Phase change oxide electronics and sensors
Creators: name: Kabir, Sumaiya
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401605
subject: Functional materials

description: With the growing demand for high-speed and high-density devices in modern electronics, phase change oxides can be a potential candidate for fast and energy-efficient electronic and optoelectronic applications. Among phase change oxides, vanadium dioxide (VO2) has drawn considerable interest in recent years as it undergoes a reversible insulator-to-metal transition (IMT) near room temperature accompanied by thermochromism in the infrared region. At a temperature lower than the transition temperature, Tc, the structure of VO2 is monoclinic (M1) and it acts as infrared-transparent insulator. Above Tc, VO2 converts into a tetragonal structure (R) with infrared-reflective metallic properties. In particular, VO2 thin film has gained much attention compared with its bulk counterpart due to its robustness, flexibility to tune electrical and optical properties, and ability to sustain several heating-cooling cycles without mechanical cracking. These thin-films triggered by multiple-stimuli, including thermal, optical, and electrical, make them suitable for a wide range of applications such as imaging, artificial synapses, neuromorphic computing, energy-saving smart windows, and so on. However, a thorough investigation of the effect of the size and structure of VO2 devices is needed to reduce the limitations and fulfil the knowledge gaps in the case of optical and electrical perturbations. Moreover, the fabrication of functional and high-performance thermochromic coatings is still challenging on large scale as the functionality and performance of VO2 are highly sensitive to annealing conditions. The high-temperature annealing requirement makes the fabrication process incompatible to temperature-sensitive surfaces. This research is divided into three parts. In the first part, IMT and photoresponse of different sizes of VO2 devices were investigated with thermal and optical excitations for optoelectronic applications such as photodetectors. This work finds that grain size and grain boundary density variation in different size devices influence the IMT temperature, hysteresis width, and switching resistance ratio during IMT. Further, VO2 thin-film based broadband (UV-Vis-NIR) photodetectors are presented in which enhancement in photoresponse is obtained in two ways – by decreasing the device size and using the IMT slope region (50-60 oC) and beyond IMT slope region for photodetection. It is found that the presence of free carriers from VO2 (R) phase significantly improves the photoresponse. The second part focuses on the influence of device size and structure impact on IMT performance in case of electrical stimulation. Three types of VO2 thin-film-based metal-insulator-metal (MIM) structures depending on electrode arrangements – offset, no offset, and overlapping- were explored experimentally and with simulation. It is noted that among the three MIM structures, overlapping one underwent IMT at the lowest voltage and no dependency is observed with the variation of electrodes width and amount of overlapping in micron-scale devices. Finally, third part investigates on the state-of-the-art solution-based techniques to achieve annealing free high-performance VO2 coatings. Using a room temperature solution-processed VO2/polymer composite films were prepared where polymer prevented oxidation and aggregation of particles, thereby improving the uniformity and thermochromic properties of the prepared films. An investigation on proper choice of polymer and effect of VO2/polymer ratio on the films’ performance has been carried out. The experimental results show that the molar ratio of VO2/polymer in composite films plays a vital role in the thermochromic performances of the prepared films. The demonstrated solution process produces functional VO2 films with improved optical properties using a diverse array of dip-coating, drop-casting, and screen-printing methods on both rigid and flexible substrates, offering great feasibility for scaling up in the future. The findings related to the size and structure of VO2 devices with optical and electrical excitations provide valuable insight towards a wide range of applications to improve integration density, scalability, and lower power consumption in high-speed electronic devices. Moreover, room-temperature solution-processed VO2 coatings using nanopowder will contribute to reducing the challenges in large-scale fabrication for thermochromic applications.