B. H. Loo

Growth of diamond films on silicon from an oxygen‐acetylene flame

Clear diamond films have been deposited on silicon substrates by a tilted oxygen‐acetylene flame operating at one atmosphere pressure in air. High quality diamond films are formed by well‐defined diamond crystallites in the octahedral and cubooctahedral shapes as well as of intermediate forms. Raman spectra of the flame deposited diamond display a peak very close to that for natural diamond with little or no broad band corresponding to graphitic bonding as well as a peak corresponding to the underlying silicon substrate. This shows that the diamond is transparent to visible light and that there are little or no interfacial materials other than silicon and diamond. Using an O2 :C2 H2 volume ratio around 0.98 high quality diamond films thicker than 10 μm have been grown on silicon in less than 30 min by an oxygen‐acetylene flame that is aimed at the silicon substrate at about 70° with respect to the direction normal to the silicon surface.

Multiple flame deposition of diamond films

A multiple flame burner has been used to deposit diamond films of up to 20 mm in diameter from an oxygen‐acetylene mixture. The burner consists of nine equally spaced linear holes each sustaining its own flame and rotates under a water‐cooled substrate. The diamond film’s quality is characterized as a function of its radial distance from the center of the film by scanning electron microscopy, Raman spectroscopy, and photoluminescence spectroscopy. Typical films exhibit variations in thickness and crystal structure with an increasing graphitic component present towards the edge of the film as evidenced by Raman analysis. Photoluminescence spectra exhibit defect bands at 1.95 eV present near the outer edge of the film and at 2.16 eV present near the center of the film. These luminescence bands are discussed and attributed to defects induced from the flame’s chemistry.

Spiral hollow cathode plasma .. assisted diamond deposition

High quality diamond films have been deposited on silicon and sapphire by means of a combined hot filament/electron beam/plasma‐assisted chemical vapor deposition technique. A spiral tantalum foil is used as the hot cathode to generate a high‐current dc discharge at a low sustaining voltage. Gas mixtures consisting of methane, hydrogen, and argon flowing through the spiral cathode towards the anode are effectively decomposed by the hot cathode and the high‐density plasma. Diamond particles and films, grown at a rate between 0.5 and 5 μm/h, have been characterized by scanning electron microscopy, x‐ray diffraction, and Raman spectroscopy.

A RAMAN SPECTROSCOPIC STUDY OF BARIUM COPPER OXIDE

Various physical treatments of BaCuO2 samples result in irreversible changes in its Raman spectrum. The prominent peaks at 576 and 628 cm−1 in the spectra of sintered pellets of BaCuO2 disappear upon further annealing in air or oxygen or upon mere regrinding of the pellets. Further annealing in air, oxygen, or vacuum does not restore these peaks. Similar but less intense peaks reappear upon exposure of pellets to laboratory environment for several weeks, but these broaden and disappear upon further exposure. These spectral characteristics contrast with those observed in Y123 Raman spectrum. It is concluded that the peaks at 582 cm−1 and 636 cm−1 in the spectra of sintered pellets of Y123 are not due to the presence of BaCuO2.

MAGNETIC PROPERTIES OF TWO NEW SUPERCONDUCTORS BiSrCuO4−y AND YSrCuO4−y

Immediately after the discovery of the 90-K multiphase Y-Ba-Cu-O superconductor, it was determined that the superconductivity was due to the YBa{sub 2}Cu{sub 3}O{sub 6+y} phase (the 123 phase). Substitution of yttrium, barium, copper and oxygen by different elements have been attempted by many workers in order to understand the role played by ions in the observed superconductivity of cuprate oxides. In particular, it was found that Y can be replaced by most of the rare-earth ions, and the T{sub c} still remains in the 90-K range in these analogous rare-earth 123 compounds. In our earlier work on ion substitution, we investigated the Bi-Sr-Cu-O system, and found that the sample with the nominal composition BiSrCu{sub 4{minus}y} (the 111 phase) exhibited a superconductivity onset at about 70K parts resistivity curve. However, the resistance did not vanish even at 4K. This bismuth 111 system was different from the 20-K superconducting Bi-Sr-Cu-O system reported by Raveau and his coworkers. In addition to the Bi system, the authors have also fabricated another 111 system YSrCuO{sub 4{minus}y}, which has a T{sub c} of 85 K. In this paper, the authors report the magnetic measurement results on these two new 111 superconductors.

INSULATING 211 PHASE-SUPERCONDUCTING 123 PHASE TRANSFORMATION IN THE YBaCuO SYSTEM

We have studied the conversion of insulating Y2BaCuO5 to superconducting YBa2Cu3Ox by means of Raman scattering, magnetization and electrical resistance measurements, scanning electron microscope and X-ray diffraction methods. The magnetic data show that as high as 12% of the conversion can be achieved.

A COMPARATIVE STUDY OF RAMAN SPECTRA AND SUPERCONDUCTING CHARACTERISTICS OF 90 K SUPERCONDUCTORS

The Raman spectra and superconducting characteristics of ReBa2Cu3O7−x where Re = Y, Nd, Sm, Eu, Gd, Dy and Ho have been studied. We have unambiguously established that the 320- and 500- cm−1 modes, consistently appearing in the 200–800 cm−1 region of the Raman spectra of good quality superconducting Re123 samples with large diamagnetic shielding values, are intrinsically belonged to the Re123 orthorhombic superconducting phase.

Study Of High Temperature Oxide Superconductors

Superconducting 123 films can be fabricated using the green 211 phase as a substrate. The superconducting characteristics of these films are better than the characteristics found when other oxide compounds are used as substrates. Using high temperature processing, 211 phase oxide can be partially converted to 123 phase. Using the same process, a new high Tc copper oxide compound with non-rare earth elements was prepared. High temperature proceEsing presents an alternative synthetic route in the search for new high Tc superconductors.