Optical properties of thallium indium disulfide (TlInS2) single crystals

Abstract

2, TlInSe2, TlInTe2, TlGaS2, TlGaSe2, and TlGaTe2 have unusual structural, optical, photoelectric, ferroelectric, and other properties and are of definite interest for semiconducting opto- and microelectronics [6–8]. The optical properties of TlInS2 near the fundamental absorption edge were studied using various methods such as transmission, luminescence, reflectance, photoconductivity, etc. [9–17]. Based on these studies, a direct-band energy structure was established and the exciton binding energy, band-gap width, and other optical characteristics of the material were determined. However, experimental optical spectroscopic data for semiconducting TlInS2 were in most instances contradictory. This indicated that further research was required in order to refine known fundamental optical parameters and to determine the unknown ones. In particular, the value of the exciton binding energy, an important parameter that defines the band-gap width, varied over wide ranges from 12.3 meV [12] to 33 meV [18]. This prevented the band-gap width Eg of TlInS2 from being accurately determined and the optical properties of dichalcogenide semiconducting compounds from being analyzed with a unified approach. Moreover, the nature of the energy-band structure of TlInS2 in the vicinity of direct allowed transitions was in need of refinement. Herein transmission and reflectance of light, photoluminescence (PL), and luminescence spectra were used to obtain new data on the optical characteristics of semiconducting TlInS2. These data may be useful for process monitoring during the production of single crystals with the maximum degree of structural perfection. Experimental. Layered TlInS2 single crystals were grown by the Bridgman–Stockbarger method [15]. Transmission was measured in thin (~2–40 μm) plane-parallel plates prepared by splitting larger samples along cleavage planes. It should be noted that the absorption coefficient of TlInS2 near the fundamental absorption edge is α ~ 5⋅10 3 cm –1 [12] and α ~ 1⋅10 3 cm –1 [18]. Cleaved thin plates (~5 × 5 mm) were yellow with a mirror-smooth surface that coincided with the crystallographic (001) plane. Non-equilibrium charge carriers in the crystals were excited during PL measurements by radiation from a He–Cd laser (λex = 325 nm, power up to 10 mW). In several instances, PL