Excitons and biexcitons in MOVPE-grown ZnS epitaxial layers☆

High quality MOVPE-grown ZnSGaAs epilayers are characterized by reflection, 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 Γ8 valence band into the heavy and light-hole subbands (Δhl=4 meV) and the transitions involving exciton ground and excited states are better seen in PL excitation spectra than in ordinary PL spectra. From the analysis of the nonlinear PL and PL excitation spectra, we can conclude the existence of biexcitons with a binding energy of about 10 to 12 meV under an intermediate intensity of excitation (a density of about 2.5×1016 to 2.5×1017 e-h excited pairs per cm3).