Vibrational Characterization of the Oxidation Products on $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$

The oxidation products on $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ are investigated at 82 K by means of high-resolution electron energy loss spectroscopy. The isotope-labeled vibrational spectra with ${}^{16}\mathrm{O}_2$, ${}^{18}\mathrm{O}_2$, and ${}^{16}\mathrm{O}{}^{18}\mathrm{O}$ show that, in the initial stage of the oxidation, an ${\mathrm{O}}_2$ molecule dissociates to form a metastable product with an O atom bonding on top of the Si adatom and the other inserted into the backbond. The metastable product is observed as a dark site in the topographic scanning tunneling microscopy (STM) image and can be transformed to a stable product by the STM manipulation. Our results are in good agreement with recent theoretical calculations.

Figure 1

EELS spectra of $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ at 82 K as a function of the exposure to ${}^{16}\mathrm{O}_2$. The exposures are (a) 0, (b) 0.008, (c) 0.015, (d) 0.03, (e) 0.07, (f) 0.1, (g) 0.2, and (h) 0.3 L. The O-derived losses are marked, and the higher three losses are labeled by ${\nu }_1$, ${\nu }_2$, and ${\nu }_3$. The spectrum (c’) is for the 0.015 L exposure followed by annealing at 200 K. ${\nu }_1$ disappears upon the annealing, indicating the metastability of the initial product. The loss intensities are normalized to the elastic peak intensities.

Figure 2

Isotope-labeled EELS spectra with (a) ${}^{16}\mathrm{O}_2$, (b) ${}^{16}\mathrm{O}{}^{18}\mathrm{O}$, and (c) ${}^{18}\mathrm{O}_2$ for ${\nu }_1$, ${\nu }_2$ (inset), and ${\nu }_3$ (inset). The oxygen exposures for ${\nu }_1$, ${\nu }_2$, and ${\nu }_3$ are 0.02, 0.2, and 0.3 L, respectively. For ${\nu }_1$ and ${\nu }_2$, the spectra of ${}^{16}\mathrm{O}{}^{18}\mathrm{O}$ show the doublet structures, which indicates that these modes are due to the Si-O stretch vibration. On the other hand, ${\nu }_3$ of ${}^{16}\mathrm{O}{}^{18}\mathrm{O}$ shows a peak at the intermediate energy of ${}^{16}\mathrm{O}_2$ and ${}^{18}\mathrm{O}_2$, indicating that this mode is assigned to the molecular O-O stretch.

Figure 3

Schematic models of the initial oxidation products on $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$. The Si and O atoms are depicted by the open and closed circles, respectively. The ${\mathrm{O}}_2$ molecules dissociate on the surface and forms the metastable ad-ins structure (left) at 82 K. Upon thermal activation at 200 K, the ad-ins species is transformed to the ins-ins structure (right). The observed vibrational energies are shown with the atomic motions depicted by the arrows.

Figure 4

(a) Topographic STM image of the initial oxidation products on $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ at 78 K. The image is taken at 0.7 V sample bias and 0.2 nA tunneling current. The reacted sites (ad-ins species) appear darker than the unreacted Si adatoms. (b)–(d) A series of STM images which demonstrate the conversion of the dark sites (ad-ins species) to the stable bright sites (ins-ins species). After imaging the dark sites at 0.7 V and 0.2 nA (b), the area is scanned at 1.2 V and 0.2 nA. Then the area is recorded at 0.7 V and 0.2 nA, which is shown in (c). The scanning at 1.2 V induces the reaction of the dark sites. The same area is recorded at 1.8 V and 0.2 nA (d), where it is found that the dark sites observed in (b) convert to the bright sites (ins-ins species) by the 1.2 V scan. The ins-ins products appear bright only when imaged at the sample bias higher than $\sim 1.8\mathrm{V}$.