Frist-Principles Calculation Study of Multiternary Semiconductors and Their Alloys
|Course||Condensed Matter Physics|
|Keywords||Diverse semiconductor Alloy semiconductors Chalcogenides Cu2ZnSnS4 Superhard materials BC2N First-principles calculations|
Diversity and alloying two important ways to design new materials, due to the increase in chemical composition and structure of degrees of freedom, pluralism materials and alloy material will exhibit a variety of rich, unique nature, greatly broadens the application range of choice of materials. Recent developments show that the function device selection semiconductor toward diversification, alloying and low-dimensional development, and thus to study the variation of the nature of this diversified and alloying process is very important. However, the degree of freedom increase also means that the more complex nature of the phase space dimension increases the difficulty surge from the theoretical and experimental studies of such materials. Fortunately, semiconductors based on zinc-blende structure and nature of the process maintained a certain degree of stability, while the first-principles calculation method of the description of this system is relatively the most mature, the two by first-principles calculation to study the change in the nature of the law in the process of diversification and alloying based semiconductor sphalerite structure, analyzing the evolution of the elements, substitutions and changes in the structure of an impact on the nature of the trend, from the microscopic on the system to understand a large number of experimental results reflect the different sides, and to provide a basis for the semiconductor has been designed with the specific performance parameters. The thesis is divided into six chapters. The first chapter introduces diverse as semiconductors and alloys in applications development phase diagram and the nature of the general laws, the emergence of the ordered structure and characteristics, as well as common semiconductor band offset. Second chapter briefly describes the calculation involved in this paper, including the band calculation method based on the tradition of single-electron approximation, the first-principles calculation method based on the density functional theory, as well as for the semiconductor alloy valence force field elastic energy . In this thesis, using first-principles calculation method, in the second chapter, also gives a comparison of the types of semiconductor alloy valence force field elastic energy methods and first-principles calculation. Chapter III First-principles calculation of system study flash the blende structure chalcogenide semiconductor by cation substitution from binary directed triple again four yuan evolution. Discovered several genetic characteristics of ternary and quaternary semiconductors law: of chalcogenide semiconductor cation counterparts in the periodic table, the structure of the ground state is always a large alpha lattice constant, relatively small quadrangular crystal The deformation parameter η = c/2α, the top of the valence band of the crystal field splitting negative, and the larger band gap; while cations in the different rows, these laws only partially correct. The analysis of the results of the calculation based on the the band ingredients and band offset: Ⅱ - Ⅵ to Ⅰ - Ⅲ - Ⅵ 2 evolution bandgap decreases from p-d hybrid valence band push high reduction of only about 20% of the contribution of the conduction band; from Ⅰ - Ⅲ - Ⅵ 2 the to I 2 - II - IV - Ⅵ 4 evolution, moving the top of the valence band is almost can ignore the IV group atoms s orbital level lower the conduction band is reduced to determine the reduction of the band gap. Found that compared with the experimental results: the ground state of the semiconductor structure of Cu 2 the ZnGeSe 4 and of Cu , 2 the ZnSnS 4 class should be the zinc yellow tin (kesterite) structure, widely observed in the experiment (stannite), yellow tin structure may be due to the X-ray diffraction patterns can not distinguish between the atomic number close to the cation or Ⅰ - Ⅱ (001) layer partially caused by disorderly experimental measurements; 4 band gap of the Cu 2 ZnSnSe should be about 1.0 eV, rather than frequently cited 1.5eV, explains the exception of the photoluminescence spectra calculations ; according to the calculations, we predict Cu 2 ZnGeSe 4 may also become the new low-cost solar cell material. Chapter $ 3 - ternary mixed, binary - ternary mixed ideas to study the structure of the two types of alloy systems, energy band structure characteristics. In CuGa-Ⅵ 2 / AgGa-Ⅵ 2 alloy research, we explained: Ag x Cu 1-x < / sub> Ga-Ⅵ 2 alloy band gap increases with the lattice constant of this anomalous band trend, and analysis of the contribution of different factors according to the band offset; through the SQS analog chalcopyrite structure ternary - ternary disordered alloys, the alloy microstructure, analyzes the source of its large band gap recessed parameters. Ⅱ - Ⅵ / I - III - Ⅵ 2 , Ⅲ - Ⅴ / II - IV - Ⅴ 2 ordered alloy research, to design a high quantum efficiency, 100 % spin-polarized electron source material for the purpose of searching the ordered structure of the two types of alloy composed of at x = 0.5, different cations, based on the results of the crystal field, the spin-orbit coupling splitting and energy stability, three The kinds of alloys as the polarization potential of the electron source cathode material. Chapter turning high hardness of BN / C 2 alloy structure, flexibility and strength of the nature of the study. BC whether 2 N hardness may range between c-BN and diamond motivated to clarify, the system of law on the nature of the different crystal structure, and quantitative proposed BC 2 key number N alloys nature the rules accordingly unrestricted structure search BN / C 2 (111) superlattice structure in the lowest energy structure of many of the same primitive cell size, structure The most dense, elastic modulus. A further elastic constants, shear modulus, the ideal tensile and shear strength, and shear strength calculated vertical condensation BC 2 > N and BC 4 N (111) the shear modulus of the super lattice structure, the ideal intensity and vertical pressurizing shear strength than other BC 2 N structure, also higher than that of c-BN, these ultra The lattice structure showed a strong ability to resist the elastic deformation and plastic deformation, and thus may have a high hardness than the c-BN. An important finding was that such alloy system, the calculation of shear modulus and shear strength in the vertical condensation was a quasi-linear relationship with the hardness of the experimental measurements, indicating that these two quantities is the prediction of the hardness of these alloys appropriate physical quantities. The chapter also discusses the BC 2 N different structural transformation of the structure, the band gap and the optical dielectric function of nature. The Chapter Review all chapters, and to discuss the lack of existing research and further improvements in the future.