본 논문은 두가지 이상의 자유도를 지님으로써 발현될 수 있는 다강성 성질을 지닌 다강체 수직정렬 나누구조체(VAN)를 희생층을 활용하여 실리콘에 전사시키는 것을 다룬다. 2상 구성에서 자기 전기 결합을 나타내는 다강성 나노 복합 박막은 데이터 저장, 에너지 하베스팅 및 센서 기술과 같은 다양한 응용 분야에서 매우 요구되고 있다. 그러나 실리콘 기판에서 이러한 필름의 제조는 수직 정렬 나노 구조(VAN)의 에피택셜 성장을 위한 단결정 페로브스카이트 기판의 요구로 인해 방해를 받고 있다. 본 연구에서는 수용성 Sr3Al2O6(SAO) 희생층을 사용하여 BiFeO3-CoFe2O4(BFO-CFO) VAN 필름을 실리콘 기판으로 성공적으로 전송할 수 있도록 함으로써 이러한 한계를 해결한다. 이 전송 프로세스는 강유전체-페로브스카이트-BiFeO3 매트릭스에 내장된 고차 페리마그네틱-스핀넬-CoFe2O4 나노 기둥의 에피택셜 성장을 초래한다. 또한 펄스 레이저 증착 중 파라미터 변조를 통해 나노 기둥 구조의 에피택셜 성장과 형태를 정확하게 제어하고 최적화할 수 있다. 중요한 것은 단결정 페로브스카이트 기판에서 성장한 것과 비교하여 전송된 장치에서 장치 성능 또는 필름 품질 저하가 관찰되지 않았으며, 이는 실제 실리콘 기반 응용 분야를 가진 차세대 다강성 소자의 실현을 향한 유망한 경로를 나타낸다.|Multiferroic nano-composite thin films exhibiting magnetoelectric coupling in a two-phase configuration are highly sought after for various applications such as data storage, energy harvesting, and sensor technologies. However, the fabrication of these films on silicon substrates has been impeded by the requirement of single crystal perovskite substrates for epitaxial growth of vertically aligned nanostructures (VANs). In this study, we address this limitation by employing a water-soluble Sr3Al2O6 (SAO) sacrificial layer, enabling the successful transfer of BiFeO3-CoFe2O4 (BFO-CFO) VAN films onto silicon substrates—this transfer process results in the epitaxial growth of highly ordered ferrimagnetic-spinel-CoFe2O4 nano-pillars embedded within a ferroelectric- perovskite-BiFeO3 matrix. Moreover, the epitaxial growth and the morphology of the nano-pillar structures can be precisely controlled and optimized through parameter modulation during pulsed laser deposition. Importantly, no degradation in device performance or film quality was observed in the transferred devices compared to those grown on single crystal perovskite substrates, indicating a promising pathway towards the realization of next-generation multiferroic devices with practical silicon-based applications.
Table Of Contents
Ⅰ. Nomenclature 1 Ⅱ. Introduction 2 2.1 Motivation 2 2.1.1 Promising functional oxide – multifunctional magnetoelectric 2 2.1.2 Engineering VANs: Demand for silicon integration 3 2.2 In this work 5 Ⅲ. Background Research 7 3.1 Multiferroic two phase thin film materials 7 3.1.1 Nanoparticles embedded in matrix 0-3 structure 8 3.1.2 Multilayers 2-2 structure 9 3.1.3 Self-assembled nanopillars embedded in matrix 1-3 structure 11 3.2 Designing self-assembled nanocomposites 14 3.2.1 Selection and growth criteria for VANs 14 3.2.2 Thermodynamic factors 16 3.2.3 Kinetic factors 19 3.3 Multiferroic properties of BFO-CFO 22 3.3.1 Structure 22 3.3.1.1 Matrix and column 25 3.3.1.2 Mechanism of the vertically aligned nanocomposite growth 25 3.3.2 Properties 28 3.3.2.1 Magnetic property 28 3.3.2.2 Electrical property 32 3.3.2.3 Magnetoelectric coupling property 33 3.4 Integration techniques of oxide thin films onto silicon 37 3.4.1 Buffer layers 37 3.4.2 Universal transferring the freestanding film methods 41 3.4.3 Sr3Al2O6 sacrificial layer wet etching method 46 Ⅳ. Experimental Method & Materials 49 4.1 Pulsed laser deposition (PLD) 49 4.2 X-ray diffraction (XRD) 51 4.3 Atomic force microscopy (AFM) 53 4.3.1 Contact mode 54 4.3.2 Piezo-response force microscopy (PFM) 55 4.4 Magnetic property measurement system (MPMS) 56 Ⅴ. Tuning the Structure and Properties of BFO-CFO Nanocomposites 57 5.1 Kinetical dependent growth of SRO bottom electrode layer 57 5.1.1 Growth of SRO with temperature dependence 58 5.1.2 Growth of SRO with oxygen partial pressure dependence 58 5.2 Kinetical dependent growth of BFO-CFO 63 5.2.1 Growth of VAN with temperature and oxygen partial pressure dependence 68 5.2.2 Epitaxial growth 69 5.2.3 Morphology 73 5.2.4 Magnetic property 75 Ⅵ. Transferring Integration of Freestanding BFO-CFO onto Silicon 77 6.1 Transferring methodology 77 6.2 Evaluating as grown BFO-CFO/SRO/SAO and integrated BFO-CFO 6.2.1 Epitaxial growth 80 6.2.2 Morphology 81 6.2.3 Magnetic property 82 Ⅶ. Conclusion 84 7.1 Summary and conclusions 84 7.2 Future work 85
