Paper detail

Real-time observation of bond-by-bond interface formation during the oxidation of (111) Si

Atomic-level structure of solids is typically determined by techniques such as X-ray and electron diffraction,1, 2, 3, 4 which are sensitive to atomic positions. It is hardly necessary to mention the impact that these techniques have had on almost every field of science. However, the bonds between atoms are critical for determining the overall structure. The dynamics of these bonds have been difficult to quantify. Here, we combine second-harmonic generation and the bond-charge model of nonlinear optics5, 6 to probe, in real time, the dynamics of bond-by-bond chemical changes during the oxidation of H-terminated (111)Si, a surface that has been well characterized by static methods. We thus demonstrate that our approach provides new information about this exhaustively studied system. For example, oxidation is activated by a surprisingly small applied macroscopic strain, and exhibits anisotropic kinetics with one of the three equivalent back-bonds of on-axis samples reacting differently from the other two. Anisotropic oxidation kinetics also leads to observed transient changes in bond directions. By comparing results for surfaces strained in different directions, we find that in-plane control of surface chemistry is possible. The use of nonlinear optics as a bond-specific characterization tool is readily adaptable for studying structural and chemical dynamics in many other condensed-matter systems.