APPLIED PHYSICS LETTERS 97, 041911 共2010兲

Fluorine-vacancy defects in fluorine-implanted silicon studied by electron
paramagnetic resonance
T. Umeda,1,a兲 J. Isoya,1 T. Ohshima,2 S. Onoda,2 N. Morishita,2 K. Okonogi,3 and
S. Shiratake3
1

University of Tsukuba, Tsukuba 305-8573, Japan
Japan Atomic Energy Agency, Takasaki 370-1292, Japan
3
Elpida Memory, Inc., Higashihiroshima 739-0198, Japan
2

共Received 29 December 2009; accepted 11 July 2010; published online 30 July 2010兲
An electron paramagnetic resonance 共EPR兲 study on fluorine-vacancy defects 共FnVm兲 influorine-implanted silicon is demonstrated. Fluorine implantation is an important technology for Si
microdevices and EPR measurements showed that this process created a variety of FnVm defects ofdifferent sizes 共V2, V4, and V5兲. In FnVm, a Si–F bond exhibited a different chemical nature compared
to a Si–H bond in hydrogen-vacancy complexes. The most primitive defect was FV2 共F0 center兲 andthe final types were FnV5 共F1 center兲 and FnV2 共F2 center兲 which increased in annealing processes
as low temperature as 200 ° C. © 2010 American Institute of Physics. 关doi:10.1063/1.3473763兴

a兲Electronic mail: umeda@bk.tsukuba.ac.jp.

0003-6951/2010/97共4兲/041911/3/$30.00

luted hydrogen fluoride treatments on the sample surfaces,
EPR spectra were measured using a Bruker Bio-Spin E500
X-band spectrometer with 100 kHz magnetic-field modulation of 0.05 to 0.1 mT width.
EPR spectra of six samples are shown in Figs. 1共a兲–1共e兲.
In the lowest-dose sample 共a兲, no significant effects of F
were detected. The spectrum is dominated by the well2(F/cm )

P3 P3

P3 P3

11

9.437GHz
120K, B||[100](a) 5×10(b) 5×10

12

F0

F0
F3

EPR intensity (arb.units)

Fluorine forms a strong bond with silicon,1 similar tohydrogen 共Si–H兲, which is useful for Si technologies.2 Fluorine also has great benefits for suppressing transient enhanced diffusion of boron atoms.1,3–6 This property is believed to be due to the formation of fluorine-vacancy defects
共FnVm兲, which suppress the activity of interstitial 共I兲-type
defects interacting with boron atoms.1,3,4 First-principles calculations predicted that the most stable FnVm defects are F4V
or F6V2 which are fully passivated by F atoms.1,3,4 On theother hand, positron annihilation spectroscopy 共PAS兲 revealed two preferential sizes of vacancies 共approximately V2
or V4兲.5,6 Secondary ion mass spectroscopy showed a highthreshold temperature 共⬃550 ° C兲 for fluorine diffusion,5suggesting that F atoms become immobile due to strong Si–F
bonds in FnVm.1
In this Letter, we present a different approach to investigate fluorine and FnVm defects using electron paramagnetic
resonance 共EPR兲, which provides a more concrete view offluorine behaviors in Si. We found at least four types ofparamagnetic FnVm defects of different sizes 共V2, V4, and V5兲
in as-implanted and in subsequently annealed Si. Using EPR,
their behavior could be studied individually as follows: 共1兲
two distinct types 共V2 and V5兲 were stable, providing microscopic models for the previous PAS data, 共2兲
a different nature of the Si–F bond was found in comparison
with the Si–H bond, and 共3兲 low-temperature motion
共⬍20 ° C兲 and diffusion 共⬃200 ° C兲 of FnVm were revealed.
The starting substrates 共0.3⫻ 0.8⫻ 0.01 cm3兲 were
phosphorus-doped float zone Si共100兲 with a high resistivity
共⬎1000 ⍀ cm兲 and integrated-circuits-grade 30-cmdiameter Czochralski-Si共100兲 with an epitaxial layer andphosphorus or boron doping 共5 – 15 ⍀ cm兲. For the former
wafers, multiple F implantation was performed on both oftheir wide faces by 5 ⫻ 1011 to 5 ⫻ 1014 F / cm2 with 7.5 to 15
MeV,
which
was
designedforhigh-sensitive
EPR measurements. A more realistic 共shallow and single兲
F implantation2,3,6 was examined on the latter wafers by 1
⫻ 1012 to 1 ⫻ 1014 F / cm2 with 20 keV. These two processes
created F profiles as shown in the inset of Fig. 1. After di-13(c) 5×10(d) 5×10

