TEMPERATURE DEPENDENCE OF IRIDIUM FIELD EVAPORATION RATE

HAL Id: jpa-00225893

https://hal.archives-ouvertes.fr/jpa-00225893

Submitted on 1 Jan 1986

HAL

is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire

destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

TEMPERATURE DEPENDENCE OF IRIDIUM FIELD EVAPORATION RATE

A. Menand, D. Blavette

To cite this version:

A. Menand, D. Blavette. TEMPERATURE DEPENDENCE OF IRIDIUM FIELD EVAPORATION RATE. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-17-C7-20. �10.1051/jphyscol:1986704�.

�jpa-00225893�

JOURNAL DE PHYSIQUE

Colloque C7, suppl6ment au n o 11, Tome 47, Novembre 1986

TEMPERATURE DEPENDENCE OF IRIDIUM FIELD EVAPORATION RATE

A. MENAND and D. BLAVETTE

U.A. CNRS, Laboratoire de Microscopie Ionique, Faculte des Sciences de Rouen, B P 67, F-76130 ~ont-Saint-Aignan, France

R6sum6 - Differents auteurs ont suggere la possibilit6 d'un effet tunnel ioni- que et quelques experiences ont conforte l'hypothkse d'un effet tunnel pour les ions W ou B 2 basse temperature.

Nous avons d6termin6, par Microscopie ionique, la dependance en temperature du flux d'ions dvapores par effet de champ (en plans Cvapor6s par seconde). Les resultats montrent deux regimes diffgrents B haute et basse temperatures, la temperature de transition etant comprise entre 35 et 40 K . A basse temperature le flux est beaucoup moins dependant de la temperature. Ceci est interpret6 comme le resultat d'une desorbtion des ions iridium par un mecanisme d'effet tunnel en dessous de 35 K.

Abstract

-

We performed FIM experiments on Iridium to determine the temperature dependence of the relative field evaporation rate (in layers per second). The results show two different regimes at high and low temperatures, with a transition tempera- ture between 35 K and 40 K. The much more smaller temperature dependence at low temperature is interpreted as being indicative of a tunneling mechanism of field desorption of Iridium ions below 35 K.

I

-

The quantum mechanical phenomenon of tunneling has been known for more than 50 years. Among the best examples of tunneling are a particle decay of nuclei /I/ and field emission of electron 121. More recently the existence cf a molecular tunneling has been reported for chemical reactions 1 3 1 .

The possibility that field desorption at low temperature would be the result of ionic tunneling has been suggested previously /4/, 151, 161, 171, 181, 1 9 1 , /lo/.

We have recently reported some experimental evidences showing that boron ions are field desorbed by ionic tunneling up to a temperature of 140 K /Ill.

At high temperature the field desorption of atoms is a full thermally activated process while at very low temperature the desorption reduces to pure tunneling.

Between these two situations a transition temperature Tc exists, defined by the equality of the two desorption rate constants.

In this paper we present a FIM study of the temperature dependence of desorption rate for iridium at low temperature where tunneling is supposed to occur.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986704

C7-18 JOURNAL DE PHYSIQUE

I1 - EXPERIMENTAL

We measured t h e d e s o r b t i o n r a t e c o n s t a n t of i r i d i u m u s i n g a r i n g c o u n t i n g t e c h n i q u e /12/. The d e s o r p t i o n r a t e s were o b t a i n e d i n l a y e r s p e r second.

The specimen was cooled down t o 20 K u s i n g a c l o s e - c i r c l e c r y o g e n e r a t o r . The tempe- r a t u r e c o u l d b e a d j u s t e d by h e a t i n g t h e c r y o t i p by means of e l e c t r i c a l r e s i s t a n c e s . The temperature was measured a t t h e base of t h e sample h o l d e r w i t h a Fe Au-chrome1 Thermocouple w i t h a r e f e r e n c e a t 77 K. The d i f f e r e n c e between t h e a c t u a l t i p tempe- r a t u r e and t h e measured one's has been e s t i m a t e d t o b e c l o s e t o 10 K. The experimen- t a l p r o t o c o l e i s a s f o l l o w s : t h e a p p l i e d p o t e n t i e l VDC i s f i r s t s e t t l e d t o a choo- sen v a l u e . Then t h e time r e q u i r e d t o e v a p o r a t e one ( o r s e v e r a l ) l a y e r i s measured.

The p o t e n t i a l i s t h e n decreased and a f t e r w a r d s t h e t i p temperature i s changed and s t a b i l i z e d b e f o r e a new measurement.

Since t h e b a r r i e r h e i g h t and width a r e f i e l d dependent i t i s n e c e s s a r y t o hold t h e f i e l d c o n s t a n t d u r i n g t h e whole measurements. Because t h e t i p r a d i u s g e n e r a l l y i n c r e a s e d u r i n g t h e experiments it i s n e c e s s a r y t o compensate t h i s phenomenon by i n c r e a s i n g t h e a p p l i e d v o l t a g e .

