Single-dot spectroscopy of boron and phosphorus codoped silicon quantum dots
Takashi Kanno, Hiroshi Sugimoto, Anna Fucikova, Jan Valenta, and Minoru Fujii
Citation: Journal of Applied Physics 120, 164307 (2016); doi: 10.1063/1.4965986
View online: http://dx.doi.org/10.1063/1.4965986
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JOURNAL OF APPLIED PHYSICS 120, 164307 (2016)

Single-dot spectroscopy of boron and phosphorus codoped siliconquantum dots
Takashi Kanno,1 Hiroshi Sugimoto,1 Anna Fucikova,2 Jan Valenta,2 and Minoru Fujii1,a)1

Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,
Rokkodai, Nada, Kobe 657-8501, Japan2
Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University,
Ke Karlovu 3, CZ-121 16 Prague 2, Czechia(Received 9 August 2016; accepted 10 October 2016; published online 27 October 2016)
Boron (B) and phosphorous (P) codoped silicon quantum dots (Si QDs) are dispersible in polar
solvents without organic ligands, and exhibit size controllable photoluminescence (PL) from 0.85
to 1.85 eV due to the electronic transitions between the donor and the acceptor states. We study the
PL spectra of the codoped Si QDs at room temperature and at 77 K. We show that the broad PL
band of codoped colloidal Si QDs (full width at half maximum is over 400 meV) is composed ofnarrower PL bands of individual QDs with different PL energies. We also show that the PL
linewidth of individual codoped Si QDs is almost twice as large as those of undoped Si QDs. In
contrast to the significant narrowing of the PL linewidth of undoped Si QDs at low temperatures,that of codoped Si QDs is almost independent of the temperature except for a few very small QDs.
These results suggest that a large number of B and P are doped in a QD and there are a number of
non-identical luminescence centers in each QD. Published by AIP Publishing.
[http://dx.doi.org/10.1063/1.4965986]

I. INTRODUCTION

Bulk silicon (Si) crystal is not suitable for light emitting
applications due to the indirect nature of the energy band
structure. Since the observation of visible photoluminescence (PL) from porous Si,1 Si quantum dots (QDs) havebeen actively studied as a promising material for optoelectronic devices2–5 as well as fluorescence markers in bioimaging and biosensing.6,7 The luminescence property of Si QDs
can be controlled by the size,8,9 surface termination,10,11
chemical doping via surface organic molecules,12 and substitutional impurity doping. Recently, the importance of thesubstitutional impurity doping to Si QDs has been recognized for the optoelectronic device applications, and thedevelopment of doped Si QDs has been reported.13–18
Furthermore, the preferential location of doped impurities
and the energy state structures of doped Si QDs have beenstudied theoretically.19–24
Impurity doping modifies not only the energy state
structures of Si QDs but also the chemical properties. For
example, simultaneously boron (B) and phosphorus (P)doped Si QDs are dispersible in polar solvents without
organic surface ligands due to the negative surface potential
induced by a heavily B and P codoped surface layer.25–27
Therefore, one can produce all-inorganic colloidal Si QDs
by B and P codoping.25,28,29 Because of the high water dispersibility, bright near infrared (NIR) luminescence, highphoto-stability, high pH stability, etc., the codoped Si QDs
are a promising alternative to organic dyes for bioimaging
and biosensing.30 In vitro fluorescence imaging of cancer
cells by codoped Si QDs has been recently demonstrated.31a)Electronic mail: fujii@eedept.kobe-u.ac.jp. Tel.: þ81-78-803-6081.

