Near infrared photoluminescence from bismuth-doped nanoporous silica
thin films
Sa chu rong gui, Kenji Imakita, Minoru Fujii, Zhenhua Bai, and Shinji Hayashi
Citation: J. Appl. Phys. 114, 033524 (2013); doi: 10.1063/1.4815939
View online: http://dx.doi.org/10.1063/1.4815939
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JOURNAL OF APPLIED PHYSICS 114, 033524 (2013)

Near infrared photoluminescence from bismuth-doped nanoporous silica
thin films
Sa chu rong gui, Kenji Imakita,a) Minoru Fujii, Zhenhua Bai, and Shinji Hayashi
Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,
Rokkodai, Nada, Kobe 657-8501, Japan(Received 14 March 2013; accepted 1 July 2013; published online 19 July 2013)
Photoluminescence (PL) properties of bismuth (Bi) doped porous silica thin films annealed atvarious temperatures and in different atmospheres were studied. The near infrared (NIR)
luminescence depended strongly on the annealing atmosphere and temperature. To reveal theorigin of the NIR luminescence, we performed comprehensive PL studies including steady state
and time-resolved PL measurements at 8–300 K in wide excitation (250–500 nm) and detection(400–1550 nm) wavelength ranges. It was revealed that multiple Bi luminescence centers, such as
C 2013 AIP Publishing LLC.
Bi3þ, Bi2þ, Biþ, and Bi dimer, are stabilized in porous silica. V[http://dx.doi.org/10.1063/1.4815939]

I. INTRODUCTION

With the rapid development of computer networks anddata-transmitting services, broadband amplifiers and tunable
lasers operating in the wavelength range covering the whole
optical telecommunication window (1250–1650 nm) havebeen required. The gain bandwidth of traditional rare earth
doped fiber amplifiers is less then 100 nm, due to the restriction of the f-f transition.1,2 Recently, ultra-broadband nearinfrared (NIR) emissions from Bi-doped materials have beendeveloped using various host materials.3–11 Bi-doped opticalfiber amplifiers were reported and a broadband and an efficient optical amplification covering the 1.2–1.6 lm region
were demonstrated.12–14 However, the involved luminescence mechanisms and the nature of the Bi NIR emission
still remain controversial.
For many years, it is believed that addition of extra elements such as Al is indispensable to obtain NIR emission
from Bi-doped bulk glasses.4,5,12 However, Razdobreev
et al. recently demonstrated that Bi doped pure silica glasscan exhibit NIR luminescence when the silica glass isporous.8 Furthermore, we have also found that Bi doped
oxidized porous Si shows NIR luminescence without additional elements.15 Oxidized porous Si is a promising material
as a host for different kinds of optical centers. Especially,
during the past years, Er-doped oxidized porous Si waveguide type amplifiers have been studied and they are considered to be promising for integrated optical filters, amplifiers/
modulators, light-emitting diodes, and other optoelectronic
devices.16–18 The broadband NIR luminescence from
Bi-doped oxidized porous silica may pave the way for thedevelopment of broadband waveguide type optical amplifiers
operating in the NIR range. However, the research on
Bi-doped oxidized porous silica is still limited and the relation between preparation parameters and photoluminescencea)Author to whom correspondence should be addressed. Electronic mail:Imakita@eedept.kobe-u.ac.jp. Tel.: þ81 78 803 6441; Fax: þ81 78 803
6441.

