Shallow P donors in 3C-, 4H-, and 6H-SiC
J. Isoya1,a, M. Katagiri1,b, T. Umeda1,c N. T. Son2,d, A. Henry2,e, A. Gali3,f,N. Morishita4,g, T. Ohshima4,h, H. Itoh4.i and E. Janzén2,j1

University of Tsukuba, Tsukuba 305-8550, Japan

2

Linköping University, S-581 83 Linköping, Sweden

3

Budapest University of Technology and Economics, Budapest, Hungary
4Japan Atomic Energy Agency, Takasaki 370-1292, Japanaisoya@slis.tsukuba.ac.jp, bkatagiri.masayuki@nims.go.jp, cumeda@slis.tsukuba.ac.jp,dson@ifm.liu.se, eahy@ifm.liu.se, eagali@eik.bme.hu, gmorishita.norio@jaea.go.jp,
hohshima.takeshi20@jaea.go.jp, iitoh.hisayoshi@jaea.go.jp, jerj@ifm.liu.se

Keywords: EPR, phosphorus donors, ion implantation.

Abstract. EPR spectra originating from phosphorus shallow donors occupying silicon sites in 3C-,4H-, and 6H-SiC are identified by using CVD grown films in which the interference from the signals
from the nitrogen shallow donors is practically absent. Phosphorus donors occupying both silicon andcarbon sites are observed in high-energy phosphorus ion implanted semi-insulating 6H-SiC which
was also free from the interference from the signals from the nitrogen shallow donors.
Introduction
Incorporation of shallow donors or acceptors which controls the type and the magnitude ofconductivity is essential for the fabrication of semiconductor devices. In silicon carbide (SiC),nitrogen (N) which is easily, even often unintentionally, incorporated during the crystal growth isused for controlling n-type conductivity. Phosphorous (P) which is an alternative shallow donor to N
in SiC is expected to have advantages over N in the high doping range where the electricalconductivity in N-doping saturates [1]. Ion implantation which provides the means of selective area
doping is required in the device fabrication process. For the electric activation of implanted ions,post-implantation annealing which removes implantation-induced defects and drives the implanted
ions into the desired lattice sites is required. For improving the doping efficiency, characterization of
the microscopic structure of implanted P ions is important. Electron paramagnetic resonance (EPR) is
useful to identify impurities and defects at the microscopic level. In applying EPR techniques toelucidate the thermal behavior of implanted ions, it is crucially important to establish the assignmentof the EPR spectra arising from the P shallow donors. The EPR spectra of isolated P shallow donors
are observed in the neutral charge state when the donor electron is bound in the impurity phosphorus
atom. Most of EPR studies of P in SiC used 6H-SiC doped by neutron transmutation of 30Si in the host
crystal to 31P [2-6]. The phosphorus related centers reported are Pa, Pb [2,3] (labeled P1, P2 in Ref.2
and sPc1, sPc2 in Ref.6) and P+V [2] (labeled sPh in Ref. 6) observed at 77 K, P1, P2 [3-5] (labeled I1, I2in Ref.3 and dPh, dPc1/dPc2 in Ref. 6) observed at T < 10 K. The Pa, Pb, and P+V spectra are observable
down to 4 K [6, 7]. The Pa, Pb spectra were also observed in P-ion (9-21 MeV, and 340 keV)
implanted 6H-SiC [8]. In these neutron-transmutation or ion-implantation doping studies, strong
signals from N donors in the substrate had prevented detailed studies. It is natural to assume that P
should substitute for silicon. However, for the case of doping by neutron transmutation, it waspointed out that a considerable fraction of P might end up in occupying the carbon sites after
high-temperature (∼1900 °C) annealing required in the doping method [7]. The Pa, Pb spectra areattributed to P at hexagonal carbon sites and P at quasi-cubic carbon sites (and PC-CSi), respectively[7]. The recoil process of the neutron transmutation creates the radiation

