Full text of DOI:10.1557/JMR.2002.0316

3
+
3+
3+
Optical properties of Ho -, Er -, and Tm -doped BaIn S
2
4
and BaIn Se single crystals
2
4
Sang-An Park
Department of Ophthalmic Optics, Chodang University, Muan Chonnam 534-701, Republic of Korea
Mi-Yang Kim and Wha-Tek Kim
Department of Physics, Chonnam National University, and Kwangju Branch, Korea Basic Science
Institute, Kwangju 500-757, Republic of Korea
a)
Moon-Seog Jin
Department of Physics, Dongshin University, Naju 520-714, Republic of Korea
Sung-Hyu Choe
Department of Physics, Chosun University, Kwangju 501-759, Republic of Korea
Tae-Young Park
Department of Physics, Wonkwang University, Iri 570-749, Republic of Korea
Kwang-Ho Park and Duck-Tae Kim
Department of Electronics, Dong-A Junior College, Youngam 526-870, Republic of Korea
(Received 2 April 2002; accepted 3 June 2002)
3
+
3+
3+
3+
BaIn S , BaIn S :Ho , BaIn S :Er , BaIn S :Tm , BaIn Se , BaIn Se :Ho ,
2
4
2 4
2 4
2 4
2
4
2
4
3
+
3+
BaIn Se :Er , and BaIn Se :Tm single crystals were grown by the chemical
2
4
2
4
transport reaction method. The optical energy gap of the single crystals was found to
be 3.057, 2.987, 2.967, 2.907, 2.625, 2.545, 2.515, and 2.415 eV, respectively, at 11 K.
The temperature dependence of the optical energy gap was well fitted by the Varshni
equation. Broad emission peaks were observed in the photoluminescence spectra of the
single crystals. They were assigned to donor–acceptor pair recombination. Sharp
emission peaks were observed in the doped single crystals. They were attributed to be
3
+
3+
3+
due to radiation recombination between the Stark levels of the Ho , Er , and Tm
ions sited in C symmetry.
1
I. INTRODUCTION
The crystal structure of the single crystals was investi-
gated by measuring their powder x-ray diffraction pat-
terns. The optical absorption near the fundamental
absorption edge and the PL spectra of the single crystals
were measured. The optical energy gap of the single
crystals was obtained. The temperature dependence of
the optical energy gap and the transition mechanism
of the emission peaks were investigated.
BaIn S and BaIn Se compounds with an orthorhom-
2
4
2
4
1,2
bic structure are wide-gap semiconductors with optical
3
energy gaps of 3.057 and 2.62 eV, respectively. They
have been expected to be useful materials applicable to
optoelectronic devices in the violet-to-visible wavelength
region. Nevertheless, except for their crystal structures,
1
,4
body color, and photoluminescence (PL) properties, no
study has been reported as yet because it is difficult to
grow single crystals, and they are destroyed by absorbing
moisture in the atmosphere. Rare-earth element-doped
BaIn S and BaIn Se single crystals have not been stud-
II. EXPERIMENTAL PROCEDURE
3
+
3+
A. Crystal growth of undoped and Ho -, Er -,
2
4
2
4
3+
ied at all.
This study investigated optical properties of undoped
and Tm -doped BaIn S and BaIn Se
2
4
2
4
single crystals
3+
3+
3+
and Ho -, Er -, and Tm -doped BaIn S and BaIn Se
3+
3+
3+
2
4
2
4
BaIn S , BaIn S :Ho , BaIn S :Er , BaIn S :Tm ,
3
+
3+
3+
2 4
2 4
2 4
2 4
3+
single crystals. Undoped and Ho -, Er -, and Tm -
doped BaIn S and BaIn Se single crystals were grown
3+
BaIn Se , BaIn Se :Ho , BaIn Se :Er , and
2
4
2
4
2
4
3+
2
4
2
4
BaIn Se :Tm single crystals were grown by the CTR
2
4
by using the chemical transport reaction (CTR) method.
method with iodine (purity 99.999%) as a transport
agent. BaS (powder, purity 99.95%), BaSe (powder, pu-
rity 99.95%), In (purity 99.9999%), S (purity 99.999%),
and Se (purity 99.9999%) were used as source materials.
