Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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In Situ Hard X‑ray Photoelectron Spectroscopy of Space Charge Layer in a ZnO-Based All-Solid-State Electric Double-Layer Transistor Takashi Tsuchiya,*,† Yaomi Itoh,† Yoshikazu Yamaoka,‡ Shigenori Ueda,§,∥ Yukihiro Kaneko,‡ Taku Hirasawa,‡ Masa-aki Suzuki,‡ and Kazuya Terabe†
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†
International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan ‡ Technology Innovation Division, Panasonic Corporation, 1006 Kadoma, Kadoma City, Osaka 571-8508, Japan § Research Center for Advanced Measurement and Characterization, NIMS, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ∥ Synchrotron X-ray Station at SPring-8, NIMS, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan S Supporting Information *
ABSTRACT: A space charge layer (SCL) at the (0001) oriented ZnO thin-film/lithium-ion conducting glass ceramics interface was investigated using in situ hard X-ray photoelectron spectroscopy (HAXPES) and Mott−Schottky analysis to understand the working mechanism of an all-solid-state electric double- layer transistor (EDLT). The EDLT showed a drain current decrement on application of a negative gate voltage, corresponding to electron depletion in ZnO. The SCL generation and steep band bending in ZnO were successfully observed through the drastic broadening of Zn 2p HAXPES spectra under reverse voltage conditions. The voltage dependence of SCL capacitance, analyzed on the basis of a Mott− Schottky plot, indicated a very thin SCL with a thickness of approximately 1−2 nm. The results indicated that a strong electric field, on average larger than 1 MV/cm, is generated in the ZnO thin film when a large reverse voltage is applied.
1. INTRODUCTION Electronic carrier doping by electric double layer (EDL), in the vicinity of electronic material/electrolyte interfaces, has been attracting much attention due to its large capacitance (several μF/cm2) and accompanying extremely high carrier density.1−14 Because the EDL technique is an electrostatic approach that, unlike chemical doping, introduces no defects in electronic materials, the technique is quite advantageous for the tuning of numerous physical properties in which disturbance of the electronic structure by chemical doping can cause serious deterioration of properties (e.g., metal−insulator transition, superconductivity, magnetism).1−12 High-density electronic carrier doping without defect generation can thus be a valuable feature for various electronic devices. Another prominent feature of the EDL technique is the huge electric field (e.g., MV/cm) of the EDL and the space charge layer (SCL) generated in electronic materials. This makes the EDL technique particularly attractive for use in electric-fielddependent physical properties (e.g., refractive index with electro-optic effect, magnetization with electro-magnetic effect), which are useful for optical switching and spintronics applications. Modulation of electronic carrier density, accompanied by that of electronic conductance or other physical properties, has been reported for various EDL devices (i.e., combinations of electronic materials and electrolytes).1−14 However, no © XXXX American Chemical Society
significant investigation of SCL behavior in electronic materials has been done because it is not straightforward to observe in situ SCL, which is buried under the electronic material/ electrolyte interface. Furthermore, the occurrence of electrochemical reduction and oxidation (i.e., redox) in a certain voltage range, which has been discussed by many researchers,3,5,7,9,15,16 is expected to decrease the electric field of SCL through charge compensation of the doped electronic carrier and the inserted ionic carrier. Therefore, the details of SCL in EDL devices and its availability for practical applications is still not clear. Here, we report in situ hard X-ray photoelectron spectroscopy (HAXPES) investigation of SCL behavior in all-solidstate EDL transistors (EDLTs) composed of the (0001)oriented ZnO thin film, which possesses good transparency and electron mobility, and is thus a typical electronic material for EDLTs.3,13 HAXPES is very useful for in situ observation of electric potential distribution near solid/solid interfaces because its excellent photoelectron probe depth, as long as 10−20 nm, enables the detection of core-level photoelectrons escaping from a buried interface.17−22 We further performed Mott−Schottky analysis of SCL capacitance, which has been Received: February 27, 2019 Revised: April 3, 2019
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DOI: 10.1021/acs.jpcc.9b01885 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. (a) Illustration of all-solid-state EDLT composed of ZnO thin film and LICGC. (b) Illustration of the metal−insulator−metal (MIM) type electrochemical cell with a ZnO film and a LICGC substrate for HAXPES. (c) Illustration of a three-terminal electrochemical cell for Mott− Schottky analysis of SCL behavior in the ZnO thin film. (d) An energy diagram in the vicinity of the ZnO/LICGC interface.
