We report a low temperature and room temperature photoluminescence (PL) study of a series of two-dimensional arrays of holes and pillars in a triangular lattice arrangement, fabricated in a 1.8 mu m thick GaN/sapphire epilayer. The sample was patterned by electron beam lithography and reactive ion etching in a CH4/H-2 plasma. Such structures with periodic variation of the dielectric constant may give rise to photonic crystals that can create a range of forbidden frequencies called a photonic bandgap. We have used the PL peak to evaluate the damage introduced onto the semiconductor by the etching procedure, and to determine the magnitude of the relaxation of the strain in the GaN film. Theoretical computations predict the photonic bandgap positions for future experimental investigations.

We have developed a model which describes the incorporation of aluminum in the solid phase of Ga1-xAlxM where M is As or N, taking into account the diffusion of the group In species through the gas phase in the vicinity of the substrate. In order to compare our model with experimental data, GaAlAs and GaAlN alloys were grown by low pressure (76 Torr) MOVPE. Art excellent agreement is obtained in the case of GaAlAs alloys, considering TEGa and TMA1 as diffusing species in a hydrogen ambient. For AlGaN alloys, we discuss here the nature of the species involved in the diffusion mechanism since the precursors pyrolysis occurs at lower temperature than the growth temperature. We thus use this model to discuss the nature of the diffusing species, which are clearly not TEGa and TMA1, but rather unstable species, like diethyl and monomethyl, resulting: from the thermal decomposition of the organometallics.

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 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.