We study low-frequency charge noise in shallow GaAs/AlGaAs heterostructures using quantum point contacts as charge sensors. We observe that devices with an Al2O3 dielectric between the metal gates and semiconductor exhibit significantly lower charge noise than devices with only Schottky gates and no dielectric. Additionally, the devices with Schottky gates exhibit drift over time toward lower conductance, while the devices with the dielectric drift toward higher conductance. Temperature-dependent measurements suggest that in devices with Schottky gates, noise is dominated by tunneling from the gates to trap sites in the semiconductor, and when this mechanism is suppressed by inclusion of a dielectric, thermally activated hopping between trap sites becomes the dominant source of noise.
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