Decomposition of tetrafluoromethane by water plasma generated under atmospheric pressure

Tetrafluoromethane (CF₄) decomposition by water plasma generated under atmospheric pressure was investigated by means of thermodynamic analyses and experiments. Thermodynamic equilibrium calculations were performed between 300 and 6000 K at atmospheric pressure. Experimental results indicated that CF₄ was completely decomposed by water plasma, and recovery of fluorine can be achieved more than 99%. Influence of factors such as arc current and additive flow rate of O₂ on CF₄ decomposition was determined. Furthermore, the decomposition mechanism of CF₄ was investigated from chemical kinetics consideration. CF_{x (x: 1-4)} was thermally decomposed above 4000 K, oxidized in the temperature range of 4000-2400 K, and removed by H radical at temperatures below 2400 K.     

An electronic vapour pressure controller

A system is described in which the vapour pressure of a liquid He⁴ bath is monitored by a capacitance gauge, the output of which is used to control the pumping rate of the vapour pump. The pressure is regulated to an accuracy of 0.1% or better over the range 3–1000 mm Hg and the system is usable down to at least 1 mm Hg. The controlled pressure is completely insensitive to magnetic field.     

An electronic vapour pressure controller

The thermal conductivity of thin gold wire of commercial purity was found to be given by λ = 0.117 T + 7 × 10^?4 T² (WK^?1 cm^?1) between 0.8 and 10 K. The process limiting the conductivity is scattering of electrons by impurities. The moderate deviation of the thermal conductivity from that predicted on the basis of the residual electrical resistivity is probably due to the effect of magnetic impurities on the transport properties.     

The cryopumping of water vapour in the continuum pressure region

Pumping speed measurements in the continuum pressure region, P>10^?3 torr, have been made for water vapour impinging on copper spheres and coils cooled to liquid nitrogen temperatures. Water vapour flow rates between 0.06 mg s^?1 and 420 mg s^?1 were used. The volumetric pumping speed was constant over the pressure range 2 × 10^?3 torr to 2 × 10^?2 torr and was, as expected, higher than that obtained in the free molecular flow region. Above 2 × 10^?2 torr the pumping speed decreased and possible reasons for this were investigated and are discussed. These included the effects of inadequate heat transfer from the liquid nitrogen refrigerant to the cryopump, a poor thermal conductivity of the cryodeposit, and an impurity, nitrogen gas, in the water vapour.     

Anomalous vapour pressure over liquid D-T

The presence of a few tenths mol % helium in hydrogen causes anomalous vapour pressures as high as 10 kPa. This effect is caused by the combination of a small cold cell, a single, long fill tube, and the insolubility of helium in liquid hydrogen. This effect is important in the handling of deuterium-tritium, as radioactive decay produces He³.     

High pressure vapour phase epitaxy of GaN

GaN epitaxial layers were grown on (1102) oriented sapphire substrates with the GaHClNH₃ technique at temperatures from 660 to 1000 °C under nitrogen pressures from 40 to 130 bar. The optimum growth temperature was 820 to 840 °C. Growth rates of 0.2 to 0.4 /um/min were obtained. The lattice constants of this material were $\text{a}\text{= 3.18}\text{A}\text{?}$ and $\text{c}\text{= 5.18}\text{A}\text{?}$, and single crystalline layers showed (1120) orientation. The layers were n-conducting with typical mobilities of 30 cm²/Vsec and with carrier concentrations from 2 · 10¹⁹ to 1 · 10²⁰ cm^?3, even when grown at 120 bar and 660 °C. Therefore, p-type conduction could not be achieved in GaN, even under these extreme conditions.     

New vapour pressure measurements for argon and nitrogen and a new method for establishing rational vapour pressure equations

When collecting the thermodynamic properties of the pure fluids, nitrogen and argon, only a few reliable experimental values were found in some ranges of the areas of interest in spite of a relatively high quantity of measured data. Unfortunately the available experimental values are based on different temperature scales. New vapour pressure measurements for argon and nitrogen have been performed with specially developed low temperature equipment, to obtain some idea of the quality of the data available in this area.In parallel with the growing number of experimental vapour pressures, different formulae for representing the vapour pressure curves have been used by many authors. These equations are tested critically. Most of the relationships tested can only moderately fulfil the demands expected from such equations, especially in the critical range. Therefore, a new method of establishing a rational vapour pressure equation is presented here. The method has been used for nitrogen and argon without any restriction as to general validity. For these two substances the coefficients of a new vapour pressure equation are also given.     

