Excitons in high-quality CuInS2 single crystals

Photoluminescence of CuInS2 single crystals grown by traveling heater and chemical vapor transport methods

“…It is noteworthy that some of these PL lines were previously observed [11][12][13][14]. Experiments on both types of crystals confirmed that lines of free and bound excitons were shifted to high energy as the temperature rose from 4.2 to 78 K. In particular, the line of free A-excitons shifted in this temperature range by 1.8-2.0 meV, which agreed with the literature [8,9,11]. The physical reason for the energy increase of free excitons and, therefore, the CuInS 2 forbidden-band gap in the temperature range 4.2-78 K, is the anomalous temperature dependence of p-d-hybridization of orbitals in A I B III C 2 VI semiconductors.…”

“…Spectra at 78 K were dominated by a line with a maximum at 1.537 eV and half-width ≈8 meV that was related to recombination of free A-excitons in the ground state (n = 1). The low-energy inflection near 1.532 eV on the path of the line for free excitons was related to line 2 of bound excitons according to the literature [8][9][10]. A weaker high-energy line separated by 15.7-16.2 meV from the line for free A-excitons was due to recombination of free A-excitons located in the first (n = 2) and second (n = 3) excited states.…”

“…A weaker high-energy line separated by 15.7-16.2 meV from the line for free A-excitons was due to recombination of free A-excitons located in the first (n = 2) and second (n = 3) excited states. Lines corresponding to these optical transitions were not resolved at 78 K. In contrast with the literature [8,9,11], it was assumed that the high-energy PL line near 1.552 eV at T ≥ 78 K was due to recombination through excited states n = 1 and 2 of free A-excitons and not to radiative recombination through degenerate BC-states of the valence band that arose from spin-orbit splitting [11].…”

“…The intensity of the line for free A-excitons was lower than lines for bound excitons for THM single crystals. We were able to resolve in PL spectra the fine structure of free excitons, lines A LPB 1.5347 eV and A UPB 1.5362 eV related to the lower and upper polariton branches of free A-excitons, for the very strong line 2 of bound excitons [8,13,18]. It is known that exciton-photon (polariton) coupling may be characterized by longitudinal-transverse splitting Δ LT A of the A-exciton ground state, the value of which corresponds to the maximum observed spectral separation between the two lines of polariton branches A LPB and A UPB .…”

“…1). Furthermore, some lines related to excitons 1, 2, 4, 5, 6, and 8, that were designated as before [8][9][10], were visible at 4.2 K in PL spectra (depending on the type of single crystal). It is noteworthy that some of these PL lines were previously observed [11][12][13][14].…”