MECHANISM OF ELECTROFERROELECTRICITY AND d-p HYBRIDIZATION IN II-VI SEMICONDUCTOR ZnO

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It is well-known that II-VI semiconductor Zinc Oxide (ZnO) shows semiconducting nature. Recently, we discovered that ZnO exhibits novel ferroelectricity by small amount of Li substitution. In this study, we investigated the mechanism of this novel electronic ferroelectricity by means of dielectric measurements and X-ray diffraction. It has been explained that ferroelectricity is induced by displacements or ordering of ions or molecules in crystals. The ferroelectricity in ZnO is the first example to show ferroelectricity originated from some electronic origin, in contrast to the well-established understandings. It is confirmed Li dopants play an important role for the appearance of ferroelectricity. We investigated precise crystal structures both of ZnO and Li-doped ZnO at rom temperature and 19K. Analyses of crystal structures and electronic density by MEM clarified that 3d electrons transfer from the Zn site to bonding region. This evidence is also confirmed by calculations by DV-Xα method. Referring the theory of electronic ferroelectricity proposed by Sham et al, localized d-holes combine with some itinerant electrons located at the bonding region or p-electrons at oxygen, which results in this novel ferroelectricity in ZnO. Therefore this ferroelectricity is mainly related to changes of the d-p hybridization, not to structural changes described in te previous theory. This study has succeeded to show the new concept of electronic ferroelectricity in the condensed matter of physics.