Investigation On Controllable Growth And Microstructure Of Nanostructrued ZnO By MOCVD

Zinc oxide(ZnO)is a semiconductor with a direct wide band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature.Therefore,ZnO is a potential candidate for applications in short-wave optoelectronic devices,such as blue/violet light emitting diodes and laser diodes.Nanostructured ZnO has a diverse group of growth morphologies,which is regarded as the richest family among all the nanomaterials.One important step for fabricating nano-optoelectronics and nanoelectronics devices is to realize the controllable growth of different ZnO nanostructures.It is also necessary to fabricated and characterize complicated structured ZnO nanomaterials to investigate possible distinguished performance in electronics,optics and photonics.In order to widen the spectral range of emission from ZnO based materials and obtain higher luminescence efficiency,alloying ZnO with Mg or Cd is imperative to modulate the band gap.In this thesis,metal organic chemical vapor deposition(MOCVD)method was used to grow nanostructured ZnO.By optimizing the growth parameter,different forms(nanorods,nanocrystals,nanodots)and different morphologies(nanowires, nanowalls,and nanotubes)of ZnO nanostructrus are fabricated.Based upon that,we successfully fabricated self-assembly ZnO-based nanostructures and the growth mechanisms were proposed.1.Different forms of ZnO nanostuctures(nanorods,nanocrystals,nanodots) have been grown by MOCVD method on Si substrates.It was demonstrated that,at small flow rate of the source materials the growth process was controlled by growth kinetics and the obtained ZnO samples transformed from nanodots to nanocrystals at elevated temperatures;while,the growth process was controlled by the growth thermodynamics and the obtained product transforming from nanocrystals to nanorods at elevated flow rate.2.Different morphologies of ZnO nanostructures have been grown on silicon substrates by MOCVD.It is proposed that the reason for the one-dimensional nanostructures to the nanowalls or nanotubes is the zinc-rich growth condition at relative high temperature.3.Lamellated ZnO nanorods have been grown on silicon substrate by MOCVD. The ZnO nanorods have single hexagonal wurtzite structure and prefered orientation along c axis direction.4.ZnMgO nanorod arrays have been synthesized by metal-organic chemical vapor deposition using diethylzinc and magnesium nitrate as the precursors.No oxidant source was used.The ZnMgO nanorods were homogeneous and uniform.The growth process was investigated in detail and a possible growth mechanism was proposed.It is believed that this growth method may avail the uniform distribution of Mg in ZnMgO nanorod.The blueshift of near-band-edge emission of PL spectrum could be readily identified as compared with ZnO crystals,indicating the band-gap engineering in the ZnMgO nanoscale system.5.ZnMgO pleated nanosheets and quasi-nanotubes were synthesized in the same region on Si substrate using[Mg(H₂O)₆](NO₃)₂ and zinc diethyl as the reactant source. The nanosheets are periodically pleated with the angles of 120°or 60°between two adjacent pleats.Some of the nanosheets fold into quasi-nanotubes.Homoepitaxial interconnections are observed at the boundaries of the pleats,and the whole pleated nanosheet is a single wurtzite crystal.The growth mechanism is discussed,which is proposed to be a combination of vapor-solid process of two-dimensional growth of the(1210)planes and their random along <1010> directions.6.ZnMgO pleated nanosheets and quasi-nanotubes with different Mg content were obtained by changing the zinc diethyl flow rate via metalorganic chemical vapor deposition.The thickness of the nanosheets decreased with the increase of Mg content in the ZnMgO alloy.The Mg incorporation caused the blue-shift of near-band-edge emission of photoluminescence,also(002)peaks position shifted to larger value in X-ray diffraction.The mechanism of the red-shift and broadening of the E2 modes of Raman spectra were discussed.Two additional bands at 387 cm^-1and 622 cm^-1were observed,which were presumably attribute to the host lattice defects induced by Mg incorporation.7.Dislocations with Burgers vector of 1/3<11(?)3> were observed by high-resolution transmission electron microscopy in the ZnMgO nanosheets with Mg content of 2.5 at.%.The Mg incorporation enhances oxygen vacancies in the crystal lattices,thus introducing the dislocations in the ZnMgO pleated nanosheets.The electrostatic potential in the vicinity dislocations was determined to be 2.38 V.