bluuebunny/crossref_metadata_2025_split huggingface dataset:
A Thermodynamic Approach to Ohmic Contact Formation to p-GaN
Authors: Liu Bo , Ahonen Mikko H. , Holloway Paul H. 
Published: September 2012

AbstractA new ohmic contact scheme for gallium nitride is presented. The use of Nitride-forming metal Over Gallide-forming metal, “NOG”, can modify the thermodynamic activity of N and Ga near the interface. This in turn can modify the near-surface point defect concentrations, particularly the vacancies of Ga and N. The principle of this contact scheme was shown to be consistent with results from Ni/Au, Ni/Zn-Au, Ta/Ti, and Ni/Mg/Ni/Si contacts. In the present study, the “NOG” scheme was used to design Ni/Ti/Au and Ni/Al/Au metallization, and addition of Ti and Al nitride-forming metals to the Ni gallide-forming metal led to lower but still high contact resistance. Ti was shown to be better than Al as the nitride-forming metal based on the decrease of resistance in as deposited contacts. Compared to Ni/Au, four times more current was measured in Ni/Ti/Au contacts to p-GaN after anneal at 300°C for 5min. However the addition of the Ti nitride-forming metal led to lower stability at 500°C.
Growth and Characterization of Piezoelectrically Enhanced Acceptor-Type AlGaN/GaN Heterostructures
Authors: Michel A. , Hanser D. , Davis R.F. , Qiao D. , Lau S.S. , Yu L.S. , Sun W. , Asbeck P. 
Published: September 2012

AbstractAcceptor (Mg)-doped AlGaN/GaN heterostructures were grown via MOVPE and compared to similarly doped GaN standard films grown in the same reactor. Chemical analysis of the films, via secondary ion mass spectrometry (SIMS), revealed comparable Mg concentrations of ∼2×1019 atoms/cm3 in all films. The Mg-doped GaN standard sample had a sheet conductance of 7-μS compared to a sheet conductance of 20-μS for an AlGaN/GaN heterostructure. The sheet conductance of the AlGaN/GaN heterostructures was higher due to piezoelectric acceptor doping and modulation doping effects in addition to conventional Mg acceptor doping.
Deep Levels in n-Type Schottky and p+-n Homojunction GaN Diodes
Authors: Hierro A. , Kwon D. , Ringel S. A. , Hansen M. , Mishra U. K. , Denbaars S. P. , Speck J. S. 
Published: September 2012

AbstractThe deep level spectra in both p+-n homojunction and n-type Schottky GaN diodes are studied by deep level transient spectroscopy (DLTS) in order to compare the role of the junction configuration on the defects found within the n-GaN layer. Both majority and minority carrier DLTS measurements are performed on the diodes allowing the observation of both electron and hole traps in n-GaN. An electron level at Ec-Et=0.58 and 0.62 V is observed in the p+-n and Schottky diodes, respectively, with a concentration of ∼3-4×1014cm−3 and a capture cross section of ∼1-5×10−15cm2. The similar Arrhenius behavior indicates that both emissions are related to the same defect. The shift in activation energy is correlated to the electric field enhanced-emission in the p+-n diode, where the junction barrier is much larger. The p+-n diode configuration allows the observation of a hole trap at Et-Ev=0.87 eV in the n-GaN which is very likely related to the yellow luminescence band.
Fermi Level Pinning at GaN-Interfaces: Correlation of Electrical Admittance and Transient Spectroscopy
Authors: Witte H. , Krtschil A. , Lisker M. , Rudloff D. , Christen J. , Krost A. , Stutzmann M. , Scholz F. 
Published: September 2012

AbstractIn GaN layers grown by molecular beam epitaxy as well as metal organic vapor phase epitaxy significant differences were found in the appearance of deep defects de-tected by thermal admittance spectroscopy as compared for deep level transient spectros-copy measurements. While, thermal admittance spectroscopy measurements which were made under zero bias conditions only show thermal emissions at activation energies between 130 and 170 meV, further deep levels existing in these GaN layers were evidenced by transient spectrocopy. This discrepancy is explained by a pinning effect of the Fermi level at the metal / GaN interface induced by high a concentration of the deep levels showing up in thermal admittance spectroscopy. We compare our results with a GaAs:Te Schottky- diode as a refernec sample. Here, both spectroscopic methods give exactly the same deep level emissions.
Low-Frequency Noise in SiO2/AlGaN/GaN Heterostructures on SiC and Sapphire Substrates
Authors: Pala N. , Gaska R. , Shur M. , Yang J. W , Khan M. Asif 
Published: September 2012

