ON CHANGES OF SECONDARY EMISSION BY RESONANT TUNNELING VIA ADSORBATES

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ON CHANGES OF SECONDARY EMISSION BY RESONANT TUNNELING VIA ADSORBATES

J. Halbritter

To cite this version:

J. Halbritter. ON CHANGES OF SECONDARY EMISSION BY RESONANT TUNNEL- ING VIA ADSORBATES. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-315-C2-317.

�10.1051/jphyscol:1984271�. �jpa-00223985�

JOURNAL D E PHYSIQUE

Colloque C2, suppl6ment a u n02, Tome 45, f6vrier 1984 page C2-315

ON CHANGES OF SECONDARY EMISSION BY RESONANT TUNNELING VIA ADSORBATES

J. Halbritter

Kernforsckungszentrum Kar~sruhe, IK 11, Postfach 3640, 0-7500 KarZsruhe, F.R.G.

Rgsurn6 - La c o n t a m i n a t i o n de l a s u r f a c e m o d i f i e l a p r o b a b i l i t k de t r a n s - m i s s i o n P ( E ) d ' k l e c t r o n s d basses g n e r g i e s , causant l ' a p p a r i t i o n de s t r u c - t u r e s de rksonance dans l e s courbes de d i s t r i b u t i o n d ' e n e r g i e . Selon l a v a r i a t i o n de P ( E ) 1 1 6 m i s s i o n d 1 6 1 e c t r o n s secondaires p e u t augmenter ou d i m i n u e r .

A b s t r a c t - S u r f a c e c o n t a m i n a t i o n s m o d i f y t h e t r a n s m i s s i o n p r o b a b i l i t y P ( E ) o f s l o w e l e c t r o n s , y i e l d i n g s t r u c t u r e s i n t h e energy d i s t r i b u t i o n curves and enhancing o r r e d u c i n g t h e secondary e l e c t r o n e m i s s i o n .

Most s u r f a c e s s t u d i e d i n secondary e l e c t r o n e m i s s i o n (SEE) a r e contaminated by se- v e r a l monolayers (ML) o f adsorbates /I/. These adsorbates n o t o n l y change t h e work f u n c t i o n by d i p o l e moments b u t a l s o change t h e t r a n s m i s s i o n p r o b a b i l i t y P ( E ) , as shown i n F i g . 1. Hz0 and hydrocarbons, which make up t h e usual s u r f a c e c o n t a m i n a t i o n , show f o r slow e l e c t r o n s ( E k i n 5 10 eV) resonances w i t h l a r g e c r o s s s e c t i o n

a > 10-16 cm2 / 2 , 3 / . These r e s o n a n t s t a t e s h y b r i d i z e w i t h c o n d u c t i o n e l e c t r o n s o f t h e m a t e r i a l m o d i f y i n g s t r o n g l y t h e t r a n s m i s s i o n p r o b a b i l i t y P ( E ) by r e s o n a n t tunne- l i n g /4,5/. So, l ML Hz0 v i a i t s l a r g e e l a s t i c c r o s s s e c t i o n eel ( E k i n = 0 ) > 10-16cm2 enhances P(clean,s=@) = 0 t o P(H20,s-@) - 1. P(Hz0) enhances SEE up t o 50% as ob- s e r v e d f o r Nb-OHHOH s u r f a c e s /1,4,5/. L i k e H20, s a t u r a t e d hydrocarbons have

a e l 10-16cm2 b e i n g l a r g e compared t o t h e i n e l a s t i c c r o s s s e c t i o n o i n / 3 / , enhancing so P ( E > @ + ~ . 2 eV) and SEE. I n contrast,unsaturated,polymeri z d hydrocarbons have 5 .

l a r g e i n e l a s t i c c r o s s s e c t i o n oin(E > 0.2 eV) > 1 0 - l 6 cm , w h ~ c h reduces

P(&>1$+0.2 eV) by about 50%. T h i s r e i k e s SEE up t o 50% /1,4-6/. V i a P(E) changes, t h e adsorbates n o t o n l y change SEE b u t a l s o e l e c t r o n energy d i s t r i b u t i o n curves (EEDC) i n p h o t o e l e c t r o n spectroscopy /I/, e s p e c i a l l y UPS / 7 / , and i n f i e l d e m i s s i o n / 4 / .

