多鐵性材料鐵酸鉍(BiFeO3, BFO)的居禮溫度(Tc)與尼爾溫度(TN)遠高於室溫，同時具有極大的鐵電極化量(約100 μC/cm2)，因此在元件應用上具有相當潛力，如鐵電隨機存取記憶體(ferroelectric RAM, FeRAM)。然而雜相和氧空缺的存在以及陽離子容易變價(如:Fe3+/Fe2+和Bi3+/Bi2+)的問題導致BFO的漏電流相當大，限制其應用上的發展，其中透過摻雜是改善BFO性能的方法之一。

本研究主要以水熱法製備BiFe1-xSbxO3 (x=0-0.02)粉體，為了提高塊材密度及後續性質量測，製備之粉體經壓錠後在大氣下以700oC燒結，最後將燒結樣品與冷壓樣品進行一系列分析並討論比較。另一部分則是在(001)取向的Nb:SrTiO3 (Nb:STO)單晶導電基板成長BiFe1-xSbxO3(x=0-0.02)磊晶厚膜，以利電滯迴線分析。整體成果概述如下。XRD結果顯示純相之BiFe1-xSbxO3 (x=00.02)粉體成功以水熱法製備得到，且粉體經過高溫燒結後並無明顯雜相生成。為了得知冷壓後和燒結後的BiFe1-xSbxO3 (x=0-0.02)樣品之晶格常數變化，樣品透過TOPAS軟體進行晶格常數與原子位置精修，結果顯示鈣鈦礦結構BFO的晶體扭曲程度是隨著Sb摻雜量增加而下降。拉曼結果顯示Sb原子是取代BFO中的Fe原子位置。XPS分析結果顯示未摻雜BFO經過燒結後Fe2+/Fe3+的比例從6.6/93.4 增加到 25.7/74.3，其電導率約提升5個數量級，從1.2x10-7 提升到 1.8x10-2 S/cm (550 K時)。在2%銻摻雜的BFO樣品中，Sb元素在燒結前的價數為3+；然而部分Sb5+在燒結後生成，並且抑制大量Fe2+的出現，因此2%銻摻雜BFO的樣品在燒結前後，其電導率僅從1.7x10-6 些微提升至 4.2x10-6 S/cm (550 K時)。對於上述樣品的氧空缺電荷補償機制，未摻雜BFO在燒結後主要是以Fe3+還原成Fe2+為主，而銻摻雜BFO經燒結後主要是以形成更多陽離子空缺佔主導機制。將純相及銻摻雜BFO樣品的電導率隨溫度變化曲線由阿瑞尼斯方程(Arrhenius equation)擬合得知，純相及銻摻雜BFO燒結前之活化能為1 eV左右，相當於氧空位捕獲(trap)電子後經受熱激發之能量。有趣的是，銻摻雜BFO在燒結後的活化能仍維持在1 eV左右，而未摻雜BFO在燒結後的活化能轉變為0.4 eV，屬於Fe2+/Fe3+之間的電子跳躍(hopping)能量。TEM選區繞射結果證實，純相與銻摻雜BFO的繞射點皆能對應到BFO之結晶面且和XRD分析結果相符。此外亦透過TEM的EDS分析證實Sb元素僅能在銻摻雜BFO的樣品中被偵測到。XRD theta-2theta分析結果顯示純相及銻摻雜BFO樣品只有觀察到(00l)繞射峰(l=1, 2, 3)，表示其晶粒是沿著c軸生長。另外以BFO的(011)繞射面做XRD φ-scans分析，其結果符合四軸對稱性，證明純相及銻摻雜BFO是以磊晶成長在Nb:STO基板上。未摻雜和1%銻摻雜BFO的厚膜具有典型的電滯迴線，其殘餘極化量分別為45和40 μm/cm2。

Among all multiferroic materials studied so far, BiFeO3 (BFO) is a single-phase multiferroic material, which is simultaneously ferroelectric and antiferromagnetic at room temperature. BFO is most promising from the practical application point of view due to its high ferroelectric Curie (TC=1103 K) and Néel (TN=643 K) temperatures and its large saturated polarization of about 100 μC/cm2. However, so far, bulk samples have failed to show interesting results of ferroelectric properties due to the poor quality of samples, in particular the large leakage current arising from a number of problems including the presence of many secondary phases, and the coexistence of mixed oxidation states of the cations (e.g. Fe3+/Fe2+ and Bi3+/Bi2+). The aim of this study was to hydrothermally synthesize Sb-doped BFO powders and thick films and then study the changes of crystal structure as well as the electrical and ferroelectric properties of BiFe1-xSbxO3 (x=0-0.02). Both cold-pressed and sintered pellets of the powders were used for characterizations.

The X-ray diffraction (XRD) results showed that BiFe1-xSbxO3 (x=0-0.02) powders of a pure phase were successfully synthesized by hydrothermal process. Furthermore, the crystal structures of BiFe1 xSbxO3 were refined using TOPAS software, which indicated that the Sb doping resulted in a reduced rhombohedral distortion of BFO from the ideal perovskite structure. Raman measurements were in favor of the assumption that the substitution of Sb at the Fe site in BFO. X-ray photoelectron spectroscopy (XPS) results showed a significant increase in the Fe2+/Fe3+ ratio from 6.6/93.4 to 25.7/74.3 in un-doped BFO after sintering and, as a consequence, the DC conductivity increased by about five orders of magnitude, from 1.2x10-7 to 1.8x10-2 S/cm (measured at 550 K). The Sb ions had a single 3+ oxidation state in the cold-pressed samples of 2% Sb-doped BFO, but a fairly large portion of Sb5+ occurred after sintering, which suppressed the formation of Fe2+ in the sintered samples of Sb-doped BFO and, as a consequence, there were only small increases in conductivity, from 1.7x10-6 to 4.2x10-6 S/cm (measured at 550 K). So, charge compensation for oxygen vacancies in un-doped BFO was achieved dominantly by the reduction of Fe3+ to Fe2+, while in Sb-doped BFO it was achieved more by cation vacancies. Temperature-dependent conductivity showed that cold-pressed samples of both un-doped and Sb-doped BFO had the similar activation energy of 1.0 eV, typical for electrons trapped in oxygen vacancies. After sintering, the activation energy of Sb-doped BFO remained almost unchanged, but the activation energy of un-doped BFO changed to 0.4 eV, which is associated to electron hopping between Fe2+/Fe3+. The sharp spots in selected area electron diffraction (SAED) patterns indicated that both un-doped and Sb-doped BFO powders are well crystallized. Moreover, EDS analyses confirmed that the Sb element is indeed present in the doped sample, whereas in the un-doped sample it is absent. XRD theta-2theta and φ scans of un-doped and 2% Sb-doped BFO thick films were performed to ensure the epitaxy on (001) Nb:SrTiO3 (Nb:STO) substrate. XRD theta-2theta scans showed that only the (00l) reflections (pseudocubic indices, l=1, 2, 3) were observed, indicating a unique out-of-plane orientation. In addition, XRD φ scans of (011) reflection showed only four peaks, matching with the four-fold rotation symmetry and, hence, confirming that the grains are in-plane aligned as well. So, the thick films grown on (001) Nb:STO are actually epitaxial. Ferroelectric measurements showed saturated hysteresis loops with the typical shape for ideal ferroelectrics. Large remanent polarizations were measured at room temperature, which were 45 and 40 μC/cm2 for the un-doped and 1% Sb-doped BFO, respectively.

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