Xiongbang Wei

Strain induced magnetic anisotropy in highly epitaxial CoFe2O4 thin films

Cobalt ferrite (CoFe2O4) thin films were epitaxially grown on (001) SrTiO3 and (001) MgO by laser molecular beam epitaxy. Microstructural studies indicate that the CoFe2O4 grown on (001) SrTiO3 with compressive strain are c-oriented island growth mode with rough surface morphology, whereas the films on (001) MgO with tensile strain become c oriented with layer-by-layer mode. Magnetic property studies reveal that the compressive strained CoFe2O4 films on (001) SrTiO3 can significantly enhance out-of-plane magnetization (190emu∕cm3) with a large coercivity (3.8kOe). In contrast, the tensile strained CoFe2O4 films on (001) MgO exhibit weak magnetic anisotropy. These results suggest that strong magnetic anisotropy is highly dependent on the lattice mismatch induced strain.

Epitaxial properties of ZnO thin films on SrTiO3 substrates grown by laser molecular beam epitaxy

Epitaxial ZnO thin films with different orientations have been grown by laser molecular beam epitaxy on (001)-, (011)-, and (111)-orientated SrTiO3 single-crystal substrates. The growth behavior was in situ monitored by reflection high-energy electron diffraction, and the epitaxial orientation relations were reconfirmed by ex situ x-ray diffraction measurements. In the case of ZnO on SrTiO3(001), four orthogonal domains coexisted in the ZnO epilayer, i.e., ZnO(110)‖SrTiO3(001) and ZnO[−111]‖SrTiO3⟨100⟩. For (011)- and (111)-orientated substrates, single-domain epitaxy with c axial orientation was observed, in which the in-plane relationship was ZnO[110]‖SrTiO3[110] irrespective of the substrate orientations. Additionally, the crystalline quality of ZnO on SrTiO3(111) was better than that of ZnO on SrTiO3(011) because of the same symmetry between the (111) substrates and (001) films. The obtained results can be attributed to the difference of the in-plane crystallographic symmetry. Furthermore, those align...

Enhanced dielectric characteristics of preferential (1 1 1)-oriented BZT thin films by manganese doping

Ba(Zr0.2Ti0.8)O3 (BZT) and 2 mol% Mn additional doped BZT (Mn–BZT) thin films were deposited on Pt/Ti/SiO2/Si by the pulsed laser deposition (PLD) technique under the same growth conditions. X-ray diffraction scans showed that both films were polycrystalline and preferentially (1 1 1)-oriented, and an enhanced crystallization effect was obtained after Mn doping. The parallel-plate capacitors of Au/BZT/Pt and Au/Mn–BZT/Pt were prepared to investigate the electric properties, respectively. The remanent polarization and the coercive electric field for Mn-doped BZT film were both smaller than those of undoped BZT film. Furthermore, Mn-doped BZT film exhibited a higher dielectric constant of 460 at zero bias, larger dielectric tunability of 69.0% and lower dielectric loss of 5.0‰ under an applied electric field of 615 kV cm−1 than those of undoped BZT film. The figure of merit for preferentially (1 1 1)-oriented BZT thin film was greatly enhanced from 94 to 138 by Mn doping. The enhanced dielectric behaviour by Mn doping could be mainly attributed to the decrease in electron concentration and oxygen vacancies and the reorientation of the defect complex.

Bipolar resistive switching behavior with high ON/OFF ratio of Co:BaTiO3 films by acceptor doping

The bipolar resistive switching characteristics have been investigated in the Co:BaTiO3 films deposited by sol-gel method. It has been demonstrated that such devices can be used as resistive random access memory cells without required electroforming. Ohmic transport and space charge limited current mechanism are dominant during the resistive switching. The ON/OFF ratio between the resistance at the high and low resistance states is more than 106, better than other perovskite films. The high ratio should be attributed to acceptor doping into the n-type semiconductor. The results imply that the ON/OFF ratio can be enhanced by controlling doping type and concentration in those insulating oxides.

Heteroepitaxial growth of ZnO on perovskite surfaces

The microstructural properties of heteroepitaxial ZnO thin films prepared by laser molecular beam epitaxy (L-MBE) were investigated on SrTiO3 substrates and BaTiO3/SrTiO3 pseudo substrates with different orientations. The interface characteristics were in situ monitored by reflection high-energy electron diffraction (RHEED), and the epitaxial orientation relations were reconfirmed by ex situ x-ray diffraction (XRD) measurements. ZnO films grown on SrTiO3(0 0 1) and BaTiO3/SrTiO3(0 0 1) contained a poly-domain structure. For the former, the lattice mismatch was about −1.7% by four types of domain growth with the epitaxial relation of ZnO(1 1 0)||SrTiO3(0 0 1) and ZnO[−1 1 1]|| SrTiO3100. For the latter, twin domains would result in a smaller mismatch of −0.8% by the epitaxial relation of ZnO(0 0 1)||BaTiO3(0 0 1) and ZnO[1 1 0]|| BaTiO31 1 0. On SrTiO3(1 1 1) and BaTiO3/SrTiO3(1 1 1), single-domain films following the c-axial direction were observed with in-plane orientation ZnO[1 1 0]||SrTiO3[1 1 0] and ZnO[1 0 0]||BaTiO3[1 1 0], respectively. This 30° rotation in the in-plane direction of the ZnO epilayer with respect to the perovskite surfaces increased the lattice mismatch from about −2% to −14.5% after inserting BaTiO3 layers. The orientation of ZnO films could be attributed to the characteristic difference of the interface energy. It is determined entirely by interface stress and crystallographic symmetry for the growth on nonpolar (0 0 1)-orientated perovskite surfaces while the competition between elastic energy and chemical energy plays an important role for that on polar (1 1 1)-surfaces.

