Materials Science Forum
ISSN: 1662-9752, Vol. 924, pp 281-284
doi:10.4028/www.scientific.net/MSF.924.281© 2018 Trans Tech Publications, Switzerland

Submitted: 2017-09-07
Revised: 2018-02-16
Accepted: 2018-02-16
Online: 2018-06-05

Oxidation-Process Dependence of Single Photon Sources Embedded in
4H-SiC MOSFETs
Yuta Abe1,a, Takahide Umeda1,b*, Mitsuo Okamoto2,c, Shinobu Onoda3,d,Moriyoshi Haruyama3,4,e, Wataru Kada4, Osamu Hanaizumi4, Ryoji Kosugi2,f,
Shinsuke Harada2,g, and Takeshi Ohshima3,h1

Institute of Applied Physics, University of Tsukuba, Tsukuba, 305-8573, Japan

2

National Institute of Advance Industrial Science and Technology, Tsukuba, 305-8568, Japan

3National Institutes for Quantum and Radiological Science and Technology, 1233 Watanuki,Takasaki, Gunma, 370-1292, Japan
4Gunma University, 1-5-1 Tenjincho, Gunma, 376-0052, Japan

as1620318@u.tsukuba.ac.jp, bumeda@bk.tsukuba.ac.jp, cmitsuo-okamoto@aist.go.jp,
donoda.shinobu@qst.go.jp, eharuyama.moriyoshi@qst.go.jp, fr-kosugi@aist.go.jp,
gs-harada@aist.go.jp, hohshima.takeshi@qst.go.jp

Keywords: single photon source, MOSFET, MOS interface, confocal microscope

Abstract. We investigated single photon sources (SPSs) in 4H-SiC metal-oxide-semiconductor
field-effect transistors (MOSFETs) by means of confocal microscope techniques. We found SPSs
only in 4H-SiC/SiO2 interface regions of wet-oxide C-face MOSFETs. The other regions ofMOSFETs such as source, drain and well did not exhibit SPSs. The luminescent intensity of the SPSs
at room temperature was at least twice larger than that of the most famous SPSs, the nitrogen-vacancy
center, in diamond. We examined four types of C-face and Si-face 4H-SiC MOSFETs with different
oxidation processes, and found that the formation of the SPSs strongly depended on the preparationof SiC/SiO2 interfaces.
Introduction
Wide-bandgap semiconductors, especially diamond, have good color centers that are suitable forsingle photon sources (SPSs) operatable at room temperature. Such “room-temperature SPSs” have agreat potential for quantum photonics and quantum sensing applications. For example, if they haveelectron spins, they are usable as a quantum sensor for electromagnetic fields and are applicable to,e.g., bio-imaging technologies. The nitrogen-vacancy (NV–) center in diamond realizes such an
imaging [1]. However, diamond is known to be a high-cost material as well as having manydifficulties for device processing.
On the other hand, it has been recently demonstrated that silicon carbide (SiC) is also a good hostmaterial for preparing room-temperature SPSs. For instance, a silicon-vacancy center (VSi) createdby electron irradiation was demonstrated as room-temperature SPSs with electron spins [2]. Carbonantisite-vacancy pair (CSiVC) in irradiated high-purity semi-insulating 4H-SiC was also found to act
as bright SPSs [3]. Room-temperature SPSs were also simply formed by a low-temperature oxidationof SiC surfaces [4]. Furthermore, room-temperature SPSs (tentatively assigned to an antisite center)
were found in 4H-SiC pn diodes, which were electrically driven by pulse currents [5]. Recently, we
have found room-temperature SPSs in the channel regions (SiC/SiO2 interfaces) of 4H-SiC
MOSFETs [6]. Their signatures were significantly different from the SPSs reported in previous
works [2-5], and hence we assigned them to a novel type of SPSs [6]. These SPSs have a possibility
to control their charge states and spin states by an electrical voltage or current of a MOSFET. We
believe that they were formed at the 4H-SiC/SiO2 interfaces; namely, they are a sort of interface
defects. Therefore, in this work, we examined oxidation-process dependences of the interfacial SPSs
by preparing four types of 4H-SiC MOSFETs with different oxidation processes.

