Critical current densities of high pressure oxygen sputtered thin films of YBa2Cu3O7−x by non-resonant rf absorption method

The critical current densities (J _c) have been measured at 77K in high pressure oxygen sputtered thin films of YBa₂Cu₃O_7−x superconductor using the non-resonant rf absorption technique. High values ofJ _c (∼ 10⁵ A/cm²) are observed in these relatively large area (∼ 1·2 cm²) films.     

退火对多晶ZnO薄膜结构与发光特性的影响

用射频反应溅射法在Si（111）衬底上制备了C轴取向的多晶ZnO薄膜，通过不同温度的退火处理，研究了退火对多晶ZnO薄膜结构和发光特性的影响.由x射线衍射得知，随退火温度的升高，晶粒逐渐变大，薄膜中压应力由大变小至出现张应力.光致发光测量发现，样品在430nm附近有一光致发光峰, 峰的强度随退火温度升高而减弱，联合样品电阻率随退火温度升高而逐渐变大的测量及能级图，推测出ZnO薄膜中的蓝光发射主要来源于锌填隙原子缺陷能级与价带顶能级间的跃迁. 关键词： ZnO薄膜 退火 光致发光 射频反应溅射     

Negative ion-containing plasma in parallel-plate radio-frequency discharge in oxygen

The properties of a plasma in a parallel-plate radio frequency (rf) symmetric discharge of 13.56 MHz in oxygen have been investigated. The plasma contains negative ions. The temperature and density of the negative ions have been determined from the slope and height of the semilog-plotted second derivative of the probe characteristics. The laser photodetachment technique is applied, and data indicating the existence of negative ions are obtained. The negative ion densities deduced by both methods are comparable. The negative ion temperature is found to be from 0.1 to 0.6 eV, depending on the pressure and rf power density. The electron energy distribution f(E) deviates from Maxwellian and has a depletion of low-energy parts, which may be due to the loss of low-energy electrons in attachments. A shifted annular-shaped distribution is fitted to the measured f(E), and the data analysis is made according to this form of f(E).     

Observation of rf SQUID characteristics in YBCO bulk at 77 K

RF SQUID behaviour due to grain boundary weak links in a bulk YBCO is observed at 77 K using modified commercial rf electronics. Porous samples with lowI _c are found to show this characteristic whereas dense samples with higherI _c do not show SQUID behaviour.V-B modulation characteristic is found to be better when the rf pumping frequency is kept slightly higher than the resonance frequency of the tank circuit. Designing of coil for tank circuit with appropriateQ has been found to be very crucial for seeing the SQUID behaviour. Estimation of parameters such as coupling constant, mutual inductance, inductance and radius of the SQUID loop, have been made and their significance is discussed. Flux noise spectrum of the bulk rf SQUID in flux locked mode is also reported.     

Reaction of hydrogen with aromatic compounds in a 13.6-MHz discharge

A series of aromatic compounds C₆H₅X (X=CH₃, Cl, NO₂, NH₂, OCH₃, CO₂CH₂CH₃, COCH₃, CN) were reacted with hydrogen in a 13.6-MHz inductively coupled glow discharge. The flow rates of aromatic and hydrogen were typically 0.5 mmol/min and 18 mmol/min, respectively. The applied power was varied from 50–200 W and the total pressure was varied from 2–14 torr. The products were collected and analyzed by gas chromatography. Three types of reactions were observed: (1) addition of hydrogen to the aromatic, (2) replacement of the group X by hydrogen, and (3) reactions characteristic of aromatic in the absence of hydrogen. The toluene reaction was studied most carefully. Methylcyclohexenes and benzene were the major products identified. The benzene was optimized by increasing the power and decreasing the pressure of either hydrogen or toluene. Reaction of toluene-d₈ with hydrogen revealed that hydrogens were sequentially exchanged for deuteria on toluene and each of the products. A new apparatus is described which allows flow rates and pressure to be preselected and controlled and which allows a series of product samples to be collected without quenching the plasma.     

Optical emission spectroscopy and actinometry in CCl4-Cl2 radiofrequency discharges

Radiofrequency discharges fed with CCl₄-Cl₂ mixtures have been studied in the pressure range 0.3 to 0.6 torr by means of emission spectroscopic actinometry with Ar, He, and N₂ as actinometers. Two different reactors, a parallel plate and a capacitively coupled tubular one, have been utilized for this study to obtain information for a large range of electron energy distributions. Analysis of the experimental results demonstrates the following: the utilization of actinometry and its range of validity, the importance of electron attachment to CCl_x species during the plasma decomposition process, and the effects of the presence of chlorine and “glowpolymer” in the discharge medium.     

