Electronic Structure and Derived Linear and Nonlinear Optical Properties of Chalcopyrites
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L. LAMBRECHT, SERGEY N. RASHKEEV,* SUKIT LIMPIJUMNONG AND
BENJAMIN SEGALL DEPARTMENT OF PHYISCS, CASE WESTERN RESERVE UNIVERSITY, CLEVELAND, OH 44106-7079 ABSTRACT The electronic band structures were calculated for a number of chalcopyrites in both the II-IV-V 2 and I-III-VI2 families using the linear muffin-tin orbital method. From these band structures, the second harmonic generation coefficients were calculated using a recently developed methodology in which a separation is made of inter- and intraband contributions. We found that the high value of d36 in CdGeAs 2 is in large part due to the fact, that in this material, unlike in the other chalcopyrites, almost no compensation occurs between inter- and intraband contributions, the former one being unusually small. For the case of ZnGeP 2 , a detailed investigation of the band structure, reveals that it has an indirect band gap rather than a pseudodirect one. The implications of this for the interpretation of the optical spectra are discussed. Finally, for the I-III-VI 2 materials, we find that the Te based materials have far higher d 36 than the selenides. Combined with their potential for non-critical phase matching, this makes AgGaTe2 an interesting compound. INTRODUCTION The ternary semiconductors with chalcopyrite structure have recently attracted renewed interest as materials for nonlinear optical frequency conversion [1]. While there has been
research on the properties of the whole class of materials in the past, mainly to be found in the Russian literature, the recent work has focused on just a few materials: ZnGeP 2 , CdGeAs 2 , [2] AgGaS 2 and AgGaSe 2 [3]. Great improvements have been made in the purity and crystalline quality, leading in turn to improvements in the performance of frequency conversion devices, such as optic parametric oscillators (OPO) and frequency doubling devices. Thus, most of the emphasis has been on improving the transparency by eliminating undesirable defect absorptions. On the other hand, one may ask: can one improve the intrinsic properties? Not only the coupling coefficients but the combination of properties such as birefringence, thermal conductivity, stability, and range of transparency, need to be considered and may have to be optimized for specific applications. For example, one might consider quaternary alloys for which the properties have so far not been well studied. To embark on this path, we first need an understanding of the trends of the properties of interest in terms of first-principles, that is in terms of the underlying band structures. Our work presents a step in this direction. First of all, we developed a methodology to calculate second order optical response functions from the band structures. Our method is based on recent developments in the fundamental theory of nonlinear optical response introduced by Sipe et al. [4, 5]. Not only does this theory remove some of the problems with *Present Address: Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235
385 Mat. Res. Soc. Symp. P
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