JOURNAL OF APPLIED PHYSICSVOLUME 84, NUMBER 8

15 OCTOBER 1998

Photoluminescence from SiO2 films containing Si nanocrystals and Er:
Effects of nanocrystalline size on the photoluminescence efficiency of Er31
Minoru Fujiia)
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai,
Nada, Kobe 657-8501, Japan

Masato Yoshida
Division of Electrical and Electronics Engineering, The Graduate School of Science and Technology,
Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan

Shinji Hayashi and Keiichi Yamamoto
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai,
Nada, Kobe 657-8501, Japan~Received 23 February 1998; accepted for publication 20 July 1998!
SiO2 films containing Si nanocrystals ~nc-Si! and Er were prepared and their photoluminescence~PL! properties were studied. The samples exhibited PL peaks at 0.8 and 1.54 mm, which could be
assigned to the electron-hole recombination in nc-Si and the intra-4 f transition in Er31 ,respectively. Correlation between the intensities of the two PL peaks was studied as functions of thesize of nc-Si, Er concentration, excitation power and excitation wavelength. It was found that the1.54 mm PL of Er31 is strongly enhanced by incorporating nc-Si in films. Furthermore, the intensityof the 1.54 mm peak was found to depend strongly on the size of the incorporated nc-Si. © 1998
American Institute of Physics. @S0021-8979~98!06320-8#transfer from nc-Si, the 1.54 mm PL of Er31 and the visibleor near-infrared PL of nc-Si are considered to correlate toeach other depending on the size of nc-Si, number of active
Er31 ions and excitation energy and power. Detailed studiesof these dependences will give deeper insight into the nc-Si
mediated excitation mechanism of Er31 .
To study these dependences of the two PL bands, stablenc-Si with well-defined size and shape as well as chemicalcomposition is indispensable. For this purpose, porous Si is
not appropriate because of its instability, complexity in its
structure and chemical composition, and resulting ambiguityin the origin of the PL. Instead of porous Si, in our previous
work, we employed SiO2 films containing nc-Si ~Refs. 10,
13, and 14! as a host of Er31 and studied the PL properties asa function of Er concentration.15 For the samples containing
both nc-Si and Er, two PL peaks corresponding to the recombination of electron-hole pairs in nc-Si and the intra-4 f transition in Er31 could be observed simultaneously. From thecorrelation of the two PL peaks, we clearly demonstrated
that the excitation of Er31 is made by the energy transferfrom nc-Si. This work is an extension of the previous work.
In order to further clarify the nc-Si mediated excitation
mechanism of Er31 , we performed systematic PL studies for
SiO2 films containing nc-Si and Er. In particular, we havestudied in detail the effects of nanocrystalline size on the PL
efficiency of Er31 . We will demonstrate that the intensity of
the 1.54 mm PL of Er31 is strongly enhanced by incorporating nc-Si in films, and that the PL intensity depends strongly
on the size of nc-Si.

I. INTRODUCTION

Recently, Er-doped Si has been attracting much interest
because of its potential application in Si-based optoelectronic
devices. The Er31 ions incorporated into Si produce lightemission from the intra-4 f transition ( 4 I 13/2 – 4 I 15/2) ataround 1.54 mm, which corresponds to the absorption minimum in silica-based glass fibers.1–5 Interest has also been
focused on Er-doped porous Si, which emits strong 1.54 mm
photoluminescence ~PL! even at room temperature and exhibits very week thermal quenching of the PL.5–9 Porous Si
consists of nanometer-size Si crystals ~nc-Si!. In porous Si,
excitation of Er31 is considered to occur through the recombination of photogenerated carriers in the nc-Si and the subsequent energy transfer to Er31 .7–9
The electronic band structure of nc-Si, as small as several nanometers, is much different from that of the bulk-Si
crystal due to the quantum confinement effects of electrons,
holes and phonons ~quantum size effects!. The most prominent feature of the quantum confinement effects is the widening of the band gap from that of the bulk-Si crystal. The
widening of the band gap is considered to result in the small
thermal quenching of the PL observed for Er-doped porous
Si.7–9 Since the electronic band structure of nc-Si depends
strongly on the size, the efficiency of the 1.54 mm PL of
Er31 excited by the energy transfer from nc-Si may also
depend strongly on the size of nc-Si.
As is well known, nc-Si as small as several nanometers
in diameter emits light at the near-infrared and visible regions due to the band-to-band transition between the widened band gap.10–12 If the Er31 is excited by the energy

II. EXPERIMENT

Samples were prepared by a rf cosputtering method. In
our previous studies,10,13,14,16 we demonstrated that nc-Si em-a!

