Full text of DOI:10.1557/JMR.2000.0387

Photoluminescence spectra of Zn1−xCdxAl2Se4−4x 4x
S
single crystals
Sung-Hyu Choe
Department of Physics, Chosun University, Kwangju 501-579, Republic of Korea
Chang-Sun Yoon
Department of Physics, Kunsan National University, Kunsan 573-800, Republic of Korea
Moon-Seog Jin
Department of Physics, Dongshin University, Naju 520-714, Republic of Korea
Seung-Cheol Hyun and Chang Dae Kim
Department of Physics, Mokpo National University, Mokpo 534-729, Republic of Korea
Choong-Il Lee
Department of Physics, Suncheon National University, Suncheon 540-742, Republic of Korea
Jae-Mo Goh, Seok-Kyun Oh, Ho-Jun Song, and Wha-Tek Kim
Department of Physics, Chonnam National University, and Kwangju Branch, Korea Basic Science
Institute, Kwangju 500-757, Republic of Korea
(Received 30 March 2000; accepted 29 August 2000)
We investigated the photoluminescence as well as the crystal structure and optical
energy gaps of the Zn1−xCdxAl2Se4−4xS4x solid solution system based on the
Al-related compounds of ZnAl2Se4, ZnAl2S4, CdAl2Se4, and CdAl2S4. The single
crystals of the system with 0.0 ഛ x ഛ 1.0 were grown by the chemical transport
reaction technique. The Zn1−xCdxAl2Se4−4xS4x crystallizes in a defect chalcopyrite
structure for a whole composition and has an optical energy gap ranging from 3.525 to
3.577 eV at 13 K. The photoluminescence spectra at 13 K showed a strong emission
band in the blue spectral region and a weak broad emission band in the visible region
due to donor–acceptor pair recombination. The composition and temperature
dependence of these bands were examined in the investigated regions. The simple
energy band scheme for the radiative mechanisms of the Zn1−xCdxAl2Se4−4xS4x is
proposed on the basis of our experimental results along with photo-induced current
transient spectroscopy measurements.
I. INTRODUCTION
␣-phase ZnAl2S4. In our previous papers,10–13 we pre-
sented the results of the blue photoluminescence spectra
observed in all the four compounds of ZnAl2Se4,
␣-ZnAl2S4, CdAl2Se4, and CdAl2S4 single crystals
grown by the chemical transport reaction (CTR) tech-
nique. Also, the several sharp emission bands due to the
Er3+ ions doping in these crystals were observed and
were analyzed by a crystal-field consideration. For prac-
tical applications in optoelectronics there is a need for
more information about these materials along with their
solid solutions. However, there has been no study of the
The AIIB2IIIC4VI ternary compound semiconductors
have been expected to be a promising material with po-
tential applications in optoelectronics.1 Among them, in
particular, the Al-related compounds such as ZnAl2Se4,
ZnAl2S4, CdAl2Se4, and CdAl2S4 attract much interest
because of their strong luminescent properties and wide
band gaps in blue-green spectral regions suitable for blue
light-emitting diode (LED) applications. Although there
have been a number of works for the optical character-
izations of these compounds,2–13 most of the studies have
been made on their photoluminescence. Kai et al.7,8 re-
ported a strong green emission band at 540 nm for the
spinel ␣-phase and a broad blue emission band at 450 nm
for the wurtzite ␻-phase in ZnAl2S4. Broussell et al.9
reported an intense ultraviolet emission band at 370 nm
of the intrinsic photoluminescence and extrinsic emission
bands due to Cr3+ impurity ions in the Co-doped spinel
6
solid solutions excepting a few studies on ZnAl2−2xGa2xS4
and Zn1−xCdxAl2Se4.14 In the present paper, we report on
the photoluminescence spectra of Zn1−xCdxAl2Se4−4xS4x
system based on four the Al-related compounds of
ZnAl2Se4, ZnAl2S4, CdAl2Se4 and CdAl2S4. The simple
energy band scheme for the radiative mechanisms in the
solid solution system is also proposed on the basis of our
experimental results.
