Structural investigation of silicon nanowires using GIXD and GISAXS: Evidence of complex saw-tooth faceting

We present the results of X-ray experiments on silicon nanowires grown on 〈111〉-oriented silicon substrate using the vapor liquid solid method. Grazing incidence X-ray diffraction shows that nanowires are in epitaxy on the substrate and have a hexagonal cross-section. The orientations of the sides are then determined. Grazing incidence small-angle X-ray scattering experiments reveal fine saw-tooth faceting of the sides of the nanowires. This fine saw-tooth faceting appears with alternating upward and downward orientations on each side of the nanowires, reflecting the trigonal symmetry of the nanowires. The crystallographic orientation of some of these facets is then determined. Finally, it is observed that large-diameter nanowires (diameter larger than 200 nm) exhibit six additional faces that truncate the edge of the usual hexagonal cross-section of the nanowires. These additional faces also show saw-tooth faceting which is tilted with respect to the horizontal and seems to be present only around the top of the nanowires.