APPLIED PHYSICS LETTERS 93, 262107 共2008兲

Very large anisotropy in the dc conductivity of epitaxial VO2 thin films grown on „011… rutile TiO2 substrates Jiwei Lu,1,a兲 Kevin G. West,1 and Stuart A. Wolf1,2 1

Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904, USA 2 Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA

共Received 9 October 2008; accepted 6 December 2008; published online 31 December 2008兲 In this letter, we reported a very large anisotropy in dc conductivity of epitaxial VO2 thin films deposited on a 共011兲 TiO2 substrate. The VO2 film grew epitaxially on TiO2 and x-ray diffraction showed that VO2 had the tetragonal symmetry due to the substrate clamping effect at room temperature. There was a compressive strain of ⫺1.2% along the c-axis of the rutile VO2. We observed a very strong angular dependence of in-plane dc conductivity. We calculated that ␴1 / ␴3 ⬃ 5.14, which was anomalously large. We attributed the drastic increase to the compressive strain along the c-axis of the rutile VO2 due to substrate clamping. This very large anisotropy disappeared above the metal-insulator transition. © 2008 American Institute of Physics. 关DOI: 10.1063/1.3058769兴 Vanadium oxides are paradigms of strongly correlated oxides and have been studied for decades because of the metal insulator transitions 共MITs兲 that several of the oxides and suboxides exhibit.1,2 In particular, VO2 has a metal semiconductor transition at ~340 K, just above room temperature.2 The low temperature semiconducting phase has a monoclinic crystal structure while the metallic phase has a rutile structure above the transition temperature. Associated with this transition, the physical properties of VO2, such as electric transport properties and optical constants, change drastically. Recently there have been reports that this metal semiconductor transition could be induced at room temperature at an extremely high speed using electric current3–5 or above gap photons,6 which makes VO2 an attractive candidate for a phase change switch and other advanced electronic devices such as sensors and memory.7–9 Better understandings of the metal semiconductor transition have been developed by studying the anisotropy in physical properties such as electric conductivity and elasticity. This is particularly true on studies performed on single crystal VO2.10,11 Recently Lysenko et al.12 reported an anisotropy of the optical properties of VO2 films grown on sapphire substrates. However, the characterization of conductivity anisotropy in VO2 thin films has not been previously explored. The epitaxy of VO2 on single crystal substrates provides a vantage point to study the anisotropy of the physical properties of VO2 thin films under the influence of substrate clamping. In this letter, we grew an epitaxial VO2 thin film on the 共011兲 TiO2 single crystal substrate and characterized the in-plane angular dependence of the transport properties. VO2 thin films were deposited by reactive sputtering from a vanadium target by the reactive ion beam bias target deposition using an 共Ar+ O2兲 gas mixture; the detailed growth conditions can be found elsewhere.5 The film thickness, determined by x-ray reflectivity, was ~40 nm. Atomic force microscopy 共AFM兲 was used to characterize the surface morphology. a兲

Electronic mail: [email protected].

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X-ray diffraction 共XRD兲 was used to determine the orientation and the lattice parameters of the VO2 films and all the measurements were performed at room temperature. To measure the conductivity, the test devices were fabricated using a one-step mask photolithographic process. There was a 15° angular rotation between adjacent contacts at the same diameter, which allowed the measurements of electric conductivity as a function of the in-plane orientation. The top contact was a 300 nm Au/20 nm Ti deposited by electron beam evaporation. The electrical conductivity at and above the room temperature was measured using a probe station equipped with a heating chuck and a semiconductor parameter characterizer. The temperature dependence of the dc conductivity was measured using a physics property measurement system 6000 from 100 to 400 K with a ramp rate of 2 K/min. The dc conductivity was then calculated according to device geometry and thickness of film. The VO2 film grows epitaxially on the 共011兲 TiO2 single crystal substrate. The reciprocal lattice mapping 关Fig. 1共a兲兴 showed that the 共011兲 peak in the VO2 diffraction was closely coupled to that of rutile TiO2. Only the VO2 共011兲 peak along with the TiO2 共011兲 peak was detected in two theta scans, which indicated that the film was free from other phases of vanadium oxides. The phi scan of the 共002兲 peak of VO2 and TiO2 again confirmed the epitaxy of the VO2 film 关Fig. 1共b兲兴, from which we estimated that the 共001兲 spacing 共c-axis length兲 of rutile VO2 was 2.844 Å. In addition, VO2 showed tetragonal symmetry in the phi scan, which was likely due to the substrate clamping effect. Therefore we will only use the notation of rutile VO2 in the following discussion. The 共011兲 spacing for the VO2 thin film was 2.3907 Å, which exhibits a compressive strain of ⫺1.2% compared to the d-spacing of bulk VO2 共2.4193 Å兲. The 共011兲 spacing of TiO2 is 2.4602 Å and, consequently, VO2 grown on top of 共011兲 TiO2 was strained due to the mismatch of lattice parameters between rutile VO2 and TiO2. As a result, the 共001兲 spacing of the VO2 film was under a compressive strain of ~0.42% compared to that of bulk VO2 共2.8557 Å兲. Figure 1共c兲 is an AFM image of 共011兲 VO2 film surface over an area of 1 ␮m2. The film surface was remarkably smooth with the

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© 2008 American Institute of Physics

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Appl. Phys. Lett. 93, 262107 共2008兲

Lu, West, and Wolf

FIG. 3. 共Color online兲 dc resistivity as function of temperature of VO2 film grown on a 共011兲 TiO2 substrate measured parallel and perpendicular to the c-axis of rutile VO2, respectively.

