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.

Identification of heavy-hole and light-hole excitonic transitions is made by reflectivity and magnetocircular dichroism in strained metal-organic vapor-phase-epitaxy-grown ZnSe/GaAs epilayers with a thickness range between 0.10 and 0.78 mum at 4.5 K. The observed splitting of the exciton transitions due to the lattice mismatch is constant up to 200 K. It is explained by the very small variation (< 2%) of the lattice mismatch strain in this temperature range. Zeeman splitting of \+/- 3/2, +/- 1/2] and \+/- 1/2, -/+ 1/2] transitions are measured at 5.5 T and effective g values g3/2 = -0. 24 and g1/2 = 0. 5 7 are determined.

We have grown short-period ZnSe-ZnTe superlattices using low-pressure MOVPE. The influence of the growth parameters was investigated in detail. Combination of optical characterization with a tight-binding calculation gives a value of 200 meV for the strain-free valence band offset.

We present for the first time a detailed investigation of the growth of ZnTe layers deposited on GaAs substrates by low pressure metalorganic vapour-phase epitaxy using the new triethylamine dimethyl zinc adduct in combination with diisopropyl telluride as zinc and tellurium precursors respectively. The influence of the growth temperature (T(g)), the VI/II molar ratio, and the overall growth pressure (P) are studied. The successful growth of ZnTe is demonstrated at temperatures as low as 300-degrees-C. For growth temperatures ranging between 300 and 450-degrees-C, the growth rate is limited by the kinetics at the growing interface, and we measure an activation energy of 28 kcal/mol. At T(g) = 375-degrees-C, we found a linear variation of the growth rate with the overall growth pressure on the one hand, and with both zinc and telluride molar flows on the other hand. Low temperature reflectance and photoluminescence measurements were performed to characterize all the samples and are used to determine the optimal growth conditions: T(g) = 350-degrees-C, VI/II = 2, with P = 40 Torr. The stress experienced by the epilayers is investigated using the reflectance measurements. The relative intensity of the arsenic related bound exciton emission at 2.367 eV decreases as T(g) decreases. On the other hand, the I(b)1 unknown acceptor related bound exciton emission at 2.356 eV decreases linearly with the growth rate.