CRYSTALLINE STRUCTURE AND DOPING LEVEL OF THIN ELECTROLUMINESCENT ZnS:Mn LAYERS

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CRYSTALLINE STRUCTURE AND DOPING LEVEL OF THIN ELECTROLUMINESCENT ZnS:Mn

LAYERS

A. Izrael, P. Thioulouse, R. Tueta

To cite this version:

A. Izrael, P. Thioulouse, R. Tueta. CRYSTALLINE STRUCTURE AND DOPING LEVEL OF THIN

ELECTROLUMINESCENT ZnS:Mn LAYERS. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-

903-C2-906. �10.1051/jphyscol:19842207�. �jpa-00223884�

JOURNAL

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Colloque C2, supplgment au n02, Tome 45, fgvrie; 1984 page C2-903

CRYSTALLINE STRUCTURE AND DOPING LEVEL OF T H I N ELECTROLUMINESCENT ZnS:Mn LAYERS

A . I z r a e l , P . T h i o u l o u s e and R. Tueta

Centre NationaZ drEtudes des T&Z&cornmunications, 196 rue de Paris, 9 2220 Bagneux, France

Resume

-

Par comparaison des p r o p r i e t e s e l e c t r o o p t i q u e s e t m i c r o s t r u c t u r a l e s des couches ~ l e c t r o l u m i n e s c e n t e s ZnS:Mn, nous avons montre que l a luminance augmente avec l a t a i l l e des g r a i n s . Le r e c u i t e f f e c t u e sous c e r t a i n e s c o n d i t i o n s s'accompagne simultanement d'un abaissement de l a tension de s e u i l de luminescence e t d'une modi- f i c a t i o n de l a r e p a r t i t i o n en profondeur du manganese dans l a couche a c t i v e .

Abstract

-

Comparing e l e c t r o o p t i c and m i c r o s t r u c t u r a l p r o p e r t i e s o f electrolumines- cent (E.L.) ZnS:Mn t h i n l a y e r s , we showed the increase o f t h e luminance w i t h t h e g r a i n s i z e ; meanwhile the annealing under s p e c i f i c c o n d i t i o n s induces two simul- taneous e f f e c t s : t h e E.L. t h r e s h o l d v o l t a g e i s lowered and t h e Mn c o n c e n t r a t i o n i n the depth o f t h e a c t i v e l a y e r i s modified.

High f i e l d ac t h i n f i l m ZnS: Mn electroluminescent devices (ACTFEL) have been r e p o r t e d as having many u s e f u l and i n t e r e s t i n g c h a r a c t e r i s t i c s f o r d i s p l a y a p p l i - c a t i o n s

)

1

)

121. However t h e m i c r o s t r u c t u r e o f t h e various l a y e r s i n such devices have been scarcely s t u d i e d 131,141,15

1 .

As the e l e c t r o o p t i c p r o p e r t i e s o f t h e devices depend upon t h e growth and annealing c o n d i t i o n s , t h i s work tends t o c o r r e l a t e them w i t h t h e m i c r o s t r u c t u r a l p r o p e r t i e s o f t h e ZnS l a y e r , deduced from STEM microscopy and microanalysis.

EXPERIMENTAL

4. Specimen growth and annealing method

W e h a v e e - i n s u l a t e d s t r u c t u r e o f ACTFEL devices shown

on Fig.1. The various l a y e r s a r e deposited under vacuum on a glass substrate by elec- t r o n beam evaporation. The ZnS:Mn a c t i v e l a y e r i s evaporated from a doped source mate- r i a l obtained by i n t i m a t e mixing between ZnS powder and various Mn compounds (MnS, Mn F2, Mn C12). During the ZnS evaporation, the substrate i s e i t h e r a t room tempe- r a t u r e o r heated a t approximately 150°C. I n some cases a t h r e e hour annealing a t 470°C i s performed under various c o n d i t i o n s given i n Table 1.

B. Specimen p r e p a r a t i o n f o r transmission e l e c t r o n microscopy

To be observed by transmission e l e c t r o n microscopy ZnS:Mn l a y e r s have t o be se- oarated from t h e o t h e r l a v e r s and thinned from the side which i s i n contact w i t h the HCI d i l u t e d etchant t o a "thickness o f l e s s than 100 nm. The u s u a l l y observed side i s

t h e growth end side, b u t n o t e x a c t l y the top, because p a r t o f the ZnS l a y e r (estimated t o about 100nm) i s taken away d u r i n g these opera-

~i(1500A) t i o n s . Cross-sections are a1 so observed. Speci- Y~O,(~OOOA) men are thinned down t o l e s s than 50um by g r i n - ZnS:Mn(6

