We report the first observation of optical quenching of photoconductivity in GaN photoconductors at room temperature. Three prominent quenching bands were found at Ev+1.44, 1.58 and 2.20 eV, respectively. These levels are related to the three hole traps in GaN materials based on a hole trap model to interpret the quenching mechanism. The responsivity was reduced about 12% with an additional He-Ne laser shining on the detector.

Variable angle spectroscopic ellipsometry (VASE) and transmission measurements have been employed to study the dielectric response of gallium nitride (GaN) thin films — an important material for light emitting diodes (LEDs) and laser diodes applications. The GaN films were grown by atomsphere pressure metal organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates (α-AI2O3). Room temperature VASE measurements were made, in the range of 0.75 to 5.5eV, at the angle of incidence of 73, 75, and 77 degree, respectively. Evidence of anisotropy is observed especially in the spectral range under the band gap (∼3.4 eV), reflecting the nature of wurtzite crystal structure of GaN. The ordinary dielectric function ε⊥(ω) of GaN were obtained through the analysis of transmission and VASE data in the range below and above the band gap. The thickness of these GaN films is also determined via the analysis.

Aluminum nitride (AlN) thin films have been synthesized by ion-beam assisted deposition method and the influence of air-exposure on the optical transmittance has been studied. The kinetic energy of nitrogen ion beam was kept at 0.1 or 1.5 keV under the constant current density. Synthesized films have been exposed to controlled air (23 °C and RH; 50%) and optical transmission spectrum from 190 to 2200 nm has been measured by UV-visible spectrometer every week. Surface morphology of the films has been observed with an optical microscope (OM). The optical transmittance has not changed drastically up to one year. Observations by OM show that round features of some microns were produced on the surface after about 25 weeks exposure. These substances seem to be reaction products between AlN and water in air.

We have investigated an optical waveguide formed by aluminum nitride (AlN) thin film on sapphire. A good quality AlN thin film on sapphire substrate was prepared by metal organic chemical vapor deposition (MOCVD) in this laboratory. A rutile prism coupler was employed to display the waveguide modes (N-lines) with wavelengths of 632.8, 532.1, 514.5 and 488.0 nm. The refractive index and thickness of the waveguide material is obtained by prism-coupler measurement. The dispersion curve of the AlN film is given and the dispersion equation is derived. The attenuation in the waveguide is evaluated by scattering loss measurements using a fiber probe. The attenuation coefficient alpha (α) is 1.5- 2.1 cm−1 depending on the sample and the different modes of waveguide. The accuracy of the measurement is discussed.

We present the results of optical studies of InxGa1-xN alloys (0<x<0.2) grown by metalorganic chemical vapor deposition on top of thick GaN epitaxial layers with sapphire as substrates. Photoluminescence (PL) and photoreflectance (PR) measurements were performed at various temperatures to determine the band gap and its variation as a function of temperature for samples with different indium concentrations. Carrier recombination dynamics in the alloy samples were studied using time-resolved luminescence spectroscopy. While the measured decay time for the alloy near-band-edge PL emissions was observed to be generally around a few hundred picoseconds at 10 K, it was found that the decay time decreased rapidly as the sample temperatures increased. This indicates a strong influence of temperature on the processes of trapping and recombination of excited carriers at impurities and defects in the InGaN alloys.

Time-resolved photoluminescence (PL) has been employed to study the optical transitions and their dynamical processes in GaN and InxGa1-xN epilayers, and GaN/GaN and GaN/ALxGa1-xN multiple quantum wells (MQW). We compare the results from both metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) grown samples. In addition, results are also compared with GaAs/ALxGa1-xAs MQW. It was found for all samples that the low temperature emission lines were dominated by radiative recombination transitions of either localized or free excitons, which demonstrates the high quality and purity of these III-nitride materials.

GaN, AlxGa1–xN, and AlN layers exhibit interference oscillations and bandgap-related features in their reflectance-difference (-anisotropy) (RD/RA) spectra. We concentrate on the interpretation of interference-related data, providing a general expression for these optical anisotropics and discussing mechanisms that originate in the layers themselves. These include anisotropic strain in the plane of the layer, a tilt of the c axis with respect to the surface normal, and non-normal-incidence illumination. We estimate the magnitudes of these contributions, and show that they are consistent with those observed. In principle these contributions can be separated by their different azimuthal dependences. The complex pattern of the data for AlxGa1–xN and AlN indicate that contributions from several layers are present.

