# Light Emission from Rare-Earth Doped Silicon Nanostructures

Authors: J. Li; O. H. Y.  Zalloum; T. Roschuk; C. L. Heng; J. Wojcik; P. Mascher
Journal: Advances in Optical Technologies
 (2008)

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## Abstract

Rare earth (Tb or Ce)-doped silicon oxides were deposited by electron cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD). Silicon nanocrystals (Si-ncs) were formed in the silicon-rich films during certain annealing processes. Photoluminescence (PL) properties of the films were found to be highly dependent on the deposition parameters and annealing conditions. We propose that the presence of a novel sensitizer in the Tb-doped oxygen-rich films is responsible for the indirect excitation of the Tb emission, while in the Tb-doped silicon-rich films the Tb emission is excited by the Si-ncs through an exciton-mediated energy transfer. In the Ce-doped oxygen-rich films, an abrupt increase of the Ce emission intensity was observed after annealing at1200∘C.  This effect is tentatively attributed to the formation of Ce silicate. In the Ce-doped silicon-rich films, the Ce emission was absent at annealing temperatures lower than 1100∘C due to the strong absorption of Si-ncs. Optimal film compositions and annealing conditions for maximizing the PL intensities of the rare earths in the films have been determined. The light emissions from these films were very bright and can be easily observed even under room lighting conditions.


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## Body

## 1. Introduction

The realization  of integrated silicon photonics
requires the development of several fundamental components including light
sources, modulators, amplifiers, and detectors [1, 2]. To achieve silicon-based
light sources has always been the most challenging task, with the first optically
pumped silicon Raman laser being reported only very recently [3]. It is well known that bulk
silicon is a poor light emitter due to its indirect bandgap and the presence of
nonradiative recombination pathways [4]. Many approaches have been attempted to
overcome these obstacles and achieve efficient emission from silicon. One of
the most promising solutions is to introduce impurities such as rare-earth
elements into materials. Of
the rare-earth elements, Er has attracted the most extensive attention because
of the coincidence between its intra-4f transition at 1535 nm and the transparency window used for telecommunications [5].
However, the demand to realize full-color light emission from silicon
structures can extend technology interests to many other rare-earth elements
such as Tb and Ce which usually emit green and ultraviolet/blue light,
respectively [6].Luminescence
from rare-earths has been studied extensively since it was discovered at the
beginning of the 20th century [7]. Today various rare-earth doped materials are
playing important roles in many areas such as displays, solid-state lasers,
detectors, and data storage [8–12]. Recently, there have arisen
enormous demands to develop optical sources and amplifiers compatible with
silicon ULSI technology leading to a large increase in the interest for one
specific type of material: rare-earth doped silicon-based materials [6].Rare-earth
elements make up the sixth row of the periodic table and have a partially
filled 4f shell.
Optically active rare-earth ions often exist as trivalent and are formed by
losing one 4f electron
and both 6s electrons. Luminescence from these ions is mainly attributed to
their intra-4f or 5d-4f transitions. Intra-4f transitions are relatively independent of
the host material, since the 4f states are shielded from outside interaction by 5d states. Note that intra-4f transitions for free ions are parity
forbidden but are partially allowed through the mixing of opposite parity wave
functions when rare-earth ions are embedded in a host material. As a result,
they have very long luminescence lifetimes and weak oscillator strengths. In
contrast, 5d-4f transitions are very sensitive to the surrounding ligands because 5d states are
directly exposed to the local environment. Thus, changes in the spectral
shapes, peak positions, and intensities may suggest the evolution of the matrix
structure. Additionally,
since 5d-4f transitions are parity permitted, they have very short luminescence lifetimes
and strong oscillator strengths [13].There are two major obstacles that have to be overcome
in order to achieve efficient emission from rare-earth doped silicon oxides:
inefficient excitation of intra-4f transitions from rare-earth ions and low solubility of optically active
rare-earth ions in silicon-based hosts.Emissions from rare-earth ions often originate from
the intra-4f transitions which are
only partially parity allowed. As a result, many rare-earth ions suffer from
constraints on excitation wavelengths and their low absorption cross-sections
of pump photons. For example, Er ions embedded in silica can only be directly
excited at several specific wavelengths including 488, 514, 800, 980, and 1480 nm and the absorption cross-section is as low as10−21 to 10−20 cm2 [5]. Various
sensitizers such as Yb ions, silicon nanocrystals (Si-ncs), silver, and organic
complexes have been found to be able to enhance rare-earth emission
significantly through a dipole-dipole
Forster-Dexter coupling
process [14].One structure
receiving particular attention is Er-doped silicon-rich silicon oxide (SRSO). The
Si-ncs precipitated upon annealing can be considered as three-dimensionally
confined structures. The “bandgap” (energy difference between the highest
occupied molecularorbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO)) of
these structures increases with a decrease in size due to quantum confinement
effects [15]. It was demonstrated that Er emission can be enhanced significantly
through exciton-mediated energy transfer between Si-ncs and Er [16].Great efforts
have been made to understand the physics of the Er:Si-ncs system. The
sensitization mechanism is now relatively well understood at the
phenomenological level. After a Si-nc absorbs a photon, a bound exciton is
generated in the Si-nc. The exciton may recombine nonradiatively by
transferring energy to a nearby Er ion, putting the ion into an excited state.
The Er ion then  decays to the ground stateby emitting a photon of
1.54μm [14]. According to the generalized rate-equation analysis by Kenyon et
al. [17], this sensitization process is strongly dependent on the distance
between the Si-nc and Er ion. The Er luminescence is thus limited by the low
number of Er ions coupled to the Si-ncs, which is due to the low Si-ncs density
and the short interaction distance.Since Si-ncs
have a high absorption cross-section and efficiently transfer energy to Er
ions, the excitation cross-section of Er ions can be increased by up to104 times [16]. Furthermore, the absorption band of Si-ncs covers the whole
visible spectrum and the utilization of broad band excitation sources becomes
possible. The successful excitation of Er in SRSO using a broad band visible
excitation source, such as a commercial camera flashgun or blue LED has been
demonstrated [18, 19].In principle,
an analogous excitation mechanism can operate in SRSO films doped with other
rare-earths as long as the “bandgap” of Si-ncs wide enough to ensure energy
transfer to the rare-earth ions [20]. In the present work, we demonstrate that by
using a 325 nm He-Cd laser the Tb emission can be excited efficiently in both
SRSO and oxygen-rich silicon oxides (ORSO) through different excitation
processes.The solubility of optically active rare-earth ions in
various silicon materials is universally low due to the mismatch of ionic radii
and the strong covalent bonding of the matrix network. Above the critical
concentrations, rare-earth ions tend to form precipitates, which results in
severe luminescence quenching through ion-ion interaction or by forming an
optically inactive phase [6]. It is very difficult to incorporate high
concentrations of optically active rare-earths in silicon materials through
equilibrium techniques such as the sol-gel method [21]. Low temperature
techniques, such as ion implantation or plasma enhanced chemical vapor
deposition (PECVD), are able to increase the solubility limit by an order of
magnitude, an effect attributed to the lower diffusivity of the dopant ions.
Particularly, electron cyclotron resonance (ECR)-PECVD can generate a uniform
distribution of rare-earth doping throughout the entire film thickness with low
defect concentrations due to the low ion energy [22].Over the past few years, we have been
conducting extensive studies on achieving highly efficient light emission from
rare-earth doped silicon nanostructures formed in thin films prepared by ECR-PECVD.
It was found that Er emission properties are highly dependent on the details of
Si-ncs and Er concentrations. The Er emission intensities can be enhanced
greatly by optimizing deposition parameters and postdeposition thermal
annealing conditions [23, 24].In this paper, we report on studies of SRSO
and ORSO doped with Tb or Ce. We show successful in situ incorporation of high concentrations of rare-earth
elements. The dependence of the emission properties on rare-earth dopant
concentration, silicon/oxygen ratio, and annealing temperature was
investigated. The emission intensities from the rare-earths were optimized. An
interpretation of the excitation mechanisms and the correlation between optical
properties and structural evolution is suggested.


