High-resolution X-ray diffraction, temperature-dependent photoluminescence and reflection spectroscopic and low-temperature selective photoluminescence spectroscopic investigations were made of single crystals which had been grown by means of solid-phase recrystallization. The high-resolution X-ray diffraction data revealed the high structural perfection of samples which exhibited rocking-curve line-widths that ranged from 15 to 20arcsec. The low-temperature photoluminescence spectra were dominated by the so-called I1deep excitonic line, a neutral-acceptor bound-exciton line, a free-exciton emission and the n = 2 excited state of the free-exciton emission. The main residual impurities were identified as being Li acceptors. Donor-acceptor pair bands were barely detectable at low temperatures, and indicated a low donor content. An important characteristic of these samples was the essential absence of the Cu-related deep emissions which usually marked the photoluminescence spectra of bulk material. Consequently, I1deep was attributed to Zn vacancy-donor complexes. From the temperature dependence of the photoluminescence emission and reflectivity, the band-gap energy of bulk material was found to decrease linearly, at temperatures above 80K, at a rate of 0.00043eV/K. The room-temperature gap was estimated to be equal to 2.720eV. The results indicated that solid-phase recrystallization produced samples with the highest structural quality and purity that were currently possible.

E.Tournié, C.Morhain, G.Neu, M.Laügt, C.Ongaretto, J.P.Faurie, R.Triboulet, J.O.Ndap: Journal of Applied Physics, 1996, 80[5], 2983-9