  Atomically precise placement of dopants in Si permits creating substitutional
P nanowires by design. High-resolution images show that these wires are few
atoms wide with some positioning disorder with respect to the substitutional Si
structure sites. Disorder is expected to lead to electronic localization in
one-dimensional (1D) - like structures. Experiments, however, report good
transport properties in quasi-1D P nanoribbons. We investigate theoretically
their electronic properties using an effective single-particle approach based
on a linear combination of donor orbitals (LCDO), with a basis of six orbitals
per donor site, thus keeping the ground state donor orbitals' oscillatory
behavior due to interference among the states at the Si conduction band minima.
Our model for the P positioning errors accounts for the presently achievable
placement precision allowing to study the localization crossover. In addition,
we show that a gate-like potential may control its conductance and localization
length, suggesting the possible use of Si:P nanostructures as elements of
quantum devices, such as nanoswitches and nanowires.
