S-C. Hyun et al.: Photoluminescence spectra of Zn1−xCdxAl2Se4 single crystals
was then added. The inner walls of the source zone of the
13 to 289 K. Figure 1 depicts the plots of the relation
between (␣ h )2 and h for a direct allowed transition at
13 K. The optical energy gap at 13 K, deduced by ex-
trapolation of the plot to (␣ h )2 ס
0, is found to be
3.082 (x ס
1.0), 3.169 (x ס
0.8), 3.258 (x ס
0.6), 3.347
(x ס
0.4), 3.436 (x ס
0.2), and 3.525 eV (x ס
0.0),
respectively. The temperature dependence of the optical
energy gaps was also investigated in the temperature
range of 13 to 285 K, and was well fitted with the
Varshni equation7
quartz tubes used were coated with carbon to prevent
aluminium of the constituent elements from reacting with
the quartz tubes at high temperatures. After clearing the
quartz walls of the growth zone,6 the source- and growth-
zone temperatures were kept at 950 and 850 °C for 7 days
for the crystal growth. The resulting single crystals
showed a colorless and transparent crystal with a dimen-
sion of about 5 × 6 × 1.5 mm3 for a whole composition.
The chemical composition of the grown single crystals
was determined by an energy-dispersive x-ray microana-
lyzer (EDAX).
Eg(T) ס
Eg(0) − (␣T2)/(T + )
,
(1)
The photoluminescence spectra were recorded using a
conventional measurement system consisting of a
double-grating monochromator (Spex 1403, U.S.A., f ס
0.85 m), a photomultiplier (RCA, C31034, Japan), and a
cryogenerator (Air Products, CSA-202B, U.S.A.). The
325-nm line of an He–Cd laser (LiConix, 3650N, U.S.A.)
was used as an excitation source. The crystal structure
and lattice constants were determined from x-ray powder
diffraction measurements (Rigaku, DMAX2000, Japan).
The optical absorption spectra were obtained by using
an ultraviolet–visible–near-infrared spectrophotometer
(Hitachi, U3501, Japan) equipped with a cryogenic sys-
tem (Air Products, CSA-202B, Japan).
where Eg(T) is the energy gap at T K, Eg(0) at 0 K, and
␣ and  are constants. The obtained parameters are given
in Table I.
The photoluminescence spectra of the Zn1−xCdxAl2Se4
single crystals were investigated in the temperature re-
gion of 13 to 289 K and in the wavelength region of 400
to 600 nm. Figure 2 represents the photoluminescence
spectra at 13 K for the Zn1−xCdxAl2Se4 in the composi-
tion region of 0.0 ഛ x ഛ 1.0. The photoluminescence
spectrum of the ZnAl2Se4 (x ס
0.0) shows a strong
broad emission band at 2.870 eV and a weak broad emis-
sion band at 2.638 eV. The same feature of these bands is
also observed for the crystals with x ס
0.2, excepting a
red shift of the bands. The red shift of the bands appears
up to x ס
0.6. However, the emission bands for the
crystals with x ס
0.8 and 1.0 are observed in higher
energy regions than the emission bands observed for the
Zn-rich crystals. Also these emission bands show a red
shift without a change of their shape with increasing x, as
in the case of the Zn-rich crystals. For the crystals with
x ס
1.0(CdAl2Se4), the emission bands are observed at
2.707 and 2.292 eV, which are considered to be due to
donor–acceptor pair recombination5.
III. RESULTS AND DISCUSSION
The analysis of the x-ray diffraction (XRD) patterns
measured for the Zn1−xCdxAl2Se4 in powder form
showed that these crystals have a defect chalcopyrite
structure for a whole composition, and revealed that the
lattice constant a increases from 5.5561 A for x ס
0.0(ZnAl2Se4) to 5.6361 A for x ס
1.0(CdAl2Se4) with
increasing x, whereas the lattice constant c decreases
from 10.8890 A for x ס
0.0 to 10.7194 A for x ס
1.0
(see Table I). The optical energy gaps of the
Zn1−xCdxAl2Se4 single crystals were obtained from the
measurements of the optical absorption spectra near the
fundamental absorption edge in the temperature region of
Figure 3 shows the temperature dependence of the
photoluminescence spectra for the Zn0.8Cd0.2Al2Se4
single crystals with x ס
0.2. The emission bands are
observed to be quenched at high temperatures. The pho-
TABLE I. Values of lattice constants, optical energy gaps, and Eg(0), ␣, and  from the Varshni equation of Zn1−xCdxAl2Se4 single crystals.
x and crystal structure
Defect
Defect
Defect
Defect
Defect
Defect
chalcopyrite
chalcopyrite
chalcopyrite
chalcopyrite
chalcopyrite
chalcopyrite
Lattice constant
a (Å)
5.5561
5.5723
5.5884
5.6045
5.6207
5.6361
c (Å)
10.8890
10.8551
10.8212
10.7873
10.7534
10.7194
Energy gap (eV)
13 K
289 K
3.525
3.312
3.436
3.241
3.347
3.172
3.258
3.103
3.169
3.034
3.082
2.965
Constant of Varshni equation
Eg(0) (eV)
␣ (eV/K)
 (K)
3.527
2.03 × 10−3
501.89
3.439
1.88 × 10−3
507.01
3.349
1.69 × 10−3
512.12
3.260
1.51 × 10−3
517.23
3.171
1.33 × 10−3
522.34
3.084
1.16 × 10−3
527.45
J. Mater. Res., Vol. 15, No. 4, Apr 2000
881
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