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Experiments
We used a home-built confocal microscope (CFM) using high-NA (NA = 0.95) air object lens anda 532-nm laser to detect photoluminescence (PL) from a single PL center. This is optimized forobserving the NV– center in diamond [7]. A confocal beam is detected by a pair of avalanche photodiodes. In this work, we measured X-Y-Z mapping images (positional resolution ≥ 1 nm) as well as
performed anti-bunching measurements with a Hanbury-Brown-Twiss interferometer [2-5]. The
laser light and PL emissions below 647 nm are cut off by a long-pass filter to highlight spots of SPSs.
Using our CFM setup, a single NV– center can be detected as a bright spot with a diameter of 0.5 μm
and a peak luminescent intensity of 80,000 counts-per-sec (80 kcps).
channel stopperwell(e)S/D(d)(b)S/DchannelFig. 1. (a) Optical microscope image of an wet-oxide C-face 4H-SiC MOSFET after removing Alelectrodes. (b) An X-Y CFM image of the channel and channel-stopper regions. The bright spotsof ~160 kcps arise from SPSs. (c) Anti-bunching behavior of a SPS in the channel region. (d) An
X-Y CFM image of the S/D and the channel regions. The S/D region exhibits no SPSs, while
many bright spots of the SPSs exist in the channel region. (e) An X-Y CFM image of the wellregion. The well region also did not show SPSs (the SPSs in the channel-stopper region are also
not visible because of a wider color-scale range). The CFM images (b), (d) and (e) were obtained
from the rectangular regions shown in (a).
The samples we measured are four types of n-channel lateral C-face and Si-face 4H-SiC MOSFETs(gate oxide thickness = 50 nm, gate length/width = 100/150 μm). The C-face samples were preparedby either wet oxidation at 1000 ºC or dry oxidation at 1100 ºC on a 5-μm epitaxial layer of a 4º-off_4H-SiC(0001) wafer. On the other hand, the Si-face ones were subjected to either the standard dryoxidation or the standard post-oxidation nitridation (NO anneal). Figure 1 (a) shows the whole imageof a MOSFET used for CFM measurements. There are four types of doped regions, “n+-type sourceand drain (S/D),” “p+-type well,” “p–-type channel,” and “p-type channel stopper.” In order toexamine these regions by CFM, metal electrodes were removed by a wet etching, as the SiO2 layers
remained as they were. The channel region was covered by a 50-nm-thick thermal oxide, while thechannel-stopper region was covered by a 500-nm-thick deposited SiO2 layer. In CFM measurements,
we focused on SiC surfaces of the S/D and well regions or SiC/SiO2 interfaces of the channel andchannel-stopper regions. All CFM measurements were carried out at room temperature and under
1-mW laser excitation.

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Results
Figure 1(b) shows a CFM image of the boundary between the channel and channel-stopper regionsof a wet-oxide C-face MOSFET. Both regions show many isolated bright spots, which are arisingfrom SPSs. The peak luminescent intensities of the spots are about 160 kcps, which are twice larger
than that of the NV– center (80 kcps). To confirm that the observed spots are surely SPSs, we carriedout the anti-bunching measurements [2-5]. Figure 1(c) shows the normalized self-correlation
function g(2)(τ) measured for a bright spot in the channel region, where τ is a delay time between
photon-detection events of the two independent photo detectors. The anti-bunching curve can be wellfitted by the case of a SPS with a three-level PL system [8], demonstrating the presence of a SPS in asingle spot. Also the bright spots in the channel-stopper region revealed similar g(2)(τ) curves as wellas the same luminescent intensities as those of the SPSs in the channel region. Therefore, we judgedthat the SPSs in the channel and channel-stopper regions are identical.
On the other hand, the S/D region and the well region were overall very bright and exhibited noSPSs [Figs. 1(d) and (e), respectively]. Instead, these regions showed high background luminescenceof ~100 kcps (S/D) and ~750 kcps (well), which were much brighter than that of the channel andchannel-stopper regions (~25 kcps). Since the S/D and well regions were subjected to high-dose ion
implantations, dopants and implantation damages may cause the bright background luminescence.
To investigate the oxidation-process dependence of the SPSs, we measured other three types ofMOSFETs. Figures 2(a) to (d) compare CFM images of the channel regions (upper half) and thechannel-stopper regions (lower half) of the MOSFETs with different gate-oxide processes. Only in(a) (C-face wet oxidation), were the bright spots of the SPSs observed. The areal density of the SPSs
was estimated to be 1 × 107 cm-2 in both regions (see Table I). In Fig. 2(b) (C-face dry oxidation),
both regions emitted overall bright background. However, the luminescent spots could be slightlyrecognized. Their areal densities were more than 20 times higher than those of (a) (see Table I), andwe could not observe anti-bunching behaviors in any those spots. Even with the high density of thespots, the dry-oxide C-face interface exhibited a lower luminescent intensity (see Table I). Therefore,
we suggest different origins for the luminescent spots in (a) and (b). The channel and channel-stopper
regions of the two Si-face SiC-MOSFETs [Figs. 2(c) and (d)] revealed spotty images more similar toFig. 2(a). The areal densities of the luminescent spots were about 1 × 108 cm-2 (see Table 1), which
we expect will enable us to observe individual SPSs. However, we could not find any anti-bunching
behaviors in the two Si-face SiC-MOSFETs. This result is contrast with the previous work thatobserved SPSs in a briefly oxidized 4H/6H/3C-SiC surfaces [3]. The CFM observations of the four
MOSFETs are summarized in Table 1, which clearly indicates that the formation of the SPSs dependson oxidation processes.
(a) C -face Wet(b) C -face Dry(c) Si -face Dry(d) Si-face Nitrided

