JOURNAL OF APPLIED PHYSICSVOLUME 94, NUMBER 3

1 AUGUST 2003

Below bulk-band-gap photoluminescence at room temperature from heavily
P- and B-doped Si nanocrystals
Minoru Fujii,a) Kimiaki Toshikiyo, Yuji Takase, Yasuhiro Yamaguchi, and Shinji Hayashi
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University,
Rokkodai, Nada, Kobe 657-8501, Japan

共Received 1 April 2003; accepted 19 May 2003兲
Photoluminescence 共PL兲 properties of heavily P- and B-doped Si nanocrystals 共nc-Si兲 are studied.
By simultaneously doping two types of impurities, nc-Si exhibit strong PL at around 0.9 eV at room
temperature. The temperature quenching of the PL is very small. Although the PL peak energy isvery close to that of dangling-bond related PL previously observed, all of the observed properties,i.e., decay dynamics, degree of temperature quenching, etc., are apparently different. The transition
between donor and acceptor states in nc-Si is the possible origin of the low-energy PL. © 2003
American Institute of Physics. 关DOI: 10.1063/1.1590409兴

I. INTRODUCTION

approach does not use luminescing species like Er3⫹ , but
controls the electronic band structure of nc-Si by shallow
impurity doping.
Doping of shallow impurities causes the formation of
impurity states in the band gap of nc-Si. The transition ofelectrons between the states and a conduction 共valence兲 bandmay exhibit PL below the energy of exciton PL of nc-Si.
However, since the conduction- 共valence- 兲 band edge isshifted to higher 共lower兲 energy by the quantum size effects,
the transition energy is expected to be larger than the bandgap energy of bulk Si crystals. In order to realize below
bulk-band-gap PL, we dope n- and p-type impurities simultaneously into nc-Si. In this system, electronic transition between donor and acceptor states is possible. Since the energy
levels of hydrogenic impurities are not strongly affected by
the size of the nc-Si,16,17 the transition between the states will
be below the bulk Si band-gap even if the size of the host Si
is in the range of nanometers.

It is well known that luminescence energy of Si nanocrystals 共nc-Si兲 can be tuned from a bulk bandgap 共1.12 eV兲to the visible region by simply controlling the size.1,2 The
temperature quenching of this luminescence is very small
and it is observed at room temperature. Based on detailed
optical studies, these near infrared to red luminescence areassigned to the recombination of excitons confined in
nc-Si.1– 6
In addition to the exciton luminescence, broad luminescence has been observed at around 0.8 to 1.0 eV for somekinds of nc-Si systems.2,7–9 Dangling bond defects at Si/SiO2
interfaces are considered to be responsible for this luminescence. The luminescence energy can also be tuned by thesize of nc-Si, although the size dependent shift is about halfthat of the exciton luminescence.2,8,9 This luminescenceshows strong temperature quenching, and luminescence intensity at room temperature is extremely weak. If efficientluminescence is realized below the band gap of bulk Si crystals at room temperature, application of nc-Si will be extended to the field of optical telecommunications, because
silica-based fiber telecommunication utilizes light in the
wavelength range of 1.3 to 1.6 ␮m.
One approach to realize room-temperature below bulkband-gap luminescence from nc-Si is to introduce luminescing species in nc-Si assemblies and indirectly excite the species by the energy transfer from nc-Si. The most important
example of this approach is Er doping. It has been demonstrated that if Er3⫹ is in the vicinity of nc-Si, it is excitedvery efficiently by the energy transfer from nc-Si, and exhibits strong photoluminescence 共PL兲 at 0.81 eV at room
temperature.10–13 Based on this approach, planer-waveguidetype optical amplifiers and electroluminescence devices havebeen realized.14,15
In this article, we report another approach to realize
room-temperature below bulk-band-gap PL from nc-Si. This

II. EXPERIMENTAL PROCEDURE

P- and B-doped nc-Si were prepared by a cosputtering
method.18 –23 Si, borosilicate glass 共BSG兲, and phosphosilicate glass 共PSG兲 were simultaneously sputterdeposited in Ar
gas, and the deposited films 共about 1 ␮m in thickness兲 wereannealed in a N2 gas 共99.999%兲 atmosphere for 30 min at
1150 °C. nc-Si were grown in borophosphosilicate glass
共BPSG兲 films during the annealing. P 共B兲 concentration infilms was controlled by changing the P2 O5 (B2 O3 ) concentration in the PSG 共BSG兲 sputtering targets. The average
concentration of P2 O5 in a whole film (C P) was determinedby an electron probe microanalysis, and that of B2 O3 (C B)
by the intensity ratio of Si–O and B–O vibration signals ininfrared absorption spectroscopy.24 In this method, the sizeof nc-Si can be controlled by changing the concentration of
Si in films or the annealing temperature. The average size ofnc-Si was estimated by cross-sectional transmission electron
microscopic 共TEM兲 observations.
Figure 1 shows a TEM image of a sample with high P
and B concentration (C P⫽1.26 mol % and C B⫽1.25 mol %兲.

a兲Author to whom correspondence should be addressed; electronic mail:fujii@eedept.kobe-u.ac.jp

0021-8979/2003/94(3)/1990/6/$20.00

1990© 2003 American Institute of Physics

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Fujii et al.

