Figure 2(c) shows the surface of S515, a sample that shows the most anisotropic mobility. The image shows oriented abrupt steps (OAS) in the [110] and [1-10] directions in addition to the hillocks. These steps always run between threading dislocations and typically end on the top of a hillock. Typically the OASs in the [ 1-1 0] direction are more numerous and longer. Fig 2(d) shows a linecut across one of these steps showing an abrupt angle of inclination of 15°. Angles across OASs up to 50^o are often observed.