2.2. Device Fabrication and Experimental Procedure of in Situ HAXPES Measurement. The metal−insulator− metal (MIM) type electrochemical cell, with a ZnO film as the semiconductor and a LICGC substrate as the electrolyte, schematically shown in Figure 1b, was used for the in situ HAXPES measurements to investigate the SCL in ZnO thin film. The working electrode (WE), consisting of 2 × 3 mm2 dimension Au, Ti, and ZnO thin films, was deposited to 6 nm in total. A 3 nm thick thin film of ZnO was deposited onto the LICGC by PLD under the same conditions described in 2.1. One nanometer thick Ti and 2 nm thick Au were deposited onto the surface of the ZnO film by EB at room temperature. The counter electrode (CE), consisting of 2 × 3 mm2 dimension LiCoO2 and Pt thin films, was deposited. A 30 nm thick thin film of LiCoO2 was deposited onto the bottom of the LICGC by PLD, using a 99.9% pure LiCoO2 ceramic target and pure O2 gas at a fixed total gas flow rate of 10 sccm to maintain 10 Pa. The substrate temperature was kept at 873 K. A 100 nm thick layer of Pt was then deposited to cover the LiCoO2 thin film. The HAXPES measurements were performed at the beamline BL15XU of the SPring-8 synchrotron radiation facility in Japan. The photon energy of the excitation X-ray was 5953.4 eV. The overall instrumental resolution of the beamline is estimated to be 0.24 eV at 5953.4 eV of photon energy. All spectra were measured at room temperature and the binding energy was referenced to the Au 4f7/2 core level of the Au thin film in the WE. The direction of the polarization is expressed as positive when the WE is anodically polarized (positive voltage is applied to the WE). Each measurement was performed after the attainment of steady state, typically waiting for at least 5 min after the application of dc bias to the sample.
widely used as a standard method for semiconductor devices but has not yet been used for EDL devices.23 Two main findings were drawn from the experimental results. One is that a strong electric field, larger than MV/cm on average, is generated in ZnO thin film under conditions of large reverse voltage. The other is that, under relatively small forward voltage applied conditions, the electric field is very low in ZnO thin film due to lithium-ion insertion. These findings indicate that the EDL technique is quite useful for tuning strong electric fields in oxide electronic materials in a certain voltage range, which is determined by the electrochemical properties of both the electronic materials and the electrolytes.
2. METHODS 2.1. Fabrication of All-Solid-State EDLT. An all-solidstate EDLT, schematically shown in Figure 1a, was fabricated on the flat surface of a 150 μm thick lithium-ion conducting glass ceramic (LICGC) (Ohara, Japan). All thin films were fabricated on the LICGC substrate by pulsed laser deposition (PLD), electron beam (EB) evaporation, and radio frequency (RF) sputtering techniques. A 10 nm thick ZnO thin film was deposited by PLD, using a sintered ZnO target pellet with 99.9% purity, under the supply of pure oxygen gas at a fixed flow rate of 10 sccm so as to keep the oxygen pressure at 10 Pa. The substrate temperature was kept at 873 K during the PLD process. The (0001) orientation of the ZnO film was confirmed by XRD (see Section S1).24−26 Source and drain electrodes were made of Au/Ti and deposited by RF sputtering with a shadow mask. The channel length and width were 75 and 50 μm, respectively. The length and width of the gate electrode were 2 and 3 mm, respectively. B
