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.

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.

We calculate the spectroscopy of is excitons in GaN epilayers grown with biaxial stress on [10(1)over-bar0]-oriented M plane. In contrast to the growth on conventional C-plane substrates, for which we predicted five radiative excitons, we now demonstrate the existence of nine radiative transitions. A, B, and C excitons are now radiative for any polarization case, due to the crossed configuration of the wurtzite crystal field and the built-in strain one. In particular, the A exciton is significantly coupled to the electromagnetic field in pi polarization (E parallel to c). Strong in-plane anisotropy of the optical response is predicted and computed.

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.

We address combined utilization of temperature dependent reflectance, photoluminescence, and Raman spectroscopy measurements to optimize the structural and electronic properties of GaN epilayers deposited on sapphire. Last, we study residual strain fields in such epilayers.

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.