Photocurrent study of the band gap and the valence band splitting in CdIn2S4 films grown by using hot wall epitaxy

CdIn2S4 (110) films were grown on semi-insulating GaAs (100) by using a hot wall epitaxy method. The photocurrent (PC) spectra in the temperature range of 30 and 10 K had three peaks in the short wavelength region. The three peaks, A-, B-, and C-excitons, were found to correspond to intrinsic transitions from the valence band states of Γ4(z), Γ5(x), and Γ5(y) to the exciton below the conduction band state of Γ1(s), respectively. A 0.122-eV crystal field splitting and a 0.017-eV spin-orbit splitting were obtained. Thus, the temperature dependence of the optical band gap obtained from the PC measurement was well described by Eg(T) = 2.712 eV - (7.65 X 10-4 eV/K)T2/(425 + T). However, the behavior of the PC was different from that generally observed in other semiconductors. The PC intensities decreased with decreasing temperature. Such a phenomenon has never been reported for a PC experiment on the bulk crystals grown by using the Bridgman method. From the relation of log Jph vs 1/T, where Jph is the PC density, two dominant levels were observed, one at high temperatures and the other at low temperatures. Consequently, trapping centers due to native defects in the CdIn2S4 film are suggested to be the causes of the decrease in the PC signal with decreasing temperature.