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High Electron Mobility of Nb-Doped SrTiO3 Films Stemming from Rod-Type Sr Vacancy Clusters Shunsuke Kobayashi, Yuki Mizumukai, Tsuyoshi Ohnishi, Naoya Shibata, Yuichi Ikuhara, and Takahisa Yamamoto ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b05720 • Publication Date (Web): 20 Oct 2015 Downloaded from http://pubs.acs.org on October 21, 2015

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High Electron Mobility of Nb-Doped SrTiO3 Films Stemming from Rod-Type Sr Vacancy Clusters Shunsuke Kobayashi1, *, Yuki Mizumukai2, Tsuyoshi Ohnishi3, Naoya Shibata4, 5, Yuichi Ikuhara1, 4 and Takahisa Yamamoto1, 2, 6, ** 1

Nanostructures Research Laboratory, Japan Fine Ceramics Center, Atsuta, Nagoya 4 56-8587, Japan 2 Department of Advanced Materials Science, The University of Tokyo, Kashiwa, Chiba 277-8561, Japan 3

National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan 4 Institute of Engineering Innovation, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan 5 PRESTO, Japan Science and Technology Agency, Saitama 332-0012, Japan 6 Department of Quantum Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan Corresponding authors *E-mail: [email protected] **E-mail: [email protected]

Keywords: SrTiO3, electron mobility, defect structure, pulsed laser deposition, strain, STEM

ABSTRACT Achieving high electron mobility in SrTiO3 films is of significant interest, particularly in relation to technological applications such as oxide semiconductors, field-induced superconductors, and thermoelectric generators. One route to achieving high electron mobility is growth of high quality SrTiO3 films with low defect concentrations. Another approach for mobility enhancement is applying a strain to the crystal. However, the maximum mobilities obtainable by these approaches are limited both

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by external and internal factors (currently available fabrication techniques, and maximum crystal strain, for example). In this paper, we demonstrate a unique crystal engineering approach to alter the strain in Nb-doped SrTiO3 films based on the deliberate introduction of Sr vacancy clusters. Nb-doped SrTiO3 films produced in this manner are found to exhibit remarkably enhanced electron mobilities (exceeding 53,000 cm2 V-1s-1). This method of defect engineering is expected to enable tuning and enhancement of electron mobilities not only in SrTiO3 films, but also in thin films and bulk crystals of other perovskite-type materials.

*Abstract graphic


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Electron-doped SrTiO3 film systems exhibit myriad interesting properties.1-4 To exploit these properties in useful applications, larger values of the electron mobility are required. Previous studies aiming to increase the electron mobility in electron-doped SrTiO3 films have focused on removing lattice defects (i.e., dislocations, planar defects and point defects)5-7 because it was thought that these interfere with electron mobility. However, even under ideal thermodynamic conditions, an equilibrium concentration of point defects is always present, so ultimately electron mobility will still be limited by intrinsic electron scattering mechanisms.8 It is thus likely that only modest improvements in the electron mobility of SrTiO3 films can be achieved using this strategy. Another well-known technique is strain management, which has been applied for many years to semiconductors such as strained Si.9 In this case, enhanced electron mobility is achieved using strain induced by an external mechanical apparatus or lattice mismatch between the in-plane lattice constant of the film and that of the surface layer on which the film is grown. The same strain management technique has also recently been shown to be effective for La-doped SrTiO3 films.10 While strain management is a very exciting approach for investigating physical properties with potential applications in current and future devices, some problems exist regarding the limit of coherent growth (i.e., a critical film thickness).11 Inducing strain mechanically is challenging because there is an intrinsic crystal strain limit and external supports are needed to generate the strain. Thus, there is strong motivation to develop another method to improve electron mobility that overcomes or avoids these limitations. In this paper we present a new approach for enhancing electron mobility without the need for mechanical treatment or the use of lattice mismatch. Our method involves engineering SrTiO3 crystals so that they contain regions of low Sr vacancy concentrations and regions with Sr vacancy clusters to induce an internal strain field. From the mass-action relationship between Sr vacancies (VSr) and oxygen


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vacancies (VO) it is known that VSr concentrations decrease and VO concentrations increase under high temperature and low oxygen pressure growth conditions.5,12 Examination of film growth under these conditions with either Sr or Ti excess compositions shows that Ti-excess films contain regions with high tensile lattice strain adjacent to the clusters and high compressive lattice strain between the clusters that act as fast transport pathways for electrons.


