Absorption of Thin ZnSe, ZnS and ZnS$\text{}_{x}$Se$\text{}_{1-x}$ Films under High Hydrostatic Pressure

We present absorption measurements on free-standing ZnSe, ZnS and ZnS$\text{}_{x}$Se$\text{}_{1-x}$ films (d = 0.4...2 μm) under hydrostatic pressure up to 15 GPa. The refraction index n(λ,P) of ZnS and ZnSe in the transparent region up to 800 nm and the pressure shift of the E0 absorption edge of ZnSe and some ZnS$\text{}_{x}$Se$\text{}_{1-x}$-compositions was investigated at 293 K. At 2 K free exciton states near the E$\text{}_{0-}$ and E$\text{}_{0}$ + Δ$\text{}_{0}$-gap are visible in absorption. Increase in the Rydberg energy under pressure was found, which is explained with k • p-theory in the framework of the hydrogen model.