Er-Doped Silica Glass Films Prepared by rf-Cosputtering Deposition:
a Cathodoluminescence Study on the Thermal Annealing Behaviour
F. Gonella1, G. Battaglin1, E. Cattaruzza1, E. Trave1, A. Leto2, G. Pezzotti3
1
Department of Molecular Sciences and Nanosystems, Ca' Foscari University of Venice, Venezia, Italy.
2
Piezotech Japan Ltd, R&D, Kyoto, Japan.
3
Kyoto Institute of Technology, Ceramic Physics Laboratory & Research Institute for Nanoscience, Kyoto, Japan.
INTRODUCTION
Erbium-doped silica glass has been drawing a great
interest in the last years for its application in optical device
technology [1–5].
In general, the optical response of the rare-earth in glass
may depend critically on several factors, such as the local
chemical environment, the presence and nature of glass
network defects, as well as on the presence, nature,
proximity and size of multimers or nanoclusters. In this
paper, preliminary experimental results are presented on
defect evolution in Er:SiO2 films prepared by an rfcosputtering deposition technique, and then annealed in the
100–1200 °C range.
550 nm, which is a contamination band, due to the
presence of H2O, C, H and so on, and the R-band, at about
650 nm, which, is related to non-bridging oxygen hole
centers (NBOHC) [6-9]. Features II, III and IV of Fig. 1
may be therefore superimposed to intrinsic silica defectrelated features, namely, B-, Y- and R-bands.
EXPERIMENTAL
Co-depositions of silica and erbia on fused silica slides
were performed in pure Ar, at the pressure of 0.40 Pa, by
the simultaneous operation of of two 13.56 MHz rf
sources. The rf power to the 2 in. diameter targets was
fixed at 200 and 15 W for silica and erbia, respectively.
Pure silica films were also prepared. The deposited films
were about 0.6 μm thick. After deposition, samples were
thermally annealed for 1 hour, in air, at temperatures
ranging from 100 to 1200 °C.
The cathodoluminescence (CL) analysis was performed
by a field-emission-gun scanning electron microscope (5
kV, 190 pA, 500 nm diameter beam). The light was
collected using a high-sensitivity CL detector unit. A
highly precise monochromator equipped with a nitrogencooled CCD camera was used. Integration time was 5 s.
Rutherford Backscattering Spectrometry (RBS) was
performed at INFN-Legnaro National Laboratories, using a
4
He+ beam at the energy of 2.2 MeV.
RESULTS
Cathodoluminescence spectra were collected for both the
Er-doped and the corresponding pure silica deposited
samples, annealed at different temperatures. Fig. 1 shows
the CL spectra for annealed Er-doped silica films.
Characteristic features are evident and the corresponding
transitions can be identified. To these transitions, silica
defect-related features must be added: the so-called Bband, in the wavelength range 460–500 nm, related to
oxygen-deficient sites; the Y-band, located at about 530–
Fig. 1. CL spectra for Er-doped silica films annealed at different
temperatures.
Table 1 reports the features attribution to Er transitions
or to silica defects.
Table 1. Features attribution to Er transitions. Position of defectrelated SiO2 bands are also shown.
Feature
Transition
4
I
II
III
IV
V
G11/2→4I15/2
H9/2→4I15/2
4
F5/2→4I15/2
4
F7/2→4I15/2
2
H11/2→4I15/2
4
S3/2→4I15/2
4
F3/2→4I13/2
4
F9/2→4I15/2
2
H11/2→4I13/2
4
I9/2→4I15/2
2
Wavelength
(nm)
380
410
450
490
520
545
635
655
785
800
SiO2
band overlap
B-band
Y-band
R-band
In general, the presence of Er3+ in the matrix is expected
to promote the formation of either NBOHC or oxygendeficient center defects [10].
