Focused-ion-beam-based synthesis of semiconductor nanowires and functional structures – an atomistic computer simulation study

We present reaction pathways of a novel method of semiconductor nanowire (NW) fabrication using a focused ion beam (FIB). This investigation is based on kinetic Monte Carlo simulations including realistic FIB implantation profiles which were determined by spatially dependent dynamic ion range calculations. A focused implantation of Si or Ge ions into SiO2 along a straight trace leads to local supersaturation of the implanted species in the dielectric substrate. During post-implantation annealing, semiconductor NWs embedded in the dielectric matrix form along the FIB implantation trace. Even complex structures involving several NWs, e.g. T- or X-junctions, may be obtained. The dominating driving force of NW formation is a free energy gain by phase separation and by reduction of high interface curvatures. The diameter of the synthesized NWs is about five times smaller than the width of the FIB implantation trace. During long-term thermal annealing, NWs disintegrate into regular chains of nanodots because of the built-up of long-wavelength interface undulations (Rayleigh instability). Crosses, corners or ends of NWs are subject to a preferential disintegration. Thus, structures suitable for single-electron-transistors may be realized where an isolated nanocluster is located in a tunnel distance from several NW contacts. Moreover, multi-gate NW field effect transistors may be fabricated by crossing FIB traces of different ion fluences.