Broadband near-infrared emission from bismuth-doped multilayer films
Satoshi Morimoto, Minoru Fujii, Hong-Tao Sun, Yuji Miwa, Kenji Imakita et al.
Citation: J. Appl. Phys. 112, 073511 (2012); doi: 10.1063/1.4757579
View online: http://dx.doi.org/10.1063/1.4757579
View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v112/i7
Published by the American Institute of Physics.

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JOURNAL OF APPLIED PHYSICS 112, 073511 (2012)

Broadband near-infrared emission from bismuth-doped multilayer films
Satoshi Morimoto,1 Minoru Fujii,1,a) Hong-Tao Sun,2,b) Yuji Miwa,1 Kenji Imakita,1
Jianrong Qiu,3 and Shinji Hayashi1

1Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,Rokkodai, Nada, Kobe 657-8501, Japan2International Center for Young Scientists, National Institute for Materials Science (NIMS), Tsukuba,Ibaraki 305-0047, Japan3
Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510640,
China(Received 17 July 2012; accepted 5 September 2012; published online 5 October 2012)
Multilayer structures consisting of bismuth (Bi)-doped silica thin films and different kinds ofspacer, i.e., Si, silica, Si-rich silica, layers are grown and the luminescence properties are studied.
When samples were annealed at a low temperature, Bi-related near infrared active centers(BRACs) were formed at interfaces between Bi-doped silica and Si-rich silica (or silicon) due tothe reduction of Bi3þ to BRACs by silicon. On the other hand, films annealed at a high temperature
showed similar emission behaviors to bulk glasses. The results demonstrated here establish a newC 2012
strategy for the control of BRACs and building peculiar Bi activated film structures. VAmerican Institute of Physics. [http://dx.doi.org/10.1063/1.4757579]
I. INTRODUCTION
1410Bi, with the electronic configuration of (Xe)4f 5d6s26p3, is one of the most thoroughly investigated maingroup elements, which has been known as “the wondermetal” owing to the easy involvement in chemical combinations for the electrons in its p orbital.1 Bi shows a variety ofoxidation states such as 0, þ1, þ2, þ3, þ5, or even states
between 0 and 1, characterized by multi-type electronicstructures. Furthermore, it shows a profound propensity toform clusters, which widely exist in molten Lewis acids andmolecular crystals.1–4 These peculiarities make them assmart optically active centers in diverse host materials. Forinstance, the emission peaks of Bi2þ occurs in the visible
range5–7 and those of Bi3þ in the ultraviolet and visible
ranges.8,9 It is noteworthy that the subvalent species, ofwhich the average oxidation states are þ1 or between 0and þ1, could show smart photophysical behaviors.3,4 The
reports of the synthesis and photophysical properties ofsubvalent bismuth date back to half a century ago.10,11
However, only in recent years Bi doped materials havebeen attracting significant attention to achieve broadband
near-infrared (NIR) optical amplification in the whole telecommunication window12–21 and to realize in vivo bioimaging in the optimal biological window,22 although the exact
emission mechanisms are still not clear in most material
systems owing to the unsuccessful establishment of thestructure-property relationships. In contrast to the applications described above, the research on Bi-doped thin films
is very limited. Development of NIR luminescent Bi-doped
silica thin films allows us to realize waveguide-type broadband optical amplifiers, which can be integrated in Si-basedoptoelectronic devices.

a)Electronic mail: fujii@eedept.kobe-u.ac.jp.
Electronic mail: timothyhsun@gmail.com.

b)0021-8979/2012/112(7)/073511/4/$30.00

In a previous work, we demonstrated the growth of thinfilms consisting of Bi doped multicomponent glass layers
and Si-rich silica layers.23 Very recently, we found that doping of excess Si in Bi-doped pure silica thin films results inefficient NIR photoluminescence (PL).24 Simplicity of Bidoped Si-rich silica thin films in their compositions, i.e.,
only Si, Bi, and O, makes it easier to be integrated in Sibased optoelectronic devices. One of the advantages of thissystem over other Bi doped bulk glasses or fibers is that wecould easily tune the excitation/emission behaviors throughadjusting Si concentrations and annealing temperatures. We
observed that NIR emissions of some films increase monotonously with increasing excitation energy, which suggeststhat low-cost optical sources such as LEDs can be used as
the excitation sources. Further systematic investigation ofthis material system is of importance not only for maximizing the luminescence efficiency, but also for understanding
the formation mechanism of Bi related NIR active centers(BRACs).
In this work, we develop a new structure which consistsof thin layers of Bi-doped pure silica and spacer. We foundthat the interface between Bi-doped pure silica films andspacer films plays an important role in the formation ofBRACs. By comparing luminescence properties of samples
with different kinds of spacer materials, we investigatedpossible luminescence mechanism.
II. EXPERIMENTAL PROCEDURE

