The formation of voids in ion-implanted Ge was studied as a function of ion implantation energy and dose. (001) Ge substrates were self-implanted at energies of 20-300 kev to doses of 1.0 x 10(13)-1.0 x 10(17) cm(-2), Transmission electron microscopy revealed clusters of voids just below the surface for implant energies <= 120 kev at a dose of 2.0 x 10(15) cm(-2) and complete surface coverage for an implant energy of 130 kev and doses >= 1.0x10(16) cm(-2). Void clusters did not change in size or density after isothermal annealing at 330 degrees C for 176 min. The initial void formation is discussed in terms of the vacancy clustering and "microexplosion" theories with a damage map detailing the implant conditions necessary to produce voids. (C) 2011 Elsevier B.V. All rights reserved.