BARRIER PROFILES I N CLEAN AND DOPED TUNNEL JUNCTIONS

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BARRIER PROFILES I N CLEAN AND DOPED

TUNNEL JUNCTIONS

R. Floyd, W. Nelson, D. Walmsley

To cite this version:

R. Floyd, W. Nelson, D. Walmsley.

BARRIER PROFILES I N CLEAN AND DOPED

JOURNAL DE P H Y S I Q ~

Colloque C6, supplkment au no 8, Tome 39, aoQt 1978,

page (26-610

B A R R I E R P R O F I L E S I N CLEAN AND DOPED TUNNEL JUNCTIONS

R.B. Floyd, W.J. Nelson and D.G. Walmsley

School of PhysicaZ Sciences, New University of Ulster, Cozeraine, Northern Ireland BT52 ISA

Rdsum6.- Nous avons f a i t l e s c a l c u l s en u t i l i s a n t l a mCthode de WKB e t nous l e s avons f a i t accorder avec l a conductance mesurge des j o n c t i o n s t u n n e l s (Al-I-Pb), l e s unes propres e t l e s a u t r e s avec l e s . impuretds. Les b a r r i s r e s dBduites pour l e s j o n c t i o n s propres s o n t asymgtriques 1,5 eV, $2 = 4,5 eV): Nous expliquons c e s r d s u l t a t s p a r une d i f f e r e n c e de c o n c e n t r a t i o n deg e s p s c e s hydroxyles ndga- t i v e s s t r a v e r s l a b a r r i s r e . Une b a r r i g r e h a u t e ( 1 10 eV) e t mince (- 2,5 A) d d c r i t l a couche organi- que des j o n c t i o n s avec l e s impuretds.

Abstract;- We have measured t h e conductances of c l e a n and doped A1-I-Pb tunnel j u n c t i o n s and f i t t e d them by a WKB c a l c u l a t i o n i n which t h e b a r r i e r parameters were v a r i e d . I n undoped j u n c t i o n s t h e bar- r i e r s o deduced i s h i g h l y asymmetric 1.5 eV, $,

-

4.5 eV). We e x p l a i n t h e r e s u l t s by a concgn- t r a t i o n g r a d i e n t of f r a c t i o n a l l y charged n e g a t i v e hydroxyl s p e c i e s i n t h e b a r r i e r . A t h i n (%2.5 A) high (% 10 eV) b a r r i e r , a s s o c i a t e d w i t h t h e o r g a n i c l a y e r , reproduces t h e measured conductances i n doped j u n c t i o n s .

F i s h e r and G i a e ~ e r / I / , i n t h e i r p i o n e e r i n g paper, determined t h e c u r r e n t - v o l t a g e response of A1-I-A1 sandwiches i n t h e range 0 t o % 2V. Theyfound a roughly l i n e a r c u r r e n t i n c r e a s e a t low (< 50 mV) b i

a

s , exponential i n c r e a s e a t high (> 200 mV) b i a s and an e x p o n e n t i a l i n c r e a s e of r e s i s t a n c e w i t h insu- l a t o r t h i c k n e s s . A l l t h e s e o b s e r v a t i o n s were shown t o f i t w e l l t o an a n a l y s i s of t u n n e l i n g due to Holm

121.

Comment was made on t h e inhomogeneity of t h e

t u n n e l b a r r i e r , which was e v i d e n t from a s l i g h t rec- t i f i c a t i o n p r e s e n t a t a l l v a l u e s of b i a s . The o n l y s u r p r i s e was a discrepancy between t h e Holm thick- n e s s r e q u i r e d t o e x p l a i n t h e r e s u l t s and t h e v a l u e s

We have measured t h e dynamic conductance of c l e a n and d e l i v e r a t e l y doped t u n n e l j u n c t i o n s and attempted t o determine t h e b a r r i e r parameters by f i t t i n g t h e r e s u l t s t o t h e conductance c a l c u l a t e d i n t h e BDR f a s h i o n . The f i t s were found by t r i a l a n d e r r o r v a r i a t i o n s of t h e parameters i n t h e c a l c u l a - t i o n .