We present the results of experimental studies of the strain effects on the excitonic transitions in GaN epitaxial layers on sapphire and SiC substrates. Photoluminescence and reflectance spectroscopies were performed to measure the energy positions of exciton transitions and X-ray diffraction measurements were conducted to examine the lattice parameters of GaN epitaxial layers grown on different substrates. Residual strain induced by the mismatch of lattice constants and thermal-expansion between GaN epitaxial layers and substrates was found to have a strong influence in determining the energies of excitonic transitions. The overall effects of the strain generated in GaN is compressive for GaN grown on sapphire and tensile for GaN on SiC substrate. The uniaxial and hydrostatic deformation potentials of wurtzite GaN were derived from the experimental results. Our results yield the uniaxial deformation potentials b1≈−5.3 eV and b2≈2.7 eV, as well as the hydrostatic components a1≈−6.5 eV and a2≈−11.8 eV.

Biaxial strains resulting from mismatches in thermal expansion coefficients and lattice parameters in 22 GaN films grown on A1N buffer layers previously deposited on vicinal and on-axis 6H-SiC(0001) substrates were measured via changes in the c-axis lattice parameter (c). Six of the films were in compression, indicating the residual strain due to lattice mismatch was not relieved. A Poisson's ratio of v=0.18 was calculated. The bound exciton energy (EBx) was a linear function of these strains. The shift in EBx with film stress was 23 meV/GPa. The role of the SiC off-axis tilt was investigated for GaN films grown concurrently on the vicinal and on-axis 6H-SiC substrates. Marked variations in EBx and c were observed, with a maximum shift of ΔEBx = 15 meV and Δc = 0.0042 Å. Threading dislocations densities of ~1010/cm2 and ~108/cm2 were determined for GaN films grown on vicinal and on-axis SiC, respectively. A 0.9% residual compressive strain at the GaN/AIN interface was observed by high resolution transmission electron microscopy (HRTEM). It is proposed that the on-axis SiC substrate does not offer a sufficient density of steps for defect formation to relieve the lattice mismatch between GaN and A1N and A1N and SiC.

Positron beam analysis has been performed on autodoped n-type, semi-insulating and Mg doped p-type epitaxially grown layers of GaN on sapphire. Doppler Broadening measurements clearly indicate the presence of vacancies in the intrinsically autodoped n-type GaN by an increase in the annihilation Doppler lineshape S-parameter of 1.04 relative to the value for the high resistivity sample. This value is typical for vacancy-type defects in compound semiconductors such as GaAs. Results of experiments with higher sensitivity to core-electrons are also presented. These two detector coincidence measurements yield information on the chemical environment surrounding the vacancies. The results are consistent with the presence of Ga vacancies in the autodoped n-type sample.

First-principles theoretical results can predict and explain a variety of materials properties of the nitride semiconductors. For n-type GaN, we summarize the current understanding about incorporation of unintentional donor impurities, as opposed to nitrogen vacancies. For p-type GaN, we discuss the cause of the limited doping levels, and the role of hydrogen. We describe the role of gallium vacancies in the yellow luminescence, and the interaction between these vacancies and donor impurities. Finally, we discuss our first-principles investigations of the atomic and electronic structure of heterojunction interfaces between the III-nitrides, and provide values for natural band lineups.

A model based on the valence-force-field (VFF) model has been developed specifically for the calculation of the irascibility gaps in III-V nitride alloys. In the dilute limit, this model allows the relaxation of the atoms on both sublattices. It was found that the energy due to bond stretching and bond bending was lowered and the solubility limit was increased substantially when both sublattices were allowed to relax to distances as large as the sixth nearest neighbor positions. Using this model, the equilibrium mole fraction of N in GaP was calculated to be 6×l0−7 at 700°C. This is slightly higher than the calculated results from the semi-empirical delta lattice parameter (DLP) model. Both the temperature dependence and the absolute values of the calculated solubility agree closely with the experimental data. The solubility is more than three orders of magnitude larger than the result obtained using the VFF model with the group V atom positions given by the virtual crystal approximation, i.e., with relaxation of only the first neighbor bonds. Other nitride systems, such as GaAsN, AlPN, AlAsN, InPN, and InAsN were investigated as well. The equilibrium mole fractions of nitrogen in InP and InAs are the highest, which agrees well with recent experimental data where high N concentrations have been produced in InAsN alloys. Calculations were also performed for the alloy systems with mixing on the group III sublattice that are so important for device applications. Allowing relaxation to the 3rd nearest neighbor gives an In solubility in GaN at 800°C of less than 6%. Again, this is in agreement with the results of the DLP model calculation. This result may partially explain the difficulties experienced with the growth of these alloys. Indeed, evidence of solid immiscibility has recently been reported. A significant miscibility gap was also calculated for the AlInN system, but the AlGaN system is completely miscible.

