InxGa1−xN multiple-quantum-well laser diode structure, which lased at 405 nm, was shown to have atomically-flat interfaces between each layer. Nanometer-probe compositional analysis showed that InN mole fraction, x, in the wells and barriers are approximately 6 % and 2 %, respectively, which agreed with the result obtained from high-resolution x-ray diffraction measurement. The Stokes-like shift (SS) at 300 K was 49 meV, being approximately 65 % of the luminescence linewidth. The localization depth, E0, of qunatum-well (QW) excitons was estimated to be 35 meV at 300 K though the compositional fluctuation in the well was as small as 1 % or less (detection limit) within adjacent 20-30 nm lateral length scale. Since the well thickness fluctuation is insufficient to reproduce SS or E0, effective bandgap inhomogeneity is attributed to be due to large bandgap bowing in InGaN. The spontaneous emission was thus assigned as being due to the recombination of QW excitons weakly localized in exponential tail-type potential minima in the QW. The size of localization is smaller than the quantum-disk [M. Sugawara, Phys. Rev. B 51, 10743 (1995)]-size. Such small bandgap inhomogeneity can be leveled by injecting high density carriers under lasing conditions, which can explain the general experimental finding that the quantum efficiency decreases with increasing carrier density in InGaN QW devices due to free carrier trapping into threading dislocations.

We have studied the properties of Be dopants in GaN using first principles calculations. Substitutional Be on a Ga site acts as an acceptor, but interstitial Be poses a potential problem because of its low formation energy and donor character. We study the diffusion of interstitial Be and find it to be highly anisotropic. We also study the formation of complexes between substitutional and interstitial Be, and between substitutional Be and hydrogen. We have calculated the Be-H vibrational modes to aid in experimental identification of such complexes.

The growth of GaNP using laser-assisted metalorganic chemical vapor deposition (LA-MOCVD) was carried out for the fabrication of a light-emitting diode (LED). We used an Ar-F laser in order to decompose the source gases at lower temperatures. Trimethylgallium (TMG), ammonia (NH3) and tertialybuthylphosphine (TBP) were used for the growth. GaNP growth was carried out at different temperatures. After that, annealing was carried out at 1273-1373 K to improve the crystal quality.

As a result, N-rich GaNP could be grown at 1123-1223 K. The surface morphologies of GaNP were improved when the growth temperature was increased to above 1173 K. We investigated the photoluminescence (PL) of GaNP. The band-edge emission of GaNP was observed at 77 K upon applying thermal annealing at 1323 K. This peak shifted to about 0.2 eV compared with the GaN band-edge emission. Furthermore, a GaNP LED was fabricated and the electoluminescence spectra were investigated. The band-edge emission at 420 nm was observed.

The effect of substrate preparation on the sublimation growth of AlN on 6H-SiC was investigated at about 1800°C and 400 torr. Short and long-time sublimation growths of AlN indicated that the nucleation, growth mode, and defects formed depended on the substrate surface preparation. Growth on an off-axis 6H-SiC substrate with 6H-SiC epilayer was in the step flow growth mode in contrast to the island growth mode on as-received substrates, while the 2-D growth was achieved on substrates first coated with an AlN epitaxial layer. Cracks due to the lattice and mainly large thermal expansion coefficient mismatch were always observed in the deposited AlN crystal, as characterized by SEM and optical microscopy.

We present a systematic theoretical study of several III-nitride (110) surfaces based on accurate parameter-free, self-consistent total energy and force calculations using the density functional theory, the local density approximation (LDA), as parametrized by Perdew and Zunger, and the generalized gradient approximation (GGA), as proposed by Perdew, Burke, and Ernzerhof, for the exchange-correlation term; we use the Full Potential Linear Augmented Plane Wave (FPLAPW) approach (WIEN-97 code) associated with the slab supercell model to simulate the (110) surface. We studied BN, AlN, GaN, InN and compared the theoretical results as related to the use of the LDA and the GGA. We conclude that although the results for both approximations are similar, differences in structural parameters may be as large as 10%.

