Methods of acoustic microscopy in investigation of high-temperature superconductors

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0038-1098/8953.00+.00 Pergamon Press plc

S o l i d SCare Communications, V o l . 72, No. 12, pp. 1177-1181, 1989. P r i n t e d in Great B r i t a i n .

METHODS OF A C O U S T I C M I C R O S C O P Y

IN INVESTIGATION

OF HIGH-TEMPERATURE

SUPERCONDUCTORS Bukhny M.A.,

Chernosatonskii Mayev R.G.

Center of A c o u s t i c Microscopy, of Physical

Khodan A.N. #,

& Soifer Y.M.*

Institute of Chemical

4, Moscow, # - Institute

L.A.,

117334,

Chemistry,

Physics,

Kosygina str.,

USSR

Leninskii prosp.,

31, Moscow,

117915,

USSR , - Institute

of Solid State Physics, (Received 12 October

Chernogolovka,

Moscow reg., USSR

1989 by V.M.Agranovich)

The new method of examination of microstructure acoustic m i c r o s c o p y - was used for studying HTSC-materials (ceramics, monocrystals and think films). The obtained high-resolution images (with resolution up to 0.4mkm) v i s u a l i z e the specimen topography, variations of the local acoustic properties, in particular, surface and subsurface defects cracks, twins, phase and structural discontinuities, peelings in films. For bulk specimens (monocrystals and ceramics) we obtained a quantitative characteristic - a local (on a section of 10xl0mkm ~) Rayleigh wave velocity, which makes it possible, in particular, to characterize individual crystallites in ceramics.

INTRODUCTION Nowadays the acoustic properties of high-temperature superconductors are measured by integral methods 1 and serve as a characteristic of the whole volume of the sample. We demonstrate the use of acoustic microscopy in investigation of both local acoustic properties of individual crystalfites and various defects in HTSC-materials. Important is that the acoustic microscopy provides for: (a) viewing the defects in the depth of a non-transparent sample, unlike the optical methods; (b) determining the Rayleigh wave velocity on a small (10mkm) section of surface (for example, on an individual crystaUite); (c) presence the defects in the presurface layer by attenuation of such waves. In our work we used a commercial acoustic microscope ELSAM (Ernst Leitz Scanning Acoustic Microscope)2. In a reflecting scanning acoustic microscope (see Fig.l) the microwave acoustic signal (1,2GHz) is converted into a plane acoustic wave, which is focused by the lens into a spot on the surface, or under the surface of the subject under investigation, reflected from it, received by the lens and convexted into an output electric signal of the microscope ~. The dimensions of the focal region (acoustic wave length in an immersion liquid - about lmkm) determine the resolving power of the acoustic microscope. The lens is scanned mechanically in the plane parallel to the subject, and image is formed on the display synchronously with its motion. The brightness of each point is proportional to signal amplitude V(Z) at the respective position of the lens in the scanning plane, Z - distance between the lens focus and the subject surface. The amplitude of the acoustic wave reflected from the subject and, consequently, V(Z) depends both on distance Z, and local acoustic properties of the subject: O~ i~2 CosO

v ( z ) = so U ( e ) R ( 8 ) e

~ 8 a O (l)

where U(O) = lens aperture function, k = wave vector in -immersion liquid, O = lens aperture, R(e) - factor of

Seen I 1. Schematic diagram of reflecting scanning acoustic microscope (SAM): HF generator, lens with scanning and synchronizing device with display, immersion liquid. Operating frequency - up to 2 GHz. reflection frov? subject, taking the latter to be isotropic in the surface plane'*. The more the difference between the acoustic properties in the given point and the properties of the immersion liquid, the higher reflection factor R(O), the brighter the respective point on the subject image; V(Z) is maximum, as a rule when Z~O (see Fig. 2). THEORETICAL CONSIDERATIONS At image, forming with Z<0, in a beam approximation one may consider that only the beam as if going from the lens focus are received effectively. For 1177

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example (see Fig.3), contribution of beam C is much higherthan that of beam A. Therefore, at focusing onto the subject surface the subsurface defects (discontinuities) will occur not in the focus, and they will be either not sharp on the image, or will be absent at all; such defects will get in the focus at an approach of the lens to the subject. For practical application determination of the depth of occurrence of the defect is important. Let us consider the case, when the defect image is created by beams repeatedly reflected from the defect, i.e. when beam rereflections between the surface and defect may be neglected. This case is realized, firstly, if the subject is sufficiently small, and the rereflected beams get onto the subject only if they propagate at a small angle to lens acoustic axis Z, secondly, when sound attenuation in the subject is sufficiently high:

