Density and deposition rate of chemical-vapour-deposited boron nitride

A study is made on the density and deposition rate characteristics of chemical-vapour-deposited boron nitride (CVD-BN) plates synthesized by use of the BCl₃-NH₃-H₂ system at a deposition temperature (T _dep) of 1200 to 2000°C and a total gas pressure (P _tot) of 5 to 60 torr. At aP _tot of 5 torr, all the CVD-BN plates synthesized at eachT _dep above 1300°C had a density greater than 2.O g cm^–3, and thus showed no noticeable dependence onT _dep. Over theP _tot range from 10 to 60 torr. on the other hand, the density of the plates reached the maximum of 2.08g cm^–3 at aT _dep of 2000° C. AsT _dep was lowered, the density decreased down to a minimum of 1.40 g cm^–3 The deposition rate varied with bothT _dep andP _tot and showed a maximum value under a certainP _tot at a givenT _dep. The value ofP _tot where the deposition rate becomes maximum changed depending on theT _dep. The maximum deposition rate was 0.6 mm h^–1 for the CVD-BN plates when the density was less than 2.0 g cm^–3, and 0.4 mm h^–1 when the density was above 2.0 g cm^–3 The effects of deposition conditions on the characteristics of density and deposition rate are discussed in terms of the structure and deposition mechanism.     

Thermal annealing-induced ageing and structural effects in YBa2Cu3O7−δ superconductors

The YBa₂Cu₃O_7– superconductor loses part of its oxygen by thermal annealing, effectively at annealing temperaturesT _a 400 °C, under reduced air pressure. The release of the oxygen decreases monotonically as a function of time, at givenT _a, and presents two different slopes; fast at the beginning and slow later. The initial slopes (dM/dt) of the isothermal mass loss curves, measured at differentT _a, follow a straight line, as In (dM/dt) is plotted against theT _a ^–1 (K^–1), suggesting the release of the oxygen to be a kinetic process with an activation energyQ ₁=0.45 eV. The oxygen vacancies probably reside along the central-cage Cu-O chains, replacing O(4) ions, which break up spinal Cu-O linkages, and consequently the highT _c superconductivity properties suffer. A YBa₂Cu₃O₇ sample annealed for 30 min at 600 °C (at 10 mbar air pressure) thus gives a considerably reducedT _c at 50 K, compared to 91 K without annealing. However, annealing, peculiarly at a higherT _a 700 °C, for a short period of 10 min, has little effect on the highT _c appearing at 70 K. We believe that the oxygen vacancies possibly populate on O(1) sites along the side Cu-O chains, at this particular temperature, and become positively charged on trapping the hole, thus helping the superconductivity.     

The effect of perpendicular electric fields on the orientation of zinc oxide crystals during the oxidation of zinc thin films

The effect of direct current (d.c.) electric fields on the crystal orientation of zinc oxide thin films was investigated. Evaporated zinc thin films were oxidized in air under the application of perpendicular electric fields (detached electrodes) at different temperatures. The application of positive fields at 550 °C improved thec-axis orientations up to 1000 V cm^–1. Further increase in field strength caused thec-axis orientations to decline to their original values (no applied field). The application of negative normal fields at 550 °C deteriorated thec-axis orientations up to 800 V cm^–1. Thec-axis orientations improved to their original values by increasing the field strength to greater than 800 V cm^–1. The crystal size remained unchanged, but the surface morphology was affected by the application of the electric fields at temperatures above the melting point of zinc. No significant change in optical properties was detected for samples that were subjected to electric fields. Whenever thec-axis orientation improved, crystals on the film surface became rounded and a more ordered microstructure was observed. On the other hand, the deterioration ofc-axis orientation was manifested by the increase in the number of whisker-shaped crystal needles on the surface of the ZnO thin films.     

Synthesis of melt-quenched Bi1.7V0.3Sr2Ca2Cu3O10+y superconducting glass-ceramics

A glass in the BSCCO system with Bi₁₇V_0.3Sr₂Ca₂Cu₃O_10+y nominal composition was prepared by the melt-quenching (glass) method. The suitability of the glass ceramic method has been assessed in terms of physical and electrical properties. Using an analysis developed for non-isothermal crystallization studies, information on some aspects of crystallization has been obtained. The result obtained indicated that substitution of vanadium for bismuth increased the activation energy compared to the unsubstituted BSCCO system but did not enhance the superconducting phase formation. The activation energy for crystallization of glass has been found, E _a=355 kJ mol^–1. The crystal structure was found to differ from that in the unsubstituted BSCCO system. Most importantly, the HT _c phase was formed by reaction between the constituent phases at lower temperatures and not directly from the glass material. The best electrical properties were obtained at T ₀=75 K and J _c=12 × 10³A cm^–2 at 4.2 K.     

