller, Stephan; Cheng, Xinhong; Zhang, Dongliang; Zheng, Li; Xu, Dawei; Yu, Yuehui; Meissner, Elke; Erlbacher, Tobias

    2018-02-01

    A novel GaN trench gate (TG) MOSFET with P-type floating islands (FLI) in drift region, which can suppress the electric field peak at bottom of gate trench during the blocking state and prevent premature breakdown in gate oxide, is proposed and investigated by TCAD simulations. The influence of thickness, position, doping concentration and length of the FLI on breakdown voltage (BV) and specific on-resistance (Ron_sp) is studied, providing useful guidelines for design of this new type of device. Using optimized parameters for the FLI, GaN FLI TG-MOSFET obtains a BV as high as 2464 V with a Ron_sp of 3.0 mΩ cm2. Compared to the conventional GaN TG-MOSFET with the same structure parameters, the Baliga figure of merit (BFOM) is enhanced by 150%, getting closer to theoretical limit for GaN devices.

  9. Synthesis and optical properties of Eu 3+ and Tb 3+ doped GaN nanocrystallite powders

    NASA Astrophysics Data System (ADS)

    Nyk, M.; Kudrawiec, R.; Strek, W.; Misiewicz, J.

    2006-05-01

    The GaN nanocrystallite powders obtained by thermal decomposition of pure and doped gallium nitrate followed by nitridation with ammonia are investigated in this paper. The evolution of the phase composition, structure and morphology was studied. The average size of GaN nanocrystallites estimated from the broadening of XRD diffraction peaks was found to be ˜9-21 nm. The photoluminescence and cathodoluminescence properties of pure and Eu 3+ and Tb 3+ doped GaN nanocrystallites were measured and analyzed. A strong emission related to f-f electron transition in Eu and Tb ions has been observed. In addition, a red/yellow emission related to a recombination in the GaN nanocrystalline grains has been observed. It has been shown that this emission strongly depends on the excitation source.

  10. Droplet heteroepitaxy of zinc-blende vs. wurtzite GaN quantum dots

    NASA Astrophysics Data System (ADS)

    Reese, C.; Jeon, S.; Hill, T.; Jones, C.; Shusterman, S.; Yacoby, Y.; Clarke, R.; Deng, H.; Goldman, Rs

    We have developed a GaN droplet heteroepitaxy process based upon plasma-assisted molecular-beam epitaxy. Using various surface treatments and Ga deposition parameters, we have demonstrated polycrystalline, zinc-blende (ZB), and wurtzite (WZ) GaN quantum dots (QDs) on Si(001), r-Al2O3, Si(111), and c-GaN substrates. For the polar substrates (i.e. Si(111) and c-GaN), high-resolution transmission electron microscopy and coherent Bragg rod analysis reveals the formation of coherent WZ GaN QDs with nitridation-temperature-dependent sizes and densities. For the non-polar substrates (i.e. Si(001) and r-Al2O3) , QDs with strong near-band photoluminescence emission are observed and ZB GaN QD growth on Si(001) is demonstrated for the first time.

  11. Boron doped GaN and InN: Potential candidates for spintronics

    NASA Astrophysics Data System (ADS)

    Fan, S. W.; Huang, X. N.; Yao, K. L.

    2017-02-01

    The full potential linearized augmented plane wave method together with the Tran-Blaha modified Becke-Johnson potential is utilized to investigate the electronic structures and magnetism for boron doped GaN and InN. Calculations show the boron substituting nitrogen (BN defects) could induce the GaN and InN to be half-metallic ferromagnets. The magnetic moments mainly come from the BN defects, and each BN defect would produce the 2.00 μB total magnetic moment. The electronic structures indicate the carriers-mediated double exchange interaction plays a crucial role in forming the ferromagnetism. Positive chemical pair interactions imply the BN defects would form the homogeneous distribution in GaN and InN matrix. Moderate formation energies suggest that GaN and InN with BN defects could be fabricated experimentally.

  12. Tuning electronic and magnetic properties of GaN nanosheets by surface modifications and nanosheet thickness.

    PubMed

    Xiao, Meixia; Yao, Tingzhen; Ao, Zhimin; Wei, Peng; Wang, Danghui; Song, Haiyang

    2015-04-14

    Density-functional theory calculations are performed to investigate the effects of surface modifications and nanosheet thickness on the electronic and magnetic properties of gallium nitride (GaN) nanosheets (NSs). Unlike the bare GaN NSs terminating with polar surfaces, the systems with hydrogenated Ga (H-GaN), fluorinated Ga (F-GaN), and chlorinated Ga (Cl-GaN) preserve their initial wurtzite structures and exhibit ferromagnetic states. The abovementioned three different decorations on Ga atoms are energetically more favorable for thicker GaN NSs. Moreover, as the thickness increases, H-GaN and F-GaN NSs undergo semiconductor to metal and half-metal to metal transition, respectively, while Cl-GaN NSs remain completely metallic. The predicted diverse and tunable electronic and magnetic properties highlight the potential of GaN NSs for novel electronic and spintronic nanodevices.

  13. Surface potential barrier in m-plane GaN studied by contactless electroreflectance

    NASA Astrophysics Data System (ADS)

    Janicki, Lukasz; Misiewicz, Jan; Cywiński, Grzegorz; Sawicka, Marta; Skierbiszewski, Czeslaw; Kudrawiec, Robert

    2016-02-01

    Contactless electroreflectance (CER) is used to study the surface potential barrier in m-plane GaN UN+ [GaN (d = 20,30,50,70 nm)/GaN:Si] structures grown by using molecular beam epitaxy. Clear bandgap-related transitions followed by Franz-Keldysh oscillations (FKO) have been observed in the CER spectra of all samples at room temperature. The built-in electric fields in the undoped cap layers have been determined from the FKO period. From the built-in electric field and the undoped GaN layer thickness, the Fermi level location at the air-exposed m-plane GaN surface has been estimated as 0.42 ± 0.05 eV below the conduction band.

  14. About Implantable Contraception

    MedlinePlus

    ... Health Food & Fitness Diseases & Conditions Infections Drugs & Alcohol School & Jobs Sports Expert Answers (Q&A) Staying Safe Videos for Educators Search English Español About Implantable Contraception KidsHealth / For Parents / About Implantable Contraception Print What ...

  15. Photoelectrochemical etching measurement of defect density in GaN grown by nanoheteroepitaxy

    NASA Astrophysics Data System (ADS)

    Ferdous, M. S.; Sun, X. Y.; Wang, X.; Fairchild, M. N.; Hersee, S. D.

    2006-05-01

    The density of dislocations in n-type GaN was measured by photoelectrochemical etching. A 10× reduction in dislocation density was observed compared to planar GaN grown at the same time. Cross-sectional transmission electron microscopy studies indicate that defect reduction is due to the mutual cancellation of dislocations with equal and opposite Burger's vectors. The nanoheteroepitaxy sample exhibited significantly higher photoluminescence intensity and higher electron mobility than the planar reference sample.

  16. Linear Distributed GaN MMIC Power Amplifier with Improved Power-added Efficiency

    DTIC Science & Technology

    2017-03-01

    Laboratories, 3011 Malibu Canyon Road, Malibu, CA 90265 Abstract: We report on a multi-octave (100 MHz ‒ 8 GHz), linear nonuniform distributed...amplifier (NDPA) in a MMIC architecture using scaled 120-nm short-gate- length GaN HEMTs. The linear NDPAs were built with six sections in a nonuniform ...MHz ‒ 8 GHz) GaN MMIC nonuniform distributed amplifier (NDPA) with built-in linearization and a gm3 cancellation method in class A and class C

  17. Flexible GaN for High Performance, Strainable Radio Frequency Devices (Postprint)

    DTIC Science & Technology

    2017-11-02

    devices on van der Waals (vdW) layers has been facilitated by the recent avail - ability of high -quality atomically smooth BN and graphene epi- taxial...AFRL-RX-WP-JA-2017-0333 FLEXIBLE GaN FOR HIGH PERFORMANCE, STRAINABLE RADIO FREQUENCY DEVICES (POSTPRINT) Elizabeth A. Moore and Timothy...2. REPORT TYPE 3. DATES COVERED (From - To) 5 April 2017 Interim 8 September 2014 – 5 March 2017 4. TITLE AND SUBTITLE FLEXIBLE GaN FOR HIGH

  18. Influence of ammonia flow rate for improving properties of polycrystalline GaN

    NASA Astrophysics Data System (ADS)

    Ariff, A.; Ahmad, M. A.; Hassan, Z.; Zainal, N.

    2018-06-01

    Post-annealing treatment in ammonia ambient is widely accepted for GaN material, but less works have been done to investigate the influence of the ammonia (NH3) flow rate for reducing the N-deficiency as well as improving the quality of the material. In this work, we investigated the influence of NH3 flow rate at 1, 2, 3, and 4 slm in improving properties of a ∼1 μm thick polycrystalline GaN layer. Our simulation work suggested that the uniformity of temperature and pressure gradient of the NH3 gas did not lead to the reduction of N-deficiency of the polycrystalline GaN layer. Instead, it was found that the mitigation of the N-deficiency was strongly influenced by the fluid velocity of the NH3 gas, which had passed over the layer. Either at lower or higher fluid velocity, the chance for the active N atoms to incorporate into the GaN lattice structure was low. Therefore, the N-deficiency on the polycrystalline GaN layer could not be minimized under these conditions. As measured by EDX, the N atoms incorporation was the most effective when the NH3 flow rate at 3 slm, suggesting the flow rate significantly improved the N-deficiency of the polycrystalline GaN layer. Furthermore, it favored the formation of larger hexagonal faceted grains, with the smallest FWHM of XRD peaks from the GaN diffractions in (10 1 bar 0), (0002) and (10 1 bar 1) orientations, while allowing the polycrystalline GaN layer to show sharp and intense emissions peak of NBE in a PL spectrum.

  19. Determination of the nitrogen vacancy as a shallow compensating center in GaN doped with divalent metals.

    PubMed

    Buckeridge, J; Catlow, C R A; Scanlon, D O; Keal, T W; Sherwood, P; Miskufova, M; Walsh, A; Woodley, S M; Sokol, A A

    2015-01-09

    We report accurate energetics of defects introduced in GaN on doping with divalent metals, focusing on the technologically important case of Mg doping, using a model that takes into consideration both the effect of hole localization and dipolar polarization of the host material, and includes a well-defined reference level. Defect formation and ionization energies show that divalent dopants are counterbalanced in GaN by nitrogen vacancies and not by holes, which explains both the difficulty in achieving p-type conductivity in GaN and the associated major spectroscopic features, including the ubiquitous 3.46 eV photoluminescence line, a characteristic of all lightly divalent-metal-doped GaN materials that has also been shown to occur in pure GaN samples. Our results give a comprehensive explanation for the observed behavior of GaN doped with low concentrations of divalent metals in good agreement with relevant experiment.

  20. Determination of the Nitrogen Vacancy as a Shallow Compensating Center in GaN Doped with Divalent Metals

    NASA Astrophysics Data System (ADS)

    Buckeridge, J.; Catlow, C. R. A.; Scanlon, D. O.; Keal, T. W.; Sherwood, P.; Miskufova, M.; Walsh, A.; Woodley, S. M.; Sokol, A. A.

    2015-01-01

    We report accurate energetics of defects introduced in GaN on doping with divalent metals, focusing on the technologically important case of Mg doping, using a model that takes into consideration both the effect of hole localization and dipolar polarization of the host material, and includes a well-defined reference level. Defect formation and ionization energies show that divalent dopants are counterbalanced in GaN by nitrogen vacancies and not by holes, which explains both the difficulty in achieving p -type conductivity in GaN and the associated major spectroscopic features, including the ubiquitous 3.46 eV photoluminescence line, a characteristic of all lightly divalent-metal-doped GaN materials that has also been shown to occur in pure GaN samples. Our results give a comprehensive explanation for the observed behavior of GaN doped with low concentrations of divalent metals in good agreement with relevant experiment.

  1. Influence of in-situ deposited SiNx interlayer on crystal quality of GaN epitaxial films

    NASA Astrophysics Data System (ADS)

    Fan, Teng; Jia, Wei; Tong, Guangyun; Zhai, Guangmei; Li, Tianbao; Dong, Hailiang; Xu, Bingshe

    2018-05-01

    GaN epitaxial films with SiNx interlayers were prepared by metal organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates. The influences of deposition times and locations of SiNx interlayers on crystal quality of GaN epitaxial films were studied. Under the optimal growth time of 120 s for the SiNx interlayer, the dislocation density of GaN film is reduced to 4.05 × 108 cm-2 proved by high resolution X-ray diffraction results. It is found that when the SiNx interlayer deposits on the GaN nucleation islands, the subsequent GaN film has the lowest dislocation density of only 2.89 × 108 cm-2. Moreover, a model is proposed to illustrate the morphological evolution and associated propagation processes of TDs in GaN epi-layers with SiNx interlayers for different deposition times and locations.

  2. Implantable Heart Aid

    NASA Technical Reports Server (NTRS)

    1984-01-01

    CPI's human-implantable automatic implantable defibrillator (AID) is a heart assist system, derived from NASA's space circuitry technology, that can prevent erratic heart action known as arrhythmias. Implanted AID, consisting of microcomputer power source and two electrodes for sensing heart activity, recognizes onset of ventricular fibrillation (VF) and delivers corrective electrical countershock to restore rhythmic heartbeat.

  3. Carrier confinement effects of InxGa1-xN/GaN multi quantum disks with GaN surface barriers grown in GaN nanorods

    NASA Astrophysics Data System (ADS)

    Park, Youngsin; Chan, Christopher C. S.; Taylor, Robert A.; Kim, Nammee; Jo, Yongcheol; Lee, Seung W.; Yang, Woochul; Im, Hyunsik

    2018-04-01

    Structural and optical properties of InxGa1-xN/GaN multi quantum disks (QDisks) grown on GaN nanorods by molecular beam epitaxy have been investigated by transmission electron microscopy and micro-photoluminescence (PL) spectroscopy. Two types of InGaN QDisks were grown: a pseudo-3D confined InGaN pillar-type QDisks embedded in GaN nanorods; and QDisks in flanged cone type GaN nanorods. The PL emission peak and excitation dependent PL behavior of the pillar-type Qdisks differ greatly from those of the flanged cone type QDisks. Time resolved PL was carried out to probe the differences in charge carrier dynamics. The results suggest that by constraining the formation of InGaN QDisks within the centre of the nanorod, carriers are restricted from migrating to the surface, decreasing the surface recombination at high carrier densities.

  4. Oxygen adsorption and incorporation at irradiated GaN(0001) and GaN(0001¯) surfaces: First-principles density-functional calculations

    NASA Astrophysics Data System (ADS)

    Sun, Qiang; Selloni, Annabella; Myers, T. H.; Doolittle, W. Alan

    2006-11-01

    Density functional theory calculations of oxygen adsorption and incorporation at the polar GaN(0001) and GaN(0001¯) surfaces have been carried out to explain the experimentally observed reduced oxygen concentration in GaN samples grown by molecular beam epitaxy in the presence of high energy (˜10keV) electron beam irradiation [Myers , J. Vac. Sci. Technol. B 18, 2295 (2000)]. Using a model in which the effect of the irradiation is to excite electrons from the valence to the conduction band, we find that both the energy cost of incorporating oxygen impurities in deeper layers and the oxygen adatom diffusion barriers are significantly reduced in the presence of the excitation. The latter effect leads to a higher probability for two O adatoms to recombine and desorb, and thus to a reduced oxygen concentration in the irradiated samples, consistent with experimental observations.

  5. Earth Observation

    2014-07-19

    ISS040-E-070439 (19 July 2014) --- One of the Expedition 40 crew members aboard the Earth-orbiting International Space Station recorded this July 19 image of wildfires which are plaguing the Northwest and causing widespread destruction. The orbital outpost was flying 223 nautical miles above a point on Earth located at 48.0 degrees north latitude and 116.9 degrees west longitude when the image was exposed. The state of Washington is especially affected by the fires, many of which have been blamed on lightning. This particular fire was part of the Carlton Complex Fire, located near the city of Brewster in north central Washington. The reservoir visible near the center of the image is Banks Lake.

  6. Hybrid device based on GaN nanoneedles and MEH-PPV/PEDOT:PSS polymer

    SciT

    Shin, Min Jeong; Gwon, Dong-Oh; Lee, Chan-Mi

    2015-08-15

    Highlights: • A hybrid device was demonstrated by using MEH-PPV, PEDOT:PSS, and GaN nanoneedles. • I–V curve of the hybrid device showed its rectification behaviour, similar to a diode. • EL peak originated by the different potential barriers at MEH-PPV and GaN interface. - Abstract: A hybrid device that combines the properties of organic and inorganic semiconductors was fabricated and studied. It incorporated poly[2-methoxy-5-(2-ethylhexyloxy)- 1,4-phenylenevinylene] (MEH-PPV) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as organic polymers and GaN nanoneedles as an inorganic semiconductor. Layers of the two polymers were spin coated on to the GaN nanoneedles. The one peak in the electroluminescence spectrum originatedmore » from the MEH-PPV layer owing to the different potential barriers of electrons and holes at its interface with the GaN nanoneedles. However, the photoluminescence spectrum showed peaks due to both GaN nanoneedles and MEH-PPV. Such hybrid structures, suitably developed, might be able to improve the efficiency of optoelectronic devices.« less

  7. Understanding the Growth Mechanism of GaN Epitaxial Layers on Mechanically Exfoliated Graphite

    NASA Astrophysics Data System (ADS)

    Li, Tianbao; Liu, Chenyang; Zhang, Zhe; Yu, Bin; Dong, Hailiang; Jia, Wei; Jia, Zhigang; Yu, Chunyan; Gan, Lin; Xu, Bingshe; Jiang, Haiwei

    2018-04-01

    The growth mechanism of GaN epitaxial layers on mechanically exfoliated graphite is explained in detail based on classic nucleation theory. The number of defects on the graphite surface can be increased via O-plasma treatment, leading to increased nucleation density on the graphite surface. The addition of elemental Al can effectively improve the nucleation rate, which can promote the formation of dense nucleation layers and the lateral growth of GaN epitaxial layers. The surface morphologies of the nucleation layers, annealed layers and epitaxial layers were characterized by field-emission scanning electron microscopy, where the evolution of the surface morphology coincided with a 3D-to-2D growth mechanism. High-resolution transmission electron microscopy was used to characterize the microstructure of GaN. Fast Fourier transform diffraction patterns showed that cubic phase (zinc-blend structure) GaN grains were obtained using conventional GaN nucleation layers, while the hexagonal phase (wurtzite structure) GaN films were formed using AlGaN nucleation layers. Our work opens new avenues for using highly oriented pyrolytic graphite as a substrate to fabricate transferable optoelectronic devices.

  8. Electron transport properties of degenerate n-type GaN prepared by pulsed sputtering

    NASA Astrophysics Data System (ADS)

    Ueno, Kohei; Fudetani, Taiga; Arakawa, Yasuaki; Kobayashi, Atsushi; Ohta, Jitsuo; Fujioka, Hiroshi

    2017-12-01

    We report a systematic investigation of the transport properties of highly degenerate electrons in Ge-doped and Si-doped GaN epilayers prepared using the pulsed sputtering deposition (PSD) technique. Secondary-ion mass spectrometry and Hall-effect measurements revealed that the doping efficiency of PSD n-type GaN is close to unity at electron concentrations as high as 5.1 × 1020 cm-3. A record low resistivity for n-type GaN of 0.16 mΩ cm was achieved with an electron mobility of 100 cm2 V-1 s-1 at a carrier concentration of 3.9 × 1020 cm-3. We explain this unusually high electron mobility of PSD n-type GaN within the framework of conventional scattering theory by modifying a parameter related to nonparabolicity of the conduction band. The Ge-doped GaN films show a slightly lower electron mobility compared with Si-doped films with the same carrier concentrations, which is likely a consequence of the formation of a small number of compensation centers. The excellent electrical properties presented in this letter clearly demonstrate the striking advantages of the low-temperature PSD technique for growing high-quality and highly conductive n-type GaN.

  9. [Two novel pathogenic mutations of GAN gene identified in a patient with giant axonal neuropathy].

    PubMed

    Wang, Juan; Ma, Qingwen; Cai, Qin; Liu, Yanna; Wang, Wei; Ren, Zhaorui

    2016-06-01

    To explore the disease-causing mutations in a patient suspected for giant axonal neuropathy(GAN). Target sequence capture sequencing was used to screen potential mutations in genomic DNA extracted from peripheral blood sample of the patient. Sanger sequencing was applied to confirm the detected mutation. The mutation was verified among 400 GAN alleles from 200 healthy individuals by Sanger sequencing. The function of the mutations was predicted by bioinformatics analysis. The patient was identified as a compound heterozygote carrying two novel pathogenic GAN mutations, i.e., c.778G>T (p.Glu260Ter) and c.277G>A (p.Gly93Arg). Sanger sequencing confirmed that the c.778G>T (p.Glu260Ter) mutation was inherited from his father, while c.277G>A (p.Gly93Arg) was inherited from his mother. The same mutations was not found in the 200 healthy individuals. Bioinformatics analysis predicted that the two mutations probably caused functional abnormality of gigaxonin. Two novel GAN mutations were detected in a patient with GAN. Both mutations are pathogenic and can cause abnormalities of gigaxonin structure and function, leading to pathogenesis of GAN. The results may also offer valuable information for similar diseases.

  10. Light Modulation and Water Splitting Enhancement Using a Composite Porous GaN Structure.

    PubMed

    Yang, Chao; Xi, Xin; Yu, Zhiguo; Cao, Haicheng; Li, Jing; Lin, Shan; Ma, Zhanhong; Zhao, Lixia

    2018-02-14

    On the basis of the laterally porous GaN, we designed and fabricated a composite porous GaN structure with both well-ordered lateral and vertical holes. Compared to the plane GaN, the composite porous GaN structure with the combination of the vertical holes can help to reduce UV reflectance and increase the saturation photocurrent during water splitting by a factor of ∼4.5. Furthermore, we investigated the underlying mechanism for the enhancement of the water splitting performance using a finite-difference time-domain method. The results show that the well-ordered vertical holes can not only help to open the embedded pore channels to the electrolyte at both sides and reduce the migration distance of the gas bubbles during the water splitting reactions but also help to modulate the light field. Using this composite porous GaN structure, most of the incident light can be modulated and trapped into the nanoholes, and thus the electric fields localized in the lateral pores can increase dramatically as a result of the strong optical coupling. Our findings pave a new way to develop GaN photoelectrodes for highly efficient solar water splitting.

  11. Fabrication and characterization of GaN nanowire doubly clamped resonators

    SciT

    Maliakkal, Carina B., E-mail: carina@tifr.res.in; Mathew, John P.; Hatui, Nirupam

    2015-09-21

    Gallium nitride (GaN) nanowires (NWs) have been intensely researched as building blocks for nanoscale electronic and photonic device applications; however, the mechanical properties of GaN nanostructures have not been explored in detail. The rigidity, thermal stability, and piezoelectric properties of GaN make it an interesting candidate for nano-electromechanical systems. We have fabricated doubly clamped GaN NW electromechanical resonators on sapphire using electron beam lithography and estimated the Young's modulus of GaN from resonance frequency measurements. For wires of triangular cross section with side ∼90 nm, we obtained values for the Young's modulus to be about 218 and 691 GPa, which are ofmore » the same order of magnitude as the values reported for bulk GaN. We also discuss the role of residual strain in the nanowire on the resonant frequency and the orientation dependence of the Young's modulus in wurtzite crystals.« less

  12. Understanding the Growth Mechanism of GaN Epitaxial Layers on Mechanically Exfoliated Graphite.

    PubMed

    Li, Tianbao; Liu, Chenyang; Zhang, Zhe; Yu, Bin; Dong, Hailiang; Jia, Wei; Jia, Zhigang; Yu, Chunyan; Gan, Lin; Xu, Bingshe; Jiang, Haiwei

    2018-04-27

    The growth mechanism of GaN epitaxial layers on mechanically exfoliated graphite is explained in detail based on classic nucleation theory. The number of defects on the graphite surface can be increased via O-plasma treatment, leading to increased nucleation density on the graphite surface. The addition of elemental Al can effectively improve the nucleation rate, which can promote the formation of dense nucleation layers and the lateral growth of GaN epitaxial layers. The surface morphologies of the nucleation layers, annealed layers and epitaxial layers were characterized by field-emission scanning electron microscopy, where the evolution of the surface morphology coincided with a 3D-to-2D growth mechanism. High-resolution transmission electron microscopy was used to characterize the microstructure of GaN. Fast Fourier transform diffraction patterns showed that cubic phase (zinc-blend structure) GaN grains were obtained using conventional GaN nucleation layers, while the hexagonal phase (wurtzite structure) GaN films were formed using AlGaN nucleation layers. Our work opens new avenues for using highly oriented pyrolytic graphite as a substrate to fabricate transferable optoelectronic devices.

  13. Earth Observation

    2014-07-25

    ISS040-E-081008 (25 July 2014) --- One of the Expedition 40 crew members aboard the International Space Station, flying 225 nautical miles above Earth, photographed this image of the Tifernine dunes and the Tassili Najjer Mountains in Algeria. The area is about 800 miles south, southeast of Algiers, the capital of Algeria. The dunes are in excess of 1,000 feet in height.

  14. Earth Observation

    2014-07-15

    ISS040-E-063578 (15 July 2014) --- One of the Expedition 40 crew members aboard the Earth-orbiting International Space Station, flying some 225 nautical miles above the Caribbean Sea in the early morning hours of July 15, photographed this north-looking panorama that includes parts of Cuba, the Bahamas and Florida, and even runs into several other areas in the southeastern U.S. The long stretch of lights to the left of center frame gives the shape of Miami.

  15. Earth Observations

    2011-05-28

    ISS028-E-006059 (28 May 2011) --- One of the Expedition 28 crew members, photographing Earth images onboard the International Space Station while docked with the space shuttle Endeavour and flying at an altitude of just under 220 miles, captured this frame of the Salton Sea. The body of water, easily identifiable from low orbit spacecraft, is a saline, endorheic rift lake located directly on the San Andreas Fault. The agricultural area is within the Coachella Valley.

  16. Earth Science

    1991-01-01

    In July 1990, the Marshall Space Flight Center, in a joint project with the Department of Defense/Air Force Space Test Program, launched the Combined Release and Radiation Effects Satellite (CRRES) using an Atlas I launch vehicle. The mission was designed to study the effects of artificial ion clouds produced by chemical releases on the Earth's ionosphere and magnetosphere, and to monitor the effects of space radiation environment on sophisticated electronics.

