S. L. Feng

Characterization of SnO2 nanowires as an anode material for Li-ion batteries

SnO2 nanowires synthesized by thermal evaporation method are investigated as a possible anode electrode for Li-ion batteries. In the first discharge process, the capacity of Li ions is 2133mAhg−1, which is much more than the theoretical total capacity of the bulk SnO2, 1494mAhg−1. During the successive 15cycles, the reversible capacity stays in the range of 1250–700mAhg−1 with a capacity fading of 3.89%percycle at a constant current density of 0.5mAcm−2. These results demonstrate that SnO2 nanowires are a promising anode material for Li-ion battery applications.

Processing and properties of Yb-doped BiFeO3 ceramics

The authors prepared Yb-doped bismuth iron oxide ceramics (Bi1−xYbxFeO3, with 0⩽x⩽0.20) by rapid liquid phase sintering method and investigated the material’s structures and electrical properties. The x-ray diffraction measurements showed that the doping of Yb has induced noticeable lattice distortion in the ceramics, and a largest distortion was observed when the concentration of Yb was 15%. By doping electrical resistivity, ferroelectric and dielectric properties of the ceramics were improved. Among all samples, BiFeO3 doped with 15% Yb was found to have the smallest leakage current density (<10−7A∕cm2) and the largest remnant polarization (8.5μC∕cm2).

ZnO-based film bulk acoustic resonator for high sensitivity biosensor applications

Zinc oxide (ZnO)-based film bulk acoustic resonator consisting of a piezoelectric element (Au∕ZnO∕Pt) and a Bragg reflector (ZnO∕Pt multilayer structure) has been fabricated by magnetron sputtering. The transmission electron microscopy and x-ray diffraction measurements revealed that all thin film layers in the device were well crystallized and highly textured. By electrical measurements, it was found that the device had a high resonant frequency (3.94GHz) and mass sensitivity (8970Hzcm2∕ng). The use of the device as a biosensor was demonstrated by comparing the resonant properties of the device with/without coatings of biospecies.

Effect of doping concentration on the performance of terahertz quantum-cascade lasers

We characterized a set of terahertz quantum-cascade lasers with identical device parameters except for the doping concentration. The δ-doping density was varied from 3.2×1010to4.8×1010cm−2. We observed that the threshold current density increased monotonically with doping. Moreover, the measured results on devices with different cavity lengths provided evidence that the free carrier absorption caused waveguide loss also increased monotonically. Interestingly, however, the observed maximum lasing temperature displayed an optimum at a doping density of 3.6×1010cm−2.

Single-crystalline tin-doped indium oxide whiskers: Synthesis and characterization

An in situ doping approach was adopted to synthesize single-crystalline Sn-doped In2O3 [indium tin oxide (ITO)] nanowhiskers. Similar synthesis strategy could be used in the production of various doped metal oxide nanowhiskers. The growth mode of ITO nanowhiskers was discussed based on a self-catalytic vapor–liquid–solid growth mechanism. At room temperature, a photoluminescence peak at 510nm was observed, which was likely related to the surface defects of ITO nanowhiskers. In air ambient, the resistivity of individual ITO whisker was measured to be 0.40Ωcm, which was much higher than that of ITO films. A possible explanation for the high resistivity was proposed based on the influence of surface oxygen absorption.

Self-organized Ge nanocrystals embedded in HfAlO fabricated by pulsed-laser deposition and application to floating gate memory

A trilayer metal-oxide-semiconductor structure containing a HfAlO tunnel layer, isolated Ge nanocrystals, and a HfAlO control layer, was obtained using pulsed-laser deposition (PLD). Self-organized Ge nanocrystals were formed by PLD at 600°C, suggesting a useful low-temperature process for fabricating Ge nanocrystals embedded in dielectric materials. The self-organized Ge nanocrystals so formed were uniform in size and distribution with a density approaching 1012cm−2. The effects of deposition temperature and growth rate in forming Ge nanocrystals were investigated and it was revealed that a relatively low temperature and growth rate are favorable for the formation of Ge nanocrystals. The memory effect of the Ge nanocrystals with storage charge density of up to 1012cm−2 has been demonstrated by the presence of hysteresis in the capacitance-voltage curves.

Performance improvement of phase-change memory cell with cup-shaped bottom electrode contact

Cup-shaped tungsten (W) bottom electrode contact (BEC) was proposed for phase-change memory (PCM) cell and compared with the same size solid BEC. The reset voltage of 2.5V and the shortest reset time of 10ns were obtained for cup-shaped W-BEC based PCM cell, which is much better than those of the solid BEC based PCM cell. The improved performance of the cup-shaped W-BEC based PCM cell is attributed to the mushroom heating region with high heating efficiency, the inner edge contact of W-BEC and Ge2Sb2Te5, and the reduced thermal diffusion from the bottom electrode, which is further verified by theoretical simulation.

Current self-oscillation and driving-frequency dependence of negative-effective-mass diodes

We have analyzed spatio-temporal current patterns and current–voltage characteristics of negative-effective-mass (NEM) p+pp+ diodes driven by dc bias and terahertz (THz) electromagnetic radiation. Interesting nonlinear dynamics are presented, including current synchronization, frequency doubling, and transition to chaos. Discussions of suppressing possible chaos in NEM semiconductor devices are included.

Self-extrusion of Te nanowire from Si–Sb–Te thin films

A crystallized Si2Sb2Te5 thin film was observed to extrude single-crystalline [0001] oriented tellurium nanowires at room temperature. The single crystalline Te nanowires nucleation and extruded outgrowth can be greatly accelerated by electron-beam-illumination (EBI) in a transmission electron microscope by an order as high as four. The EBI-enhanced outgrowth speed of Te nanowires is a function of electron beam flux and can be described as v=ku2009ln(J+m). This Te nanowires self-outflow phenomenon comes from a decomposition process of the Si2Sb2Te5 matrix and provides an interesting model and mechanism of the nanowires’ growth, which is distinctive to the vapor-liquid-solid (VLS) mechanism.

Microwave characterization of (Pb,La)TiO3 thin films integrated on ZrO2∕SiO2∕Si wafers by sol-gel techniques

Polycrystalline perovskite lead lanthanum titanate (PLT) thin films were prepared by a sol-gel method on ZrO2∕SiO2∕Si substrates. The structure of the films was studied by x-ray diffraction and scanning electron microscopy, and the microwave dielectric properties characterized on a network analyzer. A strong dependence of the dielectric constant of PLT films and, correspondingly, the resonance frequency of PLT-based interdigital capacitor on the sample preparation conditions were observed. They resulted from the structural transformation of PLT from a layered structure to a uniform film as the annealing temperature was raised from 550to700°C, suggesting a possible way to modify the device performance by controlling the layered structure of the ferroelectric film.