Broadband sensitization of 1.53 μm Er3+ luminescence in erbium-implanted alumina

Article abstract of DOI:10.1063/1.1829139

Experimental evidence of an efficient broadband sensitization mechanism in erbium-implanted alumina is presented. Alumina thin films were deposited by plasma-enhanced chemical vapor deposition using trimethyl-amine alane and nitrous oxide. The as-grown fi

Full text of DOI:10.1063/1.1829139

Broadband sensitization of 1.53 μ m Er 3 + luminescence in erbium-implanted alumina
C. E. Chryssou, A. J. Kenyon, T. M. Smeeton, C. J. Humphreys, and D. E. Hole
Citation: Applied Physics Letters 85, 5200 (2004); doi: 10.1063/1.1829139
View online: http://dx.doi.org/10.1063/1.1829139
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APPLIED PHYSICS LETTERS
VOLUME 85, NUMBER 22
29 NOVEMBER 2004
Broadband sensitization of 1.53 ␮m Er3+ luminescence
in erbium-implanted alumina
C. E. Chryssoua) and A. J. Kenyonb)
Department of Electronic & Electrical Engineering, University College London, Torrington Place, London
WC1E 7JE, United Kingdom
T. M. Smeeton and C. J. Humphreys
Department of Materials Science & Metallurgy, University of Cambridge, Pembroke Street, Cambridge,
CB2 3QZ, United Kingdom
D. E. Hole
Ion Implantation Laboratory, Department of Engineering and Design, School of Science and Technology,
University of Sussex, Falmer, Brighton BN1 9QH, United Kingdom
(Received 27 July 2004; accepted 4 October 2004)
Experimental evidence of an efficient broadband sensitization mechanism in erbium-implanted
alumina is presented. Alumina thin films were deposited by plasma-enhanced chemical vapor
deposition using trimethyl-amine alane and nitrous oxide. The as-grown films, together with
sapphire crystals, were implanted with erbium. Photoluminescence excitation spectra showed that
erbium-implanted sapphire crystals exhibit characteristic Er3+ luminescence at 1.53 ␮m only when
pumped resonantly. In contrast, erbium-implanted alumina thin films exhibit 1.53 ␮m luminescence
even when pumped at wavelengths outside Er3+ absorption bands. We postulate that the sensitizing
species is either small nanoclusters of aluminum or pairs of aluminum ions. © 2004 American
Institute of Physics. [DOI: 10.1063/1.1829139]
Rare-earth doped materials are of key importance in op-
tical communication systems, as demonstrated by the domi-
nance of the erbium-doped fiber amplifier. Rare-earth doping
of planar optical waveguides offers the promise of active
devices, such as optical amplifiers, with shorter waveguides
than fiber-based systems.1–4 Currently, there is much interest
in integrated optical components based on planar waveguides
for the implementation of wavelength division multiplexing
(WDM) in fiber-to-the-home systems.5 Opportunities for
such components include WDM sources, optical multiplex-
ing and de-multiplexing, wavelength conversion, flatband fil-
ters, optical amplifier gain equalization, routers, optical
cross-connects, and receivers.
silica.10 Potentially, this mechanism may relax requirements
on the pump wavelength for Er3+-doped optical amplifiers,
leading to the realization of broad-band pumpable optical
amplifiers at the important telecommunications wavelengths
around 1.55 ␮m; recent reports have suggested gain of up to
3 dB cm−1 from such material pumped using a blue LED.11
In addition, it has also been reported that it is possible to
sensitize erbium emission in silica in a similar fashion using
co-implantation with silver.12 For a recent review of different
mechanisms for sensitization of erbium luminescence in pla-
nar amplifiers, see Ref. 13.
Alumina has also attracted attention as a host for rare-
earth ions due to the high solubility and broad emission line-
width ͑55 nm͒ that Er3+ exhibits when incorporated in
Al2O3.3,4,14,15 Erbium-doped alumina thin films have been
fabricated using various techniques, including plasma-
enhanced chemical vapor deposition (PECVD),14 reactive
co-sputtering,4 and ion implantation.16 A net optical gain of
2.3 dB has been reported from a 4-cm-long Er3+-implanted
Al2O3 optical waveguide pumped with 9 mW of power at
1.48 ␮m.3
Recently, Er3+-doped waveguide amplifiers have found
applications in metropolitan WDM optical networks. This is
mainly due to their smaller size and reduced cost compared
to erbium-doped fiber amplifiers (EDFAs). EDFA optical to-
pologies include high-power laser diodes at either 980 or
1480 nm; these laser diodes constitute a large proportion of
the EDFA cost, and thus the development of a broadband
pumped Er3+ system would yield significant savings. Cur-
rently, pump lasers for EDFAs retail in the $500–$1000
range,6 whilst high power broadband sources such as light
emitting diodes (LEDs) can cost as little as $10–$20, with
prices falling rapidly.
Several groups have reported a coupling mechanism be-
tween luminescent rare-earth ions and semiconductor
nanocrystals.7–9 Moreover, studies have shown that this cou-
pling considerably enhances the effective absorption cross
section of the rare-earth ions. This enhancement can be up to
four orders of magnitude in the case of Er3+ in silicon-rich
In this study, we report experimental evidence of an in-
direct excitation mechanism for rare-earth ions in
Er3+-implanted alumina.
Alumina thin films 700 nm thick were deposited on Si
(100) substrates by PECVD. A detailed description of the
apparatus used for the deposition of the thin films can be
found elsewhere.17 The films were deposited using trimethyl-
amine alane (TMAA) as the aluminum precursor, and nitrous
oxide as the oxidizing agent. Argon was used as the carrier
gas for the Al precursor. The stoichiometry of the thin films