a)
e-mail: msjin@blue.dongshinu.ac.kr
J. Mater. Res., Vol. 17, No. 8, Aug 2002
© 2002 Materials Research Society
2147
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3
+
3+
3+
S-A. Park et al.: Optical properties of Ho -, Er -, and Tm -doped BaIn S and BaIn Se single crystals
2
4
2
4
TABLE I. Lattice constants, optical energy gaps at 11 K, and values of E (0), ␭, and ␤ in the Varshni equation.
g
Lattice constant
Varshni coefficients
a
b
c
Material
BaIn2S4
Structure
a (Å)
b (Å)
c (Å)
E (11) (eV)
E (0) (eV)
␭ (eV/K)
␤ (K)
g
g
−
3
orthorhombic
orthorhombic
orthorhombic
orthorhombic
orthorhombic
orthorhombic
orthorhombic
orthorhombic
21.726
21.834
21.835
21.852
22.715
22.795
22.857
22.978
21.695
21.697
21.711
21.735
22.523
22.546
22.723
22.984
13.158
13.458
13.675
13.812
13.613
13.926
14.016
14.236
3.057
2.987
2.967
2.907
2.625
2.545
2.514
2.415
3.058
2.988
2.968
2.908
2.626
2.546
2.516
2.416
3.95 × 10
535
345
264
186
557
360
276
197
3
+
−3
−3
−3
BaIn S :Ho
3.45 × 10
2.95 × 10
2.36 × 10
4.98 × 10
4.27 × 10
3.56 × 10
2.84 × 10
2
4
3
+
BaIn S :Er
2
4
3
+
BaIn S :Tm
2
4
−
3
BaIn Se
2
4
3
+
−3
−3
−3
BaIn Se :Ho
2
4
3
+
3
BaIn Se :Er
2
4
+
BaIn Se :Tm
2
4
a
Crystal structure.
Optical energy gap at 11 K.
b
c
Optical energy gap at 0 K.
Ten mole percent excess S for the BaIn S ,
with a cryogenic system (APD, SH-4) over the tempera-
ture range of 5–300 K. Photoluminescence spectra were
measured at 6 K with a conventional PL measurement
system consisting of a double monochromator (Spex
1403, f ס 0.85 m), a photomultiplier tube (RCA,
C31034), and a cryogenic system (APD, SH-4). The 325-
nm line of a He–Cd laser (LiConix, 3650N) was used as
2
4
3+
3+
3+
BaIn S :Ho , BaIn S :Er , and BaIn S :Tm single
2
4
2 4
2 4
crystals and 4 mol% excess Se for the BaIn Se ,
2
4
3+
3
+
3+
BaIn Se :Ho , BaIn Se :Er , and BaIn Se :Tm
2
4
2
4
2
4
single crystals were added to the source materials to pre-
vent deficiency of S or Se by vaporization during crystal
growth; their boiling temperatures are lower than those
of the other elements. Two mole percent Ho, Er, and Tm
3
+
3+
3+
(
metal, purity 99.9%) for the Ho -, Er -, and Tm -
doped single crystals, respectively, was added to the
source materials.
The temperatures of the growth and the source zones
were maintained for 8 days at 810 and 920 °C, respec-
3
+
3+
tively, for the BaIn S , BaIn S :Ho , BaIn S :Er , and
2
4
2 4
2 4
3+
BaIn S :Tm single crystals and at 730 and 870 °C,
2
4
3
+
respectively, for the BaIn Se , BaIn Se :Ho
,
2
4
2
4
3+
3+
BaIn Se :Er , and BaIn Se :Tm single crystals. Col-
2
4
2
4
orless and transparent single crystals with the typical
dimensions of 5.6 × 7.0 × 2.2 mm were obtained. When
the single crystals had been exposed in the atmosphere
for some time, the transparency of the single crystals
disappeared by absorbing moisture. To prevent this, the
quartz ampules were broken in a high-purity argon gas
environment, and the single crystals were dipped into
transparent silicon oil. These single crystals were used in
measurements.