  17. Earth Observation

    2011-06-27

    ISS028-E-009979 (27 June 2011) --- The Massachusetts coastline is featured in this image photographed by an Expedition 28 crew member on the International Space Station. The Crew Earth Observations team at NASA Johnson Space Center sends specific ground targets for photography up to the station crew on a daily basis, but sometimes the crew takes imagery on their own of striking displays visible from orbit. One such display, often visible to the ISS crew due to their ability to look outwards at angles between 0 and 90 degrees, is sunglint on the waters of Earth. Sunglint is caused by sunlight reflecting off of a water surface?much as light reflects from a mirror?directly towards the observer. Roughness variations of the water surface scatter the light, blurring the reflection and producing the typical silvery sheen of the sunglint area. The point of maximum sunglint is centered within Cape Cod Bay, the body of water partially enclosed by the ?hook? of Cape Cod in Massachusetts (bottom). Cape Cod was formally designated a National Seashore in 1966. Sunglint off the water provides sharp contrast with the coastline and the nearby islands of Martha?s Vineyard and Nantucket (lower left), both popular destinations for tourists and summer residents. To the north, rocky Cape Ann extends out into the Atlantic Ocean; the border with New Hampshire is located approximately 30 kilometers up the coast. Further to the west, the eastern half of Long Island, New York is visible emerging from extensive cloud cover over the mid-Atlantic and Midwestern States. Persistent storm tracks had been contributing to record flooding along rivers in the Midwest at the time this image was taken in late June 2011. Thin blue layers of the atmosphere, contrasted against the darkness of space, are visible extending along the Earth?s curvature at top.

  18. Stacking fault related luminescence in GaN nanorods.

    PubMed

    Forsberg, M; Serban, A; Poenaru, I; Hsiao, C-L; Junaid, M; Birch, J; Pozina, G

    2015-09-04

    Optical and structural properties are presented for GaN nanorods (NRs) grown in the [0001] direction on Si(111) substrates by direct-current reactive magnetron sputter epitaxy. Transmission electron microscopy (TEM) reveals clusters of dense stacking faults (SFs) regularly distributed along the c-axis. A strong emission line at ∼3.42 eV associated with the basal-plane SFs has been observed in luminescence spectra. The optical signature of SFs is stable up to room temperatures with the activation energy of ∼20 meV. Temperature-dependent time-resolved photoluminescence properties suggest that the recombination mechanism of the 3.42 eV emission can be understood in terms of multiple quantum wells self-organized along the growth axis of NRs.

  19. Intrinsic polarization control in rectangular GaN nanowire lasers

    DOE PAGES

    Li, Changyi; Liu, Sheng; Luk, Ting S.; ...

    2016-02-01

    In this study, we demonstrate intrinsic, linearly polarized lasing from single GaN nanowires using cross-sectional shape control. A two-step top-down fabrication approach was employed to create straight nanowires with controllable rectangular cross-sections. A clear lasing threshold of 444kW/cm 2 and a narrow spectral line width of 0.16 nm were observed under optical pumping at room temperature, indicating the onset of lasing. The polarization was along the short dimension (y-direction) of the nanowire due to the higher transverse confinement factors for y-polarized transverse modes resulting from the rectangular nanowire cross-section. The results show that cross-sectioned shape control can enable inherent controlmore » over the polarization of nanowire lasers without additional environment requirements, such as placement onto lossy substrates.« less

  20. Vacancy charged defects in two-dimensional GaN

    NASA Astrophysics Data System (ADS)

    González, Roberto; López-Pérez, William; González-García, Álvaro; Moreno-Armenta, María G.; González-Hernández, Rafael

    2018-03-01

    In this paper, we have studied the structural and electronic properties of vacancy charged defects in the graphene phase (honeycomb type) of gallium nitride (g-GaN) by using first-principle calculations within the framework of the Density Functional Theory. It is found that the vacancies introduce defect levels in the band gap, and these generate a total magnetization in the g-GaN system. The formation energy with different charge states for the vacancies of gallium and nitrogen were calculated, obtaining higher energies than the GaN wurtzite phase (w-GaN). Furthermore, nitrogen vacancies were found to be more stable than gallium vacancies in a whole range of electronic chemical potential. Finally, gallium and nitrogen vacancies produce a nonzero magnetic moment in g-GaN, making it a potential candidate for future spintronics applications.

  1. Hole-mediated stabilization of cubic GaN.

    PubMed

    Dalpian, Gustavo M; Wei, Su-Huai

    2004-11-19

    We propose here a new approach to stabilize the cubic zinc-blende (ZB) phase by incorporation of impurities into a compound that has a hexagonal wurtzite (WZ) ground state. For GaN, we suggest that this can be achieved by adding 3d acceptors such as Zn, Mn, or Cu because the p-d repulsion between the 3d impurity levels and the valence band maximum is larger in the ZB phase than in the WZ phase. This makes the top of the valence states of the ZB structure higher than that of the WZ structure. As holes are created at the top of the valence states by the impurities, it will cost less energy for the holes to be created in the ZB structure, thus stabilizing this phase. Our first-principles total energy calculations confirm this novel idea.

  2. [Effect of jian-gan-le on advanced schistosomiasis].

    PubMed

    He, Zheng-Wen; Wang, You-Bin; Huang, Wen-Jun

    2011-06-01

    A total of 80 cases of advanced schistosomiasis were selected and divided into an experiment group and a control group, 40 cases each group, by the random sampling method. The patients in the experiment group were administered with Jian-gan-le, and the patients in the control group received compound purple granules. In the experiment group, the curative rate was 25.0%, the improving rate was 70.0%, the inefficacy rate was 5%, and the efficiency rate was 95.0%. In the control group, the curative rate was 12.5%, the improving rate was 75%, the inefficacy rate was 12.5%. There was no statistic difference between the 2 groups (P all > 0.05). The expense was cheaper in the experiment group than in the control group.

  3. Spatial distribution of defect luminescence in GaN nanowires.

    PubMed

    Li, Qiming; Wang, George T

    2010-05-12

    The spatial distribution of defect-related and band-edge luminescence from GaN nanowires grown by metal-organic chemical vapor deposition was studied by spatially resolved cathodoluminescence imaging and spectroscopy. A surface layer exhibiting strong yellow luminescence (YL) near 566 nm in the nanowires was revealed, compared to weak YL in the bulk. In contrast, other defect-related luminescence near 428 nm (blue luminescence) and 734 nm (red luminescence), in addition to band-edge luminescence (BEL) at 366 nm, were observed in the bulk of the nanowires but were largely absent at the surface. As the nanowire width approaches a critical dimension, the surface YL layer completely quenches the BEL. The surface YL is attributed to the diffusion and piling up of mobile point defects, likely isolated gallium vacancies, at the surface during growth.

  4. Cloudy Earth

    2015-05-08

    Decades of satellite observations and astronaut photographs show that clouds dominate space-based views of Earth. One study based on nearly a decade of satellite data estimated that about 67 percent of Earth’s surface is typically covered by clouds. This is especially the case over the oceans, where other research shows less than 10 percent of the sky is completely clear of clouds at any one time. Over land, 30 percent of skies are completely cloud free. Earth’s cloudy nature is unmistakable in this global cloud fraction map, based on data collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Aqua satellite. While MODIS collects enough data to make a new global map of cloudiness every day, this version of the map shows an average of all of the satellite’s cloud observations between July 2002 and April 2015. Colors range from dark blue (no clouds) to light blue (some clouds) to white (frequent clouds). Read more here: 1.usa.gov/1P6lbMU Credit: NASA Earth Observatory NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

  5. Earth Observation

    2011-08-02

    ISS028-E-020276 (2 Aug. 2011) --- This photograph of polar mesospheric clouds was acquired at an altitude of just over 202 nautical miles (about 322 kilometers) in the evening hours (03:19:54 Greenwich Mean Time) on Aug. 2, 2011, as the International Space Station was passing over the English Channel. The nadir coordinates of the station were 49.1 degrees north latitude and 5.5 degrees west longitude. Polar mesospheric clouds (also known as noctilucent, or ?night-shining? clouds) are transient, upper atmospheric phenomena that are usually observed in the summer months at high latitudes (greater than 50 degrees) of both the Northern and Southern Hemispheres. They appear bright and cloudlike while in deep twilight. They are illuminated by sunlight when the lower layers of the atmosphere are in the darkness of Earth?s shadow. The horizon of Earth appears at the bottom of the image, with some layers of the lower atmosphere already illuminated by the rising sun. The higher, bluish-colored clouds look much like wispy cirrus clouds, which can be found as high as 60,000 feet (18 kilometers) in the atmosphere. However noctilucent clouds, as seen here, are observed in the mesosphere at altitudes of 250,000 to 280,000 feet (about 76 to 85 kilometers). Astronaut observations of polar mesospheric clouds over northern Europe in the summer are not uncommon.

  6. [Clinical trial on treatment of Parkinson's disease of Gan-Shen yin deficiency type by recipe for nourishing Gan-Shen].

    PubMed

    Zhao, Hong; Li, Wen-Wei; Gao, Jun-Peng

    2007-09-01

    To observe the curative effect of the recipe for nourishing Gan-Shen on Parkinson's disease (PD) of Gan-Shen yin deficiency type. One hundred and twenty-one PD patients were ran-domly assigned by blocking design to the control group and the treated group in the ratio of 1:1. All were treated according to the international medication guiding principle for PD treatment, but the treated group was ad-ministered with the recipe for nourishing Gan-Shen additionally. The treatment course lasted for 12 consecutive months, and the end point was the end of the 12th month. The unified Parkinson's disease rating scale (UP-DRS) score, TCM primary and secondary symptom scores were evaluated before treatment, every 3 months of treatment and at the end point. The average daily levodopa dose and the Hoehn & Yahr grading were assessed before treatment and at the end point. After treatment, UPDRS score in both groups showed an ascending trend at a slower rate in the treated groups than in the control group. At the 9th and 12th month of medication, a significant difference was found in UPDRS score between the two groups (P < 0.05), and the TCM symptom score was obviously lower in the treated group than in the control group (P < 0.05). At the end point of the trial, the average daily levodopa dose used was lower in the treated group than in the control group (P < 0.05) and there was no significant difference in the Hoehn & Yahr score between the two groups (P > 0.05). The recipe for norishing Gan-Shen can slow the ascending trend of UPDRS score in the PD patients, improve the symptoms of Gan-Shen yin deficiency, and decrease the daily levodopa dose used, showing a curative effect on PD of Gan-Shen yin deficiency type.

  7. The trap states in lightly Mg-doped GaN grown by MOVPE on a freestanding GaN substrate

    NASA Astrophysics Data System (ADS)

    Narita, Tetsuo; Tokuda, Yutaka; Kogiso, Tatsuya; Tomita, Kazuyoshi; Kachi, Tetsu

    2018-04-01

    We investigated traps in lightly Mg-doped (2 × 1017 cm-3) p-GaN fabricated by metalorganic vapor phase epitaxy (MOVPE) on a freestanding GaN substrate and the subsequent post-growth annealing, using deep level transient spectroscopy. We identified four hole traps with energy levels of EV + 0.46, 0.88, 1.0, and 1.3 eV and one electron trap at EC - 0.57 eV in a p-type GaN layer uniformly doped with magnesium (Mg). The Arrhenius plot of hole traps with the highest concentration (˜3 × 1016 cm-3) located at EV + 0.88 eV corresponded to those of hole traps ascribed to carbon on nitrogen sites in n-type GaN samples grown by MOVPE. In fact, the range of the hole trap concentrations at EV + 0.88 eV was close to the carbon concentration detected by secondary ion mass spectroscopy. Moreover, the electron trap at EC - 0.57 eV was also identical to the dominant electron traps commonly observed in n-type GaN. Together, these results suggest that the trap states in the lightly Mg-doped GaN grown by MOVPE show a strong similarity to those in n-type GaN, which can be explained by the Fermi level close to the conduction band minimum in pristine MOVPE grown samples due to existing residual donors and Mg-hydrogen complexes.

  8. GaN Based Electronics And Their Applications

    NASA Astrophysics Data System (ADS)

    Ren, Fan

    2002-03-01

    The Group III-nitrides were initially researched for their promise to fill the void for a blue solid state light emitter. Electronic devices from III-nitrides have been a more recent phenomenon. The thermal conductivity of GaN is three times that of GaAs. For high power or high temperature applications, good thermal conductivity is imperative for heat removal or sustained operation at elevated temperatures. The development of III-N and other wide bandgap technologies for high temperature applications will likely take place at the expense of competing technologies, such as silicon-on-insulator (SOI), at moderate temperatures. At higher temperatures (>300°C), novel devices and components will become possible. The automotive industry will likely be one of the largest markets for such high temperature electronics. One of the most noteworthy advantages for III-N materials over other wide bandgap semiconductors is the availability of AlGaN/GaN and InGaN/GaN heterostructures. A 2-dimensional electron gas (2DEG) has been shown to exist at the AlGaN/GaN interface, and heterostructure field effect transistors (HFETs) from these materials can exhibit 2DEG mobilities approaching 2000 cm2 / V?s at 300K. Power handling capabilities of 12 W/mm appear feasible, and extraordinary large signal performance has already been demonstrated, with a current state-of-the-art of >10W/mm at X-band. In this talk, high speed and high temperature AlGaN/GaN HEMTs as well as MOSHEMTs, high breakdown voltage GaN (>6KV) and AlGaN (9.7 KV) Schottky diodes, and their applications will be presented.

  9. Zn-dopant dependent defect evolution in GaN nanowires

    NASA Astrophysics Data System (ADS)

    Yang, Bing; Liu, Baodan; Wang, Yujia; Zhuang, Hao; Liu, Qingyun; Yuan, Fang; Jiang, Xin

    2015-10-01

    Zn doped GaN nanowires with different doping levels (0, <1 at%, and 3-5 at%) have been synthesized through a chemical vapor deposition (CVD) process. The effect of Zn doping on the defect evolution, including stacking fault, dislocation, twin boundary and phase boundary, has been systematically investigated by transmission electron microscopy and first-principles calculations. Undoped GaN nanowires show a hexagonal wurtzite (WZ) structure with good crystallinity. Several kinds of twin boundaries, including (101&cmb.macr;3), (101&cmb.macr;1) and (202&cmb.macr;1), as well as Type I stacking faults (...ABABC&cmb.b.line;BCB...), are observed in the nanowires. The increasing Zn doping level (<1 at%) induces the formation of screw dislocations featuring a predominant screw component along the radial direction of the GaN nanowires. At high Zn doping level (3-5 at%), meta-stable cubic zinc blende (ZB) domains are generated in the WZ GaN nanowires. The WZ/ZB phase boundary (...ABABAC&cmb.b.line;BA...) can be identified as Type II stacking faults. The density of stacking faults (both Type I and Type II) increases with increasing the Zn doping levels, which in turn leads to a rough-surface morphology in the GaN nanowires. First-principles calculations reveal that Zn doping will reduce the formation energy of both Type I and Type II stacking faults, favoring their nucleation in GaN nanowires. An understanding of the effect of Zn doping on the defect evolution provides an important method to control the microstructure and the electrical properties of p-type GaN nanowires.Zn doped GaN nanowires with different doping levels (0, <1 at%, and 3-5 at%) have been synthesized through a chemical vapor deposition (CVD) process. The effect of Zn doping on the defect evolution, including stacking fault, dislocation, twin boundary and phase boundary, has been systematically investigated by transmission electron microscopy and first-principles calculations. Undoped GaN nanowires show a

  10. Ion implantation in group III-nitride semiconductors: a tool for doping and defect studies

    NASA Astrophysics Data System (ADS)

    Zolper, J. C.

    1997-06-01

    Ion implantation is a flexible process technology for introducing an array of doping or compensating impurities into semiconductors. As the crystal quality of the group III-nitride materials continues to improve, ion implantation is playing an enabling role in exploring new dopant species and device structures. In this paper we review the recent developments in ion implantation processing of these materials with a particular emphasis on how this technology has brought new understanding to this materials system. In particular, the use of ion implantation to characterize impurity luminescence, doping, and compensation in III-nitride materials is reviewed. In addition, we address the nature of implantation induced damage in GaN which demonstrates a very strong resistance to amorphization while at the same time forming damage that is not easily removed by thermal annealing. Finally, we review the coupling of implantation with high temperature rapid thermal annealing to better understand the thermal stability of these materials and the redistribution properties of the common dopant (Si, O, Be, Mg, Ca, and Zn).

  11. Trends in Cochlear Implants

    PubMed Central

    Zeng, Fan-Gang

    2004-01-01

    More than 60,000 people worldwide use cochlear implants as a means to restore functional hearing. Although individual performance variability is still high, an average implant user can talk on the phone in a quiet environment. Cochlear-implant research has also matured as a field, as evidenced by the exponential growth in both the patient population and scientific publication. The present report examines current issues related to audiologic, clinical, engineering, anatomic, and physiologic aspects of cochlear implants, focusing on their psychophysical, speech, music, and cognitive performance. This report also forecasts clinical and research trends related to presurgical evaluation, fitting protocols, signal processing, and postsurgical rehabilitation in cochlear implants. Finally, a future landscape in amplification is presented that requires a unique, yet complementary, contribution from hearing aids, middle ear implants, and cochlear implants to achieve a total solution to the entire spectrum of hearing loss treatment and management. PMID:15247993

  12. Trends in cochlear implants.

    PubMed

    Zeng, Fan-Gang

    2004-01-01

    More than 60,000 people worldwide use cochlear implants as a means to restore functional hearing. Although individual performance variability is still high, an average implant user can talk on the phone in a quiet environment. Cochlear-implant research has also matured as a field, as evidenced by the exponential growth in both the patient population and scientific publication. The present report examines current issues related to audiologic, clinical, engineering, anatomic, and physiologic aspects of cochlear implants, focusing on their psychophysical, speech, music, and cognitive performance. This report also forecasts clinical and research trends related to presurgical evaluation, fitting protocols, signal processing, and postsurgical rehabilitation in cochlear implants. Finally, a future landscape in amplification is presented that requires a unique, yet complementary, contribution from hearing aids, middle ear implants, and cochlear implants to achieve a total solution to the entire spectrum of hearing loss treatment and management.

  13. Rare earth doped III-nitride semiconductors for spintronic and optoelectronic applications (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Palai, Ratnakar

    2016-10-01

    Since last four decades the information and communication technologies are relying on the semiconductor materials. Currently a great deal of attention is being focused on adding spin degree-of-freedom into semiconductor to create a new area of solid-state electronics, called spintronics. In spintronics not only the current but also its spin state is controlled. Such materials need to be good semiconductors for easy integration in typical integrated circuits with high sensitivity to the spin orientation, especially room temperature ferromagnetism being an important desirable property. GaN is considered to be the most important semiconductor after silicon. It is widely used for the production of green, blue, UV, and white LEDs in full color displays, traffic lights, automotive lightings, and general room lighting using white LEDs. GaN-based systems also show promise for microwave and high power electronics intended for radar, satellite, wireless base stations and spintronic applications. Rare earth (Yb, Eu, Er, and Tm) doped GaN shows many interesting optoelectronic and magnetoptic properties e. g. sharp emission from UV through visible to IR, radiation hardness, and ferromagnetism. The talk will be focused on fabrication, optoelectronic (photoluminescence, cathodeluminescence, magnetic, and x-ray photoelectron spectroscopy) properties of some rare earth doped GaN and InGaN semiconductor nanostructures grown by plasma assisted molecular beam epitaxy (MBE) and future applications.

  14. High active nitrogen flux growth of GaN by plasma assisted molecular beam epitaxy

    SciT

    McSkimming, Brian M., E-mail: mcskimming@engineering.ucsb.edu; Speck, James S.; Chaix, Catherine

    2015-09-15

    In the present study, the authors report on a modified Riber radio frequency (RF) nitrogen plasma source that provides active nitrogen fluxes more than 30 times higher than those commonly used for plasma assisted molecular beam epitaxy (PAMBE) growth of gallium nitride (GaN) and thus a significantly higher growth rate than has been previously reported. GaN films were grown using N{sub 2} gas flow rates between 5 and 25 sccm while varying the plasma source's RF forward power from 200 to 600 W. The highest growth rate, and therefore the highest active nitrogen flux, achieved was ∼7.6 μm/h. For optimized growth conditions,more » the surfaces displayed a clear step-terrace structure with an average RMS roughness (3 × 3 μm) on the order of 1 nm. Secondary ion mass spectroscopy impurity analysis demonstrates oxygen and hydrogen incorporation of 1 × 10{sup 16} and ∼5 × 10{sup 17}, respectively. In addition, the authors have achieved PAMBE growth of GaN at a substrate temperature more than 150 °C greater than our standard Ga rich GaN growth regime and ∼100 °C greater than any previously reported PAMBE growth of GaN. This growth temperature corresponds to GaN decomposition in vacuum of more than 20 nm/min; a regime previously unattainable with conventional nitrogen plasma sources. Arrhenius analysis of the decomposition rate shows that samples with a flux ratio below stoichiometry have an activation energy greater than decomposition of GaN in vacuum while samples grown at or above stoichiometry have decreased activation energy. The activation energy of decomposition for GaN in vacuum was previously determined to be ∼3.1 eV. For a Ga/N flux ratio of ∼1.5, this activation energy was found to be ∼2.8 eV, while for a Ga/N flux ratio of ∼0.5, it was found to be ∼7.9 eV.« less

  15. Earth Observations

    2010-09-11

    ISS024-E-014233 (11 Sept. 2010) --- A smoke plume near the northern Caspian Sea, Kazakhstan is featured in this image photographed by an Expedition 24 crew member on the International Space Station. This broad view of the north coast of the Caspian Sea shows a smoke plume (left) and two river deltas (bottom and lower right). The larger delta is that of the Volga River which appears prominently here in sunglint (light reflected off a water surface back towards the observer), and the smaller less prominent delta is that of the Ural River. Wide angle, oblique views ? taken looking outward at an angle, rather than straight down towards Earth ? such as this give an excellent impression of how crew members onboard the space station view Earth. For a sense of scale, the Caucasus Mts. (across the Caspian, top right) are approximately 1,100 kilometers to the southwest of the International Space Station?s nadir point location ? the point on Earth directly underneath the spacecraft ? at the time this image was taken. The smoke plume appears to be sourced in the dark-toned coastal marsh vegetation along the outer fringe of the Ural River delta, rather than in a city or at some oil storage facility. Although even small fires produce plumes that are long and bright and thus easily visible from space, the density of the smoke in this plume, and its 350-kilometer length across the entire north lobe of the Caspian Sea, suggest it was a significant fire. The smoke was thick enough nearer the source to cast shadows on the sea surface below. Lines mark three separate pulses of smoke, the most recent, nearest the source, extending directly south away from the coastline (lower left). With time, plumes become progressively more diffuse. The oldest pulse appears to be the thinnest, casting no obvious shadows (center left).

  16. Earth Observation

    2016-04-20

    ISS047e069406 (04/20/2016) ---Earth observation image taken by the Expedition 47 crew aboard the International Space Station. This is an oblique south-looking view of the main Bahama island chain. Cuba is across the entire top of the image, the Florida Peninsula on the right margin. In the Bahamas, the main Andros island is just distinguishable under cloud upper left of center. Under less cloud is the Abaco Islands in the foreground (middle of pic nearest camera left of center.)

  17. Earth Observation

    2014-06-24

    ISS040-E-018729 (24 June 2014) --- One of the Expedition 40 crew members aboard the Earth-orbiting International Space Station photographed this image featuring the peninsular portion of the state of Florida. Lake Okeechobee stands out in the south central part of the state. The heavily-populated area of Miami can be traced along the Atlantic Coast near the bottom of the scene. Cape Canaveral and the Kennedy Space Center are just below center frame on the Atlantic Coast. The Florida Keys are at the south (left) portion of the scene and the Gulf Coast, including the Tampa-St. Petersburg area, is near frame center.

  18. Earth Observation

    2014-05-29

    ISS040-E-005979 (29 May 2014) --- One of the Expedition 40 crew members aboard the International Space Station used a 200mm lens to photograph this image from 222 nautical miles above Earth showing Harris County and Galveston County, Texas plus several other surrounding counties, including a long stretch along the Gulf of Mexico (bottom left). The entirety of Galveston Bay is visible at bottom center. Just below center lies the 1625-acre site of NASA's Johnson Space Center, one of the training venues for all space station crew members and the nearby long-time area of residence for NASA astronauts.

  19. Earth Observation

    2013-07-31

    ISS036-E-027014 (31 July 2013) --- One of the Expedition 36 crew members aboard the Earth-orbiting International Space Station, as it was passing over Eastern Europe on July 31, 2013, took this night picture looking toward the Mediterranean Sea, which almost blends into the horizon. Also visible are the Aegean Sea, Adriatic Sea and Mediterranean Sea. Parts of the following countries are among those visible as well: Greece, Italy, Sicily, Bulgaria, Serbia, Croatia and Albania. The high oblique 50mm lens shot includes a number of stars in the late July sky. A solar array panel is visible in the darkness on the right side of the frame.

  20. Earth Explorer

    ,

    2000-01-01

    The U.S. Geological Survey's (USGS) Earth Explorer Web site provides access to millions of land-related products, including the following: Satellite images from Landsat, advanced very high resolution radiometer (AVHRR), and Corona data sets. Aerial photographs from the National Aerial Photography Program, NASA, and USGS data sets.  Digital cartographic data from digital elevation models, digital line graphs, digital raster graphics, and digital orthophoto quadrangles. USGS paper maps Digital, film, and paper products are available, and many products can be previewed before ordering.

  1. Earth Science

    1992-07-24

    A Delta II rocket carrying the Geomagnetic Tail Lab (GEOTAIL) spacecraft lifts off at Launch Complex 17, Kennedy Space Center (KSC) into a cloud-dappled sky. This liftoff marks the first Delta launch under the medium expendable launch vehicle services contract between NASA and McDonnell Douglas Space Systems Co. The GEOTAIL mission, a joint US/Japanese project, is the first in a series of five satellites to study the interactions between the Sun, the Earth's magnetic field, and the Van Allen radiation belts.

  2. Earth Science

    1996-01-31

    The Near Earth Asteroid Rendezvous (NEAR) spacecraft embarks on a journey that will culminate in a close encounter with an asteroid. The launch of NEAR inaugurates NASA's irnovative Discovery program of small-scale planetary missions with rapid, lower-cost development cycles and focused science objectives. NEAR will rendezvous in 1999 with the asteroid 433 Eros to begin the first long-term, close-up look at an asteroid's surface composition and physical properties. NEAR's science payload includes an x-ray/gamma ray spectrometer, an near-infrared spectrograph, a laser rangefinder, a magnetometer, a radio science experiment and a multi-spectral imager.

