A Molecular Dynamics Study Of Formation Of Misfit Dislocation In Epitaxial Thin Crystal

Crystal defects in thin films mostly form in the atomic deposition process,current experimental means do not have enough resolution power in both space andtime to analyze any processes at atomistic scale, while computer simulation based onmolecular dynamics offers an effective way for growth of thin films and its defectformation. Using three dimensional molecular dynamics method, and taking <111>FCC metals as prototypes, we have carried out atomistic simulation studies of theformation mechanisms of misfit dislocation in the thin film and their control. EAMmany-body potentials are adopted to calculate the inter-atomic forces. The workconducted mainly include establishing the molecular dynamics model in the epitaxialthin films, the simulation method and its atomic relaxation process and application;temperature having influence on the formation of the misfit dislocation in the epitaxialfilms; surface adatom inducing formation of misfit dislocation in epitaxial films; andthe relation between misfit dislocation in the epitaxial films and misfit property. Theresults show that(1) The formation of the misfit dislocation is more difficult in positive misfit thinfilms than in minus misfit while they exist in the same magnitude of misfit andtemperature.(2) In the same misfit magnitude, the time that misfit dislocation has been formed hasrelated to growth temperature; and higher temperature, easier in the formation ofdislocation.(3) When growth temperature is near or above aluminum melting point, the thin filmmelts more difficult under minus misfit than under positive misfit.(4) Higher temperature can promote the formation of misfit dislocation; as far as theepitaxial films at atomic scale be concerned, the thin films under the same conditions are more difficult to form dislocation; while the thin films with surfaceadatom can do.(5) Surface. adatom can induce formation of misfit dislocation in the epitaxial films,and the bigger adatom have a greater driving force. The two different processes ofdislocation nucleation are observed: one is directly called a complete edgedislocation, which got through a course of semi-atomic face being squeezed out;while the other is firstly formed extended dislocation which was clipped by twoincomplete dislocations, then gets a complete dislocation. Their Burgers vector is1/2[110] of edge dislocation paralleled to direction of misfit.(6) Misfit strain energy (beyond critical thickness corresponding misfit) under thesame thin system have a crucial role to formation of misfit dislocation in the thinfilms.(7) Nucleating by sliding under the positive misfit need many participated atomsexisted in internal and surface thin films, while nucleating squeezed out under theminus misfit need only several atoms existed in the surface and lower energy.