Title: Unveiling the nanomorphology of HfN thin films by ultrafast reciprocal space mapping

Abstract: Hafnium Nitride (HfN) is a promising and very robust alternative to gold for applications of nanoscale metals. Details of the nanomorphology related to variations in strain states and optical properties can be crucial for applications in nanophotonics and plasmon-assisted chemistry. We use ultrafast reciprocal space mapping (URSM) with hard x-rays to unveil the nanomorphology of thin HfN films. Static high-resolution x-ray diffraction reveals a twofold composition of the thin films being separated into regions with identical lattice constant and similar out-of-plane but hugely different in-plane coherence lengths. URSM upon femtosecond laser excitation reveals different transient strain dynamics for the two respective Bragg peak components. This unambiguously locates the longer in-plane coherence length in the first 15\,nm of the thin film adjacent to the substrate. The transient shift of the broad diffraction peak displays the strain dynamics of the entire film, implying that the near-substrate region hosts nanocrystallites with small and large coherence length, whereas the upper part of the film grows in small columnar grains. Our results illustrate that URSM is a suitable technique for non-destructive investigations of the depth-resolved nanomorphology of nanostructures.