• JIWEN XU

      Articles written in Bulletin of Materials Science

    • Structural evolution, electrical and optical properties of AZO films deposited by sputtering ultra-high density target

      Jiwen Xu Zupei Yang Hua Wang Xiaowen Zhang

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      Aluminum-doped zinc oxide (AZO) target was fabricated using AZO nanopowders synthesized by co-precipitation method and then the AZO films with different thicknesses were deposited on glass by d.c. magnetron sputtering at room temperature. AZO target is nodules free and shows homogeneous microstructure, ultra-high density and low resistivity. ZnAl2O4 phase appears in AZO target and disappears in AZO films. All AZO films show c-axis preferred orientation and hexagonal structure. With increasing film thickness from 153 to 1404 nm, the crystallinity was improved and the angle of (002) peak was close to 34.45°. The increase in grain size and surface roughness is due to the increase in film thickness. The decrease of resistivity is ascribed to the increases of carrier concentration and Hall mobility. The lowest resistivity is 9.6 × 10-4 𝛺.cm. The average transmittance of AZO films exceeds 80%, and a sharp fundamental absorption edge with red-shifting is observed in the visible range. The bandgap decreases from 3.26 to 3.02 eV.

    • Bipolar resistive switching behaviours in ZnMn2O4 film deposited on p+-Si substrate by chemical solution deposition

      Jiwen Xu Zupei Yang Yupei Zhang Xiaowen Zhang Hua Wang

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      ZnMn2O4 active layer for resistance random access memory (RRAM) was deposited on p+-Si substrate by chemical solution deposition. The bipolar resistive switching behaviours of the Ag/ZnMn2O4/p+-Si capacitor are investigated. The bipolar resistive switching is reproducible and shows high ON/OFF ratio of > 102 and long retention times of > 105 s. The conduction mechanism of the Ag/ZnMn2O4/p+-Si capacitor in the low-resistance state (LRS) is ohmic conduction, whereas that of the device in high-resistance state (HRS) successively undergoes Ohm’s law, trap-filled-limited and Child’s law conduction procedure at room temperature.

    • Structure and properties of (1−x)[(K$_{0.5}$Na$_{0.5}$)NbO$_3$–LiSbO$_3$]– xBiFe$_{0.8}$Co$_{0.2}$O$_3$ lead-free piezoelectric ceramics

      HUA WANG XIAYAN ZHAO JIWEN XU XIA ZHAI LING YANG

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      Lead-free piezoelectric ceramics $(1−x)$[0.95(K$_{0.5}$Na$_{0.5}$)NbO$_3$–0.05LiSbO$_3$]–$x$BiFe$_{0.8}$Co$_{0.2}$O$_{3}$(KNN–LS–$x$BFC) were prepared by a conventional sintering technique. The effect of BFC content on the structure, piezoelectricand electrical properties of KNN–LS ceramics was investigated. The results reveal that the BFC is effective in promoting the sinterability and the electrical properties of the ceramics sintering at low temperature of 1030$^{\circ}$C. Theceramics show a single perovskite structure, in which the tetragonal phase decreases while the orthorhombic phase increases with the increase of $x$. The more the BFC content is, the smaller and homogeneous grains were formed.With the increase of $x$, the $d_{33}$ and the $k_p$ increase to a maximum value and then slightly decrease, but the $Q_m$ increases continuously. As BFC content increases, the Curie temperature $T_c$ and remnant polarization $P_r$ decrease, but the diffusivity of phase transition in KNN–LS ceramics will intensify and the coercive field $E_c$ fluctuate between 1.16 and 1.51 kV mm$^{−1}$. The samples with $x =0.004$ exhibit optimum electrical properties at room temperature ($d_{33} = 268 pC$ N$^{−1}$, $k_p =52$%, $\epsilon_r = 1366$, $\tan \delta =2.11$%, $T_c = 325^{\circ}$C, $P_r = 20.4$ $\mu$C cm$^{−2}$, $E_c =1.16$ kV mm$^{−1}$).

    • Effects of Zn doping concentration on resistive switching characteristics in Ag/La$_{1−x}Zn$_x$MnO$_3$/p$^+$-Si devices

      SHUAISHUAI YAN HUA WANG JIWEN XU LING YANG WEI QIU QISONG CHEN DONG HAN

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      Ag/La$_{1−x}$Zn$_x$MnO$_3$/p$^+$-Si devices with different Zn doping contents were fabricated through sol–gel method. The effects of Zn doping concentration on the microstructure of La$_{1−x}$Zn$_x$MnO$_3$ films, as well as on the resistance switching behaviour and endurance characteristics of Ag/La$_{1−x}$Zn$_x$MnO$_3$/p$^{+}$-Si were investigated. After annealing at 600$^{\circ}$C for 1~h, the La$_{1−x}$Zn$_x$MnO$_3$ ($x = 0.1$, 0.2, 0.3, 0.4, 0.5) are amorphous and have bipolar resistance characteristics, with RHRS/RLRS ratios $>$103. However, the endurance characteristics show considerable differences; $x = 0.3$ shows the best endurance characteristics in more than 1000 switching cycles. The conduction mechanism of the Ag/La$_{1−x}$Zn$_x$MnO$_3$/p$^{+}$-Si is the Schottky emission mode at high resistance state. However, the conduction mechanism at low resistance state varies with Zn doping concentration. The dominant mechanism at $x = 0.1$ is filamentary conduction mechanism, whereas that at $x \ge 0.2$ is space-charge-limited current conduction.

