LETTERS
PUBLISHED ONLINE: 17 NOVEMBER 2013 | DOI: 10.1038/NMAT3806

A silicon carbide room-temperaturesingle-photon source
S. Castelletto1 *, B. C. Johnson2,3 , V. Ivády4,5 , N. Stavrias6 , T. Umeda7 , A. Gali4,8 and T. Ohshima3
Over the past few years, single-photon generation has beenrealized in numerous systems: single molecules1 , quantum
dots2–4 , diamond colour centres5 and others6 . The generationand detection of single photons play a central role in theexperimental foundation of quantum mechanics7 and measurement theory8 . An efficient and high-quality single-photon
source is needed to implement quantum key distribution, quantum repeaters and photonic quantum information processing9 .
Here we report the identification and formation of ultrabright,
room-temperature, photostable single-photon sources in a
device-friendly material, silicon carbide (SiC). The source iscomposed of an intrinsic defect, known as the carbon antisite–vacancy pair, created by carefully optimized electron irradiation and annealing of ultrapure SiC. An extreme brightness(2×106 counts s−1 ) resulting from polarization rules and a highquantum efficiency is obtained in the bulk without resortingto the use of a cavity or plasmonic structure. This may benefit
future integrated quantum photonic devices9 .
Silicon carbide is an important wide-bandgap semiconductor
for high-power electronics and high-temperature applications, andis a key material for next-generation photonic10 and electronic
devices11 . As such, SiC has well-established growth and device
engineering protocols12 , and 3-inch ultrapure wafers are nowcommercially available. This has allowed the development of amature platform for nanofabrication and high-Q photonic crystal
cavities, reported for a spectral region from 550 to 1,450 nm (ref. 10)
as well as microdisc resonators13 for cavity opto-mechanicalapplications. In addition, SiC is also biocompatible, especially withblood, and SiC nanoparticles (quantum dots) have been employed
as fluorescent biological labels in cell imaging14,15 .
Recently, it has been indicated16–20 that SiC could harbour
defects that may be employed as solid-state ‘qubits’. The existenceof such defects in a device-friendly material is extremely fortuitousand holds great promise for the future implementation of scalable
quantum processors17,21 . Ensembles of candidate optically active
defects, having the net angular momentum (or spin) required toact as ‘qubits’ operating at room temperature, have already been
identified in SiC (refs 18–20). Their optically detected magnetic
resonance has a lower visibility compared with the nitrogen–vacancy centre in diamond5 . However, the richness of possible
defects in the various polytypes of SiC (ref. 20) will enable further
studies and discoveries. At present, a major challenge before

the future deployment of this material in the field of quantum
technologies is to identify and form isolated defects.
In this work, we present, to the best of our knowledge, the firstobservation and engineering of an isolated defect in SiC that exhibits
single-photon emission. To isolate single defects and identify
them, we irradiated ultrapure SiC with low electron fluences andwe optimized the annealing temperature to increase/decrease thenumber of defects created from the single to the ensemble level.
This approach allows the direct correlation between ensemble
and single defects, otherwise often difficult. The emission rateof this single photon source is in the Mcounts s−1 regime, thebrightest single photon in bulk material at room temperature. In
comparison, Cr-related centres in bulk diamond yield a singlephoton count rate of 0.7 Mcounts s−1 (ref. 22). Only Cr-related
and silicon–vacancy centres in nanodiamonds can yield emission
rates above 3–4 Mcounts s−1 (refs 23,24), although for the latter caseenhancement of the spontaneous emission count rate arose fromplasmonic effects due to a metal substrate. A maximum emission
rate of 1–2 GHz for nitrogen–vacancy centres in nanodiamonds canbe achieved using a solid immersion lens to enhance the collection
efficiency although with higher pump powers the nitrogen–vacancy
centre is prone to blinking25 . The SiC single-photon source has
several zero-phonon lines and so is expected to exhibit efficient
cavity coupling at low temperatures. At present, the brightest
solid-state single-photon sources are based on advanced fibrecoupled microcavity-InAs/GaAs-quantum dots, reaching gigahertz
repetition rate single-photon emission at 4 K (ref. 26). Further
development of single-photon emission in SiC nanoparticles is
expected to provide similar rates. Our work together with thepossibility of nanostructuring this material will pave the way forfurther studies on the spin and charge state of this defect as well as
quantum emitters in this emerging material.
High-purity, semi-insulating 4H-SiC wafers were irradiated withhigh-energy electrons to different fluences to create defects andthen annealed at various temperatures (see Methods for details).
This method allows the creation of defect densities from the single
isolated level to the ensemble level.
To identify the defect, ensemble room-temperature and lowtemperature photoluminescence measurements were performed
with 532 nm excitation on a sample irradiated to a fluence of1017 e cm−2 . At 80 K the so-called AB lines27 can clearly be observedwith lines at A1 = 648.7 nm, A2 = 651.8 nm, B1 = 671.7 nm,

