Spectroscopic ellipsometry study of transparent conductive ZnO layers for CIGS solar cell applications

Al doped ZnO (ZAO) thin films were deposited by reactive magnetron sputtering from Al:Zn metallic alloy target for copper–indium–gallium–diselenide (CIGS) solar cell contacts. The effect of variation of technological parameters (power, plasma Ar/O2 ratio and target voltage) was studied.A novel approach for controllable reactive sputter deposition is suggested using a hysteresis loop in the target voltage vs. oxygen gas flow. As a result of this optimization a 1.85 × 10− 4 Ω cm layer with 90% transparency was obtained without plasma emission monitoring and substrate heating.Thickness and specific resistance were measured on the samples prior to spectroscopic ellipsometry (SE) analysis. Evaluation of SE data was done by least square fitting the Cauchy dispersion relation using polynomial functions. Subsequently a two-step “screaning” method, based on the parameters of the complex refractive index, is suggested for evaluation of the process. Furthermore, surface roughness data can also be determined by SE, as verified by comparison with SEM morphologies. Using this method SE offers a non-destructive in-line characterization tool for the deposition process.