  3. Metalorganic chemical vapor deposition growth of high-mobility AlGaN/AlN/GaN heterostructures on GaN templates and native GaN substrates

    SciT

    Chen, Jr-Tai, E-mail: jrche@ifm.liu.se; Hsu, Chih-Wei; Forsberg, Urban

    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 andmore » 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.« less

  4. Herbal Earth

    2017-12-08

    Subtle vegetation changes are visible in this year-long visualization. Large-scale patterns vary with seasons, but the local variations in green are also sensitive precipitation, drought and fire. High values of Normalized Difference Vegetation Index, or NDVI, represent dense green functioning vegetation and low NDVI values represent sparse green vegetation or vegetation under stress from limiting conditions, such as drought. The visualization was created from a year’s worth of data from April 2012 to April 2013. The information was sent back to Earth from the Visible-Infrared Imager/Radiometer Suite (VIIRS) instrument aboard the Suomi National Polar-orbiting Partnership or Suomi NPP satellite, a partnership between NASA and the National Oceanic and Atmospheric Administration, or NOAA. Credit: NASA/NOAA To read more go to: www.nasa.gov/mission_pages/NPP/news/vegetation.html NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

  5. Earth Observation

    2013-06-17

    ISS036-E-009405 (17 June 2013) --- One of the Expedition 36 crew members aboard the International Space Station, some 240 miles above Earth, used a 50mm lens to record this oblique nighttime image of a large part of the nation’s second largest state in area, including the four largest metropolitan areas in population. The extent of the metropolitan areas is easily visible at night due to city and highway lights. The largest metro area, Dallas-Fort Worth, often referred to informally as the Metroplex, is the heavily cloud-covered area at the top center of the photo. Neighboring Oklahoma, on the north side of the Red River, less than 100 miles to the north of the Metroplex, appears to be experiencing thunderstorms. The Houston metropolitan area, including the coastal city of Galveston, is at lower right. To the east near the Texas border with Louisiana, the metropolitan area of Beaumont-Port Arthur appears as a smaller blotch of light, also hugging the coast of the Texas Gulf. Moving inland to the left side of the picture one can delineate the San Antonio metro area. The capital city of Austin can be seen to the northeast of San Antonio. This and hundreds of thousands of other Earth photos taken by astronauts and cosmonauts over the past 50 years are available on http://eol.jsc.nasa.gov

  6. Earth Observation

    2011-07-06

    ISS028-E-014782 (6 July 2011) --- The Shoemaker (formerly Teague) Impact Structure, located in Western Australia in a drainage basin south of the Waldburg Range, presents an other-worldly appearance in this detailed photograph recorded from onboard the International Space Station on July 6. The Shoemaker impact site is approximately 30 kilometers in diameter, and is clearly defined by concentric ring structures formed in sedimentary rocks (brown to dark brown, image center) that were deformed by the impact event approximately 1630 million years ago, according to the Earth Impact Database. Several saline and ephemeral lakes?Nabberu, Teague, Shoemaker, and numerous smaller ponds?occupy the land surface between the concentric ring structures. Differences in color result from both water depth and suspended sediments, with some bright salt crusts visible around the edges of smaller ponds (image center The Teague Impact Structure was renamed Shoemaker in honor of the late Dr. Eugene M. Shoemaker, a pioneer in the field of impact crater studies and planetary geology, and founder of the Astrogeology Branch of the United States Geological Survey. The image was recorded with a digital still camera using a 200 mm lens, and is provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center.

  7. Zn-dopant dependent defect evolution in GaN nanowires.

    PubMed

    Yang, Bing; Liu, Baodan; Wang, Yujia; Zhuang, Hao; Liu, Qingyun; Yuan, Fang; Jiang, Xin

    2015-10-21

    Zn doped GaN nanowires with different doping levels (0, <1 at%, and 3-5 at%) have been synthesized through a chemical vapor deposition (CVD) process. The effect of Zn doping on the defect evolution, including stacking fault, dislocation, twin boundary and phase boundary, has been systematically investigated by transmission electron microscopy and first-principles calculations. Undoped GaN nanowires show a hexagonal wurtzite (WZ) structure with good crystallinity. Several kinds of twin boundaries, including (101¯3), (101¯1) and (202¯1), as well as Type I stacking faults (…ABABCBCB…), are observed in the nanowires. The increasing Zn doping level (<1 at%) induces the formation of screw dislocations featuring a predominant screw component along the radial direction of the GaN nanowires. At high Zn doping level (3-5 at%), meta-stable cubic zinc blende (ZB) domains are generated in the WZ GaN nanowires. The WZ/ZB phase boundary (…ABABACBA…) can be identified as Type II stacking faults. The density of stacking faults (both Type I and Type II) increases with increasing the Zn doping levels, which in turn leads to a rough-surface morphology in the GaN nanowires. First-principles calculations reveal that Zn doping will reduce the formation energy of both Type I and Type II stacking faults, favoring their nucleation in GaN nanowires. An understanding of the effect of Zn doping on the defect evolution provides an important method to control the microstructure and the electrical properties of p-type GaN nanowires.

  8. Piezo-generator integrating a vertical array of GaN nanowires.

    PubMed

    Jamond, N; Chrétien, P; Houzé, F; Lu, L; Largeau, L; Maugain, O; Travers, L; Harmand, J C; Glas, F; Lefeuvre, E; Tchernycheva, M; Gogneau, N

    2016-08-12

    We demonstrate the first piezo-generator integrating a vertical array of GaN nanowires (NWs). We perform a systematic multi-scale analysis, going from single wire properties to macroscopic device fabrication and characterization, which allows us to establish for GaN NWs the relationship between the material properties and the piezo-generation, and to propose an efficient piezo-generator design. The piezo-conversion of individual MBE-grown p-doped GaN NWs in a dense array is assessed by atomic force microscopy (AFM) equipped with a Resiscope module yielding an average output voltage of 228 ± 120 mV and a maximum value of 350 mV generated per NW. In the case of p-doped GaN NWs, the piezo-generation is achieved when a positive piezo-potential is created inside the nanostructures, i.e. when the NWs are submitted to compressive deformation. The understanding of the piezo-generation mechanism in our GaN NWs, gained from AFM analyses, is applied to design a piezo-generator operated under compressive strain. The device consists of NW arrays of several square millimeters in size embedded into spin-on glass with a Schottky contact for rectification and collection of piezo-generated carriers. The generator delivers a maximum power density of ∼12.7 mW cm(-3). This value sets the new state of the art for piezo-generators based on GaN NWs and more generally on nitride NWs, and offers promising prospects for the use of GaN NWs as high-efficiency ultra-compact energy harvesters.

  9. Sulfur Earth

    NASA Astrophysics Data System (ADS)

    de Jong, B. H.

    2007-12-01

    Variations in surface tension affect the buoyancy of objects floating in a liquid. Thus an object floating in water will sink deeper in the presence of dishwater fluid. This is a very minor but measurable effect. It causes for instance ducks to drown in aqueous solutions with added surfactant. The surface tension of liquid iron is very strongly affected by the presence of sulfur which acts as a surfactant in this system varying between 1.9 and 0.4 N/m at 10 mass percent Sulfur (Lee & Morita (2002), This last value is inferred to be the maximum value for Sulfur inferred to be present in the liquid outer core. Venting of Sulfur from the liquid core manifests itself on the Earth surface by the 105 to 106 ton of sulfur vented into the atmosphere annually (Wedepohl, 1984). Inspection of surface Sulfur emission indicates that venting is non-homogeneously distributed over the Earth's surface. The implication of such large variation in surface tension in the liquid outer core are that at locally low Sulfur concentration, the liquid outer core does not wet the predominantly MgSiO3 matrix with which it is in contact. However at a local high in Sulfur, the liquid outer core wets this matrix which in the fluid state has a surface tension of 0.4 N/m (Bansal & Doremus, 1986), couples with it, and causes it to sink. This differential and diapiric movement is transmitted through the essentially brittle mantle (1024 Pa.s, Lambeck & Johnson, 1998; the maximum value for ice being about 1030 Pa.s at 0 K, in all likely hood representing an upper bound of viscosity for all materials) and manifests itself on the surface by the roughly 20 km differentiation, about 0.1 % of the total mantle thickness, between topographical heights and lows with concomitant lateral movement in the crust and upper mantle resulting in thin skin tectonics. The brittle nature of the medium though which this movement is transmitted suggests that the extremes in topography of the D" layer are similar in range to

  10. GaN Initiative for Grid Applications (GIGA)

    SciT

    Turner, George

    2015-07-03

    For nearly 4 ½ years, MIT Lincoln Laboratory (MIT/LL) led a very successful, DoE-funded team effort to develop GaN-on-Si materials and devices, targeting high-voltage (>1 kV), high-power, cost-effective electronics for grid applications. This effort, called the GaN Initiative for Grid Applications (GIGA) program, was initially made up of MIT/LL, the MIT campus group of Prof. Tomas Palacios (MIT), and the industrial partner M/A Com Technology Solutions (MTS). Later in the program a 4th team member was added (IQE MA) to provide commercial-scale GaN-on-Si epitaxial materials. A basic premise of the GIGA program was that power electronics, for ubiquitous utilization -evenmore » for grid applications - should be closer in cost structure to more conventional Si-based power electronics. For a number of reasons, more established GaN-on-SiC or even SiC-based power electronics are not likely to reach theses cost structures, even in higher manufacturing volumes. An additional premise of the GIGA program was that the technical focus would be on materials and devices suitable for operating at voltages > 1 kV, even though there is also significant commercial interest in developing lower voltage (< 1 kV), cost effective GaN-on-Si devices for higher volume applications, like consumer products. Remarkable technical progress was made during the course of this program. Advances in materials included the growth of high-quality, crack-free epitaxial GaN layers on large-diameter Si substrates with thicknesses up to ~5 μm, overcoming significant challenges in lattice mismatch and thermal expansion differences between Si and GaN in the actual epitaxial growth process. Such thick epilayers are crucial for high voltage operation of lateral geometry devices such as Schottky barrier (SB) diodes and high electron mobility transistors (HEMTs). New “Normally-Off” device architectures were demonstrated – for safe operation of power electronics circuits. The trade-offs between lateral

  11. Cloudy Earth

    2015-05-08

    Decades of satellite observations and astronaut photographs show that clouds dominate space-based views of Earth. One study based on nearly a decade of satellite data estimated that about 67 percent of Earth’s surface is typically covered by clouds. This is especially the case over the oceans, where other research shows less than 10 percent of the sky is completely clear of clouds at any one time. Over land, 30 percent of skies are completely cloud free. Earth’s cloudy nature is unmistakable in this global cloud fraction map, based on data collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Aqua satellite. While MODIS collects enough data to make a new global map of cloudiness every day, this version of the map shows an average of all of the satellite’s cloud observations between July 2002 and April 2015. Colors range from dark blue (no clouds) to light blue (some clouds) to white (frequent clouds).

  12. Earth observation

    2014-08-31

    ISS040-E-113700 (31 Aug. 2014) --- This panorama view, photographed by an Expedition 40 crew member on the International Space Station, shows tan-colored dust of a major dust storm obscuring the Persian Gulf and the its northern shoreline. Strong north winds often blow in summer, churning up dust from the entire length of the desert surfaces of the Tigris and Euphrates valleys (top left). Dust partly obscures the hundreds of kilometers of Iraq’s light-green agricultural lands along these rivers (left). A line of thunderstorms is being set off by the Zagros Mountains of Iran (right), with the setting sun casting long shadows from the thunderheads. Space station crews see sixteen sunrises and sunsets every day from low Earth orbit. Here the crew captured dusk in a darkening Iranian landscape (right).

  13. Earth Observation

    2014-06-24

    ISS040-E-018725 (24 June 2014) --- One of the Expedition 40 crew members aboard the Earth-orbiting International Space Station photographed this image featuring most of the peninsular portion of the state of Florida. Lake Okeechobee stands out in the south central part of the state. The heavily-populated area of Miami can be traced along the Atlantic Coast near the bottom of the scene. Cape Canaveral and the Kennedy Space Center are in lower right portion of the image on the Atlantic Coast. The Florida Keys are at the south (left) portion of the scene and the Gulf Coast, including the Tampa-St. Petersburg area, is near frame center.

  14. Earth Observation

    2013-07-29

    ISS036-E-025908 (29 July 2013) --- One of the Expedition 36 crew members aboard the Earth-orbiting International Space Station, as it was passing over Africa, took this night picture of Sicily (center frame) and much of Italy (frame left to frame center) on July 29, 2013. The Stretto de Messina, which separates Sicily from Italy, is near frame center. The high oblique 50mm lens shot includes a scenic horizon with a number of stars in the late July sky. Barely visible in the darkness, part of the long arm of the Space Station Remote Manipulator System or Canadarm2 runs diagonally through the right one-third of the image.

  15. Earth Observation

    2011-07-15

    ISS028-E-017123 (16 July 2011) --- Separate atmospheric optical phenomena were captured in this electronic still photograph from the Inernational Space Station. The thin greenish band stretching along the Earth's horizon is airglow; light emitted by the atmosphere from a layer about 30 kilometers thick and about 100 kilometers in altitude. The predominant emission in airglow is the green 5577 Angstrom wavelength light from atomic oxygen atoms. Airglow is always and everywhere present in the atmosphere; it results from the recombination of molecules that have been broken apart by solar radiation during the day. But airglow is so faint that it can only be seen at night by looking "edge on" at the emission layer, such as the view astronauts and cosmonauts have in orbit. The second phenomenon is the appearnce of Aurora Australis.

  16. Earth Observation

    2014-11-22

    ISS042E007131 (11/22/2014) — Astronauts aboard the International Space Station captured this image of a huge crater in Africa on Nov. 22, 2014. This is the Richat Structure in northwestern Mauritania, otherwise known as the “Eye of the Sahara.” Scientists are still deciding whether this was formed by a subterranean volcano or impact from a large meteor. Deep in the Sahara Desert it is nearly a perfect circle, it is 1.2 miles (1.9 kilometers) wide, and sports a rim 330 feet (100 meters) tall. The crater sits in a vast plain of rocks so ancient they were deposited hundreds of millions of years before the first dinosaurs walked the Earth.

  17. Earth Science

    1996-01-13

    The Near Earth Asteroid Rendezvous (NEAR) spacecraft undergoing preflight preparation in the Spacecraft Assembly Encapsulation Facility-2 (SAEF-2) at Kennedy Space Center (KSC). NEAR will perform two critical mission events - Mathilde flyby and the Deep-Space maneuver. NEAR will fly-by Mathilde, a 38-mile (61-km) diameter C-type asteroid, making use of its imaging system to obtain useful optical navigation images. The primary science instrument will be the camera, but measurements of magnetic fields and mass also will be made. The Deep-Space Maneuver (DSM) will be executed about a week after the Mathilde fly-by. The DSM represents the first of two major burns during the NEAR mission of the 100-pound bi-propellant (Hydrazine/nitrogen tetroxide) thruster. This maneuver is necessary to lower the perihelion distance of NEAR's trajectory. The DSM will be conducted in two segments to minimize the possibility of an overburn situation.

  18. Nanostructural engineering of nitride nucleation layers for GaN substrate dislocation reduction.

    SciT

    Koleske, Daniel David; Lee, Stephen Roger; Lemp, Thomas Kerr

    2009-07-01

    With no lattice matched substrate available, sapphire continues as the substrate of choice for GaN growth, because of its reasonable cost and the extensive prior experience using it as a substrate for GaN. Surprisingly, the high dislocation density does not appear to limit UV and blue LED light intensity. However, dislocations may limit green LED light intensity and LED lifetime, especially as LEDs are pushed to higher current density for high end solid state lighting sources. To improve the performance for these higher current density LEDs, simple growth-enabled reductions in dislocation density would be highly prized. GaN nucleation layers (NLs)more » are not commonly thought of as an application of nano-structural engineering; yet, these layers evolve during the growth process to produce self-assembled, nanometer-scale structures. Continued growth on these nuclei ultimately leads to a fully coalesced film, and we show in this research program that their initial density is correlated to the GaN dislocation density. In this 18 month program, we developed MOCVD growth methods to reduce GaN dislocation densities on sapphire from 5 x 10{sup 8} cm{sup -2} using our standard delay recovery growth technique to 1 x 10{sup 8} cm{sup -2} using an ultra-low nucleation density technique. For this research, we firmly established a correlation between the GaN nucleation thickness, the resulting nucleation density after annealing, and dislocation density of full GaN films grown on these nucleation layers. We developed methods to reduce the nuclei density while still maintaining the ability to fully coalesce the GaN films. Ways were sought to improve the GaN nuclei orientation by improving the sapphire surface smoothness by annealing prior to the NL growth. Methods to eliminate the formation of additional nuclei once the majority of GaN nuclei were developed using a silicon nitride treatment prior to the deposition of the nucleation layer. Nucleation layer thickness was

  19. Implant technology and usability.

    PubMed

    Meadows, Paul M

    2008-08-01

    Implanted electrical stimulation technology has changed greatly in the roughly five decades of its existence. Many factors determine the viability of the technology and whether the systems and products developed ultimately go on to help the target patient population. This article describes the successful introduction of an implanted system and the challenges to implanted system development, acceptance, and usability of these systems by the patient and clinical communities.

  20. GaN Microwave DC-DC Converters

    NASA Astrophysics Data System (ADS)

    Ramos Franco, Ignacio

    Increasing the operating frequency of switching converters can have a direct impact in the miniaturization and integration of power converters. The size of energy-storage passive components and the difficulty to integrate them with the rest of the circuitry is a major challenge in the development of a fully integrated power supply on a chip. The work presented in this thesis attempts to address some of the difficulties encountered in the design of high-frequency converters by applying concepts and techniques usually used in the design of high-efficiency power amplifiers and high-efficiency rectifiers at microwave frequencies. The main focus is in the analysis, design, and characterization of dc-dc converters operating at microwave frequencies in the low gigahertz range. The concept of PA-rectifier duality, where a high-efficiency power amplifier operates as a high-efficiency rectifier is investigated through non-linear simulations and experimentally validated. Additionally, the concept of a self-synchronous rectifier, where a transistor rectifier operates synchronously without the need of a RF source or driver is demonstrated. A theoretical analysis of a class-E self-synchronous rectifier is presented and validated through non-linear simulations and experiments. Two GaN class-E2 dc-dc converters operating at a switching frequency of 1 and 1.2 GHz are demonstrated. The converters achieve 80 % and 75 % dc-dc efficiency respectively and are among the highest-frequency and highest-efficiency reported in the literature. The application of the concepts established in the analysis of a self-synchronous rectifier to a power amplifier culminated in the development of an oscillating, self-synchronous class-E 2 dc-dc converter. Finally, a proof-of-concept fully integrated GaN MMIC class-E 2 dc-dc converter switching at 4.6 GHz is demonstrated for the first time to the best of our knowledge. The 3.8 mm x 2.6 mm chip contains distributed inductors and does not require any

  1. Cochlear implant magnet retrofit.

    PubMed

    Cohen, N L; Breda, S D; Hoffman, R A

    1988-06-01

    An implantable magnet is now available for patients who have received the standard Nucleus 22-channel cochlear implant and who are not able to wear the headband satisfactorily. This magnet is attached in piggy-back fashion to the previously implanted receiver/stimulator by means of a brief operation under local anesthesia. Two patients have received this magnet retrofit, and are now wearing the headset with greater comfort and satisfaction. It is felt that the availability of this magnet will increase patient compliance in regard to hours of implant usage.

  2. Strain-mediated electronic properties of pristine and Mn-doped GaN monolayers

    NASA Astrophysics Data System (ADS)

    Sharma, Venus; Srivastava, Sunita

    2018-04-01

    Graphene-like two-dimensional (2D) monolayer structures GaN has gained enormous amount of interest due to high thermal stability and inherent energy band gap for practical applications. First principles calculations are performed to investigate the electronic structure and strain-mediated electronic properties of pristine and Mn-doped GaN monolayer. Binding energy of Mn dopant at various adsorption site is found to be nearly same indicating these sites to be equally favorable for adsorption of foreign atom. Depending on the adsorption site, GaN monolayer can act as p-type or n-type magnetic semiconductor. The tensile strength of both pristine and doped GaN monolayer (∼24 GPa) at ultimate tensile strain of 34% is comparable with the tensile strength of graphene. The in-plane biaxial strain modulate the energy band gap of both pristine and doped-monolayer from direct to indirect gap semiconductor and finally retendered theme into metal at critical value of applied strain. These characteristics make GaN monolayer to be potential candidate for the future applications in tunable optoelectronics.

  3. Fabrication of GaN doped ZnO nanocrystallines by laser ablation.

    PubMed

    Gopalakrishnan, N; Shin, B C; Bhuvana, K P; Elanchezhiyan, J; Balasubramanian, T

    2008-08-01

    Here, we present the fabrication of pure and GaN doped ZnO nanocrystallines on Si(111) substrates by KrF excimer laser. The targets for the ablation have been prepared by conventional ceramic method. The fabricated nanocrystallines have been investigated by X-ray diffraction, photoluminescence and atomic force microscopy. The X-ray diffraction analysis shows that the crystalline size of pure ZnO is 36 nm and it is 41 nm while doped with 0.8 mol% of GaN due to best stoichiometry between Zn and O. Photoluminescence studies reveal that intense deep level emissions have been observed for pure ZnO and it has been suppressed for the GaN doped ZnO structures. The images of atomic force microscope show that the rms surface roughness is 27 nm for pure ZnO and the morphology is improved with decrease in rms roughness, 18 nm with fine crystallines while doped with 1 mol% GaN. The improved structural, optical and morphological properties of ZnO nanocrystalline due to GaN dopant have been discussed in detail.

  4. Electron spin relaxation in two polymorphic structures of GaN

    NASA Astrophysics Data System (ADS)

    Kang, Nam Lyong

    2015-03-01

    The relaxation process of electron spin in systems of electrons interacting with piezoelectric deformation phonons that are mediated through spin-orbit interactions was interpreted from a microscopic point of view using the formula for the electron spin relaxation times derived by a projection-reduction method. The electron spin relaxation times in two polymorphic structures of GaN were calculated. The piezoelectric material constant for the wurtzite structure obtained by a comparison with a previously reported experimental result was {{P}pe}=1.5 × {{10}29} eV {{m}-1}. The temperature and magnetic field dependence of the relaxation times for both wurtzite and zinc-blende structures were similar, but the relaxation times in zinc-blende GaN were smaller and decreased more rapidly with increasing temperature and magnetic field than that in wurtzite GaN. This study also showed that the electron spin relaxation for wurtzite GaN at low density could be explained by the Elliot-Yafet process but not for zinc-blende GaN in the metallic regime.

  5. Acceptor binding energies in GaN and AlN

    NASA Astrophysics Data System (ADS)

    Mireles, Francisco; Ulloa, Sergio E.

    1998-08-01

    We employ effective-mass theory for degenerate hole bands to calculate the acceptor binding energies for Be, Mg, Zn, Ca, C, and Si substitutional acceptors in GaN and AlN. The calculations are performed through the 6×6 Rashba-Sheka-Pikus and the Luttinger-Kohn matrix Hamiltonians for wurtzite (WZ) and zinc-blende (ZB) crystal phases, respectively. An analytic representation for the acceptor pseudopotential is used to introduce the specific nature of the impurity atoms. The energy shift due to polaron effects is also considered in this approach. The ionization energy estimates are in very good agreement with those reported experimentally in WZ GaN. The binding energies for ZB GaN acceptors are all predicted to be shallower than the corresponding impurities in the WZ phase. The binding-energy dependence upon the crystal-field splitting in WZ GaN is analyzed. Ionization levels in AlN are found to have similar ``shallow'' values to those in GaN, but with some important differences which depend on the band structure parametrizations, especially the value of the crystal-field splitting used.

  6. Design and simulation of GaN based Schottky betavoltaic nuclear micro-battery.

    PubMed

    San, Haisheng; Yao, Shulin; Wang, Xiang; Cheng, Zaijun; Chen, Xuyuan

    2013-10-01

    The current paper presents a theoretical analysis of Ni-63 nuclear micro-battery based on a wide-band gap semiconductor GaN thin-film covered with thin Ni/Au films to form Schottky barrier for carrier separation. The total energy deposition in GaN was calculated using Monte Carlo methods by taking into account the full beta spectral energy, which provided an optimal design on Schottky barrier width. The calculated results show that an 8 μm thick Schottky barrier can collect about 95% of the incident beta particle energy. Considering the actual limitations of current GaN growth technique, a Fe-doped compensation technique by MOCVD method can be used to realize the n-type GaN with a carrier concentration of 1×10(15) cm(-3), by which a GaN based Schottky betavoltaic micro-battery can achieve an energy conversion efficiency of 2.25% based on the theoretical calculations of semiconductor device physics. Copyright © 2013 Elsevier Ltd. All rights reserved.

  7. Effects of catalyst concentration and ultraviolet intensity on chemical mechanical polishing of GaN

    NASA Astrophysics Data System (ADS)

    Wang, Jie; Wang, Tongqing; Pan, Guoshun; Lu, Xinchun

    2016-08-01

    Effects of catalyst concentration and ultraviolet intensity on chemical mechanical polishing (CMP) of GaN were deeply investigated in this paper. Working as an ideal homogeneous substrate material in LED industry, GaN ought to be equipped with a smooth and flat surface. Taking the strong chemical stability of GaN into account, photocatalytic oxidation technology was adopted in GaN CMP process to realize efficient removal. It was found that, because of the improved reaction rate of photocatalytic oxidation, GaN material removal rate (MRR) increases by a certain extent with catalyst concentration increasing. Cross single line analysis on the surface after polishing by Phase Shift MicroXAM-3D was carried out to prove the better removal effect with higher catalyst concentration. Ultraviolet intensity field in H2O2-SiO2-based polishing system was established and simulated, revealing the variation trend of ultraviolet intensity around the outlet of the slurry. It could be concluded that, owing to the higher planarization efficiency and lower energy damage, the UV lamp of 125 W is the most appropriate lamp in this system. Based on the analysis, defects removal model of this work was proposed to describe the effects of higher catalyst concentration and higher power of UV lamp.

  8. Growth of hierarchical GaN nanowires for optoelectronic device applications

    NASA Astrophysics Data System (ADS)

    Raj, Rishabh; Vignesh, Veeramuthu; Ra, Yong-Ho; Nirmala, Rajkumar; Lee, Cheul-Ro; Navamathavan, Rangaswamy

    2017-01-01

    Gallium nitride nanostructures have been receiving considerable attention as building blocks for nanophotonic technologies due to their unique high aspect ratios, promising the realization of photonic and biological nanodevices such as blue light emitting diodes (LEDs), short-wavelength ultraviolet nanolasers, and nanofluidic biochemical sensors. We report on the growth of hierarchical GaN nanowires (NWs) by dynamically adjusting the growth parameters using the pulsed flow metal-organic chemical vapor deposition technique. We carried out two step growth processes to grow hierarchical GaN NWs. In the first step, the GaN NWs were grown at 950°C, and in the second, we suitably decreased the growth temperature to 630°C and 710°C to grow the hierarchical structures. The surface morphology and optical characterization of the grown GaN NWs were studied by field-emission scanning electron microscopy, high-resolution transmission electron microscopy, photoluminescence, and cathodoluminescence measurements. These kinds of hierarchical GaN NWs are promising for allowing flat band quantum structures that are shown to improve the efficiency of LEDs.

  9. Use of GaN as a Scintillating Ionizing Radiation Detector

    NASA Astrophysics Data System (ADS)

    Wensman, Johnathan; Guardala, Noel; Mathur, Veerendra; Alasagas, Leslie; Vanhoy, Jeffrey; Statham, John; Marron, Daniel; Millett, Marshall; Marsh, Jarrod; Currie, John; Price, Jack

    2017-09-01

    Gallium nitride (GaN) is a III/V direct bandgap semiconductor which has been used in light emitting diodes (LEDs) since the 1990s. Currently, due to a potential for increased efficiency, GaN is being investigated as a replacement for silicon in power electronics finding potential uses ranging from data centers to electric vehicles. In addition to LEDs and power electronics though, doped GaN can be used as a gamma insensitive fast neutron detector due to the direct band-gap, light propagation properties, and response to ionizing radiations. Investigation of GaN as a semiconductor scintillator for use in a radiation detection system involves mapping the response function of the detector crystal over a range of photon and neutron energies, and measurements of light generation in the GaN crystal due to proton, alpha, and nitrogen projectiles. In this presentation we discuss the measurements made to date, and plausible interpretations of the response functions. This work funded in part by the Naval Surface Warfare Center, Carderock Division In-house Laboratory Independent Research program.

