Real Time Spectroscopic Ellipsometry Analysis of the Structural Evolution and Optical Properties of CuInSe₂

Thin films of the narrow bandgap solar cell material copper indium diselenide (CuInSe₂) have been deposited in one-stage and two-stage thermal co-evaporation processes on c-Si wafer substrates held at temperatures of 300-570 °C. The goal of this study is to apply real time spectroscopic ellipsometry (SE) to analyze the structural evolution and the complex dielectric functions (ε₁, ε₂) of the CuInSe₂ films at the elevated temperatures of deposition. Real time SE provides the structural parameters including the surface roughness and underlying bulk layer thicknesses as functions of time with monolayer level sensitivity during thin film nucleation and coalescence. SE measurements performed in situ during cooling of the deposited film yield (ε₁, ε₂) at different temperatures. The (ε₁, ε₂) spectra measured at the deposition temperature and upon cooling provide the room temperature band gap of the CuInSe₂ and its temperature coefficient. In two step processes, either In₂Se₃ or Cu₂Se films are deposited first and converted to CuInSe₂ upon exposure to either Cu+Se or In+Se evaporant, respectively. Real time SE has been applied to compare the structural evolution that characterizes these two routes for CuInSe₂ fabrication.