  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.
