Single period modulation doped AlxGa1−xAs/GaAs heterostructures were grown by molecular beam epitaxy using a substrate temperature range of 580–750 °C. Excellent surface morphologies were routinely obtained throughout the entire range of surface temperatures. The highest mobilities obtained for structures with the AlxGa1−xAs/GaAs on top of the GaAs were 9000, 133 000, and 282 000 cm2/V s at 300, 78, and 10 K, respectively. The sheet carrier concentration yielding the best electron mobility was about 8×1011 cm−2. While the mobilities were independent of substrate growth temperature between 600 and 675 °C, some degradation was observed outside this range. Structures with the GaAs on top of the AlxGa1−xAs showed mobilities as high as 35 400 cm2/V s at 10 K when grown near a substrate temperature of 720 °C. Mobility enhancement was not observed in samples grown at temperatures well below 700 °C.