- Email: [email protected]
Decay time of IR luminescence of ZnS:Cu
Volume 37A, n u m b e r 1
PHYSICS
DECAY
TIME
OF
IR
LETTERS
LUMINESCENCE
25 O c t o b e r 1971
OF
ZnS:Cu
A. F I L L E R a n d H . - J . S C H U L Z Institut flit Elektronenmikroskopie am Fritz-Haber-lnstilut der Max-Planck-Gesellschaft, Berlin-Dahlern
Received 21 August 1971
The r i s e and decay curves of i n f r a r e d luminescence (k ~ 1.5/xm) have been recorded d i r e c t l y with ZnS:Cu c r y s t a l s . Both a r e exponential under cathode ray excitation at '77 K and yield ~"= 4 x 10-7s - two o r d e r s of magnitude less than previously reported time constants.
T h e i n f r a r e d l u m i n e s c e n c e of Z n S : C u h a s b e e n r e f e r r e d to a 3d 9 i o n in a c r y s t a l f i e l d of t e t r a hedral symmetry with a small trigonal distortion in a hexagonal structure. The relative amounts of s p i n - o r b i t a n d J a h n - T e l l e r i n t e r a c t i o n s w h i c h may lift non-Kramers degeneracies still remain s o m e w h a t d i s p u t a b l e . A d i s c u s s i o n of t r a n s i t i o n p r o b a b i l i t i e s r e q u i r e s a k n o w l e d g e of e i g e n f u n c t i o n s . C o n f i g u r a t i o n m i x i n g w i t h 3d84p s t a t e s of t h e i m p u r i t y a n d o v e r l a p of i t s o r b i t a l s w i t h t h o s e of t h e n e i g h b o u r i n g l i g a n d s ( c o v a l e n c y ) n e e d to b e c o n s i d e r e d . O s c i l l a t o r s t r e n g t h s r e l a t e d to t h e e l e c t r i c d i p o l e m a t r i x e l e m e n t s of t h e t r a n s i t i o n s h a v e been obtained in a semiempirical analysis, by a d j u s t i n g c a l c u l a t i o n p a r a m e t e r s to d a t a f r o m optical measurements, Thus, for the transition 2 E ~ 2 T 2 , o s c i l l a t o r s t r e n g t h s of s o m e 10 - 4 in Z n S : C u h a v e b e e n s t a t e d [1]; t h e s a m e o r d e r of magnitude has been evaluated ZnO:Cu [1,2]. These numbers may be obtained also by conside r a t i o n of t h e r a d i a t i v e d e c a y t i m e in t h e s e transitions. T h e d e c a y t i m e of IR e m i s s i o n of Z n S : C u h a s been inferred "indirectly" from the frequency r e s p o n s e of e m i s s i o n s i g n a l s to i n t e r m i t t e n t e x c i t a t i o n . In t h i s w o r k [3], a s w e l l a s in a l a t e r o n e o n CdS [4], t h e v a l u e s o b t a i n e d f o r t h e t e m p e r a t u r e r a n g e of 60 K to 300 K w e r e in p a r t w e l l in e x c e s s of 1 0 - 5 s , b u t w e r e a l s o in p a r t c o n c e a l e d by t h i s l i m i t of m e a s u r e m e n t . In t h e m e a s u r e m e n t s on Z n S : C u c r y s t a l s p r e s e n t e d h e r e [5], t h e t i m e l a p s e of r i s e a n d decay has, for the first time, been recorded d i r e c t l y at 77 K a n d 4 K. T h e l u m i n e s c e n c e w a s e x c i t e d e i t h e r b y 35 k e V c a t h o d e r a y s ( w h i c h c o u l d b e s w i t c h e d on o r off b y a p p l y i n g a n a u x i l i a r y v o l t a g e to a d e f l e c t i o n c a p a c i t o r ) o r b y
f i l t e r e d l i g h t of v a r i o u s r e g i o n s in t h e v i s i b l e s p e c t r a l r a n g e . T h e i n t e g r a l of IR e m i s s i o n o v e r t h e r a n g e of 1.0 ~ m ~< ;t ~< 1.7 /~m w a s d e t e c t e d b y a G e r m a n i u m p h o t o diode, With electron excitation the rise and decay p r o c e s s e s t u r n out to b e e x p o n e n t i a l to a good approximation. The following values have been o b t a i n e d f o r b o t h t h e s e p r o c e s s e s a t a l i m i t of m e a s u r e m e n t of a b o u t 2 x 10 - 7 s: a t 77 K: 4 × 1 0 - 7 s ~= 20% a t 4 K: 1.2 × 1 0 - 6 s ~ 20%. These values only represent upper limits for t h e r e s p e c t i v e r e l a x a t i o n t i m e s of t h e Cu 2+ c e n tre, since the electron irradiation implicates a v a r i e t y of c o n s e c u t i v e p r o c e s s e s in t h e l a t t i c e . T h e s l o w e f f e c t s d u e to t r a p p i n g w e r e w e a k , h o w e v e r , f o r t h e c r y s t a l s u s e d in t h e s e e x p e r i ments. On the other hand, there is a close