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ESR and concentration quenching of EL-emission in (Zn, Cd)S:Mn phosphors
~
0038-1098/9153.00+ .00 Pergamon Press plc
Solid State Communications, Vol. 77, No. 7, pp. 513-515, 1991. Printed in Great Britain.
BSR AND CONCENTRATION QUENCHING OF BL-EMISSION IN (Zn, C d ) S : M n PHOSPHORS
3. K.Naadagatoe., P.K.Pct2~, P.R. go¢e aad R.O. Lawangox-PawoJt Solid
State
Physics
Research Laboratory, Department of U n i v e r s i t y , K o l h a p u r - 4 1 6 004, INDIA
Physics,
ShtvaJt
(Received by D.Van Dyck - October 15, 1990)
ESR investlgations of electroluminescent (Zn,Cd)S:Mn powder phosphors have been carried out for Mn-concentratious rangin8 from 0.01 wt to 3 wt ~. T h e v a r i a t i o n of EL- b r i g h t n e s s w i t h M n - c o n c e n t r a t l o n h a s a l s o b e s n s t u d i e d . A t t e m p t s h a v e b e e n made to show t h a t t h e f o r m a t i o n o f Mn + ion p a i r s e n d c l u s t e r s c a u s e s t h e c o n c e n t r a t i o n q u e n c h i n g of EL-brightness in (Zn,Cd)S:Mn p h o s p h o r s a t h i g h e r M~+ c o n c e n t r a t i o n s .
I. INTRODUCTION T h e growing i n d u s t r i a l applications end development of thin film electroluminescent devices have prompted various research w o r k e r s to s t u d y new p a r a m e t e r s of p h o s p h o r s that y i e l d high efficiency [1-3]. The electron spin resonance (ESR). is a well known technique for the investigation of defects in crystalline and non-crystalllne materials. ESR permits the chemical identification of the centers, strength of crystal fields end the environment of paramagnetlc centers. ESR also y i e l d s data about the atomtstic picture of the Mn centers and t h e i r hosts [4].The ESR i n v e s t i g a t i o n s of p o w d e r p h o s p h o r s a r e u s u a l l y carried out to a v o i d t h e d i f f i c u l t i e s i n t h i n film investigation, like incorporation of a s u f f i c i e n t q u a n t i t y o f m a t e r i a l s i n to t h e ESR cavity. Moreover, ESR characteristics of p o l y c r y s t a l l t n a powders are s i m i l a r to those of p o l y c ry s t a l l t n a thin films. The ESR powder spectrum at higher Mn-concentration was interpreted by Ishtkawa [ 5 ] considering the !sotroptc exchenge interaction between Mn ions. In the present paper BSR investigations of (Zn,Cd)S:Mn powder phosphors are employed to explain the well-known concentration quenching of electroluminescence ( EL )-brightness at higher Mn-concentratlons. 2.
RXPERIMENTAL
(Zn,Cd)S:Mn powder phosphors with v a r y i n g c o n c e n t r a t i o n of Mn (0.01 to 3 , 0 wt%) have been prepared b y h e a t i n g a m i x t u r e of 60 tool ~ of ZnS and 40 tool 96 of CdS along w l t h r e q u i s i t e amount of d o p a n t a t 9 0 0 ° C for one h o u r i n a r g o n a t m o s p h e r e . T h e ESR s p e c t r a w e r e r e c o r f l e d u s i n 9 ~ X - b a n d v a r i a n E-Is9 ESR s p e c t r o m e t e P o p e r a t e d a t room t e m p e r a t u r e e n d a t m i c r o w a v e f r e q u e n c y of 9.52 GHz. T h e f i e l d m o d u l a t i o n f r e q u e n c y was 100 Hz. TCNE was used as standard f i e l d marker with g-factor 2. 00277.
