Study On Room Temperature Ferromagnetic Property Of Mn-doped ZnO And Resistance Switching Of Metal-semiconductor

Diluted magnetic semiconductor (DMS) has been received extensive attention from researchers due to their feasibility of manipulation of both charge and spin. They are considered as one of the most promising materials for future spintronics technology. The hotspot of current research focused on searching the DMS materials and further discussing the origin of the ferromagnetism of DMS. ZnO-based DMSs with wide band gap have superior electrical and optical performances and show ferromagnetism at room temperature. So it is important to study material preparation, ferromagnetic mechanism and spin-dependent transport properties. Because of lack of comprehensive and systematic study in the Mn-doped ZnO systems, the origin of ferromagnetism of Mn-doped ZnO is contradictory.Due to potential application to next generation nonvolatile memories, resistance switching has attracted intensive attention. Many experimental groups have studied the transport property and resistance switching of metal/SrNbxTi1-xO3. However the mechanism of resistance switching still unclear and the further comprehensive studies are needed.In response to the problems described above, some studies are planned. The main content includes two parts:room temperature ferromagnetism of Mn doped ZnO and resistance switching of Pt/SrNbxTi1-xO3. The main content and conclusions are as follows:1. Room-temperature ferromagnetism investigation on Zn0.98Mn0.02O thin film annealed in N2/H2at different temperatures.In order to investigate the origin of room-temperature ferromagnetism of Mn-doped ZnO, influence of N2/H2-annealed temperature on the structure and magnetism of Zn0.9Mn0.02O film has been systemically studied. Zn1-xMnxO films were prepared on single crystal silicon substrates by radio frequency (RF) magnetron sputtering. All as-deposited films show paramagnetism behavior at room temperature by analyzing the VSM results. As-deposited films were annealed in N2/H2(99.999%) at different temperatures (600℃-750℃). When the annealing temperature (Tan) is above640℃, foamlike amorphous materials form on the surface of the films. All of the N2/H2-annealed films exhibit room temperature ferromagnetism. With increasing annealed temperature, the ferromagnetism of annealed films increase until Tan reaches750℃. By spectral decomposition of XPS and PL spectral, we found that the singly ionized oxygen vacancy has close relationships with the ferromagnetism. The origin of ferromagnetism in annealed films is consistd with two sections. The bound magnetic polaron is responsible for the ferromagnetism of (Tan≤640℃) annealed film with oxygen vacancies. The foamlike amorphous material could increase the ratio of grain-boundary area to grain volume, leading to the ferromagnetism. Magnetic amorphous material could induce the ferromagnetism of (Tan>640℃) annealed film.2. Influence of a multistep annealing on tuning the ferromagnetism in Mn-doped ZnO film.As-deposited films were pre-annealed at different temperatures (600℃-900℃) in O2and N2for3h, respectively. When pre-annealed temperature(Tpa) is not above700℃, the Mn2+ion has gradually substituted into the Zn2+ion site in Mn-doped ZnO film by annealing. At Tpa>700℃, precipitates were observed on the surface of film by observed by SEM. By analyzing experiment results, we can confirm that the precipitates are spinel ZnMn2O4. All O2pre-annealed and N2pre-annealed films are paramagnetism. All O2pre-annealed and N2pre-annealed films were annealed in N2/H2(99.999%) at630℃for1h and these samples show the room temperature ferromagnetism.700℃N2pre-annealed film annealed in N2/H2has strongest ferromagnetism. BMP model applies to ferromagnetism of annealed films as well. At last we have discussed the reason that ZnMn2O4appeared in the surface of film.3. Room temperature ferromagnetism investigation of Zn1-xMnxO nanoparticles.First, Zn1-xMnxO nanoparticles were prepared by co-precipitation technique. We study Mn doping concentration effect on the structure and ferromagnetism of Zn1-xMnxO nanoparticles. Subsequently, Zno.92Mn0.08O nanoparticles annealed in O2, Ar, CO and H2at700℃for0.5h, respectively. The absorption spectra and Raman spectra of the samples reveal that a high concentration of oxygen vacancies appears in CO-annealed and H2-annealed Zno.92Mn0.08O nanoparticles. By Gaussian fitting PL spectra of the samples, a broader green-yellow emission band decomposed into the green emission band and yellow emission band. We find the larger the ratio and integral area of singly ionized oxygen vacancies, the stronger room temperature ferromagnetism of annealed nanoparticles, providing the evidence that singly ionized oxygen vacancies may play an important role in the origin of room temperature ferromagnetism of nanoparticles。4. Resistance switching of SrTiO3:Nb single crystalThe influence of thermal treatment on the resistance switching of SrTiO3:Nb (0.05and0.5wt%Nb)(SNTO) single crystals has been investigated. The condition of thermal treatment is in O2at400℃for0.5h. Analyzing current-voltage and capacitance-frequency curves of samples demonstrate that Nb doping concentration, electroforming and thermal treatment could increase interface state (defect state) in the interface. The thermal treatment could increase interface state by improving the chemisorbed oxygen concentration on the surface. Switching time of Pt/SNTO junction with both the thermal treatment and the electroforming would be further shortened. The model of resistance switching effect is attributed to the trapping/detrapping electrons at the interface of Pt/SNTO. These results above are useful for further comprehension of resistance switching mechanism.