JOURNAL OF APPLIED PHYSICS 110, 063528 (2011)

Acceptor-related low-energy photoluminescence from boron-doped
Si nanocrystals
Hiroshi Sugimoto, Minoru Fujii,a) Masatoshi Fukuda, Kenji Imakita, and Shinji Hayashi
Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,
Rokkodai, Nada, Kobe 657-8501, Japan(Received 22 July 2011; accepted 18 August 2011; published online 27 September 2011)
Boron (B) doped Si nanocrystals (Si-ncs) dispersed in hydrofluoric (HF) acid solution are preparedby dissolving borosilicate films containing B-doped Si-ncs in HF solution. We find that the etchingrate of B-doped Si-ncs is much smaller than that of undoped Si-ncs. The difference of the etchingrate allows us to extract only doped Si-ncs in the mixture of doped and undoped Si-ncs and observe
the photoluminescence (PL) due to the transition from the conduction band to the acceptor state.
The PL was very broad with the maximum around 1.15 eV. From the analysis of the PL dataobtained for the samples prepared under different conditions and different etching time, preferential
doping sites of B atoms are estimated. The data suggests that B-doped Si-ncs consists of intrinsic
C 2011 American Institute of Physics. [doi:10.1063/1.3642952]
cores and heavily B-doped shells. V
I. INTRODUCTION

Si nanocrystals (Si-ncs) have attracted considerable
research attention for more than a decade because of theirpotential applications in optoelectronics1,2 and biology.3,4The optical and electrical transport properties of Si-ncs canbe controlled by the size, shape and surface chemistry. In
particular, the size-dependence of the optical properties has
been extensively studied experimentally5,6 and theoretically7,8 and the mechanism of the near infrared (NIR) to visible luminescence has almost been clarified.5–10
For the application of Si-ncs in optoelectronic devices,
controlling the carrier density by impurity doping is indispensable. There has been a lot of theoretical work on n- and/or p-type impurity-doped Si-ncs (Refs. 11–16). All of themdemonstrate that the energy level structures are strongly
modified by a very small number of impurity atoms. Furthermore, preferential doping sites and the formation energy of
doped Si-ncs are calculated.15,16 The formation energy isreported to be very large for doped Si-ncs.11 The large formation energy indicates that Si-ncs tend to be intrinsic bypushing impurity atoms out during the growth and explains
the so-called self-purification effect.17 A problem still
remaining is the effect of surface termination on the energy
state structures of doped Si-ncs. All theoretical work performed so far on doped Si-ncs is concerning hydrogen (H)terminated Si-ncs, and to our knowledge, no calculation has
been done on doped oxygen (O)-terminated Si-ncs. On theother hand, the most stable surface of Si is Si-O bonds.
Therefore, many of experimentally prepared doped Si-ncs
have SiO2 shells, or even if the surface is initially terminatedby H atoms or organic molecules, after a certain period, they
are partly replaced by O atoms. Since very small amount of
O atoms on the surface is expected to modify the energy
state structure of doped Si-ncs, theoretical study on the effectis highly demanded.
a)Author to whom correspondence should be addressed. Electronic mail:fujii@eedept.kobe-u.ac.jp.

0021-8979/2011/110(6)/063528/6/$30.00

In contrast to the recent progress of theoretical studiesof doped Si-ncs, the experimental ones are limited. This ismainly due to inhomogeneous broadening of experimental
data because of the large distributions of sample parameters,such as the size, shape, defect density, impurity concentration, impurity sites, and so on and due to the lack of proper
characterization techniques. Doped Si-ncs are mainly studied
by photoluminescence (PL),18–20 electron paramagnetic resonance (EPR),21–23 and Raman spectroscopy.23–25 PL properties of boron (B) and/or phosphorus (P) doped Si-ncs
samples have been studied for Si-ncs in solid matrices18,19
and those in powder form.20 From the analysis of the PL
properties, Pi et al.20 assigned preferential doping sites of P
or B in in-situ doped plasma-synthesized Si-ncs. In our previous work,21 we studied the hyper-fine structure of EPR
spectra of P-doped Si-ncs in glass matrices and demonstrated
the evidence of the quantum confinement of donors in Sincs. Stegner et al.22 studied P-doped Si-ncs powder by EPR
spectroscopy and demonstrated that doping efficiency of Sincs decreases significantly when the diameter becomes
smaller than 10 nm. For example, only about 0.2% of doped
impurities are activated when the diameter is about 10 nm.
This means that in Si-ncs assemblies, even if average doping
concentration in the whole system is very high, only a fraction of Si-ncs have active impurities.
The very small doping efficiency exerts a serious problem when we study PL properties of doped Si-ncs. The PL
quantum efficiency of doped Si-ncs is expected to be morethan three orders of magnitude smaller than that of intrinsic
Si-ncs because of the strong Auger interaction between photoexcited excitons and carriers supplied by doped impurities.26 Therefore, the PL spectra of doped Si-ncs samples
are almost totally controlled by undoped Si-ncs if small fraction of them remains in the samples and those from doped
Si-ncs are hard to be detected. As a result, PL spectral shapeof doped Si-ncs samples studied so far is very similar to
those reported for intrinsic Si-ncs samples except for small
differences in the low-energy tail region.27 In order to study
the electronic state structures of doped Si-ncs by PL

