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Current Applied Physics 11 (2011) 1001e1005
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Current Applied Physics
journal homepage: www.elsevier.com/locate/cap
Luminescent mechanism of Eu3þ-doped epitaxial Gd2O3 films grown on a Si (111)
substrate using an effusion cell
Moon Hyung Jang a,1, Yoon Ki Choi b, Kwun Bum Chung c, Hyeongtag Jeon d, Mann-Ho Cho a, *
a
Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Republic of Korea
System LSI Division, Samsung Electronics Co., LTD, Gyeonggi-Do 449-711, Republic of Korea
c
Department of Physics, Dankook University, Dongnamgu Anseodong 29, Cheonan 330-714, Republic of Korea
d
Department of Material Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 January 2010
Received in revised form
16 December 2010
Accepted 12 January 2011
Available online 27 January 2011
Eu3þ-doped epitaxial Gd2O3 (111) films with well-ordered crystalline structures were grown on oxidized Si
(111) using the physical vapor deposition method. The mole fraction (x) of Eu3þ in Gd2xO3:Eu3þx ranged
from 0.02 to 0.22. The photoluminescence characteristics, measured at an excitation wavelength of 254 nm,
showed that even at the very low Eu3þ concentration, x ¼ 0.18, the 5D0 / 7F2 transition occurred at the
maximum 612-nm emission. Based on the critical distance calculated using the decay curves at 612 nm, we
proved that the 5D0 / 7F2 transition of the Gd2O3:Eu3þ originated from an electric dipoleedipole transition. In addition, the critical distance (Rc) was greater than that reported previously due to the perfectly
crystalline film. This significantly decreases the mole fraction which maximize the photoluminescence
intensity because the non-radiative transition is much lower than that of the chemically synthesized
Gd2O3:Eu3þ.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Rare-earth phosphor
Gd2O3:Eu3þ
Luminescent mechanism
1. Introduction
Currently, rare earth-based phosphor materials are widely used
in flat panel displays, such as plasma display panels, and field
emission displays, because rare earth ions with shielded 4f electrons exhibit a high-efficiency narrow-line emission in the visiblelight region [1e8]. In particular, one of the rare earth oxide
phosphors, Gd2O3:Eu3þ, has also been used in immunoassays for
identification of biological species [6,7,9,10]. The emission mechanisms of Gd2O3:Eu3þ are identical to those of Y2O3:Eu3þ, with
a primary red-emission at 612 nm. However, the luminescent
mechanisms of Gd2O3:Eu3þ are actively studied by many groups
[11e15]. Eu3þ ions substitute for Gd3þ ions in the lattice, such that
the Eu3þ has either C2 or S6 symmetry in the bixbyite Mn (II)
structure of Gd2O3; the theoretical occupancy ratio of C2/S6 sites is
3/1 and occupation of these sites by Eu3þ ions is random [16].
Visible light is emitted due to the transition of Eu3þ on the C2 site
with inversion symmetry, while on the S6 site the energy is dissipated by a non-radiative transition.
* Corresponding author.
E-mail address: [email protected] (M.-H. Cho).
1
Present address: Department of Materials Science and Engineering, University
of Pennsylvania, Philadelphia, PA 19104, USA.
1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cap.2011.01.010
The emission that originates from the 5D0 / 7F2 transition in
Gd2O3:Eu3þ is a forced electric dipole transition. The results of
theoretical calculations show that this type of transition has RS
dependence, where R is the distance between the sensitizer
(Gd2O3) and activator (Eu3þ) and S equals 6 for the electric dipoleedipole interaction. However, reported experimental data for
Gd2O3:Eu3þ phosphors show that the observed S values are not
consistent with the theoretical value of 6 [15]. To prove the luminescent mechanism of Gd2O3:Eu3þ materials with no interference
from crystalline defects, growth of epitaxial films with perfect
crystallinity is essential. High quality epitaxial Gd2O3:Eu3þ films
were grown on a Si (111) substrate in an ultra-high vacuum
chamber using co-evaporation of Gd and Eu metals in an oxygen
atmosphere. The crystalline properties of the Gd2O3 matrix along
the film-depth direction were measured using Rutherford backscattering spectrometry (RBS) channeling. In addition, the GdeO
inter-atomic distance in the Gd2O3:Eu3þ film was observed as
a function of doping concentration using extended x-ray absorption
fine structure (EXAFS) and a fitting code. The photoluminescence
(PL) emission spectra were measured and decay curves were
obtained for analysis of the luminescent mechanism. Using these
results, we proved that the 5D0 / 7F2 transition of the Gd2O3:Eu3þ
originated from the electric dipoleedipole transition. In addition,
the critical distance (Rc) was greater than that reported previously
due to the perfect crystallinity of the film.
