Silicon nanocrystals with high boron and phosphorus concentration hydrophilicshell—Raman scattering and X-ray photoelectron spectroscopic studies
Minoru Fujii, Hiroshi Sugimoto, Masataka Hasegawa, and Kenji Imakita
Citation: Journal of Applied Physics 115, 084301 (2014); doi: 10.1063/1.4866497
View online: http://dx.doi.org/10.1063/1.4866497
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JOURNAL OF APPLIED PHYSICS 115, 084301 (2014)

Silicon nanocrystals with high boron and phosphorus concentrationhydrophilic shell—Raman scattering and X-ray photoelectron
spectroscopic studies
Minoru Fujii,a) Hiroshi Sugimoto, Masataka Hasegawa, and Kenji Imakita
Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,
Rokkodai, Nada, Kobe 657-8501, Japan(Received 26 December 2013; accepted 10 February 2014; published online 24 February 2014)
Boron (B) and phosphorus (P) codoped silicon (Si) nanocrystals, which exhibit very wide range
tunable luminescence due to the donor to acceptor transitions and can be dispersed in polar liquids
without organic ligands, are studied by Raman scattering and X-ray photoelectron spectroscopies.
Codoped Si nanocrystals exhibit a Raman spectrum significantly different from those of intrinsic ones.
First, the Raman peak energy is almost insensitive to the size and is very close to that of bulk Si
crystal in the diameter range of 2.7 to 14 nm. Second, the peak is much broader than that of intrinsic
ones. Furthermore, an additional broad peak, the intensity of which is about 20% of the main peak,
appears around 650 cm1. The peak can be assigned to local vibrational modes of substitutional B and
B-P pairs, B clusters, B-interstitial clusters, etc. in Si crystal. The Raman and X-ray photoelectron
spectroscopic studies suggest that a crystalline shell heavily doped with these species is formed at thesurface of a codoped Si nanocrystal and it induces the specific properties, i.e., hydrophilicity,
C 2014 AIP Publishing LLC.
high-stability in water, high resistance to hydrofluoric acid, etc. V[http://dx.doi.org/10.1063/1.4866497]

I. INTRODUCTION

Colloidal Si nanocrystals have been attracting significant attention because they can be a key material for
Si-based printable electronics and are expected to be moresuitable for biological applications than compound semiconductor nanocrystals due to the non-toxicity as a chemicalelement. The quality, i.e., size distribution, luminescencequantum efficiency, etc., of Si nanocrystals has been
improved rapidly1–5 and several kinds of electronic devices
have been demonstrated.6–11 In colloidal semiconductor
nanocrystals, the surface termination is an important parameter to control the electronic states as well as the chemistry. In
general, the surface of Si nanocrystals is functionalized byorganic ligands to prevent agglomeration of nanocrystals insolution by the steric barriers. However, the surface molecules act as tunneling barriers for carrier transport in films
produced from colloidal solutions and the films are usuallycapacitive. One of the approaches to realize high conductivity nanocrystal films from colloids is to replace organic
ligands with inorganic ones. Although the ligand exchange
process has been successfully applied in compound semiconductor nanocrystals,12–14 it is not applicable to Si nanocrystals because Si forms covalent bonds with capping ligands.
In Si nanocrystals, physical processes such as pulsed laser
irradiation and microplasma treatments in solutions areapplied to achieve the inorganic termination and to makethem dispersible in polar solvents.15
Recently, we have developed a new method to produce
Si nanocrystals dispersible in polar solvents without organica)Author to whom correspondence should be addressed. Electronic mail:fujii@eedept.kobe-u.ac.jp. Tel.: þ81-78-803-6081.

