Polaritonics in 2D van der waals layered TMDs : ideal contact resistance and gate tunability|RISS 상세보기

다국어 초록 (Multilingual Abstract)

Electronic and optoelectronic properties are based on the quantum mechanical effects of light on electronic materials and are among the emerging research areas for disruptive next-generation technologies. In particular, light excitations of traditional bulk semiconductors can generate numerous photoexcited particles such as photoexcited charge carriers, excitons, trions, bi-excitons, etc. These photoexcited particles drastically alter the electronic state of the material and can be utilized to develop next-generation electronic devices. The binding energy of such photoexcited particles is significantly reduced in traditional bulk semiconductors due to the screening effects from neighboring atoms in three dimensions. In contrast, the dielectric screening is notably reduced in two-dimensional materials such as transition metal dichalcogenides (TMDs), resulting in very high binding energies of the photoexcited particles up to 1 eV, much higher than the room temperature thermal excitation (~25 meV). Such high binding-energy particles can be stable under room temperature conditions and can be utilized for developing room temperature optoelectric device operations. For example, stable photoexcited excitons in TMDs can be coupled with external photons under strong coupling regimes in a microcavity structure and can result in a new type of light-matter quasiparticles called polaritons with strikingly low effective mass than the original excitons. Such a low effective mass polariton can result in intriguing electronic and optoelectronic effects and can be utilized for future electronics technologies. In this study, I focus on different optoelectronic and electronic properties of 2D TMD nanomaterials for the development of polariton-based electronic and optoelectronic device applications. Initially, the introductory concepts and characteristics are elaborately described in chapter I, including an overview of 2D vdW materials, crystal and band structure of TMDs, methods for synthesizing TMDs and their identification, van der Waals heterostructures, contact for 2D TMDs, 2D semiconductor FETs and finally, necessary details regarding exciton-polariton such as quantum well exciton, semiconductor microcavities, microcavity polaritons, strong and weak coupling, and dispersion of polariton states. Chapter II will demonstrate high electrical performance of the N-type semiconductor which is necessary to investigate polariton effects in this study. A polariton device fabricated with a traditional contact method cannot be modulated effectively for the intended polaritonics studies. Therefore, a residue-free cleaner large area TMD transfer was developed for ultra-high on/off ratio, high on-current, and low ohmic contact resistance with semimetal and semiconductor contact. Subsequently, phototransistor effects of different TMD nanomaterials were investigated by incorporating multiple photo excitations triggered by the distributed Bragg reflector (DBR) substrate and is elaborately described in Chapter III. In this study, a unique strategy was implemented in which phototransistor performance can be boosted by fabricating the device on top of a DBR. Monolayer molybdenum disulfide (MoS2) and tungsten disulfide (WS2) phototransistors were fabricated on DBR and SiO2 substrates for comparison and the optoelectronics properties were measured. Finally, electrically tunable polaritonic effects are investigated in Chapter IV. An electrically controlled MoS2 device was incorporated inside a DBR microcavity to generate exciton-polariton quasiparticles and controlled by an externally applied electrical field by using a few-layer graphene as a gate modulator. Different state of polaritons such as upper polariton branch and lower polariton branch was identified and controlled by gate modulation.

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목차 (Table of Contents)

Abstract 1
Chapter Ⅰ. INTRODUCTION 3
I.1 Overview of 2D vdW materials 3
I.2 Semiconducting Two-Dimensional (2D) Transition Metal Dichalcogenides 5
I.3. Crystal and Band structure of TMDs 6

Abstract 1
Chapter Ⅰ. INTRODUCTION 3
I.1 Overview of 2D vdW materials 3
I.2 Semiconducting Two-Dimensional (2D) Transition Metal Dichalcogenides 5
I.3. Crystal and Band structure of TMDs 6
I.4. Methods for synthesizing TMDs and their identification 9
I.4.1 Atomic force microscopy (AFM) of monolayer TMD 10
I.4.2 Photoluminescence (PL) spectra of monolayer TMDs 11
I.4.3 Raman spectra of monolayer TMDs. 12
I.5 Van der Waals heterostructures 14
I.5.1 Methods of Assembly 15
I.6 Contact for 2D TMDs. 17
I.6.1 Contact metal selection 18
I.7 Two-dimensional semiconductors field effect transistor 21
I.8 Exciton-Polariton 23
I.8.1 Quantum well Exciton 23
I.8.2 Semiconductor Microcavities 24
I.8.3 Microcavity polaritons 25
I.8.4 Strong and weak coupling 26
I.8.5 Dispersion of polariton states 28
I.9. Thesis Outline 33
Chapter II. Ohmic contact resistance and high on/off ratio in semiconducting TMDs via cleaner transfer 34
II.1. INTRODUCTION 35
II.2. Device structure 37
II.3. Device preparation and measurements 37
II.3.1 Residue free transfer of TMDs 38
II.3.2 Residue confirmation using AFM 40
II.3.3 Residue confirmation using conducting AFM (C-AFM) 42
II.3.4 PMMA residue confirmation using TEM analysis on TMD surface 43
II.3.5 DFT calculations 44
II.3.6 Photoluminescence study 48
II.4 Device fabrication 50
II.4.1 Device measurements 50
II.4.2 Contact resistance 51
II.5 Results and discussions 52
II.5.1 Contact resistance of MoS2 with different metal contacts 52
II.5.2 Performance of monolayer MoS2 and WS2 FETs with Bi, Ti contact 58
II.6 Comparative analysis of semiconductor technologies 67
II.7 Conclusions 70
Chapter III. Boosting Phototransistor Performance in Monolayer TMDs via Multiple Reflections from Distributed Bragg reflector 71
III.1. INTRODUCTION 71
III.2. DBR fabrication 72
III.3. Device fabrication and characterization 73
III.4. Results and Discussions 74
III.4.1 Phototransistor performance of 1L MoS2 on SiO2 substrate 74
III.5 Conclusions 84
Chapter IV. Gate modulated Polariton switching. 85
IV.1. INTRODUCTION 85
IV.2. Device structure 86
IV.3. Results and Discussions 87
IV.3.1. Optical characterization 87
IV.3.2. Gate modulation 89
IV.4. Conclusions 89
Chapter V. Summary and perspective 90
V.1. Summary 90
V.2. Perspective 91
References 93

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