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DIRECT OBSERVATION OF FERRO-ELECTRIC DOMAINS IN TRIGLYCINE SULPHATE (T. G. S.) USING THE SCANNING ELECTRON MICROSCOPE
(S. E. M.)
R. Le Bihan, M. Maussion
To cite this version:
R. Le Bihan, M. Maussion. DIRECT OBSERVATION OF FERRO-ELECTRIC DOMAINS IN TRIGLYCINE SULPHATE (T. G. S.) USING THE SCANNING ELECTRON MICROSCOPE (S. E.
M.). Journal de Physique Colloques, 1972, 33 (C2), pp.C2-217-C2-219. �10.1051/jphyscol:1972275�.
�jpa-00215011�
OBSERVATION DIRECTE DES DOMAINES FERROÉLECTRIQUES C2-217
FIG. 2. - La frontière de l'extrémité négative d'un domaine apparaît blanche ou noire suivant que la charge due au faisceau électronique est positive (Vo < V Z ) OU négative (VO > V2).
méthode permet d'observer le spécimen sans destruc- tion des domaines à 1800 du T. G. S. La nucléation, la tion, et d'obtenir des informations électriques sur sa croissance et la régression de domaines ferroélectriques surface, ce dernier point étant particulièrement intéres- sous l'action du faisceau d'électrons a été observée.
sant pour l'étude des cristaux ferroélectriques. Nous Nous remercions le Dr Coates et le Prof. Chapelle avons appliqué, avec succès, cette méthode à l'observa- qui nous ont fournis les cristaux de T. G. S.
DIRECT OBSERVATION OF FERRO-ELECTRIC DOMAIN S IN TRIGLYCINE SULPHATE (T. G. S.) USING
THE SCANNING ELECTRON MICROSCOPE (S. E. M.)
Abstract. - The conditions necessary for a good observation of insulating samples (without inetallization) with a S. E. M. are calculated. The application to the study of ferroelectric T. G. S.
crystals is made : observation of antiparallel domains without modification of its surface, modifica- tion and nucleation of domains by the electron beam.
We shown the possibility of observing ferroelectric crystals with S. E. M. [l]. But in the studied crystals (BaTiO, for example) the contrast could be attribute to surface potential differences or to surface topo- graphy associated with domains. In order to observe a domain contrast due only to electrical potential diffe- rence, we observed ferroelectric domains of T. G. S.
crystals by using the S. E. M. (J. S. M. 2 of J. E. O. L.
Co) in the secondary emission mode [2]. Experimental difficulties due mainly to the charging of the insulating sample by the electron-beam led us to study the most favorables conditions for observing any insulator in a S. E. M. We shall detail in first the charging mechanism in order to explain ours observations on T. G. S.
Let us consider the swept area a2 as one plate of a condenser. If we neglect boards effects, the potential between this surface and the holder is V = Qd/&a2, where Q , d and E are the surface charge, the thickness and the dielectric constant of the sample. The real accelerating voltage Va is not the cathode voltage V , but Va = V
+
V0. Let I, and I, be the primary and the secondary current. It is known that the variation of the rate of secondary electron emission (6 = IJI,,) versus accelerating voltage is a curve dependant of the mate- rial. It can be shown that if Vo is not too low, an equilibrium is reached. At this state, the quantity of the charge arriving on the surface during one scan equals that collected by the detector more that which15
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972275
C2-218 R. LE BIHAN AND M. MAUSSION
goes through the sample during the same time T. If p is the resistivity of the sample, we can so write :
In most cases EP is much greater than T and this sim- plifies in :
which gives
Eq. (1) is that of a straight line cutting the rate curve 6 versus Va in two points M l et M, and passing by the point A of coordinates (V,, 1) (Fig. 1). M, is an equili- brium point because both slopes are of opposite sign.
Let V, be the voltage at point M,. When the cathode voltage V, differs from V,, the specimen charges up to the potential V, - Vo which is positive if
FIG. 1. - Variation of the rate of secondary electron emission 6 = I , / l , versus accelerating voltage Va. Determination of the
equilibrium point.
and negative if Vo > V,. For Vo < VI, Va becomes very small and the voltage through the sample is nearly Vo. At the equilibrium point we have :
We see that for V, = V, we have 6 = 1, the specimen does not charge up.
The former caIculous is very near that which is made to get the exact curve 6 vs V, of any dielectric bom- barded continuously by an electron beam. Here the important point is that for observing any dielectric specimen (ferroelectric included of course) in a S. E. M., the cathode voltage Vo must be chosen equal to the voltage V2 definds before. For T. G. S. this value is slight higher than 2 kV. For BaTi03, which gives much more secondary electrons, it is about 5.5 kV.
These values are indicative and may Vary as we shall see below. Its experimental determination is possible when the dielectric sample is fixed in the S. E. M.
