Surface sensitivity of Auger-electron spectroscopy and X-ray photoelectronspectroscopy

A convenient measure of surface sensitivity in Auger-electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) is the mean escape depth ( MED). If the effects of elastic-electron scattering are neglected, the MED is equal to the electron inelastic mean foe path (IMFP) multiplied by the c osine of the emission angle with respect to the surface normal, and depends on the material and electron energy of interest. An overview is given hen of recent calculations of IMFPs for 50-2000 eV electrons in a range of mate rials. This work has led to the development of a predictive formula based o n the Bethe equation for inelastic electron scattering in matter from which IMFPs can be determined. Estimates show, however, that elastic-electron sc attering can significantly modify the: MED. Thus, for AES, the MED will be reduced by up to about 35%. For XPS, however, the MED can be changed by up to +/- 30% for common measurement conditions although it can be much larger (by up to a factor of 2) for near-grazing emission angles. Ratios of MED v alues, calculated with elastic scattering considered and neglected for XPS from the 3s, 3p, and 3d subshells of silver with Mg K alpha X-rays are appr oximately constant (to about 10%) over a range of emission angles that vari es from 40 degrees to 60 degrees depending on the subshell and the angle of X-ray incidence. Recommendations are given on how to determine the optimum range of emission angles for satisfactory analysis of angle-resolved XPS ( ARXPS) data. Definitions are included of three terms often used for describ ing surface sensitivity (IMFP, MED, and effective attenuation length (EAL)) , and examples are given of the varying magnitudes of these quantities for different analytical conditions. (C) 1999 Elsevier Science B.V. All rights reserved.