We report on a quantitative analysis of the band gap of hexagonal GaN epilayers in terms of the joint contributions;of the actual wurtzite symmetry on the one hand and of residual strain fields on the other hand. This investigation leads to revision of the previous modelings based on quasicubic descriptions of the valence-band physics and gives Delta(1)=10+/-0.1 meV, Delta(2)=6.2+/-0.1 meV, and Delta(3)=5.5+/-0.1 meV. Last we propose a set of deformation potentials for the hexagonal GaN semiconductor.

A series of In0.14Ga0.86As/GaAs quantum well structures have been grown by metal organic chemical vapor deposition (MOCVD). The measured dependence of photoluminescence (PL) energies on well width is compared with calculation. By the energy shift, line width, and intensity change of PL spectroscopy, critical layer thickness has been identified. The critical layer thickness obtained for MOCVD-grown material was found in agreement with theoretical value, but is smaller than material grown by molecular beam epitaxy.

Graded Index Separate Confinement Heterostructures have been grown by MOVPE using (Zn,Cd)Se and ZnSe wide bandgap semiconductors. These structures are composed of a deep Zn1-xCdxSe (x<0.23%) central well embedded between two thick (Zn, Cd)Se graded layers within which the cadmium composition varies from 0 up to 10% on one side of the well, and from 10 down to 0% on the other side. Both carriers and photon confinement occur in such structures making them suitable for blue-green stimulated emission. Reflectance and photoreflectance data, taken at 2K on a series of samples of different designs allowed us to observe excitonic transitions up to the third quantum number. The structures exhibit a strong photoluminescence line due to the recombination of the carriers in the quantum well. The photoluminescence intensity is analyzed as a function of the temperature and displays a strong thermal quenching. This quenching is due to an increase of the non-radiative recombinations through defects at low temperature and to the thermal escape of the carriers above 40-60K. The value of the activation energy for thermal escape shows that this mechanism is unipolar and concerns the heavy holes.

We report the observation of nonlinear optical properties of ZnSe-ZnTe superlattices under photo-injected carriers, at pumped liquid helium temperature. Identification of several transitions measured by transmission on samples grown by metalorganic vapour phase epitaxy and etched away from the GaAs substrate is made in the context of the envelope function approach. This reveals that the conduction to valence band line-ups are type II in these samples. This situation is invoked in order to interpret the efficiency of the exciton screening.

We have studied the temperature dependence of the photoluminescence of tellurium-doped ZnS-ZnSe superlattices. By comparing the reflectivity and photoluminescence data, we were able to identify both free exciton recombination and self-trapped exciton band energy split by 90 meV. The behaviour of the photoluminescence with temperature results from trapping versus detrapping effects which are ruled by an activation energy of 80 meV. This behaviour is analysed in the context of a configuration coordinate diagram.

Raman measurements are used to probe the built-in strain due to the large lattice mismatch in two short-period ZnSe-ZnTe superlattices. In the largest period structure, local information on confinement and strain effects in the different layers is deduced from the LO phonon frequencies. An estimate of the in-plane strain gives evidence of a partial relaxation of both layers; in this structure, folded acoustical phonons are clearly observed under resonant conditions. The Raman spectra of the shortest period superlattice are characterized by a unique structure assigned to an unusual propagative mode resulting from the overlap of the optical bands of the two constituents.

Different ZnSe/GaAs epilayers prepared by low-pressure metal organic vapour-phase epitaxy under the conditions given by Nurmikko and Gunshor (1993 Physica B 185 16) are studied through their low-temperature transmission, reflection and photoluminescence spectra. Several samples are etched out from their GaAs substrate and the properties of as-grown surfaces and etched surfaces are compared. The etched samples seem to be free of in-plane strain and to behave like bulk ZnSe. The free-exciton excited state (n = 2) is clearly observed in transmission, reflection and luminescence spectra. We also studied the thermal quenching of the photoluminescence in order to confirm the origin of some bound-exciton centres.