Novel formation and decay mechanisms of nanostructures on surface

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Abstract

For decades research on thin-film growth has attracted a lot of attention as these kinds of materials have the potential in new generation device application. It is known that the nuclei at initial stage of the islands are more stable than others and certain atoms are inert while others are active. In this paper, we will show that, when a surfactant layer is used to mediate the growth, a counter-intuitive fractal-to-compact island shape transition can be induced by increasing deposition flux or decreasing growth temperature. Specifically, we introduce a reaction-limited aggregation (RLA) theory, where the physical process controlling the island shape transition is the shielding effect of adatoms stuck to stable islands on incoming adatoms. Also discussed are the origin of a transition from triangular to hexagonal then to inverted triangular and the decay characteristics of 3D islands on surface, and connections of our unique predictions with recent experiments. Furthermore, we will present a novel idea to make use of the condensation energy of adatoms to control the island evolution along special direction.

Introduction

Nanoscale structures on surface are at the forefront of exploratory work for the next generation devices in the fields of electronic and optical industry, which rely on materials of ever increasing complexity and decreasing size. In most cases these nanostructures must be fabricated either through homoepitaxial or heteroepitaxial growth. The understanding of the kinetics involved in the formation and stability of nanostructures on surface is of importance for the fast-growing area of nanotechnology. So far, substantial experimental and theoretical studies have been focused on the evolution of morphological features on the surface and the growth modes, and also the decay of 3D islands. In this paper we will summarize the recent study of the novel formation and decay mechanisms of nanostructures on surface.

Section snippets

Reaction-limited aggregation in surfactant-mediated epitaxy

In heteroepitaxial growth, the presence of strain often leads to 3D growth mode since the equilibrium structure involves strain-relieving effects. A breakthrough was made in 1989 when Copel et al. [1] demonstrate that the use of a single layer of As can improve the heteroepitaxial growth of Ge on Si. This remarkable behavior was termed the “surfactant effect”. Since then, a large number of experimental and theoretical efforts have been devoted to understand the surfactant effect [2]. Now

Pattern election in the presence of adsorbates

In the earliest stages of Pt(1 1 1) homoepitaxy where only single-layer islands are formed, Michely et al. [12] found that the islands develop fractal or dendritic shapes at low growth temperatures, but are compact at higher temperatures. In particular, the compact islands can select triangular, hexagonal, and inverted triangular shapes as the growth temperature is increased. This set of observations on the shape evolution of the Pt islands has defied a consensus explanation for years, despite

Island evolution controlled by condensation energy

A deposited adatom on a surface will dissipate its latent condensation heat, which is of the order of several eV. Such a condensation energy is much higher than the thermal activation energy required for an adatom to diffuse on a surface, which amounts to only a few tenths of an eV. This condensation energy will assist diffusion, namely, the transient mobility [18], [19], [20], [21]. Our first attempt is to make use of the condensation energy of adatoms to control the island evolution [22].

Kinetic stability of nanostructures at surface

In order to assess the stability of 3D nanostructures, it requires an understanding of the nature of the processes responsible for the decay of nanostructures and furthermore a precise knowledge of the activation energies involved in the relevant processes. Among these the diffusion of adatoms across step edges is most important because it influences the mass transport between terraces of different height. A new effective mechanism of inter-layer mass transport was proposed in the decay of

Concluding remarks

The study on the formation and decay mechanisms is very important in the practice of electronic and optoelectronic devices. By STM and theoretical modeling, we can uncover some of the building regulations down to the atomic scale.

Acknowledgments

Financial support from the NSF of China, the Chinese Academy of Sciences, the Key Project on Basic Research (G2000067103), US DOE National and US NSF is acknowledged. We also thank I.-S. Hwang, T.T. Tsong and T. Micheley for stimulating discussions.

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