Raman scattering from localized vibrational modes of boron impurities in silicon

Reflection Raman spectroscopy has been performed on boron doped uncompensated silicon wafers. The doping of 1.5 10²⁰ cm⁻³ is obtained by diffusion. Two localized modes associated with B¹⁰ and B¹¹ are observed respectively at 642 and 619 cm⁻¹. The position of the peaks does not differ more than 2 cm⁻ from the infrared absorption results, obtained with compensated samples.     

Intrinsic localized modes and nonlinear impurity modes in curved Fermi-Pasta-Ulam chain

We explore the nature of intrinsic localized modes (ILMs) in a curved Fermi-Pasta-Ulam (FPU) chain and the effects of geometry and second-neighbor interaction on the localization and movability properties of such modes. We determine analytically the structure of the localized modes induced by an isotopic light-mass impurity in this chain. We further demonstrate that a nonlinear impurity mode may be treated as a bound state of an ILM with the impurity.      

Search for localized phonon modes inlead-based alloys by superconducting tunneling

To investigate the appearance of localized phonon modes, superconductive tunneling was performed into films of Pb_0·9M_0·1 (M = Ag, Cd, Ga, Ge, In, Mg, Mg, Sb, Sn, Te, Zn) and Pb_0·97Sn_0·03. The films were quench-condensed and later annealed at 30, 100 and 300°K. Structure in tunneling curves resulting from localized phonons was observed only for a well annealed In alloy film. For most of the impurities an absence of observable local phonon modes could be explained as due to a lack of solid solubility or of sufficient structural order in the film. For Na and Sn alloys, however, these arguments cannot be used. No modes could be seen for such alloys even in films condensed at room temperature, heat-treated slightly, and quenched directly into liquid helium.     

Deformation potential constants of the ground state of boron acceptors in silicon

The shear deformation potential constants of the ground state of boron acceptors in silicon have been determined from the piezospectroscopy of the 2p′ line; they are b′ = -(1.61 ± 0.07) eV and d′ = -(4.50 ± 0.15) eV.     

Bulk and surface localized modes on a magnetic nonlinear impurity

We study localized modes on a single magnetic impurity positioned in the bulk or at the surface of a one-dimensional chain, in the presence of a magnetic field B acting at the impurity site. The strong on-site nonlinear interaction U between two electrons of opposite spin at the impurity site, modelled here as a nonlinear local term, and the presence of the external field induce a strong correlation between parallel and antiparallel spin bound states. We find that, for an impurity in the bulk, a localized vector mode (with up and down spin components) is always possible for any given value of U and B, while for a surface impurity, a minimum value of both, U and B is needed to create a vector mode. In this case, up to two localized modes are possible, but only one of them is stable. The presence of the surface seems to destabilize the bulk mode in the parameter region U∼B, creating a “forbidden strip” region in parameter space, bounded by U=B+V and U=B−V, approximately.     

Infrared study of localized vibrations in silicon due to boron and lithium

Localized modes in silicon associated with compensated boron and lithium impurities, observed previously⁽¹⁾ have been investigated in more detail. In particular the isotopic shift of absorption peaks due to change in the lithium and boron isotopes and temperature dependence of the position, width and intensity of peaks have been measured. On the basis of these measurements the impurity and isotopic origin are given for all observed peaks. The symmetry at the impurity sites has been also deduced.Thus, two peaks at 620 and 644 cm⁻¹ are assigned to localized vibrations of isolated boron atoms, B¹¹ and B¹⁰ respectively, at the substitutional site with the local symmetry T_d. The peaks at 681 and 584 cm⁻¹ are attributed to B¹⁰, and the peaks at 653 and 564 cm⁻¹ to B¹¹ which are located at a substitutional site deformed axially by the lithium atom in the nearest vicinity (pairing effect). The lithium reduces the local symmetry at the boron site, from T_d to C_3v, and partly removes the degeneracy, so that the threefold degenerate line is split into two components, a singlet and a doublet.The peaks at 522 and 534 cm⁻¹ are associated with the localized vibration of lithium isotopes, Li⁷ and Li⁶ respectively, in an interstitial position, interacting with a boron or an oxygen atom.     

The intrinsic lifetime of low-frequency zone-centre phonon modes in silicon nanowires and carbon nanotubes

Estimates of the intrinsic lifetime of low-frequency zone-centre phonon modes in silicon nanowires and carbon nanotubes have been presented from the application of Fermi’s golden rule formula based upon an elastic continuum model for cubic anharmonicity. In particular, results have been presented for the lowest non-zero mode in both nanostructures, and also the breathing mode in the nanotube. Except for the ultrathin nanowire, the lifetime increases with size and decreases with an increase in temperature. Typically, these modes have a lifetime of the order of nanoseconds, almost a thousand times larger than the lifetimes of optical phonon modes in the corresponding bulk materials. Also, at room temperature the lifetime of the lowest non-zero mode is nearly an order of magnitude larger in the (20,20) nanotube than in the nanowire of similar thickness (width 2.2 nm).     

