KEYWORDS: Zinc oxide, Praseodymium, Zinc, Ceramics, Crystals, Scanning transmission electron microscopy, Ions, Transmission electron microscopy, Doping, Chemical species
ZnO is used in a wide variety of applications owing to the electrical properties. Polycrystalline ZnO ceramics have long
been used such as varistor, and ZnO films are currently intensively studied for transparent conductor applications. Grain
boundary (GB) in ZnO varistor is believed to be the origin of nonlinear current-voltage characteristics, and GB in ZnO
films possibly affects the electrical conductivity. It is therefore important to understand the role of ZnO GB on the
electrical properties, which should be closely related with the structure in atomic scale. With these viewpoints, we have
studied the atomistic structure of ZnO GBs, where the orientation relations of adjacent crystals are well defined. Single
GBs studied were obtained by fabricating ZnO bicrystals and the GBs were characterized by scanning transmission
electron microscopy (STEM) and theoretical calculations.
It is found that coordination number of ions change in ZnO GBs; there are underfold or overfold coordinated ions that
are unusual in bulk inside. It is calculated that these atomistic structures alters the electronic structure but would not
create deep states in the band gap. On the other hand, when praseodymium (Pr), which is known to be a key dopant
element to obtain nonlinear (I-V) characteristics, is added to the GBs, Pr strongly localizes to the GBs and occupies
specific atomic sites. Pr facilitates the formation of the acceptorlike defects such as zinc vacancies, which we think that
is an important role of Pr on generation of nonlinear (I-V) characteristics. Furthermore, atomic arrangement and
localization behavior of Pr are studied for several GBs to obtain fundamental understanding about GB structure
formation.
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