Title Page

Contents

Abstract 26

Chapter Ⅰ. Introduction 28

1-1. Introduction to van der Waals layered materials 28

1-1-1. Crystal structures of van der Waals layered materials 30

1-1-2. Electrical properties of van der Waals layered materials 38

1-1-3. 2D van der Waals heterostructures 44

1-2. Introduction to magnetic properties 44

1-2-1. The origin of magnetism; a theoretical background 45

1-2-2. Diamagnetism and paramagnetism 48

1-2-3. Ferromagnetism and antiferromagnetism 50

1-2-4. Paramagnetic ions and crystal field theory 53

1-3. Magnetic interactions 55

1-3-1. Magnetic dipolar interaction energy 56

1-3-2. Direct exchange 57

1-3-3. Superexchange 60

1-3-4. Indirect exchange 62

1-3-5. Antisymmetric exchange 62

1-3-6. Itinerant exchange 64

1-4. Magnetism in 2D van der Waals materials 66

1-4-1. Intrinsic 2D vdW magnets 67

1-4-2. Extrinsic 2D vdW magnets 69

1-4-3. Doping-induced magnetism 69

1-4-4. TMD-based DMSs 70

1-4-5. Synergetic proximity effect at 2D vdW heterostructures 71

1-4-6. Spintronic applications in Fe₃GeTe₂-based 2D vdW heterostructures 74

Chapter Ⅱ. High-mobility junction field-effect transistor via graphene/MoS₂ heterointerface 76

2-1. Introduction 77

2-2. Experimental methods 79

2-2-1. Material synthesis 79

2-2-2. Device fabrication 80

2-2-3. Optical and electrical characterization 81

2-3. Results and discussion 82

2-3-1. Schematic illustration, Raman characterization and band diagram of the heterostructure 82

2-3-2. Electrical properties at non-overlapped graphene/MoS₂ structure 85

2-3-3. Electrical properties at overlapped graphene/MoS₂ structure 91

2-3-4. Schottky barrier height estimation at graphene/MoS₂ heterointerfaces 94

2-4. Conclusion 100

Chapter Ⅲ. Proximity-induced tunable magnetic order at the interface of all-van der Waals-layered heterostructures 102

3-1. Introduction 103

3-2. Experimental methods 107

3-2-1. Synthesis of W₁₋ₓVₓSe₂ monolayer 107

3-2-2. Device fabrication process 107

3-2-3. Measurements of Hall resistance 108

3-3. Results and discussion 109

3-3-1. Structural properties of the FGT/W₁₋ₓVₓSe₂ devices 109

3-3-2. Electrical measurements of a monolayer W₁₋ₓVₓSe₂ flake 112

3-3-3. Diverse magnetic phases in the heterostructure devices 113

3-3-4. H-T Phase diagram derived from coercivity extraction. 121

3-3-5. Different magnetization switching behavior by different SOC strength 128

3-3-6. Interfacial proximity effect in an inverted heterostructure 133

3-3-7. Sample stability issue during/after measurements 136

3-4. Conclusion 137

References 140

국문초록 158

Figure 1-1. Carbon allotropes. 31

Figure 1-2. The periodic table for a list of transition metal and chalcogen elements for TMD formation. 34

Figure 1-3. The different 3 phases of MoS₂ monolayer. 36

Figure 1-4. The atomic structure of hexagonal boron nitride (h-BN). 37

Figure 1-5. The band structure of graphene and its ultra-high carrier mobility. 39

Figure 1-6. The substrate effect on the 2D materials. 41

Figure 1-7. Band gap of TMDs and dependence on the number of layers. 42

Figure 1-8. Band gap spectrum in van der Waals layered materials 43

Figure 1-9. Temperature dependence of the magnetic susceptibility in paramagnets, ferromagnets, and antiferromagnets. 52

Figure 1-10. The schematic illustration of crystal splitting of d-orbitals of a paramagnetic TM ion. 54

Figure 1-11. The spatially symmetric and antisymmetric wave functions for the H₂ molecule depending on the interatom distance. 58

Figure 1-12. The energy splitting of the spin singlet and spin triplet states for the H₂ molecule. The sign of the exchange integral J is negative, so the singlet... 59

Figure 1-13. Spin ordering in ferromagnets (J＞0) and antiferromagnets (J＜0) 60

Figure 1-14. An example of typical superexchange bond in the Mn²⁺-O²⁻-Mn²⁺ system. 61

