Branched Ta nanocolumns grown by glancing angle deposition

Periodic arrays of Ta nanocolumns, 200nm wide and 600nm tall, were grown by glancing angle sputter deposition onto self-assembled close-packed arrays of 260-nm-diameter silica spheres. Each sphere leads to the development of a single Ta column. As growth progresses, roughening of the column top surfaces causes branching of some columns into subcolumns. The measured fraction of branched columns fb decreases with increasing growth temperature, from 30% at 200°C to 4% at 700°C. This is attributed to the increased adatom mobility at elevated temperatures, leading to a larger average separation of growth mounds and, in turn, lower nucleation probabilities for subcolumns. Branching into 3 and 4 subcolumns exhibits probabilities proportional to fb2 and fb3, respectively. A fit of the data with a simple nucleation model provides an effective activation energy for Ta surface diffusion of 2.0eV.