An intermediate Si1-yCy layer in the Si1-xGex film, replacing the conventionally graded buffer layer, was used to form the high-quality relaxed SiGe substrate. With the 700 nm thick Si0.8Ge0.2 overlayer, such a Si1-xGex/Si1-yCy/Si1-xGex (x=0.2, y-0.014) heterostructure has a threading dislocation density of 5.5 x 10(5) cm(-2) and a residual strain of only 2%. The surface roughness was measured to be about 4.2 nm. The long-range misfit dislocation array was formed mainly at the interface of top Si1-xGex and Si1-yCy. Strained-Si n-channel metal-oxide-semiconductor transistors with various buffer layers were fabricated and examined. Effective electron mobility for the strained-Si device with this substrate technology was found to be 95% higher than that of Si control device. The scheme for the formation of the relaxed Si1-xGex film serving as a virtual substrate shall be applicable to high-speed strained-Si devices. (c) 2005 American Vacuum Society.