FIELD EMISSION PROPERTIES OF <110>- ORIENTED CARBIDE TIPS

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FIELD EMISSION PROPERTIES OF - ORIENTED CARBIDE TIPS

Y. Ishizawa, M. Koizumi, C. Oshima, S. Otani

To cite this version:

Y. Ishizawa, M. Koizumi, C. Oshima, S. Otani. FIELD EMISSION PROPERTIES OF - ORIENTED CARBIDE TIPS. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-9-C6-14.

�10.1051/jphyscol:1987602�. �jpa-00226805�

J O U R N A L DE PHYSIQUE

Colloque C6, suppl6ment au nO1l, Tome 48, novembre 1987

FIELD EMISSION PROPERTIES O F < ¹¹⁰ >- ^{ORIENTED CARBIDE TIPS}

Y. Ishizawa, M. ~oizumi*, C. Oshima and S. Otani

National Institute for Research in Inorganic Materials, 1-1 Namiki, Sakura-mura, Niihari-gun, Ibaraki 305, Japan

*~kashi beam Technology Corp. , ^Zama, Kanagawa 228, Japan

Abstract

We report that <IlO>-oriented Tic and NbC single crystal tips produce highly stable field emission currents after the surface- processing. The surface-processing consists of the tip heating at 900-1100°~ in the gas such as ethylene, oxygen or hydrogen sulfide, and the following continuous emission of about 10 p A for 30 minutes.

The field emission patterns and the current stability were investigated in the clean and the surface-processed tips.

1. Introduction

Cold field emission electron sources have features of their small size, high brightness and coherence. Such sources have been applied to high resolution scanning electron microscopes and other micro electron probe instruments. However, present available tungsten field emission source is not so stable that a new field emission source is expected to develope for wide-use. In this experiment, we report rield emission cha acteristics of carbide field emitters using <IlO>-oriented TiClf and NbC single crystal tips which show highly stable emission under appropriate "surface- processingu.

2. Experimental

TiCx(x=0.96) and NbCx(x=0.93) single crystals were grown by zone leveling-fl ating zone technique to get highly uniform composition. 2.37 Single crystals were cut into rectangular parallelpipeds with a cross section of 0.2 x 0.2 mm2 by a spark erosion machine. The single crystal tip which was electrolytically etched, was fixed at the Ta hairpin wire. The top radius of the tip was less than 0.1 F m .

The single crystal tip was set in the ultra-high vac um system where pumping up to 3 x 10-10 Pascal (Pa) is attainable.4Y Several kinds of gases can be introduced into this system in order to do nsurface-processing". In this experiment, field emission patterns and current stability were investigated.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987602

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Fig.1 Field emission patterns and proposed tip shapes of TiC<110>

tips.

(a),(b) : Clean TiC<110> tip.

(c),(d) : "Oxygen-processedv TiC<110> tip.

3. Results and Discussion A. TiC<110> tips

A.l Clean TiC<110> tips

Figure l(a) shows the field emission pattern from a clean surface of <IlO>-oriented TiC(TiC<110>) tip which was obtained by flash-heated up to 1500-1600°C. There is an electron beam on the center. In the case of emission patterns of TiC<100> and <Ill> tips, there are no electron beam on the center. Therefore, TiC<110> tip is important from an applicative point of view. Observed emission patterns have been interpreted by emission from strong electric field portions of the tips of which faceting structures were made by flash-heating above 1500°C. A tip shape model i s shown i n Fig.l(b).

The top of the tip consists of polyherons composed of (100) and (111) surfaces whose surface energy is low in ~ i ~ 5 ) . Field emission stability of clean TiC<110> tips was investigated. It has been found that ste and spike-like fluctuations are characteristics of TiC<110> tipsly. We observed always this type of fluctuations in emission from the clean surface.

A.2 Surface-processed TiC<110> tips

A.2.1 Surface-processing of the TiC<110> tips

We found that usurface-processedu TiC<110> tip produces highly stable emission current. The processing of the TiC<110> tip is carried through in the following procedure. The first step is ; the

TiC<110> clean surface tip is heated at about 1 1 0 0 ~ ~ with the voltage off in the a s such as oxygen(02), ethylene(C2H4) or hydrogen

sulfide(H2Sy for suitable exposure time. The second step is ; the total field emission of about 1 0 pA is maintained for 30 minutes.

