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Lateral and Vertical Electronic Transport Properties of Topological Insulator-Based van der Waals Heterostructures Grown by Molecular Beam Epitaxy
분자빔에피택시로 성장한 위상절연체 기반 반데르발스 이종구조에서의 수직 및 수평 방향 전자 수송 특성

DC Field Value Language
dc.contributor.advisor이규철-
dc.contributor.author박준영-
dc.date.accessioned2019-10-21T03:30:52Z-
dc.date.available2019-10-21T03:30:52Z-
dc.date.issued2019-08-
dc.identifier.other000000158053-
dc.identifier.urihttp://hdl.handle.net/10371/162360-
dc.identifier.urihttp://dcollection.snu.ac.kr/common/orgView/000000158053ko_KR
dc.description학위논문(박사)--서울대학교 대학원 :자연과학대학 물리·천문학부,2019. 8. 이규철.-
dc.description.abstract최근 반데르발스 물질군에 위상절연체(topological insulator; TI)라는 새로운 전자 구조를 띠는 물질이 포함됨에 따라, 반데르발스 이종구조에 대한 연구가 더욱 활기를 띠고 있다. 고품질의 위상학적 반데르발스 이종구조를 제조하기 위해서는 결함 밀도를 낮추고 계면을 원자수준으로 제어하는 것이 필수적이다. 그러나 지금까지 이에 대한 많은 연구가 있었음에도, 고도로 제어된 환경에서의 소재 성장, 새로운 특성 분석 기법, 소자제조 공정에 대한 연구가 여전히 필요한 실정이다.
본 학위논문에서는 분자빔에피택시(molecular beam epitaxy; MBE) 성장법에 반데르발스 물질 핸들링 기법, 투과전자현미경(transmission electron microscopy; TEM) 구조 분석, 나노소자 제작공정을 조합한 새로운 방법론을 제안하여, 고품질 위상절연체 기반의 원자 단위로 제어된 반데르발스 이종구조 제작에 활용하였다. 이러한 접근법을 통해, 위상절연체와 육방정계 질화붕소(hexagonal boron nitride; hBN) 간 고품질 이종에피택시 반데르발스 계면에서 일어나는 수평 및 수직 방향의 전자 수송 특성을 분석하였다.
먼저, 본 논문에서는 MBE법을 이용한 hBN 위의 고품질 위상절연체 Bi2Se3 박막 성장과 이의 구조적 및 전기적 특성을 논한다. 특히, 여기서 개발한 MBE 성장기법은 TEM과 호환성을 가지므로, 성장된 필름의 구조적 특성에 대한 빠른 피드백이 용이해진다. 다음으로는 hBN 위에 높은 균일성 및 결정성을 갖는 Bi2Se3 박막의 성장법을 제안하였다. 고분해능 TEM 연구를 통해, Bi2Se3 와 hBN 사이에 원자 수준으로 깨끗한 계면이 형성되며, 두 물질 간 에피택시 관계가 형성됨을 보였다. 또한, Bi2Se3/hBN 이종구조의 전자 수송 특성 분석을 통하여 Bi2Se3/hBN 계면에서 높은 전하이동도를 갖는 위상학적 상태가 형성됨을 보였다. 더 나아가, 앞서 제시한 성장 및 특성 분석법이 또다른 위상절연체/반데르발스물질 이종구조 조합에도 다양하게 확대 적용될 수 있음을 설명하였다.
이어지는 연구에서는 TI와 hBN을 이용한 수직 방향 반데르발스 터널 접합의 제작 및 측정을 다룬다. 원자 수준으로 얇은 두께의 hBN을 현수(suspension)시킨 뒤 이 윗면과 아랫면에 위상절연체를 각각 이종에피택시 성장함으로써 TI/hBN/TI 구조를 제조하였다. 또한 이 경우 hBN의 얇은 두께로 인해 계면의 전자 상태에 직접적 전기적 접근이 가능해진다. 본 논문은 두 종류의 TI/hBN/TI 에피택시 이종구조에 대한 터널링 분광법을 실시하여, 터널링 전자의 에너지, 운동량, 스핀 나선성 보존 조건을 분석하였다.
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dc.description.abstractResearch on van der Waals (vdW) heterostructures has recently been boosted as a new class of material called topological insulator (TI) has joined the vdW material library. While there has been a huge amount of efforts to achieve novel topological heterostructures with low defect density and precise atomic controllability of the vdW interface, it is still necessary to develop a highly controlled and engineered materials growth approach and novel characterization and device fabrication techniques.
In this dissertation, I present the novel combination of molecular beam epitaxial (MBE) growth with vdW-layer manipulation technique, transmission electron microscopy (TEM) structural analysis, and nanodevice fabrication processes to prepare atomically engineered vdW heterostructures based on TIs of high-quality. With this approach, I particularly focus on lateral and vertical topological electronic transport across the high-quality vdW heteroepitaxial interfaces between TIs and hexagonal boron nitride (hBN).
First, I discuss MBE growth and structural/electrical properties of high-quality TI Bi2Se3 thin films on hBN. The TEM-compatibility enables the MBE to receive fast feedback on structural properties of the films, thus facilitating growth optimization. I demonstrate a growth method for enhancing the uniformity and crystallinity of the films on hBN. Through the high-resolution TEM study, I show an atomically abrupt and epitaxial interface between Bi2Se3 and hBN. I investigate gate dependent magnetotransport properties of the heterostructure to demonstrate the formation of high mobility topological states at the Bi2Se3/hBN interface. In addition, I show that the growth and characterization scheme can be extended to various combinations of TI/vdW heterostructures.