J. Appl. Phys., Vol. 94, No. 3, 1 August 2003

FIG. 1. TEM image of nc-Si in a BPSG thin film. Lattice fringes corresponding to 兵111其 plans of Si can clearly be seen. Nanocrystals are almost
spherical and crystallinity is very good.

Nanocrystals are almost spherical and lattice fringes corresponding to 兵111其 planes of Si are clearly seen in Fig. 1.
TEM observations revealed that at low P and B concentrations the size of nc-Si is almost independent of impurity
concentration, while at high concentration 共e.g., ⬎1 mol %),the average size becomes larger than that of nc-Si in pure
SiO2 with the same Si concentration. Furthermore, size distribution becomes broader with increasing impurity concentration. Especially, a gradient of the size appears on the direction of the sample depth; the average size near a substrateis larger than that near a surface. The softening of matrices
by P and B doping and the resulting longer diffusion lengthof Si atoms during annealing makes particles larger. This
also makes size distribution broader because Si atoms that
diffuse to the surface of films are oxidized by residual oxygen in an annealing atmosphere. The largest average diameter of nc-Si studied in this work was about 10 nm.
The evidence that impurities are doped into substitutional sites of nc-Si and are electrically active is obtained
from infrared absorption measurements. If either P or B isdoped, broad absorption which continuously increases to alonger-wavelength region appears and this absorption becomes stronger with increasing impurity concentration.18
This absorption is assigned to the intraconduction- 共intravalence兲 band transition of carriers supplied by impurity
doping. We have studied infrared absorption spectra as afunction of P and B concentration. We found that under afixed B 共P兲 concentration, the increase of P 共B兲 concentration
results in the quenching of this absorption. This means thatcarriers are compensated in nc-Si. It is worth noting that in
nc-Si, the number of impurities is very small even if the
impurity concentration is very high, and thus it is changed
digitally. Therefore, if the average number of P and B in awhole system is nearly the same, it is possible that, in someparticles in a sample, the numbers of donors and acceptors
are exactly the same; there exists some perfectly compensated nc-Si.
Unfortunately, the number of active dopants or excess
carriers in nanocrystals cannot simply be estimated from average P or B concentrations because of the following reasons. First, it is not clear whether dopants are uniformly dis-

1991

tributed in samples or segregated in some regions, e.g., atinterfaces between nanocrystals and matrices. The distribution may also depend on the kind of impurities. Second, thenumber of excess carriers is not always the same as that of
dopants. It may depend on the size and quality, i.e., the number of defects, of nc-Si. Third, since size distribution exists
in samples, statistically, doping starts from larger nc-Si insize distribution and proceeds to smaller ones with increasing dopant concentration.
However, rough estimation is possible. In our previous
work on electron spin resonance of P-doped samples,21 withincreasing P concentration, defect-related signals became
weaker and a hyperfine structure of conduction electron signal emerged. For example, with C P⫽0.4 mol % and an average nc-Si diameter of 5.8 nm, both defect-related andconduction–electron related signals were observed. If we assume that nc-Si with the average diameter has one electrically active P and smaller ones are still pure, the average P
concentration in nc-Si with the average size is ⬃1⫻1019 cm⫺3 . In the same study, at C P⫽1.2 mol %, both thedefect-related and hyperfine structure of conduction electron
signals disappeared, and only the broad conduction electron
signal remained. This suggests that in majority of nc-Si insize distribution, more than two electrically active P exist,
corresponding to the dopant concentration of ⬃1020 cm⫺3 .
Since we cannot estimate the exact number of active
dopants in nanocrystals, we also cannot know the number ofcompensated nc-Si in the samples. Therefore, in this work,
we fixed the B concentration to some values and changed
only the P concentration. With this approach, we can find an
optimum condition to maximize the number of compensatednc-Si from the PL intensity, because the PL intensity of acompensated nc-Si is expected to be larger than that of an
uncompensated nc-Si.
PL spectra were measured using a single monochromatorequipped with an InP/InGaAs photocathode near-infrared
photomultiplier. The excitation source was the 488 nm lineof an Ar-ion laser. The excitation power was 4 mW/cm2 . The
laser beam was chopped by using an acousto-optic modulator at a frequency of 16 Hz. The spectral response of thedetection system was calibrated with the aid of a reference
spectrum of a standard tungsten lamp. The PL decay curves
were measured by pumping to steady state and chopping thelaser with the same frequency as used for the PL measurements. The overall time resolution of the system was about
40 ns. The PL and the PL decay dynamics were measured
from 4 to 295 K in a continuous-flow He cryostat.
III. RESULTS AND DISCUSSION
A. B doping

