Surface morphology during strain relaxation in the growth of InAs on GaAs(110)

Scanning tunnelling microscopy has been used to investigate dislocation-induced surface morphological changes during strain relaxation in the two-dimensional (2D) growth of InAs on GaAs(110) by molecular beam epitaxy. Two distinct classes of dislocation are required owing to the crystallographic anisotropy in the (110) plane: ideal edge dislocations (similar to 3 ML) and dislocation half-loop slip (> 5 ML) systems. Specific emphasis is on the nucleation of the edge dislocations out of the preceding pseudomorphic layer and the manner in which the slip steps accommodate the continuing growth of the epilayer and influence the surface morphology. Between 1 and 2 ML InAs thickness, a substantial redistribution of the surface material occurs, leading to highly uniform "mosaic" structures, which are either close-packed arrays of tiny islands at similar to 420 degrees C, or a linear array at similar to 480 degrees C. The closure of these fractured morphologies directly incorporates edge misfit dislocations beneath the original boundaries between the surface islands. Since a slip mechanism cannot operate for [110] strain relief, the dislocations must be located directly beneath the surface, a layer or so above the InAs-GaAs interface. Distinctive step signatures due to the slip of surface-nucleated half-loop dislocations, in terms of the screw terminations and their associated wave like topological profiles, are observed after 5 ML InAs deposition. The growth mode for the InAs layer beyond 5 ML is by propagation of the slip steps, with discrete 2D island nuclei never observed. The slip steps can therefore move across the surface in the [001] direction away from their original position, as well as increasing in length as the dislocation half-loops expand along the [<1(1)over bar>0] direction. The linear density of slip steps along [001] decreases with increasing film thickness due to interaction of the steps during growth. The length of each slip step and of each misfit segment is relatively short (less than or equal to 1000 Angstrom) in comparison with growth on (001) substrates. A related interaction between the hair-loop and preceding edge dislocations at the interface is also resolved. (C) 1998 Elsevier Science B.V. All rights reserved.