Real time spectroscopic ellipsometry analysis of the three-stages of CuIn1−xGaxSe2 co-evaporation

Real time spectroscopic ellipsometry (RTSE) has been applied for in-situ monitoring and analysis of all three processing stages in the co-evaporation of copper indium-gallium diselenide (CuIn_1-xGa_xSe₂; CIGS) for high efficiency photovoltaic devices. The first stage entails indium-gallium selenide (In_1-xGa_x)₂Se₃ (IGS) deposition at a substrate temperature of 400°C on soda lime glass coated with opaque Mo. In this stage, an accurate deposition rate and the final IGS bulk and surface roughness layer thicknesses can be obtained. In the second stage, co-evaporation of Cu and Se converts the IGS film to CIGS at an elevated substrate temperature of 570°C. A bulk layer conversion model is justified and employed to analyze the second-stage RTSE data, resulting in steady-state IGS-to-CIGS thickness and volume fraction conversion rates. Near the end of the second stage, the formation of a Cu_2-xSe layer on the CIGS surface can be tracked in terms of an effective thickness rate. The final Cu_2-xSe effective thickness at the CIGS surface is obtained in a time interval spanning the end of the second stage to the beginning of the third. Finally, in the third stage, the Cu-rich CIGS/Cu_2-xSe is converted to slightly Cu-poor CIGS by co-evaporation of In, Ga, and Se. In this stage, the thickness conversion rate, and the endpoint bulk and surface roughness layer thicknesses can be obtained. In the three stages, the thickness rates and final thicknesses yield information on the total elemental fluxes, and the roughness evolution yields information on grain growth and near-surface coalescence processes. Modeling of the dielectric functions in future studies is expected to yield compositional information and thus relative metallic fluxes. Variations in the RTSE-deduced information can yield insights into run-to-run irreproducibilities that influence the solar cell p- rformance. The application of these capabilities in the fabrication of solar cells with thick (2.5 μm) and thin (0.3 μm) absorbers is demonstrated.