Resolving spin physics in self-assembled InAs/GaAs quantum dots

Abstract

Semiconductor nanostructures and, in particular, quantum dots (QDs) have been at the forefront of solid-state research for several decades and represent attractive candidates for applications in quantum information and spintronics. QDs are characterised by their quasi-three-dimensional quantum confinement of electrons and holes which results in an ‘artificial-atom’ like structure with discrete but accessible energy levels. In addition, the spin of carriers in QDs are relatively isolated from typical spin relaxation mechanisms. This thesis investigates the spin physics of self-assembled InAs/GaAs QDs grown by molecular beam epitaxy. The design and growth of QD structures is described and it is shown how spin effects may be investigated using circularly polarised (CP) light. This work demonstrates how spin physics may be resolved through analysis of the emission polarisation of a QD ensemble. In particular, a slope in the polarisation spectrum induced by CP excitation is shown to correlate with a splitting between polarisation states of the QDs. This idea is developed to create a new technique capable of resolving spin splittings far narrower than the ensemble inhomogeneous linewidth. The technique is validated by resolving Zeeman splitting, allowing g-factors and QD fine-structure effects to be measured. The sensitivity to external magnetic field and the dynamics of the optically induced polarisation splitting are investigated. A novel QD spin memory is demonstrated with a long lifetime consistent with dynamic nuclear spin polarisation (DNSP). However, a rapid initialisation of the effect contradicts the traditional understanding of DNSP and may open the door to new spin physics in QDs. A connection is demonstrated between the polarisation splitting and the phenomenon of negative circular polarisation (NCP). It is shown that NCP, previously only observed in n-doped QDs, is in fact a general property of all QDs regardless of doping level, calling into question the currently accepted generation mechanism.