In-Plane Optical Spectra of Optimally-Doped LSCO Single Crystals

The in-plane optical spectra of an optimally doped La_1.85Sr_0.15CuO₄ single crystal were investigated over a wide range of frequency 8–30000 cm^–1, including a submillimeter wavelength region. While the in-plane conductivity shows a depression below 150 cm^–1 for T<T _c, which can be the signature of a gap formation, the spectral weight transferred to superfluid condensate is found to amount only to 48% at most of the value estimated from a London penetration depth in muon spin rotation study, suggesting the kinetic energy-driven superconductivity in this system.     

Investigation of the temperature dependence of electron and phonon Raman scattering in single crystal YBa2Cu3O6.952

Detailed Raman-scattering measurements have been performed on high-quality YBa₂Cu₃O_6.952 single crystal (T _c =93 K, T _c =0.3 K). A sharp (FWHM 7.2 cm^–1 at 70 K and 10.0 cm^–1 at 110 K) 340 cm^–1phonon mode has been observed inB _1g polarization. An electronic scattering peak at 500 cm^–1 in theB _1g polarization extends down to 250 cm^–1. These FWHM values determine the upper limit of the homogeneous linewidth of the phonon and electronic excitations. The start of the electronic spectral function renormalization and of the 340 cm^–1 mode anomalies (frequency softening, linewidth sharpening, and intensity increase) have been observed to occur approximately 40 K aboveT _c . The 340 cm^–1 mode Fano shape analysis has been performed and the temperature dependences of the Fano shape parameters have been estimated. All 340 cm^–1 mode anomalies have been explained by the electronic spectral function renormalization.This work was supported by Swedish Natural Sciences Research Council (G.B. and L.B.) and by the National Science Foundation (DMR 91-20000) through the Science and Technology Center for Superconductivity (G.B. and M.V.K.).     

Preparation of superconducting YBa2Cu3O7−x powders by a solution technique

Bulk superconducting YBa₂Cu₃O_7–x powder has been synthesized by a solution technique using a mixture of Ba-ethylenediaminetetra-acetic acid (EDTA) and [Y, Cu]-citric acid complexes. A light-blue, molecular-level, homogeneously mixed precursor was prepared, and transferred to powder form through vacuum drying. The vacuum-dried powder was decomposed at 800 °C for 4 h under flowing oxygen, then heat treated at high temperature from 850 to 950 °C for 6–12 h. The results ofT _c measurements and X-ray analysis show that the orthorhombic, superconducting phase can be formed at temperatures above 850 °C following low-temperature annealing. A sharp transition (T2 K) and high density can be achieved after 930 °C heat treatment. The 930 °C heat treated sample shows aJ _c value of 510 A cm^–2. It is concluded that this solution technique provides better stoichiometric control and lower reaction temperature than the conventional solid-state sintering process.     

Influence of deposition and annealing parameters on some properties of YBCO films prepared by spray pyrolysis

Superconducting YBCO films were prepared by chemical deposition of aerosol generated from nitrate solutions by pneumatic and ultrasonic atomizers. Single crystalline YSZ and MgO substrates were used. Two different substrate temperatures were applied, with the chemical composition of starting solutions being adjusted accordingly. Deposited precursor films were thermally processed under two basically different conditions, i.e., either in atmospheric O₂ atT900°C, or in atT600°C. The influence of these different deposition and annealing parameters onT _c andJ _c values, microstructure, and surface morphology is reported. By optimizing our technological procedure, a real possibility exists to prepare (by means of aerosol deposition) YBCO films withJ _c values of at least 10⁴–10⁵ A/cm², at processing temperatures (both deposition and annealing as well) not exceeding 600–700°C.     

