APPLIED PHYSICS LETTERS 93, 021920 共2008兲

Electron spin resonance studies of P and B codoped Si nanocrystals
Kazuyoshi Fujio,1 Minoru Fujii,1,a兲 Kazuaki Sumida,1 Shinji Hayashi,1 Masashi Fujisawa,2and Hitoshi Ohta3
1Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,Rokkodai, Nada, Kobe 657-8501, Japan2
Department of Frontier Research and Technology, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan3
Molecular Photoscience Research Center, Department of Frontier Research and Technology,Headquarters for Innovative Cooperation and Development, Kobe University, Rokkodai, Nada,Kobe 657-8501, Japan

共Received 5 June 2008; accepted 23 June 2008; published online 18 July 2008兲
P- and/or B-doped Si nanocrystals 共Si-ncs兲 embedded in glass matrices were studied by electron
spin resonance 共ESR兲 spectroscopy to investigate the origin of strong room-temperature
photoluminescence 共PL兲 of n- and p-type impurities codoped Si-ncs below the band-gap energy ofbulk Si crystals. It was shown that the intensity and width of the ESR signal depend strongly on
impurity concentrations. A clear correlation was observed between the ESR signal width and the PLintensity. The observed correlation suggests that in addition to the geometrical confinement, P andB codoping further localize carriers in Si-ncs, and the strong localization results in the characteristic
luminescence properties. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2957975兴
The luminescence properties of Si nanocrystals 共Si-ncs兲
depend strongly on the size. The high-energy shift of thephotoluminescence 共PL兲 maximum and shortening of the PL
lifetime with decreasing the size have commonly been observed for different kinds of Si-ncs samples.1,2 The PL properties are also modified by shallow impurity doping.3–6 Ingeneral, heavy doping of either n- or p-type impurities degrades the PL properties, i.e., the PL intensity decreases due
to efficient Auger recombination.7 However, the localizationof carriers by impurity doping in addition to that by thegeometrical confinement may lead to further enhancement ofoscillator strength of excitons in Si-ncs and thus impurity
doping is a fascinating approach to improve the luminescence properties if Auger recombination can be avoided. The
Auger process can be avoided if isoelectronic impurities areused. Unfortunately, Si does not have proper isoelectronic
impurities that can strongly localize excitons at room temperature. An alternative approach is to use simultaneously
n- and p-type impurities for doping of Si nanocrystals. The
codoping and compensation of carriers may result in localization of excitons without being afraid of the Auger process.
In previous papers,4,5 we demonstrated that P and B
codoped Si-ncs show strong PL in the near infrared 共IR兲range at room temperature. The PL maximum depends on thedoping levels as well as the size of Si-ncs. In intrinsic Si-ncs,
the PL energy can be controlled only above the bulk band
gap 共1.12 eV兲. In codoped Si-ncs, the range can be extended
to 0.9 eV by properly controlling n- and p-type impurities.
The low energy PL is considered to arise from optical transitions between donor and acceptor states.
The strong modification of PL spectra by impurity doping indicates that the electronic structure is strongly modified. In contrast to limited number of experimental work on
impurity doped Si-ncs, theoretically, studies on the electronic
structure have been performed intensively.8–14 In particular,
Ossicini et al.12 studied the formation energy and electronic
structures of P and B codoped Si-ncs by first-principles cala兲Author to whom correspondence should be addressed. Electronic mail:fujii@eedept.kobe-u.ac.jp.

culations. They showed that formation energy of P and B
codoped Si-ncs is much smaller than that of either P or B
doped Si-ncs. Furthermore, the calculation shows that thehighest occupied molecular orbital and the lowest unoccupied molecular orbital are localized around the impurity sites
resulting in lowering the energy gap with respect to that of
the undoped Si-nc.
The purpose of this work is to access to the electronic
structures of P and B codoped Si-ncs experimentally. In aprevious work,15 we studied P-doped Si-ncs by electron spinresonance 共ESR兲 spectroscopy. We showed that the temperature dependence of a conduction electron signal obeys Curie’s law even when the doping level was very high. Thissuggests that in P-doped Si-ncs, donor levels do not merge
into the conduction band even at very high P concentrationand that Si-ncs smaller than a certain threshold size do not
become metallic, at least when they are prepared under athermal equilibrium condition. In this paper, we extend theprevious work to B-doped Si-ncs and P and B codoped
Si-ncs. By comparing ESR and PL data, we will show thatstrong localization of carriers in codoped Si-ncs is responsible for the room-temperature low-energy PL.
P and B doped Si-ncs were prepared by the same method
as used in our previous work.3–5,16 Si chips and B2O3 tablets
were placed on a phosphosilicate glass sputtering target andthey were simultaneously sputtered in Ar gas on fused-quartz
or stainless steel substrates. After deposition, films were removed from the stainless steel substrates and ground to powder. The powder and films on fused-quartz substrates werethen annealed in a N2 gas atmosphere at 1150 ° C for 30 min
to grow P and B codoped Si-ncs in borophosphosilicate
glasses 共BPSG兲. Si-ncs are isolated each other by BPSG matrices and thus carriers are confined within Si-ncs. In thiswork, P concentration in films was almost fixed 共1.9 at. % 兲while B concentration was changed from 0 to 1.36 at. %. As
a reference, B doped Si-ncs was also prepared. The B concentration was changed from 0.37 to 1.51 at. %. Si concentration was fixed to about 40.5 at. % for all the samples.
X-band ESR spectra were measured for powder samples
by a conventional ESR spectrometer 共EMX081, Bruker兲 at0003-6951/2008/93共2兲/021920/3/$23.00
93, 021920-1© 2008 American Institute of Physics
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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Appl. Phys. Lett. 93, 021920 共2008兲

