Letter pubs.acs.org/NanoLett
Atomically Engineered Metal−Insulator Transition at the TiO2/LaAlO3 Heterointerface Makoto Minohara,*,† Takashi Tachikawa,†,‡ Yasuo Nakanishi,‡ Yasuyuki Hikita,† Lena F. Kourkoutis,§,∥ Jun-Sik Lee,⊥ Chi-Chang Kao,⊥ Masahiro Yoshita,# Hidefumi Akiyama,# Christopher Bell,† and Harold Y. Hwang†,∇ †
Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States ‡ Department of Advanced Materials Science, The University of Tokyo, Kashiwa, Chiba 277-8561, Japan § School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, United States ∥ Kavil Institute at Cornell for Nanoscale Sciences, Ithaca, New York 14853, United States ⊥ Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States # Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan ∇ Geballe Laboratory for Advanced Materials, Department of Applied Physics, Stanford University, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: We demonstrate that the atomic boundary conditions of simple binary oxides can be used to impart dramatic changes of state. By changing the substrate surface termination of LaAlO3 (001) from AlO2 to LaO, the room-temperature sheet conductance of anatase TiO2 films are increased by over 3 orders of magnitude, transforming the intrinsic insulating state to a high mobility metallic state, while maintaining excellent optical transparency. KEYWORDS: Anatase TiO2, heterointerfaces, termination layer switching, metal−insulator transition
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(3) reduction of optical transparency due to impurity states within the band gap. Here we demonstrate that the film/substrate interface termination can provide an independent method for controlling the TiO2 conductivity. Anatase TiO2 can be readily grown epitaxially on LaAlO3 (001) due to the small lattice mismatch of −0.2%. 8 At the atomic scale, TiO 2 /LaAlO 3 (001) heterostructures have two distinct interfacial stacking sequences, which we will refer to as AlO2-terminated (TiO2/AlO2− LaAlO3), and LaO-terminated (TiO2/LaO−LaAlO3), as shown in Figure 1a,b, respectively. We find 3 orders of magnitude change in the room-temperature sheet conductance by
n recent years, the ability to atomically control perovskitederived heterostructures has provided new degrees of freedom to engineer properties beyond those found in bulk form. This has led to the discovery of many unique electronic, magnetic, and orbital interface states.1 Expanding this approach to binary oxides has the potential to control their properties by manipulating the atomic boundary conditions built into the heterostructures. Anatase titanium dioxide (TiO2) is an attractive target for this concept being an oxide semiconductor with diverse photocatalytic properties.2−5 For these applications, as well as its potential as a transparent conductive oxide, control of the film conductivity is of primary importance. Usual strategies of doping by transition-metal ions or defects6,7 often suffer from (1) degradation of crystallinity due to solubility limits, (2) decrease in electron mobility due to scattering, and © 2014 American Chemical Society
Received: October 11, 2014 Published: October 24, 2014 6743
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Within both STEM images, two dissimilar crystallographic arrangements in different parts of the images are visible with similar density for both terminations. Rotating one of these areas by 90° (about the vertical axis) overlays the atomic columns directly onto the other region, suggesting the existence of antiphase domains in the films for both interfaces, arising naturally as a consequence of the symmorphic symmetry operation for the anatase structure’s crystallographic space group.9 The surface morphology of both LaAlO3 (001) substrate terminations showed clear step and terrace structures with ∼0.4 nm step heights, corresponding to a single perovskite unit cell height (see Supporting Information, Figure S1a,b), confirming a single termination type. A (1 × 1) reconstruction pattern in both cases indicates that an ideal perovskite surface is exposed before TiO2 growth (Supporting Information, Figure S2a,b). The surface morphology and the surface and bulk crystallinity of the TiO2 films were evaluated by atomic force microscopy (AFM), in situ reflection high-energy electron diffraction (RHEED), and X-ray diffraction (XRD) respectively. AFM topography images (Figure 2a,b) along with (4 × 1) surface reconstructed RHEED patterns (Figure 2c,d) show that both TiO2 surfaces are flat and single crystalline, similar to previous reports.10 The anatase TiO2 (004) XRD diffraction peaks are also similar (Figure 2e). In addition, X-ray absorption spectroscopy (XAS) measured in total electron yield mode reveals almost identical features, indicating that the surface electronic structure is termination independent (Figure 2f). Despite the structural similarities for the two TiO2/LaAlO3 heterostructures, the electronic properties were vastly different, as seen by the temperature dependent resistivity shown in Figure 3a. Here the TiO2 layer was 60 nm thick for both terminations. The TiO2/LaO−LaAlO3 heterostructure is metallic down to 70 K before a slight upturn in resistance, while the TiO2/AlO2−LaAlO3 sample is insulating with a room temperature resistivity over 3 orders of magnitude higher. Similar behavior was obtained for TiO2/LaO−LaAlO3 down to a TiO2 thickness of roughly 16 nm, below which the samples showed insulating behavior suggestive of surface depletion. Hall measurements were performed, which demonstrate that the carriers are electrons. The sheet carrier density (NSheet) and Hall mobility (μ) for various temperatures are shown in Figures 3b and 3c (note that the TiO2/AlO2−LaAlO3 could only be measured at room temperature, due to the strongly insulating character). These data demonstrate that the conductivity
Figure 1. Schematic illustration of anatase TiO2 adjacent to (a) AlO2terminated and (b) LaO-terminated LaAlO3 (001) showing the composition of each layer. (c,d) HAADF-STEM images of a TiO2/ AlO2−LaAlO3 (001) and a TiO2/LaO−LaAlO3 (001) heterostructure as shown in (a) and (b), respectively. Circles denote the atomic column positions of Ti (purple) and La (green) atoms. Red circles denote oxygen atoms in (a). Schematics in panel (c,d) show the atomic positions in the two antiphase domains.
