Impact of Large Gate Voltages and Ultrathin Polymer Electrolytes on Carrier Density in Electric-Double-Layer-Gated Two-Dimensional Crystal Transistors

Shubham Sukumar Awate; Brendan Mostek; Shalini Kumari; Chengye Dong; Joshua A Robinson; Ke Xu; Susan K Fullerton-Shirey

doi:10.1021/acsami.2c13140

Impact of Large Gate Voltages and Ultrathin Polymer Electrolytes on Carrier Density in Electric-Double-Layer-Gated Two-Dimensional Crystal Transistors

ACS Appl Mater Interfaces. 2023 Mar 29;15(12):15785-15796. doi: 10.1021/acsami.2c13140. Epub 2023 Mar 16.

Authors

Shubham Sukumar Awate¹, Brendan Mostek¹, Shalini Kumari^{2

3}, Chengye Dong⁴, Joshua A Robinson^{2

3

4}, Ke Xu^{1

5

6

7}, Susan K Fullerton-Shirey^{1

8}

Affiliations

¹ Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States.
² Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.
³ Center for 2D and Layered Materials and Center for Atomically Thin Multifunctional Materials, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.
⁴ Two-Dimensional Crystal Consortium, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.
⁵ School of Physics and Astronomy, Rochester Institute of Technology, Rochester, New York 14623, United States.
⁶ Microsystems Engineering, Rochester Institute of Technology, Rochester, New York 14623, United States.
⁷ School of Chemistry and Materials Science, Rochester Institute of Technology, Rochester, New York 14623, United States.
⁸ Department of Electrical and Computer Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States.

Abstract

Electric-double-layer (EDL) gating can induce large capacitance densities (∼1-10 μF cm^-2) in two-dimensional (2D) semiconductors; however, several properties of the electrolyte limit performance. One property is the electrochemical activity which limits the gate voltage (V_G) that can be applied and therefore the maximum extent to which carriers can be modulated. A second property is electrolyte thickness, which sets the response speed of the EDL gate and therefore the time scale over which the channel can be doped. Typical thicknesses are on the order of micrometers, but thinner electrolytes (nanometers) are needed for very-large-scale-integration (VLSI) in terms of both physical thickness and the speed that accompanies scaling. In this study, finite element modeling of an EDL-gated field-effect transistor (FET) is used to self-consistently couple ion transport in the electrolyte to carrier transport in the semiconductor, in which density of states, and therefore quantum capacitance, is included. The model reveals that 50 to 65% of the applied potential drops across the semiconductor, leaving 35 to 50% to drop across the two EDLs. Accounting for the potential drop in the channel suggests that higher carrier densities can be achieved at larger applied V_G without concern for inducing electrochemical reactions. This insight is tested experimentally via Hall measurements of graphene FETs for which V_G is extended from ±3 to ±6 V. Doubling the gate voltage increases the sheet carrier density by an additional 2.3 × 10¹³ cm^-2 for electrons and 1.4 × 10¹³ cm^-2 for holes without inducing electrochemistry. To address the need for thickness scaling, the thickness of the solid polymer electrolyte, poly(ethylene oxide) (PEO):CsClO₄, is decreased from 1 μm to 10 nm and used to EDL gate graphene FETs. Sheet carrier density measurements on graphene Hall bars prove that the carrier densities remain constant throughout the measured thickness range (10 nm-1 μm). The results indicate promise for overcoming the physical and electrical limitations to VLSI while taking advantage of the ultrahigh carrier densities induced by EDL gating.

Keywords: Nernst−Planck−Poisson; drift diffusion; electric double layer; finite element modeling; polymer electrolyte; quantum capacitance; thin electrolyte; two-dimensional materials.