  We report a detailed study of low-temperature (mK) transport properties of a
silicon double-dot system fabricated by phosphorous ion implantation. The
device under study consists of two phosphorous nanoscale islands doped to above
the metal-insulator transition, separated from each other and the source and
drain reservoirs by nominally undoped (intrinsic) silicon tunnel barriers.
Metallic control gates, together with an Al-AlOx single-electron transistor,
were positioned on the substrate surface, capacitively coupled to the buried
dots. The individual double-dot charge states were probed using source-drain
bias spectroscopy combined with non-invasive SET charge sensing. The system was
measured in linear (VSD = 0) and non-linear (VSD <> 0) regimes allowing
calculations of the relevant capacitances. Simultaneous detection using both
SET sensing and source-drain current measurements was demonstrated, providing a
valuable combination for the analysis of the system. Evolution of the triple
points with applied bias was observed using both charge and current sensing.
Coulomb diamonds, showing the interplay between the Coulomb charging effects of
the two dots, were measured using simultaneous detection and compared with
numerical simulations.
