Electric Double Layer Transistor Research Articles

Iontronics Using V2CTx MXene-Derived Metal-Organic Framework Solid Electrolytes.

Electronic applications of porous metal–organic frameworks (MOFs) have recently emerged as an important research area. However, there is still no report on using MOF solid electrolytes in iontronics, which could take advantage of the porous feature of MOFs in the ionic transport. In this article, MXene-derived two-dimensional porphyrinic MOF (MX-MOF) films are demonstrated as an electronic-grade proton-conducting electrolyte. Meanwhile, the MX-MOF film shows high quality, chemical stability, and capability of standard device patterning processes (e.g., dry etching and optical and electron beam lithography). Using the commercialized nanofabrication processes, an electric double-layer (EDL) transistor is demonstrated using the MX-MOF film (derived from V2CTx MXene) as an ionic gate and MoS2 film as a semiconducting channel layer. The EDL transistor, operated by applying an electric field to control the interaction between ions and electrons, is the core device platform in the emerging iontronics field. Therefore, The MX-MOF, confirmed as a solid electrolyte for EDL transistor devices, could have a significant impact on iontronics research and development.

Enhancing the critical temperature of strained Niobium films

The study of the high critical temperature (Tc) of hydrogen compounds under high pressure has resulted in a considerable focus on Bardeen–Cooper–Schrieffer superconductors. Nb has the highest Tc among the elemental metals at ambient pressure, so reviewing Nb films again is worthwhile. In this study, we investigated the factors that determine the Tc of Nb films by strain introduction and carrier doping. We deposited Nb films of various thicknesses onto Si substrates and evaluated the Tc variation with thickness. In-plane compressive strain in the (110) plane due to residual stress reduced the Tc. First-principles calculations showed that adjusting the density of states at the Fermi level is key for both strain-induced suppression and doping-induced enhancement of the Nb Tc. The application of hydrostatic pressure compensated for the intrinsic strain of the film and increased its Tc, which could also be enhanced by increasing the hole concentration with an electric double-layer transistor. A liquid electrolyte should be used as a pressure medium for applying hydrostatic pressure to increase the Tc of correlated materials, where this increase results from changes in material structure and carrier concentration.

Chemical pressure effect of the electron-doped FeSe films with an electric double-layer-transistor structure

We investigated chemical pressure effect of the electron-doped FeSe1−x S x and FeSe1−y Te y on LaAlO3 (x ≤ 0.25, y ≤ 0.5) with the electric double layer transistor structure. T c of all the FeSe1−x S x and FeSe1−y Te y films except y = 0.5 is increased by doping electron with gate voltage V G = +5 V. T c of the electron-doped FeSe1−x S x and FeSe1−y Te y is decreased monotonically by substituting Se for both S and Te. The behavior is similar to those of the intercalated FeSe1−x S x and FeSe1−y Te y and the electron-doped FeSe0.5Te0.5 with the solid-ion-conductor field-effect transistor structure, but quite different from that of the pristine FeSe1−x S x and FeSe1−y Te y . This difference is considered to originate from the difference of the Fermi surface topology, which suggests that the superconducting mechanism of the electron-doped FeSe is different from that of the pristine FeSe.

Controlling the magnetic anisotropy in Cr2Ge2Te6 by electrostatic gating

Electrical control of magnetism of a ferromagnetic semiconductor offers exciting prospects for future spintronic devices for processing and storing information. Here, we report observation of electrically modulated magnetic phase transition and magnetic anisotropy in thin crystal of Cr$_2$Ge$_2$Te$_6$ (CGT), a layered ferromagnetic semiconductor. We show that heavily electron-doped ($\sim$ $10^{14}$ cm$^{-2}$) CGT in an electric double-layer transistor device is found to exhibit hysteresis in magnetoresistance (MR), a clear signature of ferromagnetism, at temperatures up to above 200 K, which is significantly higher than the known Curie temperature of 61 K for an undoped material. Additionally, angle-dependent MR measurements reveal that the magnetic easy axis of this new ground state lies within the layer plane in stark contrast to the case of undoped CGT, whose easy axis points in the out-of-plane direction. We propose that significant doping promotes double-exchange mechanism mediated by free carriers, prevailing over the superexchange mechanism in the insulating state. Our findings highlight that electrostatic gating of this class of materials allows not only charge flow switching but also magnetic phase switching, evidencing their potential for spintronics applications.

