Real time spectroscopic ellipsometry of single and multistage deposition of polycrystalline films in calibrated evaporation processes: Application to In2Se3, Cu2Se, and CuInSe2

Real time spectroscopic ellipsometry (RTSE) has been applied to characterize the structural evolution of polycrystalline chalcogenide thin films deposited in a thermal co-evaporation system previously calibrated for the fluxes of metal atoms incorporated within the films. In the analysis, RTSE provides the evolution of the effective thickness, i.e., film volume per planar substrate area, and thus, the instantaneous deposition rate in terms of effective thickness. From this rate and from the constraints of known, time independent fluxes of metal atoms, a depth profile and average of the metal atom concentration within a single-stage thin film, relative to its single crystal counterpart, can be determined as a measure of the polycrystalline film density. The advanced RTSE analyses serve to identify modeling deficiencies from an inspection of the evolution of the mean square error obtained in unconstrained analyses. Applying these constraints to a thin film that evolves through different compounds or phase compositions during exposure to metal and chalcogen fluxes in the second stage of a two-stage process, the effective thickness from RTSE can provide the film composition and relative metal atom concentrations as the effective thickness evolves. The general approaches developed here have been applied to co-evaporation of first-stage In2Se3 and Cu2Se, single-stage Cu-poor CuInSe2, and second-stage conversions of either In2Se3 or Cu2Se to CuInSe2 through the predicted series of ordered defect or mixed phases, respectively. The resulting structural evolution along with the extracted optical properties provide insights into multistage fabrication of CuInSe2 as a narrow bandgap absorber for photovoltaics applications.