Atomistic is a set of tools for the simulation of nanostructures based on atomistic approaches. It contains a Module for structure relaxation of atomic structures based on Valence Force Field (Module vff) and a Module for Empirical Tight Binding calculations (Module empirical_tb). More Modules for atomistic based simulations, such as Density-Functional TB (DFTB) and Non-Equilibrium Green Function (NEGF) for quantum transport, are scheduled for the next releases of tiberCAD. All of these Modules take advantage of a tool developed in tiberCAD for the building and handling of atomistic representations of nanostructures. This built-in Atomistic Generator allows to generate an atomic basis associated with the finite element mesh which belongs to a given physical region, based on the material specifications and the growth directions defined for that region. It supports any crystal structure with fcc, bcc, cubic and hexagonal Bravais lattice and performs hydrogen passivation for any crystal structure supported. Important features of the Atomistic generator are the possibility to apply periodicity to any direction and to import an external atomic structure file in a common format. Random alloy structures can be generated.
The Empirical Tight Binding (ETB) module allows for atomistic-based calculations of electronic and optical properties of Nanowires, Quantum Dots and Quantum Wells. Eigenstates, eigenfunctions and quantum density of a given system can be obtained by solving a tight-binding Hamiltonian by means of the Module empirical_tb. The optical properties are calculated by the Module opticstb. A nearest neighbour tight-binding model is implemented, which supports accurate sp3s*d5 parameterizations for several materials, including GaN/AlGaN/InGaN systems. Combined with the built-in Atomistic Generator and the Module vff for structure relaxation based on Valence Force Field (VFF), ETB Module allows to treat in a fundamental way nanometric features in Quantum Well and Quantum Dot active regions, such as alloy fluctuations. In fact, thanks to the flexibility of the atomistic generator, both Virtual Crystal Approximation (VCA) and random alloy approach may be used to model active regions of electronic and optoelectronic devices. Random alloy representations may provide a realistic picture of a nanostructured LED active region.