FEATURED TOPIC

Synthetic Diamond for Nitrogen Vacancy Sensor and
Its Applicability
Yikang ZUO*, Tsukasa HAYASHI, Hiroshige DEGUCHI, Hiromi NAKANISHI,
Yoshiki NISHIBAYASHI, Natsuo TATSUMI, Minori TERAMOTO, and Yutaka KOBAYASHI---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Synthetic diamonds have been widely used in industrial applications due to their high purity and low crystal defects compared tonatural diamonds. It is also possible to endow specific electrical and magnetic properties by doping with different types of
elements, and is expected to be applied in various fields such as sensing and measurement. We have succeeded in producing a
diamond with the world's highest level of purity and lowest level of defects by using our unique high-pressure and hightemperature method. Furthermore, we have created a highly sensitive sensor with the nitrogen vacancy (NV-) center by electron
beam processing and ion implantation in cooperation with Nissin Electric Co., Ltd. This paper introduces the high quality diamondsynthesized for the NV sensor and its applicability.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: diamond, electron-beam processing, ion implantation, nitrogen vacancy (NV−) center, sensing

1. Introduction
Uses of diamond are not limited to jewelry. It is
known as a fundamental material for various technologies
ranging from heavy industries to semiconductors and otherleading-edge industries. Sumitomo Electric Industries, Ltd.
began research of synthetic single-crystal diamond(Sumicrystal) in the 1970s and succeeded in the world’s
first mass production of diamond (Photo 1). Sumicrystal
has high hardness and high thermal conductivity.
Moreover, our technology can decrease crystal defects anddislocations to extremely low level compared to natural
diamonds. Due to these excellent properties, Sumicrystal
has been used in a wide range of applications, such as
grinding wheels, dressers, drawing dies, cutting tools(1),drills, end mills, throwaway inserts, and heat spreaders.
Furthermore, Sumitomo Electric succeeded in developing
colorless high-purity diamond called Sumicrystal Type II
in 1995. It has been used as a material for various opticalcomponents and pressure-resistant windows.
In recent years, the NV− center in diamond has been
the focus of attention in the ultra-high-sensitivity sensor

and quantum information device fields. Since diamond-based
NV sensors are highly sensitive even at room temperature,
diamond becomes a promising material for biosensors suchas cytosensors, magnetoencephalography and magnetocardiography. Automobile sensors, and atomic-level finemagnetic field sensors have also been proposed. Moreover,diamond offers superb quantum controllability, which canbe exploited to apply it to quantum computers*1 andquantum repeaters*2 in the near future. This paper reports
on a diamond NV sensor currently in joint developmentwith Nissin Electric Co., Ltd.

2. The NV Sensor and Its Required Properties
Ultra-high-sensitivity sensors, also known as NV
sensors, can operate even at room temperature because of
their unique NV− center pair in diamond, as shown in Fig. 1.
In order to introduce the NV− center into diamond lattice, a
trace amount of nitrogen (N) is added firstly during
synthesis. Then the vacancy (V) is introduced by using
electron-beam processing* 3 or ion implantation* 4.

V

N eC

Photo 1. Synthetic single-crystal diamondFig. 1. NV− center in diamond lattice

SUMITOMO ELECTRIC TECHNICAL REVIEW · NUMBER 92 · APRIL 2021 · 73

Eventually, the NV− center can be formed by annealing the
diamond.
The NV− center is negatively (−) charged when one
electron is captured. It creates a state sensitive to magnetism, electric field, and temperature, which is known as thetriplet spin state. The state of the NV− center can bedetected sensitively by light even with a single spin, whileother methods such as electron spin resonance (ESR)
require many spins. Moreover, each carbon atom is joined
to other four carbon atoms with strong covalent bonds indiamond lattice, which makes the disturbances in diamondlattice extremely low. As a result, the NV− center can bemeasured with high-sensitivity even at room temperature.
These facts make diamond promising for very convenient
use in high-sensitivity sensors and quantum computers.
The sensitivity of the NV sensor can be expressed by
the following equation.

