Can surface-transfer doping and UV irradiation during annealing improve shallow implanted Nitrogen-Vacancy centers in diamond?

It has been reported that yield and coherence time of ion-implanted NV centers improve if the Fermi level is raised or lowered during the annealing step following implantation. Here we investigate whether surface transfer doping and surface charging, by UV light, can be harnessed to induce this effect. We analyze the coherence times and the yield of NV centers created by ion implantation and annealing, applying various conditions during annealing. Specifically, we study coating the diamond with nickel, palladium or aluminum oxide, to induce positive surface transfer doping, as well as annealing under UV illumination to trigger vacancy charging. The NV formation yield is increased by a factor of two by metal coatings. The coherence time $T_2$ varies by less than a factor of two between the samples. Both effects are weaker than previous reports, suggesting that stronger modifications of the band structure are necessary to have a pronounced effect. Among the samples, UV irradiated annealing has no impact on $T_2$ times, while fluorescence intensity and $T_2^*$ appear to degrade.