Evaluating Electric Double Layer Effects at Solid Electrolyte Interfaces with Field-Effect Transistor Utilizing p-Doped SiC (100) Single Crystal
Authors: Shibata Kaoru , Namiki Wataru , Nishioka Daiki , Terabe Kazuya , Tsuchiya Takashi 
Published: December 2024

All-solid-state batteries represent the promising avenue for next-generation battery technology, crucial for achieving carbon neutrality and possessing versatile applications, notably in electric vehicles. Despite growing anticipation regarding their high energy density and safety features, a significant impediment to their widespread adoption lies in the output reduction stemming from elevated interface resistance at the lithium solid electrolyte/electrode interface. Among the suspected factors contributing to this interface resistance is the electric double layer effect, induced by fluctuations in Li+ concentration near the interface. The electric double layer effect entails the accumulation of charged ions from the electrolyte at the electrode interface, generating a layer of positive or negative charge, with the opposing charge distributed onto the electrode, resulting in an overall charge distribution near the interface. The formation of an electric double layer near the electrode interface is well-documented in liquid electrolytes, but its occurrence in solid electrolyte systems, known for their possible mixed ion/electron conductivity, presents a complexity due to the participation of multiple charge carriers. Consequently, discerning the electric double layer effect of solid electrolytes becomes challenging.
Our research team has developed a new method to evaluate the charge of the electric double layer on the surface of a solid electrolyte by Hall measurement, utilizing the mechanism of a field-effect transistor and the characteristics of chemically inert hydrogenated diamond[1,2]. A lithium-based solid electrolyte is used for the dielectric part of the field-effect transistor, and diamond, which does not react with Li+ and does not insert or desert ions, is used for the semiconductor part. With this structure, only the charge generated by the electric double-layer effect of the solid electrolyte could be captured as a change in the electron carrier density on the diamond surface. However, hydrogenated diamond has the disadvantage that the plasma irradiation associated with sputter deposition and heat treatment at high temperatures damage the terminated hydrogen, which limits the types of electrolytes and deposition methods.
In this study, we fabricated a field-effect transistor (FET) using a highly inert SiC (100) single crystal implanted with a p-type dopant, aluminum (Figure 1). We deposit Li3PO4 (1000 nm) by the RF sputtering method at room temperature. We chose the amorphous Li3PO4 because it owns compatibility with relatively high Li+ conductivity and stability. That is the reason why the amorphous Li3PO4 is widely used as a solid electrolyte for solid-state batteries. As shown in Figure 2, the channel resistance change was confirmed by gate voltage sweep. When a positive gate voltage is applied, Li+ in the electrolyte moves towards the SiC channel. Consequently, as the holes depart from the electrolyte interface, the carrier density decreases and the channel resistance increases. In addition, hall measurements of carrier density and mobility using the SiC single crystal with doping density at 5×1018 cm-3 resulted in a hole density of 5.88×1015 cm-3 and mobility of 109 cm2/Vs at room temperature. The mobility for dopant concentration was close to the literature value of 100 cm2/Vs[3]. Regarding the hole density, the measured value was far lower than the doping density. It is reasonable by considering the relatively high activation energy of the Al acceptor in SiC (about 150-190 meV), which is considerably larger than the thermal activation energy of 25 meV. This research made it possible to evaluate the charge of the electric double layer on the solid electrolyte surface by Hall measurement for various electrolytes and deposition methods.
In Summary, we fabricated FET using p-doped SiC as the channel material and conducted measurements of channel resistance change, carrier mobility, and carrier density through Hall measurements. This research enables a comprehensive assessment of the electric double-layer effect at the electrode/solid electrolyte interface, an aspect that has not been thoroughly elucidated previously. The gate voltage dependence of channel carrier density and mobility, electrolyte dependence, and the results of Hall measurements using various electrolytes, as well as methods and principles for improving the on/off ratio of FETs will be presented on the day. This work was partly supported by GteX Program Japan Grant Number JPMJGX23S2 and JST PRESTO Grant number JPMJPR23H4.
[1] Tsuchiya, T. et al. The electric double layer effect and its strong suppression at Li+ solid electrolyte/hydrogenated diamond interfaces. Commun. Chem. 4, 117 (2021).[2] Takayanagi, M. et al. Accelerated/decelerated dynamics of the electric double layer at hydrogen-terminated diamond/Li+ solid electrolyte interface. Mater. Today Phys. 31, 101006 (2023).[3] Stefanakis, D. et al. TCAD models of the temperature and doping dependence of the bandgap and low ﬁeld carrier mobility in 4H-SiC. Microel. Eng.
116,(2014).

Figure 1