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

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。它在室温下的性能是令人满意的，但它在非易失性存储器件的应用中有限制，由于可扩展性，复杂性，CMOS兼容性等，因此，需要更简单和良性的铁电材料。另一方面，氧化铪（HfO_2）由于其CMOS兼容性（因为它已经被用作栅极电介质）、可扩展性（因为它可以被制造为具有几nm厚度的薄膜）以及最重要的铁电性而引起极大的兴趣。它显示出铁电行为与特定的阶段，斜方和菱面体。它们是亚稳相，需要特定的稳定条件，例如生长条件、后退火、诱导应变、掺杂等，目前尚不清楚哪一种占主导地位。HfO_2薄膜具有很高的矫顽场，大于1 MV cm^（-1），因此需要很高的电压来实现极化反转。此外，铁电相形成后，薄膜经历“唤醒”效应，即剩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都经历的本征性质。唤醒效果是不利的，因为一旦它被用于记忆应用，它可能会导致错误的信息存储。另外，难以控制在多晶膜中形成的相。单晶铁电相对于最小化非铁电相的贡献并实现高电容和低泄漏是理想的。在PLD系统上生长的外延膜将使得能够形成单相、单晶薄膜，这使得能够对材料的本征性质进行基本理解。为了克服上述挑战，我的目标是调查掺杂剂和应变的单独影响。我将学习如何生长具有最佳铁电特性的HfO_2薄膜，包括高饱和极化、低矫顽场及降低唤醒效应。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，离子尺寸变化，掺杂分数和电荷失配的组合。这将使用共掺杂进行研究。主要研究HfO_2上的镧和钽掺杂，它们分别具有3+和5+价电荷。通过对HfO_2的共掺杂，分别研究了体系中平均电荷和阳离子平均尺寸对晶格结构的影响。为了探索应变效应，我们将生长出由少量晶胞组成的超晶格结构。HfO_2薄膜将使用不同的氧化物晶格结构，如SrTiO_3应变。超晶格将允许我使用同步加速器方法精确地观察界面效应。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应的影响。所有的薄膜将进行测试的压电响应力显微镜（PFM）和极化电场测量，以证明铁电性。采用PUND技术可以消除漏电流对铁电性测试的影响，只考虑位移电流，从而获得掺杂、应变和界面可调的完美结构，为理解和控制HfO_2这一极具科学和工业价值的铁电体系提供了可能。这将为下一代纳米级电子产品的发展指明道路，从而提高CMOS的性能和非易失性。

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.