JOURNAL OF APPLIED PHYSICSVOLUME 92, NUMBER 7

1 OCTOBER 2002

Excitation of Tm3¿ by resonant energy transfer from Si nanocrystals
Kei Watanabe and Hiroyuki Tamaoka
Graduate School of Science and Technology, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan

Minoru Fujiia) and Shinji Hayashi
Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai,
Nada, Kobe 657-8501, Japan

共Received 16 October 2001; accepted for publication 8 July 2002兲
Photoluminescence 共PL兲 properties and PL decay dynamics of SiO2 films containing Si nanocrystals
共nc-Si兲 and Tm3⫹ were studied. The samples exhibited a broad PL at around 1.5 eV due to therecombination of excitons in nc-Si, and rather sharp PL at 1.58, 0.84, and 0.69 eV corresponding to
the intra-4 f shell transitions of Tm3⫹ . The correlation between the intensities of nc-Si and Tm3⫹
related PL was studied as a function of the Tm concentration, the size of nc-Si, and the temperature.
It was found that the intensity of Tm3⫹ related PL depends strongly on the size of nc-Si. At low
temperatures, the spectral shape of nc-Si PL was strongly modified by doping Tm. From analysis of
the modified spectral shape, a resonant energy transfer from nc-Si to Tm3⫹ is discussed. © 2002
American Institute of Physics. 关DOI: 10.1063/1.1503860兴

I. INTRODUCTION

interaction with nc-Si was demonstrated for the first time, we
believe, by Franzó et al.4 However, in their work, the size
dependence of the luminescence properties was not studied.
Since the quantum confinement effects of excitons in nc-Si
are considered to play an important role in the interaction,4
the properties of energy transfer should depend strongly on
the size of nc-Si. By analyzing size dependence, valuable
information on the mechanism of energy transfer can be obtained. Furthermore, in the previous work, no spectroscopic
evidence of the energy transfer was obtained.
In this work, the PL properties of SiO2 films containing
nc-Si and Tm3⫹ are studied in a wide spectral range from
0.45 to 1.9 eV. A comprehensive study of a wide spectral
range is essential to understand the energy transfer mechanism, because Tm3⫹ exhibits several PL bands in the nearinfrared to visible regions, and thus the intensity ratio maychange, depending on experimental parameters. We show thecorrelation of the nc-Si PL and Tm3⫹ PL as a function of the
Tm concentration, nc-Si size, and temperature. A result muchdifferent from that reported by Franzó et al. was observed in
the temperature dependence of PL intensity. We also give
spectroscopic evidence of resonant energy transfer from
nc-Si to Tm3⫹ .

Recently, it has been demonstrated that 4 f shells of rareearth ions, e.g., Er3⫹ , Yb3⫹ , Nd3⫹ , and Tm3⫹ , can be efficiently excited by energy transfer from Si nanocrystals 共ncSi兲, i.e., excitation light is absorbed by nc-Si, and excitons
generated in nc-Si recombine with the transferred energy torare-earth ions. In particular, efficient luminescence at 0.81
eV 共1.54 ␮m兲 from the system containing nc-Si and Er 共the
Er/nc-Si system兲 has attracted much interest.1–9 The efficient
0.81 eV luminescence opens up the possibility of realizing
silicon/silica-based planar optical amplifiers which can be
integrated with existing semiconductor and fiber telecommunication technology.
The excitation mechanism of Er in Er/nc-Si systems has
been studied in detail1,2,5,8 and direct spectroscopic evidenceof the energy transfer has been obtained.3 Furthermore, energy transfer rates are experimentally determined as a function of the nc-Si size. It has been demonstrated that thesmaller the size of the particles, the higher the energy transfer rate.3 In contrast to Er/nc-Si systems the detailed mechanism of the interaction between nc-Si and other rare-earth
ions is not well understood, although enhancement of theintra-4 f shell photoluminescence 共PL兲 has been demonstrated.
Among several rare-earth ions which are reported to interact strongly with nc-Si, in this work, we adopted Tm andstudied its interaction with nc-Si in silica glasses, because
Tm doped glasses have attracted a lot of attention as potential materials for future near-infrared light sources andamplifiers.10–14 The enhancement of Tm3⫹ -luminescence efficiency by adding nc-Si would provide new perspectives on
the development of silica based infrared lasers and amplifiers. This enhancement of the intra-4 f shell PL of Tm3⫹ by

