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Quantitative Analysis of Al X Ga 1-X N Thin Films by XPS

Al x Ga 1-x N thin films prepared by molecular beam epitaxy showed unexpected electrical results and it was thought that the films might have compositions different from those intended. X-ray photoelectron spectroscopy (XPS) was used to determine the Al to Ga compositions of two films. The films had been exposed to air, so they had contaminant layers on them. This paper describes the approach taken to obtain an accurate measure of the Al to Ga compositions, and compares the results with those obtained based on a routine quantitative analysis, and by lightly sputtering the surface to remove contaminants. Most analysts use a very simple approach to obtain quantitative analysis of samples using x-ray photoelectron spectroscopy (XPS). The model assumes that the atomic concentration is uniform across the surface and with depth. When films that have been grown in ultra-high vacuum, for example by molecular beam epitaxy (MBE), are exposed to air, they will be contaminated and in general such a measurement would not be expected to provide a reliable result. Ideally, the compositions of MBE grown films should be determined by transferring them under vacuum to an analysis chamber but this is not always possible. Other approaches for quantitative analysis of contaminated films include making an allowance for the attenuation of the photoelectrons emitted from the underlying film by the contaminant overlayer, or by “lightly” sputtering the surface with inert gas ions to remove the contaminants. Such approaches are not necessarily easy or reliable. In the former approach, the electron attenuation lengths [1] in the contaminant layer need to be known. When sputtering is used to remove contaminants, preferential sputtering [2] could change the surface composition of the film. The electrical properties of Si-doped Al x Ga 1-x N films grown on sapphire by molecular beam epitaxy (MBE) were being investigated as a function of Al mole fraction using temperature dependent Hall-effect measurements by colleagues in another laboratory, and the results were not as expected. The request was made to determine the Al to Ga compositions of two films with XPS. In this work, the different methods for quantitative analysis of the Al to Ga compositions of two MBE grown Al x Ga 1-x N films were compared after having been exposed to air. For the as-received samples, the choice of peaks for quantitative analysis is discussed, and the effects of “light” sputtering on the quantitative analysis are also examined. 2. Experimental conditions