| As a new material in the 21 st century,two-dimensional nanomaterials provide new options for electronic devices in the post-Moore's law era.Two-dimensional transition metal dichalcogenides(TMDCs)represented by Mo S2 are semiconductors,which have special band structures and excellent mechanical properties.They have been widely used in nanoelectronic devices,optoelectronics,and valley electronics.It has become a research focus in the field of two-dimensional materials in recent years.The energy band structure is the basis for the theoretical research of semiconductor materials and devices.The general form of the band structure requires solving the Schrodinger equation.The k·p method is a semi-empirical method for calculating the energy band structure.It can obtain accurate Hamiltonian and band dispersion relation expression(E(k))near the extreme point.It is of great significance for studying the physical properties of materials and expanding their application fields.Based on the k·p method,the paper conducts systematic research on the matrix model,energy band structure,electrical characteristics,and strain characteristics of 2D-TMDCs.The main research contents and results are as follows:(1)Based on the lattice structure characteristics and group symmetry of the 2H-TMDCs,the paper selects 11 eigenstates at the K point.With the k·p method and theory of invariants,the non-zero momentum matrix elements in the k·p Hamiltonian are determined.Under the second-order perturbation approximation,the expression of E(k)and the effective mass near the K(K?)valley are given.The rule of optical transition between energy bands in the K(K?)valley is analyzed.(2)Using the first-principles theory,the paper calculates the values of parameter in multiband k·p Hamiltonian.According to the k·p Hamiltonian,the monolayer Mo S2,WS2,Mo Se2,and WSe2 band structure and electrical properties are calculated.The results show that the effective mass is almost isotropic.Compared with Mo S2 and Mo Se2,WS2 and WSe2 have smaller effective mass and higher carrier mobility.(3)Using the method of L(?)wdin perturbation,the paper establishes a third-order perturbation 2k·p Hamiltonian including the bottom band of the conduction and the top band of valence band.The parameters in the 2k·p matrix model are described based on the multi-band k·p matrix.The numerical analysis of the k·p method is used to analyze the electron-hole asymmetry,trigonal warping,and cubic terms in the K valley.The contribution of different energy bands to these effects and the effects on the band structure and effective mass are analyzed.The results show that the top of the valence band has a more pronounced trigonal warping,and the effective mass along highly symmetrical directions in the K valley has a slight difference of about 0.02 m0 due to the trigonal warping.(4)Based on the theory of invariants and the Pikus-Bir strain Hamiltonian model,the paper establishes a multiband strain perturbation matrix model at the K valley.The paper calculates the band structure of the monolayer Mo S2,WS2,Mo Se2 and WSe2 near the K valley under uniaxial and biaxial stress.According to the band structure,the energy shift,carrier transport characteristics,and circular dichroism under different stress are calculated and analyzed.The results show that the uniaxial and biaxial tensile stress can improve the carrier mobility which can be used to modify the properties of 2D-TMDCs.Furthermore,the strain has a good regulation on the Berry Curvature.The tensile strain makes it significantly increase,and the compressive strain makes it decrease,which provides a reference for the application of its valley Hall effect.The strain does not affect the valley selectivity at the K(K?)valley.In addition,the paper also considers the effect of the strong spin-orbit coupling effect of the transition metal element,and the strain has the same band shift effect on the spin-splitting energy band as it does when unstrained. |
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