Electronic mail: fujii@eedept.kobe-u.ac.jp

0021-8979/98/84(8)/4525/7/$15.00

4525© 1998 American Institute of Physics

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J. Appl. Phys., Vol. 84, No. 8, 15 October 1998

bedded in SiO2 films can be obtained by cosputtering Si and
SiO2 and postannealing at 1100 °C or higher. In this work, in
an attempt to dope Er in the films, Er2 O3 pellets were added
as the sputtering targets.15 In actual sputtering procedures,
small pieces of Si chips 5315 mm2 in size and Er2 O3 pellets 10 mm in diameter were placed on a SiO2 target ~10 cm
in diameter! and they were cosputtered. Films of about 1 mm
in thickness were deposited onto fused quartz plates. Afterthe deposition, the samples were annealed in an atmosphereof N2 gas for 30 min at 1100 °C.
In this method, the size of nc-Si can be controlled bychanging the number of Si chips during the cosputtering. Er
concentration (C Er) can also be controlled by changing thenumber of Er2 O3 pellets during the cosputtering. The C Er
and the volume fraction of nc-Si in the films ( f Si) were determined by electron-probe microanalyses @using JCXA-733~JEOL!#. The size of nc-Si was determined by crosssectional high-resolution transmission electron microscopic~HRTEM! observations. An electron microscope @JEM-2010~JEOL!# operated at 200 kV was used. In this work, we havestudied the samples with C Er ranging from 0 to 0.11 at. %and f Si ranging from 7 to 21 vol. %. By varying f Si from 7 to
21 vol. %, the average diameter of nc-Si (d Si) was changed
from 2.7 to 3.8 nm.
The PL spectra were measured using a HR-320 ~JobinYvon! monochromator and an EO-817L ~North Coast! Ge
detector. The excitation sources were six lines of an Ar-ion
laser and a He-Ne laser with a power density of less than 1.5
W/cm2 . Spectral responses of the detection systems werecorrected by reference spectra of a standard tungsten lamp.
The temperature dependence of the PL spectra was measured
from 15 to 300 K in a CF1204 continuous-flow He cryostat~Oxford!.

III. RESULTS
A. HRTEM observation

Figure 1 shows a typical cross-sectional HRTEM imageof the SiO2 film containing nc-Si ( f Si511 vol. %!. We canclearly see lattice fringes in the image. The lattice fringes
correspond to the $111% planes of Si crystals with the diamond structure. The crystallinity of the nanocrystals is rather
good. The nanocrystals are not aggregated and are isolated in
SiO2 matrices. The average diameter of the nanocrystals estimated from the image was about 3.1 nm. It should be noted
here that the size of nc-Si is almost independent of C Er anddepends only on f Si for the samples with C Er<0.11 at. %. In
the case of an Er-implanted SiO2 film annealed at 1200 °C,
precipitates of Er as large as several tens of nanometers havebeen observed in HRTEM observations.5 In the present
samples, such large precipitates were not found in the
HRTEM images. However, we cannot neglect the possibility
that much smaller Er clusters exist in films, because Er clusters smaller than about 2 nm are considered to be not detectable by HRTEM observations due to the strong backgroundimages of SiO2 matrices.

Fujii et al.

FIG. 1. Typical HRTEM image of SiO2 films containing nc-Si ( f Si511
vol. %!. Lattice fringes corresponding to the $ 111% planes of Si can clearly
be seen. The average diameter of the nanocrystals estimated from the image
is about 3.1 nm.

