bluuebunny/crossref_metadata_2025_split huggingface dataset:
Wafer-fused n-AlGaAs/p-GaAs/n-GaN Heterojunction Bipolar Transistor with uid-GaAs Base-Collector Setback
Authors: Estrada Sarah , Champlain James , Wang Chad , Stonas Andreas , Coldren Larry , DenBaars Steven , Mishra Umesh , Hu Evelyn 
Published: April 2011

ABSTRACTRecently we reported the first AlGaAs-GaAs-GaN heterojunction bipolar transistor (HBT), a device that potentially combines the high-breakdown voltage of an n-GaN collector with the high mobility of an AlGaAs-GaAs emitter-base. Because of the high degree of lattice mismatch between GaAs (lattice constant of 5.65A) and GaN (3.19A), we formed these devices through wafer fusion, also called direct wafer bonding. Measurements on the first generations of wafer fused HBTs revealed good current modulation, with modest output current (0.83 KA/cm2) and a current gain of 1.2. Limitations to the current gain may be related to traps and defects introduced by the fusion process, or may be a consequence of the natural conduction band offset between GaAs and GaN, which is not well known. This paper describes our new HBT structure that included a thin (20nm) uid-GaAs base-collector “setback” layer. The setback layer shifted the fused GaAs-GaN interface slightly into the collector. This new HBT structure also incorporated a reduced base thickness of 100 nm. HBTs with setback layers demonstrate increased output current (1.7 KA/cm2) and increased current gain (1.9).
Growth and Characterization of Epitaxial GaN Thin Films on 4H-SiC (11.0) Substrates
Authors: Wagner Brian P. , Preble E. A. , Reitmeier Z. J. , Davis R. F. , Zakharov D. N. , Liliental-Weber Z. 
Published: April 2011

ABSTRACTGaN thin films were grown via metalorganic vapor phase epitaxy on a-plane 4H-SiC substrates on which had been deposited an AlN buffer layer. Atomic force microscopy images revealed that the microstructure of the AlN buffer layer and the subsequently deposited GaN had a highly oriented growth structure where parallel growth features propagated in the [1–1.0] direction. Scanning electron microscopy showed that the interfaces between the substrate, buffer layer, and epi-layer were continuous. Cracking was observed in GaN films having a thickness greater than 800 nm. Plan-view transmission electron microscopy analysis revealed stacking faults and threading dislocations with densities of ∼1.6×106cm-1 and ∼ 3.3×1010cm-1, respectively. X-ray diffraction confirmed that the GaN was deposited epitaxially in the same orientation as the substrate. The average on- and off-axis x-ray full-width half-maxima of the (11.0) and the (10.0) reflections were 948 arcsec and 5448 arcsec, respectively.
GaN layers re-grown on etched GaN templates by plasma assisted molecular beam epitaxy
Authors: He L. , Xie J. , Yun F. , Baski A. A. , Morkoç H. 
Published: April 2011

ABSTRACTThe growth of high-quality GaN by plasma assisted molecular beam epitaxy (MBE) is challenging, in part due to the constraint of heteroepitaxy since GaN substrates are not yet commercially available and isotropic nature of growth. Despite the large lattice and thermal mismatch between sapphire and GaN, the former is still the most commonly used substrate for the GaN-based optical devices at present. In this paper, we demonstrate a re-growth technique to obtain an improved quality GaN by MBE on GaN template on sapphire where the grossly defective regions have been removed. This GaN template is formed by MBE growth of GaN followed by wet chemical etching to selectively remove the defective region. Improved quality GaN was re-grown on such a template under Ga rich conditions to a thickness of about 1 micron. After re-growth, the surface of GaN is atomically smooth with spiral features in the short range. The low temperature PL of the re-grown GaN is superior to those of MBE GaN films directly on sapphire. Atomic force microscopy (AFM) images reveal a two-dimensional re-growth initiating in regions free of extended defects. The results show that the selectively etched GaN on sapphire can be used as a good template to improve the quality of GaN.
Properties of Crucible Materials for Bulk Growth of AlN
Authors: Slack Glen A. , Whitlock Jon , Morgan Ken , Schowalter Leo J. 
Published: April 2011

