Real time spectroscopic ellipsometry of sputtered CdTe: Effect of growth temperature on structural and optical properties

Real time spectroscopic ellipsometry (RTSE) has been applied to characterize process-property relations in polycrystalline CdTe films prepared to 3000 A by magnetron sputtering under conditions similar to those yielding 14%-efficient solar cells. In this study, the key process variable is the deposition temperature (T: 188 <= T <= 304 degrees C), which was calibrated in situ from the CdTe Eo band gap shifts relative to 15 degrees C. From analyses of the RTSE data, the time evolution of basic structural properties, including the surface roughness and bulk layer thicknesses, instantaneous deposition rate, and near-surface void volume fraction, can all be determined. As a result, depth profiles in the void fraction can be extracted. The ability to detect surface roughness with monolayer sensitivity permits study of the nucleation and growth modes of the film. In addition, analyses of the data collected after cooling the deposited sample provide the 15 degrees C dielectric functions (epsilon(1), epsilon(2)) for films prepared at the different deposition temperatures. Critical point analyses yield the fundamental and higher band gap energies and associated broadening parameters, and these provide information on film stress, defect density, and grain size.