Bi_（3.15）Nd_（0.85）Ti_3O_（12）铁电薄膜成分纵深分布及掺杂改性研究
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摘要
上世纪90年代以后,随着微电子技术和计算机工业迅速的发展,具有高存储密度和存取速度、抗辐射、非挥发等特点的铁电薄膜存储器受到了人们的广泛关注。锆钛酸铅(PZT)系铁电材料具有一系列良好的性能,如较大的剩余极化值、较低的热处理温度等,是目前铁电存储器所用的主要材料。然而,PZT薄膜存在一些致命的缺点,如在金属电极上抗疲劳性能差,含有毒元素Pb等。在越来越重视环境保护的时代,寻求新的无铅铁电材料来取代PZT变得尤为重要。Bi_(3.15)Nd_(0.85)Ti_3O_(12)(BNT)铁电薄膜由于不含铅、结晶温度较低、剩余极化较大、抗疲劳性能好,被认为是最有希望取代PZT材料而应用于非挥发性铁电存储器的铁电薄膜材料。本文通过实验制备了BNT无铅铁电薄膜,详细研究了BNT薄膜组分沿薄膜纵深的分布及其化学态,同时研究了B位Zr掺杂对BNT薄膜性能的影响。具体如下:
首先采用化学溶液沉积法(CSD)在Pt(111)/Ti/SiO_2/Si(100)基片上制备了BNT无铅铁电薄膜,并借助X射线衍射仪、原子力显微镜、X射线光电子能谱仪和铁电分析仪分别测试和分析了薄膜的成分、形貌、元素的纵向分布及化学态和电学性能。分析结果表明,所制的薄膜为铋层状钙钛矿结构多晶BNT薄膜,表面致密、无裂纹、无孔洞。BNT薄膜组分沿纵向分布明显不均匀,主要形成三个区域,即表面层、中间层和界面层。其中,在薄膜表面有大量铋元素积聚,而且薄膜表面层的成分主要是以金属氧化物的形式存在;铁电薄膜与Pt下电极之间形成厚度大约为50nm的界面层,这主要是由于BNT薄膜与Pt下电极相互热扩散造成的,同时在扩散的过程中,有少量Pt的化合物生成;薄膜中间层元素分布比较均匀。
另外,采用CSD法在Pt(111)/Ti/SiO_2/Si(100)基片上制备了Zr离子掺杂的BNT (Bi_(3.15)Nd_(0.85)Ti_(3-x)Zr_xO_(12), BNTZ_x)铁电薄膜,并研究了Zr离子掺杂浓度x对薄膜的微观结构、铁电性能、I-V特性和居里温度的影响。研究结果表明:少量的Zr离子掺杂基本没有影响BNT薄膜的铋层状钙钛矿晶体结构;BNTZ薄膜的颗粒直径大约为150-250nm;随着Zr离子掺杂浓度的增加,薄膜表面粗糙度和剩余极化分别减小和增大,而当掺杂浓度增加到一定量时,薄膜表面粗糙度和剩余极化又分别增大和减小,掺杂量x为0.1时,薄膜具有最平整光滑的表面和最大的剩余极化强度(2Pr为52.7μC/cm~2);薄膜漏电流随着Zr掺杂浓度的增加而明显减小,当掺杂量x为0.2时,在高电压区域(高于3 V),BNTZ0.2薄膜的漏电流相对BNT薄膜的漏电流低四个数量级;B位Zr离子掺杂几乎没有改变BNT薄膜的居里温度,薄膜居里温度约为450℃。
In the 1990’s, with the rapid development of microelectronics and computer industry, non-volatile ferroelectric thin film memory has attracted much attention due to its high memory density, fast read/write speed, good radiation hardness and non-volatile characteristic. At present, the materials used to prepare ferroelectric thin film memory are mainly PZT series because they have some favorable properties, such as large remnant polarization (Pr) value and low processing temperature. However, PZT films have some serious drawbacks, such as poor fatigue endurance and harm to the environment. As people are paying more and more attention to the environment, it becomes very important to find a new kind of lead-free ferroelectric material with excellent ferroelectric property to replace PZT. Bi_(3.15)Nd_(0.85)Ti_3O_(12) (BNT) is currently regarded as one of the most promising candidate materials for ferroelectric thin film memories due to its lead-free chemical composition, low processing temperature, large Pr value and excellent fatigue endurence. In this work, BNT lead-free ferroelectric thin films were deposited. The compositional depth profiles and chemical states of the elements at different film depth of prepared BNT films were investigated in detail. In addition, the effects of B-site substitution by Zr ion in BNT film were studied. The main research contents and obtained results in this thesis are summarized as follows:
BNT lead-free ferroelectric thin films were deposited on Pt(111)/Ti/SiO_2/Si(100) substrates by a chemical solution deposition (CSD) method. The compositions, surface morphologies, composition depth profiles, chemical states of film elements and electrical properties of BNT films were investigated in detail by X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy and ferroelectric test system, respectively. The prepared films are crystallized into a polycrystalline Bi-layered perovskite structure and no secondary phase