Single photon sources in 4H-SiC metal-oxide-semiconductor field-effect transistors
Y. Abe, T. Umeda, M. Okamoto, R. Kosugi, S. Harada, M. Haruyama, W. Kada, O. Hanaizumi, S. Onoda, and T.
OhshimaCitation: Appl. Phys. Lett. 112, 031105 (2018); doi: 10.1063/1.4994241
View online: https://doi.org/10.1063/1.4994241
View Table of Contents: http://aip.scitation.org/toc/apl/112/3
Published by the American Institute of Physics

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APPLIED PHYSICS LETTERS 112, 031105 (2018)

Single photon sources in 4H-SiC metal-oxide-semiconductor field-effecttransistors
Y. Abe,1 T. Umeda,1,a) M. Okamoto,2 R. Kosugi,2 S. Harada,2 M. Haruyama,3 W. Kada,3O. Hanaizumi,3 S. Onoda,4 and T. Ohshima4
1

Institute of Applied Physics, University of Tsukuba, Tsukuba 305-8573, Japan
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8568, Japan3
School of Science and Technology, Gunma University, Kiryu 376-0052, Japan
4
National Institutes for Quantum and Radiological Science and Technology (QST), Takasaki 370-1292, Japan
2(Received 5 July 2017; accepted 28 December 2017; published online 17 January 2018)
We present single photon sources (SPSs) embedded in 4H-SiC metal-oxide-semiconductor fieldeffect transistors (MOSFETs). They are formed in the SiC/SiO2 interface regions of wet-oxidation
C-face 4H-SiC MOSFETs and were not found in other C-face and Si-face MOSFETs. Their brightroom-temperature photoluminescence (PL) was observed in the range from 550 to 750 nm andrevealed variable multi-peak structures as well as variable peak shifts. We characterized a wide
variety of their PL spectra as the inevitable variation of local atomic structures at the interface.
Their polarization dependence indicates that they are formed at the SiC side of the interface. We
also demonstrate that it is possible to switch on/off the SPSs by a bias voltage of the MOSFET.
Published by AIP Publishing. https://doi.org/10.1063/1.4994241

A state-of-the-art confocal photoluminescence (PL)microscope makes it possible to observe a single luminescentdefect (PL center)1 and even control its photon state2 andspin states.3 This ability has great potential for quantum
informatics as well as quantum-sensing techniques with anextremely high sensitivity. In principle, a single PL centercan emit a single photon at a single PL event, which turns
out to be a solid-state implementation of single photon sources (SPSs).2 One of the most famous examples of SPSs is anegatively charged nitrogen-vacancy center (NV–) in diamond, which is an extremely bright and stable PL center
operatable at room temperature.1 Recently, further brighterand room-temperature-operatable SPSs have been found in
silicon carbides (SiC). For instance, bright SPSs were easily
formed in commercial high-purity semi-insulating (HPSI)
4H-SiC wafers after some treatments.3–5 Furthermore, it has
been demonstrated that bulk defects in 4H-SiC (divacancies3and silicon vacancies6) are suitable for controlling a single
spin in an SPS, similar to the case of the NV– center. Later,
Lohrmann et al. found bright SPSs in a 4H-SiC device (pþ–ndiode) and demonstrated to control them by electrical currents.7 Since 4H-SiC templates can realize a variety ofdevice structures such as diodes as well as both bipolar andunipolar transistors, it will be quite possible to combine
SPSs with various types of 4H-SiC electronic devices.
In this letter, we report SPSs in the channel region of
wet-oxidation C-face 4H-SiC metal-oxide-semiconductor
field-effect transistors (MOSFETs). The SPSs were formed
with a density of 1  107 cm2 in the SiC/SiO2 interface
regions, and they emitted very strong PL at room temperature.
Their photon emission was more than twice brighter than the
PL from a single NV– center with our confocal microscope(CFM) setup. On the contrary, we have not found any SPSs in
dry-oxidation C-face MOSFETs and two types of the standarda)umeda.takahide.fm@u.tsukuba.ac.jp0003-6951/2018/112(3)/031105/5/$30.00

