We describe an experimental setup designed to study photoconductive transients in semi-insulating materials. The method, known as photoinduced transient spectroscopy, is based on a digital signal-averaging technique. Although this digital method eliminates or reduces many of the experimental problems encountered when using an analog approach, several new problems arise. The method is described, and the difficulties are illustrated using data obtained from both Cr-doped and nominally undoped samples of GaAs.
Experimental results of photoexcitation and thermal deactivation of localized vibrational mode absorption of the oxygen-vacancy complex in GaAs are reported. Two levels within the energy gap are observed, one located at 0.14 eV and the other between 0.57 eV and 0.75 eV below the conduction-band minimum. It is proposed that this center exhibits a negative U property with the second electron ionization energy higher than that of the first electron.
An experimental arrangement is described by which emission coefficient behavior associated with deep-level traps in semi-insulating GaAs can be surveyed, from the photoconductive transient behavior following a trap-filling light pulse. This method, often referred to as PITS (for photoinduced transient spectroscopy) has analogies with the transient capacitance spectroscopy usable for conductive material, but some significant differences also. The necessary experimental precautions which can maximize the probability of getting reliable data are described, and illustrated by PITS results obtained with bulk semi-insulating samples of Cr-doped GaAs. Those results include data for five kinds of trap with activation energies from 0.3 to 0.8 eV, of which some are readily identifiable and others are not. The method described in this paper relies on analog measurement of the change in sample voltage during a specified time interval of the photoconductive decay.
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