Mercury Vapour Plasma Research Articles

Structural and Bluish-White Luminescent Properties of Li+-Doped BPO4 as a Potential Environmentally Friendly Phosphor Material

Many efforts have been devoted to exploring novel luminescent materials that not contain expensive or toxic elements, or do not need a mercury vapor plasma source. In this paper, BPO4 and Li+-doped BPO4 powder samples were prepared by the Pechini-type sol−gel (PSG) process. The structure and optical properties of the resulting samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), photoluminescence (PL) excitation and emission spectra, kinetic decay, and X-ray photoelectron spectra (XPS), respectively. It was found that PSG -derived Li+-doped BPO4 annealed at 960 °C exhibited bright bluish-white emission centered at 416 nm. The luminescence decay curves analysis indicates that each sample has two kinds of lifetimes (5.9 ns and 0.529 ms) and two types of kinetic decay behaviors which can be fitted into a single-exponential function and a double-exponential function, respectively. This can be attributed to two kinds of emission centers in samples. The luminescent mechanisms of the samples are discussed in detail.

Bluish-White Emission from Radical Carbonyl Impurities in Amorphous Al2O3 Prepared via the Pechini-Type Sol−Gel Process

Many efforts have been devoted to exploring novel luminescent materials that do not contain expensive or toxic elements, or do not need mercury vapor plasma as the excitation source. In this paper, amorphous Al2O3 powder samples were prepared via the Pechini-type sol-gel process. The resulting samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), photoluminescence (PL) excitation and emission spectra, kinetic decay, and electron paramagnetic resonance (EPR). The obtained amorphous Al2O3 powder samples annealed at 500 and 600 degrees C exhibit bright bluish-white emission centered at 430 and 407 nm, respectively. The luminescent mechanisms of the amorphous Al2O3 powder samples can be ascribed to the carbon-related impurities such as radical carbonyl species. The calculated band structure of the defective amorphous Al2O3 agrees well with the results of spectral analysis and the proposed luminescent mechanism.

Substituting low-energy (<30 eV) ion bombardment for elevated temperature in silicon epitaxy

Normally required high substrate temperature for achieving epitaxial film growth can be replaced by low-energy (<30 eV), high-current-density (∼1 mA/cm2) ion bombardment during film deposition. For this a Si substrate wafer was immersed like a large negative Langmuir probe in a low-pressure (5×10−4 Torr) mercury vapor plasma while receiving Si atoms sputtered from a Si wafer target. The Hg plasma was created by extracting a 4-A discharge current at 25-V discharge voltage from a Hg cathode spot on a liquid-Hg pool. Electron channeling patterns proved that uniform Si spitaxial films can be obtained over the whole substrate wafer area at temperatures not exceeding 300 °C. The best epitaxial films were obtained when the substrate is bombarded (by biasing) during deposition with 23-eV Hg ions. The electrical properties of the coatings indicated that the films were close, but not yet of device quality because of the impurities inherent in our non-ultrahigh-vacuum nonbakable Pyrex chamber pumped only with a 12-ℓ/s Hg diffusion pump.

Growth of high quality epitaxial Cd xHg 1- xTe films by sputter deposition

Epitaxial Cd x Hg 1- x Te films have been simultaneously grown onto several CdTe substrates (total area 20 cm 2) by cathodic sputtering in a mercury vapour plasma. The layers are grown on substrates heated up to 310°C with a deposition rate of 0.6μ/hour (total thickness ⩽ 30 μm). Crystallographic characterization has been carried out using different techniques such as: reflection X-ray topography, the measurement of the FWHM of the diffraction profile, transmission electron microscopy (TEM) and Rutherford back scattering (RBS). TEM, rocking curves and RBS results reveal a high crystallographic quality (dislocation density in the layer is approximately 10 4 cm -2). The Hall coefficient of Cd x Hg 1- x Te epitaxial layers has been measured between 4 and 300 K. At 77 the results concerning layers deposited at 285°C (Cd composition of 0.23 and thickness of 16μm) are about n = 2.7 × 10 16 cm -3 with a Hall mobility of 64000 cm 2 V -1 s -1 and for a Cd composition of 0.31 (thickness of 18 μm) we obtained n=1.6×10 16 cm -3 and a Hall mobility of 16000 cm 2 V -1 s -1. The film properties were improved by post deposition annealing in a Hg atmosphere using a two-zone furnace. An increase in electron mobility of n type films to values of those for bulk material were obtained using unoptimized annealing parameters.

