Island Formation in Ge on Si Heteroepitaxy
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ISLAND FORMATION IN Ge ON Si HETEROEPITAXY D.J. Eaglesham, H.-J. Gossmann, and M. Cerullo AT&T Bell Labs, 600 Mountain Avenue, Murray Hill,
NJ 07974.
ABSTRACT
We present a study of island formation (the transition from 2D to 3D growth) during the Stranski-Krastanow (S-K) growth of Ge on Si. The energetic driving force for S-K island formation should be the ability to relax the islands by dislocation introduction. Here, we show that Ge islands formed on Si (100) are initially dislocation-free,in the presence of a 2D "sea" that is far in excess of the equilibrium 3 monolayers (ML). We call this phase of growth "dislocation-free S-K". The 2-D sea does not collapse until dislocated islands are produced at an average coverage of = 7 ML. We call the dislocation-free island phase "coherent S-K" growth. The corresponding 2D-3D transition on Si (111) appears to reach equilibrium far faster, and we have not observed dislocation-free island formation: dislocated islands are seen at - 5 ML. As expected, the kinetics allow us to suppress island formation on (100) by reducing the growth temperature. These thick 2D films are analogous to those grown on As-covered surfaces, but have a microstructure dominated by edge dislocations.
The conventional picture of heteroepitaxial growthl[' describes three growth modes: Frank-van der Merwe (F-vdM)12], Volmer-Weber (V-M)[ 3], and Stranski-Krastanow (S-K)' 4 1, which may loosely be described as layer-by-layer (2D), island growth (3D), and layer-bylayer+islands. The growth mode adopted by a given heteroepitaxial system depends on the interface free energy terms and on the lattice mismatch. In lattice-matched systems, island formation is driven by high interface energy 'y12 and high epilayer surface energy 02: islands form provided 02 + Y12 < 01, the substrate surface energy. Changes in 02 + Y12 can only drive a transition from F-vdM to V-W: the epilayer either wets the substrate or does not. For a strained epilayer there is the additional possibility that island formation may allow the system to introduce misfit dislocations underneath the islands to relax epilayer strain. For a system with small interface energy but large lattice mismatch, initial growth is layer-by-layer, but a thicker layer has large strain energy and can lower its total energy by forming isolated thick islands in which the strain is relaxed by interfacial misfit dislocations. Thus S-K growth occurs in strained systems[l], and the primary driving force for island formation in S-K growth is the ability to introduce dislocations in the islands. (If surface free energies favoured island formation, then the system would grow in V-W mode). Here, we present a study of island formation in Ge growth on Si (the best-known S-K system). We will show that the initial stages of island formation on (100) are dislocation-free, and explain this result in terms of elastic relaxation of the island: the system is kinetically inhibited from attaining equilibrium (relaxed islands), and thus is susceptible
Mat. Res. Soc. Symp. Proc. Vol.
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