Atomic-Scale Investigation of Nano-Domain Structure in HfO2-Based Ferroelectric Thin Films by Advanced (S)TEM Analysis = 첨단 (주사)전자현미경 분석을 통한 산화 하프늄 기반 강유전 박막의 원자 수준 나노 도메인 구조 규명 연구

다국어 초록 (Multilingual Abstract)

Atomic-Scale Investigation of Nano-Domain Structure in HfO2-Based Ferroelectric Thin Films by Advanced (S)TEM Analysis Kunwoo Park School of Chemical and Biological Engineering The Graduate School Seoul National University This thesis delves into the fundamental understanding of ferroelectric properties in nanocrystalline HfO2 thin films, which are gaining significant attention in both academic and semiconductor industry circles due to their novel ferroelectric characteristics at sub-10nm thicknesses. This research not only explores the inherent properties of HfO2 films used in CMOS technology but also unravels the complex mechanisms underpinning the ferroelectric phase in these materials. Key findings include the stabilization of the ferroelectric phase through silicon doping, which leverages silicon's tendency to form strong covalent bonds with oxygen, thereby altering the phase stability in favor of the ferroelectric orthorhombic phase. Scanning Transmission Electron Microscopy (STEM) and Density Functional Theory (DFT) calculations confirm that silicon doping introduces monoclinic structural components into the orthorhombic phase, facilitating ferroelectric transition. Additionally, the study investigates the effects of oxygen deficiency on the ferroelectric switching properties of Si-doped HfO2 films. Controlled annealing experiments demonstrate that high-temperature processes enhance oxygen vacancies, thereby reducing the energy barrier for polarization flipping, as evidenced by STEM, Electron Energy-Loss Spectroscopy (EELS), and first-principles calculations. Furthermore, the research reveals that <111>-oriented grains in yttrium-doped HfO2 (Y:HfO2) films induce a high-symmetry mixed-(R3m, Pnm21) phase, which enhances polarization under in-plane compressive strain. This phenomenon, along with the cooperative effect of nanoscale orthorhombic grains and high-symmetry phase grain boundaries, is crucial in determining the overall ferroelectricity of the Y:HfO2 film. In-depth analysis of ferroelectric domain networks and oxygen position mapping elucidates that polarization is suppressed at vertical domain walls, whereas horizontal domain alignment with zero-width walls shows active polarization. This thesis provides significant insights into the complex nature of ferroelectric HfO2 thin films, offering valuable implications for the future design and application of these materials in advanced semiconductor technologies. Keywords: Ferroelectric HfO2 thin films, nanocrystalline structure analysis, doping effects, high-temperature annealing effects, electron microscopy and spectroscopy, ferroelectric domain network Student Number: 2018-23882

국문 초록 (Abstract)

본 논문은 나노 두께의 HfO2 박막이 갖는 강유전 특성에 대한 심층적인 이해를 제공한다. 특히 10nm 이하의 두께에서 나타나는 이 박막의 독특한 강
유전 특성은 학계와 반도체 산업에 큰 관심을 불러일으키고 있다. 본 연구는 CMOS 기술에서 활용되는 HfO2 필름의 기본적인 특성 뿐만 아니라, 이 재료들의 강유전 상태를 형성하는 복잡한 메커니즘을 규명한다.
이 논문의 핵심 발견 중 하나는 실리콘 도핑을 통해 강유전 상을 안정화시키는 기작을 밝혔다는 것이다. 실리콘은 산소와 강한 공유 결합을 형성하는 경
향이 있으며, 이는 강유전 상의 안정성을 변화시키는 데 기여한다. 주사 투과 전자 현미경(STEM)과 밀도 범함수(DFT) 계산을 통해 실리콘 도핑이 강유전상 내부의 단사정 구조 요소를 도입하여 강유전 상태로의 전환을 용이하게 함을 확인했다.
또한 본 연구는 산소 결핍이 실리콘 도핑된 HfO2 필름의 강유전 스위칭 특성에 미치는 영향을 분석한다. 고온 열처리 과정을 통해 조절된 산소 결핍은
분극 반전의 에너지 장벽을 낮추는 것으로 나타났으며, 이는 STEM, 전자 에너지 손실 분광법(EELS), 및 제일원리 계산으로 입증되었다.
이 연구는 또한 이트륨 도핑된 HfO2 (Y:HfO2) 필름에서 <111>-방향 결정 립이 혼합형 고대칭 상(R3m, Pnm21)을 유도하며, 이는 입면 압축 변형 하에서 분극이 강화됨을 밝혀냈다. 나노 수준의 강유전 상 결정립의 상호작용이 Y:HfO2 필름의 전체적인 강유전성을 결정하는 중요한 요소로 나타났다. 강유전 영역 네트워크와 산소 위치 매핑에 대한 정밀 분석은 수직 영역 도메인 벽에서는 분극이 억제되나, 너비가 없는 도메인 벽을 가진 수평으로 정렬된 영역에서는 활성 분극이 관찰됨을 밝혔다.
결론적으로, 이 논문은 강유전 HfO2 박막의 복잡한 특성에 대한 깊은 통찰력을 제공하며, 이를 통해 고급 반도체 기술에서 이 재료들의 미래 설계 및 응용에 대한 중요한 가이드라인을 제시한다.

