Optical Monitoring and Control of Three-Stage Coevaporated Cu(In $_{bm {1-x}}$Ga$_{bm x}$)Se $_{bf 2}$ by Real-Time Spectroscopic Ellipsometry

摘要

Real-time spectroscopic ellipsometry (RTSE) has been applied for in situ monitoring and control of thin-film copper–indium–gallium–diselenide, i.e., Cu(In $_{1-x}$Ga$_{x}$ )Se$_{2}$ (CIGS), deposition by high vacuum coevaporation in the three-stage process used for efficient photovoltaic devices. Initial studies have been performed on a ∼0.7-μm CIGS layer deposited on crystal silicon to minimize surface roughness and to develop an accurate structural/optical model of the Cu-poor-to-Cu-rich and Cu-rich-to-Cu-poor transitions that define the ends of the second (II) and third (III) stages of growth, respectively. With a better understanding of the surface achieved through this model, correlations can be made between the surface state and the unprocessed RTSE data { ${psi}(t)$, ${Delta}(t)$}. During deposition in the solar cell configuration with 2-μm-thick CIGS on a Mo-coated glass substrate, indications of the Cu poor-to-rich and Cu rich-to-poor transitions appear clearly in {${psi}(t)$ , ${Delta}(t)$}, enabling direct control of stage II and III transitions. The transition times deduced optically are in good agreement with those identified from the film/substrate emissivity by tracking the substrate heater power. It is clear, however, that RTSE can provide higher sensitivity to these transitions and is, therefore, suitable for improved control of three-stage CIGS deposition.