Lattice dynamics and polarization-dependent phonon damping in α -phase FeSi2 nanostructures

Abstract

We determined the lattice dynamics of metastable, surface-stabilized α-phase FeSi2 nanoislands epitaxially grown on the Si(111) surface with average heights and widths ranging from 1.5 to 20 nm and 18 to 72 nm, respectively. The crystallographic orientation, surface morphology and local crystal structure of the nanoislands were investigated by reflection high-energy electron diffraction, atomic force microscopy and X-ray absorption spectroscopy. The Fe-partial phonon density of states (PDOS), obtained by nuclear inelastic scattering, exhibits a pronounced damping and broadening of the spectral features with decreasing average island height. First-principles calculations of the polarization-projected Si-and Fe-partial phonon dispersions and PDOS enable the disentanglement of the contribution of the xy-and z -polarized phonons to the experimental PDOS. Modeling of the experimental data with the theoretical results unveils an enhanced damping of the z -polarized phonons for islands with average sizes below 10 nm. This phenomenon is attributed to the fact that the low-energy z -polarized phonons couple to the low-energy surface/interface vibrational modes. The thermodynamic and elastic properties obtained from the experimental data show a pronounced size-dependent behavior. arXiv:2003.02969v1 [cond-mat.mtrl-sci]

Lattice dynamics of β−FeSi2 nanorods

“…For the growth of β-FeSi 2 on Si surfaces it was experimentally observed [3] and theoretically described [77,88] that the interface energy between the two components, having a rather large lattice mismatch, can be reduced by an α-FeSi 2 interlayer. Furthermore, the z-polarized PDOS of α-FeSi 2 exhibits a distinct peak at 20 meV [86]. For other Fe-Si phases reported to form on Si surfaces, e.g., FeSi [89] or Fe 3 Si [39], this is not the case.…”

“…Using these epitaxial relations, the relative contributions of x-, y-, and z-polarized phonons to the experimental PDOS can be calculated, as briefly discussed above and described in detail in [86]. For the spectra obtained along Si[ 110], the x-ray wave vector is either parallel to (i) β-FeSi 2 [010] or (ii) β-FeSi 2 [001].…”

“…Under this assumption, the experimental spectra were modeled by a combination of the direction-projected ab initio PDOS of β-FeSi 2 , weighted by A x , A y , and A z as discussed above, and the ab initio z-polarized PDOS of α-FeSi 2 [86]. Generally, the comparison of the nanorod PDOS with the ab initio calculations for bulk β-FeSi 2 shows a broadening of the experimental PDOS features which is larger than the experimental resolution.…”

“…Generally, the comparison of the nanorod PDOS with the ab initio calculations for bulk β-FeSi 2 shows a broadening of the experimental PDOS features which is larger than the experimental resolution. This is attributed to phonon scattering at lattice defects, which are predominantly located at interfaces and surfaces [39,86,90]. The damping of the experimental PDOS can be modeled by convolution of the ab initio calculated PDOS with the damped harmonic oscillator (DHO) function [91,92].…”