First-principles Study Of Electronic Structures And Device Quantum Transport Properties Of Two-dimensional Layered Materials

With the further refinement of semiconductor manufacturing,the proportional scaling of field-effect transistors is getting closer and closer to the atomic scale.Quantum effects have become non-negligible and will dominate the physics of sub-ten nanometer devices.On the one hand,quantum confinement,quantum tunneling,etc.affect and restrict the performance of nanodevices.On the other hand,quantum mechanisms have become the working principle of some emerging devices,such as tunneling field effect transistors?TFET?,which break through the traditional Boltzmann limit,making the application of ultra-low power integrated circuits possible.Therefore,it is necessary to deeply understand the quantum mechanism in the device,and perform modeling and simulation based on quantum mechanics at the atomic scale.In addition,the ultra-thin two-dimensional?2D?layered materials represented by graphene,transition metal sulfide,and black phosphorus have become ideal channel materials for next-generation electronic devices due to their excellent electrical characteristics?high carrier mobility,moderate band gap?.Therefore,we carried out the following researches on electronic devices based on two-dimensional materials:?1?First-principles prediction of two-dimensional In-V tunneling field-effect transistor performanceTunneling field effect transistor is a kind of new mechanism quantum device which works on the principle of band-to-band tunneling,and its practical application is limited by the small on-state current.Based on the first-principles quantum transport calculation method,which combines density functional theory with non-equilibrium Green's function?DFT-NEGF?,this work took the lead in predicting the electronic structure and device transport properties of a new class of two-dimensional III-V compound InAs,InN,InP and In Sb materials.Studies have shown that a two-dimensional InSb transistor with a gate length of 15.2 nm can achieve an on-state current up to 1058?A/?m?p-type?and 880?A/?m?n-type?,and the InAs transistor can achieve a current switching ratio of 10⁷,both exhibiting excellent application prospects in the field of high-performance and low-power devices,respectively.Based on this,it is found that the shortening of gate length will cause the fluctuations of sub-threshold swing and the increase of the leakage current.In addition,the state-density bottleneck effect severely limits the performance of single-layer InN and InP transistors.This study will facilitate the experimental preparation and application of mono-layer III-V compounds.?2?Effect of defect-assisted tunneling on the performance of phosphorene tunnel field-effect transistorsWe studied the ballistic transport performance of phosphene double-gate TFET with a 8.8 nm channel length by using quantum mechanical transport modeling.Through the introduction of defect states in the band gap,an effective scheme for improving the on-state current of TFET devices is found.The simulation results show that the performance limit?on-state current and sub-threshold swing?of hydrogen adsorption and phosphorus adsorption phosphorene field-effect transistors can meet the application requirements of the international semiconductor technology roadmap for high-performance logic devices.On this basis,we also considered the effect of spin polarization on the inter-band assisted tunneling process,and discussed the influence of the position of the defect states and the value of defect density states on the transmission probability of the device.?3?Inelastic transport properties of monolayer MoS₂ nanodevicesWith the scaling of transistor size,phonon-assisted tunneling plays an increasingly important role in the performance of electronic devices.We use the STD-Landauer method based on non-equilibrium Green's function to perform atomic scale inelastic simulations of ultra-scaled single-layer MoS₂ devices.The results show that the electron phonon coupling effect at a finite temperature not only improves the band-to-band tunneling current in the MoS₂ p-i-n junction,but also significantly increases the source-to-drain direct tunneling current in the TFET,which makes the device's switching performance impaired.?4?Transport properties of two-dimensional Van der Waals heterojunction WSe₂-MoS₂ vertical tunneling field-effect transistorsHeterogeneous structure as a new degree of freedom to regulate the properties of two-dimensional materials,is causing widespread concern.This chapter theoretically studies the inter-layer tunneling mechanism and the electronic transport properties of the vertical stack of WSe₂-MoS₂ heterojunctions.The double-layer WSe₂-MoS₂heterojunction forms a type II band arrangement with an effective band gap of 0.14eV.When the source-drain bias V_DD=0.3 V,the system can achieve a current switching ratio of 3×10⁴?A/?m and an excellent sub-threshold slope of 59mV/decade.