First Observation of Ferroelectricity in ∼1 nm Ultrathin Semiconducting BaTiO3 Films

The requirements of multifunctionality in thin-film systems have led to the discovery of unique physical properties and degrees of freedom, which exist only in film forms. With progress in growth techniques, one can decrease the film thickness to the scale of a few nanometers (similar to nm), where its unique physical properties are still pronounced. Among advanced ultrathin film systems, ferroelectrics have generated tremendous interest. As a prototype ferroelectric, the electrical properties of BaTiO3 (BTO) films have been extensively studied, and it has been theoretically predicted that ferroelectricity sustains down to similar to nm thick films. However, efforts toward determining the minimum thickness for ferroelectric films have been hindered by practical issues surrounding large leakage currents. In this study, we used similar to nm thick BTO films, exhibiting semiconducting characteristics, grown on a LaAlO3/SrTiO3 (LAO/STO) heterostructure. In particular, we utilized two-dimensional electron gas at the LAO/STO heterointerface as the bottom electrode in these capacitor junctions. We demonstrate that the BTO film exhibits ferroelectricity at room temperature, even when it is only similar to 2 unit-cells thick, and the total thickness of the capacitor junction can be reduced to less than similar to 4 nm. Observation of ferroelectricity in ultrathin semiconducting films and the resulting shrunken capacitor thickness will expand the applicability of ferroelectrics in the next generation of functional devices.