The power of parallel supercomputing resources has progressed to the point where first-principles calculations involving systems up to 10³ atoms are feasible, allowing ab initio exploration of increasingly complex systems such as amorphous networks, nanostructures, and large defect clusters. Expansion of our fundamental understanding of modified Si-based materials is paramount, as these materials will likely flourish in the foreseeable cost-driven future in diverse micro- and nanotechnologies. Here, density-functional theory calculations within the generalized gradient approximation are applied to refine configurations of Si-based materials generated from Metropolis Monte Carlo simulations and study their resultant structural properties. Particular emphasis is given to the contributions of strain and disorder on the mechanical, optical, and electronic properties of modified Si-based materials in which aspects of compositional variation, phase, strain scheme, morphology, native defect incorporation, and quantum confinement are considered. The simulation strategies discussed are easily extendable to other semiconductor systems. / text

We have developed continuous random network (CRN) model based Metropolis Monte Carlo simulation tools which are capable of predicting the structural properties of amorphous semiconductor and oxide materials as well as their interface. To bolster the reliability of the CRN model, we have developed force fields based on gradient corrected density functional theory (DFT) calculations. Our in-house CRN-MMC tools have been massively parallelized, which allows us to create fairly large model structures within a reasonable computational time. Using the integrated CRN-MMC tools, we have elucidated the complex growth and structure of self-interstitial and vacancy clusters in silicon and the effect of strain on the structure and stability of the defect clusters. Our work for vacancy clusters suggests that small vacancy defects exclusively favor fourfold-coordination thermodynamically with no significant kinetic limitation rather than void-like structure formation, which has widely been adapted to explain the behavior and properties of vacancy defects. Our results also highlight the identification of stable high-symmetry fourfold-coordinated V₁₂ and V₃₂ clusters that could be expected to exist to a large extent in a vacancy rich region although its direct characterization appears impractical at present. Our work for self-interstitial clusters provides the first theoretical support for earlier experiments which suggest a shape transition from compact to elongated structures around n = 10. When the cluster size is smaller than 10, the stable I₄ and I₈ compact clusters are found to inhibit the formation of elongated defects, whereas the newly discovered fourfold-coordinated I₁₂ state is found to serve as an effective nucleation center for large extended defects. Our CRN-MMC approach also enabled us to elucidate the underlying mechanisms of synthesis and manipulation of Si rich insulators as well as the fundamental understanding of the relationship between the atomic structure and properties. We developed a valence force field based on a modified Keating model for the structure and energetics of amorphous Si rich oxide materials. In particular, our work emphasizes the importance of correctly describing the wide Si-O-Si angle distribution. Our work also suggests that the relative rigidity between Si and SiO₂ matrices is critical in determination of the Si/SiO₂ interface structure. The present potential model coupled with the CRN-MMC method can be used to create structural models (free of coordination defects) for complex a-SiO[subscript x]-based materials, which will further allow thorough studies of the properties of these materials. / text