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A simple photochemical method to synthesize Ga2O3–Dy3þ
–M3þ
thin films
and their evaluation as optical materials (where M¼Cr or Co)
G. Cabello a,n
, L. Lillo a
, Y. Huentupil a
, F. Cabrera a
, G.E. Buono-Core b
, B. Chornick c
a
Departamento de Ciencias Básicas, Facultad de Ciencias, Universidad del Bı́o-Bı́o, Chillán, Chile
b
Instituto de Quı́mica, Pontificia Universidad Católica de Valparaı́so, Valparaı́so, Chile
c
Departamento de Fı́sica, Facultad de Ciencias Fı́sicas y Matemáticas, Universidad de Chile, Casilla 487-3, Santiago 8370415, Chile
a r t i c l e i n f o
Article history:
Received 6 May 2011
Accepted 12 July 2011
Available online 22 July 2011
Keywords:
A. Amorphous materials
D. Optical properties
D. Luminescence
a b s t r a c t
We report the photochemical method to synthesize Ga2O3–Dy3þ
–Co3þ
and Ga2O3–Dy3 þ
–Cr3þ
thin
films. X-ray photoelectron spectroscopy, X-ray diffraction and photoluminescence were used to
characterize the products. These analyses revealed that as-deposited and annealed films are amor-
phous. The optical characterization of the films showed that these are highly transparent in the visible
spectrum but decrease significantly with doped and co-doped films. Under the excitation of UV light
(254 nm) the doped films (Ga2O3–Dy3þ
) show the characteristic emissions of Dy3þ
at 500, 575, 594,
605 and 652 nm corresponding to 4
F9/2-6
HJ ( J¼15/2, 13/2 and 11/2) transitions but the emissions
decrease with the co-doped films (Ga2O3–Dy3 þ
–M3þ
, where M¼Cr or Co); a possible emission
mechanism and energy transfer have been proposed.
& 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Gallium oxide (Ga2O3) is an important III–VI semiconductor,
which exhibits a wide band gap (Eg ¼4.9 eV) and it has potential
applications in optoelectronic devices including flat panel dis-
plays, solar energy conversion devices and ultraviolet emitters [1].
In general, Ga2O3 exhibits different polymorphic phases such as
rhombohedral a-, monoclinic b-, and cubic U- and s-phases and
all of them can be obtained from orthorhombic gallium oxide
hydroxide [a-GaO(OH)] simply by annealing at selective
temperature [1]. In recent years, several studies have reported
doping of this material with rare-earth (RE3 þ
) or 3d transition
metal ions; Ga2O3 thin films have also shown promising optical
and photo-luminescent (PL) properties [2]. It has been reported
[3] that b-Ga2O3 can exhibit UV, blue and green emissions,
depending on the sample preparation conditions and the nature
of defects, upon photo-excitation through the band gap. As to
photoluminescence of doped b-Ga2O3 their characteristic lumi-
nescence is produced due to d–d and f–f transitions of 3d
transition metal ions and RE3 þ
ions, respectively [3]. The photo-
excitation mechanism through which doped ions emit visible
light has been ascribed to host-activator interactions or direct
intraconfigurational electronic transitions. Therefore, understand-
ing the relationship the trap state (defects) and luminescence of
activators is of great importance in designing luminescent mate-
rials based on gallium oxide.
Various growth techniques have been used to prepare thin
films of this material: spray pyrolysis deposition [4], sol–gel
process [5], radiofrequency (RF) magnetron sputtering [6] and
metal–organic chemical vapor deposition (MOCVD) [7].
In the present paper, we report the use of photo-chemical
method to prepare gallium oxide thin films co-doped with Dy3þ
and Cr3þ
or Co3þ
. This method involves the UV irradiation of films
of suitable metal complexes precursors, which photo-decompose
on an appropriate substrate to leave on the surface thin films of
metals or metal oxides depending on the reaction conditions:
MLn(thin film)!
hv
Mn1
þnL(gas)!
O2
MOX(thin film)þsub-products
where M is the Ga, Dy, Cr or Co, L is the acac (2,4 pentandione)
and thd (2,2,6,6-tetramethyl-3,5-heptanedione).
In this paper we describe the use of the b-diketonate com-
plexes of gallium, dysprosium, chromium and cobalt to prepare
films of gallium oxide doped with Dy3 þ
and co-doped with Cr3 þ
or Co3 þ
and a preliminary study of their optical properties.
