Bulk GaN single crystals were grown from cold pressed GaN powder by su
blimation in flowing ammonia. Optical transmission measurements indica
ted that the absorption coefficient for the transparent samples is 50
cm(-1) in the wavelength region from 650 to 400 nm. Optical metastabil
ity in bulk GaN crystals was studied through time dependent photolumin
escence both at room and liquid-nitrogen temperatures. The observation
included decreasing output intensity of the ultraviolet emission attr
ibuted to the band edge and increasing output intensity of a new emiss
ion band centered at 378 nm at room temperature. At liquid-nitrogen te
mperature, the photoinduced emission band consisted of at least one LO
-phonon replica of the zero-phonon line centered at 378 nm. The ratio
of output intensities of the photoinduced band to the band edge increa
sed by a factor of 10 during 27 min of exposure time. The photoinduced
effect is attributed to the metastable nature of traps in bulk GaN. (
C) 1997 American Institute of Physics.