DOI: 10.1021/acs.jpcc.9b01885 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C The potential profiles in ZnO layer can be simulated on the basis of the Zn 2p3/2 spectral shapes and inelastic mean free path (IMFP) of Zn 2p3/2 photoelectron using Common Data Processing System (COMPRO) software, which can simulate potential profiles in materials from the PES spectra.27 The TPP2M equation is useful to calculate the IMFP in various materials.28 The calculation requires kinetic energy of photoelectron and some physical properties of ZnO as a scattering medium, as will be shown in Section 3.2. For the simulation of potential profile using COMPRO software, the attenuation length and the emission angle (the angle from the surface normal) were fixed to 7.2 nm and 10°, respectively. The exponential profiles in the 3 nm thick ZnO layer with a ratio of Lorentzian of 0.65 gave best results. Potential distribution in the Au top layer was ignored in the calculation because Au 4f spectra showed no variation in the shape under all conditions. 2.3. Device Fabrication and Experimental Procedure of Mott−Schottky Analysis. Figure 1c shows a three terminal electrochemical cell for Mott−Schottky analysis of SCL behavior in the ZnO thin film. A ZnO thin film, covered by Ti and Au thin films, was used as the WE. The CE consisting of LiCoO2 and Pt thin films and the reference electrode (RE) consisting of Al, which is known to show stable electric potential by alloying with Li, were used. The electrode and the electrolyte thin films were prepared under the same conditions described in Sections 2.1 and 2.2. Figure 1d illustrates an energy diagram in the vicinity of the ZnO/ LICGC interface. Upward band bending in the SCL of ZnO is enhanced by Li-ion migration from the interface to the LICGC bulk. This is due to the positive voltage application to ZnO, accompanied by electron depletion in ZnO. Mott−Schottky analysis was performed by using a potentiostat and a frequency response analyzer. The SCL capacitance was measured at an alternating current (ac) voltage of 10 mV (1 kHz) with respect to the electric potential of the RE. The measurement was performed by polarizing the WE from open circuit potential (0.55 V vs RE) to 2 V vs RE.
3. RESULTS AND DISCUSSION 3.1. Electric Conduction Characteristics of the ZnOBased All-Solid-State EDLT. Figure 2a shows the electrical conduction characteristic [drain current (iD) vs gate voltage (VG)] at a constant drain voltage (VD) of 0.1 V of the transistor gated by lithium-ion transport in LICGC. When iD showed a gradual decrease to relatively small value, e.g., below 1 nA, in the negative VG sweeping from 0 to −1.3 V [indicated as (I)], the VG polarity corresponds to a reverse voltage (i.e., electron depletion region) for the SCL in the n-ZnO layer in the vicinity of the ZnO/LICGC interface. The gate current (iG) was very small in the region due to the very small capacitance of the SCL under the reverse voltage application condition. With negative VG larger than −1.3 V [indicated as (II)], gradual increase in iD was observed (indicated by the green arrow). This is an artifact caused by an extremely small iD and a relatively large iG (indicated by the green arrow), as explained below. iD and iG are expressed by a sum of two contributions, respectively. iD = iDS + iDG
Figure 2. (a) iD (upper panel) or iG (lower panel) vs VG characteristic. The VG sweeping rate was 5 mV/s. (b) Variation in iD (upper) and iG (lower) with time during negative VG application kept at from −0.1 to −3 V of VG. (c) Variation in iD (upper) and iG (lower) at 1000 s with respect to applied VG.