The structure of the samples is schematically shown inFigure 1. Bi-doped silica layers (6 nm in thickness) andspacer layers (3 nm in thickness) were alternatively sputterdeposited on fused quartz substrates. Bi-doped silica layers
are deposited by simultaneously sputtering SiO2 and Bidoped silica glass (Bi2O3:SiO2 ¼ 50:50 mol. %) targets.24 As
the spacer layer, three different materials, i.e., Si-rich silica(Si:SiO2 ¼ 16.4:83.6 mol. %), Si, and silica are used. The112, 073511-1

C 2012 American Institute of Physics
V073511-2

Morimoto et al.

FIG. 1. Schematic illustration of the sample structure. Bi-doped silica(6 nm) and spacer (3 nm) layers are alternatively deposited on fused quartsubstrates. The spacer layers are sputter-deposited Si, silica or Si rich silica.

deposition process was repeated until the total number ofBi-layers reaches 100. The total thickness of deposited layer
is 300 nm. The as-deposited films were amorphous. The films
were then annealed in N2 gas at 1100 or 1300  C for 30 min.
With these annealing conditions, Si nanoclusters are grownin films.24
For PL and PL excitation (PLE) measurements, several
light sources, i.e., He-Cd laser (325.0 nm), Ar ion laser(457.9, 476.5, 488.0, 496.5, 501.7, and 514.5 nm), Ti:sapphire laser (700–800 nm), and diode lasers (403 and 641 nm)
were used as the excitation sources. The PL was detected bya liquid N2 cooled InGaAs diode array (Roper Scientific:OMA-V-SE). The spectral response of the detection system
was corrected with a reference spectrum of a standard halogen lamp. Photoluminescence decay curves were measured
using a near infrared photomultiplier tube (R5509-72, Hamamatsu Photonics) and a multi-channel scaler (SR430, Stanford Research). All the measurements were performed atroom temperature.
III. RESULTS AND DISCUSSION

Figure 2(a) shows the PL spectra of the Bi-doped silica
multilayer films with spacer layers composed of three different materials. All samples were annealed at 1100  C. As areference, the spectrum of a Bi-doped Si-rich silica single
layer film with a thickness of 300 nm is shown.24 Note thatthe thickness of the single layer sample is the same as theJ. Appl. Phys. 112, 073511 (2012)

total thickness of Bi-doped silica layers in multilayer samples (6 nm  50). The single layer sample shows a broad PL
peak around 1100 nm, which can be attributed to BRACs.24
In the multilayer samples, the spectra depend strongly on thespacer materials. It is found that when the spacer layer ispure silica (Bi-doped silica (BDS)/silica in Fig. 2(a)), thesample does not show any emission, indicating that BRACs
are not activated in Bi-doped silica. On the other hand, whenthe spacer layer is Si-rich silica (Bi-doped silica/Si-rich silica(BDS/SRS)), a broad emission originating from BRACs
occurs. Since Bi is not doped in the Si-rich silica layer,BRACs are considered to be activated at the interface
between Bi-doped silica and Si-rich silica. Probably, excess
Si in Si-rich silica acts as reducing agents to convert Bi3þinto BRACs at the interface. When the spacer layer is Si(BDS/Si), a broad and weak emission apparently different
from BRACs is observed around 950 nm. It should be
stressed here that, without Bi doping, Si nanoclusters do notshow PL around 1100 nm.
Figure 2(b) shows PL spectra of the samples annealed at1300  C. When the spacer layer is pure silica, the PL is again
not observed. Other three samples exhibit PL with the maximum around 1150 nm. The spectral shape of the sample with
the Si-rich silica spacer is very similar to that of the Bidoped Si-rich silica single layer. When the spacer layer is Si,
in contrast to the sample annealed at 1100  C, Bi-related PL
can clearly be observed at 1150 nm in addition to an
unknown PL around 1000 nm. The result of decompositionof the spectrum into two Gaussian components is shown inFig. 2(b) by dotted curves. The intensity of the 1150 nm PL
estimated after the decomposition is comparable to that of
the Bi-doped Si-rich silica single layer. When the annealing
temperature is 1300  C, the multilayer structure is considered
to be almost destroyed. The difference in the spacer layers in
BDS/SRS and BDS/Si systems appears as different amountsof excess Si in the whole films. Probably, the total amount ofexcess Si in the sample with the Si-rich silica spacer layer is
not enough to activate all Bi ions into BRACs, resulting in aweaker PL intensity than other samples.
Figure 3(a) shows PL decay curves of the Bi-doped Sirich silica single layer sample and the multilayer sample

FIG. 2. PL spectra of BDS/SRS, BDS/Si,and BDS/silica multilayer samples, and Bidoped Si-rich silica single layer sample
annealed at (a) 1100 and (b) 1300  C. The
dotted curves in (b) are the result of decomposition of the spectrum of the BDS/Si sample into two Gaussian functions.

