Electron surface layer at the interface of a plasma and a dielectric wall

TL;DR

This paper models the electron surface layer at the plasma-dielectric interface, analyzing how electron affinity influences charge distribution and potential, with specific calculations for materials like MgO, SiO2, and sapphire.

Contribution

It introduces a quasi-stationary electron surface layer model that accounts for image potential and conduction band offset at dielectric interfaces in plasma environments.

Findings

Negative electron affinity leads to surface charge formation outside the dielectric.

Positive electron affinity results in electron penetration and space charge layers inside the dielectric.

The model predicts different behaviors based on the surface's electron affinity.

Abstract

We study the potential and the charge distribution across the interface of a plasma and a dielectric wall. For this purpose, the charge bound to the wall is modelled as a quasi-stationary electron surface layer which satisfies Poisson's equation and minimizes the grand canonical potential of the wall-thermalized excess electrons constituting the wall charge. Based on an effective model for a graded interface taking into account the image potential and the offset of the conduction band to the potential just outside the dielectric, we specifically calculate the potential and the electron distribution for magnesium oxide, silicon dioxide and sapphire surfaces in contact with a helium discharge. Depending on the electron affinity of the surface, we find two vastly different behaviors. For negative electron affinity, electrons do not penetrate into the wall and an external surface charge is formed in the image potential, while for positive electron affinity, electrons penetrate into the wall and a space charge layer develops in the interior of the dielectric. We also investigate how the electron surface layer merges with the bulk of the dielectric.