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

Randall L Headrick,Peco Myint,Yang Li,Yugang Zhang,Lutz Wiegart,Karl F Ludwig,Jeffrey G Ulbrandt,Jing Wan,Andrei Fluerasu

doi:10.1038/s41467-019-10629-8

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 C60 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.

Highlights

The properties of artificially grown thin films are strongly affected by surface processes during growth
Traditional methods used to study growth in real-time such as electron diffraction[6,7] and surface X-ray scattering[8,9] are unable to provide a complete understanding of surface dynamics since they suffer from the limitation that the surface must be an almost perfectly flat single crystal
Step flow processes are observable by correlation spectroscopy when there is no change of X-ray intensity as the steps advance, and layer-by-layer oscillations are not present

Summary

Introduction

The properties of artificially grown thin films are strongly affected by surface processes during growth. Recent advances in coherent X-ray methods that utilize X-ray Photon Correlation Spectroscopy (XPCS)[10] can yield crucial information on the dynamics where the structural fluctuations about an average configuration occur This is possible since the scattering of coherent X-rays produces a speckle pattern, which depends sensitively on the detailed configuration within each coherence volume. Examples include step-edge fluctuations during annealing and evaporation[11,12], surface roughness fluctuations during deposition[13], fluctuations at polymer surfaces[14], and the dynamics of bulk phase transformations[15,16] These fluctuations are invisible to analysis with low-coherence X-rays, since those methods average over local structures within the illuminated volume.

Methods

Conclusion