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Mechanism of formation of ultrashallow thermal donors in carbon-doped oxygen-richmonocrystalline silicon preannealed to introduce hydrogen

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REGULAR PAPER

Japanese Journal of Applied Physics 54, 101302 (2015)
http://dx.doi.org/10.7567/JJAP.54.101302

Mechanism of formation of ultrashallow thermal donors in carbon-doped oxygen-richmonocrystalline silicon preannealed to introduce hydrogen
Akito Hara and Teruyoshi Awano
Faculty of Engineering, Tohoku Gakuin University, Tagajo, Miyagi 985-8537, Japan
E-mail: akito@mail.tohoku-gakuin.ac.jp
Received April 23, 2015; accepted July 25, 2015; published online September 18, 2015
We previously reported on ultrashallow thermal donors (USTDs) in carbon-doped oxygen-containing monocrystalline silicon (Czochralski-grown,
CZ-Si) crystals that were preannealed to introduce hydrogen at 1300 °C, and then annealed at 480 °C. In this study, the formation mechanism of
the USTDs was evaluated. It was observed that an increase in the intensity of UTSDs leads to a reduction in that of hydrogen-related shallow
thermal donors [STD(H)s], and the sum of the area intensities of the lines in the transmission spectra of USTDs and STD(H)s is nearly constantwhen the silicon crystals are annealed for longer than 10 h at 480 °C. We also found some thermally activated processes linked to the formation of
USTDs. We thus conclude that the mechanism is composed of the high-speed formation of STD(H)s in the ﬁrst stage and carbon modulation of theelectronic structure of STD(H)s in the second stage. © 2015 The Japan Society of Applied Physics

1.

Introduction

Table I.

We previously reported on ultrashallow thermal donors(USTDs) in carbon- and nitrogen-doped monocrystalline
Czochralski-grown silicon (CZ-Si) crystals after annealing at
480 or 600 °C.1) Many new lines between 100–300 cm−1 inthe transmission spectra, distinct from those arising fromhydrogen-related shallow thermal donors [STD(H)]2,3) andnitrogen oxygen (NO) complexes,4–7) have been attributed
to the optical transition of hydrogen-like donors from theground state to the excited state. Some of these show negative
central-cell correction and, to the best of our knowledge,
correspond to the shallowest energy levels among all previously reported hydrogen-like donors in Si crystals.1) However, it has been pointed out that the samples analyzed inRef. 1 were contaminated with hydrogen.8,9) In a anotherstudy,10) the annealing gas was carefully controlled. Table I
shows the peak positions observed in the Ref. 10, whichcorrespond to USTDs-1 to -5 in Ref. 1, hydrogen-related
thermal donors [STD(H)s]-2 and -3, and USTDs-1A and -2A.
We observed that the spectral lines identiﬁed as USTDs-1 to-5 formed at 480 °C in Ref. 1 were also formed in carbondoped monocrystalline CZ-Si, which was preannealed to
introduce hydrogen, and that the nitrogen doping status was
independent of the presence of USTDs-1 to -5.10) Additionally, USTDs-1A and -2A, which are dependent on nitrogen concentration, were also generated in the carbon- and nitrogendoped CZ-Si crystals at approximately 600 °C.10) The peak
positions of USTDs-1 to -5 were clearly diﬀerent from thoseof USTDs-1A and -2A, as shown in Table I.
Solar-cell-grade multicrystalline silicon (mc-Si) includes
high concentrations of contaminations during solidiﬁcation.
Carbon, oxygen, and nitrogen are typical light-mass contaminants in mc-Si. Hydrogen is also an important element in
mc-Si solar cells because hydrogen gas is used in thin-ﬁlm
growth methods, which are frequently applied in device
fabrication.
From the viewpoint of light-mass impurities, the majordiﬀerence between mc-Si and CZ-Si for large-scale integration (LSI) is the concentrations of carbon and nitrogen. The
carbon and nitrogen concentrations in LSI-Si are controlled
to levels below the detection limits of the elements ([C] <1016 cm−3 and [N] < 1014 cm−3). On the other hand, the1s–2p± transition of USTDs, STD(H)s and USTDs-1A and 2A.10)