Before doping P and B simultaneously in nc-Si, we summarize how the PL spectra of nc-Si is modified by B
doping.19,20 Figure 2 shows PL spectra of pure and B-dopednc-Si at room temperature. Pure nc-Si exhibits a broad PL
peak centered at around 1.35 eV. The PL is assigned to therecombination of excitons confined in nc-Si. By doping B,
the spectrum is strongly modified. First, the intensity is decreased significantly. Second, a long tail appears on the low-

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J. Appl. Phys., Vol. 94, No. 3, 1 August 2003

Fujii et al.

FIG. 2. PL spectra of B-doped Si nanocrystals. For better comparison ofspectral shape, the spectra of B-doped samples are scaled by the indicatedmultiplication factor. With increasing B concentration, PL becomes weaker
and a broad tail appears below the band gap of bulk Si crystals. The integral
intensity of the spectra are plotted in the inset.

energy side, and the tail penetrates below the band gap ofbulk Si crystals. In the inset of Fig. 2, the integral PL intensity is plotted as a function of B concentration. The intensity
is decreased down to 5% of the value of pure nc-Si at thehighest B concentration. The observed quenching is considered to be due to the small PL quantum efficiency of
B-doped nc-Si because nonradiative Auger recombination of
excitons with the interaction of holes supplied by B doping isvery efficient.19,20 However, this does not always mean that
PL observed in B-doped samples comes only from pure nc-Si
remaining undoped in samples. The fact that the PL tail penetrates into below the bulk Si band gap indicates that at least
the low-energy tail part of the spectra arises from B-dopednc-Si.
In Fig. 2, the PL peak shifts to lower energy with increasing B concentration. However, this does not always
happen. We have studied more than five series of similar
samples. Although the quenching and broadening of PL wereobserved for all the samples, the low energy shift was not
reproduced in some samples. In the case of pure nc-Si, ourpreparation method is very stable, and PL peak energy iscontrolled with good accuracy; only for heavily impuritydoped samples, the PL properties fluctuated. As just described, there exists large inhomogeneity in heavily
impurity-doped samples. This inhomogeneity is considered
to be responsible for the observed fluctuation of PL properties.
B. B and P doping

Now, we dope P and B simultaneously and see how thePL spectrum is developed with increasing P concentration.

FIG. 3. P concentration dependence of PL spectra. B concentration is fixed
to 共a兲 C B⫽0.76 mol % and 共b兲 C B⫽1.25 mol %, and P concentration isincreased up to 1.26 mol %.

Figure 3 shows PL spectra of P- and B-doped nc-Si. In Fig.
3共a兲, C B is fixed to 0.76 mol % and C P is changed from 0 to
1.23 mol %. In Fig. 3共b兲, C B is fixed to 1.25 mol % and C P ischanged from 0 to 1.26 mol %. In both cases, the PL shows
similar behavior. PL peak shifts continuously to a lower energy, crosses the band gap of bulk Si crystals and reaches
around 0.95 eV 共Fig. 4兲. The other important effect of P
doping is the enhancement of the PL intensity. In Fig. 3共b兲,the PL intensity is increased more than five times by P doping, meaning that quenching of PL by B doping is largely
recovered by adding P; the PL intensity of the sample withC B⫽1.25 mol % and C P⫽1.26 mol % is about 30% of thatof pure nc-Si.
However, here we also have a problem of reproducibility. The increase in PL intensity was observed for all series ofsamples until C P⯝0.9%. On the other hand, above that concentration, intensity was not always increased, and sometimes it was decreased. This fluctuation of the PL intensity ata high impurity concentration range may also arise from theinhomogeneity of samples.
Although there still remain difficulties in precisely controlling all sample preparation parameters, the concept of thepresent approach is very important in view of future photonic
application of nc-Si, i.e., the simultaneous doping of P and B

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J. Appl. Phys., Vol. 94, No. 3, 1 August 2003

Fujii et al.

