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

氧化物薄膜是非常广泛的电子器件中的关键部件，包括微处理器和存储器中的CMOS晶体管、压电和热电器件以及电致发光器件。在大多数情况下，我们假设氧化物本身在正常器件操作中遇到的电应力水平下是稳定的，并且已经进行了大量的工作来生长极高质量的薄膜。然而，器件和材料的最新发展导致非晶态和多晶亚化学计量比氧化物薄膜(SSOTF)的使用越来越多。这些材料从根本上不同于它们的化学计量和晶体表亲--这一事实可能会对它们在电子设备中的使用产生非常重要的影响--但人们通常认为它们的行为方式是相同的。越来越明显的是，这个假设是不正确的。最近的一些研究，其中一些是由我们进行的，已经证明了无定形亚化学计量比氧化物在器件水平的电应力下具有惊人的动力学特性。例如，在二氧化硅的情况下，我们已经证明了电应力推动氧化物偏析到具有不同氧缺乏的区域，并且这种变化可以是材料电学性质重大变化的先兆。我们的初步结果表明，氧化物微结构决定了氧分离的容易程度，在极端情况下，我们已经看到氧从薄膜中释放出来。这些变化可以是永久性的，也可以是可逆的，从而能够在两个或多个电阻状态之间循环。最终，如此大规模的更改可能会导致设备故障。因此，通过了解如何控制它们的动力学，我们既可以了解氧化物失效的早期阶段，也可以开发利用功能氧化物的动力学性质的令人兴奋的新技术。在这项研究中，我们建议结合高分辨率的实验表征和氧运动的原子模拟来研究这些变化。研究亚化学计量比的非晶氧化物薄膜是一个相当大的挑战，无论是对实验还是对模型来说，这也是人们对这些材料知之甚少的部分原因。我们将依靠实验和理论之间的密切互动，在迭代过程中开发新的模型，研究不同形貌的亚化学计量比非晶氧化物的结构及其对电应力的动态响应。这些模型将阐明控制电学变化的物理过程，我们将利用它们来生成一套材料和器件优化的设计规则。我们选择了一组具有代表性的材料进行研究，每一种材料在微电子中都有重要的应用。我们将在内部种植材料，使我们能够控制它们的组成和结构，并能够从表征和建模中快速反馈。我们的大部分表征也将在伦敦大学学院进行，但我们与两个领先的透射电子显微镜中心-Forschungszentum Jülich和新加坡材料研究与工程研究所-有着长期且富有成效的合作，这将使我们能够获得更多世界领先的显微技术来研究这些具有挑战性的材料。我们与其他领先的研发机构，包括我们的工业合作伙伴的密切合作，使我们能够获得更多最先进的设施和与工业相关的样品。

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.