0021-8979/2014/115(8)/084301/5/$30.00

ligands.16–18 The method is the formation of very high B and
P concentration layers at the surface of Si nanocrystals. The
layer induces negative potential at the surface and prevents
the agglomeration by electrostatic repulsions. The colloidal
solution of codoped Si nanocrystals is stable for years inmethanol and exhibits efficient size-controllable photoluminescence (PL) in a very wide energy range (0.85–1.9 eV) due
to the donor to acceptor transitions.19 The efficient and relatively long lifetime luminescence of codoped Si nanocrystals
suggests that majority of them are perfectly compensatedand they have no charge carriers. Therefore, codoped Si
nanocrystals can be regarded as a kind of intrinsic Si nanocrystals with additional functionalities such as extended tunable range of the luminescence energy and high solution
dispersibility. For example, codoped Si nanocrystals are dispersed in water without organic capping and exhibit efficient
PL in a biological window.20 This is an attractive feature asa contrast agent in bioimaging.
Although the strategy that solution dispersion of semiconductor nanocrystals is achieved by the formation of ahigh impurity concentration shell is unique and worth studying in detail, little is known about the structure of the high B
and P concentration shell. The purpose of this work is toclarify the structure, especially that of the shell, of B and P
codoped Si nanocrystals. Raman spectroscopy is known to
be a powerful tool to study the bonding states and symmetryof impurities in Si crystal21 and different kinds of local
vibrational modes have been identified.21–23 Raman spectroscopy is also widely used for the characterization of Si nanocrystals, because the spectral shape is sensitive to thesize.24–26 In this work, we study the structure of B and P
codoped Si nanocrystals by Raman spectroscopy and X-ray
photoelectron spectroscopy (XPS). We show that the Raman

115, 084301-1

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J. Appl. Phys. 115, 084301 (2014)

spectra of codoped Si nanocrystals are significantly different
from those of intrinsic ones. In particular, codoped Si nanocrystals exhibit relatively strong Raman signals assigned to
local vibrational modes of substitutional B, P and B-P pairs,
B clusters, B-interstitial clusters, etc., in Si crystal. The present results provide clear evidences for the existence of acrystalline shell doped with different kinds of B and P related
species at the surface of a codoped Si nanocrystal.
II. EXPERIMENTAL

Impurity-doped Si nanocrystals were prepared by a cosputtering method. Detailed preparation procedure is
described in our previous papers.16,17 Si and SiO2 are simultaneously sputter-deposited and annealed to grow Si nanocrystals in silica matrices. For the growth of B or P doped Si
nanocrystals, phosphosilicate glass (PSG) or borosilicate
glass (BSG), respectively, are added to the sputtering targets.
This results in the formation of P (B) doped Si nanocrystals
in PSG (BSG) matrices.27,28 For the growth of codoped Si
nanocrystals, Si, SiO2, PSG, and BSG were simultaneously
sputter-deposited and annealed. In this case, codoped Si
nanocrystals are grown in borophosphosilicate glass (BPSG)
matrices.29 To isolate Si nanocrystals from silica or silicate
matrices, samples are dissolved in hydrofluoric acid (HF)
solutions (46 wt. %) for 1 h. Isolated nanocrystals were thentransferred to methanol. It should be stressed here that
undoped, P-doped and B-doped Si nanocrystals agglomerate
in methanol and form large precipitates. On the other hand,in codoped samples, precipitates are hardly observed andmajority of nanocrystals are dispersed in methanol. B and P
concentration in codoped Si nanocrystals estimated byinductively coupled plasma atomic emission spectrometry(ICP-AES) measurements is 13–22 at. % and 0.8–4.4 at. %,respectively.19 No clear dependence of the concentration on
annealing temperature or size is observed.19
Raman spectra were measured using a confocal microscope (50 objective lens, NA ¼ 0.8) equipped with a single
monochromator and a charge coupled device (CCD). The excitation source was a 514.5 nm line of an Ar ion laser. The
excitation power was 1 mW. The X-ray source for XPS
measurements (PHI X-tool, ULVAC-PHI) was Al Ka. The
samples for Raman scattering and XPS measurements wereprepared by drop-coating nanocrystal-dispersed methanol ongold (Au)-coated Si wafers. TEM observations (JEM-2010,
JEOL) were performed by dropping the solution on carboncoated TEM meshes.

FIG. 1. (a) Photograph of B and P codoped colloidal Si nanocrystals (methanol solution). (b) TEM image and high-resolution TEM image (inset). The
annealing temperature is 1200  C.

