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標題: | 螢石與纖鋅礦結構之新穎鐵電超薄膜 Novel Ferroelectric Ultrathin Films Based on Fluorite and Wurtzite Structures |
作者: | Bo-Ting Lin 林柏廷 |
指導教授: | 謝宗霖(Jay Shieh) |
關鍵字: | 鐵電,薄膜,二氧化鋯,纖鋅礦,原子層沉積, ferroelectric,thin film,zirconium dioxide,wurtzite,atomic layer deposition, |
出版年 : | 2018 |
學位: | 博士 |
摘要: | 鐵電材料內部存在自發性的電極化,其極化可藉由外加電場的作用下被反轉,當電場歸零時仍保有殘餘極化量,並且具有壓電特性,因此被廣泛地應用於壓電元件、非揮發性記憶體、負電容電晶體等電子元件中。現今常見的傳統鐵電材料大多為鈣鈦礦結構的鋯鈦酸鉛,然而鈣鈦礦結構與矽的晶體結構匹配性差,因此在電子元件微型化的趨勢下,傳統鐵電材料在半導體工業的應用逐漸受到限制。近年來,以原子層沉積技術製造的二氧化鉿基鐵電奈米薄膜逐漸受到重視,其和矽的晶體結構有高度的匹配性,且為簡單的二元化合物,由於此種薄膜鐵電性的發現,使得鐵電材料在半導體工業的發展得以延續。
目前以原子層沉積技術成長的二元化合物鐵電奈米薄膜只有一種:二氧化鉿基鐵電奈米薄膜,其中包含了氧化鉿鋯鐵電奈米薄膜。本實驗室以遠程電漿增強型原子層沉積技術,首次成功製備出鐵電表現良好的氧化鉿鋯鐵電奈米薄膜。然而在氧化鉿鋯薄膜的研究中,二氧化鉿基鐵電奈米薄膜的缺點亦被發現,即需要額外的製程才能使其具備良好的鐵電性,例如後續的高溫退火處理和摻雜等,這會使材料製備的成本提高且有熱積存的問題產生。為了解決這些問題,本研究亦成功開發了兩種新穎的鐵電奈米薄膜:二氧化鋯與氮化鋁,它們不僅不需要後續高溫退火處理及摻雜,且與現今半導體工業的製程兼容,此外,它們也是簡單的二元化合物。 未摻雜的二氧化鋯奈米薄膜在不進行後續退火的情況下,首次出現顯著且穩定的鐵電性,此鐵電薄膜具有高達12 μCcm-2的殘餘極化量及鐵電結晶相。本研究使用遠程電漿增強型原子層沉積技術,成長二氧化鋯奈米薄膜在鉑電極上,藉由高解析穿透式電子顯微鏡及低掠角X光繞射相鑑定分析,證實了二氧化鋯薄膜中存在非中心對稱空間群Pbc21的鐵電Orthorhombic相。 本研究亦使用原子層沉積技術與本實驗室發展出的原子層退火技術,在300 °C的低溫下將氮化鋁奈米薄膜磊晶在氮化鎵上,形成氮化鋁/氮化鎵的異質結構。在此異質結構中,氮化鋁薄膜的鐵電性首次在電滯曲線的量測中被觀察到,當在異質結構上施加外部電壓時,氮化鋁薄膜內之自發性電極化可以被旋轉。此外,氮化鋁薄膜內的極化旋轉,可以操縱氮化鋁/氮化鎵異質結構的位能障,使此異質結構的電阻可以被調整。基於表面增強拉曼光譜、掃描穿透式電子顯微鏡之高角度暗場影像技術與X光繞射分析之倒置空間圖的結果,歸納出異質結構中的氮化鋁薄膜具有鐵電性的原因:氮化鋁薄膜受到來自氮化鎵的平面拉伸應力,產生鐵電高壓相,亦造成氮化鋁薄膜中的氮、鋁離子間距改變,使材料內部之自發性極化量增加,且容易在外部電壓的施加下被反轉。 由於二氧化鋯與氮化鋁奈米薄膜鐵電性的發現,此兩種薄膜未來預期可有效應用於非揮發性記憶體、負電容電晶體、高電子遷移率電晶體等電子元件中。在發現與鑑定二氧化鋯與氮化鋁薄膜鐵電性的過程,本實驗室建立了一套開發新型鐵電薄膜與研究鐵電性質的方法,供後人參考,以便將來發現更多更具潛力的新穎鐵電材料。 Hf0.5Zr0.5O2 (HZO) ferroelectric nanoscale thin films with significant and tunable ferroelectric properties were prepared by remote plasma atomic layer deposition (RP-ALD) in our laboratory for the first time. However, the disadvantages of HfO2-based ferroelectric thin films were also found in this study. They require additional processes to induce ferroelectricity, such as subsequent high-temperature annealing and doping. To solve these problems, this research has also successfully developed two novel ferroelectric nanoscale thin films – ZrO2 and AlN. Both of these Si-compatible films do not require post-annealing and doping to induce their ferroelectricity, which is highly beneficial to the application of the films in non-volatile memories and ultralow-power nanoelectronics. Large stable ferroelectricity in nanoscale undoped ZrO2 thin films prepared without post-annealing has been demonstrated for the first time. Remanent polarizations up to 12 μCcm−2 were obtained in the as-deposited ZrO2 thin films prepared by RP-ALD at 300 °C substrate temperature on the Pt electrode. The existence of the ferroelectric orthorhombic phase with non-centrosymmetric space group Pbc21 in the as-deposited ZrO2 thin films was confirmed by high-resolution transmission electron microscopy. Ferroelectricity in nanoscale AlN epitaxial layer was also observed for the first time. The nanoscale AlN layer was epitaxially grown on GaN by the atomic layer epitaxy at a low temperature of 300 °C using the layer-by-layer, in-situ atomic layer annealing, developed in our laboratory. As an external voltage was applied to the AlN/GaN heterostructure, a switchable electric polarization, i.e., the polarization-voltage (P-V) hysteresis loop, was observed, which clearly manifests the ferroelectricity in the nanoscale AlN epitaxial layer. In addition, the switchable electric polarization in AlN manipulates the potential barrier, leading to the modulation of the electrical resistance of the heterostructure. Based on the results of surface-enhanced Raman spectroscopy (SERS), high-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM) imaging and reciprocal space mapping (RSM) of X-ray diffraction, the reasons why ferroelectricity exists in the AlN epilayer of the heterostructure are explained as follows: The AlN layer is subjected to an in-plane tensile stress from GaN, which transits AlN to a ferroelectric high-pressure phase. This phase transition also changes the spacing of Al and N ions in the AlN epilayer, increasing the amount of the spontaneous polarization of AlN and facilitating its polarization switching. In the process of discovering and identifying ferroelectricity of the ZrO2 and AlN thin films, a set of methods for developing new ferroelectric thin films and studying ferroelectric properties was established for future reference, so that more potential novel ferroelectrics can be discovered in the future. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/78869 |
DOI: | 10.6342/NTU201804379 |
全文授權: | 有償授權 |
電子全文公開日期: | 2023-12-26 |
顯示於系所單位: | 材料科學與工程學系 |
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