It was noted that the 4.2K photoluminescence spectra of undoped bulk <100> samples which had been grown by using the liquid-encapsulated vertical Bridgman technique were characterized by 3 types of recombination peak. A peak near-bandgap energy was attributed to the recombination of bound excitons. At the low-energy side of bound excitons, a series of peaks with the same energy interval were attributed to a recombination of donor-acceptor pairs, with their longitudinal-optical phonon replicas, which was associated with the Zn acceptor. The recombination of deep levels was characterized by a broad band with a fine structure at the higher-energy side. This seemed to be associated with the recombination of native defects and their phonon replicas. The lattice relaxation of donor-acceptor recombination was smaller while the recombination intensity of deep levels was greater when the 4.2K photoluminescence spectra of the present crystals were compared with those of seed crystals which had been grown by using the liquid-encapsulated Czochralski technique. At room temperature, only a broadened peak near-bandgap energy was observed in both types of crystal, and was attributed to the recombination of band-to-bands. Photoluminescence mapping results showed that the recombination intensity of band-to-bands in the Bridgman crystals was greater than that in the other crystals. When taken together with etch-pit density data, the greater photoluminescence intensity of band-to-band recombination in the Bridgman crystals was considered to be due to lower defect densities; not only of the native defects which were responsible for the recombination of deep levels, but also of the dislocations which were responsible for etch pits.

J.Kang, F.Matsumoto, T.Fukuda: Journal of Applied Physics, 1997, 81[2], 905-9