It just consist in charging up a small area by the beam and then in observing it with a smaller magnitude.
Black or white central zone indicates respectively positive or negative charge, so that the cathode voltage must be increased in the first case and decreased in the other. This may be repeated until the contrast is inverted, V, lying therefore between the last two values of V,. The fine determination of V2 may be made by the same process and finally by increasing the detector sensibility or the primary beam current.
We must say that there is a theoretical limitation to the observation of a dielectric in a S. E. M. According to Bruining's law, 6 varies with the angle of incidence i :
6(i) = d(0) exp(- a2 cos i) and for two differents values il and i, of i :
6(i1) = 6(i2) exp( - al(cos il - cos i2) ) .
The two rate curves 6 vs Va been homothetic, M, will be divided into two and each incidence will have its own equilibrium potential V,. To prevent excessive charging, when observing a non plan dielectric in a S. E. M. it must be taken care to use : 1) mean value of V,, 2) low primary current, 3) fast scanning speed. In these conditions the specimen may be obser- ved before each of its points be charged up to its own equilibrium potential.
Except the above restriction, we are now able to observe with a S. E. M. a dielectric sample without any surface treatment. The advantages of this direct obser- vation method are evident. It prevents specimen from coating, that which is very precious in studying ferro- electric materials. A positive end of a domain will not emit so much secondary electron as a negative one, whereas after coating, electrical informations about surface disappear. At most, it keeps specimen intact for others experiments or for utilization.
Before starting the scanning, the density of charge at the surface of a T. G. S. crystal is
+
P,. When the cathode voltage is different of V,, after n scans by electron beam this charge becomes :where 6, is the rate of secondary electron emission for scan number r. This will tend more or less rapidly to o = ( V , - V,) &Id, according to the factor I, T/a2.
This explains why the contrast between two ferro- electric domains of T. G . S. disappears after few scans and why this effect is more rapid at high magnifica- tion ; after only boundaries are still visible (Fig. 2, 5 and 11 of [2]). Coates [3] shows also photographs of boundary contrast. With a correct value of Vo (Vo = V,) we have succeeded in observing steady contrast even at high magnification.
Domains's walls contrast needs more detailed expla- nations. The boundary of a same domain may be white or black, either the positive or negative end is observed.
DIRECT OBSERVATION OF FERRO-ELECTRIC DOMAINS C2-219
This contrast depends only on the sign of the charge of V,. For observing the evolution of the phenomenon introduced into the surface layer. We concluded that conditions of slow charge, as we defined for non plan the zone where polarization varies from - P, to
+
P, specimen, are required. Comparison between the pic- does not retain the surface charge, because of the hori- tures of each consecutive scan reveals growth and zontal field due to the variations of the polarization. nucleation of domains or regression (see for example Figure 2 shows the negative end of a same domain with Fig. 8 of réf. [2], where growth and nucleation areFIG. 2. - The boundary of the negative end of a same domain appears white or black ac&rding as the surface charge due to the electron beam is positive (Vo < Vz) or negative (Vo > Vz) respectively.
black or white boundary. This was obtained by invert- ing the sign of the surface charge of the sample ; cathode voltage V , being higher or lower than equili- brium voltage V,. White boundary means positive charge i. e. Vo < V,, and vice versa. This experiment is reproductible and similar with a positive end. We also observed same domains with a part of their boundary black and the rest white (see Fig. 3 of ref. [2]). In this case, the difference of charge was not due to cathode voltage, but to equilibrium voltage. A bad cleavage made two different incidences and, as we soon pointed out, two different equilibrium voltages.
The cathode voltage was between these two values so that both zone charged of opposed sign.
Modification of ferroelectric domain structures by the electron beam have been observed. To do this, cathode voltage Vo is voluntary given a value different
evident). More detailed study of this phenomenon with help of S. E. M. may contribute to ferroelectric domain switching's knowledge.
Experiments on BaTiO, using the processus des- cribed here have been also made and results will be published elsewhere.
In conclusion, we have displayed that correct value of some parameters in a S . E. M. permits direct observa- tion of any dielectric without coating. Such a method, allows to observe the specimen without destruction and to obtain electrical informations from the surface.
This last point been very interesting in the study of ferroelectric crystals. We succeeded in applying this way to observe 1800 domains of T. G . S . Nucleation, growth and regression of ferroelectric domains by the electron beam have been observed. We thankDr. Coates and Prof. Chapelle who give us T. G . S. crystals.
[Il LE BIHAN (R.) and SELLA (C.), J. of the Phys. Soc. of
Japan, Proc. of 26 Intern. meet. on ferroelectricity, 1969, 28, 377-79.
[2] LE BIHAN (R.) et MAUSSION (M.), C. R. Acad. Sci. Paris, 1971, 272, serie B, 1010.
[3] COATES (D. G.) and HAW (N. S.), Microscopie électro- nique, 1970, 1, 259.
