Deposition and growth of Ag on Si(111) surfaces studied by optical second-harmonic generation

Optical second-harmonic generation has been used to follow in situ the growth of thin Ag films deposited on Si(111) under various substrate conditions and temperatures. This method is particularly sensitive to the microscopic structure of the deposited Ag films. Thus, island formation is found to enhance the second-harmonic signal, and strongly temperature-dependent island formations were indeed observed at lower Ag coverages. For a film thickness above 25 monolayers, identical film roughness characteristics were found for growth temperatures between 200 and 450 K and for different types of substrate reconstruction. A layer-by-layer growth mode can be obtained for low substrate temperatures or for surface passivation by oxygen or hydrogen. In such cases the second-harmonic signal oscillates. The period of oscillation is 10 monolayers. These oscillations are due to the formation of quantum-well states in the film.