Transport processes of photoinduced carriers in Bi12SiO20

Abstract

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.

Conductivity mechanism analysis at high temperature in bismuth titanate: A single crystal with sillenite-type structure

“…In fact, the Ti-vacancy complex has been suggested. 15 Taking into account that the anion sublattice is fully occupied and the oxygen is relatively immobile in the structure, most of the charge carried is electronic 16,17 and correlated to the complex defect formed by the ͑Bi Ti 3+ + h 0 + ͒ pair.…”

“…As a whole, thermistor parameters derived and conductivity behavior are compatible with the conduction mechanism actuating on the Bi 12 TiO 20 that is of the hopping type, in accord with defects reported previously. [16][17][18][19] From high temperature impedance measurements, the conductivity value extrapolated at room temperature equal to 1.2 ϫ 10 −15…”

Effect of axial vibration on Bi12SiO20 crystal growth by vertical Bridgman technique

“…Generally, absorption shoulder in BSO crystal is related to lattice defects of Si vacancy and Bi ions in Si sites in the SiO 4 tetrahedron [24]. According to the earlier reports [25,26], about 10% of the Si sites are substituted by Bi ions. The reduction of the absorption shoulder is due to the concentration decrease of Si vacancy and Bi ions in Si sites.…”

Cathodoluminescence and photoluminescence in the core region of Bi12GeO20 and Bi12SiO20 crystals

“…In the first case, the oxygen vacancy acts as a doubly positive charged center, and in the second case the complexes act as quadruple negative charged centers which act as donor centers responsible for the n-type photoconductivity. 3,15,17,18 The concentration of the 640 nm related defect is higher in the core and possibly it presents different charge states outside and in the core region. This would explain the peak position shift and the irradiationinduced emission out of the core.…”