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

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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.