Si-face MOSFETs. The present SPSs are characterized by awide variety of PL spectra, suggesting that they are sensitive
to local atomic structures at the interface, like a “sensor” forthe interface. We also demonstrate a bias-controlled switchingof the SPSs by taking advantage of the MOSFET structure.
We used a home-built confocal PL microscope optimized for detecting a single NV– center in diamond.8 The PL
was excited by a solid-state 532-nm laser. A confocal beam
was detected by either a pair of independent photodiodes or
a commercial double-grating PL spectrometer. The former
detector was used for conventional X–Y–Z CFM mapping
and anti-bunching experiments with a Hanbury-BrownTwiss interferometer,3–10 where X and Y represent the coordinate axes along the sample surface, and Z indicates thedepth from the sample surface (Z  0). In this mode, thelaser line and background emissions below 647 nm were cut
off by long-pass filters to highlight spots of SPSs. For the
PL-spectrometer mode, only a long-pass filter of 561 nm wasused. We used a high-NA objective lens in air (NA ¼ 0.95).
By using our CFM setup, a single NV– center was detectable
as a bright spot with a luminescent intensity of about 80 000
counts-per-sec (80 kcps). All measurements described here
were done at room temperature.
The lateral n-channel C-face 4H-SiC MOSFETs (gatelength/width ¼ 100/150 lm) that we examined were preparedon a 5–lm-thick epitaxial layer of a 4 -off 4H-SiC(000)wafer.11 The whole image of the MOSFET is shown in Fig.
1(a). The channel region (p–-type) was formed by the epitaxial
layer with [Al]  5  1015 cm3. The source and drain (S/D)
regions were formed by phosphorous ion implantation ([P] 2  1020 cm3) followed by activation annealing at 1620  C
for a few minutes. The 50-nm-thick gate oxide was grown byeither wet oxidation at 1000  C or dry oxidation at
1100–1200  C. The wet-oxidation C-face MOSFETs were subjected to a post-oxidation anneal (POA) with H2 gas at
1100  C, while the dry-oxidation ones were annealed in Ar

112, 031105-1

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FIG. 1. (a) Optical microscopy image of a wet-oxidation C-face SiCMOSFET (gate length/width ¼ 100/150 lm) after removing top electrodes.
There are four types of regions with different doping concentrations: source/
drain (S/D) (nþ), channel (p–), channel stopper (p), and well (pþ) regions. (b)50  50-lm2 CFM image measured for the rectangular region shown in (a).
Bright spots in channel and channel-stopper regions arise from SPSs. A bright5-lm-width periphery of the S/D region was covered by an oxide layer, where
PL emission was strongly enhanced. The same periphery was also seen in thewell region (not shown in the figure). (c) CFM image of SPSs in the channel
region. (d) CFM depth profile for the SPSs. A 5-lm-thick epitaxial layer isshown as a darker layer. The SPSs are always located in the SiC/SiO2 interface region. (e) Anti-bunching experiments on spots #1 and #2 in (c) and on
the reference sample (single NV– center in diamond) after background corrections. Solid lines expressed fitting curves calculated based on the three-level
SPS system.9 Ratios of (Signal)/(Signal þ Background)10 were 0.88 for spot
#1 and 0.92 for spot #2.

ambient at the same temperature and for the same period toensure that the two C-face MOSFETs had the same thermal
history. The pþ-well region was formed by a heavy Al
ion implantation ([Al]  1  1020 cm3). Also, the channelstopper region (p-type) was prepared by moderate Al implantation ([Al]  1  1018 cm3) and was covered by a
0.5–lm-thick deposited SiO2 layer. We also prepared the standard Si-face 4H-SiC MOSFETs by either dry oxidation or dry
oxidation þ NO POA nitridation for a comparative study
between C-face and Si-face MOSFETs. The maximum valuesof the field-effect mobility (lFE) were 90 cm2V1s1 for the
wet-oxidation C-face MOSFETs, 0 cm2V1s1 for the
dry-oxidation C-face MOSFETs (no channel currents wereactivated), 2 cm2V1s1 for the dry-oxidation Si-face
MOSFETs, and 20 cm2V1s1 for the nitrided Si-face
MOSFETs.11
Before the CFM measurements, the top metal electrodes
were removed by wet etching, while we kept the SiO2 layers
as they were. When we removed the SiO2 layers, the SPSs

