Temperature dependence.of the.performance of ultraviolet detectors

We present the results of a comprehensive study of the temperature dependences of the quantum efficiency for ultraviolet detectors based on GaAs, GaP and 4H-SiC Schottky structures, and on Si GaAs p-n structures. For ultraviolet detectors based on Schottky structures, the quantum efficiency increases with increasing temperature for all photon energies, even including the semiconductor intrinsic absorption region. On the other hand, for ultraviolet detectors based on p-n structures, the quantum efficiency is practically temperature independent in the semiconductor intrinsic absorption region. The change in the quantum efficiency for the GaAs and Si detectors is less than 0.01% per degree. To explain the measurements, a variable trap occupancy model is presented. Subsurface imperfections of the semiconductor cause fluctuations in the profile of the conduction band and the valence band edges. In the presence of an electric field in the space-charge region, these fluctuations become fluctuating traps for electrons as well as holes. As a result, charge carriers of different signs become localized in the same spatial region, which may give rise to their recombination. As the temperature increases, a thermal dissociation of electron-hole pairs trapped by the caption centres occurs. The recombination losses decrease, so the density of free thermalized carriers increases and therefore the higher the temperature, the higher the quantum efficiency. This process continues until the traps are completely emptied. In p-n photodetectors the space-charge region is located in the crystal bulk and imperfections in subsurface regions do not influence the photoelectric conversion process. For 4H-SiC Schottky photodetectors a second region of increasing quantum efficiency was observed. It is connected with the conversion from the indirect to the direct optical transition and allows us to estimate the minimal energy of the direct optical transition in 4H-SiC (similar to 4.9 eV). (C) 2003 Elsevier B.V. All rights reserved.