Field-Controlled Ion Doping of Graphene

Doping technologies are needed for the development of tunnel field-effect transistors based on graphene and other single-layer two-dimensional materials. Here we utilize Li⁺ and ClO₄⁻ ions in polyethylene oxide (PEO) to enable dynamic and reconfigurable doping of graphene under electric field control.  Ions are drifted across the PEO to the graphene where they induce electron or hole conductivity. Use of PEO:LiClO₄ for ion-doping of carbon nanotubes [1], graphene [2, 3] and MoS₂[4] have all incorporated the ion conductor and field plate above the transistor channel. This requires a dry process because the developers used in photolithography attack and contaminate PEO.  This further restricts the field plate formation process to shadow evaporation or to macroscopic contacts formed by probes.  Here we report on a new dry transfer process which allows the transistor to be placed on top of the PEO and the doping to be field-controlled by the substrate back gate. In this paper we show the transfer process and characteristics of the ion motion and conductivity modulation under field-control.   

The dry transfer process is illustrated in Fig. 1. To create the top portion of the device, source and drain contacts are patterned onto wide area graphene formed by chemical vapor deposition. Polyisobutylene (PIB) and a PDMS stamp are then applied as handling layers (Fig. 1b).  The Cu substrate is then etched in ammonium persulfate (APS) and the top portion is joined to the bottom (Fig. 1e).  We have fabricated devices with both Au and graphene backgates, and initial tests were performed with an Au backgate, patterned into stripes to minimize leakage current through the electrolyte (Fig. 2). 

Doping in the graphene channel is controlled by the backgate voltage (V_bg). The conductivity, monitored by the drain current, (I_d), is varied by a factor of four by sweeping V_bgat 0.5 V/s from −10 to 10 V (Fig. 3). The shift of the current minima (Dirac points) from zero are a direct measure of the ion doping of the graphene channel.

To characterize the retention of electrostatic doping, a 5 V backgate pulse is applied for 1 second and removed for 10 seconds while monitoring the drain current (Fig. 4). The current decreases by ~10% as the ions are polarized and then released.

Acknowledgements:  This work was supported in part by the Center for Low Energy Systems Technology (LEAST), one of the six SRC STARnet Centers, sponsored by MARCO and DARPA.

[1]     C. Lu et al. Nano Lett. 4, pp. 623–627 (2004)

[2]     A. Das et al., Nat. Nanotech, 3, pp. 201-215 (2008)

[3]     D.  Efetov and P. Kim, Phys. Rev. Lett., 105, pp. 256805 (2010)

[4]     M. Lin, et al., J. Phys. D, 45, 345102 (2012).