Production of oriented nitrogen-vacancy color centers in synthetic diamond

The negatively charged nitrogen-vacancy (${\mathrm{NV}}^{-}$) center in diamond is an attractive candidate for applications that range from magnetometry to quantum information processing. Here we show that only a fraction of the nitrogen (typically $<0.5$%) incorporated during homoepitaxial diamond growth by chemical vapor deposition (CVD) is in the form of undecorated ${\mathrm{NV}}^{-}$ centers. Furthermore, studies on CVD diamond grown on $\left(110\right)$-oriented substrates show a near 100% preferential orientation of NV centers along only the $\left[111\right]$ and $\left[\overline{1}\overline{1}1\right]$ directions, rather than the four possible orientations. The results indicate that NV centers grow in as units, as the diamond is deposited, rather than by migration and association of their components. The NV unit of the ${\mathrm{NVH}}^{-}$ is similarly preferentially oriented, but it is not possible to determine whether this defect was formed by H capture at a preferentially aligned NV center or as a complete unit. Reducing the number of NV orientations from four orientations to two orientations should lead to increased optically detected magnetic resonance contrast and thus improved magnetic sensitivity in ensemble-based magnetometry.

Figure 1

(a) The ${\mathrm{NV}}^{-}$ concentration plotted against the total substitutional nitrogen concentration in diamond samples grown by CVD. The solid line is plotted for the situation where $\left(\right[{\mathrm{N}}_{\mathrm{S}}^0]+[{\mathrm{N}}_{\mathrm{S}}^{+}\left]\right)=\left[{\mathrm{NV}}^{-}\right]$. (b) The concentration of ${\mathrm{NV}}^{-}$ centers plotted against the concentration of the ${\mathrm{NVH}}^{-}$ centers. The solid line is plotted for the situation where $\left[{\mathrm{NV}}^{-}\right]=\left[{\mathrm{NVH}}^{-}\right]$. In both cases the broken line is a $y=mx$ fit to the data points. In all figures the data points marked with $\square$ were obtained from measurements on samples grown by Element Six Ltd. and deliberately doped with nitrogen, and those marked with $▵$ were on samples purchased from Apollo Diamond Inc.

Figure 2

(a) Idealized representation of an H-terminated $\left(110\right)$ diamond surface (including a growth step) in which a N atom (shown without a surface H) has been incorporated. Inset shows the orientation of the possible $\langle 111\rangle$ directions. (b) Angular dependence of the EPR lines from ${\mathrm{NV}}^{-}$ at 9.5 GHz, where the dotted horizontal line indicates a $\langle 111\rangle$ orientation. (c) and (d) Room temperature 9.5 GHz cw EPR spectra from a (110)-grown CVD diamond sample. The narrow lines close to the center of the spectrum originate from the ${\mathrm{N}}_{\mathrm{S}}^0$ defect and those further from the center from the ${\mathrm{NV}}^{-}$ defect. Optical excitation from a HgXe arc lamp was used to spin polarize the EPR spectrum from the ${\mathrm{NV}}^{-}$ defect and increase the sensitivity. In (c) the EPR spectrum was recorded with the magnetic field oriented close $(\pm 1{}^{\circ })$ to the $\left[1\overline{1}\overline{1}\right]$ crystallographic direction (i.e., approximately perpendicular to the $\left[110\right]$ growth direction), and in (d) the magnetic field was oriented approximately $(\pm 1{}^{\circ })$ along the $\left[111\right]$ crystallographic direction.

Figure 3

(a) Confocal scanning photoluminescence image of randomly oriented single NV centers in a $\left(111\right)$-polished natural diamond sample. This composite image was generated by encoding laser polarization dependence into color as described by Alegre et al.,[24] such that the four possible N-V angles are clearly distinguished. (b) A similar image obtained from a diamond homoepitaxial layer grown on a $\left(110\right)$-oriented diamond substrate. All of the observed NV centers have a similar polarization dependence. (c) Polarization dependence of the PL intensity for six NV centers in the $\left(110\right)$ sample. (d) Theoretical room-temperature polarization dependence for in-plane (lower curves) and out-of-plane (upper curve) NV centers. The maximum intensity 1 corresponds to a center with a dipole moment aligned to the excitation field and perpendicular to the collection/excitation axis. The model does not include saturation effects.