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S-H. Choe et al.: Photoluminescence spectra of Zn1−xCdx Al2Se4− 4x S4x single crystals
II. EXPERIMENTAL
III. RESULTS AND DISCUSSION
Single crystals of Zn1−xCdxAl2Se4−4xS4x solid solution
system were grown by the CTR technique using iodine as
a transporting agent. Starting materials of ZnCl2, Cd, Al,
Se, and S of the constituent elements were charged with
mole ratio in a cleaned quartz tube together with iodine
(6 mg cm−3). To obtain stoichiometric crystals, excess Se
and S of 5% to 8% were then added. The inner walls of
the source zone of the quartz tubes used were coated with
carbon to prevent aluminium of the constituent element
from reacting with the quartz tubes at high temperatures.
For the crystal growth, the ampoules sealed off under
a vacuum of 1 × 10−6 torr were placed into a horizontal
two-zone furnace, heated up to 1000 °C and held at this
temperature for 40 h to react with the charged elements.
After this preheating, a temperature inversion process
was introduced to clean the growth zone of the am-
poules.15 Finally, the temperature gradient was kept at
950 °C for the source zone and 850 °C for the growth
zone for 7 days. The resulting single crystals showed a
colorless transparency for a whole composition. The
chemical composition of the grown crystals was deter-
mined by means of inductively coupled plasma spectros-
copy (ICPS; JY 38 Plus).
Photoluminescence (PL) spectra were obtained using
a conventional measurement system consisting of a
double-grating monochromator (Spex-1403, f ס 0.85 m),
a photomultiplier (RCA-C31034) and a cryogenerator
(Air Products-CSA202B); they were investigated in the
wavelength range of 400 to 800 nm and in the tempera-
ture range of 13 to 300 K. As an excitation source, a
He–Cd laser (Liconix-3650, 325 nm line) was then em-
ployed. Optical absorption spectra near the fundamental
absorption edge were recorded by an ultraviolet–visible-
infrared (UV-VIS-NIR) spectrophotometer (Hitachi-
U3501) with a cryogenic system.
The crystal structure and lattice constants of the
Zn1−xCdxAl2Se4−4xS4x solid solution system were ob-
tained by x-ray powder diffraction measurements
(Rigaku-DMAX2000). The results are summarized in
Table I; they showed that all of the crystals in the
Zn1−xCdxAl2Se4−4xS4x system crystallized in a defect
chalcopyrite phase, although the basic compounds of the
system in this work were found to have structures dif-
ferent from each other, such as a defect chalcopyrite for
ZnAl2Se4, CdAl2Se4, and CdAl2S4 and a spinel or an
orthorhombic for ZnAl2S4.16,17 It was observed that there
exist a slight decrease of the lattice constant a and an
increase of the lattice constant c with increasing compo-
sition x.
Figure 1 represents the relation between the optical
absorption coefficient ␣ and the incident photon energy
h␯ for a direct allowed transition at 13 K, which was
obtained from the measurements of the optical absorp-
tion spectra near the fundamental absorption edge. The
optical energy gaps at 13 K were found to be 3.525
(x ס 0.1), 3.545 (x ס 0.4), 3.566 (x ס 0.8) and
3.577 eV (x ס 1.0) through the extrapolation of
(␣ и h␯)2 ס 0 and are listed in Table I together with
those at 289 K. It was also shown that the temperature
dependence of the optical energy gaps in the temperature
region of 13 to 289 K was well fitted with the Varshni
equation Eg(T) ס Eg(0) − ␣ T2/(T + ␤)18 with the con-
stants given in Table I.
