Microwave absorption in YBa2Cu3O x granular thin films at the dc magnetic fields

Localization of microwave absorption in YBa₂Cu₃O _x superconducting thin films in the presence of very low dc magnetic field was studied. The granular YBa₂Cu₃O _x thin films of submicrometer thickness had random orientation of grains, were inserted into a microwave cavity for maximum rf magnetic field. The microwave absorption in low dc magnetic field at temperatures just below the critical temperature shows properties which may be ascribed to superconducting grains. At lower temperatures the hysteretic effects and flux-trapping occur in the intergrain regions of the film.     

Chemical vapor deposition of SiO2 films by TEOS/O2 supermagnetron plasma

A TEOS/O₂ supermagnetron double electrode plasma system was used to deposit SiO₂ films. Deposition rates were measured as a function of rf power and substrate stage temperature. With an increase of rf power on both electrodes from 40 to 80W, the deposition rate increased; however, with a further increase of rf power from 80 to 120W, the deposition rate ceased to increase or decreased only a small amount. The presence of O-H bonds from bonded water in the film was evaluated using buffered HF (BHF) etching solution. With an increase of rf power from 40 to 120W, the BHF etch rate decreased; i.e., the number of O-H bonds were reduced. A minimum BHF etch rate was observed at a rf phase difference of 180° between the two rf power sources. A SiO₂ film was deposited on a trench-patterned quartz substrate. A flat surface SiO₂ layer with air gaps (voids) was formed on the high-aspect ratio (depth/width=1.5-2) trench area.     

<Emphasis Type="Italic">p</Emphasis>-AgCoO<Subscript>2</Subscript>/<Emphasis Type="Italic">n</Emphasis>-ZnO heterojunction diode grown by rf magnetron sputtering

P-type transparent semiconducting AgCoO₂ thin films were deposited by rf magnetron sputtering of sintered AgCoO₂ target. The AgCoO₂ films grown by rf sputtering were highly c-axis oriented showing only (001) reflections in the X-ray diffraction pattern unlike in the case of amorphous films grown by pulsed laser deposition (PLD). The bulk powder of AgCoO₂ was synthesized by hydrothermal process. The optical bandgap was estimated as 4·15 eV and has a transmission of about 50% in the visible region. The temperature dependence of conductivity shows a semiconducting behaviour. The positive sign of Seebeck coefficient (+220 μVK⁻¹) indicates p-type conductivity. Transparent p-n heterojunction on glass substrate was fabricated by rf magnetron sputtering of p-AgCoO₂ and n-type ZnO: Al thin films. The structure of the diode was glass/ITO/n-ZnO/p-AgCoO₂. The junction between p-AgCoO₂ and n-ZnO was found to be rectifying.     

Mass spectroscopic measuring of SiCl<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 0–2) radicals in SiCl<Subscript>4</Subscript> RF glow discharge plasma

The relative densities of SiCl _n (n = 0–2) in SiCl₄ radio frequency (rf) glow discharge plasma are measured by mass spectrometry. The effects of discharge parameters, i.e., rf power, discharge pressure, substrate temperature, and SiCl₄ flow rate on the relative densities of SiCl _n (n = 0–2) are investigated in detail. An optimum configuration of discharge parameters (low rf power, high discharge pressure, low substrate temperature, and low flow rate), which enhanced the formation of SiCl _n (n = 0–2) radicals, is searched by a great deal of measurements and discussions. In the optimum configuration of discharge parameters, we measure the spatial distribution of SiCl _n (n = 0–2) radicals in the most optimized plasma parameters. The experimental results reveal that Si and SiCl may be the dominant precursors in forming the thin film.     

Modeling of Nonlinear Surface Impedance of High-Tc Superconductors Using an Exponential Vortex Penetration Model

We have modeled surface impedance of YBa₂Cu₃O_7– thin films, using an exponential dependence on an applied rf magnetic field. For verification of the model we compared simulation results with experimental data of Nguyen et al. [2] and of Hein [14] at differing temperatures, frequencies and rf power levels. Obtained temperature dependence of the model fitting coefficients exhibited the same character in both cases.     

