Charge transfer mechanisms between some shallow-trap centres involved in the photochromism of Bi12GeO20

Abstract Charge transfers between traps in undoped Bi12GeO20 (BGO) crystals have been investigated so as to complete our previous studies concerning the mechanism of the photochromism in these crystals. Thermally stimulated current measurements as a function of both the temperature at which the crystals are photoexcited and the density of energy used to photoexcite them were carried out between 80 and 350 K. The results show that the detected electron traps, due to their behaviour, can be schematically divided into two families: the shallower ones and the deeper ones. Two of the most important traps among the latter which play the role of electron reservoirs for the fundamental state of the photochromic transitions in the visible spectral range are populated via the former. Hence, the shallower traps act as precursory levels to the filling of the deeper traps.