3þ

UV cathodoluminescence of Gd
doped and Gd3þ–Pr3þ co-doped
YAlO3 epitaxial thin ﬁlms
1

,1

,2

solidistatusphysicaPhys. Status Solidi A 212, No. 3, 703–706 (2015) / DOI 10.1002/pssa.201431778

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applications and materials science3

Yuhei Shimizu , Kazushige Ueda* , Hiroshi Takashima** , and Yoshiyuki Inaguma
1

Department of Materials Science, Graduate School of Engineering, Kyushu Institute of Technology, 1-1 Sensui, Tobata,
Kitakyushu 804-8550, Japan
2
Electronics Photonics Research Institute, National Institute of Advanced Industrial Science Technology, 1-1-1 Umezono, Tsukuba,
Ibaraki 305-8568, Japan3
Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan
Received 6 October 2014, revised 4 November 2014, accepted 10 November 2014
Published online 16 January 2015
Keywords epitaxial thin films, Gd3þ, photoluminescence, pulsed laser deposition, ultraviolet cathodoluminescence, YAlO3* Corresponding author: e-mail kueda@che.kyutech.ac.jp, Phone: þ81 93 884 3339**e-mail h-takashima@aist.go.jp, Phone: þ81 29 861 5594

Thin ﬁlms of YAlO3:Gd3þ and YAlO3:Gd3þ–Pr3þ wereprepared on LaAlO3 (001) single crystal substrates by pulsed
laser deposition (PLD). Both ﬁlms were grown epitaxially andtheir surfaces were uniformly smooth. Under UV excitation at
216 nm, YAlO3:Gd3þ ﬁlm did not show any intense photoluminescence (PL); on the other hand, YAlO3:Gd3þ–Pr3þ ﬁlm
showed intense Gd3þ PL at 314 nm through Pr3þ 4f–5d
absorption. Under electron irradiation with acceleration of
2.00 kV, both ﬁlms showed UV cathodoluminescence (CL) from
Gd3þ ions. In contrast to PL, the CL intensity of the YAlO3:Gd3þ

ﬁlm was four times greater than that of the YAlO3:Gd3þ–Pr3þ
ﬁlm. The PL excitation spectrum of YAlO3:Gd3þ in vacuum UV
region revealed the presence of an intense excitation band at
155 nm assignable to YAlO3 host absorption. This observation
implied that the energy of accelerated electrons was absorbed by
the host and then transferred to Gd3þ. Pr3þ co-doping was foundto weaken the Gd3þ emission in CL because the excitation energy
absorbed by the host was transferred to not only Gd3þ but also
Pr3þ accompanying visible emission and nonradiative transitionin Pr3þ.

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1 Introduction Some UV emitting perovskite-typeoxide phosphors such as (Y1xLux)AlO3:Ce3þ, (Y1xLux)
AlO3:Pr3þ, and YAlO3:Gd3þ have been reported in the formof single crystals or powders [1–11]. Ce3þ and Pr3þ show abroad UV emission peak attributed to 5d–4f transition.
Because their emission decay time is as short as approximately 10 ns, Ce3þ and Pr3þ doped perovskite-type oxide
phosphors were examined to use as a scintillator [1–9]. On theother hand, Gd3þ shows a sharp UV emission attributed to
4f–4f transition from 6P7/2 to 8S7/2 ground level in Gd3þ [12,13]. Using the sharp UV emission, some applications such asa phototherapy lamp for skin diseases and a transilluminatorfor DNA analysis are anticipated [14, 15]. Because the studiesof these phosphors have been mainly conducted in the form of
single crystals or powders [1–11], their thin ﬁlms have notbeen extensively studied yet in terms of both ﬁlm growth andoptical evaluation.