110, 063528-1

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spectroscopy, samples consisting only of active impuritydoped Si-ncs are indispensable.
In this work, we demonstrate that extraction of active
impurity-doped Si-ncs from Si-nc assemblies can beachieved by simply dissolving borosilicate glass (BSG) films
containing B-doped Si-ncs by hydrofluoric acid (HF) solution. We show that PL spectra changes drastically by the
etching and only the low energy region of the spectra attributable to doped Si-ncs remains. An advantage of this method
is that H-termination is guaranteed because the samples are
kept in HF solution. This can remove the uncertainty of thedata caused by partial oxidation of the surface. From thecomparison of the PL data between doped and undoped Sincs samples and from the effect of air exposure of the etchedsamples to PL spectra, we will discuss the preferential doping sites of B atoms in Si-ncs.
II. EXPERIMENTAL PROCEDURE

SiO2 films containing Si-ncs were prepared by a cosputtering method.18,24 For intrinsic Si-ncs, small pieces of Si
chips (10  15 mm2) were placed on a SiO2 sputtering target(10 cm in diameter) and they were simultaneously sputtered in
Ar gas of 2.7 Pa. For B-doped Si-ncs, Si chips (10  15 mm2)
and B2O3 pellets (10 mm in diameter) were placed on a SiO2
sputtering target. Fused silica plates were used as a substrate
for the growth of thin film samples. To prepare powder samples, the films were deposited on stainless steel plates andpeeled from the plates just after the sputtering. The films andpowder were annealed at the temperature of 1150 to 1250  C
in a N2 gas atmosphere for 30 min to grow Si-ncs in SiO2 orin BSG matrices. The average concentration of B2O3 in asample was estimated from the intensity ratio of the B-O(1400 cm 1) and Si-O (1080 cm 1) vibration peaks in
the IR absorption spectra.28 The B concentration [CB (at.%)]calculated from the ratio of SiO2, B2O3 and excess Si wasabout 0.75 at.% for all the samples studied in this work. Notethat the concentration is an average concentration in thewhole film and is not that in Si-ncs.
In order to isolate Si-ncs from matrices, the powder
samples were crushed in a mortar and 3 mg of it was dispersed in HF solution (46 wt.%, 1.5 mL) in a polymethylmethacrylate (PMMA) cell. The amount of HF is more thanenough to completely dissolve SiO2 or BSG matrices of the
3 mg samples. Photoluminescence spectra were measured in
the HF solution. The solution was continuously stirred by amagnetic stirrer during the etching and measurements. Photoluminescence spectra of thin film samples were collectedfrom the sample by a conventional PL setup. The photoluminescence was analyzed by using a single spectrometer anddetected by a liquid-nitrogen cooled InGaAs diode array.
The excitation source was a 488.0 nm line of an Ar ion laser
or a 405 nm diode laser. The spectral response of the detection system was corrected with a reference spectrum of astandard halogen lamp. Photoluminescence decay curves
were measured using a near infrared photomultiplier tube
(R5509-72, Hamamatsu Photonics) and a multi-channel
scaler (SR430, Stanford Research). All the measurements
were performed at room temperature.

