Direct observation of the nucleation and growth modes of Ag/Si(111)

Abstract

Direct observation in an ultrahigh vacuum scanning electron microscope (SEM) has revealed the growth modes of Ag on Si(111), previously deduced indirectly from analysis of Auger electron spectra, and the form and number of crystallites as a function of deposition time and temperature. At high temperatures, T s > 200°C, the silver grows definitively in a Stranski-Krastanov (layer plus island) growth mode, with a very strong dependence of the island density on the deposition temperature, varying from ≲ l0 6 to ∼ 10 10 cm −2 between T s = 500 and 200°C. The crystals have more or less regular hexagonal forms with (111) and {1 1 1} faces predominating with a height to width ratio which decreases with deposition time in the range 0.1–0.6. At temperatures below 200°C, the island density is too high to be observed directly by SEM, and silver appears to grow in a Frank-van der Merwe (layer by layer) mode. However, this uniform deposit is unstable on heating above 200°C, and annealed deposits recreate rather similar islands. An analysis of published Auger amplitude-time curves is made to estimate the initial island density produced by depositions below T s = 200° C, and it is shown that these densities constitute a very reasonable extrapolation of the higher temperature SEM values. A model of Stranski-Krastanov growth is given in outline and applied to Ag/Si(111). It is argued that the island density is determined largely by island instability at high temperatures, even though condensation is complete. At low temperatures it is argued that Stranski-Krastanov growth becomes essentially equivalent to Frank-van der Merwe growth for high enough island density, for kinetic rather than thermodynamic reasons, without any change in the basic Stranski-Krastanov mechanism.