Microstructure and optical properties of nanocrystalline CaWO4 thin films deposited by pulsed laser ablation in room temperature

Article abstract of DOI:10.1016/j.jallcom.2006.07.130

Nanocrystalline CaWO₄ films were successfully deposited by pulsed laser ablation at various background Ar gas pressures (10-100 Pa) without substrate heating or after annealing treatment. The effects of Ar pressure on microstructure, surface morphology, chemical composition and optical properties were investigated by XRD, HR-TEM, FE-SEM, XPS, UV-vis and PL analyses. The crystallite size of CaWO₄ films increased with increasing Ar pressure, which was associated with a change of surface morphology. Reduced tungsten states [W⁵⁺] or [W⁴⁺] caused by oxygen vacancies were observed at 10 Pa. However, over 50 Pa, the atomic concentration of all the constituent elements was almost constant, especially [Ca]/[W] ratio, which was nearly unity. The optical energy band-gap of CaWO₄ films was strongly dependent on the Ar pressure, i.e., decreased from 4.9 to 4.5 eV with the increase of Ar pressure from 50 to 100 Pa. The photoluminescence (PL) spectra was positioned in a blue-shifted region around 378 nm compared with emission at 420 nm of bulk CaWO₄ target, which clearly demonstrates the optical band-gap widening phenomena induced by quantum-size effect.

Full text of DOI:10.1016/j.jallcom.2006.07.130

Journal of Alloys and Compounds 441 (2007) 146–151
Microstructure and optical properties of nanocrystalline CaWO4 thin films
deposited by pulsed laser ablation in room temperature
Jeong Ho Ryua,b,∗, Sin Young Bangb, Woo Sik Kimb, Gyeong Seon Parkb, Kang Min Kimb,
Jong-Won Yoonb, Kwang Bo Shimb, Naoto Koshizakic
a Electro Materials & Device Laboratory, Cooperate R&D Institute, Samsung Electro-Mechanics Co., Ltd., 314,
Maetan3-Dong, Yeongtong-Gu, Suwon City, Gyunggi-Do 443-743, South Korea
b Department of Ceramic Engineering, Ceramic Processing Research Center (CPRC), Hanyang University,
17 Haengdang-Dong, Seongdong-Gu, Seoul 133-791, South Korea
c Nanoarchitectonics Research Center (NARC), National Institute of Advanced Industrial Science and Technology (AIST),
Central 5 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Received 17 January 2006; accepted 4 July 2006
Available online 30 October 2006
Abstract
Nanocrystalline CaWO4 films were successfully deposited by pulsed laser ablation at various background Ar gas pressures (10–100 Pa) without
substrate heating or after annealing treatment. The effects of Ar pressure on microstructure, surface morphology, chemical composition and optical
properties were investigated by XRD, HR-TEM, FE-SEM, XPS, UV–vis and PL analyses. The crystallite size of CaWO4 films increased with
increasing Ar pressure, which was associated with a change of surface morphology. Reduced tungsten states [W5+] or [W4+] caused by oxygen
vacancies were observed at 10 Pa. However, over 50 Pa, the atomic concentration of all the constituent elements was almost constant, especially
[Ca]/[W] ratio, which was nearly unity. The optical energy band-gap of CaWO4 films was strongly dependent on the Ar pressure, i.e., decreased
from 4.9 to 4.5 eV with the increase of Ar pressure from 50 to 100 Pa. The photoluminescence (PL) spectra was positioned in a blue-shifted region
around 378 nm compared with emission at 420 nm of bulk CaWO4 target, which clearly demonstrates the optical band-gap widening phenomena
induced by quantum-size effect.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Oxide materials; Thin films; Laser processing; Microstructure; Luminescence
1. Introduction
method [9,10], precipitation route [11,12], hydrothermal synthe-
sis [13] and solid-state reactions [14,15]. It is generally agreed
that a thin film phosphor has superior resolution compared to
powder because of inherently smaller grain size and less lateral
scattering. Various physical and chemical deposition methods
including sputtering deposition [16], vacuum evaporation [17],
electron beam evaporation [18], electrochemical route [19] and
spray pyrolysis [20] have been reported for preparation of the
CaWO4 films.
However, CaWO4 films prepared by the sputtering contain
Ca3WO6, Ca6WO9 and WO3 as impurity phases induced by
high vapor pressure of WO3 [16]. Moreover, CaWO4 films pre-
pared by vacuum evaporation showed no luminescence because
of poor crystallinity [17] unless subjected to heat treatment
(500–1100 ◦C), whichledtocrackingand/orpeelingofthefilms.
Such problems have rendered preparation of the CaWO4 films
difficult.
The synthesis and electro-optical properties of scheelite-type
metal tungstates have been intensively investigated during the
past half century [1–20]. Especially, calcium tungstate (CaWO4)
has attracted particular attention because it has been found to
have practical importance as MASER materials [4–6] and scin-
tillators [7,8] in quantum electronics and medical applications.
The CaWO4 single crystals and polycrystalline powder have
been prepared by several techniques such as the Czochralski
∗
Corresponding author at: Electro Materials & Device Laboratory, Cooperate
R&D Institute, Samsung Electro-Mechanics Co., Ltd., 314, Maetan 3-Dong,
Yeongtong-Gu, Suwon-City,Gyunggi-Do 443-743, South Korea.
Tel: +82 31 210 3089; Fax: +82 31 300 7900.
E-mail address: jimihen.ryu@samsung.com (J.H. Ryu).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.07.130