J. Appl. Phys. 110, 063528 (2011)

III. RESULTS AND DISCUSSION
A. B-doped Si nanocrystals in glass matrices

Before discussing PL properties of isolated B-doped Sincs, we summarize that of intrinsic and B-doped Si-ncs insolid matrices. The average diameters of the intrinsic and Bdoped Si-ncs in the matrices estimated by transmission electron microscopy (TEM) observations are 4–5 nm. Figure1(a) compares PL spectral shape of intrinsic and B-doped
samples exhibiting the PL maximum around 1.4 eV.
Although the spectral shape is very similar at the highenergy side, we can see a clear difference at the low-energyside; the B-doped sample has a broad tail below the bandgap
energy of bulk Si crystal, while the intrinsic sample does not.
Since the tail appears by doping, it is considered to arise
from the optical transitions involving the acceptor states.
Figure 1(b) compares PL decay curves of intrinsic andB-doped samples detected at 1.2 eV. The decay curve of the
intrinsic sample consists of a fast and a slow components.
The PL decay behavior of intrinsic Si-ncs has been studied
in detail.6 It has been shown that the slow component exhibits strong size, i.e., detection energy, dependence and it
becomes fast with decreasing the size. From the size dependence and the temperature dependence of the decay time,6 theslow component is usually assigned to the radiative recombination of quantum confined excitons.6 The shortening of theexciton lifetime with decreasing the size is considered to be
due to better overlap of electron and hole wavefunctions in

FIG. 1. (Color online) (a) Normalized PL spectra of thin film form intrinsic
and B-doped Si-ncs samples. (b) PL decay curves detected at 1.2 eV.

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063528-3

Sugimoto et al.

the momentum space.29 On the other hand, the fast component does not show clear size dependence and is sometimes
observed even below the bulk Si bandgap. This suggests thatthe fast component is due to defects in or on the surface of
Si-ncs. Compared to the intrinsic sample, the PL decay timeof the B-doped sample is much shorter. The decay time
obtained by fitting the decay curve by a stretched exponentialfunction is 10 lsec.
Figures 2(a) and 2(b) show PL spectra of intrinsic andB-doped samples, respectively, excited in the power range
from 70 mW to 2.0 W. In the intrinsic Si-ncs, we can see thehigh-energy shift of the peak when the excitation power isincreased. This is due to different exciton lifetimes at different photon energies; PL from larger size Si-ncs in a size distribution saturates easier than that from smaller size Si-ncs
because of the longer exciton lifetime. Similar high-energy
shift of the peak is observed for the B-doped sample [Fig.
2(b)]. The difference between the intrinsic and B-doped samples appears at the low-energy side of the spectrum. In Bdoped Si-ncs, a new band emerges in the low-energy region
when the excitation power is increased. In the inset of
Fig. 2(b), the PL intensities at 1.1 and 1.4 eV are plotted as afunction of the excitation power. We can see that the mainband saturates stronger than the low-energy tail.

FIG. 2. (Color online) Excitation power dependence of normalized PL spectra of thin film form (a) intrinsic and (b) B-doped Si-ncs samples. Inset: PL
intensities of B-doped Si-ncs sample as a function of excitation power.

J. Appl. Phys. 110, 063528 (2011)

The different excitation power dependence between thehigh-and low-energy regions suggests that Si-ncs in B-doped
samples consist of two groups. One group is Si-ncs contributing to the PL in the high-energy region. In this group, B isconsidered to be not doped or not activated because the excitation power dependence of the PL is very similar to that of
the intrinsic samples. The other group is Si-ncs withelectrically-active B. These nanocrystals show fast PL in the
low-energy region due to the optical transitions involving theacceptor states. The lifetime is consider to be determined
mainly by the nonradiative Auger process between holes
supplied by B doping and photoexcited excitons.
B. B-doped Si nanocrystals in HF solution

Before conducting optical measurements, we studied Sincs after HF etching by TEM. For TEM observations, samples were washed several times to remove HF and dispersed
in methanol. The solution was then dropped on carboncoated copper grids. Figure 3 shows a TEM image of Bdoped Si-ncs dispersed in methanol after HF etching for
30 min. We can see lattice fringes corresponding to the{111} planes of Si-ncs. The average diameter is about
4.5 nm with the standard deviation of 1.3 nm. From the image,
we can confirm that Si-ncs are isolated from matrices.
Figure 4(a) shows PL spectra of an intrinsic sample
before and during the HF etching. The PL before the etchingis measured by dispersing the crushed fine powder (3 mg) inmethanol, while that during etching is measured by dispersing the same amount of powder in HF solution by the procedure written above. Before the etching, the PL peak appears
at 1.45 eV. By the HF etching, the spectrum shifts to thehigher energy and the intensity decreases; the 10 min etching
results in the shift of the PL maximum to 1.55 eV. This shift
is explained by modification of the surface termination fromO to H and by the size decrease.10 The intensity decrease isdue to defect formation on the surface of Si-ncs by etching
and also due to induced surface charges because of the dispersion of Si-ncs in polar liquid. Photoluminescence quenching in polar solvents have been reported for porous Si(Ref. 30). When the sample is etched longer, the PL shifts
and quenches further. This is probably due to size decreaseby laser irradiation in high concentration HF solution.31