B. Dependence of PL spectra on the size of nc-Si

Figure 2 shows the PL spectra of SiO2 films containing
nc-Si and Er as a function of f Si (d Si). The inset is an expansion of the region between 1.46 and 1.65 mm. The volume fraction of nc-Si ( f Si) was changed from 0 to 21 vol. %,while C Er was fixed at about 0.04 at. %. As f Si was changed
from 7 to 21 vol. %, d Si changed from 2.7 to 3.8 nm. The
film thickness was fixed at about 1.2 mm. For the sample notcontaining nc-Si, we can see a very week peak at about 1.54mm, corresponding to the intra-4 f transition of Er31
( 4 I 13/2 – 4 I 15/2).2 By adding nc-Si to the films, the intensity of
the 1.54 mm peak increases drastically. Although the intensity increases drastically by adding nc-Si, its dependence onf Si is not simple. It was found that the intensity becomesmaximum ~about 200 times stronger than that of the sample
not containing nc-Si! when f Si'7 vol. % (d Si' 2.7 nm!. By
further increasing f Si from 7 to 21 vol. %, i.e., by increasingd Si from 2.7 to 3.8 nm, the PL intensity decreased rapidly.
In addition to the 1.54 mm peak, the samples containing
nc-Si show a peak at about 0.8 mm. From our previous PL
studies,10 the 0.8 mm peak can be assigned to the recombination of electron-hole pairs in nc-Si. In Fig. 2, we can seethat the peak energy and the intensity of the 0.8 mm peakdepend strongly on the size of nc-Si. As the size decreases,
the peak becomes intense and shifts to higher energies. The
high-energy shift is caused by the widening of the band gap
due to the quantum size effects.10 The increase in the PL
intensity is considered to be due to the increase in the oscillator strength,17 and/or the decrease in the nonradiative Auger recombination process.18
Figure 3 shows the intensities of the 1.54 and 0.8 mm
peaks as a function of d Si . The intensities are corrected by

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J. Appl. Phys., Vol. 84, No. 8, 15 October 1998

FIG. 2. PL spectra of SiO2 films containing nc-Si and Er. The inset is an
expansion of the region between 1.46 and 1.65 mm. The Er concentrationand film thickness for all the samples are fixed at 0.04 at. % and 1.2 mm,
respectively, and the concentration of the Si nanocrystals is varied from 0 to
21 vol. %.

the amount of nc-Si contained in the films by dividing theobserved intensities by f Si . For comparison purposes, theintensities are normalized at their maximum intensities obtained for d Si'2.7 nm. We can see that as d Si decreases from
3.8 to 2.7 nm, the 0.8 mm peak of nc-Si becomes about

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4527

FIG. 4. PL spectra of SiO2 films containing nc-Si and Er. The inset is an
expansion of the region between 1.46 and 1.61 mm. The size of nc-Si isabout 2.7 nm and the Er concentration is varied from 0 to 0.11 at. %.

two orders of magnitude more intense. At the same time, the1.54 mm peak of Er31 also becomes more intense dependingon the size of nc-Si. It should be stressed here that the intensity of the 1.54 mm peak of Er31 exhibits nearly the same
size dependence as that of the 0.8 mm peak of nc-Si.
C. Dependence of PL spectra on the Er concentration

Figure 4 shows the dependence of PL spectra on C Er .
The size ~volume fraction! of nc-Si is fixed at about 2.7 nm
in diameter ( f Si'7 vol. %!. The inset is an expansion of theregion between 1.46 and 1.61 mm. For the sample not containing Er, we can see a peak of nc-Si at about 0.8 mm. As
C Er increases, the intensity of the 0.8 mm peak decreases
rapidly. It is noted that, in spite of the drastic change in theintensity, the peak energy is independent of C Er . This suggests that the size of nc-Si is not affected by C Er , because
the peak energy is very sensitive to the size.10 In contrast to
the 0.8 mm peak, the 1.54 mm peak of Er31 becomes intense
as C Er increases. Figure 5 compares the intensities of the two
peaks as a function of C Er . We can see that the 1.54 mm
peak becomes intense almost linearly with C Er , while the 0.8
mm peak decreases rapidly.
D. Temperature dependence of the PL spectra
FIG. 3. PL intensities @(d) 1.54 mm peak, (s) 0.8 mm peak# as a functionof the diameter of nc-Si. The intensities are corrected by the amount of nc-Si
contained in the films by dividing the observed intensities by the Si concentration ( f Si). The intensities are normalized at their maximum intensities at
d Si52.7 nm for the comparison purposes.

