Conductivity Modulation of Gold Thin Film at Room Temperature via

Jan 27, 2017 - We demonstrated the field-effect conductivity modulation of a gold thin film by all-solid-state electric-double-layer (EDL) gating at r...
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Conductivity Modulation of Gold Thin Film at Room Temperature via All-Solid-State Electric-Double-Layer Gating Accelerated by Nonlinear Ionic Transport Tetsuya Asano,* Yukihiro Kaneko, Atsushi Omote, Hideaki Adachi, and Eiji Fujii Advanced Research Division, Panasonic Corporation, 1006 Kadoma, Osaka 571-8501, Japan S Supporting Information *

ABSTRACT: We demonstrated the field-effect conductivity modulation of a gold thin film by all-solid-state electric-doublelayer (EDL) gating at room temperature using an epitaxially grown oxide fast lithium conductor, La2/3−xLi3xTiO3 (LLT), as a solid electrolyte. The linearly increasing gold conductivity with increasing gate bias demonstrates that the conductivity modulation is indeed due to carrier injection by EDL gating. The response time becomes exponentially faster with increasing gate bias, a result of the onset of nonlinear ionic transportation. This nonlinear dynamic response indicates that the ionic motion-driven device can be much faster than would be estimated from a linear ionic transport model. KEYWORDS: electric-double-layer gating, solid-state electric double layer, solid-state ionics, nonlinear ionic transport, iontronics, nanoionics

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Recently, EDL-based carrier modulation in metals has been demonstrated using ionic liquids (ILs)11−15 in which an extremely high density of carriers, 1014−15 cm−2, was induced. Considering the real device application of this emerging technology, it is strongly desired that ILs are replaced with a solid electrolyte to realize all-solid-state ionic devices for straightforward device integration and freedom in device structure design. Studies of all-solid-state EDL gating have, so far, been mainly devoted to transistor operations for semiconductors or insulators,16−19 except for a recent report on the modulation of the Nb superconducting transition temperature by 0.06 K.20 Although organic field-effect transistors utilizing a solid polymer electrolyte21 or ion gel22 as a gate material with a conventional semiconducting channel have been demonstrated to work at room temperature with its response time in submillisecond, many of the recent studies on all-solid-state EDL gating especially for the material property modulation that cannot be attained by the conventional dielectric gating have been performed at elevated temperatures with a response speed on the order of 100 s even at 170 °C17 because of the low ionic mobility of the employed solid electrolyte. It is, therefore, critical for the all-solid-state EDL gating device to enable fieldeffect electronic property modulation of a metal at a sufficiently fast speed even at room temperature, within, for example, 1 s for applications other than electronic switching.

he heterogeneous interface of electrolyte materials, long studied and referred to as the electric double layer (EDL), has recently gained renewed attention because the high concentrations of polarized ions and electrons at the EDL can induce unforeseen material properties1,2 or device functionalities3−7 that cannot be realized in conventional bulk materials or electronic devices. For the latter, a variety of functional material properties such as superconductivity,3 magnetism,4 and the insulator-to-metal transition5,6 have been modulated using the EDL-gated field-effect due to the extremely high carrier concentration (1014−15 cm−2) under a strong electric field (1−10 MV/cm) at the vicinity of the EDL. This phenomenon opens an emerging field, referred to as “iontronics,”7 in which the electronic properties of a functional material in the vicinity of the electrolyte interface are controlled by the ionic distribution within the electrolyte material. Field-effect metallic property modulation has received little attention because the native carrier densities of metals are so high that the modulation of their carrier density by an external field has been considered to be an insurmountable challenge and also that the Thomas Fermi screening length is less than the atomic monolayer separation. Nevertheless, there have been demonstrations of the field-effect modulation of metallic properties such as catalytic properties8−10 via conventional dielectric gating. According to these studies, it is expected that injection of carrier densities one or 2 orders of magnitude higher than what can be realized by dielectric gating can alter metallic properties as the modulated carrier density approaches its density of states at the Fermi level, which in turn tunes the Fermi surface of metals. © 2017 American Chemical Society

Received: December 6, 2016 Accepted: January 27, 2017 Published: January 27, 2017 5056