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M.H. Jang et al. / Current Applied Physics 11 (2011) 1001e1005
2. Experimental
4000-Å-thick Gd2O3:Eu3þ films were deposited on a chemically
oxidized 4 vicinal Si (111) wafer by evaporation of Gd and Eu metal
sources in an ultra-high vacuum chamber with an oxygen partial
pressure of 6 106 torr. The base pressure of the deposition
chamber was less than 1 109 torr. The film thickness is confirmed
by RBS. The Eu deposition rate was controlled to achieve the proper
doping concentration, while that of Gd was held constant. During
evaporation, the substrate temperature was maintained at 700 C.
The temperature was monitored using an infrared optical pyrometer,
which was calibrated using a thermocouple. The film composition
was confirmed using particle-induced X-ray emission (PIXE), which
is a non-destructive elemental analysis technique (data not shown).
The Eu3þ-doping concentrations (x) in Gd2xO3:Eu3þx were 0.02,
0.06, 0.08, 0.10, 0.12, 0.14, 0.18, and 0.22 mol. Variation in the structural characteristics of the films with Eu3þ concentration was evaluated by measurement of change in the GdeO inter-atomic distance
using EXAFS in a synchrotron beamline 7C1 at a Pohang light source
(PLS). The EXAFS spectra of the Gd LIII edges were measured at room
temperature using the total electron yield (TEY) mode. Use of both
a double-crystal monochromator (DCM) and a collimating mirror
system in 7C1 allowed an energy region for the Gd LIII edges of
approximately 7243 eV [17]. PL emission spectra for 5D0 / 7F2
transition in Gd2O3:Eu3þ were obtained at an excitation wavelength
of 254 nm using a Perkin Elmer LS-50 photoluminescence spectrometer with a Xenon flash lamp operated with a pulse duration of
6 ns. Decay curves were measured at the emission of 612 nm using
the picosecond time-correlated single photon counting (TCSPC)
system. This system is composed of the cavity dumped dual-jet dye
laser and Nd-YAG laser which operate synchronously. The full width
at half maximum of laser pulse was 67 ps. The surface morphology of
Gd2O3:Eu3þ films are observed by JSM-6500F (Jeol, Inc) field emission scanning electron microscopy (FE-SEM) to verify the effect of
surface morphology in photoluminescence efficiency.
3. Results and discussion
The crystalline quality of the Gd2O3 matrix was evaluated using
RBS/channeling to investigate atomic ordering along the film-depth
direction. Fig. 1(a) shows the RBS channeling data for the Gd2O3 film
grown on Si(111) at 700 C. The minimum channeling yield (cmin)
obtained was approximately 14.4%, indicating that the film was
epitaxially well-grown. EXAFS and the fitting code were used to estimate the inter-atomic distance of the film as a function of Eu3þ-doping
concentration. The EXAFS spectra were analyzed with the IFFEFIT
software package [18]. The oscillatory portion c(k) of the EXAFS signal
was subtracted. Then c(k) was forward Fourier transformed to the R
space. Because the Eu LII edge appeared at 7617 eV, the k ranges were
limited to between 2.0 and 8.7 Å1. To eliminate the higher shells, the R
range was set to the first nearest neighbor. Signals from the 0.14 mol Eu
film were backward Fourier transformed and weighted by k3 for fitting
using the FEFF8 code, as shown in Fig. 1(b) [18]. The continuous line
and open circles in the figure indicate the experimental and fitted data,
respectively. The GdeO inter-atomic distances obtained from the
results of EXAFS fitting are summarized in Table 1. The inter-atomic
distances were slightly increased, by 1.7%, in the 0.14 mol Eu film. This
was the largest variation among these samples, relative to their bulk
counterpart [10]. Even at Eu3þ concentrations as high as 0.22 mol, the
effect of Eu3þ-doping concentration on the GdeO inter-atomic
distance was smalldthe distance was increased by w1%. These results
indicate that the inter-atomic distance of the films remained nearly
constant even when the Eu3þ concentration was varied. Therefore,
structural variations due to alterations in the Eu3þ-doping concentration do not affect the luminescent properties of the films.
