Exciton Absorption of GaSe Crystals in the Indirect Transition Region

Abstract

GaSe crystals belonging to A%" semiconductor compounds have layer structure /l/. Such crystals are characterized by a strong covalent bond in the layers and a weak van der Waals bond between them. Their optical axis is directed perpendicular to the layers.According to recent calculations of the band structure of GaSe /2/, both direct and indirect transitions with formation of exciton states can occur in such crystals. The difference between their energetic positions is less than 0.1 eV. In accordance with the selection rules, the direct and indirect optical transitions are allowed for thepolarization if I 8. Taking into account the spin-orbit interaction these transitions become allowed also for the polarization 8 1 b.However, the absorption intensity in this case is by far weaker than for the other polarization which practically excludes indirect transitions at the polarization 2 I E .The optical properties of gallium selenide, largely its luminescence spectra have been investigated in many papers. The absorption of GaSe due to indirect transitions was studied elsewhere /3/. The light was directed at an angle as large as possible (up to 45' ) to the z-axis. The experimental results show that for the polarization 8 tl 8 the absorption edge is somewhat shifted towards the red. This fact was attributed to absorption due to indirect transitions.Thus, up to now no data in the literature give coiivincing evidence of the presence of indirect exciton transitions in GaSe crystals. In this paper the absorption edge is measured for the first time with polarized light propagating along the crystal layers. Such an experiment provides a direct and unambiguous information on the nature of optical transitions in the absorption edge region.shown in Fig. 1. Curves 1 and 1 correspond to the polarizations 3 1 if and 3 II 8 , respectively; its comparison shows that in the latter case the absorption edge is red-shifted. The shape of the curve becomes markedly protracted. AsThe experimental results for a GaSe crystal at different temperatures are I 1) Prospekt Nauki 115, 252650 Kiev-28, USSR.

Emission of free and bound excitons in layered gase crystals

“…The energetic position of the band and also a strong polarization dependence of its intensity suggest that the band is due t o the radiative decay of an indirect exciton with emission of one TO phonon, and the band a t 2.008 eV with emission of two TO phonons of 29 meV. This is in agreement with our energy values of an indirect exciton 2.069 eV [8].…”

“…Until recently no conclusive evidence on the energet,ic position of the excitons corresponding to indirect transitions has been reported in the literature. It was shown [7] that the shape of the emission spectrum strongly depends on the concentration of structural defects, the number of which can be decreased by doping GaSe with impurities of the F e group We investigated the nature of narrow lines and wide bands of the luminescence spectra of GaSe a t different temperatures and in polarized light taking into account the data on the energy spectrum of an indirect exciton [8]. The number of structural defects in the crystals was considerably decreased by doping with 0.001 wtyo Go.…”

Composition and Surface Optical Properties of GaSe:Eu Crystals before and after Heat Treatment

“…%) increases slowly together with photon energy, from 6 × 10 3 to 4 × 10 4 cm −1 . The comparison with the results of previous research reported in [40] demonstrates that the spectral distribution of the absorption coefficient in the depth of the fundamental absorption band of GaSe single crystals weakly depends on the presence of Eu, for moderate concentrations. Figure 8a shows the PL spectrum of the single crystalline GaSe plate, which reveals that upon 405 nm excitation, a band with a narrow contoured and maximum peak located at 620 nm (2.00 eV) is formed.…”

“…As can be seen from the AFM image (Figure 6b), β-Ga2O3:Eu formations are tower-and cone-shaped and exhibit submicrometric bases. The fundamental absorption edge of ε-GaSe single crystals is well studied and is determined by the excitonic absorption mechanism [38][39][40]. The absorption coefficient (α) of radiation with wavelength λ, in both specially undoped and 3.00 at.…”

“…Since the layer formed on the surface of Eu-doped GaSe plates after heat treatment at 900 °C (β-Ga2O3:Eu) is microgranular and strongly diffuses the incident light, the absorption in the vicinity of the fundamental absorption edge of this layer was The fundamental absorption edge of ε-GaSe single crystals is well studied and is determined by the excitonic absorption mechanism [38][39][40]. The absorption coefficient (α) of radiation with wavelength λ, in both specially undoped and 3.00 at.…”