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RESULTS AND DISCUSSION The Sr/Ti ratio in the film is very sensitive to laser fluence during film growth.13-14 Systematic control of the laser fluence during pulsed laser deposition (PLD) allowed us to obtain films with compositions ranging from Sr excess to Ti excess. High-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) confirmed that this off-stoichiometry had a significant effect on the defect structures of the thin films. Figures 1a and 1b show HRTEM images of representative Sr-excess and Ti-excess SrTiO3 films, respectively, taken along the [100] zone axis (cross-sectional direction). In the images, white arrows indicate lattice defects in the off-stoichiometry films. In the case of the Sr-excess film, lines of high contrast were observed along (010) and (001) planes. The electron diffraction pattern in Figure 1a also shows streaks, indicating the presence of a planar-type defect or an extra SrO plane defect (a Ruddlesden-Popper-type defect).15-16 The HRTEM image of the Ti-excess film in Figure 1b contains high contrast lines originating from Sr vacancy clusters, which are predominantly aligned parallel to (010) and (100) planes and have been studied previously by high-angle annular dark-field (HAADF) STEM and electron energy loss (EEL) spectroscopy.17 The arrangement of defects within the Sr vacancy clusters was analyzed in detail using HAADF and low-angle annular dark-field (LAADF) STEM because the shape of the clusters is a key factor in enhancing electron mobility. HAADF STEM produces images with contrast that is directly related to the atomic number Z of elements in each atomic column (approximately as Z squared).18 LAADF STEM images are very sensitive to the strain field around lattice defects because they also record coherently scattered electrons.19 A combination of HAADF and LAADF STEM analyses thus provides information about both the atomic column positions and the strain field around a lattice defect.


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A schematic model of a vacancy cluster based on analysis of HAADF and LAADF STEM images (see Figures S1 and S2 of Supporting Information for further details) taken from two directions is shown in Figure 1c. In the model, Sr vacancies form clusters with a rod-like structure aligned parallel to the a axis. Views of the this rod-type Sr vacancy cluster model down the b and a axes are shown in Figures 1d and 1e, respectively. Figure 1f presents a HAADF STEM image of a typical vacancy cluster taken parallel to the b axis in a Ti-excess thin film, corresponding to the view of the model shown in Figure 1d. The white arrows in Figure 1f indicate a region with a high concentration of Sr vacancies. Figure 1g presents a LAADF STEM image taken from the same region as Figure 1f. Bright contrast is observed around the Sr vacancy cluster, indicating a strain field corresponding to lattice expansion around the Sr vacancy sites.20 Substantial numbers of these clusters were observed throughout all Ti-excess films. The lattice expansion in the STEM images (Figure 1f and 1g) corresponds to a large cation shift of about 0.05 nm near the Sr vacancy cluster (see Figure S1). HRTEM and STEM images show line contrast in both cross-sectional (Figures 1f and 1g) and plan-view (Figure S3) samples, suggesting that the Sr vacancy cluster is not ellipsoid-shaped but rather rod-shaped, as per our model. To confirm that the Sr vacancy cluster has a rod-type configuration, a single Sr vacancy cluster was also observed down the a axis, corresponding to the view of the model in Figure 1e. In the HAADF STEM image in Figure 1h, the Sr column intensity is weak at the center of the dotted box, indicating a lower Sr concentration than other Sr columns, as confirmed by the line profile in Figure 1i. The bright contrast around the Sr vacancy cluster in the LAADF STEM image taken from the same region (Figure 1j) shows that presence of a strain field in the region immediately surrounding the cluster. These results are consistent with vacancy clusters in Ti-excess SrTiO3 films having a rod-type structure similar to that in Figure 1c. Additional discussion about the structure of the Sr vacancy cluster is included as


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supplementary information.

