Evolution of surface width in electrochemical nucleation and growth

doi:10.1016/j.elecom.2010.01.011

Electrochemistry Communications

Volume 12, Issue 3, March 2010, Pages 431-434

https://doi.org/10.1016/j.elecom.2010.01.011 Get rights and content

Abstract

The evolution of surface width during growth of copper islands is studied in the kinetic regime, allowing control of island shape as well as island density. We show that film roughness at island coalescence is determined by island density, island shape, and the island growth kinetics. The connection between the kinetics of island growth and film growth provides the scientific basis for control of surface roughness.

Introduction

The ability to control roughness is often critically important in the deposition of thin films and nanostructures. In electrodeposition, various approaches have been developed to control surface roughness, for example by pulsed deposition or the addition of chemical additives, however, the evolution of surface roughness remains poorly understood. Here we report on the evolution of surface roughness for copper electrodeposition on ruthenium oxide, from isolated island growth to continuous film growth. The small nucleation potential for this system allows us to access the kinetic growth regime and hence control island shape and island density.

Section snippets

Disk shaped islands

Fig. 1a shows hexagonal disk-shaped copper islands after 100 s in sulfate solution at −64 mV (vs. Cu²⁺/Cu). The islands have a (111) surface normal and a random in-plane orientation [1], [2]. The island density remains constant after 20 s when nucleation is finished. Three growth regimes can be identified (Fig. 1b): (i) isolated island growth (<500 s), (ii) island coalescence (500–2000 s), and (iii) continuous film growth (>2000 s).

To analyze the evolution of surface roughness we use scaling

Conclusions

The relationship between island growth and film roughness is complex, depending on island density, island shape (aspect ratio), and the island growth kinetics. The roughness at island coalescence is given by $w_{film} = {At}_{c}^{β_{loc}}$ (Fig. 3a), where A, t_c, and β_loc can be related to deposition parameters. The pre-factor A is dictated by island shape (aspect ratio) (see Fig. 3b), and is hence dependent on solution chemistry (local adsorption). The time to island coalescence t_c is largely dependent on the

Acknowledgements

The authors gratefully acknowledge support from NSF (grant CHE-0905869).

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This approach helps to infer the main physical processes that control the growth and was already applied to deposits of various materials [15–17]. Several works have also investigated the scaling of surface fluctuations and correlations in the initial stages of heteroepitaxial film deposition [18–30,59]. Their results cannot be interpreted in the light of kinetic roughening theories because the deposits are mostly formed by isolated islands, so that height fluctuations and correlations are related to island widths, heights, and surface density.
Using kinetic Monte Carlo simulations, we relate morphological properties and microscopic dynamics during island growth, coalescence, and initial formation of continuous heteroepitaxial films. The average island width is controlled by adatom mobility on substrate. Subsequent evolution strongly depends on the Ehrlich-Schwöebel energy barrier $E_{ES}$ of the deposited material. As $E_{ES}$ decreases, islands become taller and their coalescence is delayed. For small islands and large $E_{ES}$ , the global roughness increases as $W ~ {thickness}^{1 / 2}$ and the local roughness increases at short scales (apparent anomalous scaling) before and after island coalescence. If the islands are wide, W may have a plateau for large $E_{ES}$ and has a maximum for small $E_{ES}$ when the islands coalesce. This framework is applied to atomic-force microscopy data of the initial stages of CdTe deposition on Kapton: a maximum of W during island coalescence indicates negligible ES barrier, consistently with scaling properties of much thicker films, and the diffusion coefficient $10^{- 7} — 10^{- 5} {cm}^{2} / s$ on the Kapton surface at $150 ° C$ is estimated. Applications of the framework to other materials are suggested, in which the expected roles of ES barriers are highlighted.
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Recently, so-called ‘scaling analysis’ [4] has become a popular tool for analyzing the surface roughness of electrodeposited thin films [5]. It has also been applied to the problem of island nucleation and growth [6]. Originally it was hoped that scaling analysis would give an insight into the atomic-scale processes dominating film growth [7,8] but it now appears probable that for many polycrystalline films the surface morphology evolution (and consequently the scaling) is dominated by the microstructure: the texture, the locations of grain boundaries and defects such as crystal twins [9].
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The kinetics of copper island growth on ruthenium oxide in perchlorate solution
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In contrast to the disk-shaped islands formed in sulfate solution, the hemispherical islands seen here (Fig. 1) effectively have no top surface to sustain vertical growth. The evolution of surface roughness in this system has been reported elsewhere [20]. The surface diffusion coefficient DS for Cu adatoms on the ruthenium oxide substrate can be estimated from the average island spacing and the cross-over time in island growth.
The nucleation potential for the deposition of copper on ruthenium oxide is very small and hence island growth can occur in the kinetic regime. Previously, we have reported on island growth in sulfate solution where anion adsorption results in the formation of hexagonal, disk-shaped islands. Here we report on island growth in perchlorate solution, in the absence of specific anion adsorption. All islands have a 〈1 1 1〉 surface orientation dictated by the amorphous substrate. The islands are hemispherical in shape and island growth follows power law kinetics in both lateral and vertical directions with exponents 0.50 and 0.26, respectively. From analysis of the time-dependent extended area using Avrami analysis, analysis of the near-neighbor distribution, and the correlation between the island size and the spatial distribution, we conclude that island growth is controlled by surface diffusion-mediated lateral growth.
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Evolution of surface width in electrochemical nucleation and growth

Abstract

Introduction

Section snippets

Disk shaped islands

Conclusions

Acknowledgements

Physical Review E

Journal of the Electrochemical Society

Journal of Physics a-Mathematical and General

Fractals scaling and growth far from equilibrium

Physical Review Letters

Langmuir

Journal of the Electrochemical Society