Realization of artificial synapse and inverter based on oxide electric-double-layer transistor gated by a chitosan biopolymer electrolyte

Low-voltage transparent zinc oxide (ZnO) thin-film transistor with chitosan biopolymer electrolyte as a gate dielectric was fabricated at room temperature and characterized. Due to an electric-double-layer (EDL) effect near the chitosan/ZnO interface through mobile-proton migration, the transistor exhibits a good performance with a carrier mobility of 7.88 cm2 V−1 s−1, a threshold voltage of 1.0 V, an on-off current ratio of 5 × 104, a subthreshold slope of 135 mV Dec−1, and an operating voltage of less than 3 V, respectively. Based on the EDL transistor, the excitatory post-synaptic current and paired-pulse facilitation (PPF) behavior of biological synapses are mimicked, and PPF index is related to the inter-spike interval of the paired pulse. Furthermore, the resistor-loaded inverter based on the transistor was built, showing a high voltage gain of ∼4 at a low supply voltage of 3 V, a small delay, a large noise margin and a low power consumption. The EDL transistor may find potential applications in portable electronics and synaptic electronics.

Demonstration of electric double layer gating under high pressure by the development of field-effect diamond anvil cell

We have developed an approach to control the carrier density in various materials under high pressure by the combination of an electric double layer transistor (EDLT) and a diamond anvil cell (DAC). In this study, this “EDLT-DAC” was applied to a Bi thin film, and here, we report the field effect under high pressure in the material. Our EDLT-DAC is a promising device for exploring unknown physical phenomena such as high transition-temperature superconductivity.

Soft x-ray absorption spectroscopy and magnetic circular dichroism as operando probes of complex oxide electrolyte gate transistors

Electrolyte-based transistors utilizing ionic liquids/gels have been highly successful in the study of charge-density-controlled phenomena, particularly in oxides. Experimental probes beyond transport have played a significant role, despite challenges in their application in electric double-layer transistors. Here, we demonstrate the application of synchrotron soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) as operando probes of the charge state and magnetism in ion-gel-gated ferromagnetic perovskite films. Electrochemical response via oxygen vacancies at positive gate bias in LaAlO3(001)/La0.5Sr0.5CoO3−δ is used as a test case. XAS/XMCD measurements of 4–25 unit-cell-thick films first probe the evolution of hole doping (from the O K-edge pre-peak) and ferromagnetism (at the Co L-edges), to establish a baseline. Operando soft XAS/XMCD of electrolyte-gated films is then demonstrated, using optimized spin-coated gels with a thickness of ∼1 μm and a specific composition. The application of gate voltages up to +4 V is shown to dramatically suppress the O K-edge XAS pre-peak intensity and Co L-edge XMCD, thus enabling the Co valence and ferromagnetism to be tracked upon gate-induced reduction. Soft XAS and XMCD, with appropriate electrolyte design, are thus established to be viable for the operando characterization of electrolyte-gated oxides.

High performance electric double layer transistors using solvate ionic liquids

We report electrochemical properties of a Li+-based solvate ionic liquid (SIL) and its application to gate dielectrics for electric double layer transistors (EDLTs). It is found that the electric double layer capacitance of the SIL increases with strengthening the negative polarization of the working electrode due to the possible desolvation of the cations in the SIL. The transfer characteristics of a SrTiO3-based EDLT show a dramatic improvement when the SIL is used for the gate dielectric. The present result proposes a guideline to high performance gate dielectrics for EDLTs, leading to the development of iontoronics research with semiconductor devices.