3-1 High purification technology
To improve the sensitivity of NV sensors, it is necessary to achieve a high level of purification by controllingthe amount of nitrogen and eliminating foreign elements.
Diamond synthesized through the HPHT processcommonly contains 10 to 200 ppm nitrogen in a singlesubstitutional form, which is known as the Ib type
(Photo 2). Since NV− centers are susceptible to faintmagnetic fields, controlling nitrogen impurities with spin isa challenge. We developed a technology to use titanium asa nitrogen getter during synthesis and control nitrogen to alevel of a few ppm.(4) Moreover, in addition to nitrogen,foreign elements such as boron (B) and nickel (Ni) are also
readily contained in the crystal. To avoid these inclusions,raw materials must be highly purified. High-purity
diamond thus produced is raising huge expectations about
its use in NV sensors.

.................................................... (1)where C is the contrast, η is the sensor sensitivity, nNV is
the number of NV− centers, and T2 is the electron spin
coherence time.(2)The sensor sensitivity increases with decreasing η .
Therefore, in order to develop a high-sensitivity sensor, it isnecessary to either make nNV larger or T2 longer. However, alarger nNV usually results in a shorter T2, which can be causedby the nuclear spin attributable to the nitrogen in the NV−
center, the inclusion of foreign elements, and crystal defects.
In order to optimize this trade-off, several different kinds ofmethods that make T2 longer are currently being exploredfrom material and process points of view.(3) Since the lengthof T2 is closely related to the properties of diamond, we
consider that it is effective to have a longer T2 by reducing
impurities and crystal defects in diamond lattice, thereby
reducing noise from their spins.

3. Diamond Synthesis Process for NV Sensors
Diamond synthesis processes are roughly divided into
high-pressure high-temperature (HPHT) process*5 andchemical vapor deposition (CVD) process*6.
Development of a high-sensitivity NV sensor requires
a high-quality diamond of high purity and few defects,
which is close to a perfect crystal. To synthesize diamondthrough a CVD process, it is common to adopt homoepitaxial growth using single-crystal diamond as a seed
substrate. A drawback involved in this process is that latticedefects such as dislocations are likely to occur during the
growth from the substrate interface, resulting in a short T2.
It has been difficult to synthesize high-quality diamond
since only small substrates are currently available and it isextremely difficult to obtain a seed substrate with few
dislocations and few crystal defects, as described later. In
comparison, the HPHT process synthesizes diamond in an
ultra-high-pressure and ultra-high-temperature region inwhich diamond is stable. Therefore, it is easier to yieldhigh-purity and low-defect diamond. Consequently, weused the HPHT process to synthesize diamond for NV
sensors.

74 · Synthetic Diamond for Nitrogen Vacancy Sensor and Its Applicability

Sumicrystal
Sumicrystal
Ib (Light Yellow) Type II (Colorless)Diamond for
NV sensors(Light pink)

1 mm

Photo 2. Different types of synthetic single-crystal diamond

3-2 Defect reduction technology
Diamond synthesized through the HPHT processoffers superb crystallinity and a very low level of variation
between crystals. However, close observations of crystal
defects by means of X-ray topography reveals radiallyextending needle-like dislocation defects, as shown in
Photo 3.(4)These types of defects are presented in most diamonds
synthesized through the HPHT process and affect the sensitivity of NV sensors (due to a short T2). To counter thisproblem, Sumitomo Electric’s proprietary crystal growth

Photo 3. X-ray transmission topograph of diamondtechnology was deployed, which resulted in success insynthesizing diamond with virtually no dislocation defect,
as shown in Photo 4.(5)4-1 Ion implantation
Nissin Ion Equipment Co., Ltd. has a medium-current
ion implanter compatible with 300 mm wafers in a cleanroom (Class 1). This implanter can implant various ion
species and even cluster or molecular ion species. It also
implants ion species while heating the substrate. The
company is also engaged in subcontract implantation(Table 1). Ion implantation requires expertise to set conditions because the method also produces other forms ofnitrogen than the substitutional form. Nonetheless, using
the EXCEED system in the current development, NV−centers were produced.
Table 1. Summary of Ion Implanters

3 mm
Photo 4. X-ray transmission topograph of low-defect diamond

A large defect-free region of 5 mm in diameter ispresented at the center of this diamond. The diamond thus
synthesized is the closest to a perfect crystal among
currently obtainable diamonds and has already been
provided as a sample to research institutes in Japan andabroad. Moreover, using the CVD process described at the
beginning of this chapter and this diamond as a seed
substrate, large low-defect diamond can be synthesized.

