The observed degradation of gate oxides and semiconductor-oxide-interfaces in MOS technology can be interpreted in an electrochemical reactions based framework. A theoretical study of the defects and reactions that contribute to a certain degradation mechanism must consider electronic and structural rearrangement and therefore requires a description at the atomic level.
Model structures for amorphous silica have been generated using the van Beest-Kramer-van Sandten (BKS) pairwise potential and a simple melt-quench cycle. These models have been analyzed using the Vienna Ab-Initio Simulation Package (VASP).
Unfortunately, simple potentials assume fixed charges and/or are not able to handle valency. Thus, their applicability to interface structures is quite limited. Reactive ForceFields (ReaxFF) pose a sophisticated empirical approach in the field of atomistic description. They can be seen as an extension to the well-known family of bond-order potentials. When properly trained against reference structures, ReaxFF is able to handle systems including charge transfer and bond breaking. This enables the generation of Si/SiO2 interface structures and the simulation of diffusion processes in the vicinity of the interface.
Electronic structure methods are required to calculate defect levels and evaluate defect candidates for their expected electronic behavior. Quantum mechanical studies of point defects are usually carried out on periodic supercells or isolated molecules that resemble the properties of the structure under study. Both approaches show limited capability to describe long-range adjustments in the lattice due to their boundary conditions. Improvement of accuracy of both of these methods can only be achieved by increasing the calculated region, requiring enormous computational effort. Embedded cluster calculations attempt to solve this issue by partitioning a large (nano-scale) structure into a small part that is treated quantum-mechanically and by using an environment that is described using empirical potentials. This way, relaxations up to several angstroms away from the defect can be observed. The GUESS code relies on the popular Gaussian(tm) software for DFT calculations allowing an all-electron treatment on many different levels of theory.
Pre-trained reactive forcefields have been tested for their description of reactive systems (thermal nucleation of silicon from silane, oxidation of a silicon nanoparticle). The embedded cluster method was used to investigate oxygen vacancies in their different charge states.