金属-过渡金属二硫化物异质结的第一性原理研究

FIRST-PRINCIPLES STUDY OF HETEROJUNCTIONS OF METAL-TRANSITION METAL DICHALCOGENIDE

张文珺

11849417

硕士

物理系

石兴强

2020-05-29

2020-05-29

哈尔滨工业大学

深圳

在各种二维材料中，单层过渡金属硫族化合物，因为具有可观的带隙（Eg, 1-2eV）和较高的载流子迁移率和开/关比，已显示出作为半导体材料的巨大潜力。MoS2更是由于其独特的物理特性（例如依赖于层数的直接-间接带隙转换以及原子尺度的相变等），在低功耗器件应用中受到广泛关注。然而较高的接触电阻一直限制着器件的性能，因此寻求提高接触性能的解决办法一直以来都是这一领域的一大研究重点和难点。本论文基于第一性原理计算，对金属-单层MoS2和金属-多层MoS2异质结的接触性能做了系统的研究和分析。本论文对二维层状材料和过渡金属硫族化合物的基本性质做了系统介绍。重点归纳了MoS2的两种结构相（1T相和2H相）的结构特性和电学性质，阐述了目前在场效应晶体管应用中面临的困难和国内外的研究现状。并且基于第一性原理的方法，对结构性能做了各方面的分析。在不同的金属-单层MoS2异质结中，从接触界面出发，说明较小的晶格失配更有利于异质结界面的成键和载流子迁移，而且具有合适金属功函数和一些d轨道金属更适合做金属电极。详细分析了界面费米能级钉扎是金属-单层MoS2接触的瓶颈所在，其根本原因主要有两个：界面电偶极矩的影响和Mo原子的d-轨道态的弥散性。针对不同的金属-多层MoS2异质结，我们对接触界面和MoS2层间作用分别做了分析。把金属电极根据和MoS2的接触效果分成了两种类型：一类是MoS2的第一层（和金属直接接触的一层）存在强钉扎但后续层发生层间减钉扎进而提高整体性能；另一类是第一层MoS2被金属化，后续层和被金属化层接触而提高整体性能。并且详细介绍了增加MoS2层数可以通过减小整体带隙进而减弱钉扎效应，而且还在MoS2层间实现了Ⅱ型能带排列；最后提出原子尺度的相变工程和层间掺杂都可以使接触性能得到改善。这项研究有助于对金属-过渡金属二硫化合物异质结的电学性质更系统且深入的理解，并且希望能为这一领域的应用提供一些理论依据和参考。

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 eV) 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.

MoS2 金属-半导体异质结 密度泛函理论 场效应晶体管 接触特性 肖特基势垒

MoS2 metal-semiconductor junctions density functional theory field effect transistor contact characteristics schottky barrier

中文

联合培养

南方科技大学

张文珺. 金属-过渡金属二硫化物异质结的第一性原理研究[D]. 深圳. 哈尔滨工业大学,2020.