Towards atomic-scale quantum structure fabrication in silicon and germanium

STM nanofabrication of single- and few-atom quantum devices in silicon is a well-established process. Using STM hydrogen lithography and a suitable precursor molecule, dopant atoms can be placed on the surface with nm precision, single-atom wide nanowires can be fabricated, and larger control electrodes can be patterned for ex-situ measurement. This process has also been used to fabricate phosphorus nanowires in germanium.

This project mainly explores the use of arsine as a dopant precursor molecule for germanium. Previous work in our group has shown that, unlike other dopant/substrate systems, arsenic incorporates into germanium at room temperature. We build on this by fabricating Ge:As -layers which we characterise using STM, SX-ARPES, SIMS, and electrical measurements. The -layers show superior confinement when compared to equivalent silicon samples. We also develop the STM nanofabrication process for use with the Ge:As system, demonstrating precise placement of As atoms. Thus, we show both in-plane and out-of-plane confinement of As atoms in Ge which paves the way towards Ge donor devices in the future.

Finally, one of the things limiting scale-up of STM nanofabrication process is the fact that contacting of the STM-patterned electrodes is done ex-situ, post-fabrication. The mm-scale electrodes are mapped using KPFM before Pd contact pads are placed on the surface individually for each nanodevice. This is a slow process and misalignment can cause devices to fail at the final preparation step. Pre-fabrication ion-implanted contacts are a potential solution to this, although their compatibility with the high temperature anneals required to clean silicon before STM needs to be further explored. We study ion-implanted structures in silicon using STM and KPFM, looking at several methods to distinguish the implanted structures from the substrate and how much they diffuse during the cleaning process.

The production and processing of materials accounts for 15% of UK GDP and generates exports valued at £50bn annually, with UK materials related industries having a turnover of £197bn/year. It is, therefore, clear that the success of the UK economy is linked to the success of high value materials manufacturing, spanning a broad range of industrial sectors. In order to remain competitive and innovate in these sectors it is necessary to understand fundamental properties and critical processes at a range of length scales and dynamically and link these to the materials' performance. It is in this underpinning space that the CDT-ACM fits.

The impact of the CDT will be wide reaching, encompassing all organisations who research, manufacture or use advanced materials in sectors ranging from energy and transport to healthcare and the environment. Industry will benefit from the supply of highly skilled research scientists and engineers with the training necessary to advance materials development in all of these crucial areas. UK and international research facilities (Diamond, ISIS, ILL etc.) will benefit greatly from the supply of trained researchers who have both in-depth knowledge of advanced characterisation techniques and a broad understanding of materials and their properties. UK academia will benefit from a pipeline of researchers trained in state-of the art techniques in world leading research groups, who will be in prime positions to win prestigious fellowships and lectureships. From a broader perspective, society in general will benefit from the range of planned outreach activities, such as the Mary Rose Trust, the Royal Society Summer Exhibition and visits to schools. These activities will both inform the general public and inspire the next generation of scientists.

The cohort based training offered by the CDT-ACM will provide the next generation of research scientists and engineers who will pioneer new research techniques, design new multi-instrument workflows and advance our knowledge in diverse fields. We will produce 70 highly qualified and skilled researchers who will support the development of new technologies, in for instance the field of electric vehicles, an area of direct relevance to the UK industrial impact strategy.
In summary, the CDT will address a skills gap that has arisen through the rapid development of new characterisation techniques; therefore, it will have a positive impact on industry, research facilities and academia and, consequently, wider society by consolidating and strengthening UK leadership in this field.