Nonlinear surface impedance in “low” and “high”T c superconductors

The degradation of the surface impedanceZ = R + iX with rf power is a principal limit for measurements and applications. Such nonlinearities occur for normal conducting cavities by heating or electron loading. But superconductors show a much richer spectrum of causes for nonlinearities. The nonlinearities in the surface resistance δR ∞ γ H ⁿ and surface reactance δR ∞ α H ⁿ can be classified by the ratior = δX/δ R. This ratio differs in value, temperature, or frequency dependence for the different mechanisms, allowing a unique identification of those mechanisms. This identification is a prerequisite for an improved rf cavity design and improvement of superconducting materials.     

Low refraction properties of F-doped SiOC:H thin films prepared by PECVD

Low refractive index materials which F-doped SiOC:H films were deposited on Si wafer and glass substrate by low temperature plasma enhanced chemical vapor deposition (PECVD) method as a function of rf powers, substrate temperatures, gas flow ratios (SiH₄, CF₄ and N₂O). The refractive index of the F-doped SiOC:H film continuously decreased with increasing deposition temperature and rf power. As the N₂O gas flow rate decreases, the refractive index of the deposited films decreased down to 1.378, reaching a minimum value at an rf power of 180 W and 100 °C without flowing N₂O gas. The fluorine content of F-doped SiOC:H film increased from 1.9 at.% to 2.4 at.% as the rf power was increased from 60 W to 180 W, which is consistent with the decreasing trend of refractive index. The rms (root-mean-square) surface roughness significantly decreased to 0.6 nm with the optimized process condition without flowing N₂O gas.     

Comment on pulsed NMR thermometry at very low temperatures

At temperatures below 1 mK the rf power in pulsed NMR experiments is normally kept as low as possible in order to minimize eddy current heating in the metallic samples used for thermometry. However, for metallic samples with short Korringa constants a temperature dependence enters via the spin-lattice relaxation mechanism in addition to the paramagnetic dependence. It may cause systematic errors if the temperature of the conduction electrons is raised by eddy current heating due to the action of the rf pulse to a temperatureT _e which is not very small compared to ₁, with being the Korringa constant and ₁ the rotation frequency of the nuclear moments in the rf field.     

The synthesis of Nb3Ge by RF sputtering

The conditions necessary for the formation of Nb ₃ Ge by low-pressure rf sputtering with a superconducting transition temperatureT _c >21 K have been investigated. Samples have been deposited onto cooled substrates so that the film is first amorphous and then is crystallized by a subsequent annealing, and onto hot substrates, in which case the film is crystalline upon deposition. The highestT _c samples were obtained for a substrate temperature of 735±25° C. The optimum substrate temperature is the same as the optimum annealing temperature for crystallizing films which were first deposited onto cooled substrates. Special conditions are necessary for the formation of single-phase A15 samples of Nb ₃ Ge with an optimumT _c . We have utilized the collisions that the sputtered atoms undergo with the sputtering gas molecules to thermalize the sputtered atoms. We report here on sputtering in both krypton and in argon.     

Effect of oxygen intercalation on properties of sputtered CuYO<Subscript>2</Subscript> for potential use as <Emphasis Type="Italic">p</Emphasis>-type transparent conducting films

Transparent films of copper yttrium oxide doped with 2% calcium have been prepared by rf magnetron sputtering. The films show a conductivity of 8 Scm⁻¹ on intercalation of oxygen at high pressure, which reduced the transparency in the visible region. The Ca-doped CuYO₂ films before oxygen intercalation show an average transmission of about 60% which reduces to about 45% upon oxygen intercalation. The temperature dependence of the conductivity indicates semiconductor behaviour with low activation energy of 0·59 eV at room temperature. The positive sign of Seebeck coefficient (+274 μVK⁻¹) confirms the p-type conductivity of the films. The optical bandgap of CuYO₂ was found to be 3·15 eV.     