In situ analysis of lattice relaxation by reflection high-energy electron diffraction

Reflection high-energy electron diffraction (RHEED) is a versatile technique for surface and interface analysis during film growth. Analysis of RHEED patterns during film growth is helpful for the understanding of the properties of both surface and interface. The geometrical arrangement of RHEED patterns can be derived from diffraction theory, and the in-plane and out-of-plane lattice constants are computed from the streaky or spotty patterns. During epitaxial growth of SrTiO3/SrTiO3, MgO/SrTiO3 and MgO/LaAlO3 with lattice mismatch of 0%, 7.8% and 11%, respectively, the changes of lattice constants are recorded as a function of film thickness. The in-plane lattice constant oscillates along with the oscillation of RHEED intensity in the case of SrTiO3 homoepitaxial growth. During the MgO cube-on-cube deposition on SrTiO3, the in-plane lattice constant is relaxed due to positive mismatch, and the MgO lattice is distorted into tetragonal phase at the same time. For the in-plane 45° rotation system of MgO/LaAlO3, relaxation behaviour is observed suffering from negative mismatch. The measured in-plane orientation and lattice relaxation correlate well with a two-dimensional nearly coincident site lattice model, in which the match relation of MgO/LaAlO3 growing with a 5 : 4 lattice ratio can result in negative mismatch.

Electric-field induced transition of resistive switching behaviors in BaTiO3/Co:BaTiO3/BaTiO3 trilayers

Electric-field induced transition of resistive switching behaviors has been demonstrated in BaTiO3/Co:BaTiO3/BaTiO3 trilayers. At low applied bias, the Au/BaTiO3/Co:BaTiO3/BaTiO3/Pt device shows bipolar resistive switching (BRS) behavior, whereas it converts to complementary resistive switching (CRS) at high applied bias due to the depletion of oxygen vacancies in BaTiO3 layer. The Schottky-like emission accompanied by trapping/detrapping process at the interfaces is likely responsible for the BRS and CRS effects in the trilayers. Furthermore, the operating current of the CRS devices can be reduced significantly by decreasing the doped layer.

Modeling for calculation of vanadium oxide film composition in reactive-sputtering process

A modified model describing the changing ratio of vanadium to oxide on the target and substrate as a function of oxygen flow is described. Actually, this ratio is extremely sensitive to the deposition conditions during the vanadium oxide (VOx) reactive magnetron-sputtering process. The method in this article is an extension of a previously presented Berg’s model, where only a single stoichiometry compound layer was taken into consideration. This work deals with reactive magnetron sputtering of vanadium oxide films with different oxygen contents from vanadium metal target. The presence of vanadium mixed oxides at both target and substrate surface produced during reactive-sputtering process are included. It shows that the model can be used for the optimization of film composition with respect to oxygen flow in a stable hysteresis-free reactive-sputtering process. A systematic experimental study of deposition rate of VOx with respect to target ion current was also made. Compared to experimental results, it w...

Enhanced Dielectric Characteristics Of Manganese-Doped Bzt Thin Films

Ba(Zr0.2Ti0.8)O3 (BZT) and 2 mol% Mn additional doped BZT (Mn-BZT) thin films were deposited by pulsed laser deposition technique under the same growth conditions on LaAlO3 substrates with the bottom electrodes of LaNiO3. The microstructure of the films was characterized by X-ray diffraction (XRD) in the mode of θ–2θ scan and Φ-scan. The results indicated that BZT film was (001)-oriented_with an in-plane relationship of BZT[100]//LNO[100]//LAO[100]. The Mn-BZT film exhibited higher dielectric constant of 225 at zero electric field, larger dielectric tunability of 59.4%, and lower dielectric loss of 1.8% under an applied electric field of 720 kV/cm. The figure of merit for BZT thin film increased from 19.8 to 33 by Mn doping. The enhanced dielectric behavior by Mn doping could be mainly attributed to the decrease of oxygen vacancies and the reorientation of the dipolar defect complex of

Sputtering Voltage in the Growth of Vanadium Oxide Thin Films

{\rm Mn}^{\prime\prime}_{\rm Ti} - {\rm V}_{\ddot{\rm O}}