(1) C
DOI: 10.1021/acs.jpcc.9b01885 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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This is a reasonable assumption because the anodic current acts to remove electrons from n-type ZnO. Oxygen-rich conditions are known to cause negatively charged Zn vacancy (acceptor) formation and decreases in electron density in ZnO.31 In this work, application of large positive voltages electrochemically created such an oxygen-rich atmosphere near the ZnO/LICGC interface, leading to the anodic current for Zn vacancy formation and its transport.32 3.2. In Situ HAXPES of SCL in the Vicinity of a ZnO/ LICGC Interface. Figure 3a,b shows the in situ HAXPES Zn
(2)
iDS is defined as a current from the drain electrode to the source electrode. iDG, iGS, and iGD are defined in the same manner. In conventional metal-oxide−semiconductor (MOS) transistors, iGS, iGD, and iG in eq 2 can be ignored because of the insulating property of oxide in the wide VG region.23 Then, iD in eq 1 is equal to iDS, which varies depending merely on the electric conductivity of the semiconductor channel. On the other hand, in the present case in the region (II), iG was so large that a contribution of iDG (=−iGD) to iD cannot be ignored in eq 1. Because of the symmetric structure of the transistor and the relatively small VD (0.1 V) with respect to VG, iGS and iGD are approximately equal and eq 2 is simplified as iG = 2iGD (=−2iDG). Then, eq 1 is transformed to iD = iDS − 1/2iG (1)′. Further, when iDS is very small, as in this case, iDS in eq 1′ can be ignored, resulting in the simple relation iD = −1/ 2iG (1)″. iD of 2.83 nA and iG of −4.47 nA at 3 V of VG satisfy eq 1″ in an acceptable error range of 27%, supporting the validity of our assumption that increase in iD is an experimental artifact in the mechanism. The scale out (negative iD) observed in region (III) is also understood in the mechanism with positive iG in eq 1″. The large iG in regions (II) and (III) originates in an electrochemical reaction of ZnO, the details of which will be discussed later. When VG is varied from one to another [as in Figure 2a], the observed iG is accompanied by EDL charging current because dQ/dV = Cedl. As discussed in Figure 2a, such iG can cause an artifact that prevents the correct evaluation of iD modulation. Therefore, iD was measured under constant VG conditions. The upper and lower panels of Figure 2b show variations in iD and iG during negative VG application (−0.1 to −3 V of VG). From −0.1 to −1 V, red to green curves in the upper panel of Figure 2b, iD at 1000 s gradually decreases from 500 to 40 pA. This is because of reverse voltage application to the SCL at the ZnO/ LICGC interface. iD increasing in above −1.3 V of VG corresponds to the artifact discussed in Figure 2a. The observed iG shown in the lower panel of Figure 2b, which was retained even for 1000 s, evidences that the iG is not an EDL charging current but a Faradic current for electrochemical reaction. Such a switching from electrostatic to electrochemical process was reported for EDLTs composed of ZnO or other electronic materials.3,5,7 Note that we focus on the negative VG region of the EDLT due to our main interest in the electron depletion behavior in the SCL. Please refer to Section S2 for electron doping behavior, which was switched from electrostatic to electrochemical process, in a positive VG region.29,30 To compare iD behavior with iG at 1000 s, iD and iG at 1000 s are plotted with respect to VG in Figure 2c. In the lower panel of Figure 2c, two regions are evident: between 0 to −1.3 V (A) and above −1.3 V (B). Given that 20 pA of iG is the lowest extreme (indicated by an additional line), the two regions (A) and (B) are assigned to one in which an electrochemical reaction is absent (A); and one in which an anodic reaction takes place (B). In region (A), in the upper panel of Figure 2c, iD shows significant variation in spite of extremely low iG. The iD variation is thus attributed to an electrostatic carrier doping due to a negative VG application to SCL in ZnO. In region (B), iD apparently increased to several hundred pA, but this behavior is well understood as an iG-induced artifact, as discussed above with eq 1″. This indicates that the prominent iG in region (B) causes no positive effect on iD enhancement.
Figure 3. (a) Zn 2p and (b) Au 4f HAXPES spectra measured under positive voltage applied conditions. (c) Simulated potential profiles with respect to applied voltages.
