Transport processes of photoinduced carriers in Bi12SiO20

The optical absorption edge of undoped, lightly Al-doped, and heavily Al-doped Bi12SiO20 single crystals is found to be exponential and follows Urbach's rule with σ0=0.71 at room temperature. The band edge is at 3.25 eV and is broadened by excitons and perhaps by impurities or defects. At 80 °K, the band edge is found to be shifted to 3.40 eV. The broad shoulder in the optical absorption and the secondary peak in the photocurrent excitation spectrum are attributed to the presence of a silicon vacancy complex. The longitudinal photocarrier response due to pulsed uv excitation leads to a value of the electron drift mobility of μd=0.029±0.003 cm2/V sec and a value for the range of electrons (μτ)e=8.5×10−7 cm2/V. The response times of electrons and holes (or the relaxation times) are determined to be 6.5×10−3 and 4.3 ×10−3 sec, respectively. Electrons dominate the photocurrent in undoped and lightly Al-doped crystals, while holes dominate the photocurrent in the heavily Al-doped crystals. Thermally stimulated current between 80 and 360 °K shows three major electron traps with energetic depths of 0.34, 0.54, and 0.65 eV in undoped crystals, and major hole traps at 0.26, 0.31, and 0.43 eV in the heavily Al-doped crystals. A band diagram for the undoped single crystals is proposed to explain the photocurrent kinetics and the temperature dependences of the photoluminescence emission bands at 1.95 and 1.30 eV and the temperature dependence of the photocurrent.