Donor

2p±
(cm−1)

Donor2p±(cm−1)

Donor2p±(cm−1)USTD-1

173.3

STD(H)-2

246.9

USTD-1A

216.0

USTD-2
USTD-3

175.8
179.6

STD(H)-3

241.1

USTD-2A

192.1

USTD-5202.4

carbon and nitrogen concentrations in mc-Si are on the orderof 1017 and 1015 cm−3, respectively. It has been reported thatcomplex centers or aggregative defects composed of lightmass elemental impurities degrade the yield and reliability of
Si-LSI. High concentrations of light-mass elemental impurities in mc-Si are thus also expected to have negative eﬀects
on the performance, yield, and reliability of mc-Si solar cells.
Therefore, the evaluation of reactions between these impurities in mc-Si is necessary for the development of materials
for mc-Si solar cells.11,12)
In this study, we evaluated the formation of USTDs-1
to -5 at various durations and temperatures of USTD
formation annealing, using carbon-doped CZ-Si crystals
that were preannealed to introduce hydrogen.13–15) As weevaluated the formation mechanism of USTDs using nonnitrogen-doped CZ-Si, that USTDs-1A and -2A is beyond thescope of this study.
2.

Experimental methods

Two types of III- and V-dopant-free CZ-Si crystals wereused as precursors. The ﬁrst was a low-carbon-content CZ-Si,
with [C] < 1 × 1016 cm−3 and [O] = 1.6 × 1018 cm−3 (oldAmerican Society for Testing and Materials), and the second
was carbon-doped CZ-Si with [C] = (1.8–2.3) × 1017 cm−3and [O] = 1.6 × 1018 cm−3. The carbon-doped CZ-Si wasgrown from carbon-powder-doped molten Si. Samples witha thickness of 2.0 mm were prepared; far-infrared (far-IR)
optical absorption measurements were performed on beamline 6B (resolution, 0.5 cm−1, temperature, 8–10 K) at the
UVSOR facility of the Institute for Molecular Science.
Figure 1 shows the optical transmission spectra of the
carbon-doped CZ-Si crystals. The ﬁrst sample was annealed

101302-1© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 101302 (2015)

A. Hara and T. Awano

0.8

2.8
2.7

1300°C 60min. in dry oxygen+
480°C 10h in wet oxygen

0.7

USTD G1
2p0

2.5

480°C65h
2p

2p

2.3

480°C52h

2.2

0.5

2.1

1300°C 60min. in wet oxygen+
480°C 10h in wet oxygen

2.0

480°C40h

1.9Transmittance (arb. unit)

0.4
0.3
0.2
0.1

STD(H)-3
STD(H)-2

2p0

2p

3p

1.8
1.7

480°C30h

1.6
1.5
1.4

480°C20h

1.3
1.2
480°C10h

1.1
1.0

0
100

150

200
250
Wave number (cm-1)

0.9

300

480°C5h0.8
0.7

Fig. 1. Transmission spectra for STD(H)s and USTDs. The ﬁrst Si sample
was annealed at 1300 °C for 60 min in wet oxygen, quenched in air, andfurther annealed at 480 °C for 10 h in wet oxygen (hydrogen-doped Si). The
second sample was annealed at 1300 °C for 60 min in dry oxygen, quenchedin air, and further annealed at 480 °C for 10 h in wet oxygen.

480°C3h

0.6
0.5
0.4
0.3

USTD-5

USTD-3
USTD-2
USTD-12p0

2p±

at 1300 °C for 60 min in wet oxygen, quenched in air, andthen annealed further at 480 °C for 10 h in wet oxygen. The
second sample was annealed at 1300 °C for 60 min in dry
oxygen, quenched in air, and then annealed further at 480 °C
for 10 h in wet oxygen. In these processes, the wet oxygen
was prepared by passing dry oxygen gas through boilingwater so as to include moisture in the atmosphere. The difference in the intensities of the peaks correlating to STD(H)sis evident. It has been reported that STD(H)s are donors withhydrogen-like energy levels and are generated in hydrogendoped CZ-Si crystals at approximately 450 °C.2,3,8,9) Figure 1
indicates that it is possible to introduce hydrogen into CZ-Si
by preannealing the Si crystals at 1300 °C for 60 min in wet
oxygen. Herein, monocrystalline Si prepared by the abovedescribed hydrogen doping process is referred to as hydrogen-doped Si. This hydrogen doping method is a slight
modiﬁcation of the method proposed by Martynov et al.3)
Results

