Coherence Seminar: Exploring high-energy pink-beam XPCS for in situ observation of surface dynamics during crystal growth

Exploring high-energy pink-beam XPCS for in situ observation of surface dynamics during crystal growth

By: G. Brian Stephenson

From: Materials Science Division, Argonne National Laboratory, Argonne, IL, USA

Coherent x-ray methods are providing revolutionary new capabilities for observing nanoscale dynamics and imaging atomic structure in materials. These methods are being applied to ever-more-weakly scattering systems, such as the atomic-scale morphology of surfaces. Such studies have a bright future, since synchrotron facilities worldwide are being upgraded or built to provide greatly increased coherent flux at higher x-ray energies by using a multi-bend achromat storage ring lattice. In anticipation of these developments, we are exploring x-ray photon correlation spectroscopy (XPCS) for in situ studies of surface dynamics using the current capabilities of the Advanced Photon Source.

Here I will describe XPCS measurements at higher x-ray energies (e.g. > 25 keV) using the full bandwidth of the third harmonic of the undulator ("pink beam"), to enable in situ coherent x-ray studies of surface morphology during crystal growth [1]. Using pink beam to achieve high transversely coherent flux (e.g. 9 x 10¹⁰ photons per second at 25.75 keV in a 0.85% bandwidth) allows us to begin exploring high-energy coherent x-ray methods in experiments for which such a wide bandwidth can be used. I will show that pink-beam XPCS using scattering near the specular direction can successfully reveal surface dynamics. Practical considerations will be discussed such as the effects of beamline optics on coherent flux [2]. Recent results will be presented on island nucleation arrangements during layer-by-layer growth of GaN m-plane (1 0 -1 0) by metal-organic vapor phase epitaxy [3], and on step motion during step-flow growth of TiO₂ (110) by reactive sputtering.

Work supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering.

     [1] G. Ju, M. J. Highland, A. Yanguas-Gil, C. Thompson, J. A. Eastman, H. Zhou, S. M. Brennan, G. B. Stephenson, and P. H. Fuoss, Rev. Sci. Instrum. 88, 035113 (2017).

[2] G. Ju, M. J. Highland, C Thompson, J. A. Eastman, P. H. Fuoss, H. Zhou, R. Dejus and G. B. Stephenson, J. Synchr. Rad. 25, 1036 (2018).

[3] G. Ju et al., https://arxiv.org/abs/1804.08161 (2018).