  10. Effect of different electrolytes on porous GaN using photo-electrochemical etching

    NASA Astrophysics Data System (ADS)

    Al-Heuseen, K.; Hashim, M. R.; Ali, N. K.

    2011-05-01

    This article reports the properties and the behavior of GaN during the photoelectrochemical etching process using four different electrolytes. The measurements show that the porosity strongly depends on the electrolyte and highly affects the surface morphology of etched samples, which has been revealed by scanning electron microscopy (SEM) images. Peak intensity of the photoluminescence (PL) spectra of the porous GaN samples was observed to be enhanced and strongly depend on the electrolytes. Among the samples, there is a little difference in the peak position indicating that the change of porosity has little influence on the PL peak shift, while it highly affecting the peak intensity. Raman spectra of porous GaN under four different solution exhibit phonon mode E 2 (high), A 1 (LO), A 1 (TO) and E 2 (low). There was a red shift in E 2 (high) in all samples, indicating a relaxation of stress in the porous GaN surface with respect to the underlying single crystalline epitaxial GaN. Raman and PL intensities were high for samples etched in H 2SO 4:H 2O 2 and KOH followed by the samples etched in HF:HNO 3 and in HF:C 2H 5OH.

  11. Hydrogen-surfactant-assisted coherent growth of GaN on ZnO substrate

    NASA Astrophysics Data System (ADS)

    Zhang, Jingzhao; Zhang, Yiou; Tse, Kinfai; Zhu, Junyi

    2018-01-01

    Heterostructures of wurtzite based devices have attracted great research interest because of the tremendous success of GaN in light emitting diodes (LED) industry. High-quality GaN thin films on inexpensive and lattice matched ZnO substrates are both commercially and technologically desirable. Intrinsic wetting conditions, however, forbid such heterostructures as the energy of ZnO polar surfaces is much lower than that of GaN polar surfaces, resulting in 3D growth mode and poor crystal quality. Based on first-principles calculations, we propose the use of surfactant hydrogen to dramatically alter the growth mode of the heterostructures. Stable H-involved surface configurations and interfaces are investigated with the help of our newly developed modelling techniques. The temperature and chemical potential dependence of our proposed strategy, which is critical in experiments, is predicted by applying the experimental Gibbs free energy of H2. Our thermodynamic wetting condition analysis is a crucial step for the growth of GaN on ZnO, and we find that introducing H will not degrade the stability of ZnO substrate. This approach will allow the growth of high-quality GaN thin films on ZnO substrates. We believe that our new strategy may reduce the manufactory cost, improve the crystal quality, and improve the efficiency of GaN-based devices.

  12. Ultrafast Hot Carrier Dynamics in GaN and Its Impact on the Efficiency Droop.

    PubMed

    Jhalani, Vatsal A; Zhou, Jin-Jian; Bernardi, Marco

    2017-08-09

    GaN is a key material for lighting technology. Yet, the carrier transport and ultrafast dynamics that are central in GaN light-emitting devices are not completely understood. We present first-principles calculations of carrier dynamics in GaN, focusing on electron-phonon (e-ph) scattering and the cooling and nanoscale dynamics of hot carriers. We find that e-ph scattering is significantly faster for holes compared to electrons and that for hot carriers with an initial 0.5-1 eV excess energy, holes take a significantly shorter time (∼0.1 ps) to relax to the band edge compared to electrons, which take ∼1 ps. The asymmetry in the hot carrier dynamics is shown to originate from the valence band degeneracy, the heavier effective mass of holes compared to electrons, and the details of the coupling to different phonon modes in the valence and conduction bands. We show that the slow cooling of hot electrons and their long ballistic mean free paths (over 3 nm at room temperature) are a possible cause of efficiency droop in GaN light-emitting diodes. Taken together, our work sheds light on the ultrafast dynamics of hot carriers in GaN and the nanoscale origin of efficiency droop.

  13. Exciton emission from bare and hybrid plasmonic GaN nanorods

    NASA Astrophysics Data System (ADS)

    Mohammadi, Fatemesadat; Kunert, Gerd; Hommel, Detlef; Ge, Jingxuan; Duscher, Gerd; Schmitzer, Heidrun; Wagner, Hans Peter

    We study the exciton emission of hybrid gold nanoparticle/Alq3 (aluminiumquinoline)/wurtzite GaN nanorods. GaN nanorods of 1.5 μm length and 250 nm diameter were grown by plasma assisted MBE. Hybrid GaN nanorods were synthesized by organic molecular beam deposition. Temperature and power dependent time integrated (TI) and time resolved (TR) photoluminescence (PL) measurements were performed on bare and hybrid structures. Bare nanorods show donor (D0,X) and acceptor bound (A0,X) exciton emission at 3.473 eV and at 3.463 eV, respectively. TR-PL trace modeling reveal lifetimes of 240 ps and 1.4 ns for the (D0,X) and (A0,X) transition. 10 nm gold coated GaN nanorods show a significant PL quenching and (D0,X) lifetime shortening which is tentatively attributed to impact ionization of (D0,X) due to hot electron injection from the gold nanoparticles. This is supported by electron energy loss spectroscopy that shows a redshift of a midgap state transition indicating a reduction of a preexisting band-bending at the nanorod surface due to positive charging of the gold nanoparticles. Inserting a nominally 5 nm thick Alq3 spacer between the nanorod and the gold reduces the PL quenching and lifetime shortening. Plasmonic nanorods with a 30 nm thick Alq3 spacer reveal lifetimes which are nearly identical to uncoated GaN nanorods.

  14. Infrared Reflectance Analysis of Epitaxial n-Type Doped GaN Layers Grown on Sapphire.

    PubMed

    Tsykaniuk, Bogdan I; Nikolenko, Andrii S; Strelchuk, Viktor V; Naseka, Viktor M; Mazur, Yuriy I; Ware, Morgan E; DeCuir, Eric A; Sadovyi, Bogdan; Weyher, Jan L; Jakiela, Rafal; Salamo, Gregory J; Belyaev, Alexander E

    2017-12-01

    Infrared (IR) reflectance spectroscopy is applied to study Si-doped multilayer n + /n 0 /n + -GaN structure grown on GaN buffer with GaN-template/sapphire substrate. Analysis of the investigated structure by photo-etching, SEM, and SIMS methods showed the existence of the additional layer with the drastic difference in Si and O doping levels and located between the epitaxial GaN buffer and template. Simulation of the experimental reflectivity spectra was performed in a wide frequency range. It is shown that the modeling of IR reflectance spectrum using 2 × 2 transfer matrix method and including into analysis the additional layer make it possible to obtain the best fitting of the experimental spectrum, which follows in the evaluation of GaN layer thicknesses which are in good agreement with the SEM and SIMS data. Spectral dependence of plasmon-LO-phonon coupled modes for each GaN layer is obtained from the spectral dependence of dielectric of Si doping impurity, which is attributed to compensation effects by the acceptor states.

  15. Strain engineering of atomic and electronic structures of few-monolayer-thick GaN

    NASA Astrophysics Data System (ADS)

    Kolobov, A. V.; Fons, P.; Saito, Y.; Tominaga, J.; Hyot, B.; André, B.

    2017-07-01

    Two-dimensional (2D) semiconductors possess the potential to ultimately minimize the size of devices and concomitantly drastically reduce the corresponding energy consumption. In addition, materials in their atomic-scale limit often possess properties different from their bulk counterparts paving the way to conceptually novel devices. While graphene and 2D transition-metal dichalcogenides remain the most studied materials, significant interest also exists in the fabrication of atomically thin structures from traditionally 3D semiconductors such as GaN. While in the monolayer limit GaN possesses a graphenelike structure and an indirect band gap, it was recently demonstrated that few-layer GaN acquires a Haeckelite structure in the direction of growth with an effectively direct gap. In this work, we demonstrate the possibility of strain engineering of the atomic and electronic structure of few-monolayer-thick GaN structures, which opens new avenues for their practical application in flexible nanoelectronics and nano-optoelectronics. Our simulations further suggest that due to the weak van der Waals-like interaction between a substrate and an overlayer, the use of a MoS2 substrate may be a promising route to fabricate few-monolayer Haeckelite GaN experimentally.

  16. Conversion between hexagonal GaN and beta-Ga(2)O(3) nanowires and their electrical transport properties.

    PubMed

    Li, Jianye; An, Lei; Lu, Chenguang; Liu, Jie

    2006-02-01

    We have observed that the hexagonal GaN nanowires grown from a simple chemical vapor deposition method using gallium metal and ammonia gas are usually gallium-doped. By annealing in air, the gallium-doped hexagonal GaN nanowires could be completely converted to beta-Ga(2)O(3) nanowires. Annealing the beta-Ga(2)O(3) nanowires in ammonia could convert them back to undoped hexagonal GaN nanowires. Field effect transistors based on these three kinds of nanowires were fabricated, and their performances were studied. Because of gallium doping, the as-grown GaN nanowires show a weak gating effect. Through the conversion process of GaN nanowires (gallium-doped) --> Ga(2)O(3) nanowires --> GaN nanowires (undoped) via annealing, the final undoped GaN nanowires display different electrical properties than the initial gallium-doped GaN nanowires, show a pronounced n-type gating effect, and can be completely turned off.

  17. Polarity-inverted lateral overgrowth and selective wet-etching and regrowth (PILOSWER) of GaN.

    PubMed

    Jang, Dongsoo; Jue, Miyeon; Kim, Donghoi; Kim, Hwa Seob; Lee, Hyunkyu; Kim, Chinkyo

    2018-03-07

    On an SiO 2 -patterned c-plane sapphire substrate, GaN domains were grown with their polarity controlled in accordance with the pattern. While N-polar GaN was grown on hexagonally arranged circular openings, Ga-polar GaN was laterally overgrown on mask regions due to polarity inversion occurring at the boundary of the circular openings. After etching of N-polar GaN on the circular openings by H 3 PO 4 , this template was coated with 40-nm Si by sputtering and was slightly etched by KOH. After slight etching, a thin layer of Si left on the circular openings of sapphire,but not on GaN, was oxidized during thermal annealing and served as a dielectric mask during subsequent regrowth. Thus, the subsequent growth of GaN was made only on the existing Ga-polar GaN domains, not on the circular openings of the sapphire substrate. Transmission electron microscopy analysis revealed no sign of threading dislocations in this film. This approach may help fabricating an unholed and merged GaN film physically attached to but epitaxially separated from the SiO 2 -patterned sapphire.

  18. Reduced MLH3 Expression in the Syndrome of Gan-Shen Yin Deficiency in Patients with Different Diseases.

    PubMed

    Du, Juan; Zhong, Maofeng; Liu, Dong; Liang, Shufang; Liu, Xiaolin; Cheng, Binbin; Zhang, Yani; Yin, Zifei; Wang, Yuan; Ling, Changquan

    2017-01-01

    Traditional Chinese medicine formulates treatment according to body constitution (BC) differentiation. Different constitutions have specific metabolic characteristics and different susceptibility to certain diseases. This study aimed to assess the characteristic genes of gan-shen Yin deficiency constitution in different diseases. Fifty primary liver cancer (PLC) patients, 94 hypertension (HBP) patients, and 100 diabetes mellitus (DM) patients were enrolled and classified into gan-shen Yin deficiency group and non-gan-shen Yin deficiency group according to the body constitution questionnaire to assess the clinical manifestation of patients. The mRNA expressions of 17 genes in PLC patients with gan-shen Yin deficiency were different from those without gan-shen Yin deficiency. However, considering all patients with PLC, HBP, and DM, only MLH3 was significantly lower in gan-shen Yin deficiency group than that in non-gen-shen Yin deficiency. By ROC analysis, the relationship between MLH3 and gan-shen Yin deficiency constitution was confirmed. Treatment of MLH3 (-/- and -/+) mice with Liuweidihuang wan, classical prescriptions for Yin deficiency, partly ameliorates the body constitution of Yin deficiency in MLH3 (-/+) mice, but not in MLH3 (-/-) mice. MLH3 might be one of material bases of gan-shen Yin deficiency constitution.

  19. The GAN Exonuclease or the Flap Endonuclease Fen1 and RNase HII Are Necessary for Viability of Thermococcus kodakarensis.

    PubMed

    Burkhart, Brett W; Cubonova, Lubomira; Heider, Margaret R; Kelman, Zvi; Reeve, John N; Santangelo, Thomas J

    2017-07-01

    Many aspects of and factors required for DNA replication are conserved across all three domains of life, but there are some significant differences surrounding lagging-strand synthesis. In Archaea , a 5'-to-3' exonuclease, related to both bacterial RecJ and eukaryotic Cdc45, that associates with the replisome specifically through interactions with GINS was identified and designated GAN (for G INS- a ssociated n uclease). Despite the presence of a well-characterized flap endonuclease (Fen1), it was hypothesized that GAN might participate in primer removal during Okazaki fragment maturation, and as a Cdc45 homologue, GAN might also be a structural component of an archaeal CMG (Cdc45, MCM, and GINS) replication complex. We demonstrate here that, individually, either Fen1 or GAN can be deleted, with no discernible effects on viability and growth. However, deletion of both Fen1 and GAN was not possible, consistent with both enzymes catalyzing the same step in primer removal from Okazaki fragments in vivo RNase HII has also been proposed to participate in primer processing during Okazaki fragment maturation. Strains with both Fen1 and RNase HII deleted grew well. GAN activity is therefore sufficient for viability in the absence of both RNase HII and Fen1, but it was not possible to construct a strain with both RNase HII and GAN deleted. Fen1 alone is therefore insufficient for viability in the absence of both RNase HII and GAN. The ability to delete GAN demonstrates that GAN is not required for the activation or stability of the archaeal MCM replicative helicase. IMPORTANCE The mechanisms used to remove primer sequences from Okazaki fragments during lagging-strand DNA replication differ in the biological domains. Bacteria use the exonuclease activity of DNA polymerase I, whereas eukaryotes and archaea encode a flap endonuclease (Fen1) that cleaves displaced primer sequences. RNase HII and the GINS-associated exonuclease GAN have also been hypothesized to assist in primer

  20. One-step fabrication of porous GaN crystal membrane and its application in energy storage

    NASA Astrophysics Data System (ADS)

    Zhang, Lei; Wang, Shouzhi; Shao, Yongliang; Wu, Yongzhong; Sun, Changlong; Huo, Qin; Zhang, Baoguo; Hu, Haixiao; Hao, Xiaopeng

    2017-03-01

    Single-crystal gallium nitride (GaN) membranes have great potential for a variety of applications. However, fabrication of single-crystalline GaN membranes remains a challenge owing to its chemical inertness and mechanical hardness. This study prepares large-area, free-standing, and single-crystalline porous GaN membranes using a one-step high-temperature annealing technique for the first time. A promising separation model is proposed through a comprehensive study that combines thermodynamic theories analysis and experiments. Porous GaN crystal membrane is processed into supercapacitors, which exhibit stable cycling life, high-rate capability, and ultrahigh power density, to complete proof-of-concept demonstration of new energy storage application. Our results contribute to the study of GaN crystal membranes into a new stage related to the elelctrochemical energy storage application.

  1. Study of GaN nanowires converted from β-Ga2O3 and photoconduction in a single nanowire

    NASA Astrophysics Data System (ADS)

    Kumar, Mukesh; Kumar, Sudheer; Chauhan, Neha; Sakthi Kumar, D.; Kumar, Vikram; Singh, R.

    2017-08-01

    The formation of GaN nanowires from β-Ga2O3 nanowires and photoconduction in a fabricated single GaN nanowire device has been studied. Wurtzite phase GaN were formed from monoclinic β-Ga2O3 nanowires with or without catalyst particles at their tips. The formation of faceted nanostructures from catalyst droplets presented on a nanowire tip has been discussed. The nucleation of GaN phases in β-Ga2O3 nanowires and their subsequent growth due to interfacial strain energy has been examined using a high resolution transmission electron microscope. The high quality of the converted GaN nanowire is confirmed by fabricating single nanowire photoconducting devices which showed ultra high responsivity under ultra-violet illumination.

  2. High quality self-separated GaN crystal grown on a novel nanoporous template by HVPE.

    PubMed

    Huo, Qin; Shao, Yongliang; Wu, Yongzhong; Zhang, Baoguo; Hu, Haixiao; Hao, Xiaopeng

    2018-02-16

    In this study, a novel nanoporous template was obtained by a two-step etching process from MOCVD-GaN/Al 2 O 3 (MGA) with electrochemical etching sequentially followed by chemical wet etching. The twice-etched MOCVD-GaN/Al 2 O 3 (TEMGA) templates were utilized to grow GaN crystals by hydride vapor phase epitaxy (HVPE) method. The GaN crystals were separated spontaneously from the TEMGA template with the assistance of voids formed by the etched nanopores. Several techniques were utilized to characterize the quality of the free-standing GaN crystals obtained from the TEMGA template. Results showed that the quality of the as-obtained GaN crystals was improved obviously compared with those grown on the MGA. This convenient technique can be applied to grow high-quality free-standing GaN crystals.

  3. Algan/Gan Hemt By Magnetron Sputtering System

    NASA Astrophysics Data System (ADS)

    Garcia Perez, Roman

    In this thesis, the growth of the semiconductor materials AlGaN and GaN is achieved by magnetron sputtering for the fabrication of High Electron Mobility Transistors (HEMTs). The study of the deposited nitrides is conducted by spectroscopy, diffraction, and submicron scale microscope methods. The preparation of the materials is performed using different parameters in terms of power, pressure, temperature, gas, and time. Silicon (Si) and Sapphire (Al2O3) wafers are used as substrates. The chemical composition and surface topography of the samples are analyzed to calculate the materials atomic percentages and to observe the devices surface. The instruments used for the semiconductors characterization are X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Atomic Force Microscope (AFM). The project focused its attention on the reduction of impurities during the deposition, the controlled thicknesses of the thin-films, the atomic configuration of the alloy AlxGa1-xN, and the uniformity of the surfaces.

  4. Radical Beam Gettering Epitaxy of Zno and Gan

    NASA Astrophysics Data System (ADS)

    Georgobiani, A. N.; Demin, V. I.; Vorobiev, M. O.; Gruzintsev, A. N.; Hodos, I. I.; Kotljarevsky, M. B.; Kidalov, V. V.; Rogozin, I. V.

    2002-11-01

    P-type ZnO layers with a hole mobility about 23 cm2/(V s), and a hole concentration about 1015 cm-3 were grown by means of radical-beam gettering epitaxy (the annealing of n-ZnO single crystals in atomic oxygen flux). The effect of native defects on the photoluminescence spectra of the layers was studied. The dominant bands in the spectra peaked at 370.2 and 400 nm. These bands were attributed to the annihilation of exciton localised on neutral Vzn and to electron transitions from the conduction band to singly positively charged Vzn correspondingly. The effect of annealing in atomic nitrogen flux of p-CaN:Mg films on their photoluminescence spectra and on the value of their conductivity were studied. Such annealing leads to appearance of a number of emission bands that peaked at 404.9, 390.8 and 378.9 nm and increases hole concentration from 5 × 1015 to 5 × 1016 cm-3, and the hole mobility from 120 to 150 cm2/(V s). The n-ZnO - p-GaN:Mg electroluminescence heterostructures were obtained. Their spectrum contains bands in the excitonic region of GaN at the wavelength 360.2 nm and in the edge region at wavelengths 378.9 and 390.8 nm.

  5. Intermediate Nucleation State of GaN Growth

    NASA Astrophysics Data System (ADS)

    Zheng, L. X.; Xie, M. H.; Tong, S. Y.

    2001-03-01

    Homoexpitaxial nucleation of GaN during molecular-beam epitaxy is followed by scanning tunneling microcopy (STM). We observe a metastable nucleation state, which manifests as “ghost” islands in STM images. These “ghost” islands can be irreversibly driven into normal islands by continuous STM imaging. It is further established that the “ghost” island formation is related to the presence of excess Ga atoms on the surface: Normal islands are only seen under the N-rich or stoichiometric flux condition, whereas “ghost” islands are observed under Ga-rich conditions. For intermediate excess-Ga coverages, both normal and “ghost” islands are present, however, they show distinctly different sizes, suggesting different nucleation states for the two. A growth model is proposed to account for the formation of metastable, “ghost” islands. Kinetic Monte Carlo simulation is carried out and main features of the surface are reproduced. We acknowledge financial support from HK RGC under grant Nos. 7396/00P, 7142/99P, and 7121/00P.

  6. Radiation Characterization of Commercial GaN Devices

    NASA Technical Reports Server (NTRS)

    Harris, Richard D.; Scheick, Leif Z.; Hoffman, James P.; Thrivikraman, Tushar; Jenabi, Masud; Gim, Yonggyu; Miyahira, Tetsuo

    2011-01-01

    Radiative feedback from primordial protostars and final mass of the first star Commercially available devices fabricated from GaN are beginning to appear from a number of different suppliers. Based on previous materials and prototype device studies, it is expected that these commercial devices will be quite tolerant to the types of radiation encountered in space. This expectation needs to be verified and the study described herein was undertaken for that purpose. All of the parts discussed in this report are readily available commercially. The parts chosen for study are all targeted for RF applications. Three different studies were performed: 1) a preliminary DDD/TID test of a variety of part types was performed by irradiating with 50 MeV protons, 2) a detailed DDD/TID study of one particular part type was performed by irradiating with 50 MeV protons, and 3) a SEB/SEGR test was performed on a variety of part types by irradiating with heavy ions. No significant degradation was observed in the tests performed in this study.

  7. AMIE Gan Island Ancillary Disdrometer Field Campaign Report

    SciT

    Oue, Mariko

    2016-04-01

    As part of the U.S. Department of Energy (DOE)’s Atmospheric Radiation Measurement Climate Research Facility (ARM) Madden-Julian Oscillation (MJO) Investigation Experiment (AMIE), in January 2012 a disdrometer observation took place with the second ARM Mobile Facility (AMF2), the Scanning ARM Cloud Radar (SACR), the Texas A&M SMART-R C-band radar, and the National Center for Atmospheric Research (NCAR) dual wavelength S- and Ka-bands polarimetric (SPolKa) radar on Gan Island, Maldives. In order to measure raindrop size distributions, a disdrometer of Nagoya University, Japan, was set up close to the ARM Two-Dimensional (2D) Video Disdrometer (2DVD). The SMART-R and SPolKa radars performedmore » range-height-indicator scanning in the direction of the disdrometer site. Comparing the disdrometer data with 2DVD data, the raindrop size distribution data will be calibrated. Furthermore, the analysis of the raindrop size distribution and radar data will be expected to clarify the microphysics in tropical convective clouds.« less

  8. Earth Observation

    2014-08-30

    ISS040E112662 (08/30/2014) ---- Cancún, Mexico. A long lens was used by astronauts aboard the International Space Station to take this image, and it highlights many natural and built features. The street pattern of Mexico’s tourist mecca, Cancún, contrasts with the waterways of the marinas that open into the bay and the lagoons. Brilliant blue water over coral reefs contrast with the dark waters of inland lagoons. The reefs are the second largest reef system on Earth, and draw tourists from all over the world. The wide, well developed beach on the gulf coast (image upper right) is the result of vigorous wave energy; the white sand makes the beach easily visible from space. But wave energy is reduced along Cancún’s protected shoreline (image center) and the beaches are thin or non-existant. Fair-weather cumulus clouds are scattered across the image top left. To shoot crisp mages with long lenses, astronaut photographers must learn to brace themselves against the ISS bulkhead to prevent any slight shaking that would blur or “smear” the picture. Counterintuitively, they then need to move the camera carefully retaining the target at the same point in the viewfinder (the landscape moves across the viewfinder quickly with long lenses). This is called tracking the target and requires good coordination by the photographer—again, to prevent blurring. Shorter lenses do not require this skill because the image appears to pass more slowly across the viewfinder.

  9. Earth Observations

    2010-09-20

    ISS024-E-015121 (20 Sept. 2010) --- Twitchell Canyon Fire in central Utah is featured in this image photographed by an Expedition 24 crew member on the International Space Station (ISS). The Twitchell Canyon Fire near central Utah?s Fishlake National Forest is reported to have an area of approximately 13,383 hectares (approximately 134 square kilometers, or 33,071 acres). This detailed image shows smoke plumes generated by several fire spots close to the southwestern edge of the burned area. The fire was started by a lightning strike on July 20, 2010. Whereas many of the space station images of Earth are looking straight down (nadir), this photograph was exposed at an angle. The space station was located over a point approximately 509 kilometers (316 miles) to the northeast, near the Colorado/Wyoming border, at the time the image was taken on Sept. 20. Southwesterly winds were continuing to extend smoke plumes from the fire to the northeast. While the Twitchell Canyon region is sparsely populated, Interstate Highway 15 is visible at upper left.

  10. Earth Observation

    2014-08-05

    ISS040-E-088891 (5 Aug. 2014) --- Thunderheads near Borneo, Indonesia are featured in this image photographed by an Expedition 40 crew member on the International Space Station. Late afternoon sun casts long shadows from high thunderhead anvils over southern Borneo. Crews aboard the space station have recently concentrated on panoramic views of clouds?taken with lenses similar to the focal length of the human eye. These images reveal the kinds of views crews see -- huge areas of the planet, with a strong three-dimensional sense of what it is like to fly 350 kilometers above Earth. Winds usually blow in different directions at different altitudes. High-altitude winds are clearly sweeping the tops off the many tallest thunderclouds, generating long anvils of diffuse cirrus plumes that trail south. At low levels, ?streets? of white dots -- fair-weather cumulus clouds -- are aligned with west-moving winds (lower left). Small smoke plumes from forest fires onshore are also aligned west. Storm formation near the horizon -- more than 1,000 kilometers away (center) -- is assisted by air currents rising over the central mountains of Borneo.

  11. Earth Observation

    2013-10-14

    ISS037-E-011470 (14 Oct. 2013) --- Man-made archipelagos near Dubai, United Arab Emirates, are featured in this image photographed by an Expedition 37 crew member on the International Space Station, flying at approximately 220 miles above Earth. The municipality of Dubai is the largest city of the Persian Gulf emirate of the same name, and has built a global reputation for large-scale developments and architectural works. Among the most visible of these developments -- particularly from the perspective of astronauts onboard the space station -- are three man-made archipelagos. The two Palm Islands -- Palm Jumeirah (right) and Palm Jebel Ali (out of frame further to the right) -- appear as stylized palm trees when viewed from above. The World Islands (center frame) evoke a rough map of the world from an air- or space-borne perspective. The Palm Jumeirah project began in 2001 and required more than 50 million cubic meters of dredged sand to raise the islands above the Persian Gulf sea level. Construction of the Palm Jumeirah islands was completed in 2006; for several years now they have been developed for residential and commercial housing and infrastructure. Creation of the World Islands was begun in 2003 and completed in 2008, using 320 million cubic meters of sand and 37 million tons of rock for the surrounding 27 kilometer-long protective breakwater.

