| 研究実績の概要 |
[0001] symmetric tilt GBs are systematically investigated based on the coincidence site lattice (CSL) theory in order to determine detailed atomic structures. Tilt angles (2θ) were controlled from 0° to 60° to cover various types of GBs. For this purpose, a series of single GBs were fabricated using bicrystal methods, and scanning transmission electron microscopy observation and first-principles calculation have performed.
It was found that the GBs are described by periodic arrangement of a several low-energy configurations, which are often called structure units (SUs). GBs of 2θ below 27.8° (Σ13) are described by SUs of α and/or β types that are the same as those for undoped GBs. On the other hand, GBs of the 2θ near 30° are described by new type of SU γ, which has not been found in undoped GBs. Moreover, GBs with the 2θ higher than 32.2° (Σ13) were described by SU α in zig-zag arrangements. Each type of SU has specific number of Pr at certain Zn substitution sites.
Non-low-Σ CSL GBs were observed in order to reveal relevance between those unique arrangements of SUs. The GBs were described by SUs those are a combination of SUs of neighbored CSL GBs, called GB delimitation. Based on these results, a concept of phase diagram for ZnO GB, which describes the stable GB structure as a function of 2θ can be proposed. In addition, distribution of Pr-concentration as a function of 2θ can be assumed as well.
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| 今後の研究の推進方策 |
By knowing the atomic structure, it is possible to speculate the concentration of segregated Pr at each GB. It has been reported that the varistor performance (non-linear I-V (current-voltage)coefficient (α))is determined by segregated Pr, since the Pr promotes formation of native defect such as Zn vacancies that acts like a electron trap at the GB interface state. In addition, the voltage gradient (non-linear coefficient, alpha) may also increase as concentration of Zn vacancy increases. Therefore, it would be possible to find the most effective GB orientation that accompanies the highest α value. On-going experiments suggests that 2θ = 30° tilt GB has the highest Pr concentration among 2θ[0001]GBs. By measuring I-V curve from a several GBs, a relationship between Pr-concentration and the α value can be revealed. The result would be valuable for optimizing ZnO-based varistor with minimal size but high performance.
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