Study On Phase Structure And Properties Of Multicomponent Semiconductor Alloys

ZnO materials have great application value in the field of optoelectronics.In order to meet the needs of various practical applications,the research on energy band engineering of ZnO materials is required.It is a widely effective method to use anionic or cationic doping to form multi-component alloys.At present,there is no systematic study on the electronic-structure and thermodynamic properties of ZnO based quaternary alloys.The electronic-structure and thermodynamic properties of quaternary alloys with the change of doping element content are of great significance for the study of growth conditions,control of microstructure and properties,and practical application in photoelectric field.Because of its large specific surface area and quantum effect,the two-dimensional transition metal dichalcogenides,have excellent properties different from bulk materials,and have band gap adjustable characteristics compared with graphene,which makes it widely used in optoelectronics and other fields.The theoretical study of the electronic-structure and dynamic properties of the W_xMo1-_xS₂ ternary alloys formed by doping W in 2D-MoS₂ provides the basis for the later experimental synthesis and practical application in the field of electronic devices and energy technology.In this paper,the electronic-structure and thermodynamic properties of quaternary-alloy semiconductors Be_xZn_1-xO_1-yS_y(0?x?1,0?y?1)?Cd_xZn_1-xO_1-yS_y(0?x?1,0?y?1)?Mg_xZn_1-xO_1-yS_y(0?x?1,0?y?1)and Be_xMg_yZn_1-x-yO(0?x?1,0?y?1,x+y?1)are studied by the first-principles calculations,and the electronic-structure and dynamic properties of two-dimensional W_xMo1-_xS₂ ternary alloys(0?x?1)are also studied.The main research contents and conclusions of this thesis are as follows:In the Be_xZn_1-xO_1-yS_y alloys,the compositional phase diagram of the thermodynamic equilibrium at T=0 K indicates that with the increasing the content of Be and S,the quaternary alloys undergo phase transition between wurtzite-zincblende-rocksalt.The alloys on the intersection of phase structures are two-phase coexistence,Be_0.34Zn_0.66O_0.65S_0.35 and Be_0.46Zn_0.54O_0.73S_0.27 are three-phase equilibrium points.In the quaternary alloys with different phase structures,the miscibility gap islands with the changes of temperature and composition were obtained.The results show that co-doping of Be and S can increase the solid solubility of doping elements in the alloys,reduce the destabilization decomposition,and improve the alloy quality.Moreover,the ranges of band gap regulation of WZ-,RS-and ZB-Be ZnOS quaternary alloys with composition changes were given:the adjustment range of the band gap of rocksalt quaternary alloys is 3.2 eV to 11.72 eV;for the wurtzite phase,the adjustment range of the band gap is 3.38 eV to 10.40 eV;for the zincblende quaternary alloys,the adjustment range of the band gap is 3.46 eV to 10.27 eV.In the Cd_xZn_1-xO_1-yS_y alloys,the compositional phase diagram of the thermodynamic equilibrium at T=0 K indicates that with the increasing the content of Cdand S,the quaternary alloys undergo phase transition between wurtzite-zincblende-wurtzite,the rocksalt phase is only near the CdO end.The alloys on the intersection of phase structures are two-phase coexistence.In the quaternary alloys with different phase structures,the miscibility gap islands with the changes of temperature and composition were obtained.The results show that co-doping of Cdand S can reduce the destabilization decomposition,improve the quality of alloys.Moreover,the ranges of band gap regulation of the quaternary alloys with composition changes were given:For the wurtzite phase,the adjustment range of the band gap is 2.21?4.31 eV;for the zincblende quaternary alloys,the adjustment range of the band gap is 1.54?4.30 eV;the adjustment range of the band gap of rocksalt quaternary alloys is 2.15?3.91 eV.In the Mg_xZn_1-xO_1-yS_y alloys,the compositional phase diagram of the thermodynamic equilibrium at T=0 K indicates that with the increasing the content of Mg and S,the quaternary alloys undergo phase transition between wurtzite-zincblende-rocksalt.The alloys on the intersection of phase structures are two-phase coexistence.And the three-phase equilibrium point is Mg_0.77Zn_0.23O_0.95S_0.05.In the quaternary alloys with different phase structures,the miscibility gap islands with the changes of temperature and composition were obtained.The results show that co-doping of Mg and S can reduce lattice distortion and improve the quality of alloys.Moreover,the results of regulative band gap are:the adjustment range of the band gap is 2.21?4.31 eV in wurtzite phases;the adjustment range of the band gap is 1.54?4.30 eV for the zincblende quaternary alloys;the adjustment range of the band gap of rocksalt quaternary alloys is 2.15?3.91 eV.In the Be_xMg_yZn_1-x-yO alloys,the compositional phase diagram of the thermodynamic equilibrium at T=0 K indicates that quaternary alloys are mainly wurtzite structure with the increasing the content of Be and Mg.When Be content is 0?0.07,Mg content is 0.78?0.93,the alloys show rocksalt structures;when doped Be content is 0.62?0.98,Mg content is0?0.08 and Be is 0?0.02,Mg is 0.38?0.65,the alloys are zincblende structures.With the increasing of content of Be and Mg co-doped,the band gap of the alloys can be adjusted within the range of 3.9 eV to 11.74 eV for the rocksalt structure;for the zincblende quaternary alloys,the adjustment range of the band gap is 3.64 eV to 10.26 eV;the adjustment range of the band gap of wurtzite-quaternary alloys is 3.44 eV to 10.42 eV.In this paper,the electronic-structure and dynamic properties of two-dimensional W_xMo_1-xS₂ ternary alloys(0?x?1)are also studied by cluster expansion method(CE)combined with the first-principles calculations.three intermediate ground state structures are predicted by CE:W_1/3Mo_2/3S₂?W_1/2Mo_1/2S₂and W_2/3Mo_1/3S₂;their respective order–disorder phase transition temperatures were obtained:T_1/3=102.3 K,T_1/2=87.8 K,and T_2/3=102.2 K The band gaps of two-dimensional W_xMo1-_xS₂ alloys were calculated by G₀W₀ method,the results show that the band gaps of the alloys increases nonlinearly with the increase of W doping content.All ground state structures are dynamically stable.