Fig. 2. Comparison of X-Y CFM images in the channel (upper half) and the channel-stopper (lowerhalf) regions of four types of 4H-SiC MOSFETs with (a) C-face wet-oxide, (b) C-face dry-oxide,(c) Si-face dry-oxide, and (d) Si-face dry-oxide + standard NO nitridation.

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Table I. Properties of the luminescent spots in four types of SiC-MOSFETs.
C-faceSi-face
Wet oxidation
Dry oxidation
Dry oxidation
Nitrided
*1
*1ch
ch-st
ch
ch-stchch-stchch-st
Areal density
0.1
0.1
2.9
2.3
1.2
1.0
0.7-1.0
0.3[×108 cm-2]
Spot [kcps]> 160 > 160 100-120 100-120
10080
120-130 > 160
Background20-30 20-3080-906050-60
20-30
50-60
30-40
[kcps]
Anti-bunching○○××××××behavior*1“ch” and “ch-st” mean the channel region and the channel-stopper region, respectively.
SummaryWe have found the SPSs in the channel and the channel-stopper regions of wet-oxide C-face
4H-SiC MOSFETs. They were brighter than the NV– center in diamond within our CFM setup. Inthe same MOSFET, the S/D region and the well region revealed no SPSs, and exhibited very highbackground luminescence. We also examined dry-oxide C-face MOSFETs as well as two types ofSi-face MOSFETs. The dry-oxide C-face sample did not show the SPSs, indicating a strong
dependence of the SPSs on oxidation processes. Also in the Si-face MOSFETs, we did not find anySPSs, which is contrast with the previous works (e.g., Ref. 3) that reported a simple formation ofSPSs via a brief oxidation of Si-face SiC surfaces.
Acknowledgements
This work was partly supported by the JSPS KAKENHI Grant Nos. 26286047 and 17H01056.
References[1] Hui, Y. Y., Cheng, C-L. & Chang, H-C, J. Phys. D 43, 374021 (2010).
[2] M. Widmann, S.-Y. Lee, T. Rendler, N. Tien Son, H. Fedder, S. Pirk, L.-P. Yang, N. Zhao, S.
Yang, I. Booker, A. Denisenko, M. Jamali, S.A. Momenzardeh, I. Gerhardt, T. Ohshima, A. Gali, E.
Janzén, J. Warchtrup, Nat. Mater. 14, 164 (2015).
[3] S. Castelletto, B. C. Johnson, V. Ivády, N. Stavrias, T. Umeda, A. Gali, T. Ohshima, Nat. Mater.
13, 151 (2014).
[4] A. Lohrmann, S. Castelletto, J. R. Klein, T. Ohshima, M. Bosi, M. Negri, D. W. M. Lau, B. C.
Gibson, S. Prawer, J. C. McCallum, B. C. Johnson, Appl. Phys. Lett. 108, 021107 (2016).
[5] A. Lohrmann, N. Iwamoto, Z. Bodrog, S. Castelletto, T. Ohshima, T. Karle, A. Gali, S. Prawer, J.
McCallum, B. C. Johnson, Nat. Commun. 6, 7783 (2015).
[6] Y. Abe, T. Umeda, M. Okamoto, R. Kosugi, S. Harada, M. Haruyama, W. Kada, O. Hanaizumi,S. Onoda, T. Ohshima, Appl. Phys. Lett. 112, 031105(2018).
[7] S. Onoda, M. Haruyama, T. Teraji, J. isoya, W. Kada, O. Hanaizumi, T. Ohshima, phys. stat. sol.
(a) 212, 2641 (2015).
[8] B. Lienhard, T. Schröder, S. Mouradian, F. Dolde, T. T. Tran, I. Aharonovich, D. Englund,Optica 3, 768-774 (2016).