AbstractThe low-frequency noise in GaN-based Metal-Oxide-Semiconductor Heterostructure Field Effect Transistors (MOS-HFETs) and HFETs on sapphire and n-SiC substrates were studied. Hooge parameter at zero gate bias was calculated about 8 × 10−4 for both types of the devices. The AlGaN/GaN MOS-HFETs exhibited extremely low gate leakage current and much lower noise at both positive and negative gate biases. These features demonstrate the high quality of the SiO2/AlGaN heterointerface and feasibility of this technology for high-power microwave transmitter and high-power, high-temperature switches.
Influence of Internal Electric Fields on the Ground Level Emission of GaN/AlGaN Multi-Quantum Wells
Authors: Bonfiglio A. , Lomascolo M. , Traetta G. , Cingolani R. , Carlo A. Di , Sala F. Della , Lugli P. , Botchkarev A. , Morkoc H. 
Published: September 2012

AbstractThe spectroscopic investigation of GaN/AlGaN quantum wells reveals that the emission energy of such structures is determined by four parameters, namely composition, well-width, strain and charge density. The experimental data obtained by varying these parameters are quantitatively explained by an analytic model based on the envelope function formalism which accounts for screening and built-in field, and by a full self-consistent tight-binding model.
Identification of As, Ge and Se Photoluminescence in GaN Using Radioactive Isotopes
Authors: Stötzler A. , Weissenborn R. , Deicher M. , Collaboration Isolde 
Published: September 2012

AbstractWe report on experiments which unequivocal identify the chemical nature of optical transitions related to As (2.58 eV), Ge (3.398 eV) and Se (1.49 eV) found in the photoluminescence (PL) spectra of GaN. For this purpose epitaxial GaN layers were doped by ion implantation (60 keV, 3×1012 cm−2) with the radioactive isotopes 71As and 72Se. The isotope 71As (half-life 64.28 h) decays first into 71Ge (11.43 d), which finally transmutes into stable 71Ga. The isotope 72Se decays via 72As (26 h) into stable 72Ge. These chemical transmutations were monitored with photoluminescence spectroscopy (PL). The half-lives resulting from exponential fits on our PL data are in excellent agreement with the half-lives of the isotopes. Our experiments clearly show that in each case the luminescence center involves exactly one As, Ge or Se atom. In addition to this, the results imply that no optically active GaN antisite exists.
Dislocation Arrangement in a Thick LEO GaN Film on Sapphire
Authors: Dunn Kathleen A. , Babcock Susan E. , Stone Donald S. , Matyi Richard J. , Zhang Ling , Kuech Thomas F. 
Published: September 2012

AbstractDiffraction-contrast TEM, focused probe electron diffraction, and high-resolution X-ray diffraction were used to characterize the dislocation arrangements in a 16[.proportional]m thick coalesced GaN film grown by MOVPE LEO. As is commonly observed, the threading dislocations that are duplicated from the template above the window bend toward (0001). At the coalescence plane they bend back to lie along [0001] and thread to the surface. In addition, three other sets of dislocations were observed. The first set consists of a wall of parallel dislocations lying in the coalescence plane and nearly parallel to the substrate, with Burgers vector (b) in the (0001) plane. The second set is comprised of rectangular loops with b = 1/3 [11 20] (perpendicular to the coalescence boundary) which originate in the coalescence boundary and extend laterally into the film on the (1 100). The third set of dislocations threads laterally through the film along the [1 100] bar axis with 1/3<11 20>-type Burgers vectors These sets result in a dislocation density of ∼109 cm−2. High resolution X-ray reciprocal space maps indicate wing tilt of ∼0.5°.
Dislocation Mechanisms in the GaN Lateral Overgrowth by Hydride Vapor Phase Epitaxy
Authors: Kuan T. S. , Inoki C. K. , Hsu Y. , Harris D. L. , Zhang R. , Gu S. , Kuech T. F. 
Published: September 2012

