APPLIED PHYSICS LETTERSVOLUME 85, NUMBER 7

16 AUGUST 2004

Control of photoluminescence properties of Si nanocrystals
by simultaneously doping n- and p-type impurities
Minoru Fujii,a) Yasuhiro Yamaguchi, Yuji Takase, Keiichi Ninomiya, and Shinji Hayashi
Faculty of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan(Received 24 February 2004; accepted 17 June 2004)
The effects of B and P codoping on photoluminescence (PL) properties of Si nanocrystals (nc-Si) arestudied systematically. It is shown that the PL intensity of codoped nc-Si is always higher than thatof either P- or B-doped nc-Si. The intensity is sometimes even higher than that of pure nc-Si at
relatively low P and B concentrations and low annealing temperatures. By doping P and B
simultaneously to very high concentrations, the PL peak shifts below the band gap of bulk Si. ©
2004 American Institute of Physics. [DOI: 10.1063/1.1779955]
Si nanocrystals (nc-Si) show strong luminescence in thevisible range.1–7 The luminescence is considered to arise
from the recombination of excitons across the widened band
gap of nc-Si. The luminescence energy is controlled from thebulk band gap to visible range by size.5,7 The range can beextended below the bulk band gap by properly controlling
impurities in nc-Si;8 by doping P and B simultaneously, photoluminescence (PL) appears at around 0.9 eV at room temperature. The low-energy luminescence is probably due todonor to acceptor transitions. The near-infrared (NIR) luminescence at room temperature may open up applications ofnc-Si-based materials in the optical telecommunication field,
because silica-based fiber telecommunications utilize light in
the wavelength range of to 1.3 to 1.6␮m.
The largest problem of shallow impurity doping in nc-Si
is that it is accompanied by the radiationless Auger recombination, in which the energy released by the recombinationof an exciton is consumed by giving kinetic energy to anelectron (hole) supplied by doping.9–14 The Auger recombination could be avoided if isoelectronic impurities are dopedor carriers in nc-Si are perfectly compensated by simultaneously doping n- and p-type impurities.
In nc-Si, an exciton is confined in a space comparable toor smaller than the Bohr radius, and the wave function of theexciton as well as shallow impurities are extended in a wholenanocrystal. If for example one P atom and one B atom are
doped simultaneously in a nanocrystal a few nanometer indiameter, it could be treated as a perfectly compensatedsingle system with no excess carriers, and thus the exciton
could enjoy the enhancement of oscillator strength due tofurther localization by impurity doping without being afraidof the Auger recombination. Therefore, impurity control ofnc-Si may add optical properties on nc-Si which cannot berealized by pure nc-Si, and thus is worth studying in detail.
The above model is qualitatively supported by our previous observation that the simultaneous doping of n- andp-type impurities in nc-Si recovers the very low PL intensityof either n- or p-type impurity-doped nc-Si.8 However, systematic studies on the relation between PL properties and theconcentration and valance of n- and p-type impurities havenot been made so far. In this work, we have prepared Band/or P-doped nc-Si embedded in glass matrices by controlling the B and P concentrations in a wide range, and studied
the PL properties.
a)Electronic mail: fujii@eedept.kobe-u.ac.jp

