Two-dimensional electron system with extremely low disorder

We report on the growth of modulation-doped GaAs/AlxGa1−x As heterostructures with extremely low disorder by molecular beam epitaxy. In growing these structures we employed the atomic plane doping technique and ultrathick (>1000 Å) spacer layers with graded composition. The structures have mobilities (μ) on the order of 1×106 cm2/V s (at 4.2 K) for areal densities (ns) as low as 4×1010 cm−2. Quantum transport measurements in these structures exhibit new fractional quantum Hall states and demonstrate their exceptionally high quality.