Figure 6 shows the temperature dependence of the PL
spectra for the samples with C Er'0.04 at. % and d Si'2.7
nm. At room temperature, we can see peaks at about 1.54
and 0.8 mm. As the temperature decreases, the intensity of
the 1.54 mm peak increases slightly, while that of the 0.8 mm

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J. Appl. Phys., Vol. 84, No. 8, 15 October 1998

FIG. 5. PL intensities of 1.54 and 0.8 mm peaks as a function of Er concentration. The lines are drawn to guide the eye.

peak first increases and then decreases. In addition to thesepeaks, another peak appears at about 1.2 mm below 150 K.
This peak becomes intense with decreasing temperature. The
origin of the 1.2 mm peak is considered to be the recombination of carriers trapped at P b centers at the interfaces between nc-Si and SiO2 matrices.19,20 Figure 7 shows the intensity of the 1.54 mm peak as a function of a temperature.
Although the intensity slightly decreases as the temperature

FIG. 7. The intensity of the 1.54 mm peak as a function of temperature. Theline is drawn to guide the eye.

increases, the degree of thermal quenching is very small. The
PL is quenched only about 30% from 15 to 290 K. Thisvalue is much smaller ~about two orders of magnitude! thanthat reported for the Er-doped bulk-Si crystal.4 Similar verysmall thermal quenching has been observed for all thesamples.
In Fig. 6, the energy of the 1.54 mm peak is almost
independent of the temperature, while that of the 0.8 mm
peak depends on the temperature. As the temperature decreases, the 0.8 mm peak shifts to higher energy. The degreeof the high-energy shift was nearly the same as that of the
band-gap of the bulk-Si crystal @about 45 meV ~300 to 5 K!#.
We have studied in detail the size and temperature dependence of the 0.8 and 1.2 mm peaks for the samples not containing Er. The results will be published elsewhere.
IV. DISCUSSION

FIG. 6. Temperature dependence of the PL spectra of SiO2 films containing
nc-Si and Er. The size of nc-Si is about 2.7 nm and the Er concentration isabout 0.04 at. %. The inset is an expansion of the region between 1.46 and1.61 mm.

As shown in Figs. 2 and 3, the intensity of the 1.54 mm
peak of Er31 is strongly enhanced by incorporating nc-Si
into the films. Furthermore, the intensity of the 1.54 mm peakdepends strongly on the size of nc-Si, and the size dependence is almost the same as that of the 0.8 mm peak of nc-Si,i.e., the intensity ratio of the 1.54 and 0.8 mm peaks is aconstant independent of the size of nc-Si provided that C Er is
the same. The results of Figs. 2 and 3 suggest that much
more intense 1.54 mm PL may be achieved by codoping
much ‘‘brighter’’ nc-Si. Since the PL intensity of nc-Si becomes intense as the size decreases, PL study of the sample
containing much smaller nc-Si (,2.7 nm! is promising inlearning more.
In this work, we controlled d Si by changing f Si . As f Si
increases, an interaction between nc-Si may increase, and theinteraction may affect the PL efficiency of Er31 . Therefore,
in order to study purely the size dependence, d Si should becontrolled without changing f Si . Unfortunately, until now,
we have not succeeded in controlling d Si under fixed f Si andthus cannot discuss the effects of the interactions in detail.