F3 F4 F3 F3
F2

14

F6 F5

14

(e) 1×10

n-type×10

17

F1
F2
14

(f) 5×10 with
300°C anneal

F6

F4
332

333

p-type×5

334

335

336

3

F profile (x10 /cm )9–15MeV
15
10
20keV
5
0
0
0.2 5
10
depth (µm)
225°C
337

338

339

340

Magnetic field (mT)FIG. 1. 共Color online兲 EPR spectra of F-implanted Si. F doses are 共a兲
5 ⫻ 1011 F / cm2 at 6 energies, 共b兲 5 ⫻ 1012 F / cm2 at 5 energies, 共c兲
5 ⫻ 1013 F / cm2 at 5 energies, 共d兲 5 ⫻ 1014 F / cm2 at 3 energies, 共e兲
1 ⫻ 1014 F / cm2 at 20 keV 共for p- and n-type wafers兲, and 共f兲 with isochronal annealing 共225 and 300 ° C兲. The inset shows F profiles for the 20 keV
共1 ⫻ 1013 F / cm2兲 and high-energies 共5 ⫻ 1013 F / cm2兲 implantations simulated by the SRIM code. Peak F densities are 共a兲 1.2⫻ 1016, 共b兲 1.2⫻ 1017, 共c兲
1.2⫻ 1018, 共d兲 1.2⫻ 1019, and 共e兲 1.7⫻ 1019 F / cm3. Labels of F0 to F6
indicate a series of new EPR centers originating from FnVm defects. In 共a兲
and 共b兲, a weak signal at 336.2 mT was different from F2, judging from its
isotropic angular dependence. It is probably due to a surface-damage center.