Figure 1-15. The schematic illustration of DMI in a system with large SOC. 63

Figure 1-16. RKKY exchange interaction for Mn atoms as a function of interatomic distance in Mn₂Ge₆₂. 64

Figure 1-17. Schematic representation of direct, indirect, superexchange, RKKY, and DMI. 65

Figure 1-18. The library of intrinsic van der Waals magnets. 68

Figure 1-19. Schematic illustration of inserting adatoms into nonmagnetic semiconductors for inducing ferromagnetism. 70

Figure 1-20. Interfacial engineering of 2D magnet interfaces. 73

Figure 2-1. A schematic illustration of Gr/MoS₂ heterostructure device. 82

Figure 2-2. Energy diagram at the Gr/MoS₂ heterointerface and carrier transfer direction. 83

Figure 2-3. Raman spectroscopy of Gr/MoS₂ heterojunction device. 84

Figure 2-4. Electrical characteristics of Gr/MoS₂ heterostructure device, device#1 and device #2. 86

Figure 2-5. Pure MoS₂ device for comparison and overall device characteristics compared in a single plot. 90

Figure 2-6. The overlapping device (Device #3) and its IDS-VBG characteristics.[이미지참조] 92

Figure 2-7. The comparison of individual device characteristics and model for direct tunneling at the overlapped device (Device #3). 93

Figure 2-8. Determination of y-intercept for SBH calculation. 95

Figure 2-9. The IDS-VDS extracted for SBH calculation and energy diagram schematic of the structure of Device #4.[이미지참조] 96

Figure 2-10. Determination of y-intercept for Device #3 SBH calculation. 97

Figure 2-11. The IDS-VDS extracted for SBH calculation and energy diagram schematic of the structure of Device #3.[이미지참조] 98

Figure 2-12. Estimated Schottky barrier height at Gr/MoS₂ heterointerface. 100

Figure 3-1. Device schematic and corresponding optical microscopic image. 109

Figure 3-2. The optical microscopy (upper panel) and atomic force microscopy (lower panel) images of devices. 110

Figure 3-3. Crystal structures of each sample and their temperature dependent Rₓₓ measurement results. 111

Figure 3-4. Electrical characteristics of W₁₋ₓVₓSe₂ monolayer devices. 112

Figure 3-5. Temperature-dependent Rxy(μ₀H) after the offset subtraction by anti-symmetrization.[이미지참조] 114

Figure 3-6. The representative Rxy(μ₀H) data for all devices at 2, 120, 180,and 200 K for comparison.[이미지참조] 115

Figure 3-7. Deconvolution of Rxy(μ₀H) data at various temperatures below Tc.[이미지참조] 118

Figure 3-8. Temperature-dependent Hall resistance Rxy(μ₀H) for FGT/o-FGT(yellow dots), FGT/W₀.₀₉V₀.₀₅Se₂ (green dots) and FGT/WSe₂ heterostructure...[이미지참조] 119

Figure 3-9. Determination of Rxy,0T and HC from the measured data in FGT/W₀.₉₅V₀.₀₅Se₂ at 100K.[이미지참조] 122

Figure 3-10. Field-derivative of Rxy for FGT, FGT/W₀.₉₅V₀.₀₅Se₂, and FGT/WSe₂ at various temperatures.[이미지참조] 123

Figure 3-11. Rxy,0T-T phase diagram of each device.[이미지참조] 124

Figure 3-12. Magnetic diagram for whole measuring temperature region. 126

Figure 3-13. Interlayer exchange coupling and witching behavior at T＝150 K. 128

Figure 3-14. Schematic diagram of expected magnetic order at the interface upon SOC strength with spin-flop, spin-flip, and spin-flip and inverted magnetization. 130

Figure 3-15. Field-derivative of Rxy(μ₀H) and magnetic switching speed.[이미지참조] 131

Figure 3-16. Inverted W₀.₉₅V₀.₀₅Se₂/FGT heterostructure device with the bottomelectrodes and Temperature-dependent Rₓₓ. 133

Figure 3-17. Temperature-dependent Rxy(μ₀H) for W₀.₉₅V₀.₀₅Se₂/FGT after subtracting the offset by anti-symmetrization.[이미지참조] 134

Figure 3-18. Rxy(μ₀H) for the inverted devices.[이미지참조] 135

Figure 3-19. The time-dependent measurement of the W₀.₉₅V₀.₀₅Se₂/FGT heterostructure sample. 136