Origin of switching noise in GaAs/AlxGa1-xAs lateral gated devices

We have studied switching (telegraph) noise at low temperature in GaAs/Al_xGa_1-xAs heterostructures with lateral gates and introduced a model for its origin, which explains why noise can be suppressed by cooling samples with a positive bias on the gates. The noise was measured by monitoring the conductance fluctuations around e²/h on the first step of a quantum point contact at around 1.2K . Cooling with a positive bias on the gates dramatically reduces this noise, while an asymmetric bias exacerbates it. Our model is that the noise originates from a leakage current of electrons that tunnel through the Schottky barrier under the gate into the conduction band and become trapped near the active region of the device. The key to reducing noise is to keep the barrier opaque under experimental conditions. Cooling with a positive bias on the gates reduces the density of ionized donors. This builds in an effective negative gate voltage so that a smaller negative bias is needed to reach the desired operating point. This suppresses tunneling from the gate and hence the noise. The reduction in the density of ionized donors also strengthens the barrier to tunneling at a given applied voltage. Further support for the model comes from our direct observation of the leakage current into a closed quantum dot, around 10^-20A for this device. The current was detected by a neighboring quantum point contact, which showed monotonic steps in time associated with the tunneling of single electrons into the dot. If asymmetric gate voltages are applied, our model suggests that the noise will increase as a consequence of the more negative gate voltage applied to one of the gates to maintain the same device conductance. We observe exactly this behavior in our experiments.