>~ 2d/ConE)"

(2)

( 0 = angle of refraction in subject, d = depth of occurrence of defect), i.e. when the rereflected beams are considerably attenuated due to attenuation in the medium. Then, neglecting such rereflected beams and considering the defect to be an absolutely refloc)ing one, at a paraxial approximation it may be shown ~ that if the maximum signal from the defect is attained at an offset of the lens to the subject by SZ, then at a paraxial approximation, the depth of occJrrence of the defect equals

a--

z/0- c/c )

where Ct= sound velocity in immersion liquid (water), C = sound velocity in sample. Expression, taking into account the influence of nonparaxial beams, is more precise allowing for the wide aperture O m of lens Om~16:

d=

V o l . 72, No. 12

be set forth below, show that the subsurface image in the majority of samples of perovskite ceramics under investigation are created mainly by transverse waves• Really, let us separate defect contribution (Rdef) in the reflection factor R =Rde f + R '

(5)

Rdef corresponds to beams B and C in Fig.3. Considering that the depth of penetration of Rayleigh wave is less than d, let us neglect the influence of the defect on propagation of Rayleigh wave. Therefore, Rayleigh wave, which together with beam A reflected directly from the surface, determines R'. Since longitudinal a n d transverse acoustic waves propagate in solid sample, the signal of the scanning acoustic microscope conditioned by the defect will be equal to: (~t *

o

.

.

where T I TI - factors of converslon of longitudinal wave m tmmerslon to longitudinal wave m sample and back, Tt, T t factors of conversion of wave in immersion into transverse wave in sample and back. Let us use ordinary approximation 7 U(0
conditioned by contributions of longitudinal and transverse waves in (6): O, O. .

c/c i)

The physical sense of the addend consists in the fact that the focus inside the sample is formed by beams that are intermediate between the paraxiai and extreme ones. A higher influence of the side beams can be explained by the fact that the specific contribution of beams in the output signal is proportional to the value of the respective solid angle, i.e. ~ sinO, where O is an incident beam angle. In obtaining formulas (2,3) it was supposed that the defect was an absolutely reflecting one. If rereflections between the defect and surface play an essential role, for a more precise localization of the defect in depth it is necessary to make prior assumptions relative to the nature of the defect and its dimensions (for example, defect-peeling with dimensions in plane (X, Y)>>/g filled with air, then to compute V(Z,d)dependence at different values of d as a parameter, then to solve the inverse problem of determining the value of d, comparing the computed V(Z) and experimental ones. To determine d, also can be useu V(Z)-.Jependences measured on plane-parallel samples calibrated in thickness instead of computed V(Z). Note that formulas (2,3) comprise only one sound velocity Of the sample, while even in an isotropic solid two types of waver (longitudinal and transverse) propagate. Numerical simulation and quantitative estimation, that will

For t~e transmissions coefficients in formula (7) let us use th,*. respective expressions for the transmission coefficients of a plane ~vave normally incident on the interface liquid half-spaces~. C:=3km/Fl~rs y 123-ceramics YBaCnO with velocities C=5km/s, we obtained that longitudinal waves are effectively excited in the range from 0 ° to 17° (O1=17°), while the transverse ones - from 17° to 30 ° (O t-30°), so that ra3. Consequently, in the formation of image-of subsurface defect the main contribution is made by transverse waves, and the value of C=C t should be substituted into formulas (2,3). The output signal of the SAM very greatly depends on distance Z, hence for irregular subjects only their topography can be determined: variations of function R(x,y) considerably exceed variations of Rdef(x,y), where x,y = coordinates in the scanning plane, consequently, the acoustic image contrast conditi0nedby 8 V/~x andgV/ay for irregular subjects will be determined in the first place by the topography of their surface. As it was mentioned above, the SAM can be used to obtain quantitative information on t h e si~bjeet under investigation - to measure the local velocity of the Rayleigh

I00 90 80 70 60 50 40 30 20-

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2. Output signal of scanning acoustic microscope versus distance Z between lens focus and sample surface.

3. Beam interpretation of SAM output signal generation with Z<0: A - mirror-reflected beam; B - beam transformed out of longitudinal wave in sample reflected from subsurface defect; C - beam transformed out of transverse wave; D - outfiowing Rayleigh wave.

