Evidences of a ~1 nm thick metallic-like ferroelectric BaTiO_3-δ film at room temperature

Requirements of multi-functionalities in thin-film systems have led important findings of unique physical character and degree of freedom which exist only in film forms. As growth technique gets advanced, one can decrease the film thickness even a-few nano-meter (~nm) scale where its unique physical character still appears. Among those intriguing film systems, ferroelectrics have been of interest. As a prototype ferroelectric, electrical properties of ultra-thin BaTiO₃ (BTO) films have extensively studied, which is found that ferroelectricity sustains down to ~ nm thick films as theoretically predicted. However, efforts on determination of the minimum thickness in ferroelectric films has been hindered by large leakage current. In this study, we used ~ nm thick BTO films showing metallic-like behaviour around room temperature (RT). We found that the metallic-like BTO film has ferroelectricity at RT even in ~2 unit-cells thick. Observation of such ultra-thin conducting ferroelectric will enlarge its applicable fields, leading realization of new functional devices and investigations of further physical phenomena.