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Relaxed excitonic states in CdI2 crystals
Journal of Luminescence 18/19 (1979) 19—22 © North-Holland Publishing Company
RELAXED EXCITONIC STATES IN Cd!2 CRYSTALS Hiroaki MATSUMOTO and Hideyuki NAKAGAWA Department of Electronics, Fukui University. Fukui, Japan
Intrinsic luminescence in Cd12 was studied by measuring intensity, life-time and degree of polarization as a function of temperature. Two anisotropic emission bands were observed at 2.50eV and 2.16eV. The temperature 2~I~)4-complex dependence of D of life-time and polarization is explained in terms of excitonic states of (Cd 3d symmetry.
1. Introduction Cadmium iodide, an ionic crystal of the layer structure, shows several strong luminescent emissions at low temperature with the excitation in the intrinsic region [1,21. In the present study, the intensity, life-time and degree of polarization of the intrinsic luminescence in the pure and carefully grown Cd12 crystal have been measured as a function of temperature, T, down to 10 K to clarify the structure of the relaxed excitonic states responsible for the luminescence.
2. Experimental results In fig. 1 are shown emission spectra of Cd12 observed at 4.2 K, LHeT, and 77 K, LNT. The experimental set-up is also shown in fig. 1. As shown by the solid curve, an irradiation in the excitonic absorption region gives rise to two emission bands at LHeT, the G (at 2.50eV) and the Y (at 2.16eV) bands. They exchange intensities as T rises so that the Y-emission becomes dominant at LNT. These emissions are supposed to be intrinsic ones since they are strongly stimulated only in the fundamental absorption region and highly reproducible for every sample. There are weak structures at 3.35 eV (the uv-emission) and around 3.1 eV in the emission spectra. Values of the degree of polarization, P = (iii ~±)/(~ii + Ii), obtained at LHeT are also plotted in fig. 1 by solid circles, where I~and I~are intensities of the luminescence polarized parallel and perpendicular to the c-axis, respectively. The anisotropic nature of the luminescence suggests an important influence of a layer symmetry of the crystal on the relaxed excitons. In fig. 2 is shown the temperature dependence of P. Measurements were made at 2.50 eV (G) and at 2.16 eV (Y). Values of P of the Y-emission are independent of T below 40 K and increase with T between 40 and 60 K. Apparent decrease of P for the G-emission with increasing T, below 30 K, is due to the super—
19
20
H. Matsumoto. H. Nakagawa/ Relaxed excitonic states in Cd1
2 crystals
1.0
CdI2~---~—-.._,.
~_
--EMISSION
.~
PHOTON
ENERGY
----
—
0.3 o
--
0.2
(eV
Fig. I. Emission spectra in Cd12 obtained at LHeT (solid curve) and at LNT (dashed curveL Excitation was made at 3.82 eV. Dotted curves indicate Gaussian distributions. Solid circles give the values of the degree of polarization. P. at each emission energy.
0.2 0.1
CdI2~L~
I~L~-
~
0
--~
.~
~
~-
Em.
~.--~---
0- em is. 00
..,
..~...
~
a:
.~
•..~Y-emis. 0
-0.3-
.
20
.
40 60 TEMPERATURE
.
80
100
1(K)
Fig. 2. Temperature dependence of P measured at 2.50eV (G-emis.) and at 2.16eV (Y-emis.(.
position of the Y-emission tail on the G-emission region, the intensity of the former increases with T as shown in fig. 3. The integrated intensities and the life-times of each emission are given in fig. 3 as functions of T. The intensity of the G-emission, which is dominant below 20 K, decreases rapidly with increasing T while the intensity of Y-emission increases. Fall of total intensities of these emissions below 70 K may be due to 2~impurities the neglect of the UV-emission, most of which is reabsorbed by Pb as shown in fig. I. The decay of the Y-emission leads to two life-times Tp and TS. The value of T 5
is 20 ~xsat 6 K and decreases rather sharply to 6.5 ~xsat 50 K while TF is 6 p.s at 6 K and decreases gradually as shown in fig. 3b. The decay time of the G-emission is 6.5 ~s at 6 K and decreases sharply to 1.6 p.s at 50 K.
M Matsumoto, H. Nakagawa/ Relaxed excitonic states in Cd1
2 crystals
2.0
Cdt2
-
20
Ex. at 3.82 eV
21
Cd12
\
Ex at 3.675 eV
1C
—
‘~-e~is.
N”.
~.V-emis
a ems
\~Yems
~0.1
\
\~i
N. 005
(A) liii
0
Liii
(B)
0.5 liii
50 100 TEMPERATURE
...1.....i....’
150 1(K)
Liii
0
iiii
un
50 100 TEMPERATURE
150 T(K)
Fig. 3. (A) Temperature dependence of the integrated intensities of the G- and the Y-emission bands. The sum of these intensities is plotted as “Total”. (B) Temperature dependence of life-times of the G- and Y-emissions under excitation by a N2 laser (3.678 eV). Decomposition of the decay curves into two or three exponential curves was performed graphically or by using an electronic computer.
