First Principles Study On The Electronic And Optical Properties Of Transition Metal Doped β-Ga₂O₃

In recent years,two hot fields in materials science have attracted a great deal of attention.One is diluted magnetic semiconductors(DMSs) due to their potentiality as a new functional material which open a way to introduce the freedom of spin into semiconductor devices.An essential task is to find a DMS with the Curie point Tc above room temperature.Properties ofâ…¢-â…¤andâ…¡-â…¥DMS are extensively studied and new DMSs were discovered,such as Mn-doped CdGeP₂ and zinc blende-CrAs.In the last couple of years,transition metal-doped oxide materials,in particular,titanium dioxide and zinc oxide,have come into the limelight as DMS with room temperature ferromagnetism.Another is intermediate band solar cells which have been proposed as a new approach to exceeding the efficiency of single gap solar cells.The intermediate band material is characterized by the existence of an intermediate band(IB)located within the otherwise conventional semiconductor bandgap between the valence(VB)and conduction(CB) bands.Within this framework,one of the key aspects is,therefore,the identification of intermediate band materials.Recently Ga₄P₃M and Ga₄As₃M materials as well as Ga₃₂P₃₁M and Ga₃₁P₃₂M have been studied,where M is a transition metal,as material system candidates.It was shown that the formation of the IB only takes place in some compounds.Monoclinic gallium oxide(β-Ga₂O₃)is a wide band-gap semiconductor and exhibits excellent physical and chemical stability, and thus has potential applications in optoelectronic devices and solar cells.However,whether it can be used as diluted magnetic materials and intermediate band materials for spintronic and solar cell applications has not bee explored.Therefore,it is important to study the electronic structure,spin polarization and optical properties ofβ-Ga₂O₃ doped by transition metals.In this thesis,we explore the possibilities of transition metal dopedβ-Ga₂O₃ as candidates of DMS and intermediate band materials using density functional theory pseudo-potential plane wave method and the local density approximation(LDA).The main results and conclusions are as follows.(â…°)Based on the pseudo-potential plane wave method of the first principles,the calculations of the total energy and the equilibrium lattice constants for the monoclinicβ-Ga₂O₃ have been performed with different exchange-correlation potentials(DECP)on both the generalized gradient approximation(GGA)and the local density approximation(LDA).The effects of DECP on the equilibrium lattice constants are discussed.The band structure,energy gap,the density of states and the partial density of states in the case of equilibrium are analyzed.The total energy calculations show thatβ-Ga₂O₃ is more stable thanα-Ga₂O₃.(â…±)The lattice constants,minimum energies,electronic band structures,partial and total spin density of states of Ti,V,Fe,Cr, Mn,Ni and Co dopedβ-Ga₂O₃ have been studied by using the first principles pseudo-potential plane wave method with the local density approximation(LDA).It is shown that there exists only one spin polarized state around the Fermi level for Cr,Fe,Mn,Ni and Ti dopedβ-Ga₂O₃.However,ferromagnetism is predicted only for Mn and Ni dopedβ-Ga₂O₃ because their t^a states are not filled up completely,whereas spin-glass ground states are predicted for Cr,Fe,and Ti dopedβ-Ga₂O₃ because their t^a states are either empty or full filled.The spin up and spin down components overlap and contribute near equally to each peak of the partial and total DOSs for V and Co dopedβ-Ga₂O₃ and therefore they are not ferromagnetic.From the intermediate band materials view of point for solar cell and other optoelectronic applications,only Cr,Fe,Mn and Ni dopedβ-Ga₂O₃ exhibit isolated and partially or full filled intermediate bands with finite band width.Ti,V and Co doped samples do not show isolated intermediate bands due to small splitting by the spin interaction and overlapping of the impurity bands which partially hybridize with the lowest conduction band.The real and imaginary components of dielectric functions of transition metal dopedβ-Ga₂O₃ are calculated and the changes due to doping are explained.Significant decrease in band gap occurs for transition metal dopedβ-Ga₂O₃.The abundant band structures of pure and transition metal dopedβ-Ga₂O₃ provide transition probabilities from ultraviolet to far infrared region.The novel features in band structures and DOS suggest that transition metal dopedβ-Ga₂O₃ may find new applications in optoelectronic devices such as spintronic devices and solar cells.