Physical insight into reduced surface roughness scattering in strained silicon inversion layers

TL;DR

This paper explains the increased electron mobility in strained silicon inversion layers by revealing how strain reduces surface roughness scattering through changes in electron wavefunctions, moving beyond effective mass models.

Contribution

It introduces a physical explanation for mobility enhancement by analyzing wavefunction behavior and surface interactions, surpassing traditional effective mass approximations.

Findings

Strain reduces  4 state occupancy, decreasing surface roughness scattering.

Bulging of  4 wavefunction at the surface increases scattering susceptibility.

Moving beyond effective mass approximation clarifies the physical mechanisms involved.

Abstract

A seemingly anomalous enhancement of electron mobility in strained silicon inversion layers at high sheet densities has exposed a conspicuous gap between device physics theory and experiment in recent years. We show that the root of this discrepancy is due to a bulging effect in the electron \Delta 4 wavefunction at the silicon surface. This renders \Delta 4 electrons more susceptible to perturbations in surface structure thereby increasing surface roughness scattering for these states. Strain engineering utilized by the CMOS industry reduces the relative occupancy of the \Delta 4 states resulting in less overall surface roughness scattering in the channel. We show that the origin of this effect can be explained by moving beyond the effective mass approximation and contrasting the properties of the \Delta 2 and \Delta 4 wavefunctions in a representation that comprehends full crystal and Bloch state symmetry.