SURFACE DIFFUSION OF LITHIUM ON THE W(112) PLANE BY FIELD EMISSION SPECTRAL DENSITY ANALYSIS

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SURFACE DIFFUSION OF LITHIUM ON THE W(112) PLANE BY FIELD EMISSION SPECTRAL

DENSITY ANALYSIS

T. Biernat, Ch. Kleint

To cite this version:

T. Biernat, Ch. Kleint. SURFACE DIFFUSION OF LITHIUM ON THE W(112) PLANE BY FIELD

EMISSION SPECTRAL DENSITY ANALYSIS. Journal de Physique Colloques, 1989, 50 (C8), pp.C8-

123-C8-128. �10.1051/jphyscol:1989822�. �jpa-00229920�

COLLOQUE D E P H Y S I Q U E

Colloque C 8 , supplement au n° 1 1 , Tome 5 0 , novembre 1989 C8-123

SURFACE D I F F U S I O N OF LITHIUM ON THE W ( 1 1 2 ) PLANE BY F I E L D EMISSION SPECTRAL DENSITY A N A L Y S I S

T. BIERNAT and C H . KLEINT*

Institute of Experimental Physics, Wroclaw University, 50-205 Wroclaw, CybulsTiiego 36, USSR

* S e k t i o n Physik, Karl-Marx-Universitat Leipzig, DDR-7010 Leipzig, Linnestrasse 5, D.R.G.

Abstract - T h e spectral density -functions W(f > of the field-emission

•flicker noise of lithium adsorbed on t h e tungsten (112) p l a n e were determined at different temperatures in a probe-hole field emission microscope. By an analysis in t e r m s of the Timm and van der Ziel model the surface diffusion coefficients have been obtained and their temperature dependence reveals the surface diffusion e n e r g i e s . For the two coverages of c = 0.38 and 0.75 investigated their values could b e determined to be Q = 0.56 and 1.08 eV« respectively. The diffusivity prefactors were also calculated. The results are compared with those obtained for the W(112)K system.

1 ~ INTRODUCTION

Alkali metal adsorption on m e t a l s is a subject of long-standing and current interest / l / and much effort h a s been made with different surface spectroscopies t o investigate its properties. Surface diffusion is important for many p r o c e s s e s but only a few parameters were obtained for lithium on s i n g l e planes on transition metals / 2 , 3 / . The flicker n o i s e or fluctuation m e t h o d s / 3 , 4 / Are advantageous in allowing easily to determine the coverage and even t h e directional d e p e n d e n c e / 5 , 6 / though a certain polarization contribution by t h e high voltage needed is unavoidable. From potassium investigations w e know of a strong effect of t h e coverage / 7 / and of surface structure / 8 / . Noise measurements of the spectral density in dependence on coverage of the same W(112)Li system / 9 / show clearly the influence of coherent and incoherent adlayer structures on the adatom mobility. The r e s u l t s presented here illustrate the applicability of the technique but can give only an example of the surface diffusion parameters for two selected coverages.

2 -

E^ERICIENIAL

A field emission microscope (FEM) of the Miiller probe-hole type w a s used to measure the current fluctuations from the central part of the (112) plane of a tungsten single crystal tip covered with a lithium submonolayer. T h e lithium source consisted of a Li zeolite placed on a resistively heated tantalum sheet. The temperature of the emitter was kept constant by means of a stabilized emitter loop current which w a s calibrated earlier.

The field emission current w a s analysed between 2 5 Hz and 2 0 kHz by a frequency spectrometer (Echtzeitanalys?,tor 0 1 0 1 2 , Messelektronik, D r e s d e n ) . More experimental d e t a i l s can b e found in r e f . / 9 / .

T h e flicker noise spectral density functions W(f) were measured in dependence on temperature (above room temperature) for some Li concentrations. The chosen coverages were obtained by heating the emitter covered with a thick layer of lithium - i . e . by thermal desorption of Li from the W(112) plane. The coverages were determined by measuring t h e voltage at a constant collector current from the W(112) p l a n e with and without lithium.

T h i s a l l o w s to calculate the work function, and w e obtain the adsorbate

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989822

c o n c e n t r a t i o n by t a k i n g i n t o account t h e dependence o f t h e work f u n c t i o n versus l i t h i u m d e p o s i t i o n doses which was c a l i b r a t e d by t h e c o n c e n t r a t i o n r a t i o c = n

.

/nWCii2, i n r e f . / 2 / (see also r e f . / ? / f .