Modification of surface properties of carbon-based and polymeric materials through fluorination routes: From fundamental research to industrial applications

The outstanding characteristics of fluorine gas, e.g., extreme reactivity and oxidizing power, and the utmost electronegativity of F⁻ ion, lead to very strong bonds between fluorine and most of the other elements of the periodical table. Treatments involving F₂, fluorinated gases and rf plasma-enhanced fluorination (PEF) constitute exceptional tools for modifying the surface properties of materials. Many advantages of these techniques can be indeed outlined, when compared to more conventional methods: low-temperature reactions (even at room temperature), chemical modifications limited to surface only without changing the bulk properties, possible non-equilibrium reactions. Depending on the type of starting materials and employed techniques, the improved properties may concern wettability, adhesion, chemical stability, barrier properties, biocompatibility, grafting, mechanical behavior. Several examples of surface fluorination will be given on various types of carbon-based materials, elastomers and polymers.     

Langmuir probe measurements of axial variation of plasma parameters in 27.1 MHz rf oxygen planar discharges

Langmuir probe studies have been performed on rf (27.1 MHz) discharges in O₂ under planar reactor conditions to determine the axial variation of the plasma parameters (positive ion density, electron temperature, and dc space potential) as a function of pressure (20–220 Pa) and power (10–150 W) or current (0.1–2 A). By monitoring the second derivative of the I–V probe characteristics, the suppression of the rf component in the probe circuit can be optimized. Referring to this problem, numerical studies provide relations for the determination of the residual rf component as well as of the dc component of the plasma potential at incomplete rf compensation. The positive ion density is obtained from the ion saturation currents. Here the effect of collisions between ions and neutral particles within the probe sheath (for p> 100 Pa) is considered. The electron energy distribution function is found to be of the Maxwellian type for all discharge conditions investigated here. If the pressure and the power exceed critical values, the axial charge carrier distribution is characterized by a valley formation in the bulk plasma center. This fact demonstrates that secondary electron emission due to ion impact on the electrode surfaces and following ionization by these electrons near the sheaths in front of the electrodes are significant processes for sustaining the discharge. At low pressures (p60 Pa) the dc plasma potential was found to be identical with the half-peak maintaining voltage of the discharge, in agreement with the model idea of a symmetric rf planar discharge where the rf voltage drop across the bulk plasma can be neglected. For growing pressure, however, the plasma system moves gradually toward a situation where the V-I characteristics of the discharge are significantly controlled by processes in the bulk plasma. This transition depends on the current density.     

Evaluation of CF4 plasma chemistry by power modulation

The dominant chemical reaction kinetics occurring in the plasma environment are studied by small periodic power modulation and analyzed using transfer functions. A CF₄/Ar rf plasma at 500 mTorr was chosen to validate this experimental methodology because the kinetics of the CF, system have been well studied previously.(1) The experimental results demonstrated that the modulation technique can determine dominant reactions in the plasma. The experimental results also confirmed the importance of surface recombination reactions and provided quantitative total sticking coefficients for F, CF₂ and CF: _F=0.02, _CF2=0.05, and CF0.20. The results also indicated that an activated intermediate may be a precursor to the formation of both CF₂ and CF from CF₄. Energetic considerations and excited-.state lifetime calculations suggest that this activated intermediate may be an internally excited CF₃* radical.     

Optical band gap of zinc nitride films prepared by reactive rf magnetron sputtering

Polycrystalline Zn₃N₂ films are prepared on Si and quartz glass substrates by RF magnetron sputtering at room temperature. The structural and optical properties are studied by X‐ray diffraction and double beam spectrophotometer, respectively. X‐ray diffraction indicates that the Zn₃N₂ films deposited on Si and quartz glass substrates both have a preferred orientation in (321) and (442), also are cubic in structure with the lattice constant a=0.9847 and 0.9783 nm, respectively. The absorption coefficients as well as the film thickness are calculated from the transmission spectra, and their dependence on photon energy is examined to determine the optical band gap. Zn₃N₂ is determined to be an indirect‐gap semiconductor with the band gap of 2.11(2) eV. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)