High-Resolution Synchrotron Photoemission Studies of the Electronic Structure and Thermal Stability of CH₃- and C₂H₅-Functionalized Si(111) Surfaces

Abstract

The relative coverage, thermal stability, and electronic properties of CH 3 -and C 2 H 5 -functionalized Si(111) surfaces prepared by a two-step chlorination/alkylation procedure have been compared using high-resolution synchrotron photoemission spectroscopy. Whereas the CH 3 -terminated Si(111) surface showed only one C 2s peak for the occupied σ orbitals, the C 2s spectra of C 2 H 5 -terminated Si(111) surfaces showed a symmetric splitting of the occupied σ orbitals, as expected for an ethyl moiety bonded to the surface. The C 2 H 5 termination resulted in an unpinning of the Si surface Fermi level, with a band bending of ∼0.2 eV, and produced a surface dipole potential step of -0.23(15) eV. The observed close-to-flat-band condition is similar to that of CH 3 -Si(111) and is consistent with H termination of the non-alkylated Si atop sites in the two-step chlorination/ alkylation process. The C 2 H 5 -functionalized Si(111) surfaces decomposed at temperatures >300°C, whereas CH 3 -Si(111) surfaces were stable up to at least 440°C. The data clearly highlight the similarities and identify some significant differences between the behavior of the CH 3 -and C 2 H 5 -functionalized Si(111) surfaces.

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).…”

“…[32] The downside of the stable binding to Si, 4 eV bond energy for SiÀC and 4.7 eV for SiÀO(ÀC), is that the resulting monolayer is less ordered, because diffusion of the adsorbed molecules on the surface to improve ordering is more difficult. The upside is that it is more robust against substrate oxidation, can withstand heating [33] and that metal atom diffusion during top contact deposition is less likely. [34] Amongst other binding chemistries, only CÀC bonding to graphite (or burned photo-resist, see Ref.…”

“…[44,66] Transport across very short alkyl monolayers, directly bound to Si, and characterized by spectroscopy, [33,67] was studied with Hg, [19] Au [2] and other metals as second contact. [68] In other work powerful techniques for characterizing Si MOMS were used and, in part, adapted.…”

“…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.…”

“…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. [33,43,[122][123][124] …”