Hopping Conduction and Metallic behavior in 2D Silicon Surface States induced by an Ionic Liquid

Abstract

Ionic liquids (ILs) are essentially molten salts with a melting point below room temperature. When used as the gate dielectric of a transistor, carrier densities on the order of $10^{15}\text{ cm}^{-2}$ can be achieved. These record high carrier densities are significantly higher than the maximum carrier density achievable with oxide dielectrics. The physical mechanism for inducing carriers to such a high carrier density is not well understood. Some groups have reported that the induced carriers are a result of electrostatic and electrochemical processes. Other groups have suggested that carriers induced with an IL may be entirely due to electrochemical reactions. Here we report on IL gated Si at carrier densities from $10^{11}\text{ cm}^{-2}$ to $10^{13}\text{ cm}^{-2}$. The experiment was designed to preferentially induce electrostatic carriers over electrochemical reactions. At low carrier densities, sample surface conductivity follows nearest neighbor hopping conduction. This form of conduction has also been observed in experiments where surface conductivity was induced by implanting $\text{Na}^{+}$ near the oxide surface interface. A surprising result of this work was that in some samples a 2D metallic state could be created on the surface of Si. The transition to metallic behavior occurred just below $10^{13}\text{ cm}^{-2}$. High quality Si transistors with oxide dielectric materials observe critical carrier densities around $10^{11}\text{ cm}^{-2}$. The critical carrier density observed in IL gated Si is the highest density reported to date. At carrier densities higher than $10^{13}\text{ cm}^{-2}$ it was observed that the sample conductivity decreased with increasing carrier density. The behavior was unexpected and not fully understood. Both metallic and non metallic samples show a similar reduction in conductivity that is not thought to be due to sample degradation by the IL. The reduction in the sample conductivity at high carrier densities is thought to be due to surface roughness scattering. Similar behavior has been observed in other IL gated experiments on different materials.