  The properties of artificially grown thin films are strongly affected by
surface processes during growth. Coherent X-rays provide an approach to better
understand such processes and fluctuations far from equilibrium. Here we report
results for vacuum deposition of C$_{60}$ on a graphene-coated surface
investigated with X-ray Photon Correlation Spectroscopy in surface-sensitive
conditions. Step-flow is observed through measurement of the step-edge velocity
in the late stages of growth after crystalline mounds have formed. We show that
the step-edge velocity is coupled to the terrace length, and that there is a
variation in the velocity from larger step spacing at the center of crystalline
mounds to closely-spaced, more slowly propagating steps at their edges. The
results extend theories of surface growth, since the behavior is consistent
with surface evolution driven by processes that include surface diffusion, the
motion of step-edges, and attachment at step edges with significant step-edge
barriers.
