Correlation between hydroxyl fraction and O/Al atomic ratio as determined from XPS spectra of aluminium oxide layers

A set of five different aluminium oxide layers has been investigated using XPS. The oxide layers were made by oxidizing aluminium in a vacuum, with an alkaline and acidic pretreatment and in boiling water. Hydroxyl fractions of the aluminium oxide layers ranging from 0.0 to 0.5 were determined by constrained curve-fitting of the O 1s peak. The O/Al atomic ratios of the aluminium oxide layers, ranging from 1.5 to 2.0, were determined from the O 1s and Al 2p photoelectron intensities. A method is presented to account for the attenuation of the photoelectron intensities by the contamination overlayer. For the studied oxide layers, a linear relation is observed between the hydroxyl fraction and the O/Al atomic ratio of the aluminium oxide layers. It is concluded that the results obtained by the curve-fitting procedure are reliable. Furthermore, a linear relation is observed between the hydroxyl fraction and the O is peak width. The O 1s binding energies of the O2- and OH- components of the oxide layers correspond to 531.0 +/- 0.1 eV and 532.4 +/- 0.1 eV, respectively. Only pseudoboehmite showed 0.5 eV lower binding energies for these components. Angle-resolved XPS analysis showed that most of the studied oxides are enriched in hydroxyl groups at their outermost surface. Copyright (C) 2004 John Wiley Sons, Ltd.