X-ray absorption and glancing angle reflectivity measurements in the energy range of the Nitrogen K-edge and Gallium M2,3 edges are reported. Linear muffin-tin orbital band-structure and spectral function calculations are used to interpret the data. Polarization effects are evidenced for the N-K-edge spectra by comparing X-ray reflectivity in s- and p-polarized light.

We present first-principles studies of the effect of biaxial (OOOl)-strain on the electronic structure of wurtzite GaN, and A1N. We provide accurate predictions of the valence band splittings as a function of strain, which may facilitate the interpretation of data from strained samples. The conduction and valence band effective mass tensors for A1N and GaN are also presented. The computed crystal-field and spin-orbit splittings in unstrained materials as well as the computed deformation potentials are in accord with available experimental data. We show that the numerically computed band energies can be excellently represented in terms of a 6-band k · p model. The present calculations are based on the first-principles pseudopotential method within the local-density formalism and include the spin-orbit interactions non-perturbatively.

The results of theoretical studies of the bulk and interface properties of nitrides are presented. As a test the bulk properties, including phonons of GaN at the Γ-point, are calculated and found to be in excellent agreement with the experimental data. At interfaces, the strain effects on the band offsets range from 20% to 40%, depending on the substrate. The AlN/GaN/InN interfaces are all of type I, while the Al0.5Ga0.5N on A1N zinc-blende (001) interface is of type II. Further, an interface similar to those used in the recent blue laser diodes is of type I and does not have any electronically active interface states. The valence band-offset in the (0001) GaN on A1N interface is -0.57 eV and the conduction band-offset is 1.87 eV.

We present a first-principles characterization of the initial stages of formation of AlN films on c-plane SiC substrates. Studying the competition between two-dimensional films and three-dimensional islands as a function of Al and N abundances, we find that a two-dimensional film can wet the surface in N-rich conditions. Ordered layer-by-layer growth can proceed to some extent on this wetting layer, and is improved by the formation of an atomically mixed interface which eliminates interface charge accumulation. Our results indicate that the stable AlN films grow in the (0001) orientation on the Si-terminated SiC(0001) substrate.

Full-potential linear muffin-tin orbital calculations were performed for LiGaO2 in different crystal structures in order to investigate the nature and origin of the cation ordering, structural relaxation and their effects on the band structure. It is found that the most important factor for the bonding is the exclusive occurrence of Li2Ga2 tetrahedra surrounding oxygen. Structures including LiGa3 and Li3Ga tetrahedra have significantly higher total energies and smaller bandgaps. The band-offset between GaN and LiGaO2 is estimated using the dielectric midgap approach.

We present a theoretical study of the optical properties of the GaN (1010) surface. We employed a semi-empirical tight-binding method to calculate the surface electronic structure. The parameters were adjusted to reproduce the correct band structure of the bulk wurzite GaN. These parameters were interpolated to the surface using Harrison’s rule. From the surface electronic structure the surface dielectric response was obtained. The dielectric response is analized in terms of surface-surface, and surface-bulk electronic transitions.

We present first-principles calculations of structural and electronic properties of heterova-lent SiC/GaN, SiC/AIN, and isovalent AIN/GaN heterostructures that are grown pseudo-morphically on (001) or (110) SiC substrates. For the polar interfaces, we have investigated reconstructed stoichiometric interfaces consisting of one and two mixed layers with lateral c(2 × 2), 2 × 1, 1 × 2, and 2 × 2 arrangements. The preferred bonding configurations of the reconstructed interfaces are found to be Si-N and Ga-C. With respect to vacuum, the valence band maximum is found to be highest in SiC and lowest in A1N. In these systems, the valence band offsets deviate substantially from the transitivity rule and depend sensitively on the microscopic details of the interface geometry. The SiC/AIN and AIN/GaN heterostructures are predicted to be of type I, whereas SiC/GaN heterostructure can be of type I or II.

The strain induced by lattice mismatch at the interface is responsible for the different value of the band discontinuities observed recently for the AlN/GaN (AlN on GaN) and the GaN/AlN (GaN on AlN) polar (0001) interface. We present a first-principles calculation of valence band offsets, interface dipoles, strain-induced piezoelectric fields, relaxed geometric structure, and formation energies. Our results confirm the existence of a large forward-backward asymmetry for this interface.