Review of “12th Japanese-Polish Joint Seminar on Micro and Nano Analysis(August 29­September 1, 2018)”

Manabu Ishimaru+

Department of Materials Science and Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan

“The 12th Japanese-Polish Joint Seminar on Micro and Nano Analysis” was held in Fukuoka, Japan from August 29 to September 1, 2018,and the proceedings were published in May, 2019, as a special issue of Materials Transactions (Vol. 60, No. 5). The main purpose of thisseminar is to discuss the structural analysis of materials by electron microscopy techniques. Among the papers presented at the seminar, thisarticle briefly reviews the following topics: observations of dislocations in a thick specimen by ultra-high voltage electron microscopy,suppression of geometric phase shift due to antiphase boundaries in dark-field electron holography, and structural characterization of amorphousmaterials by electron diffraction techniques. [doi:10.2320/matertrans.MT-M2021094]

(Received May 25, 2021; Accepted June 23, 2021; Published July 26, 2021)

Keywords: transmission electron microscopy, scanning transmission electron microscopy, structural analysis, chemical analysis

1. Introduction

Since the mechanical, physical, and chemical properties ofmaterials strongly depend on the atomic arrangements andthe slightly added functional elements, obtaining structuraland chemical information is of technological importance fordeveloping new structural and functional materials. In recentmaterial developments, hybridization of various materialsand introduction of nanostructures into conventional materi-als are being attempted in order to achieve high strengthand high functionality. As a result, it is difficult to evaluatematerials with nanoscale heterogeneous structures by using“average structure information” taken from a wide area.Electron microscopy can obtain “local structure information”by illuminating a nanometer-sized electron beam ontomaterials, and structural information, such as precipitatesand defects, as well as chemical information, such ascompositions and bonding states, is measurable from thesame location of an object with the high accuracy at theatomic scale. Because of their excellent spatial resolution,transmission electron microscopy (TEM) and scanningtransmission electron microscopy (STEM) are one of theimportant techniques indispensable for promoting materialsscience and engineering.

“The Japanese-Polish/Polish-Japanese Joint Seminar onMicro and Nano Analysis” was established for discussingprogresses in structural and chemical analyses using mi-croscopy and microanalysis techniques and their applicationto the field of materials science. This seminar is a biennialevent and alternately organized in Japan and Poland since1997, as summarized in Table 1. The twelfth joint seminarwas held at Kyushu University Nishijin Plaza in Fukuoka,Japan from August 29 to September 1, 2018,1) and consistedof the following sessions: “Severe Deformation”, “Deforma-tion, Stress, and Dislocation”, “Relationship betweenStructure and Functionality”, “Nanowire, Interface, andLow-dimensional Materials”, “Spectroscopy”, and “Process-ing”. Eight selected papers2­9) were published in a specialissue of Materials Transactions (Vol. 60, No. 5) under the

title of “New Trends for Structural and Chemical Analysesby Transmission Electron Microscopy”. Here, we reviewthree topics presented in the seminar.

2. Observation of Dislocations in Micrometer-ThickSpecimens

Ultra-high voltage electron microscopy (UHVEM) is oneof the world’s leading technologies in Japan. Because of thehigh transmission power, it is possible to observe microme-ter-thick specimens using UHVEM. However, the number ofinelastically-scattered electrons increases with the specimenthickness along the electron beam direction, and the qualityof images is degraded by chromatic aberration associatedwith the energy loss of the electrons. The UHVEM installedat the Ultramicroscopy Research Center, Kyushu Universityis equipped with an energy filter,10) which enables imagingusing an electron beam with a specific energy. Sadamatsuet al.11) examined a single crystalline Si including artificiallyintroduced high-density dislocations using energy-filteredUHVEM in combination with electron energy-loss spec-troscopy, and demonstrated that the dislocations are clearlyvisible even in bulk specimens over 10 µm-thick. On theother hand, Sato et al.2,12) observed the dislocations in thickspecimens by STEM using UHVEM installed at the ResearchCenter for Ultra-High Voltage Electron Microscopy, OsakaUniversity. Figure 1 shows a bright-field STEM image of awedge-shaped Si specimen with a thickness from 1µm (left-side) to 9 µm (right-side).2) The width of the dislocation linesis almost constant at 13­16 nm in the region with a thicknessof 1 to 7.5 µm, suggesting that a wedge-shaped specimenwith a thickness up to 7.5 µm can be observed in focus usingSTEM operated at an acceleration voltage of 1MV. This isbecause STEM is less susceptible to the effect of resolutionreduction due to chromatic aberration than TEM. In addition,the STEM technique is less affected by diffraction contrasts,such as thickness contours and bend contours, than TEMimaging. Because of these advantages, the UHVEM-STEMis a powerful technique to observe the defects in bulkspecimens.

+Corresponding author, E-mail: ishimaru@post.matsc.kyutech.ac.jp

Materials Transactions, Vol. 62, No. 9 (2021) pp. 1420 to 1423©2021 The Japan Institute of Metals and Materials CURRENT TRENDS IN RESEARCH

3. Suppression of Undesired Geometric Phase Shift inDark-Field Electron Holography

Electron holography can quantitatively analyze electricand magnetic field with high spatial resolution.13­20) Inordinary electron holography, a hologram is constructed byinterfering electrons transmitted through a thin film sample(object wave) with a reference wave in a vacuum. The phasechanges due to magnetic field and electric field are recordedin the hologram, and they are experimentally separable. Onthe other hand, an extra geometric phase shift occurs in thevicinity of lattice defects, which is a critical issue in dataacquisition and analysis. While ordinary holography uses atransmitted beam (the origin “000” of the reciprocal latticespace) as the object wave, dark-field electron holographyuses a diffracted beam that contains distortion information.