97, 041911-1© 2010 American Institute of Physics

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041911-2

Appl. Phys. Lett. 97, 041911 共2010兲

Umeda et al.
(a)50K
14(b)295K
2

5×10 F/cm

5×10 F/cm

13

2

100%
50

F0

20
10
5

F3/P3
Magnetic field (mT)2

5×10 F/cm
300°C anneal

340

339

nal in 共e兲. A reason for this absence will be presented later.
The F0 spectrum was not detectable above 200 K, and it
exhibited a thermally activated reorientation behavior above
60 K. Thus, in Fig. 2共a兲, the angular map of F0 is examinedat 50 K. The F0 center shows two separated patterns due to a
19
F hfs with monoclinic-I symmetry close to trigonal one.
Just in the middle of the F0’s patterns, a F2 pattern with thesame symmetry was observed. Table I shows the determined
spin-Hamiltonian 共SH兲 parameters of F0 and F2. Both centers have similar g tensors of Pb-center-like 共gX ⬇ gY ⬅ g⬜
⬇ 2.007, gZ ⬅ g储 ⬇ 2.001, the g储 axis is nearly parallel to
具111典兲, indicating neutral Si dangling-bond 共DB兲 nature forboth origins. The F2 spectrum became larger with higher F
doses 关Fig. 1共d兲兴 and still larger after annealing such that F0
decreased or vanished 关Fig. 1共f兲兴. This suggests that F2 is asubsequent defect of F0 with more accumulation of F atoms.
The major difference between F0 and F2 is 19F hfs. The 19F
hyperfine tensor 共A兲 of F0 shows an axial symmetry due toan F 2p orbital that is parallel to the g储 axis 共i.e., the DB
orbital兲. We constructed neutral FnV2 models, based on all
the data, for the origins of F0 共n = 1兲 and F2 共n = 2 – 5兲. The
respective models are shown in Figs. 3共a兲 and 3共b兲. They canreasonably account for all the facts as follows. 共1兲 FV2 关Fig.
3共a兲兴 contains a single neutral DB 共S = 1 / 2兲 as well as 共2兲 asingle Si–F bond parallel to the DB orbital. 共3兲 The V2 distance 共0.59 nm兲 allows a weak bond to form between the DB
and F atom 关Fig. 3共a兲兴, extending the spin density over the19
F atom. 共4兲 However, this weak bond will be broken if F
atoms are then added into the vacancy 关Fig. 3共b兲兴. 共5兲 Thismechanism controls the appearance of 19F hfs, resulting in
the two distinguishable FnV2 centers. 共6兲 As similarly to V2
共the G6 center兲 at 40–110 K,10 a thermally activated rearrangement can be expected for F0 among three equivalent
DB-F pairs 共Si1 – F – Si4, Si2 – F – Si5, and Si3 – F – Si6兲,11 causing the characteristic temperature dependence at 60–150 K.
At higher temperatures 共⬎200 K兲, F atoms seemed to move
inside the vacancy, resulting in the broadening and disappearance of F0 as well as a trigonal average of F2 关see Figs.
2共b兲 and 2共c兲, Table I兴. Such a motion was also observed forhydrogen in a vacancy 共⬎200 K兲.12
It is quite interesting to compare the F0/F2 centers 共neutral FnV2兲 with neutral hydrogen-vacancy defects12 such as(c)295K

14

F2

338
F2

F5

F2

2

337

336
F5
335

F4

F6

F0

334

F1

F5

F4

F4
F1

0

90

0
90
Rotation angle (°)

090

FIG. 2. 共Color online兲 Angular maps of F0–F4 centers 共FnVm defects兲 at
9.437 GHz. Magnetic field was rotated from 关100兴 共0°兲 to 关011兴 共90°兲. Graysymbols indicate experimental peak positions and their normalized intensities 共the largest peak= 100%兲 are expressed by a gray scale shown in inset.
Solid lines are simulated by SH parameters in Table I. There are still untraced angular maps 共e.g., F5 and F6兲, suggesting more variety of minor
FnVm defects.

known P3 center 共neutral 兵110其-planar V4 chain, an electron
spin S = 1兲.7–9 I-type defects such as P6 共I2兲, H8 共I-related兲,and B3/4/5 共I3–4兲 共Refs. 7–9兲 were not detected in our asimplanted samples or in the annealing study. In the next
sample 共b兲, a doublet signal with S = 1 / 2 was observed. This
doublet is ascribed as a hyperfine splitting 共hfs兲 of 19F
共nuclear spin I = 1 / 2, natural abundance= 100%兲. We named
this “F0.” Following to F0, other signals labeled “F1” to“F6” were observed in 共c兲–共e兲. These signals are most probably due to subsequent defects of F0 that are associated withmore vacancies and/or F atoms. Basically, such a formation
behavior was common to the deep implantations 共b兲⫺共d兲 andshallow implantations 共e兲, except the absence of the F3 sig-

TABLE I. SH parameters of FnVm defects and related V-type defects. Total SH is given by H = ␮BS · g · B + S · D · S + S · A · I − gn␤nI · B, where g is a g tensor, D
is a fine interaction tensor 共excluded for spin-1/2 centers兲, and A is a hyperfine tensor 共included only for F0 and S1a兲 共Refs. 7–9 and 12兲. Principal values of
A and D are expressed in mT. D and g tensors of F3 and P3 are identical. 共m兲 and 共t兲 denotes monoclinic-I and trigonal symmetries. Measured temperaturesare specified only for temperature-dependent spectra. Anneal and growth temperatures 共signal was reduced or increased in these ranges, respectively兲, are also
summarized.
CenterS

