photoluminescence, nanoclusters, erbium, Transmission electron microscopyWe have studied the 1.5 µm photoluminescence (PL) from Er ions after annealing two different sample sets in the temperature range 500 °C to 1100 °C. The different sample sets were made by magnetron sputtering from composite targets of Si+SiO2+Er and Ge+SiO2+Er respectively for the different sample sets. The annealing induces Si - and Ge-nanoclusters respectively in the different film sets. The PL peak reaches its maximum intensity after annealing at 700 °C for samples with Ge nanoclusters and after annealing at 800 °C for samples with Si. No luminescence from nanoclusters was detected in neither sample sets. This is interpreted as an energy transfer from the nanocluster to Er atoms. Transmission electron microscopy shows that after annealing to the respective temperature yielding the maximum PL intensity both the Ge and Si clusters are non-crystalline. Here we mainly compare the spectral shape of Er luminescence emitted in these different nanostructured matrixes. The PL spectral shapes are clearly different and witness a different local environment for the Er ions

Finstad T. G.

Foss S.

Heng C. L.

Klette H.

Li Y. J.

Mayandi J.

Thøgersen A.

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ENS’06 Paris, France, 14-15 December 2006
©TIMA Editions/ENS 2006 -page- ISBN:
A COMPARISON BETWEEN 1.5µm PHOTOLUMINESCENCE FROM Er-DOPED Si-RICH
SiO2 FILMS AND (Er,Ge) CO-DOPED SiO2 FILMS
J. Mayandi1), T. G. Finstad1, 2, a), C. L. Heng1), Y. J. Li 2), A. Thøgersen2), S. Foss1) and
 H. Klette3)
1)Dept. Physics, University of Oslo, Norway
2)Ctr. Mater. Sci. & Nanotechnology, University of Oslo, Norway
3)SINTEF, Mater. & Chem. Oslo, Norway
ABSTRACT
We have studied the 1.5 µm photoluminescence (PL)
from Er ions after annealing two different sample sets in
the temperature range 500 °C to 1100 °C. The different
sample sets were made by magnetron sputtering from
composite targets of Si+SiO2+Er and Ge+SiO2+Er
respectively for the different sample sets. The annealing
induces Si – and Ge-nanoclusters respectively in the
different film sets. The PL peak reaches its maximum
intensity after annealing at 700 °C for samples with Ge
nanoclusters and after annealing at 800 °C for samples
with Si. No luminescence from nanoclusters was detected
in neither sample sets. This is interpreted as an energy
transfer from the nanocluster to Er atoms. Transmission
electron microscopy shows that after annealing to the
respective temperature yielding the maximum PL
intensity both the Ge and Si clusters are non-crystalline.
Here we mainly compare the spectral shape of Er
luminescence emitted in these different nanostructured
matrixes.  The PL spectral shapes are clearly different and
witness a different local environment for the Er ions.
1. INTRODUCTION
Er3+ has attracted great interest in the photonic
technology, due to that the emitted wavelength of the
4I13/2  to 
4I15/2 transition coincides with the minimum
attenuation of silica-based optical fibers [1] and it finds
use in optical amplifiers. There is a need for a cheap and
more compact alternative to the present fiber optical
amplifier especially  to match an increasing need for
signal transfer over relatively short distances. The
approaches taken for future technological solutions to this
overlap those undertaken to meet the need for a faster
signal transfer between functional blocks on a computer
chip, between chips and systems.  Application motivated
research on Er in Si has a long history that has foreseen
the same need we see today. However luminescence from
Er in Si has several problems in particular with two major
de-excitation processes: (i) the phonon-assisted energy
back-transfer originating from the near resonance between
the Er-induced level in the band gap and the internal Er
levels [2]; and (ii) the Auger-type interaction involving
free carriers and excited Er ions [3]. The former induces a
large temperature quenching in the photoluminescence
(PL) intensity, while the latter increases with the density
of excited carriers and gives rise also to an energy
backflow mechanism. To remove or reduce these
detrimental effects, a promising approach is currently
explored through the widening of the band gap by using
SiO2 as a host material co-doped with Er ions and Si
nanoclusters (Si-nc) which are very efficient sensitizers for
Er luminescence with a two orders of magnitude increase
in efficiency as first reported by Kenyon [4].  In order to
improve on this type of structures it is necessary to
understand the systems on the nanometer scale, both
regarding the energy transfer processes occurring, their
