Island nucleation and growth in epitaxial, amorphous, and nanoparticle thin-films

The goal of this dissertation is to improve our understanding of two of the key stages in thin-film growth: (a) submonolayer island nucleation and growth and (b) multilayer growth. To explore these phenomena we have used a variety of different methods including the numerical integration of stochastic differential equations, self-consistent rate-equation calculations, and kinetic Monte Carlo simulations, as well as scaling and analytical methods.

In the first part of this thesis we study the roughening process of amorphous silicon thin film grown via low-temperature PECVD (plasma-enhanced chemical vapor deposition). By studying a continuum model which takes into account the effects of shadowing as well as the smoothing effects of surface diffusion, excellent agreement is obtained with experiments. One of our key findings is that surface diffusion plays an important role at short to medium time scales and high H₂ dilution ratio, while shadowing effects are dominant at low H₂ dilution ratio and large time scales. We also find that the initial submonolayer morphology plays a key role in determining the evolution of the surface roughness in the later stages of growth.

Motivated by these results, we have also carried out kinetic Monte Carlo simulations as well as a scaling analysis of the submonolayer growth of 3D islands. In this work we demonstrate that the scaling behavior of the island and monomer densities for 3D islands is significantly different from that for 2D islands and is also different from previous theoretical predictions. However, despite these differences we find very little or no difference in the scaling behavior of the island-size distribution between 3D and 2D islands. Self-consistent rate-equation calculations for the average island and monomer densities as a function of coverage and deposition flux are also presented and excellent agreement with simulations is obtained.

Finally, the effects of cluster-diffusion on the scaling of the island-density and island-size distribution in submonolayer growth are studied. This work is motivated by recent drop-drying experiments in which a monodisperse solution of Au nanoparticles is “deposited” at the toluene-air interface as a toluene droplet dries in air. In order to obtain a basic understanding, we have carried out extensive kinetic Monte Carlo simulations of a model of irreversible island nucleation and growth in which the cluster diffusion coefficient is assumed to exhibit power-law decay as a function of island-size with exponent µ. Results are presented for a range of possible values of the exponent µ including µ = 1/2 (corresponding to cluster diffusion via Brownian motion), µ = 1 (corresponding to correlated evaporation-condensation), and µ = 3/2 (corresponding to edge-diffusion), as well as for higher values including µ = 2, 3, and 6. In general, we find that cluster diffusion leads to a significant broadening of the island-size distribution and also alters the scaling exponent χ which describes the dependence of the island-density on deposition flux. Results are also presented for the case of reversible growth corresponding to a critical island-size larger than one. In this case, we find that the scaled island-size distribution sharpens with increasing critical island-size and appears to approach the experimentally observed island-size distribution.