Spectroscopic measurement of the vapour pressure of ice

We present a laser absorption technique to measure the saturation vapour pressure of hexagonal ice. This method is referenced to the triple-point state of water and uses frequency-stabilized cavity ring-down spectroscopy to probe four rotation-vibration transitions of at wavenumbers near 7180?cm(-1). Laser measurements are made at the output of a temperature-regulated standard humidity generator, which contains ice. The dynamic range of the technique is extended by measuring the relative intensities of three weak/strong transition pairs at fixed ice temperature and humidity concentration. Our results agree with a widely used thermodynamically derived ice vapour pressure correlation over the temperature range 0°C to -70°C to within 0.35 per cent.     

Low pressure chemical vapour deposition at quasi-high flow

A new chemical vapour deposition (CVD) technique is presented. It is especially advantageous for the deposition of compound materials. The technique improves the uniformity and reproducibility of the deposition. The economical use of gaseous reactants is improved by a factor varying between 5 and 20. This is important in the case of expensive metal-organic CVD methods. The method consists in the manifold repetition of the following sequence: evacuation, filling and deposition in a horizontal tube reactor. The filling time of 50 ms is short compared with the deposition period 1 s.The advantages of the method are demostrated with results for the deposition of undoped, phosphorus-doped and boron-doped silicon and SiO₂.     

Estimation of vapour pressure and partial pressure of subliming compounds by low-pressure thermogravimetry

A method for the estimation of vapour pressure and partial pressure of subliming compounds under reduced pressure, using rising temperature thermogravimetry, is described in this paper. The method is based on our recently developed procedure to estimate the vapour pressure from ambient pressure thermogravimetric data using Langmuir equation. Using benzoic acid as the calibration standard, vapour pressure-temperature curves are calculated at 80, 160 and 1000 mbar for salicylic acid and vanadyl bis-2,4-pentanedionate, a precursor used for chemical vapour deposition of vanadium oxides. Using a modification of the Langmuir equation, the partial pressure of these materials at different total pressures is also determined as a function of temperature. Such data can be useful for the deposition of multi-metal oxide thin films or doped thin films by chemical vapour deposition (CVD).     

Inverted forms of the new helium vapour pressure equations

Equations relating the vapour pressures of He³ and He⁴ to temperature on the ‘Échelle Provisoire de Température de 1976’ have been approved by the Comité International des Poids et Mesures. Using these definitive equations temperatures can only be calculated from measured pressures by iteration. Moreover, the equations are not amenable for use at less than full precision. Alternative equations expressing temperature as a function of pressure over somewhat reduced ranges are now presented in forms which are more easily solved for temperature and are capable of further simplification if reduced accuracy is acceptable.     

Cryogenic valve pneumatically actuated by use of vapour pressure

A new concept of a cold actuator chamber is realized to avoid using cumbersome high-pressure helium gas cylinders in space for the cold valves in AMS-2 project [Cryogenic system for a large superconducting magnet in space, Part IV, alloy]. A good repeatability was shown in the first experiment with the cold valve opened and closed.     

Stability of low pressure chemical vapour deposition amorphous silicon

The study of the influence of phosphorous doping and hydrogen content on transport properties and thermally induced metastability of low pressure chemical vapour deposition a-Si is reported. Introduction of hydrogen causes change of dominant carrier transport mechanism at room temperature. The thermally induced metastability was observed in both unhydrogenated and hydrogenated P-doped a-Si films. In this paper we report our studies on the effect of the thermally induced metastability in unhydrogenated and hydrogenated films of LPCVD a-Si, as a function of phosphorous concentration.     

