Growth of strain-relaxed Ge films on Si(001) surfaces

We have grown thin Ge films that were fully strain relaxed and had smooth surfaces on Si(001) surfaces without buffer layers by ultrahigh vacuum chemical vapor deposition. The procedure consists of layer-by-layer Ge growth with hydrogen-surfactant mediation and high-temperature $\text{(∼700\hspace{0.17em}°}\mathrm{C})$ post-growth annealing for strain relaxation. The key step is the formation of a thin (less than 1 nm thick) capping Si or SiGe layer on the layered Ge film before the annealing. This capping layer effectively suppresses clustering of Ge during the annealing, even at high temperatures. Cross-sectional transmission electron microscopy of annealed samples having a 20-nm-thick Ge film clearly revealed a periodic array of 90° full-edge dislocations with a Burgers vector of ${a}\text{/2〈110〉}$ type confined at the Ge/Si interface. This dislocation structure leads to efficient strain relaxation in the Ge film, which was also confirmed by x-ray diffraction measurement.