Cu2ZnSnSe4 thin films with different substrate temperature and Cu flux were grown by thermal co-evaporation. Raman scattering, photoluminescence, and contactless electroreflectance (ER) measurements were performed. The Raman spectra of Cu2ZnSnSe4 show two main peaks at 170 and 192 cm(-1). The photoluminescence spectrum shows a peak below 1.0 eV. Franz-Keldysh oscillations (FKOs) were observed in the ER spectra. From the analysis of the FKOs, the bandgap energy of Cu2ZnSnSe4 thin films is estimated to be 1.07 eV at 90 K and 0.99 eV at room temperature. We conclude that the bandgap energy of Cu2ZnSnSe4 thin films is around 1.0 eV. (C) 2011 Elsevier B.V. All rights reserved.