Perovskite-type oxides frequently form high quality thinﬁlms on perovskite-type substrates. Actually, the growth ofhigh quality phosphor thin ﬁlms such as AHfO3:Tm3þ,ATiO3:Pr3þ, and ASnO3:Tb3þ on SrTiO3 substrates has
been reported along with their intense visible photoluminescence (PL) [16–18]. Because oxide phosphors haveexcellent chemical stability, thin ﬁlms of the perovskite-typeoxide phosphors are suitable for some application to thinﬁlm-type optoelectronic devices.
The PL of Gd3þ doped YAlO3 was recently examined
and Pr3þ co-doping was found to enhance the UV PL of
Gd3þ [10, 11, 19]. To develop surface emitting devices orother optoelectronic application, the examinations ofcathodoluminescence (CL), as well as PL, of the Gd3þ
doped YAlO3 phosphors are important. Although Gd3þ
doped YAlO3 is a promising phosphor in PL, no
investigation on its CL has been reported yet. In this study,ß 2015 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim

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Y. Shimizu et al.: UV CL of Gd and Gd–Pr doped YAlO3 epitaxial films

ﬁne thin ﬁlms of YAlO3:Gd3þ and YAlO3:Gd3þ–Pr3þ weregrown by pulsed laser deposition (PLD), and their UV CL
properties were evaluated for future application to Gd3þ UV
emitting devices.
2 Experiments YAlO3:Gd3þ and YAlO3:Gd3þ–Pr3þ
ﬁlms were deposited on LaAlO3 (LAO) (001) pseudocubic
single crystal substrates by PLD. Polycrystalline YAlO3:
Gd3þ and YAlO3:Gd3þ–Pr3þ sintered disks with the nominal
compositions of (Y0.97Gd0.03)AlO3 and (Y0.94Gd0.03Pr0.03)
AlO3, respectively, were used as targets. The deposited thinﬁlms were annealed at 1000 8C in air to improve crystallinity
and luminescent intensities. Detailed PLD and annealing
conditions are listed in Table 1. X-ray diffraction (XRD)
patterns of the ﬁlms were measured by an X-ray diffractometer (Rigaku, RINT 2500), and surface morphology of theﬁlms was observed using an AFM (Digital Instruments,
Nanoscope III). The concentrations of Gd3þ and Pr3þ in theﬁlms were analyzed by XPS (SHIMADZU/KRATOS, AXISNOVA) evaluating Gd 3d5/2 and Pr 3d5/2 peak intensities.
Transmission spectra and PL/PL excitation (PLE) spectra
were measured using an UV/Vis/NIR spectrophotometer(Jasco, V-570) and a spectroﬂuorometer (Jasco, FP-6500),
respectively. A PLE spectrum in vacuum UV (VUV) region
was measured using a polycrystalline YAlO3:Gd3þ disk at thebeam line BL-7B of UVSOR in the Institute for Molecular
Science. CL spectra of the ﬁlms were measured using a ﬁeld
emitter with a gate electrode as an electron source and aspectroﬂuorometer (Jasco, FP-6500) with an optical ﬁber. In
the CL measurements, the acceleration voltage in the electron
emitter was varied from 0.00 to 2.00 kV.
3 Results and discussion Epitaxial growth of theﬁlms was conﬁrmed by XRD and reﬂected high-energy
electron diffraction (RHEED). An XRD pattern of the
YAlO3:Gd3þ ﬁlm is shown in the inset of Fig. 1. The XRD
pattern of the YAlO3:Gd3þ–Pr3þ ﬁlm, which is not
displayed in the inset, was nearly the same. Strong peaks
assigned to (00l) planes were observed indicating theTable 1 PLD and annealing conditions.
PLD conditions
targetsubstratelaser source
laser fluencelaser frequency
O2 partial pressure
substrate temperature(Y0.94Gd0.03Pr0.03)AlO3(Y0.97Gd0.03)AlO3
LaAlO3 (001) single crystal
ArF excimer (193 nm)~1.2 Jcm2pulse1
16 Hz
0.1 Pa740 ºCannealing conditions
atmospheretemperature
durationair1000 ºC
1hß 2015 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim

Figure 1 Transmission spectra of YAlO3:Gd3þ and YAlO3:
Gd3þ–Pr3þ thin ﬁlms. XRD pattern of orthorhombic YAlO3:Gd3þ
thin ﬁlm is shown in the inset along with the AFM image of its
surface.

