“…For those high electron densities, population of the second subband might occur. [4][5][6][7][8] The mobility determining scattering mechanisms that have to be taken into account in connection with selectively doped heterostructures are remote impurity scattering, background impurity scattering, and interface roughness scattering. 12,13 Varying the electron density in a given heterostructure, either by applying a gate voltage or, more simply, by stepwise illumination with band gap radiation exploiting the persistent photoeffect, is an established method for distinguishing among different scattering mechanisms: 12,13 For remote and background impurity scattering the mobility increases with increasing electron density, whereas it should decrease for pure interface roughness scattering.…”