similarity in t h e v i b r o n i c s t r u c t u r e s of e m i s s i o n s p e c t r a with both optical and electron beam excitations. T h i s w o u l d s e e m to e x c l u d e t h e p o s s i b i l i t y t h a t any competing secondary process specific for e l e c t r o n e x c i t a t i o n c o u l d c a u s e a l o w e r i n g of t h e l i f e t i m e . T h e t r e n d in t e m p e r a t u r e d e p e n d e n c e of t h e t i m e c o n s t a n t i s in a c c o r d a n c e w i t h a phonon interaction mechanism. O p t i c a l e x c i t a t i o n i s to b e p r e f e r e d in g e n e r a l , b e c a u s e of i t s s e l e c t i v e e x c i t a t i o n m e c h a n i s m . S i n c e t h e l i g h t b e a m w a s c h o p p e d by a r o t a t i n g d i s c , a l i m i t of o n l y 1 0 - 5 s c o u l d b e a c h i e v e d in t h e s e r i s e a n d d e c a y p l o t s . W i t h e x c i t a t i o n in t h e " y e l l o w " s p e c t r a l r e g i o n w i t h 0.52 p m ~<~t~<0.65 ~tm ( w h i c h c o r r e s j ~ o n d to t r a n s i t i o n s f r o m t h e v a l e n c e b a n d s i n t o Cu A + c e n t r e s [6]), t h e r i s e a n d d e c a y a r e b e l o w t h e l i m i t of m e a s u r e m e n t f o r t h e w h o l e t e m p e r a t u r e r a n g e of 77 K ~< T ~< 300 K. 11
Volume 37A, n u m b e r 1
PHYSICS
U n d e r " b l u e " e x c i t a t i o n w i t h 0.33 txm < ~. < 0.48 lzm t h e d e c a y i n g e m i s s i o n i s c o m p r i s e d of two c o m p o n e n t s of d i f f e r e n t t i m e c o n s t a n t s , with only the larger one (1... 3× 10-4s) being accessible for measurements. Under these conditions, however, the slow component cont r i b u t e s o n l y a s m a l l a m o u n t to t h e s t e a d y s t a t e i n t e n s i t y . U s i n g t h e s a m e r e g i o n of e x c i t a t i o n on t h e s a m e c r y s t a l s , m e a s u r e m e n t s of t h e t e m p e r a t u r e d e p e n d e n c e of l u m i n e s c e n c e i n t e n s i t y s h o w t h e w e l l - k n o w n [3, 7] i n t e n s i t y d e c r e a s e in t h e r a n g e b e t w e e n 2 0 0 K a n d 300 K a s w e l l a s a f u r t h e r t h r e s h o l d in t h e r e g i o n b e l o w 110 K. T h e l a t t e r i s p r e s u m a b l y r e l a t e d to t h e r m a l d e p o p u l a t i o n of e l e c t r o n t r a p s w h e r e a s t h e f i r s t m e n t i o n e d t h r e s h o l d h a s b e e n a s c r i b e d p r e v i o u s l y [7] to a t e m p e r a t u r e q u e n c h i n g of IR e m i s s i o n b y interaction with the valence bands. With optical excitation the slow component is r e l a t e d to t h o s e t r a p s w h i c h a l s o a f f e c t t h e t e m p e r a t u r e d e p e n d e n c e of i n t e n s i t y . T h e s e v a r y f r o m c r y s t a l to c r y s t a l a n d m a y h a v e b e e n s i g n i f i c a n t in t h e a b o v e - m e n t i o n e d i n v e s t i g a t i o n s of t h o s e a u t h o r s [4~ w h o r e p o r t e d d e c a y t i m e s of t h e o r d e r of 1 0 - ~ s . At T ~ 190 K a n d e x c i t a t i o n from the valence band, processes involving diff u s i o n of h o l e s m i g h t a l s o e n t a i l a s l o w c o m p o nent. Larger time constants than those reported h e r e w o u l d c o r r e s p o n d to t h e o s c i l l a t o r s t r e n g t h s
12
LETTERS
25 October 1971
m e n t i o n e d in t h e b e g i n n i n g of t h i s p a p e r . It w o u l d t h e r e f o r e b e s e n s i b l e to r e - c h e c k t h e i n f l u e n c e s of t h e i n t e r a c t i o n s c o n s i d e r e d h i t h e r t o [8, 9]. P a r t i c u l a r l y in t h e l u m i n e s c e n c e of Z n S : C u a n d C d S : C u , a s t r o n g l a t t i c e i n t e r a c t i o n a p p e a r s to b e i n v o l v e d in t h e r e l a x a t i o n p r o c e s s . T h i s i s a l s o i n d i c a t e d by t h e p r e v a l e n c e of v i b r o n i c t r a n s i t i o n s in t h e e m i s s i o n s p e c t r a of t h e s e s y s t e m s [10]. T h e a u t h o r s w i s h to t h a n k D r . R. B r o s e r f o r s u p p l y i n g t h e c r y s t a l s a n d P r o f e s s o r D r . I. B r o s e r for his support and stimulating discussions.