AbstractA variety of different crucible materials have been suggested and/or employed for the sublimation-recondensation growth of AlN single crystals above 2000 C. Representative materials all have melting points well above 2300 C, a reasonable degree of chemical compatibility with AlN, relatively low vapor pressures, and relatively small thermal expansion coefficients. We analyze the current state of knowledge on crucible materials such as C, W, Re, W-Re alloys, BN, HfN, HfC, NbC, TaC, Ta2C, TaN, ZrC and ZrN with respect to published bulk AlN growth conditions. Crucible materials pyrolytic graphite, pyrolytic BN, and W have integrated thermal contraction values (upon cooling from growth temperatures) that are less than that of AlN; the other materials have larger values. The lowest vapor pressure materials in a nitrogen atmosphere are W, TaC, and Re; thus they are expected to yield higher purity crystals than the other candidates. The materials C, BN, Hf, and ZrN are expected to contribute to higher impurity levels in the AlN crystals.
Influence of metal thickness to sensitivity of Pt/GaN Schottky diodes for gas sensing applications
Authors: Tilak V. , Ali M. , Cimalla V. , Manivannan V. , Sandvik P. , Fedison J. , Ambacher O. , Merfeld D. 
Published: April 2011

ABSTRACTHydrogen gas sensors based on Pt/GaN Schottky diode structures were fabricated and their responses to hydrogen were studied. These diodes were fabricated on Si doped GaN layer (ND = 1×1017). Three sets of diodes were fabricated with 80 Å, 240 Å and 400 Å of Pt for the Schottky contacts. The electronic performances of 0.25 × 0.25 mm devices were tested in up to 1 % H2 gas in synthetic air (79% N2, 21% O2) by volume. The devices were operated in constant current mode in a forward bias condition. The change in voltage was monitored with the diodes exposed to hydrogen and to dry air at varying temperatures. The responses increased as the thickness of the Schottky metal contact decreased at any given temperature up to 310 °C. The trend of increasing response with decreasing thickness was also observed in 0.5 × 0.5 mm and 1.0 × 1.0 mm size Schottky diodes. SEM studies of the microstructure showed that the thinner Pt devices had higher grain boundary densities. The increase in sensitivity with decreasing thickness points to the dissociation of molecular hydrogen on the surface, the diffusion of atomic hydrogen through the Pt grain boundaries and the adsorption of hydrogen to the surface as a possible mechanism of sensing of hydrogen by Schottky diodes.
Band-GaP Energy and Physical Properties of InN Grown by RF-Molecular Beam Epitaxy
Authors: Nanishi Yasushi , Saito Yoshiki , Yamaguchi Tomohiro , Matsuda Fumie , Araki Tsutomu , Naoi Hiroyuki , Suzuki Akira , Harima Hiroshi , Miyajima Takao 
Published: April 2011

ABSTRACTThis paper describes studies on high-quality InN growth on sapphire by RF-MBE. Critical procedures to obtain high-quality InN films were investigated and (1) nitridation process of sapphire substrates prior to growth, (2) precise control of V/III ratio and (3) selection of optimum growth temperature were found to be essential. Detailed structural characterizations by XRD, TEM, Raman scattering and EXAFS indicate that InN films obtained in this study have ideal hexagonal wurtzite structure. FWHMs of ω-2Θ mode XRD and E2(high)-phonon-mode of Raman scattering are as small as 28.9 arcsec and 3.2 cm-1, respectively. True band gap energy of InN is also discussed based on optical characterization results obtained from well-characterized hexagonal InN grown in this study. PbS, instead of InGaAs, was used as a detector for PL study in order to solve the problem coming from the cut-off wavelength of InGaAs detector. Based on these systematic studies on structural and optical property characterizations using high-quality InN, true band-gap energy of InN is suggested to be less than 0.67 eV and approximately 0.65 eV at room temperature. Single-crystalline InN films are also successfully grown on Si substrates by a brief nitridation of the Si substrates. Significant improvement of InN crystal quality on Si substrates by the insertion of an AlN buffer layer is also demonstrated.
Growth and Surface Reconstructions of AlN(0001) Films
Authors: Lee C. D. , Dong Y. , Feenstra R. M. , Northrup J. E. , Neugebauer J. 
Published: April 2011

ABSTRACTThe growth and surface reconstructions of AlN(0001) films were studied. For moderately Al-rich surfaces, the 2×6 structure is commonly observed in reflection high energy electron diffraction. It is found that this pattern consists of 2√3×2√3-R30° and 5√3×5√3-R30° reconstructions according to scanning tunneling microscopy. Similar to the Ga-rich GaN(0001) surface, these structures are determined to contain 2–3 monolayers of excess Al terminating the surface. Based on first-principles theory the structures are believed to contain a laterally contracted Al layer. At higher Al coverage a thick, flat Al film is found to form on the surface. A high density of growth spirals (associated with threading dislocations having full or partial screw character) is found to be present on the surface, although this density decreases with increasing growth temperature.
Hydrogen-related local vibrational modes in GaN:Mg grown by molecular beam epitaxy
Authors: Pastor D. , Cuscó R. , Artus L. , Naranjo F. , Calleja E. 
Published: April 2011

