Formation of nitrogen-vacancy centersin 4H-SiC and their near infrared
photoluminescence properties
Cite as: J. Appl. Phys. 126, 083105 (2019); https://doi.org/10.1063/1.5099327
Submitted: 09 April 2019 . Accepted: 27 July 2019 . Published Online: 23 August 2019
Shin-ichiro Sato (佐藤 真一郎)
泰斗)

, Takuma Narahara (楢原 拓真), Yuta Abe (阿部 裕太), Takahide Umeda (梅田 享英), Yasuto Hijikata (土方, and Takeshi Ohshima (大島 武)

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Formation of nitrogen-vacancy centers in 4H-SiC
and their near infrared photoluminescence
properties
Cite as: J. Appl. Phys. 126, 083105 (2019); doi: 10.1063/1.5099327
Submitted: 9 April 2019 · Accepted: 27 July 2019 ·Published Online: 23 August 2019

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Takuma Narahara (楢原 拓真),1,2 Yuta Abe (阿部 裕太),1,3
Shin-ichiro Sato (佐藤 真一郎),1,a)2
Yasuto Hijikata (土方 泰斗),
Takahide Umeda (梅田 享英),3
and Takeshi Ohshima (大島 武)1,3
AFFILIATIONS
1Quantum Beam Science Research Directorate, National Institutes for Quantum and Radiological Science and Technology,
1233 Watanuki, Takasaki, Gunma 370-1292, Japan

2

Graduate School of Science and Engineering, Saitama University, 255 Shimo-ohkubo, Sakura-ku, Saitama,

Saitama 338-8570, Japan
3
Tsukuba Research Center for Energy Materials Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japana)

Author to whom correspondence should be addressed: sato.shinichiro2@qst.go.jp

ABSTRACTNCV−
Si centers in SiC [nitrogen-vacancy (NV) centers], which produce near-infrared (NIR) photoluminescence (PL) at room temperature,
is expected to have applications as quantum sensors for in vivo imaging and sensing. To realize quantum sensing using NV centers,
clariﬁcation of the formation mechanism as well as control of the high-density formation is necessary. This paper reports a comprehensive
investigation on the NIR-PL properties originating from NV centers in high purity semi-insulating and nitrogen (N) contained 4H-SiC
substrates formed by ion beam irradiation and subsequent thermal annealing. It is shown that NV centers are exclusively formed by thecontained N as impurities rather than the implanted N, and also the heavier ion irradiations induce the NV center formation eﬀectively
than the lighter ion irradiations. The study on thermal annealing at diﬀerent temperatures reveals that the optimal temperature is 1000 °C.
From the results of temperature dependence on the PL intensity, it is shown that little thermal quenching of the PL intensity appears atroom temperature and the PL signal is collected even at 783 K. The formation mechanism of NV centers is also discussed based on theobtained results.
Published under license by AIP Publishing. https://doi.org/10.1063/1.5099327
I. INTRODUCTION
Defects and color centers in wide-gap semiconductors, ofwhich the spin state can be optically controlled, are of strong interest
because of their potential for applications in quantum computing(qubit)1–4 and sensing.5–8 The negatively charged nitrogen-vacancy(NV) center in diamond has been proven to be a prominent solidstate qubit which can be operated at room temperature (RT),3,9,10
and the great success of NV centers in diamond has also stimulated
further exploration of optically active spin-carrying defects. Siliconcarbide (SiC) is one of the attractive host materials for quantum
sensing and metrology because the crystal growth technology, in
addition to the device fabrication technology, are well developed andSiC-based electrically driven quantum sensors are feasible.11,12 It has

J. Appl. Phys. 126, 083105 (2019); doi: 10.1063/1.5099327
Published under license by AIP Publishing.

13–18
been demonstrated that silicon vacancies (V−and divacancies
Si)
19–26
could serve as controllable spin states(VSiVC or simply VV)
with sharp zero-phonon lines (ZPLs).
Recently, NCV−
Si centers in SiC, i.e., negatively charged pairs ofsilicon vacancy and a nitrogen atom on an adjacent carbon site,
have been proposed as another optically active defect with an electronic structure strongly resembling that of NV centers indiamond.27–31 ZPLs of the absorption between the 3A2 ground state
and the 3E excited state are located at around 1200 nm, much
longer than that of diamond-NV centers (638 nm). This opticalproperty is advantageous for in vivo imaging and sensing, since thenear-infrared (NIR) light can penetrate biological tissues such as
skin and blood more eﬃciently than visible light.32,33 Also, unlikeVVs, NCV−
Si centers produce luminescence and possibly their spin