1 School of Aerospace, Mechanical and Manufacturing Engineering RMIT University, Melbourne, Victoria 3000, Australia, 2 Centre for Quantum

Computation and Communication Technology, School of Physics, University of Melbourne, Victoria 3010, Australia, 3 SemiConductor Analysis andRadiation Effects Group, Japan Atomic Energy Agency, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan, 4 Institute for Solid State Physics and Optics,
Wigner Research Centre for Physics, Hungarian Academy of Sciences, Budapest, POB 49, H-1525, Hungary, 5 Department of Physics, Chemistry andBiology, Linköping University, SE-581 83 Linköping, Sweden, 6 School of Physics, University of Melbourne, Victoria 3010, Australia, 7 Graduate School of
Library, Information and Media Studies, University of Tsukuba, Tsukuba 305-8550, Japan, 8 Department of Atomic Physics, Budapest University ofTechnology and Economics, Budafoki út 8, H-1111 Budapest, Hungary. *e-mail: stefania.castelletto@rmit.edu.au
NATURE MATERIALS | VOL 13 | FEBRUARY 2014 | www.nature.com/naturematerials© 2014 Macmillan Publishers Limited. All rights reserved

151

NATURE MATERIALS DOI: 10.1038/NMAT3806

LETTERSab

Measurement temperature (K)Integrated
photoluminescence (a.u.)

350

kh
B2

hk
kk

650

660
Wavelength (nm)2A"

E || c

E⊥c

E⊥c

e

2A

2A'

1

hk, kh (C1h )

f

200

0.4

150
100

0.2

50
0
640

645

650
655
660
Wavelength (nm)

665

670

101
60

80 100 120
1/kT (eV¬1)

140

160

c

0.0

4

350
300

3

250
200

2

150
100

1

50
0

672

674
676
Wavelength (nm)678

Photoluminescenceintensity (a.u.)

0.6

250

Photoluminescenceintensity (a.u.)

300

AB
V1

102

Unp. 100 200 400 600 800 1,000
Annealing temperature (°C)

0.8

350

Polarization angle (°)

680

2A'

2E

hh, kk (C3v)

103

40

670

d

E⊥c

104

B1

A3

A2
640

B4

A4

A1

100

Integrated photoluminescenceintensity (a.u.)c axishh

Polarization angle (°)

Photoluminescenceintensity (a.u.)

B3

200

0

Figure 1 | Ensemble measurements of defects in electron-irradiated 4H-SiC. a, The AB photoluminescence lines observed in an electron-irradiatedsample measured at 80 K. The proposed atomic structure of the defects CSi VC , giving rise to the photoluminescence, is shown in the (1120) plane with thec axis indicated in the inset (small and large circles represent C and Si atoms; the small open circles are vacancies). The carbon antisite–vacancy pair has
four possible configurations in 4H-SiC: hh/kk axial configurations with C3v symmetry, and hk/kh basal plane configurations with C1h symmetry. h and k
denote the hexagonal and cubic sites in the lattice, respectively. b,c, The temperature dependence of the AB and V1 photoluminescence lines (b) and, theirannealing temperature dependence (c). d, Schematic diagram of the ground-state electronic structure of axial and basal plane configurations of thepositively charged carbon antisite–vacancy defect. The symmetry labels of the defect states are depicted for each configuration. Arrows represent thefeasible optical transitions with the corresponding polarization of light. e,f, The emission polarization dependence of the AB lines with the excitation beam
perpendicular to the c axis and the polarization angle θ, with θ = 0 parallel to the c axis. Unp., unprocessed.

