铁电存储器用钛酸铋材料的第一性原理计算及电导机制分析
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摘要
铁电存储器以其非挥发性(即使在电源突然断开的时候芯片仍能保存存储在其中的信息)、高写入速度、低功率消耗、低的操作电压、高重复读写能力以及优异的抗辐射等优越的性能,成为存储器领域最具发展潜力的产品。而铁电材料的选择是其实现商业化应用的关键。在铁电材料中,使用最多的是钙钛矿结构铁电如Pb(Zr_xTi_(1-x))O_3(PZT)和铋层状结构铁电如SrBi_2Ta_2O_9(SBT),但是PZT材料,它含有有毒元素铅,对环境有污染,且抗疲劳性较差;SBT材料,虽然抗疲劳性好,但是制备温度过高,很难与大规模集成器件集成。因此,寻找适合铁电存储器用的新型铁电材料成为研究热点之一。
近年来,掺杂的钛酸铋材料因其无铅特性,大自发极化,低处理温度,高居里温度,以及优异的抗疲劳特性受到了人们的广泛关注。实验上,人们提出了A位掺杂、B位掺杂,以及A,B位共掺杂的方法,来提高BIT的铁电性能,并取得了一定的成效。但其掺杂的内部的机理却并不明朗。
在BIT器件的应用过程中,实验上观测到器件的漏电流比较大,成为限制它应用的一个主要因素。人们对于BIT的漏电流属性在实验和理论方面都做了研究。实验上,A,B位共掺杂的方法,被认为是改善其漏电流属性的有效方法之一。在理论方面,研究结果表明,漏电流与Bi~(3+)离子的挥发及氧空穴有关,并且材料中存在的杂质也被认为是产生漏电流的原因之一。人们也相应地提出很多的理论模型去解释实验上观测到的实验现象,但是其真正的内在机制还是存在争议。而当器件的漏电流过大时,就会影响到极化翻转的测量,严重时甚至会导致铁电存储器件的短路,导致存储失败。因此弄清BIT的电导机制,改善漏电流属性尤为重要。
本论文采用基于密度泛函理论的第一性原理方法,结合赝势投影平面波(PP-PAW)方法,针对BIT铁电体铋层状钙钛矿结构的总能和电子结构等物理性质系统地进行了理论研究,并取得了以下主要成果:
1)总能和电子结构的计算结果表明,BIT铁电相的稳定性主要源自于Bi_2O_2层和Bi_2Ti_3O_(10)钙钛矿层的畸变以及它们之间相互耦合作用释放出来的弛豫能;BIT产生铁电性质的主要原因是源自于B位的Ti离子和O离子之间存在强烈的杂化;而A位的Bi和O之间存在的较弱的共价杂化则进一步促进了铁电相的稳定。
2)由电荷密度和AIM理论计算的迁移电荷结果表明,BIT的结构畸变和铁电性质的主要诱导因素来源于B位Ti离子和O之间以及Bi和O之间的共价杂化作用;而B位Ti离子和O之间的共价杂化是产生畸变的主导因素。
3)将第一性原理计算得到的铁电相BIT的能带结构,与MIGS理论相结合,我们计算得到了BIT与Pt电极相接触时产生的肖特基势垒的高度,其值为1.26 eV,如此大的势垒足以抑制从金属向BIT导带的发射肖特基电流,这也就意味着在BIT的电导机制中,界面限制肖特基发射机制是不占主导地位的。
4)在实验方面,我们对提取的实验数据用界面限制肖特基发射机制公式来进行拟合,证明了我们的计算结果的正确性。而肖特基发射机制是一种最接近本征的电导机制,也说明了BIT中漏电流可以通过掺杂或改善实验条件来减小。
Nonvolatile random access memories (NvRAMs) have been regarded as the most promising memories due to its nonvolatile nature (in which the stored data will not been lost even if power is interrupted), high speed, low power consumption, low operation voltage, high repeatability and excellent radiation hardness. The key for commercial applications is the choice of ferroelectric material. Among various ferroelectric materials, ferroelectric material with perovskite Pb(Zr_xTi_(1-x))O_3 (PZT) and Bi-layered structures SrBi_2Ta_2O_9 (SBT) are promising materials for NvRAMs. However, PZT contains toxic elements lead, which causes environmental pollution, and it also shows a reduction of polarization with polarity switching (fatigue). SBT, although it shows excellent endurance against polarity switching, its synthetic temperature is too high for standard LSI-device fabrication processes. Therefore, researchers are eager to find some more suitable materials.
