  Extending the spin-dephasing time (T2*) of perfectly aligned nitrogen-vacancy
(NV) centers in large-volume chemical vapor deposition (CVD) diamonds leads to
enhanced DC magnetic sensitivity. However, T2* of the NV centers is
significantly reduced by the stress distribution in the diamond film as its
thickness increases. To overcome this issue, we developed a method to mitigate
the stress distribution in the CVD diamond films, leading to a T2* extension of
the ensemble NV centers. CVD diamond films of approximately 50 \mu m thickness
with perfectly aligned NV centers were formed on (111) diamond substrates with
misorientation angles of 2.0, 3.7, 5.0, and 10{\deg}. We found that T2* of the
ensemble of NV centers increased to approach the value limited only by the
electron and nuclear spin bath with increasing the misorientation angle.
Microscopic stress measurements revealed that the stress distribution was
highly inhomogeneous along the depth direction in the CVD diamond film at low
misorientation angles, whereas the inhomogeneity was largely suppressed on
highly misoriented substrates. The reduced stress distribution possibly
originates from the reduction of the dislocation density in the CVD diamond.
This study provides an important method for synthesizing high-quality diamond
materials for use in highly sensitive quantum sensors.
