Title Page
Contents
Abstract 12
Chapter Ⅰ. INTRODUCTION 14
Ⅰ.1. Overview of 2D vdW materials 14
Ⅰ.2. Semiconducting Two-Dimensional (2D) Transition Metal Dichalcogenides 16
Ⅰ.3. Crystal and Band structure of TMDs 17
Ⅰ.4. Methods for synthesizing TMDs and their identification 20
Ⅰ.4.1. Atomic force microscopy (AFM) of monolayer TMD 21
Ⅰ.4.2. Photoluminescence (PL) spectra of monolayer TMDs 22
Ⅰ.4.3. Raman spectra of monolayer TMDs. 23
Ⅰ.5. Van der Waals heterostructures 25
Ⅰ.5.1. Methods of Assembly 26
Ⅰ.6. Contact for 2D TMDs. 28
Ⅰ.6.1. Contact metal selection 29
Ⅰ.7. Two-dimensional semiconductors field effect transistor 32
Ⅰ.8. Exciton-Polariton 34
Ⅰ.8.1. Quantum well Exciton 34
Ⅰ.8.2. Semiconductor Microcavities 35
Ⅰ.8.3. Microcavity polaritons 36
Ⅰ.8.4. Strong and weak coupling 37
Ⅰ.8.5. Dispersion of polariton states 39
Ⅰ.9. Thesis Outline 44
Chapter Ⅱ. Ohmic contact resistance and high on/off ratio in semiconducting TMDs via cleaner transfer 45
Ⅱ.1. INTRODUCTION 46
Ⅱ.2. Device structure 48
Ⅱ.3. Device preparation and measurements 48
Ⅱ.3.1. Residue free transfer of TMDs 49
Ⅱ.3.2. Residue confirmation using AFM 51
Ⅱ.3.3. Residue confirmation using conducting AFM (C-AFM) 53
Ⅱ.3.4. PMMA residue confirmation using TEM analysis on TMD surface 54
Ⅱ.3.5. DFT calculations 55
Ⅱ.3.6. Photoluminescence study 59
Ⅱ.4. Device fabrication 61
Ⅱ.4.1. Device measurements 61
Ⅱ.4.2. Contact resistance 62
Ⅱ.5. Results and discussions 63
Ⅱ.5.1. Contact resistance of MoS₂ with different metal contacts 63
Ⅱ.5.2. Performance of monolayer MoS₂ and WS₂ FETs with Bi, Ti contact 69
Ⅱ.6. Comparative analysis of semiconductor technologies 78
Ⅱ.7. Conclusions 81
Chapter Ⅲ. Boosting Phototransistor Performance in Monolayer TMDs via Multiple Reflections from Distributed Bragg reflector 82
Ⅲ.1. INTRODUCTION 82
Ⅲ.2. DBR fabrication 83
Ⅲ.3. Device fabrication and characterization 84
Ⅲ.4. Results and Discussions 85
Ⅲ.4.1. Phototransistor performance of 1L MoS₂ on SiO₂ substrate 85
Ⅲ.5. Conclusions 95
Chapter Ⅳ. Gate modulated Polariton switching. 96
Ⅳ.1. INTRODUCTION 96
Ⅳ.2. Device structure 97
Ⅳ.3. Results and Discussions 98
Ⅳ.3.1. Optical characterization 98
Ⅳ.3.2. Gate modulation 100
Ⅳ.4. Conclusions 100
Chapter Ⅴ. Summary and perspective 101
Ⅴ.1. Summary 101
Ⅴ.2. Perspective 102
추상적인 104
References 106
Table Ⅱ.1. Summary of the electrical measurements parameters for different device types 80
Figure. Ⅰ.1. The current library of 2D layered materials: (a) arranged in different color boxes according to their stability. (b) 2D materials in accordance with their bandgap. 16
Figure. Ⅰ.2. (a) The periodic table highlights the transition metals and the three chalcogen elements that primarily crystallize in these layered structures. Selective highlights for Co, Rh, Ir and Ni, Pt,... 18
Figure. Ⅰ.3. AFM of MoS₂, height profile is depicted by the red line. 21
Figure. Ⅰ.4. (a) Photoluminescence process of a direct band gap semiconductor. (b) Band structure of a direct bandgap semiconductor where Eₑ is the electronic bandgap and Eb is the...[이미지참조] 22
Figure. Ⅰ.5. (a) Monolayer MoS₂ PL Intensity vs Energy plot with different excitation power. (b) Evidence of the number of layers in MoS₂, PL intensity increases with decreasing thickness. 23
Figure. Ⅰ.6. The scattering "Jablonski" style diagram of energetic transitions involved in Raman scattering. 24
Figure. Ⅰ.7. (a) Raman spectroscopy of monolayer MoS₂ (b) Layer dependent Raman spectra in MoS₂ bulk to thin layer. 25
Figure. Ⅰ.8. Large area single layer TMDs and their heterostructure fabrication. 27
Figure. Ⅰ.9. (a) Schematic diagram of several metals with MoS₂ contact, left hand part is the actual Schottky barrier height extracted from experimental measurements. (b) Energy band... 30
Figure. Ⅰ.10. (a) Density of states (DOS) versus energy diagram in N-type semiconductor. (b) DOS of normal metal and semiconductor after contact and their band diagram (d). (c) The DOS... 31
Figure. Ⅰ.11. History of transistor technology. 32
