Measurements Of Electron Concentration Research Articles

Ballistic nanodevices for terahertz data processing: Monte Carlo simulations

By using a semi-classical two-dimensional Monte Carlo simulation, simple devices(T-branch junctions (TBJs) and rectifying diodes) based on AlInAs/InGaAsballistic channels are analysed. Initially, the model is validated by means ofHall-effect measurements of mobility and electron concentration in long (diffusive)channels. Then, quasi-ballistic transport at room temperature is confirmed in a100 nm channel. Our simulations qualitatively reproduce the experimental resultsof electric potential measured in a TBJ appearing as a result of electron ballistictransport, and in close relation with the presence of space charge inside thestructure. As examples of devices exploiting the ballistic transport of electrons,preliminary simulations of a multiplexor/demultiplexor and a rectifying diodeare presented, demonstrating their capability for terahertz operation.

A new concept of the D-region modeling at high latitudes

Previous publications of the authors on the analysis of the rocket electron concentration measurements in the high-latitude D region are summarized. The electron concentration dependence on solar zenith angle is considered. It is demonstrated that at each particular solar zenith angle χ there is a scatter of the [e] values reaching at some χ two orders of magnitude. It is also shown that there is some sort of the lower envelope of the [e] distribution which may be in a first approach taken as a model for quiet geomagnetic conditions. The electron concentration dependence on geomagnetic indices is analyzed. A model of the dependence of [e] on the ΣKp magnetic index at various χ is created. The combination of the envelope values and the dependence on ΣKp provide an empirical model for the D-region electron concentration in the auroral oval.

Измерение электронной концентрации в нижней ионосфере методом некогерентного рассеяния

Измерение электронной концентрации в нижней ионосфере методом некогерентного рассеяния

Density of heavy hole states of Hg1-xCdxTe in an isotropic nonparabolic approximation by exact measurements of electron concentration

Density of heavy hole states of Hg1-xCdxTe in an isotropic nonparabolic approximation by exact measurements of electron concentration

Comparison of the polar D-region behavior in the Arctic and Antarctic

The behavior of the polar D-region in the Arctic and Antarctic is studied on the basis of the database of the electron concentration measurements onboard the M-100B meteorological rockets launched at the Heiss Island and Molodezhnaya sites. All the measurements have been conducted by the same method, so their intercomparison is valid independently on the well known absolute calibration problem. It is found that there is a pronounced difference in the diurnal behavior of [e] in the upper D region at both sites. The [e] values demonstrate a correlation with the ΣKp magnetic index, the dependence being better pronounced in the dark period The [e] values demonstrate also a dependence on the neutral gas temperature T, the fact being a manifestation of the meteorological control of the D region even at high latitudes.

Non-destructive measurement of electron concentration in n-ZnSe by means of reflectance difference spectroscopy

Non-destructive measurement of electron concentration in n-ZnSe by means of reflectance difference spectroscopy

Non-destructive measurement of electron concentration in n-ZnSe by means of reflectance difference spectroscopy

Reflectance difference (RD) spectroscopy was employed for contactless determination of the free-carrier concentrations in n-ZnSe grown by molecular beam epitaxy (MBE). In materials systems that show the linear electro-optic (LEO) effect, the LEO-induced RD signal from the region of the E 1 and E 1 + Δ 1 transitions can be used to estimate the net doping level. In this study we have investigated n-type ZnSe for a wide range of carrier concentrations (9 × 10 16 to 5 × 10 18 cm −3). Our results show that the carrier concentration can be determined from the gauged RD spectra with an accuracy of approx. ± 20% for net donor concentrations of the order of 10 18 cm −3.

Ionosphere‐protonosphere thermal plasma fluxes estimated from Magion 2 and ionosonde data in August–September 1990

The daytime thermal plasma fluxes at the topside of the midlatitude ionosphere were assessed during a quiet period and a recovery phase of a magnetic storm in summer 1990. Using in situ measurements of electron concentration and temperature and a crude estimate of mean ion mass performed by the Magion 2 subsatellite in an altitudinal range from 1200 to 2500 km, in addition to data supplied by ground‐based ionosondes, electron concentration profiles were drawn by means of a theoretical one‐dimensional steady state ionospheric model. Latitudinal variations of daytime ion fluxes at the 1000‐km‐height level during quiet periods revealed a peak at L = 2.5 ÷ 3.0 with maximum values up to 7 × 108 cm−2 s−1. The difference in flux versus latitude behavior between quiet and poststorm periods was clearly observable for L > 3. The uncertainty in the resultant ion fluxes depends upon the accuracy of the electron temperature and ƒoF2 measurements, as well as on the neutral wind velocity approximation, and was estimated to be about 50 percent. A comparison of the ion flows obtained with either direct measurements or indirect estimations made earlier show reasonable agreement.

