Heavily indium (In)-doped Si crystals were grown by the Czochralski method under a consideration of the effects of co-doping of electrically neutral group-IV elements (C, Ge or Sn). The In concentration in In-doped Si increased with the amount of In charged into the crucible and reached 3.5 x 1017 cm(-3). The carrier concentration was at most 6 x 10(16) cm(-3), limited by the low ionization ratio of similar to 20% of In. Co-doping of C and Ge effectively enhanced the In concentration while Sn did not, which was examined in terms of the atomistic size, lattice parameter change, mutual bonding energy and solubility of groupIV elements in Si. However, no sufficient increase in carrier concentrations was detected in Si by the codoping, and formation of some clusters or complexes was suggested. (C) 2013 Elsevier B.V. All rights reserved.