physical characteristics as well as how the systems can
self-assemble on the nanometer scale and how this
possibly can be controlled. A large number of scientists
around the world have engaged in these problems of
nanoscience.   The simplified main elements of the
resulting models for photoluminescence in such systems,
consist of first absorption of electromagnetic radiation by
the nanocrystal (or other nanostructures), which gets
exited. Excitation energy is then transferred  to the Er
atom. The Er atom decays to the 1st exited level and then
emits l.5 µm radiation. The absorption properties of the
nanocrystals depends upon their size by quantum
confinement of the electrons.  The nanocrystal energy
levels will also influence which levels the Er can be
exited to in the transfer process. It is desired that the
upper exited levels gets populated followed by a rapid
decay down to the first exited level.   The energy levels of
the Si nanocrystal also determine the probability for a
transfer of energy back from the Er ion to the Si
nanocrystal, which is undesirable. The energy transfer
process   depends strongly on the interaction distance
J. Mayandi, T.G.Finstad, C.L.Heng, Y.J.Li, A. Thøgersen, S. Foss and H. Klette
A COMPARISON BETWEEN 1.5µm PHOTO LUMINESCENCE FROM Er-DOPED Si-RICH..
©TIMA Editions/ENS 2006 -page- ISBN:
between the Er ion and Si nanocrystals. The interaction
can be caused by direct wave function overlap or by
dipole-dipole interactions. The Er atom needs to be in an
environment making it optically active. An isolated Er
atom is not optically active and likewise an Er atom
bonded in an Er silicide is considered optically inactive.
The Er atom is inactive when the luminescence efficiency
is low, which can be caused by the radiative transition
being much less probable than competing de-excitation
mechanisms.  The details in the models sketched are
generally not known and they are  hard to probe directly.
Ge offers an interesting alternative to Si.  Ge has many of
the same chemical and material properties as Si and can
be expected to behave in similar ways, while there are
some important differences. A study of Ge nanostuctures
in SiO2 doped with Er can thus shine light on the
situation with silicon and our group have undertaken such
a study [5]. One interesting difference between Si and Ge
in this respect is that Ge theoretically shows a stronger
quantum confinement, due to the larger exciton Bohr
radius, for equal size of quantum dots. The atomic
mobilities in SiO2 are larger than that of Si which makes
it possible to make equal size nanoclusters at lower
temperatures with Ge than with Si. This is an advantage
for the tendency to Er clustering at increasing
temperatures, which could leave Er optically inactive.
Here we mainly intend to compare the spectral shape of
the luminescence from Er with Ge and Si nanoclusters,
respectively, in SiO2 matrixes and use that as a probe for
the local environments of Er.
2. EXPERIMENTAL DETAILS
The (Er, Ge) co-doped SiO2 film was deposited on a low
resistivity (~ 10-2 Ωcm) n-type Si (100) substrate.  Before
sputtering, the wafer was cleaned using a standard RCA
procedure and given a dip in a 10% HF solution to
remove the native oxide.  Then the film was deposited on
the substrates by sputtering an Er+Ge+SiO2 composite
target at 300 W.  The base pressure of the chamber was
less than 5x10-7 Torr.  The area ratios of the metal Er, Ge
plates, and SiO2 base in the target was 1: 3.3: 95.7.  The
working gas was Ar plasma with a gas pressure of 5.2
x10-3 Torr.  This structure was labeled sample type A and
had a film thickness of 1.1 µm.  For the preparation of
the device with Si ncs we used a composite target with
the ratio of 1:17:82 (Er: Si: SiO2) and this structure is
labeled as type B with a film thickness of about 25 nm
on a p- type Si(100) substrate.  (The structure was
originally designed for electroluminescence MOS
devices).  Here we report photoluminescence (PL)
measurements which were carried out by pumping with a
488-nm line of an Ar laser, which had a power of 100
mW and was focused to a beam spot of 6x10-2 cm2.  The
laser beam was chopped at a frequency of 72.5 Hz. A
spectrometer  consist ing of a single-grating
monochromator, a liquid-nitrogen-cooled Ge detector, and
a lock-in amplifier was used in the PL measurements.
3. RESULTS AND DISCUSSION
The annealing dependence of Er PL from both sample sets
(A and B) has been studied for annealing temperatures
from 500 °C to1100 °C in N2.  The PL spectra are shown
in Figs. 1(a) and 1(b), for sample set A and B
respectively. The peak is attributed to the 4f inner-shell
transition 4I13/2  → 
4I15/2 transition of the Er
3+ as no signal
