We have investigated layer-by-layer Ge growth methods on phosphorus (P)-doped Si(001) with our sputter epitaxy method at a growth temperature (T-G) of 350 degrees C. With the sputter epitaxy method, relaxed Ge islands are formed on P-doped 3.5 Omega cm Si with Ge-Si intermixing at the Ge/Si interface; however, a partially strained flat Ge layer is grown directly on P-doped 0.015 Omega cm Si. For comparison with a gas-source molecular beam epitaxy (GS-MBE) method using GeH4, Ge islands are formed on 0.015 Omega cm Si at T-G = 350 degrees C. It has been suggested that the P dopants together with the sputter epitaxy method effectively suppress Ge islanding and induce Ge layer-by-layer growth. (C) 2012 The Japan Society of Applied Physics