Abstract
Quantum computers require spinless, isotopically pure 28Si (or 70,72,74Ge) to provide a ‘semiconductor vacuum’ as the nuclear spin of 29Si (or 73Ge) in naturally occurring Si causes decoherence of the qubit states. 30Si (as well as 29Si) causes lattice strain that degrades qubit controllability. Currently, most Si enrichment techniques rely on the centrifugation of natSiF4 which is not widely accessible. A readily available enrichment method that makes use of current microelectronics fabrication techniques would be greatly beneficial to present research into silicon-based quantum computing and commercial production in the future; implanted layer exchange (ILE) is a new enrichment process, shown below, with the potential to fulfil this brief by using metal induced layer exchange to crystallise ion-implanted 28Si, to produce enriched 28Si layers on Si substrates suitable for quantum computers.
In this thesis, the production of high purity (>99% Si), continuous (~cm2), monocrystalline 28Si layers enriched to 99.7% through ILE with Al was demonstrated for the first time. Different annealing conditions and Al film thicknesses were investigated, and a variety of polycrystalline and epitaxial Si layers were formed. Insufficiently thick Al films become completely damaged during the implant which confounds ILE. TRIDYN modelling was used to show that the implant step of ILE is robust against ion energy and vacuum conditions and thus well suited to conventional implantation methods. To explain ILE observations, a thermodynamic diffusion and crystallisation model is presented. As ILE occurred rapidly at low temperatures (250°C), it was crucial to use short (~20s) anneal ramps up to higher temperatures (500°C) which promoted epitaxial growth. Small-area (~µm2) ILE experiments with Ge were also carried out but strong diffusion of the substrate Si limited the obtainable enrichments. The next steps for the project, including measurements of quantum properties of residual Al acceptors in ILE 28Si, are discussed.