Thin Ge films were grown which were fully strain-relaxed and had smooth surfaces. This was done on Si(001) surfaces, without using buffer layers, via ultra-high vacuum chemical vapor deposition. The procedure consisted of layer-by-layer Ge growth, with H-surfactant mediation and post-growth annealing (about 700C) for strain relaxation. The key step was the formation of a thin (less than 1nm thick) Si or SiGe capping layer on the layered Ge film before annealing. This capping layer effectively suppressed the clustering of Ge during annealing; even at high temperatures. Cross-sectional transmission electron microscopy of annealed samples with 20nm-thick Ge films clearly revealed a periodic array of 90º fully edge dislocations, with a Burgers vector of a/2<110> type, which were confined to the Ge/Si interface. This dislocation structure led to efficient strain relaxation in the Ge film; as confirmed by X-ray diffraction measurements.

A.Sakai, T.Tatsumi, K.Aoyama: Applied Physics Letters, 1997, 71[24], 3510-2