Three-pronged approach to color centers in diamond: a case study for calcium

Diamond is a material with multiple promising color centers for quantum technologies. In this work, a three-pronged approach was employed to study Ca defects in this material. Ab initio calculations, emission channeling (EC) and optical measurements were performed such that the correlation of results was possible, to achieve a better understanding of such systems. Density functional theory (DFT) was used to optimize the structure of 165 Ca defects, including multiple high symmetry sites of the diamond lattice, varying number of vacancies and five charge states. Formation, adiabatic charge transition and binding energies were calculated for all configurations. From them, it was possible to conclude that the bond-center (BC) configuration with two vacancies had the smallest formation energy, 13.36 eV for its neutral charge state. Results from β− EC measurements following low fluence implantation of 45Ca (3E12 ions/cm2, 30 keV) performed at ISOLDE were analysed. From the collected patterns, a distribution around the BC site was found in the as implanted state which is consistent with DFT results, and, after annealing, such contribution was reduced in favor of other sites. Diamond samples were also implanted with stable Ca at KU Leuven and were optically characterized. From low-temperature photoluminescence (PL), after heat treatment, it was possible to identify both charge-states of the nitrogen-vacancy defect in diamond, and also two new lines at 563 nm and 579 nm, which were attributed to self-interstitials. From the temperature dependence of the PL signal, the latter is hypothesized to be a vibronic feature of the former. It was concluded that, despite seeing effects of the implantation, no direct optical signal was found from Ca.