Experimental investigation of band structure modification in silicon nanocrystals

Abstract

Experimental studies of size-related effects in silicon nanocrystals are reported. We present investigations carried out on nanocrystals prepared from single-crystal Si:P wafer by ball milling. The average final grain dimension varied depending on the way of preparation in the range between 70 and 230 nm. The ball milling was followed by sedimentation and selection of the smallest grains. The initial grain size distribution was measured by scanning electron microscopy. Further reduction in size was achieved by oxidation at 1000°C which creates a silicon dioxide layer around a silicon core. The oxidation process was monitored by transmission electron microscopy and the growth speed of SiO 2 was estimated in order to model the grain size of nanocrystals. Crystallinity of silicon grains was confirmed by x-ray diffraction and by transmission electron microscopy using a bright/dark field method and selected area diffraction pattern. In the silicon nanocrystals the electron energy levels are shifted which was observed separately for conduction band, valence band and energy band gap. Electron paramagnetic resonance was applied to investigate variation of the conduction band minimum by monitoring its influence on the hyperfine interaction of phosphorus shallow donor. On the basis of these results an explicit expression for conduction band upshift as a function of average grain size has been derived. Information about the downshift of the valence band was obtained from measurements on a photoluminescence band related to a deep to shallow level transition. A perturbation of a few meV for grain sizes of the order about 100 nm has been observed. Internal consistency of these findings has been examined by investigation of the photoluminescence band due to an electron-hole recombination whose energy is directly related to the band gap of silicon.

Hallmarks of mechanochemistry: from nanoparticles to technology

“…Further works performed in particular by Shen et al have confirmed the crystalline to amorphous phase transition induced by ball milling. 195,196 Two amorphous phases and nanocrystalline Si were produced by ball milling of polycrystalline elemental Si. The nanocrystalline component contains defects such as dislocations, twins, and stacking faults which are typical for coarse-grained polycrystalline materials.…”

Investigation of the electronic structure of the phosphorus‐doped Si and SiO2:Si quantum dots by XPS and HREELS methods

“…2). Figure 4 shows (from top to bottom) the high-resolution electron energy loss spectra (HREELS) from the (100) surface of the Si single crystal surface in the initial state (1), after P doping (2), and the differential spectrum (3) obtained by subtracting spectrum 1 from spectrum 2. The subtraction of spectra allowed the removal of the structure of spectra tied with inelastic losses, and to amplify the input of P doping in the picture of characteristic losses.…”

“…In this system, strong visible and near-infrared photoluminescence has been revealed 1,2 that is believed to be related to silicon nanocrystals. Light emission with the energy exceeding the band gap of bulk silicon has been reported in nanocrystals of a few nanometre size.…”

Porous Silicon Formation by Mechanical Means

“…The oxidation of as-milled Si particles at higher temperature (e.g., 1,273 K for 5 min-2 h in air) leads to reduction of Si crystals size (crystal core) and SiO 2 shell formation, surrounding the Si core (Pawlak et al 2001). So, the treatment is useful in reducing the concentration of unsaturated bonds.…”

“…Any liquid agent will affect the grinding kinetics (Stevulova et al 1997). Another way for achieving the separated and well-sized defined particles is ultrasonic post ball milling treatment with lengthy particle sedimentation techniques applied after milling (Pawlak et al 2001).…”