The viscosity of nitrous oxide and tetrafluoromethane in the limit of zero density

New representations of the viscosity of nitrous oxide and tetrafluoromethane in the limit of zero density are provided. The representation for nitrous oxide extends over the temperature range 180 to 800 K, whereas that for tetrafluoromethane extends from 150 to 1100 K. The behavior of each gas is represented by an independent correlation of the appropriate effective cross section as a function of temperature. The final results are compared with experimental data as well as with representations based on a correspondingstates analysis.     

Amorphous silicon prepared by low pressure chemical vapour deposition

Amorphous silicon films, grown by low pressure chemical vapour deposition (LPCVD) in a hot-wall reactor at temperatures around and below 500°C and at pressures of 100 mTorr, were investigated using various characterization techniques, to look for possible differences between these films and films grown by LPCVD at higher temperatures and by atmospheric pressure CVD (APCVD). The emphasis was placed on morphological (scanning electron microscopy and X-ray diffraction) and physical characterization (optical absorption, reflectivity, resistivity, Hall mobility and photoconductivity), while the hydrogen profile was measured using the ¹⁵N technique. The results indicate physical properties that are quite different from those of other LPCVD and APCVD films, properties which cannot be obtained by a simple extrapolation from higher deposition temperatures and which deserve further detailed investigation.     

Ultra-low pressure chemical vapour deposition of polycrystalline and amorphous silicon

The deposition of undoped polycrystalline and amorphous silicon in an ultra-low pressure chemical vapour deposition system capable of achieving operating pressure of 0.03 Pa is discussed. It is found that at deposition temperatures in the region of 630 °C and reactor pressures of less than 1.3 Pa very large grained polycrystalline silicon films are obtained, and in this regime the growth rate is independent of the reactor pressure. Excellent uniformity is obtained and the process can be easily scaled up for large substrates and high volume batch production.     

Deposition of indium tin oxide by atmospheric pressure chemical vapour deposition

We report the deposition of indium tin oxide (ITO) by atmospheric pressure chemical vapour deposition (APCVD). This process is potentially scalable for high throughput, large area production. We utilised a previously unreported precursor combination; dimethylindium acetylacetonate, [Me₂In(acac)] and monobutyltintrichloride, MBTC.[Me₂In(acac)] is a volatile solid. It is more stable and easier to handle than traditional indium oxide precursors such as pyrophoric trialkylindium compounds. Monobutyltintrichloride (MBTC) is also easily handled and can be readily vaporised. It is compatible with the process conditions required for using [Me₂In(acac)].Cubic ITO was deposited at a substrate temperature of 550 °C with growth rates exceeding 15 nm/s and growth efficiencies of between 20 and 30%. Resistivity was 3.5 × 10^− 4 Ω cm and transmission for a 200 nm film was > 85% with less than 2% haze.     

The effect of tetrafluoromethane plasma post-treatment on the electrical property of tungsten oxide nanowires

The effects of tetrafluoromethane (CF4) plasma on the surface morphology, chemical compositions, and electrical property of tungsten oxide (W18O49) nanowires are investigated. The nanostructured tungsten oxide nanowires with average length of 250-350 nm were self-catalytically grown on Si substrate. By post-treatment with CF4 plasma for 10 min, the W18O49 nanowires on the substrate showed the highest current response. Longer CF4 plasma post-treatment time demonstrated higher etching effect which demolished the nanowires and resulted in lower conductivity of the samples. The disintegration of the W18O49 nanowires layer after CF4 plasma treatment, revealed physically by the decrease of the average thickness and chemically by the decrease of XRD peak ratio (I 23.0/I 26.0), was closely related to the overall electrical performance. The etching effect was further reveled by Raman spectra showing the evolution of O-W-O and W=O characteristics with the increased post-treatment time. Moreover, the improvement of the electrical property of W18O49 nanowires was elucidated by the exposure rate to explain the mechanism of plasma post treatment in three stages: passivation, degradation and ablation. The maximum exposure rate, corresponding to the maximum conductivity, was achieved by 10 min of CF4 plasma treatment. The time-differentiated exposure analyses confirmed the evolution of resistance of W18O49 nanowires on Si with different post-treatment time which supported the results of surface characterizations.