Appl. Phys. Lett. 112, 031105 (2018)

vanished or their luminescence became unstable. This behavior was consistent with a previous report that an oxide layer
was necessary to stabilize SPSs in oxidized HPSI 4H-SiC
wafers.5 Therefore, we believed that the SPSs were confinedto the SiC/SiO2 interface regions, similar to the case of the
Lohrmann’s study.5 However, we emphasize that our
MOSFETs revealed a different type of SPS, as shown later.
We have confirmed the presence of SPSs only in wetoxidation C-face 4H-SiC MOSFETs (Fig. 1). In Fig. 1(a),the S/D and well regions reveal bare SiC surfaces, while thechannel and channel-stopper regions were covered by eithera thin (52 nm) or thick (0.5 lm) SiO2 layer, respectively. The
rectangular region in (a) was scanned by CFM, which isshown in Fig. 1(b). The S/D regions (and also the well
region) were overall bright; hence, the recognition of SPSs
was hardly possible there. On the contrary, the channel andchannel-stopper regions showed much lower backgroundemissions (20 kcps) as well as many isolated bright spots
arising from SPSs. Figure 1(c) focuses on SPSs in the channel region. Their luminescent intensities are found to be160 kcps for normal spots (spot #1) or 250 kcps forbrighter spots (spot #2), which is at least twice larger than
the 80 kcps of a single NV– center in our setup. The SPSs
were formed in the SiC/SiO2 interface region (Z  0), as
shown by the X–Z image of Fig. 1(d). In this image, a 5–lmthick epitaxial layer is visible as a darker 1.9–lm-thick layer[the optical thickness was calculated by dividing the physicalthickness by the reflection index of SiC (2.65)]. The present
SPSs seemed to be always located at the top of the epitaxial
layer; hence, we believe that they originate from a sort ofinterface defect in SiC/SiO2 structures.
To demonstrate the correspondence between the brightspots and SPSs, we performed anti-bunching experiments.3–10
Figure 1(e) shows the normalized self-correlation functiong(2)(s) measured for the spots #1 and #2 shown in Fig. 1(c),where s is the delay time between the photon-detection events

TABLE I. Densities of luminescent spots and their luminescent intensities
in the channel regions of four types of SiC-MOSFETs. Note that luminescent intensities shown here include background emissions. We compare present data with previous work on oxidized HPSI 4H-SiC wafers.5

SampleChannel regionof 4H-SiC MOSFETs(This work)
Oxidized HPSI
4H-SiC wafers5

C-face wet
C-face dry
Si-face dry
Si-face nitrided
C-face dry
Si-face dry

Density ofluminescentspots (108 cm2)

Luminescentintensity(kcps)

0.11 (SPSa)
2.9b,c
1.2b
0.7b
8d
0.8d (SPSa)

160–250
100–120
100
120–130…
…

a
The observed spots arise from SPSs, which have been confirmed by theanti-bunching experiments.
b
After subtracting the constant background luminescence, we could find aspotty image and estimate the density of luminescent spots.
c
Because of severe overlaps of the spots, the density of the spots wasroughly estimated by assuming that a single spot should have a circular areaof 0.196 lm2.
d
We estimated the density of SPSs on the Si face using CFM images shownin Fig. 1 of Ref. 5. For the C face, it was reported that the density of SPSs
was one order of magnitude higher than that on the Si face.

031105-3

Abe et al.

of two independent photodetectors.9,10 For both the spots,
their g(2)(s) curves were well described with a three-level PL
system: g(2)(s) ¼ 1  (1 þ a)exp(jsj/s1) þ aexp(jsj/s2).9The solid lines in Fig. 1(e) were calculated with s1 ¼ 2.6 ns
and s2 ¼ 15.6 ns for spot #1 and s1 ¼ 1.8 ns and s2 ¼ 9.2 ns forspot #2. Using the same setup, we also observed g(2)(s) of asingle NV– center [see the bottom curve in Fig. 1(e)], revealing longer lifetimes (s1 ¼ 7.1 ns and s2 ¼ 39.5 ns). The shorter
lifetimes of the present SPSs imply a faster recombination ofcarriers, which may correspond to their higher brightness
compared to those of the NV– center. Similar higher brightness was commonly found in room-temperature SPSs in 4HSiC, where similar short lifetimes were observed [s1 ¼ 1.2 ns,4
s1 ¼ 2.4 ns,5 and s1 ¼ 3.3 ns (Ref. 7)]. The spots observed in
the channel-stopper region also revealed the same features as
those of the SPSs in the channel region.
The densities of the SPSs were found to be
1–2  107 cm2 in both the channel and channel-stopperAppl. Phys. Lett. 112, 031105 (2018)