Ambipolar transistor action of germanane electric double layer transistor

Germanane (GeH) is a hydrogen-terminated layered crystal of germanium. We fabricated an electric double layer transistor (EDLT) on a GeH thin film on the Ge (111) substrate and investigated its electronic properties. The EDLT showed ambipolar transfer characteristics at 240 K. The sheet resistance Rsheet at null gate bias (VG = 0 V) exceeded 1 MΩ below 100 K, which indicates an insulating behavior. For VG = −2 V (hole accumulation), a gradual increase in Rsheet up to 10 kΩ was found upon decreasing temperature to 40 K, i.e., semiconducting or even weakly insulating. Remarkably, Rsheet for VG = 1 V (electron accumulation) decreased down to 3 kΩ upon decreasing temperature to 40 K, indicating metallic conduction. Accumulation of electrons (holes) was confirmed for VG > 1 V (VG < −1 V) by Hall effect measurements. Hall mobilities of electrons and holes, which increase with decreasing temperature, were 6500 cm2 V−1 s−1 and 570 cm2 V−1 s−1, respectively, at 120 K.

Wide-voltage-window reversible control of electronic transport in electrolyte-gated epitaxial BaSnO3

The wide gap perovskite semiconductor ${\mathrm{BaSnO}}_{3}$ has attracted much interest since the discovery of room temperature electron mobility up to $320\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ in bulk crystals. Motivated by applications in oxide heterostructures, rapid progress has been made with ${\mathrm{BaSnO}}_{3}$ films, although questions remain regarding transport mechanisms and mobility optimization. Here we report on a detailed study of epitaxial ${\mathrm{BaSnO}}_{3}$ electric double layer transistors based on ion gel electrolytes, enabling wide-doping-range studies of transport in single films. The work spans an order of magnitude in initial $n$ doping ($\ensuremath{\sim}{10}^{19}$ to ${10}^{20}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, with both oxygen vacancies and La), film thicknesses from 10--50 nm, and measurements of resistance, Hall effect, mobility, and magnetoresistance. In contrast with many oxides, electrolyte gating of ${\mathrm{BaSnO}}_{3}$ is found to be essentially reversible over an exceptional gate voltage window (approaching $\ifmmode\pm\else\textpm\fi{}4\phantom{\rule{0.16em}{0ex}}\mathrm{V}$), even at 300 K, supported by negligible structural modification in operando synchrotron x-ray diffraction. We propose that this occurs due to a special situation in ${\mathrm{BaSnO}}_{3}$, where electrochemical gating via oxygen vacancies is severely limited by their low diffusivity. Wide-range reversible modulation of transport is thus achieved (in both electron accumulation and depletion modes), spanning strongly localized, weakly localized, and metallic regimes. Two-channel conduction analysis is then combined with self-consistent Schr\"odinger-Poisson and Thomas-Fermi modeling to extract accumulation layer electron densities and mobilities. Electrostatic electron densities approaching ${10}^{14}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ are shown to increase room temperature mobility by up to a factor of $\ensuremath{\sim}24$. These results lay the groundwork for future studies of electron-density-dependent phenomena in high mobility ${\mathrm{BaSnO}}_{3}$, and significantly elucidate oxide electrolyte gating mechanisms.