목차 (Table of Contents)

• Chapter 1 Introduction . 1
• 1.1. Ferroelectric HfO2-based thin film 1
• 1.2. Purpose of research . 2
• Chapter 2 Facile Ferroelectric Phase Transition Driven by Si Doping in HfO2 4
• 2.1. Introduction . 4
• 2.2. Methods . 6
• 2.2.1. Sample preparation . 6
• 2.2.2. STEM measurements . 6
• 2.2.3. Calculation method . 6
• 2.2.4. Phonon mode displacement 7
• 2.3. Results and Discussion 9
• 2.3.1. Phase stability in HfO2: impact of doping 9
• 2.3.2. Phonon mode displacement and doping effects . 10
• 2.3.3. Local bonding characteristics and structural deformation 11
• 2.3.4. Experimental confirmation using STEM analysis 13
• 2.3.5. Activation energy analysis and phase transition implications 15
• Chapter 3 Enhanced ferroelectric switching speed of Si-doped HfO2 thin film
• tailored by oxygen deficiency 34
• 3.1. Introduction 34
• 3.2. Methods 38
• 3.2.1. Sample fabrication 38
• 3.2.2. Sample characterization . 38
• 3.2.3. Scanning transmission electron microscopy 39
• 3.2.4. Grain size estimation 40
• 3.2.5. Electron energy-loss spectroscopy analyses 40
• 3.2.6. Details of switching dynamics 41
• v
• 3.2.7. Theoretical calculations . 42
• 3.2.8. Details of Monte Carlo simulations . 43
• 3.3. Results and Discussion 45
• 3.3.1. Oxygen deficiency analysis in HfO2 Films 45
• 3.3.2. Switching dynamics of HfO2 films 47
• 3.3.3. Theoretical insights and simulation studies on ferroelectric switching . 49
• Chapter 4 Atomic-scale scanning of domain network in the ferroelectric HfO2
• thin film 61
• 4.1. Introduction 61
• 4.2. Methods 65
• 4.2.1. Sample fabrication and electrical measurements 65
• 4.2.2. Release and transfer of films 65
• 4.2.3. (S)TEM, 4D-STEM, automated STEM and iDPC-STEM measurements
• . 66
• 4.2.4. STEM and TEM simulations 67
• 4.2.5. Atomic resolution displacement and strain analysis 68
• 4.2.6. Computational methods for induction of HS-phase and polarization at
• vertical domain walls 68
• 4.2.7. The interface energies between orthorhombic phase domains 70
• 4.3. Results and Discussion 72
• 4.3.1. Free-standing ferroelectric Y:HfO2 thin films 72
• 4.3.2. Inducement of strained polar phase at grain boundaries 74
• 4.3.3. Phase transformation according to thickness 80
• 4.3.4. Direct observation of ferroelectric domains . 82
• Chapter 5 Summary and Conclusions 117
• vi

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