Band Structure Engineering Of ZnO Semiconductor Material

ZnO is an attractive material for applications in photoelectric devices,photocatalysis and solar cells due to its unique physical and chemical properties.However,the band gap of ZnO is about 3.4 e V,which is in the near ultraviolet（UV）region.Greater flexibility in emission wavelengths for ZnO-based optoelectronic devices is highly demanded.The group II metal oxides and related ternary alloys are generating considerable interest,because they can provide,in principle,an accessible direct band gap range from visible light to deep UV.In this thesis,we systematically investigate the structural and electronical properties of ZnO,group II metal oxides and related ternary alloys using first-principles calculations.Firstly,after calculating the ground-state properties of wurtzite ZnO and the strain effects on them,we find that the band ordering of VBM can be changed by compressive strain,i.e.,the band Γ₇ that has the lowest energy can cross other bands.By calculating Al N and other wurtzite oxides,we find that the band crossing is related to p-d coupling,which has a particularly influence for energy level.Further calculations by using LDA+U method show that the band crossing disappears when the p-d coupling increases to a certain extent.Secondly,we have systematically investigated the structural and electronic properties of MO（M = Be,Mg,Ca,Sr,Ba,Zn,Cd）in binary rock salt（B1）,zinc-blende（B3）and wurtzite（B4）phases,including the lattice constants,bulk moduli,band gaps,and deformation potentials.Our results agree well with the experimental data and other theoretical results,and give a better understanding of the relationship between the geometric and electronic structure.By calculating the band alignment,we find that in both the B1 and B3 structures,the valence band maximum（VBM）has an obvious decrease from Be O to Mg O to Ca O,then it goes up from Sr O to Ba O to ZnO to Cd O.Finally,the lattice mismatch,band-gap bowing parameter and structure stability of ternary alloys M_xZn_1-xO are studied through the application of the special quasi-random structuremethod.The phase transition from the B3 structure to the B1 structure is predicted with decreasing of the ZnO composition.The critical point of the transition gradually increases from（Ca,Zn）O to（Mg,Zn）O to（Sr,Zn）O to（Ba,Zn）O to（Cd,Zn）O,indicating that（Ca,Zn）O can exist in the B3 structure with the lowest ZnO composition.