Led by Assistant Professor Takeo Ohsawa of NIMS and Associate Professor Taro Hitosugi of Tohoku University's AIMR, a research team has developed a new advanced system, combining a super-resolution microscope and a deposition chamber for growing oxide thin films. With this system, they successfully observed for the first time the growing metal-oxide thin films at an atomic level on the surface of single-crystal strontium titanate (SrTiO3). Based on these observations, they identified the mechanism involved in the growth of the thin films in which titanium atoms rose to the surface of the film.
(Left) Scanning tunneling microscopy image of 0.3 unit-cell SrTiO3 thin film (15 nm × 15 nm). Atomic arrangement is clearly observed to be identical between the SrTiO3 thin film (purple) and the SrTiO3 substrate underneath (blue). (Right) A growth model illustrating the formation of SrTiO3 thin film. The TiO2 layer present on the surface of the SrTiO2 substrate is transferred to the surface of the thin film. (Image: National Institute for Materials Science)
Metal oxides, including perovskite-type oxides such as SrTiO3, are commonly used due to their diverse properties, which include superconductivity, ferromagnetism, ferroelectricity and catalytic effect. In recent years, novel properties generated at the interface between two dissimilar oxides have been vigorously investigated. However, little is known about the mechanism involved in the formation of such interfaces. Understanding this mechanism is key to further research advances in this field.
The NIMS/AIMR research group developed an innovative system that combines a scanning tunneling microscope capable of identifying individual atoms with a pulsed laser deposition method that enables the growth of high-quality thin films. In addition, they also established a method for preparing a single-crystal SrTiO3 substrate on which atoms are arranged in a periodic pattern. Epitaxial thin films were grown on the surface of the substrates and the growth was observed with atomic-scale spatial resolution. In their observations, they found there was a great difference in the growth process when SrTiO3 and SrOx thin films were deposited on the surface of the substrates.
Furthermore, the team identified a phenomenon in which excess titanium atoms present on the surface of the SrTiO3 substrate rose to the surface of the thin film. These observations facilitated a clear atomic-scale understanding of the growth process regarding how oxide thin films are formed. These results may not only contribute to the understanding of the origin of interfacial properties but also lead to the creation of new electronics devices through the development of new functional materials.