Molecular beam epitaxy (MBE) was used to grow a 500-nm-thick InAs layer with a high (n++) doping concentration of 1.0 × 1020 cm−3, followed by an additional MBE growth of a 50-nm-thick InAs layer with a low (n+) doping concentration of 1.0 × 1019 cm−3. The gate oxide of 30 nm Al2O3 was prepared by atomic layer deposition (ALD) with ~400 cycles of alternating trimethylaluminum (TMA) and H2O pulses at a substrate temperature of 270°C. The estimated dose per cycle was 900 and 1200 liters for TMA and H2O, respectively, and the chamber pressure was maintained at 0.68 torr by continuous flow of dry N2. A TMA predosing step consisting of 20 alternating TMA pulsing and N2 purging cycles was performed before the main ALD process (37). This predosing step can lead to a better oxide quality by reducing the interface defect density, because TMA plays a role in passivating dangling bonds at the InAs-Al2O3 interface. The surface of the substrate can also be smoothened by having a thin monolayer of TMA without losing desirable electrical property. The top Al gratings were fabricated by standard e-beam lithography and e-beam deposition, followed by a liftoff process.

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