FIB fabrication of shallow GeV and SiV color centers and solid immersion lenses in diamonds

Diamond color centers (CC) serve as solid-state single-photon sources with optical properties that make them ideal for quantum technology applications. Group-IV impurity-related color centers are gaining increasing significance due to their narrow zero phonon line (ZPL) even at room temperature, strong optical coherence, and some of the shortest lifetimes observed among solid-state color centers [1,2]. The advancement of these emitters in technology hinges on their precise fabrication and subsequent characterization. Notably, among Group-IV defects, the SiV defect (ZPL at 738 nm) was unambiguously identified in the 1990s [3] whereas the GeV defect (ZPL at 603 nm) was first reported in 2015 [2]. Since their discovery, various fabrication processes have been explored. One of the common fabrication processes is high-energy (>100 keV) ion implantation, resulting in deep (>100 nm) color centers, where a lithography step is required to localize them in plane. This approach results in large straggling and a three-dimensional delocalization of the final defect. Among other implantation techniques, focused ion beam (FIB) tools can allow extremely localized formation of color centers in a mask-less process [4]. Using low irradiation energy (<100 keV), this method is very accurate in localization of CC if helped by a laser interferometer-controlled stage. The FIB instrument at FBK is equipped with a liquid metal alloy ion source (LMAIS) capable of delivering extremely low ion fluences of Ge, Si, and Au ions, with implantation energies ranging from 5 keV to 70 keV. In recent experimental work, we investigated the formation yield of GeV and SiV defects in diamond by implanting fluences from a few to 1000 ions at 70 keV implantation energy, resulting in production of single photon emitters and a formation yield (number of active centers per implanted ions) of 0.7% (Fig. 1) . The created defects were very close to the substrate surface with an expected depth of 35 ± 15 nm for GeV and 50 ± 15 nm for SiV. Although the promising preliminary results were obtained, the collection yield of the emitted photons is strongly hindered by the diffraction index of diamond, which induces total internal reflection for photons with an incident angle greater than 24.5° [5]. The integration of the CC in a hemispheric solid immersion lens (SIL) with a radius comparable to the defect depth leads to perpendicular incidence of the emitted photons, resulting in higher extraction yield. At higher implantation energies, the integration of CC in solid immersion lens is well-known, and SILs with radii comparable to the implantation depth (on the order of a few micrometers) have already been reported [6]. Despite recent work on micro-SILs, aligning the defects' position with the SIL is a challenging task, making nano-integration difficult [7]. The fabrication of the SIL using the same FIB equipment, can greatly improve the SIL alignment with the CC. In this work, we report the nanofabrication of SILs towards radii comparable to the expected implantation depth of Ge and Si in diamond at 70 keV, using a 70 keV Au ion beam. The alignment of the nano-SILs on very superficial defects represents a challenging nanofabrication task due to the small radius and the possible Au contamination during milling. Further investigations are needed to assess the feasibility of the process and the real enhancement of photon collection yields.