Figure. Ⅰ.12. Graphene to 2D semiconductor device progress and resolving the contact issue. 33
Figure. Ⅰ.13. (a) Excitons in 3D bulk and 2D monolayer. (b) Dimensionality of the electronic and excitonic properties represented by optical absorption. (c) Energy states of a Wannier-Mott exciton. 34
Figure. Ⅰ.14. (a) DBR cavity structure. (b) High quality microcavity reflectance spectra of an empty λ/2 cavity. 36
Figure. Ⅰ.15. Exciton-polariton dispersion (left) and their respective Hopfield coefficients (right) in three cases (a-b) when detuning energy δ＝negative energy. (c-d) δ＝0 and (e-f) δ＝positive energy. 43
Figure. Ⅱ.1. Device structure improvements for high performance MoS₂ FET. 48
Figure. Ⅱ.2. Large area TMDs transfer using (a) conventional PMMA and (b) residue free PPC supporting holder methods. 50
Figure. Ⅱ.3. Schematic and optical micrograph of residue free large area TMDs heterostructure in PPC and traditional PMMA transfer methods. TMD heterostructure fabrication in PMMA... 51
Figure. Ⅱ.4. Residue confirmation using AFM. (a) Optical micrograph and (b) AFM topography of PPC-transferred monolayer MoS₂ with high resolution AFM topography in (c). Similarly in... 52
Figure. Ⅱ.5. PMMA and PPC residue coverage comparison. AFM topography image of the monolayer MoS₂ on SiO₂ substrate transferred using conventional PMMA (a) and PPC (b) transfer... 53
Figure. Ⅱ.6. Electrical residue effects on monolayer MoS₂. Conducting AFM topography (top panel) and current mapping of the 1L MoS₂ transferred using (a) PPC, (b) PMMA methods. (c)... 54
Figure. Ⅱ.7. TEM analysis of the PMMA residue on the monolayer MoS₂ surface. (a,b) High and low magnifications TEM images. (c) EDS analysis of the PMMA residue compositions. 55
Figure. Ⅱ.8. Adsorption energy calculations using different van der Waals corrections. (a) PMMA and PPC trimer schematic diagram. (b,c) PMMA and PPC backbone and carbonyl group near to... 57
Figure. Ⅱ.9. Electronic structures and charge difference of the 1L MoS₂ with PMMA and PPC attachments using PBE + vdW (rvv10). Charge difference of (a) PMMA-backbone and carbonyl... 58
Figure. Ⅱ.10. Photoluminescence of monolayer MoS₂ (a) PL spectra of the monolayer MoS₂ under low excitation (b) high excitation power. (c,d,e) excitation power dependence PL intensity mapping. 60
Figure. Ⅱ.11. FET device and characteristics. (a) schematic of top gate FET structure. (b) Linear and logarithmic plot of typical transfer curve of FETs. (c) Typical output curves of FETs,... 62
Figure. Ⅱ.12. (a) TLM structure for contact resistance measurements. (b) Plot of Rtot versus different channel lengths with linear fitting, intercept at y-axis is 2Rc.[이미지참조] 63
Figure. Ⅱ.13. (a) Schematic model of MoS₂ FET device fabricated using PMMA (left) and PPC(right) methods along with a depiction of residues with the two methods. Top gate not shown in... 65
Figure. Ⅱ.14. Benchmark of Rc versus n2D in MoS₂ FET metal contacts for various semiconducting technologies. Black dashed line represents the quantum limit of (RC ≈ 0.026 (n2D)⁻⁰·⁵) as calculated...[이미지참조] 66
Figure. Ⅱ.15. (a) Temperature dependent output characteristics (Id-Vd) of the Device2 (R-T plot in the inset). (b) Arrhenius plots of Bi and Ti contact device. (c) Id vs T² plotted using thermionic equation.[이미지참조] 68
Figure. Ⅱ.16. Nonlinearity N versus Temperature plot, N＝0 in the PPC-Bi confirms ohmic contact between Bi and MoS₂. 68
Figure. Ⅱ.17. (a) Transfer characteristics (Id-Vg) comparison of Bi contact devices and (b) Ti contact devices. 69
Figure. Ⅱ.18. (a) Temperature dependent transfer curve between Bi and Ti (inset) contacts in monolayer MoS₂. (b) Transfer characteristics of the PPC transfer Bi contact device on h-BN... 70
Figure. Ⅱ.19. Electrical performance of the state-of-art device (Device1:h-BN/MoS₂/h-BN). (a) schematic diagram of the monolayer MoS₂ FET on h-BN substrate with top gate. (b) Transfer... 71
