Concentration Of Impurity Ions Research Articles

The stable state of charge polydisperse colloids: disordered or charge ordered?

Charge polydisperse colloidal crystals have been recently reported to turn amorphous beyond a critical charge polydispersity (CPD). The resulting disordered (amorphous) state, due to its unusual structural properties, has been conjectured to be different from that obtained from quenching of atomic liquids and is probably a stable state. In order to confirm this, Monte Carlo simulations with different initial configurations are performed and properties of equilibrium states investigated. In contrast to a monodisperse system, which always evolves to a crystalline structure irrespective of the initial configuration, the charge polydisperse system with random charge distribution is found to remain disordered. On the other hand, for the same CPD an ordered crystalline state is found to remain in equilibrium when evolved from a charge-ordered initial configuration. Simulated annealing, by gradually decreasing impurity ion concentration ni (which is equivalent to the lowering of temperature) and evolving at each ni, is also carried out to obtain the low-temperature (ground) state. From the present investigations it is concluded that beyond a critical CPD the charge polydisperse system remains in a stable disordered state unlike atomic glasses (obtained by quenching) which are known to be metastable. Based on some common properties an analogy is made between a spin glass and the amorphous state. The stability aspects of this amorphous state and charge-ordered states are also discussed.

Electrical Properties of Undoped and SnTe-Doped GaxIn1-xSb Molecular-Beam-Epitaxially Grown on GaAs

Electrical properties of undoped and SnTe-doped Gax In1-x Sb on GaAs grown by molecular beam epitaxy are studied. In the studied composition x from 0.65 to 1, source SnTe is found to be an n-type dopant. The SnTe incorporation efficiency in Gax In1-x Sb is found to have no significant difference from SnTe in GaSb. The fitted ionized impurity concentrations for GaSb increases proportionally with the sample's donor concentrations, suggesting that the charged impurities are introduced through SnTe doping. On the other hand, for Gax In1-x Sb, the fitted ionized impurity concentrations do not vary with the total donor concentration. This suggests that material inhomogeneities or imperfections in Gax In1-x Sb may be the cause of mobility values which are lower than expected. In addition, the V/III beam equivalent pressure (BEP) ratio is found to critically influence the quality of the SnTe-doped ternary layers.

Interface roughness scattering in GaAs–AlGaAs modulation-doped heterostructures

We have studied the influence of interface roughness scattering on the mobility of two-dimensional electron gas (2DEG) in GaAs–AlGaAs modulation-doped heterostructures (MDH) both experimentally and theoretically. When the background ionized impurity concentration in the GaAs layer is smaller than 2.5×1015 cm−3, our investigation shows that interface roughness scattering is the dominant scattering mechanism in the high 2DEG density (Ns≥5×1011 cm−2) GaAs–AlGaAs MDH. We also demonstrate that interface roughness scattering is about an order of magnitude stronger than alloy disorder scattering in GaAs–AlGaAs MDH if the AlGaAs/GaAs interface fluctuation is only one monolayer of GaAs.

Isotope scaling and ηi mode with impurities in tokamak plasmas

The ion temperature gradient- (ITG) driven instability, or ηi mode, is studied for discharges with hydrogen, deuterium, or tritium in a toroidal magnetic configuration. Impurity effects on the mode and the instability (impurity mode) driven by the presence of impurity ions with negative density gradient are studied. It is found that the maximum growth rate of the ηi mode scales as M−1/2i for pure hydrogenic plasmas, where Mi is the mass number of the working gas ion. With the inclusion of impurity ions, the growth rate of the ηi mode decreases in all three kinds of plasmas, with a hydrogen plasma still having the highest maximum growth rate, tritium the lowest, and deuterium in between. However, the isotope effects are weaker and scale as M−1/2eff with the presence of impurity ions, where the effective mass number, Meff=(1−fz)Mi+fzMz, with Mz and fz= Zn0z/n0e being the mass number and charge concentration of impurity ions, respectively. For the impurity mode, the scaling is similar to that of the ηi mode without impurity ions. The experimental database shows that the plasma energy confinement time scales as τE∝M1/2i for a wide range of clean plasmas. The correlation of the theoretical results with the experimental confinement scaling is discussed.

Galvanomagnetic Transport of the Two‐Dimensional Electron Gas in In0. 53Ga0. 47As Quantum Wells

AbstractGalvanomagnetic coefficients of the two‐dimensional (2D) electron gas in In0.53Ga0. 47As quantum wells are studied for classical magnetic fields by an iterative solution of the Boltzmann equation considering the degeneracy of the distribution function. Scatterings from polar optic phonons, deformation potential acoustic and piezoelectric phonons, inonized impurities, and alloy disorder are considered. The magnetoresistance coefficient is found to be more sensitive than the Hall mobility and the Hall ratio to changes in temperature, 2D carrier concentration, channel width, ionized impurity concentration, and magnetic field.

Properties Of crystalline 3C-SIC Grown From Methyl Trichlorosilane

ABSTRACTCubic silicon carbide grown by thermal decomposition of methyl trichlorosilane in hydrogen has been studied in infrared (IR) transmission, cyclotron resonance (CR), and photoluminescence (PL). Sample characteristics include: The nitrogen concentration, determined from CR and IR can be of the order of 1015 cm-3 or less. Ionized impurity concentrations determined from the linewidths of CR and IR spectra can be less than 7 × 1014 cm-3. Low temperature electron mobilities from CR are near 105 cm2 v-1 s-1. Carrier freeze-out is consistent with either uncompen-sated donors with activation energy near 50 meV or heavily compensated donors with 20 meV activation energy. The latter choice, however, is excluded by the low ionized impurity concentration. These high quality samples show little or no evidence of the apparently shallow (< ∼20 meV) donor which usually dominates the low temperature conductivity of 3C-SiC grown on Si. Nitrogen donor excitation spectra are in excellent agreement with effective mass theory. Only nitrogen donors at 54.2 meV and an unidentified effective-mass donor at 47.8 meV are observed. If material with the characteristics described above can be reliably reproduced then there is substantial expectation that high quality electronic devices based on 3C-SiC can be fabricated.

