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

TL;DR

This study uses coherent X-ray techniques to analyze step-flow dynamics during the vacuum deposition of C60 on graphene, revealing how step-edge velocity varies with terrace length and extends surface growth theories.

Contribution

It provides new insights into surface evolution mechanisms during thin film growth, highlighting the coupling between step-edge velocity and terrace spacing.

Findings

Step-edge velocity is coupled to terrace length.

Velocity varies from center to edges of crystalline mounds.

Results support models involving surface diffusion and step-edge barriers.

Abstract

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.