*o3(4OO0$ d i n g and mechanical p o l i s h i n g . Further t h i n n i n g 1.~0. i1300A) t o l e s s than 100nm i s obtained by i o n m i l l i n g .

substrate C. Microanalysis s p e c i f i c a t i o n s

Microscopic study i s performed on a dedicated scanning transmission e l e c t r o n microscope (STEM Fig. 1 VG HB5) e uipped w i t h an energy d i s p e r s i v e XRay

detector 9KEVEX)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842207

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Q u a n t i t a t i v e ~ ; i i c r o a n a l y s i s o f t h e Mn , d i s t r i b u t i o n i s p o s s i b l e t a - k i n g i n t o account t h e i o n i z a t i o n c r o s s - s e c t i o n , t h e f l u o r e s c e n c e

y i e l d and t h e w e i g h t o f l i n e o f t h e Tab. 1 X r a y peaks. L e t us c a l l :

x t h e mass c o n c e n t r a t i o n oercentacle

IDOPING

^{j ~ n b} ~ M " S ~ M ~ C I ~ ~ M ~ C I Z M ~ C I Z M ~ C I ? / M ~ C-

o f Mn i n s t o e c h i o m e t r i c Z ~ S I ( E ) t h e h a l f w i d t h i n t e g r a t e d X ray i n t e n s i t y o f t h e k p e a k e m i t t e d by t h e element E

We can show t h a t I ( M n ) / I ( Z n ) = x ~ l . 9 5 . We checked t h i s r e s u l t b y comparinq

GROWTH

TEMPERATURE ( T I

1

²⁵

1

^{2 5}

1

^{2 5}

1

¹⁵⁰

1

^{150 150}

1

¹⁵⁰

1

STEM averaged Mn c o n c e n t r a t i o n w i t h a t o m i c a b s o r p t i o n measurements.

A b s o r p t i o n c o r r e c t i o n s a r e always n e g l i g i b 1 e : i n a l l o u r measurements t h e decrease o f t h e I ( S ) / I (Zn) r a t i o

ANNEALING CONDITIONS 3 h - ^470°C

AVERAGE GAIN SIZE END OF GROWTH ( n m ) M n MASS CONCENTRATION PIe)

as compared t o i t s v a l u e w i t h o u t ^I

absor'ption i s small e r t h a n 2O%,which makes i t s u r e t h a t t h e a b s o r p t i o n

c o r r e c t i o n on t h e I ( M n ) / I ( Z n ) r a t i o i s s m a l l e r t h a n 2.5%.

A usual c o u n t i n g t i m e o f 5 t o 10 m i n u t e s and a t y p i c a l x v a l u e o f 0.4% correspond t o I(Mn) = 350 c t s I ( Z n ) = 40000 c t s

The s t a t i s t i c a l u n c e r t a i n l y upon t h e Mn c o n c e n t r a t i o n i s then : A X / X - + ( 1/350

+

1/40000)

-

^{5 6%}

With a t h i c k n e s s o f a b o u t 50nm, t h e probe s i z e a t t h e specimen o u t p u t remains smaller than 5 nm, t h e random movements o f t h e probe on t h e specimen a r e u s u a l l y s m a l l e r t h a n 5nm, so t h e s p a t i a l r e s o l u t i o n o f t h e m i c r o a n a l y s i s i s b e t t e r than 10 nm which i s a l - ways s m a l l e r than t h e g r a i n s i z e .

D. E l e c t r o o p t i c measurements

Voltage, c u r r e n t and l i g h t o u t p u t a r e measured on t h e devices d r i v e n w i t h a 5 kHz sinusoTda1 waveform. I n t h e f o l l o w i n a s e c t i o n s . t h e E.L. w i l l be s i m o l v c h a r a c t e r i z e d

- 30 OtoOl7

f i r s t 2-300nm and t h e n reaches a mean va-

by t h e t h r e s h o l d v o l t a g e and t h e s a t h a t i o n luminance as d e f i n e d on t h e luminance ver- sus a p p l i e d v o l t a g e c u r v e o f Fig.2. More s o p h i s t i c a t e d measurements,taking i n t o ac- count t h e p o l a r i t y , a l l o w t o d i s t i n g u i s h t h e

-

30 Olo013

E.L. c o n t r i b u t i o n o f each Y203/ZnS i n t e r -

face.

ld

RESULTS

A. C r i s t a l l i n i t y

lo3

As shown on t h e t r a n s m i s s i o n b r i g h t f i e l d m i c r o g r a p h o f F i g . 3 , t h e ZnS l a y e r s a r e p o l y c r i s t a l l i n e ,wi t h d i s o r d e r e d g r a i n s

-E lo2

o f v a r i o u s shapes. The e n l a r g e d m i c r o g r a p h

.

o f Fig.4 shows w i t h i n one ZnS g r a i n a black -CJ

and w h i t e s t r i p c o n t r a s t c o r r e s p o n d i n g t o

2

m i c r o t w i n s p a r a l l e l t o ( 1 11) planes,spaced

Id

by 2 n m , c h a r a c t e r i s t i c o f ZnS p o l y t y p i s m .