FIG. 3. High-resolution TEM image of B-doped Si-ncs after HF etching.

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J. Appl. Phys. 110, 063528 (2011)

The PL spectral shape of B-doped Si-ncs remaining after
the etching is qualitatively different from that of intrinsic Sincs. In general, intrinsic Si-ncs have a PL cutoff at the
bandgap energy of bulk Si crystal.6 On the other hand, PLfrom the B-doped Si-ncs extends deep below the bulk Si
bandgap. This is another evidence that the acceptor states are
involved in the optical transition. Figure 5(a) shows PL peakenergies as a function of the etching time. The PL peakenergy of the sample annealed at 1200  C is lower than thatof the sample annealed at 1150  C. In both samples, the peakshifts to higher energy with increasing the etching time.
However, the shift is much smaller than that observed for the
intrinsic sample. In general, the energy levels of hydrogenic
impurities are less sensitive to the size of Si-ncs because of
the small effective Bohr radius.32 Therefore, the conduction
band to acceptor states transition reflects only the shift of the
conduction bandedge when the size is changed, and thus
the shift is smaller than that of the band-to-band transitions.
The different PL peak energies between the samples
annealed at 1150  C and 1200  C after etching are considered
to reflect the size difference.
Figure 5(b) shows the PL intensity as a function of theetching time. Before the etching, the PL intensities of
B-doped samples are much smaller than that of the intrinsic
sample. The small PL intensity is due to the Auger recombination of photoexcited carriers with the interaction withholes supplied by B doping.33 The PL intensities of both the
intrinsic and B-doped samples decrease by the HF etching.
However, the degree of the intensity decrease is different,

FIG. 4. (Color online) PL spectra taken before and during HF etching. The
etching time is indicated in the graphs. (a) Intrinsic Si-ncs samples annealed
at 1150  C, (b) B-doped Si-ncs samples annealed at 1150  C, and (c) Bdoped Si-ncs samples annealed at 1200  C.

Figures 4(b) and 4(c) show PL spectra of B-doped samples annealed at 1150 and 1200  C, respectively. Before theetching, B-doped samples exhibit a broad PL band with the
maximum around 1.3 eV. A dip around 1.06 eV is due toabsorption of emitted light by O-H stretching vibrations ofwater and by a PMMA cell. After the HF etching, in both
samples, only the low-energy-tail region remains and themain band disappears. As a result, the PL peak shifts tolower energy. In Fig. 2, we proposed a model that the Bdoped samples consist of Si-ncs with active B atoms andthose in which B atoms are not doped. The former exhibits
short-lifetime PL in the low-energy region, while the latterlong-lived PL in the high-energy region. Figures 4(b) and4(c) suggest that after the HF etching, only B-doped Si-ncs
remain in the HF solution, and the PL due to the electronictransition between the conduction band and the acceptor
states emerges.

FIG. 5. (Color online) (a) PL peak energy and (b) peak intensity of intrinsic
and B-doped Si-ncs samples as a function of the HF etching time.

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J. Appl. Phys. 110, 063528 (2011)

i.e., the PL of the intrinsic sample quenches faster than thatof doped samples. This suggests that B-doped Si-ncs are lessvulnerable to HF than undoped Si-ncs. We will discuss themechanism later.
In Fig. 5(b), it is interesting to note that, after the etching, the PL intensities of B-doped samples annealed at
1200  C are larger than those of the sample annealed at1150  C, despite the smaller PL intensity before the etching.
This can be explained by considering different ratio of active
impurity-doped Si-ncs between the two samples. In the sample annealed at 1200  C, many of Si-ncs are B-doped. As aresult, initial PL intensity determined mainly by the numberof undoped Si-ncs is smaller, while that after the etching
determined by the number of doped Si-ncs is larger. It is viceversa, in the sample annealed at 1150  C.
C. Location of B in B-doped Si nanocrystals