ABSTRACTWe report a Raman scattering study of local vibrational modes (LVMs) on Mg-doped GaN grown by molecular beam epitaxy (MBE). Besides Mg:Ga local vibrational modes clearly observed at 262 and 565 cm−1, several peaks were detected in the spectral regions around 2200 cm−1 and 2900 cm−1. The modes in the 2200 cm−1 spectral region correspond to local modes of hydrogen complexes and hydrogen-decorated defects, and indicate the presence of a fairly high concentration of H in the samples. The peaks observed in the 2900 cm−1 region are assigned to carbon-hydrogen local modes and are indicative of the presence of C impurities in the samples. These measurements show that both C and H impurities may be present in sizable amounts not only in metal-organic chemical vapor deposition (MOCVD) samples but also in MBE grown samples, and this may have an effect on the electrical conductivity ofp-type GaN:Mg samples.
Surface Potential Measurements of doping and defects in p-GaN
Authors: Losurdo M. , Giangregorio M. M. , Bruno G. , Brown A. S. , Doolittle W. A. , Namkoong Gon , Ptak A. J. 
Published: April 2011

ABSTRACTThe interaction of Be-, Mg-, and Si- doped GaN epitaxial films with atomic hydrogen, produced by a remote r.f. hydrogen plasma, is investigated. The kinetics of the interaction is monitored in real time by spectroscopic ellipsometry through the measurement of the variation of the GaN pseudodielectric function. The passivation effect of hydrogen is inferred by surface potential measurements using scanning Kelvin probe microscopy (SKPM). It is found that the interaction of GaN with hydrogen is a strong function of both the type and level of the doping. Hydrogen treatment is shown to lead to a strong variation of the surface potential and, hence, of the Fermi level position, which is the result of p-dopant passivation by hydrogen. A different interaction of Mg and Be with atomic hydrogen is also observed and monitored in real time by ellipsometry. SKPM is also used for studying the interaction of defects in GaN with atomic hydrogen.
Electron micro-probe analysis and cathodoluminescence spectroscopy of rare earth - implanted GaN
Authors: Dalmasso S. , Martin R. W. , Edwards P. R. , Katchkanov V. , O'Donnell K. P. , Lorenz K. , Alves E. , Wahl U. , Pipeleers B. , Matias V. , Vantomme A. , Nakanishi Y. , Wakahara A. , Yoshida A. 
Published: April 2011

ABSTRACTGaN films doped with rare earth (RE) elements have attracted considerable attention due to the unique optical luminescent properties of the RE intra 4ƒ(n)-shell electron transitions which lead to sharp blue (Tm), green (Er) and red (Eu) emissions. This paper presents an overview of investigations of GaN films implanted with each of these ions using a combination of electron-beam and optical techniques. The ion implantations were performed under a wide range of conditions, covering variations in fluences, energies and temperatures and followed by different high-temperature annealing steps. The resulting RE:GaN films were analysed using an electron probe micro-analyser modified to allow cathodoluminescence (CL) spectroscopy. Elemental microanalysis data obtained by wavelength dispersive X-ray analysis (WDX) is correlated with simultaneously collected room temperature CL spectra.WDX allows the quantification of the RE elemental concentrations in GaN down to ∼ 0.03 at. % in very thin layers (∼ 100 nm deep). Furthermore, by varying the incident electron beam energy, details concerning the depth profile of RE implants can be determined. The effects of both implantation conditions and rapid thermal annealing on the depth profile and on the luminescence properties are reported. CL measurements performed on annealed samples reveal sharp visible and near IR emission lines characteristic of the RE3+ intra-4ƒ(n) atomic shell transitions.
Formation and dissociation of hydrogen-related defect centers in Mg-doped GaN
Authors: Gelhausen O. , Phillips M. R. , Goldys E. M. , Paskova T. , Monemar B. , Strassburg M. , Hoffmann A. 
Published: April 2011