Fig. 1. (a) RBS/channeling spectra of Gd2O3 grown on Si(111) at a substrate temperature of 700 C. (b) The real part of the k3-weighted EXAFS signals for the 0.14 mol
Gd2O3:Eu3þ films. The continuous line and the open circles represent the experimental
and fitted data, respectively.
The 5D0 / 7F2 primary transition at 612 nm was observed in all
films, as indicated by the dashed vertical line in the PL spectra
shown in Fig. 2. This transition is consistent with the previously
reported forced electric dipole transition, which is favored in Eu3þ
ions [19]. The PL intensity increased with Eu3þ concentrationdup
to concentrations of 0.18 mol. However, the PL intensity of the
0.22 mol film decreased dramatically, because excess Eu3þ ions
resulted in luminescence quenching. Hence, the optimal concentration of Eu3þ for maximum intensity was determined to be
0.18 mol. This value is much less than previously reported results
for chemically synthesized Gd2O3:Eu3þ [15]. However, if there are
differences in surface morphology among Gd2O3:Eu3þ films, this
luminescence intensity has to be corrected because the luminescence efficiency is increased when the surface is rough. Therefore,
to confirm the surface morphology, FE-SEM is adopted in
Gd2O3:Eu3þ films. The FE-SEM images of Gd2O3:Eu3þ films with
Eu3þ concentration of 0.02, 0.10, 0.12, 0.14, 0.18 and 0.22 mol are
shown in Fig. 3(a), (b), (c), (d), (e) and (f), respectively. The surface
morphology of the films is overall flat and has no significant
difference among films. In detail, the surface morphology becomes
Table 1
The GdeO inter-atomic distance of cubic Gd2O3:Eu3þ obtained from the EXAFS curve
fitting. The GdeO distance of cubic Gd2O3:Eu3þ bulk is 2.301 Å.
Eu3þ mol concentration
Inter-atomic distance (Å)
0.02
2.303
0.10
2.323
0.12
2.323
0.14
2.341
0.18
2.322
0.22
2.328
M.H. Jang et al. / Current Applied Physics 11 (2011) 1001e1005
1003
1/e Decay Time (msec)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.01
0.1
3+
Eu concentration (mol)
Fig. 4. Plot of 1/e decay time as a function of the Eu3þ concentration. The critical
distance (RC) was obtained from this plot.
Fig. 2. PL emission spectra of the Gd2O3:Eu3þ films at an excitation wavelength of
254 nm. The dotted vertical line indicates the peak at 612 nm corresponding to the
5
D0/7F2 transition. The intensity was maximized at a Eu concentration of 0.18 mol.
flat up to the Eu3þ concentration of 0.18 mol. This tendency is
consistent with change in the luminescence intensity. Therefore, it
is clearly shows that the PL intensity difference is originated from
the inside of films not from surface morphology difference, because
the surface flatness reflects the crystallinity of the epitaxial film.
To further investigate the difference between physically evaporated and chemically synthesized Gd2O3:Eu3þ, we calculated Rc,
which describes energy transfer during the PL transition. Because Rc
is the distance between Eu3þ ions, for which the probabilities of
undergoing either a radiative or non-radiative transition are equal, Rc
can be expressed by the following equation (Blasse’s method) [20].
"
Rc ¼ 2
3V
4PN x2c
#1=3
(1)
where xc is the critical concentration, which is the concentration in
the case of Rc, N is the number of Gd3þ ions in a unit cell, and V is the
volume of the unit cell, i.e., 1.256 1027 m3 [13]. The 1/e decay
time was extracted from the decay curves (the data is not shown
here). Fig. 4 shows the 1/e decay times for different Eu3þ concentrations. From this figure, we estimated the critical concentration
for radiative transition to be approximately 0.11 mol, indicating that
physically evaporated Gd2O3:Eu3þ film has a smaller critical
concentration than that of chemically synthesized film, which is
0.4 mol [15]. Using the above formula, we calculated the critical
distance to be 11.1 Å which is about 4 Å larger than that of chemically synthesized Gd2O3:Eu3þ [15]. This result means that the PL
from the evaporated thin film decays faster than the PL of chemically synthesized Gd2O3:Eu3þ because of a greater probability of
cross-relaxation between Eu3þ ions and a smaller probability of
non-radiative transition, even when films have the same Eu3þ
concentration. The major difference between the films used in
previously reported studies and our films is film quality, as shown
by both the RBS/channeling and EXAFS data. The chemically
synthesized films consisted of poly crystalline phases. However, our
Gd2O3:Eu3þ films were epitaxially grown. Therefore, the crystallinity of the films has an important role in the PL process. If the
films are not preferentially oriented, they have many grain boundaries and defects that can act as non-radiative absorbing centers.
