Delta-Doped GeV$^-$ in diamond with isotope resolvable optical spectra
Group IV color centers in diamond -- silicon, germanium, tin, and lead-vacancy centers -- are a promising platform for quantum networking. Typical Group-IV creation utilizes ion implantation during diamond growth that leaves the color centers with a broad depth distribution and damage-induced strain inhomogeneity. Deterministic placement and damage reduction in Group IV generation are essential for improving this platform. In this letter, we report on the localization of germanium-vacancy (GeV$^-$) to a $<$ 10 nm layer via the timed injection of GeH$_4$ during microwave plasma chemical vapor deposition. In this layer we have realized 17 ppm of Ge and a strain-free environment for the GeV$^-$. Subsequent electron irradiation and annealing have generated a sheet density of 0.12 GeV$^-$/$μ$m$^2$. The minimal damage from color center generation mitigates spectral inhomogeneity and allows spectral resolution of isotopic clusters, with the best cluster having an inhomogeneous distribution of 2.14 GHz and an average linewidth of $47 \pm 11$ MHz. Our study gives insight into formation dynamics of GeV$^-$ in delta-doped systems and estimates our vacancy to GeV$^-$ yield to be $\geq$ 90%, establishing an athermal nearest-neighbor formation model. We further characterized the spin $T_2^*$ and set a lower bound of 730 ns, limited by a paramagnetic defect.