regions (see Table I). By contrast, other MOSFETs with
C-face dry-oxidation, Si-face dry-oxidation, and Si-face nitridation did not exhibit any SPSs in both the channel andchannel-stopper regions. This is a striking contrast with theprevious work5 that not only 4H-SiC Si-face surfaces but also
6H-SiC and 3C-SiC surfaces easily form SPSs on their surfaces after a brief annealing (800  C for 5 min) in oxygenambient.
As described in Table I, our Si-face MOSFETs exhibitedisolated bright spots (100 kcps) of 1  108 cm2 under alow background emission ( 30 kcps). Even with such agood condition, we could not have found any anti-bunching
behaviors in the two Si-face SiC-MOSFETs. The overall
results indicate that the formation of the SPSs is highly sensitive to the preparation of the SiC/SiO2 interfaces and is not
so simple and easy as expected previously. We have to mention that atomic structures of our interfaces should be essentially different from those of the previous studies,5 owing to

FIG. 2. (a) Polarization dependences of the SPSs. Eachemission intensity was normalized to its highest level.
The SPSs were classified into three types of orientationsin association with a threefold symmetry. (b) Wide variation in the highest-energy peak (Peak0) of the SPSs.
Their positions were estimated via fitting analyses
described below. (c) PL spectra of individual SPSs (“A”
to “F”) and fitting results based on our proposed model.
All the SPSs revealed similar anti-bunching behaviors
as shown in Fig. 1(e). The bottom spectrum was measured at the background position (not on bright spots)
and showed the pure secondary Raman line (580 nm)
of 4H-SiC.15 Each spectrum was normalized such thatits secondary Raman line has an equal intensity. PL
spectra below 561 nm were not recorded owing to the
low-pass filter. (d) Fitting results (peak positions andrelative intensities) for the PL spectra “A” to “F.” We
used a set of the highest-energy peak (Peak0) and six
sideband peaks (Peak1 to Peak6) as is summarized inTable II. See also details of the fitting in the text.

031105-4

Abe et al.

Appl. Phys. Lett. 112, 031105 (2018)

TABLE II. A fitting model for PL spectra of the present SPSs, where we assumed a set of the highest-energy peak (Peak0) and six sideband peaks. The positionof Peak0 was adjusted for each SPS, while those of the sidebands were fixed to keep energy separations defined in this table. Peak intensities were variable.
Full width at half maximum (FWHM) of each peak is given in this table and was allowed to fluctuate by 30% in order to adjust for individual SPS. Possible
assignments of the phonon sidebands are also shown in the table.16,17

DEa (cm1)
FWHM (nm)Assignment of local vibration modes

Peak0

Peak1

Peak2

Peak3

Peak4

Peak5

Peak6

0
25
…

515
40
SiH2 Si-O

1330
40
C-H, C-O

1510
20
…

1920
35
…

2240
75
-C C-, Si-H

2630
100
…

Energy separation in cm1.

aour robust oxidations which accumulate oxidation-induced
strain12 as well as oxidation-induced interstitial atoms13 in
the interface region. Another important difference is theusage of device-grade epitaxial layers, which may preventthe easy formation of SPSs inside MOSFETs.
To examine the origins of the SPSs in wet-oxidation Cface SiC-MOSFETs, we performed polarization and PL
experiments on the SPSs. Figure 2(a) shows polarization
dependences of the SPSs by rotating a polarization angle ofan incident linear-polarized beam. As is seen in the figure,the SPSs clearly show three types of polarization behaviors
based on a three-fold symmetry. Since this symmetry coincides with that of the 4H-SiC(000) face, we concluded that
the SPSs are formed in the SiC side of the SiC/SiO2 interfaces. The probability ratio among the above three types
appeared to be 0 :120 :240 ¼ 28%:36%:36% with an errorof 69%. Therefore, the SPSs seemed to be uniformly oriented in the 4H-SiC(000) face.
In the next place, the PL spectra of individual SPSs
were measured. In Fig. 2(b), positions of the highest-energy
PL peak of each SPS are plotted (we call it “Peak0” and its
position was estimated via a fitting analysis described
below). We found a very wide variation in the peak positions
over the energy range of 0.35 eV. Similar wide variations
were also seen in zero-phonon lines (ZPL) of other SPSs in