## 2. Experimental Details

Tb or Ce
doped silicon oxides were prepared in a custom-designed ECR-PECVD system
(Figure1). Ar and O2 plasma gases were introduced from the upper dispersion
ring. Silane (SiH4) gas diluted in Ar was used as the Si source and volatile
metal-organic precursors served as the rare-earth sources. Tb(tmhd)3 or tris
(2,2,6,6-teramethyl-3,5-heptanedionato)-Tb(III), and Ce(tmhd)3 or tris (2,2,6,6-teramethyl-3,5-heptanedionato)-Ce(III)) were used as
the Tb and Ce sources, respectively. The rare-earth sources were contained in a
manifold which can be heated up to 200°C to sublime the precursor inside. Ar carrier gas then
transports the rare-earth precursor through a transmission line after which it
is introduced in the chamber alongside the SiH4 through the lower
dispersion ring. This multidopant delivery system allows the incorporation of
up to three types of rare-earth elements simultaneously. The temperatures of three rare-earth
manifold containers, the rare-earth dispersion ring, and the transmission lines
are independently controlled by five Eurotherm temperature controllers. All of the
gas flow rates can be controlled independently using a central mass flow
controller panel. The base pressure of the system was 1×10−7 Torr
and the deposition pressure was between 2 and 5 mTorr.Figure 1
ECR-PECVD system.Rare-earth doped silicon oxide films were
deposited on n-type (100) silicon wafers which sit on the substrate stage
located in the reaction chamber. In the present study, the stage temperature
was 350°C, but it
could be varied from room temperature to 800°C. The film thickness was monitored during the
deposition by an in situ ellipsometer
operating at 632.8 nm. Several deposition parameters including rare-earth cell
temperature, SiH4 and O2 gas flows, and microwave power were varied to achieve different
Si, O, and rare-earth content. A summary of the deposition parameters and their
values in this study are given in Table 1. Two types of mass flow controller
were used for different samples, therefore, there are two types of flow rate
units employed as indicated in Table 1. Each sample contains only one type of rare-earth
dopant. The thicknesses of all the samples were remeasured after the deposition
through ellipsometry and vary from 800–1100 Å. All PL intensities discussed in
this report were normalized to 1000 Å, assuming that PL intensity increases
linearly with thickness.Table 1
Summary of the
deposition parameters and their values for the depositions in this study.
Dopant
typeRE
concentration [at.%]Si concentration
[at.%]Forwarded power
[W]Reflected power
[W]SiH4 flow rateO2 flow rateAr flow rateRE cell temp [°C]Tb0.4(2)3242010724201600.8(2)3242010724301600.1(2)3660872060251600.2(2)38327272056251560.3(2)365005205625153Ce0.01(2)3351051178121000.1(3)325077440251801.0(3)325097230102000.04(2)405155257012161(2)The sample was deposited using mass flow controller 1, the corresponding flow
rate unit is mV.(3) The sample was deposited using mass flow controller 2, the corresponding flow
rate unit is sccm.The compositions of the samples were determined
quantitatively through Rutherford backscattering spectrometry (RBS) using a 1.0 MeV
He+ beam
confirming the successful incorporation of rare-earth elements into the thin
films. Figure 2 shows the RBS spectrum of an as-deposited Ce-doped sample
containing 32 at.% Si and 1.0 at.% Ce. The energies related to the surface Ce,
Ar, Si, and O atoms are as indicated. The signal from O sits on the signal from
Si in the substrate and depresses the signal from Si in the film. Si, O, and Ce
all exhibit a uniform concentration distribution throughout the film thickness.
Trace amounts of Ar are also observed in the spectrum. The majority of Ar can
be removed from the film during annealing. In the present study, the Si content
varies from 32 to 40 at.%, Tb 0.1 to 0.8 at.%, and Ce 0.01 to 1.0 at.%.Figure 2
RBS measurement of a Ce:ORSO
sample containing 32 at.% Si and 1.0 at.% Ce. The surface energies of Ce, Ar, Si,
and O are indicated by arrows.All the films were annealed in a quartz tube furnace under
flowing N2 for 1 hour. The annealing temperature varied from 800 to 1200°C. The
luminescence properties of the films were analyzed by single wavelength
laser-excited PL spectroscopy. The excitation source was a 17 mW He-Cd laser
operating at 325 nm. The PL from the films was detected by a spectrometer
employing a charge-coupled device array; these PL spectra were corrected for
system response and optics transmission and subsequently converted to
normalized photon flux. A full description of the
CCD-based PL system and the data correction methodology is given elsewhere [25].
All the measurements were performed at room temperature. The formation
of Si-ncs in the Tb doped silicon-rich film was studied by a JEOL 2010F Field Emission Gun Transmission
Electron Microscope operating at 200 keV.


