Ferroelectric thin film having superlattice structure, manufacturing method thereof, ferroelectric element, and manufacturing method thereof

#The present invention relates to a ferroelectric thin film that functions as a non-volatile ferroelectric memory and is favorable for application to a broad field of electronic information appliances such as portable telephones, mobile electronic appliances and others, and to a method for producing the ferroelectric thin film, as well as to a ferroelectric element using the ferroelectric thin film and to a method for producing the ferroelectric element.

#A ferroelectric material is a type of dielectric materials and is a material which spontaneously polarizes even in the absence of any external electric field (with electric dipoles kept aligned) and of which the polarization direction varies depending on the electric field therearound. Typical materials of the type include barium titanate BaTiO3, lead titanate PbTiO3, lead zirconate titanate Pb(Zr,Ti)O3 and the like having a perovskite structure, which are applied to ferroelectric memories, actuators and others using the spontaneous polarization characteristics and the piezoelectric characteristics that the ferroelectric material has.

#In a ferroelectric material, it is known that a larger amount of displacement of each constituent element from the normal position provides more excellent ferroelectric characteristics (spontaneous polarization, etc.).

#FIG. 1 is a schematic view of the configuration of a thin film element that includes a perovskite nanosheet thin film shown in Examples 1 to 4.

#FIG. 2 is a high-resolution transmission electron micrograph of the cross section of a layered perovskite nanosheet thin film having three layers shown in Example 1.

#FIG. 3 is a graph showing a ferroelectric hysteresis characteristic of a superlattice-structured thin film of 5 layers of Ca2Nb3O10 and LaNb2O7 alternately layered therein, shown in Example 1, and for comparison, a hysteresis characteristic of a multilayer film of 10 layers of Ca2Nb3O10 or LaNb2O7 each layered alone.

#The present inventors have assiduously investigated the optimum structure that establishes the ferroelectricity expression mechanism in an ultrathin film that “a larger amount of displacement of each constituent element from the normal position provides more excellent ferroelectric characteristics (spontaneous polarization, etc.)”, and have found a principle that even a dielectric material that could not be a ferroelectric material can show ferroelectricity when the bonding interface thereof is softly bonded with an ionic material in such a manner that the ion inside the dielectric material could be readily mobile, and further applying this finding, the inventors have attained the present invention.

#The technique of “softly bonding the bonding interface of a dielectric material with an ionic material” as referred to herein means that dielectric materials are electrostatically bonded to each other via an ionic material such as a metal ion, a polyatomic ion, a complex ion, a cluster ion or the like. This quite differs from the interface state in a conventional ferroelectric superlattice (for example, BaTiO3 and SrTiO3, etc.). More specifically, in a conventional ferroelectric superlattice, the dielectric materials firmly bond to each other via a metal-oxygen bond so that the position of each constituent atom of the dielectric material is fixed on the left, right, top and bottom thereof, while the invention is characterized in that the dielectric materials are electrostatically bonded to each other merely on the top and bottom thereof via an ionic material therebetween, but have a soft interface with a flexible freedom on the left and right thereof.

#The invention is characterized as above, and the embodiments thereof are described below.