FIG. 1. 共Color online兲 共a兲 Reciprocal lattice mapping of 共011兲 TiO2 and 共011兲 VO2 共the intensity was in log scale兲. 共b兲 Phi scan of 共002兲 TiO2 and 共002兲 VO2. 共c兲 AFM image of the 共011兲 VO2 surface. The rms roughness is ~0.93 nm.

root-mean-square 共rms兲 roughness of ~0.92 nm. There were no pin holes or nanocracks observed, thus the influence of these defects on the transport properties can be ruled out. Figure 2 shows the dc conductivity of the VO2 film measured along different in-plane directions at room temperature. With the assistance of the XRD phi scan, we translated the in-plane directions to the crystallographic orientations of the epitaxial VO2 film. The maximum conductivity 共␴max兲 was 24.21 S cm, which was parallel to the c-axis of the rutile VO2. In comparison, the minimum conductivity 共␴min兲 was 5.41 S cm and it occurred perpendicular to the c-axis of rutile VO2. Therefore, the anisotropy ratio, ␴max / ␴min, was ~4.5 in the 共011兲 plane of this VO2 thin film. The conductivity in tetragonal crystals can be described by a second-rank tensor, which is expressed as a matrix like

␴ij =

冢

␴1 ␴1 ␴3

冣

,

共1兲

where ␴1 and ␴3 are the conductivities perpendicular and parallel to the c-axis of the tetragonal crystal, respectively.

FIG. 2. 共Color online兲 Angular dependence of the conductivity of VO2 film grown on a 共011兲 TiO2 substrate. The red line is the fitting result using Eq. 共3兲. The inset is a photograph of Au top contacts with the schematic of the test structure.

According to Nye,13 the conductivity tensor in the tetragonal crystal is symmetric, thus the conductivity along any arbitrary direction can be given by

␴共␪兲 = ␴1 sin2共␪兲 + ␴3 cos2共␪兲,

共2兲

where ␪ is the angle between the arbitrary direction and the 具001典 direction of VO2 and ␴1, and ␴3 are the conductivities parallel and perpendicular to the c-axis, respectively. The angle between the 共011兲 plane and the 共001兲 plane is 57.9°. Therefore we can rewrite Eq. 共2兲 for the 共011兲 plane as

␴共␪兲 = ␴1 + 共␴3 − ␴1兲cos2共␪1兲sin2共57.9°兲,

共3兲

where ␪1 is the angle of the 共011兲 plane of VO2 against which the conductivity was plotted in Fig. 2. We used Eq. 共3兲 to fit the measured conductivity and the fitting curve was also shown in Fig. 2. We obtained ␴1 and ␴3 as 31.64 and 6.16 S cm, respectively. Thus the anisotropy ratio, ␴1 / ␴3, was ~5.14 and this was more than two and one-half times larger than that of single crystal VO2 at room temperature. Figure 3 shows the resistivity of the VO2 film as a function of temperature along two different directions. According to a separate measurement, the resistivity of the TiO2 substrate was much smaller and did not show any drastic change in this temperature range. Therefore the large change in resistivity occurring at ~310 K was due to the MIT of VO2, whose transition temperature was well below that of bulk VO2. The shift in the phase transition temperature toward room temperature was also a consequence of the compressive strain along the c-axis that agreed to a previous report.14 Below the transition temperature, the hopping mechanism determined the temperature dependence of resistivity that was observed in VO2 and Cr doped VO2 films.15,16 Strikingly, the anisotropy almost disappeared above room temperature. In contrast, single crystal VO2 still showed an anisotropic conductivity above the transition temperature10 and we observed the anisotropic conductivity in VO2 thin films deposited on the 共100兲 surface of TiO2 above the phase transition temperature that was similar to VO2 single crystals. Though further investigation is underway, we suspect that the disappearance of the conductivity anisotropy is likely related to the fact that the interface becomes more conductive above the phase transition temperature. A first principles theory study showed that the monoclinic VO2 共011兲 surface is metallic,17 but the 共011兲 interface between VO2 and TiO2 remained largely unknown. Nevertheless, to determine

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whether the disappearance of anisotropy is due to the interface and can be associated with the MIT, it is necessary to study the transport properties as a function of film thickness of VO2 in the future. In summary, an epitaxial VO2 thin film was deposited on a TiO2 共011兲 single crystal substrate. We measured the dc conductivity in the 共011兲 plane of rutile VO2 as function of in-plane direction and observed a very large conductivity anisotropy. The anisotropy disappeared above the MIT and the mechanism of this disappearance is not clearly understood, but the interface between VO2 and TiO2 may play an important role. We thank Dr. John Claassen 共Naval Research Laboratory兲 for stimulating discussions. The authors gratefully acknowledge the funding from DARPA through ARO 共Grant No. W911NF-08-1-0283兲. 1

Appl. Phys. Lett. 93, 262107 共2008兲

Lu, West, and Wolf

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