Figure 2 shows the behavior of USTDs and STD(H)sobserved in the carbon- and hydrogen-doped CZ-Si annealed
at 480 °C in wet oxygen for various durations. In this study,
we divided USTDs-1 to -5 into two groups of USTD group 1(USTD G1) and USTD group 2 (USTD G2), as shown in
Fig. 2, in order to simplify the analysis of the formation of
USTDs. Here, USTDs-1 to -3 and USTD-5 were classiﬁed
into USTD G1 and USTD G2, respectively.
We think that USTDs-1, -2, and -3 are a family of donor
species, because their energy levels are very similar. The
USTDs-1 and -2, with shallow energy levels, gradually
reduce in intensity, while USTD-3 with the deepest energy
level survives after long-term annealing. This behavior
diﬀers from conventional cluster (family) donors, such as
thermal double donors (TDDs) and NO complex donors, in

3p±

STD(H)-3 2p0
STD(H)-2

0.2

3.

USTD G2

2.4

0.6
Transmittance (arb. unit)

2p0

2.6

0.1

100

150

200

3p

2p

Wave number (cm-1)

250

300

Fig. 2. Variation in USTD and STD(H) spectral intensities in carbon- andhydrogen-doped monocrystalline CZ-Si as a function of annealing duration at
480 °C in wet oxygen.

which the donors with shallower energy levels are more
likely to survive after long annealing times. In contrast, one
strong line in the transmission spectra, corresponding to
USTD-5, dominates in USTD G2 from the initial stagesof annealing. The energy level of USTD-5 diﬀers from thatof USTD G1, thus, its the assignment to a diﬀerent group.
The STD(H) spectral line intensities are high even after
short-term annealing for 3 h, and are maximum after 10 hannealing. This indicates that the formation speed of
STD(H)s is much higher than that of USTDs. Notably, theSTD(H) lines decrease in intensity as the annealing continues
beyond 10 h.
Figure 3 shows the spectral intensity of USTD andSTD(H) lines observed in non-carbon-doped and hydrogendoped CZ-Si after annealing for 10 and 48 h at 480 °C. The
USTD lines do not appear even after long-term annealingfor 48 h, while the intensity of the STD(H) lines after 48 h
is nearly equal to that after 10 h annealing. This is markedly
diﬀerent from the annealing behavior of STD(H)s in the
carbon- and hydrogen-doped CZ-Si, as shown in Fig. 2.
Carbon impurities can thus be concluded to be necessary in
the generation of USTD spectral lines and reduction in theintensity of STD(H) spectral lines.
Figure 4 shows the annealing temperature dependenceof the intensity of USTD and STD(H) lines in carbon- andhydrogen-doped CZ-Si. The annealing duration was ﬁxed at
40 h, but diﬀerent annealing temperatures of 450, 480, and101302-2© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 101302 (2015)

A. Hara and T. Awano

1.2

1.8

480°C40h+650°C15min.

1.7
1.6

480°C48h

480°C40h+640°C15min.

1.5
1.4
1.3

0.8

0.6

Transmittance (arb. unit)

Transmittance (arb. unit)

1.0

480°C10h

0.4

0.2
2p0
STD(H)-3
STD(H)-2

150

1.1
1.0

480°C40h+600°C15min.

0.9
0.8

480°C40h+550°C15min.

0.7
0.6
0.5

2p

3p

480°C40h

0.4
0.3

0.0
100

480°C40h+620°C15min.