II. EXPERIMENT

SiO2 films containing nc-Si and Tm were prepared by acosputtering method similar to that used to prepare SiO2
films containing nc-Si and Er.1,2 Small pieces of Si chips andTm2 O3 pellets were placed on a SiO2 sputtering target andthey were simultaneously sputtered in an Ar gas atmosphere.
After deposition, in order to grow nc-Si, samples were annealed in a N2 gas atmosphere for 30 min at temperatureshigher than 1100 °C. In this work, the average size of nc-Si(d Si) was changed from 2.7 to 3.5 nm and the Tm concentration was changed from 0.17 to 1.41 at. %. PL spectra were
measured by a single grating monochromator. In the spectrala兲Author to whom correspondence should be addressed; electronic mail:fujii@eedept.kobe-u.ac.jp

0021-8979/2002/92(7)/4001/6/$19.00

4001© 2002 American Institute of Physics

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Watanabe et al.

FIG. 2. Intensities of the Tm3⫹ related peak 共0.69 eV兲 and the nc-Si related
peak as a function of the Tm concentration. The lifetime which was measured at the maximum of the nc-Si PL is also shown. The lines are to guidethe eye.

FIG. 1. PL spectra of SiO2 films containing nc-Si and Tm. The size of nc-Si
is about 2.7 nm and the Tm concentration is changed from 0.17 to 1.41
at. %. The inset shows the energy diagram of the intra-4 f shell of Tm3⫹ .

range between 0.45 and 0.80 eV, a liquid nitrogen cooled
InSb photodiode 共Hamamatsu兲 was used as a detector, and in
the spectral range between 0.80 and 1.90 eV, a liquid nitrogen cooled Ge photodiode 共North Coast兲 was used. The excitation source was a 514.5 nm line of an Ar-ion laser. For all
spectra, the spectral response of the detection system wascorrected by a reference spectrum of a standard tungsten
lamp. For the time response measurements, a 532.0 nm lineof a Nd:yttrium–aluminum–garnet laser was used as an excitation source. The pulse width was 5 ns and the repetition
frequency was 20 Hz. The decay curves were measured witha photon counting system that consisted of a photomultiplierwith an InP/InGaAs photocathode, a high speed amplifier
共1.2 ns rise/fall time兲 共SR445, Stanford Research Systems兲,and a multichannel scaler 共SR430, Stanford Research Systems兲. The time resolution of the whole system was about 80
ns.
III. RESULTS AND DISCUSSION
A. Dependence of PL spectra on the Tm concentration

Figure 1 shows the room temperature PL spectra as afunction of the Tm concentration. The size of nc-Si is fixed
at 2.7 nm and the Tm concentration is changed from 0.17 to1.41 at. %. The inset shows an energy diagram of the 4 f shellof Tm3⫹ . We can see two peaks in all the spectra. The high-energy broad PL corresponds to the recombination of excitons confined in nc-Si 共nc-Si PL兲,1,2,15 while the low-energy
one 共0.69 eV兲 corresponds to the intra-4 f shell transition ofTm3⫹ 共3 H 4 to 3 H 6 ) 共Tm3⫹ PL兲.10,11 As the Tm concentration
increases, the intensity of the Tm3⫹ PL peak increases,
whereas that of nc-Si PL decreases. In Fig. 2, the intensitiesof the two peaks are plotted as a function of the Tm concentration. We can see that the intensity of nc-Si PL decreases
rapidly with an increase in Tm concentration, while that ofTm3⫹ PL increases gradually and is saturated in the high Tm
concentration range. The intensity of the Tm3⫹ PL peak isalmost saturated at around 0.6 at. % 共92% of its saturation
value兲.
Figure 3 shows the Tm concentration dependence of the
PL decay curves detected at the maximum of nc-Si PL in Fig.
1. The decay curves are nonexponential. The PL lifetime
becomes shorter as the Tm concentration increases. The decay curves are fitted well by the modified exponential function, which is generally used to analyze the decay curves ofporous-Si and nc-Si.6,16 The lifetime determined from fitting
for the sample with the smallest Tm concentration is about
42 ␮s, while that with the largest Tm concentration is about
16 ␮s. The lifetimes obtained are shown in Fig. 2. In ourprevious work on Er-doped nc-Si, the lifetime of nc-Si decreased with an increase in Er concentration. However, inother PL studies of similar Er-doped samples, it has beenreported that the lifetime of nc-Si was independent of the Er
concentration.4,8 The reason for these contradictory results is
not clear at present.
In the case of Tm-doped SiO2 that does not contain ncSi, the Tm3⫹ PL peak is too weak to detect because of thevery low absorption coefficient of Tm3⫹ at the excitation
wavelength 共476.5 nm兲. Therefore, observation of intense
Tm3⫹ PL implies that Tm3⫹ is excited by energy transferfrom nc-Si, i.e., excitation light is absorbed by nc-Si and

