Abruptness of axial Si-Ge heterojunctions in nanowires

The process of Si-Ge heterostructures formation in nanowhiskers (NWs) grown by the vapor-liquid-solid mechanism was investigated using Monte Carlo simulation. It was demonstrated that it is impossible to grow atomically abrupt axial heterojunctions via classical vapor-liquid-solid mechanism due to gradual change of catalyst drop composition when switching the fluxes. Dependences of Ge_xSi_1-x composition in transition region on the ratio of germanium and silicon fluxes and deposition duration were investigated. The width of Si-Ge axial heterojunction was found to be dependent on NW diameter d. In adsorption-induced growth mode the width has linear dependence on d and in diffusion-induced growth mode it is either constant (when growth rate ~1/d) or proportional to 1/d (when growth rate ~1/d²).