Microstructure and intrinsic stress evolution during epitaxial film growth of an Ag0.93Al0.07 solid solution on Si(111); excessive planar faulting due to quantum confinement

D Flötotto,I J Markel,Z M Wang,S J B Kurz,E J Mittemeijer

doi:10.1063/1.4964945

Abstract

The correlation of microstructural development and the kinetics of film growth has been investigated during the epitaxial film growth of an ultrathin binary Ag0.93Al0.07 solid solution on a Si(111)-7×7 surface at 300 K by the combination of high-resolution transmission electron microscopy, X-ray diffraction, scanning tunneling microscopy, low energy electron diffraction, and real-time in-situ stress measurements. Up to a film thickness of 6 ± 2 nm, epitaxial Ag0.93Al0.07 film growth is characterized by the strikingly extensive formation of planar faults parallel to the film/substrate interface, while at larger thickness the film grows practically defect-free. As revealed by real-time in-situ stress measurements, the extensive formation of planar faults at the very initial stage of growth is not driven by the reduction of the system's elastic strain energy but is rather caused by a striking thickness-dependence of the stacking-fault energy owing to a quantum size effect of the ultrathin metal alloy film, resulting in a frequent succession of fcc and hcp stackings of close-packed layers during the initial stage of film growth. The extensive development of planar faults at the initial stage of film growth (&lt;6 ± 2 nm) is associated with the occurrence of a high density of kinks and corners at thereby atomically rough surface ledges, which strongly enhances the downward transport of adatoms from higher to lower terraces (interlayer mass transport) by a reduction of the effective diffusion barrier at the edge of surface steps and by increasing the driving force for adatoms to attach to the surface ledges. As a result, the epitaxial Ag0.93Al0.07 film initially grows in a 2D layer-by-layer type of growth and thus establishes atomically smooth film surfaces. For the practically planar-fault-free growth at thicknesses beyond 6 ± 2 nm, interlayer mass transport becomes distinctively limited, thereby inducing a transition from 2D to 3D type of film growth.

Highlights

The spatial confinement of free electrons in an ultrathin metal overlayer on a semiconductor surface results in the discretization of the energy states along the surface-normal direction and causes the total electron energy to fluctuate pronouncedly with the film thickness
The correlation of microstructural development and the kinetics of film growth has been investigated during the epitaxial film growth of an ultrathin binary Ag0.93Al0.07 solid solution on a Si(111)-7Â7 surface at 300 K by the combination of high-resolution transmission electron microscopy, X-ray diffraction, scanning tunneling microscopy, low energy electron diffraction, and realtime in-situ stress measurements
The extensive development of planar faults at the initial stage of film growth (

Summary

Introduction

The spatial confinement of free electrons in an ultrathin metal overlayer on a semiconductor surface results in the discretization of the energy states along the surface-normal direction and causes the total electron energy to fluctuate pronouncedly with the film thickness. The precise adjustment of the properties of epitaxial metal films requires a comprehensive understanding of the above discussed effects and their impact on the dynamic film growth and defect formation. From a kinetic point of view, the sufficient condition for a 2D type of growth is that under the applied growth conditions, the adatoms on an upper terrace are able to overcome the energy barrier at the edge of the terrace and descend along the atomic step to a lower terrace (i.e., downward interlayer mass transport). The present work focuses on the mechanism governing the observed extensive formation of planar faults parallel to the film surface and exclusively within a 6 6 2 nm thick region adjacent to the film/substrate interface. The here established mechanisms can have general implications for many metal-alloy thin films and thereby offer unique opportunities for the fabrication of novel metal nanostructures with tailored catalytic, magnetic, and electronic properties.

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