L L

3

-

^RESULTS

The temperature dependence o f t h e s p e c t r a l d e n s i t y f u n c t i o n i s i l l u s t r a t e d i n f i g s . 1 and 2 which correspond t o c = 0.75 and 0.38, r , e s p e c t i v e l y . (Other s p e c t r a a t i n t e r m e d i a t e temperatures and t h e same coverages n o t shown h e r e f i t w e l l i n t o t h e general t r e n d t o be described below.) The i n c r e a s e o f t h e e m i t t e r temperature l e a d s t o a change o f t h e W(f) shape i n t h e i n v e s t i g a t e d +requency range. A t room temperature (and s l i g h t l y above) W(f) can be represented by one s t r a i g h t l i n e i n a double l o g a r i t h m i c s c a l e and described by t h e equation W(f) oc f-= ! f i g s . la,b; 2a).

- FREQUENCY f [Hz1

NORMALIZED FREQUENCY up [-I

Fig. 1 - Experimental s p e c t r a l d e n s i t y f u n c t i o n s a t d i f f e r e n t temperatures f o r c = 0.75 ( c i r c l e s ) . Except f o r t h e curves a t T = 378 K and 450

K,

t h e experimental p o i n t s a r e f i t t e d by t h e o r e t i c a l curves o f t h e Timm and van der Z i e l model.

Z

2

2

l o 1 8 -

1019

gro-20-

m 5

D

lo2*

2 ^{FREQUENCY f CHzl}

1%

163

E10-17-\ T=48o K

-

- 102 ^1p" I?

18 18 ^{1 ii2}

-\

-

- -

- lo2 103 q4

NORMALIZED FREQUENCY U, 1-1

U)

N

NORMALIZED

'C

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T=480 K

FREQUENCY up C-l

1b-l I&

1b1

I

NCY f [Hz1

NORMALIZED FREQUENCY

up[-]

1b0

161 I ^I&

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ip2 lp3 lo4 _FREQUENCY t ^1p2 f E H z l ^{'93 I$}

-

FREQUENCY f[Hz]

NORMALIZED FREQUENCY

U p

[-I

Fig.2. Experimental spectral density functions at different temperatures for c = 0.38 (circles). Except f a r t h e curve at. T = 378 _{K ,} t h e experimental points are fitted by theoretical curves of t h e Timm and van der Ziel model.

lo1=

l o I 7

1u20

la2'

4-2 6 0

'

- T= 594 K

;;::\

- 19' ^{1p3 1p4}

- I$ 16' 1b0 16"

- T=633 K

-

^-0.

1 ^*----I

1Q' 1p3 IT

F o r t h e h i g h e r t e m p e r a t u r e s t h e e x p e r i m e n t a l p o i n t s are a p p r o x i m a t e d b y t h e t h e o r e t i c a l s p e c t r a l d e n s i t y f u n c t i o n o b t a i n e d b y Timm a n d v a n d e r Z i e i /10/

a n d c o r r e c t e d by G e s l e y a n d Swansan /11/. T h e s e a u t h o r s h a v e a s s u m e d a model w h e r e s u r f a c e d i f f u s i o n a f t h e a d s o r b a t e o n a s m a l l c i r c u l a r p r o b e a r e a c a u s e s f l u c t u a t i n n s i n t h e number o f a d p a r t i c l e s o n t h i s a r e a . T h e f l u c t u a t i o n s a r e d e s c r i b e d b y t h e s p e c t r a l d e n s i t y f u n c t i o n S ( u ) g i v e n b y

w h e r e LI = 2n f r 2 / D i s t h e n o r m a l i z e d f r e o u e n c y a n d T f u ) a n e x p r e s s i o n

P P

composed of K e l v i n f u n c t i o n s !cp / 6 / ) .

<

( b ~ j " Z i s t h e m e a n - s q u a r e f l u c t u a t i a n o f t h e a d p a r t i c l e number o n t h e p r o b e d a r e a w i t h r a d i u s r D is t h e , s u r f a c e

D~

d i f f u s i o n c o e f f i c i e n t a n d C t h e Fowler-Nordheim t e r m ,

F N

The t e m p e r a t u r e a b o v e w h i c h t h e c h a n g e o f t h e W f f ) s h a p e c a n b e o b s e r v e d d e p e n d s o n t h e l i t h i u m c o v e r a g e . F o r e x a m p l e a t t h e h i g h c o n c e n t r a t i o n o f c ^Z 2 . 4 5 a t e m p e r a t u r e i n c r e a s e a b o v e room t e m p e r a t u r e l e a d s a l r e a d y t o a d e c r e a s e of t h e l i t h i u m c o n c e n t r a t i o n o n t h e W(112) p l a n e ( a c c o r d i n g t a t h e v o l t a g e a t c o n s t a n t c o l l e c t o r c u r r e n t ) . A t a t e m p e r a t u r e of a b o u t 5 0 4 # c a p p r o a c h e s u n i t y . A v i s i b l e c h a n g e o f t h e Wtf 1 s h a p e t o o k p l a c e f a r c = 0.75 a n d c = 0.35 a t T = 4 6 0 K a n d T = 4 0 0 K, r e s p e c t i v e l y .