"

They a l s o appear on t h e c r o s s - s e c t i o n mi-

z

c r o g r a p h o f F i g . 5 , n e a r l y p a r a l l e l t o t h e Q

i n t e r f a c e which i n d i c a t e s a <Ill> p r e f e r r e d

Z

o r i e n t a t i o n o f t h e ZnS g r a i n s 141. F i g . 5

m i c r o g r a p h i s r e c o n s t r u c t e d f r o m s e v e r a l

2

images c o r r e s p o n d i n g t o d i f f e r e n t exposu- r e s . We n o t i c e i n t h e f i r s t 100 nm o f t h e

- 70 -

30 0 35

-

SATURATION

LU

MINANCE

-

-

-

no---

l a y e r a g r a n u l a r s t r u c t u r e (20 nm g r a i n s ) .

10'0

Then a columnar s t r u c t u r e appears:the co-

lumns grow n e a r l y p e r p e n d i c u l a r l y t o t h e

THRESHOLD VOLTAGE

Fig. 2 i n t e r f a c e . t h e i r d i a m e t e r i n c r e a s e s i n t h e

VACWM N2

' 70 70

"KUUM WITH 2ndyzo3~I\YER

0 35 1 0.35 0 35 0 3 5

l u e o f a b o u t 70 nm which remains steady up t o t h e end o f t h e grnvth The average g r a i n s i z e a t t h e end of t h e growth i s independant o f t h e Mn compound used f o r t h e sour- c e m a t e r i a l ,increases w i t h t h e s u b s t r a t e d e p o s i t i o n temperature from 30 nm a t room t e m p e r a t u r e t o 70 nm a t 150°C. U s u a l l y g r a i n s i z e i s n o t m o d i f i e d by a n n e a l i n g b u t sometimes an i n c r e a s e i s observed.

I n b o t h cases o f d e p o s i t i o n on a heated s u b s t r a t e and o f annealing, a l a r g e r mean g r a i n s i z e always induces a h i g h e r s a t u r a t i o n l u m i - nance. These r e s u l t s a r e p a r t i a l l y shown on Table 1.

B. Manqanese d i s t r i b u t i o n (see Table 1 )

As shown p r e v i o u s l y by Kun 161 b e s t r e s u l t s a r e o b t a i n e d a t room temperature w i t h ZnS:MnC12

source m a t e r i a l . The dopant i s homogeneous- l y d i s t r i b u t e d i n t h e l a y e r ( v a r i a t i o n s f r o m c e n t e r t o edges i n t h e same g r a i n o r between two g r a i n s a r e l e s s than +5%) and t h e luminance i s r e l a t i v e l y h i g h and homo- geneous on a macroscopic scale. With o t h e r Mn compounds MnS o r MnF2, t h e Mn i n s e r t i o n i s much more d i f f i c u l t and i t s d i s t r i b u - t i o n becomes v e r y

inhomogeneous:variations

f r o m one p a r t o f t h e specimen t o a n o t h e r r e a c h 100%. T h i s can e x p l a i n t h e poor l u - minance l e v e l observed w i t h these l a y e r s . However ZnS d e p o s i t i o n on heated s u b s t r a t e f r o m MnC12 doped source y i e l d s a p o o r e r homogeneity : l o c a l v a r i a t i o n s a r e now o f

? 20 %.

700nrn -

Fig. 5

A n n e a l i n g does n o t change t h e Mn homogeneity para1 1 e l 1 y t o t h e l a y e r b u t changes n o t i c e a b l y i t s d i s t r i b u t i o n i n d e p t h as shown on Fig.6. B e f o r e a n n e a l i n g t h e Mn c o n c e n t r a t i o n i s t h e same across t h e whole ZnS l a y e r t h i c k - ness:we d i d n o t n o t i c e n e i t h e r t h e presence o f Mn p r e c i p i t a t e s n o r a Mn c o n c e n t r a t i o n g r a d i e n t i n t h e l a y e r t h i c k n e s s as men- t i o n n e d by Oppol z e r 14

/ .