4. NV Sensor Fabrication Technology
The development of a defect reduction technology
approaching perfect crystals and high-purification andnitrogen concentration control technologies for controlling
nitrogen concentration to a level of a few ppm has enabled
us to provide diamond for NV sensors. Following this,Sumitomo Electric explored NV sensor fabrication andapplication technologies with Nissin Electric and its groupcompanies.
NV sensor fabrication methods include the following:1) Implantation of nitrogen ions into high-purity diamond,2) Electron-beam processing of diamond with preimplanted nitrogen, and 3) Doping nitrogen during
diamond synthesis through a CVD process. Vacancies
formed during ion implantation or electron-beamprocessing move in the diamond crystal at a high temperature of 600°C or above and bond with nitrogen, forming
the NV− centers.
Ion implantation enables controlling nitrogen concentration in the direction of depth through the control ofacceleration energy and designing a two-dimensional NV−
center arrangement through the use of a mask. Thus, further
advanced utilization of diamond is being explored.
Regarding the CVD process, it has been reported thatN-V axes were aligned on a (111) diamond substrate in a
direction perpendicular to the surface with a probability
close to 100%.(6)Model

Ion species

EXCEED
3000AH

N+, N2+, B+, BF2+, P+, As+,
In+, Sb+, etc.

CLARIS

N2 , B18Hx , C16Hx , C7Hx ,5E10~1E17
P4+, etc.

IMPHEATN+, N2+, Al+, P+, B+, etc.

+

+

Dose

+

EnergyOther

5E10~1E17 5~750 keV+

4~80 keV

Cluster

Substrate
5E10~1E17 5~960 keV temperature:
up to 500°C

4-2 Electron-beam processing
NHV Corporation provides various irradiation
services (Table 2). The company uses several transfer
methods suitably according to irradiation materials andconditions. The company can also handle indefinitely
shaped small substrates such as diamond. Using the
EPS-3000 system in the current development, irradiation
parameters for diamond a few mm in thickness were ascertained to achieve vacancy introduction into the diamondand NV− centers were formed.
Table 2. Summary of Electron-Beam Processing Systems
Model

Acceleration Max. irradiation Max. sheetingvoltage (kV)width (cm)Dimensions (cm)

Carrier

EBC-200

150~200

100−

Film･Sheet(Φ40 cm, 200 kg/m)

EBC-300

150~30060

60×60×3

Wafer, etc.
(External Carrier
Unit equipped)

EPS-750300~750120

120×80×7

Wafer, Film,
Sheet

EPS-800

400~80060

60×100×7

Wafer, Sheet,
Wire, etc.

180180×130×8

Wafer

180×90×15

Wafer,
Film･Sheet

EPS-3000

1000~3000180

4-3 Basic characteristics of NV sensors
We fabricated NV sensors by performing electronbeam processing and annealing on various synthesized
diamond substrates. Figure 2 presents changes in the fullwidth at half-maximum (FWHM) of an optically detectedmagnetic resonance* 7 spectrum against the nitrogenconcentration of diamond.

SUMITOMO ELECTRIC TECHNICAL REVIEW · NUMBER 92 · APRIL 2021 · 75

FWHM of magnetic resonance (MHz)100

Electron dose level: 1018/cm2

101

receive LD control signals, photodetector signals, andmicrowave signals, electric cables are drawn in or out. This
small sensor head with only thin cables attached enables
measurements conveniently in proximity to the magnetic
field to be measured.

