The Study On Local Electrical Characteristics Analysis And Photoelectricity Properties Of ZnO Materials

Due to the unique optical and electrical properties,zinc oxide(ZnO)materials have been the best candidate materials for the development of optoelectronic devices.As II-VI semiconductor material,ZnO has a a high exciton binding energy of 60 meV at room temperature,with wide bandgap of 3.37 eV and and has a good performance.ZnO nanomaterials can also be used as gas sensors,phosphors,energy storage devices,varistors,etc.,and have attracted great interest from researchers.In this paper,the structure and electrical characteristics of ZnO thin films grown on Si(111)by metal-organic chemical vapor deposition method were investigated.Meanwhile,synthesis and local electrical characterization of ZnO nanorods and micro walls grown on cracked GaN/Si(111)by aqueous way were also involved.The optical and corresponding local electrical properties of hexagonal ZnO microdisk on InN substrate by hydrothermal method were also investigated.Furthermore,the ultraviolet photodetector prototype devices based on ITO/MgO/ZnO structures were fabricated.The main investigation contents were listed as follow:1.ZnO nanostructures were grown on p-GaN/Si(111)and n-GaN/Si(111)by aqueous method.The structural and optical properties of the nanostructures were characterized by X-ray diffraction,scanning electron microscopy,atomic force microscopy,micro-Raman and photoluminescence.Our measurements showed the distinct appearance of ZnO nanostructures grown on different GaN substrates.The ZnO nanorod array with a hexagonal wurtzite structure was grown on the n-GaN/Si substrate,whereas the film-like structure appeared on p-GaN/Si.The difference in their morphology was due to the density of threading dislocations in the different doped GaN layers that can affect the ZnO nanorod density.Meanwhile,the ZnO nanostructures grown on n-GaN/Si showed superior optical property with lower tensile stress compared with those on p-GaN/Si.2.ZnO nanorods and microwalls were grown on cracked GaN/Si(111)using aqueous method.The cracks of GaN could direct ZnO to grow and form one-dimensional nanorods and two-dimensional microwall networks.ZnO nanostructures nucleated in the sidewall of crack and selectively grew along the crack direction on the GaN substrate.Due to the diversity of the crack,the nanostructures showed interconnected network structures.Time-dependent evolution of ZnO nanostructure morphology showed that the network nanostructures were formed via self-organized growth by interconnection of nanorods and microwalls.Meanwhile,electrical characterization of individual ZnO nanorod and ZnO microwall were performed by using C-AFM.Effect of anisotropy properties on the conductivity of a single nanorod was investigated,and enhanced conductivity for the off-axis facet plane present on as-grown ZnO microwalls was detected.The current maps of ZnO nanorods and microwalls were simultaneously recorded with the topography by C-AFM.Meanwhile,the local current-voltage characteristics showed that the sidewall had lower Schottky barrier height,indicating that the off-axis sidewalls planes were more electrically active than c-plane ZnO.3.Hexagonal ZnO microdisks were obtained by using InN/sapphire as substrate via the aqueous method.The growth mechanism of the ZnO microdisks was also proposed and demonstrated.It was speculated that In(OH)3 buffer layer was formed due to the unstable characteristics of InN template.The(0001)growth direction of ZnO was limited by In ion diffusion.Meanwhile,the lasing from the ZnO microdisk was observed.The optical characteristics and mode spacing calculation of the ZnO microdisks were investigated by micro-photoluminescence.Whispering gallery mode(WGM)lasing from the hexagonal ZnO microdisk was observed at moderate excitation density.Moreover,the C-AFM measurement presented local current-voltage(I-V)characteristics of the side facets of ZnO microdisks,however,no current was detected on the top facets of ZnO microdisks.Furthermore,the magnitude of the local current was also dependent on the height of the microdisks.4.ZnO thin films were grown on Si(111)substrates by low-pressure metal-organic chemical vapor deposition.The crystal structures and electrical properties of as-grown sample were investigated by scanning electron microscopy(SEM)and conductive atomic force microscopy(C-AFM).It can be seen that with increasing growth temperature,the surface morphology of ZnO thin films changed from flake-like to cobblestones-like structure.The thickness of the ZnO film also gradually increased,reaching a maximum at 600°C.By contact-mode C-AFM,the current maps were simultaneously recorded with the topography.Conductivity for the off-axis facet planes of ZnO grains was greatly improved.The results indicated that the off-axis facet planes were more electrically active than the c-plane of ZnO flakes or particles probably due to lower Schottky barrier height of the off-axis facet planes.5.ZnO thin films were grown on sapphire substrates with low-pressure metal-organic chemical vapor deposition method.Six types of samples were obtained by controlling the pressure of reaction chamber.The surface morphology and electrical properties of the samples were analyzed by atomic force microscopy.ZnO/sapphire samples with good quality were selected as substrates for further investigation.A simple two-step aqueous method was employed to grow MgO nanostructures on n-ZnO/sapphire at low temperature.The obtained thin MgO nanostructures were uniformly distributed on the surface of n-ZnO/sapphire substrates and showed the sheet-like structures.Meanwhile,ultraviolet(UV)photodetector prototype devices based on ITO/MgO/ZnO structures were fabricated in a simple way.The obtained UV detector based on the ITO/MgO/ZnO showed excellent UV sensing properties due to the MgO nanostructures insulating layer.