Physics-based multiscale modeling of III-nitride light emitters

The application of computer simulations to scientific and engineering problems has evolved to an established phase over the last decades. In the field of semiconductor device physics, Technology CAD (TCAD) has been regarded as an indispensable tool for the interpretation and prediction of device behavior. More specifically, TCAD modeling and simulation of nanostructured III-nitride light emitters still have challenging problems and is currently a topic under active research. This thesis devotes to the theoretical and numerical investigations of III-nitride bulk and quantum structures, following a bottom-up approach aimed at modeling and understanding photoluminescence and electroluminescence in these structures. In the first part, the calculation of electronic bandstructure is addressed, where a novel k · p model derived from Non-local Empirical Pseudopotential method(NL-EPM) is presented. Optical properties are then calculated employing both Poisson-k · p and a density-matrix based approach, gain and luminescence spectra can be extracted by solving the semiconductor-Bloch equation numerically. The last part of this thesis deals with the microscopic quantum transport, within the framework of the quantum-statistical nonequilibrium Greens function formalism(NEGF). While classical drift-diffusion models assume that bound carriers hold their coherence in the confined direction and unbound carriers are completely incoherent, NEGF does not distinguish between bound and unbound states and treats them on equal footing. In addition, NEGF also provides intuitive insights into energy-resolved carrier distributions, currents and coherence loss mechanisms. The numerical computations alongside this thesis can be computationally very involved, some code developed along with this thesis is deployed on the clusters and able to scale up to more than 1000 CPU cores, thanks to the parallel implementation technique such as OpenMP and MPI, as well as HPC infrastructures available at CINECA computing center

Component Mode Synthesis Approach for Quantum Mechanical Electrostatic and Transport Analysis of Nanoscale Structures and Devices

As the dimensions of commonly used semiconductor devices have shrunk into nanometer regime, it is recognized that the influence of quantum effects on their electrostatic and transport properties cannot be ignored. In the past few decades, various computational models and approaches have been developed to analyze these properties in nanostructures and devices. Among these computational models, the Schršdinger-Poisson model has been widely adopted for quantum mechanical electrostatic and transport analysis of nanostructures and devices such as quantum wires, metal-oxide-semiconductor field effect transistors (MOSFETs) and nanoelectromechanical systems (NEMS). The numerical results allow for evaluations of the electrical properties such as charge concentration and potential profile in these structures. The emergence of MOSFETs with multiple gates, such as Trigates, FinFETs and Pi-gates, offers a superior electrostatic control of devices by the gates, which can be therefore used to reduce the short channel effects within those devices. Full 2-D electrostatic and transport analysis enables a better understanding of the scalability of devices, geometric effects on the potential and charge distribution, and transport characteristics of the transistors. The Schršdinger-Poisson model is attractive due to its simplicity and straightforward implementation by using standard numerical methods. However, as it is required to solve a generalized eigenvalue problem generated from the discretization of the Schršdinger equation, the computational cost of the analysis increases quickly when the system\u27s degrees of freedom (DOFs) increase. For this reason, techniques that enable an efficient solution of discretized Schršdinger equation in multidimensional domains are desirable. In this work, we seek to accelerate the numerical solution of the Schršdinger equation by using a component mode synthesis (CMS) approach. In the CMS approach, a nanostructure is divided into a set of substructures or components and the eigenvalues (energy levels) and eigenvectors (wave functions) are computed first for all the substructures. The computed wave functions are then combined with constraint or attachment modes to construct a transformation matrix. By using the transformation matrix, a reduced-order system of the Schršdinger equation is obtained for the entire nanostructure. The global energy levels and wave functions can be obtained with the reduced-order system. Through an iteration procedure between the Schršdinger and Poisson equations, a self-consistent solution for charge concentration and potential profile can be obtained. In this work, the CMS approach is applied to compute the electrostatic and transport properties of a set of semiconductor devices including a quantum wire and several multiple-gate MOSFETs. It is demonstrated that the CMS approach greatly reduces the computational cost while giving accurate results

Perspectives on thermoelectrics: from fundamentals to device applications

This review is an update of a previous review (A. J. Minnich, et al., Energy Environ. Sci., 2009, 2, 466) published two years ago by some of the co-authors, focusing on progress made in thermoelectrics over the past two years on charge and heat carrier transport, strategies to improve the thermoelectric figure of merit, with new discussions on device physics and applications, and assessing challenges on these topics. Understanding of phonon transport in bulk materials has advanced significantly as the first-principles calculations are applied to thermoelectric materials, and experimental tools are being developed. Some new strategies have been developed to improve electron transport in thermoelectric materials. Fundamental questions on phonon and electron transport across interfaces and in thermoelectric materials remain. With thermoelectric materials reaching high ZT values well above one, the field is ready to take a step forward and go beyond the materials' figure of merit. Developing device contacts and module fabrication techniques, developing a platform for efficiency measurements, and identifying applications are becoming increasingly important for the future of thermoelectrics.MIT Energy InitiativeSolid-State Solar-Thermal Energy Conversion Center (funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-FG02-09ER46577)United States. Dept. of Energy (DOE Grant No. DE-FG02-08ER46516)Robert Bosch Gmb

