Investigation On Point Defect Energetics In Oxide Semiconductors By Isotopic Tracing Method

As typical transparent conductive oxide?TCO?semiconductors,ZnO and In₂O₃have both high transparency in the visible region and good electrical conductivity after donor doping?forming ITO,GZO,AZO,etc.?which are almost close to metals.Furthermore,these TCOs have been widely commercialized and applied in modern information technology fields,especially displays,photovoltaics,and smart windows.However,further exploration of ZnO and In₂O₃ applications is impeded by some key issues?the origin of unintentional n-type conductivity in In₂O₃;the controversial arguments about donor doping limit in the TCOs;the great difficulties on p-type doping of ZnO and so on.It is well known that point defects hold a major influence on the physical properties of semiconductors.Based on the in-depth study on the basic physical properties of point defects?including structure,electronic state and the activation energy,etc.?,we try to clarify the above scientific issues,which is expected to lay the foundation for defect control,electrical property modulation and device development.In this paper,ZnO and In₂O₃ isotopic heterostructures were prepared by the radio frequency plasma assisted molecular beam epitaxy?rf-MBE?.Systematic research on the energetic properties of point defects in ZnO-based materials and In₂O₃ was carried out through isotopic self-diffusion experiments combined with multiple characterization technologies,including secondary ion mass spectrometry?SIMS?,temperature-dependent Hall-effect?TDH?measurements,photoluminescence?PL?and time-resolved photoluminescence?TRPL?etc.The following three research achievements are obtained:Firstly,isotopic self-diffusion and tracing method combined with TDH and X-ray photoelectron spectroscopy?XPS?measurements was used to disclose the energetics of oxygen vacancy?V_O?defect in In₂O₃.In₂O₃ isotopic heterojunctions were prepared by rf-MBE with In-rich and O-rich limits,respectively.SIMS analysis indicates that the oxygen diffusion process is predominantly mediated by V_O.The diffusion activation energy of V_O is derived for In-rich?².2 eV?and O-rich?³.1 eV?samples,respectively.The activation of V_O in+2 charge state is observed at⁴00 K,with an activation energy of⁰.4 eV.X-ray photoelectron spectroscopy?XPS?analysis further confirms the V_O defect states in In₂O₃.The combined results establish that the oxygen vacancies are the dominant donorlike native point defects and thus responsible for the unintentional n-type conductivity and the nonstoichiometry of In₂O₃.Secondly,zinc self-diffusion experiments are adopted to disclose the nature of the dominant compensating defect in Ga-doped Zn O isotopic heterostructures.The(Ga _Zn-V_Zn)^-complex defect,instead of the isolated V_Zn^2-,is identified as the predominant compensating acceptor center responsible for the low donor doping efficiency.The comparative diffusion experiments operated by SIMS reveal a⁰.78 eV binding energy of this complex defect,which well matches the electrical activation energy derived from TDH measurements?⁰.82±0.02 eV?.Furthermore,the energy level structure and lifetime of(Ga_Zn-V_Zn)^-complex defect were firstly proposed based on PL and TRPL analysis.These findings contribute to an essential understanding of the(Ga _Zn-V_Zn)^-complex defect and may greatly pave the way towards novel optical defects derived applications.Finally,the energetics properties of V_O in Mg_xZn_1-xO were explored by designing the ¹⁸O self-diffusion experiments.Two groups of Mg_xZn_1-xO isotopic heterostructures with Zn polarity and O polarity were prepared by rf-MBE,respectively,and each group includes three Mg_xZn_1-xO samples with x=0,0.02 and 0.05.SIMS analysis indicates that moderate Mg?x?0.02?doping in ZnO would hinder the formation of V_O.However,as Mg concentration increases,due to the degraded crystal quality,a large number of defects were introduced in Mg_xZn_1-xO,which would weaken the restraing effect of Mg incorporation on V_O.On the other hand,O polarity could increase the formation energy of V_O in Mg_xZn_1-xO and also helps impeding the formation of V_O.The influence of polarity on V_O may be attributed to the polarity potential and the built-in field.The effect of doping Mg on intrinsic defect V_O will inevitably influence other donor-or acceptor-doping in ZnO.Therefore,the research on V_O in the Mg_xZn_1-xO may contribute to the promotion of device performance and even realization of p-type doping in ZnO.