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EXPERIMENTAL AND THEORETICAL RESULTS OF RECTIFICATION MEASUREMENTS IN AN STM
P. Cutler, T. Feuchtwang, Z. Huang, T. Tsong, H. Nguyen, A. Lucas, T.
Sullivan
To cite this version:
P. Cutler, T. Feuchtwang, Z. Huang, T. Tsong, H. Nguyen, et al.. EXPERIMENTAL AND THEO-
RETICAL RESULTS OF RECTIFICATION MEASUREMENTS IN AN STM. Journal de Physique
Colloques, 1987, 48 (C6), pp.C6-97-C6-100. �10.1051/jphyscol:1987616�. �jpa-00226819�
JOURNAL DE PHYSIQUE
Colloque C6, suppikment a u n O 1 l , Tome 48, novernbre 1987
EXPERIMENTAL AND THEORETICAL RESULTS O F RECTIFICATION MEASUREMENTS IN AN STM
*(I) P.H. Cutler
7
T.E. ~ e u c h t w a n ~ * ,Z.
Huang, T.T. Tsong, H. Nguyen,
A.A. ~ u c a s + , + + ( ~ ) a n d T.E. Sullivan (3)
Department of Physics, The Pennsylvania S t a t e University, University Park, Pennsylvania 16802, U.S.A.
(1) AT&T Bell Laboratories, Murray Hill, NJ 07974, U.S.A. and
Department of Physics, t h e Pennsylvania S t a t e University, University Park, Pennsylvania 16802, U.S.A.
(2) IBM Almaden Research Center, San Jose, C A 95120, U.S.A. and F a c u l t b Universitaires Notre-Dame d e l a Paix, B-5000 Namur, Belgium
(3)RCA Solid S t a t e Technology C e n t e r , Summerville, NJ 08540, U.S.A.
A b s t r a c t
-
Recent measurements of d c c u r r e n t - v o l t a g e c h a r a c t e r i s t i c s of Scanning Tunneling Microscope j u n c t i o n s have confirmed t h e i r expected high r e c t i f i c a t i o n property. We have e x p l o i t e d t h i s p r o p e r t y t o s t u d y r e c t i f i c a t i o n a t i n f r a r e d f r e q u e n c i e s . A l a s e r beam of l i n e a r l y p o l a r i z e d l i g h t is focused on an STM j u n c t i o n and t h e r e s u l t i n g d c b i a s induced a c r o s s t h e j u n c t i o n by t h e a l t e r n a t - i n g , asymmetrical t u n n e l c u r r e n t is d e t e c t e d . R e s u l t s have been obtained t h a t e x h i b i t r e c t i f i c a t i o n f o r W-Si, W-Ni, and W-Au STM j u n c t i o n s . T h e o r e t i c a l c a l c u l a t i o n s of t h e r e c t i f i c a t i o n r a t i o and I ( V ) c u r v e s u s i n g a hyperboloidal- tip-planar-base model of t h e STM j u n c t i o n show good agreement w i t h W-Au e x p e r i - mental r e s u l t s .Recc~tt c~easurenrerits[l] of dc curren.t-voltage c h a r a c t e r i s t i c s o l a Scantlitrg Tunnel- ing Elicroscope j u n c t i o n s have c o t r f i r ~ ; ~ e d t t i e i r cxpected tiigli r c c t i t i c a t i o t l property.
