4.5 Electron Microscopes and Microscopy in Japan

ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. 96

4.5 Electron Microscopes and Microscopy in Japan 4.51 Applications to Materials Science H I R O S H I FUJITA' Osaka University, Suita, Osaka 565, Japan

Applications of electron microscopy to materials science have expanded widely since dislocations in thin metal foils themselves could be observed directly. As a result, improvement of the functional features of electron microscopes has been strongly demanded to observe the atomic structures of various materials. In Japan, the functional features of electron microscopes have been very much improved by the challenge of constructing high-voltage electron microscopes in 1963-1965. Later, the use of electron microscopy was further extended into various fields of natural science, especially materials science, and indispensible applications have been carried out to obtain information dynamically by in situ experiments with high-voltage electron microscopes, and directly at the atomic scale by highresolution electron microscopy. In the present report, recent topics in the applications of electron microscopy to materials science in Japan are reviewed. I. EARLY WORK

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Before 1953, the accelerating voltage of electron microscopes in Japan was less than 100 kV, and the thinning technique was also not adequate to make thin foils. Thus, applications of electron microscopy were limited to replicas of materials surfaces and fine materials such as various sols, vapordeposited films, very fine particles, cleaved thin foils, and very fine materials grown in special atmospheres. Since the surface unevenness of materials is caused by the surface relief resulting from various shear displacements and/or chemical attack, interesting results by replica methods had been obtained on the following subjects: slip bands (Takamura, 1955; Fujita, 1955), surface reliefs of martensites (Nishiyama and Shimizu, 1956, 1958), twinned structures (Nishiyama et al., 1958), fractured surfaces (Nonaka, 1955), etch pits (Nishimura et af., 1951), subboundaries (Taoka and Aoyagi,

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Present address: Research Institute for Science and Technology, Kinki University, Kowakae, Higashi, Osaka 577, Japan. 749 Copyright 0 1996 by Academic Press. Inc. All rights of reproduction in any form reserved.

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1956, 1957), precipitates (Koda and Takeyama, 1953), paints (Terao and Sakata, 1953, 1954) and so on. Furthermore, the extraction replica method was used effectively to determine complex crystal structures of small crystallites such as precipitates and insoluble inclusions (Fukami, 1956). Microstructures of various powders, such as sols (Suito et al., 1954; Suito and Takiyama, 1955), paints (Fukami and Shiyota, 1955, 1960), fine ceramic particles (Fukami and Shiyota, 1955, 1960), and vapor-deposited ones in vacuum and low-pressure gases (Hibi, 1955), were also investigated effectively by electron microscopy. Details of these results were summarized in the book entitled Theory and Applications of Electron Microscopy, Part ZZZ-Applications to Science and Engineering, edited by K. Kubo (1960). On the other hand, it is well known that the behavior of bulk materials is very structure sensitive. It was thought that a material’s behavior is closely related to the behavior of lattice defects whose density and arrangement are very sensitive to the pretreatments of the materials. In order to make this fact clear, many trials were carried out to observe the materials themselves directly by electron microscopy, instead of by the replica method. Direct observation of thin metal foils was made first by Heidenreich (1949) using foil specimens which were thinned chemically, and such diffraction contrasts as subgrains and bend contours were observed. By using cleaved nonmetallic materials, i.e., mica films, moire fringes were observed by Mitsuishi et al. (1951). Magnified dislocation images were also observed in the moire fringes of overlapping crystals by Hashimoto et al. (1956-1957). Afterwards, the electropolishing method was used for making thin foils of metals by Bollmann (1956), and the diffraction contrast of subboundaries consisting of dislocations in aluminum was observed with a 100-kV electron microscope. Hirsch et al. (1956) also succeeded in observing the movement of dislocations in aluminum with a 100-kV electron microscope. After that, so-called direct observation of metal foils was commonly used. Direct observation of dislocations in metals was first achieved in Japan with a 50-kV electron microscope by Nishiyama and Fujita (1957), using aluminum foil specimens made by a chemical thinning method, and the subgrain grouping, or the subgrain coalescence, in recovery and recrystallization processes of deformed metals was discovered (Fujita, 1961). After that result, the accelerating voltage of Japanese-made conventional electron microscopes was also increased up to 100 kV, and various electropolishing methods were improved for making thin foils of metals and alloys. As a result, electron microscopy was widely used for metallurgical investigations, because much information could be obtained from both the diffraction contrast of microstructures and the corresponding diffraction patterns in metals and alloys. At the same time, fundamental information about diffraction contrast was provided by the kinematic theory, which was developed