By interfering the object wave transmitted through thedistorted region with the reference wave transmitted throughthe undistorted region, the extra geometric phase shift canbe recorded in the hologram. For non-magnetic materials, atwo-dimensional strain map can be extracted by analyzing thedark-field electron hologram.21)

Cho et al.6) investigated the effects of antiphase bounda-ries, where the geometric phase shift due to strain isnegligibly small, on the dark-field electron holography usinga B2-type Fe70Al30 alloy. Figure 2 shows the change of thephase shift across the antiphase boundary obtained fromreconstructed phase images of the holograms. The hologramwas taken using (a) the 000 transmitted beam, (b) 100superlattice reflection, and (c) 200 fundamental latticereflection of the electron diffraction pattern. A significantphase shift of ³2.3 rad was observed at the antiphase

Table 1 History of Japanese-Polish/Polish-Japanese Joint Seminar on Micro and Nano Analysis. The thirteenth seminar was scheduled tobe held in Poland in 2020, but was postponed to 2022 due to COVID-19.

PAS: Polish Academy of Sciences

Fig. 1 Bright-field STEM image of a wedge-shaped single crystalline Si (Ref. 2). The thickness changes from 1µm (left side) to 9 µm(right side). Artificially-generated dislocations are clearly visible up to a thickness of 7.5 µm in the specimen.

Review of “12th Japanese-Polish Joint Seminar on Micro and Nano Analysis (August 29­September 1, 2018)” 1421

boundary in Fig. 2(b), whereas no phase shift occurs inFig. 2(c). This suggests that the undesired geometric phaseshift can be suppressed in the dark-field electron holographyusing the fundamental lattice reflections. This is useful foranalyzing magnetic information at the antiphase boundariesin ferromagnetic materials.

4. Structure Analysis of Amorphous Materials byElectron Diffraction

Conventional electron microscopy techniques, such asbright- and dark-field TEM and high-resolution TEMobservations, are still useful for analyzing the materials,and several researchers reported the results of the structuralcharacterization of metals,3,5,8) semiconductors,4) and compo-site materials7,9) in the seminar. Electron diffraction canprovide scattering information over a wide range of thereciprocal lattice space, due to the short wavelength of high-energy electrons. This is useful for obtaining radialdistribution functions and atomic pair-distribution functionswhich express amorphous structures as the existenceprobability of atoms as a function of a distance from thecenter of an arbitrary origin atom. Higashiyama et al.22)

performed the structural analysis of amorphous Ge1¹xSnxvia electron diffraction radial distribution analysis.Figure 3(a) shows atomic pair-distribution functions, g(r),of sputtered amorphous thin films with different Snconcentrations. Prominent peaks appear at ³0.25 and³0.40 nm, which correspond to the first and secondcoordination shells, respectively, of the amorphous network.On the other hand, the g(r) converges to unity at the longerdistance side, indicating the lack of long-range order. Acloser examination of Fig. 3(a) reveals that the location ofthe peaks moves to the longer distance side with increasingthe Sn concentration. Figure 3(b) shows the magnified firstpeak in the g(r) of Fig. 3(a). The peak location of thespecimen with a concentration of 9.1 at%Sn is almost thesame as the bond length of Ge­Ge atomic pair. The peakheight due to the Ge­Ge atomic pair decreases withincreasing Sn concentration, while the number of Sn-relatedatomic pairs increases. On the basis of the radial distributionfunction analysis and high-resolution TEM observations, itwas suggested that Ge and Sn are mixed within the firstcoordination shell and no remarkable phase separationoccurs. Knowledge of the amorphous structures as well astheir structural changes23­25) is of technological important for

fabricating polycrystalline Ge1¹xSnx thin films which areanticipated as a channel material for high performance thinfilm transistors.26,27) Selected-area electron diffraction takenfrom a relatively wide area gives average structuralinformation of amorphous materials. On the other hand,atomic clusters embedded in the amorphous matrix aredetectable by focusing an electron beam on a material.Indeed, nanoscale inhomogeneities in amorphous materialshave been successfully detected by using a highly parallel,high-brightness angstrom beam.28,29)

Fig. 2 Phase shift at the antiphase boundary obtained from reconstructed phase images of the holograms (Ref. 6). The holograms weretaken using (a) the 000 transmitted beam, (b) 100 superlattice reflection, and (c) 200 fundamental lattice reflection of the electrondiffraction pattern. It is apparent that the phase shift is suppressed in (c).

Fig. 3 (a) Atomic pair-distribution functions and (b) magnified first peakof amorphous Ge1¹xSnx thin films with different Sn concentrations(Ref. 22). The first peak consists of Ge­Ge, Ge­Sn, and Sn­Sn atomicpairs, and its position moves to the longer distance side with increasingthe Sn concentration. (Reprinted from J. Appl. Phys. 125 (2019) 175703,with the permission of AIP Publishing.)

M. Ishimaru1422

5. Summary

In summary, this article briefly reviewed the paperspresented in the 12th Japanese-Polish Joint Seminar onMicro and Nano Analysis. In addition to the topics coveredhere, the following impressive presentations were given atthe seminar: in situ plastic deformation studied by time-resolved three-dimensional electron tomography,30) atomic-resolution two-dimensional elemental mapping by electronenergy-loss spectroscopy,31,32) and determination of latticestrain of gold nanoparticles by high-angle annular dark-fieldobservations.33) The improvement of the spatial resolution of(S)TEM and the development of highly efficient analyticalinstruments are still ongoing, and there is no doubt thatelectron microscopy techniques will become more importantin the research field of materials science and engineering.

Acknowledgements

This work was supported by JSPS Bilateral Joint ResearchProjects/Seminars.

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