Figure 2 shows the PL spectra at 13 K for x ס 0.0 and
0.6 in the Zn1−xCdxAl2Se4−4xS4x. In the case of x ס 0.0
(ZnAl2Se4), the PL spectrum consists of a relatively
strong emission band at 2.870 eV (432 nm) (named as
SD emission) and a weak broad emission band at
2.638 eV (470 nm) (named as DD emission). A similar
feature to these bands is also observed for the crystals
TABLE I. Values of lattice constants, optical energy gaps and Eg(0), ␣, ␤ from the Varshni equation of Zn1−xCdxAl2Se4−4xS4x single crystals.
Lattice
Energy
Constant of
constant
gap (eV)
Varshni equation
Crystal
x
structure
a(Å)
c(Å)
13 K
289 K
3.312
Eg(0) (eV)
3.527
␣ (eV/K)
␤ (K)
x ס 0.0
Defect
chalcopyrite
Defect
chalcopyrite
Defect
chalcopyrite
Defect
chalcopyrite
Defect
5.5561
10.8890
3.525
2.03 × 10−3
501.89
x ס 0.2
x ס 0.4
x ס 0.6
x ס 0.8
x ס 1.0
5.5567
5.5574
5.5579
5.5585
5.5592
10.7742
10.6595
10.5447
10.4301
10.3153
3.535
3.545
3.556
3.566
3.577
3.329
3.346
3.363
3.381
3.398
3.538
3.547
3.557
3.568
3.579
1.97 × 10−3
1.90 × 10−3
1.83 × 10−3
1.77 × 10−3
1.71 × 10−3
501.92
501.95
501.99
502.02
502.06
chalcopyrite
Defect
chalcopyrite
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S-H. Choe et al.: Photoluminescence spectra of Zn1−xCdx Al2Se4− 4x S4x single crystals
with x ס 0.6, excepting a position of the bands. How-
the emission band at 470 nm was seen as well, but its
intensity became weak, and the emission band at 799 nm
disappeared. At the same time the dominant emission
band was observed at 432 nm in the shorter wavelength
region, which seemed to have originated from a good
quality of the crystals grown by different growth condi-
tions, such as starting materials from the previous work.
Considering that the emission band energy (2.870 eV)
ever, the PL spectrum for x ס 0.0 shows some discrep-
ancy between our present results and previous ones.11 In
the previous work,11 we investigated the variation of the
emission bands for the ZnAl2Se4 single crystals grown
by CTR according to the various amounts of excess Se.
The PL in the stoichiometric ZnAl2Se4 single crystals
grown with 8% excess Se showed a broad blue emission
at 470 nm and a weak emission at 799 nm due to donor–
acceptor (D-A) pair recombination. In the present work,
F I G . 3 . S D a n d D D e m i s s i o n b a n d s o b s e r v e d i n
Zn1−xCdx Al2Se4 − 4x S4x single crystals.
TABLE II. Emission energy, emission mechanism, and defect energy
levels of Zn1−xCdxAl2Se4−4xS4x single crystals.
FIG. 1. Optical energy gaps at 13 K of Zn1−xCdx Al2Se4 − 4x S4x single
crystals.
Defect Energy Levels (eV)
Composition
Emission Peaks
(at 13 K)
(x)
SD1c
DD1d
DA1e
x ס 0.0 EE (eV)a
2.870
SD
2.638
DD
0.058
0.290
0.597
EMb
x ס 0.2
x ס 0.4
x ס 0.6
x ס 0.8
x ס 1.0
EE (eV)
EM
EE (eV)
EM
EE (eV)
EM
EE (eV)
EM
2.854
SD
2.852
SD
2.857
SD
2.672
SD
2.638
DD
2.647
DD
2.667
DD
2.040
DD
0.060
0.059
0.058
0.066
0.072
0.276
0.264
0.247
0.698
0.689
0.621
0.634
0.642
0.828
0.821
EE (eV)
EM
2.684
SD
2.067
DD
aEE: Emission energy
bEM: Emission mechanism
cSD1: Shallow donor level 1
dDD1: Deep donor level 1
eDA1: Deep acceptor level 1.
