Coherent X-ray measurement of local step-flow propagation during growth on polycrystalline C₆₀ thin film surfaces

Vacuum deposition of C₆₀ on a graphene-coated surface is investigated with X-ray Photon Correlation Spectroscopy in surface-sensitive conditions. Local step-flow is observed through the observation of oscillatory correlations in the later stages of growth after crystalline mounds have formed. An important aspect of the work is that coherent X-rays do not average over complex structures, and this allows us to monitor the growth on polycrystalline surfaces without loss of information. The experimental results show that the step-flow velocity must be nonuniform, and we model the velocity of each step-edge as being a simple function of the lengths of the terraces above and below it. This model predicts that the steps become almost stationary near the edges of the mounds where the local terrace length is very small, and the average slope of the surface is large. It was not previously known that such nonuniform and disordered step arrays as we have observed would follow such a simple growth law. This work shows that the use of coherent X-ray scattering provides an approach to better understand surface dynamics and fluctuations during crystal growth.