Structural dynamics of amorphous functional oxides - the role of morphology and electrical stress

Thin oxide films are critical components in a very wide range of electronic devices, including CMOS transistors in microprocessors and memory, piezoelectric and thermoelectric devices and electroluminescent devices. In most cases we assume that the oxide itself is stable under the levels of electrical stress encountered during normal device operation, and a great deal of work has gone into growing extremely high quality films. Nevertheless, recent developments in devices and materials have led to the growing use of amorphous and polycrystalline sub-stoichiometric oxide thin films (SSOTFs). These materials are fundamentally different to their stoichiometric and crystalline cousins - a fact that can have very important consequences for their use in electronic devices - but it is usually assumed that they behave in the same way. It is increasingly clear that this assumption is incorrect.

Recent studies, some performed by us, have demonstrated that amorphous sub-stoichiometric oxides are surprisingly dynamic under device-level electrical stress. In the case of silicon oxide, for example, we have shown that electrical stress drives the segregation of the oxide into regions with varying oxygen deficiency, and that such changes can be precursors to major changes in the electrical properties of the material. Our initial results suggest that oxide microstructure determines the ease with which oxygen can segregate, and we have seen, in extreme cases, emission of oxygen from the thin films. These changes can be permanent or they can be reversible, enabling cycling between two or more resistance states. Ultimately, such large-scale changes can lead to device failure. Consequently, by understanding how to control their dynamics we can both understand the early stages of oxide failure, and develop exciting new technologies that exploit the dynamic nature of functional oxides.

In this study we propose to investigate these changes using a combination of high resolution experimental characterisation and atomistic modelling of oxygen movement. Studying sub-stoichiometric amorphous oxide thin films is a considerable challenge, both for experiment and for modelling, which is partly why these materials are poorly understood. We will rely on close interaction between experiment and theory to develop, in an iterative process, new models for the structure of substoichiometric amorphous oxides of varying morphology, and their dynamic response to electrical stress. These models will shed light on the physical processes governing electrical changes, and we will use them to generate a set of design rules for material and device optimisation.

We have chosen a representative set of materials to study, each of which has important applications in microelectronics. We will grow the materials in-house, giving us control over their composition and structure and enabling rapid feedback from characterisation and modelling. The majority of our characterisation will also be performed at UCL, but we have long-standing and fruitful collaborations with two leading Transmission Electron Microscopy centres - Forschungszentrum Jülich and the Institute of Materials Research and Engineering in Singapore - which will give us access additional world-leading microscopy techniques to study these challenging materials. Our close collaboration with other leading research and development institutions, including our industrial partners, gives us access to further state-of-the-art facilities and industrially relevant samples.

Amorphous oxides are of increasing importance across a range of electronics applications, several of which have multi-billion dollar markets (the novel non-volatile memories market is predicted to be $2 billion in 2018 with a Compound Annual Growth Rate of 46%, and it is estimated that 3 trillion Multilayer Ceramic Capacitors /year based on oxides will be produced by 2020). Under normal operating conditions, many of the devices that are so critical to these technologies rely on the application of electric fields strong enough to drive significant changes in the structure of the oxide films, yet not sufficiently strong to cause them to fail catastrophically. However, our understanding of the mechanisms driving such structural changes is patchy, being largely based on studies in crystalline materials or thicker films, and our ability to either mitigate these changes or exploit them in novel technologies is correspondingly limited.

The stakeholders of this research include the academic and industrial semiconductor microelectronics communities, and more broadly those working in solid state materials science. While this is in many ways a fundamental study with a timescale of 5 to 8 years to commercialisation, there are a number of shorter-term impacts beyond the immediate academic benefits, which we hope to begin to realise within the lifetime of the project. The first of these will be the application of our new models to understanding soft and hard dielectric breakdown in transistors. In the medium term, emerging non-volatile resistive RAM (RRAM) memory technologies stand to benefit. This is a rapidly growing industry sector, with several products already in the marketplace from companies such as Panasonic, Adesto and Micron. A concern that currently limits the full exploitation of the potential of RRAM is variable device reliability and endurance. A better understanding of oxygen dynamics in these devices will therefore translate into real commercial benefits. Similarly, our models can be applied to optimise the performance of capacitors that rely on amorphous oxide dielectrics; this can be expected to have the potential for major benefits in a large industrial sector. In the longer term (5 years +), our models and techniques will help in the development of emerging neuromorphic technologies based on amorphous oxides.

Putting our work into the context of EPSRC's four Research Outcomes, our work will contribute most directly to Connected Nation and Productive Nation though the research capability of advanced materials research to drive new processes, products and sustainable solutions, and ambition C3: delivering intelligent technologies and systems.

Principal routes to commercial impact will be via IP generation, exploitation via licensing opportunities, and opportunities for start-up companies in areas of disruptive advances. This approach ensures UK impact even in areas in which the UK does not have a strong manufacturing presence - existing highly successful examples of such fab-less models include ARM and Imagination Technologies. In maximising the impact of our work on the industrial community we will rely on advice from our project partners, in particular the Aerospace Corporation and Imec, and from UCL Business, who are currently supporting the commercialisation of our amorphous oxide-based ReRAM technology via a spin-out company.