FIG. 2. 共Color online兲 ESR spectra of P and B codoped Si-ncs at 4.2 K. Si
and P concentrations are fixed at around 40.5 and 1.9 at. %, respectively. B
concentration is changed from 0 to 1.36 at. %. 共b兲 ESR signal intensity and共c兲 width as a function of temperature. Dotted curves in 共b兲 are guides forthe eyes.
FIG. 1. 共Color online兲 ESR spectra of 共a兲 P doped and 共b兲 B doped Si-ncs at
4.2 K. Si concentration is fixed at 40.5 at. %. P and B concentrations arechanged from 0 to 1.9 at. % and from 0.37 to 1.51 at. %, respectively. ESR
spectra of sputter-deposited SiO2 and B doped SiO2 are also shown as references on the upper part of 共b兲. For better comparison, the spectra arescaled by the indicated multiplication factors.

temperatures between 4.2 and 300 K in a continuous-flow
He cryostat 共ESR900, Oxford Instruments兲. The modulation
frequency and modulation amplitude were 100 kHz and 2 G,respectively. A nuclear magnetic resonance Gauss meter with10−6 G resolution was used for field monitoring. The signalintensity was corrected with Mn2+ marker by a conventional
ESR spectrometer 共JES-TE300, JEOL兲. PL spectra weremeasured for film samples by using a single grating monochromator and a liquid N2 cooled InGaAs near-IR diode array. The spectra were calibrated with the aid of a reference
spectrum of a standard tungsten lamp. The excitation sourcewas a 488 nm line of an Ar ion laser.
First, we briefly summarize ESR data of P doped Si-ncs.
Figure 1共a兲 shows ESR spectra of P doped Si-ncs at 4.2 K.
The Si concentration is 40.5 at. % and the P concentration ischanged from 0 to 1.9 at. %. For better comparison, thespectra are scaled by the indicated multiplication factors.
Three signals with different g values 共g = 2.006, 2.002, and1.998兲 are observed. The signal with g = 2.006 is due to dangling bond defects at Si– SiO2 interfaces.3 Although the intensity of this signal is very large, it vanishes quickly by P
doping. The signal with g = 2.002 can be assigned to an EX
center characterized by involved hyperfine structure of 16 G
splitting.17 The signal is also observed for sputter-deposited
pure SiO2 关upper part of Fig. 1共b兲兴. The EX center becomes
weaker with increasing the P concentration. At high P concentration 共1.0 and 1.9 at. %兲, a broad signal with g = 1.998
emerges. This signal is assigned to conduction electrons inSi-ncs.18
Figure 1共b兲 shows ESR spectra of B doped Si-ncs at
4.2 K. In contrast to P doping, the signal with g = 2.005 due
to dangling bond defects becomes strong with increasing B

