Preparation And Magnetic Exchange Mechanism Of Rare Earth Metal Doped ZnO Thin Films

Diluted magnetic semiconductors?DMSs?are regarded as new functional semiconductor materials,which can be obtained through doping transition metals or rare-earth metals.As a wide bandgap semiconductor,ZnO crystalline material has excellent optical and electrical properties and can be achieved opticl-magnetic and electrical-magnetic properties by doping.ZnO-based DMSs have always been an intense research area in DMSs research field.In the past tens of years,plenty of inspiring theoretic and experimental progess,such as room-temperature,ferromagnetism origin have been achieved in transition meatals doped ZnO.However,the origin of ferromagnetism in rare-earth metals doped ZnO is still under debate.Many issues need further study.In this paper,author performed in-depth virtual experiments based on first-principles calculation and the physical experiments including preparing rare earth metal-doped ZnO thin films by plasma-enhanced physical vapor deposition?ICP-PVD?technique and characterizing the films' performance.Main results and conclusions are summarized as follows:?1?A virtual experiment on the chemical composition,structure and magnetic properties of ZnO and rare earth metal?Er,Gd,Pm,Nd and Yb?doped ZnO crystalline materials was carried out by first-principles calculation.The electric band structures are calculated by first principle calculations.The calculation results indicate that the ferromagnetism mechanism of above rare-earth metals doped ZnO crystalline material system origins the spin polarizing of the 4f orbit elecrons of doped rare-earth ions in ZnO.The Yb-doped ZnO crystalline material system has the smallest magnetic moment,while the Gd-doped ZnO crystalline material system has the largest magnetic moment.?2?Zn_1-xYb_xO thin films with different Yb doping concentration are prepared by ICP-PVD.The XRD,AFM,SEM,UV-VIS-NIR,Hall effect and SQUID measurements are performed to characterize the structural,crystalline morphological,optical,electrical and magnetic properties of as-deposited Zn_1-xYb_xO thin films.The results show that Yb-doped ZnO thin films have good crystalline quality and optical property with an average tranmittance of above 85% in the visible wave range.All of the Zn_1-xYb_xO thin films show obvious room-temperature ferromagnetism.The Zn_0.985Yb_0.015O thin films show the strongest ferromagnetism with a saturation magnetization of 0.47emu/cm3.With increasing the oxygen partial pressure or annealed in the oxygen atmosphere,the ferromagnetism of Zn_0.985Yb_0.015O thin films becomes weaker.While,the ferromagnetism of Zn_0.985Yb_0.015O thin films becomes stronger when annealed in the hydrogen atmosphere.The ferromagnerism of Yb-doped ZnO thin films are also studied by high energy proton ions irradiation.It is found that the ferromagnetism enhances with increasing the dose up to 6 × 1015 ions/cm² and then it decreases with further increasing the dose.A strong correlation between oxygen related defects and ferromagnetism is experimentally observed in Yb-doped ZnO thin films.?3?Zn_1-xGd_xO thin films with different Gd doping concentration are prepared by ICP-PVD.The XRD,PL,AFM,TEM,Hall effect and SQUID measurements are performed to characterize the structural,crystalline morphological,optical,electrical and magnetic properties of as-deposited Zn_1-xGd_xO thin films.The results show that Gd-doped ZnO thin films have good crystalline quality and optical property with an average tranmittance of above 80% in the visible wave range.All of the Zn_1-xGd_xO thin films show obvious room-temperature ferromagnetism.The ferromagnetism and conduction properties of Zn_0.98Gd_0.02O thin films decrease with improving the oxygen partial pressure.Annealing experiments at differ atomphere and temperatures are performed.It is found that the ferromagnetism of Zn_0.98Gd_0.02O thin film decreases with improving the oxygen annealing temperature,and the ferromagnetism disappears after annealing at 850 ? for 1h.However,the ferromagnetism of Zn_0.98Gd_0.02O thin film increases with improving the hydrogen annealing temperature.A strong correlation between oxygen related defects and ferromagnetism is also experimentally observed in Gd-doped ZnO thin films.?4?The first-principles calculation was used to calculate the influence of VO,VZn and Zni defects with different charge amounts on the ferromagnetic stability of the system.The results show that only the single ionized VO defects enhance the ferromagnetic coupling exchange between rare earth metal ions.Other types of point defects do not have a significant effect on the ferromagnetic coupling exchange of the system.That VO defects can adjust ferromagnetic coupling is proved by ESR,XAS and PL measurement in Yb-doped and Gd-doped ZnO.According to the results of virtual experiments and physical experiments,the author proposed a ferromagnetic exchange model of rare earth metal-doped ZnO crystalline material system.The core idea is that the ferromagnetism of such material systems originates from the ferromagnetic coupling between the localized elecrons provided by VO defect and the 4f orbit elecrons provided by rare-earth metals.