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

    NASA Astrophysics Data System (ADS)

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

    2016-09-01

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

  13. Undoped p-type GaN1-xSbx alloys: Effects of annealing

    NASA Astrophysics Data System (ADS)

    Segercrantz, N.; Baumgartner, Y.; Ting, M.; Yu, K. M.; Mao, S. S.; Sarney, W. L.; Svensson, S. P.; Walukiewicz, W.

    2016-12-01

    We report p-type behavior for undoped GaN1-xSbx alloys with x ≥ 0.06 grown by molecular beam epitaxy at low temperatures (≤400 °C). Rapid thermal annealing of the GaN1-xSbx films at temperatures >400 °C is shown to generate hole concentrations greater than 1019 cm-3, an order of magnitude higher than typical p-type GaN achieved by Mg doping. The p-type conductivity is attributed to a large upward shift of the valence band edge resulting from the band anticrossing interaction between localized Sb levels and extended states of the host matrix.

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

    SciT

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

    2016-03-14

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

  15. Optical and structural characterisation of epitaxial nanoporous GaN grown by CVD.

    PubMed

    Mena, Josué; Carvajal, Joan J; Martínez, Oscar; Jiménez, Juan; Zubialevich, Vitaly Z; Parbrook, Peter J; Diaz, Francesc; Aguiló, Magdalena

    2017-09-15

    In this paper we study the optical properties of nanoporous gallium nitride (GaN) epitaxial layers grown by chemical vapour deposition on non-porous GaN substrates, using photoluminescence, cathodoluminescence, and resonant Raman scattering, and correlate them with the structural characteristic of these films. We pay special attention to the analysis of the residual strain of the layers and the influence of the porosity in the light extraction. The nanoporous GaN epitaxial layers are under tensile strain, although the strain is progressively reduced as the deposition time and the thickness of the porous layer increases, becoming nearly strain free for a thickness of 1.7 μm. The analysis of the experimental data point to the existence of vacancy complexes as the main source of the tensile strain.

  16. Study of GaN nanorods converted from β-Ga2O3

    NASA Astrophysics Data System (ADS)

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

    2018-05-01

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

  17. Coaxial metal-oxide-semiconductor (MOS) Au/Ga2O3/GaN nanowires.

    PubMed

    Hsieh, Chin-Hua; Chang, Mu-Tung; Chien, Yu-Jen; Chou, Li-Jen; Chen, Lih-Juann; Chen, Chii-Dong

    2008-10-01

    Coaxial metal-oxide-semiconductor (MOS) Au-Ga2O3-GaN heterostructure nanowires were successfully fabricated by an in situ two-step process. The Au-Ga2O3 core-shell nanowires were first synthesized by the reaction of Ga powder, a mediated Au thin layer, and a SiO2 substrate at 800 degrees C. Subsequently, these core-shell nanowires were nitridized in ambient ammonia to form a GaN coating layer at 600 degrees C. The GaN shell is a single crystal, an atomic flat interface between the oxide and semiconductor that ensures that the high quality of the MOS device is achieved. These novel 1D nitride-based MOS nanowires may have promise as building blocks to the future nitride-based vertical nanodevices.

  18. AB INITIO Investigations of the Magnetism in Diluted Magnetic Semiconductor Fe-DOPED GaN

    NASA Astrophysics Data System (ADS)

    Cheng, Jie; Zhou, Jing; Xu, Wei; Dong, Peng

    2014-01-01

    In this paper, we present a first principle investigation on Fe-doped GaN with wurtzite and zinc-blend structure using full potential density functional calculations. Data point out that the magnetic behavior of Fe-doped GaN system is strongly dependent on Fe doping configurations. In agreement with the experimental reports, and independently by doping, antiferromagnetism occurs in the zinc-blend structure, while in the wurtzite structure ferromagnetism depends on the Fe doping configurations. Detailed analyses combined with density of state calculations support the assignment that the ferromagnetism is closely related to the impurity band at the origin of the hybridization of Fe 3d and N 2p states in the Fe-doped GaN of wurtzite phase.

  19. Defect reduction in GaN on dome-shaped patterned-sapphire substrates

    NASA Astrophysics Data System (ADS)

    Chen, Po-Hsun; Su, Vin-Cent; Wu, Shang-Hsuan; Lin, Ray-Ming; Kuan, Chieh-Hsiung

    2018-02-01

    This paper demonstrates the behavior of defect reduction in un-doped GaN (u-GaN) grown on a commercial dome-shaped patterned-sapphire substrate (CDPSS). Residual strain inside the u-GaN grown on the CDPSS have been investigated as well. As verified by the experimentally measured data, the limited growth rate of the u-GaN on the sidewall of the CDPSS enhances the lateral growth of the GaN on the trench region while increasing the growth time. This subsequently contributes to improve the crystalline quality of the GaN on the CDPSS. The more prominent dislocations occur in the u-GaN epilayers on the CDPSS after reaching the summit of the accumulated strain inside the epilayers. Such prominent bent dislocations improve their blocking abilities, followed by the achievement of the better crystalline quality for the growth of the u-GaN on the CDPSS.

  20. Site-controlled GaN nanocolumns with InGaN insertions grown by MBE

    NASA Astrophysics Data System (ADS)

    Nechaev, D. V.; Semenov, A. N.; Koshelev, O. A.; Jmerik, V. N.; Davydov, V. Yu; Smirnov, A. N.; Pozina, G.; Shubina, T. V.; Ivanov, S. V.

    2017-11-01

    The site-controlled plasma-assisted molecular beam epitaxy (PA MBE) has been developed to fabricate the regular array of GaN nanocolumns (NCs) with InGaN insertions on micro-cone patterned sapphire substrates (μ-CPSSs). Two-stage growth of GaN NCs, including a nucleation layer grown at metal-rich conditions and high temperature GaN growth in strong N-rich condition, has been developed to achieve the selective growth of the NCs. Microcathodoluminescence measurements have demonstrated pronounced emission from the InGaN insertions in 450-600 nm spectral range. The optically isolated NCs can be used as effective nano-emitters operating in the visible range.

  1. Thermodynamic analysis of trimethylgallium decomposition during GaN metal organic vapor phase epitaxy

    NASA Astrophysics Data System (ADS)

    Sekiguchi, Kazuki; Shirakawa, Hiroki; Chokawa, Kenta; Araidai, Masaaki; Kangawa, Yoshihiro; Kakimoto, Koichi; Shiraishi, Kenji

    2018-04-01

    We analyzed the decomposition of Ga(CH3)3 (TMG) during the metal organic vapor phase epitaxy (MOVPE) of GaN on the basis of first-principles calculations and thermodynamic analysis. We performed activation energy calculations of TMG decomposition and determined the main reaction processes of TMG during GaN MOVPE. We found that TMG reacts with the H2 carrier gas and that (CH3)2GaH is generated after the desorption of the methyl group. Next, (CH3)2GaH decomposes into (CH3)GaH2 and this decomposes into GaH3. Finally, GaH3 becomes GaH. In the MOVPE growth of GaN, TMG decomposes into GaH by the successive desorption of its methyl groups. The results presented here concur with recent high-resolution mass spectroscopy results.

  2. Photophysics of GaN single-photon emitters in the visible spectral range

    NASA Astrophysics Data System (ADS)

    Berhane, Amanuel M.; Jeong, Kwang-Yong; Bradac, Carlo; Walsh, Michael; Englund, Dirk; Toth, Milos; Aharonovich, Igor

    2018-04-01

    In this work, we present a detailed photophysical analysis of recently discovered, optically stable single-photon emitters (SPEs) in gallium nitride (GaN). Temperature-resolved photoluminescence measurements reveal that the emission lines at 4 K are three orders of magnitude broader than the transform-limited width expected from excited-state lifetime measurements. The broadening is ascribed to ultrafast spectral diffusion. The photophysical study on several emitters at room temperature (RT) reveals an average brightness of (427 ±215 )kCounts /s . Finally, polarization measurements from 14 emitters are used to determine visibility as well as dipole orientation of defect systems within the GaN crystal. Our results underpin some of the fundamental properties of SPEs in GaN both at cryogenic and RT, and define the benchmark for future work in GaN-based single-photon technologies.

  3. Atomic layer epitaxy of GaN over sapphire using switched metalorganic chemical vapor deposition

    NASA Astrophysics Data System (ADS)

    Khan, M. A.; Skogman, R. A.; van Hove, J. M.; Olson, D. T.; Kuznia, J. N.

    1992-03-01

    In this letter the first switched atomic layer epitaxy (SALE) of single crystal GaN over basal plane sapphire substrates is reported. A low pressure metalorganic chemical vapor deposition (LPMOCVD) system was used for the epilayer depositions. In contrast to conventional LPMOCVD requiring temperatures higher than 700 C, the SALE process resulted in single crystal insulating GaN layers at growth temperatures ranging from 900 to 450 C. The band-edge transmission and the photoluminescence of the films from the SALE process were comparable to the best LPMOCVD films. As best as is known this is the first report of insulating GaN films which show excellent band-edge photoluminescence.

  4. Atomic layer epitaxy of GaN over sapphire using switched metalorganic chemical vapor deposition

    NASA Astrophysics Data System (ADS)

    Asif Khan, M.; Skogman, R. A.; Van Hove, J. M.; Olson, D. T.; Kuznia, J. N.

    1992-03-01

    In this letter we report the first switched atomic layer epitaxy (SALE) of single crystal GaN over basal plane sapphire substrates. A low pressure metalorganic chemical vapor deposition (LPMOCVD) system was used for the epilayer depositions. In contrast to conventional LPMOCVD requiring temperatures higher than 700 °C, the SALE process resulted in single crystal insulating GaN layers at growth temperatures ranging from 900 to 450 °C. The band-edge transmission and the photoluminescence of the films from the SALE process were comparable to the best LPMOCVD films. To the best of our knowledge this is the first report of insulating GaN films which show excellent band-edge photoluminescence.

  5. Unstable behaviour of normally-off GaN E-HEMT under short-circuit

    NASA Astrophysics Data System (ADS)

    Martínez, P. J.; Maset, E.; Sanchis-Kilders, E.; Esteve, V.; Jordán, J.; Bta Ejea, J.; Ferreres, A.

    2018-04-01

    The short-circuit capability of power switching devices plays an important role in fault detection and the protection of power circuits. In this work, an experimental study on the short-circuit (SC) capability of commercial 600 V Gallium Nitride enhancement-mode high-electron-mobility transistors (E-HEMT) is presented. A different failure mechanism has been identified for commercial p-doped GaN gate (p-GaN) HEMT and metal-insulator-semiconductor (MIS) HEMT. In addition to the well known thermal breakdown, a premature breakdown is shown on both GaN HEMTs, triggered by hot electron trapping at the surface, which demonstrates that current commercial GaN HEMTs has requirements for improving their SC ruggedness.

  6. Correlation of doping, structure, and carrier dynamics in a single GaN nanorod

    NASA Astrophysics Data System (ADS)

    Zhou, Xiang; Lu, Ming-Yen; Lu, Yu-Jung; Gwo, Shangjr; Gradečak, Silvija

    2013-06-01

    We report the nanoscale optical investigation of a single GaN p-n junction nanorod by cathodoluminescence (CL) in a scanning transmission electron microscope. CL emission characteristic of dopant-related transitions was correlated to doping and structural defect in the nanorod, and used to determine p-n junction position and minority carrier diffusion lengths of 650 nm and 165 nm for electrons and holes, respectively. Temperature-dependent CL study reveals an activation energy of 19 meV for non-radiative recombination in Mg-doped GaN nanorods. These results directly correlate doping, structure, carrier dynamics, and optical properties of GaN nanostructure, and provide insights for device design and fabrication.

  7. Botulinum toxin detection 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.; Tseng, Y.; Pearton, S. J.; Ramage, J.; Hooten, D.; Dabiran, A.; Chow, P. P.; Ren, F.

    2008-12-01

    Antibody-functionalized, Au-gated AlGaN /GaN high electron mobility transistors (HEMTs) were used to detect botulinum toxin. 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 5s when the target toxin in a buffer was added to the antibody-immobilized surface. We could detect a range of concentrations from 1to10ng/ml. These results clearly demonstrate the promise of field-deployable electronic biological sensors based on AlGaN /GaN HEMTs for botulinum toxin detection.

  8. Depletion-Mode GaN HEMT Q-Spoil Switches for MRI Coils

    PubMed Central

    Lu, Jonathan Y.; Grafendorfer, Thomas; Zhang, Tao; Vasanawala, Shreyas; Robb, Fraser; Pauly, John M.; Scott, Greig C.

    2017-01-01

    Q-spoiling is the process of decoupling an MRI receive coil to protect the equipment and patient. Conventionally, Q-spoiling is performed using a PIN diode switch that draws significant current. In this work, a Q-spoiling technique using a depletion-mode Gallium Nitride HEMT device was developed for coil detuning at both 1.5 T and 3 T MRI. The circuits with conventional PIN diode Q-spoiling and the GaN HEMT device were implemented on surface coils. SNR was measured and compared for all surfaces coils. At both 1.5 T and 3 T, comparable SNR was achieved for all coils with the proposed technique and conventional Q-spoiling. The GaN HEMT device has significantly reduced the required power for Q-spoiling. The GaN HEMT device also provides useful safety features by detuning the coil when unpowered. PMID:27362895

  9. Characterization of vertical GaN p-n diodes and junction field-effect transistors on bulk GaN down to cryogenic temperatures

    NASA Astrophysics Data System (ADS)

    Kizilyalli, I. C.; Aktas, O.

    2015-12-01

    There is great interest in wide-bandgap semiconductor devices and most recently in vertical GaN structures for power electronic applications such as power supplies, solar inverters and motor drives. In this paper the temperature-dependent electrical behavior of vertical GaN p-n diodes and vertical junction field-effect transistors fabricated on bulk GaN substrates of low defect density (104 to 106 cm-2) is described. Homoepitaxial MOCVD growth of GaN on its native substrate and the ability to control the doping in the drift layers in GaN have allowed the realization of vertical device architectures with drift layer thicknesses of 6 to 40 μm and net carrier electron concentrations as low as 1 × 1015 cm-3. This parameter range is suitable for applications requiring breakdown voltages of 1.2 kV to 5 kV. Mg, which is used as a p-type dopant in GaN, is a relatively deep acceptor (E A ≈ 0.18 eV) and susceptible to freeze-out at temperatures below 200 K. The loss of holes in p-GaN has a deleterious effect on p-n junction behavior, p-GaN contacts and channel control in junction field-effect transistors at temperatures below 200 K. Impact ionization-based avalanche breakdown (BV > 1200 V) in GaN p-n junctions is characterized between 77 K and 423 K for the first time. At higher temperatures the p-n junction breakdown voltage improves due to increased phonon scattering. A positive temperature coefficient in the breakdown voltage is demonstrated down to 77 K; however, the device breakdown characteristics are not as abrupt at temperatures below 200 K. On the other hand, contact resistance to p-GaN is reduced dramatically above room temperature, improving the overall device performance in GaN p-n diodes in all cases except where the n-type drift region resistance dominates the total forward resistance. In this case, the electron mobility can be deconvolved and is found to decrease with T -3/2, consistent with a phonon scattering model. Also, normally-on vertical junction

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

    PubMed

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

    2012-10-10

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

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

    PubMed Central

    2012-01-01

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

  12. Degradable Implantate: Entwicklungsbeispiele

    NASA Astrophysics Data System (ADS)

    Ruffieux, Kurt; Wintermantel, Erich

    Resorbierbare Implantate werden seit mehreren Jahrzehnten in der Implantologie eingesetzt. Bekannt wurden diese Biomaterialien mit dem Aufkommen von sich selbst auflösenden Nahtfäden auf der Basis von synthetisch hergestellten Polylactiden und Polyglycoliden in den 70er Jahren. In einem nächsten Schritt wurden Implantate wie Platten und Schrauben zur Gewebefixation aus den gleichen Biomaterialien hergestellt.

  13. Urinary incontinence - injectable implant

    MedlinePlus

    ... injections of material into the urethra to help control urine leakage ( urinary incontinence ) caused by a weak urinary sphincter. ... choose to have implants. Women who have urine leakage and want a ... procedure to control the problem may choose to have an implant ...

  14. Guideline on cochlear implants.

    PubMed

    Manrique, Manuel; Ramos, Ángel; de Paula Vernetta, Carlos; Gil-Carcedo, Elisa; Lassaleta, Luis; Sanchez-Cuadrado, Isabel; Espinosa, Juan Manuel; Batuecas, Ángel; Cenjor, Carlos; Lavilla, María José; Núñez, Faustino; Cavalle, Laura; Huarte, Alicia

    2018-03-26

    In the last decade numerous hospitals have started to work with patients who are candidates for a cochlear implant (CI) and there have been numerous and relevant advances in the treatment of sensorineural hearing loss that extended the indications for cochlear implants. To provide a guideline on cochlear implants to specialists in otorhinolaryngology, other medical specialities, health authorities and society in general. The Scientific Committees of Otology, Otoneurology and Audiology from the Spanish Society of Otolaryngology and Head and Neck Surgery (SEORL-CCC), in a coordinated and agreed way, performed a review of the current state of CI based on the existing regulations and in the scientific publications referenced in the bibliography of the document drafted. The clinical guideline on cochlear implants provides information on: a) Definition and description of Cochlear Implant; b) Indications for cochlear implants; c) Organizational requirements for a cochlear implant programme. A clinical guideline on cochlear implants has been developed by a Committee of Experts of the SEORL-CCC, to help and guide all the health professionals involved in this field of CI in decision-making to treathearing impairment. Copyright © 2018 Sociedad Española de Otorrinolaringología y Cirugía de Cabeza y Cuello. Publicado por Elsevier España, S.L.U. All rights reserved.

  15. Batteryless implanted echosonometer

    NASA Technical Reports Server (NTRS)

    Kojima, G. K.

    1977-01-01

    Miniature ultrasonic echosonometer implanted within laboratory animals obtains energy from RF power oscillator that is electronically transduced via induction loop to power receiving loop located just under animal's skin. Method of powering device offers significant advantages over those in which battery is part of implanted package.

  16. Implantable, Ingestible Electronic Thermometer

    NASA Technical Reports Server (NTRS)

    Kleinberg, Leonard

    1987-01-01

    Small quartz-crystal-controlled oscillator swallowed or surgically implanted provides continuous monitoring of patient's internal temperature. Receiver placed near patient measures oscillator frequency, and temperature inferred from previously determined variation of frequency with temperature. Frequency of crystal-controlled oscillator varies with temperature. Circuit made very small and implanted or ingested to measure internal body temperature.

  17. Number of implants for mandibular implant overdentures: a systematic review

    PubMed Central

    Lee, Jeong-Yol; Kim, Ha-Young; Bryant, S. Ross

    2012-01-01

    PURPOSE The aim of this systematic review is to address treatment outcomes of Mandibular implant overdentures relative to implant survival rate, maintenance and complications, and patient satisfaction. MATERIALS AND METHODS A systematic literature search was conducted by a PubMed search strategy and hand-searching of relevant journals from included studies. Randomized Clinical Trials (RCT) and comparative clinical trial studies on mandibular implant overdentures until August, 2010 were selected. Eleven studies from 1098 studies were finally selected and data were analyzed relative to number of implants. RESULTS Six studies presented the data of the implant survival rate which ranged from 95% to 100% for 2 and 4 implant group and from 81.8% to 96.1% for 1 and 2 implant group. One study, which statistically compared implant survival rate showed no significant differences relative to the number of implants. The most common type of prosthetic maintenance and complications were replacement or reattaching of loose clips for 2 and 4 implant group, and denture repair due to the fracture around an implant for 1 and 2 implant groups. Most studies showed no significant differences in the rate of prosthetic maintenance and complication, and patient satisfaction regardless the number of implants. CONCLUSION The implant survival rate of mandibular overdentures is high regardless of the number of implants. Denture maintenance is likely not inflenced substantially by the number of implants and patient satisfaction is typically high again regardless os the number of implants. PMID:23236572

  18. Axial p-n junction and space charge limited current in single GaN nanowire.

    PubMed

    Fang, Zhihua; Donatini, Fabrice; Daudin, Bruno; Pernot, Julien

    2018-01-05

    The electrical characterizations of individual basic GaN nanostructures, such as axial nanowire (NW) p-n junctions, are becoming indispensable and crucial for the fully controlled realization of GaN NW based devices. In this study, electron beam induced current (EBIC) measurements were performed on two single axial GaN p-n junction NWs grown by plasma-assisted molecular beam epitaxy. I-V characteristics revealed that both ohmic and space charge limited current (SCLC) regimes occur in GaN p-n junction NW. Thanks to an improved contact process, both the electric field induced by the p-n junction and the SCLC in the p-part of GaN NW were disclosed and delineated by EBIC signals under different biases. Analyzing the EBIC profiles in the vicinity of the p-n junction under 0 V and reverse bias, we deduced a depletion width in the range of 116-125 nm. Following our previous work, the acceptor N a doping level was estimated to be 2-3 × 10 17 at cm -3 assuming a donor level N d of 2-3 × 10 18 at cm -3 . The hole diffusion length in n-GaN was determined to be 75 nm for NW #1 and 43 nm for NW #2, demonstrating a low surface recombination velocity at the m-plane facet of n-GaN NW. Under forward bias, EBIC imaging visualized the electric field induced by the SCLC close to p-side contact, in agreement with unusual SCLC previously reported in GaN NWs.

  19. Two novel mutations in the GAN gene causing giant axonal neuropathy.

    PubMed

    Normendez-Martínez, Monica Irad; Monterde-Cruz, Lucero; Martínez, Roberto; Marquez-Harper, Magdalena; Esquitin-Garduño, Nayelli; Valdes-Flores, Margarita; Casas-Avila, Leonora; de Leon-Suarez, Valeria Ponce; Romero-Díaz, Viktor Javier; Hidalgo-Bravo, Alberto

    2018-06-06

    Giant axonal neuropathy (GAN) is a rare neurodegenerative disease transmitted in an autosomal recessive mode. This disorder presents motor and sensitive symptoms with an onset in early childhood. Progressive neurodegeneration makes the patients wheelchair dependent by the end of the second decade of life. Affected individuals do not survive beyond the third decade of life. Molecular analysis has identified mutations in the gene GAN in patients with this disorder. This gene produces a protein called gigaxonin which is presumably involved in protein degradation via the ubiquitin-proteasome system. However, the underlying molecular mechanism is not clearly understood yet. Here we present the first patient from Mexico with clinical data suggesting GAN. Sequencing of the GAN gene was carried out. Changes in the nucleotide sequence were investigated for their possible impact on protein function and structure using the publicly available prediction tools PolyPhen-2 and PANTHER. The patient is a compound heterozygous carrying two novel mutations in the GAN gene. The sequence analysis revealed two missense mutations in the Kelch repeats domain. In one allele, a C>T transition was found in exon 9 at the nucleotide position 55393 (g.55393C>T). In the other allele, a transversion G>T in exon 11 at the nucleotide position 67471 (g.67471G>T) was observed. Both of the bioinformatic tools predicted that these amino acid substitutions would have a negative impact on gigaxonin's function. This work provides useful information for health professionals and expands the spectrum of disease-causing mutations in the GAN gene and it is the first documented case in Mexican population.

  20. Axial p-n junction and space charge limited current in single GaN nanowire

    NASA Astrophysics Data System (ADS)

    Fang, Zhihua; Donatini, Fabrice; Daudin, Bruno; Pernot, Julien

    2018-01-01

    The electrical characterizations of individual basic GaN nanostructures, such as axial nanowire (NW) p-n junctions, are becoming indispensable and crucial for the fully controlled realization of GaN NW based devices. In this study, electron beam induced current (EBIC) measurements were performed on two single axial GaN p-n junction NWs grown by plasma-assisted molecular beam epitaxy. I-V characteristics revealed that both ohmic and space charge limited current (SCLC) regimes occur in GaN p-n junction NW. Thanks to an improved contact process, both the electric field induced by the p-n junction and the SCLC in the p-part of GaN NW were disclosed and delineated by EBIC signals under different biases. Analyzing the EBIC profiles in the vicinity of the p-n junction under 0 V and reverse bias, we deduced a depletion width in the range of 116-125 nm. Following our previous work, the acceptor N a doping level was estimated to be 2-3 × 1017 at cm-3 assuming a donor level N d of 2-3 × 1018 at cm-3. The hole diffusion length in n-GaN was determined to be 75 nm for NW #1 and 43 nm for NW #2, demonstrating a low surface recombination velocity at the m-plane facet of n-GaN NW. Under forward bias, EBIC imaging visualized the electric field induced by the SCLC close to p-side contact, in agreement with unusual SCLC previously reported in GaN NWs.

  1. Multiferroic GaN nanofilms grown within Na-4 mica channels

    NASA Astrophysics Data System (ADS)

    Bhattacharya, Santanu; Datta, A.; Chakravorty, D.

    2010-03-01

    Gallium nitride nanofilms grown within nanochannels of Na-4 mica structure, exhibit ferromagnetism even at room temperature due to the presence of gallium vacancies at the surfaces of the nanofilms. These nanofilms also show a ferroelectric behavior at room temperature ascribed to a small distortion in the crystal structure of GaN due to its growth within the Na-4 mica nanochannels. A colossal increase in 338% in dielectric constant was observed for an applied magnetic field of 26 kOe. The magnetoelectric effect is ascribed to magnetostriction of magnetic GaN phase.

  2. Partially filled intermediate band of Cr-doped GaN films

    SciT

    Sonoda, S.

    2012-05-14

    We investigated the band structure of sputtered Cr-doped GaN (GaCrN) films using optical absorption, photoelectron yield spectroscopy, and charge transport measurements. It was found that an additional energy band is formed in the intrinsic band gap of GaN upon Cr doping, and that charge carriers in the material move in the inserted band. Prototype solar cells showed enhanced short circuit current and open circuit voltage in the n-GaN/GaCrN/p-GaN structure compared to the GaCrN/p-GaN structure, which validates the proposed concept of an intermediate-band solar cell.

  3. p-type zinc-blende GaN on GaAs substrates

    NASA Astrophysics Data System (ADS)

    Lin, M. E.; Xue, G.; Zhou, G. L.; Greene, J. E.; Morkoç, H.

    1993-08-01

    We report p-type cubic GaN. The Mg-doped layers were grown on vicinal (100) GaAs substrates by plasma-enhanced molecular beam epitaxy. Thermally sublimed Mg was, with N2 carrier gas, fed into an electron-cyclotron resonance source. p-type zinc-blende-structure GaN films were achieved with hole mobilities as high as 39 cm2/V s at room temperature. The cubic nature of the films were confirmed by x-ray diffractometry. The depth profile of Mg was investigated by secondary ions mass spectroscopy.