0021-8979/2016/120(16)/164307/6/$30.00

The size-dependence of the energy state structure of B
and P codoped Si QDs has been studied by PL spectroscopy,32 photoemission yield spectroscopy (PYS),33 andvalence band X-ray photoelectron spectroscopy (XPS).33
These measurements reveal that the PL of codoped Si QDs
arises from the transition between the donor to acceptor
states in the bandgap, and the PL energy is several hundred
meV lower than that of undoped Si QDs of a comparable
size. The position of the Fermi level in the bandgap is also
estimated; the Fermi level of codoped Si QDs is close to
the valence band when the size is large, and it approaches
the middle of the bandgap when the size is below 3 nm.33
The PL decay dynamics are also different from those of
undoped Si QDs. The lifetime is shorter than those reported
for undoped Si QDs, and the detection energy dependence ismuch smaller.32
Although it has been previously demonstrated that impurity doping is an effective method to modify the opticalresponse of Si QDs, quantitative understanding of the energy
state structure, by comparing experimental data with theoretical calculations, has not been successful. The difficulty arises
mainly from inhomogeneous broadening of the opticalresponse of codoped Si QDs. In fact, the full width at half
maximum (FWHM) of codoped Si QDs is much larger thanthat of undoped Si QDs.34,35 This is probably due to inhomogeneity of the impurity sites and numbers as well as the size
and the shape. Different techniques have been developed totake away the inhomogeneity. Mastronardi et al. and Miller
et al. succeeded in size purification of Si QDs using density
gradient ultracentrifugation.36,37 This is a very powerful technique to remove inhomogeneous broadening by size distribution, but is not effective for the broadening arising from the
distributions of impurity sites and numbers. Spectroscopic

120, 164307-1

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TABLE I. List of samples.
RT

77 K

Sampledavea (nm)

Eensb (eV)

FWHM (meV)

Esinc (eV)

FWHM (meV)

Eensb (eV)

FWHM (meV)

Esinc (eV)

FWHM (meV)

T1000
T1050
T1100

2.0
2.7
3.5

1.76
1.59
1.43

408
426
386

1.74 6 0.05
1.58 6 0.06
1.42 6 0.04

240 6 58
240 6 50224 6 46

1.78
1.66
1.45

397
389
356

1.75 6 0.05
1.63 6 0.03
1.43 6 0.05

269 6 90244 6 47
239 6 67adave is the average QD diameter estimated by TEM observation.
Eens is the PL peak energy of an ensemble of QDs.
c
Esin is the average PL peak energy of single QDs and the standard deviation.
b

techniques such as hole burning spectroscopy,38,39 fluorescence line narrowing spectroscopy,40,41 and single-dot
spectroscopy42,43 have been applied to study homogeneous
linewidth of Si QDs. Among them, the most successful is the
single-dot spectroscopy, which observes PL from a single
QD on a low QD density substrate (1 QD/lm2) using a fluorescence microscope. FWHM of undoped Si QDs is reported
to be 100–200 meV at room temperature.44–54 It reduces to
10–30 meV at 80 K (Refs. 47 and 55) and 200 leV at
10 K.56
In this work, we perform single-dot spectroscopy for B
and P codoped Si QDs with different sizes. We show that the
FWHMs of single codoped Si QDs are almost twice larger
than those of undoped Si QDs at room temperature. We also
show that narrowing of the width at low temperature (LT),which has been observed in undoped Si QDs, is not observed
except in a few very small QDs.
II. EXPERIMENTAL