we ltave ~ , r c v i o u s l y p r o p ~ s e d e x p l o i t i ~ ; ~ t h i s property t o ( i ) s t u d y r c c t i f i c a t i o t l a t i u l r a r e d a~rd o p t i c a l f r e q u e n c i e s and t o ( i i ) a r r i v e a t a procedural d e f i n i t i o n of an c l c c t r o ~ ~ t u n n e l i n g ti1ae[2]. \de envisaged focusittg a l ~ s e r beam ot l i r r e a r l y polarized l i g h t on ail STtI j u n c t i o n and d e t e c t i n g tile dc b i a s i~~Liuced a c r o s s Lire j u l ~ c r i o ~ l by tibe a1terrlati11g, asynraetrical tunnel c u r r c l l t . Tile asyulrtetry may be or ~ ; c o l . l ~ t r i c a l , late- r i a l and/or t1lerr:ral o r i g i n . The dc b i a s sllould vanish a t s u f f i c i e n t l y high l a s e r frequency f o r f i x e d tip-to-surface d i s t a n c e o r when withdrawing t h e t i p away from the s u r f a c e f o r f i x e d l a s e r frequctlcy. Tilt average e l e c t r o n tuiltieling t h e i s then one-
half t h e i n v e r s e cut-off frequency observed i n such a l a s e r r e c t i f i c a t i o t r exparinent.
Tire r e s u l t s of t h e tunneling t i r . ~ e e x p e r i u e n t a r e discussed i l l a second paper pub- l i s h e d i n t h e s e proceedings. Siclilac expericients have a l r e a d y been conducted suc- c e s s f u l l y with metal-whisker diodes and have r e s u l t e d i n t h e r e c e n t r e d e f i n i t i o n crf t h e areter[3]. The neu o p p o r t u n i t y o f f e r e d by ST11 i s t h e co::crollability of the j u n c t i o n and i t s l~licroscopic s i z e , e n t a i l i n g hi&h r c s p o n s i v i t y j 4 1 . ;Je have obtaineci experiwental r e s u l t s t h a t e x h i b i t r e c t i f i c a t i o n Lor \;-Si, i r - N i , and \I-Au STPl junc- t i o n s . T h e o r e t i c a l c a l c u l a t i o n s of t h e r e c t i i i c a t i o n r a t i o and I(V) curves using a Ilyperboloidal-tip-planar-base model of t h e ST11 j u n c t i o n s have been aone and show good agreement with W-Au experimental r e s u l t s . Plore e x a c t three-dir*ensior.al e l e c t r o n tunneling c a l c u l a t i o n s have z l s o been done using a s c a t t e r i a g cneorecic approach r e c e n t l y developed by Lucas, e t al[Li]. These r e s u l t s can be used t o d e c r r u i n e more a c c u r a t e l y t h e geometrical and thermal asymnetry of t h e tunneling c u r r e n t , p r e d i c t e d i n t h e one-dimensional tunneling c a l c u l a t i o n s [ 5 - 7 1 .
I n t h e next s e c t i o n , we d i s c u s s b r i e f l y t h e STM a s a r e c t i f i e r and t h e mechanisms f o r r e c t i f i c a t i o n . The e x p e r i n e n t a l r e s u l t s obtained f o r a W-Au(ll0) STPI j u n c t i o n a r e compared w i t h c a l c u l a t i o n s f o r a h y p e r b o l o i d a l t i p p l a n a r base model STEL.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987616
JOURNAL DE PHYSIQUE
11. RECTIFICATION I N THE SRl
The p o s s i b i l i t y o f p o s i t i o n i n g and con- t r o l l i n g t h e t i p i n a n STM t o w i t h i n a f r a c t i o n of angstrom from t h e anode s u r -
1
