Nanofaceting of vicinal Nb(011)

We report studies of vicinal Nb(011) grown heteroepitaxially on sapphire (Formula presented) Using low-energy electron microscopy on surfaces with small vicinal miscut ∼0.1°, we record the evolution of step edges during annealing in ultrahigh vacuum. Below a temperature of about 1600 K the step edges coalesce to form (110)\ nanofacets several steps in height, due to the low surface free energy. Two distinct types of facet form, depending on the direction of miscut. For miscut along (Formula presented) facets, which intersect the (011) terraces at 90° along [100], are created by step-edge coalescence. Owing to (Formula presented) symmetry, these occur over a range of orientations around [100]. The surface develops over the course of hours into large terraces more than 10 μm long and of widths up to several μm, terminated by (Formula presented) facets tens of atomic (011) planes in height. Surfaces miscut instead along [100] develop (Formula presented) facets at 60° to the (011) terraces, which they intersect along (Formula presented) and (Formula presented) respectively. The surface ripens into long terraces bounded by serrated edges formed from alternating nanofacets about five steps in height. A variety of interesting phenomena are observed, including direct nucleation of nanofacets from step edges, growth in a sawtooth of alternating facets, and periodically faceted states in which facet sections alternate with sections in which the step edges are dissociated, to make a periodic structure. The phase diagram for nanofaceting is discussed for the limit of small miscut.