An investigation of optical and electrical properties of the TlGaSe$_2$ single crystal

In order to determine the nature of optical transitions, the authors conducted studies in the self-absorption edge of TlGaSe$_2$; transmission and reflection were measured at 300 K and 77 K, the authors also calculated the absorption coefficient. The results are in good agreement with the calculations of the Brillouin zone. It is shown that the long-wave edge of the proper absorption band is formed by indirect transitions (Y$\to$D). At 4.2 K in the spectrum, an intense absorption line $\lambda$ = 579.5 nm (2,138 eV) is observed in the absorption spectrum. It can be concluded that in TlGaSe$_2$, the self-absorption edge is formed by a direct exciton transition, which is preceded by an indirect optical transition. Studies of transitions corresponding to the TlGaSe$_2$ self-absorption edge show that the edge is formed by a transition between extreme points of the Brillouin zone that have polarization features. Transitions occur from several levels located in close proximity to each other. TlGaSe$_2$ crystallizes in a monoclinic structure with the $Cc$ space-group, which indicates that these crystals have a single element of symmetry – the reflection plane $\sigma$ perpendicular to the layers. However, the experimental results obtained are not explained by this symmetry of the crystal.