The Manipulation of Localized Carriers in ZnO Nanostructures and Their Photoelectronic Properties

Author:Li Ru Xue

Supervisor:wang xiao hua wei zhi peng


Degree Year:2018





ZnO(Eg=3.37eV)is one of typical and important wide bandgap semiconductor materials that its exciton binding energy(60meV)is far more than the thermal energy(26meV),ZnS(40meV)and GaN(25meV).These properties make ZnO as a suitable candidate for optoelectronic devices operating above room temperature.However,the surface state is very easily introduced to ZnO and gives rise to nonradiative recombination and has a very negative impact on the optical properties of ZnO material.This phenomenon is particularly prominent in ZnO nanostructures due to their high specific surface area.It is reported that the heterojunction coating is a one of effective way to eliminate the surface state,and by introducing nanoparticles or shells on the surface or intersurface of ZnO nanowires can bring some localized states to the structures.That makes the improvement of the photoelectric properties of ZnO.However,the above researches only report the improvement of the composites and devices by localized states,but the relationship between the properties of localized carriers and the material have not been disscused in systematically.In addition,the random introduction of the localized states will result in the inhomogenous potential well that is easy to cause non-radiative recombination.Thase phenomena will also give rise to a very negative impact on the optical properties of ZnO baesd devices.In this paper,we mainly study the influence of the localized states dependent the photoelectric properties of ZnO nanostructures,and the physical model as well as the theoretical model will be proposed and used to explain these phenomena.Furthermore,the optoelectronic properties of ZnO nanostructures and devices optimized under specific localized states are obtained.The main research works are carried out as follows:1.Seed layer of ZnO was prepared by ALD method on sapphire substrate,and ZnO nanowires with good crystal quality were grown by hydrothermal method.The ZnS nanoshells were coated on the ZnO nanowires by the chemical bath deposition(CBD),which the luminescence characteristics of localized states in ZnO near band are observed.2.ZnO/ZnS core-shell nanostructures prepared by hydrothermal synthesis combine with CBD method have a large localized energy that is favorable for the manipulation of localized states.The atoms at the interface of ZnO/ZnS were rearrangeed by means of rapid thermal annealing.It was found that the zigzag arrangement at the interface of ZnO and ZnS was the main reason for the formation of localized states.After the thermal treatments,the interfacial atomic arrangement is neat and the typical PL phenomenon of the localized state gradually disappear after a higher thermal-treatment temperature.This phenomenon indicates that only the introduction of appropriate localized states is conducive to enhance the ultraviolet luminescence of ZnO nanostructures.3.Three different degrees of localized states were introduced by three kinds of ZnS nanoparticles with different coverage were coated on the ZnO nanowires by different CBD growth periods.The optical properties of ZnO-based heterojunction LED devices under different degrees of localized states were obtained.The Gaussian fitting of the EL spectra and the bimolecular radiation formula of the semiconductor were used to analyze thes phenomena.The light extraction efficiency of the device has been greatly improved,and the emission near the ultraviolet 380 nm has been achieved.The performance of device has been enhanced up to 8 times,and the purpose of manipulating the localized states to improve the light emitting device of ZnO has been achieved.