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J. Appl. Phys., Vol. 92, No. 7, 1 October 2002

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FIG. 3. Tm concentration dependence of the PL decay curves detected at the
maximum of nc-Si PL shown in Fig. 1.

excitons generated in nc-Si recombine with the energy transferred to Tm3⫹ . Evidence of the energy transfer can be obtained from PL excitation spectra. Although not shown here,
the excitation spectra of Tm3⫹ PL was very broad in spite ofits discrete electronic states and was very similar to that of
nc-Si PL. A similar result has already been reported by
Franzó et al. The quenching and the shortening of the lifetime of nc-Si PL with an increase in Tm concentration,
shown in Figs. 1–3, indicate that energy transfer is mediatedby photogenerated excitons, i.e., it is not due to the absorption of light emitted by nc-Si. The energy transfer processfrom nc-Si to Tm3⫹ is the preferred nonradiative recombination process.
It is noted here that the degree of quenching of nc-Si PL
is much larger than that of the gain of the Tm3⫹ PL intensity.
In particular, the intensity of the Tm3⫹ PL peak is not proportional to the Tm concentration and is saturated. As a result, the energy conversion efficiency which represents theratio of the degree of quenching of nc-Si PL to that of thegain of the Tm3⫹ PL intensity is very small particularly inthe high Tm concentration range. Two mechanisms can beconsidered to be the origin of the small energy conversion
efficiency. The first one is the concentration quenching ofTm3⫹ PL due to the relatively high Tm concentration. Excited Tm3⫹ is relaxed by transferring energy to a neighboring Tm3⫹ and finally it is nonradiatively relaxed by meetinga nonradiative recombination center in the matrix. During theenergy transfer process, it is not known whether the energy
transfer efficiency is high or not. Another mechanism is thegeneration of defects in or at the surface of nc-Si by heavy
Tm doping. The photogenerated excitons recombine nonradiatively in nc-Si before energy transfer occurs. At present, it
is not clear which mechanism is the main cause of the smallenergy conversion efficiency. It is worth noting that in ourprevious work on Er- and nc-Si-doped SiO2 films, the total
intensity of nc-Si and Er3⫹ PL was about 80% of the PL

FIG. 4. PL spectra at room temperature as a function of the size of nc-Si
diameter. The Tm concentration was fixed at 1.41 at. %. The PL spectra arenormalized at the maximum of the nc-Si PL. The energy positions of thefirst ( 3 H 4 ), second ( 3 H 5 ), and third ( 3 F 4 ) excited states measured from theground state ( 3 H 6 ) are shown.

intensity of nc-Si in SiO2 that did not contain Er, i.e., theenergy conversion efficiency was around 80%.17

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

Figure 4 shows PL spectra at room temperature forsamples containing different size nc-Si. The intensity scale
above 0.8 eV is expanded by a factor of 4. The Tm concentration is fixed at 1.41 at. %. The spectra are normalized at
the maximum intensities of nc-Si PL. The energy position of
the first ( 3 H 4 ), second ( 3 H 5 ), and third ( 3 F 4 ) excited statesof Tm3⫹ measured from the ground state ( 3 H 6 ) are shown at
the top of Fig. 4. As the size of nc-Si decreases from 3.5 to
2.7 nm the nc-Si PL peak shifts from 1.35 eV to 1.60 eV. For
samples with average nc-Si sizes of 2.7 and 3.1 nm, a small
peak can be observed at 1.58 eV. This peak corresponds tothe 3 F 4 to 3 H 6 transition in Tm3⫹ . Figure 5 shows the intensity ratio of Tm3⫹ and nc-Si PL peaks (I Tm /I Si) as afunction of the peak energy of nc-Si PL. The inset shows anenergy diagram of the intra-4 f shell of Tm3⫹ . Figure 5
clearly shows that I Tm /I Si depends strongly on the peak energy of nc-Si PL, i.e., the size of nc-Si. I Tm /I Si increases

