THE EFFECT OF UNIAXIAL PRESSURE ON THE OPTICAL PROPERTIES OF SINGLE-CRYSTAL Tl2S IN THE REGION OF FUNDAMENTAL ABSORPTION EDGE. I. EXPERIMENT

Abstract

The effect of uniaxial pressure (p||c) on the optical properties of Tl₂S single crystal in the region of fundamental absorption edge (FAE) was first investigated. Tl₂S crystals were grown by the Bridgman-Stockbarger method. The growth axis was in the plane (0001). The structure of the obtained single crystals was investigated by X-ray HZG-4a diffractometer (CuK_α-line radiation with wavelength λ =1,5405 Å). Based on the obtained X-ray diffraction patterns, the parameters of the hexagonal [a=b=12,148(4) Å, c=18,183(8) Å, α=β=90°, γ=120°, V=2323,8(7) Å³] and rhombohedral [a=b=c=9,269(5) Å, α=β=γ=81,878°, V=774,6(7) Å³] unit cells were calculated. Samples for optical studies were obtained by cleaving Tl₂S crystals along the cleavage plane (0001). To accomplish this task, special sample holders were developed with the ability to create uniaxial pressure. The pressure varied in the range of 0-3 MPa. Studies of the effect of uniaxial pressure on the optical properties of a layered semiconductor Tl₂S in the region of the FAE were performed on several samples. This article presents the results of the longest series of optical measurements of sample No. 2-5 (sample thickness, d =55±1 μm). Studies of this sample were carried out in three stages, at which both the influence of the pressure value and the duration of the sample’s stay under pressure were studied.

The results of studies of the effect of uniaxial pressure on the optical properties of a Tl₂S layered crystal convincingly show that the exponential shape of the FAE is maintained even under the action of multiple and prolonged uniaxial pressures (up to 3 MPa) on the sample. This experimental fact makes it possible to draw the following conclusions. Firstly, the exponential shape of the FAE in a Tl₂S crystal is not due to the features of the interaction between layers in this layered semiconductor. In particular, the presence of residual mechanical stresses between the layers or the appearance of new defects in the van der Waals gaps do not change the exponential nature of the FAE in Tl₂S. Secondly, the exponential edge of fundamental absorption in this material is the properties of the Tl₂S layers themselves. Such conclusions are completely consistent with our previously proposed mechanism for the formation of an exponential edge of fundamental absorption in indirect gap semiconductors.

It was found that prolonged exposure of crystalline Tl₂S samples under uniaxial pressure (2–3 MPa) leads to the formation of an additional exponential dependence in the absorption spectra in the transparency region immediately before the FAE. The exponential nature of this part of the absorption spectrum is a consequence of the formation of a disordered defect structure in the van der Waals gaps of the Tl₂S layered crystal caused by uniaxial pressure.

Key words: fundamental absorption edge, Urbach rule, uniaxial pressure, topological disorder.