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A direct measurement of a key material parameter in photorefractivity, the charge carrier mobility, has been achieved only recently by means of a holographic time-of-flight technique. In this paper we report, as far as we know, the first successful direct determination of electron mobility using the classical time-of-flight method. This consists in measuring the velocity of a sheet of excess carriers, created by a short-duration excitation pulse of strongly absorbed photons close to the surface. These carriers drift through the sample under the action of an applied electric field. However, the technique could not be used in its original configuration. A constant background illumination was required in order to saturate traps and to prevent space charge build up. Optimal illumination conditions (wavelength below 550 nm, suitable integrated number of photons) were found under which a quasifree, nondispersive, charge transport was observed. The mobility is limited by interaction with a shallow trap, the population of which can be modulated by the additional monochromatic illumination. An unexpected high value was found as compared to results published earlier. This value is probably very close to the microscopic collision-limited mobility. This explains the relatively small spread of the results obtained with different nominally undoped BilaGeO,zo (BGO) samples. The values lie in the range 0.2-1.0 cm2 V-' s-l. They are in excellent agreement with that measured elsewhere in BSO using the holographic time-of-flight technique.

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