2p and Au 4f spectra of a two-terminal cell consisting of a Au top electrode/Ti/ZnO/LICGC/LiCoO2/Pt bottom electrode, as shown in Figure 1b, measured with a positive (a, b) voltage applied to the Au top electrode. Note that the positive polarity of voltage used in Sections 3.2 and 3.3 corresponds to the negative polarity of the gate voltage in Section 3.1. In Figure 3a, the Zn 2p peak showed remarkable broadening toward the low binding energy side as the positive voltage increased from 0 to 3 V. This means the generation of a large potential drop inside the ZnO layer. Since the Au/Ti/ZnO interface was reported to be an Ohmic contact,33 SCL is likely to be D
DOI: 10.1021/acs.jpcc.9b01885 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C generated at the ZnO/LICGC interface. Since the positive voltage polarity corresponds to a reverse voltage direction for the SCL at the ZnO/LICGC interface, the behavior is understood as a steep band bending near the interface. The Au 4f spectra in Figure 3b showed almost no variation with respect to the applied voltages in contrast to the Zn 2p spectra in Figure 3a, evidencing that both in-plane and out-of-plane potential drop in the Au top electrode can be ignored, and thus the variation in the Zn 2p spectra is attributed to the out-ofplane potential profile in the ZnO layer. The potential profiles in the ZnO layer can be simulated on the basis of the Zn 2p3/2 spectral shapes and the inelastic mean free path (IMFP) of photoelectron. In the present study with an excitation energy of 5953 eV, the kinetic energy of the Zn 2p3/2 photoelectron with the binding energy of 1023 eV is 4930 eV. The IMFP of the Zn 2p3/2 photoelectron (7.2 nm) was calculated from the kinetic energy of Zn 2p3/2 (4930 eV) on the basis of the TPP2M equation using physical parameters of ZnO (density of 5.6 g/cm3, molar mass of 81.4 g/mol, and energy gap of 3.37 eV).28 Figure 3c shows simulated potential profiles with respect to applied voltages using COMPRO software.27 The profiles are plotted as (i) a relative potential in the left axis, in which an electric potential at the Ti/ZnO interface under 0 V applied condition is taken as the basis, and as (ii) a binding energy of simulated Zn 2p peaks in the right axis. As the positive voltage increases, a steep potential slope was generated in the ZnO layer. Since the thickness of the ZnO layer is only 3 nm, electric field reaches as strong as 3.7 and 6.7 MV/cm at 1 and 3 V of applied voltage, respectively. Please refer to Section S3 for the simulated Zn 2p spectra and simulation conditions. In contrast to the positive voltage applied conditions, a negative voltage application made potential profiles flat. This is quite reasonable if we consider a flat band condition due to forward voltage application to the SCL. Please refer to Section S4 for the negative voltage application. Above −0.5 V, the potential profile became completely flat. Further increase in the negative voltage caused a parallel shift to the low-energy side. This corresponds to a Fermi level shift in the ZnO layer due to Li+ insertion (electron donation). The behavior is consistent with the iD enhancement accompanied by significant iG increase discussed in Figure S2. 3.3. Mott−Schottky Analysis of SCL Capacitance in the Vicinity of a ZnO/LICGC Interface. To verify the validity of our observation of SCL using HAXPES, Mott− Schottky analysis was performed. A three-terminal electrochemical cell composed of a ZnO working electrode and a LICGC solid electrolyte [shown in Figure 1c] was used to investigate the variation in the SCL capacitance due to EDL modulation. Figure 4a shows voltage dependence of 1/CSCL2 measured by an ac voltage of 10 mV (1 kHz) with respect to the electric potential of the RE. Although 1/CSCL2 was kept very small, below 0.8 V, it showed a linear increase above 0.8 V. The behavior is reasonable because an increase of the positive voltage corresponds to a polarization toward the reverse voltage condition for SCL in ZnO. That is, CSCL decreases as the depletion layer extends from the ZnO/ LICGC interface toward ZnO inside by reverse voltage application. Based on an intercept and a slope of the linear approximation of the 1/CSCL2 curve in the reverse voltage region (indicated by a black line), flat band voltage (VFB) and donor concentration are calculated to be 0.81 V and 9.3 × 1020 cm−3, respectively. The calculated donor concentration, which
Figure 4. (a) Voltage dependence of 1/CSCL2 measured by ac voltage of 10 mV with respect to electric potential of the RE. (b) Potential profiles calculated on the basis of a one-dimensional Poisson equation, with obtained VFB and donor concentration under various reverse voltage applied conditions.