References [1] w. Waiter, J. L. Birman, II-VI Semiconducting Compounds, ed. D. G. Thomas (W. A. Benjamin, inc., New York 1967) p. 89. [2] M. Kibter, F. Calendini, J. Physique 28 (1967) 878. [3] P . F . Browne, J. Electronics 2 (1956) 1. [4] A. F. J. Cox, W. E. Hagston, C.J. Radford, J. Phys. C[2]1 (1968) 1746. [5] A. F i l l e r , Dipiomarbeit F r e i e Universit~tt Berlin (1970). [6] I. B r o s e r , K.-H. Franke, H.-J. Schu[z, II-VI Semiconducting Compounds, ed. D. G. Thomas (W. A. Benjamin, Inc., New York 1967) p. 81. [71 H.-J. Schulz, Phys. Stat. Sol. 3 (1963) 485 IS] W . E . H a g s t o n , J. Phys, C[211 (1968) 810. [9] C.A. Bates, Phys. L e t t e r s 29A (1969) 252. [10] I. B r o s e r , H. Maier, H,-J. Schu[tz, Phys. Rev. 140 (1965) A2135.
PHYSICS
DECAY
TIME
OF
IR
LETTERS
LUMINESCENCE
25 O c t o b e r 1971
OF
ZnS:Cu
A. F I L L E R a n d H . - J . S C H U L Z Institut flit Elektronenmikroskopie am Fritz-Haber-lnstilut der Max-Planck-Gesellschaft, Berlin-Dahlern
Received 21 August 1971
The r i s e and decay curves of i n f r a r e d luminescence (k ~ 1.5/xm) have been recorded d i r e c t l y with ZnS:Cu c r y s t a l s . Both a r e exponential under cathode ray excitation at '77 K and yield ~"= 4 x 10-7s - two o r d e r s of magnitude less than previously reported time constants.
T h e i n f r a r e d l u m i n e s c e n c e of Z n S : C u h a s b e e n r e f e r r e d to a 3d 9 i o n in a c r y s t a l f i e l d of t e t r a hedral symmetry with a small trigonal distortion in a hexagonal structure. The relative amounts of s p i n - o r b i t a n d J a h n - T e l l e r i n t e r a c t i o n s w h i c h may lift non-Kramers degeneracies still remain s o m e w h a t d i s p u t a b l e . A d i s c u s s i o n of t r a n s i t i o n p r o b a b i l i t i e s r e q u i r e s a k n o w l e d g e of e i g e n f u n c t i o n s . C o n f i g u r a t i o n m i x i n g w i t h 3d84p s t a t e s of t h e i m p u r i t y a n d o v e r l a p of i t s o r b i t a l s w i t h t h o s e of t h e n e i g h b o u r i n g l i g a n d s ( c o v a l e n c y ) n e e d to b e c o n s i d e r e d . O s c i l l a t o r s t r e n g t h s r e l a t e d to t h e e l e c t r i c d i p o l e m a t r i x e l e m e n t s of t h e t r a n s i t i o n s h a v e been obtained in a semiempirical analysis, by a d j u s t i n g c a l c u l a t i o n p a r a m e t e r s to d a t a f r o m optical measurements, Thus, for the transition 2 E ~ 2 T 2 , o s c i l l a t o r s t r e n g t h s of s o m e 10 - 4 in Z n S : C u h a v e b e e n s t a t e d [1]; t h e s a m e o r d e r of magnitude has been evaluated ZnO:Cu [1,2]. These numbers may be obtained also by conside r a t i o n of t h e r a d i a t i v e d e c a y t i m e in t h e s e transitions. T h e d e c a y t i m e of IR e m i s s i o n of Z n S : C u h a s been inferred "indirectly" from the frequency r e s p o n s e of e m i s s i o n s i g n a l s to i n t e r m i t t e n t e x c i t a t i o n . In t h i s w o r k [3], a s w e l l a s in a l a t e r o n e o n CdS [4], t h e v a l u e s o b t a i n e d f o r t h e t e m p e r a t u r e r a n g e of 60 K to 300 K w e r e in p a r t w e l l in e x c e s s of 1 0 - 5 s , b u t w e r e a l s o in p a r t c o n c e a l e d by