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The effect of sapphire-substrate thickness on the curvature and stress in thick hydride vapor phase epitaxial GaN films is analyzed by high-resolution x-ray diffraction. The curvature at the growth temperature is strongly influenced by the value of the intrinsic tensile strain, which is obtained from the film/substrate thickness ratio.

The photoluminescence (PL) and cathodoluminescence (CL) spectroscopic characterization of GaN nanowires with diameters of 50-250 nm, obtained by catalyst-free molecular beam epitaxy is presented. The sub-band-gap PL and CL peaks observed were identified as phonon replicas of the free-exciton transitions, or excitons bound to structural defects or surfaces states.

The effects of surface treatment, inductively coupled plasma (ICP) etching and thermal annealing on surface and optical properties of p-type GaN were studied. This chemical treatment is effective in removing surface contaminants that can improve electrical properties of the material.

A detailed study of GaN pillar arrays by atomic force microscopy (AFM), Raman spectroscopy, and photoluminescence (PL) spectroscopy is presented. AFM is used to characterize the shape of the GaN pillars and revealed a large roughness of etched pillar surfaces.

Research into a number of multilayer contacts on GaN-Ti/Pd/Au, Ti/Al/Pd/Au and Ti/Al on n-GaN; and Ti/Pd/Au, Pd/Ti/Pd/Au and Pd/Ni/Au on p-GaN is presented. It was found that surface treatment using aqua-regia, (1)HNO3:(3)HCl is more effective for removing native oxide on GaN than (1)HCl:(1)H2O.

GaN and AIGaN/GaN heterostructure surface potential measurements were taken using scanning Kelvin probe microscopy.

Material growth method and Mg doping effects on Er3+ photoluminescence in Er-implanted GaN are examined.

GaN was grown by the two flow metalorganic chemical vapor deposition method based on monoenergetic positron beams, and defects were studied.

Fe atoms in GaN were studied to determine damage and annealing behavior and lattice site location.

Reactive rf-magnetron sputtering was used to deposit GaN films on Si(111) substrates without substrate heating.

The crystal field splitting of energy rates of Pr3+ in GaN were calculated and compared to photoluminescence and cathodoluminescence spectra of Pr3+ implanted in GaN by metal organic chemical vapor deposition, recently reported.

Time evolution with low energy electron beam was used to study cathodoluminescence of epitaxial GaN laterally overgrown on (001) sapphire substrates.

Mg doped gallium-nitride single crystals were studied to determine temperature dependence of the electrical properties.

The metalorganic chemical vapor deposition of gallium nitride on Si(111) is examined in terms of stress control and application to field-effect transistors.

GaN monolayer effects on nitridation damage of molecular beam epitaxy gown GaN on GaAs(001), have been studied.

The effect of subband gap illumination on transport properties of AIGaN/GaN heterostrcutures was investigated using the Shubnikov-de Haas and Hall effects.

Mg activation properties in thermally treated GaN:Mg films were studied.

A laser interferometer was used to measure the piezolectric coefficient d33 of aluminum nitride and gallium nitride thin films on silicon.

Raman spectroscopy was used to study quasilongitudinal optical phonons in wurtzite gallium nitride.

Pt Schottky contacts to n-GaN formed by electrodeposition and physical vapor deposition are considered.

A new study investigates the electrical properties of W contacts to p- and n-GaN.

Gas-source molecular beam epitaxy and ammonia are used to grow undoped and Si doped GaN thin solid films on Al2O3(0001).

A simple reactive evaporation technique was used to grow a single crystalline gallium nitride film on Si(111).

Research was conducted to examine in situ the kinetics changes in the growth of GaN induced by the presence of In segregated on the surface the the use of reflection high energy electron diffraction intensity oscillations. Findings demonstrate that In affected both Ga and N surface kinetics which resulted in the growth rate associated with a change in the amount of Ga and N effectively available for the GaN growth.

Research was conducted to examine the influence of threading dislocations, Mg doping and etching on the photoconductivity spectra in GaN. A significantly enhanced spectral cutoff was demonstrated in nominally undoped GaN compared to heavily Mg-doped GaN. Results illustrate that etching/polishing causes increased surface recombination and reduces the expected quantum efficiency at photon energies above the band-gap energy of GaN.

Research was conducted to achieve a low specific contact resistance to p-type GaN by developing a contact. A bi-layer Ni/Au film deposited on p-type Ga followed by heat treatment in air to transform the metallic Ni into NiO along with an amorphous Ni-Ga-O phase and large Au grains comprised the contact. A model was used to explain the mechanism for obtaining low-resistance ohmic contacts for the oxidized Ni/Au films using energy band diagrams of the Au/pp-NiO/p-GaN structure.

