Metal oxide thin films for optoelectronic applications

Cutting-edge technologies constantly evolve driven by the societal need towards development and sustainability. Device miniaturization, internet-of-things, wireless telecommunication technologies have all been highly influenced by the development of new advanced functional materials and the innovations related to their production processes. The enhanced precision and control of material properties at the nanoscale have highlighted the importance of metal oxide thin films for applications such as electronics, photovoltaics and optoelectronics.

Various deposition techniques can be employed to produce metal oxide thin films. This thesis focuses on magnetron sputtering due to its ability to deposit highly uniform thin films with high adhesion and quality, even at low temperatures. Magnetron sputtering is compatible with a wide range of target materials and allows precise control over material properties such as thickness and composition. Additionally, the use of reactive sputtering enhances the versatility of the materials that can be deposited. Consequently, magnetron sputtering is currently used in industrial production due to its scalability which makes it one of the most promising techniques for large-area optoelectronic applications such as solar cells, displays and smart windows. From the wide variety of thin films that can be produced by magnetron sputtering, metal oxide thin films are of high scientific interest due to their high versatility, including the possibility to be tuned as insulators, semiconductors or conductors.

Among the very large number of metal-oxides we focused our effort on studying three key compounds relevant to smart windows. The first is Al-doped ZnO (AZO) which is a transparent conductive oxide (TCO) used in solar cells, touch screens and various optoelectronic applications. The second is vanadium dioxide (VO₂) which is a thermochromic material known for its metal-to-insulator transition (MIT) temperature at around 68°C. The ability to transition from a dielectric state at room temperature to a metallic state above the transition temperature, has rendered VO₂ one of the most intriguing metal oxide thin films to be developed in smart windows, thermal sensors, memory devices and optoelectronic devices, among others. Finally, the study of tungsten trioxide (WO₃) was carried out due to its remarkable electrochromic performance. WO₃ can undergo a reversible change of its coloration, hence optical properties, when an electric voltage is applied due to the intercalation and deintercalation of ions such as Li⁺, H⁺, or Na⁺.

The first study covers the use of magnetron sputtering under different operation modes and their influence, along with the employed sputtering parameters, on the optoelectronic properties of AZO. Although AZO is one of the most promising candidates to replace indium tin oxide (ITO) as the most widely used TCO, it suffers from poor electrical properties uniformity related to the negative oxygen ions assisted growth. Among the investigated modes, radio-frequency magnetron sputtering was proved to provide the highest uniformity in electrical resistivity, while high power impulse magnetron sputtering (HiPIMS) provides the lowest uniformity.

HiPIMS was identified as the magnetron sputtering mode most influenced by the energetic species, attributed to the high target voltage. This effect was further investigated in relation to the electrochromic performance of WO₃, which to our knowledge has been barely studied, providing new insights on how to improve the electrochromic properties of sputtered WO₃.

In the final part, magnetron sputtering was employed to obtain state-of-the-art VO₂ thin films and then use different interface layers to assess and improve the thermochromic performance of VO₂. It was proved that by forming a stack of layers, a decrease of the transition temperature of VO₂ could be achieved to values lower than the typical 68°C.

Overall, the research carried out in this project enabled DTU Nanolab to enhance its understanding on depositing metal oxide thin films by magnetron sputtering, resulting in state-of-the-art optoelectronic properties.