Phonons of single quintuple Bi2Te3 and Bi2Se3 films and bulk materials

Phonons of single quintuple films of Bi2Te3 and Bi2Se3 and corresponding bulk materials are calculated in detail by MedeA (a trademark of Materials Design) and Vienna ab initio simulation package (VASP). The calculated results with and without spin-orbit couplings are compared, and the important roles that the spin-orbit coupling plays in these materials are discussed. A symmetry breaking caused by the anharmonic potentials around Bi atoms in the single quintuple films is identified and discussed. The observed Raman intensity features in Bi2Te3 and Bi2Se3 quintuple films are explained

Calculations of surface effects on phonon modes and Raman intensities of Ge quantum dots

Phonon modes and Raman intensities of Ge quantum dots (QDs) with two different types of surfaces, a free standing surface or a fixed surface, in a size range from five atoms to 7 nm in diameter, are calculated by using a microscopic valence force field model. The results are compared, and the effects of surfaces on phonon properties of QDs are investigated. It is found that phonon modes and Raman intensities of QDs with these two different types of surfaces have obvious differences which clearly reveal the effects of the surfaces of QDs. The calculated results agree with existing experimental observations. We expect that our calculations will stimulate more experimental measurements on phonon properties and Raman intensities of QDs

Microscopic theory of the low frequency Raman modes in germanium nanocrystals

We have studied the Raman intensities of low-frequency phonon modes in germanium (Ge) nanocrystals (NC) with varying sizes by using a microscopic valence force field model. The results are compared with the predictions of the continuum model of Lamb using a projection method. We found that the l=0 spheroidal Lamb modes are Raman active in the parallel polarization scattering geometry, while the l=2 spheroidal Lamb modes are active in the crossed polarization geometry. This result agrees with the group theory prediction that the torsional Lamb modes are not Raman active, but is in disagreement with the identification of torsional Lamb modes in the crossed polarization Raman spectra of NC suggested by many authors

Phonon modes in InAs quantum dots

Phonon modes in spherical InAs quantum dots (QDs) with up to 11 855 atoms (about 8.5 nm in diameter) are calculated by using a valence force field model, and all the vibration frequencies and vibration amplitudes of the QDs are calculated directly from the lattice-dynamic matrix. The projection operators of the irreducible representations of the group theory are employed to reduce the computational intensity, which further allows us to investigate the quantum confinement effect of phonon modes with different symmetries. It is found that the size effects of phonon modes depend on the symmetry of the modes. For zinc-blende structure, the modes with A(1) symmetry has the strongest quantum confinement effect and the T-1 mode the weakest. There could be a crossover of symmetries of the highest frequencies from A(1) to T-2 as the size of the QDs decreases. The behavior of vibration amplitudes and vibration energies of phonon modes in different symmetries are also investigated in detail. These results provide microscopic details of the phonon properties of QDs that are important to the fundamental understanding and potential applications of semiconductor QDs

Microscopic investigation of phonon modes in SiGe alloy nanocrystals

Phonon modes in spherical silicon germanium alloy (SiGe) nanocrystals containing up to 1147 atoms (3.6 nm) have been investigated as a function of the Si concentration. Microscopic details of phonon modes, including phonon frequencies and vibrational amplitudes, phonon density-of-states are calculated directly from the dynamic matrices. In particular, the dependence of phonon frequency on the configuration (such as a different ratio of Si to Ge atoms), and location (surface or interior) of clusters of atoms in SiGe alloy nanocrystals have been investigated. Low frequency surface phonons that are related to the spheroidal and torsional modes of a continuum sphere are identified and their frequency dependence on alloy concentration elucidated. The calculated results are compared with measured Raman spectra in bulk, thin films, and superlattices of SiGe alloy reported in the literature. Insights into the behavior of Raman peaks usually identified as Ge-Ge, Si-Si, and Ge-Si optical phonon modes are presented

Theoretical investigation of the surface vibrational modes in germanium nanocrystals

We have used a microscopic lattice dynamical model to study phonon modes in germanium (Ge) NC with size varying between 47 to 7289 atoms (diametersimilar to6.8 nm). By separating these atoms into bulk and surface atoms we have found that surface modes can exist in Ge NC both at low frequencies (\u3c50\u3ecm(-1)) and at high frequency (similar to260 cm(-1)). The latter mode is a resonant mode which occurs in the pseudogap between the acoustic and optical phonon branches in bulk Ge. From the low frequency surface modes we have been able to reconstruct the spheroidal and torsional Lamb modes which have been used to interpret experimental results. Finally, we found that the Lamb model starts to deviate from the lattice dynamical results for Ge NC with diameternm

Calculations on the Size Effects of Raman Intensities of Silicon Quantum Dots

Raman intensities of Si quantum dots (QDs) with up to 11,489 atoms (about 7.6 nm in diameter) for different scattering configurations are calculated. First, phonon modes in these QDs, including all vibration frequencies and vibration amplitudes, are calculated directly from the lattice dynamic matrix by using a microscopic valence force field model combined with the group theory. Then the Raman intensities of these quantum dots are calculated by using a bond-polarizability approximation. The size effects of the Raman intensity in these QDs are discussed in detail based on these calculations. The calculations are compared with the available experimental observation. We are expecting that our calculations can further stimulate more experimental measurements.Comment: 21 pages, 7 figure

Electrons and phonons at semiconductor surfaces

Typescript (photocopy).This dissertation reports four theoretical studies of electrons and phonons at nonideal semiconductor surfaces--i.e., surfaces containing defects and surfaces exhibiting geometrical reconstruction of the atoms. These studies are relevant to two of the most interesting problems in semiconductor physics: the mechanisms accounting for Schottky barriers and Ohmic contacts, and the reconstruction of polar semiconductor surfaces. (A polar surface is one consisting entirely of atoms of one species--e.g., only Ga atoms or only As atoms in the case of GaAs. The inherent instability of such a surface acts as a driving force that produces reconstruction.) The studies are the following: (1) Calculations of the deep electronic energy levels associated with dangling bonds at III-V semiconductor surfaces, using a simple model. (2) Detailed calculations of deep electronic energy levels for defect complexes at the relaxed (110) surfaces of III-V semiconductors. Defect complexes are known to occur in semiconductors, and are in fact often more important than point defects. It is therefore likely that defect complexes are in some cases responsible for Schottky barrier formation. (3) Calculations of the effects of alloy broadening on deep electronic energy levels and Schottky barrier heights for various defects at III-V semiconductor surfaces, again using a simple model. (4) Detailed calculations of surface phonons for the Tong vacancy-reconstructed model of the (111) (2 x 2) surfaces of III-V semiconductors. This is the first nontrivial reconstruction of a compound semiconductor surface to be strongly indicated by experiment. Our results show that there exist several surface phonon branches below the band of the acoustic modes and between the bands of acoustic modes and optical modes. These surface phonon modes should be observable in electron energy loss spectroscopy (EELS) measurements. They should also provide a distinctive signature of the surface geometry