Growth of low-density two-dimensional electron system with very high mobility by molecular beam epitaxy

We report on the growth of modulation-doped GaAs/Al_x Ga _1-xAs heterostructures with mobilities (μ) on the order of 1×10⁶ cm² /V s (at 4.2 K) and areal densities (n_s) below 8×10¹⁰ cm^-2. In growing these structures we employ the atomic plane doping technique and ultrathick (>1000 Å) spacer layers. The mobilities of these structures are the highest ever reported for low densities. Measurements of μ vs n_s as a function of illumination or gate voltage indicate μ∼n_s ^α behavior with α≃0.6 and, even for n_s ≃1.4×10¹⁰ cm^-2, μ has a value in excess of 0.3×10⁶ cm² /V s.