Recent Progress in Semiconductor Surface Studies by EPR - ACS

14 Recent Progress in Semiconductor Surface Studies by EPR D.

HANEMAN

Magnetic Resonance in Colloid and Interface Science Downloaded from pubs.acs.org by YORK UNIV on 12/05/18. For personal use only.

School of Physics, T h e U n i v e r s i t y of New South W a l e s , P . O . B o x 1, Kennsington, N . S . W . 2033, A u s t r a l i a

The use of electron paramagnetic resonance techniques in studies of surfaces, interfaces and adsorbed species has been described (1-3). In this paper we review some recent and current studies of interest. Perhaps the most significant advance has been the elucidation of the gas-sensitive resonance from s i l i c o n , with the highlighting of several new properties and concepts, and the concomitant evidence regarding the nature of amorphous films. There is also considerable interest in the surface of GaAs, and EPR has been useful in helping to elucidate aspects of the surface structure. It has long been wondered whether the rich many-line EPR spectrum of molecular oxygen would show detectable effects in the adsorbed state and we describe recent studies on this. In the cases of Si and GaAs, no new lines were observed but some broadening effects on the O2 lines seem to be present. Surface studies are particularly sensitive to contamination and an interesting and i n i t i a l l y unsuspected source of this was discovered in the case of semiconductor samples vacuum crushed in a container with a stainless steel lid. A few microscopic fragments of Fe 3 0 4 (magnetite) became mixed with the sample and displayed remarkable effects including a field induced phase (Verwey) transition. Direct evidence of the slowness of powders to attain equilibrium temperatures was obtained from these studies by taking advantage of the temperature sensitivity of the phase transition in the magnetite. A new category of information is obtainable from spin dependent conductivity effects using EPR techniques. At least two paramagnetic centres in the surface region of Si have been detected which are below the detection limits of conventional EPR techniques. 157

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SILICON cm n-type cm η -type

• ·

100%-Θ

( EPR inverse signal height )

—-Vr 10

10

10

OXYGEN

10

10

EXPOSURE

10

10

(torr min )

Figure 1. Effects of exposure to oxygen upon maximum photovoltage that is developed when split in Si is scanned byfinelight spot. Abo shown is effect upon inverse of EPR signal height, obtained from vacuum-crushed Si.

SILICON 100 ohm cm η - type

c• >»

Ι

Ο η

ί-

Ο

M007.

Θ

( EPR

ο < ι _j— ο > ο

Θ

Θ

inverse signal height )

α. -2 0

10

10

10"

MOLECULAR

10

10*

HYDROGEN

10

10

EXPOSURE

( torr mins ) Figure 2. Effects of exposure to hydrogen upon maximum photovoltage that is developed when split in Si is scanned byfinelight spot. Also shown is effect upon inverse of EPR signal height, obtained from vacuum-crushed Si.

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Paramagnetism

i n S i a n d Ge

I t h a s l o n g b e e n known t h a t S i g i v e s a n EPR s i g n a l (g = 2 . 0 0 5 5 , w i d t h a b o u t 0 . 6 5 mT) when c r u s h e d , a b r a d e d , c h i p p e d , c l e a v e d , c r a c k e d o r h e a v i l y i o n b o m b a r d e d (_3). When c r u s h e d o r f r a c t u r e d i n u l t r a h i g h v a c u u m , t h e s i g n a l i s a f f e c t e d by m o l e c u l a r o x y g e n , h y d r o g e n and w a t e r v a p o u r , b u t n o t n o t i c e a b l y (few p e r c e n t c h a n g e s ) u n t i l e x p o s u r e s o f o r d e r 10" * t o r r m i n a r e u s e d . This i s i n c o n t r a s t to the behaviour of surface sensitive p r o p e r t i e s s u c h as p h o t o e m i s s i o n w h i c h a r e a f f e c t e d a t e x p o s u r e s o f o r d e r 10~ t o r r min and y e t a r e o n l y s l i g h t l y a f f e c t e d by m o l e c u l a r h y d r o g e n . The l a t t e r , s u r p r i s i n g l y , h a s a s t r o n g e f f e c t o n t h e EPR s i g n a l . T h e s e p h e n o m e n a r e m a i n e d a p u z z l e f o r many y e a r s . Many a t t e m p t s w e r e made t o e x p l a i n t h e m b u t a l l w e r e open to s e r i o u s o b j e c t i o n i n the form o f i n c o m p a t i b l e experimental data. R e c e n t l y however t h e m a t t e r has b e e n c l e a r e d u p a n d t h e e x p l a n a t i o n (4) accounts f o r a wide v a r i e t y o f p r e v i o u s l y a p p a r e n t l y incompatible results. We p r e s e n t h e r e a b r i e f summary o f some o f t h e key d a t a . T h e r e l a t i v e l y s l o w g a s r e s p o n s e o f t h e EPR s i g n a l was r e m i n i s c e n t o f t h e g a s r e s p o n s e o f surface b a r r i e r s at the s u r f a c e s o f f i n e c r a c k s s t u d i e d s e p a r a t e l y p r e v i o u s l y (5). T h e p a r a m e t e r m e a s u r e d was t h e p h o t o v o l t a g e w h i c h i s t h e maximum v o l t a g e d e v e l o p e d a t o h m i c c o n t a c t s on e i t h e r s i d e o f a s p l i t w h i c h i s s c a n n e d by a f i n e l i g h t o r e l e c t r o n beam, and i s r e l a t e d to surface b a r r i e r h e i g h t . This in turn is r e l a t e d t o gas a d s o r p t i o n . F i g u r e 1 shows c h a n g e s a s a f u n c t i o n o f oxygen exposure o f both the p h o t o v o l t a g e a c r o s s a c a r e f u l l y p r e p a r e d c r a c k i n S i , and a l s o t h e i n v e r s e o f t h e EPR s i g n a l h e i g h t f r o m vacuum crushed S i . F i g u r e 2 shows a s i m i l a r c o r r e s p o n d e n c e when m o l e c u l a r h y d r o g e n e x p o s u r e s a r e u s e d . The e x p l a n a t i o n of the slow response i n the case of the cracks i s s t r a i g h t f o r w a r d - the e f f e c t i v e exposure of surfaces i n a narrow f i s s u r e i s l e s s than t h a t o f e x t e r n a l s u r f a c e s by a f a c t o r w h i c h i s r o u g h l y t h e r a t i o of the s u r f a c e s of the crack to the s u r f a c e o f t h e jaw o p e n i n g . This factor ( £ ) c a n be i n t h e r a n g e lQ t o 10* . Hence an e x p o s u r e o f 1 0 t o r r m i n may c o r r e s p o n d t o o n l y 10"" t o r r min i n the c r a c k , i.e. t h e s u r f a c e s a t t h e c r a c k a r e j u s t as s e n s i t i v e t o gas a s f r e e s u r f a c e s , b u t s e e a n e f f e c t i v e l y much l o w e r a m o u n t o f g a s p e r u n i t area t h a n e x t e r n a l surfaces. This explanation not only accounts q u a l i t a t i v e l y f o r t h e m a g n i t u d e o f t h e e x p o s u r e e f f e c t s on c r a c k surface photovoltages but a l s o accounts f o r the effects 1

