Silicon NanoWires (NWs) have attracted much attention as efficient electronic and thermoelectric devices after it was realized that length scale provides an additional degree of freedom in engineering their electronic and thermal transport properties. We use the sp3d5s*-SO tight-binding model and Boltzmann transport theory to investigate thermoelectric transport in silicon NWs. Specifically for p-type NWs a large anisotropy in the thermoelectric power factor was found for smaller diameters. As the diameter is scaled to 3nm, the power factor of the [111] orientation and secondly the [110] NWs largely increase, whereas that of the [100] NWs remains low. This behavior originates from confinement-induced curvature changes in the subbands. A similar behavior was observed for ultra-thin Si layers, in which case the power factor of the p-type (110)/[110] channel outperforms the power factor of all differently oriented channels. Furthermore, we studied the thermoelectric properties of graphene antidot lattices. The electronic structure of graphene antidot lattices has been calculated using a tight-binding Hamiltonian. Our results indicate that a direct bandgap can be achieved by properly engineering the geometrical properties of the antidots. For the phononic structure we used a fourth nearest neighbor force constant method. By introducing antidots into the graphene sheet, some phonon modes become localized and do not contribute to the thermal conductance. Thus the thermal conductivity of graphene antidot lattices decreases and the respective ZT value increases.
To model electronic transport in quantum cascade lasers we utilize the Pauli Master Equation (PME). A Monte Carlo (MC) simulator was implemented in C++ in the Vienna Schrödinger Poisson (VSP) model framework. The basis states for the PME solver are calculated using a k·p model. Perfectly matched layer boundary conditions model the openness of the quantum system. To account for the periodic heterostructure of a quantum cascade laser, three stages are considered in the model. Wave functions are assigned to a single stage using an automated routine. The resulting wave functions are then orthogonalized prior to the transport calculation. The model provides insight on macroscopic and microscopic quantities such as current- voltage characteristics, scattering rates, carrier density spectrum, subband population, and optical gain. In addition, the VSP simulation framework is used to solve the Maxwell equations. In this way the electromagnetic modes of optical cavities can be determined. Periodic boundary conditions have been implemented, which allow the study of photonic crystals in real space.
Recently, single-layer hexagonal Boron Nitride (BN), which is a wide-bandgap semiconductor, and BN nanoribbons have attracted much interest. The properties of BN nanoribbons are qualitatively different from those of hydrogen-passivated graphene nanoribbons. Carbon atoms incorporated in a BN lattice have a stable hexagonal configuration and can form a one- dimensional nanoribbon. Armchair graphene nanoribbons embedded in BN sheets become semiconductors. The bandgap opening in these structures is primarily due to the perturbation of the on- site potentials of the edge atoms. The relatively large direct bandgap of graphene nanoribbons embedded in BN makes them useful for optoelectronic applications.
We have theoretically studied for the first time the optical properties of armchair graphene nanoribbons embedded in BN.