Quantum color centers in diamond: unraveling the link between the atomic-scale structure and functionality

During the last two decades, point defects in diamond – also referred to as color centers – have arisen as excellent building blocks for a wide variety of solid state quantum devices and quantum technologies. Indeed, it has been shown that various defects can be introduced, which show bright single-photon emission at room temperature. So far, the negatively charged NV- defect, consisting of a single nitrogen impurity neighbored by a vacancy, has been the most widely studied candidate. Despite a number of particular advantages of the NV- center, the long fluorescent lifetime and a weak emission of the NV- into the zero-phonon line (ZPL) set an upper limit to the achievable photon rates of the quantum devices. As a result, there has recently been a major effort in identifying alternative color centers and investigating their optical properties for applications where high photon counts are required. A specific class of centers that show very promising properties are the so-called group IV defects, i.e., the silicon-vacancy (SiV), germanium-vacancy (GeV), tin-vacancy (SnV) and lead-vacancy (PbV). Unlike the NV centers, they exhibit strong, narrow band emission into the ZPL and limited spectral diffusion, which has been assigned to the D3d symmetric configuration of the defect structure. In nearly all cases, ion implantation is the key methodology either to introduce the impurity in a very controlled way, or to activate existing impurities via the introduction of vacancies. The advantages of implantation are multiple, e.g., excellent control of the impurity position and of the concentration, fully compatible with technological processing, etc. At the same time, the collision cascade resulting from the implantation process delivers the required vacancies to form the color centers. Whereas the NV center consists of a substitutional nitrogen atom with a neighboring carbon vacancy, the group IV vacancy centers are generally believed to exhibit a “split-vacancy”configuration, where the impurity resides at the middle of two C positions (bond-centered site), with the vacancy “split” over the two adjacent C sites. This configuration was initially suggested from ab initio calculations and confirmed indirectly from (magneto-)opticalspectroscopic measurements, although thus far not experimentally proven. Our unique approach based on emission channeling (a lattice location technique of unprecedented sensitivity) combined with photoluminescence (PL) enables us to unambiguously identify the atomic defect structure and provide a direct link to specific PL lines via radio tracer photoluminescence experiments. The lattice location studies will be performed at the ISOLDE facility at CERN, making use of radioactive probe atoms. On the one hand, based on the anisotropic emission of b particles upon decay of these implanted radioactive isotopes, we can identify and quantify the lattice site, even for extremely low fluences. Using this approach, the QSP group recently demonstrated a proof-of-principle of identifying and quantifying the split-vacancy SnV configuration in diamond. On the other hand, during the decay, the specific isotope transforms to a daughter nucleus. Consequently, the decay or growth of the corresponding PL lines enables to distinguish between luminescence originating from specific elements or specific defect configurations. These experiments using radioactive isotopes will be complemented with detailed investigations on stable isotopes, using the ion implantation and PL setups in Leuven.