Two-dimensional electron gas structures with mobilities in excess of 3×106 cm2 V−1 s−1

Abstract

We have demonstrated that the mobility of two-dimensional electron gas (2DEG) structures shows a gradual improvement with the number of uninterrupted growth runs in the molecular-beam-epitaxy system. Some, but not all, of this improvement can be attributed to the cleanup of the GaAs layers in the structure, and we suggest that there is a corresponding cleanup of the (Al,Ga)As which is also influencing the mobility. Once the system has passed the cleanup phase, a systematic trend of carrier density and mobility with undoped spacer thickness was observed, with a peak mobility at 4 K of 2.12×106 cm2 V−1 s−1 for a sheet carrier density of 2×10−11 cm−2 occurring at a spacer thickness of 800 Å. A further increase in mobility was achieved by using a thicker region of doped (Al,Ga)As, thereby moving the ionized centers in the surface depletion layer further away from the 2DEG. This has enabled us to produce a sample with mobility at 4 K of 3.1×106 cm2 V−1 s−1 (at a sheet charge of 3.1×1011 cm−2)—the first time such a value, to our knowledge, has been reported.