AFM-based Charge-Locking in Silicon Quantum Devices

We use the tip of an atomic force microscope (AFM) to charge floating metallic gates in multilayer Si/SiGe quantum dot (QD) devices. Acting as a perfect and movable cryogenic switch, the tip provides reproducible and non-destructive charge-locking with single-electron precision on the floating gate. Biasing a gate with an AFM tip allows us to reduce the footprint of a single plunger gate down to an isolated ~100 nm island. By sensing the real-time retention of the locked charge, we show that the discharging of the floating gate proceeds in discreet steps. By measuring the distribution of the single-electron leakage events, we extract the resistance of the tunnel junction between overlapping gate layers R~10^{19} Ohm – a value immeasurable by conventional means. We found the average discharge rate to be of the order of 1 electron per few seconds in overlapping gate architecture and multiple hours for single-layer devices. The random-access nature of the AFM-tip charging approach allows us to reduce the footprint of a single plunger gate down to a fundamental limit with the potential to tune a 2D array of arbitrary size.