| Mid infrared(3-20 μm)lasers have shown broad applications in the nonlinear optical detection,photovoltaic devices and laser communication,for instance,IR countermeasures and trace gas detection.Currently,the increasing interests of infrared nonlinear optical crystals have being focused on the non-oxygen system such as phosphorus compounds and sulfur compounds because of their better transparency in this spectral region and laser output which is above 4 μm,in comparison with oxide or organic materials.The ternary chalcopyrite compound ABC2,a typical non-oxygen system,has been widely investigated as a promising candidate due to its excellent optical properties.To have practical usages,the mid-IR NLO materials should generally satisfy several conditions for laser-pumped conversion devices.One of the important parameters is to have good IR transparency and high output power particularly in atmospheric windows(3-5 μm and 8-12 μm).Mid and far infrared NLO crystal materials can transform laser frequency to achieve the goal of tunable laser output by using birefringent effect and phase matching.Nonlinear optical crystal CdSiP2(CSP)is famous for its apparent birefringent effect,high nonlinear optical coefficient and good phase-matching property.And its transparent range can theoretically achieve 0.5-10μm.However,there exist nonnegligible absorption losses around 1.3 μm,2 μm and 3.5 μm for o-polarized light,and sharp peaks at 1.34μm and 1.78 μm for e-polarized light,which may seriously reduce the optical damage threshold and affect the conversion efficiency in making promising optical frequency conversion devices.Experimental tests such as EPR and ESR method is hard to directly and accurately study the intrinsic defect types and optical properties,especially the details of the link between the defects and absorption spectrum.First principles based on density functional theory(DFT)can start from the crystal structure of itself to explain the inherent laws of physics,without being confined to the experimental limitations.In this thesis,the calculations were performed within DFT-based first-principles calculations.According to specific experiment chemical environment and thermodynamics of crystal growth process,the scope of each element’s chemical potential has been determined.So the defect formation energies(DFT)of different types of charged defects have been calculated from the formula,and then,the dominated point defects and clusters have been decided.After that,by studying the main point defect geometries,electronic structures and their optical properties,the relationship between intrinsic defects and defect-induced absorption has been summarized,thus it will provide reference to defect type judgement and defect behavior comprehension in experiment.In this paper,the research content and the main innovation points are:1.The DFT-GGA and DFT-HSE methods have been applied to study the crystal geometry,electronic structure and optical property of perfect CSP material.The supercell size of 2×2×1 containing 64 atoms were started as a benchmark.The DFTs of 2x2x2,2x2x3 and 3x3x2 systems containing 128,192 and 288 atoms,respectively,are calculated to make a comparison.So the 2x2x2 system has been enough for the computing requirements with small resource consumption.By analyzing actual experiments with different chemical environment and thermodynamics of crystal growth process,the ranges of different element chemical potential have been determined.2.The DFEs of various types and different intrinsic charged defects and possible neutral defects have been calculated in different chemical environment of CSP crystal.And the distribution of DFEs with band gap have been depicted to determine the dominated defects.It can be concluded that Vcd with-2 charge state,Sicd with +2 charge state and Vsi with-4 charge state can spontaneously generated in CSP crystal because of the negative DFEs in the Cd-deficient condition.Then the main defects’ geometries,electronic structures and charge density differences have been probed as well.3.Based on the previous step,the intrinsic physic behavior of defect-induced optical properties have been put forward.Comparing the result with experiment fact,Sicd may have main responsibility for the absorption peaks at 1.34 μm and 1.78 μm,while VCd2-and Vsi can make a big contribution to the obvious red shift.4.The defect behaviors of possible neutral defect clusters and impurities have been explored.After comparing some possible neutral defect clusters,the Sicd2+-Vcd2"cluster may be the most possible charge-compensated defects,butcharged defect complexes and effective charge of point defects are still required to be more precise.Besides,the impurities such as Fe,have been found to have certain effect on the optical properties. |
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