We present a detailed study of the optical properties of short-period ZnS-ZnSe strained-layer superlattices. These superlattices have been grown by metalorganic vapor-phase epitaxy. We show that the photoluminescence exhibits a low-energy tail due to localization of the exciton to interfacial potential fluctuations. A detailed analysis of the electronic structure has been performed using the envelope-function approach to obtain the valence-band and conduction-band envelope functions and band lineups. This was completed by a self-consistent calculation of the exciton binding energies. In this calculation the marginal conduction-band offset deduced from the standard envelope-function calculation is corrected by the electrostatic deformation produced by the presence of a localized hole wave function.

We have grown ZnSe/ZnS superlattices using low pressure MOVPE. The superlattices were grown onto a relaxed ZnSe buffer and were constituted of 45 periods. The well and barrier thicknesses were chosen from the calculation of critical thicknesses for coherent and free standing situations. Following this analysis we have grown samples with individual ZnSe, ZnS thicknesses of 3 to 9 monolayers. The optical properties of the samples were studied using photoluminescence, photoreflectance and reflectivity experiments. The results of the photoluminescence and photoreflectance experiments are consistent with a free standing model of the strain state, and a typical sketch of potential profile, describing the band structure of such superlattices is proposed. The photoluminescence linewidth is analyzed in terms of interface roughness. The asymmetry of the photoluminescence peaks is modelled using a density of state with a low energy exponential tail, due to the competition between radiative recombination and non-radiative recombination mechanisms. In some samples of lower crystalline quality, bound excitons were observed in the near band edge photoluminescence, which we attribute to impurity interdiffusion, which is favoured by crystalline defects.

We have grown high-purity ZnS layers by metal organic vapour-phase epitaxy onto (100) GaAs and (111) Si substrates using triethylamine dimethylzinc. As is usually reported, the morphology and the crystalline quality assessed by the optical Nomarski microscope and by double x-ray diffraction are similar on both substrates. However, we demonstrate for the first time, using reflectivity experiments along with a theoretical fit, that the crystalline quality of the sample deposited onto Si substrates is much poorer. This observation is interpreted in terms of antiphase defects related to the lack of polarity of the Si substrate.

The optical properties of ZnTe epilayers grown by low-pressure metal organic vapour phase epitaxy on GaAs and GaSb substrates are studied. The layers are grown by using the halide-free triethylamine dimethylzinc adduct and di-isopropyl telluride as zinc and tellurium precursors, respectively. A detailed analysis of the residual strain is offered as a function of layer thickness and substrate nature via reflectivity measurements performed at pumped liquid helium temperature. This is completed by an extensive analysis of near-band-edge photoluminescence spectra in order to discriminate the contribution of residual impurities. Using these precursors ZnTe layers with extremely low contamination rates are obtained.

Thin films of high-quality ZnS were grown on (001) GaAs and (111) Si substrates by metal-organic chemical-vapor deposition. 2-K reflectivity was used to analyze various samples grown at different temperatures. The spectra show two structures at 3.801 eV corresponding to the free exciton and at 3.871 eV corresponding to the E0+Δ0 transition. Theoretical reflectivity spectra were calculated using the spatial dispersion model with two oscillators. Thus, the transverse energies, the longitudinal transversal splitting, the oscillator strengths, and the damping parameters were determined for both the free exciton and the split-off exciton of ZnS. Photoluminescence measurements were also carried out using an excimer laser (308 nm). Both light-hole and heavy-hole excitons were observed, which allow for the determination of the strain that exists in the layer. This strain is due only to the difference between the thermal-expansion coefficients of the GaAs substrate and the ZnS epilayer, and was demonstrated to be a tensile strain. This paper also presents results on the band-gap energy variation as a function of the temperature and photoluminescence spectra when the excitation was varied from weak to very high densities (15 MW/cm2).

ZnSe and ZnTe single-crystal layers have been grown onto {100} GaAs substrates by low- pressure metalorganic vapor-phase epitaxy (LP-MOVPE) using the triethylamine-dimethylzinc adduct [DMZn(NEt3)] as the zinc precursor. The selenium and tellurium precursors were H2Se (5% in H-2) and di-isopropyltellurium (DiPTe), respectively. These two semiconductors have been grown with different VI/II molar ratios, at different growth temperatures, and with an overall growth pressure ranging from 40 to 400 Torr. Optimal growth parameters have been determined by optical means for the two materials. This information was then used to grow ZnTe/ZnSe strained-layer superlattices. We have studied structures grown on both ZnSe and ZnTe relaxed buffer layers which display a drastic dependence of the Stokes shift between photoluminescence and the optical bandgap on the nature of the buffer layer. Growth interruptions have been used to optimize the optical properties of the superlattices. Theoretical modeling of superlattice band structures has been performed using results of optical and structural characterizations. Observations of zone center transitions as well as excitons associated with the miniband dispersion of the superlattices are reported, in agreement with the theoretical calculation.

Zinc selenide (ZnSe) is now a material of established interest for the design of optoelectronic devices, following the achievement of both types of conductivity by M. Haase et al. (Appl. Phys. Lett., 59 (1991) 1272). The p-type doping of this material is now well established in molecular beam epitaxy, and work is in progress in many laboratories to transpose this to metal-organic vapour phase epitaxy (MOVPE). One of the problems to be solved in MOVPE, for the successful achievement of p-type doping, is that of obtaining a material with very low electron background concentrations, to avoid compensation effects. It has previously been demonstrated, by Jones, Wright and co-workers (Semicond. Sci. Technol., 6 (1991) A36; J. Cryst. Growth, 94 (1989) 441; J. Cryst. Growth, 104 (1991) 297) that the classical zinc precursor leads to prereactions and is invariably contaminated by trace amounts of iodine. To get rid of these problems, they employ a new precursor for zinc, that is dimethylzinc triethylamine adduct (Me(2)ZnEt(3)). Their results demonstrate that the material purity can be increased. Following Wright et al. (J. Cryst. Growth, 94 (1989) 441; J. Cryst. Growth, 104 (1991) 297), we have employed this original precursor to grow extremely high purity ZnSe. We investigated the growth conditions and determined the optimum growth parameters to be a growth temperature of 280 degrees C, a VI/II molar ratio of 5 and a reactor pressure of 40 Torr. The samples were characterised by low temperature photoluminescence and reflectivity experiments. We observed that the use of Me(2)ZnEt(3) led to a dramatic decrease of the donor bound exciton intensity in the 2 K photoluminescence spectrum, along with a correlated increase of the free exciton intensity. For the first time, free excitons and excited states up to the second (3s) were observed in reflectivity spectra. The spectra were theoretically fitted using a model similar to the one of Zheng et al. (Appl. Phys. Lett., 52 (1988) 287), in the local approximation, including a ''dead layer'' as prescribed by Hopfield and Thomas (Phys. Rev., 132 (1988) 563). Using this model, we accurately deduced the transition energy positions and oscillator strengths of the observed transitions. Moreover, we deduced that the damping parameter of the HH (1s) transition is 1.4 meV. Such a sharpness clearly indicates the high crystalline quality of the material.