Two-dimensional Semiconductors And Their Heterostructures:Electronics And Edge Reconstruction

The search for semiconductor beyond the traditional silicon-based channels is helpful to solving the problem of the bottleneck in the national chip field.According to the Interuniversity Microelectronics Centre（IMEC）in 2020,there is nothing other than two-dimensional（2D）materials for further chip miniaturization.Due to the many advantages of 2D materials（flexible,ultra-thin,surface dangling bond free,and layer number adjustable）,they have potential applications in opto-and spin-electronic devices.However,there have some challenges for the application of 2D materials.On one hand,2D materials always have edges（which are analogous to the surfaces of three-dimensional（3D）materials）in preparation.The edges greatly affect the properties of a 2D material,such as semiconductor-metal transition,band gap reduction or even closing,edge magnetism,and charge/spin density waves,etc.On the other hand,the contact resistance is formed at the interface of metal-semiconductor junctions.High contact resistance and lack of p-type contact are serious problems in the application of the typical 2D semiconductor MoS₂-based devices.In this thesis,by theoretical analysis and simulation,monolayer and heterojunctions of new 2D materials,the MSi₂N₄ family,show potential applications in spintronic and opto-electronic devices.In addition,new models and deeper understanding of the edge reconstruction mechanism of MSi₂N₄ and TMDCs are presented.Moreover,to promote the application of 2D materials in complementary logical devices,a new method for reducing the p-type Schottky barrier height of metal-MoS₂ junctions is proposed and Ohmic contact is realized.The details of the thesis are as follows:Firstly,the electronic structure of H and T phases MSi₂N₄（M=Ti,Zr,Hf,V,Nb,Ta,Cr,Mo,W）monolayer are studied,and the d-level splitting（including crystal field and exchange field）and electron orbital filling are proposed to understand the electronic structure of different monolayers.Applications in spintronic devices are simulated for MSi₂N₄ monolayer and heterojunctions of the V-B group（M=V,Nb,Ta）,which exhibit rich electronic properties（such as magnetic semimetal,magnetic semiconductor,and bipolar magnetic semiconductor,etc,and have high Curie temperature）.The band alignment and momentum-space-match of MSi₂N₄heterojunctions are also studied,and showing potential applications in optoelectronic devices.Above all provides theoretical guidance for application in high（room）temperature spintronic and（opto-）electronics.Secondly,the intrinsic properties of two typical edges（armchair and zigzag nanoribbon）of MoSi₂N₄ are studied.The edge reconstruction of zigzag nanoribbon and its effect on metallization and electronic structure are investigated.It is proposed that the reconstructed model of adsorption N atoms can open a small band gap at edge in the 3×periodicities.The theoretical and edge reconstruction model of MoSi₂N₄are established.Thirdly,for deeper understand the mechanism of edge reconstruction,Mo edge reconstructions of H-Mo X₂（X=S,Se）zigzag nanoribbons are further studied,which have a large number of data and models for refere in experimental and theoretical.The deficiency of conventional electron counting rule in edge reconstruction is discussed and the mechanism of edge reconstruction is studied by using pseudo-hydrogen model.New mechanisms（the atoms on the edge not only occur structural reconstruction but electronic reconstruction,the valency increase or decrease）are proposed.And new models,which band gap is similar to the experiment results and the rationality is verified,are presented.Finally,in order to solve the bottleneck of 2D semiconductor in electronic devices,reducing the interface resistance of metal-semiconductor junctions is preliminarily studied.A binary compound（CuS）with weak metallic as the electrode is proposed to connect with the typical 2D semiconductor（MoS₂）.The contact types between different CuS surfaces and MoS₂ are simulated,and it is found that different CuS surfaces could achieve n-and p-type Ohmic contact.Moreover,the formula of p-type Schottky barrier height is modified.And it provid a theoretical model and new opportunities for p-type Ohmic contact experiments in the future.