Chemical and electronic characterization of methyl-terminated Si(111) surfaces by high-resolution synchrotron photoelectron spectroscopy

The chemical state, electronic properties, and geometric structure of methyl-terminated Si(111) surfaces prepared using a two-step chlorination∕alkylation process were investigated using high-resolution synchrotron photoelectron spectroscopy and low-energy electron diffraction methods. The electron diffraction data indicated that the methylated Si surfaces maintained a $(1\times 1)$ structure, where the dangling bonds of the silicon surface atoms were terminated by methyl groups. The surfaces were stable to annealing at 720 K. The high degree of ordering was reflected in a well-resolved vibrational fine structure of the carbon $1s$ photoelectron emission, with the fine structure arising from the excitation of C-H stretching vibrations having $h\nu =0.38\pm 0.01\mathrm{eV}$. The carbon-bonded surface Si atoms exhibited a well-defined x-ray photoelectron signal having a core level shift of $0.30\pm 0.01\mathrm{eV}$ relative to bulk Si. Electronically, the Si surface was close to the flat-band condition. The methyl termination produced a surface dipole of $-0.4\mathrm{eV}$. Surface states related to ${\pi }_{{\mathrm{CH}}_3}$ and ${\sigma }_{\mathrm{Si}\text{‐}\mathrm{C}}$ bonding orbitals were identified at binding energies of 7.7 and 5.4 eV, respectively. Nearly ideal passivation of Si(111) surfaces can thus be achieved by methyl termination using the two-step chlorination∕alkylation process.

Figure 1

Low-energy electron diffraction pattern of a $\mathrm{Si}\left(111\right)\text{‐}{\mathrm{CH}}_3$ surface, taken with primary electron energies of 40–50 eV.

Figure 2

Carbon $1s$ emission of $\mathrm{Si}\left(111\right)\text{‐}{\mathrm{CH}}_3$ excited with $h\nu =330\mathrm{eV}$. The asymmetry towards high binding energies is revealed as a vibrational fine structure. Assignment: component ${\mathrm{C}}^0={\mathrm{CH}}_3\text{‐}\mathrm{Si}$ (vibrational ground state); ${\mathrm{C}}^1,{\mathrm{C}}^2={\mathrm{CH}}_3\text{‐}\mathrm{Si}$ (C-H stretch vibrations excited, 1st + 2nd excited states); ${\mathrm{C}}^3=\text{non‐}{\mathrm{CH}}_3$ species. The fitting parameters are given in Table .

Figure 3

Si $2p$ core level spectra of $\mathrm{Si}\left(111\right)\text{‐}{\mathrm{CH}}_3$ taken with varying excitation energies. The highest surface sensitivity is obtained for $h\nu =150\mathrm{eV}$. The Si $2p$ emission is composed of a bulk component (B) and a surface component (S) which is shifted to higher binding energies by $0.30(\pm 0.01)\mathrm{eV}$. Fitting results are shown in (b), and the corresponding fit parameters are given in Table .

Figure 4

Curve fitting to the Si $2p$ emission in Fig. 3 $(h\nu =150\mathrm{eV})$ including weak (sub)oxide ${\mathrm{Si}}^{+1}$, ${\mathrm{Si}}^{+3}$, and ${\mathrm{Si}}^{+4}$ components, which have been magnified by a factor of 10 for better visibility. The intensity of the ${\mathrm{Si}}^{+1}$ component depends on the assumed background (here: Tougaard-type) but is always less than 5% of the $\mathrm{Si}\text{‐}{\mathrm{CH}}_3$ component.

Figure 5

Valence band spectra of $\mathrm{Si}\left(111\right)\text{‐}{\mathrm{CH}}_3$ excited with He I radiation $h\nu =21.2\mathrm{eV}$ (bottom) or with synchrotron radiation with $h\nu =150\mathrm{eV}$ (top). The spectra were acquired under the emission angles ${\theta }_e$ given in the figure. Through comparison to the cluster calculations (Ref. 23) and gas phase spectra (Ref. 37), the emission structures have been assigned as indicated.

Figure 6

Surface energy band diagram of the $\mathrm{Si}\left(111\right)\text{‐}{\mathrm{CH}}_3$ surface. The surface is close to flat band conditions with a residual surface band banding, $e{V}_{bb}$, of 0.1 eV. The electron affinity, $\chi$, of $\mathrm{Si}\left(111\right)\text{‐}{\mathrm{CH}}_3$ is $\approx 3.7\mathrm{eV}$, which indicates a surface dipole, $\delta$, of $-0.4\mathrm{eV}$. The position of the valence band maximum at the surface, $\mid E_F-E_v\mid =0.94\mathrm{eV}$, is inferred from the Si $2p_{3∕2}$ binding energy, where a core level-to-valence band separation ${\mathrm{BE}}_v\left(\mathrm{Si}2p_{3∕2}\right)$ of 98.74 eV was assumed (Ref. 43).