DFT study of graphene doping due to metal contacts

The experimental results of Metal\u2013graphene (M\u2013G) contact resistance (RC) have been investigated in\u2013depth by means of Density Functional Theory (DFT). The simulations allowed us to build a consistent picture explaining the RC dependence on the metal contact materials employed in this work and on the applied back\u2013gate voltage. In this respect, the M\u2013G distance is paramount in determining the RC behavior

Physics based modeling for 2D semiconductors as baseline materials for novel electron devices

Scaling CMOS technology has been the cornerstone of the continued progress in the silicon-based semiconductor industry. Nowadays, the FinFETs and nanosheet transistors are the most advanced device architectures respectively in production and under development in the industry. However, due to the many constraints posed by short-channel effects and to the limitations due to extrinsic resistive and capacitive components, the scaling of transistors has become an increasingly challenging task. Two-dimensional semiconductors are attractive materials for nanosheet transistors and for many other prospective applications, thanks to their very good intrinsic transport properties compared to 3D semiconductors at the same layer thickness. The discovery of graphene and then the development of transition metal dichalcogenides raised high expectations for a new and wide family of two-dimensional crystalline materials with remarkable electronic, mechanical, and optical properties. However, there are still many concerns about the limitations of 2D materials and several hurdles to reach the industrial maturity. In order to overcome such limitations, a physical understanding of novel electron devices based on 2D crystals is vital. After an introduction about 2D materials presented in chapter 1 of this thesis, in the second chapter we report a simulations based study of metallic contacts to 2D materials. In fact, one of the key challenges preventing the harnessing of good intrinsic transport properties of 2D crystals is the poor quality of the contacts between metals and 2D materials. First, we show that Cu and Ni largely dope graphene at the minimum energy distance, whereas a long-range interaction is predicted for Au-graphene contact. Then we discuss by using ab-initio simulations the Fermi level pinning in defects free metals to MoS2 contacts. Then by using an ab-initio transport methodology, we investigate the contact resistance between several metals and MoS2. Our results examine quantitatively the trade-off between Schottky barrier height and tunneling barrier in contacts with a buffer layer and confirmed by simulations the superior performance of the bismuth-MoS2 n-type contact. Chapter 3 is focused on sensors based on 2D materials. In this chapter, we first revisit the problem of the linearized Boltzmann transport equation for mobility calculations and the formulation of different scattering mechanisms. Then we use our mobility calculations to investigate piezo-resistance in MoS2, and in particular for the interpretation of a giant intrinsic Gauge factor experimentally observed in monolayer MoS2 This intrinsic piezoresistive can enable emerging applications in tactile sensing as well as improving the electronic transport in TMD electronics. The analysis in chapter 3 continues with the analysis and comparison with experiments for the temperature coefficient of resistance in MoS2, with fast temperature sensors as a prospective application. In the last part of chapter 3, we address some possible options for gas sensors based on the 2D materials. First we investigate the 2D Mxene as a potential ammonia sensor employing a first-principles study. Then we focus on fluorinated graphene as a potential material for humidity sensing applications. Finally in chapter 4, by utilizing a multi-valley Monte Carlo transport simulator, uniform field transport in 2D MoS2 is analyzed. Our preliminary results for the high field uniform transport regime show that the electron’s saturation velocity in monolayer MoS2 is only slightly affected by scattering with Coulomb centers and neutral defects, while the effect of surface optical phonons is more subtle and it is, at the time of writing, still partly under investigation.Scaling CMOS technology has been the cornerstone of the continued progress in the silicon-based semiconductor industry. Nowadays, the FinFETs and nanosheet transistors are the most advanced device architectures respectively in production and under development in the industry. However, due to the many constraints posed by short-channel effects and to the limitations due to extrinsic resistive and capacitive components, the scaling of transistors has become an increasingly challenging task. Two-dimensional semiconductors are attractive materials for nanosheet transistors and for many other prospective applications, thanks to their very good intrinsic transport properties compared to 3D semiconductors at the same layer thickness. The discovery of graphene and then the development of transition metal dichalcogenides raised high expectations for a new and wide family of two-dimensional crystalline materials with remarkable electronic, mechanical, and optical properties. However, there are still many concerns about the limitations of 2D materials and several hurdles to reach the industrial maturity. In order to overcome such limitations, a physical understanding of novel electron devices based on 2D crystals is vital. After an introduction about 2D materials presented in chapter 1 of this thesis, in the second chapter we report a simulations based study of metallic contacts to 2D materials. In fact, one of the key challenges preventing the harnessing of good intrinsic transport properties of 2D crystals is the poor quality of the contacts between metals and 2D materials. First, we show that Cu and Ni largely dope graphene at the minimum energy distance, whereas a long-range interaction is predicted for Au-graphene contact. Then we discuss by using ab-initio simulations the Fermi level pinning in defects free metals to MoS2 contacts. Then by using an ab-initio transport methodology, we investigate the contact resistance between several metals and MoS2. Our results examine quantitatively the trade-off between Schottky barrier height and tunneling barrier in contacts with a buffer layer and confirmed by simulations the superior performance of the bismuth-MoS2 n-type contact. Chapter 3 is focused on sensors based on 2D materials. In this chapter, we first revisit the problem of the linearized Boltzmann transport equation for mobility calculations and the formulation of different scattering mechanisms. Then we use our mobility calculations to investigate piezo-resistance in MoS2, and in particular for the interpretation of a giant intrinsic Gauge factor experimentally observed in monolayer MoS2 This intrinsic piezoresistive can enable emerging applications in tactile sensing as well as improving the electronic transport in TMD electronics. The analysis in chapter 3 continues with the analysis and comparison with experiments for the temperature coefficient of resistance in MoS2, with fast temperature sensors as a prospective application. In the last part of chapter 3, we address some possible options for gas sensors based on the 2D materials. First we investigate the 2D Mxene as a potential ammonia sensor employing a first-principles study. Then we focus on fluorinated graphene as a potential material for humidity sensing applications. Finally in chapter 4, by utilizing a multi-valley Monte Carlo transport simulator, uniform field transport in 2D MoS2 is analyzed. Our preliminary results for the high field uniform transport regime show that the electron’s saturation velocity in monolayer MoS2 is only slightly affected by scattering with Coulomb centers and neutral defects, while the effect of surface optical phonons is more subtle and it is, at the time of writing, still partly under investigation

Modeling Low and High Field Uniform Transport in Monolayer MoS2

We have developed a multi-valley Monte Carlo simulator to study uniform transport in MoS 2 monolayers. At low electric field, our solver is in excellent mutual agreement with a numerical solution of the linearized Boltzmann Transport Equation. We have then explored high field transport and analyzed the influence of different scattering mechanisms on the electron saturation velocity. Although scattering with neutral defects and Coulomb centers strongly affects the mobility, the effect on the saturation velocity is only modest. On the other hand, scattering with surface optical phonons has a significant influence on the saturation velocity, which we physically interpreted by inspecting the energy and momentum distributions of carriers

A study of metal-MoS2 contacts by using an in-house developed ab-initio transport simulator

By combining DFT calculations and an in-house developed, ab-initio transport simulator, we investigate the transport properties of n-type contacts between 3D metallic conductors and monolayer MoS2. Moreover, the impact of buffer layers between the metal and the MoS2 is also analyzed in order to reduce the Schottky barrier and approach the Ohmic behavior of contacts