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，如（Pb,Zr）TiO_3，称为PZT。它在环境温度下的性能令人满意，但由于可扩展性、复杂性、CMOS兼容性等限制，在非易失性存储器件中的应用受到限制。因此，需要更简单、更良性的铁电材料。另一方面，氧化铪（HfO_2）由于其CMOS兼容性而引起了极大的兴趣，因为它已经被用作栅极介电介质，可扩展性，因为它可以制成厚度为几纳米的薄膜，最重要的是，它具有铁电性。它具有特定相的铁电行为，具有正方和菱形相。它们是亚稳相，需要特定的条件来稳定，如生长条件、后退火、诱导应变、掺杂等，但哪一个是主导还有待证实。HfO_2薄膜具有大于1 MV cm^（-1）的高矫顽力场，因此需要高电压进行极化开关。铁电相形成后，薄膜发生“唤醒”效应，残余极化增加到一定场次。这是包括PZT在内的所有铁电材料都具有的固有特性。唤醒效应是不利的，因为一旦用于内存应用，它可能导致错误的信息存储。此外，在多晶薄膜中形成的相很难控制。单晶铁电相是理想的，以减少非铁电相的贡献，实现高电容和低漏。在PLD系统上生长的外延膜可以形成单相单晶薄膜，从而可以从根本上了解材料的内在特性。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。我将学习培养具有高饱和极化、低矫顽力场和减少唤醒效应的优化铁电性能的HfO_2薄膜。到目前为止，在HfO_2上已经测试了许多掺杂剂，但需要清楚地了解掺杂剂尺寸、离子尺寸差异、掺杂分数和电荷失配的组合。这将使用共掺杂进行研究。主要关注的是HfO_2上镧和钽的掺杂，它们分别具有3+和5+价荷。通过在HfO_2上共掺杂，分别考察了体系中平均电荷量和平均阳离子尺寸的影响。此外，应变是控制晶格结构的关键。为了探索应变效应，将会生长出一些单元胞的超晶格结构。采用SrTiO_3等不同的氧化晶格结构对HfO_2薄膜进行应变。超晶格可以让我用同步加速器的方法精确地观察界面效应。这将使我能够使用XPS了解化学态和电子态。我们还可以探索界面厚度和界面数量对结构形成和唤醒效应的影响。所有薄膜将在压电响应力显微镜（PFM）和极化电场测量上进行测试，以证明铁电性。采用正-上-负-下（PUND）技术可以消除铁电性测试中漏电流的影响，只考虑位移电流。因此，精心调整掺杂、应变和界面的完美结构将有助于理解和控制科学上和工业上令人着迷的HfO_2铁电体系。这将为下一代纳米级电子产品提供提升CMOS性能和非易失性的途径。

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.