epitaxial growth of the ﬁlms. Small lattice mismatch
between YAlO3 and LAO (2.2%) are responsible for theepitaxial growth. Streaky patterns observed by RHEED
demonstrated a high crystal quality of the epitaxial ﬁlms.
The dopant concentrations in the ﬁlm examined by XPS
were Gd 3.9% for the YAlO3:Gd3þ ﬁlm and Gd 3.9%/Pr
3.2% for the YAlO3:Gd3þ–Pr3þ ﬁlm, which were almost thesame with those in the targets. Transmission spectra of thinﬁlms and substrate are shown in Fig. 1. Drastic transmittance drop at 220 nm was ascribed to the fundamental
absorption edge of LAO [20]. Transmittance of a LAO
substrate at 314 nm was found to be high (>60%) enough totransmit Gd3þ emission. The transmittances of both ﬁlms on
LAO substrates were as high as that of the LAO substrate,
demonstrating their high transmittances. Film thicknesses
were estimated to be approximately 220 nm by interferencein the transmission spectra. Flat and smooth surface of the
YAlO3:Gd3þ ﬁlm was observed in the AFM image shown in
insets. The surface of the YAlO3:Gd3þ–Pr3þ ﬁlm was also
as ﬁne as that of the YAlO3:Gd3þ one.
Figure 2 shows PL and PLE spectra of YAlO3:Gd3þ,YAlO3:Gd3þ–Pr3þ ﬁlms, and a PLE spectrum of YAlO3:
Gd3þ bulk in VUV region. A sharp emission peak, which
was attributed to the 6P7/2–8S7/2 transition in Gd3þ, wasobserved at 314 nm in both PL spectra. Another small
emission peak attributed to the 6P5/2–8S7/2 was also observed
at 308 nm [12, 13]. In the PLE spectrum of the YAlO3:
Gd3þ–Pr3þ ﬁlm, an intense broad excitation band wasobserved around 216 nm, which was assigned to the 4f–5dtransition in Pr3þ [1-5]. The intense excitation band
indicated the excitation energy absorbed by Pr3þ wastransferred from Pr3þ to Gd3þ, resulting in the distinctive
Gd3þ emission at 314 nm. On the other hand, no obvious
excitation band was observed in the PLE spectrum of the
YAlO3:Gd3þ ﬁlm in the wavelength region above 200 nm.
Accordingly, the Gd3þ emission intensity was very weak in
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Original
Paper
Phys. Status Solidi A 212, No. 3 (2015)705

the YAlO3:Gd3þ ﬁlm, as far as the PL spectrum wasmeasured using a conventional spectroﬂuorometer with axenon lamp. The PLE spectrum of a YAlO3:Gd3þ bulk in
the VUV region revealed that an intense excitation band was
located at 155 nm (8.0 eV). The energy of the excitation
band corresponded to the band gap energy of YAlO3[21].
Accordingly, it was found that excitation of YAlO3 host by
VUV irradiation led to the intense Gd3þ emission even in
Gd3þ single doped YAlO3.
Figure 3 shows CL spectra of YAlO3:Gd3þ and
YAlO3:Gd3þ–Pr3þ thin ﬁlms at electron acceleration
voltage of 2.00 kV. An intense UV CL peak attributed to
the Gd3þ 6P7/2–8S7/2 transition was observed at 314 nm.
Another CL peak attributed to 6P5/2–8S7/2 was observed
more clearly than that in the PL spectra. Because the energyof accelerated electrons is as high as a few keV, theexcitation energy by electron irradiation in CL is ﬁrst

