Carbon nanotubes exhibiting a direct band gap have recently been studied for opto-electronic applications. An intriguing fact is that the band gap can be tuned with the tube diameter. We have analyzed the performance of infra-red photo detectors based on Carbon NanoTube Field-Effect Transistors (CNT-FET). The well established non-equilibrium Green's function formalism is applied for this purpose. The ratio of photo current to dark current is generally very low and limits the performance of such devices, as has been experimentally observed. Our theoretical study demonstrated that the dark current can be significantly suppressed by using a double gate structure. It turned out that the local scattering approximation, which is well justified in the case of deformation potential interaction, is not valid for electron-photon interaction. Instead, for accurate simulation of the photo current, a large number of off-diagonals in the self-energy matrix need to be taken into account. The simulator for carbon nanotubes has been extended to Graphene Nano Ribbons (GNRs) and the first results on transport in GNR heterostructures have been obtained.
The general purpose Schrödiger-Poisson solver VSP has been extended in several ways. To study the effect of general strain on the conduction band of silicon, a two-band k·p method has been implemented. Based on a degenerate perturbation ansatz at the X-point, this method effectively describes the effects of shear strain, non-parabolicity and warping of the conduction band valleys. The calculation of the electron sheet density involves a numerical, two-dimensional k-space integration. For this purpose, a Clenshaw-Curtis integration scheme has been implemented. Another ongoing development of VSP deals with quantum cascade structures. A density matrix approach that enables efficient simulation of cross-plane electronic transport is currently being developed.