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states can be manipulated at room temperature (RT), which are
the essential properties for life science applications. In addition,
operation at higher temperatures than RT is required for quantum
sensing applications under harsh environments such as diagnostics
of power electronics and vectorized magnetometer in space.34 For
example, devices mounted on spacecrafts for Venus exploration
missions must withstand temperatures of up to 460 °C, theVenusian surface temperature. NCV−
Si centers in SiC hold thepromise of quantum sensors which can meet these various needs.
Here, “NCV−
Si centers in SiC” is represented as “NV centers” forsimplicity unless otherwise noted.
Unfortunately, the optical spin-state manipulation [e.g., optically detected magnetic resonance (ODMR)] of NV centers has notbeen demonstrated to date. One of the reasons is that the formation
mechanism has been less well understood and the optimum fabrication procedure has not yet been established. For quantum
sensing based on optical spin-state manipulation, a higher densityof NV centers is crucial to improve sensitivity without increasing
the volume, maintaining the high spatial resolution achievable. Not
having been observed in any as-grown SiC materials, NV centers
could be introduced only by using energetic particle irradiation andsubsequent thermal annealing. For example, von Bardeleben et al.
have reported that NV centers were formed by 12 MeV proton irradiation at the ﬂuence of 1 × 1016 cm−2 and subsequent 900 °C
annealing in n-type 4H-SiC.31 Eﬀects of radiation species, ﬂuence,and irradiation temperature on the formation of NV centers should
be systematically investigated. High energy electrons and protons
produce point defects sparsely in crystals, whereas heavy ionsproduce cascade damage, i.e., localized dense defect regions, inaddition to point defects. Thus, diﬀerences among irradiated ion
species reﬂect the mechanism of NV center formation. Moreover,thermal annealing conditions after irradiation should be optimized,
since suﬃcient vacancy diﬀusion is not expected at low temperature, whereas diﬀerent types of complex defects may be formed athigh temperature. It is also interesting to explore whether nitrogen(N) as impurities or implanted N dominates NV center formation.
In this study, we systematically investigated NIR photoluminescence (PL) properties of ion irradiated 4H-SiC in order to understand the NV center formation mechanism and to control it. Twotypes of 4H-SiCs, n-type 4H-SiC with the nitrogen concentrationof 9 × 1018 cm−3 and HPSI (high purity semi-insulating) 4H-SiC,
were used in this study.
II. EXPERIMENTAL
Samples used in this study were n-type 4H-SiC and HPSI
(high purity semi-insulating) 4H-SiC substrates. Here, thesesamples are represented by N-4H-SiC and HPSI-4H-SiC, respectively. The N concentrations of N-4H-SiC and HPSI-4H-SiC were
determined to be 9 × 1018 cm−3 and 3 × 1015 cm−3 by Secondary
Ion Mass Spectroscopy (SIMS). The samples were irradiated with240 keV-H (hydrogen), 2 MeV-N (nitrogen), 4 MeV-Si (silicon),and 7 MeV-I (iodine) ion beams at diﬀerent ﬂuences at RT. Theseion beam energies were chosen so that radiation defects were introduced at the depth of about 1.5 μm from the surface. The averageion beam ﬂuxes were 2.6 × 1012 cm−2 s−1 for 240 keV-H,
1.9 × 1011 cm−2 s−1 for 2 MeV-N, 1.0 × 1011 cm−2 s−1 for 4 MeV-Si,

J. Appl. Phys. 126, 083105 (2019); doi: 10.1063/1.5099327
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FIG. 1. Vacancy concentration proﬁles in SiC (3.21 g cm−3) induced by ion
beams used in this study, calculated by TRIM.35 Lines in black, blue, pink, anddark yellow denote 240 keV-H, 2 MeV-N, 4 MeV-Si, and 7 MeV-I, respectively.
Fluences, which the highest PL intensity was obtained at, are shown in thisﬁgure (see Fig. 7).

and 5.2 × 1010 cm−2 s−1 for 7 MeV-I. Figure 1 shows the depth
proﬁles of radiation defects (vacancies) formed by these ion beams,
calculated by Monte Carlo Simulation Code, TRIM.35 The massdensity of 3.21 g cm−3 and the displacement threshold energies of
25 eV for Si and 21 eV for C were used for the calculation,36
although slightly diﬀerent values have also been reported.37,38 All
the ion implantations were performed at the Takasaki Advanced
Radiation Research Institute, National Institutes for Quantum andRadiological Science and Technology (QST). After ion irradiation,
all the samples were thermally annealed under Ar atmosphere(1 atm) using an infrared furnace. The furnace temperature rose toa set temperature in 1 min, then kept at the set temperature for 30
min, and was then naturally cooled down to RT for about 15 min.
NIR-PL properties at the irradiated region in the samples at
RT and 80 K were investigated using HORIBA LabRAM HR
Evolution (micro-PL measurement system). The excitation laser
wavelength was either 785 nm or 1064 nm. PL ranging from
850 nm to 1500 nm from the samples was collected with an objective lens (numerical aperture, NA is 0.90 at RT and 0.50 at 80 K)
and detected by an InGaAs array detector. The laser spot diameter
estimated from the used objective lens was 1.1 μm (785 nm laser,RT), 1.9 μm (785 nm laser, 80 K), 1.4 μm (1064 nm laser, RT), and2.6 μm (1064 nm laser, 80 K).
III. RESULTS AND DISCUSSION
A. Comparison of 785 nm and 1064 nm excitations
Figure 2 shows NIR-PL spectra at 80 K of HPSI- andN-4H-SiC samples through 785 nm and 1064 nm excitations. The
samples were irradiated with 2 MeV-N ions at a ﬂuence of
2.0 × 1014 cm−2 and subsequently thermally annealed at 1000 °C for
30 min under Argon (Ar) atmosphere after irradiation. The peak