14 nm by changing the annealing temperature (Ta) from 900
to 1300  C. When the annealing temperature is below
1000  C, amorphous particles are formed, while above
1050  C, crystalline ones are grown.19 In this work, we limit
the annealing temperature range from 1050 to 1300  C tofocus on crystalline particles.
Figure 2(a) shows Raman spectra of B and P codoped Si
nanocrystals annealed at different temperatures in BPSG
matrices. As references, the spectra of intrinsic and B or P
singly-doped Si nanocrystals in silica or silicate matrices
grown at 1200  C are shown. The intrinsic Si nanocrystals
exhibit a Raman peak around 520 cm1 with a tail at the
low-wavenumber side. This is a typical Raman spectral
shape of Si nanocrystals. The size dependence of the spectral
shape has been studied in detail for many years.24–26 The

III. RESULTS AND DISCUSSION

Figure 1(a) shows a photograph of codoped colloidal Si
nanocrystals. The solution is very clear due toagglomeration-free perfect dispersion of nanocrystals. Figure1(b) shows a TEM image of codoped Si nanocrystals.
Because of the perfect dispersion in solution, no threedimensional agglomerates are observed. The lattice fringes
in the high-resolution TEM (HRTEM) image (inset) correspond to (111) planes of Si crystal. The crystallinity is very
high and almost all nanocrystals are single crystal. The diameter of codoped Si nanoparticles can be controlled from 1 to

FIG. 2. (a) Raman spectra of B and P codoped Si nanocrystals in BPSG matrices. Ta is changed from 1050 to 1300  C. Spectra of intrinsic, B-doped andP-doped Si nanocrystals in silica or silicate matrices are also shown. (b)Raman spectra after removing silica or silicate matrices by HF etching.

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spectral shape of B or P singly doped Si nanocrystals is similar to that of intrinsic Si nanocrystals except for small differences in the peak wavenumbers and the widths. On the other
hand, the Raman spectra of codoped Si nanocrystals are significantly different from that of the intrinsic one. First, despite high crystallinity evidenced by HRTEM images, theRaman peak is much broader. Furthermore, a broad peak
appears around 650 cm1, which is not observed in intrinsic
and B or P singly doped Si nanocrystals.
The Raman spectra of Si nanocrystals after removing
silica or silicate matrices are shown in Fig. 2(b). The spectral
shape of intrinsic and B or P singly doped Si nanocrystals
does not change significantly by the removal of silica or silicate matrices. On the other hand, that of codoped Si nanocrystals changes in some aspects, especially when theannealing temperature is relatively low. The shape of the
520 cm1 peak after etching is like a right-angled triangle
with a gentle slope on the low-wavenumber side and a steepedge at the high-wavenumber side. The change of the spectral shape is probably due to the removal of relatively large
Si nanocrystals in the size distribution during the process of
extracting nanocrystals from matrices.16 In contrast to the
520 cm1 peak, the shape of the 650 cm1 peak does notchange significantly. In many cases, the intensity of the
650 cm1 peak with respect to that of the 520 cm1 oneslightly increases after etching. The height of the 650 cm1
peak is about 20% of that of the 520 cm1 peak, when theannealing temperature is higher than 1100  C. When theannealing temperature is 1050  C, it is weaker but stillobservable.
Since the largest phonon energy of Si crystal is
520 cm1, the 650 cm1 peak should be related to doped B
and/or P. In Fig. 3, a Raman spectrum of free-standing
codoped Si nanocrystals (Ta ¼ 1200  C) is compared with thewavenumbers of several kinds of B or P related local vibrational modes in Si crystal. One of the candidates of the
650 cm1 peak is local vibrational modes of substitutional B.

FIG. 3. Raman spectrum of free-standing B and P codoped Si nanocrystals.
The wavenumbers of substitutional B, P, and B-P local vibrational modes in
Si crystal are shown with vertical bars.

