Growth and photocurrent characteristics of the photoconductive MnAl2S4 layers grown by hot-wall epitaxy method

The epitaxial growth of the photoconductive MnAl2S4 with a layered-type rhombohedral structure was first achieved by means of the hot-wall epitaxy method. From the Hall effect measurement, two dominant scattering mechanisms on the mobility were extracted. One was an acoustic phonon scattering caused by lattice vibrations at the high-temperature range of T>100 K and the other one was an impurity ion scattering at the low-temperature range of T<100 K. Also, from the relation between the reciprocal temperature and the carrier concentration, two activation energies were evaluated to be the deep level of 83.1 and a shallow level of 14.1 meV at high-temperature and middle-temperature ranges, respectively. From the photocurrent (PC) measurement, the A, B, and C peaks were observed at whole temperature range. The electronic origin of these three peaks were caused by the band-to-band transitions from the valence band states of Γ4(z), Γ5(x), and Γ5(y) to the conduction band state of Γ1(s), respectively. Thus, the parameters of the crystal-field and the spin-orbit splitting were directly extracted by means of PC spectroscopy, and those values were 3.5 and 39.9 meV, respectively. The band gap energies estimated from PC and absorption measurements were well expressed by Eg(T)=Eg(0)−2.80×10−4T2/(T+287), where Eg(0) were found to be 3.7920, 3.7955, and 3.8354 eV at the valence band states of Γ4(z), Γ5(x), and Γ5(y), respectively.