Charge carrier and phonon transport in nanostructured thermoelectrics

There is currently no quantum mechanical transport model for charge (or phonon) transport in multiphase nano-crystalline structures. Due to absence of periodicity, one cannot apply any of the elegant theorems, such as Bloch's theorem, which are implicit in the basic theory of crystalline solids. Atomistic models such as Kubo and NEGF may assume an accurate knowledge of the interatomic potentials; however, calculations for real 3D random multi-phase systems require so large computational times that makes them practically impossible. In a multi-phase nano-crystalline material, grains and interfacial microstructures may have three distinct types as depicted in figure. In such a material, the physical processes in each individual grain no longer follow the well described classical continuum linear transport theory. Therefore, a proper model for coupled transport of charge carriers and phonons that takes into account the effect of their non-equilibrium energy distribution is highly desirable.Two new theories and associated codes based on Coherent Potential Approximation (CPA) one for electron transport and one for phonon transport are developed. The codes calculate the charge and phonon transport parameters in nanocomposite structures. These can be nano-crystalline (symmetric case) or the material with embedded nano-particles (dispersion case). CPA specifically considers multi-scattering effect that cannot be explained with other semi-classical methods such as Partial Wave or Fermi's golden rule. To our knowledge this is the first CPA code developed to study both charge and phonon transport in nanocomposite structures. The codes can be extend to different types of nano-crystalline materials taking into account the average grain size, as well as the grain size distribution, and volume fraction of the different constituents in the materials. This is a strong tool that can describe more complex systems such as nano-crystals with randomly oriented grains with predictive power for the properties of electrical and thermal properties of disordered nano-crystalline electronic materials

Dynamics of electrons and excitons in nanoclusters and molecules studied by many-body Green's function theory

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 147-150).The development of efficient and economic photovoltaic (PV) systems harvesting solar energy is one of the grand challenges for engineering and scientific researchers. The theoretical conversion limit of a single-junction solar cell is 31% according to Shockley and Queisser (SQ), which the most advanced single-junction PV devices are approaching. Thus it is important to develop new methods and devices that can exceed the SQ limit. An economic strategy that may potentially break the SQ limit is to make use of the unique properties of semiconductor nanoclusters to fabricate PV devices. The physics of semiconductor nanoclusters such as the dynamics of electrons and excitons are the fundamentals for fabricating nanocluster-based PV devices. Although the theories and numerical approaches have been long established for three-dimensional (3D) bulk materials, two-dimensional (2D) graphene-like structures and one-dimensional (ID) nanotubes, the computational methods for zero-dimensional (OD) finite systems based on the most advanced physical theory are not well established. In the thesis, the computational approaches and methods based on the many-body Green's function theory are developed for OD nanoclusters and molecules. The numerical implementations for the calculation of electronic inelastic scattering rates in nanoclusters are established. An efficient computational approach for the calculation of excitonic inelastic scattering rates in nanoclusters is also developed. Both the single-phonon and the multiple-phonon nonradiative relaxation mechanisms in nanoclusters are investigated. It is demonstrated that the nonradiative relaxation of one-particle states and two-particle states are distinctive due to the difference between the density-of-states of one-particle states and two-particle states. Based on the numerical method established in the thesis, a strategy is proposed to reduce the electron-phonon coupling in nanoclusters by pushing valence electron away from nuclei with core electrons in heavy atoms, which is demonstrated with the lead chalcogenide nanoclusters, and porphyrin molecule and a porphyrin derivative.by Yi He.Ph.D

Theory of quantum transport in nano scale structures.

In the pursuit of future nano-scale applications within the field of molecular electronics, extensive investigations into electron transport through single molecules hold significant importance. As single or multiple molecules serve as crucial building blocks for designing and constructing molecular electronic devices, comprehending their electronic and transport properties becomes imperative. Countless theoretical and experimental studies have been conducted to create molecular junctions and explore their electrical performance. This thesis focuses on fundamental aspects of transport theory, employing theoretical and mathematical approaches to investigate electron transport through junctions, particularly involving a scattering region formed by a single molecule connected to metal electrodes. The research methods used are based on a combination of density functional theory, implemented within the SIESTA code, and non-equilibrium Green's function, realized using the GOLLUM code, to delve into electrical conductance on a molecular scale. The objective of this chapter is to address a puzzling paradox concerning meta connectivity, which exhibits destructive quantum interference (DQI) in a tight binding model. However, in certain instances, DQI does not manifest in a DFT calculation on the same system. To shed light on this inconsistency, a selection of molecules is examined, focusing on the distinction between meta and para connectivity. Two different types of linkers, thiol (-SH) and methyl sulphide (-SMe), are employed to couple different molecules to Au electrodes. Through this investigation, we aim to gain insights into the underlying factors that lead to the observed quantum interferencebehaviors. In project two, we conducted a comprehensive study, combining experimental and theoretical approaches, to explore charge transport in stacked graphene-like dimers. Our findings revealed that the interaction between room-temperature quantum interference and stacking significantly influences their highly non-classical electrical conductance. Notably, for the molecule CQI-L, the electrical conductance of the dimer exceeds that of the monomer by a remarkable factor of 25, attributed to the most energetically favorable stacking interactions. Conversely, for the molecule CQI-H, the dimer's conductance is approximately 40 times lower than that of the monomer. These results unequivocally demonstrate that precise control of connectivity to molecular cores, coupled with stacking interactions between their systems, provides a versatile avenue for modifying and optimizing charge transfer between molecules. This discovery is expected to inspire further vigorous research at both macroscopic and microscopic levels

Electrical-thermal energy transfer and energy conversion in semiconductor nanowires

Ph.DDOCTOR OF PHILOSOPH