    • Effect of domains configuration on crystal structure in ferroelectric ceramics as revealed by XRD and dielectric spectrum

      JIWEN XU WEIDONG ZENG QINGNING LI LING YANG CHANGRONG ZHOU

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      It is well known that domains and crystal structure control the physical properties of ferroelectrics. The ex-situelectric field-dependent structural study, carried out in unpoled/poled crushed powder and bulk samples for (Li$_{0.5}$Nd$_{0.5}$)$^{2+}$ modified 0.95Bi$_{0.5}$Na$_{0.5}$TiO$_3$−0.05BaTiO$_3$ solid solution, established a correlation between domain configuration andcrystal structure variation. Under applying electric field, the smeared ferroelectric phase structure due to coherence diffractioneffect of nanodomains reappeared due to obsolescent coherence effect associated with the field-induced ordered nanodomains.The macroscopic characterizing techniques of domain configuration such as dielectric constant spectroscopy and X-raydiffraction measurement can provide a basis for understanding the correlation between domains configuration and crystalstructure in ferroelectric ceramics.

    • Bipolar resistive switching behaviour in Mn$_{0.03}$Zn$_{0.97}$O/amorphous La$_{0.7}$Zn$_{0.3}$MnO$_3$ heterostructure films

      HUA WANG QISONG CHEN JIWEN XU XIAOWEN ZHANG SHUAISHUAI YAN

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      Mn$_{0.03}$Zn$_{0.97}$O (MZO)/amorphous La$_{0.7}Zn$_{0.3}$MnO$_3$ (LZMO) heterostructures were deposited on p$^+$-Si substratesthrough sol–gel spin coating. Ag/MZO/LZMO/p$^+$-Si and Ag/LZMO/MZO/p$^+$-Si devices exhibit a bipolar, reversibleand remarkable resistive switching behaviour at room temperature. The ratio of the resistance at high-resistance state (HRS)to that at low-resistance state (LRS) ($R_{\rm HRS}/R_{\rm LRS}$) in the Ag/LZMO/MZO/p$^+$-Si device is approximately five orders of magnitude, and is maintained after over 10$^3$ successive switching cycles or over a period of $2\times 10^6$ s, indicating good endurance property and retention characteristics. Conversely, the ratio in the Ag/MZO/LZMO/p$^+$-Si device began to decrease after 100 successive switching cycles. The LZMO/MZO interface could play an important role in the resistive switching behaviour of the devices. The dominant conduction mechanism of the two devices is charge-trap emission.

    • Rectifying resistance-switching behaviour of Ag/SBTO/STMO/p$^+$-Si heterostructure films

      WENBO ZHANG HUA WANG JIWEN XU GUOBAO LIU HANG XIE LING YANG

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      The Sr$_{0.88}$Bi$_{0.12}TiO$_3$/SrTi$_{0.92}$Mg$_{0.08}$O$_3$ (SBTO/STMO) heterostructure films were prepared on p$^+$-Si substratesby sol–gel spin-coating technique, and the films had good crystallinity and uniform grain distribution. The heterostructure films with a structure of Ag/SBTO/STMO/p$^+$-Si exhibited a bipolar, remarkable resistance-switching characteristic, and $R_{\rm HRS}/R_{\rm LRS} \sim 10^4$. More importantly, the heterostructure films showed rectifying characteristic in the low resistance state (LRS), and the rectification ratio can reach 10$^2$ at $\pm$1 V. The dominant resistive-switching conduction mechanism of high resistance state (HRS) was Ohmic behaviour, and the LRS changed to space charge-limited current(SCLC).

    • Resistance-switching properties of Bi-doped SrTiO$_3$ films for non-volatile memory applications with different device structures

      HUA WANG WENBO ZHANG JIWEN XU GUOBAO LIU HANG XIE LING YANG

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      SrTiO$_3$ and Bi-doped SrTiO$_3$ films were fabricated with different device structures using the sol–gel method for non-volatile memory applications, and their resistance-switching behaviour, endurance and retention characteristics were investigated. SrTiO$_3$ and Sr$_{0.92}$Bi$_{0.08}$TiO$_3$ films grown on Si or Pt have the same phase structure, morphologies and grain size; however, the grain size of the Sr$_{0.92}$Bi$_{0.08}$TiO$_3$ films grown on Si is slightly larger than those of the SrTiO$_3$ films grown on Si and the Sr$_{0.92}$Bi$_{0.08}$TiO$_3$ films grown on Pt. The SrTiO$_3$ or Sr$_{0.92}$Bi$_{0.08}$TiO$_3$ films grown on Si or Pt all exhibitbipolar resistive-switching behaviour and follow the same conductive mechanism; however, the Ag/Sr$_{0.92}$Bi$_{0.08}$TiO$_3$/Si device possesses the highest $R_{\rm HRS}/R_{\rm LRS}$ of 10$^5$ and the best endurance and retention characteristics. The doping of Bi is conducive to enhance the $R_{\rm HRS}/R_{\rm LRS}$ of the SrTiO$_3$ films; meanwhile, the Si substrates help improve the endurance and retention characteristics of the Sr$_{0.92}$Bi$_{0.08}$TiO$_3$ films.

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