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FIELD ELECTRON EMISSION FROM METALLIC GLASS
M. Kanitkar, D. Joag
To cite this version:
M. Kanitkar, D. Joag. FIELD ELECTRON EMISSION FROM METALLIC GLASS. Journal de
Physique Colloques, 1986, 47 (C7), pp.C7-127-C7-132. �10.1051/jphyscol:1986723�. �jpa-00225916�
JOURNAL DE PHYSIQUE
Colloque C7, suppl6ment au n o 11, Tome 47, Novembre 1986
FIELD ELECTRON EMISSION FROM METALLIC GLASS
M . M . KANITKAR and D.S. JOAG
Department of Physics, University of Poona, Pune-411 007, India
Abstract
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The paper reports investigations o n field electron emission from Fe70Cr5SilOB15 metallic glass. Fowler-Nordheim (F-N) characteristics are obtained under different conditions of field evaporation. Linearity of t h e F-N plots and t h e nature of t h e ultra-vidlet photoelectron spectmm show t h e nearly h e electron like nature of t h e metallic glass. The observed temperature dependewe of t h e field emission current with dc and pulse field evaporation is explained o n t h e bask of compositional variation in t h e surface and its l'imperfectll nature, to a certain extent. These &ts are supplemented by field i o n micmscopic observations a d discussed in view of t h e re5
e n t atom probe work by several authors. Current densities of t h e order of 10 A/sq.cm. have been observed an3 attributed largely to t h e isotropic emission and t h e e r h a w e d field factor due to surface roughness.I
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INTRODUCTIONThe structural and electronic properties of metallic glasses have been a subject of g r e a t interest f o r t h e last few years /1,2/. X-ray and neutron dif'hction /3,4/, and extended X c a y absorption fine s t m c t u r e analysis /5/ have been extensively used to characterize t h e structure of metallic @;las;es. X-ray and ultra-violet photoelectron spectroscopy of various metallic @ a s specimers cleaned by argon ion bombardment, has been done to study t h e electronic properties / 6 / . Field i o n microscopy (FIM) has been used to study the structural c o w e h t i o r s in metallic @ses /7,8/. Atom-probe FIM studies /9,10,1 I/
of some metallic glasses have shown t h a t t h e surface composition a t t h e specimen tip strorgly deperds on t h e process of field evaporation.
Field electron e m h i o n micmscopy (FEM) is a surface sensitive tool t h a t c a n be used to study t h e electromc properties of t h e emitter material /12/. Some preliminary obser- vations on field electron e m i s i o n from metallic glass Pd 77.5 S i 16.5 Cu 6 have been reported by Heirrich e t aL /13/. This study w a s made h m t h e point of view of metallic glass as t h e field electron e m i s i o n source. However, no cleanill@; procedure was followed by t h e authors prior to t h e field emission observations. We report below, detailed inves- tigations o n field electron e m i s i o n h m metallic glass. Specimers were prepared from t h e metallic glass Fe70Cr5SilOB15, which w a s available i n wire form /14/.
I1
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EXPERIMENTALThe specimen t.qI w a s prepared by electrochemica' polishulg i n 2 % percNoric acid in 2-Butyloxy4.41anol; 24 V .dc was used with a gold ring as a cathode. Uniformly tapered tip w a s .?elected. Satisfactory t.qI shape was conf!irmed by observirg t h e tip image with t h e electron micro-probe. Field e m w o n a r d fie3.d i o n microscopy o f t h e specimen tip w a s c-g&ped out in a n all m eta1 ultra-high vacuum chamber. The chamber w a s first evacuated to 10 mbar with liquid nitrogen trapped oil diffkion pump and rotary pump combination.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986723
JOURNAL DE PHYSIQUE
-Figure 1 : FIM image (argon) of Fe70Cr5SilOB15 metallic g l a s ~ . A dark circle i n t h e cent& of the image is due to a hole i n the screen.
Figure 2 : FEM image of the specimen tip. A dark patch i n the image is a dead portion of the screen.
It was followed by pumping with sputter ion pump and titanium qtter pump. After a mild bakeout a t about 1 30° C f o r six hours, pressure below 2 x 10- mbar w a s obtained i n the chamber. Baking a t higher temperature w a s avoided t o e m e t h a t the amorphous s t a t e of the specimen did not change. The amorphicity of t h e specimen wire w a s pi-€- checked by X-ray diflhction.
The specimen tip was cooled t o liquid mtrog5n temperature and argon ion images were seen (argon m e d a t t h e pressure of 2 x 10 mbar). Figure 1 shows a typical field ion micrograph of t h e tip, recorded after substantial dc d e l d evaporation The process of field evaporation followed by field ion imaging removes the asperities and contamination on t h e microscopy. tip and leads t o a clean and urdform A l l t h e FE M observations w e r e recorded a t a preswre below 2 tip surface suitable f o r t h e field e M o n x 10 mbar.