1993

FIG. 4. PL peak energies of P and B codoped nc-Si are plotted as a functionof P concentration. B concentration is fixed to 0.76 and 1.25 mol %.

in nc-Si offers an opportunity to extend the tunability of PLenergy of nc-Si systems below bulk Si band gap without
losing its intensity very much.
C. Properties of the low-energy photoluminescence

Figure 5 shows PL decay curves of pure nc-Si, and P and
B codoped nc-Si at 5 K. The energy where the PL signal isdetected is indicated by arrows in the inset of Fig. 5 共1.18
and 0.92 eV, respectively兲. The exciton PL of pure nc-Si
shows a single-exponential decaying behavior. The lifetime
is about 8 ms. On the other hand, the decay curve of codopednc-Si is far from the single-exponential function and the lifetime is much shorter than the exciton PL of pure nc-Si. We
measured the PL lifetime at other energies on broad PL
bands. The lifetime was in the range of 100 to 500 ␮s. Figure
6 shows the temperature dependence of PL lifetime of acodoped sample. The lifetime becomes shorter with increasing temperature. The shortening may be due to an increase ofnonradiative recombination processes. In comparison, we
measured the decay curve of dangling bond PL of pure nc-Si
that is commonly observed for some kinds of nc-Si systems
at around 0.9 eV;2,7–9 the PL detection energy 共0.92 eV兲 isindicated by an arrow in the inset of Fig. 5. The PL lifetime
was of the order of ␮s and much shorter than that of codoped
samples, although detecting energy is the same.
In Fig. 7, PL peak intensities of the co-doped nc-Si 共䊏兲
and nc-Si in pure SiO2 共average diameter of about 9 nm兲 共䉱兲
are plotted as a function of temperature. The exciton PL ofpure nc-Si shows almost no temperature quenching. The
temperature quenching of PL is also very small for codoped
samples; the PL intensity at room temperature is about thehalf of that at 5 K. This small temperature quenching is

FIG. 5. PL decay curves of P and B codoped nc-Si, and pure nc-Si at 5 K.
In the inset, energy positions detecting PL decay curves are indicated byarrows.

completely different from that of dangling-bond-related PL.
The open triangles in Fig. 7 represent the PL intensity of thedangling-bond-related PL at 0.92 eV. We can see that the PL
intensity drops very rapidly with increasing temperature and

FIG. 6. Temperature dependence of PL lifetime for P and B codoped nc-Si

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1994

Fujii et al.

J. Appl. Phys., Vol. 94, No. 3, 1 August 2003

the quantum confinement effect may play a positive role.21
Furthermore, softening of matrices by introducing P and B
heavily in SiO2 is considered to be partially responsible forthe enhancement of PL intensity. Since BPSG is softer thanpure nc-Si, stress at nc-Si / matrix interfaces is smaller, resulting in a smaller defect density. This causes a smaller
number of defective 共dark兲 nc-Si, and larger number of optically active nc-Si. The observed wide spectral width could
be inhomogeneous broadening caused by the distribution of
the size and the impurity concentration.
IV. CONCLUSION

In conclusion, we have observed strong roomtemperature PL from nc-Si in BPSG thin films at around 0.9
eV. The properties of the low-energy PL were apparently
different from those of exciton PL and dangling-bond-related
PL. Since the below bulk Si band-gap PL was observed only
for P and B codoped nc-Si, the PL was assigned to be due tothe electron transition between the donor and acceptor statesin nc-Si. With this approach, the luminescence peak energyof nc-Si can be tuned from 0.9 eV to 2.0 eV by controllingthe size and the impurity concentrations.

FIG. 7. Temperature dependence of PL peak intensities for nc-Si in BPSG
共䊏兲 and exciton PL of nc-Si in pure SiO2 共䉱兲. Intensity of dangling-bondrelated PL 共䉭兲 is also plotted.

that at room temperature is about 1% of that at low temperatures. Figure 7 clearly demonstrates that the mechanism of
the low-energy PL is different from that of dangling-bondrelated PL previously observed.
The fact that below bulk-band-gap PL was observed only
for codoped samples indicates that both donor and acceptor
states are involved in the recombination process. PL originating from the transition between the conduction-band tail 共donor band兲 to the valence-band tail 共acceptor band兲 has been
observed for heavily doped, compensated bulk Si:P,B.25 In
these systems, as the total number of donor and acceptor
impurities increases, PL bands shift to lower energy and are
significantly broadened.25 The mechanism of the low-energy
PL observed in this work is considered to be essentially thesame as that observed for heavily doped compensated bulk
Si:P,B; PL arises from the electronic transition between the
conduction- and valence-band tails induced by impurity doping. However, there are some crucial differences. The largest
difference appears on the temperature dependence of the PL
intensity. In heavily impurity-doped bulk Si crystals, PL isobserved only at very low temperatures because excited electrons and holes are easily thermalized or migrated in impurity bands and recombine nonradiatively. On the other hand,in nc-Si, transportation of carriers is limited only within ncSi. If the surface of nc-Si is perfectly passivated and two
types of impurities are compensated, nonradiative recombination processes would be very small. This may result in theobserved strong PL and very small temperature quenching.
Enhancement of the ionization energy of impurities due to

ACKNOWLEDGMENTS

This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan, and in part by an
Industrial Technology Research Grant Program in ’02 from
New Energy and Industrial Technology Development Organization 共NEDO兲, Japan.
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