Highly strained Ge film with atomically flatness and thermal effect on strain relaxation and surface roughness generation in Ge film on Si(100) have been investigated. Strained Ge film is deposited by electron cyclotron resonance Ar plasma enhanced GeH4 reaction without substrate heating. At substrate temperature below 50 degrees C, the deposited Ge film grown with a microwave power of 200 W has larger strain and flat surface than the case by LPCVD at 350 degrees C. By heat-treatment at 200 degrees C, lattice strain in the deposited Ge film scarcely changes. With increasing heat-treatment temperature, strain relaxation and surface roughness generation proceed. These results indicate that, in order to obtain the highly strained Ge film with atomically flatness, lowering substrate temperature during Ge deposition and heat-treatment is necessary. (c) 2005 Elsevier B.V All rights reserved.