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Submitted on 1 Jan 1984
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EXPERIMENT ON THE DYNAMICS OF
TUNNELING THROUGH METAL OXIDE BARRIERS
H. Hübner
To cite this version:
H. Hübner. EXPERIMENT ON THE DYNAMICS OF TUNNELING THROUGH METAL OXIDE BARRIERS. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-279-C6-283.
�10.1051/jphyscol:1984947�. �jpa-00224428�
EXPERIMENT ON THE DYNAMICS OF TUNNELING THROUGH METAL OXIDE BARRIERS
H. Hubner
Institut fur Angewandte Phys-Lk, Universitat TUb-ingen, Auf Dev Movgenstelle 12, D-7400 Tubingen, F.R.G.
Résumé - Une pointe d'émission de champ en tungstène a été utilisée dans un résonateur supraconducteur à 12.. 18 GHz. Les observations indiquent que le courant émis par des pointes oxydées est finement contrôlé par la forme et aussi la charge de la barrière de potentiel de 1'oxyde.
Abstract - A tungsten field emitter has been operated in a superconduc- ting resonator at 12..18 GHz. The observations indicate that the emission current of oxidized tips is sensitively controlled by the shape and thus by the charging of the oxide barrier.
Introduction
The shape of potential barriers, formed by adsorbates or oxides is very sensitive to charging of the dielectric layer and the tunnel cur- rent through these barriers is a sensitive instrument to detect inte- grally changes of the barrier shape. There are many possibilities for the location of charges within a semiconducting or insulating layer, but each of them has a different time constant. Surface states on se- miconductors with time constants of seconds already had been measured in the 50th / 1 / . Faster states are to be expected within the adsorbate or even faster at the metal - adsorbate interface with time constants beyond the microwave frequencies. Thus the comparison of field emission currents at different frequencies seems to be able to seperate the contributions of the various charging mechanisms.
Experimental Arrangement
To learn what difficulties are to be expected, we built up a preceding experiment / 2 / : A tungsten field emitter is operated in a superconduc- ting resonator at 12..18 GHz (cf. Pig.1). The tip is mounted on the axis where a strong E-field concentration occurs with a parallel plate capacitor field configuration. Everytime the needle becomes negative with respect to the opposite wall, an electron bunch will be emitted.
The integral emission current as a function of the field strength is recorded.
In order to get FN-plots one must plot lg(l/E ' ) vs 1/E instead of lg(I/E ) vs 1/E (E = amplitude of the electric field), due to the shape of the electron bunches.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984947
C9-280 JOURNAL DE PHYSIQUE
A cleaning of the tip in this arrangement only can be achieved by heat- ing it with the aid of displacement currents in the strong high fre- quency field of the resonator.
Fig. I : 1 - resonator 2 - wave guide 3 - collector
4 - pumping tube 5 - measuring chamber 6 - feed through
Results
The freshly cleaned tips show smooth FN-plots, which are in agreement with the dc results. Expect very low work functions (around 2 eV),
which are not understood as yet and call for some further investigations.
and a m p l i t u d e s towards lower c u r r e n t s and a b o u t two o r t h r e e o s c i l l a - t i o n s p e r FN-plot. The work f u n c t i o n s a s a f u n c t i o n o f t h e d e g r e e o f c o n t a m i n a t i o n show a s a t u r a t i o n b e h a v i o r and l i e around 3 eV.
I n t h e f o l l o w i n g we w i l l a t t e m p t t o e x p l a i n t h e low work f u n c t i o n s and t h e wavy FN-plots, assamin9 a n o x i d e l a y e r t o b e p r e s e n t . The argumen- t a t i o n w i l l b e c l o s e t o t h e i n t e r p r e t a t i o n i n c a s e o f f i e l d e m i s s i o n i n MOS-structures, where o n e g e t s FN-plots w i t h e x a c t l y t h e same k i n d of m o d u l a t i o n /3/.