1

8

h

- l f

8

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of molecular hydrogen. The l a t t e r has a low s t i c k i n g c o e f f i c i e n t on f r e e s u r f a c e s and bounces o f f , but m o l e c u l e s h e a d i n g i n t o t h e c r a c k become t e m p o r a r i l y t r a p p e d , a s shown i n F i g u r e 3, w i t h a c o r r e s p o n d i n g l y longer r e s i d e n c e time i n c o n t a c t w i t h the c r a c k surfaces. The b e h a v i o u r o f t h e E P R s i g n a l i n t h e s e two r e s p e c t s ( r e l a t i v e l y slow f o r oxygen, slow but r e l a t i v e l y l a r g e f o r hydrogen) i s q u i t e analogous to t h a t o f the c r a c k s u r f a c e s b a r r i e r s and s t r o n g l y s u g g e s t s t h a t EPR c e n t r e s a r e p r e s e n t i n c r a c k s . A s i n g l e p r e p a r e d c r a c k h a s t o o l o w an EPR s i g n a l t o be detected. H o w e v e r t h e h y p o t h e s i s was t e s t e d b y c r e a t i n g a l a r g e number o f c r a c k s i n a s p e c i m e n by m u l t i p l y i n d e n t i n g i t w i t h a diamond p o i n t . T h i s p r o d u c e s some c r u s h e d p a r t i c l e s , w h i c h were removed by u l t r a s o n i c c l e a n i n g , and i n a d d i t i o n a s e t o f c r a c k s . T h e EPR s i g n a l f r o m s u c h s p e c i m e n s was m e a s u r e d a s a f u n c t i o n o f the l o a d Ρ a p p l i e d t o the p o i n t , and the results a r e shown i n F i g u r e 4 . Note t h a t the s i g n a l S i s p r o p o r t i o n a l t o P */ . Now s t u d i e s o f transparent s p e c i m e n s h a v e shown b o t h t h e o r e t i c a l l y a n d e x p e r i m e n t a l l y ( 7 ) t h a t the c r a c k r a d i u s r i s p r o p o r t i o n a l to P '. Therefore 1

3

2

S

ce p ^

3

=

( ρ Φ ) 2

oc

r

2

α

A

i . e . t h e EPR s i g n a l i s p r o p o r t i o n a l t o t h e c r a c k a r e a A. T h i s i s good c o n f i r m a t i o n o f the h y p o t h e s i s . I t was shown Ci)# b y s c a n n i n g e l e c t r o n microscope s t u d i e s o f f r a c t u r e s u r f a c e s , t h a t m i c r o c r a c k s were more p r e v a l e n t t h a n p r e v i o u s l y r e a l i s e d , and i n p a r t i c u l a r were n o t i n f r e q u e n t l y f o u n d t o be p r e s e n t u n d e r s t e p s , a s shown s c h e m a t i c a l l y i n F i g u r e 5 . By c o m p a r i n g t h e number o f s p i n s w i t h t h e s u r f a c e areas of the cracks i n the i n d e n t a t i o n experiments, the s p i n d e n s i t y was shown ( 4 J t o be i n t h e r e g i o n o f 1 s p i n p e r few c r a c k s u r f a c e a t o m s . From c o m p a r i s o n o f c r u s h e d powder s i g n a l i n t e n s i t i e s and s u r f a c e areas, one r e q u i r e s c r a c k s t o be p r e s e n t i n a s u r f a c e region of area F such that the crack surface area i s i n the region of F / 4 . From i n s p e c t i o n o f s c a n n i n g e l e c t r o n microscope p i c t u r e s , t h i s is r e a d i l y p o s s i b l e . T h e t e m p e r a t u r e b e h a v i o u r o f t h e EPR s i g n a l was close to T " which i s u s u a l l y c h a r a c t e r i s t i c of localised centres. However no s u c h c e n t r e s c o u l d be p o s t u l a t e d which f i t t e d the e v i d e n c e . I t was c o n c l u d e d t h a t the c e n t r e s were i n f a c t l o c a l i s e d s t a t e s on the s u r f a c e s o f t h e c r a c k s . The r e a s o n s f o r l o c a l i s a t i o n t a k i n g p l a c e were as f o l l o w s . 1

