GaN layers grown by MOCVD or by MBE on (0001) sapphire have been characterised by transmission electron microscopy (TEM). We make a review of the different crystallographic structures found in theses GaN layers. We comment shortly on the nitridation of the sapphire and the structure of the buffer layer (BL). We point out that the roughness of the BL can be an important parameter for releasing the residual strain of the GaN layer. We compute the Keating energies of the main inversion domain boundaries (IDBs) and translation domains boundaries (TDBs) observed in some GaN layers. The observed structures correspond to the lowest energy models. Perfect dislocations have Burgers vectors equal to a, a + c and c. The dislocation lines are generally parallel to the c-axis. a-Edge dislocations are generally not dissociated and we propose an atomic model for them. Screw dislocations with a Burgers vector equal to c, can 'open and close' during growth leaving holes (the so-called nanopipes) in the structure. (C) 1997 Elsevier Science S.A.

The growth mechanisms of GaN grown on a GaN buffer deposited onto sapphire substrates are studied here. The growth rate was measured at different temperatures versus ammonia Row and was found to decrease with increasing NH3 flow. This surprising behavior is modeled in terms of competitive adsorption of species on the growing surface. Very good agreement is obtained between the model and the experimental data. (C) 1997 American Institute of Physics.

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.

In this paper we examine a series of four GaN epilayers grown by MOVPE on sapphire substrates with different AlN buffer layer thicknesses. We examine the effect of the buffer layer thickness on the physical and optical properties of the samples via optical microscopy, cathodoluminescence imaging, photoluminescence, and cathodoluminescence spectroscopy. While the morphological and optical properties of all the films (excepting that with the thinnest buffer layer of 30 nm) are good, i.e. the films are smooth and the luminescence is dominated by excitonic luminescence, a number of circular island like features are observed in all the films whose density decrease with increasing buffer layer thickness. A large circular island present on the sample with the thinnest buffer layer and surrounded by cracks in the [11 (2) over bar 0] directions, displays some interesting acceptor related luminescence. (C) 1997 Elsevier Science S.A.

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.