NV Sensor
10100
Nitrogen (ppm)

1000

5 cm

Fig. 2. Nitrogen concentration dependence of FWHM of optically detectedmagnetic resonancePhoto 5. Exterior of pen-shaped module

The graph reveals that the FWHM increases withincreasing nitrogen concentration. It is likely that, at a highnitrogen concentration: distances between an NV− centerand its adjacent nitrogen atom or NV− center that has spin
become small; disturbances increase; and the spin coherence time becomes short.
According to Equation (1), and depending on applications, the sensitivity of an NV sensor can be enhanced
either by raising fluorescence intensity (the number of NV−
centers: nNV ) or by making the spin coherence time (T2)longer. The optimal solutions vary with specific needs.
Even so, as described above, NV sensor prototypes havemade exploration of the optimal solution possible.

5-2 Antenna-type module
To install a sensor in proximity to high-voltage power
equipment, bringing the sensor thoughtlessly close to theequipment can result in a ground fault because coaxial
cables used to transmit microwaves are made of metal. To
address this challenge, a prototype NV sensor module
transmitting and receiving microwaves was constructed toachieve electrically isolated measurement. The configuration illustrated in Fig. 3 was used to confirm for the firsttime that magnetic field signals of a simulated power line
(50 Hz alternating current of 30 A) could be measured at a
maximum distance between antennas of 10 m.

5. NV Application Technology
Sumitomo Electric and Nissin Electric are commonly
operating in the electricity sector. Taking this businesssector as an example, Chapter 5 describes forms of NV
sensor usage. Common magnetic sensors convert magnetic
signals to electrical signals and transmit them via conductors. Accordingly, they capture noise other than magnetic
signals. In contrast, NV sensors only detect the magnetic
field around them and transmit signals optically, contributing to reduced electrical noise from electric wires; inother words, their measurements are electrically isolated.
This enables the sensor to be installed in proximity to ahigh-voltage power line, providing strong signals andachieving high-performance measurements. Moreover,diamond offers superb environmental resistance and istherefore expected to function as a sensor stably for a long
period even in a harsh environment. To adapt to thesevarious applications, we constructed prototype modules
with an embedded NV sensor block.
5-1 Pen-shaped module
First, using Sumitomo Electric’s optical technology,
we constructed a prototype module of a pen-shaped NV
sensor (Photo 5). The pen-shaped casing of this model
contains diamond with NV− centers in it, as well as a laser
diode (LD), a multiplexing/demultiplexing module, amicrowave coil, and a photodetector. To transmit and

76 · Synthetic Diamond for Nitrogen Vacancy Sensor and Its Applicability

Transmitting
Antenna

Receiving
AntennaOver 4 m
Microwave(2.87 GHz)

Coil
NV
SensorOptical Fiber
AC Power Line Sensor Unit
(50 Hz)Fig. 3. Experimental system for alternating magnetic field (electric current)
measurement

Figure 4 shows the experimental results for a distance
between antennas of 4 m. Changes over time in signal
intensity were observed to correspond to changes in thealternating magnetic field. The experiment proved that NV
sensors can be installed for measurement in high-voltage
areas without the use of a metal cable.
Diamond NV sensors, as described above, areexpected to come into increasing use as convenient high-Signal Intensity (r.u.)

Over 4 m

10 msec

Time (msec)

Fig. 4. Alternating magnetic field (electric current) measurement resultssensitivity sensors in various applications and environments.

Ion implantation: A technology of irradiating a＊4	sample with high-energy ions. It can be used for
creating doped layers on semiconductors.
＊5	High-pressure high-temperature (HPHT) process: A
technology used to produce synthetic diamond fromgraphite as the start material under the conditions of
1350°C and 5 GPa, by using an ultra-high pressure
generator,.
Chemical vapor deposition (CVD) process: A＊6	technology used to produce synthetic diamond below
atmospheric pressure by decomposing source gases(hydrogen and methane) by plasma to reactchemically on a substrate.
Optically detected magnetic resonance: A＊7	phenomenon in which fluorescence intensity reflects
the resonance state of electron or nuclear spin underthe effect of a magnetic field, with the resonance state
caused by a specific microwave frequency andmagnetism.