The following part presents the fabrication and measurement of vertical vdW tunnel junctions where two epitaxially aligned TIs are separated by a-few-atom-thick hBN tunnel barriers. Here, the suspended, atomically thin hBN allows the materials growing on both sides to align heteroepitaxially and provides direct electrical access to the interfacial electronic states. Through magneto-tunneling spectroscopy on two kinds of vertical TI/hBN/TI epitaxial vdW heterostructures, I explore the properties of tunneling electrons in terms of energy, momentum, and spin helicity conservation.
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dc.description.tableofcontents1 Introduction 1
1.1 Motivation 1
1.2 Objective and Approach 2
1.3 Outline of the Thesis 5
2 Background and Literature Survey 7
2.1 MBE Growth of Topological Insulator-Based van der Waals Heterostructures 7
2.2 Novel Lateral and Vertical Electronic Transports in Atomically Engineered van der Waals Heterostructures 21
3 Experimental Techniques 27
3.1 Molecular Beam Epitaxial Growth of Topological Insulator Thin Films on van der Waals Layered Materials 27
3.1.1 The molecular beam epitaxy system used in this work 27
3.1.2 Practice: MBE growth of Bi2Se3 thin films on hBN layers 38
3.2 Preparation of MBE-, TEM-, and Nanofabrication-Compatible Mesoscopic vdW Templates 41
3.2.1 Fabrication of sub-micrometer hole array patterns in SiNx membrane window TEM grids 41
3.2.2 Suspension of few-layer vdW materials on the hole-patterned SiNx membranes 45
3.3 Morphological and Microstructural Characterizations 48
3.4 Device Fabrications for Electrical Characterizations 49
3.4.1 Hall bar and van der Pauw geometry devices 49
3.4.2 Tunneling devices based on suspended double-side vdW heterostructures 59
3.5 Electronic Transport Measurements 62
3.5.1 Lateral magnetotransport measurements 62
3.5.2 Magneto-tunneling measurements 64
4 MBE Growth of Bi2Se3 Thin Films on hBN and Their Structural and Electronic Transport Properties 66
4.1 Introduction 66
4.2 MBE Growth of Bi2Se3 Thin Films on hBN Layers 67
4.3 Structural Properties of Bi2Se3 Thin Films on hBN Layers 71
4.4 Electronic Transport Properties of Bi2Se3 thin films on hBN Layers 79
4.4.1 Gate dependencies of the longitudinal and the Hall resistances 79
4.4.2 Analysis of multiple conduction channels in Bi2Se3/hBN 82
4.4.3 Two-dimensional, two decoupled conduction channels: ruling out bulk contribution to electrical transports 85
4.4.4 Shubnikov-de Haas oscillations observed in Bi2Se3/hBN 91
4.4.5 Comparison of transport properties of Bi2Se3 thin films grown on various substrates by MBE 93
4.5 Summary 95
4.6 [Supplementary] Heteroepitaxial Growth of Topological Insulator Thin Films on Various van der Waals Materials 96
4.6.1 Telluride TIs and graphene substrates 96
4.6.2 Bi2Se3/WSe2 for TI/TMD heterostructures 98
4.6.3 Bi2Se3/α-RuCl3 for TI/quantum spin liquid candidate heterostructures 101
5 TI/hBN/TI Epitaxial van der Waals Heterostructures for Tunneling Spectroscopy Between Topological Surface States 102
5.1 Introduction 102
5.2 Double-Sided MBE Growth of Topological Insulator Thin Films on Top and Bottom Surfaces of Suspended hBN 105
5.3 Microstructural Properties of TI/hBN/TI Epitaxial vdW Heterostructures 108
5.4 Magneto-Tunneling Spectroscopy of TI/hBN/TI Junctions 113
5.5 Summary 125
6 Conclusions and Outlook 126
6.1 Concluding Remarks 126
6.2 Suggestions for Future Works 128
Appendix: Structural and Electrical Characterizations of Bi2Se3 Heteroepitaxially Grown on α-RuCl3 130
A.1 Introduction 130
A.2 Heteroepitaxial Growth of Bi2Se3 Thin Films on α-RuCl3 using MBE 131
A.3 Structural Characterizations of Bi2Se3/α-RuCl3 Heterostructures 135
A.4 Electrical Characterizations of Bi2Se3 Thin Films on α-RuCl3 Layers 142
A.5 Summary 145
List of Abbreviations 146
Bibliography 150
Abstract in Korean (국문초록) 158
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dc.language.isoeng-
dc.publisher서울대학교 대학원-
dc.subjectMolecular beam epitaxy-
dc.subjectvan der Waals heterostructure-
dc.subjectTopological insulator-
dc.subjectHexagonal boron nitride-
dc.subjectElectronic transport properties-
dc.subjectTunneling spectroscopy-
dc.subject.ddc523.01-
dc.titleLateral and Vertical Electronic Transport Properties of Topological Insulator-Based van der Waals Heterostructures Grown by Molecular Beam Epitaxy-
dc.title.alternative분자빔에피택시로 성장한 위상절연체 기반 반데르발스 이종구조에서의 수직 및 수평 방향 전자 수송 특성-
dc.typeThesis-
dc.typeDissertation-
dc.contributor.AlternativeAuthorJoon Young Park-
dc.contributor.department자연과학대학 물리·천문학부-
dc.description.degreeDoctor-
dc.date.awarded2019-08-
dc.contributor.major물리학전공-
dc.identifier.uciI804:11032-000000158053-
dc.identifier.holdings000000000040▲000000000041▲000000158053▲-
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College of Natural Sciences (자연과학대학)Dept. of Physics and Astronomy (물리·천문학부)Physics (물리학전공)Theses (Ph.D. / Sc.D._물리학전공)
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