SiC [0.17 eV (Ref. 4) and 0.34 eV (Ref. 7)] as well as in diamond [0.23 eV (Ref. 14)]. Since the present SPSs are locatedin the interface region, it is reasonably expected that their PL
spectra varied more widely than the case of SPSs in the bulk
region.4,7,14 For example, we expect that strong local strainsat the SiC/SiO2 interfaces12 contribute to the wide ZPL variation of the present SPSs. In addition to the peak shifts, thePL spectra exhibited a variety of multi-peak structures [Fig.
2(c)], which is one of the most remarkable differences fromthe Lohrmann’s study.5 In fact, the previous SPSs exhibiteda broad PL signal in the range between 530 and 670 nm.5 We
assumed them to phonon sidebands, because similar multipeak structures repeatedly appeared in the PL spectra “B” to“F” in Fig. 2(c).
We tried to fit all PL spectra simultaneously by assuming the following model. (i) The highest-energy peaks (presumably ZPLs) are shifted as mentioned in Fig. 2(b). (ii)
Multiple phonon sidebands are included, and their intensities
are allowed to vary from one SPS to another depending oneach local environment. We chose six sideband peaks as
indicated by “Peak1” to “Peak6” in Fig. 2(c) and Table II.
We should emphasize that the present model is not proven asa unique solution; however, all the PL spectra were successfully fitted, as shown in Fig. 2(c). Figure 2(d) summarizes
the fitted positions and intensities of Peak0 and six sideband

FIG. 3. A bias control of the SPSs in the wet-oxidation C-face 4H-SiC MOSFET. (a) CFM images of an nþ-drain/p-channel-stopper junction of the MOSFET
under applying drain biases. A bright region in the left edge is an electrode of the drain. The well contact was set to 0 V, and the source and gate contacts wereopened. (b) PL intensities of spots #3 and #4 as a function of the drain bias. The upper figure shows the junction currents.

031105-5

Abe et al.

peaks. Among the six sidebands, the major ones (Peak1,
Peak2, and Peak5) are assignable to the local vibration
modes of possible interfacial structures such as Si-O, Si-H2,
C-H, or C-O (see Table II). We speculate that the peak intensities reflect on local bonding structures at the interface, andhence, the present SPSs may be usable as a sensor for visualizing local interface structures. We also mention that judging
from their PL spectra, if we use an optimal long-pass filter
instead of the 647-nm filter used in this study, the luminescent intensities of the present SPSs will be largely enhanced.
Finally, a bias control of the SPSs is examined by utilizing the SPSs in the channel-stopper region of a wetoxidation C-face MOSFET. Figure 3(a) shows a series of
CFM images of an nþ-drain/p-channel-stopper junction,
where the nþ-drain region (the left edge) was covered by an
Al electrode. We applied a drain bias to the electrode both
negatively (the nþ–p junction was forward-biased) and positively (the junction was reversely biased). As is seen in the
20  20–lm2 images, many SPSs in the channel-stopper
region responded to the applied biases. We observed reversible switching behaviors of the SPSs with opposite directions(e.g., spots #3 and #4) in the narrow bias range from 2 V to
3 V, as shown in Fig. 3(b). Further increasing a negative
bias generated much larger currents [Fig. 3(b)], but no significant changes were observed for these SPSs. Also positive
biases had no impacts on them, although the magnitude of
the currents became close to that at 2 V. These observations demonstrate the ability to control the present SPSs by abias voltage, although the mechanism of the bias control aswell as their origins remains future issues to be solved.
In summary, we have found the interfacial SPSs in thechannel and channel-stopper regions of wet-oxidation C-face
4H-SiC MOSFETs. These SPSs were much brighter (160 to
250 kcps) than a single NV– center (80 kcps) in diamondwith our CFM setup. The formation of the present SPSs wasstrongly affected by the oxidation processes, suggesting thatthey originate from interface defects. In fact, their PL spectra
varied widely from one SPS to another, probably dependingon the fluctuation of local atomic structures at the interface.
This feature is useful for proving the local interface structures around SPSs. Since the present SPSs were sufficiently

Appl. Phys. Lett. 112, 031105 (2018)bright and stable at room temperature and were controllable
via bias voltages of the MOSFET, they have potential as anelectrically controlled SPS combined with SiC-MOSFETs.
We thank T. Iwasaki and M. Hatano (Tokyo Institute ofTechnology) for their fruitful advice on our experiments.
This work was partly supported by the JSPS KAKENHI
Grant Nos. 26286047 and 17H01056.
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