Steps on Steps: The Nano-structure of a Vicinal Oxide Surface

11-07-2005

Grazing-incidence X-ray diffraction, combined with Low Energy Electron Diffraction (LEED) and Scanning Tunneling Microscopy (STM), shows that annealed SrTiO3 (103) surfaces exhibit regular nano-structures on two length-scales with shorter (~ 1.2 nm) ripples bunching to form longer (~ 7 nm) waves.

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The perovskite SrTiO3 (STO) is used as substrate for the epitaxy of giant magneto-resistance and high temperature superconductor (HTS) materials. STO also exhibits superconductivity, is catalytic and photo-active, and has potential applications such as gas-sensors and insulators with high dielectric constant for microelectronics. In all cases, the STO surface structure plays a crucial role.

Annealing up to 1000°C under ultra-high vacuum (UHV) produces clean and long range ordered STO surfaces as shown by the LEED image in Figure 1a. The visible spots represent Crystal Truncation Rods, indicative of a very smooth STO(103) surface. The STM image in Figure 1b shows modulations on two lengths scales in the [100] direction.

(a) LEED and (b) STM images of the SrTiO3 (103) 
        vicinal surface.

Fig. 1: (a) LEED and (b) STM images of the SrTiO3(103) vicinal surface performed in the Surface Characterisation Laboratory (SCL) of ID32 before the X-ray measurement. The sample was transferred under UHV from the SCL to a "baby" UHV chamber, which was then mounted on the diffractometer. Grazing-incidence X-ray diffraction data were recorded at ID32. The result of the analysis is shown in Figure 2.

 

(a) Unit cell of the SrTiO3(103)-1x2 superstructure; (b) 
        Resulting model of a 7.2 nm long SrTiO3(103)-6x2 supercell.

Fig. 2: (a) Unit cell of the SrTiO3(103)-1x2 superstructure, used for fitting the data. Oxygen atoms are in red. The "site occupancy" of the Sr atoms (in blue) is decreasing from bottom to top, which is indicated by lighter colours. The variation of the population values of topmost Ti atoms (in yellow) is similar to the Sr atoms, but not indicated. (b) Resulting model of a 7.2 nm long SrTiO3(103)-6x2 supercell (oxygen atoms not shown for clarity). From left to right, surface atoms are progressively missing, leading to a corresponding reduced (average) site occupancy in the SrTiO3(103)-1x2 unit cell . Lateral relaxations along y-direction (or [010]) lead to a (1x2) weak reconstruction observed by LEED and X-ray diffraction.

The SrTiO3(103) unit cells (cf. Figure 2 (a)) are responsible for the short-range modulation (1.2 nm) and (on average) six SrTiO3 (103) unit cells (Fig.2(b)) make up the longer range periodicity (7.2 nm) observed by STM along x-direction (or [100]). The resulting surface exhibits alternating, weakly polar terminations and the highly periodic ripples and waves appear to be stabilized by electrostatic forces. This is in contrast to usual self-organization on surfaces, commonly driven by strain, and represents a new paradigm in surface science.

 

Principal Publication and Authors
X. Torrelles (a), I. Joumard (b), TienLin Lee (b), O. Bikondoa (b) , J. Rius (a), and J. Zegenhagen (b)
(a) Institut de Ciència de Materials de Barcelona (C.S.I.C.), Bellaterra (Spain).
(b) ESRF