Density Functional Theory Studies On The Electronic Structures And Optical Properties Of Chalcopyrite Ternary Compounds

The ABC2ternary compounds crystallized in the chalcopyrite structure are importantsecond harmonic generation (SHG) materials, which are potential materials for a laserfrequency conversion application in IR region especially. To get insight into the linear andnonlinear optical properties, it is necessary to understand the inner relationship betweenelectronic structures and second harmonic generation effects of compounds by theapproach of theoretical simulation. In the present thesis, density functional theory has beenemployed to study the geometries, electronic structures and optical properties ofchalcopyrites consisting of Group ⅡB, ⅣA, ⅤA Elements, ABC2(A=Zn, Cd; B=Si,Ge, Sn; C=P, As), total12crystals that have been synthesized experimentally. In addition,the origin of SHG effect for these chalcopyrite compounds has been studied.Our results indicate that follows:(1) The same series of chalcopyrite show similarband structure and density of states. In general, the band gap of system with similarstructure exhibits a graded rule, the band gap decreases in the order of Si, Ge and Sn forthe same C atom, and when B element is fixed, the band gap decreases gradually in thesequence from P to As. Except few compound, the band gap of certain CdBC2is smallerthan that of corresponding ZnBC2compound.(2) The static dielectric constant and refractive index increase when C atom variesfrom Si to Sn for the compounds with the same B element, and also increase in the order ofP, As for the same B element. The results of static birefringence show that most ofchalcopyrites (total10) are uniaxial positive crystals. From the curves of reflectivity andadsorption coefficient in the IR region, it’s clearly shown that the ZnBC2and CdBC2crystals with P element exhibit good optical characteristics, while on the contrary, the poorlight passing in the IR region is predicted for most of crystals whose B element is As.(3) The static d36coefficients of these chalcopyrites vary in a wide range. On thewhole, the opposite variation of the d36coefficients is observed comparing to the band gap,namely, for the same series compounds, the SHG coefficients increase from Si to Sn for thesame C element, and also increase from P to As for the same B element. By decomposingthe SHG coefficient into each energy band, our results indicate that the SHG response ofthe system can be attributed to the transitions from those energy bands near the top of valence band that mainly contain C p states to unoccupied bands dominated by p states ofB atoms.In sum, in these12crystals, ZnGeP2is an excellent nonlinear optical material whichexhibit a relatively large d36coefficient, an appropriate birefringence, and the goodtransmission of light in the IR region simultaneously. CdGeAs2shows a huge d36coefficient, whereas its poor linear optical performance would limit its further research andapplications in nonlinear optics. Instead, CdSiP2, ZnSnP2, CdSnP2et al. may be goodcandidates for optical frequency conversion applications.