The samples studied were nc-Si embedded in glass matrices prepared by a cosputtering method. A multitarget sputtering system was used. Four 2 in. sputtering guns with Si(No. 1), SiO2 (No. 2), a mixture of P2O5 and SiO2 (5:95 bywt. %) (phosphosilicate glass) (No. 3), and a mixture ofB2O3 and SiO2 (5:95 by wt. %) (borosilicate glass) (No. 4)
targets were operated simultaneously. The sputtering rate ofeach gun was controlled by a rf power. To keep excess Siconcentration, which is a major factor to determine the sizeof nc-Si, nearly the same for all the samples, the rf power of
the Si sputtering gun was fixed to 20 W, while the sum of thepowers of other three guns was set to 240 W. For example,
for the sample with the maximum P and B concentration, thepowers of the gun Nos. 3 and 4 were set to 120 W and No.
2 to 0 W, while for the sample with maximum P concentration without B doping, those of gun Nos. 2 and 3 to 120 Wand No. 4 to 0 W. The thickness of the film was fixed to beabout 1 ␮m for all samples. After depositing the mixture
films, a cap layer of pure SiO2 (100 nm) was deposited. Thesamples were then annealed in a N2 gas (99.999%) atmosphere for 30 min at temperatures ranging from1100 ° C to 1250 ° C to grow nc-Si.
The evidence that impurities are doped into substitutional sites of nc-Si and are electrically active was obtainedby the observation of the free-carrier absorption, i.e., intraconduction (valence)-band transitions of electrons (holes), in
infrared absorption spectra13 and by electron spin resonance(ESR) spectroscopy.14 Unfortunately, the number of active
dopants or excess carriers in each nanocrystal cannot simply
be estimated from the average P or B concentrations in awhole film. However, it is clear that the number is very small
and is changed digitally even when the impurity concentration is very high, e.g., an impurity concentration of 1
⫻ 1020 cm−3 corresponds to in average 1.4 impurities per ananocrystal 3 nm in diameter.
In Figs. 1(a)–1(d), PL spectra of pure, B-doped,P-doped, and P and B codoped nc-Si are compared. The
average impurity concentration in a whole film is written ineach figure. The annealing temperature is changed from1100 ° C to 1250 ° C. All spectra are taken with the samecondition. The intensity can thus be compared to each other.
The vertical axis is a logarithmic scale. Figure 1(a) shows PL
spectra of pure nc-Si. We can see that higher-temperatureannealing results in a lower-energy shift of the peak becauseof the increase of nc-Si size.7 The peak stays always above
the band gap of bulk Si crystals, although a weak tail, probably originating from disorder at the surface of nc-Si, pen-0003-6951/2004/85(7)/1158/3/$20.00
1158© 2004 American Institute of Physics
Downloaded 18 Nov 2004 to 133.30.106.15. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Appl. Phys. Lett., Vol. 85, No. 7, 16 August 2004

FIG. 1. Annealing temperature dependence of (a) pure nc-Si, (b) B-dopednc-Si, (c) P-doped nc-Si, and (d) B- and P-codoped nc-Si. All spectra aretaken at the same condition. A vertical line is drawn at the band-gap energyof bulk Si crystals 共1.12 eV兲.

etrates below the bulk band gap. In the present annealing
temperature range, PL intensity does not strongly depend on
the temperature.
The situation is changed if either P or B is heavily
doped. The spectra become broader and the low-energy tailgrows. In both cases, the intensity drops, especially at higher
annealing temperature. The intensity of the P-doped samples
is more sensitive to the annealing temperature; by 1250 ° C
annealing, the P-doped sample does not show any luminescence. The strong suppression of PL intensity is considered
to be due to the Auger recombination of excitons with theinteraction of electrons or holes supplied by doping. The
suppression becomes more significant at a higher annealing
temperature, because of a larger size of nc-Si and resultant
higher probability of containing electrically active impurities
in nc-Si. The different degree of PL quenching between Band P-doped samples may come from different solid solubil-Fujii et al.