The EL-brightness measurements were carried out b y u s u a l s a n d w i c h t y p e E L - c e l l u s i n g c a s t o r - o i l a s d i e l e c t r i c to d i s p e r s e t h e phosphor powder uniformly between the electrodes. High a . c . v o l t a g e s up to 1 . 5 kV a t 100 Hz - 2 . 5 kHz, o b t a i n e d from P h l l l l p s (GM-2308/90) generator coupled with wide band a m p l i f i e r (Telmex SP -100), was applied across the EL-cell. The l i g h t out put of the EL-call was measured using a photomultiplter tube (RCA-931A) coupled w t t h a s e n s i t i v e nanoammeter (NM-122). 3. RESULTS AND DISCUSSION In t h e ( Z n , C d ) S l a t t i c e Mnz+ s u b s t l t u t i e n a l l y replaces a cation b e c a u s e of s i m i l a r i t y in charge and ionic radii. T h e ESR s p e c t r a of {Zn,Cd)S:Mn powder phosphors with different M n - c o n c e n t r a t i o n s a r e s h o w n i n Fig. 1. T h e ESR s p e c t r a s h o w a c h a n g e i n l i n e s h a p e w i t h Mn-concentration. At l o w e r Mn-concentrations, up to 0 . 1 5 wt% (Fig. 1.a, b , c }, the resolved hyperfine (HF) structure with six prominent lines indicat~ t h a t t h e ESR c e n t e r is associated w l t h Mn + h a v i n g n u c l e a r s p i n I=5/2 [£]. T h e l i n e s c o r r e s p o n d to t h e a l l o w e d HF t r a n s i t i o n s with ~MI= 0 and t h e fine structure (FS) t r a n s i t i o n M =1/2 to M , = - 1 / 2 . The increase in Mn-conce~tration enh~mces the ESR l l n e i n t e n s i t y zwhich c o n f i r m s t h e p a r a m a e n e t l c c e n t e r as Mn. + F i v e weak l i n e s b e t w e e n HF l i n e s a r e d u e to t h e f o r b i d d e n transitions with AMI= +- 1 end FS t r a n s i t i o n M = 1/2 to M , = - 1 / 2 . The other FS t~nsitions c o u l d n a o t b e d e t e c t e d b e c a u s e of their small transition probabilities. The signal can b e a c c o u n t e d for i n t e r m s of an u n p a i r e d electron in 6S5/2 ground state and spin Hamiltonian,
H= 88BS + AIS where, and
At 513
8 : theBohr magneton, A : the HFS coupling constant g : • Landeg-factor.
higher
Mn-concentratlons,
around 0 . 5
wt%.
514
ESR AND CONCENTRATION QUENCHING
Vol. 77, No. 7
~00; g
/*
(c)
1"
\
) Q. O
(b)
>
\
200~
(e)
100
001
u.oo
Fig.
3ooo
1.
-~
~ood'~'~o
3boo g
4000
ESR spectra of (Zn, Cd)S:Mn phosphors with Mn-concentrations: a} 0 . 0 1 , ( b ) 0 . 1 , (c) 0 . 1 5 , d) 0.5 and (e) 1.0 wt 96 of
Fig.2.
005 0.1 ~15 ff5 1"( Mn Concentration(Wt%) V a r i a t i o n of E L - b r i g h t n e s s and p e a k p e a k e n v e l o p e w i d t h ( ~H ) with Mn-concentration in P~Zn, Cd)S Phosphors.