D i s c u s s i o n
The most s t a b l e o x i d e o f t u n g s t e n i s BJO3. I t i s a semiconductor w i t h a g a p of 2 . 5 .. 3 . 5 eV, a d i e l e c t r i c c o n s t a n t of 2 0 and a work f u n c t i o n o f 6 . 2 eV. I t grows by a CABRERA VOTT p r o c e s s t o a t h i c k n e s s o f a b o u t 2 nm. I f o n e c o n s t r u c t s a b a r r i e r w i t h t h e s e v a l u e s ( F i g . 4 a ) it seems t o b e u n l i k e l y t h a t such a b a r r i e r w i l l e m i t e l e c t r o n s . But t h i s p i c - t u r e i s t o o s i m p l e : Tungsten forms a l o t of mixed o x i d e s WxOy r a t h e r t h a n a w e l l d e f i n e d o x i d e l a y e r . Thus e x c e s s m e t a l atoms form a n-type
I vacuum
s u r f a c e l a y e r between t h e t u n g s t e n b u l k and t h e o x i d e ( F i g . 4 b ) . These donor s t a t e s c a n b e decharged i n t o t h e m e t a l and t h u s b i a s t h e b a r r i e r by some v o l t s . T h i s l e a d s t o t h e s m a l l work f u n c t i o n s @' d e s p i t e o f t h e i n i t i a l l y h i g h v a l u e o f more t h a n 6 eV a t t h e o x i d e - vacuum i n t e r f a c e . Now a second e f f e c t becomes i m p o r t a n t . The b i a s p u l l s t h e c o n d u c t i o n band below t h e Fermi l e v e l . So t h e e l e c t r o n s i n t h e f i r s t s t e p t u n n e l i n t o t h e c b , p r o p a g a t e t h e r e n e a r l y f r e e and t u n n e l l a t e r t h r o u g h t h e FN-barrier i n t o t h e vacuum. P r o v i d e d t h e mean f r e e p a t h i n t h e c b i s
long enough t o a v o i d s c a t t e r i n g , t h e r e w i l l d e v e l o p a r e s o n a n t e l e c t r o n wave f u n c t i o n . The t u n n e l c u r r e n t now i s no l o n g e r c o n t r o l l e d by t h e FN-barrier a l o n e , b u t it i s modulated by a r e s o n a n t t e r m , depending on what k i n d o f r e s o n a n c e can grow i n t h e c b of t h e o x i d e . T h i s c o n d i t i o n i s v e r y s e n s i t i v e l y c o n t r o l l e d by t h e a p p l i e d e l e c t r i c f i e l d .
C9-282 JOURNAL DE PHYSIQUE
Of course, this charging has nothing to do with the presence of an rf field. It is a purely static effect, characteristic for the metal -
metal oxide - interface being apparent at microwave frequencies too.
Finally this first experiment had two important results:
1 ) It is possible to operate a field emitter (and to apply the FN-Eq.)
with any restrictions even at microwave frequencies. This result is of interest for electron optical applications too.
2) The differences between dc and rf field emission are not weighty -
nevertheless they promise to be very informative - and call for a heighly sensitive experimental set up.
liquid nitrogen liquid helium wave guide anode view port screen
channeltron FN-plot channeltron energy
Improvements
Encouraged by the first results, we constructed a new experiment, which is outlined in Fig.5. Now the resonator not longer is submersed in LHe.
This allows a more flexible construction. E . g . it will be possible to shift axially the needle. Outside the resonator the tip can be inves- tigated by dc field emission. When it is pulled back into the resonator the same measurement can be made at three frequencies between 1 2 and 18 GHz. Both times the emission patterns can be imaged on a screen.
Below the screen an energy analyser and two channeltrons are mounted allowing the measurement of the emission currents from 0 . 1 electron per second up to 1 rnA - 17 decades without a gap. This assembly can be shifted with respect to the tip. Thus FN-plots and the energy distri- bution of the emission current of selected cristal faces can be measu- red in clean and contaminated state. In a subsequent state of the ex- periment time resolved measurements are planned.
We are grateful to the Deutsche Forschungsgemeinschaft for financial support and to the Kernforschungszentrum Karlsruhe for the temporary abandonment of the high frequency equipment.
References
/I/ C.G.B.GARRETT: Phys. Rev. 107 (57) 478 / 2 / H.HuBNER: Optik 6.1 (83) 113
/ 3 / J.MASERJIAN: J. Vac. Sci. Tech. 11 (74) 996