We have synthesized ZnMgO alloy of wurtzite (Mg content equals to 0.0=0.34) and cubic (Zn content equals to 0.0- 0.18) phases using nonequilibrium pulsed laser deposition method. Epitaxial films of ZnMgO wurtzite structure have been grown on (0001) sapphire substrates. Using JEOL-2010 field-emission transmission electron microscope equipped with STEM and Gatan image filter, we can perform atomic structure, STEM-Z, electron energy loss spectroscopy and imaging simultaneously. Such studies on the ZnO/MgZnO superlattices provide first direct evidence of phase-separation in the range 3 nm.

Although a tremendous amount of work has been done these last years on the nitride semiconductor system, a lot is still to be understood regarding the growth mechanisms of GaN. The standard GaN MOCVD growth process includes the low temperature deposition of a nucleation layer, followed by an anneal at high temperature, and the GaN layer is then deposited. The number of process parameters which can be used to tune the growth is very large (temperatures, times, thicknesses, molar flow rates and ratios …) and, due to the coupling between them, the role of each one is not clearly understood. In this paper, we present systematic series of growth experiments, where in-situ reflectance monitoring was used and correlated to ex-situ optical characterization of the samples by photoluminescence at low temperature (2K). Here, we demonstrate that the nucleation layer and its annealing have a determining effect. The nucleation layer growth temperature was not found to be a very sensitive parameter, while the amount of re-crystallization is. Surprisingly, the amount of ammonia present in the gas phase has a determining effect on the recrystallization behavior of the nucleation layer. Another interesting point is the sensitivity versus growth temperature for the main GaN layer, which was found to affect the initial stages of the growth in a drastic manner when changed by only 5°C. In-situ reflectance allowed us to tune precisely our process and to obtain GaN layers with 500 cm2/Vs electron mobility at room temperature and photoluminescence fwhm of 1.7 meV at 2K for the donor-bound exciton.

Photoanodically grown Ga2O3 layers were characterized with respect to their suitability as gate dielectrics for GaN based MOSFET Device applications. The Ga2O3 layers were produced in a photoelectrochemical cell using aqueous solutions of KOH. IV characterization of MOS structures show insulating behavior of the oxide layers and CV measurements indicate a small density of states at the oxide/GaN interface. Integrating the wet chemical oxide growth in a MOSFET device fabricating process includes tungsten as gate metal together with H2O2 as etching solution for the gate metal. Source/drain areas were made free of oxide by the alkaline developer of a conventional lithographic step and metallization was done by using the liftoff technique. MOS structures show no inversion mode but strong depletion in reverse biasing mode.

This paper presents UV imaging results for a 256×256 AlGaN Focal Plane Array that uses a back-illuminated AlGaN heterostructure p-i-n photodiode array, with 30×30 μm2 unit cells, operating at zero bias voltage, with a narrow-band UV response between 310 and 325 nm. The 256×256 array was fabricated from a multilayer AlGaN film grown by MOCVD on a sapphire substrate. The UV response operability (>0.4×average) was 94.8%, and the UV response uniformity (σ/μ) was 16.8%. Data are also presented for back-illuminated AlGaN p-i-n photodiodes from other films with cutoff wavelengths ranging between 301 and 364 nm. Data for variable-area diagnostic arrays of p-i-n AlGaN photodiodes with a GaN absorber (cutoff=364 nm) show: (1) high external quantum efficiency (50% at V=0 and 62% at V=-9 V); (2) the dark current is proportional to junction area, not perimeter; (3) the forward and reverse currents are uniform (σ/μ=50% for forty 30×30 μm2 diodes at V=−40 V); (4) the reverse-bias dark current data versus temperature and bias voltage can be fit very well by a hopping conduction model; and (5) capacitance versus voltage data are consistent with nearly full depletion of the unintentionally-doped 0.4 μm thick GaN absorber layer and imply a donor concentration of 3-4×1016 cm−3.

