Strain contrast in scanning tunneling microscopy imaging of subsurface dislocations in lattice-mismatched heteroepitaxy

In lattice-mismatched heteroepitaxy, the coherency strain in the epitaxial layers is usually relaxed by the formation of misfit dislocations at the layer/substrate interface once the critical layer thickness is exceeded. Using ultra-high vacuum scanning tunneling microscopy (STM), it is shown that the local lattice distortions present around the interfacial dislocations give rise to pronounced surface deformations on the layer surface. These surface deformations are well visible by STM, even though the misfit dislocations are usually located far below the epitaxial surface. As is shown for the case of partially relaxed EuTe epitaxial layers on PbTe (111) grown by molecular beam epitaxy, the measured STM surface profiles across these dislocations are in excellent quantitative agreement with the surface displacements derived from elasticity theory when the relaxation effects due to the existence of a free surface are taken into account. The shape of these deformations depends only on the dislocation configuration and the layer thickness, while its amplitude is given by the dislocation Burgers vector, which is usually of the order of the nearest neighbor atomic distance in the crystal lattice. In derivative STM images of such surfaces, the interfacial misfit dislocations show up as dark or white lines due to the local bending of the lattice planes above the dislocations. This strain contrast allows the direct imaging of the complicated dislocation network produced by the strain relaxation process, right after epitaxial growth without the need of any special sample preparation.