Power handling capabilities of superconducting YBCO thin films: Thermally induced nonlinearity effects

We have measured the microwave power dependence of the surface impedance Z_s of YBa₂Cu₃O_x thin films up to very high microwave power levels. Films with different crystal qualities, including one with a bicrystal Josephson junction, were investigated. The experiments included both frequency-domain and pulsed time-domain measurements using a 14 GHz TE₀₁₁ dielectric cavity. Our results demonstrate that the dissipation of heat, generated by rf currents in the superconducting film, contributes to the observed nonlinearities in the surface resistance. The relative extent of this contribution is determined primarily by the film quality. A simple Fabry-Perot resonator model, combined with a cavity heat transfer model, was used to analyze the effects of such nonlinearities on the electromagnetic response of the dielectric cavity to a pulsed input signal.     

Modeling of Nonlinear Surface Impedance of High-Tc Superconductors Using an Exponential Vortex Penetration Model

We have modeled surface impedance of YBa₂Cu₃O_7?δ thin films, using an exponential dependence on an applied rf magnetic field. For verification of the model we compared simulation results with experimental data of Nguyen et al. [2] and of Hein [14] at differing temperatures, frequencies and rf power levels. Obtained temperature dependence of the model fitting coefficients exhibited the same character in both cases.     

Dependence of the resistivity and the transmittance of sputter-deposited Ga-doped ZnO films on oxygen partial pressure and sputtering temperature

Ga-doped ZnO (GZO) thin films were prepared by rf magnetron sputtering and dependence of the electrical resistivity and the transmittance of the GZO films on the oxygen partial pressure (R = the O₂/Ar gas flow ratio) and the substrate temperature were investigated. The resistivity of the GZO film decreases first and then increases with an increase in the substrate temperature (T). A minimum resistivity obtained with a substrate temperature of 300 °C is 3.3 × 10⁻⁴ Ωcm. The resistivity nearly does not change with R for R < 0.25. The decrease in the resistivity for R < 0.25 is attributed to enhancement in crystallinity, whereas the increase in the resistivity for R > 0.25 to precipitation of gallium oxides at grain boundaries. Optical transmittance of the GZO films is enhanced by increasing R up to 0.75. This enhancement in the transmittance is due to a decrease in oxygen vacancy concentration and a decrease in surface roughness with R.     

RF plasma synthesis of YBa2Cu3O7?x powders

Fine powders of YBa₂Cu₃O_7–x have been synthesized by injecting mixed nitrate solutions of yttrium, barium, and copper into an argon rf thermal plasma. In general, the as-produced powders were dark brown and nonconducting. To obtain superconductivity, the as-produced powders were annealed either in a flowing oxygen tube furnace (at 900C) or in a lowpressure oxygen rf plasma. X-ray powder diffraction, scanning electron microscopy, and centrifugal sedimentation were used for powder characterization. For resistance measurements, bulk samples were prepared by isostatic pressing and tube furnace sintering of the annealed powders. The superconducting transition temperature (at 50% drop of resistivity) was 86 K.     

Low-threshold-current 1.2–1.5 μm laser diodes based on AlInGaAs/InP heterostructures

Separate confinement AlInGaAs/InP multiwell laser heterostructures emitting in a wavelength range of 1.2–1.5 μm have been synthesized by metalorganic vapor-phase epitaxy. The threshold current of laser diodes with a strip width of 4.5μm and a cavity length of 200μm was as low as 10 mA. With a cavity length of 1.0 mm, the threshold current density was 500–650 A/cm². The laser diodes can operate in a continuous regime without forced cooling at an ambient temperature of up to 170°C. In a temperature range from 10 to 80°C, the characteristic temperature parameter T ₀ reached up to 110 K.     

