Structural Defects and Ferromagnetic Signature of V-Doped Sb₂Te₃Thin Films Grown on SrTiO₃(001) Produced by RF-Magnetron Sputtering

Sato K., . Structural Defects and Ferromagnetic Signature of V-Doped Sb₂Te₃Thin Films Grown on SrTiO₃(001) Produced by RF-Magnetron Sputtering. ACS Omega 7, 40480 (2022); https://doi.org/10.1021/acsomega.2c05634.Thin films of V-doped Sb₂Te₃compounds were fabricated by co-deposition of Sb₂Te₃and V using rf-magnetron sputtering onto SrTiO₃(001) substrates kept at 570 K. The microstructures of the films were characterized using transmission electron microscopy (TEM) and electron diffraction. The crystal structure of the sputtered film (Sb₃₈V₂Te₆₀) is the Bi₂Te₃-type structure with lattice parameters of a = 0.44 ± 0.03 nm and c = 3.02 ± 0.02 nm. A combination of cross-sectional and plan-view TEM observations revealed the preferential orientation of the c axis in the film's normal direction. A thin amorphous layer exists between the Sb₂Te₃thin film and the SrTiO₃substrate. The interfacial amorphous layer relaxes the strain between the thin film and the substrate, and hence, it should promote the growth of a low-index atomic plane with a low surface free energy (i.e., (0001) of the Sb₂Te₃). The onset of ferromagnetic order was detected at temperatures below 70 K. A remarkable increase in magnetization was detected in the film's normal direction, which corresponds to the magnetic easy axis (i.e., c axis of the Sb₂Te₃). V³⁺ions substituting Sb sites should contribute to ferromagnetism at low temperatures

Magnetic nanoparticles: When atoms move around

Sato K., . Magnetic nanoparticles: When atoms move around. Nature Materials 8, 924 (2009); https://doi.org/10.1038/nmat2575.The degree of atomic ordering in magnetic nanoparticles decreases strongly with the particles' size. The origin of such a phenomenon has been determined by high-resolution transmission electron microscopy and tomography, which shows how correct heat treatment can lead to atomic order also in very small nanoparticles

Measurements of Antenna Surface for Millimeter-Wave Space Radio Telescope

In the construction of a space radio telescope, it is essential to use materials with a low noise factor and high mechanical robustness for the antenna surface. We present the results of measurements of the reflection performance of two candidates for antenna surface materials for use in a radio telescope installed in a new millimeter-wave astronomical satellite, ASTRO-G. To estimate the amount of degradation caused by fluctuations in the thermal environment in the projected orbit of the satellite, a thermal cycle test was carried out for two candidates, namely, copper foil carbon fiber reinforced plastic (CFRP) and aluminum-coated CFRP. At certain points during the thermal cycle test, the reflection loss of the surfaces was measured precisely by using a radiometer in the 41-45 GHz band. In both candidates, cracks appeared on the surface after the thermal cycle test, where the number density of the cracks increased as the thermal cycle progressed. The reflection loss also increased in proportion to the number density of the cracks. Nevertheless, the loss of the copper foil surface met the requirements of ASTRO-G at the end of the equivalent life, whereas that of the aluminum-coated surface exceeded the maximal value in the requirement even before the end of the cycle.Comment: 11 pages, 6 figures, accepted for publication in PAS

Domain switching dynamics in relaxor ferroelectric Pb(Mg₁⁄₃Nb₂⁄₃)O₃-PbTiO₃ revealed by time-resolved high-voltage electron microscopy

Ferroelectric domain dynamics in Pb(Mg₁⁄₃Nb₂⁄₃)O₃-PbTiO₃ single crystals have been studied by in situ biasing high-voltage transmission electron microscopy with a direct electron detection camera. We have achieved time-resolved recording of polarization switching in real space on a 2.5 ms time scale. The reversible response of micrometer-scale domains was observed by applying an electric field of 1 kV/mm. Detailed analyses on smaller sized domains 100-500 nm in size revealed that the domain switching initiated at a corner of a rectangular domain and propagated inward rapidly. The switching proceeded within 60 ms and the maximum switching rate, as fast as 6-8 μm/s, was observed. The domain switching kinetics was classified as two-dimensional nucleation and growth mode based on the Kolmogolov-Avrami-Ishibashi model.Kazuhisa Sato and Naoya Asakura, "Domain switching dynamics in relaxor ferroelectric Pb(Mg₁⁄₃Nb₂⁄₃)O₃-PbTiO₃ revealed by time-resolved high-voltage electron microscopy", Journal of Applied Physics 130, 164101 (2021) https://doi.org/10.1063/5.0064291