4 - EVALUATION AND DISSCSSION

By c o m p a r i n g t h e e x p e r i m e n t a l s p e c t r a l d e n s i t y f u n c t i o n Wff! w i t h t h e o r y t h e d i f f u s i o n c o e f f i c i e n t h a s b e e n c a l c u l a t e d . A s i t w a s d o n e i n r e f . / 7 / f o r t h e W-K s y s t e m t h e f r e q u e n c y s c a l e s t h a t c o r r e s p o n d t o t h e e x p e r i m e n t a l - M i + ) f l o w e r s c a l e i n f i g s . 1 a n d 2 ! w e r e c o m p a r e d w i t h t h e f r e q u e n c y s c a l e t h a t c o r r e s p o n d s t o t h e d i m e n s i o n l e s s n o r m a l i z e d " f r e q u e n c y " u d e f i n e d a b o v e

D

{ u p p e r s c a l e ) . From t h i s c o m p a r i s o n a n d t a k i n g t h e p r o b e d a r e a r a d i u s t o b e a b o u t c m t h e d i f f u s i o n c o e f f i c i e n t D c a n b e o b t a i n e d . F i g s . 3 a n d 4 i 1 l u s t r a t e i t s t e m p e r a t u r e d e p e n d e n c e f o r t h e W ( f

>

e x a m p l e s shown i n f i g s . 1 a n d 2, r e s p e c t i v e l y . T h e d a t a i s w e l l r e p r e s e n t e d b y a s t r a i g h t l i n e f i t .

RECIPROCAL TEMPERATURE 103/~ [ Id]

F i g . 3

-

A r r h e n i u s p l o t o f t h e d i f f u s i o n c o e f f i c i e n t s a t a c o n c e n t r a t i o n of c = 0.75

t Q = 1 . 0 8 e V ) .

F i g . 4 - A r r h e n i u s p l o t o f t h e d i f f u s i o n c o e f f i c i e n t s a t a c o n c e n t r a t i o n o f c = 0.38

( Q = 0.56 e V ) .

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L L W 0

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z 3 2 k1d0

C=0.38

\. \., ., .\.

' 0

b ,

,

^a

1.6 1.8 2.0 22 24

RECIPROCAL TEMPERATURE I@[T [K']

From the Arrhenius p l o t s of t h e d i f f u s i o n c o e f f i c i e n t s we have t h e surface d i f f u s i o n a c t i v a t i o n energy Q = 0.56 eV and t h e p r e f a c t o r D = 2.4 xlO-scmZ/s

0

a t t h e L i concentration of c = 0.38.

A t

t h e higher coverage I c = 0.752 the r e s p e c t i v e values are considerably higher: GI = 1.08 eV and D = 6.3 x10 icmz/s.

0

The L i a c t i v a t i o n energies are i n general l a r g e r than those f o r potassium on t h e W(lI2) plane /7/ which appears t o be a n a t u r a l consequence of t h e smaller s i z e o f L i atoms o r ions. (This i s not true, however, f o r the f i r s t value of Q . = 0.56 eV a t a coverage where Q was found t o be somewhat h i g h e r / 7 / . ) . A

L% K

d e t a i l e d comparison r e q u i r e s more L i data e s p e c i a l l y a t d i f f e r e n t coverages than a r e a v a i l a b l e a t present.