B. Measurement method
The composition of the single crystals was investi-
gated by energy dispersive x-ray analysis. Single crystals
with stoichiometry within the error limit of 3% were used
in measurements. The x-ray diffraction patterns of the
single crystals were measured with an x-ray diffractom-
eter (Rigaku, DMAX-2000, Japan). The crystal structure
of the single crystals was identified by analyzing the
x-ray diffraction patterns. Optical absorption spectra
were measured by an ultraviolet–visible–near-infrared
spectrophotometer (Hitachi, U-3501, Japan) equipped
FIG. 1. Optical absorption spectra near the fundamental absorption
3+
3+
edge of the (a) BaIn S , (b) BaIn S :Ho , (c) BaIn S :Er , (d)
2
4
2
4
2 4
3
+
3+
3+
BaIn S :Tm , (e) BaIn Se , (f) BaIn Se :Ho , (g) BaIn Se :Er
,
2
4
2
4
2
4
2
4
3
+
and (h) BaIn Se :Tm single crystals at 11 K.
2
4
2148
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3
+
3+
3+
S-A. Park et al.: Optical properties of Ho -, Er -, and Tm -doped BaIn S and BaIn Se single crystals
2
4
2
4
an excitation source. The PL spectra obtained in the
measurements were corrected for the spectral properties
of the monochromator and the detector.
gap decreased with increasing the temperature of the
single crystals. The temperature dependence of the opti-
cal energy gap was well fitted with the Varshni equation :
8
2
␭
T
E ͑T͒ = E ͑0͒ −
,
(2)
g
g
T + ␤
III. RESULTS AND DISCUSSION
where E (T) is the optical energy gap at T K, E (0) the
optical energy gap at 0 K, and ␭ and ␤ are constants. the
values of E (0), ␭, ␤ for the single crystals are listed in
g
g
A. Crystal structures and optical energy gaps of
3+
3+
3+
undoped and Ho -, Er -, and Tm -doped
BaIn S and BaIn Se single crystals
g
2
4
2
4
Table I.
The x-ray diffraction patterns of the BaIn S ,
2
4
3
+
3+
3+
B. Photoluminescence of BaIn S and BaIn Se
4
single crystals
BaIn Se :Ho , BaIn S :Er , BaIn S :Tm , BaIn Se ,
2
4
2
2
4
2 4
2 4
2
4
3+
3
+
3+
BaIn Se :Ho , BaIn Se :Er , and BaIn Se :Tm
2
4
2
4
2
4
single crystals were measured at room temperature. The
crystal structure of the single crystals was identified by
analyzing the x-ray diffraction patterns. The single crys-
tals crystallized into an orthorhombic structure. The val-
ues of the lattice constants a, b, and c are listed in Table I.
These values agree well with a ס 21.824 Å, b ס
Figure 4 shows the PL spectra of the BaIn S and
2
4
BaIn Se single crystals at 6 K. Two broad emission
2
4
peaks (strong red emission peaks at 656 and 677 nm,
respectively, and weak near-infrared emission peaks at
07 and 845 nm, respectively) were observed. The inten-
8
sity of the red emission peak increased with an increase
in the quantity of the excessive S and Se for the BaIn2S4
and BaIn Se single crystals, respectively. On the other
2
1.670 Å, and c ס 13.125 Å for BaIn S reported by
2 4
1
Eisenmann et al. and a ס 21.710 Å, b ס 22.479 Å, and
c ס 13.576 Å for BaIn S reported by Klee et al.
2
4
2
2
4
hand, the intensity of the near-infrared emission peak
increased with a decrease in the quantity of the excessive
S and Se.
The optical absorption spectra of the single crystals at
1 K were measured near the fundamental absorption
1
edge and are shown in Fig. 1. A rapid rise in optical
absorption appeared near the fundamental absorption
edge of each of the single crystals.
The optical energy gap E of a semiconductor with the
g
direct band structure can be deduced from the following
5
relation :
2
(
␣h␯) ∼ (h␯ − E )
,
(1)
g
where h␯ is the incident photon energy and ␣ is the
optical absorption coefficient near the fundamental ab-
sorption edge. In order to obtain the optical energy gap of
2
the single crystals, the value of (␣h␯) was plotted as a
function of the incident photon energy h␯ as shown in
Fig. 2. The optical absorption coefficient ␣ was calcu-
lated from the optical absorption spectra shown in Fig. 1.