J. Appl. Phys. 115, 084301 (2014)

In heavily B doped bulk Si crystal, substitutional B atoms
exhibit weak Raman peaks at 620 (11B) and 644 cm1(10B).21,30 The intensity ratio of the peaks reflects naturalabundance of 11B (80.2%) and 10B (19.8%) and is about 4:1.
The B local modes are also observed in B doped Si nanowires31 and nanocrystals,32 although in the present B singly
doped Si nanocrystals, the signal was below the detection
limit. Another candidate of the 650 cm1 peak is local vibrational modes of substitutional B-P pairs in Si crystal.33,34
The comparison of the spectral shape and the wavenumbersof the local modes in Fig. 3 suggests that the B and B-P local
vibrational modes partly contribute to the 650 cm1 peak.
However, they cannot explain the whole range of the broad
650 cm1 peak.
Considering extremely high B concentration in codoped
Si nanocrystals, it is very plausible that doped B atoms form
clusters, e.g., B2, and B-interstitial clusters, e.g., BI, B2I,
B2I2, etc, where BnIm refers to a cluster composed of n B
atoms with m interstitials (B or Si). B clusters andB-interstitial clusters are known to be formed by B implantation in Si crystal.22,23 Vibrational frequencies of these clusters in Si crystal have been studied experimentally andtheoretically and some of them are within the broad
650 cm1 peak.22,23 Therefore, B clusters and B-interstitialclusters are strong candidates of the peak, although it is difficult to identify the kinds of clusters.
In codoped Si nanocrystals, a large amount of P is also
doped.19 This suggests that substitutional P and/or P-related
clusters also contribute to the Raman spectra. Because of
11% larger mass of P than Si, they exhibit Raman peaks at
the low wavenumber side of the main peak. In fact, the local
vibrational mode of substitutional P in Si crystal is known to
be around 441 cm1 as designated in Fig. 3.21 Although clear
peaks are not observed, it is plausible that substitutional P
and/or P-related clusters are the constituents of the extremely
long low-wavenumber tail of the main peak.
In Fig. 4(a), the peak wavenumbers of codoped Si nanocrystals in BPSG matrices and free-standing ones are plotted
as a function of the annealing temperature. The data of
intrinsic Si nanocrystals are also shown as references.
Intrinsic Si nanocrystals in silica matrices exhibit a Raman
peak around 519 cm1, which is slightly lower than that ofbulk Si crystal (520 cm1). By etching out silica matrices,the peak shifts to lower wavenumber and reaches 516 cm1.
The low energy Raman peak of free-standing Si nanocrystals
is generally explained by the phonon confinement
effect.24–26 The effect is partly compensated by compressive
stress exerted from surrounding solid matrices.24 This resultsin different Raman wavenumber between free-standing NCs
and NCs in solid matrices.
The Raman peak wavenumbers of codoped Si nanocrystals are different from those of intrinsic ones. In BPSG matrices, the peak is in the range of 522 to 524 cm1. It shifts tolower wavenumber by removing BPSG matrices. Thisbehavior is qualitatively similar to that of intrinsic Si nanocrystals. However, the peak wavenumber of free-standing
codoped Si nanocrystals is much larger than that of intrinsic
Si nanocrystals and is very close to that of bulk Si crystal. In
Fig. 4(b), the peak wavenumber of codoped free-standing Si[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
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J. Appl. Phys. 115, 084301 (2014)

FIG. 5. (a) Si 2p, (b) B 1s, and (c) P 2p XPS spectra of B and P codoped Si
nanocrystals prepared with different annealing temperatures. The annealing
temperatures and the average diameters are shown in (a).

FIG. 4. (a) Raman peak wavenumbers of codoped Si nanocrystals in BPSG
matrices and free-standing codoped Si nanocrystals as a function of annealing temperature. The data of intrinsic Si nanocrystals are also shown. (b)Raman peak wavenumber of free-standing codoped Si nanocrystals as afunction of the diameter. The data of intrinsic ligand-stabilized Si nanocrystals taken from Ref. 24 are also shown.