  4. Characterization of GaN microstructures grown by plasma-assisted molecular beam epitaxy

    SciT

    Lo, Ikai; Pang, Wen-Yuan; Hsu, Yu-Chi

    2013-06-15

    The characterization of GaN microstructures grown by plasma-assisted molecular beam epitaxy on LiAlO{sub 2} substrate was studied by cathodoluminescence and photoluminescence measurements. We demonstrated that the cathodoluminescence from oblique semi-polar surfaces of mushroom-shaped GaN was much brighter than that from top polar surface due to the reduction of polarization field on the oblique semi-polar surfaces. It implies that the oblique semi-polar surface is superior for the light-emitting surface of wurtzite nano-devices.

  5. Simulation of optimum parameters for GaN MSM UV photodetector

    SciT

    Alhelfi, Mohanad A., E-mail: mhad12344@gmail.com; Ahmed, Naser M., E-mail: nas-tiji@yahoo.com; Hashim, M. R., E-mail: roslan@usm.my

    2016-07-06

    In this study the optimum parameters of GaN M-S-M photodetector are discussed. The evaluation of the photodetector depends on many parameters, the most of the important parameters the quality of the GaN film and others depend on the geometry of the interdigited electrode. In this simulation work using MATLAB software with consideration of the reflection and absorption on the metal contacts, a detailed study involving various electrode spacings (S) and widths (W) reveals conclusive results in device design. The optimum interelectrode design for interdigitated MSM-PD has been specified and evaluated by effect on quantum efficiency and responsivity.

  6. Ultrahigh-yield growth of GaN via halogen-free vapor-phase epitaxy

    NASA Astrophysics Data System (ADS)

    Nakamura, Daisuke; Kimura, Taishi

    2018-06-01

    The material yield of Ga during GaN growth via halogen-free vapor-phase epitaxy (HF-VPE) was systematically investigated and found to be much higher than that obtained using conventional hydride VPE. This is attributed to the much lower process pressure and shorter seed-to-source distance, owing to the inherent chemical reactions and corresponding reactor design used for HF-VPE growth. Ultrahigh-yield GaN growth was demonstrated on a 4-in.-diameter sapphire seed substrate.

  7. Modeling of radiation damage recovery in particle detectors based on GaN

    NASA Astrophysics Data System (ADS)

    Gaubas, E.; Ceponis, T.; Pavlov, J.

    2015-12-01

    The pulsed characteristics of the capacitor-type and PIN diode type detectors based on GaN have been simulated using the dynamic and drift-diffusion models. The drift-diffusion current simulations have been implemented by employing the commercial software package Synopsys TCAD Sentaurus. The bipolar drift regime has been analyzed. The possible internal gain in charge collection through carrier multiplication processes determined by impact ionization has been considered in order to compensate carrier lifetime reduction due to radiation defects introduced into GaN material of detector.

  8. GaN growth via HVPE on SiC/Si substrates: growth mechanisms

    NASA Astrophysics Data System (ADS)

    Sharofidinov, Sh Sh; Redkov, A. V.; Osipov, A. V.; Kukushkin, S. A.

    2017-11-01

    The article focuses on the study of GaN thin film growth via chloride epitaxy on SiC/Si hybrid substrate. SiC buffer layer was grown by a method of substitution of atoms, which allows one to reduce impact of mechanical stress therein on subsequent growth of III-nitride films. It is shown, that change in GaN growth conditions leads to change in its growth mechanism. Three mechanisms: epitaxial, spiral and stepwise growth are considered and mechanical stresses are estimated via Raman spectroscopy.

  9. GaN microwires as optical microcavities: whispering gallery modes Vs Fabry-Perot modes.

    PubMed

    Coulon, Pierre-Marie; Hugues, Maxime; Alloing, Blandine; Beraudo, Emmanuel; Leroux, Mathieu; Zuniga-Perez, Jesus

    2012-08-13

    GaN microwires grown by metalorganic vapour phase epitaxy and with radii typically on the order of 1-5 micrometers exhibit a number of resonances in their photoluminescence spectra. These resonances include whispering gallery modes and transverse Fabry-Perot modes. A detailed spectroscopic study by polarization-resolved microphotoluminescence, in combination with electron microscopy images, has enabled to differentiate both kinds of modes and determined their main spectral properties. Finally, the dispersion of the ordinary and extraordinary refractive indices of strain-free GaN in the visible-UV range has been obtained thanks to the numerical simulation of the observed modes.

  10. Doping Induced Structural Stability and Electronic Properties of GaN Nanotubes

    PubMed Central

    Khan, Mohammad Irfan; Tyagi, Neha; Swaroop Khare, Purnima

    2014-01-01

    The present paper discusses the effect of manganese doping on the structural stability and electronic band gap of chiral (2, 1), armchair (3, 3), and zigzag ((6, 0) and (10, 0)) single walled GaN nanotube by using density functional theory based Atomistix Toolkit (ATK) Virtual NanoLab (VNL). The structural stability has been analyzed in terms of minimum ground state total energy, binding, and formation energy. As an effect of Mn doping (1–4 atoms), all the GaN nanotubes taken into consideration show semiconducting to metallic transition first and after certain level of Mn doping changes its trend. PMID:24707225

  11. Amphoteric Be in GaN: Experimental Evidence for Switching between Substitutional and Interstitial Lattice Sites

    NASA Astrophysics Data System (ADS)

    Tuomisto, Filip; Prozheeva, Vera; Makkonen, Ilja; Myers, Thomas H.; Bockowski, Michal; Teisseyre, Henryk

    2017-11-01

    We show that Be exhibits amphoteric behavior in GaN, involving switching between substitutional and interstitial positions in the lattice. This behavior is observed through the dominance of BeGa in the positron annihilation signals in Be-doped GaN, while the emergence of VGa at high temperatures is a consequence of the Be impurities being driven to interstitial positions. The similarity of this behavior to that found for Na and Li in ZnO suggests that this could be a universal property of light dopants substituting for heavy cations in compound semiconductors.

  12. Amphoteric Be in GaN: Experimental Evidence for Switching between Substitutional and Interstitial Lattice Sites.

    PubMed

    Tuomisto, Filip; Prozheeva, Vera; Makkonen, Ilja; Myers, Thomas H; Bockowski, Michal; Teisseyre, Henryk

    2017-11-10

    We show that Be exhibits amphoteric behavior in GaN, involving switching between substitutional and interstitial positions in the lattice. This behavior is observed through the dominance of Be_{Ga} in the positron annihilation signals in Be-doped GaN, while the emergence of V_{Ga} at high temperatures is a consequence of the Be impurities being driven to interstitial positions. The similarity of this behavior to that found for Na and Li in ZnO suggests that this could be a universal property of light dopants substituting for heavy cations in compound semiconductors.

  13. Simulations of Operation Dynamics of Different Type GaN Particle Sensors

    PubMed Central

    Gaubas, Eugenijus; Ceponis, Tomas; Kalesinskas, Vidas; Pavlov, Jevgenij; Vysniauskas, Juozas

    2015-01-01

    The operation dynamics of the capacitor-type and PIN diode type detectors based on GaN have been simulated using the dynamic and drift-diffusion models. The drift-diffusion current simulations have been implemented by employing the software package Synopsys TCAD Sentaurus. The monopolar and bipolar drift regimes have been analyzed by using dynamic models based on the Shockley-Ramo theorem. The carrier multiplication processes determined by impact ionization have been considered in order to compensate carrier lifetime reduction due to introduction of radiation defects into GaN detector material. PMID:25751080

  14. Thermal Conductivity and Large Isotope Effect in GaN from First Principles

    DTIC Science & Technology

    2012-08-28

    August 2012) We present atomistic first principles results for the lattice thermal conductivity of GaN and compare them to those for GaP, GaAs, and GaSb ...weak scattering results from stiff atomic bonds and the large Ga to N mass ratio, which give phonons high frequencies and also a pronounced energy gap...66.70.f, 63.20.kg, 71.15.m Introduction.—Gallium nitride (GaN) is a wide band gap semiconductor and a promising candidate for use in opto- electronic

  15. Vacancy Defects as Compensating Centers in Mg-Doped GaN

    NASA Astrophysics Data System (ADS)

    Hautakangas, S.; Oila, J.; Alatalo, M.; Saarinen, K.; Liszkay, L.; Seghier, D.; Gislason, H. P.

    2003-04-01

    We apply positron annihilation spectroscopy to identify VN-MgGa complexes as native defects in Mg-doped GaN. These defects dissociate in postgrowth annealings at 500 800 °C. We conclude that VN-MgGa complexes contribute to the electrical compensation of Mg as well as the activation of p-type conductivity in the annealing. The observation of VN-MgGa complexes confirms that vacancy defects in either the N or Ga sublattice are abundant in GaN at any position of the Fermi level during growth, as predicted previously by theoretical calculations.

  16. Earth Observations

    2013-06-21

    ISS036-E-011034 (21 June 2013) --- The Salton Trough is featured in this image photographed by an Expedition 36 crew member on the International Space Station. The Imperial and Coachella Valleys of southern California – and the corresponding Mexicali Valley and Colorado River Delta in Mexico – are part of the Salton Trough, a large geologic structure known to geologists as a graben or rift valley that extends into the Gulf of California. The trough is a geologically complex zone formed by interaction of the San Andreas transform fault system that is, broadly speaking, moving southern California towards Alaska; and the northward motion of the Gulf of California segment of the East Pacific Rise that continues to widen the Gulf of California by sea-floor spreading. According to scientists, sediments deposited by the Colorado River have been filling the northern rift valley (the Salton Trough) for the past several million years, excluding the waters of the Gulf of California and providing a fertile environment – together with irrigation—for the development of extensive agriculture in the region (visible as green and yellow-brown fields at center). The Salton Sea, a favorite landmark of astronauts in low Earth orbit, was formed by an irrigation canal rupture in 1905, and today is sustained by agricultural runoff water. A wide array of varying landforms and land uses in the Salton Trough are visible from space. In addition to the agricultural fields and Salton Sea, easily visible metropolitan areas include Yuma, AZ (lower left); Mexicali, Baja California, Mexico (center); and the San Diego-Tijuana conurbation on the Pacific Coast (right). The approximately 72-kilometer-long Algodones Dunefield is visible at lower left.

  17. Earth Observation

    2012-07-10

    ISS032-E-006129 (10 July 2012) --- Flooding in Krymsk in the Krasnodar region of southern Russia is featured in this image photographed by an Expedition 32 crew member on the International Space Station. On the night of July 7, 2012 a major storm dumped more than a foot of water on the southern Russian area of Krasnodar, near the Black Sea. The resulting flood was likened to a tsunami, and to date, more than 170 people died, most from the city of Krymsk. The Moscow times reports that more than 19,000 people lost everything. This image taken by cosmonauts aboard the space station shows the city of Krymsk. The tan-colored areas indicate some of the regions that were flooded; the color is probably due to the mud and debris that were left by the floodwaters. Krymsk is located in the western foothills on the northern slope of the Caucasus Mountains?a range that stretches between the Black Sea and the Caspian Sea. The vast amount of rain quickly overwhelmed the small river channels that flow northward from the mountains to the Russian lowlands and the Kuban River; Krymsk, located on one of those tributaries, was directly in the pathway of the flash flood. As part of the international partner agreement to use the International Space Station to benefit humanity, crew members and other Earth observing instruments provide best-effort support to the International Disaster Charter (IDC) when it is activated by collecting imagery of areas on the ground impacted by natural events such as the flooding in Krymsk. This image was acquired July 10, 2012 in response to the IDC activation.

  18. Earth Observations

    2011-06-02

    ISS028-E-006687 (2 June 2011) --- Estuaries on the northwestern coast of Madagascar are featured in this image photographed by an Expedition 28 crew member on the International Space Station. Regions where fresh water flowing in rivers and salt water from the seas and oceans mix are called estuaries, and they are among the most biologically productive ecosystems on Earth. This photograph highlights two estuaries located along the northwestern coastline of the island of Madagascar. The Mozambique Channel (top) separates Madagascar from the southeastern coast of Africa. Bombetoka Bay (upper left) is fed by the Betsiboka River and is a frequent subject of astronaut photography due to its striking red floodplain sediments. Mahajamba Bay (right) is fed by several rivers including the Mahajamba and Sofia Rivers; like the Betsiboka, the floodplains of these rivers also contain reddish sediments eroded from their basins upstream. The brackish (mix of fresh and salty water) conditions found in most estuaries host unique plant and animal species adapted to live in such environments. Mangroves in particular are a common plant species found in and around Madagascar estuaries, and Bombetoka Bay contains some of the largest remaining stands. Estuaries also host abundant fish and shellfish species ? many of which need access to freshwater for a portion of their life cycles ? and these in turn support local and migratory bird species that prey on them. However, human activities such as urban development, overfishing, and increased sediment loading from erosion of upriver highlands threaten the ecosystem health of the estuaries. In particular, the silt deposits in Bombetoka Bay at the mouth of the Betsiboka River have been filling in the bay.

  19. Earth Observation

    NASA Technical Reports Server (NTRS)

    1994-01-01

    For pipeline companies, mapping, facilities inventory, pipe inspections, environmental reporting, etc. is a monumental task. An Automated Mapping/Facilities Management/Geographic Information Systems (AM/FM/GIS) is the solution. However, this is costly and time consuming. James W. Sewall Company, an AM/FM/GIS consulting firm proposed an EOCAP project to Stennis Space Center (SSC) to develop a computerized system for storage and retrieval of digital aerial photography. This would provide its customer, Algonquin Gas Transmission Company, with an accurate inventory of rights-of-way locations and pipeline surroundings. The project took four years to complete and an important byproduct was SSC's Digital Aerial Rights-of-Way Monitoring System (DARMS). DARMS saves substantial time and money. EOCAP enabled Sewall to develop new products and expand its customer base. Algonquin now manages regulatory requirements more efficiently and accurately. EOCAP provides government co-funding to encourage private investment in and broader use of NASA remote sensing technology. Because changes on Earth's surface are accelerating, planners and resource managers must assess the consequences of change as quickly and accurately as possible. Pacific Meridian Resources and NASA's Stennis Space Center (SSC) developed a system for monitoring changes in land cover and use, which incorporated the latest change detection technologies. The goal of this EOCAP project was to tailor existing technologies to a system that could be commercialized. Landsat imagery enabled Pacific Meridian to identify areas that had sustained substantial vegetation loss. The project was successful and Pacific Meridian's annual revenues have substantially increased. EOCAP provides government co-funding to encourage private investment in and broader use of NASA remote sensing technology.

  20. Enhanced optical output power of InGaN/GaN light-emitting diodes grown on a silicon (111) substrate with a nanoporous GaN layer.

    PubMed

    Lee, Kwang Jae; Chun, Jaeyi; Kim, Sang-Jo; Oh, Semi; Ha, Chang-Soo; Park, Jung-Won; Lee, Seung-Jae; Song, Jae-Chul; Baek, Jong Hyeob; Park, Seong-Ju

    2016-03-07

    We report the growth of InGaN/GaN multiple quantum wells blue light-emitting diodes (LEDs) on a silicon (111) substrate with an embedded nanoporous (NP) GaN layer. The NP GaN layer is fabricated by electrochemical etching of n-type GaN on the silicon substrate. The crystalline quality of crack-free GaN grown on the NP GaN layer is remarkably improved and the residual tensile stress is also decreased. The optical output power is increased by 120% at an injection current of 20 mA compared with that of conventional LEDs without a NP GaN layer. The large enhancement of optical output power is attributed to the reduction of threading dislocation, effective scattering of light in the LED, and the suppression of light propagation into the silicon substrate by the NP GaN layer.

  1. Orbitally driven low thermal conductivity of monolayer gallium nitride (GaN) with planar honeycomb structure: a comparative study.

    PubMed

    Qin, Zhenzhen; Qin, Guangzhao; Zuo, Xu; Xiong, Zhihua; Hu, Ming

    2017-03-23

    Two-dimensional (2D) materials with graphene as a representative have been intensively studied for a long time. Recently, monolayer gallium nitride (ML GaN) with honeycomb structure was successfully fabricated in experiments, generating enormous research interest for its promising applications in nano- and opto-electronics. Considering all these applications are inevitably involved with thermal transport, systematic investigation of the phonon transport properties of 2D GaN is in demand. In this paper, by solving the Boltzmann transport equation (BTE) based on first-principles calculations, we performed a comprehensive study of the phonon transport properties of ML GaN, with detailed comparison to bulk GaN, 2D graphene, silicene and ML BN with similar honeycomb structure. Considering the similar planar structure of ML GaN to graphene, it is quite intriguing to find that the thermal conductivity (κ) of ML GaN (14.93 W mK -1 ) is more than two orders of magnitude lower than that of graphene and is even lower than that of silicene with a buckled structure. Systematic analysis is performed based on the study of the contribution from phonon branches, comparison among the mode level phonon group velocity and lifetime, the detailed process and channels of phonon-phonon scattering, and phonon anharmonicity with potential energy well. We found that, different from graphene and ML BN, the phonon-phonon scattering selection rule in 2D GaN is slightly broken by the lowered symmetry due to the large difference in the atomic radius and mass between Ga and N atoms. Further deep insight is gained from the electronic structure. Resulting from the special sp orbital hybridization mediated by the Ga-d orbital in ML GaN, the strongly polarized Ga-N bond, localized charge density, and its inhomogeneous distribution induce large phonon anharmonicity and lead to the intrinsic low κ of ML GaN. The orbitally driven low κ of ML GaN unraveled in this work would make 2D GaN prospective for

  2. Influence of Palatal Coverage and Implant Distribution on Implant Strain in Maxillary Implant Overdentures.

    PubMed

    Takahashi, Toshihito; Gonda, Tomoya; Mizuno, Yoko; Fujinami, Yozo; Maeda, Yoshinobu

    2016-01-01

    Maxillary implant overdentures are often used in clinical practice. However, there is no agreement or established guidelines regarding prosthetic design or optimal implant placement configuration. The purpose of this study was to examine the influence of palatal coverage and implant number and distribution in relation to impact strain under maxillary implant overdentures. A maxillary edentulous model with implants and experimental overdentures with and without palatal coverage was fabricated. Four strain gauges were attached to each implant, and they were positioned in the anterior, premolar, and molar areas. A vertical occlusal load of 98 N was applied through a mandibular complete denture, and the implant strains were compared using one-way analysis of variance (P = .05). The palatolabial strain was much higher on anterior implants than on other implants in both denture types. Although there was no significant difference between the strain under dentures with and without palatal coverage, palateless dentures tended to result in higher implant strain than dentures with palatal coverage. Dentures supported by only two implants registered higher strain than those supported by four or six implants. Implants under palateless dentures registered higher strain than those under dentures with palatal coverage. Anterior implants exhibited higher palatolabial strain than other implants regardless of palatal coverage and implant configuration; it is therefore recommended that maxillary implant overdentures should be supported by six implants with support extending to the distal end of the arch.

  3. Mechanism of leakage of ion-implantation isolated AlGaN/GaN MIS-high electron mobility transistors on Si substrate

    NASA Astrophysics Data System (ADS)

    Zhang, Zhili; Song, Liang; Li, Weiyi; Fu, Kai; Yu, Guohao; Zhang, Xiaodong; Fan, Yaming; Deng, Xuguang; Li, Shuiming; Sun, Shichuang; Li, Xiajun; Yuan, Jie; Sun, Qian; Dong, Zhihua; Cai, Yong; Zhang, Baoshun

    2017-08-01

    In this paper, we systematically investigated the leakage mechanism of the ion-implantation isolated AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) on Si substrate. By means of combined DC tests at different temperatures and electric field dependence, we demonstrated the following original results: (1) It is proved that gate leakage is the main contribution to OFF-state leakage of ion-implantation isolated AlGaN/GaN MIS-HEMTs, and the gate leakage path is a series connection of the gate dielectric Si3N4 and Si3N4-GaN interface. (2) The dominant mechanisms of the leakage current through LPCVD-Si3N4 gate dielectric and Si3N4-GaN interface are identified to be Frenkel-Poole emission and two-dimensional variable range hopping (2D-VRH), respectively. (3) A certain temperature annealing could reduce the density of the interface state that produced by ion implantation, and consequently suppress the interface leakage transport, which results in a decrease in OFF-state leakage current of ion-implantation isolated AlGaN/GaN MIS-HEMTs.

  4. The implantation of life on Mars - Feasibility and motivation

    NASA Technical Reports Server (NTRS)

    Haynes, Robert H.; Mckay, Christopher P.

    1992-01-01

    Scientific concepts are reviewed regarding the potential formation and development of a life-bearing environment on Mars, and a potential ecopoiesis scenario is given. The development of the earth's biosphere is defined, and the major assumptions related to the formation of Martian life are listed. Three basic phases are described for the life-implantation concept which include determining whether sufficient quantities of volatiles are available, engineering the warming of the planet, and implanting microbial communities if necessary. Warming the planet theoretically releases liquid H2O and produces a thick CO2 atmosphere, and the implantation of biological communities is only necessary if no indigenous microbes emerge. It is concluded that a feasibility study is required to assess the possibilities of implanting life on Mars more concretely.

  5. Earth Observation

    2013-06-24

    ISS036-E-011843 (24 June 2013) --- Gravity waves and sunglint on Lake Superior are featured in this image photographed by an Expedition 36 crew member on the International Space Station. From the vantage point of the space station, crew members frequently observe Earth atmospheric and surface phenomena in ways impossible to view from the ground. Two such phenomena?gravity waves and sunglint?are illustrated in this photograph of northeastern Lake Superior. The Canadian Shield of southern Ontario (bottom) is covered with extensive green forest canopy typical of early summer. Offshore, and to the west and southwest of Pukaskwa National Park several distinct sets of parallel cloud bands are visible. Gravity waves are produced when moisture-laden air encounters imbalances in air density, such as might be expected when cool air flows over warmer air; this can cause the flowing air to oscillate up and down as it moves, causing clouds to condense as the air rises (cools) and evaporate away as the air sinks (warms). This produces parallel bands of clouds oriented perpendicular to the wind direction. The orientation of the cloud bands visible in this image, parallel to the coastlines, suggests that air flowing off of the land surfaces to the north is interacting with moist, stable air over the lake surface, creating gravity waves. The second phenomenon?sunglint?effects the water surface around and to the northeast of Isle Royale (upper right). Sunglint is caused by light reflection off a water surface; some of the reflected light travels directly back towards the observer, resulting in a bright mirror-like appearance over large expanses of water. Water currents and changes in surface tension (typically caused by presence of oils or surfactants) alter the reflective properties of the water, and can be highlighted by sunglint. For example, surface water currents are visible to the east of Isle Royale that are oriented similarly to the gravity waves ? suggesting that they too

  6. Dental Implant Systems

    PubMed Central

    Oshida, Yoshiki; Tuna, Elif B.; Aktören, Oya; Gençay, Koray

    2010-01-01

    Among various dental materials and their successful applications, a dental implant is a good example of the integrated system of science and technology involved in multiple disciplines including surface chemistry and physics, biomechanics, from macro-scale to nano-scale manufacturing technologies and surface engineering. As many other dental materials and devices, there are crucial requirements taken upon on dental implants systems, since surface of dental implants is directly in contact with vital hard/soft tissue and is subjected to chemical as well as mechanical bio-environments. Such requirements should, at least, include biological compatibility, mechanical compatibility, and morphological compatibility to surrounding vital tissues. In this review, based on carefully selected about 500 published articles, these requirements plus MRI compatibility are firstly reviewed, followed by surface texturing methods in details. Normally dental implants are placed to lost tooth/teeth location(s) in adult patients whose skeleton and bony growth have already completed. However, there are some controversial issues for placing dental implants in growing patients. This point has been, in most of dental articles, overlooked. This review, therefore, throws a deliberate sight on this point. Concluding this review, we are proposing a novel implant system that integrates materials science and up-dated surface technology to improve dental implant systems exhibiting bio- and mechano-functionalities. PMID:20480036

  7. Implants for orthodontic anchorage

    PubMed Central

    Zheng, Xiaowen; Sun, Yannan; Zhang, Yimei; Cai, Ting; Sun, Feng; Lin, Jiuxiang

    2018-01-01

    Abstract Implantanchorage continues to receive much attention as an important orthodontic anchorage. Since the development of orthodontic implants, the scope of applications has continued to increase. Although multiple reviews detailing implants have been published, no comprehensive evaluations have been performed. Thus, the purpose of this study was to comprehensively evaluate the effects of implants based on data published in review articles. An electronic search of the Cochrane Library, Medline, Embase, Ebsco and Sicencedirect for reviews with “orthodontic” and “systematic review or meta analysis” in the title, abstract, keywords, or full text was performed. A subsequent manual search was then performed to identify reviews concerning orthodontic implants. A manual search of the orthodontic journals American Journal of Orthodontics and Dentofacial Orthopedics (AJODO), European Journal of Orthodontics (EJO), and Angle Othodontist was also performed. Such systematic reviews that evaluated the efficacy and safety of orthodontic implants were used to indicate success rates and molar movements. A total of 23 reviews were included in the analysis. The quality of each review was assessed using a measurement tool for Assessment of Multiple Systematic Reviews (AMSTAR), and the review chosen to summarize outcomes had a quality score of >6. Most reviews were less than moderate quality. Success rates of implants ranged in a broad scope, and movement of the maxillary first molar was superior with implants compared with traditional anchorage. PMID:29595673

  8. Nanotechnology for dental implants.

    PubMed

    Tomsia, Antoni P; Lee, Janice S; Wegst, Ulrike G K; Saiz, Eduardo

    2013-01-01

    With the advent of nanotechnology, an opportunity exists for the engineering of new dental implant materials. Metallic dental implants have been successfully used for decades, but they have shortcomings related to osseointegration and mechanical properties that do not match those of bone. Absent the development of an entirely new class of materials, faster osseointegration of currently available dental implants can be accomplished by various surface modifications. To date, there is no consensus regarding the preferred method(s) of implant surface modification, and further development will be required before the ideal implant surface can be created, let alone become available for clinical use. Current approaches can generally be categorized into three areas: ceramic coatings, surface functionalization, and patterning on the micro- to nanoscale. The distinctions among these are imprecise, as some or all of these approaches can be combined to improve in vivo implant performance. These surface improvements have resulted in durable implants with a high percentage of success and long-term function. Nanotechnology has provided another set of opportunities for the manipulation of implant surfaces in its capacity to mimic the surface topography formed by extracellular matrix components of natural tissue. The possibilities introduced by nanotechnology now permit the tailoring of implant chemistry and structure with an unprecedented degree of control. For the first time, tools are available that can be used to manipulate the physicochemical environment and monitor key cellular events at the molecular level. These new tools and capabilities will result in faster bone formation, reduced healing time, and rapid recovery to function.