(Za,Cd)S. (Fig.l.d), the six hyperfine lines gradually m e r g e i n to an e n v e l o p e . T h e ESR s p e c t r u m , now, can b e c o n s i d e r e d a s t h e s u p e r p o s i t i o n of r e s o l v e d s i x HF l i n e s on an u n r e s o l v e d broad line. The resolved s i x l i n e s a r e due to i s o l a t e d Mr¢+ c e n t e r s ( r e g i o n s of low Mnconcentration) wher~ as unresolved broad line may b e d u e to Mn - + p a i r s and c l u s t e r s ( t h e regions of high Mn-concent r a t i o n ) . The e x c h a n g e c o u p l e d i o n s a r e t h e c a u s e of l i n e e n v e l o p i n g [ 5,7 ]. It is reported for ZnS:Mn phosphors that the Mn-concentration had to be smaller than 0.3 wt ~ to o b s e r v e well resolved spectra[81 w h i c h f a i r l y agrees w i t h the present result for (Zn,Cd)S:Mn phosphors. At v e r y high Mn-concentrations (around 1 . 0 wt t o r more) t h e ESR s p e c t r u m i s a broad structureless single line (Fig.l.e) and r e s o l v e d s p e c t r a c o u l d not b e o b s e r v e d . The e x c h a n g e c o u p l i n g b e t w e e n Mn z+ lo~s i s a l m o s t complete, e v e n t u a l l y i s o l a t e d Mn `+ ions are h a r d l y l e f t at v e r y high Mn-concentrations. The peak to peek w i d t h AH of envelope lines decreases w i t h pp increasing M n - c o n c e ~ t r a tion ( Fig. 2 ) which is also attributed to e x c h a n g e c o u p l i n g b e t w e e n Mn 2+ ions [5,7]. The s t u d y of v a r i a t i o n of E L - b r i g h t n e s s with Mn- c o n c e n t r a t i o n ( Fig. 2 ) a t IkHz and 1.4 kV s h o w s concentration quenching with
optimum c o n c e n t r a t i o n a r o u n d 0.15 wt t . T h e concentration q u e n c h i n g can b e e x p l a i n e d as du~ to r e s o n a n c e t r a n s f e r of e x c i t a t i o n b e t w e e n Mn ~ ions [ 9 ]. T h e e n e r g y transfer is more efficient at higher M n - c o n c e n t r a t t o n s due to spin-spin, spin-lattice exchange interactions which increase rapidly w i t h i n c r e a s i n g Mnconcentration. The maximum of peak-peak width AH for e n v e l o p e l i n e s { F i g . 2 ) a l s o falls in t~ v i c i n i t y of optimum c o n c e n t r a t i o n for EL-brightness. The ESR ~l~udy, thus reveals that the formation of Mn- ion p a i r s and clusters is the main cause of concentration quenching of EL-brightness in (Zn,Cd)S:Mn p h o s p h o r s [10 ] .
4.
CONCLUSION
At higher Mn-c~ncentrations ( around 0.15 wt ~ or more) Mn * i o n p a i r s and clusters are formed which increase the s p i n - s p i n , spinl a t t i c e exchange coupled interactions leading to concentration quenching of EL-brightness in (Zn,Cd}S:Mn phosphors.
ACKNOWLEPGENENT : A u t h o r s a r e g r a t e f u l to Prof. A.V. N a r l i k a r , Deputy Director, NPL, New Delhi, for constant encouragement and timely help.
515
ESR AND CONCENTRATION QUENCHING
Vol. 77, No. 7
REFBRENCBS
:
1.
R.Mach and G.O. Muller, S o l . ( a } 69, 11 (1982).
Phys.
Stat.
6.
W.D. Herschberger and H.N. Phys. Rev. 88, 714 (19521.
2.
Workshop on the Physics E l e c t r o l u m i n s c e n c e , J . Lure. 23, No. (1981).
of 1/2
7.
M.S.S. (1981 }.
8.
3.
W. Lehmann, 341 (1982).
Mater,
1_!,
J.Kreissl. ( 1986 }.
9.
4,
J . K r e i s s l and D . B a c k s , {a) 9__99, K 119 (1987).
Stat.
Sol.
D.E. D e x t e r and J . H . P h y s . 22, 1063 (1954).
10.
5.
Y. I s h l kawa, 1481 (1966).
J.M.Hurd. (1979).
J.
Electronic
J. P h y s .
Phys. Soc.
Japan,
2._!1,
Kolnlck,
Phys.
J.
J. Phys.
Stat.
D. I_44,
SOl(a) Schulman.
