H.M. Weng

The influence of filament temperature and oxygen concentration on tungsten oxide nanostructures by hot filament metal oxide deposition

Tungsten oxide (WOx) nanostructures were prepared by a hot filament chemical vapour deposition system and the temperature of the hot tungsten filaments was changed by steps of degrees. The morphology and average growth rate were indicated by scanning electron microscopy which showed that the morphology was highly related to the filament temperature (Tf) and the distance between the filaments and the polished Si (1 0 0) substrates (df). The influence of Tf on the crystalline nature was studied by x-ray diffraction and Raman spectroscopy. The evolution of stoichiometry and types of defects was indicated by x-ray photoelectron spectroscopy and slow positron implantation spectroscopy. When Tf was up to 1750 °C, tungsten oxide nanostructure was synthesized. A turning point of Tf was found at which the nature of crystallinity and of stoichiometry was the best. As Tf increased to 2100 °C or df decreased, the film crystallinity decreased; correspondingly, the component ratio of stoichiometry WO3 decreased and lots of vacancy agglomerates were present. In order to develop the chemical phase from substoichiometry to stoichiometry, the oxygen gas concentration in the mixture gas during deposition should be raised to an appropriate level.

Vacancy defects in epitaxial La0.7Sr0.3MnO3thin films probed by a slow positron beam

Vacancy defects in epitaxial La0.7Sr0.3MnO3 (LSMO) thin films on LaAlO3 substrates were detected using a variable energy positron beam. The line-shape S parameter of the epitaxial thin films deposited at different oxygen pressures was measured as a function of the implanting positron energy E. Our results show that the S parameter of the films changes non-monotonically with their deposition oxygen pressures. For the films deposited at lower oxygen pressures, the increase in S value in the films is attributed to the increase in oxygen vacancies and/or related defect–VO complexes, and for those deposited at higher oxygen pressures, the larger S parameter of the films is caused by the grain boundaries and/or metallic ion vacancies. The surface morphology of the films was also characterized to analyse the open volume defects in the LSMO films.

Implementation of Quantum Logic Gates by Nuclear Magnetic Resonance Spectroscopy

Using nuclear magnetic resonance techniques with a solution of cytosine molecules, we show an implementation of certain quantum logic gates (including NOT gate, square-root of NOT gate and controlled-NOT gate), which have central importance in quantum computing. In addition, experimental results show that nuclear magnetic resonance spectroscopy can efficiently measure the result of quantum computing without attendant wave-functions collapse.

Observation of slow positron annihilation in silicide films formed by solid state interaction of Co/Ti/Si and Co/Si

Slow positron beam was used to investigate the solid state reaction of Co/Si and Co/Ti/Si. Variable‐energy (0–20 keV) positrons were implanted into samples at different depths. The Doppler broadening of the annihilation γ‐ray energy spectra measured at a number of different incident positron energies were characterized by a line‐shape parameter ‘‘S.’’ It was found that the measured S parameters were sensitive to thin‐film reaction and crystalline characteristics. In particular, the S parameter of epitaxial CoSi2 formed by the ternary reaction was quite different with that of the polycrystalline CoSi2 formed by direct reaction of Co with Si.

Impact of deposition oxygen pressure on the thickness effects in epitaxial Nd0.7 Sr0.3 MnO3 thin films

Nd0.7Sr0.3MnO3 films of various thicknesses were grown on (LaAlO3)0.3(Sr2AlTaO6)0.7(001) substrates by the pulsed-laser-deposition method, and their structure, metal-insulator (ferromagnetic–paramagnetic) transition temperature TP (TC) were examined by x-ray diffraction, resistivity and magnetization measurements. To clarify the effects of in situ deposition oxygen pressure on the thickness dependence of TP (TC), the films were grown at an oxygen pressure of 21 Pa and 35 Pa, respectively. X-ray reciprocal space mapping on the films showed that they were all grown coherently on the substrates. For films grown at low oxygen pressure the transition temperature decreases more rapidly with the reduction of film thickness, while for those deposited at high oxygen pressure, with the film thickness decreasing a strain-induced decrease in TP (TC) was observed. Our results indicate that to get higher TP (TC) especially for the ultra-thin films a higher deposition oxygen pressure is indispensable, and this is crucial for understanding the thickness effect in epitaxial manganite films. The ex-situ annealing effects on the thin and thick films were also discussed in terms of their different microstructure.