B2 = 673.0 nm, B3 = 675.2 nm and B4 = 676.5 nm, as indicated inFig. 1a. These lines arise from the carbon antisite–vacancy pairdefect, CSi VC (ref. 28). We also resolve two additional peaks at
A3 = 665.1 nm and A4 = 668.5 nm, which we assign to the same
defect. The atomistic model of the CSi VC with its four possible cubicor hexagonal orientations is shown in the inset of Fig. 1a.
A study on the effect of the measurement and annealing
temperature on the AB lines (Fig. 1b,c) revealed the onset of thermal
quenching at around 150 K and the enhancement of AB lines onannealing up to 800 ◦ C. These measurements are compared tothe silicon–vacancy defect, VSi (ref. 29), which gives rise to theV1/V2 photoluminescence lines at 861.5 and 916.3 nm, respectively(see details in Supplementary Information). Here it is observedthat the AB lines grow at the expense of V1. At the temperature
at which the AB lines are optimized the V1/V2 system is almost
absent. Such behaviour is supported by ab initio calculations where
it has been shown that the energy barrier to formation of CSi VC
from VSi is small and is promoted by an increase in the annealing152

temperature30 . As will be shown, this has a large positive impact on
the photostability of the CSi VC at the single-defect level.
Ab initio supercell calculations on the CSi VC defect in 4H-SiC in
conjunction with group theory were employed to describe the AB
photoluminescence spectra (see Supplementary Information).
A schematic of the electronic structure for both axial (hh andkk) and basal plane (hk and kh) configurations is shown in Fig. 1d.
These levels arise in the bandgap owing to the dangling bonds ofthe carbon antisite as well as of the Si atoms nearest to the vacancy.
In the axial configurations (C3v symmetry), the lowest level has
2
A1 symmetry whereas the higher one has doubly degenerate 2 E
symmetry. In the basal-plane configurations, the double-degenerate
E-state splits owing to its lower C1h symmetry.
Our ab initio calculations indicate that the neutral CSi VC pairdoes not have photoluminescence in the visible, in contrast to theprevious assignment27 . Thus, our working model is the positively
charged CSi VC pair with predicted visible photoluminescence. In
the ground state, only the lowest defect level is occupied by a single
NATURE MATERIALS | VOL 13 | FEBRUARY 2014 | www.nature.com/naturematerials© 2014 Macmillan Publishers Limited. All rights reserved

NATURE MATERIALS DOI: 10.1038/NMAT3806
electron, forming a totally symmetric state. The 2 E excited state of
the axial configurations is subject to Jahn–Teller distortion whereas
the basal configurations have per se C1h symmetry. Thus, all of
the configurations may have low symmetry in their excited state,
and the resulting many-body wave functions could have either 2 A0
or 2 A00 symmetry. The latter may partially borrow the characterof the ground state owing to sharing the same symmetry. The
eight AB photoluminescence lines in 4H-SiC can naturally accountfor the four ground-state configurations where each possesses two
zero-phonon lines. It was indeed shown that B1 and B3 lines are thehigher excited states of B2 and B4 counterparts 27 separated by about
4 meV. A1 and A2 can naturally be grouped as well as the newly
found A3 and A4 states, as they are separated by about the sameenergy as B1 and B3 , or B2 and B4 .
All of the configurations can be efficiently excited by E ⊥ c light
whereas E k c light excites configurations only to the A0 excitedstate that partially shows the character of the ground state (Fig. 1d).
Decay from the excited state can be more complicated than the
optical excitation process. Direct recombination from the A00 orA0 excited states to the A00 (or A1 ) ground state is possible withthe same polarization dependence as in the absorption process.
However, non-radiative decay from the 2 E-like excited state may
occur through a 4 E-like shelving state with the aid of a spin–orbit
interaction and vibrations. From the ab initio calculations weconclude that only the single electron occupies the defect levels in
the gap in the positively charged defect; thus, a 4 E-like shelving state
may form with a hole left in the valence band edge whereas the A1level and the split E level are occupied by a single electron.
The emission polarization of the CSi VC lines at 80 K is shownin Fig. 1e,f. Here, θ = 0 corresponds to polarization parallel to thec axis. We find that the lowest excitations of the two configurations(A2 and A4 ) are partially E k c polarized whereas their higher energy
counterparts, A1 and A3 , are dominantly E ⊥ c polarized. The
other two configurations with the lowest energy (B2 and B4 ) arealso dominantly E ⊥ c polarized. However, only B1 shows partial
parallel E k c polarization, whereas B3 is again dominantly E ⊥ cpolarized. This implies that the ground-state character has only aminute contribution to the A0 excited state in the B4 defect. Our
ab initio calculations tentatively associate A2 , A4 , B2 and B4 lines
with hh,hk,kk and kh configurations, respectively.
To isolate single bright defect centres we performed photoluminescence raster scans on samples irradiated with a low electron
fluence (1 × 1013 e cm−2 ) using room-temperature confocal microscopy. Typical maps are shown in Fig. 2a,b obtained by exciting
either with 532 or 660 nm light in the same area in the sample.
The observed centres are found 1–1.5 µm below the surface (seeSupplementary Information), which is amenable for the integration of these defects into on-chip optical waveguides in photonic
circuits. The number of centres in these maps is shown in Fig. 2c
as a function of their spatial diameter at half-maximum. These arecompared with an untreated sample excited with the 660 nm laserthat also contains a number of optically active centres but at amuch reduced density. It is clear that the 532 nm excitation allows
a larger number of centres to be addressed with both A and B lines
being excited. The 660 nm laser will mostly excite only the B lines.
Furthermore, although these sources are present in these particular
untreated samples, irradiation and annealing significantly increase
their density (Fig. 2d; see Supplementary Information) similar to
the ensemble measurements presented above.
Photoluminescence spectra corresponding to the bright spots
in the confocal maps, when excited using either 660 or 532 nm
are shown in Fig. 2e. We find that all of the measured diffractionlimited spots of 300–400 nm in diameter have photoluminescenceemission between 675–700 nm, when excited at 660 nm, orwith broader emission with peaks from ∼660 to ∼705 nm
when excited at 532 nm. Such emission corresponds to that of

