H. M. Shen

ZnO light-emitting devices with a lifetime of 6.8 hours

Lithium-nitrogen doped p-type Mg0.25Zn0.75O films have been realized, and p-Mg0.25Zn0.75O/n-ZnO single-heterostructured light-emitting devices (LEDs) have been constructed. Obvious emission at around 392 nm has been observed from the LEDs under the injection of continuous current, which can be attributed to the near-band-edge emission of ZnO. The LED can work continuously for 6.8 h under a continuous current of 20 mA, revealing the good reliability of the LED. The results reported in this letter reveal that reliable ZnO-based LEDs can be realized, thus high-performance ZnO-based LEDs may be promised in the future.

Fatigue characteristics of SrBi2Ta2O9 thin films prepared by metalorganic decomposition

Polycrystalline SrBi2Ta2O9 (SBT) ferroelectric thin films were synthesized on Pt/Ti/SiO2/Si substrates by metalorganic decomposition. Electric measurements demonstrate that fatigue increases with decreasing switching voltage and frequency, and the suppressed polarization caused at a lower switching voltage can be recovered by switching at a higher voltage. This suggests that the domain walls of SBT thin films are weakly pinned and easily depinned by a higher external field. The polarization of SBT thin films annealed in air shows more degradation than that annealed in oxygen, which indicates that the oxygen vacancy also plays an important role in fatigue behavior of SBT thin films.

Reliable self-powered highly spectrum-selective ZnO ultraviolet photodetectors

Ultraviolet photodetectors (PDs) have been fabricated from p-ZnO:(Li,N)/n-ZnO structures in this Letter. The PDs can operate without any external power supply and show response only to a very narrow spectrum range. The self-power character of the devices is due to the built-in electric field in the p-n junctions that can separate the photogenerated electrons and holes while the high spectrum-selectivity has been attributed to the filter effect of the neutral region in the ZnO:(Li,N) layer. The performance of the self-powered highly spectrum-selective PDs degrades little after five months, indicating their good reliability.

Transformations in sputter-deposited thin films of NiTi shape memory alloy

Abstract Amorphous NiTi films deposited by magnetron sputtering have been studied. The results show that it is easier for precipitation to occur in crystallized films and that, in the films aged at 550 °C for 1 h, the parent phase transforms to rhombohedral phase at 27 °C and then to martensitic phase at 2 °C.

Internal friction study on CuFe2O4∕PbZr0.53Ti0.47O3 composites

The xCuFe2O4(CFO)-(1−x)PbZr0.53Ti0.47O3(PZT) composites with x=0.1, 0.2, and 0.3 have been prepared by solid state reaction method. Two separate CFO and PZT phases without mixture phase have been confirmed by X-ray diffraction. The internal friction spectra at various frequencies were measured from 90 to 700K. The internal friction peaks observed in those samples can be assigned to the contributions of ferroelectric and copper ferrite, respectively. There is no coupling of mechanical dissipation between the two components. The magnetoelectric (ME) property of the composites were also determined and the dependence of ME coefficient on CFO concentration were discussed.

The effect of grain size on domain structure in unsupported thin films

The effect of grain size on domain structure in unsupported thin films with grains (70 - 250 nm) was studied by using TEM. It is found that there probably exists a critical grain size near 200 nm, at which an abrupt change in domain structure occurs. Above this size, domains, which are lamellar microtwins, appear in almost all of the coarse grains. The domain width decreases with grain size. Below this size, however, most of the grains are single-domain ones, while some irregular and curved domain walls appear only in a small portion of the fine grains. The number of the single-domain grains increases with decreasing grain size. Such a grain-size dependence of domain structure is correlated with the anomalous behaviours of the coercive field and dielectric constant in ferroelectric thin films. This interesting result has not been reported before.

The relationship between the phase-transition-like anomalies above Tc and superconductivity in YBa2Cu3Ox

The phase-transition-like anomalies above Tc were investigated by internal friction, stress-strain curve and lattice parameter measurements. The relationship between the anomalies and superconductivity is studied. The 110-130 K anomaly is found to be closely connected with the 90 K superconducting transition, but no obvious relation was observed for the 210 K anomaly. In addition, a relaxation-type internal friction peak was measured at about 100 K.

Retention characteristics of SrBi2Ta2O9 thin films prepared by metalorganic decomposition

Polycrystalline SrBi2Ta2O9 (SBT) ferroelectric thin films were synthesized on Pt/Ti/SiO2/Si substrates by metalorganic decomposition. Electric measurements demonstrate that the polarization decay increases with increasing the write/read voltage within the first second. This could be attributed to the depolarization fields, which increases with increasing the retained polarization. However, we found that the polarization loss is insignificant with different write/read voltages over a range of 1–30 000 S. Furthermore, experiment indicates that there is weak pinning of domain walls existing in SBT, which plays an important role for SBT thin film over a range of 1–30 000 S with a low write/read voltage.

Internal friction study on low-temperature phase transitions in lead zirconate titanate ferroelectric ceramics

In this work, a bulk thermodynamics and microstructure study of lead zirconate titanate ferroelectric PbZr0.52Ti0.48O3 is performed through internal friction, Young’s modulus, differential scanning calorimetry (DSC), and electron diffraction measurements. Two internal friction peaks are observed near 261 and 225 K. The former correlates with an enthalpy change revealed by DSC data and is ascribable to a first-order tetragonal-to-monoclinic phase transition. The latter (225 K), however, showing asymmetric line shape together with a dramatic decrease in Young’s modulus, might correspond to a phase transition associated with the motion of domain walls. The emergence of new domains is evidenced from the superlattice reflections, and the direction of the superlattice is along the z-axis, as indicated by electron diffraction.

Mechanical properties related to the relaxor-ferroelectric phase transition of titanium-doped lead magnesium niobate

The dielectric properties, internal friction, and Young’s modulus of (1−x%) Pb(Mg1/3Nb2/3)O3−x%PbTiO3 (for x=13, 23, and 33) ceramics have been measured. A phase-transition-like internal friction peak associated with Young’s modulus softening has been observed at temperature TR–F, which can be attributed to the relaxor-to-ferroelectrics (R–F) phase transition. Therefore, the R–F phase transition can be explained in terms of the paraelectric-to-ferroelectric phase transition of paraelectric matrix in the materials.