THE EFFECT OF UNIAXIAL PRESSURE ON THE OPTICAL PROPERTIES OF SINGLE-CRYSTAL Tl2S IN THE REGION OF FUNDAMENTAL ABSORPTION EDGE. II. THEORETICAL ANALYSIS

Viktor Belyukh, B. Pavlyk, H. Danylyuk

Abstract


This is the second part of the article (the first part was published in the previous issue of the journal) devoted to the study of the effect of uniaxial pressure on the optical properties of a Tl2S layered crystal. In this part, a theoretical analysis of the experimental results is carried out using the model of the formation of an exponential fundamental absorption edge (FAE) in indirect-gap semiconductors. This made it possible to study the dynamics of changes in the width of the indirect band gap, Egi, of this semiconductor and to determine the pressure coefficient, which is dEgi/dp ≈ -1,07∙10-8 еV/Pа (p||c). The accuracy of determining the values of the band parameters in the course of the simulation was no worse than ± 0.002 eV. To clarify the correspondence of the obtained dynamics of changes in the band parameters Egi1, Egi2 and Egi3 to possible changes in the band structure caused by such a deformation of the crystal, ab initio calculations of the energy band structure of Tl2S were performed within the framework of the density functional theory for cases of a decrease in the parameter c of the unit cell of a Tl2S crystal by 0,5; 1,5; 5 and 10%. It was found that within 5%-th relative deformation ε, the dependence Egi=f(ε) is linear. The deformation coefficient of the indirect band gap, determined for the region of linearity of the dependence Egi=f(ε), is equal to dEgi/dc ≈ 3,6∙10-2 еV/Å (p||c). The dynamics of changes in the band parameters Egi1, Egi2 and Egi3 obtained from these calculations fully corresponds to the dynamics of changes in these quantities obtained from a theoretical analysis of the optical absorption spectra of a deformed Tl2S crystal. Thus, a mutually complementary analysis of the results of studies of the effect of uniaxial pressure on the optical properties of Tl2S single crystal and calculations of the modification of the band structure of this crystal due to such deformation has shown the applicability of the physical mechanism of the formation of exponential FAE in indirect-gap semiconductors to study the optical properties of such materials. At the same time, the conclusion was confirmed that the exponential character of the FAE in crystalline Tl2S is due to its band structure, and not to the peculiarities of the interaction between the layers in this layered semiconductor.

Key words: fundamental absorption edge, Urbach rule, indirect-gap semiconductors, tallium (I) sulfide Tl2S, uniaxial pressure.


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DOI: http://dx.doi.org/10.30970/eli.14.6

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