Single-charge tunneling in ambipolar silicon quantum dots

Abstract

Spin qubits in coupled quantum dots (QDs) are promising for future quantum information processing (QIP). A quantum bit (qubit) is the quantum mechanical analogon of a classical bit. In general, each quantum mechanical two-level system can represent a qubit. For the spin of a single charge carrier e.g., which is a natural two-level system, the basis quantum states are given by the spin-up and the spin-down state. QIP based on the spin degree of freedom requires long spin coherence times. Silicon provides an environment where spins can be controlled with minimal decoherence because of the weak hyperfine and spin-orbit interaction. So far, most experiments have focused on electron spins, but hole spins offer great potential for spin-based QIP as well. A hole-spin qubit in silicon can benefit from its finite spin-orbit coupling, because it allows efficient electric-field driven spin resonance applicable via local gate electrodes. However, it is still unclear whether the electron spin or the hole spin is most suitable as a qubit. We have developed an ambipolar metal-oxide-semiconductor field-effect transistor (MOSFET)-based device that allows the electron and the hole transport regime to be compared in one and the same nanostructure. We have investigated different types of devices. For all of them, we find the threshold voltage for the electron regime to be closer to zero than for the hole regime which, amongst others, can be ascribed to the n-type background doping of the near-intrinsic silicon wafer. We locally control the charge density by nanoscale bottom gates. Non-linear transport measurements show single-charge tunneling through QDs created underneath each bottom gate as well as between two bottom gates. Comparing the properties of the electron QD and the hole QD indicate that we load the same QD with either electrons or holes. Ambipolar QDs with single-charge occupancy can break new ground in spin-based QIP, since they have the potential to act as a qubit comparator where the suitability of electron-spin and hole-spin qubits can be evaluated in the same crystalline environment. Taking the advantages of either qubit one could think of future “quantum CMOS” technology based on ambipolar QDs.