is 3 orders of magnitude higher than the typical Li impurity (1018 cm−3) in a nondoped ZnO, strongly indicates that native positively charged defects of ZnO, such as O vacancy, Zn interstitial, and its complex,29 work as fixed donor. Potential profiles calculated on the basis of one-dimensional Poisson equation with obtained VFB and donor concentration under various reverse voltage applied conditions are shown in Figure 4b. The SCL extends from the ZnO/LiCGC interface toward ZnO inside as the reverse voltage is applied to the SCL. Although the potential profile relatively agrees with one based on HAXPES [shown in Figure 3c], there is a gap between them. Whereas the thickness of SCL in Figure 4b is less than 2 nm for all the cases, the one calculated using HAXPES results is extended to 3 nm except for the cases with forward voltage conditions, mostly because of the high donor concentration. A slight deviation of the approximation line from the experimental results observed in Figure 2c further indicates that such native defects are not homogeneously distributed in ZnO, or the voltage-driven Zn vacancy formation in the vicinity of the ZnO/LICGC interface discussed in Sections 3.1 and 3.2. Note that the saturation of the potential drop under conditions of large voltage observed in Figure 3c is absent in Figure 4b, in which the potential profiles were calculated by assuming a complete electrostatic process, that is, one without the migration of ionic carrier (e.g., Zn vacancy). The E
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the NIMS Synchrotron X-ray Station (Proposal Nos 2017A4606, 2017B4600, 2014A4604, and 2012B4602).
significant difference between Figures 3c and 4b thus strongly indicates an occurrence of the voltage-driven Zn vacancy formation under the conditions of large reverse voltage.
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4. CONCLUSIONS In situ HAXPES and Mott−Schottky analysis were performed to investigate the SCL (0001) oriented ZnO thin film/LICGC interface relevant to the carrier doping mechanism of the allsolid-state EDLT. The HAXPES of the Zn 2p core levels evidenced that the SCL in ZnO reaches as strong as 3.7 MV/ cm at 1 V of applied voltage. A Mott−Schottky plot of CSCL applied to investigate the generation of very thin, 1−2 nm, SCL in the ZnO thin film, supports the validity of the HAXPES result. In contrast, a steep band bending was observed under reverse voltage conditions, and no significant potential distribution was observed under the forward voltage condition due to electrochemical Li insertion into the ZnO thin film, which was driven by the applied voltage. The present results indicate that the operation voltage should be deliberately chosen to achieve a strong electric field in the ZnO thin film. This should be done to escape from the serious inhibition of SCL generation by electrochemical Li insertion. Such an appropriate voltage range can be strongly dependent on the thermodynamic and kinetic factors for a given combination of ionic species and electronic material (e.g., solubility of Li in ZnO). In situ control of the strong electric field in the vicinity of electronic material/solid electrolyte interface is a promising approach in developing novel nanoelectronic devices utilizing electric-field-dependent physical properties.