Electronic
Letfer,
Mater,
301
97_?, 191 J.Chem. 8,
879
0038-1098/9153.00+ .00 Pergamon Press plc
Solid State Communications, Vol. 77, No. 7, pp. 513-515, 1991. Printed in Great Britain.
BSR AND CONCENTRATION QUENCHING OF BL-EMISSION IN (Zn, C d ) S : M n PHOSPHORS
3. K.Naadagatoe., P.K.Pct2~, P.R. go¢e aad R.O. Lawangox-PawoJt Solid
State
Physics
Research Laboratory, Department of U n i v e r s i t y , K o l h a p u r - 4 1 6 004, INDIA
Physics,
ShtvaJt
(Received by D.Van Dyck - October 15, 1990)
ESR investlgations of electroluminescent (Zn,Cd)S:Mn powder phosphors have been carried out for Mn-concentratious rangin8 from 0.01 wt to 3 wt ~. T h e v a r i a t i o n of EL- b r i g h t n e s s w i t h M n - c o n c e n t r a t l o n h a s a l s o b e s n s t u d i e d . A t t e m p t s h a v e b e e n made to show t h a t t h e f o r m a t i o n o f Mn + ion p a i r s e n d c l u s t e r s c a u s e s t h e c o n c e n t r a t i o n q u e n c h i n g of EL-brightness in (Zn,Cd)S:Mn p h o s p h o r s a t h i g h e r M~+ c o n c e n t r a t i o n s .
I. INTRODUCTION T h e growing i n d u s t r i a l applications end development of thin film electroluminescent devices have prompted various research w o r k e r s to s t u d y new p a r a m e t e r s of p h o s p h o r s that y i e l d high efficiency [1-3]. The electron spin resonance (ESR). is a well known technique for the investigation of defects in crystalline and non-crystalllne materials. ESR permits the chemical identification of the centers, strength of crystal fields end the environment of paramagnetlc centers. ESR also y i e l d s data about the atomtstic picture of the Mn centers and t h e i r hosts [4].The ESR i n v e s t i g a t i o n s of p o w d e r p h o s p h o r s a r e u s u a l l y carried out to a v o i d t h e d i f f i c u l t i e s i n t h i n film investigation, like incorporation of a s u f f i c i e n t q u a n t i t y o f m a t e r i a l s i n to t h e ESR cavity. Moreover, ESR characteristics of p o l y c r y s t a l l t n a powders are s i m i l a r to those of p o l y c ry s t a l l t n a thin films. The ESR powder spectrum at higher Mn-concentration was interpreted by Ishtkawa [ 5 ] considering the !sotroptc exchenge interaction between Mn ions. In the present paper BSR investigations of (Zn,Cd)S:Mn powder phosphors are employed to explain the well-known concentration quenching of electroluminescence ( EL )-brightness at higher Mn-concentratlons. 2.
RXPERIMENTAL
(Zn,Cd)S:Mn powder phosphors with v a r y i n g c o n c e n t r a t i o n of Mn (0.01 to 3 , 0 wt%) have been prepared b y h e a t i n g a m i x t u r e of 60 tool ~ of ZnS and 40 tool 96 of CdS along w l t h r e q u i s i t e amount of d o p a n t a t 9 0 0 ° C for one h o u r i n a r g o n a t m o s p h e r e . T h e ESR s p e c t r a w e r e r e c o r f l e d u s i n 9 ~ X - b a n d v a r i a n E-Is9 ESR s p e c t r o m e t e P o p e r a t e d a t room t e m p e r a t u r e e n d a t m i c r o w a v e f r e q u e n c y of 9.52 GHz. T h e f i e l d m o d u l a t i o n f r e q u e n c y was 100 Hz. TCNE was used as standard f i e l d marker with g-factor 2. 00277.
