A quantum point contact (QPC) can be used as a detector for fast and sensitive readout of the charge and spin states in quantum structures [1]. We demonstrate the realization of such a detection scheme for nanowire-based quantum dots (QD) by integrating a QPC fabricated with Schottky gates on a GaAs/AlGaAs two dimensional electron gas (2DEG) with an InAs/InP nanowire double quantum dot heterostructure. Such a system is interesting for studies of charge traps in self-assembled nanowires and as a future read-out scheme for QD-based solid state qubits.
InAs nanowires, with three InP barrier segments, were catalytically grown using chemical beam epitaxy to form an axial double quantum dot heterostructure [2]. The wires are then transferred to a GaAs/AlGaAs substrate with a 2DEG 80 nm below the surface. Using electron beam lithography, two Ti/Au gate fingers, with a gap of 100 nm, aligned around and perpendicularly to the nanowire, are made on top of the GaAs/AlGaAs heterostructure. Applying a negative gate-voltage to these Schottky gates induces a QPC in the 2DEG below the nanowire. By separately contacting both the QPC in the 2DEG and the nanowire above, we operate the QPC to sense charging in the nanowire double quantum dot in a regime where Coulomb blockade peaks in the current through the wire are weak. We analyze the detector data to identify in which of the dots in the nanowire the charge events occur, and discuss limits of such a detection scheme.
In addition to this, we present initial results on another design in which local gate fingers are used to electrostaticly define quantum dots in an epitaxially grown homogeneous InAs wire. While local gate defined double quantum dots were previously demonstrated [3], we here combine them with a gate induced QPC in the 2DEG. The local-gate electrodes are defined on top of an insulating layer of HfO, over the nanowire and close to the QPC. Such design allows a high degree of flexibility with individual tuning of multiple dots, possibility to vary the inter-dot coupling, and sensitive detection of charge transfer events between each of the dots.
[1] J. M. Elzerman et al., Phys. Rev. B 67, 161308(R) (2003)
[2] A. Fuhrer et al., NanoLett 7, 243 (2007)
[3] C. Fasth et al., NanoLett 5, 1487 (2005) |