  Gate control of phosphorus donor based charge qubits in Si is investigated
using a tight-binding approach. Excited molecular states of P2+ are found to
impose limits on the allowed donor separations and operating gate voltages. The
effects of surface (S) and barrier (B) gates are analyzed in various voltage
regimes with respect to the quantum confined states of the whole device.
Effects such as interface ionization, saturation of the tunnel coupling,
sensitivity to donor and gate placement are also studied. It is found that
realistic gate control is smooth for any donor separation, although at certain
donor orientations the S and B gates may get switched in functionality. This
paper outlines and analyzes the various issues that are of importance in
practical control of such donor molecular systems.