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mainly by Hirsch et al. (1960). Detailed calculations of image contrasts by the dynamical theory were made by Kato (1963) and by Howie and Whelan (1961, 1962). The contribution of inelastically scattered electrons to image contrast was also clarified by Kamiya and Uyeda (1961, 1962). Extension of the diffraction theory was made on various electron microscope images by Hashimoto et al. (1962,1977) and by others in Japan by considering the absorption effect. Besides the electropolishing methods, various thinning methods such as the jet method, the spark cutting and thinning, the acid cutting and thinning, the ion thinning, etc., were improved, so that electron microscopy was widely applied to research fields of nonmetallic as well as metallic ones. Furthermore, precise tilting devices were developed to obtain the necessary contrast of microstructures in foil specimens, and some specimen treatment devices such as heating devices (Takahashi and Mihama, 1957; Nemoto and Koda, 1963), low-pressure environmental cells (Hashimoto et al., 1966), etc., were also improved. By using direct observation of foil specimens, new information was obtained on the following subjects: Dislocation structures in deformed (Fujita and Nishiyama, 1961, 1962; Takeuchi et al., 1967; Karashima et al., 1968) and annealed specimens (Takeyama and Takahashi, 1970; Furubayashi et al., 1966), boundary structures (Furubayashi, 1971), superlattice structure of ordered crystals (Ogawa et al., 1958), microstructures of alloys (Takahashi and Ashinuma, 1958,1959) and martensite crystals (Nishiyama and Shimizu, 1959,1961; Nishiyama et al., 1968,1968), G.P. zones and various precipitates (Nemoto and Koda, 1965), secondary defects in quenched (Yoshida et al., 1963; Kiritani et al., 1964; Shimomura and Yoshida, 1965) and particleirradiated specimens (Nemoto et al., 1971), stacking faults (Watanabe, 1966), and so on. These results played an important role for making clear the contributions of both microstructures and lattice defects in various phenomena occurring in metals and other materials. Furthermore, in situ experiments were tried for some research subjects such as the growth process of W-oxide with an environmental cell by using motion picture photography (Hashimoto et al., 1958), the transition from 8’ phase to 8 phase in Al-Cu alloy with a heating device (Takahashi and Mihama, 1957; Nemoto and Koda, 1963), interaction between moving dislocations and precipitates by thermal stressing (Nemoto and Koda, 1966), the growth process of Mo-oxides (Hashimoto et al., 1966), crystallization of amorphous selenium foils by electron irradiation (Shiojiri, 1967), oxidation of Mo particles and deoxidation of Moo3 (Ueda, 1959), crystal growth of Hg at -70°C (Honjio et al., 1956). The wall structures of magnetic domains in magnetic materials such as iron, Ni-Fe, and Ni-Co foil specimens