14.

HANEMAN

Figure 3.

Semiconductor

Schematic showing entry and temporary entrapment of hydrogen molecules in split

Load

0.8 1.2 2.0 3.0 1—m

Figure 4.

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Surfaces Studies

1

1

(Kgm)

4.0

5-0

60

7.0

8-0

1

ι

1

1

ι

Graph of EPR signal height, averaged per indentation, vs. 4/3 power of load of diamond point indenter

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( a ) Figure 5. Schematic of crack under step

d

(Â)

Figure 6. Wave function overlap (10) as function of spacing of nuclei, for various atomic wavefunctions. At bulk spacing of St, 0.235 nm, 2a = J.34 for sp functions. 3

Figure 7.

Schematic of crack

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Surfaces Studies

163

When two s u r f a c e s a r e p l a c e d o p p o s i t e each o t h e r i n c l o s e p r o x i m i t y as i n a c r a c k , t h e wave f u n c t i o n s o f the s u r f a c e e l e c t r o n s on one s i d e can o v e r l a p w i t h those on t h e o t h e r s i d e . The charge d e n s i t y c o n t o u r s o f o u t e r e l e c t r o n s on S i s u r f a c e atoms have been computed ( 8 , 9 ) . They become v e r y s m a l l (few p e r c e n t o f maximum v a l u e ) a t a d i s t a n c e o f about 0.25 nm from the n u c l e u s , so t h a t o v e r l a p a t a s p a c i n g o f more t h a n 0.5 nm becomes v e r y s m a l l . A s i m i l a r c o n c l u s i o n i s r e a c h e d from c a l c u l a t i o n s (10) o f wave f u n c t i o n o v e r l a p a t two s p a c i n g s performed p r e v i o u s l y . As shown i n F i g u r e 6 the o v e r l a p i s most f o r t h e c a s e o f i n - l i n e ρ o r b i t a l s , but even i n t h i s c a s e i t becomes s l i g h t a f t e r about 0.5 nm. Hence i n o r d e r f o r a p p r e c i a b l e e f f e c t s t o o c c u r , the s p a c i n g between t h e s u r f a c e s must be l e s s t h a n about 0.5 nm. (These remarks r e f e r t o whereas the i m p o r t a n t q u a n t i t y i s where Η i s the i n t e r a c t i o n H a m i l t o n i a n . The l a t t e r i s h a r d e r t o c a l c u l a t e but i t s range w i l l be s i m i l a r t o t h a t of < ψ / ψ > ) . T h i s f i g u r e e n a b l e s us t o make a q u a n t i t a t i v e c h e c k . F o r s p e c i a l c o n t r o l l e d c r a c k s s t u d i e d by X - r a y t r a n s m i s s i o n topography (6) , t h e jaws o f an a p p r o x i m a t e l y 0.5 mm l o n g c r a c k c o u l d be as l i t t l e as 1.5 nm a p a r t , due presumably t o s l i g h t s t e p mismatch ( t h e r e was a m e a s u r a b l e shear o f about 2 nm a t t h e j a w s ) , o r perhaps t o m i c r o s c o p i c ( o r d e r Angstroms) d e b r i s . The shape o f the c r a c k s i d e s f o r such c a s e s i s not r e a d i l y c a l c u l a b l e e x a c t l y , but l i e s between s t r a i g h t and parabolic. L e t h a l f the s p a c i n g o f the s i d e s be y, a t a d i s t a n c e χ from t h e c r a c k t i p , as i n F i g u r e 7. Then f o r s t r a i g h t s i d e s y = αχ where α i s a c o n s t a n t . T a k i n g the above c a s e , y = 0.8 nm a t χ = 0.5 mm, we o b t a i n α = 1.6 χ ΙΟ"" . Then a t y = 0.25 nm, χ = 0.16mm i . e . , 30% o f t h e c r a c k has a s p a c i n g o f l e s s t h a n 0.5 nm. T h i s c a s e o f s t r a i g h t s i d e s i s extreme. For a more l i k e l y p a r a b o l i c c a s e we put 6

y =

ax

2

and, w i t h t h e former boundary c o n d i t i o n , one deduces a = 3.2 χ 1 0 ~ m . A t y = 0.25 nm, χ * 0.28 mm, i . e . 60% o f the c r a c k has a s p a c i n g o f l e s s t h a n 0.5 nm. Hence i n g e n e r a l about 30-60% o f t h e c r a c k i s i n a c o n d i t i o n of wavefunction overlap. I f the microcracks are not much d i f f e r e n t from t h e c o n t r o l l e d c r a c k s , t h e r e i s t h e r e f o r e ample scope f o r an EPR s i g n a l c o r r e s p o n d i n g t o s p i n s on about 10% o f t h e c r a c k a r e a , as d i s c u s s e d , p r o v i d e d t h e r e i s something l i k e 1 u n p a i r e d s p i n per s e v e r a l s u r f a c e atoms i n t h e o v e r l a p 3

