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PECULIARITIES OF FIELD ELECTRON EMISSION FROM HIGH - TEMPERATURE

SUPERCONDUCTORS

R. Bakhtizin, S. Ghots, V. Mesyats, S. Shkuratov, Yu. Yumaghuzin

To cite this version:

R. Bakhtizin, S. Ghots, V. Mesyats, S. Shkuratov, Yu. Yumaghuzin. PECULIARITIES OF FIELD

ELECTRON EMISSION FROM HIGH - TEMPERATURE SUPERCONDUCTORS. Journal de

Physique Colloques, 1988, 49 (C6), pp.C6-495-C6-500. �10.1051/jphyscol:1988684�. �jpa-00228182�

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PECULIARITIES OF FIELD ELECTRON EMISSION FROM HIGH

-

TEMPERATURE SUPERCONDUCTORS

R.Z. BAKHTIZIN, S.S. GHOTS, V.G. MESYATS*, S.I. SHKURATOV* and Yu.M. YUMAGHUZIN

Department o f Experimental Physics, Bashkir S t a t e U n i v e r s i t y , Ufa 450074, U . S . S . R .

" ~ n s t i t u t e o f Electrophysics, Ural Department Academy o f Science, Sverdlovsk 620219, U . S . S . R .

Abstract

-

The field emission properties of the Y-Be-Cu-0 superconduc- ducting ceramics have been examined at temperature range 77

+

³⁰⁰^Kⁱⁿ

e vacuum of 5 x ~''01 Torr. The obtained results are associated nith de- gradation of subsurface region of the emitter due to the depletion of oxygen.

I. PTRODUCTIOIV

The discovery of the high-temperature superconductivity in ceramic ma.terials

/ l , 2/ gave birth to an extensive investigation of the properties of these

mnterials. The electronic nature of the superconductivity in this class of materia,ls is now firmly established /3/. It should be emphasized that in this cn.se the superconcluctivity is observed a.t relatively low values of the carri- or concentre.tion ( (2

+

⁵⁾^x^?02' C U I - ~ /4/ 1.

One of the promising wa.ys to study the electron structure of superconducting metal oxides is an approa.ch based upon the field emission experimenta.

This approa.ch is essentially a study of the one-particle tunneling of an electron from a superconductor into vacuum. A detsiled knowledge of this procest could give rise to the development of a microscopic theory of superconductivity for this class of materials.

The very first investigations of the atomic structure and chemical composition of superconducting ceramics carried out by using the field ion microsc- ope and the atom probe /5, 6 / showed a pronounced distinction between the superconducting and the normal phases of the specimens under investigation.

The present paper deals with a study of the electron subsystem of high Tc superconductors by measuring the current-voltage and noise characteristics of the field emission current and the energy distribution of field emitted electrons.

2. MPERIlJEXtTAL

The experiments were carried out nith point emitters of YBa2Cu307-x

( 0 < x

<

0.5 ). Original ceramic specimens were 3 x 3 x 10 mm in size (the temperature of superconducting phase transition, Tc = 92 $1. Monocrystalline films rere 3 x 3 m with a. thickness of up to 20 mu ( Tc = 80 K ). The poin- ts mere prepared by mechanical sharpening (or crushing) of the specimens as well 8,s by ion etching or el.ectropolishing, as described in / 5 / . To evaluate

the possible effects of these procedures on the chemical composition of the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988684

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C6-496 JOURNAL DE PHYSIQUE

subsurface region of a specimen, the SIRS-analysis of the similarly treated check samples was carried out. It turned out that electrochemical etching had considerably changed the stoichiometric composition in the subsurface region of the specimen, specifically, it led to its enrichment with Cu. The minor ef- fect on the stoichiometric composition was shown by ion etching. The monocrystalline sharply pointed tips were also preferentially prepared by spelling the specimens, followed by washing them in ethanol and mounting on the holder with a conducting epoxy.

All measurements were carried out in a vacuum of

to-' -

^lo-" lorr. To avoid loss of superconductivity, the specimens were not exposed to high temperatures while pumping out. The field emission was initially unsteble and came ge- nerally out on3.y of a few emission centers. In order that it was Igore regular the emitter surface was cleaned by means of field desorption.

3. RESULTS ARD DISCUSSIOIi

3- 1. Current-voltage characteristics

The shape of the current-voltage characteristics and its temperature depen- dence were largely determined by the technology of preparation of ceramics.

