Toward High‐Quality Epitaxial LiNbO3 and LiTaO3 Thin Films for Acoustic and Optical Applications

Over the past five decades, LiNbO3 and LiTaO3 single crystals and thin films have been studied intensively for their exceptional acoustic, electro‐optical, and pyroelectric and ferroelectric properties. Today, LiNbO3 single crystals in electro‐optics are equivalent to silicon in electronics, and about 70% of radio‐frequency (RF) filters, based on surface acoustic waves, are fabricated on these single crystals. These materials in the form of thin films are needed urgently for the development of the next‐generation of high‐frequency and/or wide‐band RF filters or tuneable frequency filters adapted to the fifth generation of infrastructures/networks/communications. The integration of LiNbO3 films in guided nanophotonic devices will allow higher operational frequencies, wider bandwidth, and miniaturized optical devices in line with improved electronic conversion. Here, the challenges and the achievements in the epitaxial growth of LiTaO3 and LiNbO3 thin films and their integration with silicon technology and to acoustic and guided nanophotonic devices are discussed in detail. The systematic representation and classification of all epitaxial relationships reported in the literature have been carried out in order to help the prediction of the epitaxial orientations in the new heterostructures. Future prospects of potential applications and the expected performances of thin film devices are overviewed, as well. Growth of LiNbO3 and LiTaO3 films, their structural and physical properties, and impact of the integration of these films on the performance of the acoustic, electro‐optical, and ferroelectronic devices are reviewed. The key issues in the growth of LiNb(Ta)O3 films, such as crystallinity, analysis and control of Li composition, orientation, morphology and roughness, and stress relaxation mechanisms, are described in detail.