Realization of a quasi-three-dimensional modulation-doped semiconductor structure

We report the realization of a modulation-doped quasi-three-dimensional electron system. The structure consists of a 2000-Å-wide undoped Al _xGa_1-xAs well bounded by undoped (spacer) and doped layers of Al_yGa_1-yAs (y>x) on both sides. The alloy composition in the well (x) is varied quadratically so that the combined potentials due to the Al_xGa_1-xAs and the electric charge in the well produce a square potential well with a nearly uniform carrier density. Magnetotransport data reveal that the system contains ≅2.5×10¹¹ cm^-2 electrons, which occupy four electric subbands and have a low-temperature mobility in excess of 1×10⁵ cm²/V s indicating the high quality of the structure.