Theory and simulations of silicon nanocomposite systems
Authors: Κλεοβούλου Κωνσταντίνος 
Published: September 2021

This thesis focuses on the study of silicon nanocomposite systems using theoryand computer simulations. The major part is dedicated on the study of siliconnanocrystals (Si-NC) embedded in amorphous silicon dioxide (a-SiO2) with great attentiongiven towards the mechanical properties and the spatial arrangement of thesystem. Such analysis is crucial not only for the stability of the system but also forunveiling the origins of the optoelectronic properties. The latter are strongly correlatedwith the mechanical response of the system to spatial and structural changes.A multiscale approach for sensing applications is developed for simulating the earlystage dry oxidation of Si(100) cantilever surface that will ultimately provide a frameworkfor science-based optimization of cantilever sensors. The multiscale formulationcouples Monte Carlo stochastic simulations employed for capturing the oxidation processand continuum level Finite Element simulations for calculating the cantileverdeflection.Continuous-space Monte Carlo simulations using the empirical potential approachare employed. For the interactions, the Tersoff empirical potential parametrizedto describe SiO2 systems is used. This potential describes well the elemental Si properties,silica polymorphs, and phase transitions between them, as well as the structureand energetics of a-SiO2. To further advocate the use of the potential for the currentapplication and ensure the validity of the results the elastic properties of the systemand its bulk components are calculated and compared with previous theoretical andexperimental measurements. surrounding NC, which amplifies considerably as they approach. Nevertheless, It isshown that the system is stable against segregation and phase separation even forclose interparticle distances. The ordering of 3D Si-NC arrangements is investigatedby comparing a conventional cubic and a hexagonal arrangement. For this, a novel computational technique for generating hexagonal arrangements whilst minimizingthe computational effort is introduced. This study, driven by the structural characteristicsand the energetics of the system in thermodynamic equilibrium does notshow preferential ordering in any of the two arrangements.The mechanical response of the system to the NC size and density variation isstudied. The problem of local rigidity is investigated. By analyzing the elastic (bulk)modulus field into atomic contributions, it is showed that it is highly inhomogeneous.It consists of a hard region in the interior of the nanocrystals, and of “superhard”and “supersoft” regions on the nanocrystal periphery. Overall, the nanocrystal bulkmodulus is significantly enhanced compared to the bulk, and its variation with sizeaccurately follows a power-law dependence on the average bond length. The bulkmodulus of the oxide matrix and of the interface region is nearly constant withsize. The average optical (homopolar) gap is directly linked to the elastic and bondlengthvariations. The MC results for the system’s mechanical response against theincrease of NC density are in good mach with the Self Consistent micromechanicsmodel predictions denoting the great importance of the NC-NC interplay.The early dry oxidation of Si(100) is simulated using a 2x1 dimer reconstructed c-Si slab. Tensile surface stresses are generated due to oxidation causing the cantileverto bent. Penetration of stress is limited to the first four layers. The oxygen absorptionreaches a plateau for concentrations higher than 20%. The cantilever deflection iscalculated by the finite element method using the surface stress calculated by theMonte Carlo simulations as a boundary (surface traction) condition. It is anticipatedthat this multiscale formulation will ultimately provide a framework for sciencebasedoptimization of cantilever sensors with improved stability, durability, operatingrange, fictionalization and sensitivity.The stress state of Si-NC/a-SiO2 is investigated and its nature and origins areunraveled and explained. This is achieved by generating detailed stress maps and bycalculating the stress profile as a function of the NC size. For normal oxide matrixdensities, the average stress in the NC core is found to be compressive in excellentagreement with experimental measurements. It drastically declines at the interface,despite the existence of several highly strained geometries. Tensile conditions prevailin NC embedded in densified silica matrices.The NC-NC interplay at various interparticle distances is examined. The decreaseof the interparticle distance introduces structural and chemical deformationon the matrix. This is attributed on the force field exerted on the matrix by the