Growth of high-quality relaxed SiGe films with an intermediate Si1−yCy layer for strained Si n-MOSFETs

High quality and thin relaxed SiGe films were grown on Si (001) using ultra high vacuum chemical vapor deposition (UHV/CVD) by employing an intermediate Si1−yCy layer. The Si1−yCy/SiGe bilayer was found to change mechanism of relaxation in the SiGe overlayer. Compared with the samples with a Si layer, the equilibrium critical thickness of top SiGe films with rough surface by introducing an intermediate Si0.986C0.014 layer are drastically reduced; this result was attributed to larger tensile stress in the inserted Si0.986C0.014 layer. With a 210-nm-thick Si0.8Ge0.2 overlayer, this Si0.8Ge0.2/Si0.986C0.014/Si0.8Ge0.2 heterostructure has a threading dislocation density (TDs) less than 1×105cm−2 and a residual strain of 30%. The root mean square (RMS) of surface roughness for this sample was measured to be about 1.8nm. In this SiGe/Si1−yCy/SiGe structure, C atoms in the intermediate Si layer will improve the relaxation of thin SiGe overlayer, however, the relaxation for the 700-nm-thick SiGe overlayer is independent of the addition of C. The point defects rich Si0.986C0.014 layer plays the role to confine the misfit dislocations, which formed at the interface of the top Si0.8Ge0.2 and the Si0.986C0.014 layer, and blocked the propagation of TDs. Strained-Si n-channel metal-oxide-semiconductor transistors (n-MOSFETs) with a 210-nm-thick Si0.8Ge0.2 overlayers as buffer were fabricated and examined. Drain current and effective electron mobility for the strained-Si device with this novel substrate technology was found to be 100 and 63% higher than that of control Si device. Our results show that thin relaxed Si0.8Ge0.2 films with the intermediate Si0.986C0.014 layer serve as good candidates for high-speed strained-Si devices.