The value of the optical energy gap E of each of the
g
single crystals was obtained by extrapolation to
2
(
1
2
␣h␯) ס 0. The value of the optical energy gap E at
g
6
,7
1 K is listed in Table I. If the energy gap of 3.95,
.03, 3.68, and 1.50 eV for BaS, In S , BaSe, and In Se ,
2
3
2
3
respectively, is taken into consideration these values are
thought to be reasonable.
To identify the temperature dependence of the optical
energy gap, optical absorption spectra were measured
near the fundamental absorption edge over the tempera-
ture range of 11–300 K. The optical energy gap of the
single crystals at each temperature was obtained by the
previously described method.
2
FIG. 2. Plots of (␣h␯) as a function of the incident photon energy h␯
3+
3+
for the (a) BaIn S , (b) BaIn S :Ho , (c) BaIn S :Er , (d)
2
4
2
4
2 4
3
+
3+
3+
Figure 3 shows the temperature dependence of the op-
tical energy gap of the single crystals. The optical energy
BaIn S :Tm , (e) BaIn Se , (f) BaIn Se :Ho , (g) BaIn Se :Er
,
2
4
2
4
2
4
2
4
3
+
and (h) BaIn Se :Tm single crystals at 11 K.
2
4
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3
+
3+
3+
S-A. Park et al.: Optical properties of Ho -, Er -, and Tm -doped BaIn S and BaIn Se single crystals
2
4
2
4
3
+
Three deep levels with activation energies of 0.242,
.595, and 0.927 eV for the BaIn S single crystal and
C. Photoluminescence of BaIn S :Ho ,
2 4
3+
3+
3+
0
0
BaIn S :Er , BaIn S :Tm , BaIn S :Ho ,
BaIn Se :Er , and BaIn Se :Tm single crystals
2 4 2 4
2
4
2 4 2 4 2 4
3+
3+
.212, 0.574, and 0.584 eV for the BaIn S single crystal
2
4
were obtained by thermoluminescence measurement.
These activation energies were determined by the general
order kinetic method. For the BaIn S single crystal, the
deep levels of 0.242 and 0.595 eV could be considered
donor levels (D1 and D2), corresponding to sulfur va-
cancies and the deep level of 0.927 eV as an acceptor
level (A1) corresponding to a metal vacancy. For the
BaIn S single crystal, the deep levels of 0.212 and
3+
Figures 5–10 show PL spectra of the BaIn S :Ho ,
2
4
3+
3+
3+
BaIn S :Er , BaIn S :Tm , BaIn Se :Ho
,
9
2
4
2
4
2
4
3+
3+
2
4
BaIn Se :Er , and BaIn Se :Tm single crystals, re-
spectively, at 6 K. The PL spectra showed that sharp
2
4
2
4
1
0
2
4
0.574 eV could be thought as donor levels (D1 and D2),
corresponding to selenium vacancies and the deep level
of 0.584 eV as an acceptor level (A1) corresponding to a
1
0
metal vacancy. Based on these experimental results, the
red and the near infrared emission peaks in the BaIn2S4
and BaIn Se single crystals could be described as
2
4
caused by donor–acceptor pair recombination between
the donor level D1 and the acceptor level A1 for the red
one and the donor level D2 and the acceptor level A1 for
the near infrared one.
FIG. 4. Photoluminescence spectra of the BaIn2S4 and BaIn Se
2
4
single crystals at 6 K.
FIG. 3. Temperature dependence of the optical energy gap of the (a)
3
+
3+
3+
BaIn S , (b) BaIn S :Ho , (c) BaIn S :Er , (d) BaIn S :Tm , (e)
2
4
2
4
2
4
2 4
3+
3
+
3+
BaIn Se , (f ) BaIn Se :Ho , (g) BaIn Se :Er , and (h)
BaIn Se :Tm single crystals.
FIG. 5. Photoluminescence spectrum of the BaIn S :Ho single crys-
2 4
2
4
2
4
2
4
3
+
tal in the wavelength region of 410–710 nm at 6 K.
2
4
2150
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3
+
3+
3+
S-A. Park et al.: Optical properties of Ho -, Er -, and Tm -doped BaIn S and BaIn Se single crystals
2
4
2
4
emission peaks overlapped with a broad emission peak
for each of the single crystals. If the sharp emission peaks
3
+
are compared with those reported in BaAl S :Ho ,
2
4
3+
3+
11
BaAl S :Er , and BaAl S :Tm single crystals, the
2
4
2 4
sharp emission peaks could be attributed to the electron
3
+
3+
3+
transitions between Stark levels of Ho , Er , and Tm ,
3
+
respectively, replaced with In in the single crystals.
