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Atomically flat reconstructed rutile TiO2(001) surfaces for oxide film growth

The availability of low-index rutile TiO2 single crystal substrates with atomically flat surfaces is essential for enabling epitaxial growth of rutile transition metal oxide films. The high surface energy of the rutile (001) surface often leads to surface faceting, which precludes the sputter and annealing treatment commonly used for the preparation of clean and atomically flat TiO2(110) substrate surfaces. In this work, we reveal that stable and atomically flat rutile TiO2(001) surfaces can be prepared with an atomically ordered reconstructed surface already during a furnace annealing treatment in air. We tentatively ascribe this result to the decrease in surface energy associated with the surface reconstruction, which removes the driving force for faceting. Despite the narrow temperature window where this morphology can initially be formed, we demonstrate that it persists in homoepitaxial growth of TiO2(001) thin films. The stabilization of surface reconstructions that prevent faceting of high-surface-energy crystal faces may offer a promising avenue towards the realization of a wider range of high quality epitaxial transition metal oxide heterostructures.


Introduction
Most transition metal dioxides (MO 2 , with M denoting a 3d, 4d, or 5d transition metal ion) have the rutile structure. They exhibit properties that are the subject of intense attention in condensed matter physics. [1][2][3][4][5][6][7] Moreover, their technological value for e.g. catalysis, [8][9][10] gas sensing, 11 and as (transparent) conductors [11][12][13] cannot be overstated. Tailoring these properties by confining these materials to thin film geometries and e.g. subjecting them to epitaxial strain requires pseudomorphic growth on suitable substrates. Indeed, the preparation of atomically flat SrTiO 3 (001), (110), and (111) substrates [14][15][16] has enabled the growth of a large range of perovskite oxide heterostructures with precisely controlled atomic thicknesses, thereby facilitating the discovery of many exciting properties. [17][18][19] Similarly, for rutile oxide film growth, the availability of different low-index atomically flat rutile TiO 2 substrates is a key requirement for advancing the utility of rutile transition metal oxide heterostructures in condensed matter and applied physics. To date, most rutile oxide heterostructures are grown on TiO 2 (110) substrates. It is very difficult to prepare TiO 2 (100) and (001) surfaces that are crystalline, atomically flat, and stable against faceting, because their surface free energies are high. 20 Rutile TiO 2 has a body-centered tetragonal unit cell (a = b = 0.458 nm, and c = 0.295 nm), Fig. 1(a), with two formula units per unit cell. The Ti atoms are octahedrally coordinated by oxygen atoms. In the [001] direction, i.e. along the rutile the c-axis, the crystal structure can be described by alternating charge neutral TiO 2 layers, as shown in Fig. 1(b). The layers stack in such a way that alternating atomic layers are rotated 90° with respect to one another. Their separation is 0.147 nm, i.e. c/2. Consequently, on an atomically flat bulk-terminated surface, steps with half-unit-cell height are expected to be present.
However, the customary substrate cleaning approach of sputtering and annealing in ultra-high vacuum (UHV), widely used for the (100), (110) and (011) orientations, 21-24 introduces either a faceted morphology on TiO 2 (001), see Fig. 1(d), [25][26][27] or small (~20 nm) terraces with a ridge-like structure. 28 The resulting surface is unsuitable for epitaxial growth of rutile films with an atomically precise thickness or a well-defined (biaxial) strain state. 29 This instability towards faceting is caused by the high surface energy of the TiO 2 (001) surface due to the large number of broken bonds on this surface. 20,29,30 While some recent publications [30][31][32] reported flat surfaces of TiO 2 (001), no evidence was presented concerning the stability of this morphology under conditions that are typical for oxide film growth. Moreover, there are no reports demonstrating the persistence of an atomically flat 'step-terrace' morphology after homoepitaxial film growth on TiO 2 (001).
Here we present a method to prepare atomically flat rutile TiO 2 (001) substrate surfaces with a step-terrace morphology. We find, using LEED, RHEED, and STM, that the surface is terminated by a two-domain c(7 2×5 2)R45° surface reconstruction, featuring stripes along the [110] and [-110] directions. The step-terrace morphology survives annealing in an oxygen plasma or in vacuum at typical oxide growth temperatures. Moreover, we have been able to grow homoepitaxial films on this surface and demonstrate that the atomically flat step-terrace morphology is retained. We conjecture that the observed stability of the step-terrace morphology of the substrates is enabled by a decrease in surface energy due to the formation of the surface reconstruction.