AbstractWe have carried out a series of lateral epitaxial overgrowths (LEO) of GaN through thin oxide windows by the hydride vapor phase epitaxy (HVPE) technique at different growth temperatures. High lateral growth rate at 1100°C allows coalescing of neighboring islands into a continuous and flat film, while the lower lateral growth rate at 1050°C produces triangular-shaped ridges over the growth windows. In either case, threading dislocations bend into laterally grown regions to relax the shear stress developed in the film during growth. In regions close to the mask edge, where the shear stress is highest, dislocations interact and multiply into arrays of edge dislocations lying parallel to the growth window. This multiplication and pileup of dislocations cause a large-angle tilting of the laterally grown regions. The tilt angle is high (∼8 degrees) when the growth is at 1050°C and becomes smaller (3-5 degrees) at 1100°C. At the coalescence of growth facets, a tilt-type grain boundary is formed. During the high-temperature lateral growth, the tensile stress in the GaN seed layer and the thermal stress from the mask layer both contribute to a high shear stress at the growth facets. Finite element stress simulations suggest that this shear stress may be sufficient to cause the observed excessive dislocation activities and tilting of LEO regions at high growth temperatures.
A Study of the Effect of V/III Flux Ratio and Substrate Temperature on the in Incorporation Efficiency in inxGa1−x/GaN Heterostructures Grown by Rf Plasma-Assisted Molecular Beam Epitaxy
Authors: O'Steen M. L. , Fedler F. , Hauenstein R. J. 
Published: September 2012

AbstractLaterally resolved high resolution X-ray diffraction (HRXRD) and photoluminescence spectroscopy (PL) have been used to assess In incorporation efficiency in InxGa1−xN/GaN heterostructures grown through rf-plasma-assisted molecular beam epitaxy. Average alloy composition over a set of InxGa1−xN/GaN superlattices has been found to depend systematically upon both substrate temperature (Tsub) and V/III flux ratio during growth. A pronounced thermally activated In loss (with more than an order-of-magnitude decrease in average alloy composition) is observed over a narrow temperature range (590–670oC), with V/III flux ratio fixed. Additionally, the V/III flux ratio is observed to further strongly affect In incorporation efficiency for samples grown at high Tsub, with up to an order-of-magnitude enhancement in In content despite only a minor increase in V/III flux ratio. PL spectra reveal redshifts as In content is increased and luminescence efficiency which degrades rapidly with decreasing Tsub. Results are consistent with In loss arising from thermally activated surface segregation + surface desorption processes during growth.
Polarity Determination for MOCVD Growth of GaN on Si(111) by Convergent Beam Electron Diffraction
Authors: Zhao L. , Marchand H. , Fini P. , Denbaars S. P. , Mishra U. K. , Speck J. S. 
Published: September 2012

AbstractThe polarity of laterally epitaxially overgrown (LEO) GaN on Si(111) with an AlN buffer layer grown by MOCVD has been studied by convergent beam electron diffraction (CBED). The LEO GaN was studied by cross-section and plan-view transmission electron microscopy (TEM). The threading dislocation density is less than 108cm−2 and no inversion domains were observed. CBED patterns were obtained at 200 kV for the <1100> zone. Simulation was done by many-beam solution with 33 zero-order beams. The comparison of experimental CBED patterns and simulated patterns indicates that the polarity of GaN on Si(111) is Ga face.
Evidence from EELS of Oxygen in the Nucleation Layer of a MBE grown III-N HEMT
Authors: Eustis Tyler J. , Silcox John , Murphy Michael J. , Schaff William J. 
Published: September 2012

AbstractThe presence of oxygen throughout the nominally AlN nucleation layer of a RF assisted MBE grown III-N HEMT was revealed upon examination by Electron Energy Loss Spectroscopy (EELS) in a Scanning Transmission Electron Microscope (STEM). The nucleation layer generates the correct polarity (gallium face) required for producing a piezoelectric induced high mobility two dimensional electron gas at the AlGaN/GaN heterojunction. Only AlN or AlGaN nucleation layers have provided gallium face polarity in RF assisted MBE grown III-N's on sapphire. The sample was grown at Cornell University in a Varian GenII MBE using an EPI Uni-Bulb nitrogen plasma source. The nucleation layer was examined in the Cornell University STEM using Annular Dark Field (ADF) imaging and Parallel Electron Energy Loss Spectroscopy (PEELS). Bright Field TEM reveals a relatively crystallographically sharp interface, while the PEELS reveal a chemically diffuse interface. PEELS of the nitrogen and oxygen K-edges at approximately 5-Angstrom steps across the GaN/AlN/sapphire interfaces reveals the presence of oxygen in the AlN nucleation layer. The gradient suggests that the oxygen has diffused into the nucleation region from the sapphire substrate forming this oxygen containing AlN layer. Based on energy loss near edge structure (ELNES), oxygen is in octahedral interstitial sites in the AlN and Al is both tetrahedrally and octahedrally coordinated in the oxygen rich region of the AlN.
Formation of BN and AlBN During Nitridation of Sapphire Using RF Plasma Sources
Authors: Ptak A.J. , Ziemer K.S. , Holbert L.J. , Stinespring C.D. , Myers T.H. 
Published: September 2012

