1.54 mm photoluminescence of Er31 doped into SiO2 films containing Si
nanocrystals: Evidence for energy transfer from Si nanocrystals
to Er31
Minoru Fujii,a) Masato Yoshida, and Yoshihiko Kanzawa
Division of Science of Materials, The Graduate School of Science and Technology, Kobe University,
Rokkodai, Nada, Kobe 657, Japan

Shinji Hayashi and Keiichi Yamamoto
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai,
Nada, Kobe 657, Japan~Received 12 February 1997; accepted for publication 30 June 1997!
SiO2 films containing Si nanocrystals ~nc-Si! and Er were prepared and their photoluminescence~PL! properties were studied. The samples exhibited luminescence peaks at 0.81 and 1.54 mm,
which could be assigned to the electron-hole recombination in nc-Si and the intra-4f transition in
Er31 , respectively. Correlation between the intensities of the two luminescence peaks was studied
as functions of Er concentration and excitation power. The present results clearly demonstrate that
excitation of Er31 occurs through the recombination of photogenerated carriers spatially confined in
nc-Si and the subsequent energy transfer to Er31 . © 1997 American Institute of Physics.
@S0003-6951~97!02835-0#

Er-doped Si has been recently attracting much interest
because of its potential application in Si-based optoelectronic
devices. The Er ions incorporated in Si in their 31 state
produce light emission from the intra-4f transition ( 4 I 13/2 to4
I 15/2! at around 1.54 mm, which corresponds to the absorption minimum in silica-based glass fibers.1–3 Interest has also
been focused on Er-doped porous Si, which emits strong
1.54 mm luminescence even at room temperature andexhibits very weak temperature quenching of theluminescence. 4–7 It is often suggested that, in porous Si,
excitation of Er31 occurs through the recombination of photogenerated carriers confined in Si nanocrystals ~nc-Si! andsubsequent energy transfer to Er31 in or near the nc-Si.5–7
If the Er31 is excited by the energy transfer from nc-Si,
the 1.54 mm luminescence and the visible or near-infrared
luminescence due to the band-to-band transition in nc-Si
should correlate to each other depending on the number ofactive Er31 , excitation energy and power, and temperature.
However, despite the numerous research, detailed studies onthese dependencies have so far not been carried out, and theenergy transfer has not been clearly evidenced. The purposeof this work is to study the correlation between the two luminescence bands and demonstrate the energy transfer from
nc-Si to Er31 .
To study the above dependencies of the two luminescence bands, the stable nc-Si with well-defined size andshape as well as chemical composition is indispensable. For
this purpose, porous Si is not appropriate, because of its
instability, complexity in its structure and chemical composition, and resulting ambiguity in the origin of the luminescence. In this work, we employed SiO2 films containing ncSi, instead of porous Si, as a host of Er31 . We have recently
succeeded in preparing spherical nc-Si with good crystallinity as small as 2.7–5.5 nm in diameter dispersed in SiO2 thinfilms.8–10 We demonstrated that these samples show a lumia!

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

nescence peak in a near-infrared ~NIR! region ~about 0.8
mm! and the peak exhibits a blue shift with decreasing thesize. From the observed size dependence, we could attribute
the NIR peak to the recombination of electron-hole pairs in
the nc-Si.10–12 In this work, we added Er in the samples andstudied the luminescence properties as functions of Er concentration and excitation power. We will demonstrate thattwo peaks corresponding to the recombination of electronhole pairs in nc-Si and the intra-4f transition in Er31 can beobserved simultaneously, and a clear correlation between theintensities of the two peaks can be observed. We will showthat the observed correlation provides clear evidence for theenergy transfer from nc-Si to Er31 .
Samples were prepared by a rf cosputtering method. In
our previous studies,8–10 we demonstrated that nc-Si embedded 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
for the cosputtering. 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 indiameter! and they were cosputtered. The films of 1 mm inthickness were deposited onto fused quartz plates. After thedeposition, the samples were annealed in a N2 ambient for 30
min at 1100 °C. In this method, the size of nc-Si can becontrolled by changing the number of Si chips during thecosputtering and the annealing temperature. Er concentration(C Er) can also be controlled by changing the number of
Er2 O3 pellets during the cosputtering. The C Er was determined by electron-probe microanalysis @JCXA-733 ~JEOL!#.
The size of nc-Si was determined by high-resolution transmission electron microscopic observation @JEM-2010~JEOL!#. Photoluminescence ~PL! spectra were measured using a HR-320 ~Jobin Yvon! monochromater and an EO-817L~North Coast! Ge detector. The excitation sources were sixlines of an Ar-ion laser with a power density of less than 1.5
W/cm2 . Spectral responses of the detection systems were1198
Appl. Phys. Lett. 71 (9), 1 September 1997
0003-6951/97/71(9)/1198/3/$10.00© 1997 American Institute of Physics
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FIG. 2. Intensities of 0.81 and 1.54 mm peaks as a function of Er concentration. The lines are drawn to guide the eye.