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FIG. 2. Typical NIR-PL spectra of HPSI- and N-4H-SiCs at 80 K: (a) HPSI-4H-SiCs excited by 785 nm, (b) N-4H-SiCs excited by 785 nm, (c) HPSI-4H-SiCs excited by1064 nm, and (d) N-4H-SiCs excited by 1064 nm. Black, red, and blue lines show as-grown, as-irradiated with 2 MeV-N at the ﬂuence of 2.0 × 1014 cm−2, and after thermal
annealing at 1000 °C for 30 min in Ar atmosphere, respectively. Arrows in the ﬁgure indicate NV-ZPLs (black), VV-ZPLs (green), and VSi-ZPLs (red). Note that the red linein (a) and the blue lines in (c) and (d) are scaled by a constant factor of 0.5, 0.05, and 0.01, respectively, to clearly show the spectra. Shaded zones in (c) and (d) denote
integration ranges of the PL intensities used in Fig. 6.

concentration of radiation induced defects appears at 1.5 μm in
depth due to 2 MeV-N irradiation and PL spectra near the surface
region were obtained. The PL intensity as a function of depth direction revealed that all luminescent centers were formed near thesurface and no signiﬁcant diﬀusion after thermal annealing wasobserved. Also, planar distribution of the PL intensity was almost
uniform for all samples.
In the case of HPSI-4H-SiC samples, a broad peak centered at
950 nm appeared by 785 nm excitation [Fig. 2(a)]. This broad peak17
Two ZPLs of
is attributable to phonon side bands (PSBs) of V−
Sis.

J. Appl. Phys. 126, 083105 (2019); doi: 10.1063/1.5099327
Published under license by AIP Publishing.

V−
Sis, which are indicated by red arrows in (a), also appeared withthe broad PSBs. After 2 MeV-N irradiation, the PL intensity from−V−
Si increased about twofold due to an increase in density of VSi.
However, the broad peak centered at 950 nm was reduced after
1000 °C thermal annealing, and a broad peak centered at 1200 nm
in addition to four sharp peaks at around 1100 nm newly appeared.
They are attributable to four ZPLs of VVs and their PSBs.26,29 Thismeans that the density of V−
Si was reduced and VVs were formed
due to 1000 °C thermal annealing. On the other hand, neither PL
signal from V−
Sis nor VVs appeared when excited by 1064 nm.

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1084.11103.61243.61243.61243.81242.81242.31180.21179.91179.91179.71180.01222.71132.01132.2

1173.41173.71173.31173.71176.41179.61130.51132.21107.61108.01130.91078.51078.11108.3

916.3
861.4

861.6

860.1

916.4

1079.1
916.1
859.1

80
80
80
80
3.5
10
5
N-4H-SiC

785
1064
785
1064
811, 930
715-1000785
HPSI-4H-SiC

Magnusson et al.26
Zargaleh et al.29
Fuchs et al.17

NV
(kh)

NV
(hk)

NV−
(hh)

1223.31223.51223.71223.21241.0NV−(kk)

