Correlations between island nucleation and grain growth for polycrystalline films
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Correlations between island nucleation and grain growth for polycrystalline films C. Eisenmenger-Sittner and A. Bergauer Institut für Angewandte und Technische Physik, Technische Universität Wien, Wiedner Hauptstraße 8-10 A-1040 Vienna, Austria
ABSTRACT For Physical Vapor Deposition (PVD) processes the nucleation and growth of stable islands on monocrystalline and even polycrystalline substrates can be described by the rate equation theory in a mean field approximation. The observable quantity which can be derived from the rate equations is the global density of stable islands. Interisland correlations and island size distributions, on the other hand, cannot be obtained from this formalism. The array of stable islands, in turn, forms the template for the subsequent growth of a polycrystalline thin film. For the initial grain shape and grain size therefore both, the global island density as well as possible interisland correlations play a key role. This paper presents the results of simulations which take into account the effects of both, the global island density as well as island/island correlations on the growth of a thick polycrystalline film. The simulations consist of two steps: first a polycrystalline template is generated from an initial distribution of stable islands by employing an algorithm for the construction of Voronoi-zones. The Voronoi zone network mimics the state of the film just after island coalescence and just before the onset of grain growth. Then the well known qstate Potts model of grain growth is employed to study the further steps of microstructural evolution. A higher initial island density leads to a faster onset of grain growth while a very homogenous distribution of islands significantly retards grain growth. The reasons for these effects are briefly discussed.
INTRODUCTION The process of film formation from the vapor phase is, in its simplest form, the random addition of single particles (atoms, molecules or clusters) to an aggregate of previously deposited atomic or molecular species. Realistic film growth, however, involves the mechanisms of particle transfer along the substrate surface or the growth front which lead to the formation of laterally correlated features. Before the complete coverage of the substrate the lateral transport of the film forming monomers leads to the formation of unstable nuclei and stable islands. These processes are well described by the rate equation theory [1-3] in their mean field approximation. In this stage feature formation is governed by adsorption, surface diffusion, re-evaporation and nucleation. After the formation and growth of stable islands the onset of coalescence leads to the formation of a polycrystalline array. If the thermal desorption of monomers can be neglected the geometry of the single crystalline domains in this array is intimately related to the details of the spatial distribution of the stable islands by the construction of the so-called Voronoi-zone network. The Voronoi Zone is the equivalent to the Wigner Seitz-cell for a random
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