J.H. Ryu et al. / Journal of Alloys and Compounds 441 (2007) 146–151
147
Recently, extensive and successful efforts on pulsed laser
ablation(PLA)havebeenundertakenandsuccessfullyemployed
in the past for thin film processing [21–24], since it is a powerful
and attractive choice for preparation of stoichiometric high-
quality complex compound films. The excellent capability of
the PLA for reactive deposition and to transfer the original stoi-
chiometry of bulk targets to the deposited film makes it suitable
for the fabrication of simple and complex oxide films [25–27].
Therefore, it is interesting challenge to synthesize thin films of
scheelite-type complex oxides by using PLA technique. Tanaka
et al. [28] reported film fabrication of CaWO4 by PLA using
targets composed of CaO and WO3 mixtures. However, no crys-
talline CaWO4 film was obtained in their conditions at substrate
temperature below 600 ◦C [28].
In this study, we report deposition and characterization of
nanocrystalline CaWO4 films using PLA without substrate heat-
ing or after-annealing treatment. Improved understanding of the
relationship between growth conditions, microstructures and
physical properties are essential to produce the film to meet
the requirements of a given technological application, because
the properties and application of the resulting CaWO4 films are
sensitive to the growth conditions employed in the film deposi-
tion. For these reasons, we focused our attention on the room
temperature growing mechanism in PLA process and effects of
background Ar pressure on the microstructure and optical prop-
erties. Moreover, this allowed us to investigate the quantum-size
effect on optical energy band-gap and photoluminescence (PL)
emission of the CaWO4 films.
2. Experimental
Fig. 1. XRD patterns of: (a) bulk target and (b) nanocrystalline CaWO4 films
deposited at various Ar pressures.
The polycrystalline CaWO4 powder was prepared by solid-state reaction
method. At first, 50 mL of ethanol (99.5%) was added to a stoichiometric mixture
of CaCO3 (purity 99.99%, Wako Chemical Co. Ltd., Japan) and H2WO4 (purity
99.99%, High Purity Chemical Co. Ltd., Japan), and subsequently mixed in
a planetary ball-mill for 12 h. The resultant mixture was then filtered, dried
and reacted in 800 ◦C for 5 h in an alumina crucible and then cooled down to
room temperature. The heat-treated powder was uni-axially cold-pressed into
a disk-shaped pellet with 300 MPa and then sintered at 1000 ◦C for 3 h in an
oxygen atmosphere. The resulting polycrystalline target was white–yellow in
color and powder XRD measurement indicated that the target was single phase
and consistent with JCPDS data (card number, 41-1431) as shown in Fig. 1(a).
An Nd-YAG laser (wavelength = 355 nm) was used for the ablation. The rep-
etition rate and pulse width of the laser were 10 Hz and 7 ns, respectively. The
quartz substrate, with typical sizes of 10 mm × 20 mm × 1 mm, was first ultra-
sonically cleaned in acetone and then mounted near the target at an off-axial
position in order to minimize the concentration of undesirable particles and
outgrowths on the film surface [29–31]. The laser beam was focused through
an optical window at a 45◦ oblique incidence onto the target, which was rotat-
ing at a rate of 30 rpm to avoid drilling. The maximum laser energy was set
at 200 mJ/pulse and total irradiation pulses were approximately 20,000. The
base pressure prior to laser ablation was about 1 × 10−7 Torr and working
pressure of background Ar gas was varied in a range from 10 to 100 Pa. All
deposition processes were carried out in room temperature without substrate
heating.
mated Al K␣ source (1486.5 eV, 100 W). The typical pass energy and the energy
resolution were 5.85 eV and 0.05 eV/step, respectively. The binding energy
values were calibrated with the C 1s line of adventitious carbon at 284.5 eV.
Optical absorption property was measured with a UV–vis spectrophotometer
(Shimadzu, UV-2100PC, Japan) and room temperature PL spectra were recorded
with a luminescence spectrophotometer (PerkinElmer, LS45, USA) using quasi-
monochromatic light dispersed by a 0.5 monochromator from a xenon lamp as
optical excitation source.
3. Results and discussion
TheX-raydiffractionpatternsofthebulktargetandnanocrys-
talline CaWO4 films deposited under various Ar pressures are
presented in Fig. 1. CaWO4 crystals exist in nature as scheelite-
type tetragonal structure. In the scheelite structure, [W]-ions are
in tetragonal [O]-ion cages and isolated from each other, and
[Ca]-ion is surrounded by eight oxygen ions [32]. At all Ar pres-
sures in Fig. 1(b), most of the prominent peaks corresponding
scheelite-type CaWO4 phase were clearly observed without any
un-reacted or additional phases such as CaO, WO3, Ca2WO6 or
Ca6WO9 phases. By increasing Ar pressure from 10 to 100 Pa,
intensity of XRD peaks increased and full width at half maxi-
mum decreased, indicating that crystallinity and crystallite size
increased. The particle sizes were calculated from (1 1 2) peaks
ThecrystallinityandmicrostructurewereexaminedbyanX-raydiffractome-
ter (XRD, Rigaku RAD-C, Japan) using Cu K␣ radiation and a high resolution
transmission electron microscope (HR-TEM, JEOL 4010, Japan) equipped with
a field emission gun operating at 400 kV. Surface morphology was observed
using field-emission scanning electron microscope (FE-SEM, Hitachi, S-4800).
The surface composition and chemical states analyses were performed with
X-ray photoelectron spectroscopy (XPS, PHI 5600ci, USA) using a monochro-