6. Conclusion
This article described high-sensitivity NV sensors
incorporating high-pressure synthesized single-crystal
diamond and their applicability. Expectations are high for
its use in various areas, including magnetoencephalography, magnetocardiography, automobile sensors, andremote measurement in harsh environments. This paper
reported on procedures used to synthesize raw material
diamond, NV− center formation, and application technologies. Going forward, for the commercialization of the NV
sensor, we intend to develop high-quality diamond products and sensor module products and explore their applications.
• ‌Sumicrystal is a trademark or registered trademark of Sumitomo ElectricIndustries, Ltd.
• ‌EXCEED, CLARIS, and IMPHEAT are trademarks or registered trademarks of
Nissin Electric Co., Ltd.

References(1)	K. Obata, “Single-Crystal Diamond Cutting Tool for Ultra-Precision
Processing,” SEI Technical Review, No. 188, pp.65-70 (2016)(2)	
J. M. Taylor, P. Cappellaro, L. Childress, L. Jiang, D. Budker,
P. R. Hemmer, A. Yacoby, R. Walsworth, and M. D. Lukin, “Highsensitivity diamond magnetometer with nanoscale resolution,” Nat.
Phys., Vol. 4, pp. 810-816 (2008)(3)	P. C. Maurer, G. Kucsko, C. Latta, L. Jiang, N. Y. Yao, S. D. Bennett,
F. Pastawski, D. Hunger, N. Chisholm, M. Markham, D. J. Twitchen,
J. I. Cirac and M. D. Lukin, “Room-temperature Quantum Bit Memory
Exceeding One Second,” Science, Vol. 336, pp. 1293-1286 (2012)(4)	H. Sumiya, N. Toda, S. Satoh, “Synthesis of High Purity DiamondSingle Crystal II,” The review of high pressure science and technology,
Vol.5, No.2, pp. 110-115 (1996)(5)	H. Sumiya and K. Tamasaku, “Large Defect-free Synthetic Type IIa
Diamond Crystals Synthesized via High Pressure and High
Temperature,” Jpn. J. Appl. Phys., Vol.51, 090102 (2012)(6)	T. Fukui, Y. Doi, T. Miyazaki, Y. Miyamoto, H. Kato, T. Matsumoto,
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Technical Terms

Quantum computer: The next-generation computer＊1	
based on quantum effect, which has overwhelming
processing power in comparison with nowadays
computers in specific calculation field. On the other
hand, the current quantum computer made withsuperconducting devices can only be operated at anextremely low temperature close to absolute zero.
Quantum repeater: A repeater means for enabling＊2	long-distance quantum information communications
based on quantum entanglement. Since theinformation sent and received is nearly impossible tobe duplicated, it is possible to extend the distance ofquantum cryptography by using multiple quantum
repeater systems.
＊3	Electron-beam processing: A technology of irradiatinga sample with high-energy electron beams. It can beused for cross-linking treatment to polymer products
or controlling the behavior of semiconductor carriers.

SUMITOMO ELECTRIC TECHNICAL REVIEW · NUMBER 92 · APRIL 2021 · 77

Contributors The lead author is indicated by an asterisk (*).

Y. ZUO*• ‌Advanced Materials Research Department

T. HAYASHI• ‌Ph.D.
Chief Senior Staff, Nissin Electric Co., Ltd.

H. DEGUCHI• ‌Chief Senior Staff, Nissin Electric Co., Ltd.

H. NAKANISHI• ‌Senior Assistant Manager, Semiconductor Innovation
Business Unit

Y. NISHIBAYASHI• ‌Ph.D.
Senior Assistant Manager, Advanced Materials
Research Department

N. TATSUMI• ‌Ph.D.
Manager, Nissin Electric Co., Ltd.

M. TERAMOTO• ‌Group Manager, Advanced Materials Research
Department

Y. KOBAYASHI• ‌General Manager, Advanced Materials Research
Department

78 · Synthetic Diamond for Nitrogen Vacancy Sensor and Its Applicability