CHEMOMECHANICAL EFFECTS IN ZnO

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CHEMOMECHANICAL EFFECTS IN ZnO

J. Ahearn, J. Mills, A. Westwood

To cite this version:

J. Ahearn, J. Mills, A. Westwood. CHEMOMECHANICAL EFFECTS IN ZnO. Journal de Physique

Colloques, 1979, 40 (C6), pp.C6-173-C6-176. �10.1051/jphyscol:1979635�. �jpa-00219051�

JOURNAL DE PHYSIQUE CoZZoque C6, suppZ6ment au n06, tome 40, juin 2979, page C6-273

J.S. Ahearn, J. J. M i l l s , and A. R. C. Westwood

Martin Marietta Laboratories, Baltimore, Mary Zand, U. S. A.

Resume.- Un t r a v a i l a n t e r i e u r a montre que des m i l i e u x a s u r f a c e a c t i v e , pouvaient i n f l u e n c e r l a dure- t G de s o l i d e s non-metalliques t e l s que l e c h l o r u r e dlargent, 1 'o x i d e de magnesium, I 'alumine, l e q u a r t z e t l e s verres Na,O-CaO. Un maximum de durete a p p a r a i t specifiquement, quand l e ~ o t e n t i e l de ces s o l i d e s e s t n u l . On a a t t r i b u e a c e t t e " c o r r @ l a t i o n - c " , l a s i g n i f i c a t i o n que l a charge de l a sur- face p o u v a i t i n f l u e n c e r de faqon importance l a durete. A f i n d'examiner directement c e t t e o o s s i b i l i t e , on a donc mesure, dans une c e l l u l e e l e c t r o l y t i q u e , l a micro durete e t l a t a i l l e des rosaces de d i s l o - c a t i o n s u r l e s surfaces (0001) e t {1010} de l ' o x y d e de z i n c , en f o n c t i o n du p o t e n t i e l applique du pH de 1 ' e l e c t r o l y t e e t du temps, chacun de ces f a c t e u r s m o d i f i a n t 1 ' 6 t a t de surface. Les r e s u l t a t s i n d i - quent qu'un maximum de durete apparait, pour l e s deux surfaces d'oxyde de zinc, non oas quand l a char- ge de l a surface e s t n u l l e (comme on s ' y a t t e n d a i t ) mais p l u t d t quand e l l e e s t legerement o o s i t i v e (courbure de l a bande vers l e bas). Comme l e s i n t e r ~ r e t a t i o n s a n t e r i e u r e s de l a cause de l a " c o r r e l a - t i o n - ~ " apparaissent maintenant inappropriees pour ZnO, on propose un a u t r e mecanisme m e t t a n t en j e u un @change de charges au voisinage des d i s l o c a t i o n s en mouvement, e n t r e l e s niveaux donneurs e t l a bande de conduction.

Abstract.- Past work has shown t h a t s u r f a c e - a c t i v e environments can i n f l u e n c e t h e hardness o f such non- metal1 i c s o l i d s as s i l v e r c h l o r i d e , magnesium oxide, alumina, quartz, and soda-1 ime glass. S o e c i f i c a l - 1 y , a maximum i n hardness occurs when t h e 5-potenti a1 o f these s o l i d s i s zero. T h i s " q - c o r r e l a t i o n "

has been taken t o i m p l y t h a t s u r f a c e charge can markedly i n f l u e n c e hardness. To examine t h i s ~ o s s i b i - l i t y d i r e c t l y , t h e r e f o r e , t h e microhardness and s i z e o f d i s l o c a t i o n r o s e t t e s on the (0001) and {10101 surfaces on ZnO were measured as a f u n c t i o n o f a p ~ l i e d ~ o t e n t i a l , e l e c t r o l y t e oH, and time i n an elec- t r o l y t i c c e l l -- a l l o f which a l t e r surface charge. The r e s u l t s i n d i c a t e t h a t , f o r b o t h ZnO surfaces, a maximum i n hardness i s produced n o t when t h e surface charge i s zero -- as e x ~ e c t e d -- b u t r a t h e r when t h e surface i s s l i g h t l y p o s i t i v e l y charged (downward band bending). Since e a r l i e r i n t e r p r e t a t i o n s o f t h e cause o f the "5- c o r r e l a t i o n " now appear t o be i n a p p r o p r i a t e f o r ZnO, an a l t e r n a t i v e mechanism i n v o l v i n g charge exchange between donor l e v e l s and t h e conduction band near moving d i s l o c a t i o n s i s suggested.

