An ion beam-assisted deposition (IBAD) technique has been attempted for molecular beam epitaxy (MBE)-grown Ge1-xCx/Si(001) to increase the substitutional C content x and to improve the crystallinity. The GeC epilayers were grown with irradiating Ar+ ions by using a differentially pumped Kaufman-type ion gun. The acceleration energy E-a of Ar+ ions was varied from 0 to 500 eV. The substitutional C content x increased from 2.5% to 3.0% with changing the E-a-value from 0 to 200 eV. The increase of x-value in Ge1-xCx epilayers was attributed to atomic mixing induced by the Ar+ ion irradiation. For E-a greater than or equal to 300 eV, the x-value remarkably decreased with increasing energy E-a. The GeC epilayers with better crystallinity were grown by increasing energy Ea under the IBAD. (C) 2003 Elsevier B.V. All rights reserved.