Electrical properties and colossal electroresistance of heteroepitaxial $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_{3}∕\mathrm{Sr}{\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}{\mathrm{O}}_{3}$ $(0.0002\ensuremath{\leqslant}x\ensuremath{\leqslant}0.02)$ Schottky junctions

Electrical properties and colossal electroresistance of heteroepitaxial $Sr Ru O_{3} ∕ Sr {Ti}_{1 - x} {Nb}_{x} O_{3}$ $(0.0002 ⩽ x ⩽ 0.02)$ Schottky junctions

T. Fujii, M. Kawasaki, A. Sawa, Y. Kawazoe, H. Akoh, and Y. Tokura

Phys. Rev. B 75, 165101 – Published 2 April 2007

Abstract

We investigated the electrical properties of heteroepitaxial oxide Schottky junctions, $Sr Ru O_{3} ∕ Sr {Ti}_{1 - x} {Nb}_{x} O_{3}$ $(0.0002 ⩽ x ⩽ 0.02)$ . The overall features agree with those of a conventional semiconductor Schottky junction, as exemplified by the rectifying current $(I)$ -voltage $(V)$ characteristics with linear $\log I − V$ relationship under forward bias and the capacitance $(C) − V$ characteristics with linear $1 ∕ C^{2} − V$ relationship under reverse bias. The $x$ dependence of the junction parameters, such as barrier height, built-in potential, and depletion layer width, can be analyzed by taking into account the band-gap narrowing due to degeneration, as well as the bias-dependent dielectric constant of depleted $Sr Ti O_{3}$ . All junctions, except for the most heavily doped $(x = 0.02)$ one, show hysteretic $I − V$ characteristics with a colossal electroresistance (CER) effect, where forward (reverse) bias stress reduces (enhances) the junction resistance. The $x$ dependence of the CER effect and the absence of hysteresis in the $C − V$ relationship suggest that the resistance switching in Schottky junctions comes from the change in conductance through additional tunneling paths rather than the change in barrier potential profile. Electron charging in or discharging from a self-trap depending on the bias polarity may account for the nonvolatility of the CER effect. This model is supported by the fact that the CER effect is completely suppressed in interface-engineered junctions with a $Sr Ru O_{3} ∕ 2 − nm$ -thick $X ∕ Sr {Ti}_{0.99} {Nb}_{0.01} O_{3}$ structure, where $X$ is either pristine $Sr Ti O_{3}$ or very heavily electron-doped ${La}_{0.25} {Sr}_{0.75} Ti O_{3}$ .

Received 14 November 2006

DOI:https://doi.org/10.1103/PhysRevB.75.165101

Authors & Affiliations

T. Fujii and M. Kawasaki

Correlated Electron Research Center (CERC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8562, Japan and Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

A. Sawa^*

Correlated Electron Research Center (CERC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8562, Japan

Y. Kawazoe

Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

H. Akoh

Correlated Electron Research Center (CERC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8562, Japan and CREST-JST, Kawaguchi, Saitama 322-0012, Japan

Y. Tokura

Correlated Electron Research Center (CERC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8562, Japan and Department of Applied Physics, University of Tokyo, Tokyo 113-8658, Japan

^*Author to whom correspondence should be addressed; electronic address: a.sawa@aist.go.jp

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Vol. 75, Iss. 16 — 15 April 2007