2. Experimental
2.1. General procedure
UV spectra were obtained with 1 nm resolution in a Perkin
Elmer Model Lambda 25 UV–vis spectrophotometer. X-ray dif-
fraction patterns were obtained using a D8 Advance Bruker X-ray
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/jpcs
Journal of Physics and Chemistry of Solids
0022-3697/$ - see front matter 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2011.07.013
n
Corresponding author. Tel.: þ56 42 253096; fax: þ56 42 253046.
E-mail address: gcabello@ubiobio.cl (G. Cabello).
Journal of Physics and Chemistry of Solids 72 (2011) 1170–1174

3. Author's personal copy
diffractometer. The X-ray source was Cu 40 kV/40 mA. X-ray
photoelectron spectra (XPS) were recorded on an XPS-Auger
Perkin Elmer electron spectrometer Model PHI 1257, which
included an ultra high vacuum chamber, a hemispherical electron
energy analyzer and an X-ray source providing unfiltered Ka
radiation from its Al anode (hn¼1486.6 eV). The pressure of the
main spectrometer chamber during data acquisition was main-
tained at ca.107
Pa. The binding energy (BE) scale was calibrated
using the peak of adventitious carbon, setting it to 284.6 eV. The
accuracy of the BE scale was 70.1 eV. Solid state photolysis was
carried out at room temperature under a low-pressure Hg lamp
(l¼254 nm) equipped with two 6 W tubes, in air. Progress of the
reactions was monitored by determining the FT-IR spectra at
different time intervals, following the decrease in IR absorption of
the complexes. The substrates for deposition of films were
borosilicate glass microslides (Fischer, 2 2 cm2
) and n-type
silicon (1 0 0) wafers (1 1 cm2
) obtained from Wafer World
Inc., Florida, USA.
Photoluminescense (PL) emission spectra measurements were
carried out in an Ocean Optics Model QE65000-FL spectrometer
with an L type setup. Excitation was done with a PX-2 pulsed
Xenon lamp (220–750 nm), and the UV light passed through a
monochromator set to 254 nm. The measurements were done at
room temperature.
2.2. Preparation of amorphous thin films
The precursors Ga(III), Co(III), Cr(III) acetylacetonate com-
plexes and Dy tris(2,2,6,6-tetramethyl-3,5-heptanedionate)
complex were purchased from Aldrich Chemical Company and
thin films were prepared by the following procedure: a silicon
chip was placed on a spin coater and rotated at a speed of
1500 RPM. A portion (0.1 ml) of a solution of the precursor
complex in CH2Cl2 was dispensed onto the silicon chip and
allowed to spread. The motor was then stopped after 30 s and a
thin film of the complex remained on the chip. The quality of the
films by optical microscopy was examined (500 magnification).
2.3. Photolysis of complexes as films on Si (1 0 0) surfaces
All photolysis experiments were conducted following the same
procedure: a film of the complex was deposited on n-type Si
(1 0 0) by spin-coating from a CH2Cl2 solution. This resulted in
the formation of a smooth, uniform coating on the chip. The
quality (uniformity, defects, etc.) of the precursor films was
determined by optical microscopy (500 ), while the thickness
was monitored by interferometry. The FT-IR spectrum of the
starting film was first obtained. The irradiation of the films was
carried out at room temperature using two low-pressure Hg
lamps (6 W, Rayonet RPR—2537 A) in air, until the FT-IR spec-
trum showed no evidence of the starting material. Prior to
analysis, the chip was rinsed several times with dry acetone to
remove any organic products remaining on the surface. In order
to obtain films of a specific thickness, successive layers of the
precursors were deposited by spin-coating and irradiated as
above. This process was repeated several times until the desired
thickness was achieved. Post-annealing was carried out under a
continuous flow of synthetic air at 800 1C for 3 h in a program-
mable Lindberg tube furnace.
3. Results and discussion
3.1. Photochemistry of Ga(III) and Dy(III) b-diketonate complexes
The photochemistry of metal b-diketonate complexes has been
widely investigated [8–10], because they absorb strongly at
readily accessible parts of the UV spectrum (250–400 nm). In
general, the irradiation of these complexes with UV light
(254 nm) leads to the photo-reduction of these complexes, upon
LMCT (ligand-to-metal charge transfer) excitation to form metals.