AbstractEvidence is presented that nitrogen plasma sources utilizing a pyrolytic boron nitride liner may be a significant source of B contamination during growth and processing. Auger electron spectroscopy analysis performed during nitridation of sapphire indicate the resulting layers contain a significant amount of BN. The formation of Al1−xBxN would explain the observation of a lattice constant several percent smaller than AlN as measured by reflection high-energy electron diffraction. The presence of cubic inclusions in layers grown on such a surface may be related to the segregation of BN during the nitridation into its cubic phase.
The Effect of Al in Plasma-Assisted MBE-Grown GaN
Authors: Zsebök Otto , Thordson Jan V. , Zhao Qingxiang , Södervall Ulf , Ilver Lars , Andersson Thorvald G. 
Published: September 2012

AbstractWe have grown GaN, with addition of a 0.10 to 0.33 % Al, on sapphire(0001) substrates by solid-source RF-plasma assisted MBE. The Al-concentration was determined by secondary ion-mass spectrometry and Auger-electron spectroscopy, while the layer quality was assessed by photoluminescence and high-resolution scanning electron microscopy. Microscopy revealed a meandering pattern and a surface roughness varying with Al-content. The smallest surface roughness was obtained at 0.10 % Al. Photoluminescence revealed two main peaks attributed to the neutral donor-bound exciton. Its energy increased slightly with Al-concentration, which established a correlation between the Al-concentration and the band gap.
Electrical Properties of Cubic InN and GaN Epitaxial Layers as a Function of Temperature
Authors: Fernandez J.R.L. , Chitta V.A. , Abramof E. , Silva A. Ferreira da , Leite J.R. , Tabata A. , As D.J. , Frey T. , Schikora D. , Lischka K. 
Published: September 2012

AbstractCarrier concentration and mobility were measured for intrinsic cubic InN and GaN, and for Si-doped cubic GaN as a function of temperature. Metallic n-type conductivity was found for the InN, while background p-type conductivity was observed for the intrinsic GaN layer. Doping the cubic GaN with Si two regimes were observed. For low Si-doping concentrations, the samples remain p-type. Increasing the Si-doping level, the background acceptors are compensated and the samples became highly degenerated n-type. From the carrier concentration dependence on temperature, the activation energy of the donor and acceptor levels was determined. Attempts were made to determine the scattering mechanisms responsible for the behavior of the mobility as a function of temperature.
Metal Organic Vapor Phase Epitaxy of GaAsN/GaAs Quantum Wells Using Tertiarybutylhydrazine
Authors: Schmidtling T. , Klein M. , Pohl U.W. , Richter W. 
Published: September 2012

AbstractGaAsN epilayers and quantum wells with a good structural quality and surface morphology were grown by low pressure metal organic vapor phase epitaxy using tertiarybutylhydrazine as a novel nitrogen source. The dependence of nitrogen incorporation on growth temperature was studied for epitaxy with arsine and tertiarybutylarsine precursors. A nitrogen content of 6.7 % was achieved using tertiarybutylhydrazine and tertiarybutylarsine at a low growth temperature of 530 °C. The observed room temperature luminescence shows an increasing redshift with increasing nitrogen contents of the wells.
TEM Study of the Morphology Of GaN/SiC (0001) Grown at Various Temperatures by MBE
Authors: Sarney W.L. , Salamanca-Riba L. , Ramachandran V. , Feenstra R.M , Greve D.W. 
Published: September 2012

AbstractGaN films grown on SiC (0001) by MBE at various substrate temperatures (600° - 750° C) were characterized by RHEED, STM, x-ray diffraction, AFM and TEM. This work focuses on the TEM analysis of the films' features, such as stacking faults and dislocations, which are related to the substrate temperature. There are several basal plane stacking faults in the form of cubic inclusions for samples grown at low temperatures compared to those grown at high temperatures. The dislocation density is greatest for the film grown at 600°C, and it steadily decreases with increasing growth temperatures. Despite the presence of various defects, x-ray analysis shows that the GaN films are of high quality. The double crystal rocking curve full width at half maximum (FWHM) for the GaN (0002) peak is less than 2 arc-minutes for all of the films we measured and it decreases with increasing growth temperature.
Threading Dislocation Density Reduction in GaN/Sapphire Heterostructures.
Authors: Kvit A. , Sharma A. K. , Narayan J. 
Published: September 2012