  9. Implants for elderly patients.

    PubMed

    Schimmel, Martin; Müller, Frauke; Suter, Valérie; Buser, Daniel

    2017-02-01

    In the developed world, the large birth cohorts of the so-called baby boomer generation have arrived in medical and dental practices. Often, elderly patients are 'young-old' baby boomers in whom partial edentulism is the predominant indication for implant therapy. However, the generation 85+ years of age represents a new challenge for the dental profession, as their lives are frequently dominated by dependency, multimorbidity and frailty. In geriatric implant dentistry, treatment planning is highly individualized, as interindividual differences become more pronounced with age. Nevertheless, there are four typical indications for implant therapy: (i) avoidance of removable partial prostheses; (ii) preservation of existing removable partial prostheses; (iii) stabilization of Kennedy Class I removable partial prostheses; and (iv) stabilization of complete prostheses. From a surgical point of view, two very important aspects must be considered when planning implant surgery in elderly patients: first, the consistent strive to minimize morbidity; and, second, the fact that coexisting medical risk factors are significantly more common in elderly patients. Modern three-dimensional cone beam computed tomography imaging is often indicated in order to plan minimally invasive implant surgery. Computer-assisted implant surgery might allow flapless implant surgery, which offers a low level of postoperative morbidity and a minimal risk of postsurgical bleeding. Short and reduced-diameter implants are now utilized much more often than a decade ago. Two-stage surgical procedures should be avoided in elderly patients. Implant restorations for elderly patients should be designed so that they can be modified to become low-maintenance prostheses, or even be removed, as a strategy to facilitate oral hygiene and comfort in the final stage of life. © 2016 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.

  10. Growth optimization and characterization of GaN epilayers on multifaceted (111) surfaces etched on Si(100) substrates

    SciT

    Ansah-Antwi, KwaDwo Konadu, E-mail: kakadee@gmail.com; Chua, Soo Jin; Department of Electrical and Computer Engineering, National University of Singapore, E4-5-45, 4 Engineering Drive 3, Singapore 117576

    2015-11-15

    The four nearest Si(111) multifaceted sidewalls were exposed inside an array of 3 μm-wide square holes patterned on an Si(100) substrate, and this patterned Si(100) substrate was used as a substrate for the deposition of a gallium nitride (GaN) epilayer. Subsequently the effect that the growth pressure, the etched-hole profiles, and the etched-hole arrangement had upon the quality of the as-grown GaN was investigated. The coalescence of the as-grown GaN epilayer on the exposed Si(111) facets was observed to be enhanced with reduced growth pressure from 120 to 90 Torr. A larger Si(001) plane area at the bottom of the etched holesmore » resulted in bidirectional GaN domains, which resulted in poor material quality. The bidirectional GaN domains were observed as two sets of six peaks via a high-resolution x-ray diffraction phi scan of the GaN(10-11) reflection. It was also shown that a triangular array of etched holes was more desirable than square arrays of etched holes for the growth high-quality and continuous GaN films.« less

  11. Kinetics of optically excited charge carriers at the GaN surface: Influence of catalytic Pt nanostructures

    SciT

    Winnerl, Andrea, E-mail: andrea.winnerl@wsi.tum.de; Pereira, Rui N.; Stutzmann, Martin

    2015-10-21

    In this work, we use GaN with different deposited Pt nanostructures as a controllable model system to investigate the kinetics of photo-generated charge carriers in hybrid photocatalysts. We combine conductance and contact potential difference measurements to investigate the influence of Pt on the processes involved in the capture and decay of photo-generated charge carriers at and close to the GaN surface. We found that in the presence of Pt nanostructures the photo-excitation processes are similar to those found in Pt free GaN. However, in GaN with Pt nanostructures, photo-generated holes are preferentially trapped in surface states of the GaN coveredmore » with Pt and/or in electronic states of the Pt and lead to an accumulation of positive charge there, whereas negative charge is accumulated in localized states in a shallow defect band of the GaN covered with Pt. This preferential accumulation of photo-generated electrons close to the surface is responsible for a dramatic acceleration of the turn-off charge transfer kinetics and a stronger dependence of the surface photovoltage on light intensity when compared to a Pt free GaN surface. Our study shows that in hybrid photocatalysts, the metal nanostructures induce a spatially inhomogeneous surface band bending of the semiconductor that promotes a lateral drift of photogenerated charges towards the catalytic nanostructures.« less

  12. Yi-gan san restores behavioral alterations and a decrease of brain glutathione level in a mouse model of schizophrenia.

    PubMed

    Makinodan, Manabu; Yamauchi, Takahira; Tatsumi, Kouko; Okuda, Hiroaki; Noriyama, Yoshinobu; Sadamatsu, Miyuki; Kishimoto, Toshifumi; Wanaka, Akio

    2009-01-01

    The traditional Chinese herbal medicine yi-gan san has been used to cure neuropsychological disorders. Schizophrenia can be one of the target diseases of yi-gan san. We aimed at evaluating the possible use of yi-gan san in improving the schizophrenic symptoms of an animal model. Yi-gan san or distilled water was administered to mice born from pregnant mice injected with polyinosinic-polycytidilic acid or phosphate buffered saline. The former is a model of schizophrenia based on the epidemiological data that maternal infection leads to psychotic disorders including schizophrenia in the offspring. Prepulse inhibition and sensitivity to methamphetamine in open field tests were analyzed and the total glutathione content of whole brains was measured. Yi-gan san reversed the decrease in prepulse inhibition, hypersensitivity to methamphetamine and cognitive deficits found in the model mice to the level of control mice. Total glutathione content in whole brains was reduced in the model mice but was restored to normal levels by yi-gan san treatment. These results suggest that yi-gan san may have ameliorating effects on the pathological symptoms of schizophrenia.

  13. Gradual tilting of crystallographic orientation and configuration of dislocations in GaN selectively grown by vapour phase epitaxy methods

    PubMed

    Kuwan; Tsukamoto; Taki; Horibuchi; Oki; Kawaguchi; Shibata; Sawaki; Hiramatsu

    2000-01-01

    Cross-sectional transmission electron microscope (TEM) observation was performed for selectively grown gallium nitride (GaN) in order to examine the dependence of GaN microstructure on the growth conditions. The GaN films were grown by hydride vapour phase epitaxy (HVPE) or metalorganic vapour phase epitaxy (MOVPE) on GaN covered with a patterned mask. Thin foil specimens for TEM observation were prepared with focused ion beam (FIB) machining apparatus. It was demonstrated that the c-axis of GaN grown over the terrace of the mask tilts towards the centre of the terrace when the GaN is grown in a carrier gas of N2. The wider terrace results in a larger tilting angle if other growth conditions are identical. The tilting is attributed to 'horizontal dislocations' (HDs) generated during the overgrowth of GaN on the mask terrace. The HDs in HVPE-GaN have a semi-loop shape and are tangled with one another, while those in MOVPE-GaN are straight and lined up to form low-angle grain boundaries.

  14. A study of the red-shift of a neutral donor bound exciton in GaN nanorods by hydrogenation

    NASA Astrophysics Data System (ADS)

    Park, Byung-Guon; Lee, Sang-Tae; Reddeppa, Maddaka; Kim, Moon-Deock; Oh, Jae-Eung; Lee, Sang-Kwon

    2017-09-01

    In this paper we account for the physics behind the exciton peak shift in GaN nanorods (NRs) due to hydrogenation. GaN NRs were selectively grown on a patterned Ti/Si(111) substrate using plasma-assisted molecular beam epitaxy, and the effect of hydrogenation on their optical properties was investigated in detail using low-temperature photoluminescence measurements. Due to hydrogenation, the emissions corresponding to the donor-acceptor pair and yellow luminescence in GaN NRs were strongly suppressed, while the emission corresponding to the neutral to donor bound exciton (D0X) exhibited red-shift. Thermal annealing of hydrogenated GaN NRs demonstrated the recovery of the D0X and deep level emission. To determine the nature of the D0X peak shift due to hydrogenation, comparative studies were carried out on various diameters of GaN NRs, which can be controlled by different growth conditions and wet-etching times. Our experimental results reveal that the D0X shift depends on the diameter of the GaN NRs after hydrogenation. The results clearly demonstrate that the hydrogenation leads to band bending of GaN NRs as compensated by hydrogen ions, which causes a red-shift in the D0X emission.

  15. A study of the red-shift of a neutral donor bound exciton in GaN nanorods by hydrogenation.

    PubMed

    Park, Byung-Guon; Lee, Sang-Tae; Reddeppa, Maddaka; Kim, Moon-Deock; Oh, Jae-Eung; Lee, Sang-Kwon

    2017-09-08

    In this paper we account for the physics behind the exciton peak shift in GaN nanorods (NRs) due to hydrogenation. GaN NRs were selectively grown on a patterned Ti/Si(111) substrate using plasma-assisted molecular beam epitaxy, and the effect of hydrogenation on their optical properties was investigated in detail using low-temperature photoluminescence measurements. Due to hydrogenation, the emissions corresponding to the donor-acceptor pair and yellow luminescence in GaN NRs were strongly suppressed, while the emission corresponding to the neutral to donor bound exciton (D 0 X) exhibited red-shift. Thermal annealing of hydrogenated GaN NRs demonstrated the recovery of the D 0 X and deep level emission. To determine the nature of the D 0 X peak shift due to hydrogenation, comparative studies were carried out on various diameters of GaN NRs, which can be controlled by different growth conditions and wet-etching times. Our experimental results reveal that the D 0 X shift depends on the diameter of the GaN NRs after hydrogenation. The results clearly demonstrate that the hydrogenation leads to band bending of GaN NRs as compensated by hydrogen ions, which causes a red-shift in the D 0 X emission.

  16. Growth behavior and growth rate dependency in LEDs performance for Mg-doped a-plane GaN

    NASA Astrophysics Data System (ADS)

    Song, Keun-Man; Kim, Jong-Min; Lee, Dong-Hun; Shin, Chan-Soo; Ko, Chul-Gi; Kong, Bo-Hyun; Cho, Hyung-Koun; Yoon, Dae-Ho

    2011-07-01

    We investigated the influence of growth rate of Mg-doped a-plane GaN on the surface morphological and electrical properties, and the characteristics of InGaN-based nonpolar LEDs. Mg-doped a-plane GaN layers were grown on r-plane sapphire substrate by metalorganic chemical vapor deposition (MOCVD). Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and cathode luminescence (CL) analysis exhibited that the surface morphology changed from stripe features with large triangular pits to rough and rugged surface with small asymmetric V-shape pits, as the growth rate increased. The Mg incorporation into a-plane GaN layers increased with increasing growth rate of Mg-doped a-plane GaN, while the activation efficiency of Mg dopants decreased in a-plane GaN. Additionally, it was found that operation voltage at 20 mA decreased in characteristics of LEDs, as the growth rate of Mg-doped a-plane GaN decreased. Meanwhile, the EL intensity of LEDs with p-GaN layers grown at higher growth rate was improved compared to that of LEDs with p-GaN layers grown at lower growth rate. Such an increase of EL intensity is attributed to the rougher surface morphology with increasing growth rate of Mg-doped a-plane GaN.

  17. Growth of GaN Layers on Sapphire by Low-Temperature-Deposited Buffer Layers and Realization of p-type GaN by Magesium Doping and Electron Beam Irradiation (Nobel Lecture).

    PubMed

    Amano, Hiroshi

    2015-06-26

    This Review is a personal reflection on the research that led to the development of a method for growing gallium nitride (GaN) on a sapphire substrate. The results paved the way for the development of smart display systems using blue LEDs. The most important work was done in the mid to late 80s. The background to the author's work and the process by which the technology that enables the growth of GaN and the realization of p-type GaN was established are reviewed. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  18. Growth of GaN on Sapphire via Low-Temperature Deposited Buffer Layer and Realization of p-Type GaN by Mg Doping Followed by Low-Energy Electron Beam Irradiation

    NASA Astrophysics Data System (ADS)

    Amano, Hiroshi

    2015-12-01

    This is a personal history of one of the Japanese researchers engaged in developing a method for growing GaN on a sapphire substrate, paving the way for the realization of smart television and display systems using blue LEDs. The most important work was done in the mid- to late 80s. The background to the author's work and the process by which the technology enabling the growth of GaN and the realization of p-type GaN was established are reviewed.

  19. The effects of GaN nanocolumn arrays and thin SixNy buffer layers on the morphology of GaN layers grown by plasma-assisted molecular beam epitaxy on Si(111) substrates

    NASA Astrophysics Data System (ADS)

    Shubina, K. Yu; Pirogov, E. V.; Mizerov, A. M.; Nikitina, E. V.; Bouravleuv, A. D.

    2018-03-01

    The effects of GaN nanocolumn arrays and a thin SixNy layer, used as buffer layers, on the morphology of GaN epitaxial layers are investigated. Two types of samples with different buffer layers were synthesized by PA-MBE. The morphology of the samples was characterized by SEM. The crystalline quality of the samples was assessed by XRD. The possibility of synthesis of continuous crystalline GaN layers on Si(111) substrates without the addition of other materials such as aluminum nitride was demonstrated.

  20. Optimization of dental implantation

    NASA Astrophysics Data System (ADS)

    Dol, Aleksandr V.; Ivanov, Dmitriy V.

    2017-02-01

    Modern dentistry can not exist without dental implantation. This work is devoted to study of the "bone-implant" system and to optimization of dental prostheses installation. Modern non-invasive methods such as MRI an 3D-scanning as well as numerical calculations and 3D-prototyping allow to optimize all of stages of dental prosthetics. An integrated approach to the planning of implant surgery can significantly reduce the risk of complications in the first few days after treatment, and throughout the period of operation of the prosthesis.

  1. The photocatalytic properties of hollow (GaN)1-x(ZnO)x composite nanofibers synthesized by electrospinning

    NASA Astrophysics Data System (ADS)

    Wang, Ding; Zhang, Minglu; Zhuang, Huaijuan; Chen, Xu; Wang, Xianying; Zheng, Xuejun; Yang, Junhe

    2017-02-01

    (GaN)1-x(ZnO)x composite nanofibers with hollow structure were prepared by initial electrospinning, and the subsequent calcination and nitridation. The structure and morphology characteristics of samples were investigated by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The characterization results showed the phase transition from ZnGa2O4 to (GaN)1-x(ZnO)x solid-solution under ammonia atmosphere. The preparation conditions were explored and the optimum nitridation temperature and holding time are 750 °C and 2 h, respectively. The photocatalytic properties of (GaN)1-x(ZnO)x with different Ga:Zn atomic ratios were investigated by degrading Rhodamine B under the visible light irradiation. The photocatalytic activity sequence is (GaN)1-x(ZnO)x (Ga:Zn = 1:2) > (GaN)1-x(ZnO)x (Ga:Zn = 1:3) > ZnO nanofibers > (GaN)1-x(ZnO)x (Ga:Zn = 1:4) > (GaN)1-x(ZnO)x (Ga:Zn = 1:1). The photocatalytic mechanism of the (GaN)1-x(ZnO)x hollow nanofibers was further studied by UV-vis diffuse reflectance spectra. The excellent photocatalytic performance of (GaN)1-x(ZnO)x hollow nanofibers was attributed to the narrow band gap and high surface area of porous nanofibers with hollow structure.

  2. High-electron-mobility GaN grown on free-standing GaN templates by ammonia-based molecular beam epitaxy

    SciT

    Kyle, Erin C. H., E-mail: erinkyle@umail.ucsb.edu; Kaun, Stephen W.; Burke, Peter G.

    2014-05-21

    The dependence of electron mobility on growth conditions and threading dislocation density (TDD) was studied for n{sup −}-GaN layers grown by ammonia-based molecular beam epitaxy. Electron mobility was found to strongly depend on TDD, growth temperature, and Si-doping concentration. Temperature-dependent Hall data were fit to established transport and charge-balance equations. Dislocation scattering was analyzed over a wide range of TDDs (∼2 × 10{sup 6} cm{sup −2} to ∼2 × 10{sup 10} cm{sup −2}) on GaN films grown under similar conditions. A correlation between TDD and fitted acceptor states was observed, corresponding to an acceptor state for almost every c lattice translation along each threading dislocation. Optimizedmore » GaN growth on free-standing GaN templates with a low TDD (∼2 × 10{sup 6} cm{sup −2}) resulted in electron mobilities of 1265 cm{sup 2}/Vs at 296 K and 3327 cm{sup 2}/Vs at 113 K.« less

  3. DEALING WITH DENTAL IMPLANT FAILURES

    PubMed Central

    Levin, Liran

    2008-01-01

    An implant-supported restoration offers a predictable treatment for tooth replacement. Reported success rates for dental implants are high. Nevertheless, failures that mandate immediate implant removal do occur. The consequences of implant removal jeopardize the clinician's efforts to accomplish satisfactory function and esthetics. For the patient, this usually involves further cost and additional procedures. The aim of this paper is to describe different methods and treatment modalities to deal with dental implant failure. The main topics for discussion include identifying the failing implant, implants replacing failed implants at the exact site, and the use of other restorative options. When an implant fails, a tailor made treatment plan should be provided to each patient according to all relevant variables. Patients should be informed regarding all possible treatment modalities following implant failure and give their consent to the most appropriate treatment option for them. PMID:19089213

  4. Leakage current reduction of vertical GaN junction barrier Schottky diodes using dual-anode process

    NASA Astrophysics Data System (ADS)

    Hayashida, Tetsuro; Nanjo, Takuma; Furukawa, Akihiko; Watahiki, Tatsuro; Yamamuka, Mikio

    2018-04-01

    The origin of the leakage current of a trench-type vertical GaN diode was discussed. We found that the edge of p-GaN is the main leakage spot. To reduce the reverse leakage current at the edge of p-GaN, a dual-anode process was proposed. As a result, the reverse blocking voltage defined at the leakage current density of 1 mA/cm2 of a vertical GaN junction barrier Schottky (JBS) diode was improved from 780 to 1,190 V, which is the highest value ever reported for vertical GaN Schottky barrier diodes (SBDs).

  5. A New Selective Area Lateral Epitaxy Approach for Depositing a-Plane GaN over r-Plane Sapphire

    NASA Astrophysics Data System (ADS)

    Chen, Changqing; Zhang, Jianping; Yang, Jinwei; Adivarahan, Vinod; Rai, Shiva; Wu, Shuai; Wang, Hongmei; Sun, Wenhong; Su, Ming; Gong, Zheng; Kuokstis, Edmundas; Gaevski, Mikhail; Khan, Muhammad Asif

    2003-07-01

    We report a new epitaxy procedure for growing extremely low defect density a-plane GaN films over r-plane sapphire. By combining selective area growth through a SiO2 mask opening to produce high height to width aspect ratio a-plane GaN pillars and lateral epitaxy from their c-plane facets, we obtained fully coalesced a-plane GaN films. The excellent structural, optical and electrical characteristics of these selective area lateral epitaxy (SALE) deposited films make them ideal for high efficiency III-N electronic and optoelectronic devices.

  6. Suppression of the self-heating effect in GaN HEMT by few-layer graphene heat spreading elements

    NASA Astrophysics Data System (ADS)

    Volcheck, V. S.; Stempitsky, V. R.

    2017-11-01

    Self-heating has an adverse effect on characteristics of gallium nitride (GaN) high electron mobility transistors (HEMTs). Various solutions to the problem have been proposed, however, a temperature rise due to dissipated electrical power still hinders the production of high power and high speed GaN devices. In this paper, thermal management of GaN HEMT via few-layer graphene (FLG) heat spreading elements is investigated. It is shown that integration of the FLG elements on top of the device structure considerably reduces the maximum temperature and improves the DC and small signal AC performance.

  7. Anisotropic carrier mobility in buckled two-dimensional GaN.

    PubMed

    Tong, Lijia; He, Junjie; Yang, Min; Chen, Zheng; Zhang, Jing; Lu, Yanli; Zhao, Ziyuan

    2017-08-30

    Developing nanoelectronic engineering requires two-dimensional (2d) materials with both usable carrier mobility and proper large band-gap. In this study, we present a detailed theoretical investigation of the intrinsic carrier mobilities of buckled 2d GaN. This buckled 2d GaN is accessed by hydrofluorination (FGaNH) and hydrogenation (HGaNH). We predict that the anisotropic carrier mobilities of buckled 2d GaN can exceed those of 2d MoS 2 and can be altered by an alterable surface chemical bond (convert from a Ga-F-Ga bond of FGaNH to a Ga-H bond of HGaNH). Moreover, converting FGaNH to HGaNH can significantly suppress hole mobility (even close to zero) and result in a transition from a p-type-like semiconductor (FGaNH) to an n-type-like semiconductor (HGaNH). These features make buckled 2d GaN a promising candidate for application in future conductivity-adjustable electronics.

  8. GalaxyGAN: Generative Adversarial Networks for recovery of galaxy features

    NASA Astrophysics Data System (ADS)

    Schawinski, Kevin; Zhang, Ce; Zhang, Hantian; Fowler, Lucas; Krishnan Santhanam, Gokula

    2017-02-01

    GalaxyGAN uses Generative Adversarial Networks to reliably recover features in images of galaxies. The package uses machine learning to train on higher quality data and learns to recover detailed features such as galaxy morphology by effectively building priors. This method opens up the possibility of recovering more information from existing and future imaging data.

  9. Incorporation of Mg in Free-Standing HVPE GaN Substrates

    NASA Astrophysics Data System (ADS)

    Zvanut, M. E.; Dashdorj, J.; Freitas, J. A.; Glaser, E. R.; Willoughby, W. R.; Leach, J. H.; Udwary, K.

    2016-06-01

    Mg, the only effective p-type dopant for nitrides, is well studied in thin films due to the important role of the impurity in light-emitting diodes and high-power electronics. However, there are few reports of Mg in thick free-standing GaN substrates. Here, we demonstrate successful incorporation of Mg into GaN grown by hydride vapor-phase epitaxy (HVPE) using metallic Mg as the doping source. The concentration of Mg obtained from four separate growth runs ranged between 1016 cm-3 and 1019 cm-3. Raman spectroscopy and x-ray diffraction revealed that Mg did not induce stress or perturb the crystalline quality of the HVPE GaN substrates. Photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopies were performed to investigate the types of point defects in the crystals. The near-band-edge excitonic and shallow donor-shallow acceptor radiative recombination processes involving shallow Mg acceptors were prominent in the PL spectrum of a sample doped to 3 × 1018 cm-3, while the EPR signal was also thought to represent a shallow Mg acceptor. Detection of this signal reflects minimization of nonuniform strain obtained in the thick free-standing HVPE GaN compared with heteroepitaxial thin films.

  10. The controlled growth of GaN microrods on Si(111) substrates by MOCVD

    NASA Astrophysics Data System (ADS)

    Foltynski, Bartosz; Garro, Nuria; Vallo, Martin; Finken, Matthias; Giesen, Christoph; Kalisch, Holger; Vescan, Andrei; Cantarero, Andrés; Heuken, Michael

    2015-03-01

    In this paper, a selective area growth (SAG) approach for growing GaN microrods on patterned SiNx/Si(111) substrates by metal-organic chemical vapor deposition (MOCVD) is studied. The surface morphology, optical and structural properties of vertical GaN microrods terminated by pyramidal shaped facets (six { 10 1 bar 1} planes) were characterized using scanning electron microscopy (SEM), room temperature photoluminescence (PL) and Raman spectroscopy, respectively. Measurements revealed high-quality GaN microcolumns grown with silane support. Characterized structures were grown nearly strain-free (central frequency of Raman peak of 567±1 cm-1) with crystal quality comparable to bulk crystals (FWHM=4.2±1 cm-1). Such GaN microrods might be used as a next-generation device concept for solid-state lighting (SSL) applications by realizing core-shell InGaN/GaN multi-quantum wells (MQWs) on the n-GaN rod base.

  11. Discussion of enthalpy, entropy and free energy of formation of GaN

    NASA Astrophysics Data System (ADS)

    Jacob, K. T.; Rajitha, G.

    2009-07-01

    Presented in this letter is a critical discussion of a recent paper on experimental investigation of the enthalpy, entropy and free energy of formation of gallium nitride (GaN) published in this journal [T.J. Peshek, J.C. Angus, K. Kash, J. Cryst. Growth 311 (2008) 185-189]. It is shown that the experimental technique employed detects neither the equilibrium partial pressure of N 2 corresponding to the equilibrium between Ga and GaN at fixed temperatures nor the equilibrium temperature at constant pressure of N 2. The results of Peshek et al. are discussed in the light of other information on the Gibbs energy of formation available in the literature. Entropy of GaN is derived from heat-capacity measurements. Based on a critical analysis of all thermodynamic information now available, a set of optimized parameters is identified and a table of thermodynamic data for GaN developed from 298.15 to 1400 K.

  12. Epitaxial GaN layers formed on langasite substrates by the plasma-assisted MBE method

    SciT

    Lobanov, D. N., E-mail: dima@ipmras.ru; Novikov, A. V.; Yunin, P. A.

    2016-11-15

    In this publication, the results of development of the technology of the epitaxial growth of GaN on single-crystal langasite substrates La{sub 3}Ga{sub 5}SiO{sub 14} (0001) by the plasma-assisted molecular-beam epitaxy (PA MBE) method are reported. An investigation of the effect of the growth temperature at the initial stage of deposition on the crystal quality and morphology of the obtained GaN layer is performed. It is demonstrated that the optimal temperature for deposition of the initial GaN layer onto the langasite substrate is about ~520°C. A decrease in the growth temperature to this value allows the suppression of oxygen diffusion frommore » langasite into the growing layer and a decrease in the dislocation density in the main GaN layer upon its subsequent high-temperature deposition (~700°C). Further lowering of the growth temperature of the nucleation layer leads to sharp degradation of the GaN/LGS layer crystal quality. As a result of the performed research, an epitaxial GaN/LGS layer with a dislocation density of ~10{sup 11} cm{sup –2} and low surface roughness (<2 nm) is obtained.« less

  13. The K 2S 2O 8-KOH photoetching system for GaN

    NASA Astrophysics Data System (ADS)

    Weyher, J. L.; Tichelaar, F. D.; van Dorp, D. H.; Kelly, J. J.; Khachapuridze, A.

    2010-09-01

    A recently developed photoetching system for n-type GaN, a KOH solution containing the strong oxidizing agent potassium peroxydisulphate (K 2S 2O 8), was studied in detail. By careful selection of the etching parameters, such as the ratio of components and the hydrodynamics, two distinct modes were defined: defect-selective etching (denoted by KSO-D) and polishing (KSO-P). Both photoetching methods can be used under open-circuit (electroless) conditions. Well-defined dislocation-related etch whiskers are formed during KSO-D etching. All types of dislocations are revealed, and this was confirmed by cross-sectional TEM examination of the etched samples. Extended electrically active defects are also clearly revealed. The known relationship between etch rate and carrier concentration for photoetching of GaN in KOH solutions was confirmed for KSO-D etch using Raman measurements. It is shown that during KSO-P etching diffusion is the rate-limiting step, i.e. this etch is suitable for polishing of GaN. Some constraints of the KSO etching system for GaN are discussed and peculiar etch features, so far not understood, are described.

  14. Emission dynamics of hybrid plasmonic gold/organic GaN nanorods

    NASA Astrophysics Data System (ADS)

    Mohammadi, F.; Schmitzer, H.; Kunert, G.; Hommel, D.; Ge, J.; Duscher, G.; Langbein, W.; Wagner, H. P.

