Auger-free luminescence in mixed alkali halides

Journal of Electron Spectroscopyand Related Phenomena79 (1996) 117-120

Auger-free

luminescence

in mixed

alkali balldes

M. Itoh, a N. O h n o , b H. Y o s h i d a , b S. H a s h i m o t o , c K. K a n ' n o d a n d M. K a m a d a e a F a c u l t y o f E n g i n e e r i n g , S h i n s h u University, N a g a n o 380, J a p a n b F a c u l t y o f E n g i n e e r i n g , O s a k a E l e c t r o - C o m m u n i c a t i o n University, N e y a g a w a 572, J a p a n C D e p a r t m e n t o f Physics, K y o t o U n i v e r s i t y o f E d u c a t i o n , Kyoto 612, J a p a n d D e p a r t m e n t o f Physics, K y o t o University, Kyoto 606, J a p a n e I n s t i t u t e for M o l e c u l a r Science, O k a z a k i 444, J a p a n

A specific k i n d o f core l u m i n e s c e n c e , called Auger-free l u m i n e s c e n c e (AFL), h a s b e e n s t u d i e d in m i x e d CsF l-xC|x s y s t e m w i t h i n t h e r a n g e x = 0-0.03 b y u s i n g s y n c h r o t r o n r a d i a t i o n a s a light source. A n e w l u m i n e s c e n c e b a n d is f o u n d to a p p e a r a t 4.5 eV in a d d i t i o n to t h e AFL o f CsF. This b a n d is a s c r i b e d to t h e r a d i a t i v e t r a n s i t i o n o f e l e c t r o n s from t h e Cl" 3/9 i m p u r i t y level to t h e Cs + 5p core b a n d . A brief d i s c u s s i o n is given o n t h e s p e c t r a l s h a p e o f t h e 4.5-eV b a n d .

I. INTRODUCTION Alkali h a l i d e s h a v e b e e n c a t e g o r i z e d i n t o t w o g r o u p s , t y p e A a n d t y p e L, w i t h r e s p e c t to t h e d e c a y m e c h a n i s m o f h o l e s p r o d u c e d in t h e o u t e r m o s t core level [ 11. T h e t y p e - A cryst a l s are c h a r a c t e r i z e d by t h e f a c t t h a t t h e i r b a n d - g a p energy Eg is s m a l l e r t h a n t h e e n ergy difference E v c b e t w e e n t h e t o p s o f t h e v a l e n c e b a n d a n d t h e o u t e r m o s t core b a n d . This s i t u a t i o n m a k e s a n o n r a d i a t i v e Auger p r o c e s s p r e d o m i n a n t , in w h i c h a core h o l e r e c o m b i n e s w i t h a v a l e n c e e l e c t r o n by k i c k i n g up another valence electron into an empty state. O n t h e o t h e r h a n d , in type-L c r y s t a l s w h e r e Eg > E v c , a core h o l e d e c a y s p r i m a r i l y through the radiative recombination with a v a l e n c e e l e c t r o n b e c a u s e o f t h e energy forbidd e n n e s s for t h e Auger process. T h e r e s u l t i n g l u m i n e s c e n c e in type-L c r y s t a l s is called Auger-free l u m i n e s c e n c e (AFL) [2] or crossl u m i n e s c e n c e [3]. T h e s p e c t r a l s h a p e o f AFL h a s b e e n d i s c u s s e d t h e o r e t i c a l l y in t e r m s of a c l u s t e r m o d e l [4] or a b a n d m o d e l t a k i n g a c c o u n t o f lattice r e l a x a t i o n [5,6]. However, n o c o n s e n s u s h a s b e e n r e a c h e d as to t h e validity o f t h e s e a p p r o a c h e s . T h e AFL t r a n s i t i o n t a k e s place after p h o t o - g e n e r a t e d core h o l e s r e l a x 0368-2048/96/$15.00 ~) 1996 ElsevierScience B.V.All rights reserved P l l S0368- 2048 (96) 02816-2

