BiFeO3 (BFO)薄膜的磁電耦與高自發極化特性。在一般情況下BFO具有斜方晶結構，然而在基板應力的影響下BFO薄膜可同時有菱長晶與長方晶兩種相態。因此本研究主要利用量測不同型式頻譜的方式，去討論長方晶(T)與混合相(T+R)的鐵酸鉍樣品內部介電機制的差異，同時每個機制都可以電路等效，再利用變溫和變電壓的方式確認樣品機制的演變。結果顯示我們可將室溫中的T-BFO以導電性的弛豫電路等效而T+R-BFO除了T相所貢獻的導電弛豫系統外，另外需要加上T與R介面上的介電弛豫電路等效。當我們對系統加反向電壓時，因為T+R相轉為純T相，造成T+R介面介電機制貢獻減少與整體介電常數的降低。在降溫實驗中，發現到T相的漏電流會隨著溫度的降低而減少，但T+R的介面電容卻會增加。因此我們可以透過頻譜術的方式，將混相樣品的晶相貢獻，一個一個分開，達到各別晶相可析的結果。

Abstract
BiFeO3 (BFO) films which possess the coupling effect between ferromagnetism and ferroelectricity and high polarization have attracted much attention. In general, the BiFeO3 (BFO) films have the rhombohedral phases. Under the compressive stress of the substrate, the strained rhombohedral (R) phase will transform to the tetragonal (T) phase. In this study, we used the impedance spectroscopy to discuss the dielectric mechanism of T-BFO and T+R-BFO. Moreover, we investigated the variation of the mechanisms represented by equivalent circuit elements with different DC bias and temperature. Results show that the dielectric contributions of T-BFO are due to conducting system dispersion (CSD), while those of the T+R-BFO films are equivalent to the combination of conducting system dispersion (CSD) and dielectric system dispersion (DSD). We conclude the CSD is originated from the T polarization state and the DSD is originated from the interface of T+R polarization state. Therefore, the dielectric constant of T+R-BFO would reduce under the electric field, because the R phase orientate to T phase in the interface. In contrast to the dielectric constant reduction of T+R-BFO under bias, the dielectric constant would increase on the cooling system. By using the impedance spectrum, we explicitly analyzed each polarization state in the T+R-BFO.

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