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Figure 1
(Color online) $I − V$ characteristics of $Sr Ru O_{3} ∕ Sr {Ti}_{1 - x} {Nb}_{x} O_{3}$ (Nb:STO; $x = 0.0002$ , 0.001, 0.002, 0.01, and 0.02) junctions. (a) and (b) are plotted in linear and logarithmic current scales, respectively. The bias voltage was swept as $0 \to + V_{\max} \to 0 \to - V_{\max} \to 0$ . The $I − V$ characteristics show rectifying properties agreeing with that of conventional $n$ -type semiconductor Schottky diode with a deep-work-function metal [see the band diagram of inset in (b) under reverse bias]. Large hysteresis is seen in the reverse-bias region except for the $x = 0.02$ junction. The forward-bias $I − V$ characteristics are magnified in Fig. 2.Reuse & Permissions
Figure 2
Doping concentration dependence of forward-bias $I − V$ characteristics for the $Sr Ru O_{3} ∕ Sr {Ti}_{1 - x} {Nb}_{x} O_{3}$ (Nb:STO; $x = 0.0002$ , 0.001, 0.002, 0.01, and 0.02) junctions. The broken straight lines are fits to the thermionic emission process. The voltage ranges of the fit are $0.8 - 1.0 V$ for the $x = 0.0002$ and 0.001 junctions, $0.9 - 1.1 V$ for the $x = 0.002$ junction, $0.8 - 1.0 V$ for the $x = 0.01$ junction, and $0.5 - 0.7 V$ for the $x = 0.02$ junction. From the fit, the ideality factor $(n)$ and barrier height $(ϕ_{B})$ can be deduced. Considerable deviation is seen from the fit at lower bias voltage, representing excess current. Hysteretic behavior is seen only in this region. The schematic diagrams at the top illustrate possible band diagrams with light (left) and heavy (right) doping. The narrower barrier width in heavily doped junctions should promote a higher tunneling conduction contribution, giving rise to the excess current.Reuse & Permissions
Figure 3
$C − V$ characteristics of the $Sr Ru O_{3} ∕ Sr {Ti}_{1 - x} {Nb}_{x} O_{3}$ (Nb:STO; $x = 0.0002$ , 0.001, 0.002, and 0.01) junctions. Because of leakage, reliable measurement could not be performed for the $x = 0.02$ junction. More heavily doped junctions showed slight deviation from the straight line at high-bias voltage $V$ due to the electric-field dependence of $ε_{S}$ in Nb:STO, as well as the leakage current. The extrapolation of the straight broken lines to $1 ∕ C^{2} = 0$ gives the built-in potential $V_{bi}$ (for values, see Fig. 5).Reuse & Permissions
Figure 4
(Color online) (a) Temperature dependence of resistivity for $Sr {Ti}_{1 - x} {Nb}_{x} O_{3}$ (Nb:STO; $x = 0.0002$ , 0.001, 0.002, 0.01, and 0.02) single-crystal substrates. The metallic behaviors suggest that the Nb:STOs for all doping levels are degenerated semiconductors. (b) Actual electron concentration $(n_{H})$ evaluated from Hall measurement as a function of nominal Nb concentration $(x)$ , which was confirmed by inductively coupled plasma emission spectroscopy. In the most lightly doped sample with $x = 0.0002$ , more than 85% of the Nb dopant is inactive, whereas the samples with $x ⩾ 0.001$ show almost 100% activation of carriers.Reuse & Permissions
Figure 5
Electron concentration $n_{H}$ dependence of (a) $V_{bi}$ , $ϕ_{B}$ , and $n$ , (b) $ε_{S} N_{D}$ , (c) $ε_{S}$ , (d) $W_{d}$ , and (e) $V_{bi} ∕ W_{d}$ . The dotted lines in (b) and (d) represent the estimated value, assuming $ε_{S}$ to be 300, independent of $n_{H}$ , which corresponds to the value of undoped $Sr Ti O_{3}$ at room temperature.Reuse & Permissions
Figure 6
$I − V$ characteristics of the interface-engineered $Sr Ru O_{3} ∕ X$ $(2 nm) ∕ Sr {Ti}_{1 - x} {Nb}_{x} O_{3}$ $(x = 0.01)$ junctions, where $X = pristine$ $Sr Ti O_{3}$ (a band insulator) and ${Sr}_{0.75} {La}_{0.25} Ti O_{3}$ [a metal whose carrier number is much larger than Nb:STO (Ref. 34)]. The CER effect is completely absent in these interface-engineered junctions.Reuse & Permissions

Physical Review B

covering condensed matter and materials physics

Electrical properties and colossal electroresistance of heteroepitaxial $Sr Ru O_{3} ∕ Sr {Ti}_{1 - x} {Nb}_{x} O_{3}$ $(0.0002 ⩽ x ⩽ 0.02)$ Schottky junctions

T. Fujii, M. Kawasaki, A. Sawa, Y. Kawazoe, H. Akoh, and Y. Tokura

Phys. Rev. B 75, 165101 – Published 2 April 2007

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Electrical properties and colossal electroresistance of heteroepitaxial SrRuO3∕SrTi1−xNbxO3 (0.0002⩽x⩽0.02) Schottky junctions

T. Fujii, M. Kawasaki, A. Sawa, Y. Kawazoe, H. Akoh, and Y. Tokura

Phys. Rev. B 75, 165101 – Published 2 April 2007

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Electrical properties and colossal electroresistance of heteroepitaxial $Sr Ru O_{3} ∕ Sr {Ti}_{1 - x} {Nb}_{x} O_{3}$ $(0.0002 ⩽ x ⩽ 0.02)$ Schottky junctions