Colloidal B and P codoped Si QDs were prepared using
the method used in our previous work.25 Borophosphosilicate
glass (BPSG) films containing B and P codoped Si QDs werefirst prepared by co-sputtering of Si, B2O3, P2O5, and SiO2
and annealing at 1000, 1050, and 1100  C for 30 min in anitrogen atmosphere. The film was dissolved in a hydrofluoric
acid (HF) solution for 1 h in order to etch out the BPSG matrices. The isolated Si QDs were separated from the HF solution
by centrifugation and then transferred to methanol. A majorityof Si QDs were dispersed in methanol. Precipitates were
removed by centrifugation, and only the supernatant liquid
was collected. Similarly, undoped Si QDs embedded in the
SiO2 matrix were fabricated by sputtering of Si and SiO2, andannealing at 1100  C in the same manner. Unlike codoped Si
QDs, undoped Si QDs form aggregates and precipitate inmethanol. The list of the samples studied is shown in Table I.
Hereafter, we refer to the samples by the name designated in
the leftmost column of the table. The number in a sample
name represents the annealing temperature.
Figure 1(a) shows a photograph of the colloidal solutionof T1100. The solution remains clear and precipitates are
not formed over several years. Figure 1(b) shows a highresolution transmission electron microscope (TEM) (JEOL,
JEM-2100F) image of Si QDs prepared by drop-casting
the solution on a carbon-coated copper grid. Lattice fringe
corresponding to {111} planes of Si crystal (0.314 nm) canbe seen. In TEM image, no three-dimensional agglomeratesof Si QDs were observed due to the perfect dispersion inmethanol. Figure 1(c) shows an atomic force microscope(AFM) (Park Systems, NX10) topography image of Si QDs
on a fused quartz substrate. The QDs are isolated on the substrate. The size of the QDs estimated from the height profiles(Fig. 1(d)) is consistent with the size distribution obtained
from TEM images. These results indicate that by simply
drop-casting diluted solution, we can prepare samples for
single-dot spectroscopy. The samples for single-dot spectroscopy were prepared by dropping 1.5 ll colloidal Si QDs(1 lg/ml) onto fused quartz substrates for room temperature measurements and onto Si substrates for low temperature measurements.
PL spectra of colloidal solutions were measured by a spectrophotometer (HORIBA, Fluorolog-3). The excitation source
was a monochromatized xenon lamp (405 nm). PL images andspectra of single Si QDs were obtained by using an imaging
spectrometer (Acton SP-2300i) equipped with a liquid nitrogencooled charge coupled device (CCD) (Princeton Instruments,
Spec-10:400B) connected to an inverted optical microscope

FIG. 1. (a) Photograph of colloidal codoped Si QDs in methanol (T1100).
(b) High-resolution TEM image of a Si QD (T1100). (c) AFM topography
image of Si QDs on a fused quartz substrate (T1050). (d) Height profile atthe position marked in the topography image. The profile is obtained by subtracting the lowest height as an offset value from the measured height.

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Kanno et al.

(Olympus IX-71, objective lens: 100/0.8 at RT and 40/0.6
at LT). The excitation source was a 405 nm semiconductor
laser (1 W/cm2 at sample). For low temperature measurements, samples were mounted on a liquid nitrogen flow cryostat (Janis, ST-500) and cooled down to 77 K. The sequence ofthe measurements is shown in the supplementary material(Figure S1). All spectra were corrected for the spectral
response of the apparatus.
III. RESULTS AND DISCUSSION

Figure 2(a) shows the PL spectra of colloidal Si QDs in
methanol. The size dependent shift of the PL can be clearly
seen. The PL energy is several hundred meV lower thanthose of undoped Si QDs of comparable size,32 and the PL isconsidered to arise from electronic transitions between donor
and acceptor states.33 The FWHM of the PL bands is around
400 meV, which is much broader than those reported for

FIG. 2. (a) The PL spectra of colloidal Si QDs dispersed in methanol. ((b),(c)) Normalized PL spectra of single codoped Si QDs (T1050) measured at(b) room temperature and (c) 77 K. The spectra are fitted by a Gaussianfunction. The black solid lines are the PL spectra obtained from larger areas
containing large numbers of Si QDs.

