- Email: [email protected]
Europium impurity as a recombination center in ZnSe lattice
LUMINESCENCE
Journal of Luminescence 53 (1992)4064)18 North-I Iolland
JOURNAL Or
Europium impurity as a recombination center in ZnSe lattice K. Swi~tekand M. Godlcwski Ins Htiels of P/ivsu’s. I’olisli Academy of .S’t,enc’e,s.,—tL Looiikow 32
,/ 46,
02—665 lfar,saw, I’o/unml
2 ground state is located within the ZnSe forbidden gap. We present the first evidence that the energ~level of Eu approximately 2.1 cV below the bottom of the conduction hand ((13). The photo—ESR and photoluminescence experinlents have been performed on high-resistivity ZnSe : Eu crystals. After illuminating the sample with lit’ 1.9 eV light a quenching of the inte nsity of Eu [SR signal has been observed. i.e. the concentration of [02 centers was found to decrease due to Eu2 + /it’ --~Eu5 ~ e~ photoionization transition.
Recently, it was evidenced that photoinization transition of some rare earth (RE) and transition metal (TM) impurities in solids may he accompanied with excitation of efficient intra-shell emtssion. This to possibility of exciton binding by was the attributed ionized center. This facts attract attention to the studies of charge states of the impurities. Contrary to practically all TM impurities only some RE ions have been assumed to introduce energy levels into the forbidden gap of lI—VI compounds [1]. Recently, we have analyzed this problem in ZnS and other sulphides doped with RE ions [2]. Until now, the only RE impurity observed in ZnSe lattice in 2 + charge state is europium [31.In this2~3~ paperenergy we present first level isthe located evidence the gap. Eu within thethat ZnSe The electron spin resonance (ESR) experiments were performed on a Bruker 418s X-hand equipped with an Oxford Instruments ESR-900 ccntinuous-gas-flow cryostat, working in the temperature range of 4—300 K. A high-pressure XBO 150 xenon lamp and a set of Carl-Zeiss interferenee filters were used for the optical excitation. The photo-ESR experiments have been performed on high-resistivity ZnSe Eu crystals. An ESR signal of Eu2~(4f7)has been observed at 4.2
K prior to illumination (fig. 1). After illuminating the sample with hi’ 1 > 1.9 eV light a quenching of the intensity of this signal has been observed, i.e., the concentration of Eu + centers was found to 5) decrease duecannot to the population of the Eu by [SR (4f state which he easily detected technique. Then the light was turned off and a small increase of the Eu2 ESR signal was ohserved. After equilibrium was reached. a secondary infrared illumination (0.9
~—-—-~.-
A ~
300
320
MAGNETIC Correspondence to: Dr. K. Swiatek, Institute of Physics. Potish Academy of Sciences. Al. Lotnikdw 32/46. (12-668 Warsaw. Poland. 0022-2313/92/SOS(S)
1992
—
~1
- .......
-~
—
FIELD
[ml]
Fig. I. The central part of the Eu2 ‘ [SR spectrum in ZnSe lattice at 4.2 K: I lyperfine-structure lines due to °‘ Eu and °3Fti isotopes are clearly visible.
