鉍鐵氧化物薄膜之製備及特性研究

Abstract

本論文在研究多鐵性鐵酸鉍（鉍鐵氧化物）材料之晶體結構與鐵電特性。首先我們利用射頻磁控濺鍍系統成長單層之鐵酸鉍薄膜於鍍有鎳酸鑭(導電材料)緩衝層之矽 (001) 單晶基板上。由高解析之Ｘ光繞射量測分析得知，其薄膜沿著(001)面擁有高度優選指向之方向性。在成長溫度範圍為400-600 oC，隨著成長的溫度提高其結晶性也跟著變佳。根據Ｘ光反射率的擬合分析結果顯示，鐵酸鉍薄膜的密度較塊材小了些許。其殘留極化值隨著成長溫度的提高而提升，此與Ｘ光繞射分析之結果相互印證，由此得知鐵電特性與結晶性及電子密度有著正相關性。此實驗結果我們得知：鐵酸鉍薄膜的鐵電特性大小的主要貢獻來自於其薄膜結晶性。 根據第一個研究主題結果顯示：對於鐵電薄膜材料而言，其薄膜的結晶性是一項相當重要的議題；為了提升鐵酸鉍薄膜之鐵電特性，成長具有磊晶品質的薄膜為一適當的研究方向。因此第二個研究主題是利用射頻磁控濺鍍系統，成長具有對稱性結構鐵酸鉍/鎳酸鑭之人工超晶格磊晶薄膜於鈮摻雜鈦酸鍶的單晶基板上。超晶格薄膜包含了6-30個週期，其單一層厚 ii 度為1.7-8.5奈米，而總膜厚則固定在100奈米，其成長溫度範圍為560-810 oC，實驗結果顯示此超晶格結構之薄膜具備了令人滿意的結晶結構及應變效應。此種鐵電材料與導電材料所形成之超晶格，可做為探討在單一超晶格結構內，應變對於鐵酸鉍薄膜層之鐵電特性的影響。利用高解析之Ｘ光繞射實驗，顯示除了主要的繞射峰兩側外皆有顯著的衛星峰；以及由Ｘ光反射率曲線之Kiessig干涉條紋中可明顯分辨出超晶格之特徵峰，證實此濺鍍薄膜確實具有超晶格結構。此薄膜的層狀結構亦由Ｘ光反射率、飛行時間二次離子質譜儀及高解析穿透式電子顯微鏡分析得到確認，而且所有的實驗皆得到一致性結果。在所有的成長溫度及不同疊層厚度下，超晶格薄膜在其鈦酸鍶(002)布拉格繞射峰周圍皆有顯著的主繞射峰與衛星峰，代表著於鈦酸鍶基板上可形成高品質之鐵酸鉍/鎳酸鑭人工超晶格結構。由高解析之Ｘ光繞射量測結果得知無論是提高沈積溫度或是減少疊層厚度，皆會提升其超晶格薄膜之結晶性，並沿著c軸方向有著拉伸應力以及沿著試片表面有著壓縮應變的存在。 超晶格薄膜之殘留極化值顯示出與單層鐵酸鉍薄膜具有相同的趨勢；其極化值隨著沈積溫度的增加而增加，在溫度660 oC時達到最大值。在量測頻率為0.5及1千赫茲時，遲滯曲線顯現出極大的漏電流存在；極化值隨著量測頻率的增加而減少。 因射頻磁控濺鍍系統有著先天上的物理極限，因此在最後的研究主題 iii 裡我們利用原子層沈積系統，成長厚度小於10奈米，具有高度(001)優選指向之超薄鐵酸鉍薄膜於鎳酸鑭緩衝層之矽(001)基板上，同時我們也利用射頻磁控濺鍍系統成長相同厚度之樣品作為對照。結合了氣流中斷及鉍與鐵前驅物之交替式反應，使原子層沈積系統為一優越成長三元化合物之方法。根據Ｘ光繞射分析，在沈積溫度低於550°C時所生成的相只有鐵酸鉍。藉由同步輻射Ｘ光繞射異常精細結構光譜，透過鐵酸鉍(001)繞射訊號可以確認其鍵結之價數。利用Ｘ光光電子能譜術可得到其薄膜之成分比，結果顯示利用原子層沈積法所成長之鐵酸鉍成分計量比射頻磁控濺鍍法來的正確，並符合繞射異常精細結構之結果。利用高解析穿透視電子顯微鏡影像鑒別薄膜之層狀結構與表面形貌，由原子層沈積法所成長之薄膜呈現相當程度的覆蓋範圍，且較射頻磁控濺鍍系統來的一致性。因此利用原子層沈積法所成長出之鐵酸鉍薄膜具有絕佳的低漏電特性，與射頻磁控濺鍍系統相比大幅的改善了一千倍之多，使得這種方法更適合用於製造鐵電隨機存取記憶體元件。由遲滯曲線可得知最大的殘留極化值為2Pr = 2.0μC cm−2 。