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were also observed by Lorentz microscopy (Tsukahara and Kawakatsu, 1966, 1972; Tsukahara et al., 1971; Watanabe and Sekiguchi, 1984). In addition, epitaxial growth process of metallic powders (Ino et al., 1956; Takahashi and Mihama, 1957), structure change of electrodeposited films (Tanabe and Kawasaki, 1972), and microstructures of fine powders deposited in vapor (Uyeda, 1942; Kimoto et al., 1963) and in low-pressure gases (Mihama and Tanaka, 1976) were effectively investigated by using specimens which were treated successively under various conditions outside the microscope. In these direct observations of foil specimens, the selected-area electron diffraction method was another advantage of electron microscopy. By using this method, crystal structures of very fine crystals and orientation relationships between two different crystals, e.g., martensite crystals and precipitates against the matrix (Shimizu and Okamoto, 1971) and multiply-twined Au particles (Ino et a/., 1972), were determined. As mentioned above, the direct observations of lattice defects and microstructures in foil specimens of materials gave valuable new information to materials science, especially metallurgical objects. Details of those results were summarized in Interpretation of Electron Microscope Images and Its Applications to Metallurgy, edited by Z . Nishiyama and S. Koda (1975). On the other hand, it was clarified that the lattice defects, especially dislocations, are very sensitive to the foil thickness, so that not only the behavior and but also the density are markedly changed in such thin foil specimens. Since material behavior is determined by the behavior of lattice defects, attention was directed to observing the same density and the same behavior, if possible, of lattice defects as those in the bulk material. Most electron microscopists, however, did not expect that the maximum observable thickness of the specimens would increase significantly with increasing accelerating voltage. Furthermore, some investigators worried about blurring of the images due to the inelastic scattering of electrons through crystals, and others about heavy damage or destruction of the specimens during observation due to electron irradiation (Fujita, 1986). Around 1960, a 300-kV electron microscope operating with a cascade transformer generator was installed in Japan by Kobayashi et al. (1963), and quantitative measurement of the energy dependence of transmissive power of electron waves was carried out by Hashimoto et al, (1964). The relativistic dynamical theory of electron diffraction was also examined by Fujiwara (1962) with a 300-kV electron diffraction instrument. These results, however, gave pessimistic information on the voltage dependence of the maximum observable thickness of the specimens. In 1963, the first separate-type 0.5-MV transmission electron microscope operating with a Cockcroft-Walton high-voltage generator was installed at the National Research Institute for Metals (NRIM), and was followed by

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construction of a few 0.5-MV electron microscopes of both separate and symmetry types, operating with the same type of high-voltage generator (Fujita, 1986). The electron microscope at NRIM was used for investigating the following phenomena in 1965-1967 (Fujita et af., 1965, 1967; Fujita, 1966): (1) voltage dependence of the maximum observable thickness for the diffraction contrast of dislocations and other lattice imperfections; and (2) the thickness effect on the behavior of lattice defects. The other instruments were used to study the voltage effect on irradiation damage in polymer specimens (Kobayashi and Ohara, 1966), the critical voltage effect on diffraction (Nagata and Fukuhara. 1967; Watanabe el af., 1968), and other questions. As a result, it was found that the same dislocation behavior as in bulk specimens can be observed at 0.5 MV for light metals whose atomic number is smaller than about 20, and in situ experiments were carried out on various phenomena in those metals (Fujita et al., 1965, 1967; Fujita, 1966). Furthermore, TV-VTR systems were also applied to image recording, so that real-time recording of the rapid motion of lattice defects became possible (Imura et af., 1969). Consequently, electron microscopy became the most effective technique for obtaining topographic information at the atomic scale directly and dynamically, giving the sensational results of both “seeing is believing” and “the materials are alive.” In the past 30 years or so, high-voltage electron microscopy and high-resolution electron microscopy have been very much improved in Japan. The challenge of constructing high-voltage electron microscopes has contributed greatly to the improvement of the functional features of electron microscopes, because the technical difficulty of keeping the same overall performance of electron microscopes, i.e., energy fluctuation of electrons, vibration proof, shielding for fluctuation of both electromagnetic field and X-ray, etc., is approximately proportional to the third power of the accelerating voltage. 11. NEWTRENDS IN APPLICATIONS TO MATERIALS SCIENCE

New trends in applications of electron microscopy to materials science in Japan will be reviewed, with reference to the Journal of Electron Microscopy, vol. 38 (Supplement), which was a special issue for the 40th anniversary of the Japanese Society of Electron Microscopy (Ura, 1989). A. Electron Holography The performance of electron holography has been very much improved by using a field-emission-type electron microscope. By this method, the

