The anharmonic lifetimes of zone-center optical phonons in C, Si, and Ge are calculated along with their temperature and pressure dependences, using third-order density-functional perturbation theory. Our basic ingredients are by-products of a standard linear-response calculation of phonon dispersions in the harmonic approximation, resulting in a similarly good agreement with experiments. The microscopic mechanisms responsible for the decay are revealed and shown to be different for different materials and to depend sensitively on the applied pressure.

We examine the dependence on size distribution of the Raman line shape of optical vibron modes in CdSe dots. A detailed comparison with the theory of electron-hole correlated Raman scattering in spherical quantum dots, which includes LO-confined modes, is carried out. The obtained CdSe vibron frequencies are found to map well on the ab initio calculated LO phonon dispersion relations of wurtzite CdSe along the [001] direction of the Brillouin zone.

In this work we present an ab initio calculation of the Raman linewidth of transverse and longitudinal phonon in zinc-blende semiconductors GaAs, AlAs, GaP, and InP. We propose a simple approximation that permits us to calculate the longitudinal linewidths with the same computational effort as the transverse ones. The microscopic mechanisms responsible for the decay are analyzed and discussed. The temperature dependence of the linewidths is computed. Our results are in good agreement with available experimental data up to room temperature for both transverse and longitudinal optical modes and give reliable predictions in materials difficult to study experimentally.

We have measured the pressure dependence of the width of the first-order Raman lines in Si and Ge at low temperature. The width increases linearly with hydrostatic pressure, which implies a decrease of the lifetime of the long-wavelength optical phonons. The results are compared with recent first-principles calculations of anharmonic decay into two phonons of lower energy, based on third-order density-functional perturbation theory. Provided the calculations are slightly adjusted so that the relevant frequencies agree exactly with the measured ones, the agreement for the linewidths between theory and experiment is excellent.

n-type doping in $\mathrm{Si}$ by shallow impurities, such as $\mathrm{P}$, $\mathrm{As}$, and $\mathrm{Sb}$, exhibits an intrinsic limit due to the Fermi-level pinning via defect complexes at high doping concentrations. Here, we demonstrate that doping $\mathrm{Si}$ with the deep chalcogen donor $\mathrm{Te}$ by nonequilibrium processing can exceed this limit and yield higher electron concentrations. In contrast to shallow impurities, the interstitial $\mathrm{Te}$ fraction decreases with increasing doping concentration and substitutional $\mathrm{Te}$ dimers become the dominant configuration as effective donors, leading to a nonsaturating carrier concentration as well as to an insulator-to-metal transition. First-principles calculations reveal that the $\mathrm{Te}$ dimers possess the lowest formation energy and donate two electrons per dimer to the conduction band. These results provide an alternative insight into the physics of deep impurities and lead to a possible solution for the ultrahigh electron concentration needed in today’s $\mathrm{Si}$-based nanoelectronics.