Full-band NEGF Analysis of Nanostructured Devices for Optoelectronics

This thesis aims to present a full-band description for the modeling and simulation of optoelectronic devices. To do this, the nonequilibrium Green's function (NEGF) formalism is used to accurately describe the quantum processes affecting carrier dynamics within the device, everything coupled through a self-consistent (SC) loop which is iterated to achieve the correct device properties under the given input conditions. Simulation of nanoscale devices should take into account several quantum processes that dominate the device behavior at these dimensions. Consequently, quantum models have acquired such a great importance nowadays in the modeling of microelectronic and optoelectronic devices, specially in the case of LED's and solar cells, whose many problems arising from the low efficiency and losses make it imperative to understand what are the real phenomena behind them. In order to obtain a high fidelity representation of the device behavior, the empirical tight-binding (ETB) basis is used to obtain a multi-band model of the device dispersion. This representation is essential so that one is able to simulate intra or inter-band transition due to quasi particle absorption/emission, which are some of the central phenomena involved in the operation of photo-detectors and LEDs. Subsequently, carrier dynamics are obtained in the NEGF formalism by solving the Dyson's equation of motion in steady-state conditions to obtain the device Green's function, and particle interactions are included in the device through the corresponding self-energies. Enormous complexity of the used model makes it necessary to keep a rather simple approach, including just the most important interactions to model scattering processes: electron-photon and electron-phonon interactions, being the later with both acoustic and optical phonons. It is remarkable that the self-energies included are enough to model most of the fundamental quantum phenomena in optoelectronics, like photogeneration, transport, relaxation and recombination of carriers. The Green's functions and the self-energies are then computed self consistently within the so called self-consistent Born approximation (SCBA), and the effects of coupling the device to semi-infinite contacts is taken into account using boundary self-energies. The theory presented in this work is used to simulate different nanostructured devices which are found in literature and which allow to clearly see all the capabilities of the NEGF formalism. In particular, three examples are considered: a AlGaAs/GaAs quantum well heterostructure, a type-two InAs/GaSb superlattice absorber, and a InAlGaAs/InGaAs interband tunnel-junction. These structures were chosen due to their rather simple configuration, which allows to perform multi-band computations within a reasonable simulation time, and due to their great importance in optoelectronic applications. Device performance is also studied by varying geometrical and simulation parameters, taking special attention to the transport mechanisms, the localization of carriers, and the formation of minibands and localized states along the device area. Finally, the results are compared with a kp NEGF implementation, revealing some important differences and similarities of these two models, and some final improvements will be suggested to be implemented in future works, which will help to speed up and enhance the computational capabilities of the present implementation.