  In diamond, nitrogen defects like the substitutional nitrogen defect (Ns) or
the nitrogen-vacancy-hydrogen complex (NVH) outnumber the nitrogen vacancy (NV)
defect by at least one order of magnitude creating a dense spin bath. While
neutral Ns has an impact on the coherence of the NV spin state, the atomic
structure of NVH reminds of a NV center decorated with a hydrogen atom. As a
consequence, the formation of NVH centers could compete with that of NV centers
possibly lowering the N-to-NV conversion efficiency in diamond grown with
hydrogen-plasma-assisted chemical vapor deposition (CVD). Therefore, monitoring
and controlling the spin bath is essential to produce and understand engineered
diamond material with high NV concentrations for quantum applications. While
the incorporation of Ns in diamond has been investigated on the nano- and
mesoscale for years, studies concerning the influence of CVD parameters and the
crystal orientation on the NVH formation have been restricted to bulk N-doped
diamond providing high-enough spin numbers for electron paramagnetic resonance
and optical absorption spectroscopy techniques. Here, we investigate
sub-micron-thick (100)-diamond layers with nitrogen contents of (13.8 +- 1.6)
ppm and (16.7 +- 3.6) ppm, and exploiting the NV centers in the layers as local
nano-sensors, we demonstrate the detection of NVH- centers using
double-electron-electron-resonance (DEER). To determine the NVH- densities, we
quantitatively fit the hyperfine structure of NVH- and confirm the results with
the DEER method usually used for determining Ns0 densities. With our
experiments, we access the spin bath composition on the nanoscale and enable a
fast feedback-loop in CVD recipe optimization with thin diamond layers instead
of resource- and time-intensive bulk crystals.