1159

ity in Si.15 The spectral shape is also slightly different between the two kinds of dopants. The spectra of the B-doped
samples consist of two broad bands, while those of theP-doped samples are composed of a single band. It should be
stressed here that although higher annealing temperaturebrings the PL peaks to a lower energy, the PL maximum isalways above the bulk band gap.
The PL properties of B- and P-codoped nc-Si is qualitatively different from those of either B- or P-doped nc-Si. PL
peak energy shifts significantly to lower energy and crosses
the bulk band gap. Furthermore, suppression of PL intensity
by high-temperature annealing is smaller in spite of the factthat the total number of dopants is nearly twice as that of Bor P-doped samples in Figs. 1(b) and 1(c).The result demonstrates that the PL property of nc-Si can be modified without
losing the intensity so much by carefully controlling theamount of simultaneously doped n- and p-type impurities.
Figure 2 shows contour plots of the PL peak energy[Figs. 2(a)–2(d)] and intensity [Figs. 2(e)–2(h)] as a functionof B and P concentration. The B concentration was changed
from 0 to 1.24 mol % and P from 0 to 0.79 mol %. In total, the data of 16 samples with different P and B concentrations are plotted in each figure after a smoothing procedure.
Annealing
temperaturesarechanged
from1100 ° C to 1250 ° C. We can see that the PL peak tends toshift to lower energy by higher-temperature annealing regardless of impurity concentration. Within the same annealing temperature, higher impurity concentration results in alower peak energy. It is noted that codoped samples exhibit
PL peaks at energies lower than either B- or P-doped ones
even if the total amount of impurities is the same, indicatingthat the energy is determined by the valance of n- and p-type
impurities as well as the total amount.
The PL intensity also depends strongly on the valance ofimpurities. In Figs. 2(e)–2(h), we can see a ridge of the intensity along the white line, where the average concentrationof B and P in a whole film is the same; if we fix the total
amount of impurities the same the intensity is the highest
when the amounts of n- and p-type impurities are nearly thesame. This behavior can clearly be seen if we look along theblack lines in Figs. 2(e)–2(h), where the sum of CB and C P is
fixed to 0.8 mol %. The observation of the ridge is the direct
evidence that impurity compensation works in nc-Si, and is avery important factor to determine the PL intensity. For thesamples containing nearly the same amount of B and P, thenumber of perfectly compensated nc-Sis is larger than others,
resulting in the ridge in Figs. 2(e)–2(h).
In Figs. 2(h) and 2(g), if we look along the white line,
the intensity first increases, reaches maximum at the B and P
concentrations of 0.2– 0.4 mol %, and then decreases. Thismeans that the PL intensity of properly impurity controlled
nc-Si samples can be larger than that of pure nc-Si samples.
Two different routes have to be considered as the origin of
the intensity enhancement. The first one is the improvementof PL quantum efficiency of an individual nanocrystal because of the shorter radiative lifetime of an exciton trapped ata donor–acceptor pair than a free exciton.8 The other one isthat number of optically active, i.e., defect free, nanocrystals
increases by doping.
After crossing the maximum intensity point, the PL intensity decreases along the ridge. This declining of the PL
intensity may be explained by the increase in the number of
doped impurities per nanocrystal. When impurity concentra-Downloaded 18 Nov 2004 to 133.30.106.15. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

1160

Fujii et al.

Appl. Phys. Lett., Vol. 85, No. 7, 16 August 2004

ability to have a perfectly compensated nc-Si is not zero. The
intensity of codoped samples is thus always much higher
than those containing the same amount of either P or B.
As discussed above, one of the reasons for the enhancement of PL intensity by impurity doping is the increase of thenumber of optically active nc-Si. Doping of B or P results inthe softening of a silica matrix. This can reduce stresses exerted on nc-Si by annealing, and prevent the formation of
dangling bonds at nc-Si/matrix interfaces. The other mechanism is the inactivation of surface dangling bonds by P doping. An electron supplied by P doping is transferred from a P
atom to a surface dangling bond, creating a lone pair which
does not trap the optically excited electron. This mechanism
has been studied in detail for P-doped nc-Si by ESR andinfrared PL spectroscopies at low temperatures.13,14,16
Although it is very plausible that these mechanisms
work in codoped samples, they cannot explain the observedridge of the intensity along nearly the same B and P concentration line [Figs. 2(e)–2(h)] or the maximum intensity atnearly the same concentration points [Figs. 2(g) and 2(h)].
Therefore, the enhancement of oscillator strength of perfectlycompensated nc-Si due to the donor to acceptor transitions
contributes to the observed high PL intensity of the codoped
samples.
In conclusion, we have demonstrated that impurity compensation really works in nanometer size Si crystals. By
properly controlling impurities, the PL intensity can exceedthat of pure nc-Si and the tunable range of the PL peakenergy can be extended out of that covered by pure nc-Si.
This work is supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science, and Technology, and by an Industrial Technology
Research Grant Program from New Energy and Industrial
Technology Development Organization (NEDO) Japan.
1