The total c u r r e n t at b i a s V is

deduced by c a p a c i t a n c e measurements. The l a t t e r were

For e v a l u a t i o n of t h e t u n n e l p r o b a b i l i t y a t r a p e z o i - g r e a t e r by a f a c t o r of t h r e e and t h e discrepancy was

d a l b a r r i e r i s chosen t o r e p r e s e n t t h e i n s u l a t o r i n r e s o l v e d by a t t r i b u t i n g an e f f e c t i v e mass of 119 t o

a c l e a n j u n c t i o n . I n doped samples an o r g a n i c bar- t h e t u n n e l i n g e l e c t r o n s .

r i e r of h e i g h t @ and t h i c k n e s s S i s added. L a t e r , more e x t e n s i v e and r e l i a b l e d a t a ena-

Tunnel sandwiches of the type A1-I-Pb were f a - b l e d Rowel1 131 t o i d e n t i f y a common f e a t u r e of near-

b r i c a t e d by w e l l - e s t a b l i s h e d techniques. A wide va- l y a l l tunnel j u n c t i o n s : t h e conductance could be

r i e t y of o r g a n i c s p e c i e s w i l l chemisorb t o t h e f r e -

_-

roughly approximated by a parabola with i t s a x i s

s h l y grown oxide (Walmsley e t a l . 151). By varying o f f s e t from z e r o b i a s . Subsequently Brinkman, Dynes

the exposure, any coverage up t o s a t u r a t i o n (% 1 and Rowel1 / 4 / ( h e r e a f t e r BDR) c a r r i e d through a nu-

monolayer) i s p o s s i b l e . The convention adopted i n m e r i c a l e v a l u a t i o n of t h e conductance f o r a trape-

t h i s work i s t h a t p o s i t i v e b i a s corresponds t o t h e z o i d a l b a r r i e r shape. A problem which BDR encounte-

l e a d (Pb) e l e c t r o d e being p o s i t i v e . r e d was t h e f a i l u r e of t h e i r model t o e x p l a i n l a r g e

An e x a m ~ l e of measured and f i t t e d conductan- (Q 250 mV) o f f s e t s of the conductance minimum. They

c e s i s shown i n f i g u r e 1. The f i t t i n g d a t a f o r a suggested a more d i s t o r t e d p o t e n t i a l was r e q u i r e d

number of samples i s shown i n Table I. P a r t i c u l a r t o account f o r t h e behaviour and noted a c o r r e l a t i o n

a t t e n t i o n was paid t o o b t a i n i n g t h e c o r r e c t abso- -

between the o f f s e t s and o r g a n i c contamination of t h e 1

l u t e v a l u e of t h e conductance (Q- ) a t zero b i a s

j u n c t i o n s . 0

and a l s o a t t h e b i a s l i m i t s .

R e f e r e n c e s ( a

I

( b l F i g . 1 : T r a p e z o i d a l and b i l a y e r p o t e n t i a l b a r r i e r s r e p r e s e n t i n g c l e a n and doped j u n c t i o n s . DOPE NONE NONE NONE NONE LAURIC A C I D ACETIC A C I D PHENOL m-CRESOL P H E N O L ( 3 0 % ) PHENOL (90%) T a b l e I

Even t h e c l e a n samples show a s t r o n g l y asymrne- t r i c b a r r i e r . I n c o n j u n c t i o n w i t h e v i d e n c e from t h e d e r i v a t i v e of t h e conductance and c a p a c i t a n c e mea- surements we a t t r i b u t e t h e asymmetry t o a concen- t r a t i o n g r a d i e n t of f r a c t i o n a l l y charged n e g a t i v e hydroxyl s p e c i e s i n c o r p o r a t e d w i t h i n t h e b a r r i e r du- r i n g i t s growth.

F i g . 2 : Measured ( s o l i d l i n e ) and f i t t e d (open cir- c l e s ) conductances of A1-I-Pb j u n c t i o n doped t o sa- t u r a t i o n w i t h l a u r i c a c i d (no = 62.2 ohms). Dashed l i n e shows asynnnetry of measured conductance.

/ I / F i s h e r , J . C . and G i a e v e r , I., J. Appl. Phys.

32

(1961) 172.

/ 2 / Holm, R , , J. Appl. Phys.

2

(1951) 569. / 3 / Rowell, J . M . , Tunneling Phenomena i n S o l i d s

(New York : Plenum) 1969.

/ 4 / Brinkman, W.F., Dynes, R.C. and Rowell, J.M., J. Appl. Phys.

9

(1970) 1915.

151 Walmsley, D.G., McMorris, I.W.N. and Brown, N. M.D., S o l i d S t a t e Comun.

Ifi

(1975) 663.

This work was s u p p o r t e d by t h e Science Rese-ar- ch Council and a S t u d e n t s h i p (RBF) from t h e Nor-