Broadening of optical transitions in polycrystalline CdS and CdTe thin films

The dielectric functions ε of polycrystalline CdS and CdTe thin films sputter deposited onto Si wafers were measured from 0.75 to 6.5 eV by in situ spectroscopic ellipsometry. Differences in ε due to processing variations are well understood using an excited carrier scattering model. For each sample, a carrier mean free path λ is defined that is found to be inversely proportional to the broadening of each of the band structure critical points (CPs) deduced from ε. The rate at which broadening occurs with λ−1 is different for each CP, enabling a carrier group speed υg to be identified for the CP. With the database for υg, ε can be analyzed to evaluate the quality of materials used in CdS/CdTe photovoltaic heterojunctions.