Engineering of Color Centers in Isotopically Purified Diamond for Quantum Technologies

Abstract

Silicon has turned from a niche laboratory-scale material into the backbone of modern electronics. Diamond functionalized with color centers could follow a similar path and grow from proof-of-concepts experiments to application-oriented quantum technologies. Quantum sensing, communication, and information processing with color centers has been proven to work in scientific experiments and enhanced sensitivities as well as novel functionalities have been reported. While the first generation of diamond quantum devices relied on color centers formed uncontrolled in natural diamond with a high statistical variation in composition and crystallinity, the situation has changed nowadays. With the emergence of high purity monocrystals and the option for isotopic purification of the carbon matrix, technological progress in materials science on the atomic-scale enabled tailoring the spin properties in diamond. But, since many color centers typically require vacancies to combine with impurities in the carbon lattice via diffusion, the generation of color centers underlies a statistical process providing only little control on the center's final position in the crystal lattice. The negatively charged nitrogen-vacancy (NV$^-$) center is the most studied optically active defect in diamond and exhibits remarkable spin properties and sensing capabilities, even on the nanoscale and at room temperature. The spin properties of NV$^-$ centers close to the surface (1\,nm - 30\,nm) have been thoroughly studied and the deeper the spins are located in diamond the less decoherence from surface defects is observed. Creating NV$^-$ centers at the surface by nitrogen ion implantation and burying them beneath a thin diamond layer via chemical vapor deposition (CVD) of diamond is reported to enhance the coherence times significantly. By separating the color center generation from controlling the resulting depth by an additional diamond layer, narrow depth profiles can be achieved by reducing the ion energy during implantation. Although the coherence properties improve, many of the implanted NV$^-$ center are reported to vanish in the hydrogen-rich plasma of growth CVD chambers. The mechanism responsible for the instability of NV$^-$ centers towards the plasma remains unclear, though. One hypothesis is the formation of nitrogen-vacancy-hydrogen centers from diffusing hydrogen and implanted NV$^-$ centers but is has not been proven, yet. Here, we demonstrate the fabrication of depth confined NV$^-$ centers by combining low energy (1\,keV - 5\,keV) ion implantation and CVD overgrowth. By increasing the thickness of the overgrown layer (6\,nm - 100\,nm) the influence of surface noise is shown to decrease leading to bulk-like coherence properties of up to $450\,\upmu$s (Hahn-echo) and $T_2^*$ dephasing times around $20\,\upmu$s. Monitoring the NV$^-$ density as a function of the overgrowth time, we analyze the kinetics of the passivation in case of implanting ion doses of 10$^{11}$ $^{15}$N $^{+}$/cm$^2$ and 10$^{12}$ $^{15}$N $^{+}$/cm$^2$. The passivation turns out to occur faster for lower doses offering insights into sub-surface hydrogen diffusion in diamond. Analyzing the spin bath before and after overgrowth reveals a drastic reduction in paramagnetic spins after the CVD process but no NVH$^-$ is found around the surviving NV$^-$ centers. Checking the chemical composition in overgrown implantation sites with secondary-ion-mass-specrometry (SIMS), however, confirms significant amounts of hydrogen in the $^{15}$N-implanted layers. Since the concentration found for hydrogen exceeds the one for nitrogen by far, we suggest that diffusing hydrogen interacts also with vacancies forming probably mostly diamagnetic defects. In the last part of this dissertation, NV$^-$ centers are generated with sub-nanometer precision below a thin $^{13}$C-enriched layer for quantum simulation. For fabricating the spin architecture starting from a $^{12}$C base layer, we overgrow implanted nitrogen with a $^{13}$C layer and bury the spins beneath a 30\,nm-thick $^{12}$C capping layer. By measuring the coupling strength between the electron and nuclear spins, we deduce a NV$^-$ placement accuracy reaching below one nanometer with respect to the $^{13}$C-layer. Regarding the spin properties of the electron spins, no reduction in $T_2$ is observed despite of the proximity to the dense $^{13}$C-bath. As a consequence, the overgrowth of NV$^-$ centers is demonstrated as a bottom-up approach to produce buried spin architectures with sharp isotopic transitions and efficient $^{13}$C-NV$^-$ coupling. Together with the potential for bulk-like coherence properties the synergy of implantation and overgrowth is a powerful tool to fabricate reproducible quantum systems with tailored properties and high depth confinement.