ABSTRACTModerately and heavily Mg-doped GaN were studied by a combination of post-growth annealing processes and electron beam irradiation techniques during cathodoluminescence (CL) to elucidate the chemical origin of the recombination centers responsible for the main optical emission lines. The shallow donor at 20–30 meV below the conduction band, which is involved in the donor-acceptor-pair (DAP) emission at 3.27 eV, was attributed to a hydrogen-related center, presumably a (VN-H) complex. Due to the small dissociation energy (<2 eV) of the (VN-H) complex, this emission line was strongly reduced by low-energy electron irradiation. CL investigations of the DAP at a similar energetic position in Si-doped (n-type) GaN indicated that this emission line is of different chemical origin than the 3.27 eV DAP in Mg-doped GaN. A slightly deeper DAP emission centered at 3.14 eV was observed following low-energy electron irradiation, indicating the appearance of an additional donor level with a binding energy of 100–200 meV, which was tentatively attributed to a VN-related center. The blue band (2.8–3.0 eV) in heavily Mg-doped GaN was found to consist of at least two different deep donor levels at 350±30 meV and 440±40 meV. The donor level at 350±30 meV was strongly affected by electron irradiation and attributed to a H-related defect.
Microstructure of Nonpolar a-Plane GaN Grown on (1120) 4H-SiC Investigated by TEM.
Authors: Zakharov D. N. , Liliental-Weber Z. , Wagner B. , Reitmeier Z. J. , Preble E. A. , Davis R. F. 
Published: April 2011

ABSTRACTPlan-view and cross-section samples of (1120) (a-plane) GaN grown on 4H-SiC substrates with AlN buffer layers were studied by transmission electron microscopy. Samples reveal the presence of a high density of stacking faults formed on the basal plane of hexagonal GaN. These stacking faults, terminated in the growth plane by threading dislocations, nucleate at the AlN/4H-SiC interface and propagate to the GaN layer surface. High resolution electron microscopy shows that the majority of stacking faults are low-energy planar defects of the type I1and I2. High energy stacking faults (E) are not observed.
Structure and electrical activity of rare-earth dopants in selected III-Vs
Authors: Filhol J.-S. , Petit S. , Jones R. , Hourahine B. , Frauenheim Th. , Overhof H. , Coutinho J. , Shaw M. J. , Briddon P. R. , Öberg S. 
Published: April 2011

ABSTRACTDensity functional theory is used to investigate Eu, Er and Tm rare earth (RE) impurities in GaAs, GaN and AlN. The most stable site is when the RE is located at a group III substitutional site but in GaN and GaAs these defects do not then possess any gap levels, unlike AlN. RE-VN defects in GaN are shown to possess levels which could act as traps for excitons. The interaction of oxygen with substitutional REs is also considered.
Identification of Carbon-related Bandgap States in GaN Grown by MOCVD
Authors: Armstrong A. , Arehart A. R. , Ringel S. A. , Moran B. , DenBaars S. P. , Mishra U. K. , Speck J. S. 
Published: April 2011

ABSTRACTCarbon doping of GaN is of great interest in part because it has been shown to result in semi-insulating (SI) behavior. However, determination of the bandgap states and hence the exact mechanism responsible for the SI behavior is, to date, an unresolved issue. A key impediment is that the presumed C acceptor levels are likely near the minority carrier (valence) bandedge of otherwise background n-type GaN, and hence their precise detection by usual methods is difficult. In this paper, we exploit the inherent ability of deep level optical spectroscopy (DLOS) to detect states near the minority carrier band edge, as well as potentially deep states associated with C in background n-type GaN. This is accomplished by comparing unintentionally doped (uid) GaN grown by atmospheric pressure (AP) MOCVD, which has residual n-type conductivity, with LP MOCVD GaN that incorporates a large concentration of C for both uid and intentionally Si co-doped conditions. The results show the emergence of a shallow state at Ec - 3.28 eV (Ev + 0.16 eV) for the LP samples with a minimum concentration of 3.6 × 1016 cm-3 that efficiently compensates Si donors for the co-doped sample, and results in semi-insulating behavior for the uid-LP sample. In contrast, this state is not observed for the AP GaN material, which incorporates a factor of ∼10 times less C, and instead only the expected residual Mg acceptor level at Ec - 3.22 eV is observed. Additionally, a state at Ec - 1.35 eV, near the theoretically expected C split-interstitial level in n-type GaN, is observed to increase significantly in concentration with increased C concentration.
Raman Characterization of Strained GaNyAs1-y and InxGa1-xNyAs1-y Epilayers
Authors: Tay Li-Lin , Lockwood David J. , Gupta James A. , Wasilewski Zbig R. 
Published: April 2011