1.2

200

250

300

0.2

Wave number (cm-1)

USTD-3
USTD-2
USTD-1
2p0

0.1
0.0

Fig. 3. Intensities of STD(H) and USTD spectral lines formed inhydrogen-doped, non-carbon-doped CZ-Si after annealing for 10 and 48 h
at 480 °C.

USTD-53p±
2p±
STD(H)-3 2p0
STD(H)-2

(0.1)

2p

3p

(0.2)
100

150

200

250

300

Wave number (cm-1)
1.3
1.2

2p0

1.1

USTDs
Group-1
Group-2

Fig. 5. Variation in the intensities of USTD and STD(H) spectral lines
formed in carbon- and hydrogen-doped CZ-Si after annealing at 480 °C for
40 h followed by 15 min annealing at higher temperatures, as a function of
wet-oxygen annealing temperature.

510°C40h

2p0

2p

2p

Transmittance (arb. unit)

1.0
0.9
480°C40h

0.8
0.7
0.6
0.5

450°C40h

0.4
0.3
0.2
0.1

USTD-3
USTD-2
USTD-1
2p0

USTD-53p±2p±2p0
STD(H)-3
STD(H)-2

0.0
100

150

200

2p

Wave number (cm-1)

3p

250

300

Fig. 4. Variation in intensities of USTD and STD(H) spectral lines formedin carbon- and hydrogen-doped monocrystalline CZ-Si after annealing for
40 h at 450, 480, and 510 °C in wet oxygen.

510 °C were used. The USTD lines for samples annealed at
450 °C appear to be equivalent to those for samples annealed
at 480 °C for 20 h, as shown in Fig. 2. On the other hand, the
USTD lines of samples annealed at 510 °C for 40 h appear
to be similar to those of the samples annealed at 480 °C for
65 h, compared with the other lines of USTDs at 480 °C.
Thus, the USTD lines of samples with shorter annealing
durations at high temperatures are similar to those of samples
with longer annealing durations at low temperatures; thisindicates that thermally activated processes are linked withthe formation of USTDs. Figure 4 also indicates that two

USTD spectral lines become dominant as the annealing
temperature increases. This is consistent with the results
shown in Fig. 2.
Figure 5 shows the variation in the intensity of USTD andSTD(H) lines in carbon- and hydrogen-doped CZ-Si after
annealing at 480 °C for 40 h followed by 15 min annealing at
higher temperatures. The ﬁgure shows that the USTDs-1 and-2 lines gradually decrease in intensity as the second-phaseannealing temperature increases, and at the highest temperatures, only USTDs-3 and -5 lines are observed. This isconsistent with the results in Figs. 2 and 4.
To investigate the generation of USTDs and the reduction of STD(H)s at 480 °C for various annealing times, we
examined the variations in area intensity of spectral peaks
correlating to USTD G1, USTD G2, and STD(H)s. For
the area intensity of UTSD G1, 1s–2p± spectral lines wereapplied, while for the area intensity of USTD G2, 1s–2p0
lines were applied. Because the 1s–3p± spectral lines of
USTD G1 and the 1s–2p0 lines of the STD(H)s both
overlap with the 1s–2p± lines of USTD G2, we selected the
1s–2p0 lines for USTD G2. According to the eﬀective-mass
theory,16) the intensity ratio of 1s–2p0 to 1s–2p± lines should
be 4 : 10. Thus, we determined the area intensity of the
1s–2p± lines of USTD G2 to be 2.5 times that of the 1s–2p0
lines of USTD G2. The area intensity of the 1s–2p± STD(H)lines was obtained using the two main peaks and ﬁve satellite
lines. Figure 6 shows a summary of the variation in area
intensity of the 1s–2p± lines of USTD G1, USTD G2, andSTD(H)s, as well as the sum of these intensities as a functionof annealing time at 480 °C. The sum of the area intensities
of USTDs and STD(H)s seems to shows a weak dependence

101302-3© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 101302 (2015)
0.5

All
STD(H)s1s-2p㼼 USTDs G1
1s-2p㼼 䠄1s-2p0 x 2.5) USTDs G2

0.4

Intensity (arb. unit)

