Abstract
Understanding how edge misfit dislocations (MDs) form in a GeSi/Si(001) film has been a long standing issue. The challenge is to find a mechanism accounting for the presence of these dislocations at the interface since they are not mobile and cannot nucleate at the surface and glide towards the interface. Furthermore, experiments can hardly detect the nucleation and early stages of growth because of the short time scale involved. Here we present the first semi-quantitative atomistic calculation of the formation of edge dislocations in such films. We use a global optimization method and density functional theory calculations, combined with computations using potential energy functions to identify the best mechanisms. We show that those previously suggested are relevant only for a low film strain and we propose a new mechanism which accounts for the formation of edge dislocations at high film strain. In this one, a 60° MD nucleates as a “split” half-loop with two branches gliding on different planes. One branch belongs to the glide plane of a complementary 60° MD and therefore strongly favors the formation of the complementary MD which is immediately combined with the first MD to form an edge MD.
Highlights
At lower film strain (i.e. at later stages of the plastic relaxation of a Ge rich GeSi/Si(001) film or during the relaxation of a Ge poor GeSi/Si(001) film), TDs of 60° MD are screw dislocations and 60° MDs nucleate through the half-loop mechanism
This study provides the first explanation of why the proportion of 90° MDs increases with Ge content20–22
The influence of the supercell size is discussed in detail in the supplementary material
Summary
Received: 11 April 2017 Accepted: 31 August 2017 Published: xx xx xxxx overlayer on Si(001). We use a global optimization method and density functional theory calculations, combined with computations using potential energy functions to identify the best mechanisms We show that those previously suggested are relevant only for a low film strain and we propose a new mechanism which accounts for the formation of edge dislocations at high film strain. In low to moderate Ge content GeSi/Si(001) films, plastic relaxation has been characterized from TEM experiments6 It occurs mainly through the nucleation from the surface of half-loop dislocations with Burgers vectors b = a/2 (a is the lattice constant) which progressively release the strain by gliding in {111} planes towards the interface.
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