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thickness distribution in the atomic resolution range, the electromagnetic field distribution, the reality of gauge fields [Aharonov-Bohm (AB) effect], etc., have been precisely determined (Tonomura et al., 1986). Figure 1 is electron holographic interferometry showing the existence of the AB effect (Tonomura et af., 1986). A tiny toroidal magnet was selected as the sample [Fig. l(a)]. The magnetic flux rotates inside the toroid and does not leak outside. Such a sample is cooled down to 5 K and the relative phase shift is measured between two electron beams, one passing through the hole and the other outside the toroid. Measurements were made for various magnetic flux values, but only two kinds of interferograms are observed, as shown in Figs. l(b) and l(c). The phase shift is either 0 or n. This phase-shift quantization implies that the magnetic flux is completely surrounded by the superconductor, and ensures that magnetic fields do not leak outside by the Meissner effect. The observed phase shift of R under ideal condition provides definitive evidence for the existence of the AB effect and also the physical reality of gauge fields.

B. High- Voltage Electron Microscopy

In order to observe the same behavior of lattice defects as in bulk materials, the specimen thickness must be greater than the mean free path of the contributing lattice defects, i.e., about 3 p m in general (Fujita et al., 1965, 1967; Fujita, 1965). Remarkable increase of the maximum observable thickness of specimens with high-voltage electron microscopes (HVEMs) shows a great advantage in the research fields of materials science. By in situ experiments which satisfy the above condition, valuable information can be obtained dynamically under various experimental conditions, such as deformation, annealing, electron and ion irradiation, various atmospheres, etc., in a wide temperature range from about 5 K to 2300 K as follows: (1) motion of each individual lattice defect, (2) interaction among the lattice defects, ( 3 ) interaction between the lattice defects and microstructures, (4) qualitative determination of physical parameters, and others. Based on the results, mechanisms of the following phenomena occurring in materials have been made clear: (1) mechanical behavior such as yielding, work hardening, fracture, creep, and fatigue deformation (Fujita, 1969; Imura et al., 1969, 1970; Imura and Saka, 1976), (2) mechanical twinning, ( 3 ) the martensitic transformation, (4) recovery and recrystallization, ( 5 ) precipitation, (6) electron irradiation damage, (7) environmental-material interaction, (8) toughening and strengthening of various ceramic composites, etc. Details of these results are summarized in In Situ Experiments with H V E M , edited by H. Fujita (1986), and in New Directions and Future Aspects of

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a

b

C

FIGURE1. Experimental confirmation of the Aharonov-Bohm effect using toroidal magnets completely covered with superconducting layers. A phase shift of n is detected between two electron beams passing inside the hole and outside of the toroid, in the case (c) of an odd number of fluxons trapped within the toroid, although the beams never touch magnetic fields. (a) Schematic of toroidal sample: (b) electron interferogram (phase shift = 0); (c) electron interferogram (phase shift = n).

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HVEM, edited by H. Fujita et al. (1991). Applications of ultra-HVEM are also summarized by Fujita (1986). In the items mentioned above, electron irradiation effects are one of the important applications of HVEM, though the rate of irradiation damage is very sensitive to the specimen orientation (Fujita, 1986). The effects have been extensively investigated by Kiritani et al. and others in regard to the following subjects: (1) mobility of point defects (Kiritani, 1991; Yoshida and Kiritani, 1975; Fujita et al., 1991); (2) secondary defects in pure metals (Shimomura et al., 1986), (3) irradiation-induced phenomena in alloys and ceramics (Takeyama et al., 1985; Kinoshita, 1985; Kinoshita et al., 1987; Yada et al., 1987). Figure 2 is an example showing successive stages of superelasticity of a monoclinic Zr02+3.9 mass% MgO crystal (Fujita, 1986, 1991). That is, the twinned structure observed in Fig. 2(a) disappears in micrograph (b) immediately after the stress is applied, and then appears again in micrograph (c) as soon as the applied stress is removed. Furthermore, with the remarkable increase of observable thicknesses of specimens, three-dimensional observation of microstructures and quantitative measurements of physical parameters have also been carried out precisely for the following subjects: determinations of (1)formation and migration energies of point defects (Kiritani, 1991), (2) three-dimensional distribution of microstructures (Kiritani, 1991), (3) internal stress, (4) stacking fault energies of crystals (Saka and Imura, 1982), ( 5 ) the stress-induced voiding in Al-lines for LSI (Takaoka and Ura, 1991; Okabayashi et af.,