Figure 1. HRTEM images of (a) Sr-excess and (b) Ti-excess SrTiO3 films taken down the [100] zone axis (cross-sectional direction). The electron diffraction patterns are shown as insets within each HRTEM image. White arrows in the HRTEM images indicate the extra SrO planes in (a) and Sr vacancy clusters in (b). A magnified image of an Sr vacancy cluster is shown as an inset in (b). (c) A schematic of the Sr vacancy cluster model in Ti-excess SrTiO3 films, with oxygen atoms removed for clarity. The a, b and c directions are no longer degenerate. (d) and (e) show the Sr vacancy model in (c) viewed down the b and a directions, respectively. (f) HAADF and (g) LAADF STEM images taken of an Sr vacancy cluster corresponding to the view of the model in (d), with white arrows in (f) indicating high concentrations of Sr vacancies. (h) HAADF STEM image taken of the Sr vacancy cluster corresponding to the view of the model in (e). (i) Intensity profile along A-A′ obtained from the HAADF STEM image


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in (h). The positions of columns with high Sr vacancy concentrations are indicated by red arrows. (j) LAADF STEM image of the region in (h).

Electron mobility measurements of the Sr-excess film showed that it is insulating (i.e., no conductivity above the detection limit of the measurement equipment). This can be rationalized as being a result of the additional SrO planes within the crystal interrupting or blocking electron conduction paths along Ti– O bonds. In contrast, the Ti-excess SrTiO3 films, which contain rod-type Sr vacancy clusters, exhibit a remarkable enhancement of the electron mobility. Figure 2a presents the electron mobility of Ti-excess films as a function of temperature for a range of lattice expansion values. The lattice expansion of each film was estimated by fitting simulated data to X-ray diffraction patterns (see Figure S4). In the film with a lattice expansion of 0.7 pm, the electron mobility was greater than 53,000 cm2 V−1s−1 at 2 K. This mobility is about seven times larger than that of the single crystal used as a target (µ = 7,652 cm2V−1s−1 and n = 1.6×1019 cm3 at 2 K). LAADF STEM images of films with different amounts of lattice expansion were taken in plan-view, as this allowed the density of the lattice defects in each film to be compared (see Figure S5). The increase in lattice expansion was thereby confirmed to correspond to increased density of rod-type Sr vacancy clusters (Figures 2c–e). Considering the close correspondence between the enhancement of electron mobility and the density of these lattice defects, it can be concluded that the high electron mobility is related to formation of rod-type vacancy clusters. Figure 2b shows the temperature dependence of the carrier concentration for Ti-excess films for a variety of lattice expansions. Not surprisingly, the carrier concentration of the near-stoichiometric film (lattice expansion = 0.2 pm, n = 2.2×1019 cm−3 at 2 K) was similar to that of the target single crystal. This indicates that the carrier concentration of the target material and the near-stoichiometric film is


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controlled by the Nb dopant concentration. The carrier concentrations are much lower in the lattice-expanded films with high electron mobility.

Figure 2. Temperature dependence of (a) electron mobilities and (b) carrier concentrations of SrTiO3 films with a variety of lattice expansions. LAADF STEM images of SrTiO3 films with lattice expansion (c) 0.2 pm, (d) 0.5 pm and (e) 0.7 pm viewed along the [001] zone axis (plan-view direction) whose properties are shown in (a) and (b).

Figure 3 shows our proposed model of how Sr vacancy clusters induce compressive strain in the crystal. Based on the strain contrast of the LAADF STEM images in Figures 2c–e and the measured lattice expanded region around the Sr vacancy clusters (approximately 3 nm, as shown in Figure S1), the lattice expanded regions adjacent to the rod-type Sr vacancy clusters do not represent the entire strain field in 9 ACS Paragon Plus Environment

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the film. Regions between the Sr vacancy clusters have a compressive strain component, resulting in chemical shifts detectable by EEL spectroscopy analysis.

Figure 3. Rod-type Sr vacancy clusters expand the spacings between neighboring lattice planes, which results in a simultaneous compression of the region between two rod-type Sr vacancy clusters.

The results in Figure 2 show that the measured electron mobility enhancement is strongly correlated to the presence of rod-type Sr vacancy clusters. To investigate the effect of the clusters on the electronic states in more detail, high-resolution EEL spectroscopy was performed. Figure 4a shows the Ti-L2, 3 edge spectra obtained from the single-crystal target material and Ti-excess films corresponding to the films imaged in Figures 2c–e. The Ti-L2, 3 edge originates from the electron transition from Ti 2p3/2 and 2p1/2 states to the unoccupied Ti 3d orbitals. In response to the ligand field around octahedrally coordinated Ti, Ti 3d orbitals split into two levels, t2g (or dε) and eg (or dγ).21 The lower symmetry case splits further; t2g splits into dxy, dyz and dzx, and eg splits into dx2-y2 and d3z2-r2. The local crystal symmetries in the films are complex because of the existence of compressive and tensile strain fields induced by the rod-type Sr vacancy clusters and the global strain field induced by the film-substrate lattice mismatch. In Figures 4a 10 ACS Paragon Plus Environment

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and 4b, peaks are labelled as t2g and eg for simplicity. Chemical shifts of the Ti-L2, 3 edges toward higher energy loss were observed in Ti-excess films with different lattice expansions and rod-type Sr vacancy clusters densities. There are two possibilities for the chemical shift of the Ti-L2, 3 edge: a change of valence state and strain field effects.19,22-23 Based on the carrier concentrations of the films (Figure 2b), the valence state of the Ti ions should be close to 4+. If the valence state of Ti were 3+, the chemical shift should be toward lower energy,22 so the change of the valence state cannot explain the chemical shift of the Ti-L2, 3 edge observed here. It is known that the band edges of a crystal are altered as a function of the strain,23 and that when a compressive strain field is present in the crystal, the Ti-L2, 3 edge shifts to higher energy because of the increasing ionicity of Ti.24 The chemical shift of the Ti-L2, 3 edge should thus be related to a compressive strain induced by the rod-type Sr vacancy clusters. In Figure 4a it can be seen that the Ti-L2, 3 edge of the Ti-excess film with a lattice expansion of 0.2 pm has a similar shape to that of the Nb-doped SrTiO3 used as the target material. This is because the film is near to stoichiometric. The shapes of the Ti-L2, 3 edge in the Ti-excess films with lattice expansions of 0.5 pm and 0.7 pm, which were high electron mobility films, are noticeably different to that of the target material, however. The shapes of the Ti-L2 edges also exhibit interesting features, as seen in Figure 4b. The t2g and eg of the Ti-L2 edge split into two peaks in the case of the 0.5 pm and 0.7 pm expanded Ti-excess films, and the split peaks of the t2g signal shift toward higher energy with increasing lattice expansion, as indicated by red arrows in Figure 4b. To confirm these trends, Ti-L2,

3

edge EEL spectra were recorded at positions in a region near a

rod-type Sr vacancy cluster (Figure 4c) in the Ti-excess film with a lattice expansion of 0.5 pm. The


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results are shown in Figure 4d. The Ti-L2, 3 edges obtained shift toward the higher energy side, which is as expected because the average Ti-L2, 3 edges of the films in Figure 4a are higher in energy than the single crystal. It can be seen that the tops of the t2g peaks shift toward a slightly higher energy in the lattice expanded region (see curves 6–11 in Figure 4d and corresponding integrated intensity profile in Figure S1), while in the lattice compressed region (curves 1–4 and 12–15 in Figure 4d) the top of the peak shifts toward even higher energies, as indicated by red arrows in Figure 4d. The peak shifts in the lattice expanded and compressed regions coincide with the split peaks of t2g shown in Figure 4b, and the sum of the split-peak spectra in Figure 4d correspond to the average Ti-L2, 3 edge of the 0.5 pm lattice expanded film in Figure 4a. In the SrTiO3 films, the t2g energy level increases with increasing compressive strain field. These results strongly suggest that compressive strain fields induced by rod-type Sr vacancy clusters alter the electron mobility.

Figure 4. (a) Ti-L2, 3 edge and (b) Ti-L2 edge EEL spectra obtained from single-crystal Nb-doped


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SrTiO3 and Ti-excess films corresponding to those imaged in Figures 2(c)–(e). The dotted lines in (a) indicate the tops of the Ti-L3 t2g peaks. The red arrows in (b) indicate the Ti-L2 t2g peaks. (c) An ADF STEM image of the region around an Sr vacancy cluster in the Ti-excess film with a lattice expansion of 0.5 pm, with white arrows indicating the location of the Sr vacancy cluster. (d) Ti-L2, 3 edge EEL spectra obtained from the regions indicated by circles in the ADF STEM image in (c). The red arrows indicate the Ti-L2 t2g peaks in the region of compressive strain.