1. I n t r o d u c t i o n and approach.- The occurrence o f a maximum i n hardness a t zero c p o t e n t i a l has been de- monstrated f o r a number o f non-metall i c m a t e r i a l s , i n c l u d i n g AgCl /I/, YgO /2/, AI203/3/, q u a r t z /4-5/, soda-lime glass /6-lo/, and S i /11/. However, though several suggestions /4/ have been offered t o e x p l a i n t h i s " < - c o r r e l a t i o n u , no d e t a i l e d understanding has y e t been achieved. One f e a t u r e common t o r e c e n t sug- gestions i s t h a t s u r f a c e - a c t i v e species adsorbed from d i l u t e e l e c t r o l y t e s a f f e c t t h e charge of t h e s o l i d surface, which i s revealed by changes i n t h e 5 p o t e n t i a l . T h i s a l t e r a t i o n o f surface charge a, then, e i t h e r a f f e c t s t h e charge on p o i n t d e f e c t s o r d i s l o c a t i o n s , o r changes t h e P e i e r l s s t r e s s . Since these f a c t o r s i n f l u e n c e d i s l o c a t i o n behavior, near- surface d i s l o c a t i o n m o b i l i t y i s a l t e r e d /2/.

To examine t h e v a l i d i t y of t h i s b a s i c concept we have r e c e n t l y examined t h e v a r i a t i o n o f hardness, H, w i t h a f o r t h e model m a t e r i a l , ZnO, f o r which t h e r e 1 a t i o n s l i i p s between o and near-surface e l e c t r o n i c band s t r u c t u r e a r e reasonably w e l l understood /12/.

I n t h i s c o n t r i b u t i o n , we summarize t h e r e s u l t s of

t h i s work.

To c o n t r o l a, t h e t e s t c r y s t a l was made a wor- k i n g e l e c t r o d e i n an e l e c t r o l y t i c c e l l , and t h e ap- p l i e d p o t e n t i a l v a r i e d a p p r o p r i a t e l y . For ZnO i n con- c e n t r a t e d e l e c t r o l y t e s , t h e e l e c t r i c a l p i c t u r e i s as -

f o l l o w s /13/ : Under s u f f i c i e n t anodic b i a s , t h e charge i s n e g a t i v e a t t h e surface, t h e bands bend up and t h e space charge r e g i o n i s depleted. The sur- face charge may be increased t o zero and then made p o s i t i v e by a p p l y i n g l e s s anodic bias. Thus, band bending i s control'led by t h e a p p l i e d p o t e n t i a l and t h i s , i n t u r n , allows c o n t r o l l e d a l t e r a t i o n o f t h e c a r r i e r d e n s i t y i n t h e conduction and valence bands, and occupancy o f d i s l o c a t i o n and d e f e c t l e v e l s i n the space charge l a y e r . The i n f l u e n c e o f these chan- ges on mechanical p r o p e r t i e s i s then s t u d i e d v i a microhardness measurements. To e l i m i n a t e any possi- b l e i n f l u e n c e o f t h e < - p o t e n t i a l , an e l e c t r o l y t e o f h i g h c o n c e n t r a t i o n was used, namely 1M KC1. This caused t h e double l a y e r t o be compressed t o t h e p o i n t t h a t t h e s l i p p i n g plane c o i n c i d e d w i t h t h e o u t e r Helmholtz plane. Hence, c ² 0 a t a1 1 values o f pH.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979635

C 6 - 1 7 4 JOURNAL DE PHYSIQUE

To determine t h e a p p l i e d v o l t a g e necessary t o produce a

0, i . e . t h e f l a t band p o t e n t i a l (FBP), t h e capaci tance-vol tage (C-V) technique used by Dewald /13/ was employed. The FB? was determined by measuring C, p l o t t i n g 1 / C 2 vs. V, and e x t r a p o l a t i n g t h e s t r a i g h t l i n e t o 1/c2 = 0. F o r f u r t h e r experimen- t a l d e t a i l s , see reference /12/.

2. Observations and discussion.- Data from (0001) surfaces a r e presented i n f i g u r e 1 f o r pH = 8.5 and

11.5. I n b o t h instances, a maximum i n hardness occurs a t - - l o 0 mV. The existence of t h i s maximum was con- f i r m e d by e t c h p i t s t u d i e s o f t h e e x t e n t o f d i s l o c a - t i o n motion, L, around hardness i n d e n t a t i o n s . I n t h i s case, L was a minimum a t - - I 0 0 mV.

B I A S VOLTAGE RELATIVE TO CALOMEL (mV)

fl::; ^rface

Fig. 1 : Hardness o f t h e (0001) ZnO surface vs.bias v o l t a g e i n an e l e c t r o l y t i c c e l l . I n d e n t a t i o n t i m e = 10 s, l o a d 10 g. (Ref. /15/).