TensorX

Y

Z共 储 兲

␪

Anneal/growth temperature

F0 共FV02兲

1/2 共m兲S1a 共HV02兲

1/2 共m兲
1/2 共t兲
1/2 共m兲F3 共FnV04兲
P3 共V04兲
F1 共FnV5兲
P1 共V−5 兲
F4

1 共m兲
1 共m兲
1/2 共m兲
1/2 共m兲
1/2 共t兲2.0070
4.23
2.00802.00812.01100.050
2.0102⫺8.34
2.00882.00912.01122.00674.15
2.00722.00812.01000.057
2.0099⫺7.92
2.01242.01272.01122.00066.27
2.0005
1.99952.00080.13
2.001016.26
2.00462.00502.001930.2°
210.4°
32.9°
35.26°
31.0°
4.5°
34.4°
−6.3°
17.5°
15.0°
35.26°

220– 280 ° C / none

F2 共FnV02兲

g共50 K兲
A共 19F兲 a
g共50 K兲
g共295 K兲
g共200 K兲
A共 1H兲 ag
D
g共295 K兲
g共320 K兲
g

a

c

b

d

Absolute values.
Reference 12.

300–over 410 ° C / 200– 300 ° C
180– 280 ° C / none b
F3: 200– 230 ° C / none
P3: 120– 170 ° C / none c
300–over 410 ° C / 200– 300 ° C
300– 460 ° C / 120– 250 ° C d
250–over 410 ° C / 200– 250 ° C

References 7 and 9.
Reference 8.

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041911-3

Appl. Phys. Lett. 97, 041911 共2010兲

Umeda et al.

35.3º(d) F1(FnV5)DB(c) F3(FnV4)

Si

DZ
-6.3º

34.4º

Z
z

(b) F2(FnV2)

1

y
[011]

θ

-Y

32.9º
3
g|| 2

(a) F0
(FV2)

F

17.5º

[111]
X

*30.4º 4

30.2º 5 A||

[100]
x

6
FIG. 3. 共Color online兲 Atomic models for F0–F3 centers drawn in the
Si共01̄1兲 plane. Cartesian coordinates 共xyz兲 and principal coordinates of SH
tensors 共XYZ兲 are also defined. Angles ␪ are shown for main principal directions 共Z or 储兲 of g, A, and D tensors. F0, F1, and F2 have a single DB
共S = 1 / 2兲, while F3 contains two DBs 共S = 1兲. In 共a兲, a weak bond between
DB and F atom is drawn by a dashed line, which generates 19F hfs. ⴱ␪ angle
from the -y axis.