above the noise level have been observed in control
samples without Er prepared in previous experiments in
this spectral range. The insets in Fig. 1 (a) and Fig. 1 (b)
show how the PL peak intensity varies with the annealing
temperature; it is relatively weak for the as-prepared
samples before annealing for both sample sets; in both
cases it increases with annealing temperature, goes
through a maximum and then decrease for the highest
temperatures.  In Fig. 1(a), the Er3+ PL peak intensity
reaches a maximum after 700 °C annealing for sample set
A while from the inset of Fig. 1(b) it is seen that the Er3+
FIG 1 The ~1.54 µm PL from the sample (a) type A
(Er+Ge+SiO2) and (b) type B (Er+Si+SiO2).  All the
structures have been annealed at the range of 500 °C -
1100 °C in N2 for 30 min. The insets in the graphs
shows the PL intensity vs annealing temperature.
J. Mayandi, T.G.Finstad, C.L.Heng, Y.J.Li, A. Thøgersen, S. Foss and H. Klette
A COMPARISON BETWEEN 1.5µm PHOTO LUMINESCENCE FROM Er-DOPED Si-RICH..
©TIMA Editions/ENS 2006 -page- ISBN:
PL peak intensity reaches a maximum at an annealing
temperature of 800 °C.
 The microstructure of the samples has been
investigated by TEM measurements and there are some
correlations between the microstructure evolution upon
annealing and the PL intensity. In the case where we
observe the maximum intensity we have a large density of
Ge nanoclusters for sample A and Si nanoclusters for
sample B.  These clusters are in an amorphous state in
both cases. This essentially confirms the observations by
others for Er doped Si-rich oxide [6,7] while having been
reported by us before [8] for  (Ge, Er) co-doped SiO2.
The density of clusters are reduced, are larger as the anneal
temperature increases and they are transformed to
nanocrystals for higher annealing temperatures.  The
correlation with PL is understood by regarding the
absorption of excitation light by the nanoclusters
provided that amorphous clusters and crystalline clusters
have similar absorption properties.  Both absorption and
loss of Er excitation by energy back transfer could
contribute to the experimental trend of PL intensity with
the described microstructure related to size and
distribution of nanoclusters.
We observe no luminescence in the visible range
neither for sample type A nor type B. It is known that Si
nanoclusters in SiO2 do produce PL in the visible range
with the current experimental setup [9].  Thus, the
quenching of this visible luminescence indicates that the
excitation energy is transferred to the Er atoms for sample
set A, and the complete quenching of a strong
luminescence indicates that the energy transfer process is
very efficient.  In the Ge case, the luminescence from Ge
nanocrystals is usually weak without Er, probably due to
de-excitation by defect states associated with the Ge nc-
/SiO2 interface or other localized defects in the close
vicinity.  The transfers of excitation energy to the Er
atoms are thus strong; It is stronger than this parasitic
energy transfer.  
How strong or efficient the excitation energy transfer
is from the semiconductor nanoclusters ( Si or Ge) to the
Er atoms will naturally be related to the distribution of Er
atoms with respect to the nanoclusters. The latter
influences the physical processes for energy transfer and
how strong they are.  The distribution of Er could be
reflected by the local environment of Er. The local
environment of Er will also influence the lifetime of  the
excited state of Er. Thus the local environment of Er is an
important factor having a large effect on the luminescence.
  The shape of PL spectrum from Er3+ will be
influenced by the local environment of Er. The 4f inner-
shell transition 4I13/2  → 
4I15/2 transition of the Er
3+ which
gives the PL peak around 1.5 µm is between the first
exited multiplet and the ground state multiplet. The
ground state and the exited state will split because of the
electrical field gradient induced from neighboring atoms
and thus intermixes other orbital momentum states to the
f state wave function. The splitting depends upon the
symmetry of the neighbors. The result is that the spectral
shape of the PL peak can depend upon the site symmetry
and neighbors for the Er3+ ion. It could be commented
that the PL is influenced  by the environment because the
atomic levels are, the 4f levels are however less influenced
by the neighbor atoms, the matrix, than s- and p- valence
orbitals.  The latter change dramatically in comparison
when their bonding environment changes. That much of
the characteristics of the PL is maintained when 4f levels
are involved makes Er3+ PL a useable probe for the
environment. In the case of matrixes containing oxygen it
