Coexistent compressive and tensile strain in Ag thin films on Si(1 1 1)-(7×7) surfaces

Growth and strain behavior of thin Ag films on Si substrates have been investigated by scanning tunneling microscopy, cross-sectional transmission electron microscopy and high resolution X-ray diffraction studies. Ag islands formed on Si at room temperature growth show strongly preferred heights and flat top. At low coverage [≳1 monolayer (ML)], Ag islands with (1 1 1) orientation containing two atomic layers of Ag are overwhelmingly formed [D.K. Goswami, K. Bhattacharjee, B. Satpati, S. Roy, P.V. Satyam, B.N. Dev, Surf. Sci. 601 (2007) 603]. A thicker (40 ML) annealed film shows two closely spaced Ag(1 1 1) diffraction peaks-one weak and broad and the other narrow and more intense. The broad peak corresponds to an average expansion (0.21%) and the narrow intense peak corresponds to a contraction (0.17%) of the Ag(1 1 1) planar spacing compared to the bulk value. This coexistence of compressive and tensile strain can be explained in terms of changes in the Ag lattice during the heating-cooling cycle due to thermal expansion coefficient mismatch between Ag and Si.