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].

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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.

Long-wavelength optical phonons of Ga1-xAlxN solid solutions have been identified in a wide compositional range by Raman spectroscopy. The Al and El polar phonon frequencies evolve continuously with x from one-member crystal to the other. The same behavior seems to hold true for the silent B-1 mode, which manifests itself by an interference with an unidentified continuum. Coupling of the longitudinal-optic (LO) modes associated with the two types of bonds, via the macroscopic electric field, is treated by a generalization of the dielectric model [D. T.Hon and W. L,. Faust, J. Appl. Phys. 1, 241 (1973)]. This approach accounts for the observed frequencies and supports the apparent one-mode behavior of the polar LO phonons. (C) 1998 American Institute of Physics.

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.

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 report here on the study of the optical properties of ZnCdSe-ZnSe quantum structures elaborated by metal-organic chemical vapour deposition on GaAs substrates. The band structure of these samples is experimentally investigated by means of photoluminescence and photoreflectance spectroscopy. This makes clear the relationship between strain, carrier confinement and excitonic effects. We have computed for the first time heavy- and light-hole excitons associated with ground and excited states sub-bands, in the context of a self-consistent two-parameter trial function. (C) 1997 Elsevier Science S,A.