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J.H. Ryu et al. / Journal of Alloys and Compounds 441 (2007) 146–151
using Scherrer’s formula. D = kλ/β cos θ, where D is the average
grain size, k a constant equal to 0.89, λ the wavelength of X-rays
and β is the corrected half-width that is obtained by using (1 1 1)
line of the pure silicon as the standard. The calculated average
crystallite sizes of CaWO4 were 8.2, 10.9, and 13.6 nm at Ar
pressures of 10, 50, 100 Pa, respectively.
Fig. 2(a) through (c) present high-resolution transmission
electron microscope (HR-TEM) images of the nanocrystalline
CaWO4 films deposited in various Ar pressures. From the elec-
tron diffraction patterns, several groups of diffraction rings are
identified and indexed in inset of Fig. 2(c). The electron diffrac-
tion rings correspond well with d spacing of scheelite-type
CaWO4. Mean particle sizes of CaWO4, calculated from about
100particlesintheTEMimages, were8.5 nmforfilmsdeposited
at 10 Pa, 11.3 nm at 50 Pa and 13.8 nm at 100 Pa. This agrees well
with the results calculated by XRD.
Fig. 3(a–c) represents surface morphology of the nanocrys-
talline CaWO4 films deposited at various Ar pressures. The
surface of the CaWO4 film deposited at 10 Pa was smooth.
However, at higher Ar pressures, the surface became rougher
resulting from the formation of nanometer-sized particulates.
This change in the surface morphology can be attributed to
the collision phenomena in the PLA process [33]. Low ambi-
ent pressures allow the ejected species to land on the substrate
for continuous film formation. However, the plume confined by
high ambient pressures induces multiple collisions, and thereby
the super-saturation and condensation of the vaporized materi-
als in the ambient gas take place. As a result, larger particulates
are deposited on the substrate at Ar pressure of 100 Pa. Accord-
ing to our previous report on preparation of perovskite oxides
using PLA in room temperature, the particle size and surface
roughness of nanocrystalline BaTiO3 and LaFeO3 increased
with increasing Ar pressure in a pressure range of 10–100 Pa
[33]. Therefore, it is considered that the growing mechanism of
nanocrystalline CaWO4 at different Ar pressures in room tem-
perature is similar to those of the perovskite oxides.
The XPS was used to infer the influence of the Ar pressure
on the chemical composition of the CaWO4 films. Fig. 4(a and
b) shows the high-resolution XPS spectra of the [Ca] 2p and
[W] 4f regions of the CaWO4 films deposited in a range from
10 to 100 Pa. The measured binding energies and branching
ratio of the [Ca] 2p1/2 and 2p3/2 spin–orbit doublet at 350.4
and 346.8 eV are typical values of oxidized states of the [Ca]
[34,35]. The [W] 4f core level spectrum recorded on samples
at 50 and 100 Pa shows the two components associated with
4f5/2 and 4f7/2 spin–orbit doublet at 37.5 and 35.4 eV, respec-
tively. These values are in agreement with those found in the
literature for [W6+] in stoichiometric films [34–36]. However,
the spectrum of the sample deposited at 10 Pa have another peak
shifted towards lower binding energy, which may be caused by
the contribution of [W5+] or [W4+] states resulting from oxygen
vacancies in the films [37]. It was reported that these states could
play an important role in the transmittance as defect centers [38].
The atomic ratio of [Ca]/[W] estimated from XPS is pre-
sented in inset of Fig. 4(b). From the XPS analysis of the films,
oxygen deficiency was observed and the [Ca] ion was rich com-
pared to the [W] ion at the low-pressure range of 10 Pa. However,
Fig. 2. HR-TEM images of nanocrystalline CaWO4 films deposited on carbon
film at: (a) 10 Pa, (b) 50 Pa and (c) 100 Pa. The selected area electron diffraction
patterns corresponding to (c) are shown in inset.
over 50 Pa, atomic concentration of all constituent elements was
almost constant, especially the [Ca]/[W] ratio, which was almost
unity. When density and energy of the ablated particles are not
very high, interaction of plume with background gas can be con-
sidered as the scattering of a molecular beam by the particles of