The measured EEL spectra suggest that the compressive strain field induced by rod-type Sr vacancy clusters is an important factor for enhancing electron mobility. To clarify the effect of the strain field, Ti-excess SrTiO3 films with various densities of rod-type Sr vacancy clusters were investigated. Figures 5a and 5b present the electron mobility and the carrier concentration, respectively, measured from the films and a single crystal at 300 K and 2 K as a function of lattice expansion, which is related to the density of rod-type Sr vacancy clusters in each film. When the lattice parameter of the film increased by approximately 0.3 pm, the electron mobility decreased and the carrier concentration increased. However, with a further increase in the lattice expansion above 0.3 pm, the electron mobility rapidly increased, while the carrier concentration decreased. These trends in electron mobility and carrier concentration can be explained by considering the lattice defects in Ti-excess films. Under the growth conditions used in this study (low oxygen pressure and excess Ti), VTi can be ignored because of its high formation energy.25 We therefore expect that the Ti-excess SrTiO3 films will include rod-type Sr vacancy clusters, as well as isolated Sr and O vacancies. For lattice expansions up to 0.3 pm, the lattice defects VSr and VO in the film cause the electron mobility to decrease and the carrier concentration to increase. This is because VSr and VO disturb the electron mobility.5, 26 Rod-type Sr vacancy clusters, having a net charge, also decrease the mobility through defect scattering. The increased carrier concentration may be the result of the VO that remain after the


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annealing process. The density of rod-type Sr vacancies increases rapidly in the films with lattice expansions greater than 0.3 pm. This might be expected to lead to a reduction in the electron mobility, as would be expected following the introduction of lattice defects, but instead is accompanied by an increase, while the carrier concentration decreases. The decrease in the carrier concentration is related to the relative concentrations of lattice defects. One possibility is that the films are not homogeneous on the atomic level, such that the nature of the conducting region may be altered by the large number of rod-type Sr vacancies. Another possibility is that there is a change in the distribution of doped Nb atoms resulting from segregation in the vicinity of the strain field. In this case, some Nb donors will not be ionized.27 The relationship between electron mobility and carrier concentrations at 2 K and 300 K is shown in Figures 5c and 5d, respectively. Previously reported data are also plotted in each figure. Power approximation profiles estimated from the present work and previous results are shown as dashed-dotted and dashed–double-dotted lines, respectively. The power approximation profiles of our results and previously reported data are clearly different at both temperatures. The change in gradient of the power approximation profiles can be attributed to the influence of the rod-type Sr vacancy clusters on the electron mobility.


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Figure 5. (a) Electron mobility and (b) carrier concentration of Ti-excess SrTiO3 films and a single crystal used as a target at 2 K and 300 K as a function of lattice expansion. Electron mobility of Ti-excess SrTiO3 films and a single crystal at (c) 2 K and (d) 300 K as a function of carrier concentration. Open triangles, squares and diamonds are data for Nb-doped SrTiO3 from Refs. 5, 26 and 28, respectively. Estimated power approximation profiles for our films are shown as dashed-dotted lines (red for 2 K and blue for 300 K), while profiles estimated from previously reported data are shown as gray dashed-double-dotted lines.

In general, electron mobility in SrTiO3 can be understood by considering an impurity scattering model in the low temperature region.26,28-29 SrTiO3 is known to be a quantum paraelectric material, i.e., it has a high dielectric constant at low temperatures.30-32 The high dielectric constant of SrTiO3 generates an effective screening of ionized impurity scattering centers and high electron mobility at low temperatures.8,33 However, the quantum paraelectric state is easily changed to a ferroelectric state34-43 and the dielectric constant of ferroelectric SrTiO3 at low temperature strongly depends on its ferroelectric state.40-43 The relationship between the dielectric constant and the electron mobility is not 15 ACS Paragon Plus Environment