A. Hara and T. Awano

0.3

450°C40h
510°C40h

0.2

0.1

0

0

20

40

60

Annealing time (h)80Fig. 6. (Color online) Variation in the intensities of spectral line areas for
STD(H)s, USTD G1, and USTD G2, and their sum as a function of annealing
duration at 480 °C. This ﬁgure includes data points of samples annealed at
450 °C or 510 °C for 40 h.

on annealing duration at 480 °C for durations greater than
10 h. The area intensities for the sum of the spectral lines of
the USTDs and STD(H)s at 450 and 510 °C after 40 h from
Fig. 4 are also plotted in Fig. 6. We noted earlier thatthe shapes of the spectral peaks corresponding to the USTDs
and STD(H)s formed by annealing at 450 °C for 40 h aresimilar to those formed by annealing at 480 °C for 20 h. On
the other hand, the spectra of samples annealed at 510 °C for
40 h are similar to those of samples annealed at 480 °C for
65 h. Therefore, the two data sets are also plotted for eachcorresponding annealing duration at 480 °C.
4.

Discussion

The formation speed of the STD(H)s is clearly very high,because the STD(H) lines become obvious after 3 h annealing
and are maximum after 10 h annealing at 480 °C, as shownin Fig. 2. Thus, we conclude that nearly all STD(H)sare generated within the ﬁrst 10 h of annealing. As the line
intensities of the USTDs increase, the line intensities of theSTD(H)s decrease; the sum of the USTD and STD(H) line
areas remains approximately constant for annealing durations
exceeding 10 h at 480 °C. This indicates that the STD(H)schange to USTDs with prolonged annealing.
According to our proposed formation mechanism, thesum of the USTDs and STD(H)s depends on the amountof STD(H)s generated during the initial annealing stage.
Because of the high formation speed of the STD(H)s, thediﬀusion of hydrogen is expected to be the rate-limiting
process for the formation of STD(H)s. The hydrogen diﬀusion coeﬃcient is very large and the activation energy for
migration is very small within CZ-Si. Thus, the number of
STD(H)s generated during the initial stage of annealing at
450 °C will be nearly the same for that generated during
annealing at 480 °C. This explains the similarity in the sumsof the signal intensities of the USTDs and STD(H)s of
samples annealed at 450 °C after 40 h and those annealed at
480 °C for 20 h.
In contrast, the sum of the signal intensities of the USTDs
and STD(H)s of samples annealed at 510 °C for 40 h islower than that of the samples annealed at 480 °C. Since
the dissolution of STD(H)s occurs as in parallel with theformation of STD(H)s with increasing annealing temperature,
the total amount of STD(H)s formed at the initial annealingstage at 510 °C decreases to less than that formed at 480 °C.
Similar phenomena have been reported for NO complex
donors, wherein the amount of NO complexes decreased
with increasing annealing temperature.6) Therefore, the sumof the line area of USTDs and STD(H)s of samples annealed
at 510 °C for 40 h is lower than that for samples annealed
at 480 °C. The eﬀects of other hydrogen reaction processes
must also be considered, especially at high temperatures,such as diﬀusion into atmosphere from the surface andgettering to oxygen precipitates formed during annealing.
These factors contribute to the mechanism of the reductionin the sum of the line area of USTDs and STD(H)s formed
at 510 °C.
The lower formation speed of USTDs indicates theexistence of a rate-limiting process. This is clearly a thermally activated process, because the formation speed of
USTDs is higher at high annealing temperatures and lower at
low annealing temperatures, as shown in Fig. 4. Carbon andsilicon are both group IV elements. Thus, it is reasonable
to assume that carbon acts as a modulator of STD(H)s by
replacing Si, which constitutes the core structure of STD(H)s.
Generally, carbon occupies substitutional lattice sites; therefore, the diﬀusion coeﬃcient of substitutional carbon (D) isvery small, as given by D = 1.9 exp(−3.2eV=kBT ) cm2=s.17)
This diﬀusion coeﬃcient limits the diﬀusion length to far
less than the atomic radius of Si when Si is annealed for 40 h
at 480 °C. However, the D for interstitial or interstitialcy
carbons is expected to be signiﬁcantly larger than the abovementioned value. It is well known that the presence of carbon
enhances the formation of SiO2 precipitates,17) which inturn eject self-interstitial Si from the host lattice. The selfinterstitial Si replaces the substitutional carbon in order tooccupy the substitutional Si site, resulting in the formationof an interstitial carbon. Thus, we can propose a reasonablemodel for the formation mechanism of USTDs. The fastforming STD(H)s, whose formation is completed during
the initial annealing stage, are transformed into USTDs
during long-term annealing by combining with carbon, which
migrates via interstitial or interstitialcy mechanisms. Thus,
the intensities of STD(H) and USTD lines simultaneously
decrease and increase, respectively. Because the migration ofinterstitial or interstitialcy carbons and the generation of suchcarbons are thermally activated processes, this mechanism
does not contradict the experimental results.
The direct observation of the existence of isolated interstitial carbons by IR analysis, photoluminescence (PL)analysis, or other methods, may support the proposed formation model. However, this observation seems to be diﬃcult,because, according to electron irradiation experiments, interstitial carbon easily forms complex defects combining withcarbon and other impurities.18,19) This behavior of interstitialcarbon also indicates that, if STD(H)s exist in Si crystals, theinterstitial carbon interacts with STD(H)s, resulting in theformation of USTDs.
Since two main strong peaks exist for STD(H)s, which
are mapped to species STD(H)s-2 and -3, it seems that the
number of main peaks of USTDs would be reduced to two if