Time resolved study of a reactive low pressure plasma: Determination of basic data on electron-fluorine collisions

Time resolved spectroscopy and electron concentration measurements were performed on an Ar–He–F2 mixture excited by an external rf antenna creating a plasma with features of a plasma source generated by a Helicon reactor commonly used in materials processing. The low pressure range and the simplified chemistry allow an appropriate modelling of some processes and lead to the determination of some fundamental data concerning atomic and molecular fluorine. We give a F2 direct dissociation coefficient by electron impact of about 2.0×10−9 cm3 s−1 for an electron temperature of 5 eV. Excitation rate coefficients of some fluorine levels (3p 2P3/2, 3p 4D7/2, and 3p′ 2F7/2) are given as well as rate coefficients of line emissions following F2 dissociation by electron impact.

High‐latitude F region electron concentration measurements near noon: A case study

A case study is presented of a ridge of enhanced F region plasma in the afternoon sector near the equatorward edge of the auroral oval. The study for September 26, 1982, principally exploits data from three ionosondes near 62° invariant latitude in the southern hemisphere separated by 5½ hours of local time but only 3½ hours of magnetic time. These observations demonstrate that the ridge structure is better ordered in magnetic time than in local time. Each station observes the ridge for about 4 hours. The poleward part of the ridge is rich in small‐scale (1–10 km) ionospheric irregularities. It is suggested that the entire ridge is formed by the mechanism described by Foster (1993) in which plasma initially corotates past noon at lower latitudes, before being entrained in the convection pattern in the afternoon sector, and brought back toward noon. The plasma thus has an extended trajectory in sunlight leading to high concentrations. The irregularities probably result from the combined effects of high electric fields and energetic particle precipitation. The contribution which this plasma is likely to make to plasma structures in the polar cap is discussed briefly.

Plasma impedance and microwave interferometric measurements of electron concentrations in dual-frequency powered sulfur hexafluoride plasmas

Recent literature shows that dual frequency rf power can give better characteristics in both plasma etching and plasma deposition. The key features of the powering system used in our experimental studies on a parallel plate plasma system is that the high frequency (HF) is an exact multiple of the low frequency (LF), and that the power from both is supplied to the same electrode. These features make it much easier to measure the plasma voltage and current waveforms with an oscilloscope. We examined 100 kHz plus 12.2 MHz discharges at 0.35 Torr with LF power from 0% to 100% of the total power of 0.05 to 0.6 W/cm2. The plasma impedance for the HF signal tended to be largest at times when the magnitude of the low frequency signal’s voltage was the largest which may be due to additional loss of electrons by extraction out of the plasma. Because of the time-varying HF impedance, we could alter when HF power was dissipated within the low frequency cycle by changing the matching network settings to effectively tune into the low or high impedance times. This effect shows that the process characteristics of the plasma might depend on the matching of the dual frequency systems for the same time-averaged HF power and LF power readings. In general as we increased the percentage of LF power, the electron concentrations decreased. No sudden changes of any plasma characteristics were observed while going through the full range of percentage LF power.

Comparison of microwave interferometry and impedance analysis measurements of electron concentrations in low-frequency sulphur hexafluoride glow discharges

Electron concentrations in low-frequency (100 kHz) sulphur hexafluoride discharges have been measured by microwave interferometry. To obtain high signal-to-noise ratios, the plasma was switched on and off with a 150 Hz pulse. Reproducible average electron concentration measurements were obtained for pressures ranging from 0.15 to 0.45 Torr and powers ranging from 13 to 92 W. The lowest time-averaged electron concentration measured is 4.3*107 cm-3 (corresponding to a line average density of 6.0*108 cm-2). Electron concentration variation within the 100 kHz cycle is obtained after correcting for response times of the interferometer. These time-averaged and time-dependent measurements are compared with measurements from an impedance analysis technique. Although the two types of measurements agree within a factor of two for all measurements, the impedance technique gives a stronger dependence of electron concentration on power and a weaker dependence on pressure. The large time-dependent variation of the electron concentration from zero to high concentrations is attributed to high attachment and ionization rates in sulphur hexafluoride discharges under these conditions.

Iron doping: a tool to improve the electrical properties of Hg1-xZnxSe

The authors present results of measurements of electron concentration and mobility in mixed crystals of Hg1-xZnxSe (0<or=x<or=0.07) doped with resonant Fe donors (0<or=nFe<or=5e1019 cm-3) from liquid helium to room temperature. The data indicate that the presence of Fe considerably improves the electrical properties of the material. The analysis of the mobility in terms of the scattering from ionized centres (with possible spatial correlation of impurity charges taken into account) and alloy scattering leads to an agreement between the measured data and the calculated values.