The critical constants of long-chain normal paraffins

Because of the recent availability of the critical constants of normal alkanes up to octadecane, some modifications in the estimation procedures for the critical constants have become necessary. It has been shown that the equation of Ambrose for the critical temperature of normal alkanes leads to the result that as n , the limiting value for the critical temperature is equal to the limiting value for the normal boiling point and the limiting value for the critical pressure is 1 atm. Currently, the CH₂ increment for the critical volume is considered constant. The recent data of Teja have shown that the CH₂ increment increases indefinitely in a homologous series until the critical volume reaches its limiting value. This has made the current procedure for estimating the critical volume obsolete. Taking into account the new measurements of Teja, we have now developed new equations for estimating the critical constants. The limiting values for an infinitely long alkyl chain for T _b, T _c, P _c, and V _c have been found to be 1021 K, 1021 K, 1.01325 bar, and 18618 cm³ · mol^–1, respectively. These new concepts have been applied to the estimation of various properties other than the critical constants.Nomenclature M Molar mass, kg·mol ^–1 - V _c Critical volume, cm³·mol^–1 - V ₁ Saturated liquid volume, cm³·mol^–1 - P _c Critical Pressure, bar - T _c Critical temperature, K - T _b Normal boiling point, K - T _B Boyle temperature, K - T _A Temperature at which the third virial coefficient is zero, K - V _c Limiting value of critical volume = 18,618 cm³ · mol^–1 - P _c Limiting value of critical pressure=1.01325 bar - T _c Limiting value of critical temperature = 1021 K - T _b Limiting value of normal boiling point = 1021 K - P _b Pressure at the normal boiling point, 1 atm - Z _c Critical compressibility factor - Z _c Limiting value for the critical compressibility factor = 0.22222 - R Gas constant, 83.1448×10^–6m³ · bar · K^–1 · mol^–1 - Acentric factor - X (T _c–T _b)/T _c - X ₁ (T _c–T)/T _c - X ₂ 1–(T _B/T)^5/4 - X ₃ 1–(T _A/T)^5/2 - Y P _c/RT _c - Surface tension, mN · m^–1 - B Second virial coefficient, cm³ · mol^–1 - B Limiting value for the second virial coefficient = –30,463 cm³ · mol^–1 - C Third virial coefficient, cm⁶ · mol^–2 - C _b Third virial coefficient at the normal boiling point, cm⁶ · mol^–2 - C _c Third virial coefficient at the critical temperature, cm⁶ · mol^–2 - C _B Third virial coefficient at the Boyle temperature, cm⁶ · mol^–2 - H _vb Enthalpy of vaporization at the normal boiling point, kJ · mol^–1 - n Number of carbon atoms in a homologous series - p Platt number, number of C-C-C-C structural elements - a, b, c, d, e, etc Constants associated with the specific equation - T _c ^* , T _b ^* , P _c ^* , V _c ^* , etc. Dimensionless variables     

Preparation of fatigue-free SrBi2Ta2O9 thin films by r.f. magnetron sputtering and their ferroelectric properties

Fatigue-free bismuth-layered SrBi₂Ta₂O₉ (SBT) films were deposited on Pt/Ti/SiO₂/Si substrates by r.f. magnetron sputtering at room temperature. The variation of structure and electrical properties were studied as a function of annealing temperatures from 750–850 °C. The films annealed at 800 °C had a composition ratio of Sr:Br:Ta = 0.7:2.0:2.0. X-ray photoelectron spectroscopy signals of bismuth show an oxygen-deficient state within the SBT films. The films annealed at 800 °C have a thickness of 200 nm and a relatively dense microstructure. The remanent polarization (2P _r), and the coercive field (2E _c), obtained for the SIBT films, were 9.1 C cm^–2 and 85 kV cm^–1 at an applied voltage of 3 V, respectively. The films showed fatigue-free characteristics up to 10¹⁰ cycles under 5 V bipolar square pulses. The leakage current density was about 7 × 10^–7 A cm^–2 at 150 kV cm^–1. The SBT films prepared by r.f. magnetron sputtering were attractive for application to non-volatile memories.     

