CHARACTERIZATION OF CZOCHRALSKI-GROWN AND HYDROTHERMAL-GROWN BI12SIO20

An investigation was made of the defect characteristics of single-crys tal Bi12SiO20 (BSO) grown by both Czochralski (Cz) and hydrothermal tr ansport methods. Only Cz-grown BSO is photorefracting using a coherent pumping source centered around 500 nm, as undoped hydrothermal BSO is transparent throughout the visible spectrum. Thermally stimulated cur rent (TSC) studies in conjunction with temperature-dependent optical-a bsorption measurements and room-temperature photoconductivity data all indicate that the hydrothermal material is near intrinsic in terms of its low defect content. TSC measurements made below room temperature indicate that concentrations of traps of activation energy <0.7 eV are a factor of 10(3) smaller in hydrothermal than in Cz BSO. At least si x different defects were identified in the TSC measurements. Temperatu re-dependent optical-absorption measurements indicate two Urbach band tails for Cz-grown materials that are not observed in the hydrothermal materials. Cz material of lower purity also possesses an impurity ban d tail which can be observed through temperature dependent optical abs orption measurements. Comparison of Cz material with hydrothermal BSO of similar impurity content suggests that the Bi(Si) defect responsibl e for the 500 nm absorption may be complexed with an impurity such as Fe or V. In the absence of this defect, however, these impurities have no effect on the absorption. An additional defect in the TSC data is also related to a transition metal impurity. The results indicate that the photoconductivity associated with the photorefractive effect in C z material must proceed via a trap-hopping conduction mechanism that i s missing in the intrinsic material.