Investigation on the influence of doping on ferroelectricity of hafnium oxide thin film grown using Pulsed Laser Deposition (PLD)

研究掺杂对脉冲激光沉积（PLD）氧化铪薄膜铁电性的影响

• 批准号: 2597614
• 金额: --
• 依托单位: University of Cambridge
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2021
• 资助国家: 英国
• 起止时间: 2021 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2597614
• 关键词: Investigation; influence; doping; ferroelectricity; hafnium

Ferroelectric materials have a wide range of applications including memory devices, energy harvesting, negative capacitance systems, etc. A well-known ferroelectric materials are perovskite structured, such as (Pb,Zr)TiO_3, known as PZT. Its performance at ambient temperature is satisfying, yet it has limitation in application to non-volatile memory devices due to scalability, complexity, CMOS compatibility, etc. Therefore, simpler and benign ferroelectric material is needed. Hafnium oxide (HfO_2), on the other hand, is of great interest due to its CMOS compatibility as it is already in use as gate dielectric, scalability as it could be fabricated as thin film with thickness of a few nm and most importantly, ferroelectricity. It shows ferroelectric behaviour with specific phases, orthorhombic and rhombohedral. They are metastable phases which requires specific conditions for stabilization such as growth condition, post-annealing, induced strain, doping, etc., and it is yet to be confirmed which is dominating. HfO_2 thin film has high coercive field, larger than 1 MV cm^(-1), so that it requires high voltage for polarization switching. Also, after ferroelectric phase is formed, the films experience "wake-up" effect, which is increase in remnant polarization to certain number of field cycles. It is an intrinsic property that all ferroelectric materials experiences including PZT. Wake-up effect is unfavourable as once it is used for memory application, it might lead to misinformation storage. Additionally, it is hard to control the phases formed in polycrystalline films. A single crystalline ferroelectric phase is ideal to minimize contribution of non-ferroelectric phases and achieve high capacitance and low leakage. Epitaxial films grown on PLD system would enable formation of single phase, single crystalline thin film, that enables fundamental understanding of the material's intrinsic property. In order to overcome aforementioned challenges, I aim to investigate separate effect of dopants and strain. I will learn to grow HfO_2 thin film with optimized ferroelectric behaviour with high saturation polarization, low coercive field, and reduction of wake-up effect. By far, many dopants have been tested on HfO_2, yet there needs to be a clear understanding of the combination of dopant size, ion size variance, doping fraction and charge mismatch. This will be studied using co-doping. Main interest being Lanthanum and Tantalum dopants on HfO_2 which have 3+ and 5+ valence charge respectively. By co-doping them on HfO_2, the effect of average charge in the system and average cation ion size will be investigated separately.Also, strain is crucial in controlling the lattice structures. In order to explore strain effect, superlattice structures of a few unit cells will be grown. HfO_2 film will be strained using different oxide lattice structures such as SrTiO_3. Superlattice will allow me to observe interface effect precisely using synchrotron methods. This will enable me to learn about chemical states and electronic states using XPS. We can also explore the influence of thickness and number of interfaces on structure formation and wake up effects. All the films will be tested on Piezo-response Force Microscopy (PFM) and Polarization-Electric Field measurement to demonstrate ferroelectricity. Using Positive-Up Negative-Down (PUND) technique will eliminate influence of leakage current during ferroelectricity testing, allowing only displacement current to be in consideration.As a result, the perfect structures with carefully tuned doping, strain and interfaces will enable understanding and control of the scientifically and industrially fascinating ferroelectric system of HfO_2. This will suggest the path to the next generation of nano-scaled electronics promoting CMOS performance and non-volatility.

铁电材料具有广泛的应用，包括记忆装置，能量收集，负电容系统等。众所周知的铁电材料是钙钛矿结构化的，例如（PB，ZR）TIO_3，称为PZT。它在环境温度下的性能令人满意，但是由于可伸缩性，复杂性，CMOS兼容性等，它在非挥发性存储器设备上的应用限制。因此，需要更简单，更良性的铁电材料。另一方面，由于其CMOS兼容性，氧化物（HFO_2）引起了极大的兴趣。它显示了特定阶段（正骨和菱形）的铁电行为。它们是亚稳态的阶段，需要特定的稳定条件，例如生长条件，解放后，诱导应变，掺杂等，并且尚待确认这是主导的。 HFO_2薄膜具有高强制场，大于1 mV cm^（-1），因此需要高电压才能进行极化开关。同样，在形成铁电期之后，薄膜会经历“唤醒”效应，这是剩余极化增加到一定数量的田间循环的效果。这是一个内在特性，包括PZT在内的所有铁电材料经历。唤醒效果是不利的，因为它用于存储器应用程序，它可能会导致错误信息存储。此外，很难控制多晶膜中形成的相。单晶铁电相是最小化非专科相的贡献并达到高电容和低泄漏的理想选择。在PLD系统上生长的外延膜可以形成单相单晶薄膜，从而使对材料的内在特性的基本了解。为了克服上述挑战，我旨在研究掺杂剂和压力的单独影响。我将学习具有具有高饱和极化，低强制场和唤醒效果的优化的铁电行为的HFO_2薄膜。到目前为止，许多掺杂剂已经在HFO_2上进行了测试，但是需要清楚地了解掺杂剂尺寸，离子尺寸方差，掺杂分数和电荷不匹配的组合。这将使用共掺杂进行研究。主要的兴趣是HFO_2上分别具有3+和5+价电荷的HFO_2上的灯笼和触觉掺杂剂。通过将它们共同掺杂HFO_2，将分别研究系统中平均电荷和平均阳离子离子大小的影响。此外，应变对于控制晶格结构至关重要。为了探索应变效应，将生长一些单位细胞的超晶格结构。 HFO_2膜将使用不同的氧化物晶格结构（例如SRTIO_3）来张紧。超级晶格将允许我使用同步加速器方法精确观察界面效应。这将使我能够使用XPS了解化学状态和电子状态。我们还可以探索厚度和界面数量对结构形成和唤醒效果的影响。所有膜将在压电反应力显微镜（PFM）和极化电场测量上进行测试，以证明铁电性。使用正上下负（PUND）技术将消除铁电测试期间泄漏电流的影响，仅允许位移电流。结果，带有精心调整的掺杂，应变和界面的完美结构将使理解和控制能够理解和控制HFO_2的科学和工业迷人的铁电体系。这将暗示通往下一代纳米级电子设备的途径，从而促进CMOS性能和非挥发性。