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ZnO在少量Li取代后表现出了新的铁电性。在这项研究中，我们研究了这种新的电子铁电性的机制，通过介电测量和X射线衍射。已经解释了铁电性是由晶体中离子或分子的位移或有序化引起的。ZnO中的铁电性是第一个表明铁电性起源于某些电子起源的例子，与公认的理解相反。证实了Li掺杂对铁电性的出现起着重要的作用。研究了ZnO和Li掺杂ZnO在室温和19 K下的晶体结构。通过对晶体结构和电子密度的分析，阐明了3d电子从Zn位向键合区转移。DV-Xα方法的计算也证实了这一点。参照Sham等人提出的电子铁电性理论，局域d空穴联合收割机与位于键合区的一些巡游电子或氧上的p电子结合，导致ZnO中出现这种新型的铁电性。因此，这种铁电性主要与d-p杂化的变化有关，而与以前理论中描述的结构变化无关。这一研究成功地揭示了凝聚态物理学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.