Increased mobility anisotropy in selectively doped AlxGa1-xAs/GaAs heterostructures with high electron densities

We have investigated the electron transport properties of a two-dimensional electron gas in a selectively doped AlxGa1-xAs/GaAs heterostructure with r educed Al content in the doped layer by recording the longitudinal resistan ce as a function of magnetic field and performing Hall measurements at low magnetic fields on the same sample. The electron density was varied by step wise illumination and we observe a dip in the mobility versus density depen dence around a density of 5.0x10(11) cm(-2). As a reason for this mobility decrease we identify by magnetotransport measurements the onset of parallel conductance in the Si-doped AlxGa1-xAl layer. With the onset of parallel c onductance, the anisotropy in the electron mobility increases significantly . We attribute this to an increase of interface roughness scattering induce d by the parallel conducting channel. (C) 2000 American Institute of Physic s. [S0021-8979(00)03213-8].