We report on the observation of free exciton photoluminescence associated with low and upper-polariton branch in GaN on sapphire. This is observed for both A and B lines. (C) 1997 Published by Elsevier Science Ltd.

We report a detailed optical study of ZnSe-based graded index separate confinement heterostructures. These structures were grown by metalorganic vapor phase epitaxy and are composed of either one or two Zn0.79Cd0.21Se central well(s) embedded between two ZnCdSe barriers which cadmium composition varies linearly from 5% near the wells to 0% at the end of the barriers. 2K photoreflectance and reflectivity experiments allow the observation of excitonic transitions involving the third electron and heavy hole confined states. The temperature dependence of the photoluminescence lines under in-well resonant excitation conditions (E(exc) = 2.661 eV) shows that the thermal quenching of the photoluminescence line is ruled by nonradiative recombinations on defects localized at the heterointerfaces at low temperature and by the thermal escape of the minority carriers at higher temperatures. Under above-barrier excitation conditions (E(exc) = 3.814 eV), the temperature dependence of the photoluminescence line from the well shows a strong influence of the mechanism of diffusion of the carriers from the barriers to the well.

We report on the growth of ZnS-ZnSe superlattices by MOVPE. These superlattices were characterized by optical spectroscopy. We have calculated the exciton binding energy in ZnSe-ZnS quantum wells in the context of the variational approach using models of varying degree of sophistication.

Graded index-separate confinement heterostructures (GRIN-SCH) based on ZnCdSe and ZnSe wide band gap semiconductors have been grown by low pressure MOVPE on (100) GaAs substrates. First, we have paid attention to the growth of thick ZnCdSe layers. A first study is carried out using selenium hydride, dimethyl cadmium and two dimethylzinc adducts as Se, Cd and Zn precursor respectively. This combination leads to intense prereactions and poor layer morphology. To limit this problem we have then used the tetrahydrothiophene:dimethylcadmium adduct. GRIN-SCH structures grown using the two Cd metalorganics are studied using photoluminescence. We have made a detailed study on the influence of the temperature on the carrier trapping and detrapping in the structures.

Gallium nitride has been grown using low pressure MOCVD and ammonia and triethylgallium as precursors. A wide range of operating conditions have been investigated : the temperature was varied between 950 and 1050 degrees C and the V/III molar ratio was changed from 1000 to 10000. The influence of the growth parameters on the optical quality is reported here. For optimized epilayers, low temperature reflectance data have been taken and analyzed in the context of a multi-polariton model using a non local description of the dielectric constant The energy positions of the resonances have been analyzed in terms of joint contributions of the actual wurtzite structure in the one hand and of residual stress fields in the other hand This enables us to give a relationship between the optical transition energies and the in-plane biaxial stress in the layers.

The optical properties of GaN epilayers grown by metal-organic vapor-phase epitaxy on (0001)-oriented sapphire are investigated by means of photoluminescence, reflectance, and differential spectroscopy. We obtain quantitative information about the intrinsic or extrinsic nature of the 2 and 300 K photoluminescence features. From detailed investigations of the reflectance properties of these layers we can quantify the residual strain field in these layers and determine the GaN deformation potentials. Comparison of these values with quantities measured on other semiconductors with wurtzite symmetry is also addressed. Last we utilize photoreflectance spectroscopy to measure exciton binding energies.

We have studied the stimulated emission of MOVPE-grown quantum wells of Zn0.78Cd0.22Se, sandwiched between two thicker, zinc-richer layers. The gain of these samples has been measured, as a function of temperature, using the variable strip-length method. Its mechanism has been shown to be the radiative recombination of an inhomogeneously broadened exciton system.