The EL-brightness measurements were carried out b y u s u a l s a n d w i c h t y p e E L - c e l l u s i n g c a s t o r - o i l a s d i e l e c t r i c to d i s p e r s e t h e phosphor powder uniformly between the electrodes. High a . c . v o l t a g e s up to 1 . 5 kV a t 100 Hz - 2 . 5 kHz, o b t a i n e d from P h l l l l p s (GM-2308/90) generator coupled with wide band a m p l i f i e r (Telmex SP -100), was applied across the EL-cell. The l i g h t out put of the EL-call was measured using a photomultiplter tube (RCA-931A) coupled w t t h a s e n s i t i v e nanoammeter (NM-122). 3. RESULTS AND DISCUSSION In t h e ( Z n , C d ) S l a t t i c e Mnz+ s u b s t l t u t i e n a l l y replaces a cation b e c a u s e of s i m i l a r i t y in charge and ionic radii. T h e ESR s p e c t r a of {Zn,Cd)S:Mn powder phosphors with different M n - c o n c e n t r a t i o n s a r e s h o w n i n Fig. 1. T h e ESR s p e c t r a s h o w a c h a n g e i n l i n e s h a p e w i t h Mn-concentration. At l o w e r Mn-concentrations, up to 0 . 1 5 wt% (Fig. 1.a, b , c }, the resolved hyperfine (HF) structure with six prominent lines indicat~ t h a t t h e ESR c e n t e r is associated w l t h Mn + h a v i n g n u c l e a r s p i n I=5/2 [£]. T h e l i n e s c o r r e s p o n d to t h e a l l o w e d HF t r a n s i t i o n s with ~MI= 0 and t h e fine structure (FS) t r a n s i t i o n M =1/2 to M , = - 1 / 2 . The increase in Mn-conce~tration enh~mces the ESR l l n e i n t e n s i t y zwhich c o n f i r m s t h e p a r a m a e n e t l c c e n t e r as Mn. + F i v e weak l i n e s b e t w e e n HF l i n e s a r e d u e to t h e f o r b i d d e n transitions with AMI= +- 1 end FS t r a n s i t i o n M = 1/2 to M , = - 1 / 2 . The other FS t~nsitions c o u l d n a o t b e d e t e c t e d b e c a u s e of their small transition probabilities. The signal can b e a c c o u n t e d for i n t e r m s of an u n p a i r e d electron in 6S5/2 ground state and spin Hamiltonian,
H= 88BS + AIS where, and
At 513
8 : theBohr magneton, A : the HFS coupling constant g : • Landeg-factor.
higher
Mn-concentratlons,
around 0 . 5
wt%.
514
ESR AND CONCENTRATION QUENCHING
Vol. 77, No. 7
~00; g
/*
(c)
1"
\
) Q. O
(b)
>
\
200~
(e)
100
001
u.oo
Fig.
3ooo
1.
-~
~ood'~'~o
3boo g
4000
ESR spectra of (Zn, Cd)S:Mn phosphors with Mn-concentrations: a} 0 . 0 1 , ( b ) 0 . 1 , (c) 0 . 1 5 , d) 0.5 and (e) 1.0 wt 96 of
Fig.2.
005 0.1 ~15 ff5 1"( Mn Concentration(Wt%) V a r i a t i o n of E L - b r i g h t n e s s and p e a k p e a k e n v e l o p e w i d t h ( ~H ) with Mn-concentration in P~Zn, Cd)S Phosphors.
