兵庫県ビームライン年報・成果集

BL08B2

2012B3345

Energy migrations and transfers between Er3+ ions in Er2 SiO5
films on SiO2 /Si substrates
Hiroo Omi1,2
1NTT Basic Research Laboratories, NTT Corporation,
2NTT Nanophotonics Center, NTT Corporation

Abstract： Optical energy migrations and transfers in erbium mono-silicate (Er2 SiO5 ) grown on Si
substrates by rf-magnetron sputtering are investigated by extended X-ray absorption fine structure (EXAFS)

and time-resolved photoluminescence measurements. The energy transfer assisted by discrete

excitation migration (hopping) among the Er3+ ions is evidenced by analyzing the lifetime of 1.5-µm
photoluminescence from the erbium mono-silicate, for which interatomic distances between Er3+ ionshave been precisely determined by EXAFS.
Keywords： Er, Extended x-ray absorption fine structure, Photoluminescencecate and energy transfers via Er3+ ions. The ef-Introduction
Erbium-doped materials, such as erbium silicatesand erbium oxides, have received much attention
as light emitters [1–3] and amplifiers [4] on siliconwafers at the telecommunications wavelength in silicon photonics. Erbium compounds such as Erbium
silicates and erbium oxides themselves have a huge
number of Er3+ ions of up to 1022 1/cm3 , which en-fect of energy migration has been actually demonstrated in a system of Erx Y2−x SiO5 poly crystals [8].
Therefore, for better control of the optical properties, it is crucial to determine the crystal structureof the erbium silicates (e.g., the distances between Erions) and its relationship to the optical properties.
However, despite their importance in silicon photonics [9, 10], the structures of the erbium silicates andables the fabrication of nanoscale optical waveguides

their optical properties, especially the mechanism ofand means they have great potential as nanoscaleenergy migration, have not been established. In thisamplifiers on Si wafers. However, several types ofwork, we performed synchrotron X-ray absorption

poly crystals, mono-silicate (Er2 SiO5 ), di-silicates

fine-structure experiments on erbium silicates grown(Er2 Si2 O7 ), and their α-, β-, γ-type phases haveby the rf-magnetron sputtering technique, which has

been reported to form in the silicates, depending on

been widely used for the fabrication of erbium sili-

the growth techniques, such as radio frequency (rf)-cates on 100-nm-thick SiO2 film on Si wafers andmagnetron sputtering, and pulsed laser deposition,subsequent thermal annealing at 1100◦ C in Ar gas

and substrates (i.e. Si or SiO2 /Si) [4–7]. The op-ambient. We determined the crystal structure of er-tical properties of erbium-doped materials strongly

bium silicates and the distances between Er – Er ionsdepend on the local structures of the crystals. Forin the erbium silicates. We also performed time-ions inresolved photoluminescence (PL) measurements on

the crystal strongly govern the photoemission effi-

the same samples. By combining the EXAFS resultsciencies. Of particular importance are that the en-and time-resolved PL measurements, we clarified theergy migration between the Er3+ ions in the sili-mechanism of energy migrations and derived opticalexample, the distances between Er

3+– Er3+

microparameters that govern PL quantum efficiency.

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Status Report of Hyogo-Beamlines with Research Results, Vol. 4 (2015)
Experimentalof erbium di-silicate (Er2 SiO7 ) (JCPDS card: No.

The XAFS experiments were performed at beamline BL08B2 in SPring-8, Japan. The Er LIII -edge
EXAFS spectra were collected at room temperaturein the fluorescence mode. The fluorescence intensity
was measured with a 19-element solid-state-detector(SSD). EXAFS spectra of Er2 O3 crystalline powders
were also collected in the transmission mode as standard references. The EXAFS data were analyzed using ATENA and ALTEMIS (ver. 6) packages. The
1.5-µmtime-resolved PL measurements were per-formed at room temperature. A near-infrared streak
camera was used for the detection of Er3+ emission
from the samples for the time-resolved PL measurements [11]. The samples were pumped by a Ti: sapphire CW laser at 532-nm wavelength and a power

48-1595), where atomic positions have been determined, but it does not well fit the experimental result. From the agreement of the fittings with themono-silicate, we can say that the thermal annealing produces the mono-silicate of Er2 SiO5 (JPCDS
card 40-384) through the interface reaction between
Er2 O3 and SiO2 , which enables us to determine thedistances between Er - O ions and between Er - Erions in the Er2 SiO5 crystals. The interatomic distances R and Debye-Waller disorder factors σ 2 aresummarized in Table 1. The Debye-Waller disorder factors σ 2 are compatible with those of Er2 O3
crystal [9], suggesting the good crystalline qualityof the mono-silicate formed by thermal annealing at
1100◦ C.

