Figure 2(a) shows the surface of S523, the highest mobility sample. The image shows many pyramidal features or hillocks. These hillocks are spiral structures that grow around a threading dislocation indicating that the layers are grown in a Burton-Cabrerra-Frank (BCF) mode. Note that they are elongated in the [ 1-1 0] direction indicating that the diffusion length is longer in this direction, as expected by the presence of continuous channels in the surface reconstruction. Fig 2(b) shows a linecut across one of these hillocks. The angle of inclination is very shallow, typically less than about 1.5°.