Supercurrent transistors, hole quantum dots and defects in silicon systems

Dr Floris Zwanenburg
The University of Twente
2:15pm Friday 28 July 2017
CQC2T Conference Room, Level 2, Newton Building J12, UNSW Kensington Campus

First, I will present results on semiconducting Ge/Si core/shell nanowires: In double quantum dots, we observe shell filling of new orbitals and corresponding Pauli spin blockade. In nanowires with superconducting Al leads we create a Josephson junction via proximity-induced superconductivity. A gate-tuneable supercurrent is observed with a maximum of ~60 nA. We identify three different regimes tuneable via backgate voltages: Cooper pair tunnelling, quasiparticle transport and finally full suppression of transport. An unknown superconducting phase is formed during thermal annealing, most likely an alloy of Al/Si or Al/Si/Ge.

Secondly, we create ambipolar quantum dots in silicon nanoMOSFETs. We use a barrier array structure to probe amphoteric defects with charging energies of ~10 meV at multiple locations in a single device. After passivation of the defects we can electrostatically define hole quantum dots up to 180 nm in length.
In recent devices, we have replaced the aluminium in our gates with palladium. We characterize the conformity of the two materials as nanoscale gates by means of transmission electron microscopy (TEM). Subsequently, we will show low-disorder quantum dots defined with Pd gates.
Finally, we have made depletion-mode hole quantum dots in intrinsic silicon. We use fixed charge in a SiO2/Al2O3 dielectric stack to induce a 2DHG at the Si/SiO2 interface. This depletion-mode design avoids complex multilayer architectures requiring precision alignment and allows directly adopting best practices already developed for depletion dots in other material systems.