DOI: 10.1007/s00339-004-3023-1
Appl. Phys. A 81, 1277–1280 (2005)
Materials Science & Processing
Applied Physics A
j.h. hao
1,2,✉
j. gao
1
A novel hydrate-based thin-film phosphor:
low-temperature growth process and properties
1
The University of Hong Kong, Department of Physics, Pokfulam Road, Hong Kong
2
Huazhong University of Science and Technology, Department of Optoelectronic Engineering,
Wuhan 430074, P.R. China
Received: 14 June 2004/Accepted: 17 August 2004
Published online: 7 October 2004 • © Springer-Verlag 2004
ABSTRACT
Thin films of barium chloride hydrate doped with
cerium were deposited by spray pyrolysis of aqueous solutions.
The cathodoluminescence (CL) spectrum consists of two peaks
at 443 and 485 nm due to 4 f –5d transitions of the cerium ion
in thin films without post-heat treatment. The CL depended
dramatically on deposition temperature between 250
◦
Cand
500
◦
C. The phenomenon is directly related to the change of
crystallinity and Cl concentration of the thin films. The CL spec-
tra of thin films of BaCl
2
· 2H
2
O:Ce were compared on various
substrates. The relaxation behavior of CL was also observed in
thin-film phosphor. Our results indicate that the material studied
as thin-film phosphor is unique since it contains water. It seems
that the 4 f –5d transition in Ce is so energetic that the O–H
phonons cannot disturb it, indicating that barium chloride hy-
drate is a novel host in which Ce has a higher efficiency.
PACS
78.20.-e; 78.60.Hk; 85.60.Jb
1 Introduction
Much attention has been paid to the study of lumi-
nescent materials for a variety of applications. In a number
of hosts, the
Ce
ion emits in the near-UV or blue spectral
range due to transitions of the
5d
to the
4 f
electronic energy
states of the
Ce
3+
ion [1]. In particular, the decay time of
Ce
3+
emission is short (
∼ 10
−8
s
) [2, 3]; a fast decay time activator
can be used to reduce ground-state depletion and improve the
saturation properties. Thus, a screen containing
Ce
3+
-doped
phosphor can be read faster, enabling high-speed imaging. It
was found that there are large differences in the quantum ef-
ficiency in various
Ce
-doped hosts [1]. Large variations in
the emission by the
Ce
ion are expected when different host
materials are used. Consequently, it is desirable to search for
efficient
Ce
-doped hosts. For alkaline earth halide lattices,
BaF
2
:
Ce
has attracted much attention as a scintillator mate-
rial [4]. Besides that, only photoluminescence in bulk of
Ce
-
doped alkaline-earth (
Ca
,
Sr
,
Ba
) chlorides had been observed
in the UV region (
330
–
400 nm
) [5]. There is little work on
the CL study of alkaline-earth chlorides. Recently, we firstly
reported that barium chloride hydrate doped with cerium is
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a blue phosphor that exhibits relatively efficient luminescence
after processing at low temperatures [6].
On the other hand, as-grown blue-emitting films of
Ce
-
doped hosts, such as
Y
2
SiO
5
and
SrGa
2
S
4
, are usually
amorphous or have poor crystallinity [7–9]. Annealing them
at relatively high temperatures was required for both crys-
tallization of the compounds and diffusion of
Ce
ions be-
fore sufficient brightness was obtained. Unfortunately, high-
temperature processes are not suitable for some applications
involving displays that require glass substrates. Considerable
research is being focused on in situ growth of
Ce
-doped thin
films. Typically, cathodoluminescence (CL) was observed
from thin films of
SrGa
2
S
4
:
Ce
deposited in situ at
472.5
◦
C
using molecular beam epitaxy [10]. Our previous study indi-
cated that the films deposited at temperatures as low as
250
◦
C
exhibit relatively efficient luminescence [6]. Obviously, im-
proving the deposition process for the thin films of existing
Ce
-activated hosts is important. In this work, we will report
the dependence of luminescence behavior on deposition tem-
perature in thin films of barium chloride hydrate doped with
cerium. Some novel properties of the thin films were found.
2 Experimental
The spray pyrolysis technique was used to deposit
thin films of
BaCl
2
· 2H
2
O
:
Ce
on Corning 7059 glass sub-
strates. The spray pyrolytic deposition system was described
in detail elsewhere [11, 12]. Briefly, the spray was developed
by an ultrasonic nebulizer and was directed towards the sub-
strate by a carrier gas of humid air at a flow rate of
1.2L/min
.
Stock precursor solutions were
0.1 M BaCl
2
(anhydrous and
99.998%
, Alfa) and
CeCl
3
(anhydrous and
99.5%
,Alfa).
The stock precursor solutions were mixed in an appropriate
ratio to provide solutions in the range of 5–50 atom %
Ce
dopant relative to
Ba
. The substrate temperature was varied at
250
–
500
◦
C
.
The analysis of the crystalline phase of the thin films was
performed on an X-ray diffractometer (XRD). The chemical
composition of the films was analyzed by using inductively
coupled plasma (ICP) atomic absorption spectroscopy and
energy-dispersive X-ray spectroscopy (EDS). The CL spec-
tra were obtained using a CL luminoscope (Relion Indus-
tries ELM-2B). Luminescence spectra were measured using
an Ocean Optics S2000 CCD spectrometer (
350
–
900 nm
).