Effect of precursor stacking order and sulfurization temperature on compositional homogeneity of CZTS thin films

In this study, Cu2ZnSnS4 (CZTS) absorber layers were grown by sequential deposition of metallic Cu, Zn, and Sn layers by DC magnetron sputtering on Mo coated glass followed by annealing in sulfur atmosphere in a rapid thermal processing (RTP) system for 3 min at 500 °C, 550 °C and 600 °C. Two different metal stacking sequences were investigated: Mo/Zn/Sn/Cu and Mo/Cu/Sn/Zn/Cu – both with a Cu-poor and Zn-rich composition. For the Mo/Cu/Sn/Zn/Cu stack, the initial Mo/Cu/Sn layer was pre-annealed to yield homogeneous Cu Sn alloys on which Zn and the final Cu was deposited afterwards. Compositional inhomogeneities were studied by measuring X-ray fluorescence (XRF) before and after KCN etching of the sulfurized samples as well as by glow discharge optical emission spectroscopy (GDOES). Additionally, crystallographic properties were analyzed by X-ray diffraction (XRD) and Raman spectroscopy. For the Mo/Zn/Sn/Cu precursor, we find that the initial stacking order is partially preserved with a Cu-rich surface and a Zn-rich bottom. In contrast, the modified stacking Mo/Cu/Sn/Zn/Cu leads to an improved compositional homogeneity. While a Zn-rich composition is still found at the bottom part of the film, after sulfurization for 3 min at 600 °C the surface and bulk part show homogeneous element distributions close to that of CZTS.