Structural, Magnetic And Transport Properties In ZnO Based Diluted Magnetic Semiconductors

Diluted magnetic semiconductors(DMS),which show ferromagnetic ordering properties, have attracted much attention for their potential in both basic researches and device applications.A key work to realize devices application is to develop DMS with a high curie temperature.The origin of ferromagnetism(FM) in DMSs remains a very controversial topic.There are many different even contrary experimental results.Some studies indicate that the ferromagnetism in transition metal doped ZnO may come from precipitation of magnetic clusters or secondary magnetic phases.On the other hand,some reports suggest the absence of magnetic clusters or secondary phases and support intrinsic ferromagnetic origin.In this thesis,we use a sol-gel method to prepare doped ZnO DMS compound powders. On the base of large amounts of processing exploration,we made a primary discussion and analysis for the observed room-temperature(RT) ferromagnetic state in the samples as well as its origin in combination with the structure analysis,the bonded state and the magnetic behavior of the samples.Firstly,a few of preparition steps to produce this kind of Zn_0.95Co_0.05O compound powders with single wurtize phase and RT ferromagnetism are explored.The experiment reveals that the powders which are 2nd sintered have large magnetization.The powders are highly insulating(resistivity:10⁶Ωcm),and thus carrier mediated ferromagnetism is not suitable to interpret the origin of the observed ferromagnetism as the absence of free carriers. According to the theoretical calculations,the observed FM in ZnO may be connected with the effect of Zn vacancies.Usually,2nd sintering effectively brings about some Zn vacancies in semiconducting oxides.For 2nd sintered powders,the absorbtion peak in the X-ray-absorption near-edge spectrum(XANES) showing a slight shift to the higher energy compared with the powders unsintered for the second time,this maybe as the result of more Zn vacancies existing in those samples after the second sintering.Such result shows that FM in our prepared powders mainly originates from Zn vacancies.Based on the study of the Co-doped ZnO samples,the structure and magnetization behaviors of Zn_0.95-xCo_0.05Cu_xO and Zn_0.95-xCo_0.05Al_xO powders had been experimentally investigated.The wurtzite structure is not changed by doping,the x-ray photoelectron spectoscopy(XPS) results enable us to conclude that the Cu is in the+1 valence state.The presence of a percentage of Cu in the+1 state which acts as a acceptor,compensates the electrons arising from Zn²⁺interstitials in the prepared ZnO.There will then be many acceptors in our codoped powders.The concentration of defect in the Zn site increased,the interaction between them increased,the magnetization is increased..However,in the Zn_0.95-xCo_0.05Al_xO powders,the saturation magnetization decreased with increasing Al doping.The resistivity decreased largely with increasing x for Zn_0.95-xCo_0.05Al_xO samples.It indicates that the Al ions play as electric dopant and contribute the creation of free carrier.The addition of electrons to the system will move the Fermi energy level up in the band gap,resulting in a decrease in acceptor concentration and a reduction in magnetization.It can be explained that acceptors in the powders are responsible for mediating ferromagnetism.The addition of electrons to the system then the concentration of defect in the Zn site decreased,the interaction between them decreased,then the magnetization is decreased..