Electrical and optical characterization of defect levels in Bi(12)GeO(20) and Bi(12)SiO(20)

Scope and method of study. The trapping states of Czochralski-grown undoped and Ga-doped {dollar}rm Bisb{lcub}12{rcub}GeOsb{lcub}20{rcub}{dollar} (BGO) and {dollar}rm Bisb{lcub}12{rcub}SiOsb{lcub}20{rcub}{dollar} (BSO) were studied using a variety of experimental techniques including optical absorption, steady-state photoconductivity, thermally stimulated conductivity, thermoluminescence, dark conductivity, time-of-flight and photorefractive measurements. The focus was placed on those states which were thought to strongly affect the photorefractive behavior.; Findings and conclusions. Undoped {dollar}rm Bisb{lcub}12{rcub}GeOsb{lcub}20{rcub}{dollar} and {dollar}rm Bisb{lcub}12{rcub}SiOsb{lcub}20{rcub}{dollar} exhibit a broad absorption band due to the optical excitation of electrons to the conduction band from a level (referred to as a "deep donor") about 2.7 eV from the conduction band. BGO and BSO show additional absorption and photoconductivity bands appear after illumination. Absorption and photoconductivity measurements show that there are a large number of closely spaced states in the forbidden band. The thermal stability of the photochromic absorption and photoconductivity bands is presented.; The spectra obtained in thermally stimulated conductivity (TSC) measurements are complex, showing the existence of many states in the band-gap. The dominant peak in the TSC spectra, however, is between the temperatures of 120 and 180 K in undoped BGO and 120 and 160 K in Ga-doped BGO. Our analysis shows that two traps cause this peak in undoped BCO and they have activation energies of 0.24 eV and 0.29 eV with distribution widths of about 0.05 eV. In Ga-doped BGO one trap causes this peak and has an activation energy of 0.29 eV and a distribution width of {dollar}sim{dollar}0.09 eV.; We measured the mobility of BGO and BSO between about 210 K and 300 K using the time-of-flight technique. We found that the mobility of undoped BGO varies from 0.11 cm{dollar}sp2{dollar}/Vs at 300 K to {dollar}rm6.1times10sp{lcub}-4{rcub} cmsp2{dollar}/Vs at 210 K. In BSO the mobility is about an order of magnitude lower. The trap limiting the mobility is the same trap which causes the TSC peak between 120 and 180 K in undoped BGO. From the temperature dependencies of the photorefractive effect we found that this trap also dominates the photorefractive behavior of BGO below 200 K.