In situ epitaxial aluminium gates in ultra-shallow GaAs/Al_xGa_1-xAs heterostructures for low noise quantum point contacts

The mobility of the two-dimensional electron gas (2DEG) in shallow GaAs/Al_xGa_1−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. In this work, we show that this native surface oxide can be eliminated by growing an epitaxial aluminium gate before removing the wafer from the growth chamber. We examine the influence of aluminium gate thickness and the use of different semiconductor wetting layers on the semiconductor-aluminium interface and correlate this with the electron mobility. Transmission electron microscope (TEM) characterisation of the different wafers shows the in-situ epitaxial aluminium is crystalline, with a near-perfect semiconductor-aluminium interface that is oxide-free. The electron mobility is found to strongly depend on aluminium thickness, as well as the wetting layer the Al is grown on. Low-temperature transport measurements show the in-situ epitaxial aluminium gate design greatly reduces surface charge scattering, with up to an 2.5× increase in mobility compared to a device with an ex-situ gate design. We demonstrate the use of epitaxial gates for quantum devices making a quantum point contact which shows robust and reproducible 1D conductance steps. Noise measurements reveal a reduction in charge noise of over an order of magnitude with respect to previous work, despite the 35-nm channel.