DOI: 10.1021/acsami.6b15662 ACS Appl. Mater. Interfaces 2017, 9, 5056−5061

Letter

ACS Applied Materials & Interfaces Herein, we report the demonstration of gold conductivity modulation via all-solid-state EDL gating with a response time as fast as on the order of less than 1 s at room temperature. The device structure we employed is the back-gated structure, namely, gold-channel/La2/3−xLi3xTiO3 (LLT) electrolyte/ SrRuO3 (SRO) back-gate electrode/SrTiO3 (STO) substrate. The LLT solid electrolyte layer is of submicrometer thickness and is epitaxially grown with its lithium ion transport planes vertically aligned for fast ionic transportation. On the basis of analysis of the transient behavior of gold conductivity modulation, it is revealed that the fast response time arises from the initiation of nonlinear ionic transport in the epitaxial LLT layer. The fabricated device structure is shown in Figure 1a, b. In contrast to previously reported EDL-gated structures,3−7,16−20 we employed a back-gate device structure. This approach enabled us to grow the epitaxial solid electrolyte layer and allowed structural design flexibility for compatibility with allsolid-state EDL-gating devices, e.g., for catalytic devices,8−10 by moving the modulated metallic layer up to the front surface. The bottom gate electrode SRO and solid electrolyte LLT were epitaxially grown on an STO (100) single-crystalline substrate using pulsed laser deposition (PLD). The approximately 5 nm thick gold channel and the Ti/Pt lead electrodes were deposited by radio frequency magnetron sputtering and ebeam evaporation, respectively, and were subsequently patterned by the standard lift-off technique. The details of the fabrication procedure are described in the Supporting Information. The LLT electrolyte was epitaxially grown in such a way that the lithium ion transport planes were vertically aligned for fast ionic transport as shown in Figure 1c. This crystallographic orientation of the LLT film was clarified by the X-ray diffraction (XRD) pattern (Figure 1d and Figure S1a) and the transmission electron microscopy (TEM) selected area diffraction (SAD) pattern (Figure 1e). It is commonly understood that the high-conductivity phase of LLT forms a perovskite structure with a doubled c-axis in which lithium-rich and lithium-poor planes are alternately stacked along the c-axis and that the lithium ions are transported within the lithium-rich plane.23,24 The absence of the superlattice c-plane peaks in the out-of-plane XRD pattern (Figure 1d) together with the polefigure measurement for the inclined (101) superlattice diffraction peak (Figure S1a) unambiguously revealed that the LLT thin film was epitaxially grown with the lithium transport planes vertically aligned as schematically shown in Figure 1c. This epitaxial orientation was further confirmed with TEM SAD patterns (Figure 1e), where superlattice peaks can be observed only in lateral directions. This orientation is consistent with a previous report of epitaxial LLT growth on an STO (001) substrate.25 The resultant lithium ion conductivity of the LLT measured by electrochemical impedance spectroscopy (EIS) in the out-of-plane was 2 × 10−5 S/cm which include both the bulk and grain boundary conductivity (see the Supporting Information). Figure 2 shows the Au-channel current (ISD) variation with applied gate bias (VG) under the constant source-drain bias (VSD) at room temperature. As shown in Figure 2, the Au channel current varies with applied gate bias. The gate current (IG) under the gate bias is more than 3 orders of magnitude smaller than the modulation of the channel current (ISD). This observation rules out the leakage of the gate current contributing to channel current modulation, and it is, therefore,

Figure 1. (a) Cross-sectional illustration of the fabricated device for Au carrier modulation via EDL gating with a back-gate structure. (b) Planar microscope image of the device. (c) Illustration of the epitaxial orientation of the LLT layer in our device. The lithium transport planes are vertically aligned. (d) Out-of-plane XRD spectrum before formation of the Au channel and electrodes, clearly showing the epitaxial growth of the SRO and LLT layers. (e, upper) Wide-view of the cross-sectional TEM image of the device. The white arrows indicate some of the planar defects. (e, lower left) Selected area electron beam diffraction pattern of LLT with (100) zone axis. Superlattice diffractions were observed in in-plane direction only. (e, lower right) The lattice image in the vicinity of the planar defect. The vertical alignment of unit cells is shifted by half of the perovskite primitive cell across the boundary as indicated by disconnected vertical yellow lines.

