LOW-TEMPERATURE PHOTOCHROMIC RESPONSE OF GALLIUM-DOPED BISMUTH GERMANIUM OXIDE

Gallium is one of several dopants that electronically compensate the deep donor responsible for the yellow coloration observed in bismuth germanium oxide (BGO). A series of Ga-doped BGO crystals were grown using the Czochralski method to confirm the formation of a trapped hole center following exposure to near band edge light at low temperatures. The deep donor absorption shoulder disappeared for Ga concentrations in the 3%-4% (melt) range. Excitation at 10 K with 3.3 eV light produces photochromic absorption bands. For low gallium concentrations the photochromic bands were the same as the ones observed in undoped BGO. For concentrations of 4% or more the photoinduced absorption consisted of overlapping bands at 1. 1, 1.55, and 2.45 eV. These bands grew uniformly with exposure time and annealed together in the 100-120 K temperature range. By comparison with earlier results on Al-doped BGO and bismuth silicon oxide (BSO), we believe that the 1.1 eV band in BGO:Ga and the 1.0 eV band in BGO:Al (and BSO:Al) are due to trapped hole centers ([GaO4]0 and [AlO4]0, respectively).