Pram~na, Vol. 15, No 1, July 1980, pp. 91-95. ~) Printed in India.
Decay characteristics of SrS :Nd phosphors
M G P A T I L a n d R D L A W A N G A R *
Department of Physics, Rajaram College, Kolhapur 416 004, India
*Department of Physics, Shivaji University, Kolhapur 416 004, India MS received 3 December 1979; revised 18 April 1980
Abstract. The phosphorescence decay of a series of strontium sulphide microcrystal- line phosphors prepared with varying amounts of neodymium as an activator has been studied at room temperature. The decay obeys the relation 1=1o t-b with b lying between 0.35 and 0-98. The trap depths have been evaluated by peeling off log 1--t curves. The results show that the distribution of trap levels is likely to be quasi-uniform and the process of retrapping during luminescence is negligible.
Keywords. SrS :Nd phosphors; decay characteristics.
1. Introduction
T h e p h e n o m e n o n o f phosphorescence decay a n d t h e r m o l u m i n e s c e n c e are used to investigate the presence o f t r a p p i n g levels in a p h o s p h o r due to their e x p e r i m e n t a l simplicity a n d the m e t h o d involved. These studies also p r o v i d e i n f o r m a t i o n a b o u t the type o f kinetics a n d hence a b o u t the process of r e t r a p p i n g during luminescence.
Recently, such studies have been reported by L a w a n g a r a n d N a r l i k a r (1975a) on C a S : B i : P d p h o s p h o r s a n d b y S h a r m a and Singh (1968) on S r S : Z r p h o s p h o r s . I n this p a p e r we r e p o r t the phosphorescence decay m e a s u r e m e n t s carried on S r S : N d phosphors. The object o f the investigation was to u n d e r s t a n d the type o f kinetics involved in the p h o s p h o r e s c e n c e decay a n d to obtain i n f o r m a t i o n a b o u t the distri- bution o f t r a p p i n g levels. T h e effect o f incorporation o f a c t i v a t o r on t r a p p i n g levels is also discussed.
2. Experimental
T h e SrS :Nd p h o s p h o r s containing varying concentrations o f n e o d y m i u m were pre- p a r e d b y the m e t h o d o f t h e r m a l reduction o f strontium sulphate (Paliwal a n d Sinha 1976). Na~SO4 ( A R grade) was used as a flux. T h e samples studied are listed in table 1.
The samples were excited to saturation using an ultraviolet source which emitted p r e d o m i n a n t l y the 3650A H g doublet. The decay intensity was m e a s u r e d using a p h o t o m u l t i p l i e r tube R C A 931 A. The output of the p h o t o m u l t i p l i e r was fed to a sensitive a u t o m a t i c p l o t t e r on which the decay curve was recorded. The decay m e a s u r e m e n t s were carried o u t at r o o m temperature (301°K).
91
92 M G Patil and R D Lawangar
Table 1. Decay constant, correlation coefficient, t r a p d e p t h as calculated from decay curves for SrS :Nd phosphors.
Sample No.
Concentra- Correla-
t i o n o f Nd Decay tion
--- gm. atom constant coefficient per mole SrS
( - - b ) ( - - r )
Trap d e p t h from peeling off o f decay curves (eV)
Slowest Middle Fastest
exponential exponential exponential
E~ E~ E,
Sll s, s, s~
&
s, S.
s,
&o
-- 0"45 0"84 0-69 0.62 0.58
0-75 x I0 -s 0"71 1-00 0-67 0"61 0.57
1.0 x 10 -8 0"72 0"99 0-68 0"62 0-58
2-5 × 10 -a 0-66 1"00 0-69 0.62 0-58
5-0 x 10 -3 0"48 0"98 0-65 0"61 0.58
7-5 x 10 -3 0"89 0"99 0"65 0"61 0"57
1 x 10 -~ 0"35 0"98 0-71 0"62 0"57
2 : 4 10 -2 0"98 0"99 0"68 0"61 0"58
3 x 10 -2 0"67 0"98 0"69 0"61 0"58
$ 6
102
101
t 0 ° l I t \ I 1 '
0 4 0 80 120 160
t (secl
Figure 1. Plot of log o f intensity of phosphorescence decay versus time--peeling off decay curve,
Decay characteristics of SrS : Nd phosphors 93
2.0
1 5
E~.o
0.5
Figure 2.
,$3
o I, ~ !
0.8 1.6 2.4
tog t
Log intensity versus log time plots for various samples.
3. Results and discussion 3.1. Mode of decay
To examine the nature of decay the logarithm of intensity (1) was plotted as a func- tion of time (t). The curves show deviation from the straight line eliminating the possibility of exponential decay (figure 1). However the plots of log I versus log t are almost linear indicating the decay to be hyperbolic. Some o f the typical plots obtained for samples containing different concentrations of Nd are shown in figure 2.
