surface states of topological insulators are protected by time reversal symmetry, which makes them impervious to nonmagnetic perturbations and restricts scattering. [1,3,4] For these reasons, topological insulators have garnered great interest. [1,2] Topological crystalline insulators (TCIs) are a subset of topological materials that are narrow band gap semiconductors in the bulk with gapless surface states; however, the topological surface states are protected by lattice symmetry rather than time reversal symmetry. [2,[5][6][7][8][9][10][11][12][13][14][15][16] Topological crystalline insulators have been theoretically predicted and experimentally realized on compounds with orbital degrees of freedom playing a similar role to spin in topological insulators; [2,[5][6][7][8][9][10][11][12][13][14][15][16] one such material is SnTe. [5][6][7][8][9][10][11][12][13][14][15][16] The key role played by surface symmetry in protecting the topological states suggests that the TCI properties can be manipulated by introducing symmetry breaking surface features. As a first step toward this goal, we have studied the epitaxial growth of SnTe films and characterized their structural and electronic properties. It will be shown that the growth process and subsequent postgrowth treatment creates characteristic defects that can alter the local electronic properties of the surface down to the atomic scale.Topological crystalline insulators (TCIs) are new materials with metallic surface states protected by crystal symmetry. The properties of molecular beam epitaxy grown SnTe TCI on SrTiO 3 (001) are investigated using scanning tunneling microscopy (STM), noncontact atomic force microscopy, low-energy and reflection high-energy electron diffraction, X-ray diffraction, Auger electron spectroscopy, and density functional theory. Initially, SnTe (111) and (001) surfaces are observed; however, the (001) surface dominates with increasing film thickness. The films grow island-by-island with the [011] direction of SnTe (001) islands rotated up to 7.5° from SrTiO 3 [010]. Microscopy reveals that this growth mechanism induces defects on different length scales and dimensions that affect the electronic properties, including point defects (0D); step edges (1D); grain boundaries between islands rotated up to several degrees; edge-dislocation arrays (2D out-of-plane) that serve as periodic nucleation sites for pit growth (2D in-plane); and screw dislocations (3D). These features cause variations in the surface electronic structure that appear in STM images as standing wave patterns and a nonuniform background superimposed on atomic features. The results indicate that both the growth process and the scanning probe tip can be used to induce symmetry breaking defects that may disrupt the topological states in a controlled way.

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The impact of ferroelectric polarization on the chemical and electronic properties of atomically thin layers of non-polar chromium oxide deposited on positively and negatively poled LiNbO3(0001) was studied. Chromium(III) oxide readily forms on LiNbO3; however, annealing at high temperatures was required to maintain well-ordered films as the thickness increased. Prolonged heating at these temperatures caused Cr diffusion into the LiNbO3 substrate. Comparing Cr 2p X-ray photoelectron spectroscopy (XPS) peak positions as a function of temperature and substrate polarization revealed no evidence of shifts from the peak positions expected for Cr2O3. The lack of any band offset between Cr2O3 on the oppositely poled surface suggests that charge compensation of the ferroelectric substrate occurs at least predominantly at the surface of the film, as opposed to the film-substrate interface. No evidence of shifts due to oxidation or reduction of the Cr was observed indicating that charge compensation did not involve a change in the ionic state of the Cr. Exposing the films to reactive oxygen species emitted from an oxygen plasma, however, caused a distinct high binding energy shoulder on the Cr 2p3/2 XPS peaks that could be associated with oxygen adsorption on surface Cr and concomitant oxidation to Cr(5+). This feature was used to gauge the concentration of O adatoms on the surfaces as a function of temperature for oppositely poled substrates; these measurements did not reveal any significant polarization dependence for oxygen desorption. Further, temperature programmed desorption measurements for a Cr2O3 film on α-Al2O3 showed a similar trend in O2 desorption. Therefore, it is concluded that the reactivity of Cr2O3 toward O is at least largely independent of substrate polarization despite data suggestive of charge compensation at the film surfaces.