Growth of thin films of Cd xHg 1− xTe solid solutions by cathodic sputtering in a mercury vapour plasma

A study of the formation of films of Cd x Hg 1− x Te solid solutions by sputtering in a mercury vapour plasma was carried out. The condensation rate and the condensed material composition were investigated experimentally as functions of the main sputtering parameters, namely the sputtering pressure, the substrate temperature, the sputtered fluxes, the target voltage and the bias voltage. The role of the mercury vapour pressure was determined, as well as the conditions of formation of single-phase or two-phase films. The study of condensed fluxes and condensation coefficients of cadmium and tellerium atoms contributed to the understanding of the growth process.

Dielectric investigation of electric discharges in a mercury vapour filled thyratron†

Abstract In this work the real and imaginary parts of the dielectric constant of mercury vapour plasma in a thyratron were determined as a function of frequency from 5 kHz to 300 kHz for different discharge currents. At the low values of discharge currents near breakdown there is a dispersion of k“ with frequency at f = 120 kHz, which is completely absent at higher values of discharge currents. The results were interpreted by relating this to the relaxation of the dipoles formed in the gas at low discharge currents. At high values of the discharge current the vapour behaves as plasma, giving negative values of k' for a certain frequency range.

Sputtering of Cd xHg 1−xTe films in mercury vapour plasma

In a previous report it has been shown that HgTe and (CdHg)Te films can be prepared by a triode sputtering technique. The resulting films are crystalline and epitaxial on single-crystal substrates (NaCl, mica and {111} oriented CdTe). However, sputtering in an argon plasma leads to layers having altered electronic properties, such as conductivity and carrier mobilities. This is thought to be caused by argon trapping in the film during sputtering. Heating the substrate to enhance crystal growth leads to considerable losses of mercury. The use of a mercury vapour plasma has two main advantages: firstly it compensates the mercury loss, and secondly, related to the first point, it avoids inert gas trapping. A stainless-steel sputtering apparatus has been designed for mercury plasma with a mercury diffusion pump which is used to avoid contamination of the film by organic impurities.

Dielectric Relaxation in Insulating Layers on Contaminated Langmuir Probes

Measurements have been made of the low-frequency admittance of contaminated Langmuir probes in a mercury-vapor plasma. It was found that the probe admittance components were frequency dependent and could be represented by an equivalent electrical circuit corresponding to a metal-insulator-plasma configuration. The frequency dispersion of the admittance components is described by the empirical equation Y=G∞−(G∞−G0)/ [1+(G0/G∞) (jωτ0)1−h], where Y is the complex admittance, G0 and G∞ are the static and infinite-frequency values of conductance, τ0 is a relaxation time constant, and h is a parameter with values between 0 and 1. The complex-plane locus of the admittance is a circular arc from which the resistance of the insulating layer on the probe surface, the plasma-sheath resistance, and the value of h are derived. The susceptance-voltage characteristic of a probe is shown to be a sensitive indicator for the presence of insulating material on the probe surface. The susceptance-voltage curve for an insulator-contaminated probe has a peak in the vicinity of plasma potential which disappears upon removal of the insulating material by sputtering.

Automatic Plotting of Langmuir-Probe Susceptance- and Conductance-Voltage Curves

Langmuir-probe susceptance- and conductance-voltage measurements have been made using a system which automatically plots the in-phase and quadrature components (simultaneously) of a two-terminal complex admittance, as a function of applied dc potential. The operation of the system, which utilizes phase-sensitive detection to separate the admittance components, is discussed. Measurements for a planar guardsleeve probe in a mercury-vapor plasma (collisionless) and for a spherical probe in an argon plasma (collisional) are presented. Positive and negative peaks observed in the low-frequency susceptance-voltage characteristic are discussed in terms of ion-response, probe-contamination, and plasma-perturbation effects.

Parametric Excitations of Ion Plasma Oscillations

Parametric excitations of ion plasma oscillations are investigated by using a simple model based on the modulation of ion density in the uniform electron background. This model is useful to calculate the higher order nonlinear terms, and gives a good interpretation to the new nonlinear phenomena observed in a mercury vapour plasma without magnetic field. The nonlinear effects observed are; (1) the frequency shift and the amplitude increase of the resonant oscillations with increasing of pumping power, (2) the existence of the resonance frequency region, and (3) the asymmetrical parametric excitation with hysteresis.

High-resolution laser interferometer for measurement of electron density in transient plasma

A laser interferometer of the Mach-Zehnder type based on the use of a single-mode frequency-stabilised He-Ne gas laser, 6329 A in wavelength and with 100 μV output, has been used to measure accurately electron densities in the range 1014–1015cm−3 in a transient mercury-vapour plasma. An ‘amplitude’ (intensity) modulation system is employed in order to obtain a wide dynamic range. The interferometer system is mounted on a massive movable table in order to obtain the spatial distribution of electron density by means of sequential discharges. The experimental results show clearly that there are definite time delays between the peak of current in the discharge and the maximum value of the (average) electron density. The results are in good agreement with the values of electron densities obtained by the extrapolation from microwave measurements.

