Brandenburgische Techn. Univ, Diss., 2000--Cottbus.

Techn. University, Diss., 2001--Berlin.

Techn. University, Diss., 2004--Berlin.

Techn. University, Diss., 2004--Berlin.

The Fröhlich coupling describes the interaction between electrons and infrared-active vibrations at long wavelength in polar semiconductors and insulators, and may result in the formation of polaronic quasiparticles. Polarons are electrons dressed by a phonon cloud, which can strongly affect the electronic properties of the crystal. Despite their ubiquitous role in a broad range of technologies, first-principles investigations of the electron-phonon interaction in polar materials are scarce. In this thesis we develop a general formalism for calculating the electron-phonon matrix element in polar semiconductors and insulators from first principles, which represents a generalization of the Fröhlich model and can be used to compute the polar electron-phonon coupling as a straightforward post-processing operation. We apply this procedure to explore an important material for photovoltaics, the hybrid lead halide perovskite CH₃NH₃PbI₃. In this case we show that the temperature dependence of emission line broadening is dominated by Fröhlich coupling. Our method is formulated in conjunction with an ab initio interpolation technique based on maximally localized Wannier functions, which allows to describe all forms of electron-phonon coupling on the same footing. We demonstrate the validity of this approach on the prototypical examples GaN and SrTiO₃. Focusing on anatase TiO₂, a transition metal oxide of wide technological interest, we establish quantitatively the effect of including the ab initio Fröhlich coupling in the calculation of electron lifetimes. The rest of the thesis is devoted to exploring the quasiparticle properties in doped oxides. In particular, we investigate angle-resolved photoemission spectra from first principles in doped anatase TiO₂ by proposing a novel framework that combines our ab initio matrix elements, including the dynamical screening arising from the added carriers, and the cumulant expansion approach. We compare our results with experimental data, and show that the transition from a polaronic to a Fermi liquid regime with increasing doping concentration originates from nonadiabatic polar electron-phonon coupling. We further validate this mechanism by calculating angle-resolved photoemission spectra in the ferromagnetic semiconductor EuO.

abstract: Thermal effects in nano-scaled devices were reviewed and modeling methodologies to deal with this issue were discussed. The phonon energy balance equations model, being one of the important previous works regarding the modeling of heating effects in nano-scale devices, was derived. Then, detailed description was given on the Monte Carlo (MC) solution of the phonon Boltzmann Transport Equation. The phonon MC solver was developed next as part of this thesis. Simulation results of the thermal conductivity in bulk Si show good agreement with theoretical/experimental values from literature. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2015

abstract: A novel Monte Carlo rejection technique for solving the phonon and electron Boltzmann Transport Equation (BTE), including full many-particle interactions, is presented in this work. This technique has been developed to explicitly model population-dependent scattering within the full-band Cellular Monte Carlo (CMC) framework to simulate electro-thermal transport in semiconductors, while ensuring the conservation of energy and momentum for each scattering event. The scattering algorithm directly solves the many-body problem accounting for the instantaneous distribution of the phonons. The general approach presented is capable of simulating any non-equilibrium phase-space distribution of phonons using the full phonon dispersion without the need of the approximations commonly used in previous Monte Carlo simulations. In particular, anharmonic interactions require no assumptions regarding the dominant modes responsible for anharmonic decay, while Normal and Umklapp scattering are treated on the same footing. This work discusses details of the algorithmic implementation of the three particle scattering for the treatment of the anharmonic interactions between phonons, as well as treating isotope and impurity scattering within the same framework. The approach is then extended with a technique based on the multivariable Hawkes point process that has been developed to model the emission and the absorption process of phonons by electrons. The simulation code was validated by comparison with both analytical, numerical, and experimental results; in particular, simulation results show close agreement with a wide range of experimental data such as the thermal conductivity as function of the isotopic composition, the temperature and the thin-film thickness. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2018

Investigamos os efeitos de tamanho e do potencial de confinamento finito V0 nas taxas de espalhamento de absorção e de emissão de elétrons interagindo com os fônons longitudinais ópticos (fônons LO) de um fio quântico cilíndrico de GaAs à temperatura ambiente. Calculamos as taxas de espalhamento inter e intra-sub-banda e a taxa de espalhamento total para uma temperatura de 300 K, pois nesta temperatura o mecanismo de espalhamento dominante em semicondutores do tipo III-V é aquele devido aos fônons LO. Qualitativamente a taxa de emissão intra-sub-banda neste sistema tem o mesmo comportamento da sua correspondente em estruturas 2D. Para a absorção encontramos uma mudança suave de comportamento da taxa de absorção intra-sub-banda quando o raio do fio é da ordem do diâmetro do polaron (ou seja, da ordem de 80 ANGSTROM). Para raios pequenos ela tem um comportamento similar ao do bulk, mas para raios maiores ela cresce até atingir um máximo e depois cai monotonicamente à medida que aumentamos a energia do portador. Vimos que, o tamanho do fio e o potencial de confinamento têm grande influência na taxa de espalhamento total. / We investigated the size effects and the effects of the finite confining potential V0 on the absorption and emission scattering rates of electron interacting with longitudinal optical (LO) phonons for a cylindrical GaAs quantum wire. We calculated the inter and intrasubband total scattering rate for a temperature of 300K, because in this temperature the dominant mechanism of scattering in semiconductors III-V is that due LO phonons. Qualitatively the intrasubband emission scattering rate in this system has the same behavior of the correspondent in 2D structures. For absorption we found a smooth change in the intrasubband absorption scattering rate behavior when the radius the wire is near the polaron diameter (ie, about 80 ANGSTROM). For small radius the scattering rate has a similar behavior as that of the bulk, but for large radius it increases until reach a maximum and after ir drops monotonicaly with increase of carrier energy. We found that the size effect and the confining potential have a large influence in the total scattering rate

