AN INTERRUPTED FIELD TIME-OF-FLIGHT (IFTOF) TECHNIQUE IN TRANSIENT PHOTOCONDUCTIVITY MEASUREMENTS

An interrupted field time-of-flight (IFTOF) technique that enables displacement-current-free measurement of the photocurrent in the conventional transient photoconductivity experiment is described and implemented. During the drift of the photoinjected charge carriers across the sample, the applied field is removed at time T1 and reapplied at time T2=T 1+ti, where ti is the interruption time. During the interruption period ti, the charge carriers interact with the deep traps so that the recovered photocurrent when the field is reapplied at T2 = T1 + ti indicates the concentration of carriers remaining in the transport band. Although the IFTOF technique has a number of distinct advantages for studying charge trapping and release processes in high-resistivity solids, it has generally not been adopted as a convenient experiment since the sample capacitance results in large displacement currents at switching. The present paper describes a Schering-type bridge network that is excited by a switchable floating high-voltage supply. Recently available high-voltage complementary TMOS transistors were used to switch voltages as high as 500 V. Trigger signals to initiate the various IFTOF events were simply and economically generated from TTL logic gates and IC timers while the required time delays were obtained via clocked digital countdown techniques. The IFTOF method was successfully applied to the examination of hole trapping processes in chlorinated a-Se:0.3%As xeroradiographic-type films for which the conventional TOF measurement indicated essentially trap-free photocurrent. Using the IFTOF technique, it is shown that over a time scale far exceeding the conventional TOF transit time, the photoinjected hole concentration under low field conditions decays almost exponentially with a well-defined trapping time τ. Furthermore, by interrupting the electric field while the photoinjected charge packet is at different locations in the film, it is shown that the IFTOF technique may be used to examine the dependence of the trapping time on the distance into the film. IFTOF is therefore a valuable technique for studying trapping inhomogeneities in amorphous semiconductor films. It is expected that the simple displacement-current-free IFTOF technique described in this paper, with further improvements, may be applied to study charge carrier trapping and release kinetics in a wide variety of high-resistivity solids.