3C-SiC, 4H-SiC, and 6H-SiC epitaxial
layers doped with P during the CVD
growth [9] were used. High-energy P
ion implantation to n-6H-SiC crystals(NA – ND ≈ 1017 cm-3) and HPSI 6H-SiC
substrates was carried out at 800 °C withnine energies in the range from 9 to 21
MeV. The fluence of 5 × 1013 cm-2 wasused for each stage. Post-implantation
annealing was performed at 1650 °C for
30 min in Ar atmosphere.
EPR measurements were carried outusing a Bruker E500 X-band
spectrometer, a Bruker Elexsys E580
spectrometer, and a home-built W-band(95 GHz) spectrometer.
Results and discussion
It should be noted that the EPR signalsof the N shallow donors were notobserved in our CVD grown epitaxial
layers. In Fig. 1, X-band echo-detected
EPR spectra of 3C-SiC and 4H-SiC areshown. In Fig. 2(d), an X-band cw-EPR
spectrum of 6H-SiC is shown. The EPR
spectra from the P shallow donors are(a)(b)

X 50349

350
Magnetic field [mT]351

Fig. 1. Echo-detected EPR spectra of P-doped CVD
grown epitaxial layers: (a) 3C-SiC:P (5 K, B//[111]) and(b) 4H-SiC (6 K, B//[0001]).

N(k1, k2)(a)

EPR signal intensity [arb. units]Experiment

Echo intensity [arb. units]

damages (vacancies) and interstitial P
atoms [7]. It is likely that a similar,
probably less severe, situation isencountered in the case of P
incorporation by ion implantation.
In our present work, we haveobtained EPR spectra from shallow P
donors at silicon site in 6H-SiC as wellas in 3C-SiC, and 4H-SiC by using thesamples in which P is introduced during
the chemical vapor deposition (CVD)
growth and which are free from theinterference by the N donor signals.
After identifying the EPR spectra of theP donors at silicon sites, we haveapplied to determine the site of P after
high-energy P ion implantation and thesubsequent high-temperature annealing
by using a high-purity semi-insulating(HPSI) 6H-SiC substrate which is also
free from the interference by the Ndonor signals.

x 40(b)PcPaPbPbPa(c)(d)

Pk2

Pk1

334 336 338 340
Magnetic field [mT]Fig. 2. EPR spectra (B//[0001]): (a) P-ion implanted
and annealed n-6H-SiC, (b) P-ion implanted andannealed HPSI-6H-SiC (60 K), (c) P-ion implanted andannealed HPSI-6H-SiC (6 K), (d) P-doped CVD grown
epitaxial layers of 6H-SiC (6 K).

Table 1. EPR parameters (the g and 31P hf tensors) of the P centers.
CenterSite

Symmetryg||g⊥

A|| (mT)A⊥ (mT)

3C-SiC:P (5 K)
4H-SiC:Pk (6 K)6H-SiC:Pk1 (5 K)
6H-SiC:Pk2 (5 K)
6H-SiC:Pa (60 K)
6H-SiC:Pb (60 K)
6H-SiC:Pc (60 K)

Si
Si(k)
Si(k1,2)
Si(k1,2)C(h) [7]C(k1,2) [7]
?

D2d
C3v
C3v
C3v
C3v
C3v
C3v

2.00512.00652.00462.00392.00392.00402.00442.00462.00062.00282.00252.00292.00272.0026

