Comparison of the layered semiconductors GaSe, GaS, and GaSe_(1-x)S_x by Raman and photoluminescence spectroscopy

摘要

The room-temperature Raman spectra of single crystals of GaSe, GaS, and mixed compounds GaSe_(1-x)S_x. with 0.02 ≤ x ≤ 0.8 were measured with a HeNe laser in confocal configuration. The changes in the spectra indicate changes of the crystal structure. The spectra of pure GaSe and of the mixed compound with x=0.02 show pronounced photoluminescence signals blueshifted from the laser line, whereas these signals do not appear for higher sulfur content. Their origin is interpreted as second-harmonic generation in the laser focus causing the formation and radiative decay of Wannier excitons. Two-photon absorption is ruled out, since the effect is absent in the centrosymmetric crystals with x > 0.02. With a green laser whose photon energy is larger than the band gap, strong photoluminescence is also observed in crystals with higher sulfur content.