It is now established that low-temperature grown buffer layers are needed to improve the structural and electronic properties of GaN layers grown on sapphire. We studied the recrystallization of AIN buffer layers grown by low-pressure MOVPE as a function of annealing time. The Warren-Averbach method was applied to the analysis of broadening and line shape of the (0 0 0 2) and (0 0 0 4) X-ray diffraction peaks. This method yielded a separation of the grain size distribution from microstrain effects. The evolution of the relative frequency distribution of the grain size with annealing is correlated with atomic force microscopy experiments. The distribution of the reflecting-planes orientation was determined by X-ray rocking-curve experiments, (C) 1998 Elsevier Science B.V. All rights reserved.

We have demonstrated a process for the fabrication of sub-micron structures using high resolution etching to transfer patterns by masking with a bilayer of Titanium and SiNX. Reactive ion etching (RIE) was performed on GaN epilayers grown on (0001) sapphire by metal organic vapour phase epitaxy (MOVPE). Various chemistries were used to etch GaN including: HBr, SiCl4/HBr, CH4/H-2, NH3/CH4/H-2 and CH4/H-2/O-2 I?Patterns including pillars, holes and gratings with feature: sizes ranging from 200 nm to 1 mu m were produced. In a CH4/H-2 plasma, quasi-vertical etching was achieved with sidewall angles greater than 81 degrees.

We report on investigations of the excitonic quantum efficiency in GaN in dependence of different buffer layer thicknesses and residual oxygen content in the crystal. The quantum efficiency of the free excitons rises with increasing buffer layer thickness and decreasing residual oxygen content. The influence of oxygen on the quantum efficiency is stronger than that of the buffer layer thickness. In general, the observed quantum efficiencies are below 20% indicating the strong impact of nonradiative relaxation and recombination processes in the excitonic range.

We address the strain state of GaN epilayers grown on C plane sapphire, C plane of 6H-SiC, (111) plane of silicon, and on A plane sapphire. We can propose the values of the six deformation potentials in GaN. We next examine the problem of exciton binding energies which are found to be near 25 meV. The A exciton has a binding energy which displays vanishingly small variations with strain. A tentative explanation of the scattering of the data in the literature is proposed in terms of the exciton polariton picture, ansi in terms of confined states in the photon-like branch.

We report on the realization of etched mirror facets in GaN cavities by chemically assisted ion-beam etching. The etching conditions are adjusted to obtain a high degree of verticality and smoothness. Optical pumping experiments and gain measurements are performed in etched GaN cavities of various geometries. Stimulated emission and lasing are observed. The study of the value of the gain at threshold as a function of the cavity length allows a determination of the reflection coefficient of the etched mirror. The measured value of 15% is in good agreement with the one expected for a perfect air-GaN interface. This demonstrates the high quality of the etched mirror facets. (C) 1998 American Institute of Physics.

We have investigated the thermal stability of GaN using Raman scattering. Noninvasive optical monitoring of structural damage to GaN by high-temperature anneals in nitrogen ambient has been demonstrated. Characteristic features in the Raman spectrum identify three thermal stability regimes. Thermal damage between 900 and 1000 degrees C results in the appearance of a broad Raman peak between the E-2 and A(1) (LO) phonon. For anneals at temperatures higher than 1000 degrees C emerging macroscopic disorder gives rise to distinct Raman modes at 630, 656, and 770 cm(-1). Below 900 degrees C no thermal damage has been observed. The evolution of the Raman spectrum of GaN with increasing annealing temperature is discussed in terms of disorder-induced Raman scattering. We find clear indications for a reaction at the GaN/sapphire interface for anneals higher than 1000 degrees C. (C) 1998 American Institute of Physics.

We have applied positron spectroscopy to study the formation of vacancy defects in undoped n-type metal organic chemical vapor deposition grown GaN, where the stoichiometry was varied. Ga vacancies are found in all samples. Their concentration increases from 10(16) to 10(19) cm(-3) when the V/III molar ratio increases from 1000 to 10 000. In nitrogen rich conditions Ga lattice sites are thus left empty and Ga vacancies are abundantly formed. The creation of Ga vacancies is accompanied by the decrease of free electron concentration from 10(20) to 10(16) cm(-3), demonstrating their role as compensating centers. (C) 1998 American Institute of Physics. [S0003-6951(98)01448-X].