Theoretical Investigation Of Structural Stabilities And Electronic Properties Of Silicon Nanocrystals

Silicon plays an important role in the semiconductor industry for its excellent electronicproperties. Silicon nanostructures, such as nanocrystals and nanowires, are greatly applied inelectronic devices and biological areas. Based on the experimental and theoretical researches,electronic and optical properties of silicon nanostructures are determined by the structuraldetails, especially size, shape and surface morphology.In this thesis, we have performed a systematic investigation of hydrogenated siliconnanocrystals （H-SiNCs） with various sizes, shapes, and surface reconstructions by modelanalysis and the first-principles calculations. We demonstrated that H-SiNCs with the smallestH/Si ratio are enclosed by （111） facets before reconstructions, and two typical surfacereconstructions will dramatically decrease the H/Si ratio, involving dimers and steps on （100）and （111） facets, respectively. With the first-principles calculations, we determined the stableH-SiNCs （up to200Si atoms） and found that the surface morphologies vary with the size andhydrogen chemical potential. In addition, we studied the electronic properties of H-SiNCsfocusing on the gap variations and charge distributions as a function of size, shape, andsurface reconstruction. The dimer and step reconstructions not only bring significantdecrement of the gap values and the H/Si ratio, but also modulate the charge distributions andinduce the spatial separation of near-gap levels.We also investigated the modulation of the defect levels of H-SiNCs. The surfacedangling bonds（SDB） are considered as the simplest defects, and the stability of the SDB arestudied in various charged systems. Then we studied the molecular adsorption at SDBs, and itshowed that H₂O and NH₃adsorption could induce the opposite tendency of defect level. Wealso discussed the substrate effect of adsorption, with different number of adsorbed molecules.The result showed that the modulation of defect level can be controlled by the number ofadsorbed molecules.