Single‐period modulation‐doped Al0.3 Ga0.7 As/GaAs heterostructures were grown by molecular beam epitaxy in a substrate temperature range of 580–750 °C. Excellent surface morphologies were routinely obtained throughout the entire range of growth temperatures. Highest mobilities (in dark) obtained were 8000, 90 000, and 196 200 at 300, 78, and 10 K, respectively. In room light, these figures improved to 9000, 110 000, and 250 000 cm2/Vs. The sheet carrier concentration yielding the best electron mobility was about 8×1011 cm−2. While the mobilities did not depend on the substrate growth temperature between 600 and 675 °C, some degradation was observed above 675 and below 600 °C.