Recently, rare-earth doped Bi_4Ti_3O_(12) (BIT) has attracted considerable attention for potential utilization because of its lead-free nature, large spontaneous polarization, low processing temperature, high Curie temperature, and excellent fatigue-free behavior. Experimentally, A site, B site and A,B sites substitution are proposed and used to improve the ferroelectric properties of BIT, and have made some achievement, but the internal mechanism of doping is not clear.
In BIT device applications, leakage current often interferes and leads to device failure. Therefore, the leakage properties of BIT films have been widely investigated in the experimental and theoretical aspects. Experimentally, A,B sites substitution is considered as the most effective method to reduce the leakage current. Theoretically, some studies revealed that the leakage current is related with the volatility of Bi3+ ions and oxygen vacancies. Impurities atoms were also considered to contribute to the leakage current. A number of mechanisms were also proposed to explain the leakage behavior observed in experiment. However, the actual conduction mechanism is still controversial. The leakage current interferes with the polarization switching and makes BIT unsuitable for the ferroelectric devices when the leakage current is large. So, it is very important to make the conduction mechanism clear.
In this paper, we have systematically studied the basic physical properties such as the electronic properties, and chemical bonding properties by pseudopotential project plane wave (PP-PAW) method based on the density functional theory. The main results are as follows.
1)The results of total energy and density of states reveals that the stability of ferroelectric phase of BIT is determined by the distortions of the Bi2Ti3O10 block and the Bi_2O_2 layer as well as the relaxation energy, which is released by the mutual coupling between distortions of the Bi_2Ti_3O_(12) block and the Bi_2O_2 layer. The ferroelectric property of BIT is largely determined by the strong Ti-O hybridizations, which is strengthened by the weak hybridization of Bi (A-site) and O.
2)The results of the charge density and the AIM theory reveals that the ferroelectric properties and lattice distortions originates from the hybridization between Ti and O and Bi and O in BIT ,and the hybridization of Ti and O is dominant.
3)The band structure and density of states of ferroelectric bismuth titanate is calculated in the framework of density functional theory. Combined with the metal induced gap state model, we calculated the Schottky barrier height of BIT on Pt electrode as high as 1.26 eV, which denotes that the Schottky effect may not be the dominant conduction mechanism in BIT.
4)Compared with the analysis of experimental data, we conclude that the leakage current behavior of BIT films is dominated by bulk limited conduction mechanisms and can be reduced by doping or better processing conditions.
近年来,掺杂的钛酸铋材料因其无铅特性,大自发极化,低处理温度,高居里温度,以及优异的抗疲劳特性受到了人们的广泛关注。实验上,人们提出了A位掺杂、B位掺杂,以及A,B位共掺杂的方法,来提高BIT的铁电性能,并取得了一定的成效。但其掺杂的内部的机理却并不明朗。
在BIT器件的应用过程中,实验上观测到器件的漏电流比较大,成为限制它应用的一个主要因素。人们对于BIT的漏电流属性在实验和理论方面都做了研究。实验上,A,B位共掺杂的方法,被认为是改善其漏电流属性的有效方法之一。在理论方面,研究结果表明,漏电流与Bi~(3+)离子的挥发及氧空穴有关,并且材料中存在的杂质也被认为是产生漏电流的原因之一。人们也相应地提出很多的理论模型去解释实验上观测到的实验现象,但是其真正的内在机制还是存在争议。而当器件的漏电流过大时,就会影响到极化翻转的测量,严重时甚至会导致铁电存储器件的短路,导致存储失败。因此弄清BIT的电导机制,改善漏电流属性尤为重要。
本论文采用基于密度泛函理论的第一性原理方法,结合赝势投影平面波(PP-PAW)方法,针对BIT铁电体铋层状钙钛矿结构的总能和电子结构等物理性质系统地进行了理论研究,并取得了以下主要成果:
1)总能和电子结构的计算结果表明,BIT铁电相的稳定性主要源自于Bi_2O_2层和Bi_2Ti_3O_(10)钙钛矿层的畸变以及它们之间相互耦合作用释放出来的弛豫能;BIT产生铁电性质的主要原因是源自于B位的Ti离子和O离子之间存在强烈的杂化;而A位的Bi和O之间存在的较弱的共价杂化则进一步促进了铁电相的稳定。
2)由电荷密度和AIM理论计算的迁移电荷结果表明,BIT的结构畸变和铁电性质的主要诱导因素来源于B位Ti离子和O之间以及Bi和O之间的共价杂化作用;而B位Ti离子和O之间的共价杂化是产生畸变的主导因素。
3)将第一性原理计算得到的铁电相BIT的能带结构,与MIGS理论相结合,我们计算得到了BIT与Pt电极相接触时产生的肖特基势垒的高度,其值为1.26 eV,如此大的势垒足以抑制从金属向BIT导带的发射肖特基电流,这也就意味着在BIT的电导机制中,界面限制肖特基发射机制是不占主导地位的。
4)在实验方面,我们对提取的实验数据用界面限制肖特基发射机制公式来进行拟合,证明了我们的计算结果的正确性。而肖特基发射机制是一种最接近本征的电导机制,也说明了BIT中漏电流可以通过掺杂或改善实验条件来减小。
