The Optical Properties Of In-doped ZnO Superlattice Nanostructures

Due to the advantageous optical and electrical properties,homogenous compounds of InMO₃?ZnO?_m?M=In,Ga,Al?have attracted a tremendous amount of attention in re-cent years.Various quasi-1D InMO₃?ZnO?m nanomaterials have been successfully syn-thesized by many groups.However,previous works largely focus on the preparation and characterization of these homogenous compounds.The research on the physical prop-erties,such as optical and electrical properties,is rather rare.Thus,the key point of the next-stage research of the homogenous compounds of InMO3?ZnO?m is to deeply study their physical properties.As described in this thesis,a series of studies were performed mainly targeted at the In₂O₃?Zn0?m?IZO?nanostructures,focusing on the optical and electrical properties of IZO planar superlattice nanoribbons,and deep defect emissions of IZO superlattice nanorods.This thesis consists of five parts,which have different themes.The first chapter is the introduction,which presents the methods of synthesis,basic optical properties of qusi-1D nanostructures,and their challenges in device appli-cations nowadays are also discussed.The second chapter discusses the optical and elec-trical properties of IZO planar superlattice nanoribbons.The third chapter describes the synthesis of the IZO superlattice nanorods,and studying of their deep defects through photoluminescence?PL?and electron paramagnetic resonance?EPR?.The fourth chap-ter includes two subsections,which are about the growth and optical properties of ZnO nanobelt-In₂O₃ octahedron complex structures,as well as the synthesis and whispering gallery mode lasing of In-doped ZnO superlattice nanorods with a layer-structured sur-face,respectively.The specific contents of chapters two through four are summarized as:1.In₂O₃?ZnO?m planar superlattice nanoribbons with small m?m=3,5?were syn-thesized by a chemical vapor transport method.The low-temperature PL spectrum of the planar superlattice nanoribbons shows only one dominant broad asymmetric exci-tonic emission with a peak at 3.37 eV,which is about 14 meV higher than the bound exciton emission in pure ZnO.For small m,the thickness of an In/Zn-O slab is slightly less than the effective Bohr radius for exciton in bulk ZnO,thus the blueshift due to quantum confinement effect exceeds the redshift resulting from the heavy doping of In,leading to the observed blueshift for our samples.From the temperature dependence of the PL intensity,the activation energy of donor bound exciton bound to In donors in our sample was derived to be around 18.3-25.7 meV.We also obtained the shrinkage param-eters of the band gap of the nanoribbons,alpha=1.0-1.2 meV/K and beta=500-520K,by using the Varshni empirical formula.Meanwhile,an interpretation of the broadening mechanism of the excitonic peak was given that an inhomogeneous broadening mech-anism was dominant at low temperature,while a homogeneous broadening mechanism was dominant at high temperature.Compared with the pure ZnO nanoribbons,the deep level emission?DLE?from native defects in IZO superlattice nanoribbons was greatly reduced,which may be related to the formation of the deep level complex due to heavy doping of In.Finally,the electrical ?-? properties of the IZO nanoribbons were prelim-inarily studied.As the bias voltage increased,the I-V curve deviated from a linear zone at around 3.7 eV and exhibited a nonlinear characteristic even under the ohmic contact measurement condition.The measured resistivity of the IZO superlattice nanoribbons was about 3.0 Q.cm,which was lower than the reported In-doped ZnO nanoribbons.2.In₂O₃?ZnO?m superlattice nanorods?m=17,19?were synthesized via a chemical vapor transport method,and their superlattice structure that the alternate layers stacked along the growth direction?c-axis?was determined by XRD,Raman,TEM and EDS.An unknown peak at around 3.325 eV marked as A was observed in the low-temperature PL spectrum of the IZO nanorods,which may be related to sturcture-telted defects.It was found that the deep level emissions may originate from VZn/VZn-R?VZn related acceptor defects?,or Oi/Oi-R defects,by the comparison of the room temperature PL spectra of the IZO nanorods annealed respectively under H2 and air atmosphere.The conclusion was further confirmed by the result of XPS which revealed the O-rich condition in the IZO nanorods,The EPR measurements of the IZO nanorods exhibited two EPR signals,g=1.9524 and 1.9443,coming from VNn-and InZn0 defects,respectively.The EPR signals of the IZO nanorods annealed respectively under H2 and air atmosphere were also obtained.Combining EPR and PL investigations of the IZO nanorods annealed under different atmospheres,it can be concluded that the DLE is mainly derived from the VZn-or VZn-R complexes,and the emission relating to Oi/Oi-R defects emerged as a red luminescence band.It is noteworthy that the intensities of the g?1.96 lines were found gradually decreasing after continuous illumination of our sample by white light,and to increase to their starting value after the light was switched off,which shows a reversible process.The intensity behavior under white light excitation suggests that initially the defects of VZn and InZn are in paramagnetic charge state before illumination,while the defect of VZn-R may be in the?InZn+-VZn-?0 paramagnetic charge state.A two-charge-state model was proposed to interpret the photoionization kinetic process of VZn.Besides,room temperature ferromagnetism in the IZO superlattice nanorods was obviously observed,which may be related to VZn induced by self-compensation.3.?1?.ZnO nanobelt-In₂O₃ octahedron complex structures were first synthesized at 1480? via a chemical vapor transport method.ZnO nanobelt are the body part of the complex structure,and its top tip is capped by an In₂O₃ octahedron particle.As per previous reports,a growth mechanism was therefore proposed that the In₂O₃ particles played a self-catalytic role in the growth process.The PL result revealed that the In₂O₃ octahedron particle could partially passivate the nanobelt surfaces,bringing about an improved near band edge emission and the suppression of the deep level defect emission.Lastly,based on the EPR result,the DLE may derive from the VO defects.?2?.In-doped ZnO superlattice nanorods with a layer-structured surface were synthesized by a chemical vapor transport method.The morphology of the nanorod shows a hexagonal cross-section with a layer-structured surface in the SEM image.There were eight sharp peaks that could be observed in the visible region of the ?-PL spectrum from individual layer-structured nanorod.By calculating the effective optical path length,we found that these peaks were attributed to a whispering gallery mode?WGM?formed in the hexagonal shaped microcavity of the individual layer-structured nanorod,rather than a normal F-P resonant mode.However,the formation of WGM may benefit from the layer structure in the nanorod.The findings mentioned above could pave the way towards the integration of the In-doped ZnO layer-structured nanorods for future visible micro-laser applications.