Band gap and optical constants of microcrystalline CdTe thin films

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Abstract
We report the results obtained from optical characterization studies of CdTe thin films grown by the standard rf sputtering technique. Films were grown at different substrate temperatures (Ts) in the 70–200 °C range. From the analysis of absorption spectra we determined the index of refraction and absorption coefficient. Employing a model of direct transitions, the fundamental absorption edge E0 was calculated. The same experiments and analyses were performed for annealed films at 400 °C in N2 atmosphere. The optical constants do not show a clear correlation with substrate temperature for Ts<200 °C. Higher substrate temperatures and/or thermal annealing does bring the optical parameters closer to the single-crystal values. An important and consistent reduction of E0 was observed for all films after thermal annealing. Defect induced absorption, transformation of the metastable hexagonal to the cubic phase of CdTe, and quantum size effects caused by the microcrystalline structure of the films are discussed as possible sources for the observed shifts of the fundamental absorption edge E0.

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