Tl-based compound semiconductor shows low-dimensional crystal structure due to the arrangement of Tl atoms. In TlInS2, TlGaSe2 and TlGaS2, Tl atoms show two-dimensional layered structure. However, It is unknown how the result of the optical properties originating from the Incommensurate phase due to the band structure and density of state. Therefore, we clarify the electronic structure and optical characteristics in Tl compound of two-dimensional layered structure. In the band structure, TlInS2 shows a direct transition type at the Γ point. TlGsSe2, TlGaS2 shows the indirect transition type. Differences in electronic structure occur due to changes in the Valence band structure. As a result of density of state, it was found that the density of state increases at the upper region of the valence band, with common property. Those results explain the optical experiments.
