APPLIED PHYSICS LETTERSVOLUME 73, NUMBER 21

23 NOVEMBER 1998

Excitation of intra-4 f shell luminescence of Yb31 by energy transferfrom Si nanocrystals
Minoru Fujii,a) Shinji Hayashi, and Keiichi Yamamoto
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai,Nada, Kobe 657-8501, Japan~Received 19 June 1998; accepted for publication 20 September 1998!
SiO2 films containing Si nanocrystals ~nc-Si! and Yb were prepared and their photoluminescence~PL! properties were studied. For the sample containing nc-Si with an average diameter of 3.1 nm,
a weak peak (;1.26 eV) attributable to the intra-4 f shell transition of Yb31 could be observed at
the low-energy side of a broad peak (;1.4 eV) of nc-Si. The intensity of the 1.26 eV peak wasfound to depend strongly on the size of nc-Si and increase rapidly with decreasing size. The
temperature dependence of the PL spectra was studied. It was found that the degree of temperature
quenching of the 1.26 eV peak depends on the size of the nc-Si and becomes small as the size
decreases. These results suggest that the band-gap widening of nc-Si due to the quantum size effects
is essential to efficiently excite Yb31 by nc-Si. © 1998 American Institute of Physics.
@S0003-6951~98!01047-X#

It is well known that rare-earth ions incorporated intosemiconductor hosts in their 31 states emit light from theintra-4 f shell transition.1–8 Er-doped Si has been mostwidely studied, because the emission band of Er31 @1.54 mm~0.81 eV!# corresponds to the absorption minimum in silicabased glass fibers.1–4 However, the 0.81 eV luminescence ofEr-doped Si often exhibits very large temperature quenching
and thus the room temperature luminescence efficiency israther low. One of the causes of the low luminescence efficiency is the large probability of the phonon-assisted backtransfer of energy from Er31 to the host Si at hightemperatures.1
The probability of back transfer is considered to dependon the band gap of the host materials and decreases as theband gap increases.1 Since the band gap of Si nanocrystals~nc-Si!, as small as several nanometers, is much larger thanthat of bulk-Si crystal,9–13 nc-Si may efficiently excite Er31.
In our previous work, we have studied in detail the nc-Si
mediated excitation mechanism of Er31. 14,15 We have foundthat the intensity of the 0.81 eV photoluminescence ~PL!
depends strongly on the size of the incorporated nc-Si andincreases about two orders of magnitude as the size decreases from 3.8 to 2.7 nm. Our data demonstrated that two
major features of the quantum size effects of nc-Si, i.e., theband-gap widening and the increase in the PL efficiency withdecreasing size, contribute to the improvement of the roomtemperature PL efficiency of Er31. 15
This work is an extension of our previous studies.14,15 In
order to further clarify the nc-Si mediated excitation processof rare-earth ions, we have studied PL properties of SiO2
films containing nc-Si and Yb. An energy level scheme ofYb31 4 f shell is very simple with only one excited state( 2 F 5/2) at about 1.26 eV from the ground state ( 2 F 7/2). 8Since the 2 F 5/22 2 F 7/2 split is larger than the band gap of abulk-Si crystal ~1.12 eV!, Yb31 cannot be excited by theenergy transfer from a bulk-Si crystal. By reducing the sizea!

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

of the host Si from the bulk to a few nanometers, the band
gap changes continuously from 1.12 eV to above 1.5 eV,9,10
and for the nc-Si with a band gap of larger than 1.26 eV,
excitation of Yb31 may become possible. Therefore, fromthe size-dependent PL studies, we can systematically study
PL properties of rare-earth ions as a function of the band gapof a host.
SiO2 films containing nc-Si and Yb were prepared by thesame method used in our previous work for SiO2 films containing nc-Si and Er.14,15 Small pieces of Si chips 5315 mm2 in size and Yb2O3 pellets 10 mm in diameter wereplaced on a SiO2 sputtering target ~10 cm in diameter! andthey were cosputtered in Ar gas atmosphere. Films of about
1 mm in thickness were deposited onto fused quartz plates.
After the deposition, in order to grow nc-Si, samples wereannealed in an atmosphere of 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 andthe annealing temperature. Yb concentration (C Yb) can also
be controlled by changing the number of Yb2O3 pellets during the cosputtering. The C Yb and the volume fraction ofnc-Si in films were determined by electron-probe microanalyses @JXA-8900 ~JEOL!#. The size of nc-Si was determined by cross-sectional high-resolution transmission
electron microscopic ~HRTEM! observations @JEM-2010~JEOL!#. In this work, C Yb was fixed to about 0.16 at. %,while the average diameter of nc-Si (d Si) was changed from
2.4 to 3.1 nm.
Photoluminescence spectra were measured using a HR320 ~Jobin Yvon! monochromator and an EO-817L ~NorthCoast! Ge detector. The excitation source was a 457.9 nm
line of an Ar-ion laser with a power density of less than
1.5 W/cm2. The spectral response of the detection system
was corrected by reference spectra of a standard tungsten
lamp. The temperature dependence of PL spectra was measured from 20 to 300 K in a continuous-flow He cryostat@Optistat ~Oxford Instruments!#.
Figure 1 shows the room-temperature PL spectra of SiO20003-6951/98/73(21)/3108/3/$15.00
3108© 1998 American Institute of Physics
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Appl. Phys. Lett., Vol. 73, No. 21, 23 November 1998

