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

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 Si2s. Si2p(3/2), Cu2s, Cu2p(3/2). Au4s. and Au4f(7/2) photoelectrons excited by Mg Kalpha 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 KL23L13, Cu M3VV, Cu L3VV, Au N7VV, and An 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, R-ID and R-MED were less than unity and varied slowly with electron emission angle a for emission angles less than 50degrees. For larger emission angles, these ratios increased rapidly with alpha. For alphaless than or equal to50degrees, average values of R-ID and R-MED varied linearly with the single-scattering albedo, omega, a simple function of the electron inelastic mean-free path and transport mean-free path. For alpha = 70degrees and alpha = 80degrees, R-ID also varied linearly with omega but R-MED showed a quadratic variation. The albedo is thus a useful measure of the magnitude of ciastic-scattering effects on the ratios R-ID and R-MED, As a result of the elastic scattering of the signal electrons, AES and XPS measurements at alpha = 80degrees are less surface sensitive than would be expected if elastic scattering had been neglected. Conversely, AES and XPS measurements made for alphaless than or equal to50degrees are more surface sensitive as a result of elastic-scattering effects. (C) 2003 American Vacuum Society.