The development of the quantum transport simulator VSP has continued. In order to account for band non-parabolicity, an effective two-band k⋅p Hamiltonian has been implemented. The implementation of an eight-band Hamiltonian is in progress. These band models are used to predict the energy levels in multi-layer hetero-structures. Applications are quantum cascade lasers in the mid infra-red and the THz range.
The quantum transport model of VSP is based on the Non-Equilibrium Green's Function (NEGF) formalism. An effective adaptive energy integration scheme, capable of resolving very narrow resonances in the energy spectrum, enabled the numerical study of tunneling currents in gate stacks by means of the NEGF formalism. This approach rigorously treats both confined and free carriers in the channel and tunneling on a rigorous basis. The effect of scattering processes on level broadening has been approximated using an optical potential. The rigorous NEGF approach has been compared with the simpler quasi-bound-state tunneling model and the Tsu-Esaki model.
Carbon NanoTubes (CNTs) have a direct band gap, which can be tuned by the tube diameter. Therefore, CNTs are suitable candidates for opto-electronic devices, especially for Infra-Red (IR) applications, due to the relatively narrow band gap. IR photo detectors based on Carbon NanoTube Field-Effect Transistors (CNT-FETs) have been reported by many groups. To explore the physics and to improve the performance of such devices, self-consistent quantum mechanical simulations have been carried out. The numerical analysis is based on the NEFG formalism. The numerical models takes into account the tight-binding model for the band structure and self-energy terms for electron-photon and electron-phonon interaction.
A three-dimensional kinetic Monte Carlo simulator for transient phenomena in amorphous organic semiconductor devices has been further developed. The device physics of polymer-based opto-electronic applications is coined by the injection, propagation, extraction and accumulation of charges in layered assemblies of about 100 nm thin polymer films at field strengths of about 1 MV/cm. The software simulates the system's time evolution as a continuous time random walk. The simulator simultaneously follows the time-evolution of both the p- and the n-conductive band. All space charge and image-force effects have been regarded with full rigor. Disorder is modeled by Baessler's well-established Gaussian Disorder Model. The transition of electrons is governed by the classical Miller-Abrahams expression, which turned out to be adequate for bulk and hetero-junction simulations. However, many organic devices exhibit contact-dominated behavior. In this case the molecular structure of the bulk is less critical for a device's electrical characteristics than the chemical and morphologic structure of the bulk-electrode-interfaces. Since the Miller-Abrahams rate turned out to be inadequate for the simulation of charge injection and extraction in real space, it is currently replaced by a more advanced rate expression.