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(1) Ueno, K.; Nakamura, S.; Shimotani, H.; Ohtomo, A.; Kimura, N.; Nojima, T.; Aoki, H.; Iwasa, Y.; Kawasaki, M. Electric-FieldInduced Superconductivity in an Insulator. Nat. Mater. 2008, 7, 855− 858. (2) Yamada, Y.; Ueno, K.; Fukumura, T.; Yuan, H. T.; Shimotani, H.; Iwasa, Y.; Gu, L.; Tsukimoto, S.; Ikuhara, Y.; Kawasaki, M. Electrically Induced Ferromagnetism at Room Temperature in Cobalt-Doped Titanium Dioxide. Science 2011, 332, 1065−1067. (3) Yuan, H. T.; Shimotani, H.; Tsukazaki, A.; Ohtomo, A.; Kawasaki, M.; Iwasa, Y. High-Density Carrier Accumulation in ZnO Field-Effect Transistors Gated by Electric Double Layers of Ionic Liquids. Adv. Funct. Mater. 2009, 19, 1046−1053. (4) Nakano, M.; Shibuya, K.; Okuyama, D.; Hatano, T.; Ono, S.; Kawasaki, M.; Iwasa, Y.; Tokura, Y. Collective bulk carrier delocalization driven by electrostatic surface charge accumulation. Nature 2012, 487, 459−462. (5) Ueno, K.; Shimotani, H.; Iwasa, Y.; Kawasaki, M. Electrostatic Charge Accumulation Versus Electrochemical Doping in SrTiO3 Electric Double Layer Transistors. Appl. Phys. Lett. 2010, 96, No. 252107. (6) Zhu, L. Q.; Wan, C. J.; Guo, L. Q.; Shi, Y.; Wan, Q. Artificial Synapse Network on Inorganic Proton Conductor for Neuromorphic Systems. Nat. Commun. 2014, 5, No. 3158. (7) Feng, P.; Du, P. F.; Wan, C. J.; Shi, Y.; Wan, Q. Proton Conducting Graphene Oxide/Chitosan Composite Electrolytes as Gate Dielectrics for New-Concept Devices. Sci. Rep. 2016, 6, No. 34065. (8) van de Burgt, Y.; Lubberman, E.; Fuller, E. J.; Keene, S. T.; Faria, G. C.; Agarwal, S.; Marinella, M. J.; Alec, T. A.; Salleo, A. A NonVolatile Organic Electrochemical Device as a Low-Voltage Artificial Synapse for Neuromorphic Computing. Nat. Mater. 2017, 16, 414− 418. (9) Wen, J.; Zhu, L. Q.; Fu, Y. M.; Xiao, H.; Guo, L. Q.; Wan, Q. Activity Dependent Synaptic Plasticity Mimicked on Indium-TinOxide Electric-Double-Layer Transistor. ACS Appl. Mater. Interfaces 2017, 9, 37064−37069. (10) Tsuchiya, T.; Terabe, K.; Aono, M. All-Solid-State ElectricDouble-Layer Transistor Based On Oxide Ion Migration in GdDoped CeO2 on SrTiO3 Single Crystal. Appl. Phys. Lett. 2013, 103, No. 073110. (11) Tsuchiya, T.; Imura, M.; Koide, Y.; Terabe, K. Magnetic Control of Magneto-Electrochemical Cell and Electric Double Layer Transistor. Sci. Rep. 2017, 7, No. 10534. (12) Tsuchiya, T.; Tsuruoka, T.; Kim, S.-J.; Terabe, K.; Aono, M. Ionic decision-maker created as novel, solid-state devices. Sci. Adv. 2018, 4, No. eaau2057. (13) Lu, A.; Sun, J.; Jiang, J.; Wan, Q. Low-voltage transparent electric-double-layer ZnO-based thin-film transistors for portable transparent electronics. Appl. Phys. Lett. 2010, 96, No. 043114. (14) Neuvonen, P. T.; Vines, L.; Yu, A.; Kuznetsov; Svensson, B. G.; Xu, X.; Tuomisto, F.; Hallen, A. Ultralow-voltage transparent electricdouble-layer thin-film transistors processed at room-temperature. Appl. Phys. Lett. 2009, 95, No. 152114. (15) Jeong, J.; Aetukuri, N.; Graf, T.; Schladt, T. D.; Samant, M. G.; Parkin, S. S. P. Suppression of metal-insulator transition in VO2 by electric field-induced oxygen vacancy formation. Science 2013, 339, 1402−1405. (16) Li, M.; Han, M. W.; Jiang, X.; Jeong, J.; Samant, M. G.; Parkin, S. Suppression of Ionic Liquid Gate-Induced Metallization of SrTiO3(001) by Oxygen. Nano Lett. 2013, 13, 4675−4678. (17) Nagata, T.; Haemori, M.; Yamashita, Y.; Yoshikawa, H.; Iwashita, Y.; Kobayashi, K.; Chikyow, T. Oxygen Migration at Pt/ HfO2/Pt Interface Under Bias Operation. Appl. Phys. Lett. 2010, 97, No. 082902.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b01885. XRD patterns of ZnO thin film/LICGC substrate, electric conduction characteristics of the transistor in a positive VG region, simulation of potential profiles using COMPRO software, in situ HAXPES of SCL in the vicinity of a ZnO/LICGC interface (negative voltage conditions) (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Takashi Tsuchiya: 0000-0002-6950-6160 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Panasonic-NIMS Center of Excellence for Advanced Functional Materials. We are grateful to K. Kobayashi, M. Kobata, and S. Ishimaru for the development of HAXPES at BL15XU at SPring-8. We also thank M. Takayanagi for assistance with the experiments. The HAXPES measurements were performed under the approval of F
