CROSS-CORRELATION MEASUREMENTS OF FIELD EMISSION FLICKER NOISE FROM COADSORPTION LAYERS K-Ni/W

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CROSS-CORRELATION MEASUREMENTS OF FIELD EMISSION FLICKER NOISE FROM

COADSORPTION LAYERS K-Ni/W

R. Blaszczyszyn, Ch. Kleint

To cite this version:

R. Blaszczyszyn, Ch. Kleint. CROSS-CORRELATION MEASUREMENTS OF FIELD EMISSION

FLICKER NOISE FROM COADSORPTION LAYERS K-Ni/W. Journal de Physique Colloques,

1986, 47 (C2), pp.C2-393-C2-397. �10.1051/jphyscol:1986261�. �jpa-00225695�

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JOURNAL DE PHYSIQUE

Colloque C2, supplement au n03, Tome 47, mars 1986 page c2-393

CROSS-CORRELATION MEASUREMENTS OF FIELD EMISSION FLICKER NOISE FROM COADSORPTION LAYERS K-Ni/W

R. BLASZCZYSZYN* a n d Ch. KLEINT

Department of Physics, Karl-Marx-University, Linnestr. 5, DDR-7010 Leipzig, D.R.G.

Abstract -The cross-correlation functions (CCFs) of the field emission flicker noise across the W(112) plane were measured with constant potassium submonolayers deposited onto different preadsorbed nickel doses. In general, the CCFs show a distinct maximum with increasing delay time

T.

The delay times

T~~

corresponding to the maximum of the CCF are exponentially increasing with increasing reciprocal temperature. The nickel coverage dependence of

Twx

reached a deep minimum at a nickel coverage corre- sponding to the work function maximum of the Ni/W (112) system. A prelim- inary interpretation is given.

I - INTRODUCTION

With the introduction of cross-correlation (CC) techniques to the investigation of field emission flicker noise (FEFN) by Dabrowski and KZeint

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new quantitative information on surface stochastic processes in adlayers seems to be available.

These investigations consist in measurements of the cross-correlation functiens (CCFs) defined by

where ix(t) and iy(t) are two time dependent field emission currents recorded at two different collectors aiming at different regions of the field emission tip. T is the total sampling time, and

T

is the time displacement or delay of one record rela- tive to the other as obtained by the data storage system of the correlator. More useful is the normalized CCF given by the expression:

(*)

On leave from Wroclaw University, Institute of Experimental Physics, ul.

Cybulsklego 36, 50-205, Wroclaw, Poland.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986261

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J O U R N A L DE PHYSIQUE

As shown in ref. 1 a large correlation exists in potassium submonolayers in par- ticular on the W(110) plane. The CC coefficient p(0) increases with substrate tem- perature and coverage apart from a sharp minimum at the adlayer work function minimum.

Subsequent investigations of CCFs across the W(112) plane of potassium submonolayers revealed many interesting features of this phenomenon (2,3) [Blaszczyszyn and Kleint, to be published (B&K) I . The mechanism of this adlayer noise correlation is not exactly known at present. Different surface processes may be responsible for it, such as: surface diffusion, spreading of very low frequency acoustic waves or soliton migration (4). In this paper the influence of preadsorbed nickel atoms on the CCFs across W(112) of potassium submonolayers has been investigated.

I1 - EXPERIMENTAL

The construction of the glass tube and the measurement system (fig. 1) was similar to those described in ref.

1.

Additionally the tube was equipped with a nickel source to precover the tungsten emitter with nickel atoms. After vacuum processing, the tube was sealed off. During the measurements the vacuum was about 10 l 0 torr (10-* Pa). The probe-holes in the screen allowed, by use of an external magnetic deflection, to direct the field emission from the chosen probe areas into the

two

identical Faraday collectors placed behind the screen. An electronic system

separated and amplified d.c. and a.c. components of the current. The correlation of the a.c. components i, and i was determined by a Stochastic Analyzer

(l),

The direct current of the probe-gales of about 13 nA was kepr constant during the corre- lation measurements. The emitter temperature was adjusted by heating of the emitter loop and calibrated by resistance measurements of a loop segment.

HIGH-VOLTAGE

c

- - - - - - - - t - - - -

l

L

ELECrnTATK I SCREEN I

I 1

!

PROBE-HOLES ¹ \ I

O.C. I

I

Fig. l - Scheme of experimental arrangement and electronics.

The CCFs were measured across the W(112) plane with probe diameters of about 100 8.

The direction between the probed regions was almost parallel to the substrate atom rows. The distance between the regions was estimated to 300 8 while the (112) plane diameter was about 150 8. The collectors, placed symmetrically to the (112) plane, viewed only vicinals of it.

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NSA - 1000 (KFKI and ENG, Hungary)

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The n i c k e l dose ( g i v e n i n minutes) was d e p o s i t e d o n t o t h e c l e a n e m i t t e r a t room tem- p e r a t u r e and t h e l a y e r was t h e r m a l l y e q u i l i b r a t e d at about 500 K. Then t h e c o n s t a n t d o s e of potassium

( @

= 0 . 4 ) was d e p o s i t e d o n t o t h e n i c k e l l a y e r and t h e CCFs were t a k e n a t d i f f e r e n t t e m p e r a t u r e s . T h i s procedure was r e p e a t e d w i t h d i f f e r e n t n i c k e l d o s e s .

111

- RESULTS

I n g e n e r a l t h e CCFs have a d i s t i n c t maximum w i t h i n c r e a s i n g d e l a y time

T

a l i k e t h o s e of K/W(112) (BLK).

