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

众所周知，II-VI族半导体氧化锌(ZnO)具有半导体性质。最近，我们发现，通过少量的Li替代，氧化锌表现出了新的铁电性。在本研究中，我们通过介电测量和X射线衍射仪研究了这种新型电子铁电性的机理。已经解释过，铁电性是由晶体中离子或分子的位移或有序化引起的。与公认的理解相反，氧化锌中的铁电性是第一个表明铁电性源于某种电子来源的例子。证实了Li掺杂对铁电性的出现起着重要的作用。在室温和19K温度下，我们研究了氧化锌和锂掺杂氧化锌的精确晶体结构。MEM对晶体结构和电子密度的分析表明，3d电子从锌位向键区转移。这一证据也被DV-Xα方法的计算所证实。参考Sham等人提出的电子铁电性理论，局域d-空穴与位于键区的一些巡回电子或氧中的p-电子结合在一起，形成了这种新颖的铁电性。因此，这种铁电性主要与d-p杂化的变化有关，而与以前理论中所描述的结构变化无关。这项研究成功地展示了物理凝聚态中电子铁电性的新概念。

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.