Urbach-Martienssen Tails in the Absorption Spectra of Layered Ternary Semiconductor TlGaS2

Abstract

The dependence of the absorption coefficient on the photon energy and sample temperature near the fundamental absorption edge (AE) was investigated for TlGaS 2 layered semiconductor crystals. The exponential absorption tails observed in the 10-340 K temperature range were interpreted as Urbach-Martienssen (U-M) tails. From the analysis of these tails, characteristic Urbach parameters were determined. The results lead to the conclusion that the temperature dependence of the absorption tails near the fundamental AE of TlGaS 2 is the result of thermal disorder.

Low-Temperature Raman Scattering in TlGaxIn1-xS2Layered Mixed Crystals: Compositional Dependence of the Mode Frequencies and Line Shapes

“…Their quasi--two-dimensionality, optical and photoconductive properties, and other features attract investigators in an effort to acquire a better insight into the physics of these compounds. Layered ternary crystals TlInS 2 and TlGaS 2 are the anisotropic crystals whose properties have recently become the subject of extensive research [1][2][3][4][5][6][7][8]. A high photosensitivity in the visible range, high birefringence in conjunction with a wide transparency range of 0.5−14 µm make these wide band gap crystals useful for optoelectronic applications [9].…”

Nanostructured CdSe Films in Low Size-Quantization Regime: Temperature Dependence of the Band Gap Energy and Sub-Band Gap Absorption Tails

“…As already implied, however, though the sub-band gap absorption has been studied a lot in the case of macrocrystalline and amorphous materials, − emphasizing many aspects as well as the influence of a number of external parameters on the UM absorption tails, the number of studies has been much smaller in the case of nanostructured semiconductors (3D QD assemblies, in particular) and also in the case of thin films. As we have already argued before, however, it is of certain importance for both fundamental aspects of physical chemistry, materials science, and the potential applications of semiconductor quantum dot thin films to follow the evolution of the sub-band gap absorption tails with both temperature and controlled particle size reduction.…”

Disorder and compositional dependences in Urbach-Martienssen tails in amorphous (GeTe)x(Sb2Te3)1−x alloys

“…3 for 1.5 × 10 3 cm −1 < α < 5 × 10 3 cm −1 . These lines for various temperatures are not parallel and they converge at a point ( E 0 ,α 0 ), called the “converging point” 45 . As the inverse slope of the approximating straight line corresponds to the Urbach energy E U , this fact evidences the temperature dependence E U (T) .…”

“…In order to estimate the temperature dependence of the Urbach tails, we applied the following expression 15 :where E U(st.) is contribution of structural disorder; E U(dyn.) is the contribution of thermally-induced disorder , k B is Boltzmann constant, Θ is characteristic Einstein temperature, X is a measure of structural disorder normalized to the zero-point uncertainty in the atomic positions 15 , and σ 0 is Urbach edge parameter of order unity, which is inversely proportional to the strength of the coupling between electrons and phonons 45 . For the purpose of this study, the characteristic Einstein temperature is a good approximation to the temperature corresponding to the average phonon energy, as reported 20 .…”