First-Principles Study Of Electronic Structures And Intrinsic Defect Properties Of Binary Compound Semiconductors

The semiconductor is the cornerstone of modern informational society,which constitutes various electronic devices.Defects have significant effects on properties of semiconductors,especially the carriers’concentration and conductivity type,thus affecting their applications.Compared with elemental semiconductors,binary semiconductors possess various structures.Recently,strongly anisotropic Ⅳ-Ⅴ layered semiconductors attracted extensive attentions.Their physical properties,including structural anisotropy,electrical transport,and photoelectric properties,etc,have been investigated.However,the poor understanding of their defect properties will limit the corresponding applications.In addition,the diffusion of defects also has an important impact on the doping,mechanical properties,and structures of semiconductors.For instance,nanosized oxide semiconductors show remarkable anelasticity,which is attributed to point defects migration.Nevertheless,the experimental verification of this mechanism has suffered from the difficulty in directly resolving point defects.Considering the questions above,first-principles calculations were conducted to study defect properties of these two kinds of binary compound semiconductors,i.e.Ⅳ-Ⅴ layered materials and Cu O:1.Layer-dependent electronic structures and defect properties were discovered in two dimensional Ⅳ-Ⅴ compounds:The electronic structures and defect properties of GeP_x（x=1,2）with different thicknesses were studied through density functional theory（DFT）calculations.The electronic band structures of Ge P and Ge P₂ with different layers depict that the valence band maximum（VBM）upshifts greatly with thickness increasing,especially in Ge P,while the conduction band minimum（CBM）downshifts little.Further electronic structure analysis shows that VBM is mainly contributed by the p_z orbital component of P and consists of antibonding states in Ge P.The formation energies of intrinsic point defects in monolayer and bulk Ge P_x were calculated,showing that the formation energies of antisites are the lowest.Ge _P is an acceptor while P_Ge is a donor.Monolayer Ge P exhibits weak n-type conductivity,while bulk Ge P transforms to strong p-type due to the large upshift of VBM.The transition is not so obvious in Ge P₂.The concentrations of antisites and carriers in Ge P_x were calculated based on the data of defect formation energies.The calculated thickness-dependent carrier concentrations are qualitatively consistent with previous experiments and in good agreement in order of magnitude.This discovery reveals that the conductivity of GeP_x is related to the number of layers.To extend our findings in the Ge-P system to all Ⅳ-Ⅴ compounds,first-principles study was conducted on Ge-As system.Electronic structures and defect properties are similar in the Ge-As and Ge-P systems.Antisites have the lowest formation energies.In the Ge-As system,the formation energies of antisites are even lower,possibly because the electronegativities of As and Ge are closer.In addition,strain engineering could alter the electronic structure of GeAs₂.2.The room temperature point defect（oxygen vacancy）diffusion mechanisms in Cu O.Diffusion of oxygen vacancy in Cu O was studied theoretically.First-principles calculations and in-situ bending experiment show that point defect diffusion as driven by the strain gradient may lead to the anelasticity of Cu O nanowires.Based on the Gorsky effect,diffusivities of point defects were estimated.In addition,it was discovered that the diffusion barriers along[1（?）0],[110]and[100]are the lowest.Finally,we find that the oxygen vacancy ordering may lead to the nucleation of the oxygen-deficient phase Cu₃O₂.Combining with the atomistic observations in experiments,the structures are determined based on the first-principles calculations.Accordingly,the calculated electronic structures depict that Cu₃O₂ is an antiferromagnetic half metal.