Thickness Effects on Charge and Discharge Characteristics of CeOx Mim Capacitors

Thickness effects on charge and discharge characteristics of a metal-insulator-metal (MIM) capacitor with ionic-oxygen-conductive CeOx layer was characterized. Transient response in the charging and discharging current and frequency dispersion suggested the movement of oxygen ions in the CeOx layer. The MIM capacitor with a thinner CeOx layer showed larger charging and discharging current and shorter time constant and suggested relation of CeOx layer thickness and ionic conductivity in the CeOx layer. Introduction On-chip decoupling capacitors with MIM structures have been used for suppress the high-frequency noise and support high-speed switching in the power supply line [1]. MIM capacitors are demanded to increase the capacitance density along with the technology node [2]. As an approach to increase capacitance density by thinner films may be limited by the leakage current through the electrodes, scaling in the insulators requires higher dielectric constant must be explored. We thought that sold electric-double-layer (EDL) capacitors with CeOx can be achieved. CeOx has a large value of dielectric constant (high-k) and suggest high-ionic conductivity at room temperature. Recently research, EDL transistors with Gd-doped CeO2 has shown that a large capacitance density of 14μF/cm2 can be achieved by the ionic conductivity in the insulators [3]. In this work, thickness effects on the charge and discharge properties of EDL capacitors with a CeOx layer are characterized. Experiments We fabricated MIM capacitors has a 10nm-thickness CeOx layer or 20nm-thickness CeOx layer. An e-beam deposited CeOx layer is sandwiched by atomic layer deposited SiO2 layers. The top and bottom electrodes are both sputter-deposited TiN. After patterning the top electrode, the capacitors are annealed at 420oC, for 30 min. The characteristics of both MIM capacitors were measured by LCR meter and the charge/discharging transient response and frequency-dependent capacitance were characterized. Results and discussions The time responses of the current after a step voltage of 4 V application (t=30s) and back to 0 V (t=150s) are shown in Fig. 1. When a voltage was applied to the top electrode, exponential decrease in the charging current was observed at both MIM capacitors. The exponential decrease was also observed in the discharging current and back to the steady-state current value at both capacitors. Those transient responses with a long time constant suggests movement of oxygen ions in the CeOx layer. Fig. 1 also showed that larger amount of charging and discharge current can be observed at the MIM capacitor with a 10nm-thickness CeOx layer than the MIM capacitor with a 20nm-thickness CeOx layer, although larger amount of leakage current was observed. Moreover, we calculated time constant from product of discharging current. We observed the short time constant at the MIM capacitor with thinner film. Those results may suggest relation of CeOx layer thickness and ionic conductivity in the CeOx layer and concluded that thinner films are suitable for large capacitance density capacitors. Fig. 2 showed the frequency-dependent capacitance under 0V. Frequency dispersion can be observed at 0.1 to 104 Hz. This behavior suggests the response of oxygen ions the CeOx layer. The obtained capacitance values are larger than the calculated value of 0.14 μF/cm2 by the dielectric constants of CeOx and SiO2. Conclusion We characterized thickness effects on charge and discharge properties of the CeOx MIM capacitors for realization of large capacitance density capacitors. Both MIM capacitors with a difference thickness layer (10nm and 20nm) showed transient responses and the frequency-dependent capacitance of the MIM capacitor with a 10nm-thickness CeOx layer showed frequency dispersion. Those results suggested ionic conductivity in the CeOx layer. Moreover, larger charging/discharge current and shorter time constant were observed at the MIM capacitor with a thinner film. This result suggests relation of CeOx layer thickness and ionic conductivity in the CeOx layer and concluded that thinner films are suitable for large capacitance density capacitors.

Improvement of the stability of an electric double-layer transistor using a 1H,1H,2H,2H-perfluorodecyltriethoxysilane barrier layer

Electric double-layer transistors (EDLTs) are capable of achieving ultra-high carrier density and very low operating voltages. However, chemical reactions cause structural change at the semiconductor–insulator interface resulting in unstable device operation and reduced device lifetime. In this work we propose a method of depositing a 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FDTS) layer which acts as a barrier between the semiconductor layer and the ionic liquid insulator. Our work shows that the effects of adding FDTS film are isolation of the chemical reaction and reduction of damage to the amorphous InGaZnO channel, thus achieving the effect of increasing device lifetime and stability.

