Electronic structure of short-period ZnSe/ZnTe superlattices grown by MOVPE at 300ºC

ABSTRACTThe ZnSe-ZnTe combination is a potential candidate for the realisation of visible light-emitting devices. The lattice mismatch between bulk ZnSe and bulk ZnTe is important (∼ 8%). Therefore, their hetero-structures are strained and high quality superlattices will only be grown if having small periods. This prescription can be fulfilled in the case of metal organic vapour phase epitaxy (MOVPE) growth by combining triethylamine dimethyl zinc adduct with di-isopropyl telluride as precursors for the growth of the ZnTe layers. The growth of high quality ZnTe can then be performed at a temperature of 300ºC , close to the best MOVPE-growth temperature for ZnSe (280ºC). Lowering the growth temperature of ZnTe to this value, we could thus obtain sharp interfaces. This work reports on ZnSe-ZnTe superlattices grown on ZnSe and ZnTe buffers deposited on GaAs substrates. We demonstrate that the stokes-shift between the reflectance and photoluminescence features ( ∼ 40 meV ) measured when the thickness of ZnSe layers does not exceed 20 Å, drastically increases for layer thicknesses beyond this critical value. This, we interpret in terms of the onset of plastic relaxation which favours tellurium diffusion in the ZnSe slices. Then photoluminescence spectra broaden ( contributions of trapped-excitons dominate), and observation of free excitons in reflectance become impossible. We have studied in detail the optical properties of the superlattices and compared our findings with the predictions of a multiband envelope function calculation. We show that both zone centre excitons as well as excitons associated with the miniband dispersions (saddle-point excitons) are observed in these superlattices.