  The electronic structure of the three-particle donor bound exciton (D$^0$X)
in silicon is computed using a large-scale atomic orbital tight-binding method
within the Hartree approximation. The calculations yield a transition energy
close to the experimentally measured value of 1150 meV in bulk, and show how
the transition energy and transition probability can change with applied fields
and proximity to surfaces, mimicking the conditions of realistic devices. The
spin-resolved transition energy from a neutral donor state (D$^0$) to D$^0$X
depends on the three-particle Coulomb energy, and the interface and electric
field induced hyperfine splitting and heavy-hole-light-hole splitting. Although
the Coulomb energy decreases as a result of Stark shift, the spatial separation
of the electron and hole wavefunctions by the field also reduces the transition
dipole. A bulk-like D$^0$X dissociates abruptly at a modest electric field,
while a D$^0$X at a donor close to an interface undergoes a gradual ionization
process. Our calculations take into account the full bandstructure of silicon
and the full energy spectrum of the donor including spin directly in the atomic
orbital basis and treat the three-particle Coulomb interaction
self-consistently to provide quantitative guidance to experiments aiming to
realize hybrid opto-electric techniques for addressing donor qubits.