Effects of 50 keV2H+-ion implantation and rapid thermal annealing on the microstructure of YBCO thin films

A YBCO film with thickness of about 200 nm was deposited on LaAlO₃[100] by direct current (d.c.) sputtering. The film, which was found to be mainlyc-axis orientated, was irradiated at room temperature with 50 keV²H⁺ ions to a dose of 1×10¹⁶ cm^–2. This implantation destroyed the superconductivity and made the film textured, with the epitaxial crystalline structure of the film being mainly maintained. Transmission electron microscope studies show that there are a considerable number of amorphous islands, which are Y rich and Cu poor compared with the 123 phase in the as-received YBCO film. These amorphous islands are found to be unstable under the irradiation, as after implantation some polycrystalline regions, rather than amorphous islands, can be seen by TEM. Rapid thermal annealing in flowing O₂ ambient can induce oxygen reordering and results in partial recovery of critical temperature,T _c . After RTA, the polycrystalline regions have not changed.     

Critical current density and microstructure of YBa2Cu3O7 thin films post-annealed at a low oxygen partial pressure

Post-annealing of thin films of YBa₂Cu₃O₇ (YBCO) has been performed at 29 Pa and 750°C. For films 0.6 m thick, a critical current density >1 MA cm^–2 is obtained at 77 K, with a sharp eddy current response at 25 MHz. Microstructural investigation of these films by crosssectional and planar transmission electron microscopy reveals that the YBCO film has thec-axis normal to the plane of the substrate in a continuous sheet of varying thickness, frequently covering the entire thickness of the film. Mutually perpendicular rods with thec-axis in the plane of the LaAlO₃ substrate are also seen. The microstructure and critical current density of these films are compared with those of previously reported films post-annealed in atmosphericpressure oxygen.     

FTIR investigation of structural modifications during low-temperature ageing of polycarbonate

Conformational changes are sought during low-temperature ageing of solution-cast films of BPA-polycarbonate, by observing the conformationally sensitive IR aromatic breathing band at 1600 cm^–1. Preliminary results using the carbonyl band at 1775 cm^–1 had shown some indication of ageing-induced changes in the distribution of conformations. The present results obtained on the 1600 cm^–1 band show no indication whatever of conformational rearrangements. This result, at variance with observations of conformational rearrangements accompanying sub-T _g annealing, lends support to the concept that sub-T _g annealing and low temperature ageing are two distinct processes.     

HighT c of liquid-quenched Bi1.6Pb0.4Sr2Ga2Cu3Ox

AlthoughT _c cannot be found for a liquid-quenched Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_x glassy sample, a highT _c is found after annealing for 24 h at 1100 K. The maximum offset temperature of the superconducting transition is 113.3 K at 2.2 × 10^–2mAmm^–2. The maximumT _{c off} is larger than that (the maximumT _{c off} is 103.4 K at 2.0 × 10^–2 mAmm^–2) of sintered specimens before liquid quenching.     

Characteristics of sol-gel derived PZT thin films with lead oxide cover layers and lead titanate interlayers

The sol-gel derived PbZr_0.53Ti_0.47O₃ (PZT) films were fabricated on the bare Pt/Ti/SiO₂/Si substrates or the same substrates coated by the PbTiO₃ (PT) interlayers. The post-deposition annealing temperature and time were optimized when the PbO cover layers and PbO vapour-containing atmosphere were compared with each other and adopted as the method to diminish the lead-loss problem during the high-temperature post-deposition annealing. The X-ray diffraction patterns, microstructures, and electrical properties such as relative permittivity, _r, remanent polarization, P _r, and coercive electrical field, E _c, were investigated in relation to the annealing conditions. The PZT films deposited on the bare Pt/Ti/SiO₂/Si substrates under the PbO vapour-containing atmosphere showed better electrical properties. This indicates that the PbO vapour-containing atmosphere may be the better method of lead-loss-prevention to process the lead-containing films rather than the PbO cover layer method. The electrical characteristics of the PZT films, _r=1150, a dissipation factor of 0.039, P _r=26 C cm^–2, and E _c=40.5 kV cm^–1 were measured at 1 kHz. When PZT films were deposited on substrates coated by the PT layers, PZT-PT films with single perovskite phase were derived by post-deposition annealing at 500 °C for 1 h. However, the relative electrical properties are very poor, i.e. E _r=160, P _r=2.0 C cm^–2 and E _c=75 kVcm^–1. The optimum combination for preparing PZT-PT films is a 40 nm PbTiO₃ interlayer and annealing conditions of 6 h at 550 °C in a PbO vapour-containing atmosphere; the derived films exhibit electrical properties of E _r=885, P _r=21.5 C cm^–2 and E _c=64 kV cm^–1. The combination of inserting a PT interlayer and annealing in a PbO vapour-containing atmosphere can prevent the formation of electrical short paths. In this case, nearly pin-hole-free PZT films can be grown on the PT (interlayer) /Pt/Ti/SiO₂/Si substrates. It is believed that it is possible to prepare the PZT films with nano-scale uniformity, reproducible quality, which may be worth considering for commercial applications.     

