Simulation tools for three different types of nano devices are under development: Silicon nanowire devices, quantum cascade lasers, and graphene nanoribbon optoelectronic devices.
For silicon nanowires we studied the electronic structure and some transport properties. A two-band k·p model for the silicon conduction band has been extended to take two-dimensional quantum confinement into account. An effective two-dimensional Schrödiger equation defined on the nanowire cross section is solved numerically. An intriguing feature of the two band k·p model is that effects of strain on the band structure can be easily taken into account by adding the respective terms to the Hamiltonian. Using the stiffness tensor for silicon, the application of stress along various axes has been investigated. Different crystallographic orientations as well as circular and rectangular cross section shapes were assumed. The results were found to be in good agreement with subband structures obtained from the tight binding method. Compared with the widely used tight-binding method, the two-band k·p model yields subband structures of similar accuracy, but as a contiuum model it is much more efficient for larger structures than the atomistic tight binding model. Besides the usage of Si nanowires for Field Effect Transistor (FET) applications, we also investiaged their potential as thermoelectric converters. Si nanowires have attracted much attention after the realization that length scale provides an additional degree of freedom in engineering the electronic and thermoelectric transport properties. So, to this aim, we adopted the sp3d5s·-SO tight-binding model. It was found that as the diameter decreases below 6nm, the so-called power factor increases due to band structure effects. Channels with a large number of subbands nearby in energy are beneficial for the power factor and enhance thermoelectric performance.
In the second project we develop a simulator for Quantum Cascade Lasers (QCLs). Cross plane transport through a heterotstructure with a large number of layers has to be conisdered. We developed a Monte Carlo simulator for electron transport incorporating a three valley model for the conduction band of group III/V compound semiconductors. Acoustic and polar optical electron-phonon as well as non-polar intervalley scattering mechanisms are included. The electronic states within a single QCL stage are evaluated using a self-consistent Schrödiger-Poisson solver. An efficient algorithm to evaluate the scattering matrix elements in momentum represenation using fast Fourier transforms has been developed. The subband distribution functions are governed by a Boltzmann-like equation, also known as the Pauli Master equation. In the steady state, only one stage within the infinite cascade of stages needs to be simulated. The current through the device is determined by the amount of interstage scattering.
The third project deals with opto-electronic properties of Graphene NanoRibbons (GNR). The direct band-gap and the tuneability of the band-gap with the nanoribbon's width render these structures as suitable candidates for opto-electronic devices, especially for infrared applications due to the relatively narrow band gap. A general analytical expression for the linear optical conductivity has been obtained, employing an orthogonal tight-binding model with nearest neighbor interaction. The optical transition matrix elements and the resulting selection rules were derived. The non-equilibrium Green's function formalism was employed to perform a comprehensive study of GNR photo detectors. The device response was studied for a wide range of photon energies. The energy conversion efficiency, as a function of the incident photon energy and the ribbon's width, was evaluated and compared to the carbon nanotube counterparts.