Xuyan Liu

Structure, magnetic, and dielectric characteristics of Ln2NiMnO6 (Ln = Nd and Sm) ceramics

In the present work, the structure, magnetic, and dielectric characteristics of Nd2NiMnO6 and Sm2NiMnO6 have been investigated with comparison to those for La2NiMnO6. The magnetic and dielectric properties of the present ceramics vary with decreasing lanthanide ionic radius (RLn). Both Nd2NiMnO6 and Sm2NiMnO6 are ferromagnetism with the monoclinic symmetry (space group P21/n). The Curie temperature TC of Nd2NiMnO6 and Sm2NiMnO6 is 194 K and 156 K, respectively, and it decreases with decreasing 〈Ni-O-Mn〉 bond angle which depends on RLn. Meanwhile, the dielectric constant e′ monotonically decreases with decreasing RLn. The similar variation tendency of magnetic and dielectric characteristics with RLn indicates that the structural origins for the magnetic and dielectric response should be closely linked with each other.

Study of the Ge Wafer Surface Hydrophilicity after Low-Temperature Plasma Activation

Plasma activation has been investigated for its ability to induce a strong bonding energy even at low-temperature annealing. In this paper, Ge, Si, and SiO 2 surface hydrophilicities with oxygen and nitrogen plasma activation are analyzed by contact angle measurement. Compared to wet chemical treatments, such as solutions containing ammonium hydroxide, the hydrophilicity of Ge wafer surface is strongly enhanced by O 2 or N 2 plasma activation. For germanium, a highly hydrophilic and smooth surface has been obtained by O 2 plasma activation only for 10 s. The contact angle measurements indicate that O 2 plasma is more remarkable than N 2 plasma in the same activation conditions. A higher surface roughness, which is only observed in the O 2 plasma activated sample, is decreased greatly after rinsing in deionized water.

Structure, magnetic, and dielectric properties of La2Ni(Mn1-xTix)O6 ceramics

In the present work, the effects of Ti-substitution for Mn upon the structure, magnetic and dielectric properties in La2NiMnO6 ceramics have been investigated. Ti-substitution for Mn strongly affects the crystallographic, magnetic, and dielectric properties of La2NiMnO6 double perovskite ceramics. Refinements of the x-ray diffraction data show that all compositions investigated here have the monoclinic perovskite structure in space group P21/n with a partially ordered arrangement of Ni and Mn/Ti cations. The magnetic nature of the present ceramics with x up to 0.8 is ferromagnetic (FM), while that is antiferromagnetic for La2NiTiO6. The Curie temperature and the spontaneous magnetization, or FM component, decrease with increasing x. These ferromagnetic states are attributed to the Ni2+-O-Mn4+ superexchange interaction. The dielectric constant monotonically decreases and the relaxor-like behavior is suppressed with x. The similar variation tendency of magnetic and dielectric properties with increasing x ca...

Effect of interface barrier between carbon nanotube film and substrate on field emission

The influence of interface barrier on field emission of carbon nanotubes (CNTs) was investigated theoretically and experimentally. A double-potential barrier model was proposed to calculate the electron tunneling probability through the interface and surface barriers. The calculation result reveals that the difference of the electron tunneling probability through the two barriers is responsible for the nonlinearity of the Fowler–Nordheim (FN) plots for the field emission of the CNTs. To verify this model, a series of the CNTs were synthesized on the Si substrates covered with different thicknesses of SiO2 layers as the interface barrier. Based on their field emission properties, it was found that the FN plots of the field emission of these CNTs deviated from the FN law when the applied electric fields were over a critical value, which was strongly dependent on the thicknesses of the SiO2 layer. Therefore, the interface barrier has an important role in determining the field emission property of the CNTs. T...

Optical transition in infrared photodetector based on V-groove Al0.5Ga0.5As/GaAs multiple quantum wire

Photoconductors based on V-grooved Al0.5Ga0.5As/GaAs multiple quantum wires (QWR) were fabricated. The geometric structure of the QWR was carefully characterized by transmission electron microscopy and spatially resolved microphotoluminescence measurements. Infrared response at 9.2 μm is observed from the photocurrent spectrum measured at 80 K. It is attributed as the intersubband transition in the quantum wire region. Due to the effective quantum confinement from the two (111)-surfaces forming the V groove, the overlapping between the ground state in the QWR and the one in the vertical quantum well is very small. This explains the weak photocurrent signal from the QWR photodetector. Theoretical design for a better wave function overlapping and optical coupling is outlined from the analysis of two-dimensional spatial distributions of the wave functions.

