Interlayer Mass Transport in Homoepitaxial and Heteroepitaxial Metal Growth

Abstract

We describe a general method for the quantitative determination of the interlayer mass transport in epitaxial growth. Through measurement of the nucleation rate on top of islands as a function of island size and temperature, the additional barrier for an adatom to descend the step edge AE, can be determined with high accuracy. This approach is applied to the growth of Ag on the (111)surfaces of Ag and Pt. In the homoepitaxial system, the barrier is found to be AE, . = 120~15 meV, whereas in the heteroepitaxial case it is substantially lowered, AE, . = 30~5 meV.PACS numbers: 61.16.Ch, 68.35.Bs The ultimate goal in epitaxial growth is the controlled fabrication of atomically thin films with smooth abrupt interfaces. In the thermodynamic limit, the growth morphology is determined by the balance of the interfacial energies involved [1]. In most cases, thin films are grown under experimental conditions far from equilibrium. Film growth and the resulting morphology will then be governed by kinetic effects. If the interlayer mass transport is sufficiently fast to allow atoms to leave the tops of growing two-dimensional islands as fast as they arrive, the growing layer will be completed before second-layer nucleation sets in and smooth layer-by-layer growth results. This is in general a delicate balance between several factors like the deposition rate, the density, size, and shape of adlayer islands, the adatom diffusion barrier, and finally the activation barrier for an adatom to descend at the edges of adatom islands. The step-edge barrier E, . accounts for the fact that the diffusion barrier of an adatom over the edge of a step is different from that on the terrace [2,3]. Usually it is harder to step down from an upper terrace than to remain on the same level, i.e. , E,~Ed, with Ed the barrier of terrace diffusion.Many microscopic details of the growth kinetics have recently been revealed by studies of Ag and Pt homoepitaxy on their (111) surfaces [4 -8]. Here, kinetic effects could be studied in their pure form, as structural misfits and electronic inhomogeneities are absent. Both systems were found to grow in a multilayer (3D) growth mode at 300 K which was attributed to the limited interlayer mass transport at this temperature, ascribed to relatively large barrier heights for an adatom to descend a step edge. While the lateral diffusion barrier Ed governing the intralayer mass transport has been quantitatively determined for numerous metal systems [9,10], quantitative measurements of the step-edge barrier E" though first postulated nearly 30 years ago, are rare [11,12].In this Letter we present a general method for the quantitative determination of the interlayer mass transport in epitaxial growth. The method is based on the measurement of the nucleation rate on top of previously grown adlayer islands as a function of island size and temperature.The additional step-edge barrier AE, = E, -Ed, i.e. , the barrier for an adatom to descend the step edge minus the surface diffusion barrier, as well as the correspondi...

Formation and morphological evolution of self-similar 3D nanostructures on weakly interacting substrates

“…In contrast, adatom descent to a lower fcc site at an A edge may occur either directly (f →f ) or via an adjacent hcp site (f →h→f ). The differences in adatom diffusion pathways, combined with different activation barriers for f →f , f →h, and h→f hops, as described below, reproduce the well-established anisotropy in edge diffusion and step-crossing rates at the two types of fcc edges [16,36,54,[60][61][62][63][64].…”

“…Moreover, BCS values are also compared to experimental results [16,54,69,[71][72][73][74][75][76][77][78], as well as barriers obtained from our nudged-elasticband (NEB) calculations using an embedded atom method interatomic potential [79]. Activation barriers for selected processes are listed in Table I.…”

“…However, vapor condensation and film growth proceed far from thermodynamic equilibrium and thus morphological evolution is primarily determined by the relative rates of competing atomistic structure-forming processes (i.e., by kinetics) [1][2][3]. Currently, the most detailed atomistic * kostas.sarakinos@liu.se description of far-from-equilibrium 3D island formation is based on homoepitaxial systems in which 3D islands (mounds) form by deposition onto existing small islands, followed by atomic-step descent limited by the Ehlrich-Schwöbel barrier [15][16][17][18][19]. However, for weakly interacting film/substrate systems-including Ag/SiO 2 [20][21][22][23][24], Pd/TiO 2 [25], Cu/ZnO [26,27], and Dy/graphene [4,28]-3D islands develop before the initially formed one-atom-high islands are large enough to efficiently capture vapor-phase deposition flux.…”

Transition-pathway models of atomic diffusion on fcc metal surfaces. II. Stepped surfaces

“…The result that the ES barriers of the diffusion on the ͑111͒ surface are much higher than those on the ͑001͒ surface is in good agreement with the experimental observations that three-dimensional islands are more easily formed on the ͑111͒ surface than on the ͑001͒ surface for most fcc metals. 6,[14][15][16] …”

Influence of strain in Ag on Al(111) and Al on Ag(100) thin film growth

“…This is in contrast to Ag on Ag͑111͒ homoepitaxy, where limited interlayer transport caused by the step edge barrier results in 3D growth. 7,8 It was proposed that the large density of kink sites originating from the boundaries of stacking-fault islands could offer channels with reduced step edge barriers to promote interlayer diffusion.…”