Switching Response of Hydrogen-Terminated-Diamond-Based All-Solid-State Electric-Double-Layer Transistor
Authors: Takayanagi Makoto , Tsuchiya Takashi , Imura Masataka , Koide Yasuo , Higuchi Tohru , Terabe Kazuya 
Published: December 2020

INTRODUCTION

High-density electronic carrier control using an electric double layer (EDL) at electrolyte/electrode interfaces has been attracting attention for the application in various electronic devices. Specifically, the use of solid electrolyte is advantageous for practical use owing to its high compatibility with other peripheral devices.
Recently, we have achieved an in situ modulation of hole density in hydrogen-terminated diamond (H-terminated diamond) using an all-solid-state electric double layer transistor (EDLT) [1]. The hole density increased from 1010 cm-2 to 1013 cm-2, a 3-order-of-magnitude difference, applying a negative gate voltage (V
G). This indicates that the two-dimensional hole gas was electrostatically modulated by EDL charging at the solid electrolyte/H-terminated diamond interface. Then, we focus on a switching response speed, which is an important parameter for EDLTs. Towards further development of all-solid-state EDLTs for practical use, high switching response is desired. Because diamond is suitable for high-frequency electronic device owing to its high carrier mobility, diamond-based all-solid-state EDLT is a candidate to exceed the response speed of conventional all-solid-state EDLTs

EXPERIMENTAL

An H-terminated diamond thin film was homoepitaxially grown on the surface of an Ib-type HPHT diamond single crystal (100) using microwave plasma chemical vapor deposition. The channel area was fabricated by photolithography, and O2 plasma ashing. Source and drain electrodes were made of Pt/Pd. Gate electrode was made of LiCoO2 (LCO) and Pt films. We chose a Li+ conducting Li2O-ZrO2-SiO2 (LZSO) amorphous thin film as a solid-state-electrolyte. To compare response speed, we also fabricated EDLT with the same structure except for the gate electrode. The gate electrode of another EDLT was made of Au. Electrochemical measurements were performed using a Keithley 4200-SCS parameter analyzer and an atmosphere/temperature tunable prober system (Nagase).

RESULTS AND DISCUSSION

Figure 1 shows the switching characteristics of EDLTs measured at the drain voltage of 0.5 V. The measurements were performed in steps of 2 K. When a pulse (V
G) is applied, drain current (i
D) instantaneously decreases from the 10 microampere order to less than 10 nanoampere order. Thus, EDLTs are switched from an on-state to off-state. As explained in the introduction, this is attributed to the effect of EDL charging at the LZSO solid electrolyte/H-terminated diamond interface. In both EDLTs, the response time is shorter than 50 ms at 298 K. Because the speed is not affected even when the type of the gate electrode was changed, the effect of EDL charge and discharge at the LZSO/LCO interface can be ignored. Therefore, this response speed mainly resulted from EDL charge and discharge processes at the LZSO/H-terminated diamond interface. The response speed becomes faster with an increase in the temperature. This is attributed to an increase in the Li+ conductivity of LZSO. The conductivity of LZSO exhibited a thermally activated behavior with an activation energy of 0.66 eV; the conductivity of LZSO increased from 8×10-9 S/cm at 298 K to 2×10-7 S/cm at 340 K. Although the conductivity of the H-terminated diamond channel changed with an increase in the temperature, the variation was about 25%, which is sufficiently small compared to the variation in that of LZSO. Therefore, the rate of the response speed in EDLTs is determined by the conductivity of LZSO. At 340 K (Li+ conductivity: 2×10-7 S/cm), EDLTs show the response time of less than 800 μs. Thus, H-terminated diamond-based all-solid-state EDLT has a potential for ultrafast switching even at room temperature if an electrolyte with better conductivity is used. Since RbAg4I5 Ag+ superionic conductor (0.12 S/cm at RT [2]) was previously reported as an electrolyte for EDL capacitors [3], RbAg4I5 is a good candidate for ultrafast EDLTs, of which the response speed is expected to exceed 1 GHz.
In the presentation, we will show the switching characteristics for H-terminated diamond-based all-solid-state EDLTs using various solid electrolytes.

Acknowledgements

This work was in part supported by JSPS KAKENHI Grant Number JP20H05816 (Grant-in-Aid for Scientific Research on Innovative Areas “Interface Ionics”), JP19K05279 and JP19J22244 (Grant-in-Aid for JSPS Fellows).

REFERENCES

[1] T. Tsuchiya, M. Takayanagi, M. Imura, Y. koide, T. Higuchi, and K. Terabe, Abstr. 22nd Intl. Conf. on Solid State Ionics, 2019, P-TUE-108.
[2] J.N. Bradley and P.D. Greene, Trans. Faraday Soc., 63 (1967) 424.
[3] J. E. Oxley, Proc. Power Sources Symp., 24, 20 (1970)
Fig. 1. Switching characteristics of H-terminated diamond-based all-solid-state EDLTs in the temperature range of 298-340 K. The gate electrodes are (a) Pt/LCO and (b) Au.

Figure 1