Measurements and modeling of linear and nonlinear effects in striplines

We present measurements of surface impedanceZ _S as a function of frequency, temperature, and rf magnetic field for high-quality epitaxial YBa₂Cu₃O_7–x thin films using a striplineresonator technique that measures theQ of the resonator vs. input power. The results have been modeled using Ginzburg-Landau theory at moderate fields, and the Bean critical state model at high fields. Good agreement has been obtained between the model and the measurements. We also discuss the current distributions in the stripline for both low power whenZ _S is linear and high power whenZ _S is nonlinear.     

Acoustic Thermometry Results from 271 to 552 K

The NIST acoustic thermometer determines the thermodynamic temperature from measurements of ratios of the speed of sound of argon in a nearly spherical cavity. We report recent results for T − T ₉₀ on 12 isotherms spanning the range 271–552 K. (T is the thermodynamic temperature and T ₉₀ is the temperature on the International Temperature Scale of 1990.) The results are in excellent agreement with recent acoustic thermometry results reported by Benedetto et al. in the range from 273 to 380 K and with our previously reported results at 303, 430, and 505 K. The combined data sets are sufficiently redundant and sufficiently distributed over the temperature range to support a re-determination of the reference function for standard platinum resistance thermometers for a future temperature scale. The isotherms were analyzed using several methods; the T−T ₉₀ results and related uncertainties are insensitive to the method chosen. The thermal expansion of the stainless-steel resonator was deduced from the frequencies of the microwave resonances of the cavity. To clearly identify two nearly degenerate eigenmodes in our nearly axially symmetric resonator, two phased coupling probes were used to control the azimuthal angle of the microwave excitation.     

RF plasma synthesis of YBa2Cu3O7−x powders

Fine powders of YBa₂Cu₃O_7?x have been synthesized by injecting mixed nitrate solutions of yttrium, barium, and copper into an argon rf thermal plasma. In general, the as-produced powders were dark brown and nonconducting. To obtain superconductivity, the as-produced powders were annealed either in a flowing oxygen tube furnace (at ～900?C) or in a lowpressure oxygen rf plasma. X-ray powder diffraction, scanning electron microscopy, and centrifugal sedimentation were used for powder characterization. For resistance measurements, bulk samples were prepared by isostatic pressing and tube furnace sintering of the annealed powders. The superconducting transition temperature (at 50% drop of resistivity) was ～86 K.     

Effects of deposition parameters on SiCln (n=0-2) densities in SiCl4 glow discharge plasma

The relative densities of SiCl_n (n=0-2) in SiCl₄ radio frequency (rf) glow discharge plasma are measured by mass spectrometry. The effects of discharge parameters, including rf power, discharge pressure, substrate temperature and SiCl₄ flow rate on the relative densities of SiCl_n (n=0-2) are investigated in detail. The experimental results demonstrate that the relative densities of SiCl_n (n=0-2) in SiCl₄ plasma are dependent strongly on these discharge parameters. An optimum configuration of discharge parameters (low rf power, high discharge pressure, low substrate temperature and low flow rate), which enhanced the formation of SiCl_n (n=0-2) radicals, was searched. Further, researching of SiCl_n (n=0-2) spatial distribution for seeking a suitable deposition condition is beneficial for understanding the deposition mechanism of thin films.     

Induced of zeeman coherence in an ensemble of cesium atoms interacting with a two-frequency (microwave+rf) magnetic field

Interference between Zeeman states corresponding to the forbidden magnetic-dipole transition ΔF=0, Δm _F=2 is reported in connection with the simultaneous interaction of 6² S _1/2 cesium atoms with a resonant microwave field and an rf field which varies at twice the Larmor frequency and is directed perpendicular to a static magnetic field H ₀=0.2 Oe. Pis’ma Zh. Tekh. Fiz. 24, 89–93 (July 26, 1998)