HV2 共the S1a center兲. Table I shows that their g tensors belong to the same class. However, interestingly, there is a big
difference between H and F. Hydrogen and DB does notform a weak bond even in a monovacancy 共HV兲. Therefore,both the HV and HV2 spectra revealed only very weak pointdipole interaction of 1H nuclear spin 共I = 1 / 2, naturalabundance= 99.9%兲,12 which is one or two orders of magnitude weaker than the 19F hyperfine interaction of FV2 关A共 1H兲
values are 0.1− 0.3 mT for HV 共Ref. 12兲, and also see Table
I for HV2 and FV2兴. The observed 19F hfs can be explainedby the direct distribution of the spin density on 19F. The
formation of a weak bond between the DB and F atom enables this. Based on the standard linear combination of
atomic orbitals analysis,13 1.4% of the spin density 共F 2s= 0.25% and F 2p = 1.1%兲 is estimated on the F atom for F0.
Note that the F0 center is one of the most primitive
defects due to F implantation. In the 5 ⫻ 1012 F / cm2 sample
关Fig. 1共b兲兴, 48% of F atoms 共48% ⫻ 5 ⫻ 1012 ⫻ 10 F / cm2兲
were consumed by F0 共FV2兲. Previous PAS studies also revealed V2-type defects 共S-parameter= 1.04兲 in their asimplanted samples.5,6With an increased F dose, we could clearly detect the F1and F3 spectra. A typical F3 spectrum was observed in the
5 ⫻ 1013 F / cm2 sample 关Fig. 1共c兲兴. Its angular pattern关Fig. 2共b兲兴 and SH parameters are indistinguishable fromthose of the P3 center 共V04兲. However, in the isochronal anneal study 共30 min at 25 ° C step兲, we found a higher thermal
stability for F3 共annealed at 200– 230 ° C兲 compared to P3
共120– 170 ° C.7,9兲 Therefore, we suggest that the F3 center
accumulates F atoms into V4. Our proposed model for F3 isshown in Fig. 3共c兲. Since S = 1 for F3, there should be twoneutral Si DBs separated by the V4 distance 共0.97 nm兲. Since
these DBs could be easily charged with doping for the caseof P3,7 the F3 spectrum could disappear in the n- and p-typesamples 关Fig. 1共e兲兴. It is notable that FnV4 共F3兲 was lessstable than FnV2 共F2兲, because the F2 spectrum remained
after F3 completely vanished 关at 225 and 300 ° C, Fig. 1共f兲兴.
FnV4 seems to be decomposed into FnV2.
In the isochronal annealing study, the most stable center
was “F1.” A typical spectrum for it appears in Fig. 1共f兲. The
angular map of F1 is clearly traced in Fig. 2共c兲. The determined g tensor of F1 was close to that of the P1 center.7,8
The P1 center has a single neutral DB 共S = 1 / 2兲 and has been
identified as a negative nonplanar V5 cluster.8 Based on the
P1 model, we assign the FnV5 model to the origin of F1, asshown in Fig. 3共d兲. In the 5 ⫻ 1014-F/ cm2 sample, the densityof F1 was maximized to 1.3⫻ 1014 / cm2 after 300 ° C anneal.
In this situation, if one assumes 50% decoration of F atoms
for F1 共six F atoms/V5兲, 52% of implanted F atoms 共52%
⫻ 5 ⫻ 1014 ⫻ 6 F / cm2兲 would accumulate into F1. Further
annealing decreased the F1 centers, however, they werestable and remained by 8 ⫻ 1014 / cm2 at 410 ° C. In previous
PAS studies, the maximum vacancy size was found to beabout V4 共S-parameter= 1.05兲 after 700 ° C annealing.5 The
F1 center is the most probable candidate for such large,
stable defects.
In summary, using EPR, we found a variety of FnVm
defects 共F0–F6兲 in F-implanted Si and in the subsequent annealing study. The most primitive center was FV2 共the F0
center兲 observed in the initial stage of F implantation andthis center revealed a characteristic 19F hfs. With increasing
the F dose or annealing the sample, other FnVm defects withmore accumulation of F atoms were observed. The moststable center was the F1 center 共FnV5兲, and the next one was
the F2 center 共FnV2兲. FnV3 defects were not found. FnV4
defects were probably detected as the F3 center.
We thank J. Ohsaki 共University of Tsukuba兲 for his help
in experimental works and also S. Sakurai 共Elpida Memory,
Inc.兲 for sample preparations.
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11
In this model, we predict a “cooperative” jump of a DB and a F atom. Thisfeature will be also as a consequence of the weak-bond formation in FV2.
In fact, the HV2 center, which has an atomic structure just analogue toFV2, did not show such a motional effect 共Ref. 12兲, most probably because
there was no weak bond between a DB and a H atom.
12
B. Bech Nielsen, P. Johannesen, P. Stallinga, and K. Bonde Nielsen, Phys.
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1
2

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