Information depth and the mean escape depth in Auger electron spectroscopy and x-ray photoelectron spectroscopy

Abstract

The information depth (ID) is a measure of the sampling depth for the detected signal in Auger-electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) while the mean escape depth (MED) is a measure of surface sensitivity. We report ID and MED calculations for Si 2s, Si 2p3/2, Cu 2s, Cu 2p3/2, Au 4s, and Au 4f7/2 photoelectrons excited by Mg Kα x rays. These calculations were made for various electron emission angles and for a common XPS configuration. Similar calculations were made for Si L3VV, Si KL23L23, Cu M3VV, Cu L3VV, Au N7VV, and Au M5N67N67 Auger transitions. The IDs and MEDs were derived from an analytical expression for the signal-electron depth distribution function obtained from a solution of the kinetic Boltzmann equation within the transport approximation. The ratios of the IDs and the MEDs to the corresponding values found if elastic-electron scattering were assumed to be negligible, RID and RMED, were less than unity and varied slowly with electron emission angle α for emission angles less than 50°. For larger emission angles, these ratios increased rapidly with α. For α⩽50°, average values of RID and RMED varied linearly with the single-scattering albedo, ω, a simple function of the electron inelastic mean-free path and transport mean-free path. For α=70° and α=80°, RID also varied linearly with ω but RMED showed a quadratic variation. The albedo is thus a useful measure of the magnitude of elastic-scattering effects on the ratios RID and RMED. As a result of the elastic scattering of the signal electrons, AES and XPS measurements at α=80° are less surface sensitive than would be expected if elastic scattering had been neglected. Conversely, AES and XPS measurements made for α⩽50° are more surface sensitive as a result of elastic-scattering effects.