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

Abstract

The chemical state, electronic properties, and geometric structure of methyl-terminated Si͑111͒ 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 ϫ 1͒ 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 = 0.38± 0.01 eV. The carbon-bonded surface Si atoms exhibited a well-defined x-ray photoelectron signal having a core level shift of 0.30± 0.01 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 eV. Surface states related to CH 3 and Si-C bonding orbitals were identified at binding energies of 7.7 and 5.4 eV, respectively. Nearly ideal passivation of Si͑111͒ surfaces can thus be achieved by methyl termination using the two-step chlorination/ alkylation process.

Molecules on Si: Electronics with Chemistry

“…SiÀC bonds on Si(111) and Si(100) have been shown to produce less than 0.2 eV band bending. [33,43,83] Furthermore, the observed inversion [20] in n-SiÀalkyl/Hg (Fig. 3) and n-SiÀOÀalkyl/Hg junctions [88] (see Section 4.2) implies that the Si surface is not pinned, supporting the idea that CÀSi and CÀOÀSi surface terminations effectively passivate the Si surface both chemically (preventing oxidation) and electronically (small density of surface states).…”

“…Among the alkyls only CH 3 can replace each H on H-terminated Si, maintaining a relaxed (1 Â 1) structure for Si (111) [83] as the Si atom surface density on the (111) face is too high for a Si:alkyl ratio of 1:1 with longer alkyls. Therefore, while a C 1 (ÀCH 3 ) layer can have nearly 100% coverage, [84] longer alkyl chains form monolayers with a density of adsorbed molecules of about 1:2, [85] with the remaining Si atoms bound to H. [77,85] The longer the alkyl chain is, the stronger is the intermolecular (van der Waals) interaction and the less commensurate is the monolayer structure with the original Si surface.…”

“…[42,83,90] Aromatic molecules are mostly electrochemically adsorbed as diazonium salts [43,62,91] or by photochemical binding of ethynylbenzene. [63] Some of these possibilities are shown schematically in Figure 1b.…”

“…The SiÀC bond decreases the effective electron affinity by 0.5 eV [20,33,43,83,121] compared to that of the H-terminated Si surface, implying that methyl is more positive than H if bound to the Si surface. However, this contradicts chemical intuition based on the electronegativity order Si < H < C. Core level XPS shows that on an atomic level, the bond charge distribution is Si þ ÀC À , as predicted by electronegativity.…”

Transmission Infrared Spectroscopy of Methyl- and Ethyl-Terminated Silicon(111) Surfaces

“…This orientation should produce diagnostic signatures and polarizations in the vibrational spectrum of such surfaces. Recent scanning tunneling microscopy 8 and low-energy electron diffraction studies 9 have revealed both short-range and long-range order on CH 3 -terminated Si surfaces. However, at present no information is available on the structure of surfaces terminated with alkyls such as C 2 H 5 -that are sterically precluded from terminating every atop site on an unreconstructed Si(111) surface.…”