Spectroscopic ellipsometry studies of 3-stage deposition of CuIn1 − xGaxSe2 on Mo-coated glass and stainless steel substrates

In order to achieve proper process monitoring and control for the deposition of thin film photovoltaic absorbers, for example CuIn1 − xGaxSe2 (CIGS), optical techniques such as spectroscopic reflectometry and polarimetry are advantageous because they can be set up in an unobtrusive manner in the manufacturing line, and collect data in-line and in-situ. The use of these techniques requires accurate optical models that correctly represent the properties of the layers being deposited. In this study, spectroscopic ellipsometry (SE) has been applied for the characterization of each individual stage of CIGS layers deposited using the three-stage co-evaporation process. Dielectric functions have been determined for the energy range between 0.7 eV and 5.1 eV. As the layers' thicknesses are in the order of 2.5 μm, thickness non-uniformity along with surface roughness has been included in the analysis. A high similarity between the optical models developed for CIGS on glass and on stainless steel substrate was observed. Critical-point line-shape analysis was used in this study to determine the critical point energies of the CIGS based layers. These results can assist in the development of optical process-control tools for the manufacturing of photovoltaic absorbers, increasing the uptime and yield of the production line.