Experimental methods
TiO 2 (001) substrates (CrysTec, miscut angle < 0.5°) with dimensions 5×5×0.5 mm 3 were first cleaned by treating them in isopropyl alcohol (IPA) ultra-sonication baths for 3 min to remove organic contaminants. Next, the substrates were rinsed in deionized (DI) water to remove any residual IPA from the surface. The substrates were then etched in buffered hydrofluoric acid (NH 3 F : HF = 10:1, JT Baker) for 10 min to aid terrace formation, followed by a DI water rinse. This effect was ascribed to impurity removal from the TiO 2 surface. 30 The TiO 2 substrates were subsequently annealed in a tube furnace in air or ultra-high purity oxygen. An alumina boat was used to load the substrates. After inspecting the surface morphology in ambient conditions with an atomic force microscope (AFM), the substrates were transferred into a UHV system for LEED, RHEED, oxygen plasma exposure and STM experiments.
During oxygen plasma exposure at a chamber pressure of 5 × 10 -5 mbar and a plasma current of 30 mA, samples were heated by a laser heater. The temperature was monitored with an infrared pyrometer calibrated using a thermocouple on a dummy sample surface. X-ray photoelectron spectroscopy (XPS) was performed in a separate system using monochromatized Al Kα x-rays. Homoepitaxial growth of TiO 2 (001) films by pulsed laser epitaxy (PLE) was conducted in a separate system to illustrate the usefulness of the atomically flat substrate for epitaxy, employing a substrate temperature of 300 °C, and an oxygen pressure of 2 × 10 -2 mbar. Additional experimental details on the PLE growth are available in Ref. 33. All AFM, STM, XPS, LEED, and RHEED data were taken at room temperature.

Results and discussion
In Figure 2 we present the evolution of the surface morphology as measured with AFM for TiO 2 (001) as a function of annealing temperature in air, systematically covering under-annealed and over-annealed conditions in order to establish clear references for future reproducibility. A temperature ramp rate of 15 °C/min was used, and the duration of the thermal annealing was kept at 5 hours. After wet chemical treatment and before the furnace anneal, the surface is smooth: no step edges are visible and the RMS roughness measured in a 25 μm 2 area is ~ 0.76 nm ( Fig. 2(a)). At an annealing temperature of 780 °C, discontinuous flat terraces emerge ( Fig. 2(b)). The area coverage of these terraces increases until the temperature reaches 820 °C where the surface is uniformly covered by atomically flat terraces with an average width of about 400 nm (Fig. 2(d)). At 850 °C, faceted islands appear, with the faceted structures showing as bright elongated features along two perpendicular directions ( Fig. 2(e)). Upon increasing the annealing temperature to 900 °C, the surface is fully covered with these faceted islands, consistent with a previous report. 29 Fig. 2(d) and (f) reveal uniform surface morphologies with RMS roughness of 0.18 nm and 1.09 nm (in a 25 μm 2 area) respectively, showing a large increase in roughness above the optimal annealing temperature. We conclude that wet chemical treatment followed by a five-hour anneal at 820 °C in air produces stepped surface morphologies with atomically flat terraces ( Fig. 2(d)) that can serve as an excellent template for subsequent film growth. These atomically flat TiO 2 (001) substrates were transferred into a UHV system. STM experiments on these surfaces are difficult due to the insulating nature of the sample, but LEED and RHEED can be performed without any further treatment. Besides the integer order spots, additional fractional order spots are visible in the LEED and RHEED patterns, although they are very faint (see Fig. S1(a) 33 showing that a surface reconstruction forms already during the annealing treatment in the furnace. This surface reconstruction is further characterized below. To ascertain whether it is contamination from the annealing treatment in air that stabilizes the observed reconstruction and step-terrace morphology, we repeated the annealing treatment using ultra-high purity oxygen gas instead of air. This resulted in LEED and RHEED patterns showing the same superstructure (Figs. S1(b) 33 ), indicating that the reconstruction most likely is not induced by e.g. carbon, nitrogen, or other contaminants from the air during the annealing treatment. Indeed, X-ray photoelectron spectra taken from an air-annealed substrate before any degassing in the vacuum system show the presence of only nitrogen and carbon, and only in small quantities that are expected after a transfer through air (Fig. S2a) 33 ). This is further corroborated by the absence of a Ti 3+ component in the Ti 2p core level spectrum of the as-loaded sample (Fig. S2(c) 33 ) showing that nitrogen doping is indeed absent, 4,33,34 and that the Ti ions are fully oxidized. Finally, a vacuum anneal at 500 °C removes nearly all N and C intensity from the XPS survey spectra (Fig. S2(a)), and does not decrease the quality of the surface diffraction pattern (Fig.   S2(b)). These results thus conclusively reveal that the reconstruction already forms during the furnace anneal, that it is a TiO 2 structure, and not due to ordered adsorbates.
After exposing the surface to atomic oxygen using an in situ oxygen plasma treatment at 500 °C for 40 minutes, the fractional order spots in the electron diffraction patterns become very clear, see Fig. 3(a) and Fig. S3. 33 The superlattice spots can all be indexed using a c(7 2×5 2)R45° reconstruction, with two 90° rotated domains as dictated by the four-fold symmetry of the (001) substrate (Fig. S4 33 ). To improve the electrical conductivity for STM experiments, we mildly degassed as-loaded samples at ~150 °C in vacuum. The STM image in Figure 3(d) shows that the as-loaded surface retains the step-terrace morphology after this degassing (and before the oxygen plasma treatment mentioned above). Line profiles distinguish steps with a height of ~ 0.15 nm (green lines in Fig. 3(d) and (f)) and Fig. 3(d) and (f)), which correspond to half and full unit cell height steps, respectively. The presence of half unit cell height steps is consistent with the crystal structure of a (001) surface plane (see Fig. 1). The RMS roughness on the terraces (as measured in an area of 2500 nm 2 ) is ~ 0.08 nm. This shows the terraces are atomically flat even though they do not reveal well-defined atomic order in STM imaging, see also  Next, we grew ~6 monolayers of TiO 2 on the c(7 2×5 2)R45° reconstructed TiO 2 (001) substrate using PLE. Since film growth was performed in a separate vacuum system, we verified that the reconstruction persists even after (re-)exposure to air, and hence confirmed that the same substrate condition existed for PLE growth (Fig. S6 33 ). The morphology of the grown film, imaged with AFM ( Fig. 4), shows the same step-terrace morphology after growth. We note that such results, demonstrating the retention of an atomically flat step-terrace morphology upon homoepitaxial overgrowth of a TiO 2 (001) substrate, have not been reported before. RHEED patterns acquired directly before and after growth (insets in Fig. 4) confirm that there are no significant differences in surface structure and morphology. The AFM and RHEED data clearly illustrate that atomically flat  Insets: RHEED patterns recorded before (left) and after (right) homoepitaxial growth.

