Nanolaminate dielectrics and GaN nanostructures for device applications

在过去的半个世纪里，技术进步的快速加速，特别是计算能力的提高，在很大程度上归功于晶体管技术的进步。这些使用二氧化硅的传统晶体管是在纳米级设计的，它们正处于经济可行性的门槛：这种类型的较小设备将导致功耗大幅增加，而计算能力的回报却微乎其微。正是出于这个原因，人们开始关注新的高k介电材料来取代二氧化硅，而且，在电力电子领域，氮化镓(GaN)和碳化硅(SIC)也被认为可以取代硅。大幅降低全球能源消耗的一个机会是通过提高电力电子技术的效率。电力电子对于最大限度地提高高压输电线路的效率和提高手机的电池寿命至关重要。它们可以在铁路、电视和节能照明中找到。控制这些系统的传统电子设备是硅基晶体管。该项目将专注于开发宽带隙(GaN，SiC)半导体器件，以产生一条降低器件功耗和成本的路线。带隙(~3 eV)比硅(1.1 eV)大得多，使器件能够在高温和高压下工作，使使用这些材料的电力电子模块的效率显著提高。耐高温的能力可以最大限度地减少防止设备过热所需的额外成本和组件。虽然高k材料为HEMT和MOSFET技术的进一步改进提供了途径，但它们需要对半导体衬底进行仔细的工程设计，特别是在界面上。该项目的目标是通过物理表征技术检查这些接口，并使用电气表征技术评估它们在晶体管技术中的可行性。X射线光电子能谱(XPS)和椭圆偏振光谱(SE)等技术将用于探测High-k介电/半导体界面的特性，而电容电压(C-V)和电流电压(I-V)以及电阻率测量将提供对电堆的电学特性的深入了解。这些测量将在内部制造的堆叠上进行，使用原子层沉积和溅射技术。本项目将重点研究纳米层状介质，特别是用于GaN增强(E)模HEMT的Al_2O_3薄膜基质中的最佳掺杂(如Sc、Ti)。后者还没有被证明具有商业应用所需的可靠性，这是一个重大的研究机会。在高介电常数(K)和频带偏移量方面，制备具有定制组成的纳米层状物可以实现所需的折衷，从而在HEMT器件中获得优异的性能。Al_2O_3已被用于GaN HEMT中，但其k值较小，约为10；掺杂Al_2O_3，例如掺入Sc(其氧化物构型为k为~26)，可获得具有宽禁带&gt；6 eV和高k的介质。此外，由于纳米材料，如纳米线、纳米管和纳米棒是无位错和无应变的，具有大的比表面积/体积比，因此对GaN纳米结构的研究也受到了广泛的关注。由于这些特性，GaN纳米结构有可能表现出比传统平面GaN更优越的性能。在这个项目中，将研究不同的GaN相(包括平面、纳米线和纳米网络)，以了解它们的材料特性和在界面形成氧化物的作用。

The rapid acceleration of technological improvement over the last half century, particularly computing power, is largely down to the improvements in transistor technology. These traditional transistors, those that use silicon dioxide are engineered at the nanometre level, are at the threshold of their economic viability: smaller devices of this type would lead to dramatic increase of power consumption for a diminishingly small return in computing capability. It is for this reason that attention has been paid to new high-k dielectric materials to replace the silicon dioxide; furthermore, silicon has been considered to be replaced by gallium nitride (GaN) and silicon carbide (SiC) for power electronics applications. An opportunity to significantly lower the global energy consumption is by increasing the efficiency of power electronics technology. Power electronics are crucial to maximizing the efficiency of high voltage transmission lines and improving the battery life of a mobile phone. They are found in railways, in TVs, in energy efficient lighting. The traditional electronic devices that control these systems are silicon-based transistors. This project will focus on developing wide band gap (GaN, SiC) semiconductor devices to generate a route that reduces power consumption and cost of the devices. A considerably larger band gap (~3 eV) than silicon (1.1 eV), permits devices to operate at high temperatures and voltages, making power electronic modules that use these materials significantly more efficient. The ability to withstand high temperatures can minimize the additional costs and components required to prevent devices from overheating. While high-k materials provide an avenue for further enhancements of HEMT and MOSFET technology, they require careful engineering with the semiconductor substrate, particularly at the interface. The goal of this project is to examine these interfaces, through physical characterisation techniques, and assess their viability for transistor technology using electrical characterisation techniques. Techniques such as X-ray Photoelectron Spectroscopy (XPS) and spectroscopic ellipsometry (SE) will be used to probe the properties of the high-k dielectric/semiconductor interface, while capacitance voltage (C-V) and current voltage (I-V) as well as resistivity measurements will give insight into the electrical properties of the stacks. These measurements will be performed on stacks fabricated in house, using atomic layer deposition and sputtering techniques. This project will focus on investigation of nanolaminate dielectrics, in particular optimal dopants (such as scandium, titanium) in Al2O3 thin film matrix for inclusion into GaN enhancement (E)-mode HEMTs. The latter has not been demonstrated with the required reliability for commercial applications and present a major research opportunity. Fabricating nanolaminates with tailored composition can achieve desired trade-offs in terms of high-dielectric constant (k) and band offsets leading to superior performance in HEMT devices. Al2O3 has been used in GaN HEMTs but suffers small k of ~10; doping it, for example with Sc (whose oxide configuration has k of ~26) can achieve a dielectric with a wide band gap >6 eV and a high-k. Furthermore, much attention has also been paid to GaN nanostructures because nanoscale materials, such as nanowires, nanotubes, and nanorods, are dislocation-free and strain-free with large surface area-to-volume ratios. Due to these characteristics, GaN nanostructures have the potential to exhibit superior performance to conventional planar GaN. In this project, differing GaN phases (including planar, nanowires and nano-networks) will be studied to understand their material properties and the role of oxide formation at the interface.