In the BaIn S and BaIn Se single crystals, In is situ-
ated in the C symmetry point. If group-theoretical
method and experimental results of the energy level
splitting of 4f transition metal ions are considered, the
2
4
2
4
1
,2
1
12
1
3
3
+
3+
3+
energy level splitting of Ho , Er , and Tm situated in
C symmetry is given as the following: The ground state
1
5
3+
I of Ho is split into 17 Stark levels, and the excited
8
5
5
5
5
3
states F , S , F , F , and K into 11, 5, 9, 7, and
5
2
4
3
8
4
3+
FIG. 8. Photoluminescence spectrum of the BaIn S :Ho single crys-
1
7 Stark levels, respectively. The ground state I
of
2 4
1
5/2
3
+
tal in the wavelength region of 430–730 nm at 6 K.
Er is split into 8 Stark levels, and the excited states
4
4
4
2
4
I , F , S , H , and F into 5, 5, 2, 6, and
9
/2
9/2
3/2
11/2
7/2
3
4
Stark levels, respectively. The ground state H of
6
3
+
FIG. 9. Photoluminescence spectrum of the BaIn Se :Er single
2
4
3
+
crystal in the wavelength region of 450–930 nm at 6 K.
FIG. 6. Photoluminescence spectrum of the BaIn S :Er single crys-
2
4
tal in the wavelength region of 450–930 nm at 6 K.
3
+
3+
FIG. 7. Photoluminescence spectrum of the BaIn S :Tm single
FIG. 10. Photoluminescence spectrum of the BaIn Se :Tm single
2 4
2
4
crystal in the wavelength region of 400–880 nm at 6 K.
crystal in the wavelength region of 400–880 nm at 6 K.
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3
+
3+
3+
S-A. Park et al.: Optical properties of Ho -, Er -, and Tm -doped BaIn S and BaIn Se single crystals
2
4
2
4
3
+
Tm is split into 13 Stark levels, and the excited states
method. The single crystals crystallized into an orthor-
hombic structure. The optical energy gaps of the
3
3
3
1
F , F , F , and G into 9, 7, 5, and 9 Stark levels,
respectively.
4
3
2
4
3
+
3+
3+
BaIn S , BaIn S :Ho , BaIn S :Er , BaIn S :Tm ,
2 4 2 4 2 4 2 4
3+ 3+
At the lower temperature, electrons are transferred
from the lowest Stark level of the excited states of 4f
transition metal ions to the split Stark levels of the
ground state. Accordingly, the five groups of the sharp
emission peaks in the wavelength regions of 487–495,
99–512, 536–554, 565–583, and 661–677 nm for the
BaIn S :Ho single crystal (shown in Fig. 5) and 495–
99, 501–518, 542–562, 575–587, and 670–681 nm for
the BaIn Se :Ho single crystal (shown in Fig. 8) could
be assigned to the electron transitions from the lowest
Stark level of the excited states F , F , F , F , and K
of Ho sited in C symmetry to the split Stark levels of
the ground state I . The five groups of the sharp emis-
BaIn Se , BaIn Se :Ho , BaIn Se :Er , and
2 4 2 4 2 4
3+
BaIn Se :Tm single crystals were found to be 3.057,
2
4
2.987, 2.967, 2.907, 2.625, 2.545, 2.515, and 2.415 eV,
respectively, at 11 K. The temperature dependence of the
optical energy gap fit well with the Varshni equation.
Broad emission peaks were observed in the single crys-
tals and thought to be caused by donor–acceptor pair
radiation recombination. Sharp emission peaks were
4
3+
2
4
4
3+
3+
3+
observed in the BaIn S :Ho , BaIn S :Er
,
2
4
2
4
2 4
3
+
3+
3+
BaIn S :Tm , BaIn Se :Ho , BaIn Se :Er , and
2 4 2 4 2 4
3+
5
5
5
5
3
BaIn Se :Tm single crystals. These sharp emission
2 4
5
2
4
3
8
3+
peaks were assigned to the electron transitions from the
1
5
3+ 3+
lowest Stark level of the excited states of Ho , Er , and
8
3
+
sion peaks in the wavelength regions of 512–530, 554–
Tm situated in C symmetry to the split Stark levels of
1
5
63, 576–582, 685–696, and 826–851 nm for the
the ground state.