The frequencies of the E2(high), A1(LO), A1(TO), E1(TO) and E1(LO) phonons of singlecrystalline bulk AlN were measured using micro-Raman spectroscopy over a temperature range from 10K to 1275K. A modeling of the temperature dependence of the AlN phonon frequencies considering the thermal lattice expansion and two-phonon decay mechanisms gave results in good agreement with the experimental data. At temperatures in excess of ∼300K an approximate linear shift of the phonon frequencies with temperature was found. In this high temperature regime, we determined a frequency shift of the E2(high) phonon of (-2.22 ± 0.02) ×10−2cm−1/K, which is very similar to values reported for bulk GaN. This suggests that similar parameters will be suitable for AlxGa1−xN alloys, commonly used in high-power high-frequency electronic devices. The results provide the basis for non-invasive local temperature monitoring in highpower III-nitride devices using micro-Raman scattering techniques.

Spectroscopic ellipsometry (SE) is employed to study the optical properties of GaAs1−yNy [0% ≤ y ≤ 3.7%] single layers for photon energies from 0.75 eV to 4.5 eV and for wavenumbers from 100 cm−1 to 600 cm−1. We provide parametric model functions for the dielectric function spectra of GaAsN in both photon energy ranges. The model functions for photon energies from 0.75 eV to 4.5 eV excellently match dielectric function data obtained from a numerical wavelength-by-wavelength inversion of the experimental data. Criticalpoint analysis of the ellipsometric data is performed in the spectral regions of the fundamental band gap and the critical points E1 and E1+δ1. The band-gap energy is red shifted whereas the E1 and E1+δ1 transition energies are blue shifted with increasing y. For y ≤ 1.65% the observed blue shift of the E1 energy is well explained by the sum of the effects of biaxial (001) strain and alloying. The GaAsN layers show two-mode behaviour in the infrared spectral range (100 cm−1 to 600 cm−1). We detect the transverse GaAs- and GaN- sublattice modes at wavenumbers of about 267 cm−1 and 470 cm−1, respectively. The polar strength of the GaN TO mode increases linearly with y. This effect can be used to monitor the nitrogen composition in GaAsN layers.

The development of a self-aligned fabrication process for small emitter contact area (2×4 um2) GaN/AlGaN heterojunction bipolar transistors and GaN bipolar junction transistors is described. The process features dielectric-spacer sidewalls, low damage dry etching and selected-area regrowth of p-GaAs(C) on the base contact or n-GaN/AlGaN on the emitter contact. Series resistance effects are still found to influence the device performance.

The development of new chemically based growth techniques has opened the range of possible GaN applications. This paper reviews some of the challenges in the chemically based growth of GaN and related materials. Ammonothermal-based growth, hydride vapor phase epitaxy and metal organic vapor phase epitaxy (MOVPE) are chemically complex systems wherein the underlying mechanisms of growth are not well understood at present. All these systems require substantial experimental and theoretical efforts to determine the nature and kinetics of GaN growth. In the case of metal organic vapor phase epitaxy, the application of computational techniques based on density functional theory have augmented the more conventional experimental approaches to determining the growth chemistry. These chemical reaction schemes, when combined with computational thermal-fluid models of the reactor environment, provide the opportunity to predict growth rates, uniformity and eve ntually materials properties.

Wide bandgap group-III nitride semiconductors are currently experiencing the most exciting development. High brightness blue and green light emitting diodes (LEDs) are commercialized, and UV and blue laser diodes (LDs), high-speed transistors (TRs) and UV photodetectors (PDs) with low dark current, which will be able to operate in harsh environments, have been demonstrated. In this paper, renaissance and progress in crystal growth and conductivity control of nitride semiconductors in the last quarter century are reviewed as the groundwork for all of those high-performance devices. My personal history of nitride research will be also introduced.

