Study On Photoluminescence And Electroluminescence Properties Of Nanostructured ZnSe/ZnS Multilayer Films

With the rapid development of the information era,the requirements for luminescent materials and display devices are getting higher and higher.Although many new breakthroughs and advances have been made in the display field,there are still some new issues that need to be resolved.Since the advent of GaN-based blue LEDs,sapphire,SiC and Si substrates LEDs have been reported,and their performance has also been improved greatly.However,there is a 16.7%lattice mismatch and a large coefficient of thermal expansion between the Si substrate and GaN.Therefore,the GaN thin film prepared by epitaxial growth on the Si substrate will lead to defects,cracks,dislocations,or bends easily.Although these problems can be substantially overcome in the technical field by introducing AIN as a buffer layer,it increases the difficulty of the preparation process.In the field of organic light-emitting diodes(OLEDs),the lifetime of devices and the stability of organic materials also restrict the development of OLEDs.In addition,quantum dot devices have become a new research hotspot based on the advantages of quantum dots with good light stability,high luminous efficiency and adjustable luminescence spectrum.In terms of synthetic materials for quantum dots,the current research is mainly based on Cd.The toxicity of Cd limits the application of quantum dots and related quantum dot luminescent devices,especially in the biological field.The preparation of non-toxic and efficient QD-LEDs is a new challenge.Therefore,the discovery of new alternative materials and the preparation of stable,non-toxic,high luminous efficiency and long life of electroluminescent devices are of great significance for the development of the display industry,especially in blue-LEDs.As a non-toxic,wide bandgap(2.7 eV)material,ZnSe has been a popular material for the development of blue-green light-emitting devices.At present,ZnSe has been widely used in the fields of laser,nonlinear optics,photodetection and luminescence.In this thesis,we prepared nanostructured ZnSe/ZnS multilayer films by electron beam evaporation technique,and then used it as a light-emitting layer to prepare blue-LEDs.It not only utilizes the properties of ZnSe,but also use the excellent properties of nanostructure.It provides a new idea for the study of blue-LEDs and makes a contribution to the research of white LEDs.The main results of our research work are as follows:(1)We studied the properties of single-layer ZnSe film.A single-layer ZnSe film was deposited by electron beam evaporation,which belonged to the cubic structure and tended to the(111)crystal orientation.Firstly,ZnSe film was annealed in N2,which showed strong absorption in the blue region and did not show blue emission of ZnSe.The effects of annealing temperature on crystal structure,transmission spectrum,refractive index,absorption coefficient,reflection coefficient,optical band gap,etc.were investigated.Secondly,ZnSe film was annealed in O2.The cubic structure of ZnSe film was completely oxidized into hexagonal wurtzite structure of ZnO film.The PL spectrum of the ZnO film consisted of a strong emission peak at 370 nm and a weak emission peak at 525 nm,which corresponded to the inter-band emission and defect-related emission of ZnO,respectively.By optimizing the annealing temperature and time,the optimal conditions were obtained.In addition,the effect of low temperature environment on the PL spectrum of ZnO film was also studied.(2)Nanostructured ZnSe/ZnS multilayer thin films were prepared by electron beam evaporation,sho,wing blue emission.The crystal structure of the multilayer thin films were cubic structure of ZnSe and wurtzite structure of ZnS.The excitation spectrum of the nanostructured ZnSe/ZnS multilayer films was located at about 350 nm.The PL spectrum contained two emission peaks:a strong and narrow emission peak at 441nm originating from the near-band emission of ZnSe;a weak and broad emission peak at 550 nm coming from defect-related emission,such as interstitials,vacancies,lattice distortion and structural defects.The photoluminescence properties of nanostructured ZnSe/ZnS multilayer films were optimized from different aspects,including ZnS layer thickness,ZnSe layer thickness,annealing temperature,annealing time,and annealing gas flow rate.In addition,the low-temperature PL spectrum and stability of nanostructured ZnSe/ZnS multilayer films were also characterized.(3)An organic/inorganic composite blue electroluminescent device with a multilayer structure(ITO/PEDOT:PSS/EML/MgO/n-Si/Ag)was prepared,where nanostructured ZnSe/ZnS multilayer films were used as light-emitting layer.The device was subjected to various characterization tests,such as XRD,SEM,PL,EL,and ?-?,and the luminescence mechanism was also studied.The electroluminescent mechanism of the device was electron and hole injection combined luminescence.The MgO layer could significantly improve the performance of the device.Firstly,it could improve the crystal quality of the emitting layer and improve the light-emitting intensity.Secondly,as a hole blocking layer,MgO could increase the hole concentration,reduce the quenching of carriers,and improve electroluminescence properties of the device.Finally,we further improved the performance of the device including annealing temperature,silicon resistivity,the thickness of MgO and p-type layer.