SURFACE SELF-DIFFUSION OF ERBIUM

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SURFACE SELF-DIFFUSION OF ERBIUM

G. Kozlowski, A. Ciszewski, W. Swiech

To cite this version:

G. Kozlowski, A. Ciszewski, W. Swiech. SURFACE SELF-DIFFUSION OF ERBIUM. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-337-C2-340. �10.1051/jphyscol:1986251�. �jpa-00225685�

SURFACE SELF-DIFFUSION OF ERBIUM

G. KOZLOWSKI, A. CISZEWSKI and W. SWIECH

Institute of Experimental Physics, University of Wroclaw, PL 50-205 Wroclaw, ul. Cybulskiego 36, Poland

Abstract

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I - INTRODUCTION

The procedure of growing epitaxial single crystals of erbium [ l ] made possible to measure the activation energy for self-diffusion of erbium. Erbium crystals were grown by vapour deposition onto a tungsten substrate in the ultra-high vacuum environment of a field electron microscope. This method growth [21 assures a very good surface cleanness and eliminates the necessity of additional cleaning before the measurements.

I1 -EXPERIMENTAL, METHOD

The measurements were done in a glass field emission microscope connected to a sputter-ion pumped metal vacuum system. Base pressure in the microscope was about 1 0 - ~ Pa, estimated from the rate of the specimen surface contamination time. The erbium used for vapour source was of about 99.9% purity. The conventional methods of field emission microscopy were used [ 3 , 4 ] .

Zero-field activation energy was determined 151 first by establishing a reproducible small deformation of the crystal shape at a certain temperature with an electric field applied and then measuring the time required to smooth the surface at various temperatures with no field applied. The deformation has to be small enough in order to make the surface diffusion be the main mechanism of transport during the course of smoothing and to avoid the influence of the bulk diffusion. The initial and the final states of the crystal shape were characterized by the relevant values of the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986251

JOURNAL DE PHYSIQUE

total field electron current at an arbitrarily fixed voltage. In order to obtain some information about the influence of the electric field on the surface diffusion processes of erbium, the measurements of the activation energy for surface self- diffusion during the "buildup" were made. The buildup was made for both directions of the electric field. The specimen temperature was controlled by an electronic tip- temperature controller. The measurements were done over the temperature range 419

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527 K. The emitter temperatures were determined by the resistance thermometry method [61.

111

-

The measurements have been done for two different orientations of the erbium crystals. The effect of kildup was different for the surfaces exposed by each orientation. When the <0110> direction of the crystal was parallel to the emitter axis, which allowed one to observe the surface consisting mainly of the {~lio},

(0111}, and {l120} planes [fig. la], the buildup resulted in the distinct growth of the diameter of these planes [fig. Ib]. Such a behaviour was not observed for the

I0001 } plane when {0001} direction of the erbium crystal was parallel to the emitter axis [fig. Ic and dl. Only a stronger emission from the higher index planes

surrounding the {0001} plane visualized the changes caused by buildup in this case.

The extent of the buildup during the activation energy measurements was much smaller than that recorded in fig. lb and Id.

The values of the activation energy for surface self-diffusion of erbium, obtained from the slope of the Arrhenius-type plots, are shown in Tables. Table 1 contains the results for the diffusion without the external electric field.

Table 1. The values of the activation energy Q for surface self- diffusion of erbium with NO electric field applied. Estimated

experimental error of the results is equal to 0.20 eV/atom.

h he

Crystal's orientation (crystallograpic direction of erbium crystal parallel to tungsten tip axis)

v

Crystallographic con- stitution of the surface

Number of erbium crystal

Straight buildup (tip biased "-") Q (eV/atom>

Reverse buildup (tip biased "+") Q (ev/atom)

