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Exploring and exploiting charge-carrier confinement in semiconductor nanostructures: heterodimensionality in sub-monolayer InAs in GaAs and photoelectrolysis using type-II heterojunctions

Research output: ThesisDoctoral Thesis

Published
Publication date23/11/2016
Number of pages229
QualificationPhD
Awarding Institution
Supervisors/Advisors
Publisher
  • Lancaster University
Original languageEnglish

Abstract

In this thesis, semiconductor nanostructures are studied, both experimentally and theoretically, to help aid the development of two diverse and important technologies.

Firstly, charge-carrier confinement in stacked sub-monolayer (SML) InAs in GaAs: SML deposition results in the formation of In-rich agglomerations within a lateral InGaAs quantum well (QW) with lower In content. Low-temperature photoluminescence (PL) and magneto-PL reveals strong vertical and weak lateral confinement, indicative of a two-dimensional (2D) excitons. Paradoxically, high-temperature magneto-PL allows excited-state peaks to become resolved, which can be fitted by a Fock-Darwin spectrum, characteristic of a zero-dimensional (0D) system. To solve this contradiction, we postulate that stacked SML InAs in GaAs forms a heterodimensional system, in which electrons are 2D, and see only the lateral InGaAs QW, whilst the heavier holes are 0D, and are confined in the In-rich agglomerations. This description is fully supported by single-particle effective-mass and eight-band k·p calculations, which show heterodimensional confinement is probable for a large variation in In content.

SML vertical-cavity surface-emitting lasers (VCSELs) — which prove to be one of the most promising candidates for datacoms applications — have been demonstrated at >20 Gb/s, and we postulate that heterodimensionality is fundamental to this high-speed operation. Efficient carrier injection is achieved by the lack of a wetting layer, along with the 2D electrons coupling to several In-rich agglomerations, making them quickly available to states that are lasing. Furthermore, the shallow confining potential of the In-rich agglomerations means that excess holes cannot build up in states that aren't lasing.
Secondly, semiconductor photoelectrolysis for the solar-powered generation of renewable hydrogen by water splitting is researched. The novel use of nanostructures at the semiconductor-electrolyte interface (SEI) in a photoelectrochemical cell (PEC) is proposed to help increase the maximum potential that can be photo-generated, thus increasing the likelihood of a given PEC being able to split water. By solving the Schrödinger, Poisson and drift-diffusion equations, we simulate the band alignment, confined carrier energy states and carrier densities for a variety of different material systems. ZnO quantum dots on InxGa1-xN show the most promising band alignment, with electron-donating and -accepting states straddling the hydrogen- and oxygen-production potentials (respectively) for small x (x < 0.3), indicating an ability to split water.