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J. Appl. Phys., Vol. 92, No. 7, 1 October 2002

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FIG. 5. Intensity ratio of the Tm related and the nc-Si related peaks(I Tm /I Si) as a function of the peak energy of nc-Si PL. The line is to guidethe eye.

with an increase in the PL peak energy of nc-Si. In particular,
the ratio increases dramatically as the nc-Si PL peak approaches 1.6 eV.
As shown in Fig. 5, the energy separation between the
ground state and the third excited state of Tm3⫹ is around1.58 eV. Therefore, the steep incline of the I Tm /I Si ratio at
around 1.58 eV indicates that energy is transferred to thethird excited state of Tm3⫹ . For the sample with average
diameter of 2.7 nm, the PL peak energy is at around 1.6 eV.
Therefore, the majority of particles in the size distribution
can contribute to energy transfer to the third excited state. On
the other hand, for samples with average diameters of larger
than 3.1 nm, only a small portion of particles in the size
distribution interacts with the third excited state, resulting ina small I Tm /I Si ratio.
It is noted here that, in our previous work on Er-dopednc-Si, the intensity ratio of Er3⫹ to nc-Si PL peaks (I Er /I Si)
increased by a factor of ⬃4 in a relatively wide range between 1.2 and 1.6 eV.3 In contrast to this, I Tm /I Si dramatically increases by a factor of ⬃18 in a narrower range between 1.45 and 1.6 eV. This indicates that the dramaticincrease of I Tm /I Si cannot simply be explained by the size
dependence of nc-Si. Excitation of the third excited state ofTm3⫹ plays an important role in the energy transfer process.
C. Temperature dependence

Figure 6 shows PL spectra of the sample with Tm concentration of 1.41 at. % at various temperatures. The intensity scale above 0.8 eV is expanded by a factor of 10. With adecrease in temperature, the spectral shape changes gradually. First, below 150 K, a new broad peak appears at around
1.0 eV. This peak is considered to be due to the recombination of excitons trapped at P b centers at the interfaces between nc-Si and SiO2 matrices.18,19 Second, a very weak

FIG. 6. PL spectra of the sample with Tm concentration⫽1.41 at. % atvarious temperatures. Part of spectra at the higher-energy side above 0.8 eVis expanded by a factor of 10. Vertical lines are drawn at 1.48 and 1.68 eV.

peak appears at around 0.84 eV, indicated by arrows. Third,
the 1.58 eV peak becomes more pronounced. The 0.84 and1.58 eV peaks correspond to the intra-4 f shell transitions ofTm3⫹ 共3 F 4 to 3 H 4 and 3 F 4 to 3 H 6 ). Fourth, the 0.69 eV peakis shifted toward lower energy 共0.69–0.67 eV兲 and becomes
sharper. Finally, the spectral shape of nc-Si PL is strongly
modified. At room temperature, the spectral shape is Gaussian like. At around 100 K, the lower energy side of the spectrum is strongly suppressed. At 5 K, both sides of the 1.58
eV PL are suppressed, and clear bends appear at around 1.48
and 1.68 eV. It should be noted here that without Tm doping,
the spectral shape is almost independent of the temperature.
Although not shown here, for samples of 3.1 and 3.3 nm, thesuppression of nc-Si PL could be observed at the same energies at low temperatures.
The suppression of the spectrum at around the 3 F 4 to3
H 6 transition PL of Tm3⫹ strongly suggests that the excitation energy of nc-Si for some particular sizes is resonantly
transferred to Tm3⫹ before the spontaneous recombinationof excitons occurs in nc-Si. To our knowledge, this is the first
spectroscopic evidence of resonant energy transfer from
nc-Si to Tm3⫹ .
If the energy transfer is made resonantly 共without emitting phonons兲 or by emitting phonons, suppression should
appear only at the high-energy side of the 1.58 eV peak.
However, not only the higher-energy side, but also the lower-

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Watanabe et al.

J. Appl. Phys., Vol. 92, No. 7, 1 October 2002

FIG. 7. Integrated intensities of the Tm3⫹ related peaks as a function of thetemperature shown in Fig. 6. The intensity is normalized at 5 K. The dashedlines are to guide the eye.