In this case no reports can be found in the literature concerning
Ga(acac)3 and Dy(thd)3 complexes. We therefore carried out
experiments to evaluate the photo-reactivity of these complexes
in solution as a film. When dichloromethane solutions of these
complexes were photolyzed with 254 nm UV light, a complete
disappearance of the absorption bands of these complexes could
be observed after 300 min of irradiation (Fig. 1). The electronic
spectra of Ga(III) and Dy(III) b-diketonate complexes exhibited
bands at 285 and 275 nm, respectively. The observed absorption
bands have been assigned to the various electronic transitions. In
dichloromethane diluted solutions, irradiation of the Ga(acac)3
complex at 254 nm provoked fast spectral changes consisting of
decreases of the absorption band at 285 nm and slight shift of the
band at 274 nm, which is characteristic of the free 2,4-pentan-
dione ligand (Hacac) associated with a p–p* (HOMO-LUMO)
transition [11,12], which also degrades (Fig. 1a).
On the other hand, the Dy(thd)3 complex exhibits a single
band at 275 nm, attributable to ligand transition of a p–p*, the
break of hydrogen intraligand bond, present in the enol form of
the ligand, and the formation of two hydrogen intermolecular
bonds with the solvent, giving an electronic situation similar to
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
t0
Absorbance
Wavelength (nm)
250 275 300 325 350 375 400 250 275 300 325 350 375 400
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
t0
Absorbance
Wavelength (nm)
Fig. 1. Changes in the UV spectrum of a solution in CH2Cl2 of (a) Ga(acac)3 complex (4.09 105
mol/L) upon 307 min irradiation and (b) Dy(thd)3 complex
(1.32 104
mol/L) upon 398 min irradiation with 254 nm light.
G. Cabello et al. / Journal of Physics and Chemistry of Solids 72 (2011) 1170–1174 1171

4. Author's personal copy
the complex. The irradiation of the Dy(thd)3 complex in dichlor-
omethane diluted solutions at 254 nm provoked complete degra-
dation of the complex, as evidenced by a gradual disappearance of
the band at 275 nm (Fig. 1b). These results demonstrate that
these complexes are highly photo-reactive in solution; the excita-
tion at 254 nm light generates a series of intermediaries or
sub-products that facilitate the photo-reduction of metal species
present in the complexes.
In order to investigate the solid-state photochemistry, films of
the Ga(acac)3 and Dy(thd)3 complexes were irradiated under air
atmosphere with a 254 nm UV lamps. This led to a decrease of the
absorptions associated with the ligand, as shown in the FT-IR
monitoring of the reaction (not shown here). These results
suggest that the complex precursors are photo-dissociated
on the surface forming volatile products, which are partially
desorbed. Exhaustive photolysis results in the loss of the all
bands associated with the starting complexes.
3.2. Characterization of Ga2O3–Dy3 þ
–M3 þ
thin films
(where M¼Cr or Co)
For the photo-deposition of Ga2O3 co-doped with Dy3 þ
and
transition metal ions thin films, solutions of Ga(acac)3 with
10 mol% proportions of the Dy(thd)3 and M(acac)3 were spin
coated on the appropriate substrate and the thin films co-doped
were irradiated for 24 h until minimal absorptions in the infrared
spectrum from the complexes were observed.
3.2.1. XPS and XRD analysis
The elemental composition of the as-deposited Ga2O3 co-
doped thin films was analyzed by XPS, in order to study the
chemical nature of the deposited surface. Fig. 2 shows the wide
scan XPS spectrum, in the binding energy (BE) range 0–1200 eV.
The spectrum shows signals from Ga 2p, Ga LMM (Auger peaks)
O 1s, Dy 4d, Cr 2p, Co 2p3 and C 1s. The carbon detected on the
as-deposited films is probably the result of an organic residue
from organic raw precursors.
