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 study the free exciton fine structure and the contribution of propagating excitations in the lower-polariton and upper-polariton branches (LPB and UPB) in the 2K fluorescence spectrum of GaN on Al2O3. The latter effect is observed for both A and B lines. A line-shape fitting of the photoluminescence was made using four Lorentzian functions. The distribution of polaritons in the UPB(A), LPB(B) and UPB(B) are found consistent with a Boltzmann electronic temperature of 29 K when exciting the fluorescence with a He-C laser at 325 nm. The longitudinal-transverse splittings can be extracted from the splittings between energies of dips in the PL bands at 3489.4 and 3497.8 meV and the values of the transverse excitons are smaller than 2 meV. (C) 1998 Elsevier Science B.V. All rights reserved.

AlGaN is an important material in the design of nitride devices. However, little is known concerning its growth with high Al contents. We have studied the growth of AlxGa1-xN epilayers on c-face sapphire by low pressure MOVPE (76 Torr), using triethylgallium, trimethylaluminum and ammonia as precursors. The solid versus gas phase composition relationship was determined experimentally and was fitted using a kinetic model. Then the structural properties of the layers (x = 0-1) were studied, using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. We demonstrate that at high Al content, the buffer layer defects are replicated into the AlGaN layer. (C) 1997 Elsevier Science S.A.

We report here on the study of the theoretical optical properties of Ga based nitrides systems. The dipolar matrix elements and hole effective masses are presented as a function of the reciprocal wavevector fur strained bulk GaN. We compute then the A. B and C exciton binding energies for strained GaN/Al0.2Ga0.8N quantum structures and present the calculated optical density for a 50 Angstrom wide well. (C) 1997 Elsevier Science S.A.

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. Using X-ray diffraction, we have studied the dependency on temperature and annealing time of the recrystallization of AIN buffer layers grown by low pressure MOVPE. The Warren-Averbach method applied to the broadening and shape analysis of the (0002) and (0004) X-ray diffraction peaks has allowed us to separate grain size distribution from micro-strain effects. The obtained evolution of the relative frequency distribution of the grain sizes with annealing conditions is correlated with atomic force microscopy experiments (dynamic mode). The angular distribution of the c-axis of the grains is determined From X-ray rocking-curves experiments. X-ray reflectometry experiments and a simulation procedure have given us access to the roughness and the chemical composition or the sapphire-buffer layer interface and to the thickness and the roughness of the AlN grown. (C) 1997 Elsevier Science S.A.

We have investigated theoretically the optical properties of strained (001) ZnSe-(Zn, Cd)Se single quantum wells with a low cadmium content (similar to 10%). Due to the band offset and strain energy, the electron-to-light-hole transitions are marginally type I. We compute the dispersion relations and joint density of states for a typical structure away from Gamma and show that the degeneracy call occur between lh(1) and hh(3) sub-bands in a large area of the Brillouin zone. This can cancel the selection rules for interband processes. (C) 1997 Elsevier Science S.A.