Fast operation of a WO3-based solid-state electrochromic transistor

Electrochromic transistors (ECTs) have attracted attention as advanced memory technology because one can use both electrochromism and switching of electrical conductivity in a nonvolatile manner. Although several solid-state ECTs have been proposed so far, their operating speed is still slow (operating time >1 min) as compared to liquid-based ECTs (∼20 s) due to their asymmetric gate-source electrode configuration. Here we demonstrate a fast operation of a solid-state ECT. We fabricated a solid-state ECT with three terminal gate-source-drain electrodes using an amorphous WO3 film as the electrochromic material and amorphous TaOx as the solid electrolyte. By the insertion of a thin ZnO layer between the source and drain electrodes to achieve pseudo symmetric gate-source electrode configuration, we greatly reduced the operation time to less than 1 s at ±3 V application while keeping the on-to-off ratio of ∼30. The present approach is effective to improve the operating speed of ECTs and may be practically used in advanced memory technologies. © 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5089604


I. INTRODUCTION
Electrochromic transistors (ECTs), which are the combinations of electrochromic displays (ECDs) and three terminal transistors, have attracted attention as advanced memory devices because ECTs have advantages against conventional data storage devices, which can store 1 bit of information as 0 or 1. 1,2 For example, WO 3 -based ECT channel turns from high resistive to low resistive state when the positive gate voltage is applied. At the same time, the color of the ECT channel turns from almost colorless transparent into dark blue. Thus, ECT can store the information of electrical resistance and color simultaneously in a nonvolatile manner. 2,3 Although many EC materials including organic materials are reported thus far, 4-6 WO 3 has been considered as one of the best EC materials to develop ECT because WO 3 has several advantages such as large on/off ratio of the electrical conductivity and the large color contrast ratio. Several researchers have investigated the structural phase transition and metal-insulator transition using a liquid electrolyte and an epitaxial WO 3 film thus far. [7][8][9][10] In 2015, Barquinha et al. demonstrated amorphous WO 3 -based ECT prepared on a paper substrate. 2 Very recently, neuromorphic transistor, 11,12 which imitated synaptic movements, has been developed using WO 3 -ECTs. Previously, most researchers used liquid electrolytes to fabricate WO 3 -based ECT, however, the use of a liquid is not suitable for the practical application due to the leakage problem. To overcome this problem, several researchers used solid electrolytes to fabricate WO 3 -based ECTs, 3,13 which are free from the leakage issues. However, the operating speed of solid-state WO 3 -based ECTs is slow (operating time >1 min 3,13 ) as compared to liquid-based ECTs (∼20 s 2 ), most likely due to that the ionic conductivity of liquid electrolytes is higher than that of solid electrolyte.
In order to overcome this issue, we compared to the device structure of WO 3 -based ECTs and WO 3 -based electrochromic displays (ECDs) because ECDs can be operated within several milliseconds. 6,14 Figure 1 schematically illustrates the structural difference between an ECD and an ECT. Both devices are composed of WO 3 , electrolyte, and NiO, which are sandwiched by two transparent conducting oxides (TCOs). Protonation of WO 3 occurs when the electric field is applied to the multilayered structure composed of the top TCO electrode, NiO, electrolyte, WO 3 and the bottom electrode. Therefore, the multilayers are sandwiched by the two ITO electrodes with fully overlapped parallel plate electrode configuration in the . Since there is a parallel plate electrode configuration, the operating speed of (a) is much faster than that of (b), of which the electrode configuration is asymmetric. Although one can use the resistance change in the case of (b), it cannot be used in the case of (a). In order to overcome this dilemma, we would like to propose the device structure (c), which is three terminal thin film transistor structure with parallel plate electrode configuration. We expected that the device (c) exhibits both fast transistor-like characteristic as well as the electrochromic display characteristics.
case of conventional ECD as shown in Fig. 1(a). On the contrary, in the case of reported three terminal ECT, the overlap area of the gate and source is very small as compared to the whole area of the multilayer as shown in Fig. 1(b). Hence the protonation occurs at the WO 3 layer near the source electrode firstly, and then it gradually proceeds to the drain electrode side. Since both ECTs and ECDs utilize the electrochemical reaction of WO 3 and proton [WO 3 + xH + + xe − ↔ HxWO 3 ], the flowing current in the whole WO 3 film dominates the operation speed. Thus, the operation speed of the reported ECTs with asymmetric electrode configuration is slower than that of ECDs and liquid-based ECTs.
Here we show a fast operation of a solid-state WO 3 -based ECT. We fabricated a three terminal thin film solid-state ECT with a pseudo parallel plate electrode configuration as shown in Fig. 1(c).
We inserted thin oxygen deficient ZnO 15,16 layer, which is known as an n-type wide bandgap (Eg = 3.4 eV) semiconductor, as the bottom TCO between the source and drain electrodes to achieve pseudo symmetric gate-source electrode configuration. As a result, we greatly reduced the operation time to less than 1 s at ±3 V application while keeping the on-to-off ratio of ∼30. The present approach is effective to improve the operating speed of ECTs and may be practically used in advanced memory technologies.

