The visible emission and excitation spectra of micro-crystals which contained single and parallel double twin planes were obtained. A band at 462.28nm (2.6813eV) was observed in the excitation spectrum. This feature was about 0.0025eV below the indirect exciton band edge. Coincident emission features were also observed. The band in the excitation spectrum of micro-crystals with 2 parallel twin planes was often split into a doublet, with splittings between 0.001 and 0.002eV. The new band was attributed to an exciton that was created at a stacking fault, and the splittings were thought to be due to tunnelling of the exciton between close twin planes. The small binding energy of the exciton to the twin plane was suggested to be in accord with the very small strain field around a twin plane.

A.Marchetti: Physical Review B, 1994, 50[16], 12164-7