J.H. Ryu et al. / Journal of Alloys and Compounds 441 (2007) 146–151
149
Fig. 4. XPS spectra of the: (a) [Ca] 2p energy region and (b) [W] 4f energy
region of the CaWO4 films deposited at various Ar pressures. The [Ca]/[W]
atomic ratio of the films is shown in inset of (b).
Fig. 5. The absorption spectra of CaWO4 films deposited at
50 and 100 Pa showed a typical absorption edge around 280
and 310 nm, respectively. The fundamental absorption mech-
anism of CaWO4 is attributed to charge-transfer transition in
which an oxygen 2p electron goes into on of the empty tungsten
Fig. 3. Surface morphology of nanocrystalline CaWO4 films deposited at: (a)
10 Pa, (b) 50 Pa and (c) 100 Pa. The roughness of the film surfaces increases
with the Ar pressure.
the background gas [39,40]. It is well known from a Monte Carlo
simulation that non-stoichiometric deposition takes place when
targetisablatedinto ambient gas withlowpressure [40]. Accord-
ing to the simulated results, the increase of the background
pressure was found to give rise to the more uniform distribution
of the deposited particles and the constant stoichiometric ratio.
This simulated calculation explains our experimental results on
the stoichiometric variation in CaWO4 films well.
Fig. 5. Optical absorption spectra of nanocrystalline CaWO4 thin films
deposited at 50 and 100 Pa (inset) and plots of (αhν) vs. (hν). The optical energy
band-gap is deduced form the extrapolation of the straight line up to (αhν)2 = 0.
The optical absorption spectra of the nanocrystalline CaWO4
films deposited at various Ar pressures are presented in inset of