to t h e t o p o f t h e b a n d . T h i s s u g g e s t s t h a t t h e AFL s p e c t r a a r e n o t s o s e n s i t i v e to t h e d e n s i t y - o f - s t a t e s o f t h e core b a n d , b u t are i n f l u e n c e d by t h e c h a n g e s in t h e v a l e n c e b a n d . In alkali h a l i d e s , t h e v a l e n c e b a n d o r i g i n a t e s from t h e p s t a t e s o f t h e h a l o g e n i o n a n d t h e o u t e r m o s t core b a n d from t h e p s t a t e s of t h e a l k a l i m e t a l ion. Therefore it is o f p r i m a r y i m p o r t a n c e to i n v e s t i g a t e t h e AFL in h a l o g e n - s u b s t i t u t e d alkali h a l i d e s . In t h e p r e s e n t w o r k we h a v e s t u d i e d t h e AFL in CsF c r y s t a l s c o n t a i n i n g a s m a l l a m o u n t of Cl" i o n s . CsF a n d CsCl are b o t h classified i n t o t h e t y p e L. T h e f o r m e r h a s a NaCl-type s t r u c t u r e , while t h e l a t t e r h a s a CsCl-type s t r u c t u r e . It is f o u n d t h a t t h e CI'i m p u r i t y AFL b a n d a p p e a r s a r o u n d 4.5 eV in a d d i t i o n to t h e AFL o f CsF. By c o m p a r i n g t h i s i m p u r i t y - a s s o c i a t e d AFL w i t h t h a t of p u r e CsC[, we get u s e f u l i n f o r m a t i o n a b o u t t h e effects o f t h e localized n a t u r e of t h e u p per filled s t a t e a n d t h e c r y s t a l s t r u c t u r e o n t h e AFL s p e c t r a .

2. EXPERIMENT T h e s a m p l e s o f CsFI.xCI x (x < 0.03) a n d CsCI u s e d in t h i s e x p e r i m e n t were p r e p a r e d

118

1.0

(a) x=O .~

I

CSFl_xClx LNT 21.4eV

_~" 0.5

0

.m ¢-

0

I

I

I

CsFl_xCl x (x=0.01) 1

v

I

LNT - ~

3.2eV

v

(b) x=0.01 1.0

oo z uJ F-

zLu Iz --

0.5

0

3

4

5

PHOTON ENERGY (eV)

0

12

16

20

24

PHOTON ENERGY (eV)

Figure 1. L u m i n e s c e n c e s p e c t r a o f CsFl.xClx measured under the core-band excitation w i t h 21.4-eV p h o t o n s a t LNT; ( a ) x = 0 a n d (b) x = 0.01.

Figure 2. E x c i t a t i o n s p e c t r a for t h e 3.2- a n d 4.5-eV b a n d s in CsFl.xClx (x = 0.01) m e a s u r e d a t LNT. E x c i t a t i o n s p e c t r u m for t h e 5.2-eV b a n d in CsCI is a l s o depicted for c o m p a r i s o n .

by cooling r a p i d l y t h e i r o w n m e l t u n d e r a n i t r o g e n a t m o s p h e r e . Here, x s t a n d s for t h e c o n c e n t r a t i o n in t h e melt. CsFI.xCIx c r y s t a l s were h a n d l e d in a dry bag, b e c a u s e t h e y are hygroscopic. We u s e d s y n c h r o t r o n r a d i a t i o n (SR) from t h e U V S O R s t o r a g e r i n g in t h e I n s t i t u t e for Molecular Science as a light source. Lumin e s c e n c e f r o m t h e s a m p l e w a s detected p h o t o e l e c t r i c a l l y t h r o u g h a J o b i n Yvon H R 320 m o n o c h r o m a t o r . The obtained spectra were c o r r e c t e d for t h e s p e c t r a l r e s p o n s e o f the detection system. The decay kinetics was measured using the correlated photoncounting method under excitation with SR p u l s e s o f a b o u t 7 0 0 - p s d u r a t i o n . T h e experi m e n t s were m a i n l y carried o u t a t liquid n i t r o g e n t e m p e r a t u r e (LNT).