## 3. Results and Discussion

### 3.1. Tb Doped Oxygen-Rich Silicon Oxides (Tb:ORSO)

The
Tb:ORSO samples have a constant Si content of 32 at.% and varying Tb content of
0.4 and 0.8 at.%, respectively. Figure3 shows the PL spectra of the 0.4 at.% Tb sample as-deposited and annealed at
various temperatures. Four strong distinct sharp bands were observed sitting on
a weak broad band for all annealing temperatures. According to the energy level
diagram of Tb3+ ions shown in Figure 4 [adapted from [26]], the four
bands centered at 487, 546, 588, and 620 nm are related to D45−Fj7⁢(j=6,5,4,3) intra-4f transitions of
Tb3+. The 546 nm band is split into two peaks,
centered at 542 and 548 nm, due to the Stark splitting of the matrix. The emissions
from the 5D3 state are absent due to the cross-relaxation
effect [27]. The observed emission spectra resemble those of Tb doped SiO2 fabricated by conventional PECVD [28]. The peak
around 546 nm has been
confirmed to originate from Tb3+ using PL decay measurements [29]. The
corresponding lifetime was around 1.3 ms. There are no appreciable changes in
the peak positions and spectral widths with annealing temperature.Figure 3
PL
spectra of the Tb:ORSO samples containing 0.4 at.% Tb, as deposited and
annealed at different temperatures in the range of 800–1200°C.Figure 4
The
electronic energy levels of Tb3+ [adapted
from [26]].The
weak broadband emission ranging from 400 to 700 nm is usually attributed to
oxide defects in the matrix [30]. If that were indeed the case, the
corresponding emission should decrease after annealing at 800°C, since the
defects in SiO2 tend to reduce during annealing process. However, in
this study the emission became stronger after annealing at 800°C. In
addition, this unidentified emission and the Tb emission have a similar
dependence on annealing temperature. Therefore, we suggest that the origin is
related to the optically active Tb3+ sites.It
is important to note that the 325 nm (3.8 eV) excitation is not resonant with
any optical absorption band of Tb3+, so the observation of the strong
Tb emission from the film implies the presence of a highly efficient indirect
excitation process. In general, there are two types of indirect excitation
mechanisms: carrier-mediated excitation and dipole-dipole Forster-Dexter
coupling [6]. A previous investigation on Tb-doped silicon oxynitrides by Jeong
etal. has suggested that Tb3+ ions can be excited efficiently by a 325 nm excitation source through carrier
recombination processes [31]. The 325 nm source is able to excite carriers into extended above-band-gap states, because the bandgap
of silicon oxynitride is only between 3.5 to 4 eV for the samples they
considered [32]. However, for the films considered here, this process is less possible,
because the bandgap of SiO2 is as large as 8 eV. In oxygen-rich
films, it is expected that no or very few Si-ncs exist. Based on this
assumption, the presence of alternate sensitizers has to be considered.One
possible sensitizer  is the organic molecule introduced during the deposition
from the metal organic precursor Tb(tmhd)3. It was found that some rare-earth
chelates have broad absorption bands and high absorption coefficients and can
serve as sensitizers to efficiently excite rare-earth ions. Many rare-earth
chelates have been synthesised to exploit the effect [14].
To  the best of our
knowledge, however, there have been no reports on the sensitizing effect of the
Tb(tmhd)3 molecules employed in the present study. Further studies on the absorption
properties of the sensitizer clearly are necessary.Figure5 shows the 546 nm peak intensities of various
samples as a function of annealing temperature between 800 and 1200°C. The
intensities generally decrease with an increase of the annealing temperature. In
comparison to the as-deposited sample, the significant enhancement of the Tb emission
after annealing at 800°C can be attributed to the reduction of defects and the activation of
optically active Tb3+ sites. The decrease of the Tb emission at
annealing temperatures higher than 800°C can be
explained by the reduction of both optically active Tb sites and available
sensitizers. X-ray absorption fine structure (XAFS) analysis has confirmed that
there are two types of Tb sites existing in Tb-doped SiO2:Tb
coordinated with two oxygen atoms (Tb-2O) or six oxygen atoms (Tb-6O), with the
formation of the latter being related to the enhancement of Tb emission [33].
At annealing temperatures higher than 800°C, Tb ions achieve enough energy to diffuse and
form clusters. So the number of oxygen atoms associated with each Tb ion is
reduced, which may lead to the transformation from Tb-6O to Tb-2O and results
in the decrease of Tb emission. Another possibility that cannot be excluded is
that the organic molecules become more unstable and begin to dissociate at
higher annealing temperatures. As a result, the available sensitizers may reduce
significantly and also lead to the decrease of Tb emission.Figure 5
PL
intensity of Tb3+ at 546 nm from two Tb:ORSO samples as a function
of annealing temperature. The inset shows an image of strong green Tb emission
from the 0.4 at.% Tb sample after annealing at 800°C taken by
camera.Since a previous study on Tb doped SiO2 showed
that the lifetime of Tb emission still remains almost constant at concentration
as high as 2.7 at.% [29], our observation of the decrease of the emission intensity
at high Tb concentration might not be due to ion-ion interaction and is possibly
attributable to the trend to form clusters which leads to the reduction of
optically active Tb-6O sites or the exhaustion of available sensitizers.In this study, the strongest Tb emission was observed from
the 0.4 at.% Tb sample annealed at 800°C. The inset of
Figure 5 shows the corresponding
picture taken with a camera. The emission was quite strong and can be easily
observed under bright room lighting conditions.


