Combined AFM and XPS analysis of complex surfaces

XPS analysis is usually limited to surfaces whose roughness is less than the inelastic mean free path of the photoelectron in the material. Furthermore, the stoichiometry of the surface to be analysed is assumed to be constant. Clearly, in many situations neither of these criteria are met. However, a model has been developed, which combines AFM and XPS techniques to allow the analysis of rougher surfaces and complex non-uniform structures. In this work, InP surfaces were processed by an RF (13.56 MHz) hydrogen plasma over a range of pressures and power densities. The higher reactivity of hydrogen with phosphorous results in preferential sputtering and therefore depletion of this element from the surface. The residual surface layer was found to consist of indium and indium oxide, with the depth of the oxide layer in the earlier stages of processing being proportional to the depth of the phosphorus-deplete region. Since the volume of the oxide is greater than the base metal, the indium oxide layer buckles as it grows, leaving a series of ‘bumps’ with cavities beneath them. This effect is exacerbated by the presence of volatile phosphorous compounds in the cavities. This complex surface was analysed by XPS and AFM. The initially spurious XPS data were re-analysed on the basis of a model developed from the AFM measurements. Together, this combined approach allowed the thickness of the indium surface oxide layer to be accurately measured using both techniques. This approach widens the application of XPS analysis to complex surfaces.