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Advanced Quantum Technologies

Efficiency Optimization of Ge‐V Quantum Emitters in Single‐Crystal Diamond upon Ion Implantation and HPHT Annealing

Elena Nieto Hernández, Elisa Redolfi, Claudia Stella, Greta Andrini, Emilio Corte, Selene Sachero, Sviatoslav Ditalia Tchernij, Fabio Picariello, Tobias Herzig, Yuri M. Borzdov, Igor N. Kupriyanov, Alexander Kubanek, P. Olivero, Jan Meijer, P. Traina, Yuri N. Palyanov, J. Forneris

Published Jun 19, 2023• 11 citations
Publisher

Abstract

Abstract The authors report on the characterization at the single‐defect level of germanium‐vacancy (GeV) centers in diamond produced upon Ge − ion implantation and different subsequent annealing processes, with a specific focus on the effect of high‐pressure‐high‐temperature (HPHT) processing on their quantum‐optical properties. Different post‐implantation annealing conditions are explored for the optimal activation of GeV centers, namely, 900 °C 2 h, 1000 °C 10 h, 1500 °C 1 h under high vacuum, and 2000 °C 15 min at 6 GPa pressure. A systematic analysis of the relevant emission properties, including the emission intensity in saturation regime and the excited state radiative lifetime, is performed on the basis of a set of ion‐implanted samples, with the scope of identifying the most suitable conditions for the creation of GeV centers with optimal quantum‐optical emission properties. The main performance parameter adopted here to describe the excitation efficiency of GeV centers as single‐photon emitters is the ratio between the saturation optical excitation power and the emission intensity at saturation. The results show an up to eightfold emission efficiency increase in HPHT‐treated samples with respect to conventional annealing in vacuum conditions, suggesting a suitable thermodynamic pathway toward the repeatable fabrication of ultra‐bright GeV centers for single‐photon generation purposes.