AbstractLarge lattice mismatch between GaN and α-Al2O3 (15%) leads to the possibility of high threading dislocation densities in the nitride layers grown on sapphire. This investigation focused on defect reduction in GaN epitaxial thin layer was investigated as a function of processing variables. The microstructure changes from threading dislocations normal to the basal plane to stacking faults in the basal plane. The plan-view TEM and the corresponding selected-area diffraction patterns show that the film is single crystal and is aligned with a fixed epitaxial orientation to the substrate. The epitaxial relationship was found to be (0001)GaN∥(0001)Sap and [01-10]GaN∥[-12-10]Sap. This is equivalent to a 30° rotation in the basal (0001) plane. The film is found to contain a high density of stacking faults with average spacing 15 nm terminated by partial dislocations. The density of partial dislocations was estimated from plan-view TEM image to be 7×109 cm−2. The cross-section image of GaN film shows the density of stacking faults is highest in the vicinity of the interface and decreases markedly near the top of the layer. Inverted domain boundaries, which are almost perpendicular to the film surface, are also visible. The concentration of threading dislocation is relatively low (∼;2×108 cm−2), compared to misfit dislocations. The average distance between misfit dislocations was found to be 22 Å. Contrast modulations due to the strain near misfit dislocations are seen in high-resolution cross-sectional TCM micrograph of GaN/α-Al2O3 interface. This interface is sharp and does not contain any transitional layer. The interfacial region has a high density of Shockley and Frank partial dislocations. Mechanism of accommodation of tensile, sequence and tilt disorder through partial dislocation generation is discussed. In order to achieve low concentration of threading dislocations we need to establish favorable conditions for some stacking disorder in thin layers above the film-substrate interface region.
Simulations of Defect-Interface Interactions in GaN
Authors: Chisholm J. A. , Bristowe P. D. 
Published: September 2012

AbstractWe report on the interaction of native point defects with commonly observed planar defects in GaN. Using a pair potential model we find a positive binding energy for all native defects to the three boundary structures investigated indicating a preference for native defects to form in these interfaces. The binding energy is highest for the Ga interstitial and lowest for vacancies. Interstitials, which are not thought to occur in significant concentrations in bulk GaN, should form in the (11 20) IDB and the (10 10) SMB and consequently alter the electronic structure of these boundaries.
Optical and Electrical Properties of MBE Grown Cubic GaN/GaAs Epilayers Doped by Si
Authors: As D.J. , Richter A. , Busch J. , Schöttker B. , Lübbers M. , Mimkes J. , Schikora D. , Lischka K. , Kriegseis W. , Burkhardt W. , Meyer B.K. 
Published: September 2012

AbstractSi-doping of cubic GaN epilayers grown by an rf plasma-assisted molecular beam epitaxy on semi-insulating GaAs (001) substrates is investigated by secondary ion mass spectroscopy (SIMS), photoluminescence (PL) and by Hall-effect measurements. SIMS measurements show a homogeneous incorporation of Si in cubic GaN epilayers up to concentrations of 5*1019 cm−3. PL shows a clear shift of the donor-acceptor emission to higher energies with increasing Si-doping. Above a Si-flux of 1*1011cm−2s−1 the near band edge lines merge to one broad band due to band gap renormalization and conduction band filling effects. The influence of the high dislocation density (≈1011cm−2) in c-GaN:Si on the electrical properties is reflected in the dependence of the electron mobility on the free carrier concentration. We find that dislocations in cubic GaN act as acceptors and are electrically active.
Efficient Acceptor Activation in AlxGa1−xN/GaN Doped Superlattices
Authors: Goepfert I. D. , Schubert E. F. , Osinsky A. , Norris P. E. 
Published: September 2012

AbstractMg-doped superlattices consisting of uniformly doped AlxGa1−xN and GaN layers are analyzed by Hall-effect measurements. Acceptor activation energies of 70 meV and 58 meV are obtained for superlattice structures with an Al mole fraction of x = 0.10 and 0.20 in the barrier layers, respectively. These energies are significantly lower than the activation energy measured for Mg-doped GaN thin films. At room temperature, the doped superlattices have free hole concentrations of 2 × 1018 cm−3 and 4 × 1018 cm−3 for x = 0.10 and 0.20, respectively. The increase in hole concentration with Al content of the superlattice is consistent with theory. The room temperature conductivity measured for the superlattice structures are 0.27 S/cm and 0.64 S/cm for an Al mole fraction of x = 0.10 and 0.20, respectively.