_1

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region. M o s t m i c r o c r a c k s a r e much s h o r t e r t h a n 0 . 5 nm, s o t h a t t h e o v e r l a p e x t e n t w o u l d be e v e n g r e a t e r . However t h e y a r e p r o d u c e d u n d e r r o u g h e r c o n d i t i o n s t h a n t h o s e u s e d i n t h e above e x p e r i m e n t s where t h e m a t e r i a l was v e r y c a r e f u l l y s e p a r a t e d . Hence i n most n a t u r a l l y o c c u r r i n g m i c r o c r a c k s , shear and o t h e r d i s t o r t i o n s a r e l i k e l y t o be g r e a t e r , s o t h a t t h e jaw o p e n i n g s m i g h t be r e l a t i v e l y l a r g e r t h a n by e x t r a p o l a t i n g f r o m the above f i g u r e s . Even s o , a s i z a b l e p r o p o r t i o n of a c r a c k must have s i d e s w i t h i n a b o u t 0.5 nm, i f i t is o n l y a s m a l l f r a c t i o n o f a mm l o n g . A. Properties of Overlap Regions. We now c o n s i d e r the p r o p e r t i e s of the o v e r l a p r e g i o n s i n detail. A t the v e r y base o f t h e s p l i t , as i n d i c a t e d schematically i n Figure 8(a), we h a v e a t r a n s i t i o n f r o m a h e a l e d r e g i o n (5>) t o o n e w i t h a f i n i t e g a p . Now t h e two s i d e s o f t h e s p l i t a r e s u b j e c t t o t h r e e effects: (a) the s e p a r a t i o n i n c r e a s e s towards the mouth; (b) t h e o r i g i n a l r e g i s t r y between them on an a t o m - t o - a t o m b a s i s becomes l o s t due t o s h e a r , F i g u r e 8(b), s i n c e even the c a r e f u l l y prepared c o n t r o l l e d s p l i t s showed m e a s u r e a b l e s h e a r i n t h e n o n - h e a l e d region. I n t h o s e c a s e s v a l u e s r a n g e d f r o m 2.4 to 8 χ 10"* r a d i a n s , g i v i n g a b o u t 3 nm d i s p l a c e m e n t a t t h e m o u t h o f a 0 . 5 mm s p l i t , a n d t h u s m o r e t h a n 0 . 1 nm o v e r most o f i t . Hence atoms a r e no l o n g e r o p p o s i t e their pre-cleavage neighbours; (c) contact regions e x i s t at the edges o f t o p o g r a p h i c a l i r r e g u l a r i t i e s such as s t e p s on the f a c e s o f t h e s p l i t , and t h e s e are c e n t e r s of pressure causing deformation of the material, Figure 8(c). The r e s u l t o f t h e s e t h r e e e f f e c t s i s t h a t , e v e n i n t h e 0 . 5 nm r e g i o n o f s e p a r a t i o n , the set of displacement v e c t o r s to opposite s u r f a c e n e i g h b o u r s f o r any s u r f a c e atom v a r i e s f r o m s i t e to s i t e . Any i n d i v i d u a l atom i s t h u s s u b j e c t to f o r c e s f r o m atoms on t h e o p p o s i t e s u r f a c e , b u t t h e s e f o r c e s v a r y from s i t e to s i t e s i n c e the shear d i s p l a c e m e n t and s e p a r a t i o n v a r y ( i n c r e a s e towards the jaw m o u t h ) , and t h e s t r e s s d i s p l a c e m e n t v a r i e s a l s o , b e i n g c e n t r e d a t somewhat r a n d o m p o i n t s a n d l i n e s . This i s a s i t u a t i o n of v a r y i n g p o t e n t i a l which i s of t h e k i n d c o n s i d e r e d b y A n d e r s o n (11) and o t h e r s . It i s hence a p o s s i b l e p r a c t i c a l example o f A n d e r s o n localised sites. 6

Β. Localised States. The c r i t e r i o n f o r A n d e r s o n l o c a l i s a t i o n has been d i s c u s s e d by v a r i o u s a u t h o r s (11-16). T h e c o n s e n s u s i s t h a t l o c a l i s a t i o n occurs i f the h a l f width of the d i s t r i b u t i o n of

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165

o o o o o o o o o ο

ο

ο

ο

ο

ο

ο

Ο

ο

ο

ο

ο

ο

(a)

Γ

/

/

/

/

/

/

/

/

/

/

/

I

(c)

(

/

(b) Figure 8. (a) Schematic of hase of crack showing transition from healed to separated region, (b) Top view of crack, showing shear of one side with respect to other, (c) End view of crack showing pressure at contact between schematic protrusion on one side and corresponding gap on other.