concentration. The different behavior of the defect density
between n- and p-type doping suggests that the defects are
positively charged and are deactivated by electrons suppliedby P doping. Note that this signal is not observed for sputterdeposited pure SiO2 and B-doped SiO2 关upper part of Fig.
1共b兲兴, and observed only when Si-ncs are grown in glasses.
In Fig. 1共b兲, in contrast to P doping, signals from carriers
共holes兲 are not observed. However, it does not mean absenceof holes. In general, ESR signal of holes in Si is difficult todetect because of the degeneracy of valence band maxima,
and resultant very fast spin relaxation, which broaden thesignal significantly. The signal can be detected by applying
an external uniaxial stress.19 Furthermore, in very high quality lightly doped crystals, random internal strains due to dislocations, imperfections, and phonons split the valence bandand hole signals are observed.20,21 In the present system, thedoping level is very high and the environment of acceptors isexpected to be very inhomogeneous due to the distributionsof the number and sites of impurities, size, and shape ofSi-ncs and strains. These inhomogeneities result in large distribution of resonant magnetic field and hole signals are considered to be too broad to be detected.
Figure 2共a兲 shows ESR spectra of P and B codoped Sincs at 4.2 K. The P concentration is fixed at 1.9 at. %, while
the B concentration is changed from 0 to 1.36 at. %. As
mentioned above, at the B concentration of 0 at. %, i.e., P
doped samples, the broad signal is assigned to conduction
electrons. The ESR intensity depends strongly on B concentration. Furthermore, the shape changes from almost symmetric at the B concentration of 0 at. % to asymmetric
共Dysonian shape兲 at 0.63 at. %, and then becomes almost
symmetric at 1.36 at. %.
In order to discuss the change of the signal shape in2detail, the intensity 共I⌬HPP兲 and the width 共⌬HPP兲, whereI and ⌬HPP are peak-to-peak intensity and distance 共width兲,respectively, are estimated. The EX center signal is subtracted before estimating I and ⌬HPP. In Figs. 2共b兲 and 2共c兲,Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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2
FIG. 3. 共Color online兲 共a兲 ESR signal intensity 共I⌬HPP
兲, 共b兲 width 共⌬HPP兲,and 共c兲 PL intensity as a function of B concentration. Inset is a PL spectrum
for the sample with the B concentration of 0.63 at. %.

the intensity and the width are plotted as a function of temperature. For all the samples, the intensity is nearly inversely
proportional to the temperature except for a very low temperature range below 15 K. This Curie-like paramagnetism
indicates that Si-ncs do not become metallic even in aheavily doped condition. The mechanism of this phenomena
is discussed in our previous paper.15 In Fig. 2共c兲, we can seethat the signal width of codoped Si-ncs is always much larger
than that of P doped Si-ncs 共䊏兲.
In Figs. 3共a兲 and 3共b兲, the intensity and the width at
25 K are plotted as a function of B concentration. Similargraphs can also be obtained at other temperatures above
20 K. In Fig. 3共a兲, the intensity is nearly constant up to the B
concentration of 0.63 at. % and increases drastically by further increasing the B concentration. On the other hand, thewidth shows different behavior. The width is the smallest for
P doped Si-ncs, although it is several times larger than that ofP doped bulk Si crystals.15 The width increases significantly
by B doping and reaches maximum when the B concentration is 0.63 at. %. Further increase in B concentration resultsin the decrease in the width.
The increase in the ESR intensity by B doping is mainly
due to significant increase in the width. A possible mechanism of the broadening is the enhancement of interaction
between confined carriers and impurities by simultaneous
P and B doping. In heavily doped and compensated bulk Si
crystals, broadening of ESR signals due to impurity field isreported.22 Similarly, in codoped Si-ncs, electrons 共holes兲 aregiven repulsion by negatively charged acceptors 共positivelycharged donors兲 by Coulomb interaction, which results in thestrong localization. The stronger localization of carriers incodoped Si-ncs than in singly doped 共P-doped兲 Si-ncs is considered to be responsible for the significant broadening of theESR signal.
In Fig. 3共c兲, PL peak intensity is plotted as a function ofB concentration. When B concentration is zero, i.e., only P isdoped, the PL intensity is very small due to efficient Auger
process between photoexcited excitons and excess electrons.
The intensity increases when B is simultaneously doped, andthen decreases again when B concentration is further increased. Figure 3共c兲 suggests that the number of compensated Si-ncs is the largest when B concentration is 0.63 at. %within the present series of samples. In the inset of Fig. 3, PL
spectrum of the sample with the highest PL intensity isshown. The PL peak energy is below the band gap energy ofbulk Si crystals. By comparing Figs. 3共b兲 and 3共c兲, we cansee a correlation between the ESR width and the PL intensity. This correlation suggests that carriers are strongly localized in compensated Si-ncs and the strong localization
causes efficient PL at room temperature.
In higher B concentration range 共1.03 and 1.36 at. %兲 inFig. 3共c兲, the PL intensity decreases significantly and theESR signal intensity increases abruptly 关Fig. 3共a兲兴. This correlation suggests that PL quenching is due to nonradiative
Auger process between photoexcited excitons and holes supplied by B doping. However, this is in contradiction to thedata in Fig. 1共b兲 in which signals from holes are not detectedin B doped Si nanocrystals. Probably, energy band structureof P and B codoped Si-ncs is significantly modified from thatof B doped Si-ncs and the degeneracy of the valence band islifted.
In conclusion, ESR and PL studies of P and B codoped
Si-ncs indicate that in addition to geometrical confinement,
further localization of carriers is possible by properly controlling n- and p-type impurities in Si-ncs. If carriers areperfectly compensated, the localization may result in the enhancement of exciton radiative recombination rate without
being afraid of the nonradiative Auger process.
This work is supported by a Grant-in-Aid for Scientific
Research from the MEXT, Japan, and by a grant from department of frontier research and technology, Kobe
University.
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123Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp