Low frequency noise analysis in 2D FET and diode : black phosphorus and rhenium disulfide

다국어 초록 (Multilingual Abstract)

Silicon-based semiconductors have undergone the following changes (Planar FET → Fin FET → GAA FET → MBC FET) as devices become smaller. This is an attempt to gradually change the structure to increase the gate control, and while getting smaller, the device must satisfy better performance, high integration, and lower power consumption. In order to make the device smaller with these various advantages, the limit has been reached with only silicon material, and 2D materials with proper bandgap begin to emerge as an alternative to break through this. Since silicon basically has sp3 orbital, dangling bonds are formed at the interface or edge part, which has a greater effect as the device becomes smaller. On the other hand, in the case of 2D materials, most of them have sp2 bonds, so theoretically, there are almost no defects, so a stable structure is achieved. From this point of view, it is more advantageous because the scattering of mobility is relatively small up to the channel thickness up to 1nm.
Van der Waals 2D materials are composed of transition metal dichalcogenides (TMDs) and mono-elemental materials, and are attracting great interest in research because of their various properties. Physical phenomena such as quantum Hall effect, exciton condensation, quantum phase transition, and quantum oscillation and excellent device characteristics were confirmed for various thickness from monolayer to bulk. Nevertheless, studies on degradation, carrier scattering mechanism, channel migration, and heterojunction interface trap remain unsolved mysteries.
In particular, the device was fabricated using BP and ReS2 among various 2D materials, and the channel and junction were mainly studied through noise measurement. In the channel, the effective tunneling length according to the channel movement was mainly studied through noise measurement using ReS2, which has higher interlayer resistance compared to other laminated materials. The size of noise contains information about scattering, trap and defect, and it changes according to the effective channel distance. That is, it has been proven that the channel moves through the tunneling effective length. For interface analysis, BP/ReS2 devices were fabricated in a stacked structure. The origin of fluctuation according to carrier transport mechanism (BTBT, DT and FNT) was investigated through noise measurement. In particular, when the carrier transport mechanism changes, it was confirmed that the fluctuation mechanism changed from carrier number fluctuation to mobility fluctuation.
Here, we first studied the degradation of black phosphorus (BP) materials that are promising as optoelectronic devices and have direct bandgap. BP reacts with O2 and H2O molecules in the air to form bubbles, and electrical properties were evaluated over time. Various parameter such as threshold voltage (VTH), carrier mobility (μ), contact resistance (RCT), and channel resistance (RCH) were estimated using the Y-function method over time. It is found that RCT reduces and then, increases with time, whereas the behavior of RCH is vice versa.
Second, the channel migration phenomenon in multilayer ReS2 was described as a combination of Thomas fermi charge screening length and interlayer resistance. It is a phenomenon that occurs in all 2D materials, and the weaker the interlayer coupling is, the better it is. Direct evidence of channel migration, two peaks of transconductance (gm) - VBG curve, was observed, indicating the formation of top and bottom channels. Channel movement was verified through LF noise measurement, and simulation was conducted based on the theoretical resistor network model. In addition, the change in the degree of coulomb scattering was also shown for the interlayer resistance that varies depending on the drain voltage.
third, a study was conducted on BP/ReS2 heterojunction, and how fluctuation varies according to the current mechanism in the gate tunable diode was explained. When the drift current is dominant and is therefore not affected by the trap. However, in the case of direct tunneling, since it has to pass through the interface and the BP, it has to pass through internal traps, inducing fluctuation. In addition, it was observed that the trap causes carrier number fluctuation or mobility fluctuation according to the fermi level.
In conclusion, the process of contact and channel change as it degrades in air in BP among 2D materials was studied in connection with electrical characteristics and noise measurement. In ReS2, the channel movement phenomenon by Rint and Rch, which changes according to the gate and drain voltages, was demonstrated. Additionally, a decrease in coulomb scattering caused by changing Rint was also demonstrated. Here, we experimentally revealed the channel shifting phenomenon through the two-peak transconductance curve for the first time. Through this phenomenon, 2D-based FET has different characteristics from conventional silicon, and it has been demonstrated that effective contact resistance and the degree of scattering within the device change when the channel moves from bottom to top. This is a very critical point and cannot be interpreted as a conventional metal-semiconductor phenomenon. Finally we have demonstrated the current distribution in 2D materials, which we hope can serve as a milestone in the fabrication of promising next-generation semiconductors considering device structure, thickness and materials.

목차 (Table of Contents)

• 1. Introduction 6p
• 1.1 Two-dimensional (2D) material 6p
• 1.2 Potential application of two-dimensional (2D) material 7p
• 1.3 Thesis outline 11p
• 2. Principle and electrical characterization of 2D based devices 19p
• 2.1 Introduction 19p
• 2.2 Principle of field effect transistor (FET) and diode 20p
• 2.3 Where does the current flow in multilayer system? 26p
• 2.4 Electrical characterization 30p
• 2.5 Low frequency noise analysis 33p
• 2.6 Conclusion 40p
• 3. Black phosphorus (BP): structure, transport and degradation 43p
• 3.1 Introduction 43p
• 3.2 The structure and properties 43p
• 3.3 Degradation mechanism and electrical properties. 49p
• 3.4 Low frequency noise analysis of BP FET 56p
• 3.5 Conclusion 60p
• 4. Rhenium disulfide (ReS2) : structure, transport, channel migration 63p
• 4.1 Introduction 63p
• 4.2 The structure and properties 63p
• 4.3 Channel migration effect via transfer curve 67p
• 4.4 Channel migration effect via output characteristic curve 78p
• 4.5 Conclusion 83p
• 5. Tunneling noise analysis in BP/ReS2 hetero junction 86p
• 5.1 Introduction 86p
• 5.2 Type of tunneling mechanism and thickness dependent heterojunction 88p
• 5.3 Electrical properties of heterojunction (BP/ReS2) 93p
• 5.4 Tunneling low frequency noise analysis 99p
• 6. Conclusion 110p
• 7. Journal of publication 112p
• 8. Appendix (Python code) 113p