Luminescence Properties Of ZnCdO Thin Film And Corresponding Heterojunction Devices Modified By Ag Nanoparticles

ZnO is a transparent oxide semiconductor material with a band gap of 3.37 eV at room temperature.By doping the Cd element into ZnO to form the ZnCdO alloy,the energy gap of the ZnO can be effectively reduced,with the photoluminescence being adjusted from the ultraviolet to the green region.However,ZnO-based heterojunction light emitting diodes are usually fabricated by using p-type GaN rather than p-type ZnO because of the difficulty on preparing p-type ZnO.Moreover,due to the low carrier concentration and mobility in p-GaN,the luminescence of n-ZnO/p-GaN heterojunction devices mainly occurs on the p-GaN side.In order to solve the weak light-emission problem in the n-ZnCdO layer and improve the light-emitting efficiency of the n-ZnCdO/p-GaN heterojunction device,we have attempted to exploring two main aspects by i)accelerating the carrier recombination rate in the n-ZnCdO layer by inserting a MgO high-resistance layer into the n-ZnCdO/p-GaN heterostructure,and ii)improve the light-emitting efficiency of the heterojunction device by decorating Ag nanoparticles.The main contents of the thesis include the following parts:（1）The Ag film was evaporated by adopting a vacuum thermal evaporation method on a sapphire substrate（c-Al₂O₃）,and the Ag nanoparticles were then obtained through a high-temperature vacuum annealing process.It has been found that the morphology and arrangement of the Ag nanoparticles can be controlled by changing the thickness and the annealing time of the Ag film.Ag nanoparticles with good circular symmetry could be obtained by annealing at the temperature of 300°C for 45 min.Also,with the increase of the thickness of the Ag film,the extinction spectrum peak corresponding to the dipolar resonance in the Ag nanoparticle gradually redshifted,and at the same time the half-height width was widened.（2）ZnCdO alloy films were prepared through the PLD method,at the temperature of300°C on the c-Al₂O₃ substrate at different oxygen pressures by using a Zn_0.8Cd_0.2O ceramic target as the source material.It has been found that the Cd component in the thin film increases by raising the oxygen pressure,and the emission peak of the ZnCdO in the photoluminescence（PL）spectrum at room temperature shifted from the ultraviolet to the green light at⁴95 nm.Enhanced luminescence（about 3-fold）of the ZnCdO thin films could be obtained when the dipole resonance peak of the Ag nanoparticles matched with the PL emission peak position of the ZnCdO as a result of the localized surface plasma（LSP）resonance coupling.（3）The n-ZnCdO thin film was prepared on the p-GaN substrate by PLD method.The high-resolution transmission electron microscope indicates a good heterojunction interface,however,the light-emitting efficiency of the constructed n-ZnCdO/p-GaN heterojunction device was still low,and the light emission mainly comes from the p-GaN layer.Light emitting from the n-ZnCdO layer was realized by inserting a MgO limiting layer into the heterostructure interface,indicating that the diffusion of electrons from the n-ZnCdO to the p-GaN was effectively blocked by the MgO layer.Furthermore,the electroluminescence of the ZnCdO could be tuned from ultroviolet to green band by adjusting the film growth conditions.The effect of the thickness of the MgO-limiting layer on the light-emitting performance of the device was studied,and it was found that the appropriate MgO thickness（20 nm）could effectively limit the diffusion of electrons from the ZnCdO to the p-GaN.However,when the MgO layer was too thick（83 nm）,the device could not realize the injection of the carriers,and the luminescence mechanism turned to the interfacial defect-induced light-emitting by the high field.（4）The influences of in-situ annealing on the morphology and the extinction spectrum of Ag nanoparticles were studied,and through which an optimized processing condition was established for achieving enhanced luminescence of the ZnCdO thin films via the LSP-resonance coupling effect of decorated Ag nanoparticles.Then the Ag nanoparticles were fabricated through this process for the design of the n-ZnCdO/Ag NPs/MgO/p-GaN heterojunction light-emitting devices.Compared with the device without Ag decoration,electroluminescence enhancement effect has been obviously observed,which indicates that the Ag LSP resonance coupling can accelerate the carrier recombination process and improve the radiation recombination rate,leading to the improved luminescence efficiency of the heterojunction devices.