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clear from the present data and is left as a topic for future investigations. A change in the effective carrier mass may be an important factor higher electron mobility. It has been reported that strain in a distorted SrTiO3 crystal improves mobility by reducing the band gap and interband scattering, which suggests that the effective mass is reduced.24,44-45 For example, La-doped SrTiO3 under compressive strain (−0.3 %) showed an electron mobility enhancement of approximately three times (exceeding 120,000 cm2V−1s−1).10 In our films, the strain field is generated by two sources: the rod-type Sr vacancy clusters and the lattice mismatch between the expanded film and the substrate. In the case of the Ti-excess films (µ = 53,734 cm2V−1s−1), the lattice mismatch estimated using the out-of-plane lattice parameter and the Poisson ratio of SrTiO3 (ν = 0.232)46 is −0.15 %, which seems too small to explain the seven times larger electron mobility of Ti-excess film compared to the target crystal. The rod-type Sr vacancy clusters must therefore provide a larger strain field than the global strain field induced by the lattice mismatch. To determine the magnitude of the strain field induced by rod-type Sr vacancy clusters, cluster densities in each film were estimated by counting the number of clusters from low magnification LAADF STEM images (see Figure S6). The measured densities of rod-type Sr vacancy clusters were 2.8 × 1016 cm−3, 6.7 × 1016 cm−3, and 2.6 × 1017 cm−3 for the 0.2 pm, 0.5 pm, and 0.7 pm expanded films, respectively. These measurements give average distances between clusters in the three films of 33.1 nm, 24.6 nm, and 15.7 nm for the 0.2 pm, 0.5 pm, and 0.7 pm expanded films, respectively. The average magnitude of the compressive strain field can be approximated using a simplified model of two rod-type Sr vacancy clusters parallel to each other along their long axes, as illustrated in Figure 4. The average lattice parameter in the lattice expanded regions was 0.396 nm and the average lattice parameter in all other regions was assumed to be the same as that of the substrate (0.3905 nm). The compressive strains


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estimated in the region between two Sr vacancy clusters were −0.30 %, −0.44 %, and −0.85 % for the 0.2 pm, 0.5 pm, and 0.7 pm expanded films, respectively. This simple quantitative analysis confirms that the strain field induced by rod-type Sr vacancy clusters is larger than the global strain field induced by the lattice mismatch. The compressive strain region induced by rod-type Sr vacancy clusters is expected to facilitate high electron mobility. The above strain field analysis can also explain the high electron mobility at room temperature. Previous reports on electron mobility in n-type SrTiO3 at room temperature suggest a mobility of approximately 6 cm2V−1s−1 over a wide range of carrier concentrations.26,28,47-48 Figure 5d shows that the electron mobility in the Ti-excess film exhibits different behavior. The electron mobility at room temperature is dominated by longitudinal optical (LO) phonon scattering.8,26,33,49 In contrast to a previous report on strained La-doped SrTiO3 films showing that the electron mobility did not change at room temperature,10 our results suggest that the strain in the Ti-excess film has an effect even at room temperature. The lattice expansion at which the electron mobility increased in our Sr vacancy cluster films varied from 0.3 pm at 2 K to 0.7 pm at 300 K, as shown in Figure 5a. These results indicate that the enhancement of electron mobility at room temperature requires a larger strain field than at low temperatures. The mobility enhancement at room temperature could be a result of changes to the effective mass and LO phonon frequency because of the strain. It is known that strain decreases the effective mass because of changes to the shape and degeneracy of the conduction band.24,44-45 Also, strain can lower the LO phonon frequency and reduce scattering.49 To overcome the negative effects of the lattice defects and optimize electron mobility at room temperature, the magnitude of the strain may need to be increased further to alter the effective mass, the LO phonon frequencies, or both.24,44-45,49


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Previous studies have shown how lattice mismatch-induced strain effects can be used for tuning, controlling and, indeed, enhancing properties in other perovskite materials.50-54 It is also likely that introducing lattice defects similar to rod-like Sr vacancy clusters into single crystals of other materials will result in similar improvements in carrier mobility without the need for mechanical treatment or multilayer film formation, opening up new possibilities for a wide variety of applications.

