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

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.