鉍鐵氧薄膜材料之溫度響應結構對稱性轉變對鐵電極化方向與壓電性質影響之研究

Abstract

本實驗研究LaAlO3基板上磊晶之BiFeO3 (BFO/ LAO)薄膜，著重於結構、熱力學穩定與鐵電/壓電性質的量測與討論。首先，利用倒晶格空間影像技術(Reciprocal space maps, RSM)進行不同溫度BFO薄膜的晶體結構對稱性的分析；此外，利用可升溫之壓電力顯微鏡(Piezoelectric force microscopy, PFM)量測BFO薄膜之水平面鐵電極化方向(In-plane polarization, IPP)和薄膜壓電係數(d33)。 RSM結果指出，當溫度從室溫上昇至150oC時，由於晶格常數的改變，BFO相結構即由單斜MC對稱(a = 3.81 Å, b = 3.76 Å, c = 4.64 Å, βMC = 88.5° )轉變成單斜MA對稱(a = 3.80 Å, b = 3.79 Å, c = 4.66 Å, βMA = 86.8°)。此外，由PFM-IPP影像分析得知，此薄膜之鐵電極化方向亦受溫度影響，從單斜結構之[11 ]方向旋轉至[10 ]。最後由壓電係數(d33)量測得知，當BFO薄膜在低溫穩定相MC與高溫穩定相MA相時，可量測到穩定的d33，分別約為50與60 pm/V；然而，在150oC 時，壓電係數d33劇升至98.1 pm/V。此一現象可理解為晶格結構與鐵電極化方向之轉變提供了額外的壓電響應自由度所造成的結果。 綜合以上結果可知，受溫度影響之BFO薄膜單斜結構對稱性轉換現象，不僅造成薄膜之鐵電極化方向旋轉，亦進而提昇了壓電性能。此一結果除了提供更多的BFO薄膜之鐵電性質資訊，亦有助於未來開發高壓電係數薄膜材料之相關研究。

We report a study on epitaxial BiFeO3 thin films on LaAlO3 substrates with focuses on their structural nature, thermodynamic stability and ferro-/piezoelectric properties. First, we use Reciprocal space maps (RSM) to confirm the structure symmetry of BFO thin film at different temperature. Then, we use temperature-variable piezoelectric force microscopy (PFM) and observed a rotational transition of in-plane ferroelectric polarization (IPP-PFM) and piezoelectric coefficient (d33). Form RSM data, we found the dominant phase at room temperature possess an MC symmetry (a = 3.81 Å, b = 3.76 Å, c = 4.64 Å, βMC = 88.5° ) but switches to an MA symmetry (a = 3.80 Å, b = 3.79 Å, c = 4.66 Å, βMA = 86.8°) at 150oC . Furthermore, IPP-PFM images reveal that temperature of BFO thin film will affect a rotational transition of in-plane ferroelectric polarization from [100] to [110]. Also, in stable phase of either MC or MA have a steady d33 around 50 or 60 pm/V, but at 150 oC, a significant increase in d33 to 98.1 pm/V is observe. This phenomenon can be understood as both the lattice structure and ferroelectric polarization additional degrees of freedom to respond to the external electrical field. In summary, the symmetry structure change is accompanied by a ferroelectric polarization rotation, and the piezoelectric response is also enhanced considerably at the same temperature. These results provide key insights into the ferroelectric materials and could help future engineering of highly piezoelectric thin films.