Characterization of Cu(In,Ga)(Te,S)2 thin films grown on stainless steel foil substrates

Abstract

In this study, Cu(In,Ga)(Te,S)2 (CIGTS) thin films with [Ga]/([In] ± [Ga]) atomic ratios in the ranges of 0.22–0.28 and 0.50–0.67 were fabricated using a two-stage technique. During the first stage of the technique, in one set of samples, Cu, In and Ga layers were deposited by electrodeposition on a Mo coated stainless steel (SS) foil substrate forming a SS/Mo/Cu/In/Ga precursor structure. For another set of samples, a Te layer was also deposited by e-beam evaporation on the SS/Mo/Cu/In/Ga structure forming a SS/Mo/Cu/In/Ga/Te precursor structure. During the second stage, SS/Mo/Cu/In/Ga and SS/Mo/Cu/In/Ga/Te stacks were reacted using rapid thermal annealing (RTA) for 5 min at 600 °C with or without presence of S vapors to produce CIGTS series thin films. SS/Mo/Cu/In/Ga stack under S atmosphere yielded CuInGaS2 with a Ga-In gradient across the thickness by RTA process. SS/Mo/Cu/In/Ga/Te stack reacted without S in the reaction atmosphere yielded the CuInGaTe2 compound. When S was present, the same stack with top Te layer yielded only CuInGaS2 compound. When, however, already formed CuInGaTe2 compound layers were heated in S environment at 400 °C, some Te could be retained in the films in the form of elemental Te. Gallium and In grading in various reacted films were evaluated by x-ray diffraction, secondary-ion mass spectrometry and EDS. CIGTS films showed highly (112) preferred oriented chalcopyrite phase and with the increase of Ga content, shifts were observed in the XRD peak positions demonstrating Ga inclusion in the lattice. Gibbs free energy calculations were used to explain the preferred reaction of S with metallic constituents when both S and Te were present for reaction.