Photo-Hall effect and photoconductivity in p-type epitaxial GaAs:Cr

Measurements of the photo-Hall effect, AC photoconductivity, and pulsed-laser photoconductivity have been carried out on epitaxial p-type GaAs:Cr grown by MOCVD. The temperature range investigated was principally 80 to 300K, although photo-Hall measurements extended down to 10K. The samples remained p-type under illumination with light in the wavelength range 1 to 3 mu m. Optical thresholds near 0.8 and 0.4 eV were observed and these were interpreted as Cr 3+ /Cr 2+ and Cr 4+ /Cr 3+ transitions respectively. The temperature dependence of the higher threshold was of the form (0.831+or-0.008-2.8*10 -4 T eV). The lower threshold was at (0.42+or-0.01)eV in the range 100 to 160K. The behaviour of the photoconductivity of steadily illuminated samples as a function of temperature exhibited four distinct regimes. A detailed explanation of this behaviour was made on the basis of a model involving the conversion of Cr 3+ to Cr 2+ and Cr 4+ as the temperature was lowered. One consequence of this conversion was the profound weakening of the electrical screening of surface charges. The degeneracy ratio for the Cr 4+ /Cr 3+ level could be only roughly determined: 0.5