3. Discussion It is believed in cadmium halide crystals that the relaxed excitonic molecular states are 2X~)4-complex well-approximated the excited states of the diagram (Cd ions, where X is a by halogen ion [3, 4]. An energy of this complex ion of D3d symmetry is depicted schematically in fig. 4a for the case of Cd! 2. The Gand Y-emissions would be associated with the parity forbidden transitions from the lowest lying excited states Eg(Eg) and Eg(Tig) to the ground A16 states where the corresponding states in °h symmetry are given in parentheses. It is obviously impossible to explain the temperature dependence of life-time in fig. 3 by (A)
~
(B)
5pM) .5s(Cd)K//____~_~~
jA:
4d(Cd), 5s(Cd) (hoIee[ectron)\~~
A5 Oh
~g(Aug)
Ru
~
~ ~ ~Aux D,s
D~d
•exchange
2~1~,)~-complex molecular ion. In the left side are shown Fig. 4. the (A) possible Schematic excitonic energy states. diagram(B)ofThe (Cdinitial states of the Y-emission, ‘Eg and 3Eg. taking account of the electron—hole exchange interaction. Radiative transition probabilities from these levels to the ground Aig state are indicated as P,, and Prs and phonon assisted transition probabilities between them as S,, and S,,.
22
H. Matsurnoto. H. Nakagatva/Relaxed excitonic states in Cd!
2 crystals
considering only lattice vibrations to break the parity forbiddenness. Another effective mechanism, such as an off-center effect, should be taken into account as well as lattice vibrations. By introducing an exchange interaction between an electron and a hole of the exciton, the 3E initial states of the Y-emission. E6(E~),would split off slightly into ‘Eg and 5 as shown in fig. 4b. It is possible to explain the temperature dependence of Tp and TS in fig. 3 in terms of the rate equations concerning these two closely lying levels and the ground state by making use of parameters indicated in the figure. Anomalous behavior of P between 40 and 60 K in 3Egfig. will2 may also be understood by assuming that above 40 K most excitons at decay to the ground state indirectly through ‘Eg.
References Ill l.M. Bolesta, Ukr. Fiz. Zh. 21(1976) 28. 121 A.B. Lyskovich. N.K. Gloskovskaja and l.M. Bolesta, Ukr. Fiz. Zh. 21(1976)89. [3] H. Nakagawa. K. Hayashi and H. Matsumoto: J. Phys. Soc. Japan 43 (l977( 1655. 141 H. Nakagawa. K. Hayashi and H. Matsumoto. Proc. Int. Conf. on Defects in insulating crystals. Gatlinhurg (1977) Conf. Abstracts P. 304.
RELAXED EXCITONIC STATES IN Cd!2 CRYSTALS Hiroaki MATSUMOTO and Hideyuki NAKAGAWA Department of Electronics, Fukui University. Fukui, Japan
Intrinsic luminescence in Cd12 was studied by measuring intensity, life-time and degree of polarization as a function of temperature. Two anisotropic emission bands were observed at 2.50eV and 2.16eV. The temperature 2~I~)4-complex dependence of D of life-time and polarization is explained in terms of excitonic states of (Cd 3d symmetry.
1. Introduction Cadmium iodide, an ionic crystal of the layer structure, shows several strong luminescent emissions at low temperature with the excitation in the intrinsic region [1,21. In the present study, the intensity, life-time and degree of polarization of the intrinsic luminescence in the pure and carefully grown Cd12 crystal have been measured as a function of temperature, T, down to 10 K to clarify the structure of the relaxed excitonic states responsible for the luminescence.
2. Experimental results In fig. 1 are shown emission spectra of Cd12 observed at 4.2 K, LHeT, and 77 K, LNT. The experimental set-up is also shown in fig. 1. As shown by the solid curve, an irradiation in the excitonic absorption region gives rise to two emission bands at LHeT, the G (at 2.50eV) and the Y (at 2.16eV) bands. They exchange intensities as T rises so that the Y-emission becomes dominant at LNT. These emissions are supposed to be intrinsic ones since they are strongly stimulated only in the fundamental absorption region and highly reproducible for every sample. There are weak structures at 3.35 eV (the uv-emission) and around 3.1 eV in the emission spectra. Values of the degree of polarization, P = (iii ~±)/(~ii + Ii), obtained at LHeT are also plotted in fig. 1 by solid circles, where I~and I~are intensities of the luminescence polarized parallel and perpendicular to the c-axis, respectively. The anisotropic nature of the luminescence suggests an important influence of a layer symmetry of the crystal on the relaxed excitons. In fig. 2 is shown the temperature dependence of P. Measurements were made at 2.50 eV (G) and at 2.16 eV (Y). Values of P of the Y-emission are independent of T below 40 K and increase with T between 40 and 60 K. Apparent decrease of P for the G-emission with increasing T, below 30 K, is due to the super—
19
20
H. Matsumoto. H. Nakagawa/ Relaxed excitonic states in Cd1
2 crystals
1.0
CdI2~---~—-.._,.
