Temperature dependence of phonon-defect interactions: phonon scattering vs. phonon trapping.

The interactions between thermal phonons and defects are conventionally described as scattering processes, an idea proposed almost a century ago. In this contribution, ab-initio molecular-dynamics simulations provide atomic-level insight into the nature of these interactions. The defect is the Si|X interface in a nanowire containing a δ-layer (X is C or Ge). The phonon-defect interactions are temperature dependent and involve the trapping of phonons for meaningful lengths of time in defect-related, localized, vibrational modes. No phonon scattering occurs and the momentum of the phonons released by the defect is unrelated to the momentum of the phonons that generated the excitation. The results are extended to the interactions involving only bulk phonons and to phonon-defect interactions at high temperatures. These do resemble scattering since phonon trapping occurs for a length of time short enough for the momentum of the incoming phonon to be conserved.

Impact of impurities on the thermal conductivity of semiconductor nanostructures: first-principles theory.

The thermal conductivity kappa(T) of Si nanostructures containing impurities is calculated from first-principles using nonequilibrium molecular dynamics simulations in thermally "prepared" periodic supercells. For a given concentration of impurities, kappa exhibits strongly nonlinear variations with the mass of the impurity. There is a narrow range of conditions for which kappa is substantially reduced relative to that of the pure material. This suggests that the kappa of nanowire could be controlled with impurities and that nanoregions with a desired kappa could be implanted on chips.

Isotope dependence of the lifetime of the vibration of oxygen in silicon.

By simply changing the isotopes of the Si atoms that neighbor an oxygen Oi atom in crystalline silicon, the measured decay rate tau of the asymmetric-stretch vibration (nu3=1136 cm-1) of oxygen (Oi) in silicon changes by a factor of approximately 2.5. These data establish that nu3 decays by creating one nu1 symmetric-stretch, local-vibrational mode of the Si-Oi-Si structure. If the residual energy (nu3-nu1) is less than the maximum frequency num of the host lattice, as for 28Si-16O-28Si in natural silicon, then it is emitted as one lattice mode, and tau depends on the density of one-phonon states at nu3-nu1. If (nu3-nu1)>num, as for 16O in single-isotope 30Si silicon, two lattice modes are created in addition to nu1, increasing tau. Prediction of tau for a particular defect clearly requires a detailed knowledge of that defect.

First-principles calculations of vibrational lifetimes and decay channels: hydrogen-related modes in Si.

The vibrational lifetimes and decay channels of local vibrational modes are calculated from first principles at various temperatures. Our method can be used to predict the temperature dependence of the lifetime of any normal mode in any crystal. We focus here on the stretch modes of H2*, H+(BC), and VH x HV in Si. The frequencies are almost identical, but the lifetimes vary from 4 to 295 ps. The calculations correctly predict the lifetimes for T > 50 K and illustrate the critical importance of pseudolocal modes in the decay processes of high-frequency local vibrational modes.

First-principles calculations of pseudolocal vibrational modes: the case of Cu and Cu pairs in Si.

Pseudolocal vibrational modes (pLVMs) are defect-related vibrational modes which are localized despite being below the phonon maximum. Such modes are sometimes seen as phonon replicas in photoluminescence spectra. The pLVMs associated with two copper-related defects are calculated from first-principles density-functional theory in periodic supercells. The localization of the pLVMs is quantified using the magnitude of the eigenvectors of the dynamical matrix.

Unexpected dynamics for self-interstitial clusters in silicon.

Ab initio molecular-dynamics simulations of self-interstitial clusters in Si show that I2 and the most stable of the I3 (" I(a)(3)") clusters diffuse extremely fast. In these clusters, the I's share a single bond-centered (BC) site. The metastable I(b)(3) cluster involves three adjacent BC sites. Simulations show that the three I's exchange sites with each other, but the center of the defect remains at the same place. Simulations with I1 and I4 show no diffusion on the same time scale.