Structural and Optical Properties of Two-Stage CuInSe2 Thin Films Studied by Real Time Spectroscopic Ellipsometry

CuInSe2 (CIS) thin films ~ 500-650 Å in thickness have been deposited on c-Si substrates by two-stage thermal co-evaporation starting either from In2Se3 [according to In2Se3 + (2Cu+Se) → 2(CuInSe2)] or from Cu2-xSe [according to Cu2Se + (2In+3Se) → 2(CuInSe2)]. The design of such processes is facilitated by accurate calibrations of Cu and In2Se3 growth rates on substrate/film surfaces obtained by real time spectroscopic ellipsometry (RTSE). The two-stage deposited CIS films were also studied by RTSE to deduce (i) the evolution of film structure upon conversion of the starting In2Se3 or Cu2-xSe films to CIS via Cu+Se or In+Se co-evaporation, respectively, and (ii) the complex dielectric functions of the starting films as well as the resulting CIS. The goal is to fabricate CIS that develops large grains as early as possible during growth for high quality materials in tandem solar cell applications. Results indicate that by depositing Cu2-xSe in the first stage and exposing the film to In+Se flux in the second stage [as in the third stage of a three-stage CIS process] well-defined bandgap critical points with no detectable subgap absorption are noted in films as thin as 650 Å.