L’objectif de cette thèse est de comprendre la dynamique des excitations électroniques, thermiques et acoustiques dans une structure hybride composée d’un film métallique noble et d’un film ferromagnétique. L’analyse des processus est possible grâce à l’excitation d’un film métallique par une impulsion laser ultra brève qui génère des impulsions acoustiques. L’étude s’appuie sur la technique femtoseconde pompe-sonde et les simulations numériques. Beaucoup de processus intéressants comme l’excitation des électrons chauds et génération des impulsions acoustiques peuvent être étudiés. Le fait de mieux comprendre le mécanisme de ces processus amène à la possibilité de contrôler les propriétésélectroniques, mécaniques et magnétiques des matériaux. Cela permettra d’introduire des concepts innovants pour des nouveaux dispositifs ou d’optimiser la performance des dispositifs qui existent déjà. Le point de départ de ce travail est l’observation expérimentale d’impulsions acoustiques de durée très courte (2 ps) générées dans un film de cobalt de 30nm via excitation d’un film d’or (de 50 nm jusqu’à 500 nm). Le processus majeur en jeu est le transport ultrarapide de l’énergie par les électrons super diffusifs à travers la couche d’or. Pour étudier le transport diffusive des électrons on utilise le modèle à Deux Températures. Ce nouveau concept a été confirmé par des mesures magnétiques : excitation de la précession de la magnétisation par les électrons super diffusifs. De plus, notre analyse des courbes de réflectivité amontré la possibilité de retrouver la résistance aux l’interfaces (résistance de Kapitza) pour les interfaces métal- diélectrique et métal-métal. / This thesis aims to understand the ultrafast dynamics of electronic, thermal and acoustic excitations in noble metal -ferromagnet multilayer structures. We start from the initial stage of ultrafast laser excitation up to the detection of thegenerated picosecond acoustic pulses by using the femtosecond pump-probe technique and numerical simulations. Various interesting transient physical processes are taking place on intermediate timescales. Understanding and explaining their mechanisms leads to the possibility of controlling electronic, mechanical and magnetic properties of materials. Thus, it becomes possible to introduce innovative concepts of new devices and optimize the performances for already existingtechnologies. The starting point of this work is the experimental observation by reflectivity measurements of 2-ps acousticpulses generated in a 30 nm thin cobalt layer when the ultrashort laser pulse excites a much thicker gold overlayer of varying thicknesses from 50 to 500 nm. If we take into account the fact that the optical energy deposition by a visible light in gold is limited by a tiny (~10nm) skin depth, the only reasonable explanation of the acoustic strain generation in adjacent cobalt layer can be provided by considering ultrafast energy transport by super-diffusive hot electrons through the gold layer. The hot electron diffusion is studied within the Two Temperature Model. This novel concept was corroborated by complementarymeasurements of magnetization precession in cobalt induced by hot electrons initially generated in gold. In our analysis we were able to fit both thermal and acoustic components of transient reflectivity measurements and retrieve the values of the Kapitza resistances at the metal-metal and metal-dielectric interfaces.

Thesis advisor: David Broido / We present fundamental studies of phonon and electron transport in semiconductors. First principles density functional theory (DFT) is combined with exact numerical solutions of the Boltzmann transport equation (BTE) for phonons and electrons to calculate various transport coefficients. The approach is used to determine the lattice thermal conductivity of three hexagonal polytypes of silicon carbide. The calculated results show excellent agreement with recent experiments. Next, using the infinite orders T-matrix approach, we calculate the effect of various neutral and charged substitution defects on the thermal conductivity of boron arsenide. Finally, we present a general coupled electron-phonon BTEs scheme designed to capture the mutual drag of the two interacting systems. By combining first principles calculations of anharmonic phonon interactions with phenomenological models of electron-phonon interactions, we apply our implementation of the coupled BTEs to calculate the thermal conductivity, mobility, Seebeck and Peltier coefficients of n-doped gallium arsenide. The measured low temperature enhancement in the Seebeck coefficient is captured using the solution of the fully coupled electron-phonon BTEs, while the uncoupled electron BTE fails to do so. This work gives insights into the fundamental nature of charge and heat transport in semiconductors and advances predictive ab initio computational approaches. We discuss possible extensions of our work. / Thesis (PhD) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.