Nonvolatile random access memories (NvRAMs) have been regarded as the most promising memories due to its nonvolatile nature (in which the stored data will not been lost even if power is interrupted), high speed, low power consumption, low operation voltage, high repeatability and excellent radiation hardness. The key for commercial applications is the choice of ferroelectric material. Among various ferroelectric materials, ferroelectric material with perovskite Pb(Zr_xTi_(1-x))O_3 (PZT) and Bi-layered structures SrBi_2Ta_2O_9 (SBT) are promising materials for NvRAMs. However, PZT contains toxic elements lead, which causes environmental pollution, and it also shows a reduction of polarization with polarity switching (fatigue). SBT, although it shows excellent endurance against polarity switching, its synthetic temperature is too high for standard LSI-device fabrication processes. Therefore, researchers are eager to find some more suitable materials.
Recently, rare-earth doped Bi_4Ti_3O_(12) (BIT) has attracted considerable attention for potential utilization because of its lead-free nature, large spontaneous polarization, low processing temperature, high Curie temperature, and excellent fatigue-free behavior. Experimentally, A site, B site and A,B sites substitution are proposed and used to improve the ferroelectric properties of BIT, and have made some achievement, but the internal mechanism of doping is not clear.
In BIT device applications, leakage current often interferes and leads to device failure. Therefore, the leakage properties of BIT films have been widely investigated in the experimental and theoretical aspects. Experimentally, A,B sites substitution is considered as the most effective method to reduce the leakage current. Theoretically, some studies revealed that the leakage current is related with the volatility of Bi3+ ions and oxygen vacancies. Impurities atoms were also considered to contribute to the leakage current. A number of mechanisms were also proposed to explain the leakage behavior observed in experiment. However, the actual conduction mechanism is still controversial. The leakage current interferes with the polarization switching and makes BIT unsuitable for the ferroelectric devices when the leakage current is large. So, it is very important to make the conduction mechanism clear.
In this paper, we have systematically studied the basic physical properties such as the electronic properties, and chemical bonding properties by pseudopotential project plane wave (PP-PAW) method based on the density functional theory. The main results are as follows.
1)The results of total energy and density of states reveals that the stability of ferroelectric phase of BIT is determined by the distortions of the Bi2Ti3O10 block and the Bi_2O_2 layer as well as the relaxation energy, which is released by the mutual coupling between distortions of the Bi_2Ti_3O_(12) block and the Bi_2O_2 layer. The ferroelectric property of BIT is largely determined by the strong Ti-O hybridizations, which is strengthened by the weak hybridization of Bi (A-site) and O.
2)The results of the charge density and the AIM theory reveals that the ferroelectric properties and lattice distortions originates from the hybridization between Ti and O and Bi and O in BIT ,and the hybridization of Ti and O is dominant.
3)The band structure and density of states of ferroelectric bismuth titanate is calculated in the framework of density functional theory. Combined with the metal induced gap state model, we calculated the Schottky barrier height of BIT on Pt electrode as high as 1.26 eV, which denotes that the Schottky effect may not be the dominant conduction mechanism in BIT.
4)Compared with the analysis of experimental data, we conclude that the leakage current behavior of BIT films is dominated by bulk limited conduction mechanisms and can be reduced by doping or better processing conditions.
引文
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