Bound excitons in Cu2 O: Efficient internal free exciton detector

We report the spectroscopic observation of bound excitons in natural Cu2 O samples, which are generated by impurity capture of free excitons. Due to the broken symmetry of a bound exciton, its radiative recombination rate increases, causing a greatly enhanced luminescence intensity compared with that for a free exciton. Moreover, the bound exciton luminescence serves as an internal monitor of the free ortho- and free para-exciton densities, whereas the free para-exciton luminescence is allowed only via weak Γ25- phonon emission. At high excitation levels, the bound exciton luminescence shows a density-dependent loss of excitons that is well explained by a recently proposed biexciton formation model. The temperature dependence of the bound ortho- and bound para-exciton luminescence indicates a T-1 -dependence for the exciton-impurity capture rate.