ABSTRACTPseudomorphically strained epitaxial films of the ternary alloy GaNyAs1-y have been grown on GaAs(100) with y ranging from 0 to 0.05. The optical phonon Raman spectrum of the alloy displays a two-mode behavior. The GaAs-like first order modes are represented at y = 0.05 by the strong longitudinal optic (LO1) mode at 288.5 cm-1 and the weaker transverse optic (TO1) mode at 268.3 cm-1, while the GaN-like LO2 mode is observed at 474.8 cm-1. Two very broad disorder-induced acoustic bands are evident at 80 and 170 cm-1 due to atomic disorder within the crystalline network. Raman studies show that as the nitrogen concentration increases, the GaAs-like LO1 band shifts linearly towards lower wavenumber while the GaN-like LO2 phonon band displays a sub-linear increase in wavenumber. Raman results for the unstrained quaternary alloy In0.06Ga0.94N0.02As0.98 are compared with those of GaN0.02As0.98.
Luminescence Properties of Eu ion-implanted GaN
Authors: Uekusa Shin-ichiro , Tanaka Isao 
Published: April 2011

ABSTRACTThe Eu ion was implanted at an energy of 300keV with a dose of 1×1015cm-2 at room temperature. Photoluminescence (PL) and PL lifetime were characterized and studied on thermal quenching process. We calculated the activation energy (E1) of temperature dependent PL and the value of E1 was 5.68meV. E1 was affected the luminescence intensity in the temperature range from 15K to 70K. The activation energy (Ea) of PL lifetime was calculated and the value of Ea was 8.5meV. The non-radiative recombination in the transition from the 5D0 to 7F2 of Eu was dominated in the temperature range from 15K to 100K. We found that the thermal quenching occurred in both the electron emission from RE-trap and the non-radiative recombination in the transition on Eu in the temperature range from 15K to 70K.
Origins of Light Emission and Efficiency Saturation of the Photoluminescence of GaN Nanocrystallites
Authors: Chen Xiang-Bai , Morrison John L. , Penner Margaret K. , Elle Jennifer , Bergman Leah , Purdy Andrew P. 
Published: April 2011

ABSTRACTThe photoluminescence (PL) properties of GaN nanorods were studied utilizing UV micro-photoluminescence. The room temperature PL of the GaN nanorods exhibits one strong emission line. The PL intensity as a function of the laser power was investigated in order to determine whether this emission originates from an excitonic or a bandgap recombination process. Our analysis indicates that the PL of the rods is excitonic-like and very similar to the behavior of the free exciton A in GaN thin films. However, for a relatively large and compact ensemble of rods, the PL intensity exhibits a significant saturation occurring already at relatively low laser power. We attribute the intensity saturation to the laser heating and heat trapping which takes place in the enclosure of the ensemble.
Polarization-dependent spectroscopy of the near-bandgap excitonic emission in free standing GaN
Authors: Paskov P. P. , Paskova T. , Holtz P. O. , Monemar B. 
Published: April 2011

ABSTRACTWe report a comparative study of the exciton emission in free standing HVPE layer for all polarization configurations. A noticeable difference between the emission spectra polarized perpendicular and parallel to the c-axis of the crystal is observed. The spectra for E┴c and EIIc are found to be dominated by the emissions of the donor-bound exciton and exciton-polariton both arising from the A and B valence band, respectively, which clearly reveals the optical selection rules in wurtzite GaN. The temperature evolution of the emission spectra is also examined and the thermal redistribution of the excitons at different polarization is discussed.
Structural and Optical Characterization of InGaN Layers Grown by MOMBE
Authors: Singh P. , Aderhold J. , Graul J. , Davydov V. Yu. , Gourbilleau F. , Ruterana P. 
Published: April 2011

ABSTRACTThe investigated InGaN ternary films were grown by MOMBE, with the In fraction varying from 6% to 100%. Using XRD and TEM, we determine the crystalline state, the nature of defects inside the layers and their distribution as related to the In concentration. The band gap of the alloys was studied through PL measurements, it is below 1 eV for InN.
Growth and Characterization of GaPNAs on Si
Authors: Geisz John , Olson J. M. , McMahon W. E. , Hannappel T. , Jones K. , Moutinho H. , Al-Jassim M. M. 
Published: April 2011

ABSTRACTThe dilute nitrogen alloy GaPNAs can be lattice matched to silicon with band gaps ranging from 2.3 eV to less than 1.7eV making it of special interest for photovoltaic applications. We have studied the growth and structural quality of the alloy grown on vicinal Si(001) and GaP(001) substrates by MOCVD. Using a particular nucleation scheme, we have deposited 1-μm thick layers that are crack-free and exhibit narrow x-ray line widths. The FWHM of the (004) x-ray reflection from a GaP1−xNx epilayer decreases dramatically from ∼300 arcsec for x=0 to 18 arcsec for x = 0.021. The band gap of this alloy is 1.96 eV. With the addition of As (and more N), the x-ray line widths tend to increase slightly to 27 arcsec.