L. T. Canham, Appl. Phys. Lett. 57, 1046 (1990).
P. D. J. Calcott, K. J. Nash, L. T. Canham, M. J. Kane, and D. Brumhead,
J. Phys.: Condens. Matter 5, L91 (1993).
3
Light Emission in Silicon: From Physics to Devices, edited by D. J. Lockwood (Academic, San Diego, 1998).
4
A. G. Cullis, L. T. Canham, and P. D. J. Calcott, J. Appl. Phys. 82, 909(1997).
5
D. Kovalev, H. Heckler, G. Polisski, and F. Koch, Phys. Status Solidi B
215, 871 (1999).
6
M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan, and C. Delerue, Phys.
Rev. Lett. 82, 197 (1999).
7
S. Takeoka, M. Fujii, and S. Hayashi, Phys. Rev. B 62, 16820 (2000).
8M. Fujii, K. Toshikiyo, Y. Takase, Y. Yamaguchi, and S. Hayashi, J. Appl.
Phys. 94, 1990 (2003).
9C. Delerue, M. Lannoo, G. Allan, E. Martin, I. Mihalcescu, J. C. Vial, R.
Romestain, F. Muller, and A. Bsiesy, Phys. Rev. Lett. 75, 2228 (1995).
10C. Delerue, M. Lannoo, G. Allan, and E. Martin, Thin Solid Films 255, 27(1995).
11G. Allan, C. Delerue, M. Lannoo, and E. Martin, Phys. Rev. B 52, 11982(1995).
12A. Mimura, M. Fujii, S. Hayashi, and K. Yamamoto, Solid State Commun.
109, 561 (1999)
13
A. Mimura, M. Fujii, S. Hayashi, D. Kovalev, and F. Koch, Phys. Rev. B
62, 12625 (2000).
14
M. Fujii, A. Mimura, S. Hayashi, Y. Yamamoto, and K. Murakami, Phys.
Rev. Lett. 89, 206805 (2002).
15
S. M. Sze, Physics of Semiconductor Devices (Wiley-Interscience, NewYork, 1981).
16
Z. Zhou, R. A. Friesner, and L. Brus, J. Am. Chem. Soc. 125, 15599(2003).
2

FIG. 2. (Color) Contour plots of PL peak energy [(a)–(d)] and intensity[(e)–(f)] as a function of B and P concentrations. Annealing temperatures are
1100 ° C [(a) and (e)], 1150 ° C [(b) and (f)], 1200 ° C [(c) and (g)], 1250 ° C[(d) and (h)]. P concentration was set to 0, 0.10, 0.40, and 0.79, and B
concentration to 0, 0.30, 0.62, and 1.24 mol %. The data of 16 samples areplotted in each graph after a smoothing procedure. On the white lines, CBand C P are the same, while on the black lines, the sum of CB and C P is fixedto be 0.8 mol %.

tion is larger than a certain value, typically the total concentration of larger than about 1 mol %, even if the annealing
temperatures is the same, the size of nc-Si becomes larger
because of the softening of glass matrices and resultant
longer diffusion length of Si during annealing.8 The numberof solvable impurities in a nanocrystal will thus increase. As
a result, the number of impurities in each nanocrystal canfluctuate and the probability to have a perfectly compensatednc-Si will be small, even if total number of P and B in awhole sample is the same. Since n- or p-type nc-Si fall victim to Auger recombination, the total PL intensity will drop.
The effect is more prominent for higher-temperature annealed samples because of the larger size. However, it should
be stressed here that even for relatively large nc-Si, the prob-

Downloaded 18 Nov 2004 to 133.30.106.15. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp