EXCITONS AND BIEXCITONS IN MOVPE-GROWN ZNS EPITAXIAL LAYERS

High quality MOVPE-grown ZnS/GaAs epilayers are characterized by refle ction, transmission and linear photoluminescence (PL) spectroscopy at 2 K. The GaAs substrate of a part of each ZnS layer is removed and the optical properties of as-grown and etched surfaces are compared. The splitting (caused by the thermally induced strain) of the topmost Gamm a(8) valence band into the heavy and light-hole subbands (Delta(hl)=4 meV) and the transitions involving exciton ground and excited states a re better seen in PL excitation spectra than in ordinary PL spectra. F rom the analysis of the nonlinear PL and PL excitation spectra, we can conclude the existence of biexcitons with a binding energy of about 1 0 to 12 meV under an intermediate intensity of excitation (a density o f about 2.5x10(16) to 2.5x10(17) e-h excited pairs per cm(3)).