STUDY OF LANTHANUM ON TUNGSTEN FIELD EMITTER - II

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STUDY OF LANTHANUM ON TUNGSTEN FIELD EMITTER - II

G. Kozlowski, S. Surma

To cite this version:

G. Kozlowski, S. Surma. STUDY OF LANTHANUM ON TUNGSTEN FIELD EMITTER - II.

Journal de Physique Colloques, 1987, 48 (C6), pp.C6-27-C6-31. �10.1051/jphyscol:1987605�. �jpa-

00226808�

STUDY OF LANTHANUM O N TUNGSTEN FIELD EMITTER

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^I1

G. Kozlowski and S. Surma

Institute of Experimental Physics, University of Wroclaw, 50-205 Wroclaw, Poland

Abstract.- The adsorption and surface diffusion of lanthanum on a surface of a single crystal of tungsten was investigated at a low coverage. The average work function passes through a minimum of about 4 eV with increasing deposition time. A value of activation eneryy for the diffusion of lanthanum surveyed on the vicinals of the W(111) plane has been found to be 2.9 k 0.4 eV and it drops to 1.18 & 0.1 eV .for the diffusion observed in an applied electric field.

I - INTRODUCTION

In a previous FEN study [I] of the lanthanum surf ace self-diffusion made on an epitaxial crystal; of lanthanum we have found that the activation energy for this process decreases after the build-up has been made in a reverse high electric field at a temperature slightly above the /

4

phase transition point of lanthanum (583 K)

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^{Such a}

"field memory" effect, not detected for single crystals built-up in the straight electric field, was explained for erbium 121 as due to a modification of the binding energy of the surface atoms of the epitaxial crystal, which could be introduced by the high electric field directed outside the surface. It seemed that interesting would be to additionally examine the behaviour of lanthanum atoms adsorbed on different clean single-crystal surfaces of the field emitter showing a high anisotropy of the binding energy of an adatom, and to observe the effect of the surface diffusion of the adsorbed lanthanum. This paper presents results of the measurement of the activation energy for the surface diffusion of lanthanum on the tungsten field emitter, which was carried out for a submonolayer coverage under UHV conditions.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987605

JOURNAL DE PHYSIQUE

I 1

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EXPERIMENTAL

The background pressure in the field emission microscope was about 2x10'9 Pa and it maintained below 3 x 1 0 ~ ~ Pa during the deposition of lanthanum from vapour. A sputter-ion p u m p maintained the UHV continuously. We did not use a Bayard-Alpert gauge to avoid additional flashing and the UHV conditions were checked by observing the FEM pattern and the stability of the field emission current. Temperature of the tungsten field emitter was stabilized and controlled by means of an electronic device by using the temperature dependence of a supporting loop segment and it was necessary to recalibrate this dependence after each cleaning of the emitter tip by high-temperature flashes. We estimate the temperature calibration error to be 220 K at 800 K and the stability of the temperature during the course of experimentation to be within 2 5 K.

I 1 1

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RESULTS

Fig. 1 shows the field elgctron emission voltage U (at a constant integral current of 2x10- A ) vs the time of a unilateral evaporation of lanthanum species onto the W emitter held above the melting point of lanthanum (1193 K). Such curves were taken to select the dose of the La deposit which would be suitable for experimentation with the surf ace diffusion of lanthanum.

Fig. 1

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Field emission voltage taken at a constant current vs the deposition time of lanthanum on tungsten.

experiment). Additional measurements of the average work function by taking the Fowler-Nordheim characteristics were made at 690 K.

Deposition of such a dose onto the clean surface of tungsten (shown in fig. 2a) raised the average work function from the assumed 4.50 eV for the clean tungsten to 4.73 & 0.1 eV and the effect, which was fully reproducible, can be seen in fig. 2b. We believe the observed rise of the work function corresponding to the change of the tungsten pattern was caused by the smoothing effect of lanthanum on rough regions of the surface rather than by arrival of electronegative impurity species up the emitter shank.