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INVESTIGATION

OF HIGH-TEMPERATURE

wave. It is possible to show 10 that for a homogeneous isotropie subject the expression for the Rayleigh wave velocity l~ tt~tl: where Z - the period of oscillations on V(Z)-curve, = wave length in immersion. An experimental criterion of smoothness of a subject and its homogeneity in depth is the homogeneity of image at focusing onto the surface and into the depth of the subject, i.e. ~ r / d x = ~ r / a J - 0 for different z <0. EXPERIMENTS Ceramics We have investigated 123-ceramics - A B a 2 C u 3 0 x (A=Y, Yo, Ho), BiSrCaCuO, both hot-pressed up to 70 kbar and ordinary ones. Water heated up to 333K or methanol at a room temperature was used as an immersion liquid. Fig.4 illustrates a fragment of the surface of coarse-grained 123ceramics prepared by the usual sintering method. Regions of crystallites are seen as bright sections, dark sections - are pores. The focus being brought onto the bottom of the pores by the maximum of the acoustic wave reflected from them, makes it possible to determine their depth accurate to 0.05mkm. Usually it lies within 1 to 3mkm. Using the ELIAS image processing system of the ELSAM microscope, we can carried out an automatic count of the quantity of pores and crystallites, plotted histograms of their distribution over the area. For example, on the image of high-dense hot-pressed 123-ceramics (Fig.5) pores occupy5.5% of the area, the average ar~a of the crystallites was approximately equal to 190mkm~;. High-density ceramics feature a small percentage of the pore area (see Figs 4 and 5). The value of Rayleigh velocity V R was determined at a frequency of 1.7GHz according to V(z)-dependence at variation of Z within 0 to 20mkm, consequently, the maximum distance covered by the Rayleigh wave made up d=Zxtg0 10mkm 5, therefore measured V R characterizes the surface section with'a diameter of about 10mkm. Of course, V R is an average Rayleigh velocity in all directions in the plane of the sample. Measurements of V R were taken at sufficiently large crystallites, so that their boundaries do not

4. Low-density ceramics YBaCuO. Frequency - 1.7 GHz

SUPERCONDUCTORS

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influence t h e propagation of the Rayleigh wave. On an area of about 2ram 2 there were selected 10-15 crystallites, on which V(z)-curves were taken. The obtained curves were compared in a computer, and the majority of them (70-80% for some samples) coincided. It indicates an absence of occasional factors influencing the value of the microscope output signal: boundaries of crystallites, subsurface defects, small thickness of crystailites that could cause a reflection of the a:oustic wave from the crystallite rear surface, which, in turr, would inevitably cause an accidental distortion of the V(z)-curve due to random variations of the crystallite thickness. Thus, on different crystallites of the same sample velocity V R was found to be the same. Since it is know that in the perovskite structures under investigation there is a considerable difference between the sound velocity along axis c and the sound velocities. . in planes. (a,b): V[100] V[010 0.1kR, taking the signal-to-noise ratio for the ELSAM t o b e equal to 20 dB 2. Such an attenuation may be caused by a great number of microcraks, even if the value of their exposure (for example, 0.01mkm) is far beyond, the range of the microscope resolving power (0.4mkm). This is associated with the fact that water does not penetrate into mierocracks, and the hypersound of the used gigahertz frequencies practically fully gets attenuated in the air at the wave length. Such an estimation of*t R characterizes the integral quantity of attenuation of the Rayleigh wave on a section with a diameter of 10mkm, and is rather sensitive to the magnitude of density of crack distribution on the crystallite. Such high magnitudes of absorption were observed on microcrystallites of many ceramics, especially of high-density ones. In Fig.5 cracks are invisible, since they are "polished" (the sample is not pickled), besides, part Of the cracks are invisible since they are too small: the characteristic magnitude of crack exposure, as it is seen from the electronmicroscopic photograph of the same sample, but already pickled, not to exceed 1000.,~, their presence causes an abnormally high attenuation of the Rayleigh wave. It should be pointed out that the accuracy of determining of magnitude was about 50%, since the magnitude of the side maxima determining angle greatly depends on the instrument radio engineering parameters, in particular, on the magnitude of delay of the beginning of reception of the reflected pulse with respect to the transmitted pulse (the so-called "gate"). Not the total value ot attenuation of the Rayleigh wave is a matter of practk.al interest, fer low-defective and not too porous samples it is mainly determined by the rate of its outflow, but that partot s of the whole absorption R that is conditioned by the acoustic attenuation and structural scattering. Attenuation ~ = ~ g - U s caused by the wave outflow proper can oe estsmatea, using the magnitudes for CI, C t gsven above, and taking water as an immersion liquid. Using for "1 expression 13