4005

even at room temperature due to the large energy mismatch.
This may result in the rather small temperature quenching of
the PL observed.
The observed temperature dependence of the PL intensity is qualitatively different from that reported by Franzó etal. for similar samples. In their report, the PL peak due to thetransition from 3 H 4 to 3 H 6 states is at around 0.75 eV atroom temperature. With a decrease in temperature, the PL
intensity decreased and the PL disappeared below 100 K.
The reason for this qualitative disagreement is not clear atpresent. One possible explanation is an artificial effect. Theirdata suggest that the measurement system does not have sensitivity below 0.72 eV. It might be that they measured only ahigh-energy tail of the spectrum. Since the 3 H 4 to 3 H 6 transition PL shifts toward lower energy and becomes sharp witha decrease in temperature, the signal intensity of the spectral
region, where the measurement system has sensitivity, decreases with a decrease in temperature. This might have resulted in considerable underestimation of the PL intensity at
low temperatures.
IV. CONCLUSION

energy side of the spectra is strongly suppressed. The suppression of the low-energy side can be explained by taking
into account the indirect band-gap nature of nc-Si. As reported previously, the indirect-gap nature of bulk Si crystal ishighly preserved in nc-Si even if its size is a few nanometerin diameter, i.e., radiative recombination of excitons in nc-Si
requires the emission of momentum conserving phonons at ⌬minima.20 Since the transverse optical 共TO兲 phonon energy
at ⌬ minima is about 57 meV, nc-Si whose exciton recombination energy exactly coincidences with the energy of thethird excited state of Tm3⫹ 共1.58 eV兲 shows PL at around1.58 eV–57 meV if energy transfer does not occur. Therefore, the appearance of spectral suppression at the lowenergy side of the 1.57 eV PL is direct evidence of resonant
energy transfer from nc-Si to Tm3⫹ .
It is worth noting that the PL spectra of nc-Si become
weak in the entire spectral range with an increase in Tm
concentration, i.e., not only the spectral positions that satisfy
the resonant energy transfer condition, but also other regionsin broad PL bands of nc-Si are quenched. This indicates that
there is a dominant nonresonant energy transfer process inaddition to the resonant one. The energy conservation ruleduring energy transfer is satisfied by emitting various combinations of optical and acoustic phonons.
Figure 7 shows the integrated intensities of the 0.69 eV
PL peak as a function of the temperature. The intensities of
the 0.69 eV peak decrease slightly with an increase in temperature, although there is small fluctuation. The origin of
the small fluctuation has not been identified. The cause ofsmall temperature quenching is considered to be due to thesmall energy back transfer rate. In the present samples, excitation of Tm3⫹ is mainly to the third excited state due tothe large band gap of nc-Si. Tm3⫹ excited to this higher state
relaxes promptly to the lowest excited state ( 3 H 4 ) before
energy back transfer occurs. The probability of back transferfrom the lowest excited state to nc-Si is negligibly small

The interaction between excitons in nc-Si and 4 f -shellelectrons in Tm3⫹ was studied by PL spectroscopy. We demonstrated that 4 f -shell electrons in Tm3⫹ can be excited byenergy transfer from nc-Si. The intensity of Tm3⫹ PL at 0.69
eV depended strongly on the size of nc-Si. As the PL energyof nc-Si approached the difference in energy between thethird excited state and the ground state of Tm3⫹ 共1.58 eV兲,
the intensity of Tm3⫹ PL increased dramatically. This result
indicates that the third excited state interacts more strongly
with excitons confined in nc-Si than other energy states ofTm3⫹ . Tm3⫹ showed a very weak peak at around 1.58 eV in
addition to the 0.75 eV peak. At low temperatures, broad PL
spectra of nc-Si were suppressed at both sides of the 1.58 eV
PL of Tm3⫹ . This suppression is spectroscopic evidence ofresonant energy transfer from nc-Si to Tm3⫹ .
ACKNOWLEDGMENT

The authors are grateful to Keisuke Nakajima for his
valuable assistance in this work. This work was supported by
a Grant-in-Aid for Science Research from the Ministry of
Education, Science and Culture, Japan, and a Grant for Research for the Future Program from Japan Society for the
Promotion of Science 共Grant No. JSPS-RFTF-98P-01203兲.
One of authors 共K.W.兲 would like to thank the Japan Society
for the Promotion of Science for financial support.
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