  We present a systematic study of various ways (top gates, local doping,
substrate bias) to fabricate and tune multi-dot structures in silicon nanowire
multigate MOSFETs (metal-oxide-semiconductor field-effect transistors). The
carrier concentration profile of the silicon nanowire is a key parameter to
control the formation of tunnel barriers and single-electron islands. It is
determined both by the doping profile of the nanowire and by the voltages
applied to the top gates and to the substrate. Local doping is achieved with
the realisation of up to two arsenic implantation steps in combination with
gates and nitride spacers acting as a mask. We compare nominally identical
devices with different implantations and different voltages applied to the
substrate, leading to the realisation of both intrinsic and doped coupled dot
structures. We demonstrate devices in which all the tunnel resistances towards
the electrodes and between the dots can be independently tuned with the control
top gates wrapping the silicon nanowire.