To estimate the degree o f linearity between log I and log t, the correlation coefficient (r) was calculated by the method of least squares. The calculated value of r for all samples is close to unity with a negative sign indicating decrease of log I with increase of log t. Moreover, this also indicates a fairly close linear relationship between log I and log t.
The hyperbolic decay observed could be represented by the equation of the form (Randall and Wilkins 1945)
t - - - x 0 t -b, (1)
where I is the intensity at time t, X o is at the beginning of the decay, b, the decay constant indicates the decay rate and provides information about the relative popu-
94 M G P a t i l a n d R D L a w a n g a r
lation of traps at various depths. This value was calculated by the method of least squares, using the relation
b - - .Sx.S,y - - n C x y (2)
( 2:x)~ --- n ~ x ~
where x -- log t, y =: l o g / , and n is the number of observations. The values thus obtained are listed in table 1. The b values vary from 0.35 to 0.98 with added acti- vator concentration. However the variation is not systematic. For all samples studied b is less than unity implying that the distribution of trapping levels is not uniform but likely to be quasi-uniform (Lawangar and Narlikar 1972).
3.2. A c t i v a t i o n energies
The hyperbolic decay can be explained on the basis of the monomolecular super- position theory suggested by Randall and Wilkins (1945). Such a decay results due to superposition of various exponentials corresponding to different traps and is expressed by the equation.
I := 1 o t -b,
-= 101 exp (--pxt) 4- 102 exp ( - - p ~ t ) + ...
. . . q- Io, exp (--p, t), ...
(3)
where I0, is the phosphorescence intensity due to electrons in the traps of energy E n , p , = S exp ( - - E n / k T ) is the probability of an electron escaping from a trap, K the Boltzmann constant, and S is the escape frequency factor. It is possible then to split each decay curve into a set of exponentials by the ' peeling o f f ' procedure as has been followed by Bube (1950). In the present case all the decay curves were split into three exponentials. An example of the procedure is illustrated in figure 1 where each straight line corresponds to each exponential and is a plot of log I versus t for the decay resulting from traps having single depth. Trap depths corresponding to these exponentials were calculated from slopes of straight lines. The E values thus obtained are summarised in table 1. The S values used in the above calculations were taken from the thermoluminescence studies carried on the same samples and were of the order of 109 see -1. (Patil and Lawangar 1980).
From table 1 it is seen that the trap depths corresponding to the slowest exponen- tial vary from 0.65 to 0.71 eV, the middle exponential from 0.61 to 0.62 eV and the fastest exponential from 0.57 to 0.58 eV with the addition of an activator. The varia- tion observed in E values is negligibly small and also unsystematic. This implies that the addition of the activator does not introduce new trapping levels but only modifies the relative importance of traps responsible for the phosphorescence decay. Thus the trapping levels in the present phosphor system may be attributed to defects in host lattice which are likely to be the sulphur vacancies (Lawangar and Narlikar 1975b). The contention is supported by the variation in b values with activator con- centration.
D e c a y characteristics o f S r S : N d phosphors 95 3.3 Kinetics o f l u m i n e s c e n c e
The kinetics o f luminscence m a y be inferred from the nature o f the decay. I f the decay is exponential, the kinetics is m o n o m o l e c u l a r (first order) and a semilog p l o t between I a n d t gives a straight line. Power law decay results due to bimolecular process (second order) a n d a plot between log I a n d log t is linear with a slope equal to - - 2 (Curie 1963; H o o g e n s t r a a t e n 1958). The decay observed in the present investi- gation is n o t o f either form. However it could be explained on the basis o f the m o n o - molecular superposition theory suggesting that the kinetics o f luminescence is m o n o - molecular. The existence o f the first order kinetics implies that the process o f retrapping is negligible and traps are situated quite close to luminescence centres so that the two m a y be regarded as a single unit ( M o r a n d Bhawalkar 1970).
Acknowledgements
The authors are grateful to the authorities o f Shivaji University, K o l h a p u r for financ- ing a research scheme under which the w o r k is carried. They are also thankful to D r C S S h a l g a o n k a r for helpful discussions.
References
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Curie D 1963 Luminescence in crystals tLondon: Methuen and Company) 13. 163 Hoogenslraaten W 1958 Philips Res. Rep. 13 515
Lawangar R D and Narlikar A V 1972 lnd;an J. Pure AppL Phys. 10 617 Lawangar R D and Narlikar A V 1975a J. Lumin. 11 135
Lawangar R D and Narlikar A V 1975b Y. Mater. Sci. 10 1251 Mor S L and Bhawalkar D R 1970 Indian J. Pure Appl. Phys. 8 320 Paliwal S S and Sinha O P 1976 Phys. Status Solidi A38 73 Patil M G and Lawangar R D 1980 (to be published)
Randall J T and Wilkins M H F 1945 Proc. R. Soc (London) A184 366 Sharrna D and Amac Singh 1968 Indian J. Pure AppL Phys. 6 346