A f t e r vacuum a n n e a l i n g w i t h t h e ZnS l a y e r as t h e e x t e r n a l l a y e r o f t h e device, t h e Mn c o n c e n t r a t i a l decreases i n a r e g i o n l a y i n g between 100 and 200 nm f r o m t h e e x t e r n a l s u r f a c e and i n c r e a s e s

C2-906

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near t h i s s u r f a c e . C o r r e l a t e d w i t h t h e

Mn c o n c e n t r a t i o n g r a d i e n t n e a r t h e se-

-

^- -BEFORE ANNEALING cond YpO3/ZnS i n t e r f a c e , one observes 2 0 . 5 0 h

a decrease o f 50 Vrms o f t h e t h r e s h o l d ^' -AFTER VACUUM ANNEALING v o l t a g e and a c l e a r asymmetry i n t h e

e l e c t r i c a l response o f t h e d e v i c e w i t h

'

t h e p o l a r i t y o f t h e a p p l i e d v o l t a g e . None o f t h e s e phenomenon appears when

5

a n n e a l i n g i s performed a f t e r t h e se-

L L I ~ . ~ ~

cond d i e l e c t r i c l a y e r d e p o s i t i o n , where

2

no Mn m i g r a t i o n i s observed. However

8

i n t h e case o f a n n e a l i n g under n i t r o g e n f l o w w i t h o u t t h e second d i e l e c t r i c l a -

3

y e r , t h e r e s u l t s a r e i n t e r m e d i a t e and

I

need f u r t h e r i n v e s t i q a t i o n s : w e o b s e r v e

a M n m i g r a t i o n , a s m a l l t h r e s h o l d v o l -

1

0Ib

,&, ^2h0nm

t a g e decrease (25 Vrms), an unusual l w

r e s i s t i v i t y and no asymmetry. DISTANCE FROM THE TOP OF THE

ZnS

LAYER

CONCLUSIONS ^{Fig. 6}

T h i s work c o r r o b o r a t e s t h e way o f improving E.L. p r o p e r t i e s o f ACTFEL

devices. Luminance increases w i t h Cl presence i n t h e source m a t e r i a l and w i t h b i g g e r g r a i n s i z e o b t a i n e d by h e a t i n g t h e s u b s t r a t e d u r i n g d e p o s i t i o n . High temperature

(470°C) vacuum a n n e a l i n g b e f o r e t h e d e p o s i t i o n o f t h e second Y203 1 a y e r induces a decrease o f t h e t h r e s h o l d voltage, which i s perhaps e x p l a i n e d by t h e d i f f e r e n t e l e c - t r o n i n j e c t i o n c o n d i t i o n s a t t h i s i n t e r f a c e due t o t h e Mn c o n c e n t r a t i o n i n c r e a s e . A t t h e same t i m e p o o r e r d i e l e c t r i c p r o p e r t i e s and u n d e s i r e d p o l y c r i s t a l l i n e m i c r o s t r u c - t u r e o f t h e annealed Y2O3 l a y e r have been observed. These two t o p i c s need f u r t h e r i n - v e s t i g a t i o n s as w e l l f r o m t h e m i c r o s c o p i c a l p o i n t o f view as f r o m t h e e l e c t r i c a l one.

ACKNOWLEDGEMENT

The a u t h o r s would l i k e t o acknowledge t h e u s e f u l d i s c u s s i o n s h e l d w i t h R. MENN.

Thanks a r e a l s o due t o

P.

GABELOTAUD f o r specimen growth and R. BERTRAND f o r mechani- c a l t h i n n i n g .

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(11 T. INOGUCHI,M. TAKEDA,Y. YAKIHARA and M. YOSHIDA, SID I n t . Symp. Dig. Tech. P a p r s (1974) 86

121 D. THEIS and H. HUBER, Siemens Forsch-u E n t w i c k l - B e r

2

(1980) 357

131 H. VENGHAUS, D. THEIS, H. OPPOLZER and S. SCHILD, J. Appl

.

^Phys.

⁵³

(1982) 4146 141 H. OPPOLZER, S. SCHILD, H. VENGHAUS and D. THEIS, 1 0 t h Cong. on Elec. M i c r o s c .

Hambourg (1982) 421

151 R. MENN, R.J. TUETA. A. IZRAEL and M. BRAGUIER. IEEE T r a n s a c t i o n s on E l e c t r o n

, ,

devices-- ( 1 983) -460

161 Z.K. KUN, D. LEKSELL, P.R. MALMBERG, 3. MURPHY and L.J. SIENKIEWICZ,J. E l e c t r o n . Mat. 10 (1981) 287.