Design and Multi-State Tunneling Characteristics of Perovskite Ferroelectric Ultrathin Films With Low-Driving Fields
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摘要: 铁电隧穿结通常为金属-超薄铁电薄膜-金属三明治结构，利用铁电极化状态调控量子隧穿效应获得不同电阻态，实现数据存储功能. 其因读写速度快、功耗低、存储密度高及非易失性存储等特点，成为了新一代信息存储技术重要发展方向. 然而，这种超薄铁电薄膜因极化翻转电场大、速度高，往往存在局部温度升高、稳定性降低等问题，因此，进一步降低铁电薄膜驱动电场对铁电隧穿器件设计至关重要. 研究表明，铁电薄膜可通过调控衬底应变使其处于多畴共存状态，各畴之间翻转驱动电场随着能量势垒的降低而大幅降低. 该文基于WKB近似的电子隧穿理论并结合Landau唯象理论，研究了衬底应变对铁电驱动电场、量子隧穿特性及隧穿电阻开关比的影响. 计算结果表明：通过衬底应变调控，经典钙钛矿铁电薄膜PbTiO3和BaTiO3同时存在面外向上、向下极化以及面内极化3种电阻状态，有效驱动电场可降低至25 MV/m，比单畴铁电隧穿结驱动电场减少了76%. 研究结果为低能耗、多阻态铁电存储器件设计提供了理论基础.
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关键词:
- 铁电多态隧穿 /
- 低能耗 /
- 共存畴 /
- Landau唯象理论 /
- 衬底应变
Abstract: The ferroelectric tunneling junction, with a metal-ferroelectric ultra-thin film-metal structure, has different tunneling resistance states through polarization manipulation, leading to potential applications in next-generation information storage devices with low-power consumption, fast reading/writing speed, high storage density, and non-volatility. However, the ferroelectric thin films still experience high-temperature rises with reduced stability due to high driving fields, and reducing the driving electric field is crucial for designing ferroelectric tunneling devices. The ferroelectric thin films with coexisting domains have lowered barriers and decreased driving electric fields for domain switching, which are achieved through substrate manipulation. Herein the substrate effects on the driving field, the tunneling resistance switching ratio and the tunneling properties, were studied based on the WKB approximation combined with the Landau phenomenological theory. The results show that, the ferroelectric tunnel junction with coexisting domains exhibits 3 resistive states corresponding to out-of-plane and in-plane polarizations. The effective driving electric field can be reduced to 25 MV/m, which is 76% lower than that with 2 resistive single domains. The proposed theoretical framework provides a fundamental understanding of the formation of multi-state and reduction of the driving field for low-energy, multi-resistance ferroelectric storage devices. -
图 1 多畴共存的铁电隧穿模型示意图
注 为了解释图中的颜色，读者可以参考本文的电子网页版本，后同.
Figure 1. Schematic diagram of the ferroelectric tunneling junction with coexisting domains
图 3 衬底对畴结构和自发极化的影响(PbTiO3)
Figure 3. Substrate strain-dependent spontaneous polarization of different domains in the PbTiO3 thin film
图 4 衬底对多畴自由能、体积分数的影响(PbTiO3)
Figure 4. Substrate strain-dependent total free energy and the fractions of different domains in the PbTiO3 thin film
图 5 衬底对不同畴结构自发极化的影响(BaTiO3)
Figure 5. Substrate strain-dependent spontaneous polarization of different domains in the BaTiO3 thin film
图 6 衬底对多畴自由能、体积分数的影响(BaTiO3)
Figure 6. Substrate strain-dependent total free energy and the fraction of different domains in the BaTiO3 thin film
图 7 面外压电系数随衬底的变化(PbTiO3)
Figure 7. Substrate strain-dependent out-of-plane piezoelectric coefficient in the PbTiO3 thin film
图 8 面外压电系数随衬底的变化(BaTiO3)
Figure 8. Substrate strain-dependent out-of-plane piezoelectric coefficient in the BaTiO3 thin film
图 9 多畴铁电隧穿结的3种电阻状态示意图
Figure 9. The schematic of 3 resistance states of the ferroelectric tunnel junction with coexisting domains
图 10 应变为-1.5%时PbTiO3薄膜的I-V曲线
Figure 10. Current-voltage curves of the PbTiO3 thin film under a substrate strain of -1.5%
图 11 应变为+0.5%时PbTiO3薄膜的I-V曲线
Figure 11. Current-voltage curves of the PbTiO3 thin film under a substrate strain of +0.5%
图 12 应变为+1.8%时PbTiO3薄膜的I-V曲线
Figure 12. Current-voltage curves of the PbTiO3 thin film under a substrate strain of +1.8%
图 13 应变为-0.3%时BaTiO3薄膜的I-V曲线
Figure 13. Current-voltage curves of the BaTiO3 thin film under a substrate strain of -0.3%
图 14 应变为+0.3%时BaTiO3薄膜的I-V曲线
Figure 14. Current-voltage curves of the BaTiO3 thin film under a substrate strain of +0.3%
图 15 衬底应变对最大电阻开关比的影响(PbTiO3)
Figure 15. Substrate strain-dependent RTER in the PbTiO3 thin films
图 16 衬底应变对最大电阻开关比的影响(BaTiO3)
Figure 16. Substrate strain-dependent RTER in the BaTiO3 thin films
表 1 铁电薄膜的Landau系数、电致伸缩系数和刚度系数
Table 1. The Landau coefficient, the stiffness coefficient, and the electromechanical coefficient of ferroelectric thin films
PbTiO3 BaTiO3 a1/(C-2·m2·N) 3.8×(T-752)×105 3.8×(T-383)×105 a11/(C-4·m6·N) -7.3×107 3.6×(T-448)×106 a12/(C-4·m6·N) 7.5×108 4.9×108 a111/(C-6·m10·N) 2.6×108 6.6×109 a112/(C-6·m10·N) 6.1×109 2.9×109 Q11/(C-2·m4) 0.089 0.11 Q12/(C-2·m4) -0.026 -0.043 s11/(m2·N-1) 8.0×10-12 8.3×10-12 s12/(m2·N-1) -2.5×10-12 -2.7×10-12 
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表 2 铁电隧穿结的读写电压设计(PbTiO3薄膜)
Table 2. The design of reading and writing voltages of the ferroelectric tunnel junction (PbTiO3)
misfit strain um/% coercive field Ec/(MV/m) writing voltage Vw/V reading voltage Vr/V -1.5 120 0.25 -0.05~0.05 +0.5 28.5 0.06 -0.05~0.05 +1.8 200 0.4 -0.05~0.05
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表 3 铁电隧穿结的计算参数
Table 3. Coefficients of the ferroelectric tunnel junction
component of FTJ relative coefficient ferroelectric layer εb/(8.854×10-12) 90 φ1/eV 0.5 κ3/eV -4.5 μ33 10 ν 0.3 electrode (SrRuO3) ls1/m 6×10-11 εe1/(8.854×10-12) 8.45 m0/kg 9.109 4×10-31
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