The peaks of Ga 2p1/2 and Ga 2p3/2 are located at the binding
energy of 1144.8 and 1118.0 eV, respectively, and that corresponding
to Ga–O bonding, which are consistent with other experimental
results [6,13,14]. The core level of gallium has a positive shift from
elemental gallium [13], which indicates that the gallium present in
the films is in compound state, not in elemental state. The O 1s peak
centers at 531.1 eV, generally, the O 1s peak has been observed at the
binding energy of 528–535 eV [13]. The peak around 530.5–531.8 eV
is usually attributed to oxygen in non-stoichiometric oxides, while
the peak of chemisorbed O2 is located in the region of 532–535 eV
[15]. The binding energy of Dy 4d is situated at 154.1 eV, associated
with a Dy(III) species; similar values have been reported by other
authors [16,17]. According to the literature data, the Cr 2p3/2 peak of
pure Cr2O3 is at a BE of 576 eV, whereas that of pure chromium
metal is at a BE of 574 eV [18,19]. In this case the Cr 2p3/2 is located
at 576.8 eV. On the other hand it has been established that for the Co
2p signal the Co 2p3/2 and Co 2p1/2 spin–orbit components are
located at 780 and 796 eV, respectively [20]. For the Co 2p3/2 peak
has been identified in several phases at 779.5, 780.7, 781.6 and
777.5 eV corresponding to Co3O4, CoO, Co2O3 and metallic Co [21].
Our studies indicate that the Co 2p3/2 peak is situated at 780.1 eV
associated with the Co3O4 formation.
In order to investigate the effect of thermal annealing at 800 1C
for 3 h on the Ga2O3–Dy3þ
–M3þ
thin films, we have performed an
XRD analysis on the co-doped films, the XRD patterns were
featureless, except for the reflection from Si(1 0 0) located at
2y¼33.11
, thereby indicating the amorphous nature of the films
(Fig. 3). Similar results were obtained using Ga(acac)3 as precursors
for the deposition of Ga2O3 thin films by MOCVD at 700 1C [22].
3.2.2. Optical measurements
The thickness of the films used for optical measurement was
about 400 nm. Fig. 4 shows the transmittance (%) spectrum for
Ga2O3, Ga2O3–Dy3 þ
and Ga2O3–Dy3 þ
–M3 þ
thin films. In general
the undoped Ga2O3 shows a transmittance of about 90% in the
wavelength range from 400 to 900 nm, and this is slightly
reduced as it enters in the UV region. However, the transmittance
of the doped and co-doped thin films decreases significantly.
A high optical transmission in the visible region is required for
luminescent materials to obtain the effective emission.
On the other hand, the high transparency of the annealed thin
films indicates fairly smooth surfaces and low thickness due to
relative film homogeneity.
To determine the optical band gap of these films, from the (ahv)2
vs. hv has been plotted and extrapolated and the linear portion of
the curve has been extrapolated to a¼0 (inset of Fig. 4), in which a
and hv are the absorption coefficient and photon energy, respec-
tively. The absorption coefficient increases rapidly at the photon
energy range around 4.0–5.0 eV. The result indicates that the
1200 1000 800 600 400 200
0
50
100
150
200
250
Dy
4d
Dy
4p3
0
Ga
2p3
Ga
2p1
Cr
LMM
1
O
KLL
Cr
2p1
Cr
2p3
Co
LMM
Co
2p3
Ga
3p
Dy
4d
Dy
4p3
Ga
LMM
1
Ga
LMM
O
1s
Intensity
(a.u.)
Binding Energy (eV)
Fig. 2. XPS survey scan of an as-deposited of (a) Ga2O3 (b) Ga2O3–Dy3 þ
–Cr3þ
and
(c) Ga2O3–Dy3 þ
–Co3 þ
thin films obtained by UV irradiation at 254 nm on Si(1 0 0)
substrates.
30 35 40 45 50 55 60 65 70
0
5
10
15
20
25
30
35
40
45
Si (100)
Intensity
(a.
u.)
2 theta (degree)
Fig. 3. XRD patterns of (a) Ga2O3–Dy3 þ
–Cr3þ
and (b) Ga2O3–Dy3þ
–Co3þ
thin
films annealed at 800 1C for 3 h on Si (1 0 0) substrates.
G. Cabello et al. / Journal of Physics and Chemistry of Solids 72 (2011) 1170–1174
1172

5. Author's personal copy
absorption in this energy region is due to the direct transition. The
band gap was estimated to be 4.870.1 eV for undoped Ga2O3 films,
which agrees with the values reported [5,23]. Our measured optical
band gap values of Ga2O3–Dy3þ
and Ga2O3–Dy3þ
–M3þ
thin films
for the as-deposited and annealed films are shown in Table 1. In
general the band gap values decrease slightly with the doping and
co-doping of the Ga2O3 films. These values are too large to absorb
and utilize visible light. It has been reported that incorporation of
metal ions into the Ga2O3 thin films resulted in shifts in the
absorption spectra. This observation suggests that impurity levels
are introduced between the intrinsic bands, which subsequently
generate new band gaps [24]. The effects that dopants have on the
performance of Ga2O3 are mainly associated with modifying the
microstructure as well as reducing the band gaps. However, our
results are not showing significant changes in the values of the
band gaps because, our films still have an amorphous structure
after subjecting them to heat treatment.