nanocrystals is plotted as a function of the diameter. For
comparison, the data of ligand-stabilized free-standing
intrinsic Si nanocrystals are shown.24 In the ligand-stabilized
Si nanocrystals, size-dependent low energy shift of the peakis clearly observed. The size dependence is well-explainedby the phonon confinement effect. On the other hand, incodoped free-standing Si nanocrystals, the peak shifts only
slightly from 520 cm1 to 519 cm1, when the average diameter is decreased from 14 to 2.7 nm. The very small size dependence of the peak energy strongly suggests that, evenafter removing BPSG matrices, codoped Si nanocrystals
have a hard shell and the low wavenumber shift of theRaman peak by the phonon confinement effect is compensated by compressive stress exerted from the shell.
In order to study the atomic structure of the shell ofcodoped Si nanocrystals, we measure the XPS spectra.
Figure 5 shows XPS spectra of codoped free-standing Si
nanocrystals prepared at different annealing temperatures. In
the Si 2 p core signal (Fig. 5(a)), a peak assigned to Si nanocrystal cores (Si0) and that to surface native oxides areobserved around 99.8 and 102.6 eV, respectively. The binding energy of the oxides is smaller than that of stoichiometric
SiO2 (103.8 eV, Si4þ in Figure 5(a)) and is close to the valueof Si3þ (102.7 eV).35 This suggests that only Si atoms at theoutermost surface have bonds with oxygen (O) atoms andthe thickness of the oxide layer is less than a monolayer. The
intensity of the oxide signal increases with decreasing thesize. This can be explained qualitatively by the increase of
the ratio of surface Si atoms within the escape depth of photoelectrons (2 nm).36
Figure 5(b) shows the B 1 s signals. Boron metal and boron oxide (B2O3) exhibit XPS peaks at 187–188 and 193 eV,
respectively.37 The main peak in Fig. 5(b) is around 188 eV,
indicating that majority of B atoms exist in Si nanocrystals
in non-oxidized states. A tail towards higher energy suggests
slight oxidation. Although the intensity of the 188 eV peakdecreases and the spectrum becomes broad with decreasing
the size, majority of B atoms are not oxidized even for thesmallest nanocrystals. Similar results are obtained for the P
2p signals. In Figure 5(c), the main peak around 130 eV canbe assigned to non-oxidized P atoms and the broad tail tosuboxides. In contrast to the B 1s signal, the oxide-related
signal is stronger than that of the non-oxidized one in thelowest temperature annealed samples. From the data in
Fig. 5, we estimated the ratio of B to P in the shell. When theannealing temperature is higher than 1150  C, the ratio is in
the range of 3 to 4 and has no clear dependence on theannealing temperature. Below 1100  C, the signal is too
weak and noisy for quantitative discussion.
The data in Fig. 5 are obtained for samples one day after
preparation. When the samples are kept in methanol for along period, e.g., a year, oxidation slowly proceeds. The
XPS peak of surface oxidized Si at 102.6 eV shifts to 104 eV(Si4þ) and the intensity with respect to that of the Si0 peakincreases. The thickness of oxides estimated from the intensity ratio of Si4þ and Si0 peaks after one year storage inmethanol is about 1 nm. Slight oxidation after long term storage is also observed for B and P. However, even after one
year storage in methanol, signals from non-oxidized B and P
are stronger than those of oxidized ones except for the P 2p
peak of the sample annealed at 1050  C.

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The results of the XPS measurements, i.e., large
amounts of non-oxidized B and P exist on and/or near thesurface of codoped Si nanocrystals, are consistent with ourmodel that crystalline Si shells heavily doped with B and/or
P atoms are formed at the surface of nanocrystals.19
However, the XPS data do not provide information on thebonding states of B and P in the shell because of small chemical shifts of borides and phosphides.
In B doped bulk Si crystal, it is well-known that saturated B-rich layers (BRL) are formed at the interface
between B2O3 and Si after thermal treatments.38,39 The BRL
is hydrophilic and has high resistance to HF solution. These
properties of BRL are similar to those of the shells incodoped Si nanocrystals. The shell is thus considered to be akind of BRL. What is unknown at present is the role of P for
the formation of the shell. In the present preparation procedure, doping of P in addition to B is indispensable for theshell formation.18 One plausible explanation is that codopingof P stabilizes larger amount of B at the surface by charge
compensation. Further research is necessary to fully understand the interplay of P and B for the formation of the shell.
IV. CONCLUSION

We demonstrate that B and P codoped Si nanocrystals
exhibit Raman spectra significantly different from that of
intrinsic Si nanocrystals. The Raman peak energy of freestanding codoped Si nanocrystals is almost independent of
the size and is close to that of bulk Si crystal (520 cm1) inthe diameter range of 2.7 to 14 nm. Furthermore, the shapeof the 520 cm1 peak is very much different from that of
intrinsic Si nanocrystals. In addition, codoped Si nanocrystals have a broad Raman peak around 650 cm1, which isconsidered to arise from local vibrational modes of substitutional B and B-P pairs, B clusters, B-interstitial clusters, etc.
XPS measurements demonstrate the existence of large
amounts of non-oxidized B and P on and/or near the surfaceof codoped Si nanocrystals. The present results demonstratethat a thin hard crystalline shell containing large amounts of
B and P related species are formed at the surface of acodoped Si nanocrystal and it induces the specific properties
in solution.
ACKNOWLEDGMENTS

This work was supported by KAKENHI (Grant Nos.
23310077 and 24651143).
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