These non-radiative absorbing centers make the energy transfer
between Eu3þ ions difficult because the energy to be transferred is
captured by these grain boundaries and defects while the energy is
transferred much longer in epitaxially grown films [21,22]. In
Fig. 3. The FE-SEM images of the Gd2O3:Eu3þ films with Eu3þ concentrations of (a) 0.02 mol, (b) 0.10 mol, (c) 0.12 mol, (d) 0.14 mol, (e) 0.18 mol and (f) 0.22 mol. All images are
magnified with same condition.
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M.H. Jang et al. / Current Applied Physics 11 (2011) 1001e1005
Fig. 5. (a)e(d) lnðlnðIðtÞ=I0 Þ ðt=sÞÞ versus lnðt=sÞ3 plot for various Eu3þ concentrations. The slope corresponds to the inverse of the S value of the Gd2O3:Eu3þ films. (e) S values as
a function of Eu3þ concentration.
addition, the non-radiative transition from Eu3þ ions to defects is
a very slow process compared with radiative transition, which
explains the slow decay in chemically synthesized Gd2O3:Eu3þ.
Unfortunately, the energy transfer mechanism cannot be
explained by experimental data for the chemically synthesized
Gd2O3:Eu3þ. Although the theoretical calculation predicts the
5
D0 / 7F2 transition, the transition data in the PL spectra were not
consistent with the predicted values. However, the energy transfer
mechanism was analyzed using the highly consistent transition
spectra. We calculated the S values from the Gd2O3:Eu3þ decay
curves for the emissions at 612 nm. To estimate the interaction
type of the energy transfer, Inokuti and Hirayama’s equation can be
adopted as follows:
" 3=S #
IðtÞ
t
C
3
t
G 1
¼ exp s
s
I0
C0
S
(2)
where I(t) is the emission intensity as a function of time, I0 is the
initial emission intensity, C is the number of Eu3þ ions per unit cell,
C0 is the critical volume, i.e., 4PR3c =3, and s is the lifetime of an
M.H. Jang et al. / Current Applied Physics 11 (2011) 1001e1005
isolated Eu3þ ion with a very low doping concentration. The S value
indicates the type of electric multipolar interaction during the
energy transfer. When the interaction is dipoleedipole, quadrupoleedipole and quadrupoleequadrupole, the S values are 6, 8
and 10, respectively.
Equation (2) can be rearranged as follows:
3
IðtÞ
t
1
t
¼ ln
ln ln
þconst:
s
s
I0
S
(3)
The S values were obtained from the slope of equation (3) by
fitting the decay curves, as shown in Fig. 5(a)e(d). The slope
represents the inverse of the S value for various Eu3þ concentrations.
Because the films with Eu3þ concentrations of less than 0.12 mol
were too disperse to estimate the S values, we limited the S value
evaluations to films with Eu3þ concentrations greater than 0.12 mol.
For all films, the S values as a function of the Eu3þ concentration
approached 6, as shown in Fig. 5(e), which implies the electric
dipoleedipole interaction was within the 10% range of error.
Compared with results for the chemically synthesized Gd2O3:Eu3þ,
of which only one-half exhibited the ideal S ¼ 6, the result obtained
for our films is remarkable [15]. Therefore, we conclude that the
energy transfer occurs via the electric dipoleedipole transition
mechanism. The characteristics of the Gd2O3:Eu3þ film were nearly
identical to those of the ideal crystalline Gd2O3:Eu3þ, which has no
non-radiative absorbing defects.
4. Summary
In summary, the luminescent properties of the physically evaporated Gd2O3:Eu3þ films were investigated using the PL emission
spectra and decay curves at 612 nm that originated from the
5
D0 / 7F2 transition. The Rc values of the epitaxially grown films
were much larger than that of the chemically synthesized phosphors. This was due to the difference in crystal quality between
epitaxial films and poly crystalline films, which were evaluated
using RBS/channeling and EXAFS. The number of non-radiative
absorbing centers in the Gd2O3:Eu3þ films was much less than that
of the chemically synthesized ones, indicating that the electric
multipolar interaction of the film was determined by the electric
dipoleedipole interaction.
Acknowledgement
This work was partially supported by the Post Doctorate
Program of Yonsei University and the Joint Program for Yonsei
Univ.-Samsung Semiconductor Co.
1005
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