3+
BaIn S ;Er single crystal (shown in Fig. 6) and 522–
2
4
5
36, 562–571, 585–591, 694–703, and 831–858 nm for
3+
the BaIn Se :Er single crystal (shown in Fig. 9) could
2
4
ACKNOWLEDGMENT
be assigned to the electron transitions from the lowest
4
4
4
2
This work was supported by Korea Research Founda-
tion Grant No. KRF-99-042-D00053-D2008.
Stark level of the excited states I , F , S , H11/2,
9
/2
9/2
3/2
4
3+
and F7 of Er situated in C symmetry to the split
Stark levels of the ground state I15/2. The four groups of
the sharp emission peaks in the wavelength regions of
/2
1
4
4
69–478, 650–665, 672–698, and 794–841 nm for the
REFERENCES
3+
BaIn S :Tm single crystal (shown in Fig. 7) and 481–
4
BaIn Se :Tm single crystal (shown in Fig. 10) could be
assigned to the electron transitions from the lowest Stark
2
4
1. B. Eisenmann, M. Jakowski, W. Klee, and H. Schafer, Rev. Chim.
89, 661–674, 680–703, and 799–851 nm for the
3+
Min. 20, 255 (1983).
2
4
2
3
. W. Klee and H. Schafer, Z. Anorg. Allg. Chem. 479, 125 (1981).
. W-T. Kim, S-A. Park, and M-S. Jin (unpublished).
3
3
3
1
3+
level of the excited states F , F , F , and G of Tm
4. P.C. Donohue and J.E. Hanlon, J. Electrochem. Soc., Solid State
4
3
2
4
sited in C symmetry to the split Stark levels of the
Sci. Technol. 121, 137 (1974).
. J.J. Pankov, Optical Processes in Semiconductors (Dover, New
York, 1971), Ch. 3.
. H. Nakanishi, S. Endo, and T. Irie, Jpn. J. Appl. Phys. 20, 1481
1
3
5
ground state H .
6
As shown in Figs. 5–10, broad emission peaks were
observed at 682, 693, 717, 708, 721, and 761 nm for
6
(
1981).
3
+
3+
3+
the BaIn S :Ho , BaIn S :Er , BaIn S :Tm ,
7. M. Peressi and A. Balderesch, J. Appl. Phys. 83, 3092 (1998).
8. Y.P. Varshni, Physica 34, 149 (1967).
. S.W.S. McKeever, Thermoluminescence of Solids (Cambridge
Univ. Press, New York, 1985), p. 90.
0. R.H. Bube, Photoconductivity of Solids (John Wiley and Sons,
New York, 1960), pp. 158–171.
1. J-M. Goh, W-T. Kim, M-S. Jin, S-H. Choe, H-G. Kim, and
T-Y. Park, J. Appl. Phys. 88, 4117 (2000).
2. G.F. Koster, J.O. Dimmack, R.G. Wheeler, and H. Statg, Proper-
ties of 32 Point Groups (MIT, Cambridge, MA, 1963).
3. G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in
Crystals (John Wiley and Sons, New York, 1968).
2
4
2
4
2 4
3
+
3+
3+
BaIn Se :Ho , BaIn Se :Er , and BaIn Se :Tm
2
4
2
4
2
4
9
single crystals, respectively. The broad emission peaks
could be thought to correspond to the broad emission
1
1
1
1
peaks observed in the BaIn S and BaIn Se single crys-
2
4
2
4
tals (shown in Fig. 4). The peak position shift is attrib-
3
+
3+
uted to structure distortion by doping Ho , Er , and
3+
Tm impurities.
IV. CONCLUSION
3
+
3+
3+
BaIn S , BaIn S :Ho , BaIn S :Er , BaIn S :Tm ,
2
4
2 4
2 4
2 4
3+
3
+
BaIn Se , BaIn Se :Ho , BaIn Se :Er , and
2
4
2
4
2
4
3+
BaIn Se :Tm single crystals were grown by the CTR
2
4
2152
J. Mater. Res., Vol. 17, No. 8, Aug 2002
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