High electron mobility and low noise quantum point contacts in an ultra-shallow all-epitaxial metal gate GaAs/Al_xGa_1−xAs heterostructure

The mobility of the two-dimensional electron gas (2DEG) in shallow GaAs/$AlxGa1−xAs$ heterostructures is strongly suppressed by unwanted Coulomb scattering from surface charge, likely located in native surface oxides that form after the wafer is removed from the crystal growth system. Here, we show that this native surface oxide can be eliminated by growing an epitaxial aluminum gate before removing the wafer from the growth chamber. We fabricate accumulation mode devices on two wafers with nearly identical structures and growth conditions: one with an epitaxial aluminum gate 35 nm above the channel and another with an ex situ metal gate deposited on an aluminum oxide dielectric. Low temperature transport measurements show that the epitaxial gate design greatly reduces surface charge scattering, with up to $2.5×$ increase in mobility. Despite the ultra-shallow 2DEG (35 nm), the mobility remains high even at low carrier densities. Finally, we show that the epitaxial aluminum gate can be patterned to make nanostructures by fabricating a quantum point contact that shows robust and reproducible 1D conductance quantization, with extremely low charge noise.