Studies On The Structure And Properties Of CuAl₅Se₈, I₂-II-IV-VI₄, And AlGaN Alloys

For having the characteristics of photonic materials advance and electronicmaterials stability, photoelectric materials has a broad application prospect inmicroelectronics, photoelectron, communication technology and other high-techfield. Therefore, investigation on the electro-optical properties of optoelectronicmaterials in optoelectronic devices design and manufacture is particularly necessary.The electronic and optical properties of photoelectric materials are characterized bythe band structure, density of states and all kinds of optical constants. Now theinvestigation of photoelectric materials is focused on the diversification and alloy.However, the increased component makes it not easy to obtain a stoichiometric ratioof crystal sample in experiment, and the increment of the degrees of freedom alsomeans that the properties of these materials become more complex, which makes itdifficult to accurately measure the properties of material s. Based on the densityfunctional theory, the materials properties were simulated using the first-principlesmethod. This method could accurately get the information of the structures andvarious properties of the existing materials, and even could predict the photoelectricproperties of new materials.In this paper, the current popular new multiple photoelectric materialsincluding ternary semiconductors CuAlSe₂and CuAl₅Se₈, quaternarysemiconductors I₂-II-IV-VI₄and ternary alloy semiconductor AlxGa1-xN are studied.First, it gives a brief introduction of the development of these novel photoelectricmaterials in the fields of optoelectronic devices. And at the same time the simulationmethods used in the study are introduced. On these bases, the main research contentswere summarized. The contents and conclusions are as follows:Firstly, on the basis of analysis of the defect stability, the defect formationenergy of defect pair (2V_Cu^-+Al_Cu⁺²) in CuAl₅Se₈is studied using first-principlesmethods and the result proves that this defect pair maybe exist. Then the six elasticconstants of CuAl₅Se₈are investigated and it could found that the results satisfiedwith the Born stability criteria for tetragonal chalcopyrite structure. The analysis ofthe energy band structures and density of states shows that CuAlSe₂and CuAl₅Se₈are the direct gap materials, and the band gap of the defect crystal is bigger than thatof the perfect one. In addition, the optical properties are also studied by calculationsof the dielectric function, absorption spectra, and reflectivity of CuAlSe₂andCuAl₅Se₈. From these results we proposed that the amplitudes of the opticaltransitions for CuAl₅Se₈are always smaller than those of CuAlSe₂in the range of0⁴.5eV. Compared with the perfect crystal, the spectra including absorption coefficient and reflectivity for the defect crystal have a blue-shift. And theabsorption coefficient and reflectivity for CuAl₅Se₈also decreased with the decreaseof Cu3d states near the valence band maximum. The above analysis show that thedefect pair could affect the electronic structure s and optical properties of materials.Moreover, the geometric structures, electronic and optical properties of thequaternary semiconductor compounds I₂-II-IV-VI₄in three different structures arestudied using first-principles methods, the specific contents are as follows:（1） Byanalyzing the defect stabilities of V_Cu and CuZnin kesterite Cu₂ZnSnS4, we foundthat the difference of formation energy between these two point defects is very smalland then they can coexist in the Cu₂ZnSnS₄crystal. From the electronic structures ofCu₂ZnSnS4and the defect semiconductors we found that low concentration defect ofCuZnor VCucould not change the properties of material completely.（2） Theelectronic structures and optical properties of stannite-type Cu₂CdGeSe₄andwurtzite-stannite-type Ag₂HgSnSe₄are also predicted.（3） From the calculatedproperties of the above three quaternary materials, the following conclusions can bedrawn: I₂-II-IV-VI₄are all the direct gap semiconductors, and both the valence bandmaximum and the conduction band minimum exist at Γ point in the Brillouin zone.The valence band maximum is mainly dominated by the I d and VI p states, whilethe conduction band minimum is dominated by IV s and VI p states.Finally, the influence s of Al concentration and pressure on the properties ofzinc-blende AlxGa1-xN alloys are discussed as well. The results of the electronicstructures show that the band gap energy changes from the direct to indirect with theincrease of Al mole fraction（x=0.62）. In order to understand the bonding behaviorbetween atoms ulteriorly, the Mulliken charge populations are also studied forzinc-blende AlGaN alloys. The result shows that the alloys have covalent nature andthe covalent bonding strength of the Al-N bond is stronger than the Ga-N bond. Inaddition, due to the blue-shift of conduction band edge, all the optical peaks ofalloys have a tendency of shifting to the higher energy with the increase of Alconcentration. We also analyze the direct and indirect band gaps for alloys underdifferent pressures. The results show that AlGaN alloys keep zinc-blende structurewithin the pressure range of0¹5GPa, and the band gap increases with theenhancement of the pressure.In order to verify the correctness of the theoretical predictions, the theoreticalresults have been compared with our and previous experimental results. The phasestructure s of thin films are investigated and analyzed by XRD. And the electricaland optical properties of these films are investigated by ellipsometry andspectrophotometry. We found that the experimental results are in good agreement with the calculated results. It laid a solid and reliable theoretical foundation fortheoretical research.