*~ ~ C ~ i C l / ? C l ) ( K ~ / a ~ r ) ,

(9)

where ~- dense of the sample, we obtain o/R-0.01K R. Thus,

5. Hot-pressed ceramics Y~BaCuO, 300x200 mkm. Frequency 1.7 GHz.

the presence of great number of microcrac~'s results in an essential increase of attenuation. Observation of separate sections of crystallites (Fig.5) reveals sections that are notable for brightness, which may correspond to a changed phase or structural composition. Different ceramic phases feature different acoustic properties and, consequently, different factors of reflection from them.

1180

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Let us estimate the magnitude of this effect, using for reflection factor R a simplest expression for reflection of,.9 normal incident longitudinal wave from a liquid half-space e. Then, for instance, for phases YBa2Cu30¢_5 and YBa2Cu307. whose longitudinal velocities are equal to 4.9kmls and 4.4kin/s, respectively 9, we obtain the difference in the reflection factors equal only to 2%. Thus, for intensively reflecting subjects the difference in the reflection factors is low, therefore, for visualization of different phases use can be of the so-called quasi-three-dimensional image, at which the oscilloscope beam while scanning on the screen, besides variation in brightness, deflects from the scanning line proportionally with the magnitude of the output signal of the SAM. Fig.6 illustrates a quasi-three-dimensional image of high-density 123-ceramics, on which phase inclusions look like peaks. We also observed a twin structure on some crystallites, which, generally speaking, should not be observed with the aid of a spherical lens featuring an axial symmetry. Apparently, such a picture is associated with formation of strained regions near the twinning boundaries. This is confirmed by the fact that the twin structure is invisible with Z=0 (Fig.5), and it becomes visible only when Z<0 (Fig.7),when the lens starts receiving the outflowing Rayleigh wave. Fig.8 explains the probable mechanism of visualization of the" twins boundaries. While propagating over the monocrystallite surface, the Rayleigh wave is reflected from the twinning boundary because of the discontinuity of the latter, the wave outflow near the boundary increases. As a result, the twinning boundarY is outlined by a brighter line and becomes visible on the image, even if the cross-sectional dimensions of the boundary are much less than the wave length; a similar effect of outlining occurs for any boundaries with Z<0. To improve the contrast range of the picture of the twin structure, use should be made of an asymmetric lens, for instance, with an offset or elliptic transducer. It will allow to distinguish the components of the twin structure, different in orientation, according to the difference of the reflection factor, similarly to the picture obtained in polarized light 14. Films YBaCuO obtained on different 'substrates were also investigated. Fig.9 illustrates the image of the YBaCuO film applied by target evaporation with the aid of a pulsedperiodic laser on a ZrOx substrates at 1200K. Weak variations

,,i// (a,B)

(B,a)

8. Mechanism of twins visualization by means of Rayleigh wave.

9a. Film YBaCuO applied by laser spraying on substrate ZrO, 300x200 mkm. Frequency - 1.8GHz.

9b. SEM-photo of the same film.

6. Quasi-three-dimensional image of surface of hot-pressed ceramics YBaCuO, 200x130 mkm. Phase 1236 is separated with higher sound velocity. Frequency - 1.7 GHz.

7. Same as in Fig.5 Subsurface (Z=-5.5 mkm). Twins are visible.

of the acoustic signal corresponding to the dome-shaped relief of film 123 are seen on the image. Separate microdrops, 1-2 mkm in diameter, formed due discontinuous spraying of materials (the so-called "sputtering" associated with a nonstationary evaporation) are also well seen on the image. Films obtained by successive tr, ermal deposition of Y, Ba and Cu o,: a silicon substrate at 720K in '.he oxygen atmosphere were investigated. The obtained multilayer film was then burnt off in the oxygen atmosphere at l120K during 4 minutes to form a superconducting phase. Such a method of obtaining of films was described in more detail in works 15,x6. Investigation of the obtained films has shown that they are multi-phase ones (Fig.10). Along with the YBaCuO phase there are sections of Y2BaCuOs - "green" phase (dark spots) and inclusions of CuO, YzO3 and BaCuO 2 (dark points). As to the kind of V(z)-dependences, phases 123 and 211 are identified unambiguously. In Fig.12 one can also see light spots with interference rings-peelings caused by an insufficient adhesion and by the difference of the thermal coefficients of expansion at a thermal cycling. Crystals Beside ceramic samples, we investigated also separate crystals YBaCuO and BiSrCaCuO. We revealed on them both surface (Fig.ll) and subsurface (Fig.12) defects of structure. Twinning with a period of about l m k m was observed on YBa2Cu307 samples. The values of V R measured on planes (a,b) of monoerystals were close to V K measured on non-strained crystallites of the respective ceramtcs. Thus, it was shown that the acoustic microscopy features rather wide opportunities to investigate and control

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10a. Film YBaCuO applied by thermal evaporation on substrate Si. A multiphase film is obtained. Darker spots inclusions of "green" phase 211. Light spots - peelings.

