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，包括存储器件、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的，例如(Pb,Zr)TiO_3，称为PZT。其在环境温度下的性能令人满意，但由于可扩展性、复杂性、CMOS兼容性等原因，其在非易失性存储器件的应用中受到限制。因此，需要更简单且良性的铁电材料。另一方面，氧化铪 (HfO_2) 因其 CMOS 兼容性（因为它已被用作栅极电介质）、可扩展性（因为它可以制造为厚度为几纳米的薄膜）以及最重要的是铁电性而受到极大关注。 。它表现出具有特定相、斜方晶系和菱方晶系的铁电行为。它们是亚稳态相，需要特定的条件才能稳定，例如生长条件、后退火、诱导应变、掺杂等，但尚未确定哪种条件占主导地位。 HfO_2薄膜具有高矫顽场，大于1 MV cm^(-1)，因此需要高电压来进行偏振切换。此外，铁电相形成后，薄膜会经历“唤醒”效应，即残余极化增加到一定数量的场循环。这是包括 PZT 在内的所有铁电材料所具有的固有特性。唤醒效果不佳，一旦用于内存申请，可能会导致错误信息存储。此外，难以控制多晶薄膜中形成的相。单晶铁电相非常适合最小化非铁电相的贡献并实现高电容和低泄漏。在 PLD 系统上生长的外延薄膜将能够形成单相、单晶薄膜，从而能够从根本上了解材料的固有特性。为了克服上述挑战，我的目标是研究掺杂剂和应变的单独影响。我将学习生长具有优化铁电行为的 HfO_2 薄膜，具有高饱和极化、低矫顽场和减少唤醒效应。到目前为止，许多掺杂剂已经在 HfO_2 上进行了测试，但需要清楚地了解掺杂剂尺寸、离子尺寸方差、掺杂分数和电荷失配的组合。这将使用共掺杂进行研究。主要兴趣是 HfO_2 上的镧和钽掺杂剂，它们分别具有 3+ 和 5+ 价电荷。通过将它们共掺杂在HfO_2上，分别研究系统中平均电荷和平均阳离子离子尺寸的影响。此外，应变对于控制晶格结构至关重要。为了探索应变效应，将生长一些晶胞的超晶格结构。 HfO_2 薄膜将使用不同的氧化物晶格结构（例如 SrTiO_3）进行应变。超晶格将使我能够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 了解化学态和电子态。我们还可以探索厚度和界面数量对结构形成和唤醒效应的影响。所有薄膜都将通过压电响应力显微镜 (PFM) 和极化电场测量进行测试，以证明铁电性。使用正上负下（PUND）技术将消除铁电测试期间漏电流的影响，只允许考虑位移电流。因此，精心调整掺杂、应变和界面的完美结构将能够理解和控制科学和工业上令人着迷的 HfO_2 铁电系统的研究。这将为促进 CMOS 性能和非易失性的下一代纳米级电子产品指明道路。