Wavelength-dependent photoconduction effects on the second sub-band occupancy in (Al, Ga)As/GaAs heterojunctions

The authors have used magneto-transport measurements with the magnetic field aligned perpendicular to, and then parallel to, the interface of a number of (Al, Ga)As/GaAs two-dimensional electron-gas samples, to determine the populations of the first and second sub-bands in the interface potential well. The total carrier density in the samples has been gradually increased by pulsed illumination either with a red (hv=1.95 eV) or infrared (hv=1.43 eV) LED, and it has been found that the carrier density at which the second sub-band is first occupied is lower when the red LED is used. This difference is ascribed to the effect of electron-hole pairs generated in the GaAs altering the depletion charge, and hence the sub-band spacing; such a mechanism does not operate when the infrared LED is used. Supporting evidence has been obtained from spectroscopic studies of the persistent photoconduction effect, and from mobility measurements.