Development and characterisation of site-controlled quantum dot arrays

The development of methods for the epitaxial growth of semiconductor materials and nano-crystals has revolutionised modern technology. Molecular beam epitaxy (MBE) has allowed ultra-high quality materials to be produced and underpins a vast amount of research in the field of semiconductor photonics. Constant improvements to the technology have meant that the development of quantum circuits and therefore quantum computers has become a realistic goal. Semiconductor quantum dots (QDs) embedded within nanophotonic device structures can form the components of these circuits, which can create or manipulate quantum bits (qubits). A significant challenge remains if this goal is to be achieved, in that the use of randomly positioned QDs in these structures does not easily allow scale-up of the systems. Site-controlled QD growth, where the nucleation position of emitters is deterministically controlled, offers a solution to scale-up challenges. However, the quality of these QDs does not yet match that of randomly grown QDs and best results are often achieved using growth structures that are unsuitable for integration into the single-mode photonic devices that would be the building blocks of these circuits. This work seeks to develop a method for deterministically controlling the growth position of QDs by fabricating regular arrays of nanoholes. Additionally, the fabrication and growth structures must be produced in a scalable manner that facilitates incorporation into single-mode photonic devices. First, an atomic force microscope (AFM) assisted local anodic oxidation (LAO) nanohole fabrication method will be presented. The nanohole arrays are fabricated on an InP substrate, for the growth of site-controlled InAs QDs that emit in the telecom C-band. Preliminary results for site-controlled droplet epitaxy via MBE are presented, as this growth method has been shown to produce QDs with low fine structure splitting (FSS). An alternative application for LAO structures will be demonstrated, where the oxides are shown to tune to the resonant frequency of a 2D photonic crystal cavity in simulation. The subsequent parts of this work focus on the development of fabrication, in-situ surface preparation, and MBE growth methods for the site-control of InAs QD arrays on GaAs substrates. Nanohole array fabrication methods focus predominantly on using electron beam lithography (EBL) and inductively coupled plasma – reactive ion etching (ICP-RIE). This method is easily scaled up and produces reproducible results. An atomic hydrogen cleaning method is developed that allows surfaces to be thoroughly decontaminated in situ, in a manner that preserves the nanohole nucleation sites and does not damage the semiconductor surface. Finally, results demonstrating the production of site-controlled QD arrays are presented. Both optical and AFM imaging measurements are used to characterise the QD arrays, which are grown on nanoholes fabricated using the EBL/ICP-RIE and LAO. The methods are directly compared in the same growth run. Very narrow linewidths for QDs grown on nanoholes fabricated using EBL/ICP-RIE and LAO of 26 micro eV and 33 micro eV are observed respectively, in addition to low FSS of 16 micro eV.