Fig. 2

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Stages of La adsorption on W illustrating the curve of fig. 1. a) clean W , b) after 30 s deposition, c) for decreasing part of the curve, d ) near the minimum,

e )

near the saturation value.

Note: the weak horizontal wedge in the patteras is an artefact of the screen caused by evaporation of lanthanum.

The progress in adsorption with higher and higher coverage is illustrated in fig. 2c-e. A short inspection reveals that most of the adsorption induced changes are related to the rough regions around the W(111) and W(100) poles, as should be expected. It is worth noting that in stage c with a decreased work function at a coverage believed to be half the saturation value (stage $, =4.33 eV cf. also fig. 1 ) - the relatively weak emitting regions of the surface correspond to the flat, low electric field areas. At about the minimum of the work function (stage d , 9 = 3 . 9 8 eV) the terraced

1221b and {332] areas anew contribute to the emission image.

JOURNAL DE PHYSIQUE

The first lanthanum dose, illustrated in fig. 2b. as we believe ensured a coverage of 0.05-0.10 under conditions of the above described experiment, was deposited at room temperature for the measurements of the diffusion intervals of lanthanum on tungsten in the temperature range 860 K - 965 K with no applied electric field and 765 K

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945 K for diffusion in an imaging high electric field.

The used procedure was the one described by Schmidt and Gomer [31.

The initial state of the diffusion was chosen to be the one after the deposition of lanthanum an'd just after the onset of a given temperature, and the progress in diffusion was watched via the field emission. imaging. The final state was defined as the one when the relative intensities of the emission image of a two 12213 small regions would become equalized. This is shown in fig. 3a-c which illustrates the end of the diffusion which was made with no applied

Fig. 3 - Final states of surface diffusion of La on W. a) no applied field, b) applied straight electric field, c) after action of reverse field. (FEM patterns taken with a magnet, 300 K).

field, state in pattern a, and with a straight electric field.

pattern b. Pattern c corresponding to a similar end state for the diffusion examined in the reverse electric field is presented for comparison. Testing d-fusion times in the reverse field at 800 K - 820 K were shorten than those in the straight field , by an order of magnitude. In all the cases of watching the diffusion patterns on the screen of the field emission microscope under all the three field conditions we noticed the lanthanum approaching the terraces of the (100) plane first, which manifested in a speedy formation of the relativelly bright wreaths seen in these photos around the (100) pole of the tungsten. The computed Arrhenius plots with the diffusion intervals defined and measured in the described way are presented in fig. 4 . The observed behaviour of La on W should be expected for an electropositive metal and is akin to a picture of the adsorption and migration of copper on tungsten reported earlier by Melmed 141. Morever, possible formation of a surface alloy should also be considered.

IV

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CONCLUSION

An 0.5 eV reduction of the average work function by lanthanum adsorbed at 640 K on tungsten has been observed and a value of the activation energy for the surface diffusion, of 2.90 eV per atom, found.. A 1.72 eV reduction of this value under conditions of a high electric field and temperature seems to be an effect of the surface

o

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applied field +

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no field

Fig. 4

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Arrhenius plots for surface diffusion of La on W at 765 K

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965 K .

Acknowledgement

The authors wish to thank Dr. R. Blaszczyszyn for many stirnulatirig discussions during the course of this work.

Work sponsored by the Polish Ministry of Science and Schools of Academic Rank within the Central Project of Basic Research CPBP 01.08.Al.

REFERENCES

[ l I . G. Kozlowski, W. Swiech and S. Surma, J . P h y s i q u e 47 (1986) C7-101.

[ 2 ] . G. Kozlowski, A . C i s z e w s k i a n d W. Swiech, J . P h y s i q u e 47 (1986) C2-337.

[ 3 ] . L. Schmidt and R. Gomer, J . Chem. P h y s . 42 (1965) 3573.

[ 4 ] . A . J . Melmed, J . Chem. P h y s . 4 3 (1965) 3057.