Photoresponse-behavior analysis of ZnAl2Se4 layers by using photocurrent spectroscopy

The photocurrent (PC) variation of photoconductive ZnAl2Se4 layers was investigated as a function of temperature. Three PC peaks corresponding to band-to-band transitions were observed. From the relation between the temperature and the PC-peak position, the temperature-dependent band-gap energy E g was well described by E g (T) = E g (0) − (1.94 × 10−3)T 2/(468 + T). Also, E g (0) was estimated to be 3.5268, 3.6148, and 3.7178 eV at the valence band states of Γ4(z), Γ5(x), and Γ5(y), respectively. A characteristic splitting of the valence band governed by the selection rule was first observed through the method of PC spectroscopy. We noted that the crystal-field splitting and spin-orbit splitting were 0.088 eV and 0.103 eV, respectively. With decreasing temperature, however, the PC intensities decreased, contrary to our expectation. From the log J ph vs. 1/T plot, a dominant level of 37.8 meV was observed in the high-temperature region. Thereby, we suggest that this trapping center due to native defects and impurities limits the PC response with decreasing temperature.