Correlation of slow rates of rise of current with times to pinch in transient mercury-vapour plasma

Measurements of time of maximum pinch in a mercury-vapour-plasma discharge over a range of voltages up to 5kV are not, at first sight, in agreement with values calculated by using the Leontovich-Osovets theory, which has been shown to apply in similar experiments with noble gases. The influence of the external circuit involving the ignitron switch, measuring shunt and associated stripline connecting links, is deduccd from the oscillographic records of instantaneous v and i for the plasma discharge. The ratios v/i for both the external circuit and the plasma discharge are plotted against time to show the times at which the minima and the pinch maximum are reached. A graph showing measured times to pinch is compared with values calculated using the Leontovich-Osovets formula, using the initial rate of rise of current, and shown to be in poor agreement. A further plot of calculated values, obtained by using the mean rate of rise of current up to the pinch point as a fixed value, is shown to be in good agreement with the experimental results.

Free-Fall Decay of a Mercury Vapor Plasma in the Early Afterglow

A free-fall theory is developed for the decay of the average electron number density and of the electron temperature in a pulsed cylindrical mercury vapor discharge plasma using moment equations for the ions with a collision term for momentum loss, and assuming Maxwellian electrons. The plasma assumption was used. Good agreement is obtained with experimental results on four tubes of varying diameter.

Determination of the ambipolar diffusion coefficient of mercury-vapour plasma in the late-afterglow region

Microwave measurements of electron density in the afterglow of a transient mercury-vapour plasma discharge, by the oblique-incidence method, indicate distinct responses prior to the main response. Such responses appear shortly before the central part of the column becomes transparent to the microwave probing signal and correspond to multiple reflections at the plasma boundary. The time constant of electron decay is deduced from the responses which appear at definite times, and it is shown that the principal electron-removal process is ambipolar diffusion from calculations based on an assumed electron-density profile. Extrapolation back to the time corresponding to peak current indicates an extremely high degree of ionisation.

Parametric Excitation of Ion Plasma Oscillations

Parametric excitations of the ion plasma oscillations are investigated in the mercury vapour plasma. The ion plasma oscillation is excited by the pumping oscillation at 2 f p i and 2/3 f p i , where f p i is the ion plasma frequency. The simultaneous parametric excitation of the ion plasma oscillation and low frequency oscillations, whose frequencies are given by f 0 ± f p i where f 0 is the pumping frequency, is also studied experimentally.

Ion Oscillations in a Plasma

ABS>The oscillation intensity in a hog-cathode mercury vapor plasma was observed as a function of discharge current. The frequency of the strongest oscillation noted increased with the discharge current and was observed simultaneously with the electron oscillation frequency. This main oscillation was identified as the ion oscillation, which proved to be unaffected by a weak magnetic field. It was also coexistent with the other large fluctuation. (D.C.W.)

Electron and Ion Density Hysteresis Effects in a Low-Pressure Mercury-Vapour Plasma carrying a Current of Varying Instantaneous Value

AN experimental single-anode mercury arc valve was constructed containing a low-heat-resistance pool cathode of the molybdenum-cup type1 to anchor the cathode spot and a plasma probe inserted with its tungsten tip at the axis of the valve at a distance about one-third from anode to cathode. The valve was operated in a resistance-loaded single-phase half-wave rectifier circuit, thus carrying a half-period sinusoidal current at a frequency of 50 c/s. The cathode temperature of the valve, and hence its saturated vapour pressure, was adjusted by a control of air flow from a fan below the cathode, and the pressure was thus varied over a range of 2.5–17.5 µm mercury. The crest value of current through the valve was altered by changing the load resistance. Further on, a modified version of plasmo-graph2 was constructed to obtain on a cathode-ray tube screen a trace in semi-logarithmic form of the probe characteristic at any chosen instant of the wave-shape of the current pulses passing through the valve. Probe characteristics were obtained to construct sets of characteristics of the plasma parameters as functions of the instantaneous values of current ia for different values of load resistance at a fixed cathode temperature, and for different values of cathode temperature at a fixed value of load resistance.

Impedance Characteristics of a Mercury-Vapor Plasma

Measurements have been made of the impedance/frequency characteristics of a hot-cathode, dc, mercury-vapor plasma at a pressure of approximately 1 μ. The experimental tube contained a directly-heated cathode from a commercial mercury-vapor rectifier and provided a discharge column 80 cm long. Bridge techniques were used to obtain impedance loci extending to 2 Mc/sec for several values of anode current. These appeared to normalize, the magnitudes of the components being approximately inversely proportional to current. A basic equivalent circuit can be deduced, its parameters being dependent on physical processes occurring in the discharge. Prominent frequency components of the low-frequency voltage fluctuations present in the tube appear to be closely related to the impedance characteristics.