Effect of heat treating metallic constituents on the properties of Cu2ZnSnSe4 thin films formed by a two-stage process
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In this study Cu2ZnSnSe4 (CZTSe) thin films were grown by a two-stage process that involved sputter deposition of a Cu/Sn/Zn/Cu metallic stack, annealing the stack at various temperatures for 30 min, evaporation of a Se cap over the metallic stack thus forming a precursor layer, and subjecting the precursor layer to a final high temperature reaction step at 550 °C. Different samples were prepared with annealing temperatures of the metallic stacks ranging from 200 °C to 350 °C. The results showed that heat treatment of the metallic stacks did not cause much change in their morphology and elemental composition, however their phase content changed noticeably when the anneal temperature was raised to 250 °C. Specifically, while the metallic films were dominated by CuSn and Cu5Zn8 phases at low temperatures, the dominant phase shifted to Cu6Sn5 at the annealing temperature of 250 °C and higher. Also formation of a distinct Cu3Zn2 phase was observed upon annealing at temperatures at or above 250 °C. After reaction with Se, the CZTSe layer obtained from the metallic film, which was annealed at 250 °C was found to be the best n terms of its composition, crystalline quality and purity, although it contained a small amount of CuSe. The other layers were found to contain small amounts of other secondary phases such as SnSe, CuSe2, ZnSe and Cu2SnSe3. SEM micrographs showed denser structure for CZTSe layers grown from metallic films annealed at or above 250 °C. Optical band gap, resistivity and carrier concentration of the best quality CZTSe film were found to be about 0.87 eV, 2 Ω-cm and 4 × 1017 cm− 3, respectively.