First-principles Study Of Heterojunctions Of Metal-transition Metal Dichalcogenide

Among various two-dimensional（2D）materials,monolayer transition-metal dichalcogenide（mTMD）have shown great potential as semiconductor materials for their considerable band gap（Eg,1-2 e V）and high carrier mobility and on/off ratio.MoS₂ is getting wide attention in low-power digital applications because of its unique physical characteristics（such as indirect-direct band gap transitions,atomic phase transitions,etc.）.However,performances of the devices have been always limited by the relative high contact resistance.Therefore,finding a solution to improve the contact performance is a hotspot research topic and yet to be studied.In this thesis,firstly,the basic properties of 2D materials,especially TMDs are introduced.The structural and electrical properties of different structural phases（1T and 2H phase）of MoS₂ are mainly summarized.The difficulties faced in the application of MoS₂ field effect transistors and the current research status are expounded.And based on the first-principles approach,analyzing various aspects of structural performance.For the metal-monolayer MoS₂ heterojunction,it is explained that the smaller lattice-mismatch is more conducive to bonding and high carrier mobility in the heterojunction interface.Our results show that metals with suitable metal work function and some d-orbital are more suitable for metal electrodes.Finally,the detailed analysis shows that the Fermi-level pinning at metal-semiconductor interface is the bottleneck of the metal-monolayer MoS₂ contact;there are two basic reasons: the interface dipole and the delocalization of Mo-d orbitals.For different metal-multilayer MoS₂ heterojunction,the interface of metal-MoS₂ and the MoS₂-MoS₂ interlayer interaction are analyzed respectively.The metal electrodes are divided into two types according different contact performance,type Ⅰ: the first layer of MoS₂（direct contact with metal）is strongly pinned but depinning between MoS₂ layers;type II: the first layer is metallized,and subsequent semiconductor layers are in close contact with the metallized layer to improve of the whole structure’ s performance.Then it is introduced in detail that increasing the number of semiconductor layers can help depinning by reducing the band gap,and type II band alignment is achieved between MoS₂ layers.Finally,it is proposed that both phase engineering and interlayer doping can improve the contact performance.This research will be helpful for a more systematic and thorough understanding of the electrical properties of metal-TMD heterojunctions,hoping to provide some theoretical basis and reference for the application in this field.