Determination of band gap energy (Eg) of Cu2ZnSnSe4 thin films: On the discrepancies of reported band gap values

We demonstrate experimental data to elucidate the reason for the discrepancies of reported band gap energy (E-g) of Cu2ZnSnSe4 (CZTSe) thin films, i.e., 1.0 or 1.5 eV. E-g of the coevaporated CZTSe film synthesized at substrate temperature (T-sub) of 370 degrees C, which was apparently phase pure CZTSe confirmed by x-ray diffraction (XRD) and Raman spectroscopy, is found to be around 1 eV regardless of the measurement techniques. However, depth profile of the same sample reveals the formation of ZnSe at CZTSe/Mo interface. On the other hand, E-g of the coevaporated films increases with T-sub due to the ZnSe formation, from which we suggest that the existence of ZnSe, which is hardly distinguishable from CZTSe by XRD, is the possible reason for the overestimation of overall E-g. (C) 2010 American Institute of Physics. [doi:10.1063/1.3457172]