High mobility electron gas in selectively doped n:AlGaAs/GaAs heterojunctions

Selectively doped n/GaAs heterojunctions have been grown by molecular beam epitaxy, employing a slow growth technique, at a substrate temperature of 600°C. The effect of the undoped AlGaAs spacer thickness on carrier density and Hall mobility was investigated. Mobilities as high as 9200, 200 000, and 1 060 000 cm2/Vs at 300,77, and 4.2 K, respectively, were measured in the dark for a spacer thickness of ∼180 Å and an areal carrier density of ∼2.2×1011 cm -2. Surprisingly, samples with spacer thicknesses of 80 Å had 4-K mobilities of ∼800 000 cm2/Vs, higher than expected theoretically from the structural parameters.