A single-grid UHV-compatible ion source was used to provide partially-ionized accelerated In+ dopant beams during Si growth by molecular beam epitaxy (MBE). Indium incorporation probabilities in 800 °C MBE Si(100). as measured by secondary ion mass spectrometry, ranged from < 10−5 (the detection limit) for thermal In to values of 0.02–0.7 for In+ acceleration energies EIn, between 50 and 400 eV. Temperature-dependent Hall-effect and resistivity measurements were carried out on Si films grown at 800 °C with EIn = 200 eV. Indium was incorporated substitutionally in electrically active sites over the entire concentration range examined. 1016— 1019 cm−3, with an acceptor level ionization energy of 165 meV. The 111 meV level associated with In-C complexes and the 18 meV “supershallow” level reported for In ion-implanted Si were not observed. Roomtemperature hole mobilities μ were higher than both annealed In-ion-implanted Si and Irvin's values for bulk Si. Phonon scattering was found to dominate at temperatures between 100 and 330 K and μ varied as T−22.