Morphology of ion sputtered Cu(0 0 1) surface: Transition from unidirectional roughening to bidirectional roughening

We have previously shown that under off normal ion sputtering, ripples parallel to the beam with a periodicity of hundreds of nanometers can be formed on a Cu(0 0 1) surface. This ripple formation behavior can be well explained by the Bradley and Harper (BH) instability mechanism. However, by lowering the temperature at a constant flux, we find that the resultant morphology transitions from a unidirectional modulation to a bidirectional modulation. The lower temperature morphology is attributed to the presence of diffusion barrier across step edges. The time evolution of this morphology is also studied and the bidirectional modulation morphology is observed to change back to ripple morphology as the ion fluence increases.