Bound-biexciton photoluminescence in CuCl thin films grown by vacuum deposition

Abstract

We report on the photoluminescence properties of CuCl thin films with the thicknesses of 20 and 100 nm under intense-excitation conditions produced with a pulsed nitrogen laser. We have clearly observed two photoluminescence bands with a superlinear (almost quadratic) excitation-power dependence. The high-energy band is attributed to the well-known free-biexciton photoluminescence. The excitation-power dependence of the low-energy band exhibits a saturation behaviour when the free-biexciton band grows remarkably. In addition, the energy and shape of the low-energy band do not change with the excitation power, which is contrary to the characteristics of the free-biexciton photoluminescence reflecting the thermal distribution in the energy dispersion. From these facts, we conclude that the origin of the low-energy band is a bound biexciton. Furthermore, we find from the temperature dependence of the PL properties that the thermal dissociation of the bound exciton induces the instability of the bound biexciton.