    2017-12-01

    We studied the emission of bare and aluminum quinoline (Alq3)/gold coated wurtzite GaN nanorods by temperature- and intensity-dependent time-integrated and time-resolved photoluminescence (PL). The GaN nanorods of ˜1.5 μm length and ˜250 nm diameter were grown by plasma-assisted molecular beam epitaxy. Gold/Alq3 coated GaN nanorods were synthesized by organic molecular beam deposition. The near band-edge and donor-acceptor pair luminescence was investigated in bare GaN nanorods and compared with multilevel model calculations providing the dynamical parameters for electron-hole pairs, excitons, impurity bound excitons, donors and acceptors. Subsequently, the influence of a 10 nm gold coating without and with an Alq3 spacer layer was studied and the experimental results were analyzed with the multilevel model. Without a spacer layer, a significant PL quenching and lifetime reduction of the near band-edge emission is found. The behavior is attributed to surface band-bending and Förster energy transfer from excitons to surface plasmons in the gold layer. Inserting a 5 nm Alq3 spacer layer reduces the PL quenching and lifetime reduction which is consistent with a reduced band-bending and Förster energy transfer. Increasing the spacer layer to 30 nm results in lifetimes which are similar to uncoated structures, showing a significantly decreased influence of the gold coating on the excitonic dynamics.

  15. Emission dynamics of hybrid plasmonic gold/organic GaN nanorods.

    PubMed

    Mohammadi, F; Schmitzer, H; Kunert, G; Hommel, D; Ge, J; Duscher, G; Langbein, W; Wagner, H P

    2017-12-15

    We studied the emission of bare and aluminum quinoline (Alq 3 )/gold coated wurtzite GaN nanorods by temperature- and intensity-dependent time-integrated and time-resolved photoluminescence (PL). The GaN nanorods of ∼1.5 μm length and ∼250 nm diameter were grown by plasma-assisted molecular beam epitaxy. Gold/Alq 3 coated GaN nanorods were synthesized by organic molecular beam deposition. The near band-edge and donor-acceptor pair luminescence was investigated in bare GaN nanorods and compared with multilevel model calculations providing the dynamical parameters for electron-hole pairs, excitons, impurity bound excitons, donors and acceptors. Subsequently, the influence of a 10 nm gold coating without and with an Alq 3 spacer layer was studied and the experimental results were analyzed with the multilevel model. Without a spacer layer, a significant PL quenching and lifetime reduction of the near band-edge emission is found. The behavior is attributed to surface band-bending and Förster energy transfer from excitons to surface plasmons in the gold layer. Inserting a 5 nm Alq 3 spacer layer reduces the PL quenching and lifetime reduction which is consistent with a reduced band-bending and Förster energy transfer. Increasing the spacer layer to 30 nm results in lifetimes which are similar to uncoated structures, showing a significantly decreased influence of the gold coating on the excitonic dynamics.

  16. Optical regulation of protein adsorption and cell adhesion by photoresponsive GaN nanowires.

    PubMed

    Li, Jingying; Han, Qiusen; Zhang, Ying; Zhang, Wei; Dong, Mingdong; Besenbacher, Flemming; Yang, Rong; Wang, Chen

    2013-10-09

    Interfacing nanowires with living cells is attracting more and more interest due to the potential applications, such as cell culture engineering and drug delivery. We report on the feasibility of using photoresponsive semiconductor gallium nitride (GaN) nanowires (NWs) for regulating the behaviors of biomolecules and cells at the nano/biointerface. The GaN NWs have been fabricated by a facile chemical vapor deposition method. The superhydrophobicity to superhydrophilicity transition of the NWs is achieved by UV illumination. Bovine serum albumin adsorption could be modulated by photoresponsive GaN NWs. Tunable cell detachment and adhesion are also observed. The mechanism of the NW surface responsible for modulating both of protein adsorption and cell adhesion is discussed. These observations of the modulation effects on protein adsorption and cell adhesion by GaN NWs could provide a novel approach toward the regulation of the behaviors of biomolecules and cells at the nano/biointerface, which may be of considerable importance in the development of high-performance semiconductor nanowire-based biomedical devices for cell culture engineering, bioseparation, and diagnostics.

  17. 10kW TWT Transition to GaN IRE

    DTIC Science & Technology

    2015-03-31

    tubes in high power radar and Electronic Warfare (EW) applications. GaN transistors, using evaluation boards, were tested and analyzed, supplementing...29 Appendix B. High Power Amplifier Testing Data...19 Figure 11. High Power RF Amplifier Test Set ............................................................................. 22 Figure 12

  18. RBS/Channeling Studies of Swift Heavy Ion Irradiated GaN Layers

    SciT

    Sathish, N.; Dhamodaran, S.; Pathak, A. P.

    2009-03-10

    Epitaxial GaN layers grown by MOCVD on c-plane sapphire substrates were irradiated with 150 MeV Ag ions at a fluence of 5x10{sup 12} ions/cm{sup 2}. Samples used in this study are 2 {mu}m thick GaN layers, with and without a thin AlN cap-layer. Energy dependent RBS/Channeling measurements have been carried out on both irradiated and unirradiated samples for defects characterization. Observed results are compared and correlated with previous HRXRD, AFM and optical studies. The {chi}{sub min} values for unirradiated samples show very high value and the calculated defect densities are of the order of 10{sup 10} cm{sup -2} as expectedmore » in these samples. Effects of irradiation on these samples are different as initial samples had different defect densities. Epitaxial reconstruction of GaN buffer layer has been attributed to the observed changes, which are generally grown to reduce the strain between GaN and Sapphire.« less

  19. Fine structure of the red luminescence band in undoped GaN

    SciT

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

    2014-01-20

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

  20. Fabrications and application of single crystalline GaN for high-performance deep UV photodetectors

    SciT

    Velazquez, R.; Rivera, M.; Feng, P., E-mail: p.feng@upr.edu

    2016-08-15

    High-quality single crystalline Gallium Nitride (GaN) semiconductor has been synthesized using molecule beam epitaxy (MBE) technique for development of high-performance deep ultraviolet (UV) photodetectors. Thickness of the films was estimated by using surface profile meter and scanning electron microscope. Electronic states and elemental composition of the films were obtained using Raman scattering spectroscopy. The orientation, crystal structure and phase purity of the films were examined using a Siemens x-ray diffractometer radiation. The surface microstructure was studied using high resolution scanning electron microscopy (SEM). Two types of metal pairs: Al-Al, Al-Cu or Cu-Cu were used for interdigital electrodes on GaN filmmore » in order to examine the Schottky properties of the GaN based photodetector. The characterizations of the fabricated prototype include the stability, responsivity, response and recovery times. Typical time dependent photoresponsivity by switching different UV light source on and off five times for each 240 seconds at a bias of 2V, respectively, have been obtained. The detector appears to be highly sensitive to various UV wavelengths of light with very stable baseline and repeatability. The obtained photoresponsivity was up to 354 mA/W at the bias 2V. Higher photoresponsivity could be obtained if higher bias was applied but it would unavoidably result in a higher dark current. Thermal effect on the fabricated GaN based prototype was discussed.« less

  1. Design of Low Inductance Switching Power Cell for GaN HEMT Based Inverter

    SciT

    Gurpinar, Emre; Iannuzzo, Francesco; Yang, Yongheng

    Here in this paper, an ultra-low inductance power cell is designed for a three-Level Active Neutral Point Clamped (3LANPC) based on 650 V gallium nitride (GaN) HEMT devices. The 3L-ANPC topology with GaN HEMT devices and the selected modulation scheme suitable for wide-bandgap (WBG) devices are presented. The commutation loops, which mainly contribute to voltage overshoots and increase of switching losses, are discussed. The ultra-low inductance power cell design based on a fourlayer Printed Circuit Board (PCB) with the aim to maximize the switching performance of GaN HEMTs is explained. The design of gate drivers for the GaN HEMT devicesmore » is presented. Parasitic inductance and resistance of the proposed design are extracted with finite element analysis and discussed. Common mode behaviours based on the SPICE model of the converter are analyzed. Experimental results on the designed 3L-ANPC with the output power of up to 1 kW are presented, which verifies the performance of the proposed design in terms of ultra-low inductance.« less

  2. Piezo-phototronic effect on electroluminescence properties of p-type GaN thin films.

    PubMed

    Hu, Youfan; Zhang, Yan; Lin, Long; Ding, Yong; Zhu, Guang; Wang, Zhong Lin

    2012-07-11

    We present that the electroluminescence (EL) properties of Mg-doped p-type GaN thin films can be tuned by the piezo-phototronic effect via adjusting the minority carrier injection efficiency at the metal-semiconductor (M-S) interface by strain induced polarization charges. The device is a metal-semiconductor-metal structure of indium tin oxide (ITO)-GaN-ITO. Under different straining conditions, the changing trend of the transport properties of GaN films can be divided into two types, corresponding to the different c-axis orientations of the films. An extreme value was observed for the integral EL intensity under certain applied strain due to the adjusted minority carrier injection efficiency by piezoelectric charges introduced at the M-S interface. The external quantum efficiency of the blue EL at 430 nm was changed by 5.84% under different straining conditions, which is 1 order of magnitude larger than the change of the green peak at 540 nm. The results indicate that the piezo-phototronic effect has a larger impact on the shallow acceptor states related EL process than on the one related to the deep acceptor states in p-type GaN films. This study has great significance on the practical applications of GaN in optoelectronic devices under a working environment where mechanical deformation is unavoidable such as for flexible/printable light emitting diodes.

  3. Insufficiency of the Young's modulus for illustrating the mechanical behavior of GaN nanowires.

    PubMed

    Kouhpanji, Mohammad Reza Zamani; Behzadirad, Mahmoud; Feezell, Daniel; Busani, Tito

    2018-05-18

    We use a non-classical modified couple stress theory including the acceleration gradients (MCST-AG), to precisely demonstrate the size dependency of the mechanical properties of gallium nitride (GaN) nanowires (NWs). The fundamental elastic constants, Young's modulus and length scales of the GaN NWs were estimated both experimentally, using a novel experimental technique applied to atomic force microscopy, and theoretically, using atomic simulations. The Young's modulus, static and the dynamic length scales, calculated with the MCST-AG, were found to be 323 GPa, 13 and 14.5 nm, respectively, for GaN NWs from a few nanometers radii to bulk radii. Analyzing the experimental data using the classical continuum theory shows an improvement in the experimental results by introducing smaller error. Using the length scales determined in MCST-AG, we explain the inconsistency of the Young's moduli reported in recent literature, and we prove the insufficiency of the Young's modulus for predicting the mechanical behavior of GaN NWs.

  4. Low loss GaN waveguides at the visible spectral wavelengths for integrated photonics applications.

    PubMed

    Chen, Hong; Fu, Houqiang; Huang, Xuanqi; Zhang, Xiaodong; Yang, Tsung-Han; Montes, Jossue A; Baranowski, Izak; Zhao, Yuji

    2017-12-11

    We perform comprehensive studies on the fundamental loss mechanisms in III-nitride waveguides in the visible spectral region. Theoretical analysis shows that free carrier loss dominates for GaN under low photon power injection. When optical power increases, the two photon absorption loss becomes important and eventually dominates when photon energy above half-bandgap of GaN. When the dimensions of the waveguides reduce, the sidewall scattering loss will start to dominate. To verify the theoretical results, a high performance GaN-on-sapphire waveguide was fabricated and characterized. Experimental results are consistent with the theoretical findings, showing that under high power injection the optical loss changed significantly for GaN waveguides. A low optical loss ~2 dB/cm was achieved on the GaN waveguide, which is the lowest value ever reported for the visible spectral range. The results and fabrication processes developed in this work pave the way for the development of III-nitride integrated photonics in the visible and potentially ultraviolet spectral range for nonlinear optics and quantum photonics applications.

  5. Dental Implant Surgery

    MedlinePlus

    ... supplements. If you have certain heart conditions or orthopedic implants, your doctor may prescribe antibiotics before surgery ... trademarks of Mayo Foundation for Medical Education and Research. © 1998-2018 Mayo Foundation for Medical Education and ...

  6. Cochlear Implants (For Parents)

    MedlinePlus

    ... nerve, and send it to the brain. The cochlear implant package is made up of: a receiver-stimulator that contains all of the electronic circuits that control the flow of electrical pulses into the ear an antenna ...

  7. Superelastic Orthopedic Implant Coatings

    NASA Astrophysics Data System (ADS)

    Fournier, Eric; Devaney, Robert; Palmer, Matthew; Kramer, Joshua; El Khaja, Ragheb; Fonte, Matthew

    2014-07-01

    The demand for hip and knee replacement surgery is substantial and growing. Unfortunately, most joint replacement surgeries will fail within 10-25 years, thereby requiring an arduous, painful, and expensive revision surgery. To address this issue, a novel orthopedic implant coating material ("eXalt") has been developed. eXalt is comprised of super elastic nitinol wire that is knit into a three-dimensional spacer fabric structure. eXalt expands in vivo to conform to the implantation site and is porous to allow for bone ingrowth. The safety and efficacy of eXalt were evaluated through structural analysis, mechanical testing, and a rabbit implantation model. The results demonstrate that eXalt meets or exceeds the performance of current coating technologies with reduced micromotion, improved osseointegration, and stronger implant fixation in vivo.

  8. Breast Reconstruction with Implants

    MedlinePlus

    ... implants is a complex procedure performed by a plastic surgeon. The breast reconstruction process can start at ... doctor may recommend that you meet with a plastic surgeon. Consult a plastic surgeon who's board certified ...

  9. Implantable cardioverter defibrillator - discharge

    MedlinePlus

    ... defibrillation. This device can also work as a pacemaker. What to Expect at Home When you leave ... pubmed/23265327 . Swerdlow CD, Wang PL, Zipes DP. Pacemakers and implantable cardioverter-defibrillators. In: Mann DL, Zipes ...

  10. Ultra High p-doping Material Research for GaN Based Light Emitters

    SciT

    Vladimir Dmitriev

    2007-06-30

    The main goal of the Project is to investigate doping mechanisms in p-type GaN and AlGaN and controllably fabricate ultra high doped p-GaN materials and epitaxial structures. Highly doped p-type GaN-based materials with low electrical resistivity and abrupt doping profiles are of great importance for efficient light emitters for solid state lighting (SSL) applications. Cost-effective hydride vapor phase epitaxial (HVPE) technology was proposed to investigate and develop p-GaN materials for SSL. High p-type doping is required to improve (i) carrier injection efficiency in light emitting p-n junctions that will result in increasing of light emitting efficiency, (ii) current spreading inmore » light emitting structures that will improve external quantum efficiency, and (iii) parameters of Ohmic contacts to reduce operating voltage and tolerate higher forward currents needed for the high output power operation of light emitters. Highly doped p-type GaN layers and AlGaN/GaN heterostructures with low electrical resistivity will lead to novel device and contact metallization designs for high-power high efficiency GaN-based light emitters. Overall, highly doped p-GaN is a key element to develop light emitting devices for the DOE SSL program. The project was focused on material research for highly doped p-type GaN materials and device structures for applications in high performance light emitters for general illumination P-GaN and p-AlGaN layers and multi-layer structures were grown by HVPE and investigated in terms of surface morphology and structure, doping concentrations and profiles, optical, electrical, and structural properties. Tasks of the project were successfully accomplished. Highly doped GaN materials with p-type conductivity were fabricated. As-grown GaN layers had concentration N{sub a}-N{sub d} as high as 3 x 10{sup 19} cm{sup -3}. Mechanisms of doping were investigated and results of material studies were reported at several International conferences

  11. Measurement of second order susceptibilities of GaN and AlGaN

    NASA Astrophysics Data System (ADS)

    Sanford, N. A.; Davydov, A. V.; Tsvetkov, D. V.; Dmitriev, A. V.; Keller, S.; Mishra, U. K.; DenBaars, S. P.; Park, S. S.; Han, J. Y.; Molnar, R. J.

    2005-03-01

    Rotational Maker fringes, scaled with respect to χ11(2) of crystalline quartz, were used to determine the second order susceptibilities χ31(2) and χ33(2) for samples of thin AlxGa1-xN films, a thicker GaN film, and a free-standing GaN platelets. The pump wavelength was 1064nm. The AlxGa1-xN samples, ranging in thickness from roughly 0.5to4.4μm, were grown by metalorganic chemical vapor deposition (MOCVD) and hydride vapor-phase epitaxy (HVPE) on (0001) sapphire substrates. The Al mole fractions x were 0, 0.419, 0.507, 0.618, 0.660, and 0.666, for the MOCVD-grown samples, and x =0, 0.279, 0.363, and 0.593 for the HVPE-grown samples. An additional HVPE-grown GaN sample ˜70μm thick was also examined. The free-standing bulk GaN platelets consisted of an HVPE grown film ˜226μm thick removed from its growth substrate, and a crystal ˜160μm thick grown by high-pressure techniques. For the AlxGa1-xN samples, the magnitudes of χ31(2) and χ33(2) decrease roughly linearly with increasing x and extrapolate to ˜0 for x =1. Furthermore, the constraint expected for a perfect wurtzite structure, namely χ33(2)=-2χ31(2), was seldom observed, and the samples with x =0.660 and x =0.666 showed χ31(2) and χ33(2) having the same sign. These results are consistent with the theoretical studies of nonlinear susceptibilities for AlN and GaN performed by Chen et al. [Appl. Phys. Lett. 66, 1129 (1995)]. The thicker bulk GaN samples displayed a complex superposition of high- and low-frequency Maker fringes due to the multiple-pass interference of the pump and second-harmonic generation beams, and the nonlinear coefficients were approximately consistent with those measured for the thin-film GaN sample.

  12. How localized acceptors limit p-type conductivity in GaN

    NASA Astrophysics Data System (ADS)

    Lyons, John L.

    2013-03-01

    Despite the impressive development of GaN as an optoelectronic material, p-type conductivity is still limited. Only a single acceptor impurity, magnesium, is known to lead to p-type GaN. But Mg is far from a well-behaved acceptor. Hydrogen is known to passivate Mg, necessitating a post-growth anneal for acceptor activation. In addition, the ionization energy is quite large (~ 200 meV in GaN), meaning only a few percent of Mg acceptors are ionized at room temperature. Thus, hole conductivity is limited, and high concentrations of Mg are required to achieve moderately p-type GaN. Other acceptor impurities have not proven to be effective p-type dopants, for reasons that are still unresolved. Using advanced first-principles calculations based on a hybrid functional, we investigate the electrical and optical properties of the isolated Mg acceptor and its complexes with hydrogen in GaN, InN, and AlN.[2] We employ a technique that overcomes the band-gap-problem of traditional density functional theory, and allows for quantitative predictions of acceptor ionization energies and optical transition energies. Our results allow us to explain the deep or shallow nature of the Mg acceptor and its relation to the optical signals observed in Mg-doped GaN. We also revisit the properties of other group-II acceptors in GaN. We find that all cation-site acceptors show behavior similar to MgGa, and lead to highly localized holes. The ZnGa and BeGa acceptors have ionization energies that are even larger than that of Mg, making them ineffective dopants. All acceptors cause large lattice distortions in their neutral charge state, in turn leading to deep, broad luminescence signals that can serve as a means of experimentally verifying the deep nature of these acceptors. This work was performed in collaboration with Audrius Alkauskas, Anderson Janotti, and Chris G. Van de Walle. It was supported by the NSF and by the Solid State Lighting and Energy Center at UCSB.

  13. Crew Earth Observations

    NASA Technical Reports Server (NTRS)

    Runco, Susan

    2009-01-01

    Crew Earth Observations (CEO) takes advantage of the crew in space to observe and photograph natural and human-made changes on Earth. The photographs record the Earth's surface changes over time, along with dynamic events such as storms, floods, fires and volcanic eruptions. These images provide researchers on Earth with key data to better understand the planet.

  14. Why Earth Science?

    ERIC Educational Resources Information Center

    Smith, Michael J.

    2004-01-01

    This article briefly describes Earth science. The study of Earth science provides the foundation for an understanding of the Earth, its processes, its resources, and its environment. Earth science is the study of the planet in its entirety, how its lithosphere, atmosphere, hydrosphere, and biosphere work together as systems and how they affect…

  15. NASA Earth Day 2014

    2014-04-22

    NASA Administrator Charles Bolden speaks to students who attended the NASA sponsored Earth Day event April 22, 2014 at Union Station in Washington, DC. NASA sponsored the Earth Day event as part of its "Earth Right Now" campaign, celebrating the launch of five Earth-observing missions in 2014. Photo Credit: (NASA/Aubrey Gemignani)

  16. Biomaterials in cochlear implants

    PubMed Central

    Stöver, Timo; Lenarz, Thomas

    2011-01-01

    The cochlear implant (CI) represents, for almost 25 years now, the gold standard in the treatment of children born deaf and for postlingually deafened adults. These devices thus constitute the greatest success story in the field of ‘neurobionic’ prostheses. Their (now routine) fitting in adults, and especially in young children and even babies, places exacting demands on these implants, particularly with regard to the biocompatibility of a CI’s surface components. Furthermore, certain parts of the implant face considerable mechanical challenges, such as the need for the electrode array to be flexible and resistant to breakage, and for the implant casing to be able to withstand external forces. As these implants are in the immediate vicinity of the middle-ear mucosa and of the junction to the perilymph of the cochlea, the risk exists – at least in principle – that bacteria may spread along the electrode array into the cochlea. The wide-ranging requirements made of the CI in terms of biocompatibility and the electrode mechanism mean that there is still further scope – despite the fact that CIs are already technically highly sophisticated – for ongoing improvements to the properties of these implants and their constituent materials, thus enhancing the effectiveness of these devices. This paper will therefore discuss fundamental material aspects of CIs as well as the potential for their future development. PMID:22073103

  17. Short dental implants: an emerging concept in implant treatment.

    PubMed

    Al-Hashedi, Ashwaq Ali; Taiyeb Ali, Tara Bai; Yunus, Norsiah

    2014-06-01

    Short implants have been advocated as a treatment option in many clinical situations where the use of conventional implants is limited. This review outlines the effectiveness and clinical outcomes of using short implants as a valid treatment option in the rehabilitation of edentulous atrophic alveolar ridges. Initially, an electronic search was performed on the following databases: Medline, PubMed, Embase, Cochrane Database of Systematic Reviews, and DARE using key words from January 1990 until May 2012. An additional hand search was included for the relevant articles in the following journals: International Journal of Oral and Maxillofacial Implants, Clinical Oral Implants Research, Journal of Clinical Periodontology, International Journal of Periodontics, Journal of Periodontology, and Clinical Implant Dentistry and Related Research. Any relevant papers from the journals' references were hand searched. Articles were included if they provided detailed data on implant length, reported survival rates, mentioned measures for implant failure, were in the English language, involved human subjects, and researched implants inserted in healed atrophic ridges with a follow-up period of at least 1 year after implant-prosthesis loading. Short implants demonstrated a high rate of success in the replacement of missing teeth in especially atrophic alveolar ridges. The advanced technology and improvement of the implant surfaces have encouraged the success of short implants to a comparable level to that of standard implants. However, further randomized controlled clinical trials and prospective studies with longer follow-up periods are needed.

  18. A new system for sodium flux growth of bulk GaN. Part I: System development

    NASA Astrophysics Data System (ADS)

    Von Dollen, Paul; Pimputkar, Siddha; Alreesh, Mohammed Abo; Albrithen, Hamad; Suihkonen, Sami; Nakamura, Shuji; Speck, James S.

    2016-12-01

    Though several methods exist to produce bulk crystals of gallium nitride (GaN), none have been commercialized on a large scale. The sodium flux method, which involves precipitation of GaN from a sodium-gallium melt supersaturated with nitrogen, offers potentially lower cost production due to relatively mild process conditions while maintaining high crystal quality. We successfully developed a novel apparatus for conducting crystal growth of bulk GaN using the sodium flux method which has advantages with respect to prior reports. A key task was to prevent sodium loss or migration from the growth environment while permitting N2 to access the growing crystal. We accomplished this by implementing a reflux condensing stem along with a reusable capsule containing a hermetic seal. The reflux condensing stem also enabled direct monitoring of the melt temperature, which has not been previously reported for the sodium flux method. Furthermore, we identified and utilized molybdenum and the molybdenum alloy TZM as a material capable of directly containing the corrosive sodium-gallium melt. This allowed implementation of a crucible-free system, which may improve process control and potentially lower crystal impurity levels. Nucleation and growth of parasitic GaN ("PolyGaN") on non-seed surfaces occurred in early designs. However, the addition of carbon in later designs suppressed PolyGaN formation and allowed growth of single crystal GaN. Growth rates for the (0001) Ga face (+c-plane) were up to 14 μm/h while X-ray omega rocking (ω-XRC) curve full width half-max values were 731″ for crystals grown using a later system design. Oxygen levels were high, >1019 atoms/cm3, possibly due to reactor cleaning and handling procedures.

  19. Ion Beam Assisted Deposition of Thin Epitaxial GaN Films.

    PubMed

    Rauschenbach, Bernd; Lotnyk, Andriy; Neumann, Lena; Poppitz, David; Gerlach, Jürgen W

    2017-06-23

    The assistance of thin film deposition with low-energy ion bombardment influences their final properties significantly. Especially, the application of so-called hyperthermal ions (energy <100 eV) is capable to modify the characteristics of the growing film without generating a large number of irradiation induced defects. The nitrogen ion beam assisted molecular beam epitaxy (ion energy <25 eV) is used to deposit GaN thin films on (0001)-oriented 6H-SiC substrates at 700 °C. The films are studied in situ by reflection high energy electron diffraction, ex situ by X-ray diffraction, scanning tunnelling microscopy, and high-resolution transmission electron microscopy. It is demonstrated that the film growth mode can be controlled by varying the ion to atom ratio, where 2D films are characterized by a smooth topography, a high crystalline quality, low biaxial stress, and low defect density. Typical structural defects in the GaN thin films were identified as basal plane stacking faults, low-angle grain boundaries forming between w-GaN and z-GaN and twin boundaries. The misfit strain between the GaN thin films and substrates is relieved by the generation of edge dislocations in the first and second monolayers of GaN thin films and of misfit interfacial dislocations. It can be demonstrated that the low-energy nitrogen ion assisted molecular beam epitaxy is a technique to produce thin GaN films of high crystalline quality.

  20. Exploiting Dragon Envisat Times Series and Other Earth Observation Data

    NASA Astrophysics Data System (ADS)

    Marie, Tiphanie; Lai, Xijun; Huber, Claire; Chen, Xiaoling; Uribe, Carlos; Huang, Shifeng; Lafaye, Murielle; Yesou, Herve

    2010-10-01

    Earth Observation data were used for mapping potential Schistosomiasis japonica distribution, within Poyang Lake (Jiangxi Province, PR China). In the first of two steps, areas suitable for the development of Oncomelania hupensis, the intermediate host snail of Schistosoma japonicum, were derived from submersion time parameters and vegetation community indicators. Y early maps from 2003 to 2008 indicate five principally potential endemic areas: Poyang Lake National Nature Reserve, Dalianzi Hu, Gan Delta, Po Jiang and Xi He. Monthly maps showing the annual dynamic of potential O. hupensis presence areas were obtained from December 2005 to December 2008. In a second step human potential transmission risk was handled through the mapping of settlements and the identification of some human activities. The urban areas and settlements were mapped all around the lake and fishing net locations in the central part of Poyang Lake were identified. Finally, data crossing of the different parameters highlight the potential risk of transmission in most of the fishing nets areas.