Athermal Solid Phase Reaction in Pt/SiOx Thin Films Induced by Electron Irradiation

Microelectronics based on Si requires metal silicide contacts. The ability to form platinum silicide (Pt₂Si) by electronic excitation instead of thermal processes would benefit the field. We studied the effects of electron irradiation on Pt₂Si formation in composite films-composed of Pt and amorphous silicon oxides (a-SiOx)-by transmission electron microscopy and electron diffraction. Pt₂Si formed in Pt/a-SiOx bilayer and a-SiOx/Pt/a-SiOx sandwiched films by 75 keV electron irradiation, at 298 and 90 K. The reaction is attributable to dissociation of SiOx triggered by electronic excitation. In a-SiOx/Pt/a-SiOx sandwiched films, reflections of pure Pt were not present after irradiation, i.e., Pt was completely consumed in the reaction to form Pt₂Si at 298 K. However, in Pt/a-SiOx bilayer films, unreacted Pt remained under the same irradiation conditions. Thus, it can be said that the extent of the interfacial area is the predominant factor in Pt₂Si formation. The morphology of Pt islands extensively changed during Pt₂Si formation even at 90 K. Coalescence and growth of metallic particles (Pt and Pt-Si) are not due to thermal effects during electron irradiation but to athermal processes accompanied by silicide formation. To maintain the reaction interface between metallic particles and the dissociation product (i.e., Si atoms) by electronic excitation, a considerable concomitant morphology change occurs. Elemental analysis indicates that the decrease in Si concentration near Pt is faster than the decrease in O concentration, suggesting formation of a Si depletion zone in the amorphous silicon oxide matrix associated with formation of Pt₂Si.Kazuhisa Sato and Hirotaro Mori, Athermal Solid Phase Reaction in Pt/SiOx Thin Films Induced by Electron Irradiation, ACS Omega, 2021, 6 (33), 21837-21841

Nanocrystal growth and morphology of PbTeSe-ZnSe composite thin films prepared by one-step synthesis method

The microstructure of polycrystalline PbTe₁₋ₓSeₓ-ZnSe composite thin films has been studied by scanning transmission electron microscopy and electron diffraction. The films were prepared by the one-step synthesis method using simultaneous evaporation of PbTe and ZnSe. The nanocrystals of PbTe₁₋ₓSeₓ are formed in a ZnSe matrix. Tellurium concentration can be tuned by controlling the PbTe evaporation source temperatures between 753 K and 793 K. Binary PbSe nanocrystals were formed at 753 K, while ternary PbTe₁₋ₓSeₓ nanocrystals were formed at 793 K. The nanocrystals grow in a granular shape at the initial stage of film growth, and the morphology changes to nanowire-shape as the film grows, irrespective of the Te concentration. The ternary PbTe₁₋ₓSeₓ nanocrystals were composed of two phases with different Te concentration; Te-rich (Se-poor) granular crystals were formed near the bottom half parts of the film and Te-poor (Se-rich) nanowires were formed at the upper half parts of the film. Columnar ZnSe crystals contain high-density {111} stacking faults due to the low stacking fault energy of ZnSe. A balance of deposition and re-evaporation on the substrate during the film growth will be responsible for the resultant nanocrystal morphology.Kazuhisa Sato and Seishi Abe, "Nanocrystal growth and morphology of PbTeSe-ZnSe composite thin films prepared by one-step synthesis method", Journal of Applied Physics 120, 155301 (2016) https://doi.org/10.1063/1.496487

Athermal Crystal Defect Dynamics in Si Revealed by Cryo-High-Voltage Electron Microscopy

Sato K., Yasuda H.. Athermal Crystal Defect Dynamics in Si Revealed by Cryo-High-Voltage Electron Microscopy. ACS Omega 5, 1457 (2020); https://doi.org/10.1021/acsomega.9b03028.Low-temperature crystal defect dynamics in Si has been studied by a newly developed cryo-high-voltage electron microscopy. The planar {113} defects of self-interstitial atoms were introduced at 94 K by 1 MeV electron irradiation with damage higher than 0.42 displacements per atom (dpa), unlike past findings. The defects once grew and then shrunk during the observation. We show that the nucleation and the dissociation dynamics of the {113} defects can be attributed to an athermal process, which is deduced from anomalously fast diffusion of self-interstitial atoms at a low temperature