Research was conducted to examine the electronic structure of, and the effects of Cs and O adsorption on, the p-GaN(001) surface using photoemission spectroscopy. The surface states were investigated and eliminated by comparing bulk- and surface-sensitive photoemission spectra and by exposing the surface to oxygen. Results indicate that the electron affinity of the clean surface was equal to 3.3 plus or minus 0.2 eV.

Research was conducted to examine Schottky gate electron tunneling in polarization induced AlGaN/GaN high electron mobility transistors which show promise for ultrahigh power microwave amplification. The polarization fields in the Ga-face, wurtzite structures were employed to engineer the conduction band profile in the absence of any doping. Results suggest that the tunneling electron current flow from metal to semiconductor is the dominant current transport mechanism under reverse bias at pinchoff and for additional drain-gate electric field in the barrier.

The measurement of carrier concentration and mobility of metalorganic chemical vapor deposited GaN thin films on the sapphire substrate by an infrared reflection method is described. Parameters of the lattice vibration oscillators and of the plasmon were obtained by fitting with the experimental data. A comparison of data from the Hall measurement reveals the good accuracy for the carrier concentration obtained from IR measurement while the infrared carrier mobility is lower than that from the Hall measurement by a factor of about 0.5.

Research was conducted to examine two metalorganic chemical vapor deposition grown samples and two hydride vapor phase epitaxy grown samples which are unintentionally doped n type for studying the temperature dependence of electron mobility for four different carrier concentrations. Comparisons with experimental results confirm the present calculation over a large range of electron concentrations and temperatures.

Research was conducted to prove that the photoinduced modifications of undoped GaN thin films are more complex than those of earlier experiments. Previous experiments focused on the photoluminescence efficiencies of the yellow and blue bands during ultraviolet laser irradiation of samples of GaN thin films grown on sapphire. Mechanisms that may explain the luminescence pathways and defects involved are presented.

Research was conducted to measure the Raman spectra of a GaN epitaxial layer grown on MgAl2O4 substrate in several backscattering and right angle scattering configurations at room temperature. The symmetry-allowed optical phonon modes were observed except the low frequency mode. The quasitransverse and quasilongitudinal modes were also observed in the x(zx)z and x(yy)z configurations. Results are in good agreement with the polarization selection rules.

Research was conducted to examine the structural and vibrational features of device quality pure GaN substrate grown using a lateral epitaxial ovvergrowth method by x-ray diffraction, Brillouin, Raman and infrared spectroscopy. Lattice constants were observed to be close to bulk GaN and relaxed epilayers on sapphire. Results indicate extremely good structural quality of the GaN substrate with structural coherence length of as long as 1.9 micron along the c axis.

Both Fourier transform infrared (FTIR) grazing incidence reflectivity and FTIR transmission techniques were employed to investigate GaN films grown on alpha-Al2O3 (0001) substrates by atmospheric pressure metal-organic chemical vapor deposition and low pressure metal-organic chemical vapor deposition. Findings revealed that in the frequency range of 400 to 3500/cm the signal-to-noise ratio of the FTIR grazing incidence measurement is much larger than that of the FTIR transmission measurement.

Research was conducted to examine the configuration of dislocations in lateral overgrowth GaN films. Transmission electron microscopy and cathodoluminescence were used to investigate the microstructure of sublimation lateral overgrowth GaN. Results demonstrate that the dislocations also affect the yellow emission and not just the band edge emission. The dislocations were also shown to propagate laterally in the overgrowth region.

Research was conducted to examine the surfaces of GaN films deposited by metalorganic chemical vapor deposition and molecular beam epitaxy using atomic force microscopy. The objective was to explain how the surface structures form and how they can be controlled. Results demonstrate that the spiral hillock formation was dependent on the critical radius of curvature of the step-line and the miscut terrace width.

Research was conducted to examine the photoluminescence pressure dependence of heteroepitaxial GaN thin films on sapphire substrates. A laser assisted substrate liftoff process was used to separate GaN thin films from its sapphire growth substrate to derive free-standing GaN membranes. A comparison of the GaN membranes to as grown GaN on sapphire substrates allowed a direct observation of the influence of the substrate on the GaN epilayer.

Research was conducted to determine the refractive index and extinction coefficient of cubic GaN in the energy range of 1.5-3.7 eV using combined reflectivity and spectroscopic ellipsometry studies of layers determined by molecular beam epitaxy on GaAs(001). The reflectivity measurements were utilized to determine the effects of surface roughness and the substrate-film interface. Results indicated that the refractive index of cubic GaN was higher in the transparent region than that in the hexagonal modification.

