Theoretical Study On Interfacial Properties And Performance Of Field-effect Transistors Based On Two-dimensional Materials

The field-effect transistors（FETs）,as an important part of integrated circuit,play a vital role in the the field of information science and technology.With the size of FETs into the nanometer,the short-channel effect is more and more prominent,resulting in the conventional Si-based FETs reaching their physical limits.Two-dimensional（2D）materials own atomic-thickness structures and dangling-bond-free surfaces,which makes them show superior electrostatic gate ability.Therefore,2D semiconductors are promising to supplement or even replace the traditional Si-based semiconductors.There are still challenges and problems when using 2D semiconductors as channel materials for short-channel FETs.Firstly,due to their ultra-thin thickness,no suitable and controllable doping methods for 2D materials have been proposed so far.Thus,the 2D semiconductors usually contact with metal materials directly for carrier injection.However,Schottky barriers are usually generated at the 2D semiconductor-3D metal interfaces,which greatly weaken the performance of the FETs.How to reduce or even eliminate the Schottky barrier is an urgent problem to be solved.Secondly,what is the performance limit and how to improve the performance of the 2D FETs,and whether the 2D FETs can be applied in many aspects are still needed to be studied.In this thesis,we have systematically investigated the above problems by using the density functional theory combined with the nonequilibrium Green’s function method.In the third,fourth and fifth chapters of this thesis,we study the problems in the first acpect;In the sixth and seventh chapters of this thesis,we focus on the problems in the second acpect.The main research contents and results are as follows:1.The interfacial properties of the 4-h T² graphdiyne and metalsThe 4-h T²graphdiyne with porous structure,sp¹ and sp² hybridization is a semiconductor with suitable band gap and high carrier mobility,which shows great application prospects in many fields.We explore the interfacial properties of the 4-h T² graphdiyne and metal electrodes（Cu,Au,Ni,Al,Ag,Cr₂C,Ta₂C and V₂C）with different work functions.The results demonstrate that the4-h T²graphdiyne can form ideal Ohmic contact with all considered metals at both vertical and lateral interfaces,and the weak Fermi level pinning.Taking the Au electrode as an example,the I-V characteristics of the FET are calculated.The current linearly changes with the voltage change,further demonstrating the Ohmic contact formed in these systems.The results show that 4-h T²graphyne can be used as an ideal channel material.2.The interfacial properties and the regulation methods of the anisotropic C₂N and metalsBecause of its suitable band gap and high carrier efficiency,the monolayer C₂N shows extensive application prospects in the field of next-generation optoelectronic devices.We systematically study the interfacial properties and regulation methods of the anisotropic monolayer C₂N and different metal materials（Al,Ti,Ni,Cu,Ag,Pt,V₂C,Cr₂C and graphene）.The results show that the interfacial properties are related to the work function of the electrode materials,the electronic properties of the C₂N,and the strength of the Fermi level pinning.In addition,the regulation methods of the interfacial properties are studied.The Fermi level pinning can be effectively weakened by inserting h-BN/graphene into the C₂N-Pt system with Schottky contacts,and thus,forming ideal Ohmic contact.This work provides theoretical guidance for understanding and controlling the interface properties of short channel FETs based on C₂N.3.The interfacial properties and sub-5 nm FETs of blue-phosphorene phaseⅣA-ⅥA semiconductors and MXenesAt the 2D semiconductor-2D metal interfaces,the weak interaction is usually formed,beneficial to avoiding formation of the Fermi level pinning.The blue-phosphorene phaseⅣA-ⅥA semiconductors are a kind of semiconductors with suitable band gap and high carrier mobility,which are expected to become channel materials for FETs.2D MXenes materials with different saturated groups exhibit strong stability and metallic properties,and are a class of qualified electrode materials.We study the vertical and lateral interfacial properties of the MXenes with different saturation groups andⅣA-ⅥA semiconductors in blue-phosphorene phase.In addition,based on the systems with Ohmic contact,we study the performance of the sub-5 nm FETs.The results show that-OH terminated MXenes and Ti₃C₂O₂are in favor of forming Ohmic contact with semiconductors.And,the 5 nm gate-length FETs show large on/off ratio.After introducing optimizing strategies,the 3 nm gate-length FETs also exhibits large on/off ratio,and the gate control ability improves obviously with the swing slope reducing below the thermal limit.The results provide theoretical guidance for the realization of Ohmic contact and high-performance short-channel FETs in two-dimensional nanoelectronics.4.The novel 2D Janus WSi₂P₂As₂ based sub-5 nm FETsSearching or designing competent 2D semiconductors for realizing high-performance FETs is crucial to device miniaturization.Here,based on the newly synthesized Mo Si₂N₄in 2020,we predict a new 2D Janus material WSi₂P₂As₂,and explore its application in the short-channel FETs.The results demonstrate that the monolayer WSi₂P₂As₂ is a semiconductor with appropriate band gap and carrier mobility,and the performance of the sub-5 nm WSi₂P₂As₂-based FETs can fulfill the International Technology Roadmap for Semiconductor（ITRS）for high-performance（HP）standards.This work provides a theoretical reference for designing the 2D materials for short-channel FETs.5.The switching and gas sensing functions based on the Mo Si₂As₄ FETsThe Mo Si₂As₄,as a new 2D semiconductor with appropriate band gap and high mobility,exhibits a broad application prospect in many fields.We propose a multifunctional device based on the monolayer Mo Si₂As₄,which can realize switch and gas sensing functions.The key performance of the single-gate 5 nm gate-length Mo Si₂As₄ can satisfy the ITRS for HP requirements,and the FET also shows large on/off ratio.Moreover,the Mo Si₂As₄-based FET-type gas sensor also exhibits the high sensitivity to NO₂ and NH₃,and the sensitivity can be further improved by gate voltage.Our results provide theoretical guidance for manufacturing of multifunctional devices with high performance in experiments.This thesis consists of the following eight chapters.The first chapter is the introduction,which briefly introduces nano-electronics and several kinds of 2D materials.In chapter 2,the theoretical calculation methods are introduced,including density functional theory used in the calculation of bulk properties and density functional theory combined with non-equilibrium Green’s function method used in the calculation of device transport properties.Chapter 3 to chapter 8 briefly introduce the content and results of works,including the calculation model,methods and the relevant results.The third chapter introduces the interfacial properties of 4-h T² graphdiyne contacted with metals.Chapter 4 gives the interfacial properties of anisotropic C₂N contacting with different metals,and the modulation method to improve the interfacial properties.In the fifth chapter,we select the 2D metallic MXenes with different terminal groups as the metallic electrode,and studied the interface properties of them with blue-phosphorene phaseⅣA-ⅥA semiconductors.In addition,based on the Ohmic contact systems,we further study the performance of its sub-5 nm FETs.In the sixth chapter,we design and study the stability and electronic structure of the new 2D Janus WSi₂P₂As₂,and calculate the performance limit of the sub-5 nm FETs.In chapter 7,we study the multifunction of the single-gate sub-5 nm FET,which integrates the switching and gas sensing functions.Finally,chapter 8 is the conclusion and prospect,which is the comprehensive and detailed conclusion of the thesis,and the developments of the 2D short-channel FETs in the future.