Spectroscopic investigation of buried interfaces and liquids with soft X-rays

When, in general, two entities interact, they do it by forming an interface. The properties of such interfaces are determined not only by the properties of the two interface partners, but also to a large degree by the peculiarities of the interface-formation process itself. This is of particular importance in solid-state devices composed of two or more different materials. Unfortunately, the investigation of such interfaces is very difficult for two reasons. First they are, by their nature, buried. Secondly, interfaces generally form a thin layer within a larger ensemble and thus give very weak signals. Nevertheless, a few experimental techniques are available to study such buried interfaces. This report demonstrates that a combination of soft-X-ray spectroscopies (X-ray emission, photoemission, and X-ray absorption) is extremely well suited for this task. As examples, the electronic and chemical properties of several material systems are discussed, including II-VI semiconductors, Cu(In,Ga)(S,Se)(2) thin-film solar cells, organic thin films, and liquids.