As a specimen waa cooled to 77 K field emission became more stable, the spre- ad in the values of current decreased and the characteristics became reversi- ble. In most cases the current-voltage chare.cteristics obeyed the Fowler- Nordheim law (Fig. 1 a).

Fig. I. (a)- The Fovrler-Nordheim plot from a sample of YBa2Cu307-x. (b)

-

Corresponding oscillograms of the field emission current p ~ l s e s ; f , / ~ is the time of rise of the field emission current relative to the beginn-

6

ing of the voltage pulse to a half of its peak value, andrs is the time of rise of the field emiasion current from the moment

2'

to its stee.dy-state value.

The kinetics of the development of field emission current on application of a high-voltage rectangular pulse to the emitter was also investigated (the pulse duration was 10

+

¹⁵⁰^pa). ^Asis seen from the current waveform (Fig. 1 b) the field emission from ceramics at low values of anode voltage develops not instantly, as in the case of meta.ls, but it occurrs with a con- siderable delay; the la-tter refl.ects the dynamics of esta.blishment of the

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characteristic times: ZtI2 and

Ts.

!Phis is believed to be associated with the capture of current carxiers by both the traps and the surface states of the high Tc superconductor, as well as with rather a low mobility of the cur- rent carriers. In our experiments

T112

end

Ts

ranged between 0.2 and 20 ps and between 30 and t20ps, respectively.

3-2. Investigation of field emission current fluctuations

The noiae of the field emission current from the surface of the high Tc su- perconductor is due to fluctuations of the local charge density in the sub- surf ace region.

Stationary mode of measurement. In the frequency range f (10 Hz the noise spectral power density at both T = 300 K and T 77 K has the following form

(Pig. 2 1 % if

S(f)=So/f

,

^{( 1 )}

where the index of the spectral power density is 1.1

<

I(

<

1.7. With decre- asing the temperature the noise level decreases, which may be associated with the weakening of the migration processes on the emitter surface. We have she- w n earlier /7/ that the spectra1 density of fluctuations may be defined as in ( 1 at _{So =}_{d - I}2 _/f

if

_.N

,

( 2 ) where I is the average field emission current, I? is the number of emission

centers (conducting micrograins) and c( = ( 2

+

⁸⁾^x is the Rooge coef- ficient. Measurements in the frequency band 0.02

+

¹⁰He have shown that the

Fig. 2

-

Spectral density function Fig. 3

-

The density probability dis- of the field emission current noise tribution functions of the emission for YBa2Cu307-, emitter. noise from YBa2Cu307-xemit ter at vasi-

ous currents compared to a normal dis- tribution (two straight lines); n is the number of the channel proportional to the amplitude of the fluctuation.

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number N lies stithin the range of 706

+

⁹⁰⁷^(Fig.²⁾which by 2 f

4

orders of magnitude exceeds those values for semiconductor field emitters /7/. Du- ring the emitter operation the value of N shows a tendency to aecreaae by I $ 3 orcPsrs of mragnitufte, vhich is assumedl to be releted to the emitter de- gradntion, specifically, due to a release of the oxygen from ceremics an8 8n increase of the degree of non-uniformity of the paint tip.

The experimental smplitude distribution of current fluctucptions is similar in its form to the Gaussisn distribution for different vaLues of the field emiasion current an3 different temperetures (Fig. 3). The inxignificent variati- ons of the normalized central moments (thrst of skewnes~.

ae3,

8116 kurtosis,

z 4 ) / 8 / with increasing the fiela emission current a,re indicative of s smal.1 deviation of the distribution of low-frequency f1uctuetion.s from the normel one (Fig. 4).

Pulse mode of meeauremen*. This mode alllows measuring the temporal characte- rietics of low-frequency field emission current fluctuwtions. By using the stroboscopic method of'regiatretion ancl consjclering the possible suectrel Ciatartion@ in the course of ssmpling /9/ there have been obtsined the spect- re nf f i e l d e m j wion current fluctuations Prom YBa2Cu307-,@mi tter (Fig. 5).

FIEU) EMISSION CURRENT,

Fig.4

-

The measured current de- Fig.5

-

Spectral density functions measu- pendences o f coefficientsae 8n8 red in pulse mode at ;pule@ duration tOOl(e

ae4

for YBe2Cuj07-x field sktter. clock frequency 1.5 HE, deley time of sampling relative to the beginning of the pulse T d 53

+

⁹⁰^ps.