In the case of oxygen gas-processing of exposure time, x seconds, at 1 1 00' c in the vacuum of 1.33 x 10-4 Pa. We denote this surface- processing to O ~ ( X L , ~ ~ O O ~ C ) . Double or triple stage processing was also tested using two or three kinds of gases. For example, double stage processing using ethylene and oxygen gases is C ~ H ~ ( I OOL, 11 00' C) + 02(20L, 1 1 0o0c). Triple stage processing using ethylene, hydrogen sulfide, and oxygen gases is also possible.

A.2.2 Effect of the surface-processing

After this procedure, the effect of the surface-processing appeares, that is, the first effect is the change in the field emission pattern, the second effect is the increase of emission current under constant voltage condition, and the third effect is stabilization of field emission currents.

Figure l(c) shows the observed field emission pattern from the oxygen-processed TiC<110> tip. The central beam becomes much stronger than that of the clean TiC<110> tip. Such an emission pattern change occurs only in the case of oxygen, ethylene or hydrogen sulfide-processing. This pattern change is interpreted by sharpening the central portion of the tip whose model is shown in Fig.l(d). This interpretation is supported by following data and analysis.

Figure 2 shows Fowler-Nordheim plots of double stage surface- processed tip using ethylene and oxygen gases. Open circles are data of the clean surface tip flashing at 1 6 0 0 ~ ~ . Open and solid squares are data of surface-processed tip flashing at 950'~ and 1 1 5 0 ~ ~ , respectively. The gradient of the surface-processed tip is clearly smaller than that of the clean surface tip. The ratio of gradient of the surface-processed tip to clean surface tip is 0.4 in this case. When we flashed the surface-processed tip at 1 2 0 0 ~ ~ ,

Fig.2 Fowler-Nordheim plots of the double stage surface-processed TiC<110> tip at each flashing temperatures Tf,

Open circles ; Tf = 1600°c, open squares ; Tf = 950'~

solid squares ; Tf = 1 150°C, solid circles ; Tf = 1200'~

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then, experimental points, solid circles, returned to the state of clean surface tip. This indicates that the surface state of the surface-processed tip changes back to that of the clean surface tip by flashing at 1200' C.

Gradient of Fowler-Nordheim plot is roughly proportional to r p 312 where r and are the tip radius and the work function, respectively. If we assume the tip radius of the surface-processed tip to be almost the same as that of the clean surface tip, then we found the work function of the surface-processed tip decreases to about 60 % of the clean surface tip, that is 2.3 eV. This value is too small to the carbide surfaces. Therefore, the tip radius changes in the surface-processed tip. Sharpening of the tip happens at the surface-processed tip. Increase of emission currents due to the surface-processing is also interpreted by sharpening of the tip.

It has been found that step and spike-like fluctuations of the carbide tip can be reduced much by the surface-processing. Ethylene, oxygen and hydrogen sulfide-processed tips show highly stable

emission. In the most stable emission, current fluctuations are below 0.2 %. Especially, it has been clearly observed that decrease in emission currents is very small to be below 0.1 % per hour. This driftless current is a new feature of carbide field emission sources.

Here, we define "stable emission currentsM. This is the maximum current whose fluctuation width is below 1% at the initial 20 minutes after applying the voltage. In the case of single stage processing, this "stable emission currents" are several PA at the pressure of the order of 10-9 Pa. "Stable emission currentsn of double or triple stage processed tips are bigger than those of single stage processed tips. Emission stability of double stage processing using ethylene and oxygen gases is best among multi-stage processings. Figure 3 shows the emission current of about 10,uA from a double stage processed tip using ethylene and oxygen gases at the vacuum of 2 x 10-9 Pa. We have continuous emission of 1 0 p A for 5 hours at the extremely high vacuum condition.

Pressure dependences of "stable emission currentsn were

investigated to obtain information on current fluctuation mechanism.

Time ( min .)

Fig.3 Field emission currents with time after flashing the surface-processed TiC<110> t-ip at 1 1 00° C.