Average value of Q

<0li0>

{~ligj ,

planes

l*

<

-1

<0001>

{0001} and higher index planes of its surroundings

2

1.08

1.23

3

1.34

1.55

1.15

4

1.48

1.77

1.53

*

spond t o t h e i n i t i a l forms of buildup, b and d

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The a n i s o t r o p y i n t h e erbium c r y s t a l behaviour i s a p p a r e n t l y s t r o n g enough t o be d e t e c t e d even by such an "averaging" method of measurement a s t h e one a p p l i e d i n t h i s i n v e s t i g a t i o n , which f o l l o w s from t h e comparison of t h e v a l u e s obtained f o r t h e s u r f a c e s with d i f f e r e n t c r y s t a l l o g r a p h i c s t r u c t u r e s . When t h e thermal b l u n t i n g was preceded by t h e buildup a t t h e f i e l d d i r e c t e d o u t s i d e t h e s u r f a c e ( a r e v e r s e b u i l d u p ) , t h e a c t i v a t i o n energy had h i g h e r values than i n t h e c a s e of t h e o p p o s i t e d i r e c t i o n of t h e f i e l d ( a s t r a i g h t buildup). Although t h e d i f f e r e n c e of t h e s e v a l u e s i s c l o s e t o t h e e s t i m a t e d e r r o r of t h e measurement, t h e permanent appearance of i t f o r each erbium c r y s t a l i n d i c a t e s t h a t t h i s " f i e l d memory" e f f e c t should not be a c c i d e n t a l . It r a t h e r e s t a b l i s h e s evidence t h a t t h e buildup process changes t h e binding energy between atoms i n t h e upper p a r t s of t h e built-up erbium c r y s t a l . These changes remain conserved a f t e r removal of t h e f i e l d , a f f e c t i n g t h e v a l u e of t h e a c t i v a t i o n energy f o r s u r f a c e d i f f u s i o n without an e x t e r n a l e l e c t r i c f i e l d , o r r a t h e r they a f f e c t only t h i s term of t h e a c t i v a t i o n energy which, i n t h e vacancy model of d i f f u s i o n 171, is c a l l e d t h e energy of formation Qf. The f i e l d d i r e c t e d o u t s i d e t h e s u r f a c e a c t s upon t h e erbium atoms s t r o n g e r t h a n t h e o p p o s i t e one.

This i s confirmed by t h e r e s u l t s obtained f o r t h e d i f f u s i o n with an e l e c t r i c f i e l d a p p l i e d , which a r e shown i n Table 2.

A c o n s i d e r a b l e d e c r e a s e i n t h e v a l u e of t h e a c t i v a t i o n energy, caused by t h e elec- t r i c f i e l d , remains i n good agreement w i t h t h a t observed e a r l i e r f o r dysprosium and gadolinium [ 8 ] . It is not c l e a r why t h e r e s u l t s obtained f o r t h e same <0001>

JOURNAL DE PHYSIQUE

Table 2. The values of the activation energy Q for surface self-diffusion of erbium WITH an electric field applied. Estimated experimental error of the

results is equal to 0.10 eV/atom. There existed a noticeable difference in the shape between crystal Nos. 3 and 4

orientation but for different erbium crystals differ so much. Most probably the different shapes of the crystals caused the different distribution of the local field strength during the buildup.

Crystal orientation

Number of erbium crystal

Straight field (tip biased "-"l

Q (eV/atom)

Reverse field (tip biased

"+")

Q (evlatom)

IV - CONCLUSIONS

The activation energy for surface self-diffusion of erbium appears to be strongly anisotropic. Two different values of it have been obtained for two crystallographi- cally different surfaces of erbium crystals. The electric field affects con- siderably the process of surface diffusion. Evidence of the probable changes of the binding energy between erbium atoms during the buildup was found. These changes may influence the results of the measurements of the activation energy for the sur- face self-diffusion with no field applied. It is expected that the error caused by this effect should not be much greater than the error from the method of measurement.

<0l'i0>

2

0.80

0.69

ACKNOWLEDGMENT S

The authors would like to thank Dr. Allan J. Melmed for the erbium used in this experiment and also for stimulating and helpful discussions during preparation of this manuscript. This work was supported by the Polish Academy of Sciences under Project No. MR.I.9.

<0001>

REFERENCES

3

1.20

0.95

Ciszewski, A., Kozlowski, G. and Swiech, W., Acta Universitatis Wratislav, 847 (1985) 33.

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Melmed, A. J., J. Chem. Phys.,x (1963) 1444; J. Appl. Phys.,s (1965) 3585.

Good, R. H. Jr. and ~Gller, E. W., Handbuch der Physik, Springer-Verlag, Berlin (19561, Vol. 21, pp. 176231.

Gomer, R., Field Emission and Field Ionization, Harvard University Press, Cambridge (1961).

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2

Workowski, C. J., J. Phys. (1977) 538.

Blakely, J. M., Progress in Materials Science, Vol. 10, Pergamon Press, (19831, Russian translation.

Ciszewski, A. and Melmed, A. J., Surface Sci.

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4

0.75

0.60