### 3.2. Tb-Doped Silicon-Rich Silicon Oxide (Tb:SRSO)

The
Tb:SRSO samples have a constant Si content of 36 at.% and varying Tb content
of 0.1, 0.2, and 0.3 at.%, respectively. Figure6 shows the PL spectra of the
0.3 at.% Tb sample as-deposited and annealed at various temperatures. A broad
emission band centered at 650 nm was observed from the as-deposited sample.
Annealing at temperatures between 800 and 1100°C results in
an increase in its intensity with a red shift of the peak position. This is
characteristic of Si-ncs emission. The enhancement of the emission intensity
indicates the formation of more Si-ncs, while the red shift of the peak
suggests the growth of the Si-ncs sizes.Figure 6
PL spectra of the Tb:SRSO samples containing
0.3 at.% Tb, as-deposited or annealed at various temperatures in the
range of 800–1200°C.A
weak emission band peaking at 546 nm emerged after annealing at 1100°C. After
annealing at 1200°C, the Si-ncs
emission decreases abruptly. At the same time, the four sharp peaks centered at
487, 546, 588, and 620 nm become dominant. Both the weak 546 nm peak observed
after annealing at 1100°C and the four strong sharp peaks observed after annealing at 1200°C originate
from intra-4f transitions of Tb3+ ions.As
mentioned in the previous section, the 325 nm excitation is not resonant with
any absorption band of Tb3+ ions and the observation of Tb emission
indicates the existence of (an) indirect excitation mechanism(s).  The
simultaneous quenching of the Si-ncs emission and the increase of the Tb emission
intensity after annealing at 1200°C suggests the efficient energy transfer from Si-ncs to
Tb3+ ions. Here, we attribute the observed Tb emission to the
exciton energy transfer process. This mechanism may appear questionable, since
the PL of Si-ncs after annealing at 1200°C is peaking around 800 nm (1.55 eV), while at
least 2.5 eV (488 nm) energy is required to excite Tb3+. However, it
has been suggested that when the sizes of Si-ncs reduce to less than 3 nm, the
infrared PL of Si-ncs is due to the recombination of the excitons trapped in
oxygen-related localized states. Therefore, the actual “bandgap” of an Si-nc might be much
higher than that indicated by its emission wavelength [15].
Figure 7 shows a high-resolution transmission
electron microscopy (HR-TEM) image of a Tb:SRSO sample containing 0.3 at.% Tb
annealed at 1100°C. The clear lattice fringes shown in the circle
areas reveal the formation of Si-ncs. The sizes of
Si-ncs are around 2 nm. The “bandgap” of a 2 nm Si-nc can be as high as 2.5 eV
as a result of quantum confinement effects [15], which is sufficient for the
excitation of Tb3+.Figure 7
HR-TEM image of a Tb:SRSO sample containing
0.3 at.% Tb annealed at 1100°C.Although
the organic sensitizer-Tb (tmhd)3 may also be present in the silicon-rich
samples, the absence of the Tb emission from the samples as-deposited and after
annealing between 800 and 1200°C indicates that the corresponding excitation is not
very efficient. One possible explanation is that the Si-ncs are dominant in the
absorption of excitation photons due to their relatively high absorption
coefficient [6]. In particular, the organic sensitizers are less likely to be
responsible for the strong Tb emission after annealing at 1200°C, since the
Tb emission intensity of the silicon-rich sample containing 0.3 at.% Tb is 50
times higher than that of the oxygen-rich sample containing 0.4 at.% Tb after
annealing at the same temperature.Figure8 shows the emission intensities of Si-ncs in the samples with various Tb
concentrations as a function of annealing temperature. The Si-ncs emission was
observed to decrease with the increase of Tb concentration at annealing temperatures greater
than 900°C. Although Tb
emission was only appreciable from the samples annealed at 1100 and
1200°C, this
decrease is indicative that the energy transfer between Si-ncs and Tb3+ ions
is also occurring at 1000°C,
thereby suppressing the Si-ncs PL.
The inset of Figure 8 shows that after annealing at 1200°C, the
intensity of the Tb3+  546 nm emission increases with the Tb
concentration, which can be explained by an increase of optically active Tb3+ sites.Figure 8
PL
peak intensities of Si-ncs from the Tb:SRSO samples with various Tb
concentrations as a function of annealing temperature. The inset shows the
PL intensity of Tb3+ at 546 nm from Tb: SRSO samples after annealing
at 1200°C as a
function of Tb concentration.


