A POSSIBLE PHYSICAL MECHANISM FOR THE FORMATION OF THE EXPONENTIAL FUNDAMENTAL ABSORPTION EDGE IN INDIRECT GaP SEMICONDUCTORS

Abstract

Based on a mutually complementary analysis of the results of optical studies of layered semiconductors Tl₂S and GaSe and calculations of the energy band structure of these crystals, a physical mechanism is proposed for the formation of the exponential fundamental absorption edge (FAE) in indirect gap semiconductors. The essence of this mechanism is the sequential activation of indirect transitions of electrons from the valence band to the conduction band with increasing energy of quanta of electromagnetic radiation incident on the sample. Thus, the absorption coefficient a is defined as the sum of the absorption coefficients due to the corresponding indirect transitions of electrons with absorption and emission of phonons. The number of such indirect transitions of electrons depends on the energy band structure of a particular semiconductor. The resulting superposition of indirect transitions leads to the formation of an exponential FAE. Such a model allowed us to explain the exponential shape of the FAE of Tl₂S and GaSe crystals in the entire spectral range of measurements. The simulation results showed a good agreement between the theoretical optical absorption spectra and the experimental spectra of these semiconductors. The numerical values of the band parameters determined during the simulation are in satisfactory agreement with the values of these parameters obtained from calculations of the energy band structures of Tl₂S and GaSe crystals. This gives grounds to use the proposed model as a peculiar method for determining the band gap, E_gi, in indirect gap semiconductors in the case of an exponential shape of the FAE. Based on the results of studies of the optical properties of a layered GaSe semiconductor, an assumption was made about the possible universality of such a physical mechanism for the formation of the FAE in indirect gap semiconductors. However, to verify this assumption, further studies of the optical properties of other semiconductors are necessary.

Key words: fundamental absorption edge, Urbach rule, indirect gap semiconductors, band gap.