AVS 58th Annual International Symposium and Exhibition
    Surface Science Division Monday Sessions
       Session SS2-MoM

Paper SS2-MoM4
Determination of the Structure and Vibrational Dynamics of Methyl-Terminated Si(111) Using Helium Atom Scattering

Monday, October 31, 2011, 9:20 am, Room 110

Session: Surface Chemical Dynamics
Presenter: Ryan Brown, University of Chicago
Authors: R.D. Brown, University of Chicago
S.J. Sibener, University of Chicago
Correspondent: Click to Email
MMethyl-terminated Si(111) interfaces are of great interest in the fields of solar energy, solar fuels, and electronics due to their superior electrochemical stability and device performance. Recent advances in synthetic techniques allow for the fabrication of high-quality, passivated organic-semiconductor hybrid interfaces on single-crystalline silicon substrates. This termination maintains the bulk electronic behavior of the underlying silicon, but eliminates surface trap states which hinder the performance of any device. We directly examined the structural and dynamical characteristics of the CH3-Si(111) and CD3-Si(111) interfaces using helium atom scattering. Helium atom scattering is a uniquely surface-sensitive technique which probes both atomic structure and dynamics. These results are the first measurements of this nature for an organic-semiconductor hybrid interface. Helium atom diffraction confirmed a high quality (1x1) methyl termination of the Si(111) substrate. We observed unusual dynamical characteristics for these interfaces through Debye-Waller attenuation measurements of helium atom diffraction peaks. These measurements characterized the thermal motion of the interface, and elucidated the surface Debye temperature and gas-surface interaction well depth. The Debye Waller measurements yielded temperature dependencies of the surface mean-square displacements displaying polarization dependence, and were similar to the thermal motion of local molecular vibrational modes. The effective surface Debye temperatures were much higher than expected, and correspond to the local vibrational modes of the C-Si bond. These two unusual results indicate that termination with the simplest monolayer organic film initiates a transition away from phonon dominated surface dynamics to dynamics dominated by local molecular vibrational modes. We also performed a series of high precision inelastic time-or-flight measurements to characterize the phonon band structure of these interfaces. Our measurements have clearly resolved multiple single phonon surface modes on CH3-Si(111), and are the first observations of this nature on an organic-semiconductor hybrid interface.