To study the crystal structure and ferroelectric properties of multiferroic material BiFeO3 (BFO), we first investigated the BFO thin film as a single layer grown on a Si (001) substrate coated with LaNiO3 (LNO) with a RF magnetron sputtering (RF-sputtering) system. The measurements of high-resolution X-ray diffraction (HRXRD) showed a highly orientated crystal structure along (001) direction. The crystal quality increases with increasing deposition temperature in a range of 400-600 oC; according to the fitted results of X-ray reflectivity (XRR), the densities of BFO thin films were slightly less than their bulk values. Increasing the deposition temperature increases the remnant polarization, and is consistent with the results of XRD, indicating that the ferroelectric property has a positive relation with the crystal quality and electron density. The results indicate that the crystal quality dominates the ferroelectric property of BFO thin films. According to the first theme, the crystal quality is an important issue of a ferroelectric material; an epitaxial thin film is a proper direction to improve the ferroelectric property of a BFO thin film. The second theme is about an artificial superlattice of multiferroic BFO and conductive LNO grown epitaxially, with a v RF-sputtering system, on a single-crystal substrate of (001) Nb-doped SrTiO3 (STO) in a range 560-810 oC. The superlattice structure contains the features of a satisfactory crystal structure and the strain effect in a single thin-film structure. The superlattice contained 6-30 periods of symmetric BFO/LNO bi-layers with each layer of thickness 1.7-8.5 nm. The total thickness of the films was fixed at ~100 nm. A ferroelectric and conductive superlattice material serves for an investigation of the strain dependence of the ferroelectric properties of BFO layers in a superlattice structure. The formation of the superlattice structure is confirmed from the appearance of satellite peaks on both sides of the main peak from the HRXRD pattern. The appearance of superlattice peaks separated by Kiessig fringes in the XRR curve. The layer structure is confirmed with XRR, a time-of-flight secondary-ion mass spectrometer (TOF-SIMS), and a high-resolution transmission electron microscope (HRTEM). All results are consistent with one another. The clearly discernible main peak and satellite peaks on both sides of the substrate peak around the (002) STO Bragg peak indicate the high-quality BFO/LNO artificial superlattice structure formed on a STO substrate at all deposition temperatures and with various thicknesses of sublayer. HRXRD measurements showed that these superlattice films become subject to greater tensile stress along the c-axis and compressive strain in the in-plane direction, and possess increased crystalline quality with both increasing temperature of deposition and decreasing thickness of the sublayer. The remanent polarization of the superlattice showed the same behavior as the single-layer BFO thin film; the polarization increases with increasing deposition and approaches a maximum at 660 oC. The hysteresis loops showed a large current leakage at frequencies 0.5 and 1 kHz; the polarization decreases with increasing measuring frequency. vi Because of the physical limitation of RF-sputtering, the final theme is about highly (001)-oriented ultrathin BiFeO3 films of total thickness less than 10 nm deposited on Si (001) substrates via deposition of atomic layers (ALD) with a LaNiO3 buffer. A radio-frequency (RF)-sputtered sample of the same thickness was prepared for comparison. The ALD, combined with interrupted flow and an exchange reaction between Bi and Fe precursors, provides a superior method to grow ternary compounds. According to X-ray diffraction, upon deposition at a temperature less than 550 °C, the only phase in the film is BiFeO3. Anomalous fine structure from synchrotron X-ray diffraction certifies the valence bonding through the (001) diffraction signal. The stoichiometric ratio of BiFeO3 obtained from X-ray photoelectron spectra indicates that ALD has a proportion much improved over the RF preparation, in agreement with the diffraction anomalous fine structure. The use of high-resolution transmission electron and atomic-force microscopes shows that the layer structure and morphology from ALD presented a satisfactory coverage, more conformal than that with the RF method. The BiFeO3 thin film deposited with ALD shows improved leakage at least 1000 times with respect to the RF preparation, making this method suitable for the fabrication of ferroelectric random-access memory devices. From the hysteresis loop, the largest remanent polarization is 2Pr = 2.0 μC cm−2.