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p o t e n t i a l s i s about g r e a t e r than t h e w i d t h o f t h e band r e s u l t i n g i f a l l p o t e n t i a l s were t h e same. The normal s t a t e band has a w i d t h o f about 0.3eV (8). The p o t e n t i a l d i s t u r b a n c e due t o t h e v a r y i n g o v e r l a p can be e s t i m a t e d f o r comparison w i t h t h i s f i g u r e . A t t h e base o f t h e c r a c k t h e o v e r l a p i s s t r o n g , c l o s e t o b u l k , and w i l l be i n t h e r e g i o n o f t h e s i n g l e bond energy i n b u l k S i , namely 2.37eV (Γ7). When t h e s e p a r a t i o n o f the c r a c k s i d e s i s more than 0.5-0.6 nm, t h e o v e r l a p approaches z e r o . Hence t h e range o f p o t e n t i a l d i s t u r b a n c e i s over 2eV, which e a s i l y exceeds 0.3eV and thus f i t s t h e l o c a l i s a t i o n c r i t e r i o n . Although the d i s t u r b a n c e v a r i e s somewhat m o n o t o n i c a l l y as one moves i n a d i r e c t i o n χ a l o n g t h e s p l i t , s i n c e s e p a r a t i o n and shear d i s p l a c e m e n t a r e b o t h f u n c t i o n s o f x, t h i s i s broken up by t h e v a r i o u s p r e s s u r e s c o n t a c t p o i n t s d i s t r i b u t e d over t h e s p l i t a r e a . At the stress o r i g i n , e l a s t i c d i s p l a c e m e n t up t o about 0.1 nm a r e p o s s i b l e , corresponding t o order l e V p o t e n t i a l disturbance. T h i s r e d u c e s r o u g h l y r a d i a l l y from t h e c e n t e r , f a l l i n g t o z e r o a t a d i s t a n c e o f about 0.2 mm. (5). However, t h e s t r e s s c e n t e r s a r e spaced more c l o s e l y than t h i s , so t h a t every s i t e i s s u b j e c t to a d i f f e r e n t r e s u l t a n t s t r e s s . Hence t h e s i t e p o t e n t i a l s a t a g i v e n χ a r e no l o n g e r i d e n t i c a l . I f t h e above c o n c e p t s a r e c o r r e c t , then t o agree w i t h experiment they must l e a d t o a temperature dependence o f t h e paramagnetism χ t h a t i s c l o s e t o T ~ . T h i s was c o n s i d e r e d i n d e t a i l ( £ ) . Without r e p e a t i n g the m a t h e m a t i c a l d e t a i l s , t h e r e s u l t (18) was o b t a i n e d , t a k i n g i n t o account c o r r e l a t i o n c o r r e c t i o n s t o one electron theory, that 1

where w i s t h e s e l f i n t e r a c t i o n energy o f two e l e c t r o n s on t h e one s i t e ( c o r r e l a t i o n c o r r e c t i o n ) , Δ i s the w i d t h o f t h e (assumed c o n s t a n t - d e n s i t y ) band, g i s t h e g v a l u e , u t h e Bohr magneton and N t h e number of states (orthogonal, l o c a l i s e d ) . The r e s u l t h o l d s a l s o when i n t e r a c t i o n s between e l e c t r o n s on n e i g h b o u r i n g s i t e s a r e t a k e n i n t o a c c o u n t (19) , p r o v i d e d t h i s term i s n o t u n r e a s o n a b l y l a r g e . S i n c e w /kT i s much g r e a t e r t h a n 21n2, t h e f o r m u l a g i v e s t h e paramagnetism as i n v e r s e l y p r o p o r t i o n a l t o T, as r e q u i r e d by t h e measurements. A v a r i e t y o f o t h e r p r o p e r t i e s o f t h e s i g n a l were a l s o e x p l a i n e d such as i t s s l i g h t v a r i a b i l i t y i n w i d t h between samples, which i s now due t o d i f f e r e n c e s i n 0

B

s

0

14.

