Development and Application of Non-Equilibrium Doping in Amorphous Chalcogenides

In the 20th century, the development of silicon-based electronics revolutionised the world, becoming the most pervasive technology behind modern-day life. In the 21st century, it is envisaged that technology will move to the use of light (photons) together with, or in place of, electrons, providing a dramatic increase in the speed and quantity of information processing whilst also reducing the energy required to do so. Making this transition to an all optical 'photonic' technology has proved to be a complex task, as the material of choice for electronics, silicon, is limited in its ability to control light. In the search for alternative materials, a class of glasses called amorphous chalcogenides (a-ChGs) have shown remarkable promise, to the point where they have been referred to as the 'optical equivalent of silicon'. Chalcogenides are materials which contain one or more of the elements sulfur, selenium or tellurium as a major constituent. These materials are already widely used in applications such as photovoltaics, memory (e.g. DVDs), advanced optical devices (e.g. lasers), and in some thermoelectric generation systems. It is accepted that the move to all-optical technologies will require an intermediate stage where information processing is undertaken using a hybrid 'optoelectronic' system. This provides a strong and compelling argument for the development of a-ChGs, as they can be deposited on Si to form a hybrid approach en-route to their use as an all-optical platform.

Whilst the optical properties of a-ChGs may be controlled and modified it has proved to be extremely difficult to modify their electronic properties during the material preparation, which has typically involved melting at high temperatures. Any impurities that are added to these materials in order to change the electronic behaviour are ineffective under these conditions due to the ability of the ChG material to reorder itself when melted, and so negate the desired doping effect. We have successfully pioneered a method to modify their properties by introducing dopants into a-ChGs below their melting temperature, thus not allowing the material to reorder, using ion-implantation. This method of doping allows precise control of the type of impurity introduced and is widely used in silicon technologies. As a result of this work, we have demonstrated the ability to reverse the majority charge carrier type from holes (p-type) to electrons (n-type) in a spatially localised way. This step-changing achievement enabled us to demonstrate the fabrication of optically active pn-junctions in a-ChGs, which will act as the enabling catalyst for the development of future photonic technologies.

In this project we will seek to develop a full understanding of the process of carrier-type reversal on the atomic scale, and use this information to optimize it, and the materials that are to be modified, so as to add further functionality. We will also develop the required advanced engineering methods which relate to the control and activation of dopants introduced using ion-implantation into a-ChGs. Together, these will enable the demonstration of a series of optoelectronic devices demonstrating the key functionalities required to build an integrated optoelectronic technology.

This programme will consolidate the position of the UK as the world leader in the field of non-equilibrium doping of chalcogenides. We will, in this way, champion these materials in the world's transition to beyond CMOS technology and therefore directly contribute to the continuing growth of the knowledge economy. We will train the next generation of scientists and engineers in state-of-the-art techniques to ensure that the UK maintains the expertise base required for this purpose, aim to ensure that the impact of this work is maximised and accelerated where possible, and communicate the results widely, including to all stakeholders in this research.

Who will benefit from the research?

The development of advanced materials and future technologies in optoelectronics and photonics is critical for enabling future advances in communications and computing, information handling and the 'internet of things', energy harvesting and storage, healthcare, sensing and even quantum technologies where the control and distribution of entangled light is required.

a-ChGs are already utilised by industry in thin-film and optical fibre waveguides, for light switching, as an optically pumped laser host, whilst from an electronic aspect they are a key component in phase-change memory. In research environments they are at the forefront of the development of optical memory and logic. Our recent work has significantly advanced their potential and importance in terms of functionality with the ability to form pn-junction devices providing a step-change in potential capability. We strongly believe that the work that we propose will be of direct benefit to both industry and researchers alike. Initially, industrial benefit will be in the photonics and micro-electronics fields and related industries where our patent filing[3] may be exploited. Micro-electronics is a vast, pervasive market that has continued to grow at 5% each year since 2000. As the benefits of photonic technologies, a $375 billion global market in 2013, are further realised, new disruptive innovations are required for their full potential to be reached. It is said that the 20th century was the 'century of the electron' and that the 21st century can become the 'century of the photon'. Indeed, the United Nations has declared 2015 as the International Year of Light and Light-Based Technologies.[38] A-ChGs have been described as the 'optical equivalent of Si' and therefore may provide the enabling platform technology to achieve this aim.

Such broad-ranging impact will be of benefit to a wide range of stakeholders and end users, outside of academia and industry, including funders of research and policy makers, government, healthcare institutions and those delivering it, education, the 'third sector' and the wider public, and the UK.

How will they benefit from this research?

Having initiated and been involved in defining the challenges we are addressing, funding bodies and stakeholders that they represent will benefit from the progress we make. The outcome of our research will inform and provide direct support for future research-programme prioritisation relating to these challenges. As we develop new materials and technologies, other funders of research in different discipline areas will also benefit as the outcomes are translated into applications elsewhere.

Those developing future technologies and undertaking research and development in industry will benefit from this project. The a-ChG devices and integrated optical chips which we will demonstrate will provide the evidence required to justify new research programmes offering a novel approach to the development of new technologies. The realisation of the materials, effects, and devices envisaged within the programme will be transformational on the field in the short-term, setting the agenda internationally. On a longer timescale as all-optical integrated photonic systems are realised and move into production, the impact on quality of life, manufacturing, and the economy will be dramatic, as reflected by the high level of interest in enabling all-optical technologies at both national and international levels.

The UK will benefit from the research undertaken not only through the above contributions (and those described in the pathways to impact) to policy, industry, education etc., but by the consolidation of its current position in leading the development of next-generation a-ChG technologies via a world-leading research programme. This will translate into the wider benefits of increased competitiveness in research and development, attracting further talent to the UK.