It turned aut thot the fluctuations have the S(f) spectrum which can be veil

~,pgsaximeted by the I/f function end it is establi~hed I not immer2iately after the high-voltage pulse a.pglicstion. Such a behavfour of S(f) is quite under- standab3e if one assumes thet emission centers with shorter lifetimen are lltriggeredlt quicker than those with l o v r ones and that they are predominant at the beginning of the pulse, therefore, the Pow-frequency fluctuations will be -~eakened. From the S ( f ) function, corresponding to the index 5 ?, effec-

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I - ^loe9

A was found to be equal to 80 pa. The above value was used to evs- luate the Hooge coefficient for the material under investigation

d

-

~(f)-f(:.~~~~/~-e 9 2.5 x 1 v 3 , ( 3 ) where e 3.6 x ?0-" K is the electrbn charge. The obtained value is in gooa agreement nrith that for such fluctu&tions in the bulk of semiconductors /7/.

3-3. Energy distribution of field emitted electrons

According to different authors, the magnitude of the order parameter in the high T, supekconductore (i. e. the energy gap 2

n(0)

⁾is approximately

2 0 + 100 meV; so that the energy gap can be resolved by the presently availa-

ble energy analysers. Field emiaeion electron energy distributions for T = 300,sad 85 K were taken by a dispersion energy analyzer. The instability of field emiesion manifested itself in that both one-peaked and multi-peaked epectra oould be obtained as well as in spectrum shifts relative to the Fermi level. To diminish the instability effects a single fast scanning of the spectrum ( 0 , 3 f 0.5 sec) at low field emission currents was used.

The field emission electron energy distributions are similar in shape both for ceramic emitters and monocrystalline ones at T = 300 K an8 they are shif- ted by several eV below the Fermi level as compared with similar curves for metals (Fig. 6). The spectral half-width of the distribution (FWHM), which is

Fig. 6

-

Experimental field emission energy distribution plots for

ceramic emitter at va- rious emission currents.

equal to 300 meV (curve 1 1 , is closein its magnitude to the respective pem- meter for metals but it is considerably less than that for semiconductor field emitters. The FWKM value for monocrystalline emitters at T E 85 K is 200 meV. The value of FWHILT had a tendency to increase with increasing the ve- lue of the field emission current ( F i g , 6 , the curves ? , 2 , 3 and 4).

In the course of measurements a gradual increase o f FWHM from 300 meV to 700 meV and Its graduel shift into the low-energy region relative to the Fer- mi level (up to 10.2 eV) was observed (Fjg, 7). Such a beh~viour might be eo-- gociated with dielectrisation of the emitter surftice region due to the dell-

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beration of oxygen through the ceramic-vacuum interface with the presence of a high electric field and current flow. It should be noted that at T = 85 K the energy spectrum for ceramic emitters strongly differs in shape from that for metals, as plotted in accordance with the free electron model (Fig.7 a).

Fig.7

-

Energy spectra for YBa2Ca307-x field emitters (slat the beginning of measurements; (b) after 6 hours of operation at field emission current I = 2.4 x lo-' A.

For comparison the sane a w e (dashed line) was plotted in a semilog scale:

the slope of a steep section of the latter was used to determine the value of kT, which exactly corresponded to the temperature of the emitter. Thus, the above section is believed to be the Boltsmann tail of the field emission electron distribution.

The obtained results in their,totality are quite typical for the given class of materi~ls. A more detailed interpretation requires a further extensive study, perticulrzrly, for the temperatures considerably below T = 77 K.

REFERENCES

/1/ G,Bednorz and K.A.Muller. Z.Phys.

%,

189 (1986).

/2/ C, W. Chu, P.H.Hor, B. L.leng, L. Gao, 2. J.Huang and Y. &.Wan& Phys. Rev. Let.

s,

405 (1987).

/3/ G.E.Giogh et al. Nature,

326

(1987) 30.

/4/ A. J. Panson et al. Preprint Westingh. Res. Dev. Center, USA, 2 Feb., 1987.

/5/ G. L.Kellogg and S. S.Bremer. Appl.Phys.Lett., 51(1987) 1851.

/6/ A. J. Melmed, R. D. Shull, C, K. Chiang, H. A.Powler. Science, 239 (7988) 176.

/7/ R. 2. Bekhti~in, S. S. Ghats, ~.'~.~hernin-~ekhnuk. J.de Phys. (1987) C 6

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^203,

/8/ J. S.Benda t

,

^A.^G.Persol. Random Data Analysis end Measurement Procedure.

WiLey, New York. 1971.

/9/ R. Z.Bakntizin. Acta Univ. Wratisl., No 1025 (7988) 47.

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