Physics of sample charging during x-ray photoelectron spectroscopy: Insights from experiments with thin film insulators

Abstract

Sample charging during x-ray photoelectron spectroscopy measurements of poorly conducting samples is a widely recognized concern that seriously complicates the analysis of chemical bonding. The high complexity owing to many instrument- and sample-determined variables involved in the process is likely responsible for the fact that no comprehensive theory of charging exists. The present study aims to describe the development of charging for the case of thin insulating films supported on conducting substrates. Such systems are particularly well suited for studies of charging phenomena as they provide a unique opportunity to separate the effects that operate on different length scales and allow one to investigate the role of charge supplied from the bottom contact. Two inherently insulating oxides, SiO2 and WO3, with the thickness varying by more than three orders of magnitude (from 1 to 5000 nm) are chosen to serve as model systems for insulators with, respectively, low and high x-ray-induced conductivity. The key role of low-energy secondary electrons (SEs), x-ray attenuation length, sample work function, and the insulator SE yield in the development of surface charging is demonstrated. Based on these findings, a conceptual model is presented to serve as a starting point for the interpretation and discussion of charging phenomena in specific cases.