formed. The surfaces of fabricated thin films are dense and crack free. The composition depth profiles show that there are three distinct regions formed in films, which are surface layer, middle layer and interface layer. The surfaces of films are found to consist of one outermost Bi-rich region and be made up of metal-oxide mainly. An interfacial layer with thickness of about 50 nm appeares in the BNT-Pt interface region, which is formed through the ion diffusion between the BNT film and Pt electrode mainly. With the ion diffusion, a small amount of platinum compound is formed as well. The distribtution of component elements are nearly uniform within the middle layer of thin films.
Zr-doped BNT (Bi_(3.15)Nd_(0.85)Ti_(3-x)Zr_xO_(12), BNTZ_x) ferroelectric thin films were fabricated on Pt/TiO_2/SiO_2/Si(100) substrates by CSD. Then the effects of Zr substitution concentrations on the microstructures, ferroelectric properties, leakage current-voltage (I-V) and phase transformations behaviours of BNTZ films were studied in detail. The experimental results show that a low content of Zr doping in BNT film does not affect the Bi-layered perovskite structure. BNTZ films are composed of fine grains of about 150-250 nm in diameter. With the increment of Zr content, the film surface roughness decreases initially and then increases, while the 2Pr value increases initially and then decreases. BNTZ0.1 film has the largest 2Pr of 52.7μC/cm~2. The leakage current of BNTZ film decreases with the increase of Zr content obviously, and the leakage current of the BNTZ0.2 film is smaller nearly four orders of magnitude than that of BNT film at high voltage region (higher than 3 V). B-site Zr substitution in BNT film almost does not change the structural phase transition behaviour. The Curie temperature of BNTZ film is about 450℃.
首先采用化学溶液沉积法(CSD)在Pt(111)/Ti/SiO_2/Si(100)基片上制备了BNT无铅铁电薄膜,并借助X射线衍射仪、原子力显微镜、X射线光电子能谱仪和铁电分析仪分别测试和分析了薄膜的成分、形貌、元素的纵向分布及化学态和电学性能。分析结果表明,所制的薄膜为铋层状钙钛矿结构多晶BNT薄膜,表面致密、无裂纹、无孔洞。BNT薄膜组分沿纵向分布明显不均匀,主要形成三个区域,即表面层、中间层和界面层。其中,在薄膜表面有大量铋元素积聚,而且薄膜表面层的成分主要是以金属氧化物的形式存在;铁电薄膜与Pt下电极之间形成厚度大约为50nm的界面层,这主要是由于BNT薄膜与Pt下电极相互热扩散造成的,同时在扩散的过程中,有少量Pt的化合物生成;薄膜中间层元素分布比较均匀。
另外,采用CSD法在Pt(111)/Ti/SiO_2/Si(100)基片上制备了Zr离子掺杂的BNT (Bi_(3.15)Nd_(0.85)Ti_(3-x)Zr_xO_(12), BNTZ_x)铁电薄膜,并研究了Zr离子掺杂浓度x对薄膜的微观结构、铁电性能、I-V特性和居里温度的影响。研究结果表明:少量的Zr离子掺杂基本没有影响BNT薄膜的铋层状钙钛矿晶体结构;BNTZ薄膜的颗粒直径大约为150-250nm;随着Zr离子掺杂浓度的增加,薄膜表面粗糙度和剩余极化分别减小和增大,而当掺杂浓度增加到一定量时,薄膜表面粗糙度和剩余极化又分别增大和减小,掺杂量x为0.1时,薄膜具有最平整光滑的表面和最大的剩余极化强度(2Pr为52.7μC/cm~2);薄膜漏电流随着Zr掺杂浓度的增加而明显减小,当掺杂量x为0.2时,在高电压区域(高于3 V),BNTZ0.2薄膜的漏电流相对BNT薄膜的漏电流低四个数量级;B位Zr离子掺杂几乎没有改变BNT薄膜的居里温度,薄膜居里温度约为450℃。
In the 1990’s, with the rapid development of microelectronics and computer industry, non-volatile ferroelectric thin film memory has attracted much attention due to its high memory density, fast read/write speed, good radiation hardness and non-volatile characteristic. At present, the materials used to prepare ferroelectric thin film memory are mainly PZT series because they have some favorable properties, such as large remnant polarization (Pr) value and low processing temperature. However, PZT films have some serious drawbacks, such as poor fatigue endurance and harm to the environment. As people are paying more and