High-performance Electronic and Optical Devices Based on Two-dimensional Metal Oxides

The advent of two-dimensional (2D) materials has opened a new page for the development of material science. Their distinct electronic and optical properties, apart from that of conventional counterparts, have attracted significant research interest in multidiscipline. This is mainly due to their unique high surface-to-volume ratios and quantum-confined electronic structures. 2D metal oxide material is one of them with unique tuneable optoelectronic properties by controlling dopant concentration and type variations. This emerging class of 2D metal oxides (MOs) have been demonstrated to show many advances in electronic transport behaviours and optical tunability which in turn will benefit substantial research opportunities in advanced nano-enabled electronic and optical applications. However, the research exploration of 2D metal oxides synthesis methods as well as their devices is still in the early stages. Therefore, a fundamental understanding of the layered oxide growth and transfer method, and the exploring of their optical and electronic properties and practical applications can be both challenging and beneficial. Thus, this PhD project focuses on the study of high-quality 2D metal oxide (MOs) from the synthesis methods to the fundamental analysis of their electronic and optical properties, consequently to the demonstration of high-performance electronic and optical devices for different applications. The research work starts from germanium dioxide (GeO2) which is synthesised by the method based on a recent publication from an author’s research group. 2D layers of metal oxide materials such as GeO2 can grow from the Ge chunk where the oxygen-deficient environment and an elevated temperature are provided. GeO2 layers are exfoliated from the parental Ge chunk by simply “stamping” the polished germanium (Ge) surface onto a suitable substrate. By using this mechanical exfoliation method, few-layer hexagonal GeO2 (h-GeO2) are obtained. Monolayer h-GeO2 is also obtained through repeated and optimised material synthesis and transfer progress. The prepared materials feature a large lateral dimension and a high degree of smoothness, which are shown under an optical microscope. An atomic force microscopy (AFM) is used to measure the morphology of h-GeO2 nanosheets, which demonstrates a monolayer thickness of 0.76 nm as well as few-layer thicknesses of 10 nm at maximum. Based on the typical transmission electron microscopy (TEM) characterisation, the exfoliated h-GeO2 shows a multilayer stacking and translucent appearance. More importantly, it exhibits an appearance with a homogeneous distribution of the planar honeycomb-like hexagonal crystal structure, implying unique electronic properties compared with those of conventional non-layered phase. Therefore, such an exciting set of electronic properties has great potential in enabling high-performance electronic applications. Then the author continues to study the potential of other crystal structures of Ge oxides. 2D α-quartz type trigonal (hexagonal) phase GeO2 (α-GeO2) can be obtained by heating Ge under an oxygen-deficient environment after the Ge films are deposited onto sapphire, silicon dioxide (SiO2) and glass substrates via physical vapour deposition (PVD). With characterisations upon the prepared 2D α-GeO2, the films possess a large lateral size and can realise large-area transfer. From the TEM, the morphology of the α-GeO2 specimens grown on the sapphire substrate shows a better homogeneity structure than the ones grown on the other two substrates. To confirm its crystal structure, the lattice match is observed, showing a polycrystalline 2D α-GeO2. After the study of material synthesis methods, the author explores the potentials of 2D hexagonal GeO2 for field-effect transistors (FETs) due to its unique crystal structure and band gap, which has been predicted based on density functional theory (DFT) calculations to be a high mobility wide bandgap p-type semimetal oxide. The back-gated FET is realised by transferring the 2D h-GeO2 onto the Si/SiO2 substrate with sequential electron-beam deposition of titanium and gold as the metal contact to the material. Different material thicknesses, including monolayer, bilayer and few layer, are applied to the FETs. The results show that the majority of the FETs possess excellent mobility, on/off ratio and subthreshold swing under both room and low temperatures. Such a FET outperforms the state-of-art FET counterparts. Moreover, the high mobility found in the sample material paves the way for other 2D MOs-based high mobility electronic applications. At the final stage, the author mainly focuses on the study of characterisations of the optical properties of as-synthesised α-GeO2 together with their light-matter interaction applications. Optical absorption of α-GeO2 films is measured in the wavelength range of 250nm – 1400 nm, showing the absorption peak at 250 nm and 544 nm. As α-GeO2 is also known as a photoluminescence (PL) material, a prominent peak is observed at the 680nm wavelength through photoluminescence measurements. As a result, it provides a potential for use in optoelectronic applications. The prepared 2D quartz-type GeO2 grown on the sapphire or Si/SiO2 substrate is applied to form a photodetector under the incident laser signal with different wavelengths of 473 nm and 532 nm. The quantum confinement in the 2D crystal structure leads to sharp peaks in the density of states near the valence and conduction bad edges, showing high light absorption efficiency. The as-fabricated photodetector shows a better performance in responsivity and detectivity under the 473 nm illumination region. In brief, the author successfully obtains a few breakthroughs in the studies of MOs with different crystal structures from fundamental mechanisms to their practical applications during the course of his PhD program. The outcome of this research project will provide an additional pathway in developing 2D materials-enabled high-performance electronic and optical devices.