SiGe superlattice nanocrystal pure and doped with substitutional phosphorus single atom: Density functional theory study

摘要

Ab initio density functional theory is used to simulate electronic structure of hydrogenated SiGe nanocrystal superlattice pure and doped with substitutional P single atom. The results of electronic structure calculations are compared to the same size silicon and germanium nanocrystals. The comparison reveals that the energy gap of the three kinds of nanocrystals is nearly the same in non-relativistic and relativistic limits. Because of large width of gap in the present small nanocrystals the relativistic corrections are not as much important as in the case of bulk crystals. The doping of SiGe nanocrystal with P single atom introduced an impurity level at 4 eV below original conduction band edge. This result is much larger than comparable silicon bulk and nanocrystal doping with P atoms. Results also show that the deep internal angles and bonds in SiGe nanocrystals reach approximately the angles and structure of bulk crystals after nearly three surface layers. A double positively charged layer is located at the Ge terminated surface of SiGe nanocrystal. This layer is enhanced and is accompanied with a large increase of the dipole moment of the nanocrystal in the case of P doped nanocrystal. Due to oscillatory lattice potential in SiGe superlattice, density of states show that bands are broken up to sub-bands in comparison with silicon nanocrystal density of states especially at the conduction band.