Rotation of the misfit dislocation grid as a function of the thickness in epitaxial FeSe thin films

Misfit dislocations appear in epitaxially-grown films and serve as a strain-relief mechanism. Previously, well-ordered dislocation grids have been observed in rock-salt structure films grown on substrates with a similar lattice structure, for example, PbTe/PbSe. Here, we use molecular beam epitaxy to synthesize FeSe thin films on a disimilar substrate SrTiO₃. Using low temperature scanning tunneling microscopy, we observe well-ordered misfit dislocation networks in multilayer FeSe films. Surprisingly, the orientation of the dislocation network rotates by 45 degree from the 4 monolayer thick film to the 8 monolayer thick film. Moreover, a secondary grid is found in the 4 monolayer dislocation network which has the same orientation as the 8 monolayer network. Interestingly, the spacing of the secondary grid can be nicely fitted with the Matthews-Blakeslee model. The origin of the rotation of the dislocation networks will be discussed.