Disorder-induced phonon self-energy of semiconductors with binary isotopic composition

Self-energy effects of Raman phonons in isotopically disordered semiconductors are deduced by perturbation theory and compared to experimental data. In contrast to the acoustic frequency region, higher-order terms contribute significantly to the self-energy at optical phonon frequencies. The asymmetric dependence of the self-energy of a binary isotope system $m_{1-x} M_x$ on the concentration of the heavier isotope mass x can be explained by taking into account second- and third-order perturbation terms. For elemental semiconductors, the maximum of the self-energy occurs at concentrations with $0.5<x<0.7$, depending on the strength of the third-order term. Reasonable approximations are imposed that allow us to derive explicit expressions for the ratio of successive perturbation terms of the real and the imaginary part of the self-energy. This basic theoretical approach is compatible with Raman spectroscopic results on diamond and silicon, with calculations based on the coherent potential approximation, and with theoretical results obtained using {\it ab initio} electronic theory. The extension of the formalism to binary compounds, by taking into account the eigenvectors at the individual sublattices, is straightforward. In this manner, we interpret recent experimental results on the disorder-induced broadening of the TO (folded) modes of SiC with a $^{13}{\rm C}$-enriched carbon sublattice. \cite{Rohmfeld00,Rohmfeld01}Comment: 29 pages, 9 figures, 2 tables, submitted to PR

Carbon antisite clusters in SiC: a possible pathway to the D_{II} center

The photoluminescence center D_{II} is a persistent intrinsic defect which is common in all SiC polytypes. Its fingerprints are the characteristic phonon replicas in luminescence spectra. We perform ab-initio calculations of vibrational spectra for various defect complexes and find that carbon antisite clusters exhibit vibrational modes in the frequency range of the D_{II} spectrum. The clusters possess very high binding energies which guarantee their thermal stability--a known feature of the D_{II} center. The di-carbon antisite (C_{2})_{Si} (two carbon atoms sharing a silicon site) is an important building block of these clusters.Comment: RevTeX 4, 6 pages, 3 figures Changes in version 2: Section headings, footnote included in text, vibrational data now given for neutral split-interstitial, extended discussion of the [(C_2)_Si]_2 defect incl. figure Changes version 3: Correction of binding energy for 3rd and 4th carbon atom at antisite; correction of typo

Phonons and related properties of extended systems from density-functional perturbation theory

This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudo-potential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long wave-length vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.Comment: 52 pages, 14 figures, submitted to Review of Modern Physic

Dependence of phonon widths on pressure and isotopic mass: ZnO

Considerable attention has been devoted recently to the dependence of the widths of the Raman phonons of semiconductors on pressure and on isotopic mass. The dependence on pressure is usually small and monotonic unless the phonon happens to be close to a singularity of the two-phonon density of states (DOS) which determines its width. In the latter case, strong nonmonotonic dependences of the phonon width on pressure and on isotopic mass can appear. We have investigated the E high 2 phonons of ZnO crystals with different isotopes and observed a wide range of FWHM depending on isotopic masses. Ab initio calculations of the two-phonon DOS provide an explanation for this variation of the FWHM: the E high 2 frequency falls on a sharp ridge of the 2-DOS corresponding to combinations of TA and LA phonons. Changes in isotopic mass result in a motion of the E high 2 frequency up and down that ridge which produces the changes in FWHM. These phenomena suggest a decrease of the FWHM with pressure which seems to be present in existing data obtained at 300 K. Similar phenomena are discusse