f a c e h a s r e c e n t l y allowed w p e r i m e n t a l - ists t o o b t a i n a c c u r a t e s t a t i c I-V char- a c t e r i s t i c of t h e j u n c t i o n f o r a c o n t r o l - l e d s e r i e s of t i p - t o - s u r f a c e d i s t a n c e s [ l ] . Nore r e c e n t l y , Kuk and c o l l a b o r a t o r s a t B e l l L a b o r a t o r i e s have o b t a i n e d s t a t i c I-V c h a r a c t e r i s t i c s f o r b o t h metal-metal and 'metal-semiconductor STM j u n c t i o n s . F i g u r e 1 i l l u s t r a t e s two s u c h I-V c u r v e s f o r a k'(tip)-Au ( l l O ) ( b a s e ) SR1 j u n c t i o n . The c u r v e s a r e f o r two d i f f e r e n t v a l u e s of t h e t i p - s u r f a c e s e p a r a t i o n "s". X t is s e e n t h a t t h e e l e c t r o n c u r r e n t always f l o w s e a s i e r i n f o r w a r d (FB, i.e.. t i p n e g a t i v e ) t h a n i n t h e r e v e r s e o r backward b i a s (BB) d i r e c t i o n . Whereas h a r d l y any asymmetry i s s e e n i n t h e I-V c u r v e f o r s = 8H i n t h e b i a s v o l t a g e range ( V ~ < I V , t h e FB/BB c u f r e n t r a t i o i s a p p r o x i m a t e l y Fig. 1. The measured I-V c h a r a c t e r i s - 2 f o r s = 10A a tI v I
= 3 V . Measured I-V t i c s f o r a W - ~ ~ ( 1 1 0 ) STM j u n c t i o n . The c h a r a c t e r i s t i c s f o r W-Ni(100) and c u r r e n t i is i n a r b i t r a r y u n i t s . The U-Si(II1) STM j u n c t i o n s e x h i b i t s i m i l a r tip-base d i s t a n c e s f o r each c u r v e was c u r r e n t asymmetry, w i t h t h e h i g h e s t rec- deduced from t h e s l o p e of t h e e x p e r i - t i f i c a t i o n r a t i o o b t a i n e d f o r t h e metal- m e n t a l I ( V ) c u r v e and t h e c u r r e n t - scnriconductor combination. These r e s u l t s v o l t a g e t u n n e l i n g r e l a t i o n s h i p . d e n i o n s t r a t e t h a t a s t a t i c a l l y unbiased o rb i a s e d STM j u n c t i o n c a n a c t as a d i o d e , c a p a b l e o t r c c t i l i c a t i o n , Iiar~;loiric g e n e r a t i o n and d e t e c t i o r ~ . \le rlvw c x a a ~ i ~ i e tlhc pos- s i b l e c:luses of tile observed asynlclietry i n t h e s t a t i c 1-V c u r v e s .
Iti t h e Sl'bl, t h e c h a r g e t r a n s p o r t i s due t o e l e c t r o n s t u n n e l i n g a c r o s s tllc c l a s s i c a l l y f o r b i d d e n vacuunl b a r r i e r b c t w e e ~ i t i p a n d . b a s e . Thc t u n n e l i n g p r o c e s s i t i v o l v e s cssen- t i a l l y t l i r e a c o ~ n p o ~ ~ e n t s : i ) t h e i i l i t i a l and f i n a l s t a t e s which depend upon t h e band s t r u c t u r e of t h e u r a t c r i a l , i i ) t h e i r o c c u p a t i o n a l p r o b a b i l i t i e s , w l ~ i c l ~ a r c tenlpera- t u r e d e p e ~ ~ d e r ~ t and i i i ) t h e s h a p e of t h e t u n n e l b a r r i e r wl~icli i t i t u r n i s a f u i l c t i o ~ ~ o t tlic s h a p e and geometry of t h e t i p and base e l e c t r o d e s . C o n s e q u e a t l y , rite c u r r e t i t osyn1l:retry observed a t a f i x e d gap w i d t h s ntust o r i g i n a t e fro111 one o r titore of t h e t h r e e p o s s i b l e c a u s e s d i s c u s s e d below, naniely m a t e r i a l , g e o u ~ e t r i c a l and thermal asyawctry.
A j u n c t i o n c o n s i s t i n g of two d i f f e r e n t m a t e r i a l s ( i . e . , d i f f e r e n t work f u n c t i o n s ) i n - p l i e s c h a t t h e b a r r i e r s h a p e is asymmetric a t z e r o b i a s and hence is d i f f e r e n t f o r PB and BH. T h i s c a u s e s d i f f e r e n c t r a n s c ~ i s s i o n p r o b a b i l i t i e s f o r t h e sawe b i a s v o l t a g e l e a d i n g t o a s y m n e t r i c I-V c h a r a c t e r i s t i c s . T h i s e f f e c t is expected t o be more pro- nounced f o r a m e t a l - s e n i c o n d u c t o r combination because t h e energy g a p i n t h e seuicon- d u c t o r r e s u l t s i n a s t a t e d e n s i t y d i f f e r e n t from t h e m e t a l f o r e n e r g i e s which c o n c r i - b u t e dominantly t o t h e t u n n e l i n g c u r r e n t [ 4 ] .