LETTERSthe room-temperature photoluminescence of the AB lines. We
therefore identify the defect responsible for this emission with the
CSi VC pair. Some variability is observed in the photoluminescencepeak position (see Supplementary Information) possibly as a resultof addressing defects of a specific configuration.
To confirm that single defects are being addressed, a Hanbury Brown–Twiss (HBT) interferometer was used to verify singlephoton emission. We observed that all defects exhibiting photoluminescence between 648–710 nm behaved as single photon sources;that is, the second-order photon correlation function at zero delay
is g (2) (0) < 0 (Fig. 2f). We analysed g (2) (τ ) at different excitation
powers to study the photo-physics and the population dynamics
from the ground state to the excited state.
To assess the properties of the single-photon emission accurately, the variability of their kinetics was established. Many
photostable emitters (more than 100) were measured with the twodifferent excitation wavelengths and analysed. From this analysis we
deduced a typical excited-state lifetime of 1.2 ns, the existence of ametastable state with a short lifetime and a quantum efficiency ofthe defect of 70%. The shelving state is predicted by our theoreticalmodel and is an intrinsic part of the centre’s electronic structure.
The observed short metastable-state lifetime can be due to effective
scattering from the sub-states of the shelving state.
The saturation properties of the defects also exhibit a remarkablevariability, with optical saturation powers observed between 0.2 and3.3 mW and saturation count rates found from 2.5 × 105 counts s−1
to the maximum observed of 2 × 106 counts s−1 . This variability
is associated with the metastable-state lifetime variation. The highquantum efficiency and the most efficient excitation of axial defects(excitation orthogonal to the c axis) allows the maximum collection
efficiency with the optical configuration of the confocal systemused, yielding the high count rate measured for this defect.
We monitored the photoluminescence emission stability of these
defects in a sample irradiated and annealed at 300 ◦ C. It wasobserved that 45% of the emitters measured were stable over a long
period of time. An example of such a single centre is shown in Fig. 3aalong with its associated Gaussian count rate histogram in Fig. 3b.
Interestingly, the remaining centres showed photoluminescenceblinking with some eventually photo-bleaching, even if notpermanently. Blinking centres in samples annealed at higher
temperatures (500 ◦ C) were almost absent even though the singlephoton source density was greater. By annealing up to 800 ◦ C, 65%of the emitters were stable for days at 532 nm excitation.
This suggests that the photostability may be compromised by
the density of non-radiative pathways that are thermally unstable
and annihilate on annealing. An emitter with on/off fluorescence,
between two photoluminescence states, is shown in Fig. 3c,d.
By measuring all on/off-time durations in a single trace, thecorresponding probability of occurrence P(τon,off ) can be calculated.
The distribution of the ‘on’ and ‘off’ times are plotted on asemi-logarithmic scale in Fig. 3e together with the fits. Both P(τon )and P(τoff ) show an exponential trend following the equation:P(τon,off ) ∝ exp[−τ /(τ on,off )]with the time constant τon usually much greater than τoff (Fig. 3f)
and very sensitive to the excitation power. This is in contrast to
the more common power-law dependence usually observed forblinking. We attribute the observed blinking to either a photoinduced charge conversion of the addressed defect or to a charge
exchange with other defects in close proximity, acting as charge
traps or donors, probably induced by irradiation.
Samples annealed at higher temperature revealed a largely
reduced number of blinking defects, indicating that the blinking
can be controlled. In addition, the defects were more stableusing 660 nm excitation.

