Huizhong Zeng

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.

Electrical and photovoltaic characteristics of MoS2/Si p-n junctions

Bulk-like molybdenum disulfide (MoS2) thin films were deposited on the surface of p-type Si substrates using dc magnetron sputtering technique and MoS2/Si p-n junctions were formed. The vibrating modes of E12g and A1g were observed from the Raman spectrum of the MoS2 films. The current density versus voltage (J-V) characteristics of the junction were investigated. A typical J-V rectifying effect with a turn-on voltage of 0.2 V was shown. In different voltage range, the electrical transporting of the junction was dominated by diffusion current and recombination current, respectively. Under the light illumination of 15 mW cm−2, the p-n junction exhibited obvious photovoltaic characteristics with a short-circuit current density of 3.2 mA cm−2 and open-circuit voltage of 0.14 V. The fill factor and energy conversion efficiency were 42.4% and 1.3%, respectively. According to the determination of the Fermi-energy level (∼4.65 eV) and energy-band gap (∼1.45 eV) of the MoS2 films by capacitance-voltage curve and ...

Epitaxial growth and metal-insulator transition of vanadium oxide thin films with controllable phases

Vanadium oxide thin films with well controlled phases such as rhombohedra V2O3 and monoclinic VO2 were synthesized on Al2O3 (0001) substrates by optimizing the processing parameters of a polymer assisted deposition technique. X-ray diffraction and high-resolution transmission electron microscopy studies revealed that both V2O3 and VO2 films can be well controlled with good epitaxial quality. The temperature dependency of electrical resistivity demonstrated sharp metal-insulator transitions (MITs) for V2O3 and VO2 films. The crystallinity and the strains in the films are believed to play critical roles in determining the MIT properties.

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.

Enhanced thermal conductivity of polycrystalline aluminum nitride thin films by optimizing the interface structure

The growth-temperature dependency and interface structure effects on the thermal conductivity of the highly textured AlN thin films on (001) Si substrates were systematically studied by characterizing the crystal structures, surface morphologies, interface structures, chemical compositions, and thermal conductivity using x-ray diffraction analysis, atomic force microscopy, high resolution transmission electron microscopy, x-ray photoelectron spectroscopy, and 3-omega method, respectively. By optimizing the interface microstructure and the growth temperature, thermal conductivity of polycrystalline AlN thin films can be greatly enhanced from 9.9 to 26.7 W/mK, when the growth temperature increases from 330 to 560 °C. This achievement is considered to be associated with the diminishment of the amorphous and disordered layer at the AlN/Si interface.

Electron trap memory characteristics of LiNbO3 film/AlGaN/GaN heterostructure

LiNbO3 film (LNO)/AlGaN/GaN heterostructure was fabricated and its memory characteristics were studied. The heterostructure exhibited a wide range clockwise hysteresis (0.3–12.1 V) likely due to the electrons trapping and distrapping from the Li vacancies in the LNO film. After 10 years retention, 10% of the window could remain. In addition, a slight decrease for the memory window happened after 105 cycles. These results indicated that LNO film combined with AlGaN/GaN would hold promise for next-generation nonvolatile memory devices. Possible operating mechanism for the memory effect in the heterostructure was explained qualitatively by the energy band diagram.

Epitaxial fabrication and memory effect of ferroelectric LiNbO3 film/AlGaN/GaN heterostructure

A metal-ferroelectric-semiconductor structure was fabricated epitaxially by depositing a LiNbO3 film on the surface of the AlGaN/GaN template with two dimensional electron gas (2DEG). The capacitance-voltage characteristics were studied. Counterclockwise memory windows could be observed clearly. The size of the window first increased with increasing forward bias (Vmax) and reached a maximum of 2.5 V when Vmax=6 V. This was attributed to the switchable ferroelectric polarization modulating on 2DEG. When Vmax exceeded 6 V, the window decreased due to electron injection. These results indicated that ferroelectric films combined with AlGaN/GaN would hold promise for next-generation memory devices.

Growth Dynamics of Barium Titanate Thin Films on Polycrystalline Ni Foils Using Polymer-Assisted Deposition Technique

Polymer-assisted deposition (PAD) technique was developed to fabricate ferroelectric BaTiO(3) (BTO) thin films directly on polycrystalline nickel foils. The growth dynamics was systematically studied to optimize the single-phase BTO films with good dielectric properties. It is critical to pretreat nickel foils with hydrogen peroxide (H(2)O(2)) solution to form thin nickel oxide layers on the surfaces for the growth of BTO films. Both the concentration of H(2)O(2) solution and the pretreated time were found to strongly affect the dielectric constant of BTO films, which may be associated with the oxygen diffusion from nickel oxide buffer layers to BTO layers during annealing. The BTO thin films with optimized growth conditions have good crystal structure and electrical properties, suggesting that the as-grown BTO films by PAD technique can be utilized for new devices development and energy storage applications.

Rectifying filamentary resistive switching in ion-exfoliated LiNbO3 thin films

In this letter, we report the resistive switching properties of ion-exfoliated LiNbO3 thin films. After annealing in Ar or in vacuum, electro-forming has been observed on the thin films, and the oxygen gas bubbles can be eliminated by tuning the annealing conditions in order to prevent the destruction of top electrodes. The thin films show rectifying filamentary resistive switching after forming, which is interpreted by a simplified model that the local filament does not penetrate throughout the LiNbO3 thin film, resulting in asymmetric contact barriers at the two interfaces. The well controlled electro-forming step and the highly reproducible switching properties are attributed to the more homogeneous distribution of defects in single crystalline materials and the specific geometry of filament.