CZTS layers formed under sulfur-limited conditions at above atmospheric pressure

Abstract

In this study CZTS thin films were grown by a two-stage process that involved sequential sputter deposition of metallic Cu, Zn, and Sn layers on Mo coated glass substrates followed by RTP annealing in a sulfur atmosphere at background gas pressures in the range of 1-2 atm. Sulfurization was carried out in a mini reaction volume that provided a relatively S-limited environment Reacted films were characterized using XRD, EDX, SEM, photoluminescence and Raman spectroscopy. It was found that, under the S-limited regime provided in these experiments the Cu-S secondary phase formation was most extreme in the sample grown at 1.5 aim, whereas films grown at lower and higher pressures showed much smaller degree of phase separation. Reaction at 2 atm yielded a compound film that was the closest to the initial precursor in terms of its composition. SEM micrographs showed rough morphology and polycrystalline structure that changed with the sulfurization pressure. the optical band gap of the films as determined by photoluminescence was found to be about 1.37 eV. These experiments demonstrated the importance of the sulfurization pressure as well as the size of the reactor internal volume in determining secondary phase formation in two-stage processed CZTS layers.