Study Of The Effect Of Defects And Their Diffusion Dynamics On The Properties And Stability Of New-Type Semiconductors

Modern semiconductor technology promotes the rapid development of human society,while needs from different fields inspire researchers to develop new semiconductor materials with various functions.The combination of advanced experimental instruments and high-performance theoretical calculations also makes targeted developments of new semiconductor materials more and more convenient,and various new materials have come out consecutively.Many experimental and theoretical studies on the properties of defects in new semiconductor materials have also emerged.However,there are still some unexplained points in the understanding of the interaction between defects and materials:1)How to understand the change in the diffusion behavior of the electrode metal atoms within semiconductor during the transition of dimension from three-dimension to two-dimension in the device;2)The role of the Cu atom 3d levels in the anharmonicity and ultrafast ion diffusion of cuprous chalcogenides;3)The origin of the distinct difference in ion diffusion rate variation between organic and inorganic perovskites under light;4)How to enhance the Curie temperature of two-dimensional ferromagnetic materials by doping engineering.Therefore,understanding the effect of defects on the properties and stability of the material is critical for the development of new-type semiconductors.The properties and diffusion dynamics of defects in four new-type semiconductors are investigated by density functional theory in this dissertation.The main research contents and results are as following:(1)In order to elucidate the microscopic mechanism,which changed the diffusion behavior of noble metal atoms in the transformation of the device from a three-dimensional system to a two-dimensional system,the diffusion behaviors of Cu and Ag atoms are investigated in bulk and monolayer Mo S₂.The calculation results show that the competition between the electronic energy and the strain energy determines the diffusion rates of Cu,Ag,Na and K atoms in bulk Mo S₂.Strain energy is dominant in bulk Mo S₂.Low strain energy cost in the diffusion path makes Cu atoms with small atomic size have the fastest diffusion rate.However,there is no strain energy cost for atom diffusion on the surface of two-dimensional materials.The electronic energy difference,which caused by the strong d-d coupling,results in the largest diffusion energy barrier for Cu atoms.The study of the diffusion behaviors of noble metal atoms contributes to understanding the influences of electrodes on the performance of van der Waals layered devices.(2)By calculating the phonon spectrum,phonon lifetime,phonon group velocity,specific heat capacity and diffusion energy barrier of the material,the origin of the low lattice thermal conductivity in cuprous chalcogenides is investigated.The room temperature lattice thermal conductivities of Cu₂S,Cu₂Se and Cu₂Te are 0.28,0.02,and0.27 Wm^-1K^-1,respectively.The large layer spacing,complex crystal structure,and low symmetry in Cu₂Se and Cu₂Te lead to the sizeable optic bandwidth,the small frequency spacing of vibration modes,and the flat-band-like phonon dispersion.These conditions result in excellent three-phonon scattering conditions and low absolute values of phonon group velocities.The symmetry-controlled coupling mechanism and the high3d level of Cu atoms lead to the anharmonicity and ultra-fast ion diffusion rate in Cu₂S,which results in the ultra-low lattice thermal conductivity in Cu₂S.(3)The different behavior of the photo-enhanced ion diffusion rates in CH₃NH₃Pb I₃ and CsPbI₃ are reported experimentally.Based on density functional theory,the diffusion behaviors of iodine vacancies in CH₃NH₃Pb I₃ and CsPbI₃ are simulated without and with illumination.The excitation energy difference,created by deformation around the iodine vacancy and the rotatingPbI₆ octahedron under light,reduces the diffusion barrier of iodine vacancies in CH₃NH₃Pb I₃ to 0.15 e V.This value is 42%lower than that in the dark environment.However,CsPbI3 without significant structural deformation still retains a high barrier of 0.45 e V.Furthermore,the hopping rate,which is proportional to the vacancy diffusion rate,is investigated.Calculated results show that hopping rates of iodine vacancies in CsPbI₃before and after light exposure are consistently three orders of magnitude lower than that in CH₃NH₃Pb I₃.Therefore,the ion diffusion in CH₃NH₃Pb I₃is more sensitive to light,while the ion diffusion rate in inorganic perovskite is always slow.The photo-enhanced ion transport behavior in CH₃NH₃Pb I₃ is explained from the electronic structure perspective,and our dissertation provides a theoretical basis for the study of long-term photostability in perovskite solar cells.(4)Too low Curie temperatures of two-dimensional ferromagnetic materials Cr I₃and Cr₂Ge₂Te₆ directly restrict their applications in low dimensional spintronics.Therefore,researchers focus on finding a two-dimensional ferromagnetic material with a higher Curie temperature.The effects of intrinsic point defects on two-dimensional ferromagnetic NiCl₂ are investigated from three dimensions:formation energy,magnetism and diffusion behavior.Theoretical calculations show that the Cl vacancy is the best defect for improving the ferromagnetism of the system.The lower formation energy and slow diffusion rate facilitate the formation of uniform high-concentration doping,and Cl vacancy effectively increases the Curie temperature of the NiCl₂ from140 K to 173 K.This work predicts the effect of defects on the magnetism from a theoretical point of view,and provides an effective strategy for increasing the Curie temperature of monolayer NiCl₂ by doping engineering.In this dissertation,the effects of defects and their diffusion dynamics on electrode selection,thermal conductivity,stability and magnetism of the material are studied in detail:the origin of the novel diffusion behavior of Cu atoms in Mo S₂ is elucidated;the contribution of the 3d orbital of Cu atom to the ultra-low lattice thermal conductivity in Cu₂S is revealed;the significant photo-enhance ion diffusion rate in CH₃NH₃Pb I₃ is explained;Cl vacancy is found to be effective in increasing the Curie temperature of monolayer NiCl₂.These results provide theoretical supports for the understanding and development of excellent new semiconductor materials and devices.