Theoretical investigation of structural stability and electronic properties of hydrogenated silicon nanocrystals: Size, shape, and surface reconstruction

By model analysis and the first-principles calculations, we have performed a systematic investigation of hydrogenated silicon nanocrystals (H-SiNCs) with various size, shape, and surface reconstruction. We demonstrated that H-SiNCs with the smallest H/Si ratio are enclosed by (111) facets before reconstruction, and two typical surface reconstructions will dramatically decreases the H/Si ratio, involving dimers and steps on (100) and (111) facets, respectively. With the first-principles calculations, we determined the stable H-SiNCs (up to 200 Si atoms) and found that the surface morphologies vary with the size and hydrogen chemical potential. In addition, we also studied the electronic properties of H-SiNCs focused on the gap variations and charge distributions as a function of size, shape, and surface reconstruction. The dimer and step reconstructions not only bring significant decrement of the gap values and the H/Si ratio, but also modulate the charge distributions and induce the spatial separation of near-gap levels.