ELECTRON-TRANSPORT IN SHALLOW HETEROSTRUCTURES WITH ALGAAS AND ALAS BARRIERS

Two series of shallow GaAs heterostructures, with AlGaAs and AlAs barr iers respectively and both delta-doped with around 4 x 10(16) m(-2) Si donors, have been studied using low-temperature magnetotransport tech niques. The electrons in these structures were confined against interf aces 28 nm from the surface. The AlGaAs barrier samples depleted at a uniform rate with bias under a Schottky gate, but the carrier mobility was considerably greater at all biases than predicted assuming random ly positioned donors. The depletion and mobility data for the AlAs bar rier samples could only be explained by postulating the existence of a pool of electrons around the doping plane, which screened the donors to produce high mobilities in ungated samples but which could be remov ed by the application of gate bias. Bias cooling experiments on the Al GaAs samples showed that a proportion of the donor centres were occupi ed when samples with as few as 4 x 10(16) m(-2) donors were cooled wit hout bias. The mobility data from such samples are discussed assuming correlations between the positions of these occupied donors.