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Field evaporation was done, i n vacuum, for about five mimtes f o r cleaning the tip surface.
The polarity of the high voltage supply to t h e tip was reversed and field electron emission pattern w a s seen. This process of field evaporation followed by field electson imaging, was repeated tjIl a uniformly i l l u m h t e d image was obtained. Further field evaporation did not cause any significant variation i n t h e value of field electron emission (FEE) current f o r a particular voltage. Figure 2 shows a typical FEM pattern of the tip. Due t o t h e use of microchannel plate image i n t e r d l e r , it w a s p o m l e t o obtain FEM image a t suf'flciently low voltages (about 1 kV). This reduces the possibility of t h e tip damage caused by residual gas ion bom bardment. The FEE c w e n t r v d t a g e (I-V) characteristics were recorded and repeated a t least four times. While measuring the FEE current t h e microchannel plate and screen were grounded. For cleardng t h e tip both dc and pulse field evaporation w a s used. For pulse field evaporation a variable puke height (500 V t o 2.5 kV) high voltage nanosecond laytmn pulser /15/ was used. The tip was cleaned by field evaporation prior to recording the I-V characteristics. T h a e were found t o be highly reproducible. Figure 3 shows the F-N plots obtained fkom I-V data taken under different conditions of tip temperature and evaporation.
The metallic glass specimen w a s also subjected t o ultra-violet photoelectron spectroscopy (UPS) using Helium I (21.2 eV) source. A special sample holder w a s used for_ e metallic glass wlre sample, and the measuremerts were done a t a pressure below 10 ''mbar. The sample was hre-eleaned by argon ion bombardment (5 keV) f o r 30 miw.
: F-N plots recorded under different corditiors;
a With pulse fleld evaporation alld tip a t room t e mperatwe (300 K),
F
(b)+
With pulse Seld evaporation alld tip a t liquid nitrogen temperature (c) A With dc fleld evapwation and tip a t liquid nitrogen temperature (dl a With dc field evaporation and tip a t room temperature.Figure 4 : UPS spectrum of Fe70Cr5Si10B15 metallic glass.
C7-130 JOURNAL DE PHYSIQUE
The m t u r e of t h e F-N plots taken under different corditiors w a s found to be linear W a t i r g a f r e e electron like behaviour. This is expected as t h e band &xwcture e f f e c t s are not seen i n t h e F-N plots /16/. The UPS spectmm of t h e metallic glass specimen, depicted i n t h e Figure 4, also supports this view.
It was found t h a t d o p e of t h e F-N plot is very much sersitive to t h e field evaporation con3itiors t h a t are used f o r c l e a n k g t h e tip. Field emission observatiors with t h e tip held a t room temperature (300 K) and a t liquid nitrogen temperature u d e r different field evaporation coxlitions (Table 1) lead to t h e following results :
TABLE 1
Field ekciznn emission aarerk obwvations
Field Tip Field F-N parameters k=m/V
e v a p m t i o n temperature emission h 3 J
m ode current Slope B (X lo3) A/cm 2
( ~1 A) at m 1.71 kV
300 K 3.25 -0.36 -0.12 0.210 I 109
liquid mtrogen 4.00
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0.37 0.216 0.9x
lo9pulse 300 K 7.15
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0'25+
0.06 0.146 8 X lo9liquid n i b g e n 5.10
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0.28 0.160 6 X lo9 B = log (A/ A ), where A , A are t h e pre-exponential factors of t h e F-N plots a t 300 K and a t liquid nitr0"gen tempera& respectively.i) W h e n d c f i e l d e v a p o r a t i o n w a s u s e d f o r t h e t i p d l e a ~ , i t w a s f o u n d t h a t F E E current a t liquid mtrogen temperature w a s higher than t h a t a t room temperature.
i3 When pulse field evaporation (with approximately 20% pulse h c t i o n ) was used f o r b p cleardng, t h e FEE current a t liquid nitrogen temperature w a s found to be lower than a t room temperature.
iii) Under both t h e c o n l i t o r s of field evaporation rn signif'icant change i n t h e FEM image was seen, either a t room temperature o r at liquid nitrogen temperature.