1.9×10-2
0.294
0.113
0.607
5.51
5.09
0.91

−4.6×10-3
0.21
0.023
0.220
5.43
5.09
0.66

labeled P in 3C-SiC, Pk in 4H-SiC, and Pk1 and Pk2 in 6H-SiC, respectively, based on their assignmentof the silicon sites to be described later. Pk1 and Pk2 correspond to P1 and P2, respectively.
The observation of the resolved 31P (I=1/2, 100%) hyperfine (hf) structure in the case of Pk1, Pk2 in
6H-SiC and Pk in 4H-SiC and the observation of 31P ENDOR in the case of P in 3C-SiC by using
pulsed ENDOR technique confirm that the centers are related to P. The symmetry of the P donors wasdetermined by the angular dependence of the EPR line positions.
While each silicon and carbon has two inequivalent lattice sites called hexagonal (h) andquasi-cubic (k) sites in 4H-SiC, only one kind of EPR spectrum was observed, which we assign to bearising from P shallow donors at Si(k) site (Pk). An ab initio calculation using a 576-atom supercell
has predicted that A1 is lower than E for Pk, while E is lower than A1 for P at the Si(h) site (Ph). The
calculation has predicted that the spin density on the axial carbon is much larger than that of the threebasal carbon atoms for the ground state A1 of Pk, while the spin density on the three basal carbon
atoms is much larger than that on the axial carbon atom for the ground E state of Ph. The observationof one set of 13C hf satellite arising from one nearest-neighbor carbon atom as shown in Fig. 1(b)supports the assignment that the observed EPR spectrum is arising from Pk. The absence of the EPR
spectrum from Ph is ascribed to the degenerate ground state E which is likely to be broadened by localstrains.
As shown in Fig. 2(d), two doublets, Pk1 and Pk2 (Pk2a) are observed in 6H-SiC for B//[0001]. Withrotating the magnetic field away from [0001], the intensity of the With rotating from [0001], theintensity of the Pk2a doublet decreases and the Pk2b doublet appears. The intensity of the Pk2a/Pk2bdoublets strongly depends on the direction of the magnetic field. The ESEEM (electron spin echo
envelope modulation) which is arising from the interference between the allowed (∆MS=±1, ∆mI=0)
and forbidden (∆MS=±1, ∆mI=±1) transitions confirmed that the Pk2a/Pk2b doublets in 6H-SiC (i.e.
sPc1/sPc2 [6], which were previously assigned to two P centers at the two quasi-cubic sites [8]) belong
to the allowed and forbidden transition of the same P center (Pk2). The analysis including the nuclear
Zeeman interaction provides a satisfactory explanation for the angular dependence of the intensity of
the Pk2a/Pk2b doublets. The observation of 13C satellites in Pk1, Pk2 suggests that P occupies silicon site.
From the analogy to Pk in 4H-SiC, the Pk1 and Pk2 doublets are assigned to the P shallow donors atSi(k1, k2) sites.
The assignment of P in 3C-SiC to be arising from P at Si site is based on the similarity of thetemperature dependence of the EPR signals to that of Pk in 4H-SiC and Pk1, Pk2 in 6H-SiC. The EPRparameters obtained in the present work are listed in Table I. The 31P hf-tensors of P in 3C-SiC andPk1 in 6H-SiC were determined by pulsed-ENDOR and that of Pk2 in 6H-SiC by ESEEM.
In Figs. 2(b) and 2(c), EPR spectra of HPSI 6H-SiC implanted with high-energy P ions andsubsequently annealed are shown. In contrast to the case of n-6H-SiC substrate shown in Fig. 2(a), theabsence of the signals from the N shallow donors allows us to examine the presence of the bound P
donors at silicon sites. In the spectrum taken at 60 K, the Pa, Pb, and P+V (here, we tentatively label
the spectrum Pc in Figs. 2(b) and 2(c) since the microscopic origin has not been established). In thespectrum taken at 6 K, the signals Pk1 and Pk2 arising from the P shallow donors at the silicon sites arenoticed overlapped with a broad signal in the central part of the spectrum. The intensity of the signals
from P shallow donors substituting C (Pa, Pb) is much stronger than that of the P shallow donors
substituting Si (Pk1, Pk2). The observable temperature range of the EPR signals suggests that Pk1, Pk2
are shallower than Pa, Pb. It should be noted that the predominant intensity of Pa, Pb over Pk1, Pk2
might be either due to the much preferable incorporation into the carbon sites in the doping process ordue to the relative position between the Fermi level and (+|0) causes most of the P donors at siliconsites take the charge state of EPR-inactive. Our results indicate that ion implantation and subsequent
high-temperature annealing (1650 °C) also drive a considerable fraction of P into the carbon sites.
Summary
The use of P-doped CVD grown epitaxial layers which were practically free from N impurities
allowed us to identify the EPR spectra originating from P shallow donors at silicon sites, P in 3C-SiC,
Pk in 4H-SiC, and Pk1 and Pk2 in 6H-SiC. In the high-energy P-ion implanted HPSI 6H-SiC which wasalso free from the interference of the signals of the N shallow donors, EPR signals of P shallow
donors both at carbon and silicon sites were observed, with the former with much stronger intensity.
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