3.2.3. Photoluminescence study
Fig. 5 shows the photoluminescence (PL) spectra of Ga2O3
doped and co-doped samples under excitation at 254 nm. It is
known that the luminescence of trivalent dysprosium Dy3 þ
mainly consists of narrow lines in the blue region (470–500 nm)
and yellow region (570–600 nm). The two emissions have origi-
nated from 4
F9/2-6
H15/2 and 4
F9/2-6
H13/2 transitions of Dy3 þ
[25]. The 4
F9/2-6
H13/2 transition is hypersensitive and its inten-
sity strongly depends on the host, in contrast to a less sensitive
4
F9/2-6
H15/2 transition of Dy3 þ
[26].
Our experimental results in Fig. 5 show PL from 450–700 nm
of all samples. No signals associated to undoped Ga2O3 thin films.
However for the films doped with Dy3 þ
(Ga2O3–Dy3 þ
) show a
series of peaks located at 500, 575, 594, 605, 622 and 652 nm. The
signal at 500 nm is assigned to the transition of Dy3 þ
ion from the
4
F9/2 excited state to the 6
H15/2 ground state. The signals at 575,
594 and 605 nm correspond to 4
F9/2-6
H13/2 transition and the
peak situated at 652 nm is attributed to the 4
F9/2-6
H11/2 transi-
tion of Dy3 þ
ion. On the other hand, for the co-doped films
(Ga2O3–Dy3 þ
–Cr3 þ
and Ga2O3–Dy3 þ
–Co3 þ
), it is possible to
observe the same spectral pattern of the signals but their
intensities decreased significantly. There are no signals that can
be assigned to Cr3 þ
or Co3 þ
ions.
It should be noted that both as-deposited and annealed
samples showed the same pattern of spectral behavior (see
Fig. 5a and b). It has been reported [27] that local structural
environments and spatial distribution of dopants when develop-
ing a nano-sized luminescent material activated by RE elements
because absorption and emission properties of intra-4f-config-
urational transitions of RE ions are sensitive to their chemical and
structural status in a host. In this case given the amorphous
nature of all as-deposited and annealed films, there was no
significant structural change to produce a noticeable change in
the PL emissions.
However the PL of samples annealed at 800 1C (Fig. 5b) shows
that their emission intensity slightly decreases in comparison
with the signals from the as-deposited samples (Fig. 5b), we have
obtained similar results in other works [28,29] where the anneal-
ing samples at high temperature decrease the excitation effi-
ciency and the corresponding emission due to the loss of oxygen
weakly absorbed on the surface of the films that can act as a
bridge to enhance the bonding state of activator (Dy3 þ
ions) with
O–Ga of the material host and thus improve the energy transfer
process. When the samples are further annealed at 800 1C, the
host defect of band gap excitation suddenly disappears and the
luminescence quenches.
However, it is observed that co-doping of transition metals
remarkably influences PL properties of the Ga2O3–Dy3 þ
films; in
this case the concentration of 10 mol% of co-dopant decreases the
PL properties of the Ga2O3–Dy3 þ
films; the presence of Co3 þ
or
Cr3 þ
ions at 10 mol% may act as PL killers due to the energy
transfer from excited Dy3 þ
to Co3 þ
or Cr3 þ
ions. On the other
hand, co-doping at this level of concentration leads to an increase
of surface defects that will be involved and the non-radiative
relaxation rate will increase. PL studies of TiO2:Sm3 þ
prepared by
the sol–gel method have shown that on co-doping with different
amounts of Br
[30] or Bi3 þ
or Zr4 þ
[31] the PL intensities
decrease when the co-dopant concentration is 41 mol%.