ensured that the conductivity modulation of the gold film was due to field-effect. The gate capacitance value measured by EIS reached as high as 7 × 10−6 F/cm2 (Figure S2), indicating that the all-solid-state EDL was formed and acted as a gate at the interface between the LLT and gold channel. The channel current increased (decreased) when the lithium ions in the electrolyte moved toward (away from) the Au-channel under positive (negative) gate biases (inset in Figure 2). There has been controversy in terms of the mechanism of the conductivity modulation of gold via IL EDL-gating whether arising from either electrostatic carrier injection12 or the electrochemical redox of the gold.13 In Figure 2, the channel conductivity is shifted both up and down depending on the polarity of the gate 5057

DOI: 10.1021/acsami.6b15662 ACS Appl. Mater. Interfaces 2017, 9, 5056−5061

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ACS Applied Materials & Interfaces

Figure 2. Au channel current (top) and gate current (bottom) of the Au channel/LLT/SRO structure as a function of the applied gate bias. (Inset) Illustration of the polarization of the ionic and electronic carriers under positive and negative gate bias.

bias. This indicates that the conductivity modulation is due to electrostatic carrier injection or extraction; if the gold conductivity modulation were due to the electrochemical redox reaction, the gold conductivity would either increase or decrease, but not both, with respect to the zero gate bias.13 Note also that there was no reduction of Ti4+ in the LLT electrolyte that causes the electronic conductivity in LLT, as confirmed by the hard X-ray photoelectron spectroscopy (HAXPES) spectrum with and without gate bias (Figure S3). The possibility of lithium intercalation into/out of Au film contributing to the channel current modulation within the applied bias range VG = ± 1.5 V can be ruled out because (a) no lithium intercalation/deintercalation behavior26 was observed in the IG − VG curve (bottom of Figure 2), and (b) no reduction of the LLT indicates the potential of the Au film is far higher than the lithium intercalation potential (redox potential of LLT is about 1.5 V vs Li/Li+ and the Li−Au intercalation potential is about 0.1 and 0.2 V vs Li/Li+ 26). Therefore, we can conclude that the Au conductivity was modulated by field-effect carrier injection via an all-solid-state EDL-gating at room temperature. To explore the dynamic behavior of the EDL-gated carrier modulation of the Au film, we measured the time-dependent channel current variation under different gate biases at room temperature (Figure 3a). In Figure 3a, the gate bias was instantaneously applied at time t = 0, and the channel current evolution after the applied bias was monitored. Interestingly, the rise time of the channel current dramatically varied with the gate bias. At VG = 0.5 V, it took several hundreds of seconds to asymptotically approach the saturated current value. The rise time of the channel current decreased sharply with increasing applied gate bias, becoming as fast as the order of seconds at VG = 1.5 V. Such a dramatic variation of the response speed has not previously been reported in conventional dielectric-gated field-effect devices and is a unique feature of the ionic motiondriven field-effect using thin film solid electrolytes as discussed below. We parenthetically note that the transient current behaviors under negative gate bias were unlike those under positive bias (not shown here), presumably due to the dissimilar dynamics of the lithium ions flowing into the

Figure 3. (a) Transient modulated current of the Au channel with various gate biases. The gate bias was applied at t = 0. Red dot lines are the fitted curves using eq 2 with n = 2. (b) Equivalent circuit model for our EDL-gated device. (c) Fitted parameters of eq 2. Red dotted lines are fitted curves of the time constants using a nonlinear ionic current equation represented by eq 3. (d−f) Schematic representation of the potential landscape of the lithium ions under (d) no bias, (e) applied bias, and (f) applied bias with high grain boundary potentials.

interface for VG > 0 and being swept away from the interface for VG < 0. To gain more quantitative insights into the transient current behavior in Figure 3a, we modeled the device with the equivalent circuit as described in Figure 3b and Figure S4. We assume that the modulated conductivity of the gold channel is proportional to the sheet carrier density stored at the EDL capacitance, namely ΔISD(t ) = αqEDL(t )