~0.3 nm (blue lines in
In conclusion, we have identified a narrow thermal annealing temperature window to prepare atomically flat and relatively stable TiO 2 (001) surfaces, in spite of their high surface energy and the associated instability towards faceting. During this annealing the surface reconstructs into a superstructure with a c(7 2×5 2)R45° unit cell that is stable towards faceting up to at least 600 °C. We conjecture that this surface reconstruction decreases the intrinsically high surface energy of the

Pulsed Laser Epitaxy growth parameters
We ablated a sintered TiO 2 target using a KrF excimer laser (λ = 248 nm, repetition rate 10 Hz, laser fluence 1 J/cm 2 ). The oxygen pressure and substrate temperature were optimized at 2 × 10 -2 mbar and 300 °C, respectively. The distance between target and substrate was kept at 45 mm, and the laser spot size on the target was approximately 0.49 mm × 0.80 mm. Film growth started after pre-ablating the target for 1000 pulses. The deposition rate was determined as 111 pulses/ML. Figure S1. LEED patterns at 100 eV beam energy and RHEED patterns along the [010] direction for air-annealed (a) and O 2 -anealed (b) 'as-loaded' TiO 2 (001) substrates. There was no further surface treatment after loading samples to the UHV system. Integer order spots are indexed in yellow circles. Faint fractional order spots of the c(7 2 ×5 2 )R45° reconstruction are already visible after both furnace annealing treatments. Figure S2. Survey X-ray Photoelectron spectra (a) and LEED patterns at 100 eV (b) of an as-loaded and subsequently UHV annealed TiO 2 (001) substrate. (c) Ti 2p core level spectrum of the as-loaded sample. The survey spectrum does not show sufficient nitrogen, carbon or other species to conclude that N or C species are incorporated in the unit cell of the observed reconstruction (compare to Figs. 1(b) in Refs. 4 and 34. The Ti 2p core level spectrum (c) can be well fitted using a single core level component, indicating there is no reduced Ti 3+ on the surface and the surface is fully oxidized to Ti 4+ . These results verify that the c(7 2×5 2)R45° reconstruction is not stabilized by impurities introduced during the furnace anneal. Figure S3. LEED pattern at 82 eV beam energy after oxygen plasma exposure at 500 °C for 40 minutes. The incident electron beam is slightly off-normal so as to reveal the fractional order spots near the (0,0) position. A unit cell of the c(7 2×5 2)R45° superstructure is indicated. Figure S4. Simulated reciprocal space lattice of the c(7 2×5 2)R45° reconstruction based on the square (p4/mmm) 1×1 substrate lattice. Blue and red dots originate from the two orthogonal domains, as dictated by the substrate symmetry. A unit cell of the c(7 2×5 2)R45° superstructure is indicated by a blue dashed rectangle. Figure S5. STM image (+5.0 V, 5.0 pA, 50 nm × 50 nm) of the as-loaded TiO 2 (001) surface after degassing. Figure S6. LEED patterns of TiO 2 (001) substrates for an as-loaded sample (a); after annealing in an O 2 plasma at 500 °C for 40 min (b); after subsequent exposure to air and reloading into the vacuum system (c); and finally after a mild degassing in UHV (d). These data show that the c(7 2×5 2)R45° reconstruction persists even after exposure to air.