Origin of the deep center photoluminescence in CuGaSe2 and CuInS2 crystals

Abstract

Photoluminescence (PL) of CuGaSe2 and CuInS2 single crystals, either as grown or Cu annealed, reveals a broad and clear deep emission band at hν≈Eg−0.6 eV. In both of these as-grown materials this band has a similar doublet structure with the two D1,D2 subbands separated by about 100 meV. After the Cu annealing all samples became highly compensated and an additional deep PL band (W band) appeared on the high energy side of these D bands. This suggests a closely similar origin of the emission for the both materials. By a straightforward model calculation we show that the changes in the shape and intensity of these emission bands—due to variation of temperature, excitation intensity or due to the Cu annealing—are well explained if we assume that the D1 and D2 PL subbands originate in the recombination between the closest and the second closest donor–acceptor pairs, with the essential ingredient of the emission center being an interstitial donor defect, i.e., either Cui or Gai in CuGaSe2 and Cui or Ini in CuInS2. The W band in both compounds appears to be due to the recombination of an electron from this deep donor level with a hole in a deep localized state of the valence band tail.