was controlled by varying the Ar/TMAA to N2O flow rate
ratio. The as-deposited thin films, together with single sap-
phire crystals, were then implanted with erbium. In order to
achieve a uniform erbium depth distribution in the alumina
layer, two implantation energies were employed: 50 and
a)Present address: Southampton Photonics, Inc., 3 Wellington Park, Tollbar
Way, Hedge End, Southampton, U.K.
b)Author to whom correspondence should be addressed; electronic mail:
t.kenyon@ee.ucl.ac.uk
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Appl. Phys. Lett., Vol. 85, No. 22, 29 November 2004
Chryssou et al.
5201
FIG. 2. Electron energy loss spectrum of an erbium-implanted PECVD-
grown alumina film.
FIG. 1. Photoluminescence excitation spectra of erbium-implanted sapphire
and erbium-implanted PECVD-grown alumina. Laser power at the sample
Ϸ50 mW, spot size=1 mm, monochromator resolution=1 nm. Dotted lines
are guides for the eye.
the alumina host and the Er3+ ions. This effect is similar to
the coupling that exists between Er3+ and Si nanocrystals in
silicon-rich silica thin films.7–9
150 keV (implantation fluences ranged from 6.0
ϫ1014 to 2.5ϫ1015 Er/cm2). The peak erbium concentra-
tion was measured by Rutherford backscattering spectrom-
etry, and was found to be 0.5 at. % ͑1.3ϫ1020 cm−3͒. The
implanted samples were thermally annealed at 850 and
950 °C for 90 min in nitrogen in order to optically activate
the Er3+ and enhance the photoluminescence intensity at
1.53 ␮m by removing implantation-induced defects. The
samples were analyzed structurally using Auger emission
spectroscopy (AES), Fourier transformed infrared spectros-
copy (FTIR), transmission electron microscopy, and electron
energy-loss spectroscopy. Photoluminescence (PL) and PL
excitation (PLE) spectra were measured at room temperature
using eight visible lines from an Ar+ laser, a single grating
monochromator, and standard lock-in techniques. An InGaAs
photodiode and a photomultiplier tube were used to detect
near-infrared and visible spectra, respectively.
Figure 2 shows an electron energy loss spectrum (EELS)
of an erbium-implanted alumina sample. The main plasmon
peak at around 23 eV corresponds to Al2O3, and an addi-
tional shoulder is present at an energy loss of 14–15 eV. The
full width at half maximum of the zero-loss peak was 1 eV
in the EELS measurements. The shoulder is present in spot-
EELS spectra recorded from throughout the alumina layer,
and from some locations a separate peak is resolved at this
energy. There is no clear correlation between the strength of
the peak and structural features in the film. The aluminum
and erbium plasmon excitations are both expected to be close
to 14–15 eV, so these EELS spectra may indicate the pres-
ence of a low concentration of metallic clusters.
It is known that implantation of alumina with metallic
ions can in some cases lead to the formation of aluminum
precipitates via a reduction mechanism.18 In a study by Hunt
et al., it was determined that aluminum clusters can be pre-
cipitated if the free energy of the following reaction, where
X is a rare-earth ion, is negative:
The AES depth profile of as-grown, undoped alumina
thin films deposited by PECVD showed them to be slightly
aluminum rich (ϳ4 at. % excess aluminum) with very low
carbon contamination. A detailed account of the structure and
the surface quality of the undoped PECVD-deposited thin
films can be found elsewhere.17
2X + Al2O3 → 2Al + X2O3.
Using values for enthalpies of formation and entropies
given in standard reference texts,19 we calculate from Eq. (1)
that the Gibbs free energy for the reduction of alumina by
erbium is negative ͑−218.6 kJ mol−1͒, so it is feasible that
some aluminum clusters are formed.
Figure 1 shows photoluminescence excitation spectra of
an erbium-implanted sapphire crystal and an erbium-
implanted PECVD-deposited alumina thin film, both ther-
mally annealed at 900 °C for 90 min following implantation.
Care was taken to maintain constant laser power for each
pump wavelength. All samples were implanted with the same
erbium concentration ͑0.5 at. %͒ under identical conditions.
It is evident that the erbium-implanted sapphire crystal ex-
hibits PL only when pumped at wavelengths that correspond
to Er3+ absorption bands (488 and 514.5 nm). Emission is, in
this case, due to direct optical excitation of the Er3+ ions
from the ground state ͑4I15/2͒ to the 4F7/2 and 2H11/2 levels via
the absorption of pump photons at 488 and 514.5 nm, re-
spectively. In contrast, erbium-implanted PECVD-grown
alumina thin films exhibit PL even when pumped at wave-
lengths far away from Er3+ absorption bands, although there
is still a strong element of direct absorption (note the PL
⌬G = ⌬H − T⌬S.
͑1͒
However, in the present case no definitive evidence of
the existence of nanoclusters in the alumina matrix could be
found, though the AES depth profile and FTIR indicate that
the film is slightly aluminum-rich. It should be noted that in
the study reported in Ref. 18, the peak concentration of im-
planted rare-earth ions was very high—approximately
9 at. %. This is much greater than the concentration used in
our study, and may explain the difficulty in discerning a clear
peak in the EELS spectra attributable to aluminum clusters.
Figure 3 shows visible PL spectra for films as-grown and
following thermal annealing at 850 and 950 °C after implan-
tation with erbium. From this figure, it is evident that the
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peak at 488 nm). This behavior indicates coupling between
defect-related peak at 2.2 eV has been considerably in-
138.51.150.161 On: Fri, 28 Nov 2014 20:41:51

