Yi-Yu Zhang

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.

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.

Synthesis and terahertz transmission properties of nano-porous vanadium dioxide films

Vanadium dioxide (VO2) films were prepared by the sol–gel method on high-purity silicon substrates, in which cetyltrimethyl ammonium bromide (CTAB) was used as a functional additive to form nano-porous structure in the VO2 films. The morphology, crystalline structure and stoichiometry of the films were investigated by field emission scanning electron microscopy, x-ray diffraction and x-ray photoelectron spectroscopy. Furthermore, the effects of nano-scaled grain size and pores on the THz transmission properties across the phase transition in the VO2 films were studied. The results indicated that the film modified with CTAB can form a nano-porous VO2 structure with a uniform grain size of about 30 nm. The nano-porous VO2 film exhibited significantly broader hysteresis loops and a slight decrease in THz transmission reduction across the phase transition, compared with the common film without a nano-porous structure. A tentative interpretation is given for these phenomena.

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.

Amorphous InGaMgO Ultraviolet Photo-TFT with Ultrahigh Photosensitivity and Extremely Large Responsivity

Recently, amorphous InGaZnO ultraviolet photo thin-film transistors have exhibited great potential for application in future display technologies. Nevertheless, the transmittance of amorphous InGaZnO (~80%) is still not high enough, resulting in the relatively large sacrifice of aperture ratio for each sensor pixel. In this work, the ultraviolet photo thin-film transistor based on amorphous InGaMgO, which processes a larger bandgap and higher transmission compared to amorphous InGaZnO, was proposed and investigated. Furthermore, the effects of post-deposition annealing in oxygen on both the material and ultraviolet detection characteristics of amorphous InGaMgO were also comprehensively studied. It was found that oxygen post-deposition annealing can effectively reduce oxygen vacancies, leading to an optimized device performance, including lower dark current, higher sensitivity, and larger responsivity. We attributed it to the combined effect of the reduction in donor states and recombination centers, both of which are related to oxygen vacancies. As a result, the 240-min annealed device exhibited the lowest dark current of 1.7 × 10−10 A, the highest photosensitivity of 3.9 × 106, and the largest responsivity of 1.5 × 104 A/W. Therefore, our findings have revealed that amorphous InGaMgO photo thin-film transistors are a very promising alternative for UV detection, especially for application in touch-free interactive displays.