(Za,Cd)S. (Fig.l.d), the six hyperfine lines gradually m e r g e i n to an e n v e l o p e . T h e ESR s p e c t r u m , now, can b e c o n s i d e r e d a s t h e s u p e r p o s i t i o n of r e s o l v e d s i x HF l i n e s on an u n r e s o l v e d broad line. The resolved s i x l i n e s a r e due to i s o l a t e d Mr¢+ c e n t e r s ( r e g i o n s of low Mnconcentration) wher~ as unresolved broad line may b e d u e to Mn - + p a i r s and c l u s t e r s ( t h e regions of high Mn-concent r a t i o n ) . The e x c h a n g e c o u p l e d i o n s a r e t h e c a u s e of l i n e e n v e l o p i n g [ 5,7 ]. It is reported for ZnS:Mn phosphors that the Mn-concentration had to be smaller than 0.3 wt ~ to o b s e r v e well resolved spectra[81 w h i c h f a i r l y agrees w i t h the present result for (Zn,Cd)S:Mn phosphors. At v e r y high Mn-concentrations (around 1 . 0 wt t o r more) t h e ESR s p e c t r u m i s a broad structureless single line (Fig.l.e) and r e s o l v e d s p e c t r a c o u l d not b e o b s e r v e d . The e x c h a n g e c o u p l i n g b e t w e e n Mn z+ lo~s i s a l m o s t complete, e v e n t u a l l y i s o l a t e d Mn `+ ions are h a r d l y l e f t at v e r y high Mn-concentrations. The peak to peek w i d t h AH of envelope lines decreases w i t h pp increasing M n - c o n c e ~ t r a tion ( Fig. 2 ) which is also attributed to e x c h a n g e c o u p l i n g b e t w e e n Mn 2+ ions [5,7]. The s t u d y of v a r i a t i o n of E L - b r i g h t n e s s with Mn- c o n c e n t r a t i o n ( Fig. 2 ) a t IkHz and 1.4 kV s h o w s concentration quenching with
optimum c o n c e n t r a t i o n a r o u n d 0.15 wt t . T h e concentration q u e n c h i n g can b e e x p l a i n e d as du~ to r e s o n a n c e t r a n s f e r of e x c i t a t i o n b e t w e e n Mn ~ ions [ 9 ]. T h e e n e r g y transfer is more efficient at higher M n - c o n c e n t r a t t o n s due to spin-spin, spin-lattice exchange interactions which increase rapidly w i t h i n c r e a s i n g Mnconcentration. The maximum of peak-peak width AH for e n v e l o p e l i n e s { F i g . 2 ) a l s o falls in t~ v i c i n i t y of optimum c o n c e n t r a t i o n for EL-brightness. The ESR ~l~udy, thus reveals that the formation of Mn- ion p a i r s and clusters is the main cause of concentration quenching of EL-brightness in (Zn,Cd)S:Mn p h o s p h o r s [10 ] .
4.
CONCLUSION
At higher Mn-c~ncentrations ( around 0.15 wt ~ or more) Mn * i o n p a i r s and clusters are formed which increase the s p i n - s p i n , spinl a t t i c e exchange coupled interactions leading to concentration quenching of EL-brightness in (Zn,Cd}S:Mn phosphors.
ACKNOWLEPGENENT : A u t h o r s a r e g r a t e f u l to Prof. A.V. N a r l i k a r , Deputy Director, NPL, New Delhi, for constant encouragement and timely help.
515
ESR AND CONCENTRATION QUENCHING
Vol. 77, No. 7
REFBRENCBS
:
1.
R.Mach and G.O. Muller, S o l . ( a } 69, 11 (1982).
Phys.
Stat.
6.
W.D. Herschberger and H.N. Phys. Rev. 88, 714 (19521.
2.
Workshop on the Physics E l e c t r o l u m i n s c e n c e , J . Lure. 23, No. (1981).
of 1/2
7.
M.S.S. (1981 }.
8.
3.
W. Lehmann, 341 (1982).
Mater,
1_!,
J.Kreissl. ( 1986 }.
9.
4,
J . K r e i s s l and D . B a c k s , {a) 9__99, K 119 (1987).
Stat.
Sol.
D.E. D e x t e r and J . H . P h y s . 22, 1063 (1954).
10.
5.
Y. I s h l kawa, 1481 (1966).
J.M.Hurd. (1979).
J.
Electronic
J. P h y s .
Phys. Soc.
Japan,
2._!1,
Kolnlck,
Phys.
J.
J. Phys.
Stat.
D. I_44,
SOl(a) Schulman.
Electronic
Letfer,
Mater,
301
97_?, 191 J.Chem. 8,
879