Fujii, Hayashi, and Yamamoto

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FIG. 2. Temperature dependence of the PL spectra for the sample withd Si52.4 nm and C Yb50.16 at. %.
FIG. 1. Room-temperature PL spectra of SiO2 films containing nc-Si andYb. Inset is an expansion of the region between 1.235 and 1.285 eV. The
size of nc-Si is varied from 2.4 to 3.1 nm, while Yb concentration and film
thickness are fixed at about 0.16 at. % and 1.2 mm, respectively. For comparison purposes, the spectra are normalized at their maximum intensity.

films containing nc-Si and Yb as a function of d Si . The inset
is an expansion of the region between 1.235 and 1.285 eV.
For comparison purposes, the spectra are normalized at theirmaximum intensity. In Fig. 1, we can see two peaks. The
broad one around 1.5 eV corresponds to the radiative recombination of electron–hole pairs in nc-Si.9,10 This peak exhibits a clear blueshift with decreasing the size due to quantum
size effects. The relative intensity of this peak is shown inFig. 1 ~a larger value corresponds to a larger PL intensity!.
The intensities are corrected by the amount of nc-Si contained in the samples ~f ! by dividing the observed intensities
by f. We can see that the band-edge PL becomes intense as
the size decreases.9,10 At the low-energy tail of the peak, aweak peak is observed around 1.26 eV. This peak can be
assigned to the intra-4 f shell transition of Yb31
( 2 F 5/22 2 F 7/2). The intensity of the 1.26 eV peak becomeslarge as the size of nc-Si is decreased ~as the band-edgerelated peak of nc-Si shifts to higher energies!, although Yb
concentration is nearly the same for all the samples. We havealso studied samples containing nc-Si with d Si.3.1 nm.
However, the 1.26 eV peak could not be observed for thesesamples.
Figure 2 shows the temperature dependence of the PLspectra for the sample with d Si'2.4 nm. At room temperature, we can see peaks at around 1.26 and 1.6 eV. As thetemperature decreases, the 1.6 eV peak shifts monotonously
to higher energies. The degree of the high-energy shift was
nearly the same as that of the band gap of the bulk-Si crystal@about 45 meV ~300–20 K!#.10 The intensity of the 1.6 eVpeak first increases and then decreases. The intensity at 20 K
is comparable to that at 293 K. Although the reason for thecharacteristic temperature dependence of the PL intensity isnot clear, a similar temperature dependence has commonly
been observed for nc-Si in SiO2 matrices.10 At 20 K, the 1.6
eV peak is very broad with a long tail at the low-energy side.
The origin of the tail is considered to be the recombination ofcarriers trapped at P b centers at the interfaces between nc-Siand SiO2 matrices.10,16,17 In contrast to the 1.6 eV peak, the1.26 eV peak of Yb31 increases monotonously with decreasing the temperature. The small peak appearing at low temperatures at around 1.2 eV is due to the crystal field splittingof the 2 F 5/2 and 2 F 7/2 bands.8 Figure 3 shows the intensity of
the 1.26 eV peak as a function of a temperature for thesamples with d Si'2.4, 2.7 and 3.1 nm. The intensities arenormalized at 20 K. We can find that the degree of temperature quenching of the PL depends strongly on the size ofnc-Si. The sample with d Si'3.1 nm exhibits the largest temperature quenching, and as the size decreases, the quenching
becomes small.
Until now, PL properties of Yb31 have intensively beenstudied for Yb-doped InP.5–8 Yb forms an acceptor-like electron trap level at 30 meV below the bottom of the conduction
band.5 Electron–hole pairs generated by visible light excitation are captured by this trap and form a bound exciton. The
recombination energy of the bound exciton is subsequently
transferred to Yb31 and excites the intra-4 f shell. At relatively high temperatures, the reverse process of the energy
transfer is also possible. The excited 4 f shell relaxes nonradiatively, transferring its energy to the host by producing
electron–hole pairs ~back transfer!. Since the back transfer isFIG. 3. Intensity of the intra-4 f shell PL of Yb as a function of temperature
for the samples with d Si52.4, 2.7 and 3.1 nm. For comparison purposes, theintensity of the intra-4 f shell PL of Er is also shown as a function of atemperature (d Si52.7 nm). The curves are drawn to guide the eye.
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Appl. Phys. Lett., Vol. 73, No. 21, 23 November 1998

