X‐Ray Photoelectron Spectroscopy in Analysis of Surfaces

X-ray photoelectron spectroscopy (XPS) is an analytical technique that uses photoelectrons excited by X-ray radiation (usually Mg Kα or Al Kα) for the characterization of surfaces to a depth of 2–5 nm. Elemental identification and information on chemical bonding are derived from the measured electron energy and energy shifts, respectively. The use of ultrahigh vacuum (UHV) during analysis requires special sample handling. Depth profiling is possible using ion sputtering. In contrast to the most popular surface analytical technique, Auger electron spectroscopy (AES), nonconducting material can be investigated, weak material damage occurs, and the chemical shifts are easier to interpret. However, the lateral resolution of the complementary AES technique (typically down to 20 nm) is much better than for XPS (50 µm for standard equipment, down to lower than 3 µm for dedicated instruments). The detection limit (of about 0.1 at%) is not as low as for mass spectroscopic techniques, but the quantification from XPS peak area measurements is more reliable, even disclaiming standard sample measurements. This article briefly discusses the principles of the technique, sample requirements, and the typical measuring strategy. Possible information sources (elements, chemical bonding, depth and lateral distributions) are described and their quantification principles are summarized. The main part deals with typical applications relating to several material classes (metals, semiconductors, insulators, and polymers) to give an impression of the capabilities of the method over a wide range of research and technological problems. A comparison with further complementary analytical techniques is given as a summary. Technological developments in electronics, nanotechnology, polymers, biotechnology, and medicine are all concerned with surface-related phenomena, suggesting sustained interest in XPS in the foreseeable future.