The surface-processing is C2H4 100L,llOOO~)

4

Figure 4(a) to (d) show the "stable emission currentsn vs pressure relation in four kinds of surface processings. Data of solid circles (a) and solid squares (b) are of a hydrogen sulfide- processed tip and an oxygen-processed tip, respectively. Data of open triangles (c) are of a double stage processed tip using

ethylene and oxygen gases. Data of open circles (d) are of a triple stage processed tip using ethylene, hydrogen sulfide and oxygen gases. These results show that current fluctuations are

proportional to the product of the emission currents and the pressure.

B NbC<110> tips B.1 Clean NbC<l10> tips

The field emission pattern from the clean surface of NbC<110>

tip is similar to that of clean TiC<110> tip. Moreover, it has been clarified that emission currents from the clean NbC<110> tip is unstable. Therefore, we investigated the effect of the surface- processing on NbC<110> tips.

B.2 Surface-processed NbC<110> tips

We confirmed the effect of the surface-processing on emission properties of NbC<110> tips. The emission pattern changed and the current stabilization occurred by the similar surface-processing as Tic. The Fowler-Nordheim plot also shows the similar behavior as that of Tic.

P (Pa)

Stable emission currents vs pressure relation of the surface-processed TiC<110> and NbC<I 10) tips.

(a) TiC<110> tip : H2S(10~,11000~) (b) TiC<110> tip : 0 ~ ( 2 0 ~ , 1 1 0 0 ~ ~ )

(c) TiC<110> tip : C2H ( 3 0 0 ~ , 1 0 5 0 ~ ~ ) + ~ ~ ~ ( 2 0 ~ ~ 1 0 5 0 ~ ~ ) + 02f20L,11000~)

(d) TiC<110> tip : C2H4(300L,11000~) + 0 ~ ( 2 0 ~ , 1 1 0 0 ~ ~ ) (e.) NbC<110> tip : C~H,I+(~ 0 0 ~ , 1 0 0 0 ~ C ) + 0 ~ ( 2 0 ~ , 1 0 0 0 ~ ~ )

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Figure 5 shows the

"stable emission currents" of the double stage processed tip using ethylene and oxygen gases at about 2 x lo-* Pa whose stability is higher than that of the surface-processed Tic tip. Figure 4(e) shows the

"stable emission currents"

vs pressure relation of the double stage surface-processed NbC<110> tip using ethylene and oxygen gases. Generally speaking, the surface- processed NbC tip is more stable than the surface- processed Tic tip as shown in Fig.4.

Time ( min.)

Pig.5 Field emission currents with time after flashing the

surface-processed NbC<110> tip at 1000° C. The surface-process is ~ ~ ~ ~ ( 1 0 0 ~ , 1 0 0 0 ~ ~ ) + o ~ ( ~ o L , I o o o ~ C ) . P=1.9 x 10-8 Pa C. Current fluctuation mechanism

We refer to current fluctuation mechanism. Pressure dependence of "stable emission currents1! from the surface-processed Tic and NbC<110> tips indicate that ionization of residual gases due to emitted electrons plays an important role in causing current fluctuatons. Current fluctuations of the surface-processed tip appeared at the larger current and poorer vacuum conditions arise from impinging ions entered into the emitting region. We propose that a main cause of step and spike-like fluctuations are due to impinging ion induced-surface atom migration.

References

1. Y. Ishizawa, S. Aoki, C. Oshima and S. Otani, Proc. XIth Int.

Cong. on Electron Microscopy, Kyoto, (1986) 223.

2. S. Otani, S. Honma, T. Tanaka and Y. Ishizawa, J. Crystal growth 61 (1983) 1.

3. S. Otani, T. Tanaka and Y. Ishizawa, J. Crystal Grouth 6 2 (1983) 211.

4. Y. Ishizawa, S. Aoki, C. Oshima and S. Otani, J. Vac. Soc.

Jpn. 29 (1986) 544 (in ~ a ~ a n e s e )

5. S. Zaima, Y. Shibata, H. Adachi, C. Oshima, S. Otani, M. Aono and Y. Ishizawa, Surf.Sci. 157(1985) 380.