F i g . 1 - The t r a n s i t i o n of the conduction band Ec(x) t o t h e vacuwn l e v e l 41 y i e l d s a transnrission probability P(E) for hot eZectrons of energy r approaching, the surface from the l e f t . The p l o t s show P I E ) of a clean surface and an adsorbate covered sur- face where e l e c t r o n i c resonances w i t h n e g l i g i b l e i n e l a s t i c processes have been assu- med. The transmission probability l i n k s the i n t e r n a l d i s t r i b u t i o n function Niisl t o

dNtiel dNiisl due i s l

= d s P ( E ) and ~ j = ids' - d E , bzing transmitted i n t o the uacuwn.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984271

C2-316 JOURNAL DE PHYSIQUE

Unsaturated, p o l y m e r i z e d hydrocarbon adsorbates develop o u t o f s a t u r a t e d hydrocarbons /5/ by n e u t r a l d e s o r t i o by t h e impact o f s l o w ( 5 10 eV) e l e c t r o n s , because o f t h e i r l a r g e o 2 lom1! cmq. T h i s phenomenon e x p l a i n s t h e r a d i a t i o n damage and t h e c h e m i s o r p t i o n o f hydrocarbons of t h e background vacuum. I n poor vacuum e l e c t r o n o r photon impact y i e l d s c h e m i s o r p t i o n of hydrocarbons i n excess o f 10 mm t h i c k n e s s /8/.

I - SURFACE CONTAMINATION

Surfaces handled i n p o o r vacuum o r a i r show a d s o r p t i o n l a y e r s c o n s i s t i n g m a i n l y of H 0, CO .. and e s p e c i a l l y hydrocarbons. Such d i r t y s u r f a c e s cannot b e s t u d i e d w i t h t 6 e u s u a l s u r f a c e methods, because t h e UHV and t h e p a r t i c l e s o r photons used, desorbe and modify t h e a d s o r p t i o n l a y e r s d r a s t i c a l l y . So o n l y a s m a l l number o f " d i r t y "

system - see F i g . 2 - have been s t u d i e d s y s t e m a t i c a l l y /1,7/, where one s h o u l d keep i n mind, t h a t t h e UHV and t h e p h o t o n impact had a l r e a d y caused d e s o r p t i o n and r a d i a t i o n damage.

Water i s a common a d s o r b a t e b u t r a r e l y f o u n d i n UHV on s u r f a c e s a t room temperature.

B u t some s u r f a c e d e f e c t s b i n d H20 n o n d i s s o c i a t i v e l y - l i k e l y by hydrogen bonds -

s t r o n g l y enough. So, e.g., on Nb205 a t 300 K, an o r d e r e d Nb-OHHOH l a y e r has been i n f e r r e d from XPS and SEEDC measurements /1,4,5/.

Hydrocarbons, e.g., an o i l f i l m , i s commonly f o u n d a t 300 K on s u r f a c e s i n UHV systems h a v i n g a t h i c k n e s s o f a b o u t 2 ML o r more, depending on vacuum and p a r t i c l e impact. Whereas atom impact s p u t t e r s adsorbates away, e l e c t r o n s and photons cause l o c a l i z e d e l e c t r o n i c e x c i t a t i o n s e s p e c i a l l y v i a secondary e l e c t r o n s because o f t h e l a r g e c r o s s s e c t i o n a > 1 0 - l 6 cm2 o f such e l e c t r o n a t t a c h m e n t s t a t e s /2,3/. I n un- s a t u r a t e d , p o l y m e r i z e d hydrocarbons t h e s e e x c i t a t i o n s a r e d e l o c a l i z e d and t h u s de- s o r p t i o n o f H..., i . e . r a d i a t i o n damage, i s u n l i k e l y / 5 / . I n c o n t r a s t , i n s a t u r a t e d hydrocarbons t h e l o c a l i z a t i o n o f e x c i t a t i o n s f a v o r s desorp ? o n es e c i a l l y o f t h e 8

most m o b i l e species, namely H, w i t h a y i e l d up t o 1. So, 10f5 e'/cm c o n d i t i o n , i . e . , p o l y m e r i z e and dehydrogenate 1 ML o f hydrocarbons as d i s c u s s e d i n more d e t a i l i n Ref. 5. T h i s r a d i a t i o n damage depends on temperature and on chemical s u r r o u n d i n g b l o c k i n g c h e m i c a l l y o r space w i s e t h e d e s o r p t i o n of atoms /4,5/. A s p e c i a l con- sequence o f t h e r a d i a t i o n damage o f weakly adsorbed hydrocarbons i s t h e p r o d u c t i o n o f r e a c t i v e p r o d u c t s becoming t h e n permanently adsorbed. Because o f l a r g e r o v a l u e s t h i s i s s t r o n g e r f o r l o n g c h a i n hydrocarbons and slow e l e c t r o n s /5/. So, i n p o o r vacuum, secondary e l e c t r o n s produce a hydrocarbon l a y e r above 10 nm t h i c k n e s s /5,8/.

I n f o r m i n g t h e more r a d i a t i o n r e s i s t a n t hydrocarbons,oin and t h e e l e c t r o n a f f i n i t y i n c r e a s e /1,5/.