J. Appl. Phys. 120, 164307 (2016)

undoped Si QDs.34,35 In Fig. 2(b), the PL spectra of Si QDs
on a quartz substrate at room temperature is shown (T1050).
The spectrum labeled “ensemble” is obtained by detecting
a large area, which contains about 100 Si QDs, while theothers are those of single Si QDs. The spectra of single Si
QDs are fitted by Gaussian functions. The PL peak energy(1.59 eV) and the FWHM (426 meV) of the “ensemble” arealmost the same as those of the colloidal solution. This indicates that the PL property of codoped Si QDs is not strongly
modified by drying in air (in contrast to some organicallypassivated QDs). The ensemble PL spectrum shifts to higher
energy (1.66 eV) at 77 K. However, no significant change of
the FWHM (389 meV) is observed.
The PL spectra of single Si QDs are much narrower thanthat of the ensemble; the PL FWHM is from 223 to 290 meV,nearly the half of that of the ensemble. The PL peak energy
varies from QD to QD, although all the spectra are locatedwithin the broad PL band of the ensemble. Figure 2(c) shows
the PL spectra of single codoped Si QDs and the ensemble at
77 K. Similar to the room temperature data, the FWHM of single Si QDs is much narrower than that of the ensemble. The
PL spectra of other samples are shown in the supplementarymaterial (Figures S2 and S3).
Figures 3(a)–3(c) show the histograms of PL peak energies of codoped Si QDs together with the PL spectra of theensembles. The data are obtained for more than 50 single
QDs. The distributions of the PL peak energies are in good
agreement with the PL spectra of ensembles, indicating thatthe broad PL of the ensembles is due to the distribution of
the PL peak energies of individual QDs. Figures 3(d)–3(f)are the data obtained at 77 K. There is a slight narrowing anda tiny blue-shift of the PL peak distributions.
Figure 4 shows the histograms of PL FWHMs of single
codoped Si QDs. For all the samples, the FWHM is distributed
in a wide range. At room temperature, the largest FWHM
is around 350 meV and the smallest one is around 100 meV.
The average FWHM is 250 meV, which is larger than those

FIG. 3. Histograms of PL peak energy of codoped Si QDs measured at((a)–(c)) room temperature and ((d)–(f)) 77 K. ((a), (d)) T1000, ((b), (e))
T1050, ((c), (f)) T1100. Solid lines are the PL spectra obtained from larger
areas containing large number of Si QDs.

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J. Appl. Phys. 120, 164307 (2016)

FIG. 4. Histograms of PL FWHM of codoped Si QDs measured at ((a)–(c))
room temperature and ((d)–(f)) 77 K. ((a), (d)) T1000, ((b), (e)) T1050, ((c),(f)) T1100.

reported for undoped Si QDs luminescing in the same energy
range.45–47,50,53 For T1050 and T1100, no significant difference can be seen between room temperature and 77 K. On theother hand, in T1000, several QDs have the FWHM smaller
than 100 meV at 77 K. This will be discussed later.
Figure 5(a) shows the relationship between the FWHM
and the energy of the PL at room temperature. The data of
undoped Si QDs obtained in this work and those taken from literature are also shown. The gray area represents the typicalrange of the FWHM of undoped Si QDs, which is 50–200 meV
independent of the PL energy.46–54 On the other hand, the
FWHM of B and P codoped Si QDs is distributed from 100 to
350 meV. The average FWHM is almost twice as large incodoped Si QDs as that in undoped Si QDs. In undoped Si
QDs, the PL linewidth is considered to be determined by the
coupling of the optical transitions with the momentum conservation phonons (TO phonon at the X point: 56 meV and at theC point: 64 meV).57 In this model, the largest width was limited to 130 meV, and thus, very large FWHM of codoped Si
QDs can not be explained. One possible reason for the large
width may be that there is more than one non-identical luminescence center in each QD due to the doping of large numbersof B and P atoms. In Ref. 32, the average number of B and P
atoms per a Si QD is estimated. In the present samples, morethan 10 B and P atoms are doped in each QD, and a number ofthem are considered to be doped in the substitutional sites.26
Theoretical calculations demonstrate that the energy state
structures of Si QDs depend strongly on the location of the
impurity sites and the number.21,22,58,59 Therefore, doping of alarge number of impurities and existence of a number of luminescent centers in a QD may result in a significant broadeningof the PL band.
In Figure 5(a), a few QDs in T1000 have the FWHM
comparable to those of undoped Si QDs. The average diameterof T1000 is 2 nm. At this size, one impurity atom corresponds
to the doping concentration of 2.4  1020 cm3, which iscomparable to bulk solid solubility of P (4  1020 cm3). It is
plausible that, in some QDs in T1000, there are no active