Calculations of localized modes on surface and impurity layer embedded in a semi-infinite Heisenberg ferromagnet

We investigate the magnetic excitations for the magnetic problem arising from the absence of magnetic translation symmetry in one dimension due to the presence of an impurity layer embedded within a semi-infinite ferromagnet. A Heisenberg model is employed to investigate the possibility that localized modes can occur with an impurity layer implanted within a semi-infinite ferromagnet. No electronic effects are considered. The theoretical approach employs the matching procedure in the mean field approximation and determines the propagating and evanescent spin amplitude fields including the contribution due to an applied field. The results are used to calculate the energies of localized modes associated with the impurity layer and with the surface. Numerical examples of the modes are given and they are found to exhibit various effects due to the interplay between the impurity layer and surface modes. It is shown that more localized modes can occur and the modification of the spin wave spectra can be signaled by the appearance of surface and impurity modes, besides the bulk excitations. Also, the bulk spin fluctuations field, the spin waves localized on the surface as well as on impurity layer depend are shown to depend on the nature of the exchange coupling between spin sites, the values of spin sites and the position of the impurity layer from the surface.     

Unconventional phonon spectra and obstructed edge phonon modes

Based on the elementary band representations(EBR), many topologically trivial materials are classified as unconventional ones(obstructed atomic limit), where the EBR decomposition for a set of electronic states is not consistent with atomic valenceelectron band representations. In the work, we identify that the unconventional nature can also exist in phonon spectra, where the EBR decomposition for a set of well-separated phonon modes is not consistent with atomic vibration band representations(A...     

On phonon focussing in silicon

Phonon flux density in silicon as a function of direction is calculated. It is observed that the flux density corresponding to (quasi-)longitudinal polarization branch exhibits moderate dependence on direction. But in the case of the two (quasi-)transverse branches, the density vs direction curves are characterized with sharp peaks indicating strong concentration of phonons in certain directions, and correspondingly there is marked deconcentration in other directions.     

Infra-red absorption due to localized modes of vibration of impurity complexes in ionic and semiconductor crystals

A brief account is given of the theory of the vibrations of an imperfect crystal containing isolated impurities and pairs of impurities, and the strength of the induced infra-red absorption is discussed. An analysis is then made, by the methods of group theory, of the energy levels and expected optical transitions of an anharmonic localized oscillator vibrating in a static potential well with various point symmetries. These results are compared with the available experimental data for pairs of impurities and more complicated impurity aggregates in ionic crystals with the fluorite and rocksalt structures, and also for similar defects in the covalent crystals silicon, germanium and some III–V compound materials.     

ZnSe—ZnTe应变层超晶格的纵光学声子模

本文首次利用室温非共振喇曼后向散射测得具有7.3％晶格失配的ZnSe-ZnTe应变层超晶格限制在ZnSe层中的纵光学声子模。计算了限制效应引起的声子模频率的红移,以及弹性应变引起的声子模的移动,它比前者大得多。ZnSe层所受拉伸应变引起声子频率红移,ZnTe层所受压缩应变引起声子频率蓝移。同时在喇曼光谱中观察到由于这种效应导致出现的纵光学声子折叠模。     

Splitting of transverse optical phonon modes localized in GaAs quantum wires on a faceted (311)A surface

The energy splitting of fundamental localized transverse optical (TO1) phonon modes in GaAs/AlAs superlattices and quantum wires grown by molecular-beam epitaxy on a faceted (311)A GaAs surface is observed by Raman spectroscopy. The form of the Raman scattering tensor makes it possible to observe the TO_x and TO_y modes separately, using different scattering geometries the y and x axes are the directions of displacement of the atoms and are directed parallel and transverse to the facets on the (311)A surface). Enhancement of the splitting of the TO1_x and TO1_y modes is observed as the average thickness of the GaAs layers is decreased from 21 to 8.5 Å. The splitting is probably due to the effect of the corrugation of the GaAs/AlAs (311)A hetero-interface on the properties of localized phonon modes. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 45–48 (10 July 1997)     

Infrared absorption spectrum of beryllium acceptors in silicon

Beryllium, when introduced into silicon by thermal diffusion, gives rise to several acceptor impurity centers. The infrared absorption spectrum of beryllium in silicon has been measured with a Fourier-transform spectrometer. The absorption spectrum observed clearly shows five different beryllium acceptor centers in silicon.     

Temperature dependent phonon confinement in silicon nanostructures

Temperature dependent variation in Raman line-shape from silicon (Si) nanostructures (NSs) is studied here. Asymmetry and red-shift in room temperature Raman spectrum is attributed to phonon confinement effect. Raman spectra recorded at higher temperatures show increase in FWHM and decrease in asymmetry ratio with respect to its room temperature counterpart. Theoretical Raman line-shape analyses of temperature dependence of phonon confinement is done by incorporating the temperature dependence of phonon dispersion relation. Experimental and theoretical temperature dependent Raman spectra are in good agreement.