Surface dynamics of ordered Cu3Au(001) studied by elastic and inelastic helium atom scattering

The surface structure and vibrational dynamics of CH 3 -Si͑111͒ and CD 3 -Si͑111͒ 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 ͑ i = f ͒ decay revealed perpendicular mean-square displacements of 1.0ϫ 10 −5 Å 2 K −1 for the CH 3 -Si͑111͒ surface and 1.2ϫ 10 −5 Å 2 K −1 for the CD 3 -Si͑111͒ surface, and a He-surface attractive well depth of ϳ7 meV. The effective surface Debye temperatures were calculated to be 983 K for the CH 3 -Si͑111͒ surface and 824 K for the CD 3 -Si͑111͒ 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 7.1ϫ 10 −4 and 7.2ϫ 10 −4 Å 2 K −1 for the CH 3 -Si͑111͒ and CD 3 -Si͑111͒ surfaces, respectively. The observed increase in thermal motion is consistent with the interaction between the helium atoms and Si-CH 3 bending modes. These experiments have thus yielded detailed information on the dynamical properties of these robust and technologically interesting semiconductor interfaces.

Section: Materials and Experimental Methodsmentioning

confidence: 99%

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

Becker

Brown

Johansson

et al. 2010

“…24 The crystal-to-ionizer distance was however shortened from 1.0234 m to 0.5705 m, which when added to a constant chopper-to-crystal distance of 0.4996 m, produced a total flight path of 1.0701 m (shortened from a total distance of 1.5230 m for the long mode). Inelastic helium time-of-flight experiments (described below) were performed with the detector in both configurations, providing greater angular resolution in the long mode, whereas greater intensity was produced in the short mode.…”

Section: Methodsmentioning

confidence: 99%

Vibrational dynamics and band structure of methyl-terminated Ge(111)

Hund

Nihill

Campi

et al. 2015

Articles you may be interested inThe interaction of organic adsorbate vibrations with substrate lattice waves in methyl-Si (111) A combined synthesis, experiment, and theory approach, using elastic and inelastic helium atom scattering along with ab initio density functional perturbation theory, has been used to investigate the vibrational dynamics and band structure of a recently synthesized organic-functionalized semiconductor interface. Specifically, the thermal properties and lattice dynamics of the underlying Ge (111) semiconductor crystal in the presence of a commensurate (1 × 1) methyl adlayer were defined for atomically flat methylated Ge (111) surfaces. The mean-square atomic displacements were evaluated by analysis of the thermal attenuation of the elastic He diffraction intensities using the Debye-Waller model, revealing an interface with hybrid characteristics. The methyl adlayer vibrational modes are coupled with the Ge(111) substrate, resulting in significantly softer in-plane motion relative to rigid motion in the surface normal. Inelastic helium time-of-flight measurements revealed the excitations of the Rayleigh wave across the surface Brillouin zone, and such measurements were in agreement with the dispersion curves that were produced using density functional perturbation theory. The dispersion relations for H-Ge(111) indicated that a deviation in energy and lineshape for the Rayleigh wave was present along the nearest-neighbor direction. The effects of mass loading, as determined by calculations for CD 3 -Ge(111), as well as by force constants, were less significant than the hybridization between the Rayleigh wave and methyl adlayer librations. The presence of mutually similar hybridization effects for CH 3 -Ge(111) and CH 3 -Si(111) surfaces extends the understanding of the relationship between the vibrational dynamics and the band structure of various semiconductor surfaces that have been functionalized with organic overlayers. C 2015 AIP Publishing LLC.[http://dx

“…26 The apparatus consisted of three regions: a differentially pumped beam source, a UHV sample chamber, and a rotatable detector arm with a total flight path of 1.0778 m (chopperto-crystal distance of 0.4996 m, crystal-to-ionizer distance of 0.5782 m). A nearly mono-energetic beam of atoms ( v/v ≤ 1%, FWHM) was produced by the supersonic expansion of helium through a nozzle source, the center of which was captured by a conical skimmer and collimated by two apertures into a 4 mm spot at the sample.…”

Section: B Helium Atom Scattering Techniquesmentioning

confidence: 99%

The interaction of organic adsorbate vibrations with substrate lattice waves in methyl-Si(111)-(1 × 1)

Brown

Hund

Campi

et al. 2014

A combined helium atom scattering and density functional perturbation theory study has been performed to elucidate the surface phonon dispersion relations for both the CH3-Si(111)-(1 × 1) and CD3-Si(111)-(1 × 1) surfaces. The combination of experimental and theoretical methods has allowed characterization of the interactions between the low energy vibrations of the adsorbate and the lattice waves of the underlying substrate, as well as characterization of the interactions between neighboring methyl groups, across the entire wavevector resolved vibrational energy spectrum of each system. The Rayleigh wave was found to hybridize with the surface rocking libration near the surface Brillouin zone edge at both the ${\rm \bar M}$M¯-point and ${\rm \bar K}$K¯-point. The calculations indicated that the range of possible energies for the potential barrier to the methyl rotation about the Si-C axis is sufficient to prevent the free rotation of the methyl groups at a room temperature interface. The density functional perturbation theory calculations revealed several other surface phonons that experienced mode-splitting arising from the mutual interaction of adjacent methyl groups. The theory identified a Lucas pair that exists just below the silicon optical bands. For both the CH3- and CD3-terminated Si(111) surfaces, the deformations of the methyl groups were examined and compared to previous experimental and theoretical work on the nature of the surface vibrations. The calculations indicated a splitting of the asymmetric deformation of the methyl group near the zone edges due to steric interactions of adjacent methyl groups. The observed shifts in vibrational energies of the -CD3 groups were consistent with the expected effect of isotopic substitution in this system.