Nanofaceting of vicinal Nb(011)

We report studies of vicinal Nb(011) grown heteroepitaxially on sapphire (1120). 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 [011],(011) facets, which intersect the (011) terraces at 90^∘{} along [100], are created by step-edge coalescence. Owing to $(2mm)$ symmetry, these occur over a range of orientations around [100]. The surface develops over the course of hours into large terraces more than 10 {μ}m long and of widths up to several {μ}m, terminated by (011) facets tens of atomic (011) planes in height. Surfaces miscut instead along [100] develop (110) facets at 60^∘{} to the (011) terraces, which they intersect along [111] and [111], 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.