Abstracts Astronomy & Space Science

Electrons and Holes in Semiconductor Nanostructures

by Ruoyu Li

Over the past 35 years, much effort has gone into the development of semiconductor nanostructures. A key application is to use quantum states, e.g. electron or hole spins, for information processing. This thesis investigates the underpinning physics of spin-orbit interactions in nanostructures, which could be used for fast spin-state operation, and electron-hole recombination in ambipolar devices, which could be used to convert spin states to photon polarizations. Initially, we report on the spin-orbit interaction induced electron g-factor anisotropy in InAs nanowire quantum dots. The quantum dot structure is defined by random potential fluctuations along the nanowire. By fine-tuning the gate bias, we can resolve regular Coulomb diamonds, characteristic of a single quantum dot. By monitoring the Coulomb peak spacing for different magnetic field orientation, we mapped the anisotropy of the electron g-factor. The following two chapters investigate heavy holes in silicon quantum dots for high fidelity spin state operation. The strong spin-orbit interaction of heavy holes promises high-speed spin rotations with purely electrical signals. The silicon host crystal could minimize the effect of hyperfine interactions. Based on a versatile multi-layer gate metal-oxide-semiconductor (MOS) structure, we could define a stable single hole transistor operating from the many hole regime down to the few hole regime. Furthermore, we used the flexibility of the MOS structure to also define a double quantum dot system. We report the first observation of Pauli spin blockade of heavy holes, which is used to convert spin states to electric charges. By mapping the transport current under different magnetic field, and inter-dot energy detuning, we could identify the leading spin relaxation mechanism, and how it depends on the inter-dot tunnel coupling. The results obtained here are important milestones for realizing hole spin quantum bits. Finally, we investigate the possibility of spin state telecommunication with photons. We demonstrate radiative recombination of electrons and holes in a GaAs ambipolar field-effect transistor of a similar design used to make quantum dots. The device structure uses two top gates for independent electron and hole density control, allowing a p-i-n junction to be formed without doping. By tuning the top gates bias, we could shift the recombination zone to the unperturbed intrinsic region, allowing high light-emission efficiency. Advisors/Committee Members: Hamilton, Alex, Physics, Faculty of Science, UNSW.