Frist-Principles Calculation Study Of Multiternary Semiconductors And Their Alloys

As a result of the increased chemical and structural freedom,multiternary and alloy semiconductors exhibit novel and special properties which give rise to potential application as functional materials.The increased freedom also makes the study of these materials so complicated that the dimension of the phase space increases with the number of elements and atomic positions in the primitive cell. The rapidly-developing first-principles calculation methods make it possible to study the changes of properties in the mutation from simple to multiternay and alloy semiconductors theoretically,to show the trend in the element substitution and structural transformation,understand various experimental results and give the theoretical guidelines for knowledge-based design of new semiconductors.The thesis is composed of six chapters.The first chapter gives an introduction to the recent development in the application of semiconductors,the phase diagram and properties of alloys,the ordered structures,and the band offsets of popular semiconductors.The second chapter devotes to the simulation methods used in the study,including conventional band structure theory,density functional theory and valence force field method.The third chapter discusses the sequential cation cross-substitution in zincblende chalcogenide semiconductors,from binary to ternary to quaternary compounds. Several universal trends are found for the ternary and quaternary chalcogenides with common-row cations,the lowest-energy structure always has larger lattice constant a,smaller tetragonal distortion parameterη= c/2a,negative crystal field splitting at the top of the valence band,and larger band gap compared to the metastable structures.The band offsets and band character decomposition analysis show,the gap decreases fromâ…¡-â…¥toâ… -â…¢-â…¥₂ are mostly due to the p-d repulsion in the valence band,while the decreases fromâ… -â…¢-â…¥₂ toâ… ₂-â…¡-â…£-â…¥₄ are due to the downshift in the conduction band caused by the wavefunction localization on the groupâ…£cation site.We propose that common-row-cationâ… ₂-â…¡-â…£-â…¥₄ compounds are more stable in the kesterite structure, and the band gap of Cu₂ZnSnSe₄ is on the order of 1.0 eV,which are both misunderstood by previous experimental study,and we also propose Cu₂ZnGeSe₄ has a ideal band gap as a single-junction solar cell absorber.The forth chapter deals with two ternary-ternary,binary-ternary alloys.We first explained the band structure anomaly of Ag_xCu_1-xGa-â…¥₂ alloy that the gap increases with the lattice constant,and then using the SQS to describe the disordered alloys,we studied the local structure in the alloy and the reason of large band gap bowing parameter.In the second part,we studiedâ…¡-â…¥/â… -â…¢-â…¥₂,â…¢-â…¤/â…¡-â…£-â…¤₂ ordered alloys at x = 0.5,to design high efficiency,100% spin-polarized electron source materials.A series of ordered alloys with different cations are screened based on the crystal field splitting,spin-orbital splitting and energy stability,three of which are proposed as candidates.The fifth chapter concentrates on the superhard BN/C₂ alloys.To resolve the debate if BC₂N could be harder than c-BN,we systematically studied different crystal structures and gave a quantitative bond counting rule,based on which we performed an unconstrained structure search and identified BN/C₂(111) super-lattices as the lowest-energy,most-compact,most-volume-incompressible struc-tures with the same cell size.The further calculation of elastic constants,shear modulus,ideal strength,and shear strength with normal compression showed that the(111) superlattices possess stronger resistance to elastic and plastic deformation than other BC₂N structures as well as c-BN,thus could be harder than c-BN.The calculated shear modulus and shear strength under normal compression are linearly related with the measured Vickers hardness for BN/C₂ alloys, hence could be used as a predictor of hardness.The structural transformation, band gap and optical dielectric function are also studied for different BC₂N structures.The sixth chapter summarizes all the research and discusses the potential improvement.