controlling the LaAlO3 termination, resulting in insulating and metallic behavior for AlO2- and LaO-terminated substrates, respectively. Despite this dramatic conductivity difference, the same high optical transparency was found for both structures. Figure 1c,d shows cross-sectional high-angle annular darkfield scanning transmission electron microscopy (HAADFSTEM) images of representative AlO2- and LaO-terminated heterostructures grown by pulsed laser deposition. Periodic arrays of La atoms (green circles in the schematic insets) in the LaAlO3 substrate and the dumbbells formed by the Ti atoms (purple circles) in the TiO2 film are clear and epitaxially related. For both heterostructures, chemically abrupt interfaces on the scale of one unit cell were found with no observable structural difference.
Figure 2. AFM images of anatase TiO2 films deposited on (a) AlO2-terminated and (b) LaO-terminated LaAlO3 (001) substrates. RHEED patterns for (c) TiO2/AlO2−LaAlO3 and (d) TiO2/LaO−LaAlO3. (e) XRD patterns for TiO2/AlO2−LaAlO3 (black curve) and TiO2/LaO−LaAlO3 (red curve). TiO2 thickness is 60 nm in both cases. Ti L2,3-edges XAS spectra of anatase TiO2 films on (f) AlO2- and LaO-terminated LaAlO3 (black and red, respectively). 6744
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contributions to the carrier generation in the TiO2/LaAlO3 heterostructures. Many transition metal oxide lattices can support oxygen vacancies, and there are several possible origins in these heterostructures. Kinetic bombardment of the substrate and the growing film by the plasma can in principle remove oxygen from the surface. However, parallel electronic conductivity from the substrate can be ruled out; LaAlO3 is a far more robust insulator than SrTiO3 in the LaAlO3/SrTiO3 case.17−19 The TiO2 films were fabricated under identical thermodynamic and kinetic conditions, optimized using AlO2-terminated substrates to reduce the density of oxygen vacancies below ∼1017 cm−3, resulting in insulating films.20 We can therefore exclude growth and substrate dependent factors. The interfacial chemical composition, especially the electron affinity of the different cation species, can also drive oxygen vacancy formation.21,22 Considering the constituents’ Pauling’s electronegativity (Ti, 1.54; Al, 1.61; and La, 1.1), it is possible that the LaO-terminated interface may more strongly getter oxygen from TiO2 than the AlO2, giving metallic conductivity. To test this scenario, we inserted an even less electronegative cation, Sr (electronegativity 0.95), next to TiO2, which would be expected to give metallic character in this scenario. In contrast, we find that TiO2/SrO is insulating (Figure 3a), ruling out a dominant conductivity contribution from interfacial oxygen affinity differences. Another general possibility is interdiffusion23 and resultant cation doping of the TiO2 from the substrate. In this regard, we note that Al and La doping have been shown to be ineffective as n-type dopants,24,25 and it is therefore an unlikely source of the conductivity observed. Remaining is the possibility of an electrostatic origin to the asymmetry of these two heterointerfaces, also discussed in organic systems.26 TiO2/LaAlO3 is the reverse of the LaAlO3/ SrTiO3 system, the substrate is the polar layer, but the electrostatic boundary condition problem is related. The electrostatic scenario provides a consistent picture of the LaO and AlO2 terminated conductivity data shown in Figure 3. The SrO inserted sample is also consistent because the final polar layer in that heterostructure is AlO2. Following a similar technique used to tune the surface termination in manganite/ titanate junctions with SrMnO3 insertion,27 we can insert one unit cell of LaTiO3 between AlO2-terminated LaAlO3 (001) and TiO2 (Supporting Information, Figure S3), leading to the same structure and conductivity as the LaO-terminated sample but by a different growth approach. This result strongly supports the single termination of the surface LaAlO3 substrate for both terminations just before TiO2 growth. Although experimentally the termination-dependent metal− insulator transition is robust, the interface stoichiometry and the detailed mechanism of carrier formation require further investigation, much as the case for ongoing research of the LaAlO3/SrTiO3 interface. The polar LaAlO3 (001) substrate surface must start from a reconstructed state; this may be accommodated purely by oxygen stoichiometry,28 or may also involve the cations. Preliminary analysis of the B-site intensity in HAADF-STEM results at the interface suggests the possibility of microscopically mixed termination. This should be examined in detail to understand the microscopic origins of the conductivity contrast we observe. In conclusion, we have experimentally demonstrated the ability to tune between a metal and an insulator in TiO2/ LaAlO3 (001) interfaces using termination control with