of 30 mW. The erbium oxides were deposited on
100-nm-thick thermal SiO2 /Si(100) wafers and subsequently annealed at 1100◦ C in Ar gas ambient.
Results and Discussion
Figure 1 shows the EXAFS spectra obtained fromthe sample annealed at 1100◦ C. In the radial profiles [Fig. 1(a)], we can see two main peaks at 0.2
and 0.3 nm. Theoretical best fittings are also shownin Figs. 1(a) and (b) as red solid lines. As can beseen in Figs. 1(a) and (b), the experimental profiles are in good agreement with the simulations (Rfactor = 0.0444).

The best fits are obtained forthe mono-silicate of Er2 SiO5 when the atomic positions of Y2 SiO5 (JCPDS card: No. 52-1810) areused instead as initial parameters for the fittings.
Note here that we assume the atomic positions of Y,Si, and O in Y2 SiO5 as those of Er, Si, and O in
Er2 SiO5 for the profile fittings, since the atomic positions of Er2 SiO5 (JCPDS card: No. 40-384) havenot been precisely determined at present. The assumption is validated by the facts that the ionic radius of Er and Y are almost same (Er: 0.89Å, Y:Fig. 1. Er LIII -edge EXAFS spectrum of Er3+ in thesample. Red solid line is the fitting by the theory. χ,of Er2 SiO5 are almost the same as those of Y2 SiO5 .

R, and q represent X-ray absorption, distance, andwavenumber, respectively. (b) Fourier-transformed
XAFS spectrum of the sample (see Table 1 for de-

As a reference, we also simulated an EXAFS profiletails).

0.9Å) and unit cells and that the lattice constants– 32 –

兵庫県ビームライン年報・成果集

Table 1. Interatomic distances (R) of Er – O, Er– Si and Er – Er in the unit cell up to3.5Å and the Debye-Waller disorder factor(σ 2 ), determined by EXAFS. The suffixes
represent the different sites of O and Er

high concentration of Er3+ ions in the mono-silicate,as described below.
Figure 2(b) shows the decay curve of the peak at
1528 nm from the Er2 SiO5 film grown on Si(100).
As seen in this figure, the experimental curve decays
non-exponentially and agrees well with the simula-atoms in the unit cell.

tion on the basis of the model in which energy trans-R(Å)σ 2 (Å2 )Er1-O1Er1-O22.25512.27020.0141
0.0539

Er1-O32.2866

0.0629model, PL intensity I(t) can be expressed I(t) =Er1-O4
Er1-O52.29372.30740.00240.0724I0 exp(−t/τR − γt1/2 − W t), where τR is the intrinsic

Er1-O6
Er1-O72.7754
3.11320.01470.0142

acceptor energy transfer parameter (direct transfer

Er1-Si
Er1-Er1

3.292
3.45650.01550.0148

Er1-Er2

3.49720.009

fer is accelerated by energy migration through hopping proposed by Burshtein [13]. In the hopping

radiative lifetime of erbium ions, γ is the donor-towithout migration), W is the rate parameter related

Figure 2(a) shows the PL spectrum obtained fromthe sample.

It is evident that there are a mainpeak at 1528.8 nm, which is the transition between
4I13/2 (X1 ) −4 I15/2 (Z1 ) states of 4f -levels of Er3+

ions in Er2 SiO5 films, and some additional peaks dueto Stark splitting. The shape and positions of thePL are in good agreement with previous results [4].
The decay profile of the peak and the correspondingln(− ln(t/τR ) − t/τR − (ln(t/τR ))3 plots are shownin Fig. 2(b) and the inset, respectively, where τRis the intrinsic fluorescence decay time of the donor.
According to the Inokuti-Hirayama (I-H) equation[12], the slope of the plots in the inset corresponds
to 3/s, where s is multipole interaction parameter(s = 6, 8, and10 for dipole-dipole, dipole-quadrupole,and quadrupole-quadrupole interactions). From theslope in the inset, we obtained s = 4.4, which isfairly close to s = 6, indicating that the energy
transfer is caused by the dipole-dipole interactions
between Er3+ ions. It can be seen, however, thatthe experimental plots are not completely in agreement with the I-H equation; they deviate for shorterand longer decay time ranges. This could be because
the energy transfer from donors to acceptors is as-Fig. 2. (a) PL spectrum obtained from the sample. (b) Normalized decay curve of 4 I13/2 (X1 )−4I15/2 (Z1 ) emission under pulsed excitation for theEr2 SiO5 . The red solid line is the theoretical

fit by the Burshtein equation. Inset shows theln(− ln(t/τR ) − t/τR − (ln(t/τR ))3 plot of the decay
curve in (b).