a phonon-assisted process, the probability becomes large as
the temperature is increased.5
In the present experiments, no PL signal was detected
for the sample not containing nc-Si ~Yb-doped SiO2). Since
Yb31 has only one excited state ( 2 F 5/2) in the near-infrared
region, to excite Yb31 by visible light, energy transfer from ahost material is indispensable. The 1.26 eV PL of Yb31
shown in Fig. 1 is thus considered to be excited by the energy transfer from nc-Si by a process similar to that of Ybdoped InP, although we have at present no definite idea
about a Yb-related trap center.
Two possible quenching processes are often considered
to explain the PL properties of rare-earth ions doped into
semiconductor hosts. One is the dissociation of excitons
bound to a rare-earth-related trap center before the energy istransferred to the rare-earth ions, and the other is the energy
back transfer from rare-earth ions to a host. Since bothquenching processes require the annihilation of phonons,
they become more probable at high temperatures. A decision
between these two possibilities is made through a study ofPL decay dynamics. The dissociation process will not affectthe PL lifetime, while the back transfer process will decrease
both the PL intensity and lifetime. In the case of Yb-doped
InP ~Ref. 5! and Er-doped Si,1 the temperature dependenceof the PL lifetime has been studied in detail. It has been
demonstrated that the back transfer process is the primarycause of the temperature quenching at high temperatures.
The band-edge-related PL of the present nc-Si appears ataround 1.4–1.6 eV ~Fig. 1!. This value is very similar to thatof InP (;1.4 eV). We thus assume that the back transfer
process is also the dominant quenching process in the presentsamples. Under the assumption, the observed size and temperature dependences can be explained as follows. The probability of the back transfer depends on the band gap of ahost, and decreases as the band gap increases, because theback transfer requires multiple phonons. In Fig. 1, we haveshown that as the size of nc-Si decreases ~as the band-edgerelated PL shifts to higher energies!, the 1.26 eV PL intensity
increases. We believe that the enhancement of the 1.26 eV
PL with decreasing the size is mainly caused by the reduction of the back transfer probability due to the band-gap
widening. The suppression of the temperature quenchingwith decreasing the size shown in Fig. 3 can also be explained by the widening of the band gap of nc-Si.
In Fig. 3, we also show the temperature dependence of
the 0.81 eV PL ~intra-4 f shell transition of Er31) intensity ofa Er-doped sample with d Si'2.7 nm. We can see that the0.81 eV peak exhibits very small temperature quenching. We
have performed the same experiments for Er-doped samples
with various sizes (d Si'2.7– 3.8 nm). However, even withthe largest size ~3.8 nm!, the temperature quenching wasvery small and no remarkable size dependence was observedon the degree of temperature quenching. The small temperature quenching is considered to be caused by the large energy difference between the band gap of nc-Si and the emission band of Er31.
As described above, the decay dynamics of PL should bestudied to definitely determine the PL quenching process ofthe present samples. However, the decay dynamics of the1.26 eV PL of Yb31 for the present samples are difficult toFujii, Hayashi, and Yamamoto

measure, because the 1.26 eV PL is on the tail of the bandedge PL of nc-Si. In the case of Yb-doped InP, evidence forthe back transfer is demonstrated from the PL decay dynamics of an InP host.5 It was demonstrated that at temperatures
above 100 K, a slow component appears in the decay curveof the band-edge PL. The appearance of the slow component
suggests that free electrons and holes are generated by theenergy back transfer from the excited Yb31 to the InP host,and then the electrons and holes recombine and contribute to
the band-edge PL. Similar studies seem to be useful in discussing the PL quenching process of the present samples.
In summary, we have studied PL properties of SiO2 films
containing nc-Si and Yb. By reducing the size of nc-Si toabout 3.1 nm, we could successfully excite Yb31 and observe a PL peak at around 1.26 eV ( 2 F 5/2 – 2 F 7/2 transition!.
The 1.26 eV PL intensity increased rapidly by further decreasing the size. The temperature dependence of the PL
spectra was studied. It was found that the degree of the temperature quenching depends on the size of nc-Si and becomes small as the size decreases. These results clearly demonstrate that excitation of Yb31 is made by the energy
transfer from nc-Si. Furthermore, the band-gap widening ofnc-Si due to the quantum size effects is essential to efficiently excite Yb31 by nc-Si.
The authors are grateful to Kei Watanabe for his valuable assistance in this work. This work was partially supported by a Grant-in-Aid for Scientific Research from theMinistry of Education, Science, Sports and Culture, Japan.

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