FIGURE 2. Pseudo-elasticity of a Zr02-3. 88 mass% MgO composite. (a) Fine twin structures under no applied stress. (b) When the shock stress is applied at around room temperature, the twin structure disappears immediately. (c ) Twin structure appears again as soon as the stress is removed.

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1991), etc. Details of these results are summarized in the two books mentioned above on the applications of HVEM. Besides these applications, the magnetic domain structures of materials have been investigated by using the Lorentz force of magnetized regions by high-voltage electron microscopy (Watanabe et al., 1984). The critical voltage effect has also been used effectively for determination of the structure factor of complex crystal structures (Eguchi et al., 1987; Tomokiyo et al., 1990). Furthermore, a high-voltage STEM with a field-emission gun was constructed in Japan, and new advantages such as usefulness as an analytical microscope, weak damage of biological specimens, etc., have been clarified (Kuroda et al., 1991). New high-resolution HVEMs have also been constructed (Matsui et al., 1991), and the resolving power is expected to approach about 0.1 nm, as mentioned later. By using the great advantages of high-voltage electron microscopy as mentioned already, a new research field called “microlaboratory” has been introduced. This means that high-voltage electron microscopy is not only a powerful tool for both characterization and identification of materials, but is also an indispensable “microlaboratory” in which various sorts of specimen treatments, including formation of nonequilibrium phases, can be carried out precisely on the atomic scale (Fujita, 1990, 1991). Typical examples are electron irradiation-induced phenomena as follows: (1) crystalline-amorphous solid transition (Mori and Fujita, 1982), and (2) foreign atom implantation in solid materials (Fujita and Mori, 1988). In the amorphization process, the electron irradiation-induced method is superior to earlier methods, such as liquid quenching, ion implantation, etc., in controlling conditions of amorphization. A precise study of amorphization has been carried out by this method (Mori and Fujita, 1982), and the general rules for amorphization were clarified (Fujita, 1990, 1991). Figure 3 is an example showing electron irradiation-induced foreign atom implantation (Fujita and Mori, 1988), in which a vertical arrow indicates the irradiating direction and a dark particle is a Pb precipitate. Pb atoms were implanted into an A1 substrate by 2-MeV electron irradiation, and Fig. 3(b) was taken by rotating the specimen 90” after irradiation at a dosage of 1.08 X e/m2. Under the irradiation, the precipitate decreases in size and Pb-implanted regions ( 7 ) are produced within the substrate underneath the precipitate, as shown in Fig. 3(b). The implanted Pb atoms form a supersaturated solid solution of aluminum, but the faint contrast of the implanted region results mainly from the difference in the scattering factor between the two elements. After annealing of the specimen at 573 K for 36 ks, the Pb atoms congregate into small particles, whose size is 2-10 nm in diameter, as seen in Fig. 3(c). The distribution of these small precipitates reflects the concentration profile of lead in the implanted region

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FIGURE 3. Electron irradiation-induced implantation of Pb atoms into an A1 crystal. Accelerating voltage, irradiation temperature, and flux are 2 MV, 175 K, and 1.2 X loz4 elm’s, respectively. Micrograph (a) was taken before irradiation, in which a dark particle is a target Pb precipitate. The same area after irradiation to 1.09 X 1Oz8e/mm2is depicted in (b). After irradiation, the implanted Pb atoms form a supersaturated solid solution in the Al matrix. These Pb atoms congregate into small particles whose size is 2-10 nm in diameter after annealing at 573 K for 36 ks, as shown in (c).

