Helium atom diffraction measurements of the surface structure and vibrational dynamics of $CH3–Si(111)$ and $CD3–Si(111)$ surfaces

The surface structure and vibrational dynamics of $CH3–Si(111)$ and $CD3–Si(111)$ surfaces were measured using helium atom scattering. The elastic diffraction patterns exhibited a lattice constant of 3.82 Å, in accordance with the spacing of the silicon underlayer. The excellent quality of the observed diffraction patterns, along with minimal diffuse background, indicated a high degree of long-range ordering and a low defect density for this interface. The vibrational dynamics were investigated by measurement of the Debye–Waller attenuation of the elastic diffraction peaks as the surface temperature was increased. The angular dependence of the specular $(θi=θf)$ decay revealed perpendicular mean-square displacements of $1.0×10−5Å2K−1$ for the $CH3–Si(111)$ surface and $1.2×10−5Å2K−1$ for the $CD3–Si(111)$ surface, and a He-surface attractive well depth of $∼7meV$⁠. The effective surface Debye temperatures were calculated to be 983 K for the $CH3–Si(111)$ surface and 824 K for the $CD3–Si(111)$ surface. These relatively large Debye temperatures suggest that collisional energy accommodation at the surface occurs primarily through the Si–C local molecular modes. The parallel mean-square displacements were $7.1×10−4$ and $7.2×10−4Å2K−1$ for the $CH3–Si(111)$ and $CD3–Si(111)$ surfaces, respectively. The observed increase in thermal motion is consistent with the interaction between the helium atoms and $Si–CH3$ bending modes. These experiments have thus yielded detailed information on the dynamical properties of these robust and technologically interesting semiconductor interfaces.