Background subtraction of digital coincidence Doppler broadening spectra

A new method of subtracting the chance coincidence background is presented. The two-dimensional (2D) background spectrum is discussed in detail. We found that the Compton scattering boundary is not parallel to the axis in the 2D spectrum. The conclusion has been demonstrated by some experiments.

Identities of the deep level defects E 1/E 2 in 6H silicon carbide

E1/E2 (EC-0.36/0.44eV) are deep level donors generally found in ion-implanted, electron and neutron irradiated n-type 6H-SiC materials. Their configurations are controversial and have been related to a negatively charged carbon vacancy, a divacancy or a VSi-complex . With positron lifetime technique, we have identified VSi and VCVSi in the Lely grown n-type 6H-SiC sample, with VSi annealed out at 650 o C. Concentration of VCVSi persists at 1400 o C annealing and significantly decreased after the 1600 o C annealing. Considering the deep level transient spectroscopic (DLTS) results on the neutron irradiated n-type SiC epi sample that E1/E2 completely disappeared after the 1400 o C annealing, E1/E2 is not the VCVSi defect. With positron annihilation techniques, Rempel et al [1] have shown the energy dependence of vacancy generated by electron irradiation. With low irradiation energy of 0.3MeV, only VC was generated and at higher energy (0.5MeV), Si vacancy was detected. With focus to find the minimum energy for generating E1/E2, we have performed DLTS studies on n-type epi 6H-SiC materials irradiated by electrons with varying energies. Our results suggest that E1/E2 have microstructure related to a carbon vacancy or a carbon interstitial.

Positron annihilation study in SmFeAsO and SmFeAsO0.82F0.18

SmFeAsO1−xFx polycrystalline samples were first studied by positron annihilation lifetime spectroscopy and Doppler-broadening spectroscopy, combined with the calculated results of positron lifetime. The experimental results agree well with the calculated positron bulk lifetime in SmFeAsO and SmFeAsF crystals. The temperature dependence of S-parameter shows a remarkable difference between the parent and superconductor. An abrupt jump of S-parameter is detected around 150 K in parent sample, however, two different slopes were shown below and above superconducting transition. The linearity S-T plot determines one-type of defects through the superconducting transition which must play an important role in superconductivity.

Tungsten Oxide Nanostructures Growth in HFCVD System by Slow Positron Beam

Tungsten oxide (WOx) nanostructures were prepared by a hot filament chemical vapor deposition (HFCVD) system. Two series of samples were synthesized by adjusting filaments temperature (Tf) and oxygen gas concentration (OGC). The crystallinity and stoichiometry were highly related to Tf and OGC. The evolution of stoichiometry and types of defects was illuminated by slow positron implantation spectroscopy (SPIS). A turning point of Tf=1750°C was found that at this point the crystallinity and stoichiometry natures were the best. In order to develop the chemical phase from substoichiometric to stoichiometric, the oxygen gas concentration in the mixture gas during deposition should be raised to an appropriate level.

Slow Positron Annihilation in Silicide Films Formed by Solid State Interaction of Co/Ti/Si and Co/Si

Slow positron beam was used to investigate the solid state reaction of Co/Si and Co/Ti/Si. Variable-energy (0-20 keV) positrons were implanted into samples at different depths. The Doppler broadening of the annihilation γ-ray energy spectra measured at a number of different incident positron energies is characterized by a line-shape parameter, S. It was found that the measured S parameters are sensitive to thin film reaction and crystalline characteristics. In particular, the S parameter of epitaxial CoSi2 formed by the ternary reaction is quite different from that of the polycrystalline CoSi2 formed by direct reaction of Co with Si.