shown in Fig. 3(b). One point worth noting in Fig. 3(b) is the absence of strain contrast within and around the Pb-implanted region in spite of the large difference in atomic size between Pb and A1 atoms (e.g., the former is 22% larger than the latter in diameter). Postirradiation annealing experiments revealed that this is due to the relaxation of lattice distortion by an effective coupling of lead atoms with vacancies (Fujita and Mori, 1988; Fujita, 1990). In this implantation method, various advantages in the use of a highly accelerated electron beam as the primary irradiation beam can be fully utilized. For example, owing to the high penetration power of electrons, implantation into solids can be carried out without any serious temperature

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rise or surface damage. Furthermore, by using a microbeam of electrons, the implantation can be confined to a narrow region of the order of 2-3 nm diameter by the electron channeling phenomenon (Fujita, 1986). Therefore, by controlling the irradiation conditions, a variety of nonequilibrium states can be induced in the objects. A systematic study on the stability of artificially produced nonequilibrium solid phases was also made. C. High-Resolution Electron Microscopy

The use of high-resolution electron microscopy has been extended into various fields of materials science by the development of the resolving power of electron microscopes. In Japan, some HVEMs have been developed for improving image resolution, and 300- and 400-kV electron microscopes are now conventional high-resolution electron microscopes. These results are summarized in Characterization of Advanced Materials by High Resolution Electron Microscopy and Analytical Electron Microscopy, edited by S. Nagakura (1990). The resolving power of these electron microscopes approaches about 0.1 nm with HVEMs. The many-wave imaging method has been used widely in these research fields, and the many-wave lattice fringes, i.e., so-called atomic structures, of various organic (Uyeda et al., 1972), inorganic (Cowley and Iijima, 1972; Horiuchi and Matsui, 1974), and metal compounds (Hashimoto et al., 1977; Izui et al., 1978-1979) have been investigated by this method combined with computer-simulated images. Figure 4 shows many-wave lattice fringes of (110) diamond taken with a 300-kV electron microscope (Fujita and Sumida, 1986), and Fig. 5 is a so-called structure image of (110) silicon taken with a 1300-kV electron microscope in comparison with a computer-simulated one (Horiuchi et al., 1991). In these micrographs, the minimum image separation is about 0.1 nm. Figure 6 is an example showing molecular images of chlorinated Cu-phthalocyanine taken with a 500-kV high-resolution electron microscope and computer-simulated images (Kobayashi and Uyeda, 1988). The specimen used was a thin epitaxial film prepared with vacuum deposition on KCl. This kind of work has been carried out by Suito et al. (1958), Uyeda et al. (1970, 1972), and others in Japan. High-resolution electron microscopy has been applied to determine atomic structures of the following materials: (1) lattice defects in molecular crystals (Kobayashi and Uyeda, 1988), (2) atomic structures of various grain boundaries and interfaces (Ichinose and Ishida, 1981, 1989; Ichinose et al., 1987), (3) microstructures of iron-based alloys (Nagakura, 1986), (4) ordered structures and intermetallic compounds (Hirabayashi, 1983; Kitano and Komura, 1985), ( 5 ) ichosahedral structures of materials (Hiraga

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4. Electron micrographs showing atomic structures of diamond and silicon crysFIGURE tals. The micrographs were taken with a 300-kV high-resolution electron microscope.

et al., 1985), (6) porous alumina films (Akahori, 1965), (7) microstructures and crystal growth of inorganic materials (Kakibayashi et al., 1987; Takeda, 1991), (8) amorphous solids (Hirotsu and Akada, 1984; Ishida et al., 1981, 1985; Hamada and Fujita, 1986), (9) industrially important materials (Hiraga et al., 1986, 1987, 1990), (10) atom clusters (Iijima, 1985), (11) carbon nanotubes (Iijima, 1991), etc. In term (4), the cause of incommensu-

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FIGURE5. A structure image of silicon crystal. The micrograph was taken with a highresolution 1300-kV HVEM with the electron beam along the [110] axis. Each site of Si atoms is imaged as dark spots. The dark spot images of dumbbell-like pattern correspond to the structure image of silicon [110] incidence.

