Thermopower analysis of metal-insulator transition temperature modulations in vanadium dioxide thin films with lattice distortion

Takayoshi Katase, Kenji Endo, and Hiromichi Ohta

Phys. Rev. B 92, 035302 – Published 13 July 2015

Abstract

Insulator-to-metal (MI) phase transition in vanadium dioxide $(V O_{2})$ thin films with controlled lattice distortion was investigated by thermopower measurements. $V O_{2}$ epitaxial films with different crystallographic orientations, grown on $(0001) α − A l_{2} O_{3}$ , $(11 \bar{2} 0) α − A l_{2} O_{3}$ , and $(001) Ti O_{2}$ substrates, showed significant decrease of absolute value of Seebeck coefficient $(S)$ from $\sim 200$ to $23 μ V K^{- 1}$ , along with a sharp drop in electrical resistivity $(ρ)$ , due to the transition from an insulator to a metal. The MI transition temperatures observed both in $ρ (T_{ρ})$ and $S (T_{S})$ for the $V O_{2}$ films systematically decrease with lattice shrinkage in the pseudorutile structure along the $c$ axis, accompanying a broadening of the MI transition temperature width. Moreover, the onset $T_{S}$ , where the insulating phase starts to become metallic, is much lower than the onset $T_{ρ}$ . This difference is attributed to the sensitivity of $S$ for the detection of hidden metallic domains in the majority insulating phase, which cannot be detected in $ρ$ measurements. Consequently, $S$ measurements provide a straightforward and excellent approach for a deeper understanding of the MI transition process in $V O_{2}$ .

Received 5 February 2015

DOI:https://doi.org/10.1103/PhysRevB.92.035302

Authors & Affiliations

Takayoshi Katase^*, Kenji Endo, and Hiromichi Ohta^†

Research Institute for Electronic Science, Hokkaido University, N20W10, Sapporo 001-0020, Japan

^*katase@es.hokudai.ac.jp
^†hiromichi.ohta@es.hokudai.ac.jp

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Vol. 92, Iss. 3 — 15 July 2015

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Images

Figure 1
(a)–(c) Out-of-plane and (d)–(f) in-plane XRD patterns at RT for $V O_{2}$ films grown on single crystalline substrates of (a) and (d) $(0001) α − A l_{2} O_{3}$ , (b) and (e) $(11 \bar{2} 0) α − A l_{2} O_{3}$ , and (c) and (f) $(001) Ti O_{2}$ . Crystalline phases and diffraction indices are noted above the corresponding diffraction peaks. Insets of (a)–(c) show out-of-plane rocking curves $(2 θ$ -fixed $ω$ scans) and those of (d)–(f) show in-plane rocking curves ( $2 θ_{χ}$ -fixed $ϕ$ scans) for each diffraction peak of $V O_{2}$ film.
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Figure 2
(a)–(c) Topographic AFM images and (d)–(f) schematic epitaxial relations of monoclinic (m-) $V O_{2}$ epitaxial films grown on substrates of (a) and (d) $(0001) α − A l_{2} O_{3}$ , (b) and (e) $(11 \bar{2} 0) α − A l_{2} O_{3}$ , and (c) and (f) $(001) Ti O_{2}$ . RHEED patterns of each $V O_{2}$ film are superimposed in (a)–(c).
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Figure 3
(a) and (b) Temperature dependences of electrical resistivity $(ρ)$ and Seebeck coefficient $(S)$ for $V O_{2}$ epitaxial films grown on $(0001) α − A l_{2} O_{3}$ (blue), $(11 \bar{2} 0) α − A l_{2} O_{3}$ (red), and $(001) Ti O_{2}$ (green), respectively. (a) $ρ - T$ curves normalized by $ρ$ at $35^{\circ} C (ρ / ρ_{35^{\circ} C})$ and (b) $S - T$ curves measured by increasing $T$ from RT. Temperature derivative plots of (c) $d [log ρ / ρ_{35^{\circ} C}] / d T$ and (d) $d S / d T$ . The arrows indicate the positions of $T_{ρ}$ and $T_{S}$ . The onset and offset temperatures of the transition from an insulator to a metal are marked by open and closed triangle symbols, respectively, and the color bars indicate the temperature range of the MI transition.
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Figure 4
The relations of (a) $T_{ρ}$ and (b) $T_{S}$ with estimated $c$ axis lattice parameters in the pseudorutile structure for $V O_{2}$ epitaxial films. Those of the monoclinic- $V O_{2}$ bulk are also shown for comparison [1, 25, 34]. $T_{ρ}$ and $T_{S}$ of the $V O_{2}$ films decreased with shrinkage of the $c$ axis lattice parameters from those of bulk $V O_{2}$ .
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Figure 5
Schematic illustration of the metallic and insulating domain configuration at each temperature (a)–(d) around the MI transition [15], and corresponding changes in $ρ - T$ and $S - T$ . Majority phase changes from (a) an insulator to (d) a metal with increase of temperature, through the distribution of (b) locally separated metallic domains and (c) connected metallic domains in insulating domains. Open and closed triangle symbols indicate onset and offset temperatures of the transition from an insulator to a metal. Since $S$ can sensitively detect the metallic phase remaining in the majority insulating phase, which cannot be detected in resistivity, $| S |$ starts to decrease from (b).
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