5202
Appl. Phys. Lett., Vol. 85, No. 22, 29 November 2004
Chryssou et al.
quenched due to structural rearrangement compensating the
dangling bonds in a similar way to that seen in silicon-rich
silica.20 This suggests that the sensitization is not due to
implantation-related defects in the alumina matrix—a con-
clusion supported by the lack of sensitization in implanted
single crystal sapphire.
Figure 4 shows the PL intensity from the Er3+-implanted
PECVD-deposited alumina thin films as a function of pump
power. Two pump wavelengths are chosen: 488 nm corre-
sponding to direct optical excitation, and 476 nm, which is
predominantly indirect excitation. When pumped nonreso-
nantly, the Er3+ PL saturates at approximately 4.0
ϫ1019 photons/cm2 s. In contrast, resonant pumping does
not produce saturation even at
a
flux of 6.0
ϫ1020 photons/cm2 s. This implies either a small concentra-
tion of the sensitizing species, or a large effective absorption
cross section for indirect excitation.
FIG. 3. Photoluminescence spectra of a PECVD-grown alumina film as-
grown and following annealing at 850, 900, and 950 °C. Laser power at the
sample Ϸ50 mW, spot size=1 mm, monochromator resolution=2 nm.
In summary, a broadband sensitization mechanism for
erbium luminescence in polycrystalline alumina films has
been demonstrated. Although there is no conclusive evidence
of aluminum clusters in this material, we speculate that the
sensitization may be either due to such nanoclusters present
at a very low concentration, or else to a species generically
similar to that seen in silver-activated erbium-doped borosili-
cate glass, in which absorption is thought to be due to pairs
of silver atoms.12 Further work is needed to conclusively
identify the sensitizing species; nevertheless, this mechanism
is another promising route to broadband excitation of
erbium-doped optoelectronic devices.
creased by annealing at 850 °C. This can be explained by an
increase of the number of dangling bonds in the alumina
matrix after the reduction of –OH that takes place at 650 °C.
After annealing at 900 and 950 °C, the peak at 2.2 eV is
The authors are grateful to Peter Cherns for his assis-
tance with obtaining the EELS results.
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FIG. 4. Photoluminescence at 1535 nm as a function of pump laser power
for an erbium-implanted PECVD-grown alumina film. (a) Pump
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