~_
--EMISSION
.~
PHOTON
ENERGY
----
—
0.3 o
--
0.2
(eV
Fig. I. Emission spectra in Cd12 obtained at LHeT (solid curve) and at LNT (dashed curveL Excitation was made at 3.82 eV. Dotted curves indicate Gaussian distributions. Solid circles give the values of the degree of polarization. P. at each emission energy.
0.2 0.1
CdI2~L~
I~L~-
~
0
--~
.~
~
~-
Em.
~.--~---
0- em is. 00
..,
..~...
~
a:
.~
•..~Y-emis. 0
-0.3-
.
20
.
40 60 TEMPERATURE
.
80
100
1(K)
Fig. 2. Temperature dependence of P measured at 2.50eV (G-emis.) and at 2.16eV (Y-emis.(.
position of the Y-emission tail on the G-emission region, the intensity of the former increases with T as shown in fig. 3. The integrated intensities and the life-times of each emission are given in fig. 3 as functions of T. The intensity of the G-emission, which is dominant below 20 K, decreases rapidly with increasing T while the intensity of Y-emission increases. Fall of total intensities of these emissions below 70 K may be due to 2~impurities the neglect of the UV-emission, most of which is reabsorbed by Pb as shown in fig. I. The decay of the Y-emission leads to two life-times Tp and TS. The value of T 5
is 20 ~xsat 6 K and decreases rather sharply to 6.5 ~xsat 50 K while TF is 6 p.s at 6 K and decreases gradually as shown in fig. 3b. The decay time of the G-emission is 6.5 ~s at 6 K and decreases sharply to 1.6 p.s at 50 K.
M Matsumoto, H. Nakagawa/ Relaxed excitonic states in Cd1
2 crystals
2.0
Cdt2
-
20
Ex. at 3.82 eV
21
Cd12
\
Ex at 3.675 eV
1C
—
‘~-e~is.
N”.
~.V-emis
a ems
\~Yems
~0.1
\
\~i
N. 005
(A) liii
0
Liii
(B)
0.5 liii
50 100 TEMPERATURE
...1.....i....’
150 1(K)
Liii
0
iiii
un
50 100 TEMPERATURE
150 T(K)
Fig. 3. (A) Temperature dependence of the integrated intensities of the G- and the Y-emission bands. The sum of these intensities is plotted as “Total”. (B) Temperature dependence of life-times of the G- and Y-emissions under excitation by a N2 laser (3.678 eV). Decomposition of the decay curves into two or three exponential curves was performed graphically or by using an electronic computer.
3. Discussion It is believed in cadmium halide crystals that the relaxed excitonic molecular states are 2X~)4-complex well-approximated the excited states of the diagram (Cd ions, where X is a by halogen ion [3, 4]. An energy of this complex ion of D3d symmetry is depicted schematically in fig. 4a for the case of Cd! 2. The Gand Y-emissions would be associated with the parity forbidden transitions from the lowest lying excited states Eg(Eg) and Eg(Tig) to the ground A16 states where the corresponding states in °h symmetry are given in parentheses. It is obviously impossible to explain the temperature dependence of life-time in fig. 3 by (A)
~
(B)
5pM) .5s(Cd)K//____~_~~
jA:
4d(Cd), 5s(Cd) (hoIee[ectron)\~~
A5 Oh
~g(Aug)
Ru
~
~ ~ ~Aux D,s
D~d
•exchange
2~1~,)~-complex molecular ion. In the left side are shown Fig. 4. the (A) possible Schematic excitonic energy states. diagram(B)ofThe (Cdinitial states of the Y-emission, ‘Eg and 3Eg. taking account of the electron—hole exchange interaction. Radiative transition probabilities from these levels to the ground Aig state are indicated as P,, and Prs and phonon assisted transition probabilities between them as S,, and S,,.
22
H. Matsurnoto. H. Nakagatva/Relaxed excitonic states in Cd!
2 crystals
considering only lattice vibrations to break the parity forbiddenness. Another effective mechanism, such as an off-center effect, should be taken into account as well as lattice vibrations. By introducing an exchange interaction between an electron and a hole of the exciton, the 3E initial states of the Y-emission. E6(E~),would split off slightly into ‘Eg and 5 as shown in fig. 4b. It is possible to explain the temperature dependence of Tp and TS in fig. 3 in terms of the rate equations concerning these two closely lying levels and the ground state by making use of parameters indicated in the figure. Anomalous behavior of P between 40 and 60 K in 3Egfig. will2 may also be understood by assuming that above 40 K most excitons at decay to the ground state indirectly through ‘Eg.
References Ill l.M. Bolesta, Ukr. Fiz. Zh. 21(1976) 28. 121 A.B. Lyskovich. N.K. Gloskovskaja and l.M. Bolesta, Ukr. Fiz. Zh. 21(1976)89. [3] H. Nakagawa. K. Hayashi and H. Matsumoto: J. Phys. Soc. Japan 43 (l977( 1655. 141 H. Nakagawa. K. Hayashi and H. Matsumoto. Proc. Int. Conf. on Defects in insulating crystals. Gatlinhurg (1977) Conf. Abstracts P. 304.