more attention to the environment, it becomes very important to find a new kind of lead-free ferroelectric material with excellent ferroelectric property to replace PZT. Bi_(3.15)Nd_(0.85)Ti_3O_(12) (BNT) is currently regarded as one of the most promising candidate materials for ferroelectric thin film memories due to its lead-free chemical composition, low processing temperature, large Pr value and excellent fatigue endurence. In this work, BNT lead-free ferroelectric thin films were deposited. The compositional depth profiles and chemical states of the elements at different film depth of prepared BNT films were investigated in detail. In addition, the effects of B-site substitution by Zr ion in BNT film were studied. The main research contents and obtained results in this thesis are summarized as follows:
BNT lead-free ferroelectric thin films were deposited on Pt(111)/Ti/SiO_2/Si(100) substrates by a chemical solution deposition (CSD) method. The compositions, surface morphologies, composition depth profiles, chemical states of film elements and electrical properties of BNT films were investigated in detail by X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy and ferroelectric test system, respectively. The prepared films are crystallized into a polycrystalline Bi-layered perovskite structure and no secondary phase formed. The surfaces of fabricated thin films are dense and crack free. The composition depth profiles show that there are three distinct regions formed in films, which are surface layer, middle layer and interface layer. The surfaces of films are found to consist of one outermost Bi-rich region and be made up of metal-oxide mainly. An interfacial layer with thickness of about 50 nm appeares in the BNT-Pt interface region, which is formed through the ion diffusion between the BNT film and Pt electrode mainly. With the ion diffusion, a small amount of platinum compound is formed as well. The distribtution of component elements are nearly uniform within the middle layer of thin films.
Zr-doped BNT (Bi_(3.15)Nd_(0.85)Ti_(3-x)Zr_xO_(12), BNTZ_x) ferroelectric thin films were fabricated on Pt/TiO_2/SiO_2/Si(100) substrates by CSD. Then the effects of Zr substitution concentrations on the microstructures, ferroelectric properties, leakage current-voltage (I-V) and phase transformations behaviours of BNTZ films were studied in detail. The experimental results show that a low content of Zr doping in BNT film does not affect the Bi-layered perovskite structure. BNTZ films are composed of fine grains of about 150-250 nm in diameter. With the increment of Zr content, the film surface roughness decreases initially and then increases, while the 2Pr value increases initially and then decreases. BNTZ0.1 film has the largest 2Pr of 52.7μC/cm~2. The leakage current of BNTZ film decreases with the increase of Zr content obviously, and the leakage current of the BNTZ0.2 film is smaller nearly four orders of magnitude than that of BNT film at high voltage region (higher than 3 V). B-site Zr substitution in BNT film almost does not change the structural phase transition behaviour. The Curie temperature of BNTZ film is about 450℃.
引文
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