Stabilization and heteroepitaxial growth of metastable tetragonal FeS thin films by pulsed laser deposition

Pulsed laser deposition, a non-equilibrium thin-film growth technique, was used to stabilize metastable tetragonal iron sulfide (FeS), the bulk state of which is known as a superconductor with a critical temperature of 4 K. Comprehensive experiments revealed four important factors to stabilize tetragonal FeS epitaxial thin films: (i) an optimum growth temperature of 300 °C followed by thermal quenching, (ii) an optimum growth rate of ∼7 nm min−1, (iii) use of a high-purity bulk target, and (iv) use of a single-crystal substrate with small in-plane lattice mismatch (CaF2). Electrical resistivity measurements indicated that none of the films exhibited superconductivity. Although an electric double-layer transistor structure was fabricated using the tetragonal FeS epitaxial film as a channel layer to achieve high-density carrier doping, no phase transition was observed. Possible reasons for the lack of superconductivity include lattice strain, off-stoichiometry of the film, electrochemical etching by the ionic liquid under gate bias, and surface degradation during device fabrication.

Particulate Generation on Surface of Iron Selenide Films by Air Exposure

Nanometer-sized particular structures are generated on the surfaces of FeSe epitaxial films directly after exposure to air; this phenomenon was studied in the current work because these structures are an obstacle to field-induced superconductivity in electric double-layer transistors using FeSe channel layers. Chemical analyses using field-effect scanning Auger electron spectroscopy revealed no clear difference in the chemical composition between the particular structures and the other flat surface region. This observation limits the possible origins of the particulate formation to light elements in air such as O, C, H, and N.

Fabrication of the electric double layer transistor with (La,Pr,Ca)MnO3 nanowall wire channel

In the scaling down of electronic devices, functional oxides with strongly correlated electron system provide advantages to conventional semiconductors. We report the fabrication of the electric double layer transistor (EDLT) with the (La,Pr,Ca)MnO3 (LPCMO) epitaxial nanowall wire (NW) channel whose width was less than 100 nm. The width controlled LPCMO NWs were fabricated using an original nanofabrication technique: 3D nanotemplate pulsed laser deposition. The LPCMO NW EDLT was produced by employing 80 nm width NWs as channel. The produced LPCMO NW EDLT showed low leakage current. The metal-insulator transition property was successfully modulated by gating effect on the LPCMO NW EDLT.

Understanding Electric Double-Layer Gating Based on Ionic Liquids: from Nanoscale to Macroscale.

In electric double-layer transistors (EDLTs), it is well known that the EDL formed by ionic liquids (ILs) can induce an ultrahigh carrier density at the semiconductor surface, compared to solid dielectric. However, the mechanism of device performance is still not fully understood, especially at a molecular level. Here, we evaluate the gating performance of amorphous indium gallium zinc oxide (a-IGZO) transistor coupled with a series of imidazolium-based ILs, using an approach combining of molecular dynamics simulation and finite element modeling. Results reveal that the EDL with different ion structures could produce inhomogeneous electric fields at the solid-electrolyte interface, and the heterogeneity of electric field-induced charge distributions at semiconductor surface could reduce the electrical conductance of a-IGZO during gating process. Meanwhile, a resistance network analysis was adopted to bridge the nanoscopic data with the macroscopic transfer characteristics of IL-gated transistor, and showed that our theoretical results could well estimate the gating performance of practical devices. Thereby, our findings could provide both new concepts and modeling techniques for IL-gated transistors.

Pulse Dynamics of Electric Double Layer Formation on All-Solid-State Graphene Field-Effect Transistors.