Figure 3. (a) Temperature dependent resistivity of TiO2/AlO2− LaAlO3 (black), TiO 2 /LaO−LaAlO 3 (red), TiO 2 /SrO/LaAlO 3 (green), and TiO2/LaTiO3/LaAlO3 (blue) heterostructures. The temperature dependences of (b) the sheet carrier density and (c) the Hall mobility for the two metallic films as well as roomtemperature properties for the two insulating films shown in (a).
difference is primarily due to carrier density variations rather than mobility. Notably μ for the LaO-termination is comparable to the highest values obtained in transition metal doped TiO2 films at any density and significantly larger than films at this density, based on the total film thickness.6,7,10−13 In addition, the room temperature mobility of 25 cm2V−1s−1 is significantly higher than for the LaAlO3/SrTiO3 system.14 Room-temperature optical transmittance was measured using samples grown on double-side polished substrates. The transmittance spectra of the heterostructures as well as a bare LaAlO3 substrate are shown in Figure 4. The optical
Figure 4. Transmittance spectra for TiO2/AlO2−LaAlO3 (black) and TiO2/LaO−LaAlO3 (red) heterostructures and a bare LaAlO3 (001) substrate (green). Inset photographs show the TiO2 films on AlO2and LaO-terminated LaAlO3 substrates (left and right, respectively), placed on top of printed text. Lateral sample size is 10 mm by 10 mm.
transparency of both heterostructures is essentially the same, showing only a small reduction from the substrate, as demonstrated by the photographs (inset of Figure 4). Thus, termination control is remarkably successful introducing high mobility carriers without sacrificing optical transparency. Considering these results, we note the structural analogies to the LaAlO3/SrTiO3 (001) interface, which also shows a termination-dependent metal to insulator transition14,15 but in an inverted sense; at the TiO2/LaAlO3 interface the polar layer is the substrate rather than the film. The possible mechanisms of conduction in the LaAlO3/SrTiO3 case have been widely debated16 and we use that framework to evaluate the possible 6745
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comparable or better optical transparency than via chemical doping.6 The conductivity is comparable with practical amorphous indium tin oxide (ITO) but with enhanced transparency.29 The central result here is consistent with theoretical expectations of a profound electronic asymmetry for these two interface structures.30,31 This suggests a new approach using the interface boundary conditions to exceed the best mobilities observed in amorphous ITO at room temperature. In particular, this allows the carrier density to be tuned without impacting the electron mobility via dopant scattering and crystal defects, bypassing some of the difficulties found in conventional chemical doping.
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ASSOCIATED CONTENT
S Supporting Information *
Materials and methods. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address
(M.M.) Photon Factory, Institute of Materials Structure Science (IMSS), High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank H. Hosono and K. S. Takahashi for discussions and H. Takagi and M. Lippmaa for experimental support. This work was primarily supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering Division, under contract DE-AC02-76SF00515 (M.M., T.T., Y.H., C.B., J.L., and H.Y.H.). M.M. acknowledges partial support from the ONR-MURI number N00014-12-10976. X-ray absorption spectroscopy studies were carried out at the SSRL, a Directorate of SLAC and an Office of Science User Facility operated for the US DOE Office of Science by Stanford University.
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