### 3.3. Ce-Doped Oxygen-Rich Silicon Oxides (Ce:ORSO)

The
Ce:ORSO samples have a constant Si content of 32 at.% and varying Ce contents of 0.01,
0.1, and 1.0 at.%, respectively. Figure9 shows the PL spectra of the 1.0 at.% Ce sample as-deposited or after annealing
at 800 and 1200°C, respectively.
All spectra show an intense broad peak with a pronounced shoulder at longer
wavelengths. The main peaks were observed at 400, 422, and 460 nm from the
samples as-deposited or annealed at 800 and 1200°C,
respectively. The other two samples containing 0.01 and 0.1 at.% Ce have
similar spectral shapes and peak positions.Figure 9
PL spectra of a Ce:ORSO sample
containing 1.0 at.% Ce, as-deposited or after annealing at 800 and 1200°C, respectively.Cerium
can exist in both trivalent and tetravalent forms when embedded in the host
materials by losing two 6s electrons and either one or two 4f electrons. Ce4+ is optically
inactive due to the lack of 4f electrons. Although the 4f states are shielded by 5s and 5p states, the 5d states into which the 4f electron will be excited are exposed to
the surrounding ligands. As previously discussed, the PL properties of Ce ions
are highly dependent on the surrounding environment. Therefore, the changes in the
spectral shapes, peak positions, and intensities strongly suggest the evolution
of the film structure [13].Unlike
Tb, Ce3+ usually has a broad absorption band around 330 nm. The
observed emissions originate from 5d-4f transitions of the Ce3+ in the films. Similar PL
features have been observed
from Ce doped SiO2 prepared by the sol-gel method [34]. As the films
were deposited under oxygen-rich conditions, most of the Ce ions exist as quadrivalent
and possibly in the form of CeO2. Since the Ce4+ ions are
the dominant species in the as-deposited samples and only few optically active
Ce3+ ions exist, the corresponding emission intensity is low.Figure10 shows the integrated normalized PL intensity in the range of 350–700 nm for the
above three Ce:ORSO samples as a function of annealing temperature. At all of annealing
temperatures—except for 700 and 1000°C—the PL intensity increases with increasing Ce
concentration. And a similar dependence of the PL intensities on annealing
temperatures was observed despite of the Ce concentrations. The PL intensities
increase with annealing temperature up to 800°C and then decrease
with annealing temperature up to 1000°C. At 1100°C, the intensities begin to increase again and reach the
greatest value at 1200°C.Figure 10
Integrated PL intensities in the range of 350–700 nm from the Ce:ORSO
samples with various Ce concentrations as a function of annealing temperature.
The inset shows an image of strong blue emission from the 1.0 at.% Ce sample
after annealing at 1200°C taken by camera.When
annealed in an inert ambient at 700 and 800°C, the increase
in emission intensities can be attributed to the increasing Ce3+/Ce4+ ratio. Previous studies on CeO2 have shown that at high temperatures,
crystallized stoichiometric CeO2 tends to form Ce6O11 in order to lower the O/Ce ratio through the migration of oxygen
vacancies [35]. It appears that under these conditions, Ce3+ ions
are more stable than Ce4+ ions. Furthermore, due to the
concentration difference between the film and surrounding environment, some
oxygen may be released from the film during the annealing process, which
possibly leads to a local oxygen deficiency facilitating the deoxidization of
Ce4+ ions.It
was reported that Ce3+ can form two types of optical centers
depending on whether their surrounding environment was composed mainly of Ce4+ or Ce3+ ions. The former were observed to exhibit a shorter
emission wavelength than the latter [36]. Our results are consistent with this
finding, with a red shift of emission wavelength being observed for samples
after annealing at 800°C
or greater.The decrease in
emission intensity between anneals from 800 to 1000°C can be
attributed to the clustering of Ce3+ ions leading to a quenching of
the luminescence as this quenching is more profound in the samples with greater
Ce concentrations. At these temperatures, the SiOx matrix begins to
form a more uniform and stable structure. Correspondingly, the number of
nonbridging oxygen (NBO) defects is reduced significantly. As the temperature
further increases, more Ce3+ ions form clusters to
share the limited amount of NBO, and emissions from the Ce3+ ions
are quenched by transferring their energy to other Ce3+ ions in the
vicinity [37].Several groups have observed similar UV/blue light emission from CeOx/SiOx or CeOx/Si films annealed at 1000°C or greater in
inert or reducing ambients [35, 38, 39]. However, the origin of the emission
still remains controversial. Morshed et
al. [35] observed  the emission from CeO2/Si films after rapid
thermal annealing in Ar at 1000°C peaking at 400 nm. This emission was attributed to the
formation of Ce6O11 around the CeO2/Si
interface. Choi etal.  [38]
observed a weak 388 nm emission from a CeOx/Si film and an intense
358 nm emission from (CeOx + Si)/Si and (CeOx + Si)/SiOx/Si
films after annealing in N2 at 1100°C. Significant
diffusion of Si from the substrate into CeO2 and the formation of
two cerium silicate phases, Ce2Si2O7 and Ce4.667(SiO4)3O, at the interface were observed. The 358 nm
emission was attributed to Ce2Si2O7 phase, and
the weak 388 nm emission to the Ce4.667(SiO4)3O
phase. Kępiński etal. [39] observed emission peaking at 400 nm
from CeO2/SiO2 samples subjected to anneal in H2 at 1050 and 1100°C and assigned
it to an unidentified cerium silicate with some structural similarity to
tetragonal Ce2Si2O7 silicate. Therefore, in our study, the abrupt increase of the
emission intensity at annealing temperatures greater than 1100°C may also suggest the formation of Ce
silicate. At this high temperature, Ce3+ ions may acquire enough energy to interact with the surrounding Si and
O atoms and reorganize to form a more stable structure.A
detailed study of the effects of high-temperature annealing on the structural
evolution of these samples is in progress. Preliminary results from Fourier transform
infrared spectroscopy show the emergence of new sharp absorption bands after the
sample was annealed at 1200°C, similar to the peaks characteristic of Ce2Si2O7 silicate, while there are still
differences in both numbers and positions of absorption bands between them (data not shown).The
greatest intensity was observed from the 1.0 at.% Ce sample subjected to a
1200°C anneal which has the highest Ce concentration in the samples we studied. The inset of
Figure 10 shows the emission picture taken by a camera. The Ce emission from this
sample was 230 times greater than that of the as-deposited sample. In fact,
after annealing at 800 or 1200°C, both the 0.1% Ce and 1.0% Ce samples exhibit such strong
emissions that they can be easily observed under very bright room lighting
condition. Considering that the excitation power is as low as 17 mW, the
emission intensities are quite outstanding.


### 3.4. Ce-Doped Silicon-Rich Silicon Oxide (Ce:SRSO)

The
Ce:SRSO sample has Si and Ce content of 40 and 0.04 at.%, respectively.
Figure11
shows the PL spectra of the sample as deposited or annealed at various
temperatures. There is no appreciable emission from the as-deposited sample.
After annealing at 800°C,
the sample shows a broad emission band centered at 800 nm, the emission
intensity of which increases with annealing temperature until 1100°C. After annealed
at 1200°C, the
emission decreases by a factor of 8 and another weak broad emission band
centered at 450 nm emerges.Figure 11
PL spectra of a Ce:SRSO sample
containing 0.04 at.% Ce, as-deposited or annealed at various temperatures in
the range of 800–1200°C.It
is evident that the 800 nm emission originates from the precipitated Si-ncs
upon annealing. We attribute the 450 nm emission to the Ce3+ ions in
the film. The enhancement of the Si-ncs emission intensity with annealing
temperature indicates the formation of more Si-ncs in the same manner as was observed
in Tb:SRSO samples. The absence of Ce emission at annealing temperatures lower
than 1200°C is also
possibly due to the competition between Si-ncs and Ce ions in the absorption of
excitation photons.