0021-8979/2013/114(3)/033524/5/$30.00(PL) properties is not fully clarified. Furthermore, the originof the NIR PL is not fully elucidated.
In this paper, we perform comprehensive PL studies ofBi-doped oxidized porous silica thin films annealed in different atmospheres at different temperatures in wide excitation(250–500 nm) and detection (400–1550 nm) wavelength
ranges. We also study the temperature dependence of the
NIR PL spectra and the lifetime for the NIR PL bands.
Based on these results, it is shown that the temperature
quenching of the NIR PL is very small, and two different
kinds of Bi NIR active centers (Biþ and Bi dimer) co-exist in
the Bi-doped porous silica. The ultra-broadband of the NIR
luminescence with the lifetime of few hundreds microseconds suggests that the porous Si thin film is a promisingmaterial for a Si based waveguide type optical amplifier at
optical telecommunication wavelengths.
II. EXPERIMENT(100) oriented p-type Si wafers with the resistivity of
20 mX cm were used as initial substrates for the preparationof porous Si. Porous Si monolayers were prepared byelectrochemical anodization in the mixture of hydrofluoric
acid (HF) (46 wt. % in water) solution and ethanol(HF:ethanol ¼ 1:1) with the anodic current density of
70 mA/cm2. At the end of the etching process, a high current
pulse (300 mA/cm2, 1.6 s) was applied to detach the porous
Si layer from the substrate. The porosity of the porous Si
estimated from the effective refractive index is about
58%.15,19,20 Porous silica thin films were obtained by oxidizing porous Si at 1000  C for 1 h in air. For Bi doping, porous
silica thin films were soaked in aqueous solution ofBi(NO3)35H2O for 48 h at room temperature. The pH valueof the solution was controlled to be about 3.0 by adding
HNO3 to increase the solubility of Bi(NO3)35H2O. The
concentration of Bi(NO3)35H2O in aqueous solution is
1.5 mM/l. For the activation of Bi, the samples wereannealed in air or N2 gas at the temperatures ranging from1000 to 1300  C. Bi concentration in the final products was

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C 2013 AIP Publishing LLC
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033524-2

Rong gui et al.

below the detection limit of energy-dispersive x-ray spectroscopy (EDS) measurements (0.1 at. %).
NIR PL spectra were measured by using a single grating
monochromator equipped with a liquid-nitrogen-cooled
InGaAs diode array. Samples were excited at 488 nm using
an Ar ion laser. Three dimensional (3D) mappings of the PL
intensity versus excitation and emission wavelengths wasmeasured by the spectrophotometer (Horiba Jobin Yvon,
Flourolog) equipped with a photomultiplier and an InGaAs
photodiode as detectors and a Xe lamp (450 MW) as an excitation source in wide excitation (250–500 nm), and detection(400–1550 nm) wavelength ranges. For all the PL measurements, the spectral response of the detection system was corrected by the reference spectrum of a standard tungsten
lamp. The decay time measurements were performed bydetecting the modulated luminescence signal with a photomultiplier tube (Hamamatsu, R5509-72), and the signal was
analyzed with a photon-counting multichannel scaler. The
excitation source for the decay measurements was 488 nm
light from an optical parametric oscillator pumped by thethird harmonic of a Nd:YAG laser. The measurements of
NIR PL and decay time were performed in the temperature
range of 8–300 K.
III. RESULTS AND DISCUSSIONS

Figure 1(a) shows a 3D mapping of the visible to NIR
PL intensity of the sample annealed at 1300  C in N2 gas.

J. Appl. Phys. 114, 033524 (2013)

The ordinate is the excitation wavelength and the abscissa isthe emission wavelength. Three PL emission bands, one in
the visible range and two in the NIR range, can be seen. For
the PL emission band around 606 nm, the PL excitation
bands appear around 352 and 475 nm. For the NIR PL emission band around 845 and 1410 nm, two PL excitation (PLE)bands are seen around 374 and 420 nm. The PL emission
bands around 845 and 1410 nm are similar to those observedfor Bi doped pure silica glass fiber preform.8,13
In contrast to the sample annealed in N2 gas, the sample
annealed in air at 1300  C did not show NIR PL (Figure1(b)). It exhibits a PL emission band around 530 nm with a
PL excitation band around 290 nm. Bi-related luminescenceis known to be very sensitive to the annealing atmosphere.
Recently, Bi doped alkali borosilicate porous silica glass
annealed in various atmosphere including oxygen, argon,
and hydrogen gases has been reported.9 It revealed that only
the glass annealed in oxygen did not exhibit NIR luminescence. The present results are thus consistent with those in
the previous reports.7,9,21
For more detailed analysis, normalized PL spectra of thesample annealed in N2 excited at various wavelengths areshown in Figure 1(c). For comparison, PL spectrum of thesample annealed in air (orange) is also shown. When excitation wavelengths are shorter than 300 nm, both samples
show broad PL around 530 nm. The PL and the PL excitation
features shown in Figure 1(b) are very similar to those of Bi
doped germinate glass and it is in general assigned to Bi3þ.22

FIG. 1. (a) and (b) are the 3D plots of
the PL intensity versus excitation andemission wavelengths of Bi doped porous silica annealed in N2 and air. (c)Normalized visible to NIR PL spectraof Bi doped porous silica under thevarious excitation wavelengths.

