JOURNAL OF APPLIED PHYSICS 101, 033504 共2007兲

Electron spin-resonance studies of conduction electronsin phosphorus-doped silicon nanocrystals
Kazuaki Sumida, Keiichi Ninomiya, Minoru Fujii,a兲 Kazuyoshi Fujio, and Shinji Hayashi
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University,
Rokkodai, Nada, Kobe 657-8501, Japan

Masafumi Kodama
The Graduate School of Science and Technology, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan

Hitoshi Ohta
Department of Frontier Research and Technology, Headquarters for Innovative Cooperation
and Development, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan

共Received 23 August 2006; accepted 22 November 2006; published online 2 February 2007兲
The properties of conduction electrons in P-doped Si nanocrystals embedded in insulating glass
matrices have been studied by electron spin-resonance spectroscopy. For heavily P-doped samples,a broad conduction electron signal is observed at low temperatures. The width of the signal is foundto be much broader than that of P-doped bulk Si crystals. The temperature dependence of the signal
intensity obeys the Curie law even when the P concentration is very high. This suggests that in
P-doped nanocrystals donor levels do not merge into the conduction band even at very high P
concentration, and also provides evidence that Si nanocrystals smaller than a certain threshold size
do not become metallic, at least when they are prepared under an equilibrium condition. © 2007
American Institute of Physics. 关DOI: 10.1063/1.2432377兴
I. INTRODUCTION

Si nanocrystals 共Si-nc’s兲 show strong luminescence inthe visible range.1–3 The luminescence arises from the recombination of electron-hole pairs confined in the zerodimensional quantum confined system. The luminescenceenergy is changed from the band-gap energy of bulk Si crystals to the visible range by controlling the size.2–4 The luminescence property can be modified by shallow impurity
doping.5–7 Doping of either n- or p-type impurities results inthe quenching of the photoluminescence 共PL兲 due to the efficient Auger interaction between photoexcited electron-holepairs and carriers supplied by the impurity doping.7,8 The PL
quenching is effectively suppressed when n- and p-type impurities are doped simultaneously because of the compensation of carriers in Si-nc’s.6,7 The impurity codoped Si-nc’s
exhibit PL at energies lower than that of the bulk Si bandgap, due probably to the transition between donor and acceptor states.7
The strong modification of PL spectra by impurity doping indicates that the electronic structure is modified. Electron spin resonance 共ESR兲 is one of the powerful experimental techniques to investigate the electronic structure ofsemiconductors and has routinely been used to study theproperties of conduction electrons in shallow-impurity-doped
bulk Si crystals. The ESR spectra depend strongly on
the doping level. In lightly P-doped bulk Si crystals
共ⱕ1017 cm−3兲, a hyperfine structure 共splitting energy of
42 G兲 caused by the interaction between the donor electron
spin and the P nucleus spin appears.9 In this doping level,a兲Author to whom correspondence should be addressed; electronic mail:fujii@eedept.kobe-u.ac.jp

0021-8979/2007/101共3兲/033504/5/$23.00

donor electrons are localized at the P donor sites and thedonor level is well separated from the bottom of the conduction band. When the P concentration is increased, the hyperfine structure disappears and a conduction electron signal
共g = 1.998兲 appears. The signal obeys the Curie law when thedonor concentration is in the 1016 − 1018 cm−3 range.10,11 In
this P concentration range, donor electrons transfer between
donor states via overlapped wave functions of P atoms.11
When the P concentration is larger than 2 ⫻ 1019 cm−3, thePauli paramagnetism appears, i.e., the integrated intensity of
the conduction electron signal is independent of temperature
due to the semiconductor-to-metal transition by the overlapof the donor band with the conduction band and the penetration of the Fermi level into the conduction band.11–13
The hyperfine structure has also been observed in
P-doped Si-nc’s when the P concentration is moderate.14
However, the spectrum was much different from that of
P-doped bulk Si crystals. The most significant difference wasthat in P-doped Si-nc’s, the splitting energy is more than
twice as large as that of bulk Si crystals. Furthermore, thesplitting energy depends strongly on the size of Si-nc’s. The
spatial confinement of P atoms into the region smaller thanthe Bohr radius in bulk Si crystals is responsible for theenhancement of the hyperfine splitting.
The hyperfine structure is considered to be observed
when, on average, one P atom is doped into one Si-nc. If
more than two P atoms are doped, the interaction between
the P atoms smears the structure and only a broad conduction
electron signal appears. In fact, a broad signal with the gvalue of 1.998 has been observed for heavily P-doped Si-nc’s
at low temperatures.14 However, detailed studies on the conduction electron signal have not been made despite the importance of the knowledge on conduction electrons to realize