absorbed by the YAlO3 host. Therefore, the observation of
the intense Gd3þ CL peak suggested that the energy transferfrom the YAlO3 host to Gd3þ occurred smoothly, which issimilar to the intense PL in the YAlO3:Gd3þ bulk observed
under VUV excitation. Inset of Fig. 3 shows acceleration
voltage dependence of the CL intensity at 314 nm. The ﬁlms
started to exhibit the Gd3þ CL peak at the acceleration
voltage above 0.75 kV. With increasing the acceleration
voltage, the CL intensity of the ﬁlms increased. At a 2 kVacceleration, the CL intensity of the YAlO3:Gd3þ ﬁlm wasfour times greater than that of the YAlO3:Gd3þ–Pr3þ ﬁlm.
In the PL spectra, the YAlO3:Gd3þ–Pr3þ ﬁlm gave more
intense Gd3þ emission than the YAlO3:Gd3þ ﬁlm. On thecontrary, in the CL spectra, the YAlO3:Gd3þ ﬁlm showed
more intense Gd3þ emission than the YAlO3:Gd3þ–Pr3þ
ﬁlm. To understand the contrast between PL and CL, the CL
spectra of these ﬁlms are compared, along with the PL
spectrum of the YAlO3:Gd3þ–Pr3þ ﬁlm, in the wide
wavelength range as shown in Fig. 4. The intensities of
the PL spectrum for the YAlO3:Gd3þ–Pr3þ ﬁlm werenormalized as the PL intensity of Gd3þ emission agreed
with its CL intensity. In CL originating from YAlO3 hostexcitation, the YAlO3:Gd3þ ﬁlm showed only Gd3þ peaks
in UV region, whereas the YAlO3:Gd3þ–Pr3þ ﬁlm showed
not only the Gd3þ peaks but also Pr3þ peaks in visible
region. The CL peaks at 490 and 540 nm were assigned to3
P0–3H4 and 3P0–3H5 transitions in Pr3þ, respectively [22].
In PL originating from Pr3þ 4f–5d excitation, the YAlO3:
Gd3þ–Pr3þ ﬁlm showed no peaks in visible region but the
Gd3þ peaks in UV region. The observation of the Pr3þvisible emissions in CL revealed that the excitation energy
absorbed by the host was transferred to not only Gd3þ but
also Pr3þ, and the energy transfer from Pr3þ to Gd3þ was not
dominant in contrast to PL. The contrast of Gd3þ emission
intensity is probably attributed to not simply the differenceof excitation source, PL or CL, but also the differenceof excitation target, Pr3þ 4f–5d or host excitation.

Figure 3 CL spectra of YAlO3:Gd3þ and YAlO3:Gd3þ–Pr3þ thinﬁlms. Inset shows acceleration voltage dependence of CL intensity
at 314 nm.

Figure 4 CL spectrum of YAlO3:Gd3þ and CL and PL spectra of
YAlO3:Gd3þ–Pr3þ thin ﬁlms in UV and visible region. Inset shows
CL mechanisms of YAlO3:Gd3þ–Pr3þ.

Figure 2 PL/PLE spectra of YAlO3:Gd3þ and YAlO3:Gd3þ–Pr3þ.
Solid lines exhibit spectra of YAlO3:Gd3þ and YAlO3:Gd3þ–Pr3þ
thin ﬁlms. The dotted line exhibits spectrum of YAlO3:Gd3þ bulk.

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Y. Shimizu et al.: UV CL of Gd and Gd–Pr doped YAlO3 epitaxial films

Furthermore, because concentration quenching of Pr3þemissions in YAlO3:Pr3þ has been reported to occur at Pr3þconcentration over 1% [23], Pr3þ co-doping at 3% in the
YAlO3:Gd3þ–Pr3þ ﬁlm implies the presence of nonradiative transitions as well as visible f–f transitions in
CL. Although further studies are necessary to analyze thedetailed PL and CL mechanisms, it became clear that Pr3þco-doping was ineffective in CL and decreased the overall
efﬁciency of Gd3þ emission. Namely, the intense Gd3þ CL
peaks were observed in Gd3þ single doped YAlO3 because
the excitation energy absorbed by the host was concentrated
in only Gd3þ.
4 Conclusion Epitaxial thin ﬁlms of YAlO3:Gd3þ
and YAlO3:Gd3þ–Pr3þ were grown on LAO (001) substrateby PLD. Regardless of dopants, the transmittance of theﬁlms was high and their surface morphology was uniformly
smooth. In the CL measurements, YAlO3:Gd3þ was foundto give an intense Gd3þ emission at 314 nm assigned to6
PJ –8S7/2 transition. Because Gd3þ–Pr3þ co-doping caused
the energy leakage from the host to Pr3þ, it was concluded
that Gd3þ single doping was appropriate in CL to transfer
the absorbed energy to Gd3þ only.
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