The effects of inductively-coupled plasma etching on the quality of ohmic contacts to etched n- GaN are reported. A high density chlorine plasma and a range of rf bias power levels (incident ion energies) are employed to etch n-GaN films with varying doping levels. Resulting plasma damage degrades contact ohmicity in all cases examined. At moderate levels of rf bias power the extent of this degradation for nitride layers with lower doping levels (mid-1017 cm−3) is similar to the degradation for nitride layers with higher doping levels. With increased rf bias, the degradation becomes more severe in films with higher carrier concentration. Annealing at 700 °C rapidly improves the contact quality. Studies of cumulative annealing time, up to 160 seconds, show that the improvement in contact quality takes place within the first twenty seconds of annealing.

We have synthesized new cubic phase of ZnxMg1−xO alloy, which can be grown epitaxially on MgO (100) by lattice-matching epitaxy, and on Si (100) substrate by our domainmatching epitaxy for integration with silicon microelectronic devices. Cubic ZnxMg1−xO films on MgO (100) and Si (100) substrates were grown using a rotating target in a single chamber “insitu” pulsed-laser deposition system. Integration of ZnxMg1−xO films with silicon was accomplished via titanium nitride (TiN) buffer layers where four lattice constants of TiN match with three of the silicon during epitaxial growth via domain epitaxy. Rutherford backscattering/ion channeling techniques were used to determine chemical composition and crystalline quality of the films for x = 0.0-0.18. Detailed X-ray diffraction and transmission electron microscopy studies confirmed the epitaxial nature of ZnMgO/MgO (100) and ZnMgO/TiN/Si (100) heterostructures, and showed the formation of the Mg2−xZnxTiO4 spinel at the interface with TiN. Using optical transmission measurements, the band gap of cubic Zn0.18Mg0.82O film was estimated to be approximately 6.7 eV. The potential use of these alloys for optical devices in the ultraviolet range is discussed.

Using the density-functional theory within the Full Potential Linear Augmented Plane-Wave (FP-LAPW) method, we have calculated, in this work, the equation of state, the effective charges and the phonon dispersion along [100] and [111] directions for the cubic Indium Nitride(c-InN), including hydrostatic strains dependence. A good agreement with the micro-Raman scattering experiment is obtained for the phonon modes at ã. Our results show that the apparent divergence between the known experimental results is a consequence of hydrostatic effects on the sample due to differences of the used Raman methods.

We have theoretically studied optical properties of a quantum well (QW) in which the well region is constructed from a binary alloy semiconductor A1−xBx in the coherent potential approximation (CPA). A tight binding model is used for a single particle (electron, hole, Frenkel exciton) in the well composed by a rectangular array of NxxNyxNz sites. The effect of the diagonal randomness is reduced to the coherent potential σ(E), which is assumed to be the same for all sites, and is selfconsistently determined with the average Green's function. For a slab (∞, ∞, Nz) and wire (∞, Ny, Nz), the density of states (E) is composed of Nz (or NyxNz) subbands, each shows the two (one)-dimensional van-Hove singularity. When x (or 1−x) is small, a B (A) impurity-band always appears at the lower (higher) energy side of the lowest (highest) host-band. The change of (E) and the absorption spectrum by changing the well-width and the dimensionality is discussed in detail.

Epitaxial Al1−xInxN films (thickness ∼150 nm) with 0 ≤ × ≤ 1 have been grown by Plasma Source Molecular Beam Epitaxy on Sapphire (0001) at a low substrate temperature of 375°C and were characterized by reflection high energy electron diffraction (RHEED), x-ray diffraction (XRD), and atomic force microscopy (AFM). Both RHEED and XRD measurements confirm the c-plane growth of Al1-xInxN films on sapphire (0001) with the following epitaxial relations: Nitride [0001] ∥ Sapphire [0001] and Nitride < 0110 > ∥ Sapphire <2110>. The films do not show any alloy segregation. However, the degree of crystalline mosaicity and the compositional fluctuation increases with increasing In concentration. Further, AFM measurements show an increased surface roughness with increasing In concentration in the alloy films.