Based on the results obtained by other authors [30–32], similar
to those obtained in this work, we proposed a model of energy
transfer to explain why the co-doping in the films causes a
reduction in the PL emissions of Dy3 þ
ion. This co-doping system
is proposed in Fig. 6. (1) under 254 nm irradiation (band
gap excitation) the generation process of excitations from the
valence band (VB) to the conduction band (CB) is produced;
20
30
40
50
60
70
80
90
100
(4)
(3)
(2)
(1)
(4)
(3)
(2)
(1)
(4) Ga O -Dy -Co
(3) Ga O -Dy -Cr
(2) Ga O -Dy
(1) Ga O
Transmittance
(%)
Wavelength (nm)
200 300 400 500 600 700 800 900
200 300 400 500 600 700 800 900
10
20
30
40
50
60
70
80
90
100
(1)
(2)
(3)
(4)
(1) Ga O
(2) Ga O -Dy
(3) Ga O -Dy -Cr
(4) Ga O -Dy -Co
Trasmittance
(%)
Wavelength (nm)
Fig. 4. Transmission spectra for the Ga2O3–Dy3þ
–M3 þ
thin films. (Inset) Square of absorption coefficient as a function of photon energy for Ga2O3–Dy3þ
–M3þ
(a) as-deposited and (b) annealed thin films.
Table 1
Measured optical band gap (eV).
Sample Ga2O3 Ga2O3–Dy3þ
Ga2O3–Dy3 þ
–Cr3 þ
Ga2O3–Dy3þ
–Co3þ
As-deposited 4.870.1 4.770.1 4.570.1 4.570.1
Annealed 4.970.1 4.770.1 4.670.1 4.570.1
G. Cabello et al. / Journal of Physics and Chemistry of Solids 72 (2011) 1170–1174 1173

6. Author's personal copy
(2) the electrons move freely in the CB and finally relaxes to the
donor band (oxygen vacancies) or directly to 4
F9/2 excited state
level of Dy3 þ
; (3) the emission process occurs due to the
transitions from the excited state 4
F9/2 to the ground levels of
6
H15/2, 6
H13/2 and 6
H11/2 from Dy3 þ
. These emission processes are
reduced by (4) the presence of defects caused by co-doping of
Cr3 þ
or Co3 þ
that produces the non-radiative energy transfer
from the excited state of Dy3 þ
to defects of the transition metals,
leading to the decrease of emission intensity (5) and process of
non-radiative transitions of transition metal (M).
4. Conclusions
The Ga2O3–Dy3þ
–Co3þ
and Ga2O3–Dy3þ
–Cr3þ
thin films, were
successfully prepared by a simple photochemical method. The
compositional and structural characterization of the samples using
XPS and XRD, respectively, revealed the Ga2O3 formation for the
host material and Dy(III), Cr(III) or Co(III) as activators and the
amorphous structure of the films. The emission spectra of the Dy3þ
doped samples showed characteristic 4
F9/2-6
HJ (J¼ 15/2, 13/2 and 11/2)
transitions, but these emissions decreased with the co-doped films.
A study of decay times or decay processes of PL is necessary to verify
the proposed mechanism in the present preliminary study.
Acknowledgments
The authors are grateful to the financial support of the
FONDECYT (National Fund for Scientific and Technological Devel-
opment), Chile, Grant no. 1100045 and Direccion de Investigacion
de la U. del Bıo-Bıo, DIUBB proyect. Grant no. 092509 3/R.
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0
10000
20000
30000
40000
50000
60000
70000
80000
(2)
(1)
(3)
(4)
652
nm
622
nm
605
nm
594
nm
575
nm
500
nm
Intensity
(a.u.)
Wavelength (nm)
450 500 550 600 650 700 450 500 550 600 650 700
0
10000
20000
30000
40000
50000
60000
652
nm
(4)
(3)
(2)
(1)
605
nm
594
nm
622
nm
575
nm
500
nm
Intensity
(a.
u.)
Wavelength (nm)
Fig. 5. PL spectra of (a) as-deposited and (b) annealed films at 800 1C of the samples: (1) Ga2O3; (2) Ga2O3–Dy3þ
–Co3þ
; (3) Ga2O3–Dy3þ
–Cr3 þ
and (4) Ga2O3–Dy3 þ
thin films.
Fig. 6. A simple model illustrating the emission process in Ga2O3–Dy3þ
thin films
and the energy transfer from Ga2O3–Dy3þ
–M3þ
thin films (M¼Cr or Co).
G. Cabello et al. / Journal of Physics and Chemistry of Solids 72 (2011) 1170–1174
1174