Bales-Zangwill meandering instability observed in homoepitaxial step-flow growth

The growth of Cu on vicinal Cu templates has been investigated with helium-atom beam scattering. Step Bow on Cu (1,1,17) below room temperature forces steps to strongly meander collectively in phase, leading to the appearance of facets parallel to the average step direction. We identify this ''fingering'' with the meandering instability predicted by Bales and Zangwill, resulting from the presence of an adatom uphill current. In contrast to Cu (1,1,17), step flow above room temperature on Cu (1,1,5) leads to a destabilization of the step train perpendicular to the step direction. Conceivable origins of this type of faceting are discussed.