Electric doublelayer (EDL) dynamics in graphene field-effect transistors (FETs) gated with polyethylene oxide (PEO)-based electrolytes are studied by molecular dynamics (MD) simulations from picoseconds to nanoseconds and experimentally from microseconds to milliseconds. Under an applied field of approximately mV/nm, EDL formation on graphene FETs gated with PEO:CsClO4 occurs on the timescale of microseconds at room temperature and strengthens within 1 ms to a sheet carrier density of nS ≈ 1013 cm-2. Stronger EDLs (i.e., larger nS) are induced experimentally by pulsing with applied voltages exceeding the electrochemical window of the electrolyte; electrochemistry is avoided using short pulses of a few milliseconds. Dynamics on picosecond to nanosecond timescales are accessed using MD simulations of PEO:LiClO4 between graphene electrodes with field strengths of hundreds of mV/nm which is 100× larger than experiment. At 100 mV/nm, EDL formation initiates in sub-nanoseconds achieving charge densities up to 6 × 1013 cm-2 within 3 nanoseconds. The modeling shows that under sufficiently high electric fields, EDLs with densities ∼1013 cm-2 can form within a nanosecond, which is a timescale relevant for high-performance electronics such as EDL transistors (EDLTs). Moreover, the combination of experiment and modeling shows that the timescale for EDL formation ( nS = 1013 to 1014 cm-2) can be tuned by 9 orders of magnitude by adjusting the field strength by only 3 orders of magnitude.

Biological Band-Pass Filtering Emulated by Oxide-Based Neuromorphic Transistors

Temporal filtering supports a variety of neural coding and signal processing in our brain, such as sensory adaptation, dynamic input compression, and selective communication. In this letter, neuromorphic devices and circuits based on indium-zinc-oxide (IZO) electric-double-layer (EDL) transistors are proposed for biological filtering emulation. For the first time, biological band-pass filtering is emulated by a simple circuit with only two IZO EDL transistors. The resonant frequency can be modulated by the device and circuit parameters. Our results demonstrate the potential application of oxide-based EDL transistors in neuromorphic systems and brain-like computation.

Low-Voltage, Flexible IGZO Transistors Gated by PSSNa Electrolyte

A carving, cutting, and flip-chip bonding process is proposed for the fabrication of flexible electric double layer transistors (EDLTs) with low cost. Solution processed poly(styrenesulfonic acid sodium salt) is used as a gate dielectric. The large EDL-specific capacitance ( $4.5~\mu \text{F}$ /cm2 at 20 Hz) can induce very high charge carrier density in the InGaZnO (IGZO) channel layer, enabling the EDLTs to operate at a single-battery-drivable low voltage of 1.0 V with a high on-current of 10−4A. The effect of IGZO layer thickness on the performance of EDLTs was investigated. The flexible EDLT with optimized IGZO thickness of 100 nm has achieved a high on/off ratio of $1.4 \times 10^{7}$ , a low threshold voltage of 0.51 V, a saturated field-effect mobility of 1.14 cm2/Vs, and high positive gate bias stress stability. Furthermore, the achieved subthreshold swing, 76 mV/dec, is very close to the theoretical ideal minimum value.

Electric Field Induced Permanent Superconductivity in Layered Metal Nitride Chlorides HfNCl and ZrNCl**Supported by the National Natural Science Foundation of China under Grant No 11704403, the National Key Research Program of China under Grant No 2016YFA0401000 and 2016YFA0300604, and the Strategic Priority Research Program (B) of Chinese Academy of Sciences under Grant No XDB07020100.

Devices of electric double-layer transistors (EDLTs) with ionic liquid have been employed as an effective way to dope carriers over a wide range. However, the induced electronic states can hardly survive in the materials after releasing the gate voltage VG at temperatures higher than the melting point of the selected ionic liquid. Here we show that a permanent superconductivity with transition temperature Tc of 24 and 15 K is realized in single crystals and polycrystalline samples of HfNCl and ZrNCl upon applying proper VG’s at different temperatures. Reversible change between insulating and superconducting states can be obtained by applying positive and negative VG at low temperature such as 220 K, whereas VG’s applied at 250 K induce the irreversible superconducting transition. The upper critical field Hc2 of the superconducting states obtained at different gating temperatures shows similar temperature dependence. We propose a reasonable scenario that partial vacancy of Cl ions could be caused by applying proper VG’s at slightly higher processing temperatures, which consequently results in a permanent electron doping in the system. Such a technique shows great potential to systematically tune the bulk electronic state in the similar two-dimensional systems.