Recent estimates suggest there are now over 3 billion mobile phones and 1 billion personal computers in use worldwide. The total energy consumption associated with such devices is growing and is predicted to triple by 2030, becoming equivalent to the current residential electricity consumption of the US and Japan combined (Gadgets and Gigawatts - Policies for Energy Efficient Electronics, 2009). Given the environmental costs associated with energy generation and storage, improving the energy efficiency of electronic devices is now an urgent priority.
The key to reducing the energy consumption of electronic devices is better control of the electric currents flowing within them. Crucially, this is often dependent on the properties and robustness of thin metal-oxide (MO) films. For example, insulating MO films are used to separate metallic and semiconducting electrodes in transistors. During operation, the voltage applied between the electrodes causes current to leak through the MO film, causing wasteful energy consumption. Over time, leakage current can grow and lead to a more terminal problem whereby the MO film abruptly becomes highly conducting, a process known as breakdown. These deleterious effects are becoming increasingly important as transistors are ever further miniaturised to meet consumer demand for increasingly powerful devices. On the other hand, the reversible switching of a MO film between insulating and conducting states by applying voltages has recently received interest as the basis for a non-volatile and low-power memory technology. For transistors, memristors and many other oxide-based electronic devices there is speculation that electron trapping by defects, polycrystallinity, electric fields and redox reactions at the electrode, all play important roles, however, there are few theoretical models which take these factors into account.
The main aims of this fellowship are to learn how structure and composition are related to the electrical properties of thin MO films sandwiched between conducting electrodes, and to understand the mechanisms responsible for the transformation of these properties by application of a voltage. This will provide a framework for understanding leakage current and resistive switching in MO films, and allow strategies to control these effects to be investigated. Materials modelling can play a crucial role in addressing these aims by elucidating processes taking place over a wide range of time- and length-scales, and identifying the critical material parameters. The usual modelling approach is first to determine the equilibrium structure, then to calculate the corresponding electronic properties and current. However, this does not allow for the possibility that the non-equilibrium flow of electrons can modify the structure of the material, e.g. by field driven ion diffusion and local heating. Considering such non-equilibrium effects is essential to be able to model breakdown and resistance switching, and is also important for other processes involving correlated electron-ion dynamics, such as radiation damage. Therefore, the development of a new integrated approach is proposed that can describe the feedback between electron and ion dynamics consistently, resulting in dynamically evolving non-equilibrium structure and properties. It will combine several levels of theoretical modelling to describe the polycrystalline film structure, including defects and interfaces, the associated electronic and thermodynamic properties, and the coupled non-equilibrium dynamics of both electrons and ions. Through close collaboration with project partners, models will be tested and refined. Ultimately, this will feed into the electronics industry, leading to the design of more efficient and more reliable devices. In the later stages of the project the methodologies developed will be extended to address related materials challenges for applications including solid oxide fuel cells and batteries.
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