Wurtzite GaN grown onto an A-plane sapphire exhibits a uniaxial strain because the thermal expansion coefficient of the substrate is anisotropic. Measuring the dependence of the transition energies and of the oscillator strengths on the polarization of light, we deduced the value of the spin-exchange energy in wurtzite GaN: y approximate to 0.6+/-0.1 meV.

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.

Time-resolved photoluminescence studies have been performed on GaN/Ga0.93Al0.07N multiple quantum wells grown by molecular-beam epitaxy on a sapphire substrate. Radiative recombinations of excitons in both the wells and barriers exhibit average decay times of similar to 330 ps, at T = 8 K, with a significant spectral distribution of times. This distribution is interpreted in terms of localization of carriers by potential fluctuations due to alloy disorder and to well width and depth variations. The temperature dependence of the radiative lifetime tau(r). Of excitons in the wells is deduced from the measurement of both the photoluminescence decay time and intensity. We find that tau(r) increases linearly with temperature dependence partial derivative tau(r)/partial derivative T=20.5+/-0.7 ps K-1, for T <60 K. From this result, we deduce the radiative lifetime of free excitons in the low-temperature limit (related to the oscillator strength) to be of 2.4 ps, i.e., half one order of magnitude smaller than for GaAs/Ga-Al-As quantum wells. [S0163-1829(98)50916-2].

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 perform a careful study of the combined influence of the quantum well width and light polarization on the optical properties of strained GaN-AlGaN quantum wells oriented along the [0 0 0 1] direction. This is made in the context of a six-band envelope function approach together with utilization of the most recent experimental values of crystal field splitting, spin-orbit interaction and deformation potentials. We calculate the holes confinement, exciton binding energies and absorption spectra. We show that under sigma polarization conditions for chosen quantum wells, the only noticeable features can be attributed to enhanced A and B excitons; the C exciton process cannot be stimulated due to inter-valence band mixings. (C) 1998 Elsevier Science B.V. All rights reserved.

Numerical simulation of light reflection from a lambda/2 GaN microcavity with Ga0.8Al0.2N/Ga0.5Al0.5N Bragg mirrors grown on the A surface of Al2O3 revealed a Rabi splitting of the order of 50 meV and remarkable optical anisotropy. These effects are originated from the giant exciton oscillator strength in GaN and a pronounced uniaxial strain in the structure. (C) 1998 American Institute of Physics.

(Al,Ga)N/GaN quantum wells have been studied by temperature-dependent luminescence and reflectivity. The samples were grown by molecular beam epitaxy on (0001) sapphire substrates, and well widths were varied from 3 to 15 monolayers (ML's) with a 2-ML increment, thus providing a reliable data set for the study of the well width dependence of transition energies. The latter shows a strong quantum confined Stark effect for wide wells, and an internal electric-field strength of 450 kV/cm is deduced. X-ray diffraction performed on the same samples shows that the GaN layers are nearly unstrained, whereas the (AI,Ga)N barriers are pseudomorphically strained on GaN. We conclude that the origin of the electric field is predominently due to spontaneous polarization effects rather than a piezoelectric effect in the well material. [S0163-1829(98)50944-7].