Crystallographic structure and ferroelectricity of epitaxial hafnium oxide thin films

Devices using silicon-based materials have been studied and developed by the semiconductor industry. With silicon-based materials reaching their performance limit, there have been attempts to develop and discover alternative materials. Recently, HfO2 thin films have been considered a candidate material because of their diverse characteristics and potential for application in future memory devices. High-k-gate dielectric-based HfO2 thin films can replace silicon-based gate oxide layers. Moreover, HfO2 has been reported to possess ferroelectric properties in polycrystalline films, as also seen in memory devices. Hence, it is important to analyze the phase, structure, and crystallinity of HfO2 to confirm its ferroelectric properties; however, it has been challenging to do the same for pure HfO2 thus far. HfO2 thin films are ferroelectric in their orthorhombic or rhombohedral phase. The epitaxial growth of HfO2 thin films makes it possible to analyze the properties of each phase. Following the first report in 2015 on the epitaxial growth of HfO2 films, researchers have extensively studied their growth methods, structural and ferroelectric properties, phases, and application potential for future memory devices. This review summarizes the crystal structure, phases, deposition methods, and epitaxial growth mechanism of HfO2 thin films, as well as devices based on them. The findings will aid in next-generation device research.