Unidentified

1089.9scitation.org/journal/jap

VV
(kk)
VV
(hh)
VV
(hk)
VV
(kh)
V−
Si
(V2)
V−
Si
(V1)
Temperature
(K)
Laser
(nm)
SampleA broad peak centered at 1300 nm and four sharp peaks at around
1200 nm appeared after 1000 °C thermal annealing [Fig. 2(c)].
They are attributable to four ZPLs of NV centers and their
PSBs.26,29 Prior to thermal annealing, the two sharp peaks observed
at 1160.6 nm and 1186.5 nm were assigned to be Raman scatteringof the E2 planar optical (PO) and the A1 longitudinal optical (LO)phonons, respectively.39 The diﬀerence of PL spectra between
785 nm and 1064 nm excitations suggests that NV centers can beeﬀectively excited by 1064 nm, whereas VVs can be excited by
785 nm. In the case of N-4H-SiCs, no PL was observed fromas-grown samples and the PL originating from V−
Sis appeared after
2 MeV-N irradiation when excited by 785 nm. After 1000 °C
thermal annealing, the PL originating from NV centers appearedby exciting at both 785 nm and 1064 nm. No PL from VVs wasobserved by exciting at either wavelength. Also, only the E2 PO
phonon mode appeared at 1160.6 nm in the N-4H-SiCs because
the LO phonon mode is reduced by the coupling between phononsand plasmons.40,41
Wavelengths of the observed ZPLs, which appeared in the PL
spectra at 80 K by both 785 nm and 1064 nm excitations, are summarized in Table I. Two V–Si-ZPLs, four VV-ZPLs, and four
NV–-ZPLs have previously been identiﬁed by other groups17,26,29
and we identiﬁed ZPLs in Fig. 2 by comparing these previous
values. Two V–Si-ZPLs have been labeled as V1 and V2, which havebeen assigned to be V−
Si in the horizontal (h) and cubic (k) sites of
4H-SiC lattice, respectively.14 However, contradictory results havealso been reported.42 There are four diﬀerent VV-ZPLs dependingon the conﬁguration of VC and VSi. They have been identiﬁed askh (VC at k site and VSi at h site, the same hereinafter), hk, hh, andkk sites in descending order of ZPL energy. kh and hk sites are thebasal conﬁgurations, and hh and kk sites are the axial conﬁgurations. Four NV-ZPLs have also been identiﬁed as hk, hh, and kk
sites in descending order of ZPL energy as with the case of
VVs27,29 In the case of HPSI-4H-SiC irradiated with 2 MeV-N
ions, two VSi-ZPLs and four VV-ZPLs appeared by 785 nm excitation, whereas four NV-ZPLs dominantly appeared by 1064 nm
excitation. A similar trend has also previously been reported.29 We
believe that VV-ZPLs from the hh and kk sites overlapped and thestrong peak at 1130.9 nm appeared by 785 nm excitation. On thecontrary, only four NV-ZPLs appeared in the case of N-4H-SiCs
after thermal annealing. Neither VSi-ZPLs nor VV-ZPLs have beenclearly identiﬁed. Slight discrepancies of the obtained ZPL wavelengths compared to previous reports are thought to be due to aneﬀect of lattice strains induced by ion irradiations and thermal
annealing.
Interestingly, no VV-ZPL and only NV-ZPLs were observedin the N-4H-SiC samples. This indicates that NV centers weremore preferentially formed than VVs This is due to the lower
formation energy of NV centers than that of VVs43 Also, if VVs
are created, they are immediately transferred to NV centers by
the reaction with nitrogen-carbon split interstitial, (NC)+C:
44
On the(NC)+C + VCVSi + 2e− → NCV−
Si (to be discussed later).
other hand, VVs were still formed in the HPSI-4H-SiC samples
and were excited preferentially by 785 nm, but not excited at1064 nm. Only NV centers were excited by 1064 nm. It has beenclariﬁed that the excitation eﬃciency of VVs dropped above
980 nm.26 However, only PL from NV centers were observed at RT

ARTICLETABLE I. Summary of ZPLs (nm) observed in Fig. 2. The nature of ZPLs was identiﬁed by the comparison to previous reports by other research groups17,26,29 listed in the table. Weakly observed ZPLs
are shown in italics. Note that hh and kk denote the axial conﬁguration of VVs and kh and hk are the basal conﬁgurations of VVs (see text in detail).

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in both the HPSI- and N-4H-SiC samples because the PL fromVVs was negligibly weak at RT. Figure 3 shows the PL spectra of
HPSI- and N-4H-SiC samples at RT. Unlike PL spectra at 80 K,
only broad peaks originated from PSBs and no ZPLs appeared.
Sharp peaks observed at 1088.2 nm, 1160.6 nm, and 1186.4 nm,
when excited by 1064 nm, were assigned to be Raman scatteringsof the E2 planar acoustic (PA), the E2 PO, and the A1 LO phonons,
respectively.39 Again, the A1 LO phonon modes did not appear in
the N-4H-SiCs because of the phonon-plasmon coupling. The
broad peaks centered at 950 nm and 1300 nm are attributable to PL
from V−Si and NV centers, respectively. These variations were in

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agreement with the variation at 80 K. Figure 4 shows the PL integrated intensity as a function of the distance between the focal
point and the sample surface at RT. All luminescent centers wereformed near the surface and no signiﬁcant diﬀusion after thermal
annealing was observed. The similar trend was also found at 80 K
and at 1064 nm excitation.
The PL intensity at diﬀerent laser power densities was measured to investigate the excitation eﬃciency of NV centers. Figure 5
shows the PL integrated intensity as a function of excitation laser
power density. The PL intensity increased monotonically withincreasing excitation power density and no obvious saturation

FIG. 3. Typical NIR-PL spectra of HPSI- and N-4H-SiCs at RT: (a) HPSI-4H-SiCs excited by 785 nm, (b) N-4H-SiCs excited by 785 nm, (c) HPSI-4H-SiCs excited by1064 nm, and (d) N-4H-SiCs excited by 1064 nm. Black, red, and blue lines show as-grown, as-irradiated with 2 MeV-N at the ﬂuence of 2.0 × 1014 cm−2, and after thermal
annealing at 1000 °C for 30 min in Ar atmosphere, respectively. Note that the blue lines in (c) and (d) are scaled by a constant factor of 0.1 and 0.01, respectively, toclearly show the spectra. Shaded zones in (c) and (d) denote integration ranges of the PL intensities used in Figs. 4, 5, 6, 7, and 10.

