Surface roughness scattering in multisubband accumulation layers

TL;DR

This paper develops a theory for surface roughness scattering in multisubband accumulation layers at high electron concentrations, showing that the 2DEG remains metallic without a metal-insulator transition, contrary to previous conjectures.

Contribution

The authors extend surface roughness scattering theory to multisubband high-density layers, revealing new behavior of conductivity saturation and metallicity.

Findings

Surface conductivity decreases as n^{-6/5} with increasing electron density.

Conductivity saturates at high densities, remaining much larger than the quantum of conductance.

No re-entrant metal-insulator transition occurs at high concentrations.

Abstract

Accumulation layers with very large concentrations of electrons where many subbands are filled became recently available due to ionic liquid and other new methods of gating. The low temperature mobility in such layers is limited by the surface roughness scattering. However theories of roughness scattering so far dealt only with the small-density single subband two-dimensional electron gas (2DEG). Here we develop a theory of roughness-scattering limited mobility for the multisubband large concentration case. We show that with growing 2D electron concentration $n$ the surface dimensionless conductivity $\sigma/(2e^2/h)$ first decreases as $\propto n^{-6/5}$ and then saturates as $\sim(da_B/\Delta^2)\gg 1$, where $d$ and $\Delta$ are the characteristic length and height of the surface roughness, $a_B$ is the effective Bohr radius. This means that in spite of the shrinkage of the 2DEG width and the related increase of the scattering rate, the 2DEG remains a good metal. Thus, there is no re-entrant metal-insulator transition at high concentrations conjectured by Das Sarma and Hwang [PRB 89, 121413 (2014)].