t h i s l i m i t of m e a s u r e m e n t . In t h e m e a s u r e m e n t s on Z n S : C u c r y s t a l s p r e s e n t e d h e r e [5], t h e t i m e l a p s e of r i s e a n d decay has, for the first time, been recorded d i r e c t l y at 77 K a n d 4 K. T h e l u m i n e s c e n c e w a s e x c i t e d e i t h e r b y 35 k e V c a t h o d e r a y s ( w h i c h c o u l d b e s w i t c h e d on o r off b y a p p l y i n g a n a u x i l i a r y v o l t a g e to a d e f l e c t i o n c a p a c i t o r ) o r b y
f i l t e r e d l i g h t of v a r i o u s r e g i o n s in t h e v i s i b l e s p e c t r a l r a n g e . T h e i n t e g r a l of IR e m i s s i o n o v e r t h e r a n g e of 1.0 ~ m ~< ;t ~< 1.7 /~m w a s d e t e c t e d b y a G e r m a n i u m p h o t o diode, With electron excitation the rise and decay p r o c e s s e s t u r n out to b e e x p o n e n t i a l to a good approximation. The following values have been o b t a i n e d f o r b o t h t h e s e p r o c e s s e s a t a l i m i t of m e a s u r e m e n t of a b o u t 2 x 10 - 7 s: a t 77 K: 4 × 1 0 - 7 s ~= 20% a t 4 K: 1.2 × 1 0 - 6 s ~ 20%. These values only represent upper limits for t h e r e s p e c t i v e r e l a x a t i o n t i m e s of t h e Cu 2+ c e n tre, since the electron irradiation implicates a v a r i e t y of c o n s e c u t i v e p r o c e s s e s in t h e l a t t i c e . T h e s l o w e f f e c t s d u e to t r a p p i n g w e r e w e a k , h o w e v e r , f o r t h e c r y s t a l s u s e d in t h e s e e x p e r i ments. On the other hand, there is a close similarity in t h e v i b r o n i c s t r u c t u r e s of e m i s s i o n s p e c t r a with both optical and electron beam excitations. T h i s w o u l d s e e m to e x c l u d e t h e p o s s i b i l i t y t h a t any competing secondary process specific for e l e c t r o n e x c i t a t i o n c o u l d c a u s e a l o w e r i n g of t h e l i f e t i m e . T h e t r e n d in t e m p e r a t u r e d e p e n d e n c e of t h e t i m e c o n s t a n t i s in a c c o r d a n c e w i t h a phonon interaction mechanism. O p t i c a l e x c i t a t i o n i s to b e p r e f e r e d in g e n e r a l , b e c a u s e of i t s s e l e c t i v e e x c i t a t i o n m e c h a n i s m . S i n c e t h e l i g h t b e a m w a s c h o p p e d by a r o t a t i n g d i s c , a l i m i t of o n l y 1 0 - 5 s c o u l d b e a c h i e v e d in t h e s e r i s e a n d d e c a y p l o t s . W i t h e x c i t a t i o n in t h e " y e l l o w " s p e c t r a l r e g i o n w i t h 0.52 p m ~<~t~<0.65 ~tm ( w h i c h c o r r e s j ~ o n d to t r a n s i t i o n s f r o m t h e v a l e n c e b a n d s i n t o Cu A + c e n t r e s [6]), t h e r i s e a n d d e c a y a r e b e l o w t h e l i m i t of m e a s u r e m e n t f o r t h e w h o l e t e m p e r a t u r e r a n g e of 77 K ~< T ~< 300 K. 11
Volume 37A, n u m b e r 1
PHYSICS
