Modification of electronic states of √3×√3-Ag structure by strained Ge/Si(111) substrate

To make clear the relationship between strain and electronic stricture, the strain-induced √3×√3-Ag structure formed on the Ge/Si(111) surface was investigated by using scanning tunneling microscopy and angle resolved ultraviolet photoelectron spectroscopy. The compressive strain is induced by epitaxial growth of Ge on the Si(111) substrate. The interatomic spacing in the surface plane is altered with the coverage of the Ge, and the strain of the surface is also varied. The stress reaches to a maximum value when the Ge layer covers up the surface completely. The band dispersion of the two dimensional metallic state on the √3×√3-Ag surface becomes steeper with the compressive strain, which means a reduction in the effective mass. It is confirmed that the electronic states of the √3×√3-Ag surface could be modified by using the lattice strain.