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d i s t r i b u t i o n s o f c r a c k s between s a m p l e s . Since l o c a l i s e d s t a t e s s t i l l h a v e a s p r e a d o v e r a number o f s i t e s , the explanation i s c o n s i s t e n t with the spread n a t u r e o f t h e wave f u n c t i o n o f t h e E P R c e n t r e a s deduced from the absence o f d i s c e r n i b l e h y p e r f i n e structure [ e x p e c t e d f r o m 5.7% a b u n d a n t S i (29)] and t h e e f f e c t s o f a l l o y i n g Ge i n t o t h e S i (20). I n t h e c a s e o f a m o r p h o u s f i l m s t h e EPR s i g n a l (21), b o t h f o r Ge a n d S i , i s u s u a l l y i d e n t i c a l w i t h t h a t from crushed samples. T h i s i s now e x p l a i n e d a s d u e to l o c a l i s e d c e n t r e s on the s u r f a c e s of the m i c r o s c o p i c a g g r e g a t e s making up t h e f i l m s . Here of course l o c a l i s a t i o n i s endemic t h r o u g h the b u l k whereas i n the c r y s t a l l i n e m a t e r i a l s the l o c a l i s a t i o n on the c r a c k s u r f a c e s i s due t o t h e s p a t i a l l y v a r y i n g o v e r l a p o f f o r c e s between o p p o s i t e faces. Gallium Arsenide Some t i m e a g o i t was f o u n d t h a t v a c u u m c r u s h e d s a m p l e s o f G a A s a n d A l S b , when e x p o s e d t o o x y g e n a t l i q u i d n i t r o g e n t e m p e r a t u r e s , d i s p l a y e d a n EPR s i g n a l due t o 01 i o n s a d s o r b e d o n t h e s u r f a c e s (22) . In the c a s e o f A l S b , h y p e r f i n e s t r u c t u r e was d e t e c t a b l e , which e n a b l e d i d e n t i f i c a t i o n o f the a d s o r p t i o n s i t e s as A l , a n d t h e c o n c l u s i o n t h a t t h e d a n g l i n g b o n d wave f u n c t i o n o n t h e A l was o v e r 90% ρ l i k e . A l t h o u g h h f s was n o t d e t e c t a b l e f r o m t h e G a A s , t h e s i g n a l was o v e r a l l v e r y s i m i l a r t o t h a t from A l S b , and s i m i l a r c o n c l u s i o n s were made. We h a v e a t t e m p t e d s i m i l a r e x p e r i m e n t s o n s i n g l e c r y s t a l GaAs c l e a v e d a t uhv and e x p o s e d t o o x y g e n a t low t e m p e r a t u r e s . A s i g n a l was a t f i r s t n o t d e t e c t a b l e but a f t e r a low p r e s s u r e microwave f r e q u e n c y d i s c h a r g e o c c u r r e d i n t h e o x y g e n , a n EPR s i g n a l was d e t e c t e d , w h i c h l o o k e d s i m i l a r t o t h a t f r o m GaAs powder b u t a l i t t l e n a r r o w e r . Some p r e l i m i n a r y r e s u l t s , y e t t o be c o n f i r m e d , s u g g e s t h y p e r f i n e s t r u c t u r e a l s o was o b s e r v a b l e . T h e 01 s i g n a l o c c u r s r e a d i l y o n η t y p e c r u s h e d GaAs b u t o n l y w e a k l y o n ρ type m a t e r i a l , which i n d i c a t e s s t r o n g l y t h a t the Fermi l e v e l a t the s u r f a c e i s d i f f e r e n t f o r η and ρ t y p e material. Recent evidence suggests t h a t i t i s pinned w i t h r e s p e c t to the band edges f o r η t y p e but not f o r ρ type m a t e r i a l (23). Our r e s u l t s are t h u s c o n s i s t e n t w i t h t h i s s i n c e t h e 01 e n e r g y l e v e l i s a p p a r e n t l y c l o s e t o the F e r m i l e v e l on η type m a t e r i a l , about midway b e t w e e n t h e b a n d e d g e s a t t h e s u r f a c e . Hence e l e c t r o n s can t r a n s f e r from the bulk to the O2 * but n o t i n ρ t y p e m a t e r i a l where t h e F e r m i l e v e l a t t h e

168

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s u r f a c e m u s t b e w e l l b e l o w t h e 01 l e v e l , i . e . near t h e v a l e n c e band e d g e . The l a t t e r i s e x p e c t e d if t h e r e i s n o p i n n i n g f o r ρ t y p e m a t e r i a l , a n d t h i s was concluded from e m i s s i o n data (23). R e c e n t l y v a r i o u s t h e o r e t i c a l c a l c u l a t i o n s have b e e n c a r r i e d o u t f o r GaAs (110) surfaces. (These are the e a s y c l e a v a g e s u r f a c e s , and p r e s u m a b l y p r e d o m i n a t e in crushed m a t e r i a l ) . A self consistent pseudopotential c a l c u l a t i o n f o r an u n r e c o n s t r u c t e d s u r f a c e (24) f i n d s t h e d a n g l i n g b o n d c h a r g e t o be l o c a l i s e d o n t h e As s u r f a c e and t h e empty s t a t e o n t h e G a , w h i c h t h u s c a n a c c e p t t h e 01 a d s o r b a t e . However t h e w a v e f u n c t i o n was m o r e s t h a n ρ l i k e , c o n t r a r y t o t h e E P R d a t a (22), and o t h e r e s t i m a t e s b a s e d o n r e c o n s t r u c t i o n (25,26). I t seems f r o m t h i s t h a t r e c o n s t r u c t i o n p r o b a b l y o c c u r s o n GaAs s u r f a c e s a n d i t w o u l d b e u s e f u l t o c a r r y o u t a s e l f c o n s i s t e n t c a l c u l a t i o n f o r such a c a s e . Molecular