CONCLUSIONS Enhancement of the electron mobility in Nb-doped SrTiO3 films was achieved by introducing a unique defect structure, the rod-type Sr vacancy cluster, into their crystals through careful control of the composition and synthesis conditions. We found that these vacancy clusters create a compressive strain field in the SrTiO3 crystal that is larger than the strain field induced by lattice mismatch between the film and the substrate. The compressive strain field induced by the vacancy clusters results in a high electron mobility at low (including room) temperatures (e.g., over 53,000 cm2 V-1s-1 at 2 K). It is possible that the carrier mobility of Ti-excess films may decrease beyond some density of rod-type Sr vacancy clusters because of defect scattering or poor crystallinity, so further optimization of the defect density and carrier density in Ti-excess SrTiO3 films to obtain the highest possible electron mobility is necessary. The method we have outlined for enhancing mobility using a defect-induced strain field, however, should also be applicable to other perovskite materials, and represents a new strategy for crystal engineering of nanoelectronic materials.


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METHODS Sr-excess and Ti-excess SrTiO3 thin films were prepared using pulsed laser deposition (PLD). Nb-doped (0.1 at. %) and undoped SrTiO3 single crystals were used for the target and substrate, respectively. The (001) surface of each substrate was treated to have an atomically flat step-terrace structure.55 The growth temperature was set at 1150 °C, and the chamber base pressure was set to approximately 7×10−7 Pa. The PLD setup used a KrF excimer laser (248 nm) with a laser frequency of 2 Hz. Film thicknesses of approximately 100 nm were achieved by controlling the total pulse counts. The Sr/Ti ratio of the films was controlled by systematically tuning the laser fluence

13, 14

which was

optimized using a reflection-type variable attenuator system without any change of spot size or laser discharge voltage. After film growth, the sample was annealed in air at 400 °C for 12 h to remove oxygen vacancies. The lattice parameters of the prepared films were estimated by simulated data to X-ray diffraction (XRD) patterns (ATX-G, Rigaku Co.). Resistivity and Hall coefficient measurements were carried out using a physical property measurement system (Quantum Design Inc.) using a standard four point method and van der Pauw geometry,57 respectively. Contacts to the films were made using Al wire bonding. The defect structure of each film was investigated using HRTEM (EM-002BF, Topcon Co.) and aberration-corrected (CEOS GmbH) STEM (ARM-200F, JEOL Ltd.). HRTEM and STEM were performed with accelerating voltages of 200 kV. Cross-sectional TEM samples were prepared by the conventional method. First, an undoped single crystal of SrTiO3 was glued onto the film surface so that it had the same crystal orientation as the substrate. The glued sample was then cut in a cross-sectional direction, ground, polished and ion-milled to electron transparency. Plan-view TEM samples were prepared using the wedge polishing method.56 19 ACS Paragon Plus Environment

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The probe-forming aperture semiangle in STEM was 22 mrad. HAADF and LAADF STEM images were primarily recorded with 81–228 mrad and 27–110 mrad detectors, respectively. An EEL spectrometer (Tridiem ERS, Gatan, Inc.) was attached to a monochromated aberration corrected STEM (JEM-2400FCS, JEOL Ltd.), which had a Wien filter monochromator and operated at 200 kV. EEL spectra were obtained from TEM samples of thin films in plan-view as well as crushed particles of Nb-doped (0.1 at. %) SrTiO3 single crystals supported on holey carbon films. EEL spectra were recorded within a rectangular area (about 25 × 25 nm) and line scan in STEM mode, using 0.1 eV per channel and an energy resolution of 300 meV (full-width at half-maximum of zero-loss peak) to reduce electron beam damage. The background signal of the EEL spectra was subtracted by power law fitting.

Supporting Information Supporting Information Available: Details of estimation for cation shifts, XRD, interpretation of LAADF STEM images and analysis for strain field. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The authors thank A. Tsukazaki and C. A. J. Fisher for useful discussions. Part of this work was supported by a Grant-in-Aid for Scientific Research for Innovative Areas "Nano Informatics" (Grant No. 25106003) and a Research Activity Start-up Grant (No. 26889078) from the Japan Society for the Promotion of Science (JSPS). A part of this work was supported by “Nanotechnology Platform” (project No. 12024046) sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. A part of this work was performed using facilities of the Cryogenic Research Center, The University of Tokyo.


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