Numerical study of anharmonic vibrational decay in amorphous and paracrystalline silicon

TL;DR

This study numerically investigates anharmonic vibrational decay in amorphous and paracrystalline silicon, revealing picosecond lifetimes and decay pathways, and discusses implications for experimental relaxation observations.

Contribution

It provides the first detailed numerical analysis of vibrational decay rates in realistic models of amorphous and paracrystalline silicon, including crystalline nanostructures.

Findings

Vibrational lifetimes are on picosecond timescales.

Decay rates in p-Si are similar to a-Si.

Localized modes on crystalline clusters decay mainly to diffusons.

Abstract

The anharmonic decay rates of atomic vibrations in amorphous silicon (a-Si) and paracrystalline silicon (p-Si), containing small crystalline grains embedded in a disordered matrix, are calculated using realistic structural models. The models are 1000-atom four-coordinated networks relaxed to a local minimum of the Stillinger-Weber interatomic potential. The vibrational decay rates are calculated numerically by perturbation theory, taking into account cubic anharmonicity as the perturbation. The vibrational lifetimes for a-Si are found to be on picosecond time scales, in agreement with the previous perturbative and classical molecular dynamics calculations on a 216-atom model. The calculated decay rates for p-Si are similar to those of a-Si. No modes in p-Si reside entirely on the crystalline cluster, decoupled from the amorphous matrix. The localized modes with the largest (up to 59%) weight on the cluster decay primarily to two diffusons. The numerical results are discussed in relation to a recent suggestion by van der Voort et al. [Phys. Rev. B {\bf 62}, 8072 (2000)] that long vibrational relaxation inferred experimentally may be due to possible crystalline nanostructures in some types of a-Si.