The electronic and elastic properties of BN, AlN and GaN in both zinc-blende (ZB) and wurtzite (WZ) structures were examined through first-principles total energy calculations using the molecular dynamics methods with variable cell shapes. The study's findings for lattice constants and the elastic constants are consistent with experimental data and/or other calculations. The Berry phase approach was used to calculate the piezoelectric constants of WZ and ZB crystals.

Research was conducted to examine the properties of molecular-beam epitaxy-grown GaNAs using photoluminescence, optical absorption and Raman spectroscopy. Nitrogen content in the layers was determined using secondary-ion-mass spectrometry and x-ray diffraction. Results demonstrate that the position of the luminescence in the As-poor GaNxAs1-x layers is in agreement with an estimation of the band-gap energy in GaNxAs1-x.

Research was conducted to examine the photoluminescence of undoped GaN grown on a sapphire substrate. The layers were grown using the rotating disk metal organic chemical vapor deposition technique. Excitonic transitions were observed from conduction band to spin-orbit split valence bands. The activation energy of the free exciton was derived and the optical band gap of GaN was deduced.

Micro-Raman spectroscopy, photoluminescence, x-ray diffraction and reflection high-energy electron diffraction are used to examine molecular beam epitaxy of cubic gallium nitrate on silicon carbide films deposited by chemical vapor deposition on silicon. Results showed that the wurtzite/zinc-blende ratio is influenced by N/metal ratio and the initial substrate roughness.

The donor-acceptor pair (DAP) recombination process was examined by performing temperature-dependent photoluminescence measurements in Mg-doped GaN epitaxial layers grown on sapphire substrates via plasma-assisted molecular beam epitaxy. Results indicated the existence of two Mg-related traps, one shallow and the other deep level. The ionization energies are 126 meV for the shallow and 160 meV for the deep level.

Research was conducted to identify the heterojunction valence band discontinuity at the (0001) GaN/AlN interface utilizing X ray and ultraviolet photoelectron spectroscopies. Results showed that the type I discontinuity values for GaN grown on AlN at 800 degrees centigrade are 0.8+-0.2 eV. On the other hand, the values for GaN grown on AlN at 650 degrees centigrade reached 0.8+-0.2 eV.

Research was conducted to examine the temperature dependence of the current-voltage characteristics of Ni-GaN Schottky barriers. In this study, test substrates were grown in a metal organic vapor phase epitaxy system. Results indicated that the large scatter observed in the measured Richardson constant may have resulted from the enhanced tunneling component in the transport current of metal-GaN Schottky barrier contacts.

Temperature- and frequency-dependent admittance spectroscopy were utilized to examine nominally undoped GaN layers grown via molecular beam epitaxy. This approach is advantageous as seen from the development of Schottky contacts on high-resistivity GaN layers. Results indicated two deep defect levels with thermal activation energies of (0.45+-0.04) and (0.63+-0.04) eV found at the MBE-grown GaN layers.

The electron transport characteristics of GaN for temperatures ranging from 300 to 6000 K and in the applied electric range from 0 to 500 kV/cm are presented. The two lowest energy conduction bands of the pseudopot ential ban structure measured for wurtzite phase GaN are selected as a basis of an analytical multivalley conduction band model. Results give rise to an analytic expression that accounts for ionized impurity concentration, temperature and compensation ratio dependences of the drift mobility in bulk GaN.

The quality of epitaxial GaN layers can be enhanced by using compliant silicon-on-insulator (SOI) substrates rather than Si substrates. When SOI substrates are used, there is better surface morphology and crystal uniformity and a lower number of threading dislocations. A rise in growth temperature leads to a significant fall in the full width at half maximum value of the GaN (0002) x-ray diffraction spectroscopy peak. This indicates an improvement of the crystal quality of the epilayer.

Amorphous SixNy formed at the GaN/Si interface during the nucleation stage of beta-GaN on Si(001) is responsible for the phase mixture. A buffer layer can thus be used to prevent the formation of SixNy at the GaN/Si interface. This brings improved phase purity of the beta-GaN films grown on GaAs/Si(001). GaN films grown on the GaAs/Si(100) structure are mainly cubic and show the characteristic band-edge photoluminescence of beta-GaN up to room temperature.

Research was conducted to study the application of cathodoluminescence in evaluating cross-sectional samples of GaN epitaxial films grown on sapphire. The films were characterized by an 8 micron-m thick GaN buffer layer grown by the hydride vapor phase epitaxy method and were mounted in the specimen holder of the scanning electron microscope. Results indicated that improvements in the 430 and 545 bands takes place in two epilayers.