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J.H. Ryu et al. / Journal of Alloys and Compounds 441 (2007) 146–151
1
5d orbitals, i.e., the transitions between A1 ground state and
1T2 excited state within [WO42−] complex [1–3]. The abnor-
mal flat absorption curve in 10 Pa can be explained by surface
scattering effect in oxygen defect sites, which was investigated
in XPS analysis. The optical energy band-gap, Egap, for the
nanocrystalline CaWO4 films were determined form the sharply
increasing absorption region. According to Tauc and Menth’s
law [41], the absorption coefficient has the following energy
dependence, α(hν) = A(hν − Egap)m, where A is a proportion-
ality constant, m another constant, which is different for the
transition types (m = 1/2, 2, 3/2 or 3 for allowed direct, allowed
indirect, forbidden direct and forbidden indirect electronic tran-
sitions, respectively), hν is photon energy, and Egap is the Tauc
optical energy band-gap. The plots of (αhν)2 versus photon
energy (hν) for the CaWO4 nanocrystalline films deposited at
various Ar pressures are displayed in Fig. 5. In low energy
region of the edge, the absorption spectrum deviated from the
straight line plot. However, in high energy region of the absorp-
tion edge, (αhν)2 varied linearly with photon energy (hν). This
straight line behavior in the high energy region was taken as
prime evidence for the direct band-gap. The optical band-gap
was therefore determined by extrapolating the linear portion of
the plot relating (αhν)2 versus hν to (αhν)2 = 0 (see Fig. 5).
The optical energy band-gap was estimated to be about 4.9 and
4.5 eV for films deposited at 50 and 100 Pa, respectively. It was
reported that CaWO4 has direct band-gap of 4.09 eV in Γ direc-
tion calculated by using the linearized-augmented-plane-wave
method [42]. The band-gap widening of nanocrystalline CaWO4
films compared to the simulated theoretical values (4.09 eV)
is considered mainly due to quantum-size effects. It is clear
that the optical energy band-gap is higher for films deposited
at 50 Pa, where the grain size is relatively lower than that of
100 Pa. When the average crystal dimension becomes small,
the quantum-size effect results in widening of the optical band-
gap. The widening of optical energy band-gap with reduction
in size was also reported for several other complex oxide films
[43–45].
The room temperature photoluminescence (PL) emission
spectra of the CaWO4 films deposited at Ar pressure of 50 and
100 Pa are shown in Fig. 6(a and b). The PL emission spectra
of the CaWO4 films were obtained at excitation of 240 nm. It is
well known that CaWO4 shows emission in the blue or green
regions depending on excitation wavelength or energy. Espe-
cially, optical band-edge excitation yields a strong blue emission
and near-band-edge excitation yields a green emission. Most of
the studies on room temperature PL for CaWO4 crystals reported
a blue emission around 420 nm for excitation between 240 and
280 nm and a green emission around 510 and 540 nm for exci-
tation between 300 and 315 nm [1–8]. For the CaWO4 films
prepared by electron beam evaporation [18], electrochemical
method [19] and PLA of oxide mixtures in high temperature
[28], only blue emissions at 420–460 nm were obtained at room
temperature.
Fig. 6. PL emission spectra of the CaWO4 films deposited at: (a) 50 Pa and (b)
100 Pa. The decomposed Gaussian curves are given with PL spectrum. The inset
shows the room temperature PL spectrum of the bulk target for comparison with
nanocrystalline films.
target is shown together in inset of Fig. 6(a). It is noted that
the nanocrystalline CaWO4 films deposited by PLA have blue
shifted PL emission spectra about 40 nm compared to bulk target
at room temperature. This blue shift of PL spectra and band-gap
widening at the films with relatively small grain size may be
considered mainly due to the quantum-size effect. In the films
prepared at 50 and 100 Pa as shown in Figs. 2 and 3, there exist
very small nanocrystallites, grain boundaries and imperfections,
which lead to larger free carrier concentrations and the existence
of potential barriers at the boundaries. Therefore, electric fields
are formed and this results in band-gap widening. In particular,
the film deposited at 50 Pa had two Gaussian components shifted
to higher energy range shown in Fig. 6(a), which are attributed
to the presence of smaller nanocrystallites. Moreover, there was
another Gaussian component in a lower energy range for the
film deposited at 100 Pa as shown in Fig. 6(b), which shows that
the PL spectra shifted to a lower energy range with an increase
of particle size. It is very clear that the optical energy band-gap
is much wider for films deposited at lower Ar pressures, where
the crystallite size is relatively smaller than that of higher Ar
pressures.
Consisted with the optical band-gap widening estimated from
the Tauc plot in Fig. 5, both nanocrystalline CaWO4 films
deposited 50 and 100 Pa exhibited a sharp blue emission peak
near 378 nm. For comparison, a PL spectrum of the bulk CaWO4

J.H. Ryu et al. / Journal of Alloys and Compounds 441 (2007) 146–151
151
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Nanocrystalline CaWO4 films were successfully deposited
by PLA in room temperature with controlling the Ar pressures
from 10 to 100 Pa. The films had a single scheelite phase with
an average crystallite size around 10 nm. The crystallite size and
surface roughness tended to increase with Ar pressure, which
was due to the formation of nanoparticulates. The Ar gas pres-
sure was found to influence the stoichiometric ratio of the films
and the [Ca]/[W] ratio was close to unity at Ar pressure above
50 Pa, which ascertained that the increase of Ar pressure resulted
in uniform stoichiometrical ratio of the films. The estimated
optical band-gap of nanocrystalline CaWO4 films deposited at
50 Pa was estimated to 4.9 eV and decreased to 4.5 eV with the
increase of Ar pressure to 100 Pa. The optical band-gap widen-
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films compared with bulk target was attributed mainly to the
quantum-size effect induced by very small crystallite size. It is
worthy to note that this is the new approach for room temper-
ature deposition of nanocrystalline CaWO4 films using PLA.
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widening and blue-shift of PL emission spectra can be helpful
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Acknowledgement
This work was supported by the Korea Science and Engi-
neering Foundation (KOSEF) through the Ceramic Processing
Research Center (CPRC).
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