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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 calculate the excitonic exchange interaction in alpha-GaN, and find it to be about 2 meV. A theoretical modelling of excitons is performed, and the oscillator strengths of radiative levels are calculated as a function of strain for (0001)-grown epilayers. In particular, we find that the strength of optical transitions are extremely sensitive to the residual strain field in view of the small value of the spin-orbit interaction. Our calculation shows agreement with low-temperature reflectance investigations in GaN epilayers grown on sapphire substrates.

Law pressure MOVPE has been used to grow a series of GaN epilayers, deposited onto (0001) sapphire substrates. All the growth parameters have been kept constant, except for the V/III molar ratio, which has been changed from sample to sample by changing the ammonia flow rate. The growth rate has been measured vs the V/III molar ratio at both low temperature (550 degrees C, growth of the buffer layer) and normal process temperature. A strong dependence of the growth rate vs the V/III molar ratio is found at both temperatures. Systematic photoluminescence and reflectivity experiments have been performed at 2 K on all samples. The photoluminescence is dominated by the donor bound exciton and donor acceptor pair are visible on samples grown under the lowest V/III molar ratio, indicating higher impurity incorporation. An optimum V/III molar ratio of 5000 is proposed, which corresponds to the highest growth rate observed and to the lowest full width at half maximum for the photoluminescence line. (C) 1997 Elsevier Science Ltd.