sisted by energy migration among donors due to the– 33 –

Status Report of Hyogo-Beamlines with Research Results, Vol. 4 (2015)
to the donor-to-acceptor energy transfer assisted by(Ref. 17), in which exciton trapping efficiency is

discrete excitation migration (hopping) among donor

taken into account. Note that, on the other hand,states. In this case, the energy transfer is referred tothe diffusion-limited mechanism has been suggested

as migration-accelerated. From the theoretical fit-in the Erx Yx−2 SiO5 (0.06 ≤ x ≤ 2) crystals [6],ting, we obtained γ = 150.4 with standard errors(SE) of 12.7 s

1/2and W = 19172.4 with SE =which is not in accordance with the results describedabove.

at τR = 7.4 ms [14]. Subsequently, theRegarding RDA , the distance is nearly equal to

microparameters of energy transfer from donor to ac-

the average Er – Er ions distance of REr−Er = 4.6Å,ceptor CDA and that from donor to donor CDD canwhich is derived from the EXAFS analysis, indicat-

be estimated from the value of γ and W by the fol4lowing relationships [15–17]: γ = π 3/2 NA CDA ,3 5/22πW = Kh = π
NA ND CDA CDD , Kd =31/4 3/4[16π 2 /(3×23/4 )]NA ND CDA CDD , where NA and ND

ing that the energy transfer is mainly caused by

are the concentration of acceptors and donors in thewith thermal oxide films by ion-beam sputtering

erbium mono-silicate, and Kd is the energy trans-[18]. Additionally, we derived the average lifetime

fer rate, which is associated with diffusion and trap-by normalizing to unity the PL decay curve at timeping derived from the random walk treatment. Wet = 0 and integrating over the entire decay curve for2910.2 s−1cross-relaxations between Er3+ ions, such as cooperative up-conversions. Actually, evidence of suchup-conversions has been found in waveguides of polycrystalline Erx Yx−2 SiO5 films grown on Si covered

3.74 × 10−42 cm6 /s, which lead to the critical radii

non-exponential
decay according to the expression
 ∞I(t)/I0 dt, in which I0 is the PL intenτef f =time th = 81.6 µs. The critical radii indicate thatis nearly equal to the value obtained by Miritello etthe energy transfer and migration can occur amongal. [6]. The relative quantum efficiency η is estimatedions located within these distances. The rates ofto be 0.21% by the equation η = τef f /τR .

−42obtained CDA = 1.94 × 10

6cm /s and CDD =RDA = 4.95Å and RDD = 6.03Å, and hopping

0sity at t = 0. We obtained τef f = 15.95 µs, whichcross-relaxation (PDA ) and that of energy migration (PDD ) are estimated to be 132 and 78 s−1 , re-Conclusion

6spectively, using the equations PDA = CDA /RDAIn conclusion, we have investigated the optical en-6and PDD = CDD /RDD
. According to Ref. 17, S

ergy transfers and energy migrations between Er+3

is a good indictor for distinguishing the migration

ions in Er2 SiO5 grown on SiO2 /Si substrates by rf

mechanisms, where S = W/Kd . Owing to this cri-magnetron sputtering and subsequent thermal an-teria, the diffusion mechanism satisfies S = 1 in-nealing at 1100◦ C in Ar gas ambience. We deter-dependent of the value of CDD /CDA [16] and themined the local structures of Er2 SiO5 (JCPDS card:hopping mechanism works for S < 1 [13], depend-No.40-384) by EXAFS and quantitatively clarified

ing on CDD /CDA . From the fitting, we obtained

from time-resolved PL measurements that the en-

S = 0.544 and CDD /CDA = 3.25, indicating thatergy migrations between Er3+ ions in the mono-silicateenergy migration is due to energy hopping between

can be explained by the modified Burshtein’s hop-3+

ions [see Fig. 6 in Ref. 17], but not due toping model and that the energy transfers are possi-

the diffusion-limited mechanism where S = 1. Fur-bly due to cooperative up-conversions between Er3+

thermore, when we plot our result on Fig. 6 in Ref.

ions in the Er2 SiO5 film on Si.

Er17, we find that the plot rides on the solid curve ofthat figure. This indicates that the hopping mechanism of our system is in good accordance with themodified hopping model proposed by Jagosich et al.

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兵庫県ビームライン年報・成果集

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