DETERMINATION OF THE MOBILITY AND TRANSPORT-PROPERTIES OF PHOTOCARRIERS IN BI12GEO20 BY THE TIME-OF-FLIGHT TECHNIQUE

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, a s far as we know, the first successful direct determination of electro n mobility using the classical time-of-flight method. This consists in measuring the velocity of a sheet of excess carriers, created by a sh ort-duration excitation pulse of strongly absorbed photons close to th e surface. These carriers drift through the sample under the action of an applied electric field. However, the technique could not be used i n its original configuration. A constant background illumination was r equired in order to saturate traps and to prevent space charge build u p. Optimal illumination conditions (wavelength below 550 nm, suitable integrated number of photons) were found under which a quasifree, nond ispersive, charge transport was observed. The mobility is limited by i nteraction with a shallow trap, the population of which can be modulat ed by the additional monochromatic illumination. An unexpected high va lue was found as compared to results published earlier. This value is probably very close to the microscopic collision-limited mobility. Thi s explains the relatively small spread of the results obtained with di fferent nominally undoped Bi12GeO20 (BGO) samples. The values lie in t he range 0.2-1.0 cm2 V-1 s-1. They are in excellent agreement with tha t measured elsewhere in BSO using the holographic time-of-flight techn ique.