Ultrasonic study of photoconductivity in undoped n-GaAs at low temperatures

Abstract The velocity of piezoelectrically-active, ultrasonic shear waves was measured before, during, and after sub-band gap illuminations of undoped, n-type GaAs at temperatures between 1.5 K and 68 K. Unlike semi-insulating or “ordinary” semiconducting material, the excess donors in our sample had a thermal ionization energy of 0.22 eV. Our results revealed photoconductivity during illumination with 1.26, 1.10, or 0.95 eV narrow band light, partial reduction of such photoconductivity by the addition of Ge-filtered light, and decaying and persistent conductivity components after cessation of illumination. Interpretation of these effects is given which includes photoexcitation of electrons into the conduction band from a large fraction of near mid-gap levels of the EL2 center. The results were very important for our interpretation of the photo-ultrasonic behavior recently reported by us for oxygen-doped, n-GaAs at low temperatures.