rate structures has been investigated directly by high-resolution electron microscopy (Nagakura, 1986;Hirabayashi, 1983; Kitano and Komura, 1985), while these studies were carried out only by the electron diffraction method. Relationships between the amorphous structures and microcrystallites in item (8) have been discussed by using various models based on contrast change of observed images (Hirotsu and Akada, 1984; Ishida et al., 1981; Hamada and Fujita, 1986). Furthermore, microstructures of high-T, oxide ceramics (Hiraga etal., 1987,1990) are involved in item (7), and microstructures of Sm-Co and Nd-Fe-B permanent magnets are involved in item (9) (Hiraga et al., 1986). Figure 7 is an example of item (2), and shows the atomic structure of a superlattice of GaAs/AIAs (Tanaka et al., 1987). In the superlattice, the thickness of AlAs is changed from one to 20 molecular layers artificially, as shown in Fig. 7(b). Micrograph (a) is an enlargement of the circled region in micrograph (b) in which each AlAs part consists of only a monomolecular layer. It is recognized in Fig. 7(a) that the interface is flat on the lower side of each monomolecular AlAs layer but rough on the upper side. The difference in the roughness of the AlAs interface coincides with the result of RHEED investigation. Figure 8 is an example showing the atomic structure of icosahedral quasicrystal and the corresponding diffraction pattern of an Al-Li-Cu alloy (Hir-

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6. Molecular images of chlorinated Cu-phthalocyanine taken with a 500-kV highFIGURE resolution electron microscope and computer-simulated images. The specimen was a thin epitaxial film prepared with vacuum deposition on KCI.

aga, 1991). The micrograph was taken with the incident beam parallel to the fivefold symmetry axis. Characteristic image contrasts are composed of a bright ring and 10 bright dots surrounding the ring, and they correspond to icosahedral atom clusters forming the structure of the quasi-crystal. Surface science is one of the important applications of high-resolution electron microscopy, and an ultrahigh-vacuum, high-resolution electron microscope has been developed for such subjects in Japan (Yagi and Honjio,

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GaAs

FIGURE 7. High-resolution electron micrograph of monomolecular AlAs layers in GaAs crystals. The thickness of the AlAs layers was changed from 1 to 20 molecular layers in GaAs crystals, as shown in (b). Electron microscope image (a) is an enlargement of a circled region in (b), and shows the atomic structures of the monomolecular (001) plane. The GaAs/AIAs interface is flat at the lower side of each monomolecular AlAs layer, but it is rough at the upper side.

1976). Figure 9 is an example of a profile TEM image of the 5 X 1 reconstructed structure of a gold (001) surface (Takayanagi et al., 1987). Figure 10 is a high-resolution REM image of the 7 X 7 reconstructed surface of Si (111) crystal with the corresponding R H E E D pattern, in which the 2.3-nm fringes (dark lines) of 7 X 7 superlattice and the dark lines of the surface steps are clearly resolved (Takayanagi el al., 1987). High-resolution REM has been used effectively for studying adsorption and film growth (Takayanagi et al., 1987). Since REM has an advantage

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FIGURE8. Electron micrograph and corresponding electron difraction pattern of an AlLi-Cu icosahedral quasi-crystal. Note that the atom clusters are arranged along straight lines parallel to the fivefold directions indicated by arrows.

that bulk specimen surfaces are observable at the atomic scale, this method is applicable to various fields of materials science. Dynamic information has also been obtained by in situ experiments with high-resolution electron microscopes in the following cases: (1) change of local atomic structures during growth of both stacking faults and twins in Au crystals, (2) behavior of ultrafine particles and atom clusters of Au (Iijima, 1985; Iijima and Ichihashi, 1986; Takayanagi et al., 1987), ( 3 ) melting processes of small particles embedded in a crystalline matrix (Saka et d., 1985),

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FIGURE9. Profile TEM image of the 5

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1 reconstructed structure of gold (001) surface.

