Minimum energy path for the nucleation of misfit dislocations in Ge/Si(001) heteroepitaxy

TL;DR

This study uses atomic scale calculations to identify a minimum energy path for the nucleation of misfit dislocations in Ge/Si(001) heteroepitaxy, revealing the stepwise formation of 60° dislocations leading to 90° dislocation formation.

Contribution

It introduces a detailed atomic-scale pathway for misfit dislocation nucleation, highlighting the sequential nucleation of 60° dislocations and their energy interactions.

Findings

First 60° dislocation nucleates at a higher energy barrier.

Second 60° dislocation nucleation is facilitated by the first, with 30% lower activation energy.

The approach provides insight into complex defect formation mechanisms.

Abstract

A possible mechanism for the formation of a 90{\deg} misfit dislocation at the Ge/Si(001) interface through homogeneous nucleation is identified from atomic scale calculations where a minimum energy path connecting the coherent epitaxial state and a final state with a 90{\deg} misfit dislocation is found using the nudged elastic band method. The initial path is generated using a repulsive bias activation procedure in a model system including 75000 atoms. The energy along the path exhibits two maxima in the energy. The first maximum occurs as a 60{\deg} dislocation nucleates. The intermediate minimum corresponds to an extended 60{\deg} dislocation. The subsequent energy maximum occurs as a second 60{\deg} dislocation nucleates in a complementary, mirror glide plane, simultaneously starting from the surface and from the first 60{\deg} dislocation. The activation energy of the nucleation of the second dislocation is 30% lower than that of the first one showing that the formation of the second 60{\deg} dislocation is aided by the presence of the first one. The simulations represent a step towards unraveling the formation mechanism of 90{\deg} dislocations, an important issue in the design of growth procedures for strain released Ge overlayers on Si(100) surfaces, and more generally illustrate an approach that can be used to gain insight into the mechanism of complex nucleation paths of extended defects in solids.