CsFl.xClx, (a) x = 0 a n d (b) x = 0.01, o b t a i n e d b y t h e c o r e - b a n d e x c i t a t i o n w i t h 21.4-eV p h o t o n s . In Fig. 1 (a), o n e c a n see a n i n t e n s e b a n d a t 3.2 eV a n d s o m e w e a k b a n d s a t 3.7 a n d 4.0 eV, in g o o d a g r e e m e n t w i t h t h e r e s u l t o f ref. [1]. T h e s e b a n d s h a v e b e e n a s s i g n e d t o t h e AFL d u e to r a d i a t i v e t r a n s i t i o n o f e l e c t r o n s from t h e F" 2 p v a l e n c e b a n d to t h e Cs + 5 p core b a n d [ l ]. T h e origin o f t h e 5. l-eV b a n d is n o t yet clear, b u t it m a y be a h i g h e s t e n e r g y b a n d o f t h e A F L o f CsF. F r o m Fig. l(b) it is a p p a r e n t t h a t t h e i n t r o d u c t i o n o f s m a l l a m o u n t s o f Cl" i o n s i n t o CsF gives rise to a l u m i n e s c e n c e b a n d p e a k i n g a t 4.5 eV. It w a s c o n f i r m e d t h a t t h e i n t e n s i t y o f t h e 4.5eV b a n d i n c r e a s e s w i t h x w i t h i n t h e c o n c e n t r a t i o n r a n g e i n v e s t i g a t e d (x < 0.03). F u r t h e r m o r e , t h e l u m i n e s c e n c e s p e c t r a were n o t very s e n s i t i v e to t h e s a m p l e t e m p e r a t u r e in t h e r a n g e 77- 300 K. For c o m p a r i s o n , we a l s o m e a s u r e d t h e AFL o f CsCI, a r i s i n g from t h e r a d i a t i v e t r a n s i t i o n b e t w e e n t h e Cl" 3p v a l e n c e b a n d a n d t h e Cs +

3. R E S U L T S

Figure 1 s h o w s l u m i n e s c e n c e s p e c t r a o f

119

105 CsFl_xClx (x=O.01) LNT 21.4eV

104 ¢-

103 eV

N 10 e z l.u tz_ 101

5.2eV

100 I

0

I

I

t

4 TIME (ns)

I

8

Figure 3. D e c a y profiles o f t h e 3.2- a n d 4.5-eV b a n d s in CsFl.xClx (x = 0.01) a n d t h e 5.2-eV b a n d in CsCl, e x c i t e d w i t h 21.4-eV p h o t o n p u l s e s a t LNT.

1.0 c-I

CsFI_xCIx (x=O.01) LNT | 21.4eV ~

=2 l ,l

v

\

0.5 zIII Z

CsCI - ~ / I / r

I

\j

l | I I l l I l

4. DISCUSSION

I

4 5 PHOTON ENERGY (eV)

5p core b a n d . T h e o b t a i n e d s p e c t r u m c o n s i s t s o f t w o b a n d s a t 4.6 a n d 5.2 eV, in satisfactory agreement with the previous data [1,3]. T h e r e s u l t will be s h o w n l a t e r ( b r o k e n curve in Fig. 4). In Fig. 2 are s h o w n e x c i t a t i o n s p e c t r a for t h e 3.2- a n d 4.5-eV b a n d s in CsF l-xClx w i t h x = 0.01. T h e e x c i t a t i o n s p e c t r u m for t h e 5.2eV b a n d o f CsCl is a l s o d e p i c t e d in Fig. 2. T h e 4.5-eV b a n d is i n d u c e d by t h e p h o t o e x c i t a t i o n a b o v e 14.2 _+ 0.2 eV, w h i c h is t h e s a m e energy a s t h e 3.2-eV b a n d o f CsF a n d t h e 5.2-eV b a n d o f CsCl. T h i s e x c i t a t i o n t h r e s h o l d c o r r e s p o n d s to t h e o n s e t o f t h e t r a n s i t i o n from t h e Cs + 5p core b a n d to t h e c o n d u c t i o n b a n d , i.e., Eg + E v c . D e c a y b e h a v i o r s o f t h e 3.2- a n d 4.5-eV b a n d s in t h e x = 0.01 s a m p l e are s h o w n in Fig. 3, t o g e t h e r w i t h t h a t o f t h e 5.2-eV b a n d o f CsCl. T h e 3.2-eV b a n d (CsF) a n d 5.2-eV b a n d (CsCI) h a v e a lifetime o f z = 2.6 + 0.2 n s a n d 1.2 + 0.2 n s , respectively. T h e s e valu e s are fairly c o n s i s t e n t w i t h t h e d a t a o f ref. [7]. T h e 4.5-eV b a n d e x h i b i t s a f a s t d e c a y c o m p o n e n t h a v i n g z = 0.8 _+ 0.2 n s , followed by a w e a k c o m p o n e n t o f z = 1.5 _+ 0.3 ns. Although the appearance of two components is n o t fully u n d e r s t o o d , s u c h f a s t d e c a y b e h a v i o r o f l u m i n e s c e n c e is a c h a r a c t e r i s t i c f e a t u r e o f AFL [2].

