Blistering in Cu2ZnSnS4 thin films: correlation with residual stresses

An investigation is presented on the blistering of Cu2ZnSnS4 (CZTS) thin films grown by sulfurization of co-sputtered quaternary precursors. SEM cross-sectional images of post-annealed samples show film delamination occurring at the CZTS/substrate interface, with formation of blisters whose diameter typically ranges between 10 and 60 mu m and height between 3 and 10 mu m. Two main competing mechanisms suggested by the literature on different materials have been considered to explain blistering in CZTS, respectively based on the relief of compressive stress or on the formation of over-pressurized trapped-gas bubbles. Intrinsic residual stress in as-sputtered precursor layers was measured by substrate-curvature and by X-ray diffraction techniques. Results show that by increasing the sputtering pressure both compressive stress in precursors and blistering extent in the corresponding CZTS film decrease. A mechanism based on stress-driven viscoplastic deformation is proposed to explain blistering in CZTS films, while trapped-Ar pressure seems to play just a secondary role. (C) 2016 Published by Elsevier Ltd.