$(\sqrt{13}\times \sqrt{13})R13.9°$ and $(\sqrt{7}\times \sqrt{7})R19.1°$ reconstructions of the polar ${\mathrm{SrTiO}}_3\left(111\right)$ surface

Nb-doped ${\mathrm{SrTiO}}_3\left(111\right)$ samples are annealed in UHV at $850°\mathrm{C}$ for $30\min$ and investigated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). STM images show that both $(\sqrt{7}\times \sqrt{7})R19.1°$ and $(\sqrt{13}\times \sqrt{13})R13.9°$ reconstructions coexist on the surface. Step heights of $0.21\pm 0.02\mathrm{nm}$ on the surface are equivalent to the $d_{111}$ lattice parameter, which is the distance between two adjacent, similar (111) planes in the bulk crystal. The calculated LEED pattern for this co-reconstruction corresponds to the observed LEED pattern, which resembles a six-petal flower. AES analysis indicates no detectable impurities, and shows the surface to be Ti and Sr enriched and O deficient compared to the bulk stoichiometry. This change in surface composition is proposed to provide the stability for the polar surface.

Figure 1

(Color online) (a) Unit cell of ${\mathrm{SrTiO}}_3$ with the ${{\mathrm{SrO}}_3}^{4-}\left(111\right)$ plane indicated. Sr ions at the center of the unit cell are green (medium gray), Ti ions, which are situated at the corners of the unit cell, are blue (light gray). O ions are red (dark gray) and are found at the center of the edges of the unit cell. (b) Bulk-terminated ${\mathrm{SrTiO}}_3\left(111\right)$ surface viewed from the side. The material is made up of alternating ${\mathrm{Ti}}^{4+}$ and ${{\mathrm{SrO}}_3}^{4-}$ layers. A step is shown from a Ti-terminated layer to a ${\mathrm{SrO}}_3$-terminated layer $(1∕2d_{111})$ and between two ${\mathrm{SrO}}_3$ layers $\left(d_{111}\right)$.

Figure 2

(a) Image showing domains of the $(\sqrt{13}\times \sqrt{13})R13.9°$ and $(\sqrt{7}\times \sqrt{7})R19.1°$ reconstructions and irregularly shaped step edges. Disorder can be seen between the reconstructed domains (image size $150\times 150{\mathrm{nm}}^2$, sample bias $+2.0\mathrm{V}$, tunneling current $0.2\mathrm{nA}$). (b) Topography trace of the image in (a) in a straight line between the two arrows. Step heights are equal to the $d_{111}$ lattice parameter $\left(0.23\mathrm{nm}\right)$.

Figure 3

(a) Domains toward the top and bottom $\left(\alpha \right)$ are $(\sqrt{13}\times \sqrt{13})R13.9°$, and those at the right- and left-hand sides $\left(\beta \right)$ are $(\sqrt{7}\times \sqrt{7})R19.1°$. The two $\beta$ regions show two different rotational domains, whereas the $\alpha$ regions show the same rotation from the underlying bulk lattice (image size $65\times 65{\mathrm{nm}}^2$, sample bias $+2.9\mathrm{V}$, tunneling current $0.3\mathrm{nA}$). (b) Image of the $(\sqrt{13}\times \sqrt{13})R13.9°$ reconstruction with the unit cell indicated (image size $12\times 12{\mathrm{nm}}^2$, sample bias $+3.6\mathrm{V}$, tunneling current $0.12\mathrm{nA}$). (c) Image of the $(\sqrt{7}\times \sqrt{7})R19.1°$ reconstruction with the unit cell indicated (image size $12\times 12{\mathrm{nm}}^2$, sample bias $+2.14\mathrm{V}$, tunneling current $0.1\mathrm{nA}$).

Figure 4

Illustration of the unit cells of the two rotational domains of the $(\sqrt{7}\times \sqrt{7})R19.1°$ (gray) and $(\sqrt{13}\times \sqrt{13})R13.9°$ (black) reconstructions.

Figure 5

Generic and observed LEED patterns for the $(\sqrt{7}\times \sqrt{7})R19.1°$ and $(\sqrt{13}\times \sqrt{13})R13.9°$ reconstructions. All calculated patterns were obtained using LEEDPAT (Ref. 27). (a) Calculated generic LEED pattern for the two rotational domains of a $(\sqrt{7}\times \sqrt{7})R19.1°$ reconstruction. (b) Generic LEED pattern of the two rotational domains of a $(\sqrt{13}\times \sqrt{13})R13.9°$ reconstruction. (c) Superposition of the calculated generic LEED patterns of coexisting $(\sqrt{7}\times \sqrt{7})R19.1°$ and $(\sqrt{13}\times \sqrt{13})R13.9°$ reconstructions. (d) Observed LEED pattern for the annealed surface. The gray scale of the LEED pattern has been inverted (electron energy $51\mathrm{eV}$). This pattern is the same as in (c).

Figure 6

(a) Auger spectrum from the $(\sqrt{7}\times \sqrt{7})R19.1°\text{−}(\sqrt{13}\times \sqrt{13})R13.9°$ surface. The spectrum has been normalized so that it has a value of 1000 counts at $1100\mathrm{eV}$ (a point within the featureless background). (b) High-resolution Auger spectra of the $(\sqrt{7}\times \sqrt{7})R19.1°\text{−}(\sqrt{13}\times \sqrt{13})R13.9°$ reconstruction (black) and a UHV cleaved surface (gray). This region shows the characteristic Ti [380 and $413\mathrm{eV}$ $\left(LMM\right)$] and O [492 and $511\mathrm{eV}$ $\left(KLL\right)$] peaks. (c) Difference plot for the cleaved spectrum in (b) subtracted from the $(\sqrt{7}\times \sqrt{7})R19.1°\text{−}(\sqrt{13}\times \sqrt{13})R13.9°$ spectrum in (b). Positive peaks indicate a greater concentration of a species in the $(\sqrt{7}\times \sqrt{7})R19.1°\text{−}(\sqrt{13}\times \sqrt{13})R13.9°$ surface compared with that of the stoichiometric cleaved surface. Negative dips indicate a lower concentration. The concentration of Ti in the $(\sqrt{7}\times \sqrt{7})R19.1°\text{−}(\sqrt{13}\times \sqrt{13})R13.9°$ surface is greater, and that of O is lower, in comparison to the stoichiometric cleaved surface. (d) High-resolution spectra of the $(\sqrt{7}\times \sqrt{7})R19.1°\text{−}(\sqrt{13}\times \sqrt{13})R13.9°$ (black) and cleaved (gray) surfaces. The Sr peaks are at 1644 and $1711\mathrm{eV}$ $\left(LMM\right)$. (e) Difference plot for the cleaved spectrum in (d) subtracted from the $(\sqrt{7}\times \sqrt{7})R19.1°\text{−}(\sqrt{13}\times \sqrt{13})R13.9°$ spectrum in (d). A peak is present at $1644\mathrm{eV}$, indicating a greater Sr concentration for $(\sqrt{7}\times \sqrt{7})R19.1°\text{−}(\sqrt{13}\times \sqrt{13})R13.9°$ relative to the cleaved sample.