Studies of Photo-Excited and Trapped Electrons in Cubic BISMUTH(12) Silicon OXYGEN(20)

We present experimental and theoretical studies of charge transport processes in cubic n-type Bi _{12}SiO_{20 } (n-BSO). We first study the room-temperature photocurrent response to short-pulse illumination in two n-BSO samples called CT1 and SU1 in previous publications. These experiments suggest that drifting electrons spend much time in shallow traps. They allow us to estimate the corresponding trap-limited mobility and to measure the electron lifetime in the conduction band and the dwell time in shallow traps. In sample CT1, we also study the transient photocurrent behavior below room temperature: we find that the charge transport is limited by two sets of shallow traps with energy depths equal to 410 +/- 50 meV and 650 +/- 80 meV. In sample SU1, we directly measure the trap-limited mobility and find it is equal to 0.24 +/- 0.07 cm^2V ^{-1}s^ {-1} at room temperature. We then describe what we believe to be the first measurement of the pure conduction band mobility in n-BSO which we find to be 4.4 +/- 1.3 cm^2V ^{-1}s^ {-1} in SU1. We describe the novel holographic "time-of-flight" technique we developed for this measurement in which we observe the average time for a photoexcited charge carrier to drift in the dark (because of a strong applied electric field) over the period of a grating of charged traps created in the crystal by two interfering short laser pulses. We also use this technique to study the temperature dependence of the mobility. These results suggest the existence of shallow traps of energy depth equal to 320 +/- 40 meV. We also derive an analytical solution to the standard material equations which describes the build-up of the photorefractive grating in the dark after an initial low-energy, spatially -sinusoidal, short-pulse excitation. It is the first short -pulse solution to be developed in a band transport model containing both deep photoexcitable traps and shallow thermally excitable traps. The build-up of the space-charge field includes two components which approach their stationary values with complex time constants. Not only does the oscillatory part of this solution fit well our mobility data, the predicted damping of these oscillations fits well the damping seen in all experiments. (Copies available exclusively from Micrographics Department, Doheny Library, USC, Los Angeles, CA 90089-0182.).