FIG. 5. Relation between the PL FWHM and the PL peak energy measured
at (a) room temperature and (b) 77 K. Filled symbols represent experimental
data in this work. Open symbols represent the data of undoped Si QDs
obtained from Refs. 46–54 in (a) and Refs. 47, 55, 56, and 60 in (b).

impurities, which results in the PL spectra with the FWHM
comparable to that of undoped ones.
Figure 5(b) shows the relationship between the FWHM
and the PL peak energy at 77 K. As shown by the shadow,
the FWHM of undoped Si QDs is smaller than 50 meV,which is much smaller than those at room temperature. On
the other hand, the FWHM of the codoped Si QDs does notstrongly depend on the temperature. This supports the modelthat the PL width of codoped Si QDs is determined by the
distribution of the impurity sites in individual QDs.
Although the FWHM of almost all codoped Si QDs isinsensitive to the measured temperature, there are a few
exceptions in T1000. In Figure 5(b), several QDs have the
FWHM smaller than 50 meV at 77 K, which is comparable
to that of undoped Si QDs. In these QDs, impurities are considered to be absent. As discussed above, in Si QD ensembles, the PL peak energy of codoped QDs is smaller than thatof undoped ones. If impurities are not doped in Si QDs withnarrow PL bands, the PL energy should be higher than otherswith broad PL bands. Although we do not have enough number of narrow spectra for the statistical analyses, the PL energies of QDs with narrow bands are at almost the highest endof the distribution (Fig. 5(b)).
Figure 6 shows an example of the sharp PL. The widthof the peak is about 15 meV. The peak seems to have a broad
satellite at the low energy side of the main peak. Similar

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J. Appl. Phys. 120, 164307 (2016)

ACKNOWLEGMENTS

The authors thank Y. Fukuda and A. Otsuki forcontributions to this work. This work was partly supported by
the 2015 JST Visegrad Group (V4)-Japan Joint Research
Project on Advanced Materials (MSMT Project No. 8F15001,
cofinanced by the International Visegrad Fund) and JSPS
KAKENHI Grant No. 16H03828. T.K. and H.S. acknowledge
the support from Grant-in-Aid for JSPS Research Fellow(27-4138 and 26-3120).
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FIG. 6. A PL spectrum of a single Si QD (T1000) at 77 K. The spectrum isfitted by two Gaussian functions.

spectra have been observed in undoped Si QDs at low
temperatures.55 Usually, the narrow band is assigned to azero-phonon transition and the low-energy one to a phononassisted transition with the emission of a momentum conservation TO phonon.55,60 Similar explanation might be applied
to the spectrum in Fig. 6, although the low-energy bandis much broader than those reported in undoped Si QDs.
Another possible explanation of the broad satellite band is
the overlap of PL spectra of one undoped and one codoped
QDs.
IV. CONCLUSION

We have studied the PL spectra of B and P codoped Si
QDs by single-dot spectroscopy. Similar to the case of
undoped Si QDs, the very broad PL bands of codoped Si
QDs observed for the colloidal solution and the ensemble
were partly due to the wide variation of the PL peak energy
between QDs. However, we also found that the PL width ofindividual codoped QDs was nearly twice larger than that ofundoped Si QDs at room temperature. The large PL width ofindividual QDs may suggest that there are several luminescent centers in each QD, and the variation of the local environment of the impurity sites results in the broadening. The
difference of the PL width between undoped and codoped Si
QDs was enhanced at low temperatures. In undoped Si QDs,
the width decreases significantly due to the lower probabilityof phonon absorption. On the other hand, in codoped Si
QDs, the width is almost independent of the measured temperature except for a few very small QDs. The temperature
insensitiveness supports the model that a number of nonidentical luminescence centers exist in each QD. In a few
very small QDs, a very sharp PL peak with a phonon satellite, similar to that reported for undoped Si QDs, is observed.
In these QDs, impurities are expected to be absent.
SUPPLEMENTARY MATERIAL

See supplementary material for method of single-dot
spectroscopy of single Si QDs and PL spectra from different
single Si QDs.

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