(1)

where ΔISD(t) = ISD (t) − ISD (t = 0), α is a prefactor, and qEDL is the charge stored at the EDL. Based on the circuit model described in Figures 3b and Figure S4, the time-dependence of the modulated current can be described as follows ⎡ ΔISD(t ) = αqEDL(t ) = αVGC EDL⎢1 − ⎢⎣

n

⎛ t ⎞⎤ ⎟⎥ ⎝ τi ⎠⎥⎦

∑ xiexp⎜− i=1

(2)

where n is the number of RC parallel impedance components as shown in Figure S4b, τi is the time constant of the i-th RC parallel impedance component, xi is the prefactor that satisfies i ∑n xi = 1, and CEDL is the interfacial capacitance (see the Supporting Information for the derivation). Needless to say, the 5058

DOI: 10.1021/acsami.6b15662 ACS Appl. Mater. Interfaces 2017, 9, 5056−5061

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smaller than that of the previously reported ionic conducting glasses (F0 ≈ 122 kV/cm)28 because of the long apparent jump distances of the LLT epitaxial film. From the TEM image shown in Figure 1e, the planar defects are observed (some of which are marked by white arrows) and their separations are ∼30−80 nm, which approximately coincides with the values of the aforementioned apparent jump distances of the epitaxial LLT thin film. The TEM lattice image (Figure 1(e, lower right)) revealed that the lithium conducting path was disrupted at the planar defects by the disordered atomic arrangement (∼1.8 nm width) across which the c-plane of LLT was shifted by half the perovskite primitive cell. It has been known that grain boundaries of LLT act as high potential barrier for Li-ion transport.30 Therefore, the low threshold field strength, F0, for the nonlinear ionic transport of the LLT epitaxial film is most likely a result of its domain structure where easily mobile crystalline domains are bounded by the high potential planar defects. Under the applied bias, the voltage drop mainly occurred at the grain boundary, yielding to the reduced barrier height (Figure 3f). Regarding the response speed of all-solid-state EDL-gated transistors, Tsuchiya et al. noted the linear scaling rule for response speed as a function of the ionic conductivity and transport distance of a solid electrolyte.17 Our findings indicate that the applied bias can be added as another dimension to the scaling rule in the case where nonlinear ionic transport takes place with appropriate selection of the ionic conducting material; in this added dimension, the response speed exponentially improves. This finding of nonlinear dynamics of the EDL-gating device, therefore, can be an important pathway to overcome its typical slow response. In summary, we demonstrated the conductance modulation of Au thin film by all-solid-state EDL gating using an epitaxially grown lithium ion conductor, LLT, with a back-gated structure. The Au conductance was modulated at room temperature with its time constant less than a second at VG = 1.5 V. The response time became exponentially faster with increasing applied gate bias, indicating that nonlinear ionic transport occurs in the submicron-thick LLT epitaxial film. This finding of the exponential nonlinear dynamics of an all-solid-state EDLgated device provides the pathway to overcome its typical slow response speeds with appropriate device structure design and material selection.

