鉍鐵氧薄膜之X光吸收能譜學與光電子激發能譜學研究

X-ray Absorption Spectroscopy and Photoemission Spectroscopy Studies of BiFeO3 Thin Films

袁志瑋(Chih-Wei Yuan)

指導教授 ： 黃迪靖 ； 朱英豪

國立清華大學/理學院/物理學系/碩士(2013年)

摘要

過渡金屬氧化物於近年來被許多科學團隊廣泛地研究成為新一代電子元件的重要材料，其內部擁有相當有趣的物理特性，主要是來自於電子本身的基本屬性。而鉍鐵氧(BiFeO3)是少數在室溫下即擁有鐵電-反鐵磁耦合的多鐵材料，並具備電控磁的磁電耦合效應，然而此材料所擁有磁與電的特性在現今資訊與電子科技發達的年代扮演著重要角色，如非揮發性記憶體、自旋電子元件及記錄媒體等。因此本實驗主要使用脈衝雷射沉積法(PLD)沉積優良取向性的BiFeO3薄膜。利用軟X光吸收光譜技術(XAS)來量測BiFeO3薄膜之鐵L-edge的線性二向性(LD)、線性磁二向性(MLD)實驗，由光譜圖能了解BFO薄膜的電子結構、軌域與反鐵磁等特性。同時搭配磁圓偏振二向性(MCD)光譜與超導量子干涉儀(SQUID)來探討樣品的磁特性來源。此外，本實驗採用高能量解析力的硬X光光電子能譜技術(HXPES)能直接探討BFO薄膜深層的佔有態電子結構，實驗結果表示BFO薄膜的光電子激發光譜圖於Fe-2p3/2峰值有雙重峰值的產生，我們解釋是因為薄膜內部存在的三價態(Fe3+)鐵離子之多重態結構是由多重峰值所組成的，進而使得Fe-2p3/2峰值有分裂的現象，其峰值分裂的能量大小為1.15 eV。另外，我們將製備完畢的BFO薄膜在降溫時去改變不同的氧氣壓力，100 Torr、9*10-2 Torr下製備的BFO薄膜在光電子激發光譜圖看不出其電子結構變化。最後，將BFO薄膜沉積於薄膜具有拉伸、壓縮應力的各種基板上，當各種基板的晶格常數與BFO薄膜本身晶格常數之間的匹配性越差時，光電子激發光譜圖於Fe-2p3/2峰值的雙重峰值的光電子激發強度、峰值間的能量差距均有差異，本部分實驗結果只能看到實驗現象，尚未解釋其物理原因為何，尚需搭配進一步的理論計算或實驗來幫助了解。

關鍵字

多鐵物理 ； 鉍鐵氧 ； X光吸收能譜學 ； 光電子激發能譜學

並列摘要

Magnetoelectric coupling in multiferroic materials has attracted much attention because the exciting application potential in multiply controlled devices. Among the few room-temperature single-phase magnetoelectric multiferroics reported, BiFeO3 shows the strong coupling between antiferromagnetic and ferroelectric types of order. Hence these multiferroic matericals are of interest for memory and logic device applications. First, the (001)-oriented BiFeO3 thin film is synthesized by the pulsed laser deposition technique (PLD). We unravel the charge, orbital, and the antiferromagnetic properties of BiFeO3 films using soft X-ray absorption spectroscopy (XAS) based on X-ray linear dichroism (LD) and magnetic X-ray linear dichroism (MLD). Simultaneously we have been pursuing approaches to understand the origin of the magnetic properties using X-ray magnetic circular dichroism (XMCD) and SQUID measurements. And hard x-ray photoemission spectroscopy (HXPES) is a powerful method can direct unravel the electronic structure. The HXPES results show the peak of Fe-2p3/2 was separated at the BiFeO3 thin films. We explain these double peaks are closely related to the multiplet structure of Fe3+. The difference in energy between the peaks was 1.15 eV. In addition, the comparison between data from the normal BFO and BFO which was cooled under a reduced O pressure (90 m torr) indicates that the HXPES data is no different. We also spent a lot of effort to measure HAXPES of BFO grown on different substrates as planned. It is clear that the HAXPES data of BFO grown differ much; we are doing calculations to extract physics out of these data. The preliminary results are quite encouraging.

並列關鍵字

Multiferroic Material ； BiFeO3 ； X-ray Absorption Spectroscopy ； Photoemission Spectroscopy

參考文獻

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