Yunjun Rui

Size-dependent electroluminescence from Si quantum dots embedded in amorphous SiC matrix

Si quantum dots (QDs) were formed by thermal annealing the hydrogenated amorphous silicon carbide films (a-SiCx:H) with different C/Si ratio x, which were controlled by using a different gas ratio R of methane to silane during the deposition process. By adjusting x and post annealing temperature, the QD size can be changed from 1.4 to 4.2 nm accordingly, which was verified by the Raman spectra and transmission electron microscopy images. Size-dependent electroluminescence (EL) was observed, and the EL intensity was higher for the sample containing small-sized Si QDs due to the quantum confinement effect (QCE). The EL peak energy as a function of the Si QDs size was in good agreement with a modified effective mass approximation (EMA) model. The calculated finite barrier potential of the Si QDs embedded in SiC matrix is 0.4 and 0.8 eV for conduction and valence band, respectively. Moreover, the current-voltage properties and the linear relationship between the integrated EL intensity and injection current i...

Hydrogen-induced recovery of photoluminescence from annealed a‐Si:H∕a‐SiO2 multilayers

a‐Si:H∕SiO2 multilayers prepared by plasma-enhanced chemical-vapor deposition exhibit a luminescence band around 760nm, which is quenched after a dehydrogenation process. Subsequent hydrogen plasma annealing (HPA) treatments are carried out, and the luminescence is then recovered. The effects of HPA are investigated as functions of hydrogen annealing time and temperature. Fourier transform infrared spectroscopy and Raman-scattering spectroscopy are used to study the change of the microstructures and bonding configurations due to the HPA treatments. It is indicated that the atomic hydrogen cannot only diffuse into the film to reduce the density of nonradiative recombination centers but can also relax the film network and improve the microstructure order of the a‐Si:H sublayers. All these factors are believed to contribute to the recovery of the luminescence signals of the a‐Si:H∕SiO2 multilayers.

Characterization of Hg-Ba-Ca-Cu-O 1223 superconductor

Abstract The HgBaCaCuO 1223 superconductors have been characterized in this work. XRD and SEM analyses indicate that Hg-Ba2Ca2Cu3O8+δ is the major superconducting phase in the shape of a bar or plate in the samples. The transition temperature Tc of one sample is 129 K by resitivity, AC and DC susceptibility measurements. Magnetic measurements including the magnetic critical current density and magnetization relaxations indicate that a strong thermally activated flux motion also exists in the Hg-based system of high-temperature superconductors.

Synthesis and X-ray powder diffraction analysis of the high-Tc superconductor HgBa2Ca2Cu3O8+δ

Abstract The third member (Hg-1223) of the recently-discovered homologous series HgBa 2 Ca n −1 Cu n O 2 n +2+δ , which having a T c of 118 K (assynthesized sample) and 133 K (oxygen-annealed sample), was successfully prepared by solid state reaction and short time annealing technique using the mixtures of metal oxides HgO, BaO, CaO and CuO with starting composition HgBa 2 Ca 2 Cu 3 O y . We found that the combination of very short annealing time with optimum temperatures 715–725°C gives rise to a very effective sintering of the constituents and avoids an excessive loss of Hg. This also means that the reaction should be followed by X-ray powder diffraction analysis of the material at each step. Sample so obtained displays a sharp superconducting transition determined both magnetically and resistively. The X-ray diffraction patterns of the 118 K phase and the 133 K phase both indicate a nearly single phase, corresponding to the tetragonal structure of space group of P 4/ mmm and with lattice parameters a = 0.385 nm and c = 1.585 nm. Energy dispersive X-ray spectroscopic analysis (EDX) data of several flat grains of the Hg-1223 sample shows that the cation stoichiometry is an agreement with the proposed chemical formulae.

Flux-creep mechanism studied in extended windows for the HgBa2Ca2Cu3Oy superconductor

Flux-creep mechanisms are examined by the current density J dependence of the effective barriers U(J), which were determined to be U(J) alpha J(-mu) by three independent measurements i.e. AC susceptibility (ACS), magnetic relaxation (M-t) and hysteresis loop (M-H) for polycrystalline HgBa2Ca2Cu3Oy. It is found that, as J is increased, the U(J) of M-N decreases monotonically and gradually crosses over from mu = 0.43, which connects smoothly with the ACS one with J approximate to 10(2) A/cm(2), to the one with mu = 1.03, which just overlaps with the M-t one with J approximate to 10(6) A/cm(2). This result demonstrates the consistency of the data of the three methods and is considered to be evidence of a crossover between the CDW type and large bundle creep in the vortex-glass phase or collective-creep theories.

Structural ordering in amorphous silicon thin film due to post hydrogen plasma annealing

The effect of hydrogen plasma annealing on the microstructural transition from disorder to order in amorphous silicon film is studied in this paper. Combined with the Fourier Transmit Infrared spectroscopy, Raman scattering and absorption spectra, it is found that there exists two steps for the reaction between atomic hydrogen and Si network. It is shown that the hydrogen plasma treatment conditions strongly influence the microstructures of the amorphous Si films.

Correspondence between magnetic time relaxation and field sweeping rate dependence of magnetic hysteresis

Abstract The dependence of magnetization in flux creep experiments M(t) and field sweep rate dependence of magnetic hysteresis M(dH/dt) are derived and found to correspond with each other in Taylor expansions of the effective activation energy U eff (J). The agreement of U eff (J) extracted from the magnetic relaxation and magnetic hysteresis loops of a polycrystalline HgBa 2 Ca 2 Cu 3 O 8+δ sample is a strong support of this correspondence.

Electroluminescence from Si nanocrystals by annealing amorphous silicon carbide films

In this work, a-SiC:H films have been fabricated in plasma enhanced chemical vapor deposition system by controlling the gas flux ratio R of methane to silane and subsequently annealed in N2 atmosphere for 1 h at the temperature of 1000°C. Raman spectra showed the formation of Si nanocrystals embedded in amorphous SiC matrix after annealing. Room temperature visible electroluminescence was achieved due to the recombination of electron-hole pairs in Si nanocrystals for the annealed samples. The current-voltage relationships were also investigated and the tunneling mechanism was discussed based on the carrier transport properties.

Visible photoluminescence originated from various mechanisms during step-by-step annealing

Light emission variated from violet-blue to red-infrared is observed from a-Si:/SiO2 multilayers during step by step annealing. The peak of photoluminescence (PL) changes in the range between 417nm and 750nm under excitation of 325nm light as a function of annealing temperature, which is strong enough to be observed by naked eyes. Combined with Fourier Transform Infrared (FTIR) measurements, the correlation of the red PL excited by 488nm Ar+ laser and the structural evolution during the step by step annealing is detailed studied. The origins of different PL bands and the role of hydrogen and oxygen in microstructures of a-Si:/SiO2 multilayers are discussed

Nonlinear U(J) relationship in single crystal YBa2Cu3O6+x

Abstract In this paper, a new method to extract U ( J ) relationship in high-temperature superconductors is described without prior assumptions about the functional form of U ( J ), which is based on the Arrhenius rate equation. This method is applied to a single crystal YBa 2 Cu 3 O 6+ x sample and we found that U ( J , T = 0 K ) follows a power law U ∝ J − ϱ with μ = 1.2 for H ∥ ab and μ = 0.87 for H ∥ c .