Near-field coupling effect in normal-incidence absorption of quantum-well infrared photodetectors

Experiments have shown a significant photoresponsivity in quantum well (QW) infrared photodetectors (QWIP) with normal incidence without optical grating. Different explanations based on the energy band structures have been given, but no concrete conclusions can be drawn at the present stage. We develop a theory based on the optical near-field effect in the QW active region to explain the observed photoresponsivity in the normal incidence condition. Our theory indicates that the near-field effect results in an evanescent wave which induces a nonzero electrical component in the optical field along the QWIP growth direction in the quantum well region inducing the observed photoresponsivity.

Wavelength Tuning of GaAs/AlGaAs Quantum-Well Infrared Photo-detectors by Proton Implantation Induced Intermixing

Proton implantation induced intermixing was used to tune the quantum well infrared photo-detector (QWIP) response wavelength, which may provide a method of fabricating two-color QWIP devices. Obvious red shift of response wavelength as large as 0.6 µm was observed relative to the reference sample that was annealed without implantation, from 7.7 µm to 8.3 µm. Two response wavelengths for the highest-dose implanted sample were observed in the photo-current spectra. In the highest dose case (5×1015 H/cm2), the dark current increased by an order of magnitude and peak responsivity decreased by a factor of 3 for the implanted and annealed sample. This has been attributed to the raising of the ground state energy level in the intermixed quantum well and residual damage in the implanted and annealed QWIPs.

OPTICAL PROPERTIES OF GAAS/ALGAAS NEAR A SURFACE QUANTUM WELL

Abstract We have investigated the photo-modulated reflection spectra of GaAs quantum wells where the top barrier is confined by thin Al 0.3 Ga 0.7 As layers. The optical transitions spectra of the first heavy hole hh1 and the first light hole lh1 states to the first electron sublevel e1 are observed. Due to the increase of the confinement potential in the near-surface quantum well by varying the Al 0.3 Ga 0.7 As top barrier thickness, we observe a significant blue-shift of the transition lines (hh1 to e1 and lh1 to e1) compared to the transition lines of the quantum well with a thick semiconductor barrier. The experimental observation on the energy shift and the variation of the strength ratio between the transitions e1–hh1 and e1–lh1 can be modeled by a rectangular well combined with a vacuum potential and built-in field.

Tuning of detection wavelength of quantum-well infrared photodetectors by quantum-well intermixing

Abstract Proton implantation followed by rapid thermal annealing was used to tune the spectral response of the quantum-well infrared photodetectors (QWIPs). Various implantation schemes, such as single low energy implantation, multiple energy implantation, and single high energy implantation were compared for tuning the wavelength of QWIPs. Large redshift of the QWIPs’ response wavelength with either narrowed or broadened spectrum was obtained after interdiffusion. Comparatively, the single high energy scheme is the simplest and most effective method. Furthermore, by applying a two-step implant–anneal sequence to single high energy implantation, the device wavelength was further shifted with improved photocurrent signal, making it promising to achieve the fabrication of multi-color detectors.

Germanium surface hydrophilicity and low-temperature Ge layer transfer by Ge–SiO2 bonding

Wafer bonding and layer transfer are two fundamental technologies in the fabrication of advanced microsystems. In the authors’ experiments, prior to Ge wafer bonding, the hydrophilicity of the germanium surface after wet chemical treatment and O2/N2 plasma activation is evaluated by contact angle measurement. The effects and mechanism of wet or dry treatments on the Ge surface roughness are also characterized. The results are used to tailor the Ge–SiO2 direct bonding process. Finally, oxygen plasma activation for 10 s and B+/H+ coimplantation are employed to facilitate Ge–SiO2 direct bonding and Ge layer transfer at a low temperature. In comparison with hydrogen only ion implantation using the same fluence, coimplantation of B+ and H+ decreases the layer transfer temperature from over 400–320 °C.