Non-equilibrium electron-ion dynamics in thin metal-oxide films

最近的估计表明，目前全球正在使用的手机超过30亿部，个人电脑超过10亿台。与这类设备相关的总能耗正在增长，预计到2030年将增加两倍，相当于目前美国和日本的住宅用电量的总和(电子产品和千兆瓦--《节能电子政策》，2009)。考虑到与能源生产和储存相关的环境成本，提高电子设备的能效是当前的当务之急。降低电子设备能耗的关键是更好地控制流动在其中的电流。至关重要的是，这往往取决于金属氧化物(MO)薄膜的性能和坚固性。例如，绝缘的MO膜被用来分离晶体管中的金属电极和半导体电极。在操作过程中，电极之间施加的电压会导致电流通过MO膜泄漏，从而造成浪费的能源消耗。随着时间的推移，泄漏电流可能会增长并导致更极端的问题，从而使MO膜突然变得高度导电，这一过程称为击穿。随着晶体管进一步小型化，以满足消费者对日益强大的设备的需求，这些有害影响变得越来越重要。另一方面，通过施加电压使MO膜在绝缘和导通状态之间的可逆切换最近引起了人们的兴趣，作为非易失性和低功率存储技术的基础。对于晶体管、忆阻器和许多其他基于氧化物的电子器件，人们推测缺陷、多晶性、电场和电极上的氧化还原反应对电子的捕获都起着重要的作用，然而，很少有理论模型考虑这些因素。本文的主要目的是了解夹在导电电极之间的MO薄膜的结构和组成与其电学性质的关系，并了解通过施加电压来改变这些性质的机制。这将为理解MO膜中的泄漏电流和阻性开关提供一个框架，并允许研究控制这些影响的策略。材料模型可以通过阐明在广泛的时间和长度尺度上发生的过程，并确定关键的材料参数，在实现这些目标方面发挥关键作用。通常的建模方法是首先确定平衡结构，然后计算相应的电子性质和电流。然而，这不允许电子的非平衡流动可以改变材料的结构，例如通过场驱动的离子扩散和局部加热。考虑这样的非平衡效应对于能够对击穿和电阻切换进行建模是必不可少的，对于涉及相关电子-离子动力学的其他过程也是重要的，例如辐射损伤。因此，提出了一种新的综合方法，它可以一致地描述电子和离子动力学之间的反馈，从而导致非平衡结构和性质的动态演变。它将结合多个层次的理论建模来描述多晶膜的结构，包括缺陷和界面，相关的电子和热力学性质，以及电子和离子的耦合非平衡动力学。通过与项目合作伙伴的密切合作，将对模型进行测试和改进。最终，这将进入电子行业，导致设计出更高效、更可靠的设备。在该项目的后期阶段，将推广所开发的方法，以解决包括固体氧化物燃料电池和电池在内的应用所面临的相关材料挑战。

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.