Photoelectronic properties of high-resistivity GaAs : Cr

The photoelectronic properties of high-resistivity n-type GaAs : Cr are quite similar to those of high-resistivity n-type GaAs : O described in the preceding paper. In the low-temperature region, intrinsic photoconductivity is n type, increases exponentially with 1/T, and can be consistently described by a simple one-level Shockley-Read recombination model with a level lying 0.66 eV below the conduction band and a density that is remarkably constant over a variety of different materials. Two levels characteristic of the Cr doping lie at 0.86 eV below the conduction band and at 0.9 eV above the valence band. Low-frequency photocurrent oscillations are associated with levels at 0.86 and 1.25 eV below the conduction band. The extrinsic photoconductivity at 0.86 eV is produced via two steps: (i) electrons are photoexcited from the ground state to the excited state of Cr+2 (d4) center, and (ii) they are then thermally excited to the conduction band.