Figures 4 and 5 strongly suggest that B-doped Si-ncs areless vulnerable to HF than undoped Si-ncs. A possible explanation of this effect is that B is doped near the surface of Sincs. It is well known that the segregation coefficient of B at
the Si/SiO2 interface is small (0.3) (Ref. 34). Therefore,
during the growth of Si-ncs in SiO2 by annealing, B atoms
are precipitated near the interface. Since Si-B bonds arereported to be much more resistant to HF than Si-Si bonds,35
if B atoms are doped in the sub-surface, the etching rate of
B-doped Si-ncs is expected to be much smaller than that of
intrinsic Si-ncs.
In order to confirm the model that B atoms are doped in
the sub-surface of Si-ncs, we performed the following
experiments. After etching 30 min in HF solution, the samples were washed and stored in methanol. The PL spectra of
the samples in methanol are shown in Fig. 6(a) with that of
the sample before etching. The spectra are normalized at the
maximum intensity. The PL spectrum of the sample kept one
day in methanol is very similar to that in HF solution in Fig.
4(c). The dip at 1.03 eV is attributable to absorption of emitted light by the overtones of C-H stretching vibrations ofmethanol. The spectral shape changes drastically by keeping
the sample 16 days in methanol. The PL peak returns almost
to the original position before the etching and the spectral
shape is very similar to that of intrinsic Si-ncs samples. After
three months, although further drastic change of the spectral
shape is not observed, the intensity below the bandgap
energy of bulk Si crystal continuously decreases and almost
disappears. The drastic change of the PL spectrum suggeststhat active B atoms in B-doped Si-ncs are inactivated by justkeeping the samples in methanol. In order to confirm that B
atoms are inactivated, we measured PL lifetimes of the samples kept three months in methanol and compared them withthose of an intrinsic sample. Figure 6(b) shows the PL lifetimes as a function of the detected photon energy. The inset
shows the PL decay curve of the B-doped sample at 1.2 eV.
The decay curve is completely different from that shown in
Fig. 1(b). The decay curve is a single exponential function
and the lifetime is about 360 lsec. The lifetime is very close
to that of the intrinsic sample in the whole energy range studied. Therefore, B is indeed inactivated in methanol at room

FIG. 6. (Color online) (a) Normalized PL spectra of B-doped Si-ncs samples
measured in methanol before HF etching and kept one day, 16 days and3 months in methanol after HF etching. The HF etching time is 30 min. (b)PL lifetimes of the sample kept three months in methanol and an intrinsic
sample as a function of photon energy. Inset: PL decay curve of the sample
kept three months in methanol detected at 1.2 eV.

temperature. This strongly suggests that active B atoms exist
very close to the surface of Si-ncs.
The most plausible explanation of the room temperature
inactivation of B atoms is the oxidation of the Si-nc surface.
In H-terminated Si-ncs, the Si-Si back-bonds are easily oxidized in air or in water at room temperature. In the present samples, when they are kept in HF solution, the surface is always
H-terminated. If the samples are kept in methanol for a long period, the Si-Si bonds below the Si-B bonds are oxidized and B
atoms are incorporated into surface oxide layers. This results ininactivation of B and drastic change of the PL spectra.
IV. CONCLUSION

We have succeeded in observing the acceptor state
related PL from B-doped Si-ncs. We found that B-doped
Si-ncs consist of a high B concentration shell and an intrinsic
core and the shell prevents etching of doped Si-ncs from HF.

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063528-6

Sugimoto et al.

This structure allows us to extract B-doped Si-ncs from theassembly of doped and undoped Si-ncs and observe the PLfrom H-terminated B-doped Si-ncs. The B-doped Si-ncs
show a very broad PL with the maximum around 1.1 to
1.2 eV. The large width is considered to be due to the distributions of the size of Si-ncs and number of active B atoms in
Si-ncs. Further detailed studies are required to fully understand the relation between the PL spectra and these parameters. The present result is also important from the application
point of view. Since B atoms exist very close to the surfaceof Si-ncs, they are very easily deactivated by oxidation atroom temperature. To avoid the effect, B-doped Si-ncs
should be kept in oxygen-free environment until used assource materials for optoelectronic devices.
ACKNOWLEDGMENTS

This work is supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan and by Research
Grants in the Natural Sciences from Mitsubishi Foundation.
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