The d e l a y t i m e

Tmax

c o r r e s p o n d i n g t o t h e maximum of t h e CCF e x h i b i t s a pronounced n i c k e l coverage dependence: a deep minimum a p p e a r s a t a n i c k e l dose of about 2.5 min ( f i g . 2 ) .

DEPOSITION TIME OF NICKEL t

Fig. 2 - Delay time

^Tmax

of t h e c r o s s - c o r r e l a t i o n s ' maxima a t 365 K a s measured p a r a l l e l t o t h e atom rows v e r s u s t h e d e p o s i t i o n time of n i c k e l t ~ i . Average potassium coverage 4 ( 0 . 4 .

The CCFs depend a l s o on t h e t e m p e r a t u r e T. The s t r e n g t h of t h e CC i n c r e a s e s w i t h i n c r e a s i n g T and t h e

Tmx

v e r s u s T dependence was s e p a r a t e l y i n v e s t i g a t e d . F i g u r e 3 shows t h a t

i t

can be d e s c r i b e d by an A r r h e n i u s - l i k e dependence:

where

Q

is t h e a c t i v a t i o n energy of the CCF maximum.

A

s i m i l a r t e m p e r a t u r e behaviour was observed f o r t h e K/W (112) system (B&K).

I V - DISCUSSION

Adsorption of n i c k e l on t u n g s t e n does n o t change t h e work f u n c t i o n s o much a s potassium does (5-9). Moreover t h e m o b i l i t y of n i c k e l is expected t o be much s m a l l e r t h a n t h a t of potassium on t u n g s t e n a t t h e same t e m p e r a t u r e because of t h e d i f f e r e n c e i n a c t i v a t i o n e n e r g i e s f o r n i c k e l and potassium on W ( 5 , 8 , 1 0 , 1 1 ) . The FEFN of t h e n i c k e l l a y e r can be n e g l e c t e d w i t h r e s p e c t t o t h a t of potassium.

Although i t is d i f f i c u l t t o e s t i m a t e t h e e f f e c t of t h e n i c k e l m o b i l i t y on t h e

c u r r e n t f l u c t u a t i o n s from t h e c o a d s o r p t i o n l a y e r s K - N ~ / W ( 1 1 2 ) , we b e l i e v e t h a t they

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JOURNAL DE PHYSIQUE

RECIPROCAL TEMPERATURE Ipf

Fig. 3 - Arrhenius plot of the CCFs' maxima delay times parallel to the

(

112) atom rows for-dif f erent deposition times of nickel t~i. Average potassium coverage

Q

0.4. OEWSlTlON TIME OF NICKEL t

Fig. 4 - Activation energy of rmax along the substrate atom rows versus the deposition time of nickel t ~ i . Average potassium coverage

- 4 (

E 0.4.

were mainly generated by the mobilig of K atoms on the topmost layer of the Ni/W system. The potassium coverage of

E,

0.4 was applied following ref. 5 because at this coverage the CCFs of the potassium layer across the W(112) plane weakly depend on

Q

(B&K).

The adsorption of nickel on W(112) is connected with the appearance of a maximum in

the work function

1 2 (8,9).

It was checked that the

1 2

maximum corresponded in

our experiments to a Ni "dose" of t

=

2.5 min. Figure 2 shows that the minimum of

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the

,,T

coverage dependence occurs just at t

=

2.5 min. From the definition in eq. 1

,,T

can be interpreted as some sort of average spreading time of the CC signal between the two probed regions (2--4) (B&K). If

T-,

reflects roughly the velocity v of the correlation signal-transmission the Ni coverage dependence of this velocity increases first to a maximum and then decreases, The dependence of the activation energy Q on nickel coverage obtained from eq. 3 reveals a minimum (fig. 4) which also occurs at t

=

2.5 min.

On the other hand, due to the interpretation of the work function maximum

+ 1 1 2

given by Kolaczkiewicz and Bauer (g), a smoothing effect takes place with increasing Ni coverage. The small Ni atoms successively fill the troughs in the rows of W(112) at first in a random distribution up to the maximum work function where all tungsten atom sites are occupied by Ni atoms. With further hi deposition a compressed Ni layer appears with increasing roughness and also alloying occurs especially with higher temperatures. The measurements of the cross-correlation function support very well this atomic picture. A maximum spreading velocity is to be expected for the smooth W(112) where the troughs are filled completely with Ni atoms. Increasing roughening thereafter causes the signal velocity to decrease and the activation energy to increase with further dosing.

The Arrhenius-like behaviour of

,,T

(fig. 3) indicates a thermally activated character of the correlation mechanism. While surface diffusion is certainly ther- mally activated, other arguments point to a soliton spreading mechanism (4) which, according to theory, also seems to be thermally activated. Therefore a complete interpretation of Q and

,,T

is not possible at present, but it is clear from the presented results that the cross-correlation is very sensitive to the "substrate"

structure.

REFERENCES

(1) Dabrowski, A. and Kleint, Ch., Surface Sci. 119 (1982) 118.

(2) Blaszczyszyn, R . and Kleint, Ch., paper presented at 7th Seminar on Surface Physics, Wroclaw-Karpacz 1983.

(3) Blaszczyszn, R., Proc. Solid State Surface Physics Symposium- 20 Years Cooperation Wroclaw and Leipzig Universities, Wroclaw 1984, to be published in Acta Universitatis Wratislaviensis.