Real-time observation of bond-by-bond interface formation during oxidation of H-terminated (111)Si by second-harmonic generation

Structure of solids is typically determined at the atomic level by techniques such as X-ray and electron diffraction, which are sensitive to positions of atomic nuclei. However, structure is determined by bonds between atoms, which are difficult to measure directly. We combine second-harmonic generation and the bond-charge model of nonlinear optics 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. Oxidation is activated by 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. This also leads to transient changes in bond directions.~Strain is known to increase oxidation rate of Si for thermal oxides, however its affects at the microscopic scale has not been studied at the bond level. By comparing results for surfaces strained in different directions, we show 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.