The Study On Characteristics Of ZnO-based Materials And Detectors

As a new kind of photoelectrical material,ZnO has tremendous development potential and is being paid much attentions in many aspects,such as material growth,doping modification,device fabrication,and so on.Recently,heavy rare earth element yttrium?Y?has been playing an important role in doping modification of ZnO by interacting with the native defects like zinc interstitial?Zn_i?.Thus,it is necessary to further study the interaction mechanism between yttrium and Zn_i in ZnO.In addition,ZnO material,which has strong band gap engineering and other advantages compared to III–V materials,has been one of the promising candidates for QCDs of mid-infrared regions and even the near-infrared.To these mentioned above,we study the Y and/or Zn_i doped ZnO systems and the ZnO/MgZnO QCD in this master's thesis.The details are as follows:1.The geometric structures of ZnO,yttrium doped ZnO?YZO?,Zn interstitial doped ZnO?ZIO?and yttrium doped ZnO with Zn interstitial?YZIO?systems are investigated using first-principle plane-wave ultrasoft pseudopotential method.All calculations are operated in CASTEP module.First,the crystal structures of YZO with different Y doping concentrations are calculated,which confirms the rationality of the YZO structure models.Second,the crystal structures and doping stability of ZIO,in which the Zn_i atoms occupy different interstitial positions,are investigated.It is found that the center of octahedron surrounded by zinc atoms(oct_Zn)is the easiest position for Zn_i to take up in ZIO.And the results about formation energy of Zn_i in ZIO and YZIO are compared and show that doping yttrium can decrease the formation energy of Zn_i defects adjacent to yttrium.Finally,selecting the center of oct_Zn as the position of Zn_i,the ZIO(Zn_1.0625O),YZO(Zn_0.9375Y_0.0625O)and YZIO with varying Y doping concentrations(ZnY_0.0625O and Zn_0.9375Y_0.125O)are created.The electronic structures and optical properties of these systems are further obtained.The results show that the bandgap of YZIO is bigger than that of ZnO and will increase with the increase of Y doping concentration.Compared with ZnO,YZO and ZIO systems,there is an impurity level?E_r?near the center of the forbidden band in YZIO,which will enhance the luminescence peak in yellow region.Besides,the absorption property of YZIO system is improved in almost all of the visible and UV regions.2.The ZnO/Mg_0.3Zn_0.7O QCD consisting of thirty periods of ZnO active absorber quantum wells and phonon cascade is designed.The active absorbers are responsible for absorbing photons and phonon cascades are used to transport electrons.The effect of temperature on the properties of the ZnO/Mg0.3Zn0.7O QCD is studied through the established theoretical model.First,the conduction band energy,subband energy levels and wave functions of the system are calculated by solving the Schr?dinger-Poisson equation self-consistently.A temperature dependent bandgap and an energy dependent effective mass that accounts for non-parabolicity have been included in the conduction band model.The Fermi levels of the detector under different biases and temperatures are derived from the condition of charge neutrality.Second,the dark current,R₀A,and responsivity of the ZnO/Mg_0.3Zn_0.7O QCD is investigated by simulating the bound-to-bound electronic transport processes under dark and illumination circumstances.Finally,the detectivities are calculated considering Jonson noise and photon noise due to blackbody background radiation.The results show that,with the temperature increasing from 50 to 300K,the dark current increases,and R₀A decreases,so that the detectivity would decreases too.The responsivity of the ZnO/Mg_0.3Zn_0.7O QCD shows good temperature stability without obvious redshift and the peak responsivity only drops 14%.The research could provide some theoretical references for fabricating ZnO-based materials and detectors.