Looking at fig. 1, the general trend is characterized by
the increase with T of the emission at 480 nm and at 660
nm (progressive rise up to 1200 °C). On the other hand, at
high annealing T, the 550 nm feature drops down, being
overlaid by the Y-band related component. A similar trend
is observed by the Er-related features at 400 and 780 nm,
suggesting possible Er clustering events (either Er metallic
or Er oxide phases) upon high temperature treatment, with
reduction of the luminescence activity [11–14].
Spectra are strongly characterized by the behavior of the
545 nm feature, observed in the whole 0–1200 °C
annealing T range, that can be ascribed mainly to the Er3+
ion luminescence activity (4S3/2→4I15/2 transition) along
with the component at 520 nm related to the 2H11/2→4I15/2
transition, with a possible contribution from silica Y-band.
The comparison between the annealed samples and the
corresponding co-deposited pure silica ones is shown in
Fig. 2, where the features at 480 and 660 nm are related to
the silica B- and R-bands, respectively.
conversion and relaxation phenomena, therefore leading to
PL emission quenching.
Fig. 3. RBS simulated spectra for the samples annealed at 200 °C
and at 1200 °C. Energy edges for the elements are shown.
Fig. 2. Comparison between CL spectra for Er-doped silica and
pure silica co-deposited films, annealed at 400 °C and at 1200 °C.
The matrix recovery, as the progressive annihilation of
the silica defective centers, promotes the activation of nonradiative channels for the Er3+ de-excitation and gives rise
to the characteristic features in the silica CL spectrum.
Fig. 3 shows the RBS simulated spectra for the samples
annealed at 200 °C and at 1200 °C.
RBS analysis indicates a homogeneous doping of
erbium, at a concentration close to 1020 atoms/cm3
throughout the deposited layer up to about T=1000 °C of
annealing. After that temperature, erbium starts to
penetrate into the glass matrix, while clustering
phenomena may occur in the near-surface region, as
evidenced in Fig. 3 where the signal corresponding to Er
atoms (channels 440–460) exhibit peaks corresponding to
an increase of the local concentration, with indication of an
erbium in-depth diffusion only for the deepest doped
region. Erbium aggregation then gives rise to up-
In conclusion, strong optical modifications were
observed with annealing temperature of the samples,
connecting the peculiar features of the Er luminescence
activity to those of the silica matrix, namely, the density of
non-bridging oxygen hole centers, of oxygen-deficient
sites and -OH groups. In general, a complex
phenomenology was observed, involving major local
rearrangements of the Er environment. RBS data supported
the optical ones, and allowed to evidence migration
phenomena, resulting in a compositional change of the
films, thus indicating the progressive modification of the
system as well as the possible formation of Er nanoclusters
for high temperatures.
[1] A. Polman, J. Appl. Phys. 82 (1997) 1.
[2] N. Daldosso et al., IEEE J. Sel. Topics Quantum Electron.
12 (2006) 1607.
[3] F. Gonella, Rev. Adv. Mater. Sci. 14 (2007) 134.
[4] Y. Jestin et al., Appl. Phys. Lett. 91 (2007) 071909.
[5] E. Cattaruzza et al., Thin Solid Films 519 (2011) 5376.
[6] W.J. Miniscalco, IEEE J. Lightwave Technol. 9 (1991) 234.
[7] L. Skuja, J. Non-Cryst. Solids 239 (1998) 16.
[8] A. Leto at al., J. Appl. Phys. 101 (2007) 093514.
[9] G. Pezzotti et al., Phys. Rev. Lett. 103 (2009) 175501.
[10] S. Cueff et al., J. Appl. Phys. 108 (2010) 113504.
[11] A. Polman et al., J. Appl. Phys. 70 (1991) 3778.
[12] S. Cueff et al., IOP Conf. Ser.: Mater. Sci. Eng. 6 (2009)
012021.
[13] X.J. Wang et al., J. Mater. Res. 18 (2003) 2401.
[14] E. Cattaruzza et al., Opt. Mater. 33(2011) 1135.