Growth and electronic properties of nanolines on TiO2-terminated SrTiO3(001) surfaces

Abstract

Surfaces of homoepitaxially grown TiO 2 -terminated SrTiO 3 (001) were studied in situ with scanning tunneling microscopy and spectroscopy. By controlling the Ti/Sr ratio, two-dimensional domains of highly ordered linear nanostructures, so-called "nanolines", are found to form on the surface. To further study how the surface structure affects the band structure, spectroscopic studies of these surfaces were performed. Our results reveal significantly more band bending for surfaces with the nanolines, indicative of an acceptor state associated with these features.Additionally, an in-gap state is observed on nanoline surfaces grown under high oxygen deficient conditions. This state appears to be the same as that observed previously, arising from the (++/+) transition level of surface oxygen vacancies. I. INTRODUCTIONAs a common complex-oxide substrate, SrTiO 3 has been intensely investigated for several decades. Since 2004 in particular, the SrTiO 3 surface has drawn particular attention from researchers owing to a discovery of a 2D electron gas (2DEG) at the interface of a complex-oxide heterostructures, i.e. LaAlO 3 (001)/SrTiO 3 (001). [1][2][3][4][5][6] In such heterostructures, where novel properties are found at the interface, it is important to be able to distinguish any contribution from the substrate, SrTiO 3 (STO), from those that are produced by formation of the epitaxial overlayer, LaAlO 3 (LAO). Therefore, it is crucial to have a complete material concerning the surface of the substrate immediately prior to the overlayer growth. Castell et al. demonstrated that by argon-ion sputtering and ultra-high-vacuum (UHV) annealing 7,8,9 (as well as by Ti deposition and subsequent oxidation), 10 the STO surface will form close-packed domains of linear features, denoted as nanolines. These features were shown to be composed of TiO 2 -derived complexes, and they thus provide new means for developing technologies for photocatalysis. 9 Those workers studied the electronic properties of the nanolines by x-ray photoelectron spectroscopy and density functional theory, 9 but a study of such properties by scanning tunneling spectroscopy (STS) has not been performed to date.It is well known that homoepitaxial growth by molecular beam epitaxy (MBE) can yield surfaces of high surface quality, and it enables fine control of their stoichiometry. In this work, we study MBE-grown TiO 2 -terminated SrTiO 3 (001) surfaces in situ using scanning tunneling microscopy (STM) and STS. With the combination of STM