Oxygen

Spectra

T h e EPR s p e c t r u m o f m o l e c u l a r o x y g e n c o n t a i n s o v e 100 l i n e s d u e t o t r a n s i t i o n s i n r o t a t i o n a l l e v e l s . Some o f t h e s e may b e a f f e c t e d i f t h e g a s i s i n t h e adsorbed s t a t e . A n a t t e m p t was made t o c h e c k t h i s b u t u n e q u i v o c a l e f f e c t s c o u l d n o t be e s t a b l i s h e d . However by w o r k i n g a t p r e s s u r e s l o w e r t h a n t h o s e u s e d p r e v i o u s l y , many new l i n e s w e r e d i s c o v e r e d (27) . In a l l , over 220 l i n e s w e r e o b s e r v e d a t 1 0 " t o r r ~ T n the magnetic f i e l d r a n g e 0-1 T e s l a . Some o f t h e s e b r o a d e n e d r a p i d l y as t h e p r e s s u r e i n c r e a s e d , a c c o u n t i n g f o r t h e i r non d e t e c t i o n i n p r e v i o u s e x p e r i m e n t s a t h i g h e r pressures. I t h a s b e e n r e p o r t e d (28) t h a t f o r oxygen adsorbe o n a m o r p h o u s c a r b o n , t w o new l i n e s w i t h t h e same g value but greater l i n e w i d t h s appeared at the p o s i t i o n s of the p r e v i o u s f r e e oxygen l i n e s . The authors a t t r i b u t e d t h e s e t o c h e m i s o r b e d and p h y s i s o r b e d oxygen. However no s u c h e f f e c t s were f o u n d w i t h crushed s i l i c o n . A r e a s s e s s m e n t o f t h e r e s u l t s on c a r b o n showed t h a t t h e o b s e r v e d e f f e c t s c o u l d b e e x p l a i n e d by t a k i n g i n t o a c c o u n t t h e b r o a d e n i n g o f o x y g e n l i n e s when t h e g a s i s i n t h e f i n e p o r e s o f t h e carbon, together with the e f f e c t s of overmodulation. 3

Magnetic-Field-Induced Fragments

ultra small

Transitions

i n Small

Magnetite

I n some E P R e x p e r i m e n t s o n s e m i c o n d u c t o r s i n h i g h vacuum, a s t a i n l e s s s t e e l cap covered a g l a s s bowl i n which a g l a s s - c o a t e d r o d c r u s h e d

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a specimen w h i l e i n u l t r a h i g h vacuum. The powder was t h e n t i p p e d o u t i n vacuo i n t o a q u a r t z appendage tube p r o t r u d i n g i n t o a microwave c a v i t y . On some o c c a s i o n s a l a r g e background s i g n a l was o b s e r v e d (29) w i t h peculiar properties. I t was t r a c e d e v e n t u a l l y t o contaminant m i c r o s c o p i c p a r t i c l e s coming from t h e s t e e l cap. The p a r t i c l e s t u r n e d o u t , somewhat u n e x p e c t e d l y , t o be F e 0 i * , m a g n e t i t e . An example o f the u n u s u a l phenomena i s shown i n F i g u r e 9. Note t h a t a p o r t i o n o f t h e b r o a d r e s o n a n c e t o t a l l y d i s a p p e a r s a t a c e r t a i n c r i t i c a l v a l u e o f the sweep magnetic f i e l d . T h i s c r i t i c a l v a l u e v a r i e s over a l a r g e range f o r a few degrees change i n t e m p e r a t u r e . The phenomena were e x p l a i n e d (29) as due t o a Verwey t r a n s i t i o n , i . e . the o r d e r i n g o f F e and F e i o n s on o c t a h e d r a l s i t e s below a c r i t i c a l t e m p e r a t u r e , giving ferrimagnetism. This t r a n s i t i o n i s apparently f i e l d dependent. The m a g n e t i t e (Fe^O^) presumably forms d u r i n g vacuum bakeout o f t h e Fe203 n o r m a l l y p r e s e n t on the s t a i n l e s s s t e e l s u r f a c e s , and t i n y p a r t i c l e s a r e knocked o f f by c o n t a c t w i t h t h e semi c o n d u c t o r powder. T h i s i s o f i n t e r e s t s i n c e t h e n a t u r e o f the o x i d e on s t a i n l e s s s t e e l s u r f a c e s i n baked vacuum systems has not p r e v i o u s l y been determined. The temperature s e n s i t i v i t y o f t h e v a l u e o f the c r i t i c a l f i e l d made i t p o s s i b l e t o e s t i m a t e when e q u i l i b r i u m temperature was a t t a i n e d i n powder samples c o n t a i n i n g t h e m a g n e t i t e f r a g m e n t s . Volumes o f S i o r GaAs o f about 0.1 cc took a t l e a s t 20 o r more minutes t o e q u i l i b r a t e when c o o l e d by about 50 Κ degrees v i a a sapphire rod. 3

2 +

3 +

S p i n Dependent C o n d u c t i v i t y i n S i l i c o n In the p r e s e n c e o f a magnetic f i e l d b o t h c u r r e n t c a r r i e r s and paramagnetic c e n t r e s a r e s u b j e c t t o Zeeman energy s p l i t t i n g . The i n t e r a c t i o n between c u r r e n t c a r r i e r s and f i x e d c e n t r e s t h e n c o n t a i n s a s p i n - s p i n term which depends on whether t h e i n c i d e n t and f i x e d spins i n v o l v e d a r e p a r a l l e l o r a n t i p a r a l l e l . I f microwave r a d i a t i o n i s used t o a l t e r t h e p o p u l a t i o n ( s p i n up and s p i n down) o f t h e c u r r e n t c a r r i e r s t h e n the p a r a l l e l and a n t i p a r a l l e l c o n t r i b u t i o n s t o the i n t e r a c t i o n a r e a l t e r e d i n r e l a t i v e magnitude, l e a d i n g t o a s l i g h t change i n t h e c o n d u c t i v i t y ( 3 0 ) . By m o n i t o r i n g the c o n d u c t i v i t y as a f u n c t i o n o f magnetic f i e l d , w h i l e i r r a d i a t i n g w i t h microwaves and u s i n g o p t i c a l pumping t o g e t s u f f i c i e n t c u r r e n t c a r r i e r s , i t was found p o s s i b l e t o d e t e c t

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Figure 9. Temperature variation of broad signal (12 db). fa) 103°K, (b) 98°K. Note signal disappearance at critical field, (c) 95° K. Critical field has shifted, (a) For Τ = 90°K, no signal was observed, i.e. critical field shifted to near 0 Kg.

