Real Time Measurement, Monitoring, and Control of CuIn_1−xGa_xSe₂ by Spectroscopic Ellipsometry

Abstract

Spectroscopic ellipsometry (SE) using a rotating compensator multichannel instrument has been applied in real time for characterization of the three stages of the coevaporation process for copper indium-gallium diselenide (CuIn_1−xGa_xSe₂ ; CIGS ) thin films. This absorber material and the coevaporation process yield the highest small area efficiency among inorganic polycrystalline thin film solar cells . In the first deposition stage of the process, indium-gallium selenide (In_1−xGa_x)₂Se₃ (IGS) thin film deposition is performed via coevaporation of In, Ga, and Se on a substrate consisting of Mo-coated soda lime glass held at a temperature of 400 °C. In this stage, Mo/IGS interface roughness filling can be characterized along with the subsequent evolution of the IGS bulk and surface roughness layer thicknesses. An accurate rate for the effective thickness (or volume/area) can be determined from these results, and alloy compositional analysis of the IGS has been demonstrated at the first stage endpoint. In the second deposition stage, the IGS film is converted to CIGS via coevaporation of Cu and Se fluxes at an increased substrate temperature of 570 °C. A bulk conversion model provides the best fit to the real time SE data, and this model is employed in the analysis of the second-stage data. The results include the evolution of the content of CIGS within the film, along with the thicknesses of the bulk and surface roughness layers. The formation of a copper selenide (Cu_2−xSe ) component phase on the CIGS surface is detected just before the completion of the second stage. Subsequently, the evolution of the near-surface Cu_2−xSe content is followed in terms of its effective thickness, spanning the time interval from the end of the second stage through much of the third stage. In the third stage of the deposition process, In, Ga, and Se coevaporation serves to convert the Cu-rich CIGS/Cu_2−xSe to slightly Cu-poor CIGS. In this stage, the thickness evolution can be obtained along with bulk and surface roughness layer thicknesses at the overall deposition endpoint. In the three stages, the deduced deposition rates and final thicknesses provide information on the total metallic elemental fluxes, and the roughness evolution provides information on crystalline grain growth and near-surface crystallite coalescence in the polycrystalline films. Variations in the information deduced from real time SE can lead to insights into run-to-run irreproducibility that influences the performance of the resulting solar cells. The application of these capabilities is demonstrated for the fabrication of thin film solar cells in continuous and discontinuous (shuttered) processes for standard thickness (~2.5 μm) CIGS absorber layers as well as a shuttered process deemed necessary for adequate control of the deposition of thin (~0.3 μm) absorbers.