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When the excitation wavelength is 320 nm, a PL peak
appears around 606 nm for the sample annealed in N2 gas. In
Figure 1(a), we can see the PL excitation bands around 352
and 475 nm for the PL emission band at 606 nm. This orange
emission has been reported for various materials, e.g.,
SrB4O7, and is usually assigned to Bi2þ.23 In addition tothese visible PL assigned to Bi3þ and Bi2þ, the sample
annealed in N2 at 1300  C exhibits NIR PL around 845 and1410 nm. These PL emission bands have similar excitation
bands around 374 and 420 nm.
To elucidate the mechanism of NIR luminescence, wemeasured the excitation wavelength dependence of the PL
intensity as shown in Figure 2. At the emission wavelengthof 1150 nm, we can see two broad PL excitation bands at
500 and 700 nm. This is very similar to those of Biþ inmulti-component glasses.5–7,9 On the other hand, at the emission wavelengths of 850 nm and 1450 nm, sharp PL excitation bands at about 374 and 420 nm were observed. Thesebands are similar to the absorption bands in Bi doped porous
silica glass.8 Very recently, the similar absorption bands
were reported in crystals and glasses, in which the absorption
was ascribed to Bi dimer.24,25 Thus, Bi dimer can be considered to be the origin of the PL emission bands at 850 nm and1450 nm. It should be noted that pure porous silica has beenreported to exhibit a PL band around 845 nm with the full
width with half maximum of about 150 nm.26–28 This is very
similar to that observed in this work. However, the PL properties of pure porous silica are much different from those of
Bi-doped porous silica. Our previous work revealed that the
PL intensity of pure porous silica is more than 300 times
weaker than that of Bi doped porous silica.15 In addition, theshape and peak position of the PL from the Bi active centeris very insensitive to oxidization conditions of porous silicon. This is in a sharp contrast with that the peak position of
PL from porous silica strongly depends on the oxidation
condition.26,28 Thus, the origin of the PL at 845 nm can be
ascribed to Bi NIR active center.
Figure 3(a) shows the PL spectra of Bi-doped porous
silica annealed in N2 from 1100 to 1300  C. The excitation
wavelength is 488 nm. We can see two broad PL bands. BothPL bands are strongest when the annealing temperature (Ta.)
is 1200  C. The full width at half maximum of the long

FIG. 2. Excitation wavelengths dependence of PL intensity of the sample
annealed in N2 at 1300  C. Scale up the intensity of PL for 1150 nm.

J. Appl. Phys. 114, 033524 (2013)

FIG. 3. (a) NIR PL spectra of the sample at various annealing temperatures
1100–1300  C in N2 gas. (b) and (c) are the normalized spectra of NIR PL
bands.

wavelength side PL spectra at 1410 nm (annealing temperature is 1300  C) is comparable to those reported for Bi-doped
multi-component bulk glasses.5,9 Figures 3(b) and 3(c) are
the normalized spectra of the two PL bands. It is interestingthat the shape of the shorter wavelength PL peak around
850 nm is independent of the annealing temperature, whilethat the longer wavelength PL peak shifts from 1310 to1410 nm with increasing the annealing temperature. This
result suggests that the PL intensity of Biþ (at around
1270 nm) decreases while that of Bi dimer (at around1420 nm) increases with increasing the annealing temperature. This implies that Bi ions aggregate and becomes Bi
dimer when the annealing temperature is high.
In order to clarify the valence state of Bi, the X-ray photoelectron spectroscopy (XPS) spectra were measured.
Figure 4 shows the XPS spectra of Bi-doped porous silica
annealed in N2 from 1100 to 1300  C. The samples show
Si4þ (2s) peak and Bi3þ (4f7/2) peak, which reveals that 3þ
is the dominant valence state of Bi in these samples. Other
valence states of Bi were under the detection limit.
Unfortunately, due to the low Bi concentration, it is difficult
to obtain more information on the valence states of Bi active
centers. The XPS intensity of Bi3þ peak decreases withincreasing the annealing temperature. This is probably due tothe evaporation of Bi during the annealing at high temperatures.7 As shown in Figure 3(a), the PL intensity at 845 and
1450 nm of our samples is the strongest when the annealing
temperature is 1200  C. Probably, this optimum annealing
temperature is determined by the trade-off between the activation rate of Bi NIR active centers and evaporation of Bi
during the high temperature annealing.
For further study of the NIR PL properties, the NIR PL
spectra of the sample annealed at 1300  C in N2 atmosphere
was measured as a function of temperature. Figure 5(a)Downloaded 21 Jul 2013 to 101.142.7.78. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions

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J. Appl. Phys. 114, 033524 (2013)

FIG. 4. XPS spectra of Bi doped porous silica annealed in N2 from 1100 to
1300  C.

shows the results. For all the temperatures, we can see broad
NIR PL spectra. The PL spectra can be decomposed into two
Gaussian bands centered at 1270 and 1425 nm. The temperature dependence of the PL intensities of the two PL bands isshown in Figure 5(b). Both the PL intensities first increases
and then decreases. Qualitatively similar results werereported in a previous work on the NIR PL from Bi4Ge3O12
crystal.29 In that work, the PL intensity increased as the temperature increases from 8 to 100 K, then the intensity
dropped by 90% as the temperature raised to 300 K. On theother hand, in our present work, it drops by only 40%. This
small temperature quenching is a promising feature for applications such as optical amplifiers at optical telecommunication wavelengths.

FIG. 6. (a) Decay curves of the sample annealed in N2 at 1300  C measured
at 8 and 300 k, detection and excitation wavelengths are 1450 and 488 nm,
respectively. (b) Temperature dependence of slow component’s lifetimes atvarious detection wavelengths under the excitation wavelength of 488 nm.

To draw more information from the NIR PL properties,
time resolved PL is measured as a function of temperature.
Figure 6(a) shows decay curves of the sample annealed in N2
at 1300  C measured at 8 and 300 K. The detection andexcitation wavelengths are 1450 and 488 nm, respectively.
The PL decay curves consist of two components, i.e., fastand slow components. The lifetime of the fast components isshorter than 4 ls, which is independent on the detection
wavelengths (data were not shown). This component probably arises from defects in porous silica. The lifetime of theslow component at room temperature is 675 ls, which iscomparable to those reported for Bi doped glasses andcrystals.3–9 The lifetime increases to 857 ls at low temperature (8 K). In Figure 6(b), the lifetime of the slow component
is plotted as a function of the temperature at various detection wavelengths (1200–1550 nm). At all the detection wavelengths, the lifetime decreases gradually with increasing thetemperature. The decrease in the lifetime in the temperature
range from 8 K to 300 K was about 30%. This value is muchsmaller than that of Bi4Ge3O12 crystal, which has beenreported to be about 90%.29 The observed small temperaturedependence of the lifetime is consistent with the small temperature quenching of the PL.
IV. CONCLUSION

FIG. 5. (a) NIR PL spectra upon photo-excitation with a wavelength of
488 nm at various temperatures 10–300 K of the sample annealed in N2 at
1300  C. (b) Temperature dependence of integrated intensity of the two
Gaussian peaks at 1270 and 1420 nm in (a).

Bi doped porous silica thin films were annealed at various temperatures in air or N2 gas atmosphere. NIR PL ofthese samples was measured as a function of excitation
wavelengths. It was found that the annealing in N2 is necessary for the activating NIR emission from Bi in pure porous

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033524-5

Rong gui et al.

silica thin films. The multiple Bi centers would be stabilizedin various forms as Bi3þ, Bi2þ, Biþ, and Bi dimer.
Robustness of NIR PL intensity against temperature is shownby measuring the steady state and time-resolved PL at 8 to
300 K.
ACKNOWLEDGMENTS

This work was partly supported by KAKENHI(23310077).
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