II. EXPERIMENTAL
The ECT devices were fabricated on an alkaline-free glass substrate (Corning EAGLE XG, 10 × 10 × 0.7 mm) by pulsed laser deposition (PLD) with KrF excimer laser (248 nm, 10 Hz) protonated WO 3 (∼70 mS), was deposited on the substrate as the bottom TCO layer. The electrical conductivity, carrier concentration, and Hall mobility of the ZnO layer were ∼50 S cm −1 , ∼5 × 10 −19 cm −3 , and ∼6.6 cm 2 V −1 s −1 , respectively, at room temperature. Then the source and drain electrodes (a-ITO), a-WO 3 active layer (100 nm), a-TaOx electrolyte (250 nm), NiO counter layer (20 nm), and gate electrode (a-ITO) were deposited sequentially using the stencil masks [ Fig. 2(a)]. The active channel size was 800 µm in length and 400 µm in width as schematically shown in Fig. 2(b). The ITO, WO 3 and TaOx films were amorphous, whereas the ZnO and NiO films were polycrystalline [ Fig. 3], which was analyzed by glancing incidence (ω-fixed 2θ scan) X-ray diffraction (Cu Kα 1 , ATX-G, Rigaku Co.). Since we inserted a thin ZnO layer as a thin TCO, the electrical resistance switching ON/OFF ratio should be lower than reported without bottom TCO layer ECT device (ON/OFF ratio ∼10 6 ). The ON/OFF ratio can be controlled using the following equation, which describes the relationship between t WO3 and t TCO : log(ON OFF) = log(t WO3 t TCO ) + log(σ WO3 σ TCO ). Figure 2(c) shows the calculated ON/OFF ratio as a function of the thickness ratio of t WO3 /t TCO . Since we used a 30-nm-thick ZnO (∼50 S cm −1 ), the maximum ON/OFF ratio was calculated be ∼50 (filled red circle). It should be note that there is a trade-off relationship between the ON/OFF ratio and the operating time. In order to increase the operating speed while keeping moderate ON/OFF ratio, we used a 30-nm-thick ZnO film.

III. RESULTS AND DISCUSSION
The present ECT with the TCO layer could be operated very fast. Figure 4 Pauw electrode configuration. The Rs of the ECT without the TCO layer is also shown for comparison (dotted line). When we applied Vg = +3 V, the Rs of the present ECT dropped from ∼3000 Ω sq −1 to ∼100 Ω sq −1 within 1 s. The ON/OFF ratio was ∼30, which is similar to the maximum ON/OFF ratio described above. On the other hand, the ECT w/o TCO showed very slow decay of Rs; the Rs changed slowly taking > 50 s from ∼10 7 Ω sq −1 to ∼90 Ω sq −1 . Thus, the operating speed was ∼20 µm s −1 , which correspond to the time to protonate from the edge of source electrode to the edge of the drain electrode. In case of deprotonation, the present ECT showed fast recovery of Rs within 1 s. It should be noted that the operation of the present ECT device obeys Faraday's laws of electrolysis (Fig. 5). 3,17 These results clearly indicate that the present ECT with TCO layer can operated at a much faster speed as compared to the ECT without the TCO layer. Then, we measured the optical transmission change of the present ECT [ Fig. 4(b)]. The transmission (T) of the deprotonation state was 67 % at the wavelength (λ) of 550 nm and has a high overall transmittance in the visible region. After a gate voltage application of + 3V, the transmittance dramatically decreased to 28 % at λ = 550 nm, and the average transmittance in the visible region was greatly reduced (see the video of the optical transmission spectrum during the device operation at supplementary material). To check the cyclability of the present ECT, we applied alternating Vg = ±3 V at 4 s period (i.e. 2 s at +3 V and 2 s at −3 V) and monitored the sheet resistance (Fig. 6). Cyclability over 90 % was achieved after 500 repetitions, which ensures the good reversibility of the present ECT device. Figure 7 shows a comparison between the present WO 3 -based ECT and the reported WO 3 -based ECTs (solid-state ECTs 3,13,18 and liquid electrolyte ECTs 2,7-10,19,20 ). The operating voltage and the operating time of the present ECT were ±3 V and ∼1 s, respectively. Since there is an inverse proportional relationship between the operating voltage and the operating time, our result shows that the insertion of a thin TCO in solid-state WO 3 -based ECTs is effective to greatly reducing both operating voltage and speed. Although current operating time (∼1 s) is difficult to use as the memory cell of the flash memory (∼µs), the operating time would be enough fast for smart display applications. Thus, the present approach may be practically used in advanced memory technologies.

IV. CONCLUSIONS
In conclusions, we have demonstrated that a solid state WO 3based ECT with TCO bottom layer can be operated faster (∼1 s) than the reported WO 3 -based ECTs (solid-state: ∼1 min, liquid electrolytes: ∼20 s) at low operation voltage (±3 V) keeping the ON/OFF ratio ∼30. Since all fabrication processes were performed at room temperature, the present ECT can be fabricated not only on glass substrates but also on flexible substrates such as plastics, which will have significant economic advantages. We envision that this new EC device structure will move the EC technology forward and be utilized in advanced memory devices.

SUPPLEMENTARY MATERIAL
See supplementary material for the movie showing fast operation of the electrochromic transistor.