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

Vapor deposition of C₆₀ on a graphene coated surface is investigated in real-time by utilizing coherent hard X-rays. X-ray Photon Correlation Spectroscopy is performed in grazing incidence to achieve surface-sensitivity. Local step-flow is monitored through the observation of oscillatory correlations in the later stages of growth after crystalline mounds have formed. Coherent X-rays do not average over complex structures, and this allows us to monitor growth on polycrystalline surfaces without loss of information. The results show that the step-flow velocity is 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. Sensitivity to local step-flow is enhanced by coherent mixing of X-rays scattered from the average mound structure with those scattered from the step array. This effect is a version of heterodyne scattering, where the scattering from the average step array can be considered to be a quasi-static reference signal. This work shows that the use of coherent X-ray scattering provides an approach to better understand surface dynamics and fluctuations during crystal growth.