Hopping Conduction via Ionic Liquid Induced Silicon Surface States

TL;DR

This paper investigates how ionic liquids induce surface states in lightly doped silicon, leading to hopping conduction via bound holes, and compares the findings to known behaviors in Na+ implanted Si MOSFETs.

Contribution

It demonstrates the formation of surface acceptor states by ionic liquids at low hole concentrations and analyzes the resulting hopping conduction mechanism.

Findings

Surface acceptors bind holes at the Si interface.

Hole localization length agrees with binding energy calculations.

Hopping conductivity resembles that in Na+ implanted Si MOSFETs.

Abstract

In order to clarify the physics of the gating of solids by ionic liquids (ILs) we have gated lightly doped $p$-Si, which is so well studied that it can be called the "hydrogen atom of solid state physics" and can be used as a test bed for ionic liquids. We explore the case where the concentration of induced holes at the Si surface is below $10^{12}\text{cm}^{-2}$, hundreds of times smaller than record values. We find that in this case an excess negative ion binds a hole on the interface between the IL and Si becoming a surface acceptor. We study the surface conductance of holes hopping between such nearest neighbor acceptors. Analyzing the acceptor concentration dependence of this conductivity, we find that the localization length of a hole is in reasonable agreement with our direct variational calculation of its binding energy. The observed hopping conductivity resembles that of well studied $\text{Na}^{+}$ implanted Si MOSFETs.