has been observed that there are two main 4I13/2  →  
4I15/2
transitions; with peak energies around 1.550 µm and
1.538 µm [10]. These characteristics are also present in
Fig.1, with a slight sample type dependant wavelength
shift, which we will return to, but with the individual
contributions being merged into each other and being
unresolved. These main features/transitions  can be
resolved at lower temperatures and/or high resolution in
the case of Er with Si nanocluster in SiO2 [1].
      In Fig. 2 we compare the shape of the Er3+ PL spectra
for samples A and B for annealing temperatures when
they give the maximum peak PL intensity. For this
comparison the peaks have been normalized in order to
notice their differences  readily. In both cases the main
features of the shape can be described  as a peak
consisting of two main unresolved contributions and a
wide “socket” or “bell-rim” surrounding the main peaks.
There are some statistical significant differences. These are
(1) the peak positions are  slightly different: ~1.534 µm
for sample A (Er + Ge + SiO2) and ~1.538 µm for sample
B (Er + Si + SiO2). (2) the width of the distribution,
parameterized by the full width at half maximum
(FWHM) for the two spectra is also different: 50.5 nm for
sample A  while for the sample it  is 43 nm.(3) the broad
“bell rims” at low and high wave lengths is the most
striking difference in the shape of the spectra for sample A
and B.  These differences clearly indicates that the
FIG. 2 Normalized ~1.54 µm PL peak from two samples:
A (Er+Ge+SiO2 and B (Er+Si+SiO2) for the annealing
temperature which gives the maximum intensity in each
case: 700 °C and 800  °C respectively.
J. Mayandi, T.G.Finstad, C.L.Heng, Y.J.Li, A. Thøgersen, S. Foss and H. Klette
A COMPARISON BETWEEN 1.5µm PHOTO LUMINESCENCE FROM Er-DOPED Si-RICH..
©TIMA Editions/ENS 2006 -page- ISBN:
environment of the Er atoms are different for the different
cases.  From an intuitive expectation that the energy
transfer would be most efficient when the Er ions are
close to the nanocluster which excite them, this seems
natural, and could indicate that the Er ions are surrounded
with the atoms from the cluster. This is however different
than the common assumption for Si, at least in the case
of nanocrystals, it is assumed that the Er would be
optically inactive if inside the nanocrystal [1].  The
present comparison with Ge could open up for another
view. In any case it indicates that the local environment
for Er is different for Ge-Er co-doped SiO2 than for Si-rich
SiO2 with Er. In both cases, the relatively wide peak is
indicative of a distribution of Er atom environments; they
are not all the same in either case.
       We have observed that the Er and Ge are spatially
correlated [8] by employing high resolution TEM with
energy dispersive X-ray analysis. This is somewhat
unexpected from equilibrium phase diagrams and simple
chemical bond analysis, since Er-O bonds and Er-Si
bonds would be stronger than Er-Ge bonds. We can
speculate that Er and Ge could be correlated  in the
present case because the amorphous clusters containing Er
may have a large configuration  entropy. The differences
observed in the Si and Ge cluster case is in agreement
with this. On the other hand, presently it appears that the
similarity in PL shapes for different annealing
temperatures with the same cluster element put this
explanation for the Er and Ge correlation in some doubt,
if it is not kinetically controlled, which it may.
     Other hyphothesises which should be addressed, or
perhaps tried synthesized are amorphous clusters which
may contain oxygen and Er. Thus the absorbing clusters
can be optimally close to the Er ion it transfer the energy
to and the environment for the Er is optimal for the first
exited to ground level transition.
4. SUMMARY
We have compared the 1.54 µm PL from (Er, Ge) co-
doped SiO2 films and Er-doped Si-rich SiO2 films both
deposited by using magnetron sputtering a composite
target in Ar plasma.  The PL intensities from these two
films reach their maxima after annealing the films at 700
°C and 800 °C for 30 min, respectively.  The comparison
of the spectral shape shows a considerable difference in
the peak position and FWHM.  Analysis of the
experimental results confirms that amorphous Ge and Si
nanoclusters play more effective role in exciting the Er3+
luminescence than the nanocrystals.
ACKOWLEDGEMENT
Financial support from N.R.C. is appreciated.
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A COMPARISON BETWEEN 1.5µm PHOTOLUMINESCENCE FROM Er-DOPED Si-RICH SiO2 FILMS AND (Er,Ge) CO-DOPED SiO2 FILMS

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