073511-3

Morimoto et al.

J. Appl. Phys. 112, 073511 (2012)

FIG. 3. (a) PL decay curves of BDS/SRS
multilayer and Bi-doped Si-rich silica single
layer samples annealed at 1100  C. The excitation wavelength is 488 nm and the detection wavelength is 1100 nm. (b) PL decay
curves of BDS/SRS and BDS/Si multilayer
samples and Bi-doped Si-rich silica single
layer sample annealed at 1300  C. Thedetection wavelength is 1150 nm. The decay
times estimated by fitting the curves bystretched exponential functions are shown inthe figure.

with Si-rich silica spacer annealed at 1100  C. The excitationand detection wavelengths are 488 and 1100 nm, respectively. The decay curves are not a single exponential function, suggesting that the local environment of BRACs is
distributed in a wide range. The PL mean lifetimes(sm) areÐ1
calculated from following equation, sm ¼ t0 ½IðtÞ=I0 dðtÞ,where I(t) is the luminescence intensity as a function of time
t. I0 is the initial intensity of I(t) at t0. The values of sm arecalculated as 380 ls for the multilayer sample and 280 ls forthe single layer sample. The longer lifetime of the multilayer
sample is consistent with the stronger PL intensity (Fig.
2(a)). When the annealing temperature is 1300  C (Fig.
3(b)), the decay curves become single exponential. The lifetimes become longer, and exceed 700 ls for all the samples.
Since this value is comparable to those of Bi-doped multicomponent glasses,12–18,22–24 it is considered to be an intrinsic lifetime of BRACs. As we described above, themultilayer structures are almost destroyed when they wereannealed at 1300  C. The similar PL lifetimes of the samples
in Fig. 3(b) indicates that the local environments of BRACs
are almost identical for these samples even though the initialstructure and excess Si concentrations are different. This

supports the previous conclusion that the intensity differencein Fig. 2(b) is due to different amounts of the centers.
In Figure 4(a), the intensities of the PL maxima are plotted as a function of the excitation wavelength for Bi-doped
silica/Si-rich silica (BDS/SRS) multilayer and Bi-doped Sirich silica single layer samples annealed at 1100  C. In bothcases, the spectra increase almost monotonously to shorter
wavelengths, although there are small bumps around 500
and 700 nm. The broad background in Fig. 4(a) is similar to
those reported for Er doped Si-rich silica.25 In that case, 4f
electrons of Er3þ are excited by the energy transfer from Si
nanoclusters. The similarity in spectral shape suggests thatthe excitation of BRACs is also governed by indirect excitation by Si nanoclusters in Si-rich silica via energy transfer.23,24 Figure 4(b) shows the spectra of the samples
annealed at 1300  C. The spectra have two distinctive peaks
at 500 and 700 nm due to direct absorption by BRACs, as
revealed in aluminosilicate12,17,18,23 and aluminogermanate
glasses.21 The contribution of the indirect absorption is verysmall. These results confirm that the multilayer structures arealmost destroyed and Si nanoclusters were not formed in thefilms.

FIG. 4. PL peak intensity as a function ofexcitation wavelength for BDS/SRS multilayer and Bi-doped Si-rich silica single
layer samples annealed at (a) 1100 and(b) 1300  C. The curves are drawn for theguide to the eyes.

073511-4

Morimoto et al.

IV. CONCLUSION

We study PL properties of multilayer films consisting ofBi-doped silica and different kinds of spacer layers, i.e., Si,silica, Si-rich silica layers. We demonstrate that, when theannealing temperature is relatively low, Bi-related NIR PL isobtained only when Si-rich silica is used as a spacer. This suggests that BRACs are formed at the interface between Bidoped silica and spacer when the amounts of Bi, Si, and O at
the interface are in an optimum range. The Si-rich silica layers
act also as sensitizer for the excitation of BRACs via energy
transfer from Si nanoclusters. The sensitization significantly
extends the excitation band of BRACs.
ACKNOWLEDGMENTS

This work is partly supported by KAKENHI (23310077).
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