NATURE MATERIALS | VOL 13 | FEBRUARY 2014 | www.nature.com/naturematerials© 2014 Macmillan Publishers Limited. All rights reserved

153

NATURE MATERIALS DOI: 10.1038/NMAT3806

LETTERSac

8

6 × 1045
4
2
1
0532 nm5 × 105
4

1 × 1013 (532 nm)
1 × 1013 (660 nm)
Control (660 nm)

107

Number of centres (cm¬2)

3

b

106

105

3
2

¬

300

400

500

6

4

2

104

1

10 μm

Annealing temperature (°C)

Number of centres (106 cm¬2)

660 nm

d

500

0

1,500
1,000
FWHM (nm)

0

0

1013
Fluence (e cm¬2)

1014

f
2.0
TO
LO

1.5
(i)

g2 (τ )

Normalized intensity (a.u.)

e

2,000

1.0

0.5
(ii)

500

600

700
800
Wavelength (nm)

900

1,000

0.0
¬50

0
τ (ns)

50

Figure 2 | Confocal microscopy of single-defect localization in 4H-SiC. a,b, Confocal maps of the low electron fluence sample excited with 660 nm (a)and 532 nm (b) lasers. The diffraction-limited spots all correspond to single-photon emitters. The map (a) has been scanned using a 10 nm full-width at
half-maximum (FWHM) 694 nm filter. Note the difference in colour scale. c, Histogram of the number of centres observed as a function of the spatial
FWHM measured and counted from a,b. The histogram from a confocal map of an unprocessed sample is also shown (black filled circles), indicating aclear increase in the defect density after irradiation and annealing at 300 ◦ C. d, The number of diffraction-limited optical centres measured from single
defects as a function of the annealing temperature (top axis) and fluence (bottom axis). e, Normalized room-temperature photoluminescence of asingle-photon source in the irradiated sample excited with 532 nm (i) and 660 nm (ii) lasers. The laser wavelengths are indicated by the dashed lines. The
transverse and longitudinal optic Raman modes (TO, LO) are also indicated. f, The corresponding anti-bunching curves for e (i) and (ii).

Our atomistic model of the defect (see Supplementary Information) indicates that the nature of the shelving state may also
be responsible for the blinking properties (for example, a nearby
defect can effectively capture the loosely bound hole and hinder
the luminescence). This is consistent with our observation thatwith the appropriate anneal the defect density is reduced andblinking is minimized. Blinking may also arise in our working
model by the fact that if the defect was originally neutral thenit can be efficiently ionized to the luminescent positively charge
state; in this case, the electrons should be continuously pumped
to the conduction band to maintain the luminescence; 660 nm
excitation can ionize the neutral defect but it has less probability
to ionize the defect further (doubly positive charge state), and thus
it can show greater photostability than with 532 nm excitation.
Given that the blinking can be controlled with laser power, wavelength or annealing, we conclude that blinking is not an intrinsic
property of the defect.
In our working model the spin state is 1/2 for the positively
charged CSi VC defect. Clearly, further theoretical modelling andexperiments are required to determine whether this charge state
could have an optically detected magnetic resonance (ODMR). We

note that our working model indicates a relatively low ODMR signal
because its metastable shelving state has low symmetry with a strong
mixture of mS = ±1/2 and mS = ±3/2 substates. With a small
magnetic field the ground state can be split but it is not understoodwhether different decay rates to these two states will be different
enough to observe ODMR. The metastable state however might
be used for spin readout. We further note that the spin-1/2 CSi VC
defect may cause decoherence of silicon–vacancy spins where thesilicon–vacancy is created with optically polarizable spins together
with the CSi VC defect. The CSi VC spin may communicate withsilicon–vacancy spins through dipole–dipole interactions affecting
the coherence time of those spins20,29 . Thus, the manipulation of
the ground and excited states of the CSi VC defect at the singledefect level might be useful in the manipulation of other opticallyaddressable nearby spins.
In conclusion, this ultrabright single-photon emitter working
at room temperature provides a new building block in SiCbased opto-electronic devices and future integrated quantum
photonics9 . We envisage that SiC will constitute an ideal platform
for future integrated photonic circuits comprising photonic
cavities and waveguides, under fabrication at present. Our

154

NATURE MATERIALS | VOL 13 | FEBRUARY 2014 | www.nature.com/naturematerials© 2014 Macmillan Publishers Limited. All rights reserved

