  We investigate monocrystalline InAs nanowires (NWs) which are grown by
molecular beam epitaxy (MBE) and induced by focused ion beam (FIB) implanted Au
spots. With this unique combination of methods an increase of the aspect ratio,
i.e. the length to width ratio, of the grown NWs up to 300 was achieved. To
control the morphology and crystalline structure of the NWs, the growth
parameters like temperature, flux ratios and implantation fluence are varied
and optimized. Furthermore, the influence of the used molecular arsenic
species, in particular the As$_2$ to As$_4$ ratio, is investigated and
adjusted. In addition to the high aspect ratio, this optimization results in
the growth of monocrystalline InAs NWs with a negligible number of stacking
faults. Single NWsx were placed site-controlled by FIB implantation, which
supplements the working field of area growth.
