Investigation on semi-insulating GaAs detectors using laser-induced current pulses

Current pulses produced by absorption of picosecond near-infrared light pulses have been used in order to analyze the shape of the electric field and charge transport properties in semi-insulating (ST) GaAs Schottky barrier X- and gamma-ray detectors. Diodes with square pads of side 0.2-1 mm on 200, 600 and 1000-mum-thick SI GaAs substrates have been illuminated from the front (Schottky) contact, from the back (ohmic) side and laterally between the contacts. Current pulses have been recorded with a fast digital sampling oscilloscope, at room temperature, without using any signal amplification. Current and charge pulse shapes have been analyzed as a function of reverse bias voltage, positioned on the detector surface and optical wavelength around the near-infrared absorption edge of GaAs. The wavelength tuning allows for increasing light penetration lengths. Signal analysis gives the peak current, total collected charge and the charge collection time, which are related to the average strength of the electric field and the drift velocity field, while pulse shape depends on the local value of these fields. The experimental results show that a complete detector characterization is feasible with this optical technique, in a manner analogous to that commonly done with gamma- or X- or alpha-ray sources, with the advantage of analyzing the charge collection time. An additional advantage is represented by focusing the light beam on the detector surface, for 1D or 2D scan.