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Influence of copper composition and reaction temperature on the properties of CZTSe thin films
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In this study Cu2ZnSnSe4 (CZTSe) compound layers were grown using a two-stage technique that involved deposition of metallic precursors (Cu, Zn, and Sn) and Se in the first stage, followed by reaction of all the species at temperatures between 525 °C and 600 °C, during the second stage of the process. Two sets of samples, one with Cu-poor, Zn-rich and the other with Cu-rich, Zn-rich compositions, were prepared and their structural, optical and electrical properties were measured. XRD analyses showed the characteristic peaks of CZTSe regardless of the Cu content and the processing temperature. However, for samples reacted at temperatures of 575 °C and 600 °C a Cu2-xSe secondary phase separation was detected for all films suggesting that the reaction temperatures should be limited to values below 575 °C in a two-stage process such as ours. Excessive Sn loss was also present in samples processed at the highest temperatures. Raman scattering measurements confirmed formation of the CZTSe kesterite structure, and also indicated a small ZnSe phase, which could not be detected by XRD. Scanning electron micrographs demonstrated dense film structure with the Cu-rich films having smoother morphology. Optical characterization showed that increasing the Cu content in the compound layers caused a reduction in the optical band gap values due to increased interaction between the Cu-3d orbital electrons and the Se-4p orbital electrons. Electrical measurements showed that the carrier concentration increased with Cu content.