An XPS and ellipsometry study of Cr–O–Al mixed oxides grown by reactive magnetron sputtering

Cr–O–Al thin film mixed oxides grown on Si (100) substrates by reactive magnetron sputtering using different target compositions from 90% Cr (10% Al) to 10% Cr (90% Al) and oxygen fluxes in the range from 0 to 15 sccm have been investigated using ex situ XPS, XPS depth profiles and ARXPS. The chemical information obtained with XPS as well as the observed chemical shift of the Cr 2p, Al 2s and O 1s bands points to the formation of mixed substitutional Me2O3 oxides (Me = Al + Cr) instead of the formation of single oxide phases. Compositions and stoichiometries obtained from concentration depth profile measurements (CDP) simultaneously using XPS and Ar+ bombardment confirm the formation of such a type of substitutional mixed oxides. ARXPS allows ruling out oxygen preferential sputtering during Ar+ bombardment. Finally, it is shown that the optical properties of the films like their refractive index can be controlled through their chemical composition.