J.A. Xia

Clear quantum-confined luminescence from crystalline silicon/SiO2 single quantum wells

Crystalline silicon single quantum wells (QWs) were fabricated by high-temperature thermal oxidation of ELTRAN® (Epitaxial Layer TRANsfer) silicon-on-insulator (SOI) wafers. The Si layer thicknesses enclosed by thermal SiO2 range from 0.8 to 5 nm. Luminescence energies from such QWs vary from 1.77 to 1.35 eV depending on the Si layer thickness, without evidence for interface-mediated transition seen in earlier work. The ability to detect quantum-confined luminescence seems to arise from the use of ELTRAN SOI wafers, from suppressed interface state luminescence by high-temperature oxidation and, possibly, from interface matching by crystalline silicon oxide.

Structure and characteristics of ZnO:Al/n-Si heterojunctions prepared by magnetron sputtering

Abstract Aluminium-doped zinc oxide (ZnO:Al) films were deposited by RF magnetron sputtering on single-crystal n-Si substrates to form ZnO:Al/n-Si heterojunctions. The structure of the ZnO:Al films was analysed by X-ray diffraction spectroscopy and scanning electron microscopy. The structural properties of the ZnO:Al/n-Si junction were studied by transmission electron microscopy. The electrical junction properties were characterised by current–voltage (I–V) and capacitance–voltage (C–V) methods. The conductance measured shows a room-temperature turn-on voltage of 0.2–0.4 V. The ideality factor and the junction built-in potential deduced from I–V and C–V plots are 1.62 and 0.56 V at room temperature, respectively. Temperature-dependent dark forward current vs. voltage measurements suggest that trap-assisted multistep tunnelling is the dominant carrier transport mechanism in this heterojunction.

Enhanced critical current density in melt-textured (Y1−xPrx)Ba2Cu3Oy

Abstract We have investigated the dependence of the magnetisation critical density J c of Pr-substituted YBa 2 Cu 3 O y on the percentage praseodymium content (for 0, 2, 4 and 8% substitution) using a vibrating sample magnetometer. Although the critical temperatur of these pseudo-quaternary compouds Y 1− x Pr x Ba 2 Cu 3 O y decreases gradually as x increases, a maximum in the critical current density is observed for x = 0.04. At this composition the critical current density is about three times higher than that in a pure YBa 2 Cu 3 O y sample prepared in the same way. From a discussion of the possible mechanisms it is suggested that J c increases as a result of pinning force enhancement due to Pr substitution. This may be associated with the fact that the atomic volume of Pr is larger than that of Y. It is also possible that Pr-containing compound particles (123 or 211) are finely dispersed in the Y 123 superconducting phase leading to increased pinning, and consequently to higher J c values.

Critical current density and the irreversibility line in melt textured YBa2Cu3Oy and YBa2Cu3OyAg superconductors

Abstract A series of textured YBa 2 Cu 3 O y Ag superconductors were fabricated by the MTG (melt-texture-growth) method. The effect of the silver dopant on YBCO superconductors was investigated by examination of the critical current density, the magnetic irreversible behaviour, and the microstructures of these materials. All samples have highly dense structures and excellent grain alignment. The undoped 123 samples show large magnetic hysteresis and the value of the critical current density J c , estimated by using the Bean critical state model, exceeds 4×10 4 A/cm 2 at 77 K in 10 kOe. However, the J c is reduced by doping silver and the irreversibility line is degenerated as well. Microstructural analyses show that silver-doping reduces the volume fraction of Y 2 BaCuO 5 particles, therefore reducing flux pinning. Comprehensive analyses indicate that the silver-doping has no significant effect on improving the J c and the flux pinning.

Atomistic structure of SiO2∕Si∕SiO2 quantum wells with an apparently crystalline silicon oxide

Thermal oxidation of a silicon-on-insulator substrate produces evidence that an ordered SiO2 structure can exist on thermally oxidized SiO2–Si interfaces. An apparently ordered SiO2 layer was observed by a high-resolution transmission electron microscope (HRTEM) when a thin silicon layer enclosed by SiO2 was less than 3.0nm thick. X-ray diffraction of the ultrathin Si (<3nm) samples showed diffractions from an ordered SiO2 phase, first-order Bragg reflection peaks with a lattice spacing of 4.1±0.15A, and second-order Bragg reflection peaks with 2.03±0.15A, in addition to the peaks from the Si substrate and the thin Si layer. Even in samples with thick Si layers enclosed by SiO2, which did not show the apparently ordered silicon oxide layer by the HRTEM, x-ray results showed a weak diffraction as if from a crystalline silicon oxide. The disappearance of the second-order Bragg reflection at higher energies indicates that the lattice structure of any crystalline SiO2 phase is far from perfect.