## 3.1. Tb Doped Oxygen-Rich Silicon Oxides (Tb:ORSO)

The
Tb:ORSO samples have a constant Si content of 32 at.% and varying Tb content of
0.4 and 0.8 at.%, respectively. Figure3 shows the PL spectra of the 0.4 at.% Tb sample as-deposited and annealed at
various temperatures. Four strong distinct sharp bands were observed sitting on
a weak broad band for all annealing temperatures. According to the energy level
diagram of Tb3+ ions shown in Figure 4 [adapted from [26]], the four
bands centered at 487, 546, 588, and 620 nm are related to D45−Fj7⁢(j=6,5,4,3) intra-4f transitions of
Tb3+. The 546 nm band is split into two peaks,
centered at 542 and 548 nm, due to the Stark splitting of the matrix. The emissions
from the 5D3 state are absent due to the cross-relaxation
effect [27]. The observed emission spectra resemble those of Tb doped SiO2 fabricated by conventional PECVD [28]. The peak
around 546 nm has been
confirmed to originate from Tb3+ using PL decay measurements [29]. The
corresponding lifetime was around 1.3 ms. There are no appreciable changes in
the peak positions and spectral widths with annealing temperature.Figure 3
PL
spectra of the Tb:ORSO samples containing 0.4 at.% Tb, as deposited and
annealed at different temperatures in the range of 800–1200°C.Figure 4
The
electronic energy levels of Tb3+ [adapted
from [26]].The
weak broadband emission ranging from 400 to 700 nm is usually attributed to
oxide defects in the matrix [30]. If that were indeed the case, the
corresponding emission should decrease after annealing at 800°C, since the
defects in SiO2 tend to reduce during annealing process. However, in
this study the emission became stronger after annealing at 800°C. In
addition, this unidentified emission and the Tb emission have a similar
dependence on annealing temperature. Therefore, we suggest that the origin is
related to the optically active Tb3+ sites.It
is important to note that the 325 nm (3.8 eV) excitation is not resonant with
any optical absorption band of Tb3+, so the observation of the strong
Tb emission from the film implies the presence of a highly efficient indirect
excitation process. In general, there are two types of indirect excitation
mechanisms: carrier-mediated excitation and dipole-dipole Forster-Dexter
coupling [6]. A previous investigation on Tb-doped silicon oxynitrides by Jeong
etal. has suggested that Tb3+ ions can be excited efficiently by a 325 nm excitation source through carrier
recombination processes [31]. The 325 nm source is able to excite carriers into extended above-band-gap states, because the bandgap
of silicon oxynitride is only between 3.5 to 4 eV for the samples they
considered [32]. However, for the films considered here, this process is less possible,
because the bandgap of SiO2 is as large as 8 eV. In oxygen-rich
films, it is expected that no or very few Si-ncs exist. Based on this
assumption, the presence of alternate sensitizers has to be considered.One
possible sensitizer  is the organic molecule introduced during the deposition
from the metal organic precursor Tb(tmhd)3. It was found that some rare-earth
chelates have broad absorption bands and high absorption coefficients and can
serve as sensitizers to efficiently excite rare-earth ions. Many rare-earth
chelates have been synthesised to exploit the effect [14].
To  the best of our
knowledge, however, there have been no reports on the sensitizing effect of the
Tb(tmhd)3 molecules employed in the present study. Further studies on the absorption
properties of the sensitizer clearly are necessary.Figure5 shows the 546 nm peak intensities of various
samples as a function of annealing temperature between 800 and 1200°C. The
intensities generally decrease with an increase of the annealing temperature. In
comparison to the as-deposited sample, the significant enhancement of the Tb emission
after annealing at 800°C can be attributed to the reduction of defects and the activation of
optically active Tb3+ sites. The decrease of the Tb emission at
annealing temperatures higher than 800°C can be
explained by the reduction of both optically active Tb sites and available
sensitizers. X-ray absorption fine structure (XAFS) analysis has confirmed that
there are two types of Tb sites existing in Tb-doped SiO2:Tb
coordinated with two oxygen atoms (Tb-2O) or six oxygen atoms (Tb-6O), with the
formation of the latter being related to the enhancement of Tb emission [33].
At annealing temperatures higher than 800°C, Tb ions achieve enough energy to diffuse and
form clusters. So the number of oxygen atoms associated with each Tb ion is
reduced, which may lead to the transformation from Tb-6O to Tb-2O and results
in the decrease of Tb emission. Another possibility that cannot be excluded is
that the organic molecules become more unstable and begin to dissociate at
higher annealing temperatures. As a result, the available sensitizers may reduce
significantly and also lead to the decrease of Tb emission.Figure 5
PL
intensity of Tb3+ at 546 nm from two Tb:ORSO samples as a function
of annealing temperature. The inset shows an image of strong green Tb emission
from the 0.4 at.% Tb sample after annealing at 800°C taken by
camera.Since a previous study on Tb doped SiO2 showed
that the lifetime of Tb emission still remains almost constant at concentration
as high as 2.7 at.% [29], our observation of the decrease of the emission intensity
at high Tb concentration might not be due to ion-ion interaction and is possibly
attributable to the trend to form clusters which leads to the reduction of
optically active Tb-6O sites or the exhaustion of available sensitizers.In this study, the strongest Tb emission was observed from
the 0.4 at.% Tb sample annealed at 800°C. The inset of
Figure 5 shows the corresponding
picture taken with a camera. The emission was quite strong and can be easily
observed under bright room lighting conditions.


