量研学術機関リポジトリ「QST-Repository」は、国立研究開発法人 量子科学技術研究開発機構に所属する職員等が生み出した学術成果（学会誌発表論文、学会発表、研究開発報告書、特許等）を集積しインターネット上で広く公開するサービスです。 Welcome to QST-Repository where we accumulates and discloses the academic research results(Journal Publications, Conference presentation, Research and Development Report, Patent, etc.) of the members of National Institutes for Quantum and Radiological Science and Technology.
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1. Introduction
Negatively charged nitrogen-vacancy (NV-) center in diamond is expected as quantum sensor to measure small changes in physical quantities, such as magnetic and electric field, temperature, strain, etc. The quantum sensors with high sensitivity require diamond containing high concentration of NV centers. Electron irradiation is a good method to create high concentration of NV centers. The high energy electrons knock carbon atoms out of the diamond lattice, creating vacancies. Annealing an irradiated diamond causes all the vacancies to become mobile and subsequently trapped by substituted nitrogen (P1 centers), forming NV centers. It is known that the process of NV center creation depends on the initial P1 concentrations, and the irradiation fluence dependence at low initial P1 concentrations below 1 ppm is still uncleared. In this study,
we irradiate samples with different initial P1 concentrations, especially low concentration, and estimate the fluence dependency of the charge state and the amount of created NV centers.
2. Experiments
The diamonds synthesized by High Pressure and High Temperature (HPHT) and Chemical Vapor Deposition (CVD) were used. The initial concentration of P1 was in the range from 40 ppb to 100 ppm. 2 MeV electrons were irradiated with a fluence up to 2E18 cm-2 at room temperature. Then, the samples were annealed at
1000℃ for 2 hours to create NV center. The concentration of P1 was measured by Electron Spin Resonance (ESR) using JES-X330 (JEOL). The ratio between NV0 and NV- was estimated from photoluminescence (PL) spectrum measurement obtained using LabRAM HR Evolution (HORIBA). All these measurements were
performed at room temperature.
3. Results & Discussion
In the case of type Ib HPHT diamond containing initial P1 concentration of ~100 ppm, the most of NV centers are thought to be NV-, and PL measurement revealed that only NV- was detected when the fluence of 2E18 cm-2. P1 center acts as electron donor, and provide electron to NV0. In contrast to high P1 concentration,
an appreciable fraction of NV center is present as NV0 for samples with low initial P1 concentration (~ 10 ppm). We examined the relationship between the concentration of P1 and the fluence, using HPHT diamonds with initial P1 concentration of ~0.8 ppm. The concentration of P1 decreased to ~0.3 ppm when the fluence of 1.5E17 cm-2. This result suggests that P1 centers of ~0.5 ppm change to either NV center or positively charged substitutional nitrogen. PL spectrum when the fluence of 1.5×1017 cm-2, shows that both NV0 and NVare
created. The ratio of NV- to NV0 decreased as irradiation fluence, to be 0.70 after irradiated with 1.0×1017 cm-2 and 0.53 after irradiated with 1.5E17 cm-2. It is noted that NV0 was formed even though P1 center which can act as electron donor to NV0 center remains. Similar results were observed for CVD diamonds with initial P1 concentration of ~40 ppb. The adequate fluence to maximize NV- concentration for diamonds containing low P1 concentration will be discussed by examining the fluence dependency of the charge state and total amount of NV center.
Effect of P1 concentration on NV centers created by electron beam irradiation
