J. Phys. Chem. C 2007, 111, 13053-13061
13053
Investigation of the Reactions during Alkylation of Chlorine-Terminated Silicon (111) Surfaces Sandrine Rivillon Amy,* David J. Michalak, and Yves J. Chabal Department of Chemistry and Biomedical Engineering, Rutgers UniVersity, Piscataway, New Jersey 08854
Leszek Wielunski Department of Physics and Astronomy, Rutgers UniVersity, Piscataway, New Jersey 08854
Patrick T. Hurley† and Nathan S. Lewis Beckman Institute and KaVli Nanoscience Institute, DiVision of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 ReceiVed: March 5, 2007; In Final Form: May 29, 2007
Absorption infrared spectroscopy (IRAS) and Rutherford backscattering (RBS) have been used to investigate the reaction of chlorine-terminated Si(111) surfaces with organometallic molecules (Grignard reagents). Although the predominant reaction leads to alkylation, with formation of covalent Si-C bonds, evidenced by a 678 cm-1 feature assigned to the Si-C stretch mode, solvents typically used during alkylation (tetrahydrofuran and methanol) can also react with Cl/Si(111) surfaces, either during the alkylation reaction or during the rinsing/cleaning process to form Si-OCnH2n+1 as observed by the presence of a SiO-C stretch mode at 1090 cm-1. We also address the origin of some silicon oxidation observed after the methylation or ethylation reactions.
Introduction Controlling the reactivity of semiconductor surfaces while maintaining their structure is particularly important for applications such as hybrid devices (organic/inorganic, biological/ inorganic) and micromechanical systems (MEMS).1-3 Since small concentrations (parts-per-million) of defects at surfaces can greatly impact the electrical properties of devices, it is important to assess the chemical and structural perfection of surfaces after wet chemical treatment for microelectronics applications.4,5 Moreover, the ability to modify semiconductor surfaces by covalent linkage of unsaturated organic molecules with various functionalities (-CH3, -NH2, -COOH, -SH, ...) is necessary for many new technological applications and presents interesting fundamental issues. For example, attachment of organic species on a silicon substrate could potentially prevent the diffusion of oxygen atoms at the interface during either the growth of the insulating layer or the post-annealing process in the formation of high-κ dielectrics on silicon. Therefore, the formation of Si-C bonds is particularly attractive due to its kinetic inertness compared to Si-O or Si-H bonds. The Si-C bond is chemically more stable than Si-O bonds on partially oxidized Si surfaces, because it is unpolarized and thus less susceptible to nucleophilic substitution reactions.6 This is in contrast to surface Si-O bonds that tend to decompose and diffuse under further device processing steps to form subsurface oxidation, which may compromise the electrical quality of devices. Functionalization of silicon surfaces through the formation of Si-C bonds has already been achieved by alkylation of both * Corresponding author. E-mail:
[email protected]. † Now at Air Products, 7201 Hamilton Bd, Allentown, PA 18195-1501.
hydrogen- and chlorine-terminated silicon surface atoms1,7-15 through the use of organometallic reagents or alkene/alkyne molecules, respectively, in thermal activation,12 electrochemistry,16,17 light/UV irradiation,9,15,18,19 metal-catalyzed reduction, or mechanochemical reaction.20 In general, the alkyl coverage of the silicon surfaces is lower than 50% (i.e.,
