PHOTO-HALL STUDIES OF HIGH-PURITY GAAS

High-purity GaAs layers which are nearly or fully depleted of carriers at low temperatures, due to surface and interface depletion effects, can be characterized electrically by utilizing the phenomenon of persistent photoconductivity. To facilitate electrical measurements of such layers, at 4.2 K the sample is momentarily illuminated by above band-gap light, which causes a reduction in the surface and interface depletion region. After illumination the effects of the photoinduced charge neutral region persist until at some higher temperature, the charge distribution in the sample relaxes back to its original equilibrium state. Results of variable temperature Hall-effect measurements performed under these conditions show that the sheet carrier concentration is increased as compared to measurements obtained in the dark but that the mobility is unchanged. The increase in the sheet carrier concentration after illumination results from the decrease of the surface and interface depletion widths. Such measurements can provide a method for judging the quality of layers that are fully depleted of carriers. The mobility of a high-purity GaAs layer which becomes fully depleted of carriers at low temperatures in the dark, measures 180 000 cm2/V s at 77 K by this method. Photocapacitance-voltage profiling measurements clearly show the change in the depletion widths. A model is presented which accurately depicts the temperature dependence of the band-bending potentials at the surface and interface after illumination with above band-gap light.