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

Abstract

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 the sillenite structure. An electron trapped at such a MoGe center could act as a small polaron and give rise to the bands. The photorefractive response of Mo-doped BGO was measured at 514nm as a function of temperature down to 116K. Doping with Mo causes a significant slowing of the grating growth rate. The growth rate slowed as the temperature was lowered. The diffraction efficiency of the gratings in Mo-doped BGO increased significantly as the sample was cooled down to 216K. Below that temperature the growth rate was so slow that the gratings did not reach saturation for the 1s “write” time. In undoped BGO the growth rate of the gratings slowed as the temperature was lowered. However, they remained much faster than in the Mo-doped samples and they continued to reach saturation down to the lowest temperatures studied. The dark decay of index gratings in Mo-doped BGO was also modified. At room temperature the dark decay was much faster than in undoped BGO. The dark decay slowed as the sample was cooled and for temperatures below about 200K is nearly the same as that of undoped BGO.