Figure. Ⅱ.20. (a) Optical micrograph of the MoS₂ FET (LCH＝1 μm) on SiO₂ substrate, (Device-2: SiO₂/MoS₂/h-BN). (b) Transfer curve at room temperature. (c) Output characteristics at room... 72
Figure. Ⅱ.21. (a) Optical micrograph of the PMMA transfer Bi-MoS₂ device. Transfer curve at room temperature (b) and low temperature at 15 K in (c). Output characteristics at room... 73
Figure. Ⅱ.22. (a) Ultra high on current in PPC-Bi state-of-the-art device at 15 K (LCH＝200 nm). (b) On current in PPC-Bi device on SiO₂ substrate (LCH＝200 nm). (c) Measured hysteresis of...[이미지참조] 74
Figure. Ⅱ.23. (a) On/off ratio vs. T plot for the Bi contact devices (PPC and PMMA). Bi contact device (Device1) shows an ultrahigh on/off ratio of 1.4x10¹¹ at 15 K due to residue-free interfacial... 75
Figure. Ⅱ.24. Electrical performances of the PPC transfer Bi contact WS₂ device. (a) device schematic and optical micrograph (b) of WS₂ device on SiO₂ substrate. (c) Output characteristics... 76
Figure. Ⅱ.25. (a) Two-probe field effect mobility (μFE) comparisons between Bi and Ti contact devices (PPC and PMMA). (b) Two-probe and four-probe mobility for Bi and Ti contact devices... 77
Figure. Ⅱ.26. Benchmark of on/off ratio versus RC in MoS₂ FET compared to different metal contacts used in semiconductor technologies.[이미지참조] 78
Figure. Ⅱ.27. Benchmark of the maximum on-current (Iₒₙ₋ₘₐₓ) versus on/off ratio of the MoS₂ FET(LCH＝200 nm) for PPC-Bi contact compared to black phosphorus (BP), MoS₂, metal oxides, and...[이미지참조] 79
Figure. Ⅲ.1. (a) Schematic of the Bottom DBR substrate. (b) Cross section SEM image of the Bottom DBR. (c) Reflectance spectra of the bottom DBR substrate demonstrates the optical... 84
Figure. Ⅲ.2. Device schematic of the h-BN/MoS₂/h-BN heterostructure with its optical micrograph image. 85
Figure. Ⅲ.3. (a) Schematic diagram of the device structure on SiO₂ substrate. (b) Optical micrograph image of the device. (c) Transfer characteristics (Id-Vg) of the device under dark...[이미지참조] 86
Figure. Ⅲ.4. (a) Subthreshold swing (SS) variation of the device under dark condition. (b) SS variation under light conditions. 87
Figure. Ⅲ.5. (a) Device schematic of the MoS₂ device on the bottom DBR substrate. (b) Optical micrograph of the device. (c) Transfer characteristics of the device under dark condition. (d)... 88
Figure. Ⅲ.6. (a) Subthreshold swing (SS) variation of the device under dark condition. (b) SS variation under light conditions on the bottom DBR substrate device. 89
Figure. Ⅲ.7. (a) Photocurrent and responsivity ratio comparison at 17 K on both DBR and SiO₂ substrate device. (b), (c), (d) Responsivity ratio variations at different temperature (100, 200, and... 90
Figure. Ⅲ.8. (a) Photocurrent, (b) photocurrent ratio, (c) responsivity, and (d) responsivity ratio variations at different temperatures for both DBR and SiO₂ substrates devices. 91
Figure. Ⅲ.9. (a) Mobility versus carrier density comparison at 17 K under dark and light conditions on the DBR-MoS₂ device. (b) Corresponding mobility variations with Vg. (c)...[이미지참조] 92
Figure. Ⅲ.10. (a) Schematic diagram of the monolayer WS₂ sample on a DBR substrate. (b) Optical micrograph of the device. 93
Figure. Ⅲ.11. Photocurrent and responsivity ratio comparison of all samples in different Vg on both substrate devices.[이미지참조] 94
Figure. Ⅳ.1. Polaritons of 2D vdW materials. Plasmon polariton in graphene and black phosphorus(BP), oscillating electrons at the surface of metal which creates a surface-plasmon polariton.... 97
Figure. Ⅳ.2. (a) Schematic of a DBR based cavity with monolayer Gr/h-BN/MoS₂/h-BN at the center of the cavity. (b) Optical micrograph of the device. 97
Figure. Ⅳ.3. DBR cavity: (a) PL spectra of the monolayer MoS₂. (b) 12.5 pairs of bottom DBR reflectance spectra. (c) Reflectance from the empty cavity. (d) PL spectra of the cavity loaded... 99
Figure. Ⅳ.4. (a) Transfer characteristics of the cavity-embedded FET with different source-drain voltages. (b) Output characteristics of the cavity-embedded FET with different Vgs.[이미지참조] 100