6

Figure 4. C o m p a r i s o n o f t h e 4.5-eV b a n d in CsFl.xClx (x = 0.O1 ) w i t h t h e AFL b a n d in CsCl. T h e solid line w a s o b t a i n e d b y s u b t r a c t i n g t h e s p e c t r u m o f Fig. l(a) from t h a t o f Fig. l(b).

W h e n s m a l l a m o u n t s o f CI" i o n s a r e a d d e d i n t o t h e m a t r i x o f CsF, t h e C I 3 p s t a t e s ( i m p u r i t y b a n d ) will be s i t u a t e d 1-2 eV a b o v e t h e t o p of t h e F" 2 p v a l e n c e b a n d . It is t h u s a n t i c i p a t e d t h a t a n a d d i t i o n a l AFL b a n d a p p e a r s o n t h e h i g h - e n e r g y side o f t h e AFL b a n d o f CsF, b e c a u s e CsCl b e l o n g s to t h e type L. All t h e p r e s e n t r e s u l t s i n d i c a t e t h a t s u c h a b a n d is a c t u a l l y o b s e r v e d in t h e reg i o n o f a b o u t 4.5 eV w h e r e p u r e CsF d o e s n o t emit. T h e n e w l y o b s e r v e d 4.5-eV b a n d is a t t r i b u t e d to t h e r a d i a t i v e e l e c t r o n i c t r a n s i t i o n Cl" 3p i m p u r i t y level -* Cs ÷ 5p core b a n d . It is i n t e r e s t i n g to c o m p a r e t h e C l ' - i m p u rity AFL b a n d w i t h t h a t o f p u r e CsCI, To do so, we s u b t r a c t t h e l u m i n e s c e n c e s p e c t r u m

120 o f Fig. l(a) from t h a t o f Fig. l(b) a f t e r n o r m a l i z i n g t h e i r i n t e n s i t i e s a r o u n d 4.0 eV. T h e r e s u l t is s h o w n by solid line in Fig. 4, t o g e t h er w i t h t h e AFL s p e c t r u m ( b r o k e n curve) o f p u r e CsCI. There are t w o i m p o r t a n t p o i n t s to be d i s c u s s e d in Fig. 4. First, t h e s p e c t r a l w i d t h o f t h e 4.5-eV b a n d is m u c h n a r r o w e r t h a n t h a t o f t h e CsC1 AFL b a n d . A c c o r d i n g to a simple b a n d m o d e l [2], t h e t o t a l w i d t h o f AFL reflects a p p r o x i m a t e l y t h e w i d t h o f t h e u p p e r filled b a n d . Therefore, t h i s f a c t m a y be u n d e r s t o o d by c o n s i d e r i n g t h a t t h e CI" 3 p s t a t e h a s t h e localized n a t u r e in m i x e d crystals, w h e r e a s it h a s t h e b a n d n a t u r e in p u r e crystals. It is still a n o p e n q u e s t i o n a s to h o w l a t t i c e - r e l a x a t i o n effects c o n t r i b u t e to t h e b r o a d e n i n g o f t h e AFL b a n d . S e c o n d ly, t h e i m p u r i t y - a s s o c i a t e d b a n d is singlep e a k e d , in c o n t r a s t to a d o u b l e - p e a k e d structure of the pure band. Since the space lattice o f CsCl is s i m p l e cubic, t h e Cs + i o n h a s eight Cl" i o n s a s n e a r e s t n e i g h b o r s . O n t h e o t h e r h a n d , in CsF c r y s t a l o f f a c e - c e n t e r e d cubic, e a c h Cs + i o n is s u r r o u n d e d by six n e a r e s t n e i g h b o r s o f F" ions, o n e o f t h e m being r e p l a c e d by a Cl" i o n in m i x e d CsFl.xClx. It is t h u s s u p p o s e d t h a t t h e s p e c t r a l s t r u c t u r e o f AFL is very s e n s i t i v e to t h e i m m e d i a t e a n i o n e n v i r o n m e n t a r o u n d t h e core h o l e p r o d u c e d o n a c a t i o n site. It is w o r t h n o t i n g t h a t a s i n g l e - p e a k e d AFL b a n d h a s a l s o b e e n o b s e r v e d a r o u n d 4.5 eV in KChCs a n d RbChCs s y s t e m s u n d e r X - r a y i r r a d i a t i o n [8]. In t h e s e s y s t e m s , a n alkali m e t a l i o n (K+ or Rb +) is r e p l a c e d by a Cs + i o n in t h e NaCl-type c o n f i g u r a t i o n . T h e o b served AFL b a n d s h a v e b e e n a s c r i b e d to t h e CI" 3p v a l e n c e - b a n d - t o - C s + 5p i m p u r i t y - l e v e l transition. In s u m m a r y , t h e AFL in m i x e d CsFI.xCl x s y s t e m w a s s t u d i e d w i t h u s e o f S R a s a light source. A n e w l u m i n e s c e n c e b a n d w a s f o u n d to arise from t h e r a d i a t i v e d e c a y o f core h o l e s into the impurity states located above the valence band, and was compared with the AFL o f p u r e CsCI. T h e s p e c t r a l s h a p e of AFL h a s b e e n s t u d i e d t h e o r e t i c a l l y in refs.[4-6].

