## 3 Thin-Film Materials and Their Applications
### Complex Metal Oxides  
In contrast with binary oxides, complex oxides have higher dielectric constant, which can be utilized as high-K materials. It is found that some complex oxides have ferromagnetic, ferroelectric, and magnetoresistive effects, which allows complex oxides to be implemented into important applications in the field of microelectronics. For example, amorphous a-IGZO [103] and perovskite metal-oxides [104], have been exploited to facilitate the advancement of microelectronics under the bottleneck of Moore's law. In 2019, [PERSON] et al. presented water-soluble TFTs and circuits based on a-IGZO, which was fabricated with water-soluble materials including SiN\({}_{x}\), SiO\({}_{x}\), molybdenum, and polyvinyl alcohol (PVA) [105]. Firstly, the a-IGZO TFTs were fabricated on Si substrate by photolithography, etching, and deposition. Then, the a-IGZO TFTs were transferred onto degradable PVA substrates. They explored the dissolution kinetics for a-IGZO films in deionized water, bovine serum, and phosphate buffer saline solution. The obtained results suggested that the proposed water-soluble a-IGZO TFTs have great application possibilities in biological and environmental resorbable technologies and for temporary biomedical implants. In the same year, [PERSON] et al. proposed an amorphous Al-doped zinc tin oxide (a-AZTO) film instead of the a-IGZO film acting as a resistive-switching layer [106]. Compared with In and Ga elements, Al and Sn are relatively safe elements and are common dopants in ZnO materials, which is contributed to environmental-friendly production. Moreover, the replacement of IGZO with AZTO film can effectively reduce the consumption of in element (a toxic and rare metal), prominently improving environmental safety. Perovskite materials (ABX\({}_{3}\)) including oxide-perovskite and halide-perovskite are a special kind of inorganic film, whose special characteristic comes from its unique properties. The most remarkable feature is that it can produce a photothermal effect, which means that the conversion of light and electricity in the environment can be realized through these films [107; 108]. For example, Pr\({}_{x}\)Ca\({}_{I-x}\)MnO\({}_{3}\), BaTiO\({}_{3}\), SrTiO\({}_{3}\), Fe-doped SrTiO\({}_{3}\), DyMn\({}_{2}\)O\({}_{5}\), and BiFeO\({}_{3}\) have been broadly investigated for the environmental-friendly sensors and electronic devices [109; 110]. [PERSON] et al. prepared Ag-doped BaTiO\({}_{3}\)-CuO mixed oxide thin film, which was used as a carbon dioxide sensor for environmental pollution and monitoring applications [76]. More complex oxide-perovskite like La\({}_{0,6}\)Sr\({}_{0,4}\)Co\({}_{0.2}\)Fe\({}_{0,8}\)O\({}_{3}\) perovskite were fabricated by [PERSON] et al. for a stable anode catalyst for direct methane solid oxide fuel cells [111]. Among complex oxides, there is a family of material known as piezoelectrics (such as perovskite materials) that can interconvert between electrical and mechanical energies. In other words, inputted strain signals can produce an electrical signal for sensors; inverse, inputted electric fields can produce a mechanical deformation. [PERSON] et al. reviewed the piezoelectric thin films for piezoelectric microelectromechanical systems (MEMS) capable of applying to sensors, actuators, and energy harvester [112]. They investigated the influence of composition, orientation, and microstructure on the piezoelectric properties of perovskite thin films (e.g., PbZr\({}_{1-x}\)Ti\({}_{x}\)O\({}_{3}\) (PZT)) in detail. The materials of this structure can not only achieve photovoltaic thermal effects to obtain energy but also have important applications in bionics. For example, PZT materials combined with ZnO have a good piezoelectric effect, which can realize high-speed biological vibration on the silicon-based plane. Traditional PZT thin films require complicated fabrication technologies and contain a toxic component (lead (Pb)), which limits their potential use in flexible and wearable devices as well as in medical implants. [PERSON] et al.