Excitation photon energy dependence of photoluminescence in CdTe thin films

We report the photoluminescence spectra of CdTe polycrystalline thin films at 10 K as a function of the exciting photon energy in the 1.59 to 1.71 eV range. Our results indicate that the ratio of the maximum intensity of the bound exciton band (I_b at ≈ 1.59 eV) to that of the defect band (I_d at ≈ 1.45 eV) shows resonances in this range. Above the band-gap region (E_g = 1.606 eV) and near E_g + (latin small letter h with stroke)ωLO (with (Latin small letter h with stroke)ωLO the LO-phonon energy) a first resonant feature is detected. This behavior is attributed to the efficient emission of LO-phonons during the thermalization process of the created photocarriers in their corresponding bands. Below and near E_g a second and lower resonance is also present, which can be explained in terms of the direct formation of bound excitons.