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.

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 Show that many-body effects and band-gap renormalization can be easily produced in strained-layer quantum wells with internal built-in piezoelectric fields, under photoexcitation. Our observation was made at low temperature by comparing the behavior of Ga0.92In0.08As-GaAs strained-layer quantum wells grown along the (111) and (001) directions and tuning the densities of photoinjected carriers over several decades. Comparison between experimental data and the results of Hartree-calculations including the charge space effects-reveals that many-body interactions are photoinduced.

We show that manybody-effects and bandgap renormalization can be easily produced in strained-layer quantum wells with internal built-in piezo-electric fields, under photo excitation. Our observation was made at low temperature by comparing the behaviour of Ga0.92In0.08As-GaAs strained layer single and double quantum wells grown along the (001) and (111) directions when the densities of photoinjected carriers is tuned over several decades. Comparison between experimental data and the results of a Hartree calculation including the space charge effects reveals that manybody interactions are efficiently photo-induced in the (111)-grown samples. Moreover, in the case of double quantum wells, additional transitions appear in the photoluminescence spectra, due to the tunnelling of the two first excited heavy-hole levels for moderate densities.

We present low-temperature-reflectance experiments, performed on GaAs-Ga1-xAlxAs asymmetrical double quantum wells, under in-plane uniaxial stress. rhe results, when compared to what is obtained from single quantum wells, exhibit some peculiar behaviors, which are typical effects of the asymmetry of the structures. A careful examination of these results is proposed in light of theoretical predictions about subband-to-subband transition energies and oscillator strengths. Our conclusion is that asymmetry makes it possible to obtain novel couplings between excitonic states, which cannot be accounted for without an accurate theoretical treatment of the Coulombic interaction, including intersubband mixings of valence wave functions.

A non-conventional technique of thermally detected optical adsorption (TD-OA) is used to study a strained InAs/InP quantum well with thickness between two and three monolayers. The spectrometer and TD cell are briefly described and the TD-OA spectra are presented. Three signals due to the InAs well are identified. The simple envelope-function approach is sufficient for the interpretation of these, which are attributed to transitions involving the heavy-hole levels. However, for the light-hole-related peak, a more sophisticated theory, taking into account the coupling between the light hole and spin-orbit split-off states, is necessary to fit the result.