Elsevier Science Publishers By. All rights reserved
K. Swiqrek, M Godlewski
~
~i
—
1.0
J
/ Europium
ization. solving the appropriate kinetic equations [4] By it can be shown that r t(hp) is proportional to the optical cross-section o~(ho)for this transition. Thus, can the be spectral determined. dependence Since for of the Eu2~ionization
hI~
>-
V
407
impurity as a recombination center in ZnSe lattice
hv
off ha1
1 ~ z
08 07~
C
description of a smooth band observed in a nar-
off
proach row energy is notformula range justified, an[5]:we elaborate used thetheoretical simple Kopyaplov—Pikhtin 2(ho)3. (1) o(E.0~,ho) (ho E0~1)t/ Because in a partly ionic compound the change of a center’s charge state is accompanied by a relaxation of the lattice around the ionized ion, this formula was extended to account for the electron—phonon interaction in the manner proposed
on
W06.~ -
2
50
0
50
100
TIME
Es
~
150
200
Fig. 2. The sequence of steps in the photo-ESR experiment, Kinetics of the intensity of the Eu2~ ESR signal under the primary ho 1 = 2.2 eV and secondary ho2 = 1 eV illumination are shown,
2~ signal intensity. Still a small, further inEu crease occurred after the light was turned off. The sequence of steps in the experiment is shown in fig. 2. Before each measurement, a primary
—
by Langer et al. [6]: —~ ff(ho)=~t/2fdZffet(Eopt,ho+FZ)
(2a)
X(1 +Fz/hv) exp(—z2), /3
=
(ho
—
E 00~)/F,
ho2 illumination was applied to ensure the same initial conditions. The spectral distribution of the photoquenching rate constant (T t) normalized to constant light intensity is shown in fig. 3. We attribute the first transition > 1.9 eV) 2~+ho 3~+(hot e~ photoionto the direct Eu 1 Eu —~
(2b)
‘°o F 1= ____12(EOPt_Elh)hwt) ~
1
‘~°~o 1 1/2 -
Wex
(2c) From the fit we have obtained the thermal and optical ionization energies of Eu2~impurity: Elh 1.9 ±0.1 eV and E 001 2.1 ±0.1 eV, respectively. The complementary photo-neutralization tran=
=
—~
5.
°
o z z
w
2-
0 0
~
0 1.8
J 1.9
2.0
2.1
ENERGY
2.2
served 2~ESR for 0.7 ~signal ho2 intensity. 1.1 eV as an part increase of This of meathe Eu surements is less accurate due to competition of sition Eu3~+ho 2~+hvB has been obmany extra processes leading to both stimulation 2 Eu and quenching of Eu2~,i.e., the capture of electrons ionized from donors into the conduction band, capture of holes generated in the valence band in the ionization process of chromium and copper—related defects (Cr and Cu are contaminants in our samples). The photosensitivity of the Eu2~charge state must mean that Eu3~ is present, or can be obtamed in the ZnSe host. A separate problem is still the proper excitation mechanism of the Eu3~ .~
2.3
[eVE
Fig. 3. The spectral dependence of the Eu2~photoquenching rate measured at 4.2 K. The solid curve represents the fit to the experimental data with formulas (1) and (2).
408
K. Swiqtek, M. (4odlew,ski
3
/
Europium impurity as a reco,nhinait’o,i center in ZnSe lattice
associated with the presence of Cu impurity, namely the green (2.34 eV). red (1.95 eV) and
~
2
jf
\
References B~int989)pl))
0 —J
1.2
r-~~T~ 1.4 1.6 ENERGY
E~] K 1.8
2.0
2.2
[eV~
Fig. 4. The photoluminescence spectrum of Cu-related emissions in ZnSe: Eu crystal under 488 nm excitation measured at 4.2 K.
luminescence, recently discussed by us for ZnS : Eu [9]. The efficient charge transfer between Eu2~and Cr~indicates that europium can act as a recombination center of free carriers generated in conduction and valence band of ZnSe. In the photoluminescence (fig. 4) experiments we observed well known emission bands
Swi4tek A Suchoiki md M Cjodlcwski AppI Phys l,ett. 56(1991)1195.
[3] 5. Ihuki. H. Komiya. M. Nakada. H. Masui and LI. Kimura, .1. Lumin. l&2 (1970) 797. [4] M. Godlewski. Phys. Stat. Sol. (a) 9)) (1985) Il. [5] A.A. Kopylov and AN. Pikhtin. Fiz. Tverd. TeD 6 (1974) 1837.
[6] U. Piekara. J.M. I.,anger and B. Krukowska-Fulde. Solid State Commun. 23 (1977) 583; J.M. Langer. Leet. Notes Phys. 122 (1980) 123. [7] S. Shionoya. in: Luminescence of Inorganic Solids, ed. P. Goldberg (Academic Press, New York, 1966) Chap. 2, p. 2))6. [8] M. Godlewski, WE. Lamb and B.C. Cavenett, Solid State Commun. 39(1981) 595 [9] K. Swi~tek,M. Godlewski, and D. Hommel, Pbs’s. Rev. B 42 (1990) 3628.