101, 033504-1© 2007 American Institute of Physics

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033504-2

K. Sumida and M. Fujii

J. Appl. Phys. 101, 033504 共2007兲

FIG. 1. PL peak intensities of P-doped Si-nc’s as a function of P concentration. P concentration is changed from 0 to 1.9 at. %. In the inset, PL spectraof Si-nc’s for the samples with the Si concentration of 35.6 and 40.5 at. %are shown.

Si-based optoelectronic devices. The purpose of this work is
to study in detail the property of conduction electrons inheavily P-doped Si-nc’s. We will show from the temperatureand P concentration dependence of ESR spectra that the behavior of the conduction electron signal is significantly different from that of P-doped bulk Si crystals.
II. EXPERIMENT

Samples were prepared by the same method as used in
our previous work.5–7,14 Small pieces of Si chips were placedon a phosphosilicate glass 共PSG兲 sputtering target and they
were simultaneously sputtered in Ar gas on a fused quartz
substrate. The films were then annealed in a N2 gas atmosphere for 30 min at 1150 °C to grow P-doped Si-nc’s in
PSG matrices. In the present samples, Si-nc’s are isolated
from others by PSG matrices and thus carrier transport between nanocrystals is prohibited. Si and P concentration in awhole film was determined by an electron probe microanalysis 共JXA8900: JEOL兲. P concentration was changed from0 to 1.9 at. %.
PL spectra were measured by using a single grating
monochromator and a liquid N2 cooled InGaAs near-infrared
diode array. The spectral response of the detection system
was calibrated with the aid of a reference spectrum of astandard tungsten lamp. The excitation source was a
488.0-nm line of an Ar ion laser. Continuous-wave X-band
ESR was measured by a conventional ESR spectrometer
共EMX 8/2.7: Bruker兲. The spectra were measured in the temperature range between 4.2 and 300 K in a continuous-flow
He cryostat 共Optistat: Oxford Instruments兲. The modulation
frequency and modulation amplitude were 100 kHz and 2 G,
respectively. A nuclear magnetic resonance Gauss meter with
10−6 G resolution was used for field monitoring.
III. RESULTPL spectra for the samples with the Si concentration of
35.6 and 40.5 at. % are shown in the inset of Fig. 1. The PL

FIG. 2. ESR spectra of P-doped Si-nc’s at 4.2 K. Lower part: Si concentration is 40.5 at. % and P concentration is changed from 0 to 1.9 at. %. Upper
part: Si concentration is 35.6 at. % and P concentration is 1.5 at. %. For
better comparison of the spectral shape, the spectra are scaled by the indicated multiplication factors.

arises from the recombination of electron-hole pairs confinedin Si-nc’s. The lower Si concentration results in higher PL
peak energy because of the smaller size of Si-nc’s and thestronger quantum confinement effect. In Fig. 1, PL peak intensity is plotted as a function of P concentration for twoseries of samples with the Si concentration of 35.6 and 40.5
at. %. For the sample with the Si concentration of 35.6 at. %,the PL peak intensity first increases with increasing the P
concentration and then decreases very rapidly, while for thatwith the Si concentration of 40.5 at. %, it decreases monotonously.
Figure 2 shows ESR spectra of P-doped Si-nc’s obtained
at 4.2 K. P concentration is changed from 0 to 1.9 at. %,while Si concentration is fixed to about 40.5 at. %. For better
comparison of the spectral shape, the spectra are scaled by
the indicated multiplication factors. The ESR signal at
g = 2.006 for the sample without P doping is due to dangling
bond defects at Si-SiO2 interfaces.5 This signal becomes
weaker by P doping. The P concentration dependence of thesignal and the relation with the PL intensity have been discussed in detail in our previous papers.5,14 After the quenching of the g = 2.006 signal, a signal with g = 2.002 emerges.
This signal can be assigned to an EX center characterized byinvolved hyperfine structure of 16 G splitting.15 By further
increasing P concentration, the EX center is also quenched.
This suggests that P doping softens the glass matrices andthat stress at Si-SiO2 interfaces is reduced.6,16 At the P concentration of 1.0 and 1.9 at. %, a broad signal with