Time-resolved measurements of electron and ion concentrations in low-frequency sulfur hexafluoride discharges

Electron and ion concentrations in low-frequency SF6 discharges were measured with an impedance technique in which a small high-frequency power perturbation is added to the low-frequency powering signal. The impedances for the high frequency signals are measured and interpreted with an electrical analog of the plasma that gives bulk ion concentration and electron concentration with time within the low frequency cycle. Ion concentrations, which are about 5×1011 cm−3 at 0.3 W/cm2, 2 cm electrode spacing and 350 mTorr, increase approximately with (power)0.8 but change little with pressure from 250 to 450 mTorr. Electron concentrations change from essentially 0 to about 109 cm−3 during the low frequency cycle and increase approximately with (power).1.5 An overall negative ion balance in which the creation of negative ions by electron attachment equals loss by ion–ion recombination is used to compare the ion and electron concentrations. The calculated ion concentrations as determined from measured electron concentrations and literature-reported rate constants are within 25% of the measured concentrations without using any adjustable fitting parameters.

Effect of substrate temperature on the concentration of point defects in vapour phase epitaxial GaP: N,S

The effect of substrate temperature on the concentration of electrically active point defects in vapour phase epitaxial layers of GaP : N,S has been investigated using electron concentration measurements and deep level transient spectroscopy (DLTS). Incorporation of shallow donors and acceptors is discussed. A thermochemical model is presented to explain the formation of a nitrogen related deep-level defect whose activation energy for electron thermal emission is 0.44±0.02 eV. Experimental evidence is given that another electron trap with an activation energy of 0.24±0.01 eV, presumably related to the phosphorus vacancy, is responsible for degradation of material luminescence properties due to an increase of substrate temperatue in the range of 833 to 858°C.

Nontraditional methods for measurement of plasma characteristics and their practical realization

Methods for approximate measurements of electron concentration and mean energy, rates of various types of inelastic reactions with electron participation, and populations of lower levels of excited atoms directly from probe current-voltage characteristics (CVCs) are presented. Also described is a measurement and computation complex based on a personal computer, intended for automation of probe CVC processing by the methods described. Results of applying this approach to determination of discharge parameters in Ne are offered.

Unbiased method for signal estimation in electron energy loss spectroscopy, concentration measurements and detection limits in quantitative microanalysis: Methods and programs

We present an unbiased method for the estimation of the background in EELS spectra. It looks for the maximum likelihood of a power law model whose two dependent parameters A and R are estimated together with their variances and covariance. Applying the usual rules of error propagation, core-loss characteristic signals and thier variances are estimated. Confidence intervals for the relevant atomic concentrations are then established. Special attention is paid to the first- and second-kind risks when detection limits are attained: they respectively correspond to the risk of stating that a concentration is positive when actually it is zero, and to the risk of stating that a concentration is zero when actually it is positive. A special algorithm, based on a chi-square test, is developed giving the weighted mean of successive estimations of the same unknown. This test can select significant data from a set of experimental results. We also present an algorithm which estimates the effective angle of collection β∗ in the case of convergent beam EELS experiments. These methods and algorithms are used in a set of five programs, written in DEC-FORTRAN 77. These programs can be obtained, upon request, from the author.

Comparison of mesospheric VHF radar echoes and rocket probe electron concentration measurements

Refractive index irregularities in the equatorial mesosphere have been investigated using both the Jicamarca VHF radar and a rocket‐borne Langmuir probe launched from Punta Lobos, Peru. On February 27, 1983, a single layer of turbulence was observed in the upper mesosphere by both experiments. There is very good agreement between the observed radar echo power and the radar scattering cross section calculated from the rocket data when these are interpreted in the context of isotropic turbulence. The inner and outer scales of turbulence have been calculated from both the radar and the rocket data, and good agreement is found. The radar data show indications of large‐scale vortices in the layer of irregularities. Rocket data show that the inner scale of turbulence in the upper mesosphere is a few tens of meters and that the Jicamarca radar Bragg wavelength (3 m) is well within the viscous subrange of turbulence in this altitude range. The spectral index in the inertial subrange is close to −5/3, changing to about −7 at higher wave numbers. Energy dissipation rate in the layer was calculated to be 0.05 W kg−1, in good agreement with previous estimates.

Electron concentration measurement in the high voltage spark using stark broadening of H β emission

The applicability of Stark broadening of hydrogen atomic emission as a probe of electron concentrations in the high voltage spark discharge is critically evaluated. Instrumental limitations, broadening theory applicability and limitations due to discharge fluctuations are discussed. Comparison to literature time resolved, spatially resolved data indicates those regimes in which accurate electron concentration measurements can not be made using this technique. Suggestions are made for improvements in instrumentation which would allow faster throughput and more insight into discharge processes using Stark broadening as a diagnostic.

Rocket measurements of mesospheric ionization irregularities

The Langmuir-probe technique for measurement of electron concentration in the mesosphere is capable of excellent altitude resolution, of order 1 m. Measurements from nine rocket flights frequently show small-scale ionization structures in the altitude region 60–90 km. These are believed to be identical with regions of strong coherent backscatter seem by VHF radars at Jicamarca, Peru and Urbana, Illinois. They are believed to represent intermittent turbulence attributable to nonlinear interaction of waves in the mesosphere. Parameters of the turbulent regions are estimated.