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J. Appl. Phys., Vol. 84, No. 8, 15 October 1998

Although the intensity ratio of the 1.54 and 0.8 mm
peaks does not depend on the size, it depends strongly on C Er
as can be seen in Figs. 4 and 5. The intensity of the 1.54 mm
peak increases as C Er increases, while the 0.8 mm peak decreases. By assuming an energy transfer from nc-Si to Er31 ,these results can be explained as follows. First, the excitation
light is absorbed mainly by nc-Si and the electron-hole pairs
are generated in the nanocrystals. A part of the recombination energy of the electron-hole pairs is then transferred to
Er31 . The amount of energy transferred to Er31 increases as
C Er increases. This results in growth of the 1.54 mm peakand quenching of the 0.8 mm peak with increasing C Er .
Since the intensity of the 1.54 mm peak in Fig. 5 depends nearly linearly on C Er , the energy transfer from nc-Si
to Er31 is considered to be limited by the number of optically active Er31 in the films ~i.e., almost all optically active
Er31 ions are excited by the energy transfer!. In this case, we
can expect much different excitation power ( P ex) dependence for the Er31 and nc-Si peaks. Since the amount of theenergy transferred to Er31 is limited by the number of Er31 ,the intensity of the 1.54 mm peak will not depend linearly on
P ex but will be saturated at rather low P ex , while that of the
0.8 mm peak will not be saturated. The saturated intensity of
the 1.54 mm peak will depend on C Er .
Figure 8 shows the P ex dependence of the intensities of
1.54 mm @Fig. 8~a!# and 0.8 mm @Fig. 8~b!# peaks for the
samples with C Er of 0.02 and 0.07 at. %. The average diameter of nc-Si is about 2.7 nm. For comparison purposes, thePL intensity of the sample not containing nc-Si and with C Er
of 0.07 at.% is also shown in Fig. 8~a!. Since the PL intensityof the sample not containing nc-Si is very weak, the raw
intensity is multiplied by a factor of 30. We can see that the1.54 mm peak of the samples containing nc-Si exhibits verysmall P ex dependence and is almost saturated @Fig. 8~a!#,while that of the 0.8 mm peak increases monotonously @Fig.
8~b!#. As is expected, the saturated intensity of the sample
with C Er of 0.07 at. % is much larger than that of the sample
with C Er of 0.02 at. %. It should be noted here that the PL
intensity of the sample not containing nc-Si is much smaller
than those containing nc-Si and saturation was not observed@Fig. 8~a!#.
To further investigate the nc-Si mediated excitation
mechanism of Er31 , we also studied the excitation wavelength (l ex) dependence of the 1.54 and 0.8 mm peaks. Figure 9 shows the l ex dependence of the intensities of the 1.54
and 0.8 mm peaks for the sample with d Si of 2.7 nm and C Er
of 0.11 at. %. For comparison purposes, the data for the
sample not containing nc-Si is also shown ~multiplied by afactor of 10!. First, we compare the l ex dependence of the
1.54 mm peak with that of the 0.8 mm peak for the samples
containing nc-Si and Er31 . We can see that, as l ex increases,
the intensities of the two peaks decrease almost monotonously, and the overall l ex dependence of the two peaks isvery similar except for a dip at 488.0 nm for the 1.54 mm
peak.
We now compare the l ex dependence of the 1.54 mm
peak between the samples containing and not those containing nc-Si. We can see a completely different l ex dependence
between the two samples. For the sample not containing nc-

Fujii et al.

4529

FIG. 8. PL intensities of ~a! 1.54 and ~b! 0.8 mm peaks as a function of
excitation power for the samples with Er concentration of 0.02 and 0.07
at. %. The diameter of nc-Si for both the samples is about 2.7 nm. For
comparison purposes, the PL intensity of the sample without nc-Si is also
shown. The lines are drawn to guide the eye.

Si, the 1.54 mm PL was detectable only when the film was
excited by 488.0 and 514.5 nm light, and no signal was detected with other l ex for the power density ~1.5 W/cm2 ) usedin the experiment. The 488.0 and 514.5 nm correspond to the4
I 15/2 – 4 F 7/2 and 4 I 15/2 – 2 H 11/2 transitions of Er31 ,respectively.2 On the other hand, the sample containing nc-Si
shows a dip at 488.0 nm. Although the origin of the dip is
not clear at present, one possible explanation is as follows.
At 488.0 nm excitation, Er31 is excited both by the energy
transfer from nc-Si and by the direct absorption of the excitation light. The direct absorption causes filling of the excited states, and thus the efficiency of the energy transfer
may be reduced. This may result in a dip at the 488.0 nm
excitation. The results of Fig. 9 clearly demonstrate that theexcitation of Er31 is not due to a direct absorption by Er31 ,but made by the energy transfer from nc-Si.