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033504-3

K. Sumida and M. Fujii

J. Appl. Phys. 101, 033504 共2007兲

FIG. 3. Temperature dependence of ESR spectra. P concentration is 共a兲 1.0
and 共b兲 1.9 at. %. Si concentration is 40.5 at. %.

g = 1.998 appears. The signal is assigned to conduction electrons in Si-nc’s.17 In the case of the lower Si concentration
sample 共35.6 at. %兲, even at the highest P concentration
共1.5 at. %兲, the conduction electron signal was not observed
共upper part of Fig. 2兲.
Figures 3共a兲 and 3共b兲 show temperature dependence of
ESR spectra for the samples with the P concentration of 1.0
and 1.9 at. %, respectively. Si concentration is 40.5 at. %.
The conduction electron signal intensity depends on the temperature. In order to quantitatively discuss the temperaturedependence of the intensity and linewidth, the spectra werefitted by the Lorenzian function. In Fig. 4, the linewidth 共a兲
and the integrated intensity 共b兲 obtained by the fitting areplotted as a function of temperature. For the sample with the
P concentration of 1.0 at. %, the linewidth decreases slightly
by raising the temperature up to 15 K and then increases,
while for the sample with the P concentration of 1.9 at. %, it
increases monotonously from 4.2 to 120 K. The solid line in
Fig. 4共a兲 is the ESR linewidth obtained for P-doped bulk Si
crystals.13,18 The linewidth of the present samples is significantly larger than that of bulk Si crystals. In Fig. 4共b兲, theintensity decreases with increasing the temperature in both
samples except for a very low temperature range 共⬍15 K兲
for the sample with the P concentration of 1.0 at. %.
IV. DISCUSSION

The integrated intensity of the ESR signal is proportional
to the static susceptibility 共␹兲. P-doped bulk Si crystals show
the Curie or Pauli paramagnetism depending on the doping
level. When the P concentration is in the 1016 − 1018 cm−3

FIG. 4. 共a兲 Linewidth and 共b兲 integrated intensity of the conduction electron
signal as a function of temperature. The solid line in 共a兲 represents linewidthof a conduction electron signal in P-doped bulk Si crystals. The solid curves
in 共b兲 are the results of fitting by the weighted sum of Curie and Pauliparamagnetism.

range, the electrons stay at their nondegenerate states andelectron transport is made by hopping. In this situation, theelectron statistics are governed by the Maxwell-Boltzmann
distribution and the total number is constant in a wide temperature range, showing a ␹ ⬀ T−1 behavior 共Curie
paramagnetism兲.10,13,19 On the other hand, when the P concentration is larger than 2 ⫻ 1019 cm−3, the paramagnetic
susceptibility of the degenerated electrons at sufficiently
low temperatures is temperature independent 共Pauliparamagnetism兲.13 In the P concentration range of1018 − 1020 cm−3, the temperature dependence of the ESR signal intensity is usually fitted by the sum of the Curie andPauli paramagnetism.13
Following the procedure usually employed for P-doped
bulk Si crystals, we attempt to fit the temperature dependence of the ESR signal intensity by the weighted sum of theCurie and Pauli paramagnetism, ␹ = A␹Curie + B␹Pauli, where A
and B are the fitting parameters. The solid curves in Fig. 4共b兲show the results of the fitting. For the sample with the P
concentration of 1.0 at. %, the intensity can be fitted only by
the Curie paramagnetism, while for the sample with the P
concentration of 1.9 at. % both the Curie and Pauli paramagnetism should be considered, although the contribution of thePauli paramagnetism is very small 关B / 共A + B兲 = 0.016兴.