This c a n b e achieved i n a s i m p l e way s i n c e f o r a c o n s t a n t f i e l d t h e r e i s a l i n e a r r e l a t i o n s h i p between t h i s p o t e n t i e l i n c r e a s e V1

-

V,

-

with ^{I L}

AV ( h k l ) = EBa s i n a12 h 2 ^+kZ +1* I - s i n a / 2

where a i s t h e cone a n g l e of t h e t i p , a t h e l a t t i c e parameter, E t h e a p p l i e d f i e l d and B t h e f i e l d p r o p o r t i o n a l i t y f a c t o r d e f i n e d by E = V/BR w i t h R t h e t i p r a d i u s . The v a l u e of AV ( h k l ) c a n b e a c c u r a t l y determined w i t h a p r e l i m i n a r y experiment. As t h e a p p l i e d v o l t a g e i s slowly i n c r e a s e d t h e number of f i e l d evaporated planes i s recorded. The f i g u r e 1 shows t h e r e s u l t s o b t a i n e d f o r two d i f f e r e n t t i p s . The number n of f i e l d evaporated planes i s p l o t t e d v e r s u s t h e a p p l i e d v o l t a g e V. The l i n e a r r e l a t i o n s h i p between n and V i s w e l l v e r i f i e d . The cone a n g l e s c a l c u l a t e d from t h e s e d a r a a r e r e s p e c t i v e l y equal t o 4.5" and 8.8". As mentionned by D. B l a v e t t e I131 t h e AV ( h k l ) parameter i s only dependent upon t h e cone a n g l e and t h e c r i s t a l l o g r a p h i c p l a n e , i t does n o t depend on t h e i n c r e a s e v o l t a g e r a t e .

I11

-

The f i g u r e 2 g i v e s t h e r e s u l t s o b t a i n e d f o r t h r e e d i f f e r e n t a p p l i e d f i e l d s t r e n g t h s . The l o g a r i t h m s of e v a p o r a t i o n r a t e s (measured i n l a y e r p e r second) a r e p l o t t e d v e r s u s 1 IT.

For t h e f i r s t experiment conducted a t t h e lower f i e l d we d i d not succeed t o go down i n t e m p e r a t u r e below 58 K because t h e time r e q u i r e d t o e v a p o r a t e one l a y e r was y e t g r e a t e r t h a n h a l f an hour. For t h i s experiment p o i n t s a r e a l i n e d on a s t r a i g h t l i n e corresponding t o an a c t i v a t i o n energy ^Qn = 0.106 eV.

--

The experiments 2 and 3 were performed a t h i g h e r f i e l d s ( f o r i n s t a n c e t h e i n c r e a s e i n f i e l d i s about 6 % between experiment 1 and 2 ) . A s h a r p change i n t h e s l o p e occurs w i t h i n 35-40 K . Two d i s t i n c t s regimes can b e r e a d i l y observed : above 40 K and below 40 K.

S e v e r a l e x p l a n a t i o n s have t o b e c o n s i d e r e d i n t h e i n t e r p r e t a t i o n of such a change i n t h e e v a p o r a t i o n r a t e s . Wada 1141 suggested t h e p o s s i b i l i t y of two d i f f e r e n t s t h e r m a l l y a c t i v a t e d p r o c e s s e s : m i g r a t i o n of a n atom from a s i t e t o a n o t h e r p r i o r e v a p o r a t i o n . A t low temperature below Tc t h e observed d e s o r b t i o n r a t e would b e c o n t r o l l e d by t h e m i g r a t i o n r a t e w h i l e above Tc t h e e v a p o r a t i o n process would become r a t e - c o n t r o l l i n g .

However we t h i n k t h a t t h e a c t i v a t i o n energy which i s observed a t low temperature i s s o s m a l l

(a

Figure 1

-

Figure 2

-

C7-20 JOURNAL DE PHYSIQUE

Tsong / 7 / and Kingham / l o / e s t i m a t e s g i v e a c r i t i c a l t e m p e r a t u r e Tc l y i n g between 50 K and 75 K f g r i r i d i u m and f o r a b a r r i e r h e i g h t and w i d t h r e s p e c t i v e l y e q u a l t o 0.1 eV and 0.1 A.

C o n s i d e r i n g a p a r a b o l i c s h a p e f o r t h e b a r r i e r

and L a r e t h e b a r r i e r h e i g h t and w i d t h and M t h e i o n mass. I f we t a k e a s e b a r r i e r h e i g h t s observed above Tc f o r t h e r m a l a c t i v a t e d d e s o r p t i o n , th:

c r i t i c a l t e m p e r a t u r e 35-40 K i s c o n s i s t e n t w i t h b a r r i e r w i d t h s of 0.1 and 0.13 A f o r r e s p e c t i v e l y

R

Of c o u r s e t h e s e e x p e r i m e n t s were performed w i t h t h e p r e s e n c e of imaging g a z . K e l l o g g 1151 p o i n t e d o u t t h a t t h e i n f l u e n c e of gaz upon a c t i v a t i o n e n e r g y was s i g n i f i c a n t even a t a low background p r e s s u r e of 5 x 1 0 - l o T o r r . T h i s p r o b a b l y e x p l a i n s t h e low a c t i v a t i o n e n e r g i e s observed above Tc compared t o K e l l o g r e s u l t s .

REFERENCES

/ I / Gamov, G. Z . , Z . P h y s i k

51,

121 Fowler,R. H. and ~ o r d h e i m , P r o c . Roy. Soc. A119 (1928) / 3 / G o l d a n s k i i , V. I . , N a t u r e 7 9 , 1 0 9 (1979)

/ 4 / Gomer, R. and Swanson, L. W . , J . Chem. Phys.

2,

14,

/ 6 / E h r l i c h , G. and K i r k , C . F., J. Chem. Phys.

48,

10,

/ 8 / Wada, M . , K o n i s h i , M. and Nishikawa, O . , S u r f . S c i . 100, 439 (1980) / 9 / Chibane, K., F o r b e s , R. G., S u r f . S c i .

122,

/ l o / Kingham, D. R., S u r f . S c i .

116,

/11/ Menand, A. and Kingham, D. R., J. Phys. C 18, 4539 (1985)

1121 Tsong, T. T. and Miiller, E. W., Phys. ~tat.~ol. ( a )

1,

497, Almqvist and W i k s e l l , Stockholm 1141 Wada, M., S u r f . S c i .

145,

1151 K e l l o g g , G. L., Phys. Rev. B

2,