FIG. 2. Photoluminescence spectra at 13 K of Zn1−xCdx Al2Se4 − 4x S4x
(x ס 0.0, 0.6) single crystals.
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S-H. Choe et al.: Photoluminescence spectra of Zn1−xCdx Al2Se4− 4x S4x single crystals
was much smaller than the band gap energy (3.525 eV at
The energy of the emission bands observed in the
Zn1−xCdxAl2Se4−4xS4x is plotted in Fig. 3 as a function
of composition x. In the composition range of
0.0 ഛ x ഛ 0.6, the SD emission shifted to lower energy
regions with increasing x, whereas the DD emission
shifted to higher energy regions, as can be seen in Fig. 3.
However, the SD emission and the DD emission in the
range 0.8 ഛ x ഛ 1.0 were lower in energy than those in
0.0 ഛ x ഛ 0.6, and showed the behavior shifting to
higher energy regions with increasing x.
13 K), the dominant emission was assumed to be due to
D-A pair recombination as in the case of the 470 nm
emission band.
We now propose the simple energy band scheme for
the radiative mechanisms of Zn1−xCdxAl2Se4−4xS4x on
the basis of our experimental results along with the re-
sults of photo-induced current transient spectroscopy
(PICTS) measurements. The PICTS measurement was
made by exchanging the polarity of electric field applied
to the samples illuminated in proximity to a contact19 and
gave an information on the depth of electron traps as
donors and hole traps as acceptors in the forbidden band
gap. The results are summarized in Table II. The values
of the shallow donor levels in Table II are in good agree-
ments with the activation energies obtained from the
thermal quenching of the SD emission bands observed in
Zn1−xCdxAl2Se4−4xS4x. For the crystals with x ס 0.6, the
thermal quenching of the SD emission at 2.857 eV is
typically shown in Fig. 4 and reveals the activation en-
ergy of ⌬E ס 0.058 eV. In the case of the crystals with
x ס 1.0 (CdAl2S4), however, the SD emission band at
2.684 eV showed the anomalous temperature depend-
ence that its intensity decreased with increasing tempera-
ture from 13 to 150 K and increased from 150 to 300 K
as in Ref. 13 (not shown in the figure). This observation
is considered to be related to a metastable state in the
forbidden band gap of the CdAl2S4.13 Figure 5 represents
the simple energy band scheme introduced two donor
levels (SD1 and DD1) and one acceptor level (DA1) in
the forbidden band gap. The proposed energy band
scheme permits us to assign the observed emission
bands.
FIG. 4. Thermal quenching of the 2.857 eV emission band in
Zn1−xCdx Al2Se4 − 4x S4x (x ס 0.6) single crystals.
IV. CONCLUSION
In summary, we investigated the composition depend-
ence of the PL spectra as well as the crystal structure and
the optical energy gaps of the Zn1−xCdxAl2Se4−4xS4x
solid solution system grown by CTR technique. The
Zn1−xCdxAl2Se4−4xS4x was a defect chalcopyrite struc-
ture for a whole composition and showed the optical
energy gap ranging from 3.525 to 3.577 eV at 13 K. The
temperature dependence of the optical energy gaps was
satisfied with the Varshni equation. The PL spectra at
13 K showed a strong emission band in the blue spectral
region and a weak broad emission band in the visible
spectral region for a whole composition. These emission
bands were attributed to D-A pair recombination be-
FIG. 5. Proposed energy band scheme for the radiative mechanism of
Zn1−xCdx Al2Se4 − 4x S4x single crystals.
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S-H. Choe et al.: Photoluminescence spectra of Zn1−xCdx Al2Se4− 4x S4x single crystals
6. V.V. Ursaki, I.I. Burlakov, I.M. Tiginyanu, Y.S. Raptis,
tween two donor levels and one acceptor level in the
forbidden band gap of the energy band scheme proposed
on the basis of our experimental results.
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ACKNOWLEDGMENT
This work was supported by a Korea Research Foun-
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