Investigation On Nitrogen Doped ZnO Films:Defect Regulation And Optoelectric Properties

ZnO is an attractive semiconductor with direct bandgap of about 3.37 eV at room temperature,high exciton binding energy of 60 meV and excellent optoelectronic performance.These outstanding properties make it a preferred material for the application to ultraviolet?UV?light-emitting diodes,low-threshold UV lasers,and UV detectors.Thus,ZnO is considered as another frontier research hotspot in the field of optoelectronic materials after GaN,and has received extensive attention from researchers.However,similar to the early GaN,ZnO also suffers from the asymmetric doping problem:applicable n-type ZnO is easy to prepare while the stable and reliable p-type ZnO is extremely difficult to achieve,which has become a bottleneck problem that hinders the further development of ZnO materials and devices,and directly lead to the p-type doping study entering a trough.However,figuring out the origin and key influence factors of p-type conductivity of ZnO must require us to start from more basic defects.In this dissertation,we focuses on the influence of defects on the optoelectronic properties of ZnO,looking forward to obtain the regulation method of defects and provide possible ways or clues for solving p-type doping problem.We first studied the oxygen vacancy?V_O?defects as well as optical,electrical and magnetic properties in ZnO single crystals annealed at different temperatures.Then,N doped,In-N codoped,and In-Mg codoped ZnO films were prepared by RF magnetron sputtering,ion implantation and post-annealing treatment.With the help of XRD,Raman,XPS,PL,SIMS,Hall,transmission spectra and first-principles calculation,we have studied the V_O,N related defects,Zinc interstitial?Zn_i?and evolution behavior of N defect in ZnO materials,and explored the influence mechanism of defects on the optical,electrical and magnetic properties of ZnO materials.The main specific contents and results are as follows:?1?The effects of annealing temperature on the V_O defects as well as optical,electrical and magnetic properties in ZnO single crystals are systematically investigated.We find that ZnO single crystals have typical room-temperature ferromagnetic saturation behavior,the content of oxygen related defects can be effectively adjusted by annealing,and the observed d⁰ room-temperature ferromagnetism is closely related to surface V_O defects.Additionally,the conductivity of ZnO single crystal increases with increasing annealing temperature,attributed to the formation of Zn_i-V_O shallow donor complex rather than the activation of V_O defects or H impurities during annealing.And the optical band gap first decreases and then increases with the increase of annealing temperature,which is caused by the competition between the V_O defects and the Burstein-Moss effect.?2?The reproducible p-type conductivity for N-implanted ZnO thin films is achieved by appropriate post-annealing treatment.The p-type region exists in a certain annealing temperature/time window.The p-type region is located at 850⁹50? when the annealing time is 5 min.Especially,the films annealed at 850? for 5 min possess the optimal p-type conductivity with the hole concentration of 2.64×10¹⁶ cm^-3,the mobility of 1.37 cm²V^-1s^-1 and resistivity of 172.6?·cm,and the p-type conductive behavior can last for 1 month.However,when the annealing temperature is too high,the N impurities will continuously diffuse outward,resulting in the generation of V_O donor defects and the decrease of hole concentration.Additionally,we reveal that non-axially distributed N_O-V_Zn shallow acceptor complex with an acceptor ionization energy of about 127 meV is a new configuration for p-type conductivity of N-ion implanted ZnO films.This work confirmed that the N ion implantation is an effective approach to prepare p-type N-doped ZnO thin films.?3?The effect of surface C on the crystal quality and optoelectric properties of ZnO:N films was investigated.The surface C causes a large tensile stress in the ZnO:N film resulting in a decline in crystal quality,but hardly affects the optical band gap of the film.C easily combines with N in the local N-rich region to form C-N bond,thereby reducing the concentration of N-related acceptors.This work indicates the adsorbed C at the grain boundaries and surfaces are likely to be one of the important reasons for the difficulty in achieving highly stable p-type ZnO:N films.?4?By performing first-principles calculation,the effect of Zn_i defects on the conductivity stability of p-type N doped ZnO films was investigated.It is found that the Zn_i defect easily occupies the octahedral sites and can"freely"migrate into the Zinc vacancy(V_Zn)position at room temperature,thereby destroying the N_O-V_Zn acceptor complex,which is extremely unfavorable for the p-type conductivity and stability of N-doped ZnO material.This work provides a theoretical basis for understanding the influence of Zn_i defects on the p-type conductivity and stability of N-doped ZnO materials.?5?We systematically discussed the thermal stability and migration behavior of N impurity in In-N codoped ZnO films,the evolution of N-related defects and the interaction mechanism between defects by experiments and first-principles calculations.It is found that N impurities have poor thermal stability,which has a significant impact on N local chemical states.Especially,two distinct temperature ranges can be distinguished.The low temperature annealing?T_A?600??promotes the interaction between N_O acceptor and split-N_i,which not only leads to the formation of additional?N₂?_O compensatory donors but also reduces the N_O acceptors.In contrast,high temperature annealing?T_A>600??favors energetically the generation of abundant V_O near the surface,which dominates the serious out-diffusion of N impurity.Although the Indium impurity can improve the thermal stability of N to a certain extent,it cannot withstand high temperature annealing.Ultimately,high temperature annealing results in the dissociation of?N₂?_O donors and a local nitrogen deficiency environment.?6?The effects of Mg content on the structure,optical and electrical properties of In-Mg codoped ZnO?IMZO?films were investigated.Indium impurity can induce abundant Zn_i-related shallow donors,which provide additional electron concentration,greatly improving the n-type conductivity of film.The introduction of Mg can partially suppress the concentration of Zn_i-related shallow donors,but it also reduces crystal quality of IMZO films and enhances the impurities scattering,thereby reducing the mobility.Starting from the influence of annealing temperature on the intrinsic defects in ZnO,this dissertation deeply studied the thermal stability and evolution process of N-related defects,discussed in depth the influence of Zn_i,V_O and surface C on N_O-V_Znn complex defect and revealed that Mg doping has the effect of reducing the concentration of Zn_i defects in ZnO films.These work provide a theoretical basis for in-depth understanding the influence of Zn_i,Vo and surface C on the p-type conductivity and stability of N doped ZnO materials,and are of great scientific significance for the future exploration and preparation of high quality p-type ZnO:N materials.Even so,it is worth noting that how to improve the stability of N impurities is still a core issue to achieve high stability and high quality p-type ZnO semiconductors,which needs further exploration.