Surface diffusion: A computer study of its effects on thin film morphology.
AuthorSargent, Robert Bruce.
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PublisherThe University of Arizona.
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AbstractA two-dimensional hard-disk model of thin-film deposition is described; it is of the type originally introduced by Henderson, Brodsky, and Chaudhari (1974). We have implemented a simple (and necessarily approximate) way to incorporate the effects of surface diffusion in our model, and a means to connect the input parameters of the computer algorithm to the evaporation parameters of substrate temperature and evaporation rate. In the limit of no surface diffusion (low substrate temperature), the model predicts a dendritic structure with large voids; this is the Henderson model. With sufficient surface diffusion (higher substrate temperature), a structure of closely packed crystallites is predicted, and the root-mean-square surface roughness is less than half that predicted by a Henderson-type simulation. This dependence of microstructure on substrate temperature is similar to a zone transition originally described by Movchan and Demchishin (1969) in metal and oxide films. We consider the effect of changing the angle of vapor incidence from normal to oblique. As this angle is increased, a certain critical angle is reached, at which the film density drops and the surface roughness rises precipitously. Both effects result from large columnar voids that develop; the structure of the material that comprises the columns between the voids is similar to the structure of depositions simulated at normal vapor incidence. In a separate study, we simulate the growth of a thin film in two dimensions with a computer implementation of the molecular dynamics (MD) method. The system consists of a krypton substrate maintained at a temperature of 10 degrees Kelvin, toward which argon atoms are periodically directed (with a velocity corresponding to 120 degrees Kelvin). The resulting argon film follows the (horizontal) spacing of the krypton lattice until the thickness approaches an average thickness of about ten monolayers. As deposition proceeds, the configuration of the film changes to incorporate an edge misfit dislocation at the film-substrate interface; this relieves the interfacial stress. We also apply the MD method to study the relaxation of thin-film structures predicted by the hard-disk growth model.
Degree ProgramOptical Sciences