Abstract
Temperature-dependent photoresponse variation on the CuAlSe2 layers was investigated by photocurrent (PC) spectroscopy as a function of temperature. Three peaks of A, B, and C caused by the band-to-band transition were observed. From these PC peaks, the temperature-dependent band-gap energy was well described by Eg(T) = 2.8382 − (8.68 × 10−4)T2/(155 + T). Thus, the parameter of the split valence band on CuAlSe2 was directly extracted out by means of the PC spectroscopy. The crystal field splitting and the spin-orbit splitting were estimated to be 0.1641 and 0.1778 eV at 10 K, respectively. However, the PC intensities decreased with decreasing temperature. It is against our expectation that the PC intensities increase with decreasing temperature. From the relation of log Jph versus 1/T, where Jph is the PC density, two dominant levels were observed, one at high temperatures and the other at low temperatures. We found out that the activation energy of 48.8 meV at high temperatures is related to the binding energy of the free exciton, whereas the PC-intensity variation at a low temperature below 100 K remains constant. This suggests that trapping centers due to native defects and impurities in CuAlSe2 limit the photoresponse with decreasing temperature.
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