Ultra-Shallow All-Epitaxial Aluminum Gate GaAs/AlxGa1-xAs Transistors with High Electron Mobility

The electron mobility in shallow GaAs/AlxGa1-xAs heterostructures is strongly suppressed by charge wafer surface, which arises from native surface oxide layers formed when the wafer is removed from the crystal growth system. Here an in situ epitaxial aluminum gate, grown as part of the wafer, is used to eliminate surface charge scattering. Transmission electron microscope characterization shows that the in situ epitaxial aluminum is crystalline, and the wafer surface is free of native oxide. The influence of Al thickness and the use of different semiconductor wetting layers at the semiconductor-aluminum interface are examined and correlated with electron mobility. The electron mobility is found to strongly depend on aluminum thickness. For 8 nm thick aluminum, the electron mobility is also influenced by the wetting layer, with aluminum grown on GaAs producing higher mobility compared to AlAs or Al0.33Ga0.67As wetting layers. The suppression of surface charge scattering in these all-epitaxial devices allows for high mobilities across a wide density range despite the shallow conduction channel (35 nm below the gate). These measurements also provide a uniquely sensitive method of determining the electrical quality of the semiconductor-metal interface, relevant to the formation of hybrid semiconductor-superconductor devices.