time-dependent development of the built-in potential at the interfaces with evolution of the EDL is inherently taken into account, which, in turn, account for the time-dependent reduction of the electric field within the electrolyte layer under the constant gate bias, VG (see the Supporting Information Section 5). The transient current modulation properties of Figure 3a can be well-fitted by eq 2 with n = 2, but not with n = 1. The fit parameters, αVGCEDL, τ1, and τ2, are plotted as a function of the applied gate bias in Figure 3c. Interestingly, both the time constants τ 1 and τ 2 are exponentially reduced with applied bias. The slower process, τ2, is almost 10 times slower than the faster process, τ1, under all the measured gate biases. The coefficient αVGCEDL linearly increases with gate bias VG. This linearity validates the first order approximation represented in eq 1 and is also a clear indication that the mechanism of the Au conductivity modulation in our device is due to the electrostatically accumulated charges at the EDL12 rather than the electrochemical redox reaction of the gold.13 Such an exponential dependence of the time constants, τ1 and τ2, on the gate bias, VG, is the manifestation of nonlinear behavior of the ionic transport in the epitaxial LLT thin film in our device. In the case of the standard linear circuit model, the time constants for EDL charging should not depend on the gate bias (see Supporting Information) but should depend only on the RC values of the circuit elements as shown in eqs S6 and S7. Conversely, the exponential dependence of the time constants on the gate bias indicates that the “effective resistance” of the LLT thin film reduced exponentially with the applied bias. Such nonlinear ionic transport within a solid electrolyte has been observed under large applied electric fields.27−29 Ionic transport is a thermal activation process in which mobile ions are bound to the potential valleys exerted from the surrounding ions (Figure 3d). Under a sufficiently large electric field, the potential landscape is deformed, the binding potential for the ions is reduced by the applied bias (Figure 3e), and the conductivity of the ionic transportation becomes field-dependent as approximately expressed as follows27−29 ⎛ qdappF ⎞ INLI ∝ sinh⎜ ⎟ = sinh(F /F0) ⎝ 2kBT ⎠

(3)

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where INLI is the nonlinear ionic current, q is the charge of the mobile ion, dapp is the apparent jump distance, and F is the mean electric field strength, kB is the Boltzmann constant, T is temperature, and F0 is the characteristic field strength for nonlinear ionic transport defined by F0 ≡ 2kBT/qdapp. Under a low electric field, F ≪ F0, Tailor expansion of eq 3 yields linear ionic transportation, which can be represented by Ohm’s law. Under a high electric field, however, Ohm’s law is no longer valid. In such a case, the resistance for the ionic transportation is effectively exponentially dependent on applied bias, RNLI(VG) = VG/INLI ∝ VG/sinh(VG), where RNLI is the “effective resistance” under nonlinear ionic transport. That is, the time constants τi are inversely proportional to the exponential of the applied bias (eqs S6 and S7). By fitting Figure 3c with τ (VG) ∝ VG/INLI using eq 3, the characteristic field for the nonlinear ionic transport, F0, and apparent jump distances, dapp, for the epitaxial LLT thin film are obtained as F0 = 9.7 kV/cm and dapp = 52 nm (±10 nm) for τ1 and F0 = 14.4 kV/cm and dapp = 35 nm (±5.5 nm) for τ2. These values of the characteristic field are 1 order of magnitude

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15662. Experimental details, analysis of the crystal structure and the orientation of the LLT epitaxial film, electrochemical impedance spectroscopy analysis of the LLT film and the interfaces, measurements of the redox state of LLT in the vicinity of Au/LLT interface, mathematical derivation of the transient behavior of the EDL charging using the equivalent circuit model (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tetsuya Asano: 0000-0002-4988-6928 Yukihiro Kaneko: 0000-0003-0000-5840 5059

DOI: 10.1021/acsami.6b15662 ACS Appl. Mater. Interfaces 2017, 9, 5056−5061

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ACS Applied Materials & Interfaces

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Atsushi Omote: 0000-0001-7137-4374 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. T.A. designed, conducted, and analyzed the experiments. Y.K. contributed to the device process, and H.A. contributed to the epitaxial growth of the back gate SRO and solid electrolyte LLT layers. A.O. and E.F. supervised this research project. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate Dr. Kawanowa at Ion Technology Center Co., Ltd., for TEM imaging of the device. We also acknowledge Mr. Kurooka, Mr. Yamada, and Mr. Kozaki at Panasonic Corporation and Mr. Umemoto at SPring-8 Service Co., Ltd. for their support on HAXPES measurements at BL16XU at SPring-8 in Japan.

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ABBREVIATIONS EDL, electric double layer IL, ionic liquid LLT, La2/3‑xLi3xTiO3 SRO, SrRuO3 STO, SrTiO3 PLD, pulsed laser deposition XRD, X-ray diffraction TEM, transmission electron microscopy SAD, selected area diffraction EIS, electrochemical impedance spectroscopy HAXPES, hard X-ray photoelectron spectroscopy

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DOI: 10.1021/acsami.6b15662 ACS Appl. Mater. Interfaces 2017, 9, 5056−5061