Point defects in Ce-doped Y
3
Al
5
O
12
crystal scintillators
C. L. Wang,* D. Solodovnikov, and K. G. Lynn
Center for Materials Research, Washington State University, Pullman, Washington 99164-2711, USA
Received 28 December 2005; revised manuscript received 16 May 2006; published 16 June 2006
Defect properties of Ce-doped and undoped Y
3
Al
5
O
12
YAG crystals were studied by Doppler broadening
of positron annihilation rays and thermoluminescence TL as a function of temperature 25–300 °C. The
positron diffusion length L
+
was evaluated mainly from the S parameter versus positron energy. Compared with
undoped YAG, Ce-doped YAG has a smaller positron diffusion length, due to its higher density of defects. L
+
in Ce-doped YAG decreases with increasing temperature up to 100 °C, and then increases with temperature.
The TL intensity in Ce-doped YAG shows the opposite behavior to L
+
. The results indicate that point defects
probed by positrons may be responsible for the energy transfer to Ce ions and TL intensity. Possible defects
detected by positrons are negatively charged or neutral defects related to impurity antisites, cation vacancies,
and vacancy complexes.
DOI: 10.1103/PhysRevB.73.233204 PACS numbers: 78.70.Bj
I. INTRODUCTION
Recently there has been intense interest in searching for
scintillation materials for radiation detection and medical
imaging.
1–3
Optical materials, e.g., halides, oxides, and chal-
cogenides, were traditionally used as -ray and x-ray
scintillators.
1
Undoped crystals such as alkali-metal halides
were often applied; while metal-ion-doped such as Ce
3+
,
Eu
3+
crystals are more favored as fast scintillators due to
their shorter luminescent decay times of 10– 100 ns.
Yttrium aluminum garnet Y
3
Al
5
O
12
, YAG doped with
rare-earth or transition metals makes excellent laser and op-
tical crystals. Cerium-doped YAG Ce:YAG has been used
as electron imaging sensors on scanning electron
microscopes
4
and radiation scintillators.
5
It was speculated
that the luminescence intensity at 550 nm in Ce-doped YAG
increases with the concentration of defects such as oxygen
vacancies or F centers.
6
These defects also lead to a change
in the rise and decay transient profiles of luminescence.
6,7
Positron annihilation spectroscopy PAS is a sensitive
method for studying point defects in materials. In this tech-
nique, a positron in a solid material is rapidly thermalized
and diffuses until it annihilates with an electron, producing
two quanta that are almost collinear. The energy spectrum
of the rays is Doppler shifted from 511 keV the rest mass
of electrons, due to momentum conservation in the annihi-
lation processes. If positrons are efficiently trapped at defects
such as vacancies and voids, they annihilate with electrons
with lower momenta and produce a narrower energy spec-
trum of rays compared with positron annihilation in the
bulk. Therefore, the sharpness parameter S can be used to
characterize the defects.
8,9
On the other hand, S versus the
wing parameter W can provide independent information on
the number of layers with different defect properties,
10,11
where W is due to positron annihilation with high-
momentum electrons core electrons. Furthermore, the pos-
itron energy dependence of S or W can give us the positron
diffusion length and therefore information on defect proper-
ties in materials.
In this work, defects in cerium-doped YAG were studied
by PAS and thermoluminescence TL. The correlation be-
tween TL intensity and defect concentration is discussed. It
was shown that controlling defect concentration is crucial for
improving luminescence intensity and probably scintillation
efficiency.
II. EXPERIMENTS
Three YAG samples grown by the Czochralski Cz
method were obtained from VLOC a subsidiary of II-VI,
Inc: sample R9 with 0.15 at. % Ce, sample R20 with
1.0 at. % Ce and 1.0 at. % Er, and an undoped YAG sample
R0. Before positron and TL measurements, they were
etched in phosphoric acid 85% concentration at 200 °C to
reduce surface damage, and were radiated by a Xe lamp so
that the TL intensity was enhanced.
12
The TL measurement was performed on a heat stage with
a heating rate of 12 °C/min after the sample was radiated
with a Xe lamp for 15 min. A thermocouple monitored the
temperature of heating stage. A photospectrometer Ocean
Optics, Inc. was used to collect the thermal luminescence.
The luminescence spectrum in the region of 500– 700 nm
shows a peak at around 550 nm. The integrated intensity
between 500 and 700 nm was calculated as a function of
temperature.
Doppler broadening spectra of positron annihilation
quanta were measured with a variable-energy positron beam
with a flux of about 5 10
5
/ s cm
2
, a diameter of 5 mm, and
an energy range of 0.1– 12 keV. Thermocouples monitored
temperatures at the surface of the sample and the surface and
bottom of the heater. Only the temperature on the surface of
the samples is used in the positron results. The shape param-
eters S and W are used in the analysis.
8
As an example, the
measured SE and SW relations
10,11
at room temperature
for three samples are shown in Fig. 1.
In Fig. 1b, the good linear relation between S and W at
room temperature indicates that a single layer is adequate to
describe the positron behavior,
10
as do the SW linear rela-
tions at higher temperatures. We used the VEPFIT program
13
to fit SE by a single-layer model with an epithermal posi-
tron state in the surface. The diffusion lengths had large er-
rors after fixing the bulk S or epithermal parameters at cer-
PHYSICAL REVIEW B 73, 233204 2006
1098-0121/2006/7323/2332044 ©2006 The American Physical Society 233204-1