Raman measurements on ZnSe/GaAs heterostructures show unusual behaviour of the optical phonon modes from ZnSe. High epitaxy rates are responsible for both poor structural quality at the interface and Fermi level pinning by carrier traps, The first effect gives rise, beyond a critical thickness, to polycrystalline growth associated with the activation of a theoretically forbidden TO-ZnSe mode, The second effect induces a spectacular enhancement of the interfacial LO-ZnSe mode strength, The competition between the latter local electric field effect and absorption in the upper disoriented part of the layer may result in the quasi-total extinction of the LO-ZnSe mode. (C) 1997 John Wiley & Sons, Ltd.

Wurtzite GaN on (0001) sapphire is studied by means of reflectance and magnetocircular dichroism up to 5.5 T at 2 K. This very powerful technique allows us to determine the Zeeman splittings to be about 0.05 meV/T for the X-A and X-C excitons and almost zero for the X-B exciton. Reflectance and dichroism are interpreted with a model of Gaussian dispersion of the excitonic energies and are in excellent agreement with previously proposed band-edge models.

When additional carriers are introduced in a material with a non uniform concentration, they tend to diffuse on a scale given by their diffusion length. This parameter can be measured by different methods. Depending on the conditions, different values can be found as the recombination mechanisms differ. In this paper, we present the situation in GaN with various experiments including the photocarrier grating method, photoluminescence and the spectral response in photoconductors. We show that the diffusion length varies from 0.1 mu m to a few mu m depending on experimental conditions. The interpretation is given based on the diffusion equations and on the analysis of the recombinations. (C) 1997 Elsevier Science S.A.

Photostimulated emission (PSE) of epitaxial GaN layers is readily obtained at photoexcitation densities of order MW cm(-2) at temperatures up to room temperature. We report surprising spectral differences between PSE obtained using surface and edge emission geometries for a particular sample. While the peak energy of PSE from the surface of this sample is downshifted from the GaN band edge by only a small amount, the photon energy of the edge-emitted light is reduced by similar to 100 meV. Exactly the same downshift is observed in the edge emission spectrum of a commercial Nichia light emitting diode. The origin of the anomalous PSE peaks revealed ill these measurements is discussed with reference to recent literature. (C) 1997 Elsevier Science S.A.

The epitaxial growth of both cubic and hexagonal GaN epilayers is considered here with the aim of comparing their physical properties. In particular, the growth mechanisms at the first stages of growth will be dealt with together with the quality of the growth front. The optical characteristics of the epilayers will be compared by reference to the structure of the defects present within the different types of layers. (C) 1997 Elsevier Science S.A.

We present the fabrication of mirror facets by chemically assisted ion beam etching. Optical pumping experiments are performed to characterize the facets. The reflection coefficient is measured and its value confirms the high quality of the etched mirrors. Optical resonators are then realized in a double heterostructure composed of InGaN and GaN. Stimulated and laser emissions are investigated in this heterostructure and we show that population inversion can occur in both materials. (C) 1997 Elsevier Science S.A.

Wurtzite GaN on (0001) sapphire is studied by mean of reflectance and magnetocircular dichroism up to 5.5 T at 2 K. This very powerful technique allows us to determine the Zeeman splittings, for the first time to our knowledge, to be about 0.05 meV/T for the X-A and X-C excitons and almost zero for the X-B exciton. (C) 1997 Elsevier Science S.A.