The 1990’s trend towards the miniaturization of functional devices driven by the microelectronic industry led to the development of thin film materials integrated with semiconductor substrates, able to use their properties in micro and nanodevices with high integration densities and low operation voltages. However, since the beginning of this century, the electronic industry is demanding cost-efficient, soft-portable and high-tech devices. This has pushed the advance of flexible electronics, where the thin film is deposited on cheap flexible substrates (e.g., polymers, paper or textile).
The degradation temperature of these substrates is always below 400°C. Therefore, flexible electronics is calling for low-temperature thin-film fabrication methods, in addition to materials that can be processed at these temperatures. Hence, organic and amorphous metal oxide semiconductors are the most widely used materials in flexible electronics. However, other active layers different from semiconductors are demanded because of the need of enlarging the performance of the forthcoming flexible devices. This is an opportunity for ferroelectric oxide thin films since their intrinsic multifunctionality (e.g., ferroelectric, pyroelectric, piezoelectric, multiferroic or photoferroic) would make possible multiple operations in the flexible device. However, ferroelectric oxide films are conventionally crystalized at temperatures that exceed by far the thermal stability of the most favorable flexible substrates.
In this regard, solution deposition methods are the best positioned today to integrate metal oxide thin films with flexible substrates, as large-area, low-cost, high throughput fabrication techniques. In addition, they offer the opportunity of tailoring the solution chemistry to achieve some properties that can accelerate the formation of the ferroelectric oxide thin film at low temperatures.
This work shows the first results obtained in our laboratory about the fabrication of BiFeO3 thin films at low temperatures by using disruptive solution deposition methods that are based on the use of photochemistry and sonochemistry. The combination of these strategies has not been explored before for the fabrication of metal oxide films at low temperatures. We show here that they are potential methods for the successful deposition of functional crystalline metal oxide thin films on flexible plastic substrates.