Journal of Luminescence 53 (1992)4064)18 North-I Iolland
JOURNAL Or
Europium impurity as a recombination center in ZnSe lattice K. Swi~tekand M. Godlcwski Ins Htiels of P/ivsu’s. I’olisli Academy of .S’t,enc’e,s.,—tL Looiikow 32
,/ 46,
02—665 lfar,saw, I’o/unml
2 ground state is located within the ZnSe forbidden gap. We present the first evidence that the energ~level of Eu approximately 2.1 cV below the bottom of the conduction hand ((13). The photo—ESR and photoluminescence experinlents have been performed on high-resistivity ZnSe : Eu crystals. After illuminating the sample with lit’ 1.9 eV light a quenching of the inte nsity of Eu [SR signal has been observed. i.e. the concentration of [02 centers was found to decrease due to Eu2 + /it’ --~Eu5 ~ e~ photoionization transition.
Recently, it was evidenced that photoinization transition of some rare earth (RE) and transition metal (TM) impurities in solids may he accompanied with excitation of efficient intra-shell emtssion. This to possibility of exciton binding by was the attributed ionized center. This facts attract attention to the studies of charge states of the impurities. Contrary to practically all TM impurities only some RE ions have been assumed to introduce energy levels into the forbidden gap of lI—VI compounds [1]. Recently, we have analyzed this problem in ZnS and other sulphides doped with RE ions [2]. Until now, the only RE impurity observed in ZnSe lattice in 2 + charge state is europium [31.In this2~3~ paperenergy we present first level isthe located evidence the gap. Eu within thethat ZnSe The electron spin resonance (ESR) experiments were performed on a Bruker 418s X-hand equipped with an Oxford Instruments ESR-900 ccntinuous-gas-flow cryostat, working in the temperature range of 4—300 K. A high-pressure XBO 150 xenon lamp and a set of Carl-Zeiss interferenee filters were used for the optical excitation. The photo-ESR experiments have been performed on high-resistivity ZnSe Eu crystals. An ESR signal of Eu2~(4f7)has been observed at 4.2
K prior to illumination (fig. 1). After illuminating the sample with hi’ 1 > 1.9 eV light a quenching of the intensity of this signal has been observed, i.e., the concentration of Eu + centers was found to 5) decrease duecannot to the population of the Eu by [SR (4f state which he easily detected technique. Then the light was turned off and a small increase of the Eu2 ESR signal was ohserved. After equilibrium was reached. a secondary infrared illumination (0.9
~—-—-~.-
A ~
300
320
MAGNETIC Correspondence to: Dr. K. Swiatek, Institute of Physics. Potish Academy of Sciences. Al. Lotnikdw 32/46. (12-668 Warsaw. Poland. 0022-2313/92/SOS(S)
1992
—
~1
- .......
-~
—
FIELD
[ml]
Fig. I. The central part of the Eu2 ‘ [SR spectrum in ZnSe lattice at 4.2 K: I lyperfine-structure lines due to °‘ Eu and °3Fti isotopes are clearly visible.
