First-principles Study On Transparent Conductive Materials From Wide Bandgap Semiconductors

With the rapid development of science and technology,people pay much attention to the advancement and optimization of electronic device materials.Due to the unique combined properties of the transparency of glass and the conductivity of metals,transparent conductive materials have a wide range of applications in the field of optoelectronic devices.Wide bandgap semiconductors have high light transmittance and can be used as the parent materials for transparent conductive materials.It is a hot topic at present to seek n-type and p-type transparent conductive materials with low prices and excellent performance.Compared with traditional wide bandgap semiconductors,ZnGa₂O₄ as an emerging wide bandgap semiconductor with multi-functional applications has presented outstanding optoelectronic properties;SrSnO₃ is a wide bandgap semiconductor with perovskite structure and has high transparency and low price;And Cu-based wide bandgap semiconductors have widely been studied as the p-type transparent conductive materials.CuBSe₂ as a kind of wide band gap semiconductor has been reported recently.In this dissertation,we have carried out the first-principles studies on the three kinds of ternary wide bandgap semiconductors to understand their electronic structures and optoelectronic properties and screen the suitable doping elements to improve the conductivity.The details are as follows:1.The electronic structure of ZnGa₂O₄ have been described.The defect formation energies of the intrinsic defects and extrinsic defects on Zn,Ga,and O sites have been calculated.Our calculation results show that ZnGa₂O₄ is a wide bandgap semiconductor with good light transmittance.According to the calculations on defect formation energy,it is found that the intrinsic defect Ga_Znis responsible for the n-type conductivity under the condition of Metal-rich and O-poor conditions.The impurity elements In,Si,and F doped at Zn,Ga and O sites respectively,have shallow transition energy levels and low defect formation energies.Based on the study on the energy positions of the band edges,it is found that ZnGa₂O₄has a relatively low conduction band edge and then is easy to realize n-type doping.Therefore,ZnGa₂O₄ is suggested as an n-type transparent conductive material.2.The electronic structure of SrSnO₃ has been presented.The defect formation energies of intrinsic defects and external defects have been calculated.Our calculation results show that SrSnO₃ is a wide bandgap semiconductor with good light transmittance.Through the calculations on defect formation energy,it is found that the impurity elements Sb and As substituting atSnsite in SrSnO₃can promote the n-type conductivity under Metal-rich and O-poor conditions.Based on the study on the energy positions of the band edges,it is found that the energy position of the valence band maximum of SrSnO₃ is-7.5 e V while that of the conduction band minimum is located at-4 e V,which stand for SrSnO₃has a relatively low conduction band edge and then is easy to realize n-type doping.Therefore,SrSnO₃ is a very promising candidate for n-type transparent conductive material.3.The electronic structure and the defect formation energies of three types of intrinsic point defects（i.e.substitution,vacancy,and interstitial）in CuBSe₂have been calculated.Meanwhile,we have also calculated the defect formation energies of Be,Mg,Ca,Sr,Ba,Zndoped at Cu or B sites in CuBSe₂,respectively.The calculation results show that CuBSe₂ is an intrinsic p-type semiconductor,and the p-type conductivity is derived from V_Cu.Znsubstituting at B site in CuBSe₂ has a shallow transition energy level,which can promote p-type conductivity.Based on the study on the energy positions of the band edges,it is found that CuBSe₂ has a relatively high valence band edge of-5.07 e V and then is easy to realize p-type doping.Therefore,CuBSe₂ can be regarded as a good p-type transparent conductive material.Through the detailed researches on the above three wide bandgap semiconductor materials,it is found that among the wide bandgap semiconductors,the semiconductors with high valence band maximum are easy to achieve p-type doping,while those with low conduction band minimum are easy to realize n-type doping.However,wide bandgap semiconductor is hard to possess a low conduction band minimum and a high valence band maximum at the same time owing to the wide bandgap,it is unlikely to achieve both n-type and p-type doping in the same wide band gap semiconductor.Finally,we demonstrated that the band-edge energy positions are important for selecting n-type and p-type transparent conductive materials.