First-principles Study Of Curvature Tunes Mobility In Two-dimensional Semiconductors

Two-dimensional semiconductors are excellent candidates for replacing silicon-based semiconductors due to their atomic-level thickness and excellent electrical properties,providing a good idea for continuing the development trend of transistor performance in the post-Moore era.An important property of them is the flat and dangling-bond-free surface,which provides a solid platform for the carrier ballistic transport.It is also supported by the deformation potential approximation（DPA）theory that ignores optical phonon scattering.However,the carrier mobility obtained in practical experiments often shows an order of magnitude decrease compared to the theory.The first-principles calculations considering the electron-phonon interaction show that two-dimensional semiconductors are more susceptible to optical phonons than bulk materials,so that the material itself can cause valley scattering or impurity defect backscattering on the carrier transport speed severely restricted.Carrier mobility,as an important transport property of semiconductor materials,directly affects the switching speed and power consumption of devices in applications.It is very necessary to systematically study the influence of 2D semiconductor electron-phonon interaction and explore the regulation means of interaction.In this thesis,density functional theory（DFT）and Vanier function representation are used to solve first-principles electron-phonon coupling.Based on density general function theory（DFT）and Wannier function representation to solve first principle electroacoustic coupling,the scattering of carriers by acoustic and optical phonons is investigated based on a review of theoretical and experimental studies of carrier mobility in two-dimensional semiconductors,represented by two-dimensional layered transition metal-sulfide compounds（TMDCs）,boron arsenide（BAs）and single-element arsenene（As）and antimonene（Sb）patterns,and the effects of intra-layer stress and curvature modulation on electro-acoustic coupling behavior as well as mobility were investigated in depth,as follows:（1）The electro-acoustic coupling calculations（EPW）under the Wannier surface were used to compare the electro-phonon scattering and its electronic energy band structure for two-dimensional MX₂（M=W,Mo;X=S,Se）TMDCs as well as two-dimensional As,Sb and BAs.The electron and phonon linewidth calculations reveal that inter-energy valley scattering involving long-wave vector optical phonons has a crucial effect on the carrier mobility of two-dimensional semiconductors.This is due to the deviation of the conduction band bottom and valence band top of several materials mentioned above from theΓpoint,which generates multiple valleys and multiple valley scattering due to the characteristic inverse spatial symmetry of the crystal.The electroacoustic scattering of several structures of 2D TMDCs,As and Sb all exhibit further enhancement when deviating from the bottom of the conduction band and the top of the valence band,from which it is inferred that they are not only affected by symmetry-induced multi-valley scattering,but also because there are other neighboring valleys in the energy range of phonon frequencies,and the inter-valley jumps are further enhanced;Two-dimensional BAs are only affected by multi-valley scattering caused by symmetry,and the unique valley structure of BAs allows them to close other interval scattering channels in addition,obtaining high carrier mobility.The electron mobility（2853 cm²/Vs）and hole carrier mobility（4605 cm²/Vs）of 2D BAs at room temperature were finally predicted,and it was also revealed that eliminating inter-valley scattering and suppressing inter-valley leap can effectively reduce the electro-phonon scattering intensity,thus obtaining high carrier mobility materials.（2）In addition,a method to increase the carrier mobility of 2D As and Sb by biaxial strain was designed.It was found that the scattering between the conduction band valleys was greatly reduced and the carrier relaxation time was greatly increased under strain,resulting in the increase of the electron mobility of 2D As and Sb from 58cm²/Vs and 49 cm²/Vs to 1629 cm²/Vs and 928 cm²/Vs,respectively.On this basis,the effects of curvature modulation and in-plane strain on the mobility of As are compared,and the changes of electrical properties and electron-phonon scattering with the change of nanotube curvature are investigated in depth.Comparing the effects of in-plane strain,the results show that the bond lengths of the two semiconductor materials do not change significantly as the nanotube diameter decreases and the curvature increases,and the strain due to curvature is mainly manifested in the change of bond angles.Therefore,curvature modulation does not achieve the same effect of in-plane stress on carrier transport in most cases,but instead causes an increase in effective mass and a decrease in mobility due to the change in inter-atomic hopping between the sub-nearest neighbors.