STRUCTURAL PHASE TRANSITIONS AND PROPERTIES OF ZnO1-xSx AND ZnO1-xSex UNDER EXTREAME CONDITIONS

Abstract

Structures and high pressure phase transitions in ZnO0.5S0.5, ZnO0.5Se0.5 and Zn16O1S15 have been investigated using density functional theory calculation. The previously proposed structures of ZnO0.5S0.5 and ZnO0.5Se0.5 which are Chalcopyrite, Rocksalt, Zincblende, Wurtzite and CuAu-I have been fully investigate Stability of these materials has been systematically studies up to 30 GPa for ZnO0.5S0.5 and ZnO0.5Se0.5 using various approached. In this thesis, we have confirmed the stability of Chalcopyrite structure up to at least 10 GPa where CuAu-I structure have been previously proposed. However, our calculation revealed that the CuAu-I is not a stable structure under this conditions which could explain the failure in several attempts to fabricate this materials under such conditions. We have also examined the pressure dependence of the bandgap and electronic structure up to 30 GPa. For Zn16O1S15, two candidate structures namely Wurtzite and Zincblende were examined theirs properties and structures stability up to 20 GPa. The two exists phase which occurred during the difference film growth conditions was explained. Phonon dispersion and the Born criteria reveal that Zincblende is only be stable up to 10 GPa. Besides, Wurtzite structure yield no imaginary phonon frequencies and also satisfy the elastic constants sufficiency condition up to 20 GPa. The electronic structure and PDOS were fully investigated using HSE06. The electronic structure calculation provided mid O-3 state between fundamental ZnS band gap. The pressure effect on their electronic structure has been investigated for possibly adjustable optoelectronic device.