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FIG. 4. The PL integrated intensity (1150–1450 nm) of N-4H-SiC samples as afunction of the distance between the focal point and the sample surface. The PL
measurement was performed through 785 nm excitation at RT. Black, red, andblue lines denote the results of as-grown, as-irradiated with 2 MeV-N at the
ﬂuence of 2.0 × 1014 cm−2, and after 1000 °C thermal annealing for 30 min in
Ar atmosphere, respectively. “Depth = 0” means the surface. The value is negative when the focal point is above the surface (air) and positive when below thesurface. Note that the blue line is scaled by a constant factor of 0.01 to clearlyshow the data.

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FIG. 5. The PL integrated intensity at RT (1150–1450 nm) as a function of excitation laser power density (785 nm and 1064 nm). Black closed and blue open
symbols denote the N- and HPSI-4H-SiCs irradiated with 2 MeV-N at the
ﬂuence of 2.0 × 1014 cm−2, respectively.

appears that N-related complex defects like (NC)xVSi are preferentially formed at above 1000 °C43,44 because of the high N-impurity
concentration ([N] = 9 × 1018 cm−3), resulting in the reduction of
the NV center concentration. It is known that higher annealing
temperatures than 1500 °C are required to annihilate the N-related

behaviors were observed in all cases within the power densities
investigated in this study. The PL intensity by 1064 nm excitation
was more than 10 times higher than 785 nm excitation, suggestingthat 1064 nm laser excites NV centers more eﬃciently than 785 nm
laser. The similar trend was also found at 80 K. According to thetheoretical calculation by Weber et al.,2 the absorption energy of
NV centers in 4H-SiC is 1.15 eV, which is fairly close to thephoton energy of 1064 nm (1.165 eV). On the other hand, in thecase of 785 nm excitation, we speculate that the photon energy(1.58 eV) is high enough to induce charge transfer to valence/conduction band and other defects, resulting in the reduction of PL
intensity. Charge state transfer has been reported in other defects
such as diamond-NV centers and VVs in 4H-SiC.19,20,45
B. Thermal annealing temperature dependence
The N- and HPSI-4H-SiC samples were irradiated with
2 MeV-N ions at a ﬂuence of 2.0 × 1014 cm−2 and thermallyannealed at temperatures from 700 °C to 1100 °C for 30 min under
Ar atmosphere. The excitation wavelength of PL measurement was
1064 nm. Figure 6 shows the results of N- and HPSI-4H-SiC
samples, respectively. The ordinate is the relative PL integrated
intensity from 1150 nm to 1450 nm at RT and from 1215 to1235 nm at 80 K (NV-ZPL at hh site), which are represented byshaded zones in Figs. 2 and 3, although both data showed the same
trend. In the case of N-4H-SiC, the PL integrated intensity showed
the highest at 1000 °C and was rather reduced at above 1000 °C. It

J. Appl. Phys. 126, 083105 (2019); doi: 10.1063/1.5099327
Published under license by AIP Publishing.

FIG. 6. The PL integrated intensity as a function of thermal annealing temperature measured at RT (closed squares) and 80 K (open circles). Black and red
symbols denote the results of N- and HPSI-4H-SiCs irradiated with 2 MeV-N at
the ﬂuence of 2.0 × 1014 cm−2, respectively. Thermal annealing was performedfor 30 min in Ar atmosphere after irradiation. The ordinate is the PL integrated
intensity (RT: 1150–1450 nm, 80 K: 1215–1235 nm) and is normalized by thevalue of N-4H-SiC at 1000 °C. Solid lines in blue are drawn to guide the eye.

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complex defects and to obtain perfect recovery of crystallinity.46,47
On the other hand, the PL integrated intensity increased withincreasing temperature and showed a saturation trend above
1000 °C in the case of HPSI-4H-SiC. It should be noted that relatively larger ﬂuctuation of PL intensity appeared in the
HPSI-4H-SiC because of their lower PL intensity. Finally, additional thermal annealing for 30 min at the same temperatures as
Fig. 6 was performed for all the samples, although no signiﬁcantchange was observed.
According to theoretical calculations, there are mainly two
processes to form NV centers. One is the simple process:−43−
and the other is the multiple process via
N+C ± V−Si ± e → NCVSi+(NC)C split interstitial and VCVSi: (CC)C ± N+C → (NC)+C and44
Here, the former and latter pro(NC)+C ± VCVSi ± 2e− → NCV−
Si.
cesses are represented by processes (1) and (2), respectively. Defectkinetics of V−
Si is thought to play an important role in the process(1). Although the annealing occurs in several stages at 150 °C,
350 °C, and 750 °C, more than 1450 °C is required to be annealed
completely.48 This annealing behavior is explained by the formation of stable nitrogen-vacancy complexes. In our study, both the
reduction of V−
Si and the formation of NV centers appeared due tothermal annealing at 700–1100 °C. This fact may indicate that thedirect formation process (1) contributed to the creation of NV
centers. However, interstitial carbons (IC) are more mobile thanvacancies and form (CC)C and then form NV centers via theprocess (2). We believe that process (2) is more favorable thanprocess (1) as will be discussed in Subsection III C.
C. PL intensity variations due to different ion beams
and ﬂuences
Since the sensitivity of quantum sensing and metrology isdirectly related to the density of NV centers, we systematicallyclariﬁed the eﬀects of ion irradiation conditions (ion species andﬂuences) on the PL intensity from NV centers. Figure 7 shows theirradiation ﬂuence dependence of PL integrated intensity (1150–
1450 nm) with diﬀerent ion beams (240 keV-H, 2 MeV-N, 4 MeV-Si,
and 7 MeV-I ions), measured at RT through 1064 nm excitation.
The same trend was observed in the PL measurement at 80 K. Theseion beams induced similar vacancy proﬁles, as shown in Fig. 1, andNV centers were thought to be mainly formed at the depth of 1.5–
2 μm from the surface after thermal annealing. When focusing on
the N-4H-SiC samples, the highest PL integrated intensity appearedat the intermediate ﬂuence in all the cases, and the peak value wasenhanced with increasing atomic mass of ion beams (I > Si > N > H).
The highest PL integrated intensity was obtained using 7 MeV-I irradiation at 6.0 × 1012 cm−2. Vacancy concentration increases withincreasing irradiation ﬂuence and also the amount of NV centersformed after thermal annealing increases. If an excess amount ofvacancies is introduced by irradiation, however, suﬃcient recovery of
the crystallinity is not expected even after thermal annealing. This
results in the reduction of NV center formation and/or quenching ofNV centers due to neighboring defects. In addition, the eﬀects of
implanted atoms as well as induced defects have to be consideredwhen the irradiation ﬂuence is high. According to the TRIM calculation, hydrogen concentration after the 240 keV-H irradiation at the
ﬂuence of 1.0 × 1016 cm−2 reaches 7.5 × 1020 cm−3 at the Bragg peak.

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Most of the residual implanted hydrogen is thought to be desorbed
after thermal annealing at above 700 °C even though Si–H and C–Hbonds were once formed.49 We believe that the eﬀects of H-related
defects on NIR-PL properties are not signiﬁcant, since no unique
NIR-PL spectrum appeared from 240 keV-H irradiated samples
compared to other irradiation conditions. No NIR-PL properties for
H-related defects in SiCs have previously been reported to ourknowledge. However, the possibility that a portion of implantedhydrogens form thermally stable complex defects after thermal
annealing up to 1100 °C cannot be excluded.
To gain further insight into the relationship between the NV
center formation and the radiation induced defects, the abscissa in
Fig. 7 was rescaled by the amount of vacancies induced by irradiations. The number of vacancies induced by single ion beams wasestimated by TRIM calculation35 and the product of ﬂuence andnumber of vacancies was deﬁned as vacancies per area (cm−2).
Figure 8 shows the PL integrated intensity of N-4H-SiC samples asa function of vacancies per area. It is plausible that all the regionsin which vacancies were induced by irradiations were observed in
the measurement system of this study, since the ideal axial resolutionof measurement system was estimated to be more than 5 μm when
the refractive index of SiC is 2.6.50 The result shows that the highest
PL integrated intensity appeared at around 1017 vacancies cm−2 independent of ion species, indicating that the amount of NV center formation was determined by the amount of vacancies induced. The PL
intensity was rather reduced above around 1017 vacancies cm−2.
Radiation damage studies have clariﬁed that the critical dose of SiC
for amorphization was about 0.2 dpa (displacement per atom) at
RT,51,52 although the critical dose also depends on ion species, ion
energy, and irradiation temperature. The value of 0.2 dpa roughly

FIG. 7. PL integrated intensity (1150–1450 nm) at RT of N- and HPSI-4H-SiCs
irradiated with different ions. The excitation wavelength was 1064 nm. Closed
and open symbols denote the results of N- and HPSI-4H-SiCs, respectively.
Squares in black: 240 keV-H, diamonds in blue: 2 MeV-N, triangles in pink:
4 MeV-Si, and circles in dark yellow: 7 MeV-Si irradiations. Thermal annealing of1000 °C for 30 min in Ar atmosphere was performed after irradiation. The ordinate is normalized by the value of N-4H-SiC at 2.0 × 1014 cm−2.

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FIG. 8. PL integrated intensity of the N-4H-SiC samples as a function of vacancies per area (cm−2), estimated by TRIM calculation.

corresponds to around 1018 vacancies cm−2 in this study and thus
the reduction of PL intensity at above 1017 vacancies cm−2 is related
to the appearance of amorphization. Since the (local) amorphization
occurred at above 1017 vacancies cm−2, the NV center formation
was suppressed by the insuﬃcient recovery of SiC lattice structure
after the thermal annealing at 1000 °C. Regarding the correlation
between the peak PL intensity and the atomic mass of ion
beams, cascade damage by heavy ions is seemingly related tothe enhancement of PL intensity. This can be explained byconsidering the defect kinetics as follows. According to
Gerstmann et al.,44 process (2) is more favorable than process(1) because of the lower activation energies. In process (2), more
defects (IC, VC, and VSi) are required than the process (1), andthus the cascade damage (localized dense defect region) has thepotential to form NV centers eﬀectively.
When focusing on the 2 MeV-N irradiations, the highest PL
integrated intensity appeared at the ﬂuences of 2.0 × 1014 cm−2 inthe N-4H-SiC and 1.0 × 1015 cm−2 in the HPSI-4H-SiC. In the caseof HPSI-4H-SiC, the peak might appear at the ﬂuence between
2.0 × 1014 cm−2 and 1.0 × 1015 cm−2 as those values were fairly
close. The fact that the PL intensity of HPSI-4H-SiC was drasticallylower than that of N-4H-SiC (4.3% at 2.0 × 1014 cm−2) indicatesthat the contained N atoms as impurities exclusively formed NV
centers while the implanted N atoms less contributed to the NV
center formation. The peak concentration of implanted N is estimated to be 1.2 × 1019 cm−3, which is larger than the N-impurity
concentration ([N] = 9 × 1018 cm−3), and hence much higher PL
intensity would have been observed in the HPSI-4H-SiC samples if
the implanted N atoms had preferentially formed NV centers. Thisﬁnding is also supported by the fact that in the peak PL intensities
of N-4H-SiC irradiated with 4 MeV-Si and 7 MeV-I irradiations
were higher than that of 2 MeV-N irradiation. This is probably
because a thermal annealing temperature of 1000 °C is too low to
diﬀuse implanted N atoms into lattice site (NI + VC → NC, where
NI the interstitial nitrogen) and NV centers are exclusively formed

J. Appl. Phys. 126, 083105 (2019); doi: 10.1063/1.5099327
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FIG. 9. PL spectra of the HPSI-4H-SiCs irradiated with 240 keV-H at
1.0 × 1016 cm−2 (black) and 2 MeV-N at 2.0 × 1014 cm−2 (blue) at RT. Observed
four sharp peaks are attributable to the Raman scattering (see text).

from N atoms in the lattice site. It cannot be excluded, however,0that not NCV−
Si centers (single negatively charge state) but NCVSi
centers (neutral state) were formed in the HPSI-4H-SiC samples
because donors which could supply electrons were extremely low.
Nevertheless, implanted N atoms undoubtedly contribute to
the NV center formation. As shown in Fig. 7, the NIR-PL integrated intensity of 2 MeV-N ion irradiated HPSI-4H-SiCs wasmuch higher than the other HPSI-4H-SiCs. Figure 9 shows the PL
spectra at RT of HPSI-4H-SiC samples irradiated with 2 MeV-N at
2.0 × 1014 cm−2 and 240 keV-H at 1.0 × 1016 cm−2. Only extremely
weak luminescence ranging from 1100 nm to 1400 nm appeared in
the 240 keV-H irradiation even though the comparable amount ofvacancies was introduced (see Fig. 1). Peaks observed at 1088.0 nm,
1160.2 nm, 1163.0 nm, and 1186.2 nm were assigned to be Raman
scatterings of the E2 PA, the E2 PO, the E1 transverse optical (TO),and the A1 LO phonons, respectively.39 This result strongly indicates that the NV centers formed in the HPSI samples were mainly
originating from not a minute amount of N impurities but
implanted N atoms. Therefore, we conclude that the NV centerswere created from both implanted and contained N atoms after
1000 °C thermal annealing, although most of NV centers werecreated from contained N atoms. In addition, unlike the N-4H-SiC
samples, the NIR-PL integrated intensity increased with increasing
ﬂuence in the ranges considered in this study. This implies that theN-impurity concentration strongly aﬀects the NV center formation
yields. Since these highest irradiation ﬂuences were close to the critical dose of amorphization, further irradiation might reduce the
NIR-PL intensity. Additional investigation is required to clarify theeﬀects of impurity N concentration on the NV center formation.
Here, the creation yield of NV centers by N ion implantationis roughly estimated. The lowest PL integrated intensity was
obtained from the HPSI-4H-SiC irradiated with 2 MeV-N at the
ﬂuence of 3.0 × 1012 cm−2, although the plane distribution of PL
intensity on the samples was also uniform. Taking into account thatlateral resolution of the micro-PL measurement system was estimated126, 083105-8

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to be 0.60 μm (=0.51 × 1064/0.90) when λ = 1064 nm and NA = 0.90
were used, this fact suggests that at least more than one NV center
were formed within the area of 2.9 × 10−9 cm2 on average. This value
corresponds to the areal density of 3.5 × 108 cm−2. Thus, the creation
yield of NV centers by 2 MeV-N irradiation was estimated to be
more than 0.012% (=100 × 3.5 × 108/3.0 × 1012) at least.