101302-4© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 101302 (2015)

A. Hara and T. Awano

the proposed USTDs formation model is correct. Only two
USTD lines, USTDs-3 and -5, were dominant after long-term
annealing at 480 °C or at high-temperature annealing above
480 °C (see Figs. 2, 4, and 5); this indicates a correlation
between STD(H)s and USTDs and supports the proposed
formation model.
The intensity of USTD-5 lines continues to increase
with increasing annealing duration, while that of USTD-3
lines decreases with annealing durations exceeding 40 h, asshown in Fig. 6. This may indicate a correlation between
species USTDs-3 and -5. However, more experimentationis necessary to determine the interactions between them.
Here, we discuss another formation mechanism of USTDs.
It is well known that carbon oxygen (CO) complexes20) areformed in carbon-doped CZ-Si crystals; thus, the following
process may be a candidate for the formation mechanismof USTDs. In the initial stage, CO complexes form slowly;
subsequently, the hydrogen atoms in STD(H)s dissolve, andﬁnally, the dissolved hydrogen atoms migrate to the COcomplexes, modifying them into USTDs. However, thismodel can be excluded because the amount of STD(H)s does
not change even after long-term annealing at 480 °C, as
shown in Fig. 3.
Åberg et al.21) also observed the existence of USTDs by
using deep-level transient spectroscopy (DLTS), a methodvery sensitive to surface defects. They found the existenceof defects with shallow energy levels of 25 meV below theconduction band around the surface region. If we assume that
the USTDs investigated here and in the previous report ofÅberg et al. are identical, we must conclude that the samples
investigated by Åberg et al. were contaminated with carbon
and hydrogen; both of these impurities are indispensable
to the formation of USTDs. However, we cannot speculate
further on the origin of USTDs observed by the separate
research groups, because both the properties of the initial Si
and the annealing atmosphere used in the two studies diﬀered.
5.

Conclusions

We proposed a formation mechanism for USTDs in carbonand hydrogen-doped monocrystalline CZ-Si crystals during
annealing at 480 °C. In the ﬁrst stage, STD(H)s form at a highrate; in the second stage, carbon diﬀuses via an interstitial

or interstitialcy mechanism and modulates the electronic
structure of STD(H)s by combining with them to form the
USTDs.
Acknowledgements

This study was partly conducted under the Joint Studies
Program of the Institute for Molecular Science and the Japan
Society for the Promotion of Science [Grant-in-Aid for
Scientiﬁc Research (C), No. 25420339].

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