240

220

The v a r i a t i o n s i n hardness o f a (1070) surface w i t h b i a s v o l t a g e a t pH = 8.5, 9.5 and 11.5 a r e shown i n f i g u r e 2. I n a d d i t i o n t o t h e hardness maximum o c c u r r i n g at, f o r example, -350 mV f o r a pH of 8.5, t h e r e appears t o be a maximum i n o v e r a l l hardness a t pH = 9.5. This r e s u l t may be r a t i o n a l i z e d from c o n s i d e r a t i o n s of t h e pH-dependence of ZnO s o l ubi 1 i -

ty. D i s s o l u t i o n may be enhanced near a hardness i m - p r e s s i o n bv l o c a l mechanical s t r a i n .

pH

8.5

- 10 g Load

-

BIAS VOLTAGE RELATIVE TO CALOMEL tmV1

200 -

180 -

I I

-

Fig. 2 : Hardness o f t h e (1070) ZnO surface vs.bias v o l t a g e i n 1 Y KC:. I n d e n t a t i o n t i m e = 10

l o a d 10 g pH = 8.5, /12/, pH = 9.5 and 11.5 ( r e f . /15/).

T h i s would enlarge t h e impression and lower t h e ap- parent hardness. Since the pH i n f l u e n c e s t h e disso- l u t i o n r a t e , and a minimum i n s o l u b i l i t y i s expected a t pH = 9.5, t h e zero p o i n t o f charge of ZnO /14/, any d i s s o l u t i o n e f f e c t should be l e a s t i n s o l u t i o n s o f t h i s pH, and t h e "average hardness" w i l l appear t o be l e s s f o r ZnO surfaces exposed t o environments o f pH = 9.5, as observed.

The p r i n c i p a l r e s u l t s o f t h e ZnO experiments a r e summarized i n t a b l e I.

Table I : ZnO i n 1 Y KC1

--

Predicted Range Surface Electrolyte Position of of Vnax fmn Charge FBP

PH Hmax (my) Excharlge H e 1 (mV1 (mV)

These, and t h e data o f f i g u r e s , 1 and 2 demonstrate

t h a t the microhardness o f a ZnO c r y s t a l can be s i g n i -

f i c a n t l y changed by a l t e r i n g i t s s u r f a c e charge v i a

an a p p l i e d p o t e n t i a l . Thus, t h e existence o f a r e l a -

t i o n s h i p between surface charge and hardness, pre-

sumed i n e a r l i e r work o f chemomechanical e f f e c t s ,

i s now v e r i f i e d f o r t h e ZnO-aqueous e l e c t r o l y t e sys-

J . S . Ahearn et al. C6-175

tem.

O f more p a r t i c u l a r i n t e r e s t , however, i s the o b s e r v a t i o n t h a t the data o f t a b l e I r e v e a l t h a t ma- xima i n hardness, Hmax, occur not a t t h e FBP, b u t a t p o t e n t i a l s producing a s l i g h t l y p o s i t i v e surface charge on b o t h (0001) and (10IO) surfaces. Thus, a l - though t h e r e s u l t s c o n f i r m t h e importance o f surface charge i n chemomechanical e f f e c t s , they do not - - a t l e a s t f o r ZnO -- c o n f i r m t h e < - c o r r e l a t i o n as e a r l i e r conceived, namely t h a t hardness maxima occur when

< = 0

= o .

As a b a s i s f o r developing an e x p l a n a t i o n f o r t h i s somewhat unexpected r e s u l t , i t i s u s e f u l t o ap- p r e c i a t e what t h e e l e c t r o n i c s t r u c t u r e o f ZnO i s li- k e l y t o be under a p p l i e d p o t e n t i a l s employed i n our experiments. Consideration o f t h e l i k e l y e l e c t r o n i c s t r u c t u r e o f ZnO r e v e a l s t h a t both z i n c i n t e r s t i t i a l donor l e v e l s and t h e conduction band near t h e c e n t e r of t h e B r i l lo u i n zone may w e l l change occupancy under s p e c i f i e d a p p l i e d voltages near t h e FBP. Accordingly, t h e f o l l o w i n g "charge exchange" hypothesis has been proposed /12/ t o e x p l a i n t h e r e s u l t s .

F i r s t , assume t h a t both conduction and valence bands are d i s t o r t e d near a d i s l o c a t i o n , F i g u r e 3(a).