## 3.2. Tb-Doped Silicon-Rich Silicon Oxide (Tb:SRSO)

The
Tb:SRSO samples have a constant Si content of 36 at.% and varying Tb content
of 0.1, 0.2, and 0.3 at.%, respectively. Figure6 shows the PL spectra of the
0.3 at.% Tb sample as-deposited and annealed at various temperatures. A broad
emission band centered at 650 nm was observed from the as-deposited sample.
Annealing at temperatures between 800 and 1100°C results in
an increase in its intensity with a red shift of the peak position. This is
characteristic of Si-ncs emission. The enhancement of the emission intensity
indicates the formation of more Si-ncs, while the red shift of the peak
suggests the growth of the Si-ncs sizes.Figure 6
PL spectra of the Tb:SRSO samples containing
0.3 at.% Tb, as-deposited or annealed at various temperatures in the
range of 800–1200°C.A
weak emission band peaking at 546 nm emerged after annealing at 1100°C. After
annealing at 1200°C, the Si-ncs
emission decreases abruptly. At the same time, the four sharp peaks centered at
487, 546, 588, and 620 nm become dominant. Both the weak 546 nm peak observed
after annealing at 1100°C and the four strong sharp peaks observed after annealing at 1200°C originate
from intra-4f transitions of Tb3+ ions.As
mentioned in the previous section, the 325 nm excitation is not resonant with
any absorption band of Tb3+ ions and the observation of Tb emission
indicates the existence of (an) indirect excitation mechanism(s).  The
simultaneous quenching of the Si-ncs emission and the increase of the Tb emission
intensity after annealing at 1200°C suggests the efficient energy transfer from Si-ncs to
Tb3+ ions. Here, we attribute the observed Tb emission to the
exciton energy transfer process. This mechanism may appear questionable, since
the PL of Si-ncs after annealing at 1200°C is peaking around 800 nm (1.55 eV), while at
least 2.5 eV (488 nm) energy is required to excite Tb3+. However, it
has been suggested that when the sizes of Si-ncs reduce to less than 3 nm, the
infrared PL of Si-ncs is due to the recombination of the excitons trapped in
oxygen-related localized states. Therefore, the actual “bandgap” of an Si-nc might be much
higher than that indicated by its emission wavelength [15].
Figure 7 shows a high-resolution transmission
electron microscopy (HR-TEM) image of a Tb:SRSO sample containing 0.3 at.% Tb
annealed at 1100°C. The clear lattice fringes shown in the circle
areas reveal the formation of Si-ncs. The sizes of
Si-ncs are around 2 nm. The “bandgap” of a 2 nm Si-nc can be as high as 2.5 eV
as a result of quantum confinement effects [15], which is sufficient for the
excitation of Tb3+.Figure 7
HR-TEM image of a Tb:SRSO sample containing
0.3 at.% Tb annealed at 1100°C.Although
the organic sensitizer-Tb (tmhd)3 may also be present in the silicon-rich
samples, the absence of the Tb emission from the samples as-deposited and after
annealing between 800 and 1200°C indicates that the corresponding excitation is not
very efficient. One possible explanation is that the Si-ncs are dominant in the
absorption of excitation photons due to their relatively high absorption
coefficient [6]. In particular, the organic sensitizers are less likely to be
responsible for the strong Tb emission after annealing at 1200°C, since the
Tb emission intensity of the silicon-rich sample containing 0.3 at.% Tb is 50
times higher than that of the oxygen-rich sample containing 0.4 at.% Tb after
annealing at the same temperature.Figure8 shows the emission intensities of Si-ncs in the samples with various Tb
concentrations as a function of annealing temperature. The Si-ncs emission was
observed to decrease with the increase of Tb concentration at annealing temperatures greater
than 900°C. Although Tb
emission was only appreciable from the samples annealed at 1100 and
1200°C, this
decrease is indicative that the energy transfer between Si-ncs and Tb3+ ions
is also occurring at 1000°C,
thereby suppressing the Si-ncs PL.
The inset of Figure 8 shows that after annealing at 1200°C, the
intensity of the Tb3+  546 nm emission increases with the Tb
concentration, which can be explained by an increase of optically active Tb3+ sites.Figure 8
PL
peak intensities of Si-ncs from the Tb:SRSO samples with various Tb
concentrations as a function of annealing temperature. The inset shows the
PL intensity of Tb3+ at 546 nm from Tb: SRSO samples after annealing
at 1200°C as a
function of Tb concentration.