U n d e r " b l u e " e x c i t a t i o n w i t h 0.33 txm < ~. < 0.48 lzm t h e d e c a y i n g e m i s s i o n i s c o m p r i s e d of two c o m p o n e n t s of d i f f e r e n t t i m e c o n s t a n t s , with only the larger one (1... 3× 10-4s) being accessible for measurements. Under these conditions, however, the slow component cont r i b u t e s o n l y a s m a l l a m o u n t to t h e s t e a d y s t a t e i n t e n s i t y . U s i n g t h e s a m e r e g i o n of e x c i t a t i o n on t h e s a m e c r y s t a l s , m e a s u r e m e n t s of t h e t e m p e r a t u r e d e p e n d e n c e of l u m i n e s c e n c e i n t e n s i t y s h o w t h e w e l l - k n o w n [3, 7] i n t e n s i t y d e c r e a s e in t h e r a n g e b e t w e e n 2 0 0 K a n d 300 K a s w e l l a s a f u r t h e r t h r e s h o l d in t h e r e g i o n b e l o w 110 K. T h e l a t t e r i s p r e s u m a b l y r e l a t e d to t h e r m a l d e p o p u l a t i o n of e l e c t r o n t r a p s w h e r e a s t h e f i r s t m e n t i o n e d t h r e s h o l d h a s b e e n a s c r i b e d p r e v i o u s l y [7] to a t e m p e r a t u r e q u e n c h i n g of IR e m i s s i o n b y interaction with the valence bands. With optical excitation the slow component is r e l a t e d to t h o s e t r a p s w h i c h a l s o a f f e c t t h e t e m p e r a t u r e d e p e n d e n c e of i n t e n s i t y . T h e s e v a r y f r o m c r y s t a l to c r y s t a l a n d m a y h a v e b e e n s i g n i f i c a n t in t h e a b o v e - m e n t i o n e d i n v e s t i g a t i o n s of t h o s e a u t h o r s [4~ w h o r e p o r t e d d e c a y t i m e s of t h e o r d e r of 1 0 - ~ s . At T ~ 190 K a n d e x c i t a t i o n from the valence band, processes involving diff u s i o n of h o l e s m i g h t a l s o e n t a i l a s l o w c o m p o nent. Larger time constants than those reported h e r e w o u l d c o r r e s p o n d to t h e o s c i l l a t o r s t r e n g t h s
12
LETTERS
25 October 1971
m e n t i o n e d in t h e b e g i n n i n g of t h i s p a p e r . It w o u l d t h e r e f o r e b e s e n s i b l e to r e - c h e c k t h e i n f l u e n c e s of t h e i n t e r a c t i o n s c o n s i d e r e d h i t h e r t o [8, 9]. P a r t i c u l a r l y in t h e l u m i n e s c e n c e of Z n S : C u a n d C d S : C u , a s t r o n g l a t t i c e i n t e r a c t i o n a p p e a r s to b e i n v o l v e d in t h e r e l a x a t i o n p r o c e s s . T h i s i s a l s o i n d i c a t e d by t h e p r e v a l e n c e of v i b r o n i c t r a n s i t i o n s in t h e e m i s s i o n s p e c t r a of t h e s e s y s t e m s [10]. T h e a u t h o r s w i s h to t h a n k D r . R. B r o s e r f o r s u p p l y i n g t h e c r y s t a l s a n d P r o f e s s o r D r . I. B r o s e r for his support and stimulating discussions.
References [1] w. Waiter, J. L. Birman, II-VI Semiconducting Compounds, ed. D. G. Thomas (W. A. Benjamin, inc., New York 1967) p. 89. [2] M. Kibter, F. Calendini, J. Physique 28 (1967) 878. [3] P . F . Browne, J. Electronics 2 (1956) 1. [4] A. F. J. Cox, W. E. Hagston, C.J. Radford, J. Phys. C[2]1 (1968) 1746. [5] A. F i l l e r , Dipiomarbeit F r e i e Universit~tt Berlin (1970). [6] I. B r o s e r , K.-H. Franke, H.-J. Schu[z, II-VI Semiconducting Compounds, ed. D. G. Thomas (W. A. Benjamin, Inc., New York 1967) p. 81. [71 H.-J. Schulz, Phys. Stat. Sol. 3 (1963) 485 IS] W . E . H a g s t o n , J. Phys, C[211 (1968) 810. [9] C.A. Bates, Phys. L e t t e r s 29A (1969) 252. [10] I. B r o s e r , H. Maier, H,-J. Schu[tz, Phys. Rev. 140 (1965) A2135.