The Room-temperature Ferromagnetism Of Un-doped ZnO

In the last decades, diluted magnetic semiconductors (DMSs) haveattracted considerable attention due to the possible applications in spintronicdevices. Tremendous efforts have been devoted to the magnetic ordering aboveroom-temperature in materials doped with transition metal (TM) ions. Althoughroom-temperature ferromagnetism (RTF) has been achieved in these TM dopedmaterials, the formation of magnetic clusters and/or secondary phases by TMdopants is detrimental to the applications of DMSs. Recently, one of the mostintriguing discoveris in DMSs is observation of RTF in non-TM doped or evenun-doped wide band-gap semiconductors. However, its origin is still unclear andneed to be clearified by reseachers.ZnO based DMSs serves as a model system so far. In this paper, the RTFwas achieved in un-doped ZnO thin films and single crystals by thermalannealing in argon flow, respectively. By investigating the kinds andconcentration of defects created during the thermal annealing process, we triedto clearify the origin of the RTF in un-doped ZnO and thus to manipulate it.First, we used pulsed electron deposition (PED) to prepare the high qualityZnO thin films and investigated the influnce of substrate temperature andworking gas pressure on the evoltution of [0002] texture.The optimum conditions were chosen to deposite ZnO thin films on (100)Si as well as quartz substrates. After thermal annealing in argon flow, theferromagnetic property of ZnO thin films was greatly improved for both twokinds of substrates. It revealed that the concentration of zinc vacancy in ZnOwas no different in these samples, while the oxygen vacancy was created duringthe thermal annealing process. Therefore, the oxygen vacancy induced the RTFin un-doped ZnO thin films. Furthermore, by comparison of the saturationmagnetization and the amount of oxygen vacancy of ZnO thin films depostiedon these two kinds of substrates, we get a roughly estimation of atomic magneticmoment for per singly charged oxgyen vacancy (i.e. F+defect).ZnO single crystals do not exist the strain and grain boudary problmes aswell as thin films and thus are suit for clarification the origin of the defects induced RTF. By study the ferromagntic property and defect status before andafter thermal annealing in argon flow, it suggested that the RTF in ZnO singlecrystals was also originated from F+defect which was the same as ZnO thinfilms.And then, in order to further manipulate the defect’s concentration, anexternal strong magnetic field was applied during thermal annealing. It wasproved that the RTF could be controlled by adjusting the concentration ofoxygen vacancy.