FIG. 1. PL spectra of SiO2 films containing nc-Si and Er. Inset is an expansion of the region between 1.46 and 1.61 mm.

corrected by reference spectra of a standard tungsten lamp.
All the measurements were made at room temperature.
Figure 1 shows the dependence of PL spectra on
C Er~at.%! for SiO2 films containing nc-Si about 2.7 nm indiameter ~volume fraction of the nc-Si is about 5%!. Inset isan expansion of the region between 1.46 and 1.61 mm. For
the sample not containing Er, we can see only one peak at
about 0.81 mm. From previous PL studies for nc-Si,10 thispeak can be assigned to the recombination of electron-holepairs in nc-Si. As the C Er increases, the intensity of the peakdecreases rapidly. It should be noted here that, in spite of thedrastic change in the intensity, the peak position is not affected by the C Er . In our previous paper, we demonstrated
that the PL peak of nc-Si is size sensitive and shifts from
0.87 to 0.80 mm as the size decreases from 3.8 to 2.7 nm.10
Therefore, the size of the nc-Si is not affected by the C Er andis considered to be almost identical for all the samples ~about
2.7 nm!.
For the samples containing Er, we can see another peakat 1.54 mm, corresponding to the intra-4f transition of Er31
( 4 I 13/2 to 4 I 15/2).2 In contrast to the 0.81 mm peak, this peak
becomes intense as C Er increases. Figure 2 compares theintensities of the two peaks as a function of C Er . In spite of
the very small amount of Er, the intensities of the two peaks
change drastically with C Er . We can see that the 1.54 mm
peak becomes strong with increasing C Er , while the 0.81 mm
peak decreases rapidly.
Although not shown here, the intensity of the 1.54 mm
peak in Fig. 2 is more than 50 times stronger than those of
SiO2 films not containing nc-Si and containing the sameamount of Er. This suggests that nc-Si enhances the 1.54 mm
luminescence of Er31 . If the energy transfer from nc-Si to
Er31 is assumed, the results of Figs. 1 and 2 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 energies of theelectron-hole pairs are then transferred to Er31 . The amountof the energy transferred to Er31 increases as C Er increases.
This results in the growth of the 1.54 mm peak and thequenching of the 0.81 mm peak with increasing C Er .
Since the intensity of the 1.54 mm peak in Fig. 2 depends nearly linearly on C Er , the energy transfer is considered to be limited by the number of optically active Er31 in
the films ~i.e., almost all optically active Er31 is excited by
the energy transfer!. In this case, we can expect much different excitation power ( P ex ) dependence between the two
peaks. Since the amount of the energy transferred to Er31 islimited by the number of Er31 and is not proportional to
P ex , the intensity of the 1.54 mm peak will not depend linearly on P ex but will be saturated at rather low P ex , whilethat of the 0.81 mm peak will not be saturated. The saturated
intensity of the 1.54 mm peak will depend on C Er . Figure 3
shows the P ex dependencies of the PL intensities for the
sample with C Er of 0.02 at.%. We can see that the intensityof the 1.54 mm peak shows very small P ex dependence andis almost saturated, while that of the 0.81 mm peak increases
monotonously. It should be noted here that the saturation
was not observed for the samples not containing nc-Si. The
results of Fig. 3 combined with Figs. 1 and 2 strongly sug-

FIG. 3. Intensities of 0.81 and 1.54 mm peaks as a function of excitation
power for the sample with the Er concentration of 0.02 at. %. The lines are
drawn to guide the eye.

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Fujii et al.
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FIG. 4. Intensity of 1.54 mm peak as a function of excitation energy forsamples containing nc-Si and not. The lines are drawn to guide the eye.

gest that the excitation of Er31 occurs through the energy
transfer from nc-Si.
To investigate the excitation mechanism of Er31 further,
we also studied the excitation energy (l ex ) dependence ofthe 1.54 mm peak. Figure 4 compares the l ex dependence forthe sample containing nc-Si with that not containing nc-Si.
Since the intensity of the sample without nc-Si is very weak,a factor of 100 is multiplied. We can see the completely
different l ex dependencies between the two samples. The
sample not containing nc-Si exhibits a distinct peak at 488
nm, corresponding to the 4 I 15/2 to 4 F 7/2 transition of Er31 .2 It
is noted that, in this power density ~1.5 W/cm2 !, no signal
was detected with other l ex . On the other hand, the sample
containing nc-Si shows no distinct structure, indicating thatthe excitation is not a direct absorption by Er31 , but theabsorption by nc-Si and the subsequent energy transfer to
Er31 .
In the above, we did not discuss whether the Er31 responsible for the strong 1.54 mm luminescence is locatedinside or outside the nc-Si. Since the shape of the 1.54 mm
peak is considered to depend on the local environment of
Er,4 we compared spectral shape of the samples containing
nc-Si with that not containing nc-Si. Although the PL intensity becomes more than 50 times stronger by incorporatingnc-Si, the spectral shape was almost identical for all thesamples. This result and a relatively low solid solubility of
Er in Si (;1018 /cm3 !13 suggest that Er31 responsible for thestrong luminescence is around the interface region between
nc-Si and SiO2 , although a few may exist in the nanocrystals.
In this letter, we restricted our discussion for nc-Si with

only one average size ~2.7 nm!. We also studied the size
dependence of the PL spectra and found that the PL intensity~1.54 mm! decreases with increasing the size of nc-Si. The
results of the size and the temperature dependencies of the
PL properties will be published elsewhere.
In summary, we have studied luminescence properties of
SiO2 films containing nc-Si and Er. Peaks attributable tonc-Si ~0.81 mm! and Er31 ~1.54 mm! were observed simultaneously. Er concentration, excitation power, and excitationenergy dependencies of the two peaks were studied. It was
found that the 1.54 mm peak increases almost linearly withC Er , while the 0.81 mm peak decreases rapidly. The 1.54 mm
peak did not depend linearly on P ex but was saturated atrather low P ex . The l ex dependence of the 1.54 mm peak did
not have any distinct structure corresponding to the directtransition in Er31 . These results provide clear evidence forthe energy transfer from nc-Si to Er31 .
This work was partially supported by a grant from Foundation Advanced Technology Institute and a Grant-in-Aid
for Scientific Research from the Ministry of Education, Science, Sports and Culture.
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