Zinc Oxide Doped With First-principles Study Of The System Electronic Structure

In the past decades,with the development of photoelectric information technology,the study on the ultraviolet(UV) luminescence device has become more and more important.ZnO is a wide-band semiconductor material,and the important photoelectric material due to its stimulated emission in UV band.Much attention has been paid on the research of ZnO.The as-grown ZnO is an n-type semiconductor with many donor defects such as O vacancy(V_o) and interstitial Zn(Zn_i),so it is very difficult to dope ZnO to a p-type semiconductor because of the compensation by the donor defects.Thus,the development and application of ZnO-based optoelectronic devices are greatly limited due to the lack of ZnO p-n junctions,so the p-type doping of ZnO has become a focus task.Besides,ZnO based optoelectronic devices,such as UV detector,generally work in the solar-blind region(220～280nm).The cut-off wavelength of solar-blind UV detectors should be shorter than 280nm(4.SeV),a value much larger than the intrinsic bandgap(3.37eV) of ZnO.So ZnO films with wider bandgap are desired.The bandgap of ZnO system can be enhanced by doping Be and Mg;on the other hand,Be doping will lead to large lattice mismatch because of the big difference between the ionic radius of Be²⁺ and Zn²⁺, and phase segregation has been observed in the Mg doped ZnO system.Computational materials and materials design combined with computer techniques play an important role in materials science.In this thesis,the doping problems of wurtzite ZnO have been studied by the first-principles approach based on the density functional theory(DFT).The main contents are as follows:(1) The structure,electricity properties,optical properties,intrinsic defects,and the application and developments of ZnO are given.The calculation tool-CASTEP and its theoretical foundation-DFT are briefly discussed.(2) The electronic structure of ZnO has been investigated by generalized gradient approximation(GGA).The electronic density of states and the partial density of states of ZnO have been analyzed.The calculated results indicate that ZnO is a direct bandgap semiconductor material with gap of 0.97eV.(3) Three kinds of defects,the substitution of Zn by C,O by C,and two kinds of substitution coexisting in ZnO,have been studied.The electronic structures of the systems have been calculated.The obtained results show that when Zn is substituted by C,the system becomes an indirect bandgap semiconductor from the direct bandgap semiconductor,and the donor levels are formed;when O is substituted by C,the acceptor levels are formed nearby the top of the valence band,thus the p-type transformation of the system is achieved,and high acceptor concentration is unfavorable for the p-type transition,when the two kinds of substitution coexist, the acceptor levels are compensated for all cases,which is unfavorable for the p-type transformation of the system.(4) The electronic structures of Be and Mg doped ZnO and Be-Mg co-doped ZnO systems have been calculated respectively,and the mechanism for bandgap broadening has been discussed.A comparative study on lattice parameters,formation energies,and bandgap of the three doping systems has been given.The obtained results show that the band of the system can both be enlarged by doping Be or Mg,but the systems become unstable with the increase in impurity concentration.Especially the radius of Be²⁺ is much smaller than Zn²⁺,which leads to a large difference of the lattice parameters between the doped ZnO system and pure ZnO system. For Be-Mg co-doped ZnO,the lattice parameters are more closer to pure ZnO,the systems become more stable compared with Be doped ZnO,and the bandgap of Be-Mg co-doped ZnO system is larger than Mg doped ZnO system in the same doping concentration.For certain doping concentration,a solar-blind region bandgap can be achieved.