DOI
10.1002/qute.202300010

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Abstract The authors report on the characterization at the single‐defect level of germanium‐vacancy (GeV) centers in diamond produced upon Ge − ion implantation and different subsequent annealing processes, with a specific focus on the effect of high‐pressure‐high‐temperature (HPHT) processing on their quantum‐optical properties. Different post‐implantation annealing conditions are explored for the optimal activation of GeV centers, namely, 900 °C 2 h, 1000 °C 10 h, 1500 °C 1 h under high vacuum, and 2000 °C 15 min at 6 GPa pressure. A systematic analysis of the relevant emission properties, including the emission intensity in saturation regime and the excited state radiative lifetime, is performed on the basis of a set of ion‐implanted samples, with the scope of identifying the most suitable conditions for the creation of GeV centers with optimal quantum‐optical emission properties. The main performance parameter adopted here to describe the excitation efficiency of GeV centers as single‐photon emitters is the ratio between the saturation optical excitation power and the emission intensity at saturation. The results show an up to eightfold emission efficiency increase in HPHT‐treated samples with respect to conventional annealing in vacuum conditions, suggesting a suitable thermodynamic pathway toward the repeatable fabrication of ultra‐bright GeV centers for single‐photon generation purposes.
Negatively charged germanium vacancy centers (GeV) in diamond are a promising platform for quantum computing and quantum communication. However, these applications require the precise incorporation of GeV centers with good optical properties inside of nanophotonic structures. In this work, we demonstrate the highly efficient local creation of GeV centers in diamond via focused-ion-beam implantation, followed by high-temperature annealing. We report the successful creation of GeV centers over the depth range of 5.5 - 30 nm. Implantation at low fluence enables the creation of single GeV centers. The formation yield strongly depends on implantation energy and fluence, reaching up to 33% at energies of 35 and 70 keV. This method, therefore, enables the efficient creation of GeV centers within a small, well-defined local sample volume and offers a potential means of incorporating them into photonic structures.
Abstract The nanoscale fabrication of μm-spaced single-photon emitter arrays is crucial for the development of integrated photonic chips. We report on the fabrication and systematic characterization of germanium-vacancy (GeV) color centers arrays in diamond obtained upon ion implantation at the nanoscale. Ge2+ ion implantations at 35 keV and 70 keV energies were carried out using a focused ion beam (FIB) equipped with a liquid metal alloy ion source. The arrays of emitters are subsequently aligned to ø300 nm nanopillar waveguiding structures, fabricated using a combination of electron-beam lithography and plasma etching. The photon collection efficiency and photoluminescence (PL) signal-to-background ratio increased by a factor 8 with respect to the unstructured sample. The photophysical properties of the GeV emitters fabricated by this approach were unaltered with respect to those found in unprocessed diamond. The efficiency of the overall manufacturing process to fabricate individual GeV centers was assessed. Up to 33% of the fabricated nanopillars, depending on ion implantation parameters, were found to contain single emitters.
Diamond point defects have been applied to various quantum devices, including quantum interfaces, quantum memory, and quantum sensing. Recently, group IV-V centers, which consist of group IV ions such as silicon (Si), germanium (Ge), and tin (Sn), along with atomic vacancies (V), are getting high hopes for potential application in quantum devices. This is because group IVV centers exhibit a longer spin relaxation time T2 on the order of milliseconds at cryogenic temperatures and have bright zero phonon line (ZPL) emission intensities1 . The formation process of group IV-V centers can be roughly classified into two steps: (1) generation of interstitial atoms and vacancies by ion implantation, and (2) diffusion of vacancies and substitutional atoms by hightemperature annealing. High-temperature annealing is thought to graphitize the diamond substrate and is typically performed under high pressure. In this presentation, we focus on optimizing the annealing processes in an argon atmosphere at 1800oC. While the optimization of the annealing processes has been actively studied for nitrogen-vacancy (NV) centers, it has not been thoroughly explored for group IV-V centers. Group IV-V centers are constructed with an atom between split vacancies, whereas an NV center is constructed with an interstitial atom and an atomic vacancy. These structural differences could lead to variations in the elementary processes of their formations. Therefore, the optimization processes of annealing conditions need to be re-examined. In this study, we investigated atmospheric annealing processes using HPHT (grown by NIMS) and CVD (Element 6 electric grade) diamonds. Each diamond sample was irradiated with 18 MeV Si, Ge, and Sn ion beams and implanted at about 3 μm from the sample surface. We report efficient formation processes of group IV-V centers at atmospheric pressure, and in a very short time for high-temperature processes compared to the conventional process. In addition, we will discuss the interpretation of the examination result that the number of group IV-V centers formed does not necessarily increase monotonically with annealing time, as shown in Figure 1.
We report on the metrological characterization of the emission from a germanium-vacancy center in diamond under a microfabricated solid immersion lens in a confocal laser-scanning microscope setup. Ge ions were implanted into a synthetic diamond at 3 MeV, and germanium-vacancy centers were then formed by subsequent annealing. Afterward, solid immersion lenses were fabricated in a focused ion beam scanning electron microscope. The photoluminescence was investigated at room temperature in terms of the spectral distribution, the excited state lifetime, the second-order correlation function, and the saturation behavior, proving simultaneous high single-photon purity and high brightness. Two methods were exploited to minimize the residual multi-photon probability: spectral filtering and temporal filtering. According to these results, we assume that Raman scattered photons and emission from neighboring color centers play an important role in the residual multi-photon emission probability. The system efficiency of the single-photon source was investigated and found to be in accordance with the value calculated from all sources of loss in the setup. The branching ratio of the germanium-vacancy center for the decay into the ground state and into metastable state was calculated. The results enable the usage of the single-photon source in future quantum radiometric experiments.
ABSTRACT With unique optical and spin properties, the negatively charged nitrogen‐vacancy (NV − ) center in diamond has become a leading physical platform for quantum information processing. Fabrication of NV centers involves three steps: introducing nitrogen atoms, creating lattice vacancies, and combining these two types of defects. The final step is typically achieved by high‐temperature annealing, as vacancy migration in diamond is thermally activated above 600°C. Compared to low‐pressure high‐temperature (LPHT) annealing, high‐pressure and high‐temperature (HPHT) treatment allows for much higher annealing temperatures and longer durations, providing new opportunities to optimize key metrics of NV centers. This short review summarizes recent experimental progress in this direction. Dense NV centers with good spin coherence and single NV centers with excellent optical properties have been produced through HPHT growth and post‐treatment of single‐crystal diamonds. In addition, structural imaging, optical characterization, and DFT calculations provide further insights into the dynamics of defect migration under high‐temperature annealing. These results offer new perspectives on the fabrication of color centers in diamond and other wide‐gap semiconductor materials, including silicon carbide and hexagonal boron nitride, for advanced quantum applications.
A systematic photoluminescence (PL) investigation of the spectral emission properties of individual optical defects fabricated in diamond upon ion implantation and annealing is reported. Three spectral lines at 620, 631, and 647 nm are identified and attributed to the SnV center due to their occurrence in the PL spectra of the very same single‐photon emitting defects. It is shown that the relative occurrence of the three spectral features can be modified by oxidizing the sample surface following thermal annealing. The relevant emission properties of each class of individual emitters, including the excited state emission lifetime and the emission intensity saturation parameters are reported.