Experimental investigation of band structure modification in silicon nanocrystals - art. no. 115308

Experimental studies of size-related effects in silicon nanocrystals are re ported. We present investigations carried out on nanocrystals prepared from single-crystal Si:P wafer by ball milling. The average final grain dimensi on varied depending on the way of preparation in the range between 70 and 2 30 nm. The ball milling was followed by sedimentation and selection of the smallest grains. The initial grain size distribution was measured by scanni ng electron microscopy. Further reduction in size was achieved by oxidation at 1000 degreesC which creates a silicon dioxide layer around a silicon co re. The oxidation process was monitored by transmission electron microscopy and the growth speed of SiO2 was estimated in order to model the grain siz e of nanocrystals. Crystallinity of silicon grains was confirmed by x-ray d iffraction and by transmission electron microscopy using a bright/dark fiel d method and selected area diffraction pattern. In the silicon nanocrystals the electron energy levels are shifted which was observed separately for c onduction band. valence band and energy band gap. Electron paramagnetic res onance was applied to investigate variation of the conduction band minimum by monitoring its influence on the hyperfine interaction of phosphorus shal low donor. On the basis of these results an explicit expression for conduct ion band upshift as a function of average grain size has been derived. Info rmation about the downshift of the valence band was obtained from measureme nts on a photoluminescence band related to a deep to shallow level transiti on. A perturbation of a few meV for grain sizes of the order about 100 nm h as been observed. Internal consistency of these findings has been examined by investigation of the photoluminescence band due to an electron-hole reco mbination whose energy is directly related to the band gap of silicon.