Optical metastability in bulk GaN single crystals

Abstract

Bulk GaN single crystals were grown from cold pressed GaN powder by sublimation in flowing ammonia. Optical transmission measurements indicated that the absorption coefficient for the transparent samples is 50 cm−1 in the wavelength region from 650 to 400 nm. Optical metastability in bulk GaN crystals was studied through time dependent photoluminescence both at room and liquid–nitrogen temperatures. The observation included decreasing output intensity of the ultraviolet emission attributed to the band edge and increasing output intensity of a new emission band centered at 378 nm at room temperature. At liquid–nitrogen temperature, 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 increased 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.