Preparation Of Low-dimensional ZnO Nanostructures And Heterostructure Ultraviolet LED

Zinc oxide(ZnO)material is a typical third-generation wide bandgap semiconductor with a wide and direct bandgap of 3.37 eV.ZnO has a high exciton binding energy and acts as an alternative to achieve efficient ultraviolet exciton luminescence using GaN material in the room temperature or higher operating temperature.Compared with ZnO thin films and bulk materials,ZnO nanostructures show a variety of morphological features and the diversity of preparation,showing different optical properties.By employing the low-dimensional ZnO nanostructured materials in light-emitting diodes field,it will be great to reduce the cost of preparation of the device.Because the stable and reliable p-type ZnO has not been made a breakthrough,the development of ZnO-based ultraviolet light-emitting devices is the main content.By introducing the low-cost one-dimensional ZnO nanorods and zero-dimensional ZnO quantum dots,the heterostructure of ZnO acts as the entry point.Meanwhile,the structure and properties of low-dimensional ZnO nanostructures are studied systematically.Combined with their structural characteristics,the different device structures are constructed to realize the efficient short-wavelength ultraviolet emission of ZnO.First of all,ZnO seed layer is produced on the Si substrate by using the liquid droplet overlay method for the subsequent growth of ZnO nanorods,a large-area close packed monolayer polystyrene(PS)microspheres thin film is prepared by a self-assembly drift method.Meanwhile,non-close packed PS template obtains by the reactive ion etch technology.Then TiO2 sol fills the non-close packed PS microspheres with the appropriate concentration,and toluene removals PS microspheres to obtain a large-area inverse opal template.The ZnO nanorods are grown by hydrothermal method using the inverse opal template.Finally,the one-dimensional ZnO nanorod arrays with vertical structure are fabricated.The effects of reaction time and reaction concentration on the morphology of ZnO nanorods are investigated.The growth mechanism of ZnO nanorods by hydrothermal method is discussed through analyzing the growth morphology evolution of ZnO nanorods.Patterned and ordered ZnO nanorods array is consistent in diameter and height,the nanorods grow along[0001]direction.Sharp and strong near band edge emission and weak deep level luminescence intensity of ZnO nanorods array indicates that the ZnO nanorods have the good optical properties.Meanwhile,B1(low)model has been absent in the Raman test,it shows that the patterned ZnO nanorod arrays have a good crystal quality.Secondly,GaN quantum dots are grown on p-GaN by Ga droplet method.The effects of different annealing temperature,NH3 flow rate in growth and cooling process on the diameter,height and density of GaN quantum dots are studied.Due to the nanodrill effect,GaN quantum dots appear the nonohole morphology in the growth process.Using the characteristics of high surface energy of GaN quantum dots,the growth of one-dimensional ZnO nanorods by hydrothermal method using GaN quantum dots as the template is studied.The experimental results show that the density of ZnO nanorods can be controlled by adjusting the density of GaN quantum dots,and it shows that the GaN quantum dots play a nucleation role in the growth of ZnO nanorods.In addition,we find that the growth of ZnO nanorods with nanohole morphology by using the GaN quantum dots as a template will appear on the two different locations,they are located on the quantum ring and ring on the wall,and we explore the mechanism of growth of ZnO nanorods using the GaN quantum dots as a templateThirdly,one-dimensional ZnO nanorods are prepared by Al-doped ZnO(AZO)transparent conductive glass.In order to avoid the effect of ZnO seed layer with crystal defects on the luminescent properties of LED devices and take full advantage of the single crystal properties of one-dimensional ZnO nanorods,a new direct surface bonding structure is proposed.The high-quality heterojunctions are fabricated between p-GaN and the n-ZnO nanorods,and the near-ultraviolet emission centered at 390 nm.There is no defect visible light in the electroluminescence spectrum of the device.It is found that the near-ultraviolet emission of the device is derived from the n-ZnO nanorods,p-GaN and the interface,the luminescence mechanism of the device is given by combining the energy band structure.Fourthly,well-distributed 0D ZnO QDs with an average diameter of about 7 nm are prepared via NaOH and acetate dehydrate through low-temperature solution method.MgO is intruduced as the electron blocking layer between the ZnO layer and the GaN layer so as to improve the recombination in ZnO,and we design all-inorganic the n-ZnO QDs/MgO/p-GaN LED.From the test of the optoelectronic performance of the devices,we find that the device in the forward bias(n-ZnO as negative)has no light emission,the EL spectra show that light emission can only be detected when the LEDs are applied with reverse bias.With the increase of reverse bias,deep level ZnO defect related visible light is suppressed.When the reverse voltage achieves to 25 V,the device obtain the pure 370 nm UV emission,the width at the half maximum is only 7.5 nm.We use the energy band diagram of the device under the reverse bias to explain the causes of this phenomenon.For the evaluation of the stability of the device,the device is placed in the ambient air to exposure for one month,the experimental results show that near band edge emission in ZnO decreases,while the deep level visible emission increases,and the adsorption of O2 and OH-(water vapor)on the surface of quantum dots as the acceptor and donor type affect the properties of the device,it increase the surface radiative recombination and intermediary deep level recombination,thus resulting in the decreased efficiency of the devices.At last,to further improve the quantum efficiency of heterojunction LEDs,the LSP of Ag nanoparticles are incorporated into the ZnO quantum dots.By combining the advantages of both quantum confinement effect and metal LSP,we look forward to improve of the emission efficiency of UV LED.A facile synthetic method to produce Ag nanoparticles with controllable shapes using Poly(vinylpyrrolidone)(PVP)as surfactant is introduced.The Ag nanoparticles with a diameter of about 40 nm have a LSP peak located at 400 nm By optimizing the MgO thickness,which can suppress the nonradiative Forster resonant energy transfer between Ag and ZnO,a 4.3-fold EL enhancement is achieved.We put forward the heterojunction interface charge transfer model to explain the luminescence enhancement.The emission enhancements mainly result from the increased spontaneous emission rate and internal quantum efficiency of ZnO quantum dots.