Lian Guo

Island growth in electrodeposition

Electrochemical deposition of metals onto foreign substrates usually occurs through Volmer–Weber island growth. The mechanism of island nucleation and growth dictates the shape, orientation and number density of islands, and ultimately, the structure and properties of thin films. With increasing emphasis on deposition of ultrathin films and nanostructures, it is critically important to understand the kinetics of nucleation and growth. Here we provide a comprehensive review of island growth in electrodeposition and summarize methods for mechanistic analysis in both the kinetic and diffusion limited regimes.

Electrodeposition of Copper on Oxidized Ruthenium

Ruthenium is an alternative liner material for copper metallization. We report on the nucleation and growth of copper on ruthenium in acidified copper sulfate solution and explore the influence of the PEG-Cl-SPS additive system. We show that the physical vapor deposition ruthenium has an oxide layer approximately 1 nm thick. The additives and copper concentration have a relatively small influence on island density but a significant influence on the island morphology. Without additives, both randomly oriented hemispherical islands and disk-shaped islands with a (111) orientation are observed. The distribution of islands is dependent on potential. In the presence of additives, the islands are irregular in shape due to renucleation that becomes more pronounced at higher concentrations and lower overpotentials. Analysis of the island size distributions implies that nucleation is fast in comparison to growth. The island density is exponentially dependent on potential increasing from 10 7 cm -2 at low overpotentials to almost 10 11 cm -2 at larger overpotentials. The distribution of islands at the surface is shown to be consistent with complete spatial randomness.

Tunnel barrier photoelectrodes for solar water splitting

Tunnel barrier photoelectrodes, with a thin inorganic tunnel barrier layer that isolates the semiconductor electrode from the electrolyte while allowing current flow across the interface, are a possible solution to the problem of photocorrosion in solar water splitting. In this approach, selection of the semiconductor for the light absorber is decoupled from selection of the tunnel barrier material that provides chemical stability. Here we demonstrate a proof-of-principle of this approach with TiO2/MgO tunnel barrier photoelectrodes.

Anisotropic island growth: a new approach to thin film electrocrystallization.

The electrochemical deposition of metals onto foreign substrates usually occurs through Volmer-Weber island growth, and hence the structure and properties of thin films are critically dependent on the mechanism of nucleation and growth. For example, high nucleus densities are essential for achieving island coalescence at small thickness. Here we demonstrate a new approach to controlling thin film microstructure through the control of island geometry. By promoting anisotropic island growth, film coalescence can be achieved at smaller thickness and with lower island densities.

Formation of a Core/Shell Microstructure in Cu–Ni Thin Films

Electrodeposition of Cu-Ni thin films can result in phase separation characterized by the formation of nodular features that exhibit a uniform columnar core/shell structure with a copper-rich core and nickel-rich shell. Here, we show that the core/shell microstructure is the result of differences in the nucleation and growth rates of the two components. In the potential range where the core/shell structure is observed, copper deposition is fast, resulting in the formation of a relatively low density of large hemispherical islands. Nickel deposition is characterized by slower kinetics, resulting in the formation of a high density of small islands surrounding the copper islands. These results provide a basis for understanding the formation of this core/shell microstructure in binary alloy systems.

Simulations of Island Growth and Island Spatial Distribution during Electrodeposition

Recently we have developed a kinetic Monte Carlo model of nucleation and island growth during bulk electrodeposition. In this article we report on the growth rate of individual islands and the spatial distribution of islands on the surface. The growth curves of individual islands show two power law regimes with exponents of 1/2 and 1/6, respectively, and we show that the time associated with the transition between the two regimes is dependent on the local environment. We also show that ion depletion in the vicinity of a growing island increases the spatial ordering of the island distribution.