This d i s t o r t i o n m i g h t r e s u l t from : ( i ) a p o s i t i v e l y o r n e g a t i v e l y charged d i s l a c a t i o n w i t h a surrounding screening c l o u d o f conduction e l e c t r o n s o f i n t e r s t i - t i a l z i n c i o n s ; ( i i ) c o u p l i n g o f t h e s t r a i n f i e l d around t h e d i s l a c a t i o n and t h e energy bands through t h e d e f o r n a t i o n p o t e n t i a l , o r ( i i i ) t h e p i e z o e l e c t r i c e f f e c t s . Now i f , f o r example, a d i s l o c a t i o n induces an upward band d i s t o r t i o n , then motion o f the d i s l o - c a t i o n w i l l a l i o r e q u i r e movement o f t h e band d i s t o r - t i o n . Under f l a t band conditions, o n l y rearrangement o f t h e conduction e l e c t r o n s would be i n v o l v e d when t h e d i s l o c a t i o n moves, and so i t s motion would be r e l a t i v e l y easy, and t h e c r y s t a l would be r e l a t i v e - l y s o f t . As t h e hands a r e bent downward i n response t o environmental changes, however, and EC and Ed (see f i g u r e 3) approach EF, t h e Zn+ donors away from t h e d i s l a c a t i o n become n e u t r a l i z e d . Therefore, d i s - l o c a t i o n motion now n e c e s s i t a t e s e x c i t a t i o n o f e l e c - t r o n s from t h e n e u t r a l donor l e v e l s t o unoccupied l e v e l s i n t h e conduction band, f i g u r e 3(b). Such ex- c i t a t i o n r e q u i r e s about 50 meV/danor ( t h e energy bet- ween t h e conduction band edge and t h e Zn+ donor l e - v e l ) . The consequence should be a decrease i n d i s l o - c a t i o n m o b i l i t y and a concomitant i n c r e a s e i n hard- ness, because t h i s a d d i t i o n a l amount o f energy must be s u p p l i e d b e f o r e d i s l o c a t i o n motion can occur.

With f u r t h e r downard band bending, i l l u s t r a t e d

i n f i g u r e 3(c), e s s e n t i a l l y a l l o f t h e donors i n the space charge l a y e r become n e u t r a l i z e d .

a) Flat Bands: A l l Donors Ionized

.

+ + Ec

++ +++ ^50mV +++++++

++++++Ed

Spatial 225 mV

Position of

Dislocation 1

EF b) Positive Surface Charge: Donors Near Dislocation Ionized

Charge k k c h a n g e

' ^donors

donors NEUTRAL 1 ^{ON lZED} I donors NEUTRAL Direction ot Dislocation Motion c) Increased Positive Surface Charge: All Donors Neutralized

Fig. 3 : Schematic o f t h e d i s t o r t i o n i n e l e c t r o n i c l e v e l s i n t h e neighborhood o f a basal d i s l o c a t i o n near t h e edge of t h e conduction band.

Now t h e d i s l o c a t i o n may again move w i t h o u t e x c i t i n g donors. Hence, d i s l o c a t i o n m o b i l i t y w i l l be l e s s f o r t h e s i t u a t i o n i l l u s t r a t e d i n f i g u r e 3 ( b ) than those shown i n f i g u r e s 3(a) o r ( c ) .

A s i m i l a r l i n e o f argument can be developed when t h e d i slocation-induced band d i s t o r t i o n i s down- ward.

The degree of environmentally-induced band ben-

d i n g necessary t o produce a maximum i n hardness de-

pends on t h e l o c a l d i s t o r t i o n o f t h e conduction bands,

AE, shown i n f i g u r e 3 (a). And t o p r e d i c t the p o s i -

t i o n o f Hmax on t h e p o t e n t i a l a x i s , i t i s necessary

t o assume a value f o r AE. Agreement between theory

and experiment can be achieved by s e l e c t i n g AE =

200 meV f o r t h e presumed n e g a t i v e l y charged basal

C6-176 J O U R N A L DE PHYSIQUE

(6 t y p e ) d i s l o c a t i o n s c o n t r o l l i n g t h e hardness of t h e (1070) s u r f a c e , and -50 meV f o r t h e presumed unchar- ged p r i s m a t i c d i s l o c a t i o n s c o n t r o l l i n g t h e hardness o f (0001) s u r f a c e s /12,15/.

Acknowledgements.- The experimental a s s i s t a n c e o f 3s. D.A. K a l i v o d a i s g r e a t l y a p p r e c i a t e d . T h i s work was supported i n p a r t by t h e U.S. N a t i o n a l Science Foundation under Grant DMR 75-05443.

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