(4)mechanisms of phenomena in which the mean free path of contributing lattice defects is extremly small, such as behavior of solute atoms and point defects at low temperature (Fujita and Lu, 1992), and ( 5 )spontaneous alloying of atom clusters (Mori and Yasuda, 1993; Yasuda eta!., 1992,1993). In these investigations, ultrahigh-vacuum, high-resolution electron microscopes are necessary. Figure 11 shows behavior of a Au-ultrafine particle on a Si02crystal surface, in which both the shape and the atomic arrangement of the particle are frequently changed by multitwinning as in a liquid drop (Iijima, 1986). Figure 12 is a similar in situ experiment showing growth of Au-atom clusters on a graphite film (Takayanagi et al., 1987). It is noted in Fig. 12 that the atomic structure also frequently changes during coalescence between different clusters. The results show that the behavior of atom clusters markedly differs from that of the corresponding bulk materials. Namely, in situ experiments give indispensible information about the peculiar behavior of atom clusters.

FIGURE 10. High-resolution REM image of the 7 crystal.

X

7 reconstructed surface of Si (111)

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FIGURE11. Change of atomic structure of an ultrafine Au particle: Electron micrographs showing various shapes of a Au particle 2 nm in diameter reproduced from a VTR tape. The lattice fringes correspond to 0.235 nm of d l l , spacing. The particle in (a), ( d ) , and (i) is a single twin. A single crystal with a cuboctahedral shape is seen in (e), ( f ) , and (i). It is concluded from the size of cuboctahedron ( j) that the particle theoretically contains 459 Au atoms. The particle also transformed into a multiply twinned icosahedral particle, (b) and (h).

111. OTHER TOPICS Convergent-beam electron diffraction has also been used by Tanaka (1986) and Tanaka et af. (1987, 1991) and others as a useful analytical tool for studying the space groups and the lattice constants of crystals, characteristics of lattice defects, lattice strain, and composition change in the vicinity of grain boundaries. In order to increase further the reliability of atomic information, electron microscopy has been combined with other methods such as EELS, Auger valence electron microscopy, STM, etc. Microcharacterization is one example. Microcharacterization is carried out based on the analysis of local chemical composition and atomic structure. Figure 13 is an example showing microcharacterization of a silicon aluminum oxynitride polytype, 15RSiA1402N4(Bando et af.,1986,1989). This microcharacterization was carried out by the combination of covergent-beam electron diffraction (a), EDS (b), EELS (c), and high-resolution electron microscopy combined with a calculated image contrast (d). All these results were obtained from the observed area in Fig. 13(d). In this crystal, a true space group was not determined uniquely by both X-ray and the convergent selected-area electron diffraction method. From the convergent-beam ED pattern of Fig. 13(a), the space group is assigned as R3m. From the EDS spectrum, Fig. 13(b), the molar ratio of Si to A1 is assigned to be 1:4, The quantitative

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FIGURE12. (a-j) Electron microscope images of Au atom clusters. An atom cluster is growing on a graphitized carbon surface which is vertical to the plane of the sheet.

analysis of the EELS spectrum, Fig. 13(c), gives the molar ratio of 0 to N as 2 :4. These results show that the chemical composition of the 15R type is determined to be SiAI4O2N4.Furthermore, many-wave lattice fringes of the 15R, Fig. 13(d), in which calculated image contrast is inserted, shows that cation sites appear well resolved as dark spots. The possible structure model is derived from the structural analogy of AlN, and then the validity of the structure model obtained is confirmed by image calculation. Thus, it is concluded that the combination of both lattice image and spectroscopic microanalysis leads to accurate determination of crystal structure.

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FIGURE 13. Crystal structure and chemical composition analysis of silicon aluminum oxynitride polytype, 1SR-SiAb02N4, by a combination method. The analysis was carried out by a combination of convergent beam electron diffraction (a), EDS (b), EELS (c). and highresolution electron microscopy (d). A calculated image contrast is inserted in (d).

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