14.

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Figure 10. Lock-in amplifier detection of two peaks in conductivity of illuminated Si sample during magnetic field scan. (Microwave power modulated at 35 Khz, 500 nV amplifier sensitivity).

paramagnetic r e c o m b i n a t i o n c e n t r e s i n v e r y low c o n c e n t r a t i o n a t S i s u r f a c e s . We have used t h i s method on samples o f d i f f e r e n t s u r f a c e t r e a t m e n t and found a t l e a s t two d i f f e r e n t c e n t r e s t o be p r e s e n t , as shown by t h e s p l i t peaks i n F i g u r e 10. N e i t h e r of t h e c e n t r e s i s d e t e c t a b l e by normal EPR t e c h n i q u e s . A number o f i n t e r e s t i n g phenomena have been found i n the b e h a v i o u r w i t h s t r o n g i l l u m i n a t i o n which have n o t y e t been c l a r i f i e d , b u t t h e e v i d e n c e shows t h a t t h e c e n t r e s a r e a t o r v e r y near t o t h e s u r f a c e , p o s s i b l y a s s o c i a t e d with t o p o g r a p h i c a l i r r e g u l a r i t i e s induced by t h e s u r f a c e f i n i s h i n g t r e a t m e n t . Acknowledgements E x c e l l e n t e x p e r i m e n t a l work was c a r r i e d o u t by B.P. Lemke and J . Menendes-Cortinas. T h i s work was s u p p o r t e d by t h e A u s t r a l i a n Research Grants Committee and t h e U.S. Army Research and Development Group (Far E a s t ) under Grant No. DA-CRD-AFE-S92-544-71-6168.

Literature Cited 1.

Haneman, D. "Characterization of Solid Surfaces", Chapter 14, p. 337. ed. P.F. Kane and G.B. Larrabee (Plenum Press, 1974).

172

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

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Adrian, F . J . , J. Colloid. Interface S c i . (1968), 26, 317. Haneman, D., Jap. J. Appl. Phys. Suppl. (1974), 2, (Pt.2), 371. Lemke, B.P. and Haneman, D., submitted to Phys. Rev.; Phys. Rev. Lett (1975), 35, 1379. Haneman, D., Grant, J.T.P. and Khokhar, R . L . , Surface S c i . (1969), 13, 119. Khokhar, R.U. and Haneman, D., J. Appl. Phys. (1973). 44, 1231 Lawn, B.R. and Fuller, E.R., J. Mat. S c i . (1975), 10, 2016. Schluter, M. Chehkowsky, Louie, S.G. and Cohen,M.L. Phys. Rev. (1975), B12, 4200. Appelbaum, J.A. and Hamann, D.R., Phys. Rev. Lett. (1974), 32, 225; Phys. Rev. (1973), B8, 1777. Haneman, D. and Heron, D . L . , "The Structure and Chemistry of Solid Surfaces", Chapter 24, ed. G.A. Somorjai, (Wiley, New York, 1969). Anderson, P.W., Phys. Rev. (1958), 109, 1492. Mott, N . F . , P h i l . Mag. (1970), 22, 7; (1969), 19, 835. Cohen, M.H., J . noncryst. Solids, (1970), 4, 391 Ziman, J., J. Phys. C. (1969), 2, 1230. B a l l , M.A., J. Phys. C. (1971), 4, 1747. Cohen, M.H., Fritzsche, H. and Ovshinsky, S.R., Phys. Rev. Lett. (1969), 22, 1065. Sanderson, R.T., "Chemical Bonds and Bond Energy", (Academic Press, New York, 1971). Kaplan, T . A . , Mahanti, S.D. and Hartmann, W.M., Phys. Rev. Lett. (1971), 27, 1796. M i l l e r , D . J . , to be published. M i l l e r , D.J. and Haneman, D., Surface S c i . (1972) 33, 477. Brodsky, M.H., Title, R.S., Weiser, K. and Pettit, G.D., Phys. Rev. (1970), B1, 2632. M i l l e r , D.J. and Haneman, D . , Phys. Rev. (1971), B3, 2918. Gregory, P.W. and Spicer, W.E., Phys. Rev. (1971), B13, 725. Chelikowsky, J. and Cohen, M.H., Phys. Rev. Lett. (1976). Pandy, K.C. and P h i l l i p s , J . C . , Phys. Rev. L e t t . , (1975), 34, 1450. Harrison, W.A., Surface S c i . (1976), 55, 1. Lemke, B.P. and Haneman, D. to be published. Seymour, R.C. and Wood, J . C . , Surface S c i , (1971), 27, 605. Lemke, B.P. and Haneman, D., J. Magn. Res. (1976), 22. Lepine, D . , Phys. Rev. (1972), B6, 436.

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