Chemical and Electrical Passivation of Single Crystal Silicon Surfaces Through Covalently Bound Organic Monolayers

Citation

Nemanick, Eric Joseph (2006) Chemical and Electrical Passivation of Single Crystal Silicon Surfaces Through Covalently Bound Organic Monolayers. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/GNSW-GJ84. https://resolver.caltech.edu/CaltechETD:etd-09292005-152710

Abstract

The formation of and passivation by alkyl monolayers on Si(111) and Si(100) surfaces was studied. Crystalline Si(111) and Si(100) surfaces were alkylated in a two-step chlorination/alkylation process using both straight chain alkyl groups CH₃, C₂H₅, C₄H₉, and C₈H₁₇, as well sterically bulky alkyl groups such as (CH₃)₂CH-(iso-propyl), (CH₃)₃C-(tert-butyl), and C₆H₅-(phenyl) moieties to form well defined alkyl monolayers on the surface. X-ray photoelectron spectroscopic (XPS) data in the C 1s region of such surfaces exhibited a low energy emission at 283.6 binding eV, consistent with carbon bonded to Si, demonstrating a Si-C covalent bond. The C 1s XPS data indicated that larger alkyl groups were present at lower coverages than methyl groups on both the CH₃-terminated Si(111) surfaces as well as CH₃-terminated Si(100) surfaces. Despite the lower monolayer percent coverage, no Cl was detected after alkylation on either surface. Functionalization with even the bulky alkyl groups effectively inhibited the oxidation of the Si surfaces in air and produced low (< 100 cm s⁻¹) surface recombination velocities, indicating a low density of electronically active surface trap states. Transmission infrared spectroscopy indicated that the Si(111) surfaces were partially H-terminated after the functionalization reaction for groups larger than CH₃. High resolution soft X-ray photoelectron spectroscopic (SXPS) measurements of the Si 2p region of the alkylated Si(111) and Si(100) surfaces show monolayer coverage of SiCl and SiCH₃ on the Si(111) surface, with mixtures of species on H-term Si(111) as well as H-term Si(100) surfaces. Bulkier alkyl groups such as C₂H₅ and phenyl on both Si(111) and Si(100) surfaces as well as on CH₃-terminated Si(100) show broad Si 2p peaks with binding shifts indicative of hybrid surfaces composed of both Si-R groups and SiH/SiH₂ species. To model surface packing of these alkyl groups on the Si(111) surface, molecular dynamics modeling was employed using Cerius. The energies for various packing densities and packing patterns were calculated and referenced versus the reactant energy for each surface. From this it was concluded that 100% of the Si(111) surface should be CH₃-terminated, with lower packing densities for C₂H₅ between 50-80%, 50-66.7% for C₈H₁₇, 33.3-40% for tert-butyl, 33.3-50% for iso-propyl, and 50-66.7% for phenyl. A single electron transfer (SET) mechanism for the reaction of alkyl Grignards with the Cl-terminated surface is proposed. Application of a reducing potential, -2.5 V vs. Ag⁺/Ag, to Cl-terminated Si(111) electrodes in tetrahydrofuran resulted in the complete elimination of Cl, as measured by XPS. The data are consistent with a mechanism in which the reaction of alkyl Grignard reagents with the Cl-terminated Si(111) surfaces involves electron transfer from the Grignard reagent to the Si, loss of chloride to solution, and subsequent reaction between the resultant silicon radical and alkyl radical to form a silicon-carbon bond. Sites sterically hindered by neighboring alkyl groups abstract a H atom to produce Si-H bonds on the surface.

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