11. Surface of BiCaSrCuO monocrystal, 60x40 mkm.

Si (b)

12. Same as in Fig.ll. Subsurface(-6.6 mkm). Subsurface defects are visible. Y(I.826)

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10b. Electron microprobe X-ray spectrum from "211" phase area. the qualities of HT-superconductors, provides for revealing the specific characteristics of the microstructure, determining the local acoustic properties and various defects in crystallites, crystals and films.

ADDITIONAL CRYOGENIC EQUIPMENT For examination of specimens in an acoustic microscope within a temperature range from 85 to 180K we created a cryogenic thermostated chamber providing an accuracy of 0.1K in the specimen area, The chamber operates on the principle of a circulating cryostat with a two-step temperature control. The working space of the chamber is filled with liquid propane, which is used as an immersion. The microscope acoustic lens is inserted into the chamber installed om the microscopic stage after the optical adjusting the latter. The results, obtained on this device will be published in another paper. ACKNOWLEDGMENTS The authors are much grateful to G.S.Abilov for assistance in preparing the cryogenic chamber.

References 1. Horie Y., Terashi Y. & Mase S. Ultrasonic studies and phonon modes of (RE)Ba2Cu307. J. of Phys. S0c. of Japan., 58, 279-290, 1989 2. M.Hoppe & J.Bereiter-Hahn. Applications of scanning acoustic microscopy - Survey and new aspects.1985,fEEE, SU32, no.2, pp.289-301 3. C.F.Ouate, A.Atalar & H.K.Wikramasinghe, "Acoustic microscopy with mechanical scanniug.-A review." Proc.IEEE, 67(8), pp.1092-1113. 1979. 4. A.Atalar, J.Appl.Phys., 49, 5130, 1978. 5. A.Atalar, Penetration depth of the scanning acoustic microscope. 1985. I?~EE, SU-32, no.2, pp.164-167. 6. M.A.Bukhny, O.V.Kolosov, T.A.Senjushkina. Layers elastic properties characterisation in transmission acoustic microscope. //Proe. of conf. of young scientists of ICP AS USSR. Soy. dep. VINITI. 15.02.88. N1226-1388.-Moscow, 1988 (in Russian). Also to be published in JASA. 7. H.K.Wikramasinghe. Contrast and imaging performance in the s~:anning acoustic microscope. J.Appl.Phys., 50(2), pp.6472, 1979. 8. Brekhovskikh L.M, Godin O.A. Acoustics of layered media. Moscow: Nauka, 1989, p.29 (in Russian).

9. T.J.Kim et.al. J. of Magn. and Magn.Mat. 76&77, 1988, pp.604-606. 10. W.Parmon & H.L.Bertoni, "Ray interpretation of the material signature in the acoustic microscope", Electron. Lett., vo1.15, pp.684-686, Oct.1979. 11. H.C.Gupta. Sol. St. Com. 65(6), 495-496. 1988. 12. R.D.Weglein, "Rayleigh wave absorption via acoustic microscopy", Electron.Lett., vol.48, pp.20-21, Jan.1982. 13. Dransfeld K. & Salzmann I. 1970. Excitation, detection and attenuation HF elastic surface waves. Physical acoustics. Principles and methods. Edited by V.P.Mason & R.N.Thurston . Academic Press. vol.7. Chapter 4. 14. Vlasko-Vlasov V.K., Indenbom M.V., Osip'yan J.A.,1988 Polarized-optical contrast in HTSC YBaCuO. Pisma v JETF, 47, 312-315 (in Russian) 15. Q.Y.Ma, T.J.Licata, X.Wu et.al. "High Tc superconducting thin films by rapid thermal annealing of C u / B a O / Y 2 0 3 layered structures", Appl.Phys.Lett., 1988, 53(22), pp.22292231. 16. Chin-Au Chang et.al. Appl.Phys.Lett., 1988, 53(10), pp.916918.