Interband transitions in bismuth germanate crystals grown from the melts of several [Ge]/[Bi] ratios

We investigated bismuth germanate (Bi12GeO20) single crystals grown from melt with several GeO2 mole percentages, 100 x [GeO2]/([GeO2]+[Bi2O3]) = 8, 12, 14.3, 20, 24, as an aid to understanding the stoichiometric dependence of their optical and electrical properties. Although no observable differences in their lattice constants or GeO2 concentrations could be detected, their crystalline properties depended strongly on the melt composition. Their optical absorption increased almost linearly with the increase of bismuth concentration in the melt, and the dark conductivity increased in crystals grown from germanium-rich melts. The crystal grown from the stoichiometric melt with 14.3-mol. % GeO2 exhibited the largest photoconductivity, which was measured optically by grating-decay experiments. The photoconductivity was quenched in crystals grown from bismuth melts that were either richer or poorer than the stoichiometric melt. Photoluminescence emission spectra displayed two broad bands, with maximum intensities at 1.9 eV (intense) and 2.9 eV (very weak) for the crystal grown from the bismuth-rich melt (8-mol. % GeO2). Only one band, at 2.9 eV, was observed for all other crystals. Based on these optical and electrical results, a three-level transition model is suggested to interpret the stoichiometry-dependent transport mechanism. (C) 1999 Optical Society of America [S0740-3224(99)00208-8].