Development and Application of Non-Equilibrium Doping in Amorphous Chalcogenides

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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.