Simulation tools for various types of nanoelectronic devices have been developed. Current research projects focus on the modeling of NanoWire (NW) devices, Quantum Cascade Lasers (QCL), and Graphene Nano-Ribbon (GNR) devices. Silicon 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. Thermoelectric devices traditionally show low efficiency. Nanostructured thermoelectric devices with enhanced thermopower compared to their bulk counterparts, however, have recently been reported. Silicon NWs with thermopower two orders of magnitude higher than that of bulk silicon have been demonstrated. To study the electronic structure of semiconductor nanostructures, self-consistent codes employing the sp3d5s*-SO tight-binding model and the k·p model have been developed. Quantum confinement in one, two, and three dimensions can be assumed. The effect of strain can be considered in the band structure calculations. Linearized Boltzmann transport theory including all relevant scattering mechanisms was used to investigate the transport properties of silicon NWs. It was found that size quantization below 10nm can severely effect the electronic properties of NW channels by changing the effective masses, changing the curvature of the bands, and altering degeneracies through valley and subband splitting. The length scale and orientation degrees of freedom in engineering silicon NW channels offer optimization possibilities for nanostructured electronic and thermoelectric devices. It has been shown that the hole mobility, in particular, can be enhanced considerably by size quantization in suitably oriented Si nanowires and ultrathin Si layers.
Another activity in the group is the development of a simulator for QCLs. Cross plane transport through a heterostructure with a large number of layers has to be considered. The electronic states within the QCL stages are evaluated using a self-consistent Schroedinger-Poisson solver based on a k·p Hamiltonian. Electronic transport is governed by the Pauli Master equation. To solve this equation a Monte Carlo simulator incorporating a three valley model for the conduction band of group III/V compound semiconductors has been developed. The relevant electron-phonon scattering mechanisms, as well as scattering by interface roughness, ionized impurities and alloy disorder are taken into account. Simulation results are in good agreement with measured I/V data of QCLs fabricated at the TU-Wien.
Several applications of graphene have been studied, including electronic conduction, thermal conduction, and the opto-electronic properties of GNRs. The direct band-gap and the tunability of the band-gap with the nano-ribbon's width render these structures as suitable candidates for opto-electronic devices, especially for infrared applications due to the relatively narrow band gap. The non-equilibrium Green's function formalism was employed to study various effects in GNR devices, including the photoelectric response of GNR photo detectors, conductance degradation and localization due to line edge roughness, and thermal conductivity reduction in graphene anti-dot lattices.