Differential Reflectance Spectroscopy in Analysis of Surfaces

Differential reflectance spectroscopy (DRS) is a surface analytical technique. It uses ultraviolet (UV), visible, or infrared (IR) light as a probing medium. The interaction of light with “strongly absorbing materials”, such as metals, alloys, semiconductors, etc. occurs in the first 10–20 nm. Thus the differential reflectometer probes 50–100 atomic layers into nontransparent solid surfaces. Because of the specific probing depth of light, DRS fills the gap between other surface techniques such as ion-scattering, Auger spectroscopy, and ESCA (electron spectroscopy for chemical analysis), which probe 1, 5, or even 20 monolayers, and X-ray diffraction (XRD) which probes as deep as 1–50 µm into a bulk material. The information gained by DRS is somewhat different from that obtained by the other surface techniques mentioned. A “differential reflectogram” reveals details about the electron structure around the Fermi surface. Specifically, the instrument allows the exact measurement of the energies which electrons absorb from photons as they are raised into a higher, allowed energy states. Since each material has a specific electron-band structure the measurement of the characteristic energies for electron “interband transitions” serves as a means for identifying these materials. Furthermore, investigation of any shift of these characteristic energies, which may be caused by the addition of solute elements to a solvent, by transformations, lattice defects, ordering, or the like, provides a deeper insight into the nature of the solid state. The application of DRS is, of course, not restricted to strongly absorbing materials such as metals, alloys, or semiconductors. Its strength has likewise been demonstrated in the identification and characterization of transparent or semitransparent surface layers as observed in thin-film corrosion products on metal substrates. DRS is used to scan two samples whose properties differ slightly. Thus, the main advantage of DRS over conventional optical techniques lies in its ability to eliminate any undesirable influences of oxides, windows, electrolytes, instrumental variations, and the like, owing to the differential nature of the technique. No vacuum is needed unless measurements in the vacuum UV are desired. Thus, the formation of a surface layer due to environmental interactions can be studied in situ. The measurements are fast: a complete differential reflectogram, that is, an automatic scan from the UV through the visible into the IR region, is accomplished in about 3 min.