It is e x p e c t e d t h a t t h e s e t h e o r e t i c a l s t u d i e s are e x t e n d e d to i m p u r i t y - a s s o c i a t e d AFL bands[9]. T h e r e s u l t s o b t a i n e d in h e a v i l y d o p e d c r y s t a l s were r a t h e r c o m p l i c a t e d p r o b a b l y b e c a u s e o f a n y i r r a d i a t i o n effect a n d / o r s e g r e g a t i o n o f m i x e d s a m p l e s . This r e q u i r e s s e p a r a t e e x p e r i m e n t s to solve t h e s e problems.

ACKNOWLEDGMENTS T h e a u t h o r s a r e g r a t e f u l to Mr. H. H a r a for h i s g e n e r o u s a s s i s t a n c e in e x p e r i m e n t . This w o r k w a s s u p p o r t e d by t h e J o i n t S t u d ies P r o g r a m o f t h e I n s t i t u t e for M o l e c u l a r Science, O k a z a k i .

REFERENCES I. S. K u b o t a , M. Itoh, J. R u a n ( G e n ) , S. S a k u r a g i a n d S. H a s h i m o t o , Phys. Rev. Lett., 6 0 (1988) 2319. 2. M. Itoh, S. K u b o t a , J. R u a n ( G e n ) a n d S. H a s h i m o t o , Rev. Solid S t a t e Sci., 4 (1990) 467. 3. J. L. J a n s o n s , V. J. K r u m i n s , Z.A. R a c h k o a n d J. A. Valbis, Phys. S t a t u s S o l i d i ( b ) 1 4 4 (1987) 835. 4. |. F. Bikmetov, A. B. S o b o l e v a n d Ya. A. Valbis, Soy. Phys. Solid S t a t e , 3 3 (1991) 1715. 5. Y. T o y o z a w a , Core-Level S p e c t r o s c o p y in C o n d e n s e d S y s t e m s , edited by J. K a n a m o r i a n d A. K o t a n i , Springer-Verlag, Berlin, 1988, pp. 2 3 1 - 2 3 3 . 6. Y. K a y a n u m a a n d A. K o t a n i , t h i s c o n ference. 7. S. K u b o t a , J. R u a n ( G e n ) , M. Itoh, S. Hashimoto and S. Sakuragi, Nucl. I n s t r u m . M e t h o d s Phys. Res., Sect. A 2 8 9 (1990) 253. 8. A.S. V o l o s h i n o v s k i i , V.B. Mikhailik, S.V. S y r o t y u k a n d P.A. Rodnyi, Sov. Phys. Solid S t a t e , 3 4 (1992) 1022. 9. A.B. S o b o l e v , Phys. Solid S t a t e , 3 5 (1993) fill.