Molecular Precursors for the CVD of Gallium and Indium Oxides

本研究的目标是开发新的高挥发性CVD前驱体，以沉积无污染(如C，F)的氧化镓和氧化铟薄膜，并详细研究所得薄膜的气敏和TCO(导热氧化物)性能。氧化镓被认为是最理想的高温薄膜气敏材料之一。它是热稳定的，在室温下是电绝缘体，但在400oC以上是半导体的。在900℃以上的温度下，电导率随氧浓度的变化而变化，因此可以检测到氧浓度。氧气传感器在监测和控制汽车尾气、废气和化工过程中的氧浓度方面有实际应用。高于400oC的Ga2O薄膜作为表面控制型传感器，可还原CO和乙醇等气体。因此，可以根据温度切换传感器的功能。氧化铟薄膜既对可见光透明，又具有导电性(TCO)。掺杂剂(如锡)可用于提高薄膜的导电性，使其更适合应用于固态光电子器件。第13族氢化物物种具有几个显著的特征，使得它们作为固体材料的前驱体具有吸引力。首先，金属-碳键的缺乏有可能减少最终材料中的碳杂质的数量，并且由于金属-氢化物键的热脆弱，加工温度可能会降低。其次，13族氢化物作为前体很有吸引力，因为它们比烷基衍生物挥发性大得多。因此，将开发一系列新型的挥发性氢化镓和铟的醇盐络合物以及异构性醇氧化物。我们将研究用低压化学气相沉积(LP)CVD和气溶胶辅助(AA)CVD方法合成的新型前驱体Ga2O_3和In_2O_3薄膜的沉积，并对薄膜的气敏性能进行评估。通过利用广泛的前驱体和沉积技术，我们将能够产生不同的微结构，并开发微结构和气敏响应之间的关联图景。氧化铟镓(GaxInyO3)是一种用于TCO应用的特殊材料，具有超过所有其他氧化物的绝对透明度/以及极高的电荷迁移率。采用AACVD/APCVD方法，在In_2O_3基质中用组合大气压化学气相沉积(AP-CVD)法生长GaxInyO_3薄膜，并在主体In_2O_3中混合纳米颗粒Ga_2O_3。我们有能力使用一种新的组合APCVD反应器来铺设薄膜，以制备梯度成分的薄膜。这种新的反应器可以在一次CVD实验中在一块平板上制造多达400种不同的化合物。这一点很重要，因为它将使我们能够快速筛选镓-铟氧化物系统中的组成空间，并为气体传感和TCO应用提供理想和优化的组成。在单一的CVD实验中优化成分从而优化性能的能力将展示组合技术的力量。此外，我们还有一种新的反应堆设计，可以用来制造嵌入纳米颗粒的氧化铟--比如氧化镓。在该系统中，气溶胶流进入APCVD气流下方的沉淀室，这有利于制备复合薄膜，其中存在或在气溶胶液滴中生成的纳米颗粒嵌入APCVD主膜中。这种结合的方法将使我们能够研究不同的纳米颗粒密度、大小和形状以及这些因素对气敏性能的影响。

The goal of this study is to develop new highly volatile CVD precursors to deposit gallium oxide and indium oxide films free from contamination (e.g. C, F) and for a detailed investigation of the gas sensing and TCO (thermally conductive oxide) properties of the resulting films. Gallium oxide (Ga2O3) is considered to be one of the most ideal materials for application as thin-film gas sensors at high temperature. It is thermally stable and an electrical insulator at room temperature but semiconducting above 400 oC. At temperatures above 900 oC the electric conductivity changes depend on the concentration of oxygen, hence the oxygen concentration can be detected. Oxygen gas sensors have practical use in monitoring and controlling oxygen concentrations in exhaust gases of automobiles, as well as waste gases and chemical processes. Above 400 oC Ga2O3 thin-film operates as a surface-control-type sensor to reducing gases, e.g. CO and EtOH. Therefore, it is possible to switch the function of the sensor with temperature. Indium oxide films are both transparent to visible light and conductive (TCO). Dopants (e.g. Sn) can be used to increase the conductivity of the films and to make them more suitable for applications such as in solid-state optoelectronic devices. Group 13 hydrido species possess several notable characteristics that result in them being attractive as precursors to solid-state materials. Firstly, the lack of metal-carbon bonds has the potential to reduce the amount of carbon impurities in the final material and processing temperatures can potentially be reduced due to the thermally frail metal-hydride bonds. Secondly, group 13 hydrides are attractive as precursors as they are considerably more volatile than alkyl derivatives. Thus, a range of novel volatile hydrido-gallium and indium alkoxide complexes as well as heteroleptic alkoxides will be developed. The deposition of Ga2O3 and In2O3 thin-films from the novel precursors synthesised in this programme via low pressure chemical vapour deposition (LP)CVD and aerosol assisted (AA)CVD will be investigated and the gas sensor properties of the films will be assessed. By utilising a wide range of precursors and deposition techniques we will be able to produce different microstructures and develop a correlation landscape between microstructure and gas sensing response. Indium gallium oxide (GaxInyO3) is an exceptional material for TCO applications with absolute transparency that exceed all other oxides / coupled with extremely high charge mobility. Thin-films of GaxInyO3 will be grown using combinatorial atmospheric pressure (AP)CVD and mixed nanoparticulate Ga2O3 inside host In2O3 by AACVD/APCVD from the novel precursors. We have the ability to lay down thin films using a new combinatorial APCVD reactor to make films of graded composition. This new reactor enables upto 400 different compositions to be made on a single plate in one CVD experiment. This is important as it will enable us to rapidly screen composition space in the gallium-indium oxide system and make idealised and optimised compositions for gas sensing and TCO applications. The ability to optimise composition and hence performance in a single CVD experiment would demonstrate the power of the combinatorial technique. Further we have a new reactor design for making indium oxide with embedded nanoparticles- such as gallium oxide. In this system the aerosol flow enters the deposition chamber below the APCVD gas flow, this has the benefit of allowing composite films to be made in which nanoparticles either present or generated in the aerosol droplet are embedded in the APCVD host film. This combined approach will enable us to investigate different nanoparticle densities, sizes and forms and how these effect the gas sensing properties.