Incipient ferroelectricity: A route towards bulk-terminated SrTiO3

Abstract

Perovskite oxides attract increasing attention due to their broad potential in many applications [1][2][3]. Understanding their surfaces is challenging, though, because the ternary composition of the bulk allows for multiple stable surface terminations [4]. We demonstrate a simple procedure for preparing the bulk-terminated (001) surface of SrTiO 3 , a prototypical cubic perovskite. Controlled application of strain on a SrTiO 3 single crystal results in a flat cleavage with micrometer-size domains of SrO and TiO 2 . Distribution of these two terminations is dictated by ferroelectric domains induced by strain [5] during the cleavage process. Atomically-resolved scanning tunneling microscopy/atomic force microscopy (STM/AFM) measurements reveal the presence of point defects in a well-defined concentration of (14±2)%; Sr vacancies form at the SrO termination and complementary Sr adatoms appear at the TiO 2 termination. These intrinsic defects are induced by the interplay between ferroelectricity, surface polarity, and surface charge.SrTiO 3 is the prototypical cubic perovskite oxide, interesting for its catalytic and photocatalytic properties [6], potential use in oxide electronics [7][8][9][10], and fundamental questions including the appearance of twodimensional electron gas at its surfaces [11][12][13] and interfaces [3]. The SrTiO 3 (001) surface plays a key role in all these functionalities, but the available surface studies of this facet illustrate a general problem of approaching perovskite materials: A plethora of various surface terminations can form, depending on the preparation technique and on slight changes in the surface stoichiometry. The centre of interest typically lies in bulk-terminated perovskite surfaces, because wet chemical preparation methods typically provide surfaces with a (1×1) diffraction pattern [9]. Yet this issue is contoversial due to the possible presence of amorphous structures [14] or contaminants [15].Cleaving a single crystal would be an obvious solution for obtaining a pristine surface. Despite numerous attempts [16-18], we are not aware of any successful work leading to atomically flat bulk-terminated SrTiO 3 . The main impediment is that cubic perovskites typically do not possess a natural cleaving plane. Instead they exhibit so called conchoidal fracture (see Figure S1 in Supplementary Information). An alternative technique for surface preparation is applying cycles of sputtering and high-temperature annealing -a standard procedure used for preparing oxide surfaces in ultrahigh vacuum (UHV). For perovskite surfaces, however, this methods results into a series of complex reconstructions [4,19,20], which are very stable and chemically inert; exactly the opposite to the behaviour of perovskites in most applications.Our previous study on KTaO 3 [21] indicated that ferroelectricity induced in a perovskite can create a natural cleavage plane oriented perpendicular to the electric polarization. SrTiO 3 is an incipient ferroelectric material: Its ferroelectric Curie temperature lies...

Single spin-polarized Fermi surface in SrTiO3 thin films

“…6(f). Whether this observation fully explains the differences in band filling requires further investigation with microscopy and spectromiscroscopy techniques [39].…”

The Electronic Properties of Extended Defects in SrTiO3—A Case Study of a Real Bicrystal Boundary

“…Figure 3c shows a comparison of line profiles obtained from the current maps in Figure 3e,f that illustrate the In order to obtain an insight into the conductivity in all dimensions, we investigated the bicrystal after cleaving it perpendicular to the boundary. As SrTiO 3 crystallizes in the cubic perovskite structure, it does not possess a defined cleavage plane, resulting in a shell-like fracturing [34]. Hence, the topography map of the cleaved face (Figure 4a) is relatively rough and different facets can be observed.…”

Electrically driven ferroelastic domain walls, domain wall interactions, and moving needle domains

“…The left and bottom surfaces have A atom terminations, while the upper and right surfaces have B atom terminations. Such configurations may also be observed in bulk materials on a fairly local scale [50].…”

“…The biggest movement of the needle tip is seen when the sample surface terminations are either A or B atoms. This implies that the sample surface is charged, which may occur in thin films or in bulk material under special cleavage conditions or light illumination [50,[59][60][61]. An applied electric field induces surface strain by attracting and repulsing the surface atoms.…”