AC susceptibility in Y123 and Ag-doped Y123 compounds prepared by the melt-textured-growth method

Abstract Bulk Y123 and Y123+4.5wt.% Ag samples with high Jc were prepared by a melt-textured-growth process. AC susceptibility measurement have been carried out and the temperature dependence of χ″ in these materials is presented. The results show that the “peak temperature” Tp of χ″ (T) shifts to lower temperatures as the AC field amplitude (hAC) is increased. The slopes of the Tp-h 2 3 AC lines for hAC parallel to the c-axis are larger than those with hAC perpendicular to the c-axis. We believe that the higher slope corresponds to a strong intergranular coupling. The presence of silver has no effect on the coupling in the a-b plane of the material, but is found to enhance the coupling parallel to the c-axis direction.

Evidence for crystalline silicon oxide growth on thin silicon

Thermal oxidation of a silicon-on-insulator (SOI) substrate gives evidence that an ordered silicon oxide structure exists on the upper, thermally oxidised SiO/sub 2/-Si interface. The ordered structure of silicon oxide was observed by transmission electron microscopy (TEM) in samples of thin monocrystalline Si film less than 3 nm thick encased by thermal SiO/sub 2/. Based on TEM measurement, the ordered silicon oxide has 1.9 A/spl ring/ spacing along the [110] direction of the Si structure and 17 A/spl ring/ thickness along the (100) direction of the Si structure. Its existence may be related to the fact that the silicon layers were very thin, which may have influenced factors such as their pliancy, since ordered oxide phases were not observed on our samples with silicon layers thicker than 3 nm.

Compensation effect, impurity scattering and superconductivity in 123 compounds

Abstract A series of samples of 123-compound with the composition of Cu(1)1−xMxBa2Y1−yCayCu(2)2Oz, Cu(1)1−xMxBa2YCu(2)2−yZnyOz (M=Al and Co), and Cu(1)Ba2YCu(2)2−x−yZnxNiy have been synthesised and investigated. It is found that the dopants can be mainly classified into two types: For the first type, they merely influence the charge-transfer of the system; for the second type, they destroy directly the correlation between electrons and hence suppresses intrinsically the superconductivity in the cuprates.

Microstructures of high-Jc melt-textured YBa2Cu3O7−x/Ag superconductors

Silver has been previously added to the melt-textured YBa2Cu3O7−x in order to increase the critical current density (Jc) of these materials. However, the effect of this addition on theJc is presently unclear. The purpose of this study is to investigate the effect of silver on both critical current density and the microstructure of the melt-textured YBa2Cu3O7−x superconductors by means of X-ray diffraction, optical polarized microscopy, and transmission electron microscopy (TEM). TheJc of the MTG YBCO/Ag samples is more than 104A/cm2 under the 5 kOe magnetic field. It has been shown that as the concentration of silver increases, the fraction of the 211 phase dispersed within the 123 matrix decreases. Therefore, theJc slightly decreases. These results, together with the effect of the 211 phase, dislocations, and other structure defects on flux pinning, are described in this paper.

A study on the microstructural defects along the a-b plane in the melt-textured YBa2Cu3Oy superconductor

Abstract The microstructural defects of the melt-textured growth (MTG) YBa 2 Cu 3 O y superconductor were studied by transmission electron microscopy using a sample prepared such that the electron beam was parallel to the a - b plane of the material. A special planar defect with a highly disordered atomic arrangement in it was observed. A large density of defects (10 9 –10 10 cm −2 ), in the forms of dislocations and stacking faults, was found in the material. [001]-type edge dislocations, and localised crystal-plane distortion with a surrounding region of strain field were also observed. Many of the defects observed may act as flux-pinning centres and thus increase the critical current density of the superconductor. In addition, stacking faults which were identified as the YBa 2 Cu 4 O 8 (124) phase were found. Some amorphous layers were to also noted on grain boundaries.