Photocurrent Spectroscopic Study of Temperature-Dependent Photoresponse and Valence-Band Splitting in MnAl2S4 Layers

摘要

The temperature-dependent photoresponse characteristics of MnAl2S4 layers have been investigated, for the first time, by use of photocurrent (PC) spectroscopy. Three peaks were observed at all temperatures. The electronic origin of these peaks was associated with band-to-band transitions from the valence-band states I"(4)(z), I"(5)(x), and I"(5)(y) to the conduction-band state I"(1)(s). On the basis of the relationship between PC-peak energy and temperature, the optical band gap could be well expressed by the expression E (g)(T) = E (g)(0) - 2.80 x 10(-4) T (2)/(287 + T), where E (g)(0) was estimated to be 3.7920 eV, 3.7955 eV, and 3.8354 eV for the valence-band states I"(4)(z), I"(5)(x), and I"(5)(y), respectively. Results from PC spectroscopy revealed the crystal-field and spin-orbit splitting were 3.5 meV and 39.9 meV. The gradual decrease of PC intensity with decreasing temperature can be explained on the basis of trapping centers associated with native defects in the MnAl2S4 layers. Plots of log J (ph), the PC current density, against 1/T, revealed a dominant trap level in the high-temperature region. By comparing PC and the Hall effect results, we confirmed that this trap level is a shallow donor 18.9 meV below the conduction band.