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4530

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J. Appl. Phys., Vol. 84, No. 8, 15 October 1998

Er in Si (;1018/cm3 ) 21 suggest that the Er31 responsible
for the strong PL is around the interface region between
nc-Si and SiO2 , although a few Er31 ions may exist in the
nanocrystals.
V. CONCLUSION

FIG. 9. PL intensities of 1.54 and 0.8 mm peaks as a function of excitation
wavelength for samples with an Er concentration of 0.11 at. % and a nc-Si
diameter of about 2.7 nm. For comparison purposes, the PL intensity of thesample without nc-Si is also shown.

As shown in Figs. 6 and 7, the thermal quenching of the
1.54 mm peak of the present samples is much smaller thanthat of the Er-doped bulk-Si crystal.4 In the case of the Erdoped bulk-Si crystal, the large thermal quenching is considered to be due to the increase in the probability of the phonon assisted back transfer of the energy from Er31 to host Si
crystals4. In the present samples, the band gap of nc-Si ismuch larger than the bulk band gap, and thus the probabilityof the back transfer becomes much smaller because the backtransfer requires multiple phonons. The band gap wideningof nc-Si due to the quantum size effects is thus considered tobe the primary cause of the small thermal quenching observed.
Similar small thermal quenching has been reported for
Er-doped porous Si5,6,8 and semi-insulating polycrystalline
silicon ~SIPOS!.4 The small quenching of Er-doped
porous-Si is often explained in the same manner as that ofthe present samples, because it consists of nanometer-size Si
grains.8 However, since porous Si is very complex in its
structure and chemical components, PL mechanism of Erdoped porous Si is not fully understood. In particular, someauthors claim that Er-related PL from Er-doped porous Si is
not related to the quantum size effects in nc-Si but due to Er
atoms incorporated into amorphous-Si:H:O regions in porous
Si.5,6 In the case of SIPOS, a large band gap due to a highoxygen content ~typically several tens of at. %! is considered
to be the cause of the small quenching.5
A question arises as to whether the Er31 responsible forthe strong 1.54 mm PL is located inside or outside the nc-Si.
Since the shape of the 1.54 mm peak depends on the local
environment of Er31 ,6 we compared the spectral shape of thesamples containing nc-Si with that not containing nc-Si. Although the PL intensity became stronger by incorporatingnc-Si, the spectral shape was almost identical for all thesamples. This result and a relatively low solid solubility of

We have studied PL properties of SiO2 films containing
nc-Si and Er. Peaks attributable to nc-Si ~0.8 mm! and Er31
~1.54 mm! were observed simultaneously. The correlation
between the intensities of the two PL peaks was studied as afunction of the size of nc-Si, Er concentration, excitation
power and excitation wavelength. The most interesting result
observed was that the 1.54 mm PL of Er31 is strongly enhanced by incorporating nc-Si into the films, and the PL
intensity depends on the size of nc-Si. Furthermore, we
found that the intensity ratio of the 1.54 and 0.8 mm peaks is
a constant independent of the size of nc-Si, provided that the
Er concentration in the films is fixed. This implies that incorporation of ‘‘brighter’’ nanocrystals results in the stronger
1.54 mm PL. Since the nc-Si becomes ‘‘brighter’’ as the size
decreases, the 1.54 mm PL will be more intense as the size ofthe nc-Si incorporated decreases. The 1.54 mm PL exhibitedvery small thermal quenching. The small thermal quenchingis considered to be due to the widening of the band gap by
the quantum size effects. Therefore, we can conclude that
two major features of the quantum size effects of nc-Si, i.e.,the band gap widening and the increases in the PL efficiency
with decreasing the size, contribute to the improvement ofroom temperature PL efficiency of Er31 .
ACKNOWLEDGMENT

This work was partially supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Science,Sports and Culture.
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58, 2797 ~1991!.
19

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