DOI: 10.1021/acs.jpcc.9b01885 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C (18) Nagata, T.; Haemori, M.; Yamashita, Y.; Yoshikawa, H.; Iwashita, Y.; Kobayashi, K.; Chikyo, T. Bias Application Hard X-ray Photoelectron Spectroscopy Study of Forming Process of Cu/HfO2/ Pt Resistive Random Access Memory Structure. Appl. Phys. Lett. 2011, 99, No. 223517. (19) Williams, J.; Yoshikawa, H.; Ueda, S.; Yamashita, Y.; Kobayashi, K.; Adachi, Y.; Haneda, H.; Ohgaki, T.; Miyazaki, H.; Ishigaki, T.; et al. Tunable terahertz left-handed metamaterial based on multi-layer graphene-dielectric composite. Appl. Phys. Lett. 2012, 100, No. 051902. (20) Tsuchiya, T.; Miyoshi, S.; Yamashita, Y.; Yoshikawa, H.; Terabe, K.; Kobayashi, K.; Yamaguchi, S. Room Temperature RedOx Reaction by Oxide Ion Migration at Carbon/Gd-Doped CeO2 Hetero-Interface Probed by an In-Situ Hard X-ray Photoemission and Soft X-ray Absorption Spectroscopy. Sci. Technol. Adv. Mater. 2013, 14, No. 045001. (21) Tsuchiya, T.; Miyoshi, S.; Yamashita, Y.; Yoshikawa, H.; Terabe, K.; Kobayashi, K.; Yamaguchi, S. Direct Observation of Redox State Modulation at Carbon/Amorphous Tantalum Oxide Thin Film Hetero-Interface Probed by Means of In Situ Hard X-ray Photoemission Spectroscopy. Solid State Ionics 2013, 253, 110−118. (22) Ueda, S.; Katsuya, Y.; Tanaka, M.; Yoshikawa, H.; Yamashita, Y.; Ishimaru, S.; Matsushita, Y.; Kobayashi, K. Present Status of the NIMS Contract Beamline BL15XU at SPring-8. AIP Conf. Proc. 2010, 1234, 403. (23) Sze, S. M.; Kwok, K. N. Physics of Semiconductor Devices; Wiley: New York, 2006. (24) Miyake, A.; Yamada, T.; Makino, H.; Yamamoto, N.; Yamamoto, T. Effect of substrate temperature on structural, electrical and optical properties of Ga-doped ZnO films on cycro olefin polymer substrate by ion plating deposition. Thin Solid Films 2008, 517, 1037−1041. (25) Miyake, A.; Yamada, T.; Makino, H.; Yamamoto, N.; Yamamoto, T. Structural, electrical and optical properties of Gadoped ZnO films on cyclo-olefin polymer substrates. Thin Solid Films 2009, 517, 3130−3133. (26) Osada, M.; Sakemi, T.; Yamamoto, T. The effects of oxygen partial pressure on local structural properties for Ga-doped ZnO thin films. Thin Solid Films 2006, 494, 38−41. (27) COMPRO, version 12, 1989. http://www.sasj.jp/COMPRO. (28) Tanuma, S.; Powell, C. J.; Penn, D. R. Calculation of electron inelastic mean free paths (IMFPs) VII. Reliability of the TPP-2M IMFP predictive equation. Surf. Interface Anal. 2003, 35, 268−275. (29) Knutsen, K. E.; Johansen, K. M.; Neuvonen, P. T.; Svensson, B. G.; Kuznetsov, A. Y. Diffusion and configuration of Li in ZnO. J. Appl. Phys. 2013, 113, No. 023702. (30) Lander, J. J. Reactions of Lithium as a donor and an acceptor in ZnO. J. Phys. Chem. Solids 1960, 15, 324−334. (31) Oba, F.; Choi, M.; Togo, A.; Tanaka, I. Point defects in ZnO: an approach from firstprinciples. Sci. Technol. Adv. Mater. 2011, 12, No. 034302. (32) Erhart, P.; Albe, K. Diffusion of zinc vacancies and interstitials in zinc oxide. Appl. Phys. Lett. 2006, 88, No. 201918. (33) Kim, H. K.; Han, S. H.; Seong, T. Y.; Choi, W. K. Lowresistance Ti/Au ohmic contacts to Al-doped ZnO layers. Appl. Phys. Lett. 2000, 77, 1647.
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DOI: 10.1021/acs.jpcc.9b01885 J. Phys. Chem. C XXXX, XXX, XXX−XXX