T h i s e f f e c t w i l l o c c u r when t h e r e i s a temperature d i f f e r e n c e between t h e two e l e c t r o d e s and hence a d i f f e r e n t e l e c t r o n o c c u p a t i o n of t h e s t a t e s i n v o l v e d i n tun- n e l i n g from t h e v i c i n i t y of t h e Fermi level. T h e r e f o r e , t h e f o r w a r d c u r r e n t would exceed t h e backward one i f , s a y , t h e t i p t e m p e r a t u r e was h i g h e r t h a n t h e base. F o r l a r g e t e m p e r a t u r e d i f f e r e n c e s between t i p and b a s e , t h e s i t u a t i o n i s analogous t o
o r d i n a r y t h e r m i o n i c e m i s s i o n . Thermally a s s i s t e d f i e l d e m i s s i o n , t h a t i s a combina- t i o n of t h e r m a l and g e o m e t r i c a l asymmetry e f f e c t s , was invoked[5,6,9-111 a s a n expla- n a t i o n f o r t h e r e c t i f i c a t i o n mechanism i n m e t a l w h i s k e r s d i o d e s .
C. GEOMETRICAL ASYMMETRY.
It h a s p r e v i o u s l y been s h o w n [ 5 , 6 , l l ] t h a t t h e geometry of a p o i n t - c o n t a c t o r w h i s k e r d i o d e l e a d s t o a n a s y m n e t r i c a l t u n n e l i n g b a r r i e r even i n t h e a b s e n c e of a n y m a t e r i a l asymmetry. The e f f e c t i s understood by n o t i n g t h a t t h e s t a t i c e l e c t r i c f i e l d i s s t r o n g e r n e a r t h e p o i n t e d convex s u r f a c e of t h e w h i s k e r t i p t h a n a t t h e p l a n a r s u r f a c e of t h e base. T h i s means t h a t t h e n o r m a l l y t r i a n g u l a r - l i k e vacuum t u n n e l b a r r i e r f o r a p l a n a r - p l a n a r j u n c t i o n w i l l t e n d t o b u c k l e inward f o r FB and b a l l o o n outward f o r BB[12] t o always m a i n t a i n h i g h e r e l e c t r i c f i e l d s a t t h e t i p . Hence, t h e t u n n e l b a r r i e r on FB is e f f e c t i v e l y t h i n n e r t h a n i n BH, l e a d i n g t o a s y m m e t r i c a l t u n n e l i n g p r o b a b i l i t i e s . The i m p o r t a n c e of t h i s e f f e c t has a l r e a d y been d e m o n s t r a t e d f o r two g e o m e t r i e s , namely t h e h y p e r b o l o i d a l t i p mode1[5,6,9-111 and f o r t i p modeled by a h e m i s p h e r i c a l p r o t r u s i o n on a p l a n a r s u r f a c e [ l ] . I n t h e f o r m e r c a s e t h e r e s u l t i n g asymmetry of t h e a x i a l t u n n e l c u r r e n t was c a l c u l a t e d i n a s i m p l i f i e d one-dimensional t u n n e l i n g model and shown t o be non-neg1:gible e v e n f o r r a t h e r weak g e o m e t r i c a l asymmetry of j u n c t i o n s w i t h t i p r a d i i (S100A) l a r g e compared t o t y p i c a l S W v a l u e s (220A). I n R e f e r e n c e 8, t h e new three-dimensional t u n n e l i n g t h e o r y was used t o c a l c u l a t e t h e c u r r e n t f o r a s p h e r i c a l p r o t r u s i o n model of t h e STE1.
The r e s u l t s o b t a i n e d f o r f r e e e l e c t r o n models of t h e e l e c t r o d e s show t u n n e l i n g cur- r e n t s s h a r p l y peaked a l o n g t h e t i p a x i s and w i t h a n a n g u l a r d i s p e r s i o n a b o u t e q u a l t o t h e t i p r a d i u s . The p o t e n t i a l s c a t t e r i n g f o r m a l i s v f o r t u n n e l i n g through l o c a l - i z e d non-separable t h r e e - d i m e n s i o n a l b a r r i e r s s h o u l d p r o v i d e more a c c u r a t e q u a n t i t a - t i v e i n f o r m a t i o n on t h e role and i m p o r t a n c e of g e o m e t r i c a l asymmetry i n r e a l i s t i c models of t h e a c t u a l STPl m i c r o s c o p i c j u n c t i o n s .
We have used t h e s i m p l i f i e d one-dimensional t u n n e l i n g t h e o r y w i t h t h e h y p e r b o l o i d a l model of t h e tip[5,6.9-111 t o c a l c u l a t e r e c t i f i c a t i o n p r o p e r t i e s and I-V c h a r a c t e r i s - t i c s of W-W, W-Au and W-Ni STM junctions:To compare w i t h t h e e x p e r i m e n t a l r e s u l t s o f f i g . 1 , we e x h i b i t i n f i g u r e s 2a and Zb, t h e c a l c u l a t e d I-V c u r v e s f o r a W-Au junc-
Fig. 2. C a l c u l a t e d I-V c h a r a ~ t e r i s t i c s f o r t h e h y p e r b o l o i d a l t i p - p l a n a r base model.
The g a p d i s t a n c e i s ( a ) s = 5A, and T = 300°K f o r e a c h e l e c t r o d e ; f o r W, $ = 4.5eV, FF = 7.88 e V and f o r Au, $ = 4.48 eV and EFo= 5.52 eV. Theocurves correspond t o t h r e e v e l u e s of t i p r a d i u s , p = 5 &), 10 A (---) and 20 A (....-)
.
I n Fig. b , s = 1 0 A.C6-100 JOURNAL DE PHYSIQUE
t i o n w i t h d i f f e r e n t t i p r a d i i P and gap ~ e p a r a t i o n ~ s . Although t h e I-V curves are a l l asymmetric f o r even t h e c l o s e s p a c i n g of s = 5A, t h e e f f e c t of t h e g e o m e t r i c a l o asymmetry is d r a m a t i c a l l y enhanced when t h e tip-base s e p a r a t i o n i s increased t o IOA.
Moreover, t h e r a t i o of FB/BB i s a b o u t 2.5 a t a b i a s
[vI
= 3V f o r s = 10A and a t i p r a d i u s of 10A. T h i s i s i n good agreement w i t h e x p e r i m e n t a l o b s e n r a t i o n s . T h i s q u a l i t a t i v e and s e m i - q u a n t i t a t i v e agreement based o n t h e s i m p l i f i e d one-dimensional t u n n e l i n g model s u g g e s t s i) t h a t t h e f u l l . m u l t i p 1 e image i n t e r a c t i o n s [ 6 , 1 1 1 , which a r e i n c l u d e d i n t h e model a r e i m p o r t a n t f o r a more a c c u r a t e form of t h e tunneling b a r r i e r ; i i ) t h a t t h e c u r r e n t i s h i g h l y l o c a l i z e d a l o n g t h e axis, a r e s u l t which is s u b s t a n t i a t e d by t h e three-dimensional t u n n e l i n g c a l c u l a t i o n s [ 8 ] , and iii) t h a t t h e r e c t i f i c a t i o n i n t h e s e s e r i e s of e x p e r i m e n t s i s p r i m a r i l y g e o m e t r i c a l i n o r i g i n . T h i s f o l l o w s because t h e work f u n c t i o n s of U and Au used i n t h e c a l c u l a t i o n s a r e n e a r l y e q u a l (i.e.,4w
= 4.5 e V and @AU = 4.48 eV) and T-300°K was assumed f o r each e l e c t r o d e , t h u s e f f e c t i v e l y e l i m i n a t i n g material and t e m p e r a t u r e asymmetry as c o n t r i - b u t i o n s t o t h e mechanfsm f o r r e c t i f i c a t i o n .F i n a l l y , t h e f o l l o w i n g experiments are c u r r e n t l y i n p r o g r e s s o r are proposed f o r f u r t h e r s t u d y of r e c t i f i c a t i o n c h a r a c t e r i s t i c s of a n STU j u n c t i o n under c o n t r o l l a b l e c o n d i t i o n s :
1. Do i n u l t r a - h i g h vacuum.
2. U s e i d e n t i c a l m e t a l s f o r t i p and anode b a s e , e.g., W, Mo, Au o r N i . 3. I l l u m i n a t e t h e t i p a n d / o r j u n c t i o n w i t h l a s e r r a d i a t i o n from t h e i n f r a -
r e d t o t h e v i s i b l e , and h i g h e r i f p o s s i b l e . 4. Measure t h e I-V c h a r a c t e r i s t i c $ a s a f u n c t i o n .
i ) d c b i a s v o l t a g e i i ) l a s e r i r r a d i a n c e
i i i ) p o l a r i z a t i o n of t h e r a d i a t i o n
i v ) g e o m e t r i c a l p a r a m e t e r s of t h e j u n c t i o n .
One of u s (H.N.) wishes t o g r a t e f u l l y acknowledge t h e guidance and a s s i s t a n c e of D r . Young Kuk and M r . Paul Silverman d u r i n g t h e e x p e r i m e n t a l phase of t h i s work.
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Rev. B: Rapid Commun.
35,
7774 (1987).3. K. M. Evenson, "Frequency Measurements from t h e Microwave t o t h e V i s i b l e , t h e Speed of L i g h t and t h e R e d e f i n i t i o n of t h e Meter," i n Quantum Metrology and Fundamental P h y s i c a l C o n s t a n t s , ed. by P. H. C u t l e r and A. A. Lucas, NATO AS1 S e r i e s B-98, (Plenum P r e s s , New York, 1983).
4 . See, f o r example, P. K. Hansma and J. T e r s o f f , J. Appl. Phys.
2,
R1 (1987) o r G. Binnig and H. Rohrer, S u r f . S c i . 152/153, 17 (1985).5. A. A. Lucas, A. Moussiaux, M. Schmeits and P. H. C u t l e r , Commun. Phys.
2,
169 (1977).6. N. M. Miskovsky, P. H. C u t l e r , T. E. Feuchtwang and A. A. Lucas, I n t e r n . J.
I n f r a r e d and M i l l i m e t e r Waves
2,
739 (1981).7. H. Q. Nguyen, P. H. C u t l e r , T. E. Feuchtwang, N. Miskovsky and A. A. Lucas, Surf. S c i . &I(1985).
8. A. A. Lucas, H. Morawitz, G. R. Henry, J. P. Vigneron, Ph. Lambin, P. H. C u t l e r and T. E. Feuchtwang, B u l l . Am. Phys. Soc. 32, 648 (1987), Abs I 0 7 and t o be p u b l i s h e d , Phys. Rev. B.
9. A. A. Lucas and P. H. C u t l e r , S o l i d S t a t e Comm. l3, 361 (1973).
10. T. E. S u l l i v a n , P. H. C u t l e r and A. A. Lucas, Surf. S c i . 54, 561 (1976);
62,
455 (1977).
11. N. M. Miskovsky, S. J. Shepherd, P. H. C u t l e r , T. E. S u l l i v a n and A. A. Lucas, Appl. Phys. L e t t .
25,
560 (1979);37,
189 (1980).12. See, f o r example, Figs. 2 and 3 i n T. E. Feuchtwang, P. H. C u t l e r , N. M.
Miskovsky and A. A. Lucas i n Quantum Metrology and Fundamental P h y s i c a l C o n s t a n t s , ed. by P. H. C u t l e r and A. A. Lucas, NATO AS1 S e r i e s B-98 (Plenum, New York, 1983).
