The low-temperature photochromic and photorefractive response of bismuth germanium oxide doped with molybdenum

Doping the photorefractive material bismuth germanium oxide, Bi12GeO20 (BGO), with the 4d atom molybdenum introduces only a small shift to lower energy in the absorption cutoff in as-grown or well-annealed samples. Exposing a Mo-doped sample to blue-green light at 80K superimposes an additional well-resolved photochromic absorption band at 1.22eV and a possible weak band at 1.75eV upon the regular photochromic spectrum of undoped BGO. The 1.22eV band has a major anneal stage in the 125–140K range, which is accompanied by a smaller decrease in the regular photochromic bands. Similar to undoped BGO the remaining photochromic spectrum anneals when the sample is warmed above 200K and completely disappears above 250K. The photosensitivity of the additional bands is the same as that of the regular photochromic bands in undoped BGO. Since undoped BGO is an n-type photoconductor the bands are most likely due to an electron trapped at the Mo defect. A simple possible defect would be a Mo substituting for a Ge in t...