Strained Ge overlayer on a Si(001)-(2X1) surface

The epitaxial growth behavior and the strain distribution of Ge overlayer grown on Si(001)-(2x1) surface were studied with medium energy ion scattering spectroscopy and scanning tunneling microscopy. We were able to grow flat Ge overlayers up to ten monolayers, thicker than the known critical thickness of Stranski-Krastanov growth mode, with hydrogen surfactant, as suggested by recent theories. By comparing the dips of Ge overlayer and Si bulk in the angular scan of ion blocking, we found that the flat Ge overlayer is uniformly strained in the direction perpendicular to a surface while the overlayer with three-dimensional islands is fully relaxed at the thickness of ten monolayers.