Quantized Vortices and Diamagnetic Precursor to the Meissner State in High-T c Superconductors

We report the magnetic imaging for underdoped and optimally-dopedLa_2–x Sr _x CuO₄ (LSCO) thin films on single substrates and nearly optimallydoped YBa₂Cu₃O_7–x (YBCO) thin films on tricrystal substrates in the temperature range both below and above T _c using scanning SQUID microscopy. Below T _c, clear integer- and half-integer quantized vortices were observable. Above T _c, however, the inhomogeneous diamagnetic domains appeared. The local diamagnetic domains that led to the Meissner state were found in the broad temperature range for underdoped samples and in the narrow limited temperature range for optimally-doped samples. The results provide evidence that local diamagnetic domains are closely related to the pseudogap state. The continuous connection of the domain state above T _c with the state of a half-integer vortex at the tricritical point in the YBCO film below T _c also indicates that the diamagnetic domains are also closely related to the occurrence of d_x ^{2-y 2}-wave superconductivity.     

Properties of indium oxide/tin oxide multilayered films prepared by ion-beam sputtering

The preparation and characterization of indium oxide (InO _x )/tin oxide (SnO _y ) multilayered films deposited by ion-beam sputtering are described and compared with indium tin oxide (ITO) films. The structure and the optoelectrical properties of the films are studied in relation to the layered structures and the post-deposition annealing. Low-angle X-ray diffraction analysis showed that most films retained the regular layered structures even after annealing at 500° C for 16 h. As an example, we obtained a resistivity of 6×10^–4 cm and a transparency of about 85% in the visible range at a thickness of 110 nm in a multilayered film of InO _x (2.0 nm)/SnO _y (0.2 nm)×50 pairs when annealed at 300° C for 0.5 h in air. Hall coefficient measurements showed that this film had a mobility of 17 cm² V^–1 sec^–1 and a carrier concentration (electron density) of 5×10²⁰ cm^–3.     

In situ grown superconducting YBCO films on buffered silicon substrates for device applications

Thin films of YBa₂Cu₃O_7– (YBCO) have been grownin situ on silicon single-crystal (100) substrate by using SrTiO₃ as a buffer layer. The deposition has been carried out by on-axis rf magnetron sputtering method. The deposition condition have been optimized by studying the plasma characteristics and correlating them with the superconducting performance of the film. Films deposited at substrate temperature in the range of 680–700°C from stoichiometric YBCO targets in an argon + oxygen mixture (31) are superconducting and showc-axis epitaxy. Compositional confirmation has been carried out using Rutherford backscattering. Scanning tunneling microscopy of the films reveal formation of well-defined layered structure with some defects in the initial stages ofin situ growth of the films. Films grown on SrTiO₃ substrates have excellent crystalline quality (XRD), transition temperatureT _c0=81 K and the critical current densityJ _c >2×10⁵ A/cm² for unpatterned films at 77 K. On silicon substrates using buffer layers thein situ deposited YBCO films shows a higher transition width andT _c0 is also slightly less (71 K).     

Monitoring phase formation of Pb-substituted Bi-Sr-Ca-Cu-O superconducting samples at different preparative stages using Raman spectroscopy and X-ray diffraction

Raman spectroscopy and X-ray diffraction were used to follow structure and phase formation at various stages in order to study the effect of Pb substitution in (Bi_1–x Pb _x Sr₃Ca₃Cu₄O _y samples. We found that major reactions involving Bi₂O₃ occurred at around 700°C and that reactions with Ca, Cu, and Pb started at a lower temperature. The amount of Ca₂PbO₄ formed increases as a function of lead concentration and annealing temperature (up to 800°C), but excess lead substitution (50%) destroys the superconductivity. The high-temperature superconducting phase (2223) is only observed in the 15% and 20% leaded samples. These two samples exhibit a higher amount of Ca₂PbO₄ during intermediate processing stages, which suggests that the presence of the Ca₂PbO₄ is important for the formation of the high-T _c phase. It was found that theT _c increases with lead concentration (up to 20% Pb).