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K. Sumida and M. Fujii

If we assume that P is doped uniformly into the samples,i.e., uniformly doped in Si-nc’s and PSG matrices, P concentration of 1.0 and 1.9 at. % corresponds to that of 5.0⫻ 1020 and 9.5⫻ 1020 cm−3, respectively, in Si-nc’s. As described above, bulk Si crystals become metallic in these P
concentration ranges; the donor states are overlapped with
conduction bands and ESR signal intensity is almost independent of temperature. Therefore, the observed results are
qualitatively different from that of P-doped bulk Si crystals.
In Si-nc’s, the energy band structure is strongly modified
from that of bulk Si crystals due to the quantum size effect.
The modified energy band structure is considered to be responsible for the observed difference. The conduction bandedge of Si-nc’s is shifted to higher energy, while the energyof the donor states measured from the vacuum level is almost
independent of the size of nanocrystals.20–22 Therefore, as thesize decreases, the energy difference between the ground donor state and the conduction band edge, i.e., ionization energy, is increased, and thus the overlap of the conduction
band and donor states is not realized even at very high P
concentration. In other words, the P donor level in Si-nc’s isno longer “shallow” but “deep.”23 In this situation, the Curie
paramagnetism is expected. Furthermore, even if the Fermi
level is in the conduction band by heavy doping, the conduction band is quantized and electrons occupy the quantizedlevels. If the width of the quantized levels is very narrow, theCurie paramagnetism is also expected.24
Another possible explanation of the Curie paramagnetism is that P concentration in Si-nc’s is lower than the average P concentration in a whole film. In this work, Si-nc’s areprepared by annealing Si-doped PSG. The as-deposited films
are amorphous and during the annealing Si-nc’s are grown;
during the growth of Si-nc’s P atoms are incorporated. Although doping of P into substitutional sites of Si-nc’s is unambiguously demonstrated from previous ESR14 and IR
absorption25 measurements, it is not clear whether solid solubility of P in Si-nc’s is the same as that in bulk Si crystals or
not. It may be possible that the maximum possible P concentration in Si-nc’s is lower than that of bulk Si crystals. In
fact, the formation energies of P- or B-doped Si clusters arecalculated to be much larger than those of pure Si clusters.26
This may suggest that a part of P atoms are pushed out ofnanocrystals during annealing and cooling the samples.
As described above, for the sample with the P concentration of 1.9 at. %, the Pauli paramagnetism is slightly contained. This probably arises from Si-nc’s in the large end of
the size distribution. In a previous paper,27 we demonstrated
that the size distribution becomes broader when P concentration is increased. This is because of the longer diffusion
length of Si atoms during annealing due to the softening ofglass matrices by P doping. The longer diffusion length
makes Si-nc’s larger and the size distribution broader. The
present result seems to suggest that there is a threshold sizeof Si-nc’s; nanocrystals larger than the threshold size canexhibit the Pauli paramagnetism when heavily doped, whileothers not.
In Fig. 4, below 15 K, the linewidth increases and theintensity decreases with decreasing temperature for the
sample with the P concentration of 1.0 at. %. These proper-

ties can be explained by the localization of donor electrons.
At low temperatures, the hopping of electrons between P
sites is suppressed and the effect of motional narrowing reduces, resulting in the broadening of the spectra. The localization makes the spin-lattice relaxation time longer, whichcauses the saturation of the signal at relatively low microwave power. This effect appears as the decrease of the intensity at low temperatures. If the localization of donor electrons at P sites is suppressed by shortening the distance
between P atoms by further increasing P concentration, theseeffects become small. In fact, for the sample with the P concentration of 1.9 at. %, the broadening and quenching of thesignal at low temperatures are not observed.
V. CONCLUSION

The conduction electron signal in ESR spectra of
P-doped Si-nc’s was studied in detail as functions of P concentration and temperature. Despite very high P concentration, the intensity obeys Curie law, suggesting that Si-nc’s do
not become metallic by shallow impurity doping. The
present results indicate that there is a threshold size of Sinc’s, below which they do not become metallic by shallow
impurity doping. Therefore, similar studies for the samples
with narrower size distribution of Si-nc’s seem to be very
interesting. Although the mechanism of Curie paramagnetism for heavily P-doped samples is not fully clarified, themost important conclusion obtained in this work is that Sinc’s do not exhibit semiconductor-to-metal transition evenwhen they are annealed in the sea of P, i.e., in PSG.
In Si-nc’s, carriers are confined in all three dimensions,and thus the quantum confinement effect appears most significantly. Since the properties of shallow impurities are expected to depend strongly on the confinement dimension, it
is very interesting to compare the present results with thoseof one-dimensional Si nanostructures such as Si nanowires.
The electronic properties of impurity-doped Si nanowires
have been discussed based on the current transportmeasurements.28 However, the diameter of nanowires studied
so far was rather large 共several tens to 100 nm兲 and in thissize range, the quantum size effects are not expected. We
believe that ESR studies of impurity-doped Si nanocrystals
and nanowires with comparable sizes provide valuable information to fully understand the properties of conduction electrons in low-dimensional Si structures.
ACKNOWLEDGMENTS

This work is supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. This work is partly supported by the project in Department of Frontier Research andTechnology, Kobe University.
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