NATURE MATERIALS DOI: 10.1038/NMAT3806

LETTERSb1

02

Counts per 10 ms bin (105)Counts per 10 ms bin (105)

a

0

5

10

15

1

0

20

0

Time (s)c2Counts per 10 ms bin (105)Counts per 10 ms bin (105)

d

1

0

0

5

10

1

0

20

15

0

Time (s)

e

f
102

5

10

4

8

Occurrence of τon

101

3

6

2

4

100

1

2

1
Time interval (s)

2

0
0

2

4
τ (s)

6

Occurrence of τ off

Occurrence

1,000
Occurrence

On time, τ on = 0.33 s
Off time, τ off = 0.038 s

0

2,000
Occurrence0

Figure 3 | Photo-stability of single-photon sources in SiC. a,b, A 20 s fragment of a photoluminescence time-trace sampled into 10 ms bins from a stablesingle-photon source (a) with its corresponding count rate histogram (b). c,d, A similar time trace is shown for a blinking centre. e, The on and off times ofc are shown on a semi-logarithmic scale. f, Histogram of the measured τon and τoff of the 55 emitters analysed. 45% of the emitters in this sample werestable and did not show blinking behaviour.

defect assignment yields good agreement with observations. The
counterparts to this intrinsic defect can exist in other SiC polytypes
and may also possess single-photon emission characteristics. The
incorporation of these bright defects in SiC nanoparticles and SiC
quantum dots could provide an exciting opportunity for biologicalimaging and microscopy.
Further optimization of the defect formation and excitation
is expected to improve the photostability, its brightness andunderstanding of its properties. Electrical excitation of the centre isalso a very realistic possibility that may result in SiC light-emitting
diodes with active quantum systems31 .

Frenkel pair annihilation and reduce the possibility of contamination. After
irradiation, anneals between 200 and 1,100 ◦ C in an Ar ambient for 30 min werecarried out. Previous ensemble electron spin resonance measurements were also
used to optimize anneals to enhance particular defects32 . An unprocessed sample
from the same wafer was also characterized to establish the effectiveness of theirradiation procedure employed.

Sample preparation. The material under study is high-purity semi-insulating
4H-SiC purchased from CREE. According to the manufacturer, the as-received
samples contained B (1014 cm−3 ), C interstitials (1014 cm−3 ) and N (<1014 cm−3 ).
Samples were irradiated with 2 MeV electrons to fluences of 1 × 1013 , 1 × 1015
and 1 × 1017 e cm−2 at a flux of 2.82 × 1012 cm−2 s−1 . The samples were held attemperatures below 80 ◦ C during irradiation in a nitrogen ambient to inhibit

Photoluminescence and photon correlation measurements. A HBTinterferometer was used to identify optically active single defects and measure thetime correlation of the centre emission. In the confocal microscope the samples
were excited using a 0.2–1.5 mW continuous-wave laser operating at 532 nm(Coherent, model: Compass 315 M-100) and a continuous diode laser operating
at 665 nm. The laser was focused onto the sample using a ×100 infinity-correctedoil-immersion objective lens with a NA of 1.4 and the luminescence was collectedconfocally through a 50 µm pinhole. A spectrometer (Princeton Instruments
Acton 2500i, Acton) with a cooled CCD (charge-coupled device) (Camera Pixis
100 model: 7,515–0,001 Princeton Instruments) was used to characterize theluminescence and a HBT interferometer with single-photon-sensitive avalanche
photodiodes (Perkin Elmer SPCM-AQR 14) was used to measure the photon
statistics. Photon counting and correlation was carried out using a time-correlated
single-photon-counting module (PicoHarp 300, PicoQuant GmbH).

NATURE MATERIALS | VOL 13 | FEBRUARY 2014 | www.nature.com/naturematerials

155

Methods© 2014 Macmillan Publishers Limited. All rights reserved

NATURE MATERIALS DOI: 10.1038/NMAT3806

LETTERSThe 4H-SiC samples were mounted on a piezoelectric XYZ stage allowing
100 × 100 µm2 scans. The unwanted residual laser line was eliminated bya 660-nm long-pass filter. A filter with 694 ± 10 nm was placed in front of
the HBT interferometer to observe single photons during the 660-nm laser
excitation. The spectroscopy was done using only a long-pass filter. All of thesingle-photon/single-defect measurements were performed at room temperature.
A Renishaw MicroRaman Spectrometer equipped with a Linkam stage fortemperature control was used for the ensemble photoluminescence measurements.
A 532-nm laser was directed through a ×20 long-working-distance objective with a
0.4 NA onto the sample. The laser power at the sample was approximately 0.5 mW.
The spectrometer has a working distance of 0.25 m and uses an 8 µm slit and a
1,200 g mm−1 grating. The detector is a fan-cooled Si CCD.
Atomistic simulations. Theoretical calculations were carried out using density
functional theory and group theory. A 576-atom supercell with 0-point samplingof the Brillouin zone was used in the supercell calculations on the CSi VC defects
in 4H-SiC. A 0-point calculation is sufficient to provide convergent charge densityand enabled us to study the degeneracy of the defect levels. We applied HSE06
hybrid density functional theory that is able to reproduce the experimental bandgapof 4H-SiC (≈3.2 eV) and defect levels in the fundamental bandgap33,34 . We notethat we were able to identify the electron paramagnetic spectrum of the negatively
charged and positively charged CSi VC pair using supercell density functional theory
methods in refs 32,35.

Received 15 December 2012; accepted 7 October 2013;
published online 17 November 2013

References
1. Basché, T., Moerner, W. E., Orrit, M. & Talon, H. Photon anti-bunching in
the fluorescence of a single dye molecule trapped in a solid. Phys. Rev. Lett. 69,
1516–1519 (1992).
2. Michler, P. et al. Quantum correlation among photons from a single quantum
dot at room temperature. Nature 406, 968–970 (2000).
3. Santori, C., Fattal, D., Vuckovic, J., Solomon, G. S. & Yamamoto, Y.
Indistinguishable photons from a single-photon device. Nature 419,
594–597 (2002).
4. Shields, A. J. Semiconductor quantum light sources. Nature Photon. 1,
215–223 (2007).
5. Kurtsiefer, C., Mayer, S., Zarda, P. & Weinfurter, H. Stable solid-state sourceof single photons. Phys. Rev. Lett. 85, 290–293 (2000).
6. Kolesov, R. et al. Optical detection of a single rare-earth ion in a crystal.
Nature Commun. 3, 1029 (2012).
7. Peruzzo, A., Shadbolt, P., Brunner, N., Popescu, S. & O’Brien, J. L. A quantum
delayed-choice experiment. Science 338, 634–637 (2012).
8. Lundeen, J. S., Sutherland, B., Patel, A., Stewart, C. & Bamber, C.
Direct measurement of the quantum wavefunction. Nature 474,
188–191 (2011).
9. Politi, A., Matthews, J. C. F. & O’Brien, J. L. Shor’s quantum factoringalgorithm on a photonic chip. Science 325, 1221 (2009).
10. Yamada, S., Song, B-S., Asano, T. & Noda, S. Silicon carbide-based photonic
crystal nanocavities for ultra-broadband operation from infrared to visible
wavelengths. Appl. Phys. Lett. 99, 201102 (2011).
11. Madar, R. Materials science: Silicon carbide in contention. Nature 430,
974–975 (2004).
12. Nakamur, D. et al. Ultrahigh-quality silicon carbide single crystals. Nature 430,
1009–1012 (2004).
13. Lu, X., Lee, J. Y., Feng, P. X-L. & Lin, Q. Silicon carbide microdisk resonator.
Opt. Lett. 38, 1304–1306 (2013).
14. Fan, J. et al. 3C–SiC nanocrystals as fluorescent biological labels. Small 4,1058–1062 (2008).
15. Beke, D. et al. Silicon carbide quantum dots for bioimaging. J. Mater. Res. 28,
205–209 (2013).
16. Gali, A. Excitation spectrum of point defects in semiconductors
studied by time-dependent density functional theory. J. Mater. Res. 27,
897–909 (2012).
17. Weber, J. R. et al. Quantum computing with defects. Proc. Natl Acad. Sci. USA
107, 8513–8518 (2010).
18. Koehl, W. F., Buckley, B. B., Heremans, F. J., Calusine, G. & Awschalow, D. D.
Room temperature coherent control of defect spin qubits in silicon carbide.
Nature 479, 84–87 (2011).
19. Baranov, P. G. et al. Silicon vacancy in SiC as a promising quantum system forsingle-defect and single-photon spectroscopy. Phys. Rev. B 83, 125203 (2011).

156

20. Falk, A. L. et al. Polytype control of spin qubits in silicon carbide. Nature
Commun. 4, 1819 (2013).
21. DiVincenzo, D. Quantum bits: Better than excellent. Nature Mater. 9,
468–469 (2010).
22. Aharonovich, I. et al. Chromium single-photon emitters in diamond fabricatedby ion implantation. Phys. Rev. B 81, 121201 (2010).
23. Aharonovich, I. et al. Two-level ultrabright single photon emission fromdiamond nanocrystals. Nano Lett. 9, 3191–3195 (2009).
24. Neu, E. et al. Single photon emission from silicon-vacancy colour centresin chemical vapour deposition nano-diamonds on iridium. New J. Phys. 13,025012 (2011).
25. Schröder, T., Gädeke, F., Banholzer, M. J. & Benson, O. Ultrabright andefficient single-photon generation based on nitrogen-vacancy centres in
nanodiamonds on a solid immersion lens. New J. Phys. 13, 055017 (2011).
26. Ates, S. et al. Improving the performance of bright quantum dot single
photon sources using temporal filtering via amplitude modulation. Sci. Rep. 3,1397 (2013).
27. Steeds, J. W. Photoluminescence study of the carbon antisite-vacancy pair in
4H-and 6H–SiC. Phys. Rev. B 80, 245202 (2009).
28. Steeds, J. et al. Transmission electron microscope radiation damage of 4H and
6H SiC studied by photoluminescence spectroscopy. Diamond Relat. Mater.
11, 1923–1945 (2002).
29. Soltamov, V. A., Soltamova, A. A., Baranov, P. G. & Proskuryakov, I. I. Roomtemperature coherent spin alignment of silicon vacancies in 4H-and 6H–SiC.
Phys. Rev. Lett. 108, 226402 (2012).
30. Bockstedte, M., Mattausch, A. & Pankratov, O. Ab initio study of the annealingof vacancies and interstitials in cubic SiC: Vacancy-interstitial recombination
and aggregation of carbon interstitials. Phys. Rev. B 69, 235202 (2004).
31. Fuchs, F. et al. Silicon carbide light-emitting diode as a prospective room
temperature source for single photons. Sci. Rep. 3, 1637 (2013).
32. Umeda, T. et al. Identification of the carbon antisite-vacancy pair in 4H-SiC.
Phys. Rev. Lett. 96, 145501 (2006).
33. Deák, P., Aradi, B., Frauenheim, T., Janzén, E. & Gali, A. Accurate defect levels
obtained from the HSE06 range-separated hybrid functional. Phys. Rev. B 81,
153203 (2010).
34. Ivády, V., Gällström, A., Son, N. T., Janzén, E. & Gali, A. Asymmetricsplit-vacancy defects in SiC polytypes: A combined theoretical and electron
spin resonance study. Phys. Rev. Lett. 107, 195501 (2011).
35. Umeda, T. et al. Identification of positively charged carbon antisite-vacancy
pairs in 4H-SiC. Phys. Rev. B 75, 245202 (2007).

Acknowledgements
S.C. acknowledges partial financial support from the Australian Research Council,
Centre of Excellence Engineered Quantum Systems (CE110001013). B.C.J. acknowledges
financial support from the Japanese Society for the Promotion of Science (JSPS;
Grant-in-aid for Scientific Research, 22.00802) and the Australian Research Council
Centre for Quantum Computation and Communication Technology (CE110001027).
This study is also partially supported by the Ministry of Education, Science, Sports
and Culture, Grant-in-Aid for Challenging Exploratory Research, 2012, 24656025.
A.G. acknowledges the EU FP7 Grant No. 270197 (DIAMANT), the Hungarian
OTKA Grant Nos K101819 and K106114 and the support from the Knut and Alice
Wallenberg Foundation.

Author contributions
S.C. and B.C.J. proposed the idea and the strategy for the experimental design and data
analysis and wrote the paper. S.C. performed the optical single-photon characterization
experiments. B.C.J. irradiated and annealed the samples and made the low-temperature
photoluminescence measurements. N.S. performed some of the low-temperature
photoluminescence measurements. S.C. and B.C.J. coordinated the experiments andanalysed the data. V.I. and A.G. performed the theoretical model of the centre. S.C.,
B.C.J. and A.G. contributed to writing the manuscript. All authors discussed the results
and commented on the manuscript.

Additional information
Supplementary information is available in the online version of the paper. Reprints andpermissions information is available online at www.nature.com/reprints. Correspondence
and requests for materials should be addressed to S.C.

Competing financial interests
The authors declare no competing financial interests.

NATURE MATERIALS | VOL 13 | FEBRUARY 2014 | www.nature.com/naturematerials© 2014 Macmillan Publishers Limited. All rights reserved