We report on the growth by molecular beam epitaxy of modulation-doped GaAs-(Ga,Al)As heterostructures with low-temperature hole mobility exceeding 1.2×106 cm2 V−1 s−1 with carrier concentrations as low as 0.8×1011 cm−2: The highest value observed at such low densities. We also report the first observation of persistent positive photoconductivity in a two-dimensional hole gas. An analysis of the number density and temperature dependence of the mobility leads us to conclude that the mobility is limited by phonon scattering above ∼4 K and interface scattering at lower temperatures.

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