Elsevier Science Publishers By. All rights reserved
K. Swiqrek, M Godlewski
~
~i
—
1.0
J
/ Europium
ization. solving the appropriate kinetic equations [4] By it can be shown that r t(hp) is proportional to the optical cross-section o~(ho)for this transition. Thus, can the be spectral determined. dependence Since for of the Eu2~ionization
hI~
>-
V
407
impurity as a recombination center in ZnSe lattice
hv
off ha1
1 ~ z
08 07~
C
description of a smooth band observed in a nar-
off
proach row energy is notformula range justified, an[5]:we elaborate used thetheoretical simple Kopyaplov—Pikhtin 2(ho)3. (1) o(E.0~,ho) (ho E0~1)t/ Because in a partly ionic compound the change of a center’s charge state is accompanied by a relaxation of the lattice around the ionized ion, this formula was extended to account for the electron—phonon interaction in the manner proposed
on
W06.~ -
2
50
0
50
100
TIME
Es
~
150
200
Fig. 2. The sequence of steps in the photo-ESR experiment, Kinetics of the intensity of the Eu2~ ESR signal under the primary ho 1 = 2.2 eV and secondary ho2 = 1 eV illumination are shown,
2~ signal intensity. Still a small, further inEu crease occurred after the light was turned off. The sequence of steps in the experiment is shown in fig. 2. Before each measurement, a primary
—
by Langer et al. [6]: —~ ff(ho)=~t/2fdZffet(Eopt,ho+FZ)
(2a)
X(1 +Fz/hv) exp(—z2), /3
=
(ho
—
E 00~)/F,
ho2 illumination was applied to ensure the same initial conditions. The spectral distribution of the photoquenching rate constant (T t) normalized to constant light intensity is shown in fig. 3. We attribute the first transition > 1.9 eV) 2~+ho 3~+(hot e~ photoionto the direct Eu 1 Eu —~
(2b)
‘°o F 1= ____12(EOPt_Elh)hwt) ~
1
‘~°~o 1 1/2 -
Wex
(2c) From the fit we have obtained the thermal and optical ionization energies of Eu2~impurity: Elh 1.9 ±0.1 eV and E 001 2.1 ±0.1 eV, respectively. The complementary photo-neutralization tran=
=
—~
5.
°
o z z
w
2-
0 0
~
0 1.8
J 1.9
2.0
2.1
ENERGY
2.2
served 2~ESR for 0.7 ~signal ho2 intensity. 1.1 eV as an part increase of This of meathe Eu surements is less accurate due to competition of sition Eu3~+ho 2~+hvB has been obmany extra processes leading to both stimulation 2 Eu and quenching of Eu2~,i.e., the capture of electrons ionized from donors into the conduction band, capture of holes generated in the valence band in the ionization process of chromium and copper—related defects (Cr and Cu are contaminants in our samples). The photosensitivity of the Eu2~charge state must mean that Eu3~ is present, or can be obtamed in the ZnSe host. A separate problem is still the proper excitation mechanism of the Eu3~ .~
2.3
[eVE
Fig. 3. The spectral dependence of the Eu2~photoquenching rate measured at 4.2 K. The solid curve represents the fit to the experimental data with formulas (1) and (2).
408
K. Swiqtek, M. (4odlew,ski
3
/
Europium impurity as a reco,nhinait’o,i center in ZnSe lattice
associated with the presence of Cu impurity, namely the green (2.34 eV). red (1.95 eV) and
~
2
jf
\
References B~int989)pl))
0 —J
1.2
r-~~T~ 1.4 1.6 ENERGY
E~] K 1.8
2.0
2.2
[eV~
Fig. 4. The photoluminescence spectrum of Cu-related emissions in ZnSe: Eu crystal under 488 nm excitation measured at 4.2 K.
luminescence, recently discussed by us for ZnS : Eu [9]. The efficient charge transfer between Eu2~and Cr~indicates that europium can act as a recombination center of free carriers generated in conduction and valence band of ZnSe. In the photoluminescence (fig. 4) experiments we observed well known emission bands
Swi4tek A Suchoiki md M Cjodlcwski AppI Phys l,ett. 56(1991)1195.
[3] 5. Ihuki. H. Komiya. M. Nakada. H. Masui and LI. Kimura, .1. Lumin. l&2 (1970) 797. [4] M. Godlewski. Phys. Stat. Sol. (a) 9)) (1985) Il. [5] A.A. Kopylov and AN. Pikhtin. Fiz. Tverd. TeD 6 (1974) 1837.
[6] U. Piekara. J.M. I.,anger and B. Krukowska-Fulde. Solid State Commun. 23 (1977) 583; J.M. Langer. Leet. Notes Phys. 122 (1980) 123. [7] S. Shionoya. in: Luminescence of Inorganic Solids, ed. P. Goldberg (Academic Press, New York, 1966) Chap. 2, p. 2))6. [8] M. Godlewski, WE. Lamb and B.C. Cavenett, Solid State Commun. 39(1981) 595 [9] K. Swi~tek,M. Godlewski, and D. Hommel, Pbs’s. Rev. B 42 (1990) 3628.