## 3.3. Ce-Doped Oxygen-Rich Silicon Oxides (Ce:ORSO)

The
Ce:ORSO samples have a constant Si content of 32 at.% and varying Ce contents of 0.01,
0.1, and 1.0 at.%, respectively. Figure9 shows the PL spectra of the 1.0 at.% Ce sample as-deposited or after annealing
at 800 and 1200°C, respectively.
All spectra show an intense broad peak with a pronounced shoulder at longer
wavelengths. The main peaks were observed at 400, 422, and 460 nm from the
samples as-deposited or annealed at 800 and 1200°C,
respectively. The other two samples containing 0.01 and 0.1 at.% Ce have
similar spectral shapes and peak positions.Figure 9
PL spectra of a Ce:ORSO sample
containing 1.0 at.% Ce, as-deposited or after annealing at 800 and 1200°C, respectively.Cerium
can exist in both trivalent and tetravalent forms when embedded in the host
materials by losing two 6s electrons and either one or two 4f electrons. Ce4+ is optically
inactive due to the lack of 4f electrons. Although the 4f states are shielded by 5s and 5p states, the 5d states into which the 4f electron will be excited are exposed to
the surrounding ligands. As previously discussed, the PL properties of Ce ions
are highly dependent on the surrounding environment. Therefore, the changes in the
spectral shapes, peak positions, and intensities strongly suggest the evolution
of the film structure [13].Unlike
Tb, Ce3+ usually has a broad absorption band around 330 nm. The
observed emissions originate from 5d-4f transitions of the Ce3+ in the films. Similar PL
features have been observed
from Ce doped SiO2 prepared by the sol-gel method [34]. As the films
were deposited under oxygen-rich conditions, most of the Ce ions exist as quadrivalent
and possibly in the form of CeO2. Since the Ce4+ ions are
the dominant species in the as-deposited samples and only few optically active
Ce3+ ions exist, the corresponding emission intensity is low.Figure10 shows the integrated normalized PL intensity in the range of 350–700 nm for the
above three Ce:ORSO samples as a function of annealing temperature. At all of annealing
temperatures—except for 700 and 1000°C—the PL intensity increases with increasing Ce
concentration. And a similar dependence of the PL intensities on annealing
temperatures was observed despite of the Ce concentrations. The PL intensities
increase with annealing temperature up to 800°C and then decrease
with annealing temperature up to 1000°C. At 1100°C, the intensities begin to increase again and reach the
greatest value at 1200°C.Figure 10
Integrated PL intensities in the range of 350–700 nm from the Ce:ORSO
samples with various Ce concentrations as a function of annealing temperature.
The inset shows an image of strong blue emission from the 1.0 at.% Ce sample
after annealing at 1200°C taken by camera.When
annealed in an inert ambient at 700 and 800°C, the increase
in emission intensities can be attributed to the increasing Ce3+/Ce4+ ratio. Previous studies on CeO2 have shown that at high temperatures,
crystallized stoichiometric CeO2 tends to form Ce6O11 in order to lower the O/Ce ratio through the migration of oxygen
vacancies [35]. It appears that under these conditions, Ce3+ ions
are more stable than Ce4+ ions. Furthermore, due to the
concentration difference between the film and surrounding environment, some
oxygen may be released from the film during the annealing process, which
possibly leads to a local oxygen deficiency facilitating the deoxidization of
Ce4+ ions.It
was reported that Ce3+ can form two types of optical centers
depending on whether their surrounding environment was composed mainly of Ce4+ or Ce3+ ions. The former were observed to exhibit a shorter
emission wavelength than the latter [36]. Our results are consistent with this
finding, with a red shift of emission wavelength being observed for samples
after annealing at 800°C
or greater.The decrease in
emission intensity between anneals from 800 to 1000°C can be
attributed to the clustering of Ce3+ ions leading to a quenching of
the luminescence as this quenching is more profound in the samples with greater
Ce concentrations. At these temperatures, the SiOx matrix begins to
form a more uniform and stable structure. Correspondingly, the number of
nonbridging oxygen (NBO) defects is reduced significantly. As the temperature
further increases, more Ce3+ ions form clusters to
share the limited amount of NBO, and emissions from the Ce3+ ions
are quenched by transferring their energy to other Ce3+ ions in the
vicinity [37].Several groups have observed similar UV/blue light emission from CeOx/SiOx or CeOx/Si films annealed at 1000°C or greater in
inert or reducing ambients [35, 38, 39]. However, the origin of the emission
still remains controversial. Morshed et
al. [35] observed  the emission from CeO2/Si films after rapid
thermal annealing in Ar at 1000°C peaking at 400 nm. This emission was attributed to the
formation of Ce6O11 around the CeO2/Si
interface. Choi etal.  [38]
observed a weak 388 nm emission from a CeOx/Si film and an intense
358 nm emission from (CeOx + Si)/Si and (CeOx + Si)/SiOx/Si
films after annealing in N2 at 1100°C. Significant
diffusion of Si from the substrate into CeO2 and the formation of
two cerium silicate phases, Ce2Si2O7 and Ce4.667(SiO4)3O, at the interface were observed. The 358 nm
emission was attributed to Ce2Si2O7 phase, and
the weak 388 nm emission to the Ce4.667(SiO4)3O
phase. Kępiński etal. [39] observed emission peaking at 400 nm
from CeO2/SiO2 samples subjected to anneal in H2 at 1050 and 1100°C and assigned
it to an unidentified cerium silicate with some structural similarity to
tetragonal Ce2Si2O7 silicate. Therefore, in our study, the abrupt increase of the
emission intensity at annealing temperatures greater than 1100°C may also suggest the formation of Ce
silicate. At this high temperature, Ce3+ ions may acquire enough energy to interact with the surrounding Si and
O atoms and reorganize to form a more stable structure.A
detailed study of the effects of high-temperature annealing on the structural
evolution of these samples is in progress. Preliminary results from Fourier transform
infrared spectroscopy show the emergence of new sharp absorption bands after the
sample was annealed at 1200°C, similar to the peaks characteristic of Ce2Si2O7 silicate, while there are still
differences in both numbers and positions of absorption bands between them (data not shown).The
greatest intensity was observed from the 1.0 at.% Ce sample subjected to a
1200°C anneal which has the highest Ce concentration in the samples we studied. The inset of
Figure 10 shows the emission picture taken by a camera. The Ce emission from this
sample was 230 times greater than that of the as-deposited sample. In fact,
after annealing at 800 or 1200°C, both the 0.1% Ce and 1.0% Ce samples exhibit such strong
emissions that they can be easily observed under very bright room lighting
condition. Considering that the excitation power is as low as 17 mW, the
emission intensities are quite outstanding.


## 3.4. Ce-Doped Silicon-Rich Silicon Oxide (Ce:SRSO)

The
Ce:SRSO sample has Si and Ce content of 40 and 0.04 at.%, respectively.
Figure11
shows the PL spectra of the sample as deposited or annealed at various
temperatures. There is no appreciable emission from the as-deposited sample.
After annealing at 800°C,
the sample shows a broad emission band centered at 800 nm, the emission
intensity of which increases with annealing temperature until 1100°C. After annealed
at 1200°C, the
emission decreases by a factor of 8 and another weak broad emission band
centered at 450 nm emerges.Figure 11
PL spectra of a Ce:SRSO sample
containing 0.04 at.% Ce, as-deposited or annealed at various temperatures in
the range of 800–1200°C.It
is evident that the 800 nm emission originates from the precipitated Si-ncs
upon annealing. We attribute the 450 nm emission to the Ce3+ ions in
the film. The enhancement of the Si-ncs emission intensity with annealing
temperature indicates the formation of more Si-ncs in the same manner as was observed
in Tb:SRSO samples. The absence of Ce emission at annealing temperatures lower
than 1200°C is also
possibly due to the competition between Si-ncs and Ce ions in the absorption of
excitation photons.


## 4. Conclusions

Rare-earth (Tb or Ce) doped silicon oxides (silicon-rich
or oxygen-rich) were deposited by ECR-PECVD. The successful in situ incorporation of high
concentrations of optically active rare-earth ions was demonstrated. The
dependence of the PL properties on Si/O ratio, doping concentration, and
annealing temperature was investigated.Green Tb emission was observed in both oxygen-rich and
silicon-rich samples. The excitation wavelength is not resonant with any
optical absorption band of Tb3+. We propose that in oxygen-rich
films, the organic ligands introduced from the deposition process may serve as
the sensitizers to excite Tb emission while Si-ncs serve as the sensitizers in
silicon-rich films.Violet/blue Ce emission was observed in both
oxygen-rich and silicon-rich films. An abrupt increase of Ce emission intensity
in oxygen-rich films after annealing at 1200°C is possibly due to the formation of Ce silicate.
In silicon-rich films, the Ce emission was absent at annealing temperatures
lower than 1200°C.It was found that by choosing appropriate compositions
and annealing temperatures, the Ce and Tb PL
intensities can be enhanced
significantly. The highest Ce PL
intensity was observed from the sample containing 32 at.% Si and 1.0 at.% Ce
annealed at 1200°C which has the highest Ce
concentration in the samples we examined. The highest
Tb PL
intensity was observed from the sample containing 32 at.% Si and 0.4 at.% Tb annealed at 800°C.
Those emissions were very bright and can be easily observed under room lighting
condition.



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*Source: 295601-2008-05-13.xml*