Coverage-dependent adsorption behavior of benzene on Si(100): A high-resolution photoemission study

The adsorption of benzene molecules on Si(100) is investigated by high-resolution core-level photoelectron spectroscopy using synchrotron radiation as well as ultraviolet photoelectron spectroscopy (UPS) for valence bands. The C $1s$ photoelectron spectra of benzene adsorbates are found to consist of two different components, which represent carbon atoms with or without a direct $\mathrm{Si}-\mathrm{C}$ bond. The population ratio of these two components exhibits a drastic change upon increasing the molecular coverage. This is explained by the coverage-dependent change of the adsorbate structure, which is corroborated by the noticeable variation of the molecular orbitals in the valence bands as observed by UPS. From these results, it is deduced that the population of the di-$\sigma$ structure increases significantly at a high coverage over the tetra-$\sigma$ structure which is energetically favored at a low coverage. This indicates the importance of the interaction between the adsorbate molecules at a high coverage.

Figure 1

Schematic illustration of the structure models for a benzene adsorbate on Si(100). (a) Butterfly di-$\sigma$ and (b) tilted tetra-$\sigma$ structure. The double $\mathrm{C}\mathrm{C}$ and single $\mathrm{C}-\mathrm{C}$ bonds are depicted.

Figure 2

Si $2p$ and C $1s$ photoelectron spectra obtained from the $\mathrm{Si}\left(100\right)2\times 1$ surface exposed to benzene of $0–5\mathrm{L}$ at $200\mathrm{K}$. The photon energy and photoelectron emission angle for C $1s$ (Si $2p$) measurements are $330\left(135\right)\mathrm{eV}$ and 0° (60°), respectively.

Figure 3

Decompositions of the Si $2p$ core-level spectra shown in Fig. 2.

Figure 4

Decompositions of the C $1s$ photoelectron spectra shown in Fig. 2.

Figure 5

C $1s$ photoelectron spectra for $1.0\mathrm{L}$ benzene on Si(100) at room temperature and after subsequent flash annealings at the specified temperatures. The spectra are taken at a photon energy of $330\mathrm{eV}$ along the normal emission. The decompositions by curve fittings are given together.

Figure 6

Valence band spectra taken using He-II for benzene/Si(100) with benzene exposures of $0–5.0\mathrm{L}$. The VB spectrum taken at $h\nu =135\mathrm{eV}$ for the condensed benzene multilayers on Si(100) at the substrate temperature of $100\mathrm{K}$ is given for comparison.