English: A scanning tunneling microscopy image of the surface of a 4 um-thick GaSb film grown by molecular beam epitaxy on GaAs(001). The image, 0.5 um x 0.5 um, was acquired in situ following growth of the film. Spiral-like structures, such as the one seen here, grow around threading dislocations in the film caused by the 7% lattice mismatch with the substrate. Note the pair of dislocations emerging from the surface at the top of the spiral, along with a third emerging half-way down on the side; each one creates a 0.3 nm-height "step." In situ atomic-scale characterization is an important component in the development of growth procedures that minimize the dislocation density in heteroepitaxial layers.