  Understanding surface dynamics during epitaxial film growth is key to growing
high quality materials with controllable properties. X-ray photon correlation
spectroscopy (XPCS) using coherent x-rays opens new opportunities for in situ
observation of atomic-scale fluctuation dynamics during crystal growth. Here,
we present the first XPCS measurements of 2D island dynamics during
homoepitaxial growth in the layer-by-layer mode. Analysis of the results using
two-time correlations reveals a new phenomenon - a memory effect in island
nucleation sites on successive crystal layers. Simulations indicate that this
persistence in the island arrangements arises from communication between
islands on different layers via adatoms. With the worldwide advent of new
coherent x-ray sources, the XPCS methods pioneered here will be widely
applicable to atomic-scale processes on surfaces.
