INVESTIGATION OF SUBBAND-EDGE DISORDER IN A 2-DIMENSIONAL ELECTRON-GAS USING MAGNETIC DEPOPULATION

The resistivity and thermopower of a high-mobility two-dimensional electron gas at a heterojunction have been measured as a function of in-plane magnetic field in the He-3 temperature range. The sharp negative peak previously found as the upper subband just empties is identified with phonon drag; at low temperatures it is replaced by a sharp positive peak which is due to diffusion thermopower. By assuming that the measured resistivity can be used to obtain the density of states of the upper subband, the form and magnitude of the diffusion peak can be calculated and the results are found to be in good agreement with experiment. This suggests that the density of states obtained in this way is quantitatively accurate and that the method gives a good measure of both the form of the disorder broadening, as well as the energy width. Following sample illumination, the phonon-drag peak is found to be time dependent, more so than any other quantity measured; this might suggest that the peak is connected with disorder, but we have no consistent explanation of the behavior. The effect of slight misalignment of the field on both the thermopower and resistivity has also been investigated.