Temperature dependence of PL intensity can be generally
expressed as the following equation:

D. Temperature dependence on the PL intensity

where I, T, kB, EA, and A are the PL intensity, the temperature, theBoltzmann constant, the activation energy of thermal quenching,
and the ﬁtting constant, respectively. The experimental results were
well ﬁtted by Eq. (1) (solid lines in Fig. 10) and the activationenergy was obtained to be 0.14 eV in the N-4H-SiC and 0.19 eV in
the HPSI-4H-SiC. Carrier escape and/or energy transfer from NV
centers to other neighboring defects are thought to be the cause ofthermal quenching. These values might be related to the diﬀerenceof energy levels between the excited state of NV centers and theother defects which act as recombination centers. VSi and VC are
one of the candidates for the cause of thermal quenching, since
their excited levels are located at about 0.2 eV above the excitedlevels in NV centers.26

Highly ﬂuorescent PL signals are required at RT for applications of quantum sensors in the ﬁeld of life science. In addition,high temperature operation of quantum sensors is also expected for
applications in harsh environments like space.34 Hence, temperature dependence on the PL intensity is one of the most importantfactors for their practical use as quantum sensors. The temperaturedependence of the PL intensity originating from NV centers
ranging from 80 K to 773 K is shown in Fig. 10. The N-4H-SiC
irradiated with 7 MeV-I at 6.0 × 1012 cm−2 and the HPSI-4H-SiC
irradiated with 2 MeV-N at 2.0 × 1014 cm−2 were measured. The
abscissa is the reciprocal of temperature multiplied by 1000 and theordinate is the PL integrated intensity from 1150 nm to 1450 nm
normalized by the value of N-4H-SiC at 80 K. The PL intensities at
RT and 683 K were reduced to be 75% and 9.8% for the N-4H-SiC
and 94% and 29% for the HPSI-4H-SiC, respectively. The PL spectrum originating from NV centers was observed up to 773 K,
which was the temperature limit of the measurement system. The
same trend was obtained when the integrated range was from1215 nm to 1235 nm where NV-ZPL at hh site appeared.

FIG. 10. PL integrated intensity (1150–1450 nm) as a function of temperature.
Black squares and red circles denote the N-4H-SiC irradiated with 7 MeV-I at
the ﬂuence of 6.0 × 1012 cm−2 and the HPSI-4H-SiC irradiated with 2 MeV-N at
2.0 × 1014 cm−2, respectively. Solid lines denote the ﬁtting results by Eq. (1).
Thermal annealing of 1000 °C for 30 min in Ar atmosphere was performed after
irradiation. The excitation wavelength was 1064 nm. The abscissa is the reciprocal of temperature multiplied by 1000 and the ordinate is the PL integrated
intensity normalized by the value of the N-4H-SiC at 80 K. The vertical dashedline in blue shows RT (300 K).

J. Appl. Phys. 126, 083105 (2019); doi: 10.1063/1.5099327
Published under license by AIP Publishing.

I/

1

,
EA
1 þ A exp 
kB T

(1)

IV. CONCLUSIONS
We studied PL properties of NV centers in 4H-SiCs, whichwere irradiated with various conditions and were then thermallyannealed at diﬀerent temperatures. The eﬀect of excitation wavelengths, power densities, and temperatures on PL properties was systematically investigated and the following conclusions were obtained.
It was shown from the results of the temperature dependenceof PL spectra that the broad spectrum ranging from 1100 nm to1500 nm at RT is attributable to the PL from NV centers. NV
center ZPLs were clearly evident when studied at a temperature of
80 K. The PL from NV centers mainly appeared by 1064 nm excitation in both the N- and HPSI-4H-SiC samples, whereas the PLfrom V−Sis and VVs also appeared through 785 nm excitation in the
HPSI-4H-SiC samples. NV centers are preferentially formed rather
than VVs in the N-4H-SiC samples and the PL from VVs were notsigniﬁcantly observed at 80 K even by 1064 nm excitation, since theformation energy of NV centers are lower than that of VVs In
addition, it was shown from the results of laser power density
dependence that the NV centers were more eﬃciently excited at1064 nm than 785 nm. This is because the absorption energy of
NV centers is close to the photon energy of 1064 nm (1.165 eV). It
was shown from the thermal annealing temperature dependence
that the optimal temperature for creating NV enters was around1000 °C. It would appear the NV center formation is rather suppressed at above 1000 °C due to the formation of other types ofN-related complex defects. Comparing the PL intensities among
diﬀerent ion irradiation conditions under the same amount of radiation induced defects, the heavier ion irradiation provides more
eﬀective NV center formation than the lighter ion irradiation. Thissuggests that the cascade damage eﬀectively contributes to NV
center formation. It was also clariﬁed that NV centers were mainly
formed from contained N atoms as impurities rather than
implanted N atoms.

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The results of temperature dependence of PL intensities show
the weak thermal quenching property, unlike the PL from VVs The
PL intensities at RT were comparable to that at 80 K in both theN- and HPSI-4H-SiC samples, and the PL from NV centers
appeared even at 773 K. Considering this fact, in combination withthe other reports on the spin properties based on EPR (electron paramagnetic resonance) studies,31 quantum sensing using NIR-PL fromNV centers in SiC operating at RT (or even high temperature), which
is desirable for life science and space applications, is highly feasible.
ACKNOWLEDGMENTS
This study was supported by the JSPS KAKENHI under Grant
No. 17H01056. The authors would like to thank Professors Brant
C. Gibson and Andrew D. Greentree of RMIT University for theirfruitful discussion.
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