Functional Oxides in Optical and Electronic Applications by Atmospheric Pressure Spatial Chemical Vapour Deposition

Oxide thin films possess a variety of electronic, optical, and magnetic properties, making them valuable in multiple fields, particularly in electronics and memory technology. These thin films are used in resistive random-access memory (RRAM) due to their ability to alter resistance states, which is crucial for non-volatile memory applications. With the growing demand for efficient data storage, and processing driven by post-CMOS AI hardware, linked to Big Data and the Internet of Things, RRAM is being developed to achieve high-density integration while being compatible with silicon-based technology. One prominent oxide, tungsten oxide, or WO3, stands out in academic research because it is CMOS-compatible and is a binary oxide making it easier to process. It has many other potential applications in gas sensing, photocatalysis, smart windows, water splitting and etc. However, WO3 does suffer from a diverse of phases and states, making it hard to control electronic conductivity, and variable oxygen and cation valence states for RRAM application. Furthermore, for electronics, either high-quality epitaxial or amorphous systems are preferred in terms of achieving uniform properties. WO3 thin films can be prepared using multiple deposition techniques, including sputtering, thermal evaporation, and spray pyrolysis. However, the use of Atmo spheric Pressure Spatial Chemical Vapour Deposition (AP-SCVD) has emerged as a promising method due to its simplicity and scalability. Unlike conventional methods that require vacuum, AP-SCVD allows for the production of high-quality, conformal films over large areas in open-air conditions, making it suitable for commercial-scale production. This technique is also CMOS compatible and deposit films with proper ties comparable to those made by traditional CVD, allowing industrial addaptation of oxide thin films, particularly WO3, in advanced optical and electronic applications. In this thesis, AP-SCVD was widely explored for making controlled WO3 thin films, where learning how film properties relate to growth variables. A non-pyrophoric W(CO)6 precursor was used as this is easy to handle. Both polycrystalline and 4 epitaxial WO3 films were grown at relatively low temperatures (350°C) in an open atmosphere. The first results chapter presents the development of deposition parameters for polycrystalline WO3 films on silicon substrates. This work investigates how various deposition parameters influence the orientation, morphology, and stoichiometry of the films. Furthermore, the polycrystalline films were fabricated into devices such as UV detectors, photoelectrochemical water-splitting cells, and photocatalysts, demonstrating how orientation tuning can significantly impact device performance. The second results chapter extends the WO3 film research by examining deposition on single-crystal substrates (SrTiO3, LaAlO3, and YAlO3) with both tensile and compressive strain, enabling epitaxial growth. The deposition process, conducted at 350°C, challenges the traditional requirements of high-vacuum and high-temperature conditions for epitaxial oxide films. This advancement highlights the potential of AP-SCVD for applications in cost-effective electronic devices. The third results chapter explores a novel application of WO3 films, focusing on the stabilisation of a polar phase with ferroelectric properties at room temperature. This work demonstrates compressive epitaxial strain on a (110)-oriented YAlO3 substrate, with the addition of a conductive buffer layer, achieving ferroelectric switching observed through piezoresponse force microscopy. Notably, reducing the film thickness to 6 nm enabled the observation of distinct ferroelectric domains and conductivity variations, indicating potential for memory storage applications. The final results chapter investigates the fabrication of non-volatile resistive switch ing devices for RRAM development using WO3 films. Variable performances and conduction mechanisms were observed with different substrates used, electrodes, annealing conditions and barrier layers. Epitaxial WO3 on Nb-doped SrTiO3 an nealed at 350°C exhibited resistive switching behaviour with an ON/OFF ratio of 102 and high endurance and retention. A CMOS-compatible W on Si substrate was also used to achieve a much larger ON/OFF of 105 as well as exhibiting multilevel switching behaviour. However, the device suffers in endurance and retention. Overall, AP-SCVD has been shown to an excellent deposition method, even rivalling vacuum-based deposition techniques in terms of material properties and device performance perspectives for a range of potential applications of WO3. The open atmospheric tool can be taken another step further into the industry and target low-budget applications with high throughput.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Sun, Zhuotong

Advisor dc:contributor.advisor

• Driscoll, Judith

• Spatial Chemical Vapour Deposition
• Tungsten oxide
• Resistive switching
• photo-detector
• Ferroelectricity
• Epitaxy

Licence

• https://www.repository.cam.ac.uk/bitstreams/57b55619-72b7-4e6d-8374-45e0c5baeef8/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

Author Identifier

0000-0002-6951-7265

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/395698