According t o f i g s . 1 and 2 t h e experimental s p e c t r a l d e n s i t y f u n c t i o n s are f i t t e d r a t h e r w e l l by t h e t h e o r e t i c a l curves o f t h e T i m m and van der Z i e l model is,) except i n t h e lower temperature range. There Wtf) can be described by t h e spectrum W f f ot f-= which r e v e a l s a s t r a i g h t l i n e i n a double l o g a r i t h m i c p l o t . With increasing temperature small discrepancies develop a t t h e h i g h frequency p a r t o f t h e measured spectra (only a p a r t of them i s shown i n f i g s . 1 and 2). F i n a l l y t h e curves of t h e two-dimensional (2D) f l u c t u a t i o n model Sm ( c f . / I T / ) describe t h e spectra as mentioned above. I t seems t h a t t h i s temperature dependence can be explained i n t h e f o l l o w i n g way: Near room temp=erature we observe i n f a c t a one-dimensional d i f f u s i o n along t h e raws o f t h e bcc ( 1 12) plane o f tungsten. The adequate ID-model g i v e s t h e spectrum

s * ~

derived by Gesley and Swanson 1 This f u n c t i o n p l o t t e d i n f i g . 1 of r e f . / l l / v e r s u s t h e n o r m a l i z e d f r e q u e n c y u = w r 2 / D i s n o t e x a c t l y a s t r a i g h t 1 i n e b u t can be approximated for,. say, - 192 o r 2 frequency decades by s t r a i g h t l i n e s w i t h a slope between E = 1 and 1.5. Considering t h e approximate d e s c r i p t i o n of the r e a l d i f f u s i o n by t h e ID-model one can i n f a c t e x p l a i n t h e spectra by these arguments.

A t higher temperatures, however. we f i n d some d e v i a t i o n s a t low frequencies ( f i g . 2h) and another mechanism might be responsible f o r them. We expect a phase t r a n s i t i o n +rom t h e ordered phase t o t h e disordered but t h e t h e o r i e s developed so f a r disregard t h i s t r a n s i t i o n . The beginning of an observable s u b s t r a t e atom m o b i l i t y might a l s o show up a t these temperatures /12/.

F i g 5

-

Spectral d e n s i t i e s W(f . ) J versus r e c i p r o c a l temperature f o r a concentration of c = 0.75.

V) ~ t ~ r l : : ~ r l : r t ~ I

25 2D 2.5 3.0

RECIPROCAL TEMPERATURE 1 0 7 ~ [K']

Fiq.6

-

Spectral d e n s i t i e s W(f .!

1

versus r e c i p r o c a l temperature f o r a concentration of c = 0.38.

The temperature dependence o f t h e s p e c t r a l d e n s i t i e s ( f i g s . 5 and 6 ) r e f l e c t s t h e general t r e n d o f t h e complete spectra: w i t h r i s i n g temperature t h e n o i s e power a l s o increases b u t we have a t t h e same t i m e d i m i n i s h i n g low frequency p a r t s o f t h e W l f ) ' s according t o t h e change from t h e ID- t o t h e 2D- spectra. namely S iD changing t o S,,. Therefore, a t t h e coverage o f c = 0.38

( f i g . 6 ) we observe a t f i r s t a W(l kHz) maximum f o l l o w e d by t h e W(10 kHz) maximum. A s i m i l a r behaviour a t c = 0.75 ( f i g . 5) i s expected a t h i g h e r temperatures because o f t h e f u l l o w i n g reason. From t h e Arrhenius p l o t s o f t h e d i f f u s i o n c o e f f i c i e n t s we found t h e s u r f a c e d i f f u s i o n a c t i v a t i o n energy Q = 0.56 and 1.08 eV a t t h e L i c o n c e n t r a t i o n o f c = 0.38 and 0.75 r e s p e c t i v e l y . Therefore one expects i n f a c t t h e maximum o f f i g . 5 a t h i g h e r temperatures.

I t i s i n t e r e s t i n g t o n o t e t h a t a s i m i l a r dependence o f t h e s p e c t r a l d e n s i t i e s w i t h r e c i p r o c a l temperature was observed a l s o f o r c l e a n /12/ and h e a v i l y covered /13/ tungsten t i p s as w e l l as f o r e p i t a x i a l K c r y s t a l s /14/.

The l i t h i u m i n v e s t i g a t i o n s and t h e i r i n t e r p r e t a t i o n i n terms o f s u r f a c e d i f f u s i o n presented above may a l s o e x p l a i n t h e general f l u c t u a t i o n behaviour o f t h e t h r e e mentioned s u r f a c e s (cp a l s o /15/) i f t h e number o f mobile atoms i s n o t t o o l a r g e .

ACMNDWLEDGEHENTS

Discussions w i t h Professor R.Meclewski on t h i s s u b j e c t a r e q r a t e + u l l y acknowl edged.

Work sponsored by t h e P o l i s h M i n i s t r y o+ N a t i o n a l Education w i t h i n t h e C e n t r a l P r o j e c t o f Basic Research CPBP 01.08. A.

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