I n particular, t h e haziness and uniformity of t h e pattern remained umhanged.
iv) The h c t i o m l change i n t h e FEE currents ( a t a particular applied voltage a t room temperature and a t Q u i d mtrogen temperature) A I/I (where I is t h e FEE current a t liquid nitrogen temperature), was found to be abouto10 to 15%:
v) No signELcant variation w a s found in B = log A/A,?where A and A are t h e pre- exponentid factors of t h e F-N plots, a t room temperature and a t budi mtrogen temperature respectively (urder t h e same coxiftion of field evaporation).
d
T h e m e t h o d g i v e n b y V a n O o s t r o m / 1 7 / w a s u s e d f o r c a l c u l a t i r g t h e e m ~ o n c u r r e r r t density "J", h m t h e value of k=
m/V (where, m is t h e slope of t h e F-N plot and V is t h e applied voltage). For these calculatiors t h e value of t h e work f u m t i o n w a s assumed to be 4.4 eV, based o n t h e values of work f u m t i o n of t h e constituents.The values of t h e current dersities, thus computed have been listed i n Table 1.
Field ion m i c m o p i c obsenratiors, with neon a& argon as i m a g i r g gases, were made under similar conditiom of field evaporation. The atom probe studies /9,10,1 I/ o n some
metallic @ a s specimers show t h a t t h e pulse field evaporation with a pulse h c t i o n of 20%, resilts i n near stoichiometric composition a t t h e tip M a c e . We have fourd t h a t t h e d c field evaporation leads to t h e image co- a large m m b e r of very bright spots. The m m b e r of such i n t e r s e image spots w a s considerably reduced when pulse field evaporation w a s carried out. This is in agveement with t h e atom-probe &ts. This e f f e c t was promirently seen with neon as i m a e j l l g gas.
FiRure 5 : FIM images (Neod recorded a f t e r
(A) dc field evaporation, (B) Pulse field evaporation.
Figure 5 clearly shows t h e e f f e c t of pulse field evaporation. Under d c field evaporation, due to t h e preferential evaporation of metallic species, a metalloid rich tip surface is obtained. The intense image spots seen i n FIM images seem t o be due to t h e protrudirg metalloid atoms o r their clusters. Although such sharp points appear as i n t e r s e image spots in t h e FIM images, they are not s e e n i n t h e FEM image due to lower resdlution of t h e technique. The FEE current, however, does reflect t h e situation.
The imrease in t h e FEE current on coolirg t h e tip to liquid mtrogen temperature under dc field evaporation condition and its decrease with pulse field evaporation condition may, a t this juncture, be attributed to t h e "imperfectft nature of t h e surface o n t h e atomic scale leadirg to t h e surface compositional variation under different conditiors. I n UPS an3 XPS experiments t h e argon ion etct-iu?g, used f o r c l e a n k g t h e specimen surface of some metallic glasses, w a s found to produce t h e preferential sputterirg of some of t h e comtituent species /2/. The process of field evaporation is thought to produce a result similar to t h a t due to t h e ion bombardment cleanitg. Thus it is w e s t e d t h a t t h e field evaporation can lead to t h e observed behaviow of t h e FEE currents. It appears that in case of t h e metallic glass field emitter, it is difficult to achieve a high degree of c o m p o s i t i o d uniformity of t h e surface owin@; to its tTimperfecttt mture.
The average e m h i o n c m n t d e n s i t e s for t h e metallic glass tip &ace were found to be very high as compared to t h e average emission current demities gerwally obtained f h m t h e si@e crystal emitter tips. Although t h e above method is valid f o r low current demit;es, w e have fourd it convenient to apply it in this case to g e t m a h estimation of t h e current derdties. Even if t h e estimation is wrorg by a n order of magmtude, t h e values of t h e current densities are stiu corsiderably large. The observed high current dersities may be attributed to t h e isotropic e m i s i o n Prom a l l over t h e tip &ace a d to t h e hgh degree of surface roughness.
IV
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SUMMARYi) The FEE studies show the h e electron like behaviow of t h e metallic glass.
ii) The field evaporation con3itiom a r e found to have a strong influence o n t h e surface composition.
C7-132 JOURNAL DE PHYSIQUE
hi) The roughness of the emitter surface and t h e isotropic e m h i o n are thought to yield t h e observed high emission current demities.
ACKNOWLEDGEMENT
The flmrcial asiistance by t h e Department of Atomic Energy, Govt. of India is gmtefLilly acknowledged.
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Thanks are due t o Prof. Y. Waseda f o r providitg t h e metallic glass w i r e samples. Facilities provided by t h e Head of t h e physics ~ e ~ a r % m e n t , Poona ~ r r i v e m i t ~ , are g r a t e m y acknowledged. Authors aka thank Prof. P.L. Kanitkar f o r cooperation and help. Thanks are due t o D r . S. Banerji, BAR C, Mumbai, f o r t h e electron probe i m a g i r g . Authors are tharkf'd to Prof. A S . Nigavekar and Dr. Mrs. S.K. Kulkarm f o r t h e UPSREFERENCES
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