I 1 - ENHANCED EMISSION BY RESONANT TUNhELING

As i n d i c a t e d i n F i g . 1, molecules w i t h a >> crin(~) a d j a c e n t t o a m a t e r i a l w i t h e l e c t r o n i c s t a t e s a t E w i l l enhance t h e $ r a n s m i s s i o n c o e f f i c i e n t P ( E ) . Hz0 has, l i k e o t h e r molecules w i t h a d i p o l e moment l a r g e r t h a n 1.6 Oebye, a resonance a t E k i n " 0

F i g . 2 - XPS spectrwn of a NbjSn sur- face coated w i t h about 3 nm oxide

Sn 3D and about 2 ML of oxygen and carbon

: *

C I S (SE) contain 80% o f a22 photoeZec-

~ b 3 ~ trons and are i n t h i s "dry" Nb205

Nb3P t y p i c a l for emission o u t of a con- duction band E, about 4 eV below +.

The work function $I i s defined by

+ 1117 1165 1163 1161 1139 1137 ' ^{7b0 660 560 660 360 260 160 6 the intercept the steepest}

eE [ e ~ ] with the a x i s .

> 10-15 cm2 / 2 / . F o r Nb o x i d i z e d i n H20, on t h e r e s u l t i n g Nb2O5 a Nb-OHHOH :;:0hrFe!o; system e x i s t s a c c o r d i n g t o YPS measurements /1,5/. T h i s l a y e r y i e l d s t h e i n F i g . 3 shown SEEDC /5/, w h i c h has t h e same shape as a e l ( E ) o f H20 /2/ and y i e l d s an enhancement of t h e secondary y i e l d up t o 50% and a r e d u c t i o n o f I$ by about 0.3eV /1,5/. Small, s a t u r a t e d hydrocarbon molecules have l o c a l i z e d e l e c t r o n attachment s t a t e s w i t h a,!(> 0.2 eV) - 10-16 cm2 > o i n . Hence a l s o such contaminants enhance P ( ~ - p 0 . 2 e V ) , I n l i n e w i t h experiments / 6 / . T h i s P ( E ) enhancement i s s m a l l e r t h a n t h e i n c r e a s e o f P(E=@) by 1, which occurs f o r Hz0 a d s o r p t i o n .

I 1 1 - REDUCED EMISSION BY RESONANT CAPTURING

As d i s c u s s e d above, i m p i n g i n g e l e c t r o n s o r photons v i a t h e i r secondary e l e c t r o n s c r e a t e a s u r f a c e l a y e r o f u n s a t u r a t e d , p o l y m e r i z e d hydrocarbons The e l e c t r o n a t t a c h - ment s t a t e s i n such hydrocarbons a r e n a r r o w l y spaced and have l a r g e i n e l a s t i c c r o s s s e c t i o n o i n ( E k j n > 0 . 2 ) = eel = 10-15 cm2, which reduces P ( E ) up t o 50% f o r 1 ML.

The l a r g e ^{G i n} d e s c r i b e t h e energy d i s s i p a t i o n by v i b r o n i c p r o g r e s s i o n , which ends i n t h e l o w e s t attachment s t a t e . T h i s l o w e s t s t a t e has an energy s e v e r a l eV below t h e vacuum l e v e l , e.g. f o r g r a p h i t e @ - 4.7 eV. Hence t h e h o t r e s o n a n t l y c a p t u r e d e l e c - t r o n s d i s s i p a t e t h e energy needed t o escape i n t o t h e vacuum r e d u c i n g so t h e secon- d a r y y i e l d by 50%, as shown i n F i g . 3. Because eel ( > 0.2 eV) o f such hydrocarbons i s monotonic depending on E, t h e SEEDC w i l l be reduced smoothly, as shown i n F i g . 3.

References

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9 C 1 L J I

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/8/ BOLLER K., HAELBICH R.-P., HOGREFE H., J m K W., KUNZ C., N u c l . I n s t r . Meth. - 208 (1983) 273

Fig. 3 - SEEDC of Nb which has been oxidized i n H2O. This Nb i s coated w i t h about 3 nm Nb205 and 2 nrn C and 0 containing absorba- t e s . The 2 peaks a t @ and @+8 eV are due t o t h e transmission co- e f f i c i e n t P ( E ) of SE peaking a t

' '

t h e resonant s t a t e s of HZO.

With e l e c t r o n impact (200 eV, 100 p ~ c i n ~ ) the SEEDC's decrease w i t h dose. The decrease i s due t o i n e l a s t i c processes of the e l e c t r o n i c e x c i t a t i o n s i n t h e polymerized /5/, unsaturated hydrocarbons, which are produced by e l e c t r o n impact.

WI 1147 1145 1143 1141 1139 1137

B.E. IeVJ