The effect of carrier densities and compensation ratios on the electron mobility of InAs xP 1−x

The electron mobility of InAs x P 1− x has been calculated for an ionized impurity concentration between 10 14 cm −3 and 2 × 10 17 cm −3 at 300 K and 77 K by using Matthiesson rule formalism and the variational method. These results are shown to be in reasonably good agreement with experimental mobilities. The electron mobilities of InAs 0.13P 0.87, InAs 0.5P 0.5 and InAs 0.80P 0.20 with temperature are also calculated for a range of ionized impurity concentration. Since the compensation ratio of acceptor and donor doped impurities, the inhomogeneity of the samples and other effects are not taken into account, these results represent the upper theoretical limit of electron mobility in InAs x P 1− x . The dependence of electron mobility of InAs x P 1− x on carrier concentration and compensation ratio are also studied for these alloys. The compensation ratio does not have a significant effect on mobility at room temperature, particularly for low carrier density where ionized impurities are relatively unimportant, while at 77 K, the effect of compensation is very significant. The upper theoretical electron mobilities for x = 0.13, 0.5 and 0.8 are 4500, 6500 and 13,000 cm 2 V −1 s −1 at 300K and 50,000, 50,000 and 150,000 cm 2 V −1 s −1 at 77 K, respectively. The various accepted methods used in the calculation of electron mobility are briefly discussed.

Determination of intrinsic mid-infrared absorption of a fluoride glass

The intrinsic mid-infrared optical absorption of a ZrF(4)-BaF(2)-LaF(3)-AlF(3)-NaF (ZBLAN) glass has been determined by combining laser calorimetry and spark-source mass spectrographic analysis. The total optical absorption of four samples of varying purity was measured at 3.9 microm with laser calorimetry. The extrinsic contribution to the total absorption for each glass was calculated from the measured concentration of rare-earth impurity ions and known extinction coefficients. The intrinsic absorption of this glass at 3.9 microm determined from the difference between the total measured absorption and the calculated extrinsic absorption is 6.11 (+/-0.61) x 10(-4) cm(-1).

Some empirical relations for electron mobility in II–VI compound semiconductors

Electron mobility has been calculated in a number of binary II–VI compound semiconductors using a displaced Maxwellian distribution function and taking the various scattering mechanisms into consideration at different lattice temperatures and for various amounts of ionized impurity concentrations. It is observed that the low field mobility values can be expressed by a cubic power relationship with lattice temperature and with ionized impurity concentration using a least mean square fit technique with an accuracy better than 5 per cent. Similarly, the field dependence of mobility can also be expressed as a power series of the applied electric field. It is suggested that these equations can be profitably used for a quick estimation of mobility values as a check on experiments and also as sufficiently accurate formulae for simulation and modelling purposes.

Absolute concentrations of light impurity ions in tokamak discharges measured with lithium-beam-activated charge-exchange spectroscopy

We demonstrate the application of Li-CXS (fast lithium-beam-activated charge exchange spectroscopy) for measuring spatially and temporally resolved impurity ion concentrations in the TEXTOR tokamak edge plasma. After briefly describing the method of Li-CXS and its capabilities, we present a model for attenuation and atomic state population of an injected Li beam due to collisional interactions with the background plasma particles, taking into account up to eight excited LiI states for considering stepwise excitation-ionization processes.

On the effect of non-degenerate doping of polysilicon gate in thin oxide MOS-devices—Analytical modeling

A one-dimensional model of the polysilicon-gate-oxide-bulk structure is presented in order to analyze the implanted gate MOS-devices. The influence of the ionized impurity concentration in the polysilicon-gate near the oxide and the charge at the polysilicon-oxide interface on the flat-band voltage, threshold voltage, inversion layer charge and the quasi-static C– V characteristic is quantitatively studied. The calculations show a considerable degradation of the inversion layer charge due to the voltage drop in the gate, especially in thin oxide devices. The calculated quasi-static C– V curves agree with the recently published data of implanted gate devices.

Electron transport in ZnS

Transport characteristics of N-ZnS are calculated by solving the Boltzmann transport equation using a variational method (including all major scattering mechanisms and screening). The dependence of electron mobility on carrier concentration, for a range of compensation ratio and ionized impurity concentration, are given at both 300 and 77 K. This provides a rapid means for determining material quality. Mobility limits of 230 cm2/V s (n∼1019 cm−3) and over 3000 cm2/V s (n<1014 cm−3) are calculated at 300 and 77 K, respectively. The temperature dependence of the mobility is calculated and agrees favorably with experimental data.

Review of Wall Conditioning and Wall Materials for Fusion Research Devices

Wall materials for fusion research must meet stringent requirements. The purity of a fusion plasma is critical to obtaining meaningful results during fusion plasma experiments. The concentration of impurity ions in the plasma must be kept low because impurities result in radiative losses that at best decrease the efficiency of the plasma “burn” and at worst prevent ignition completely. Radiative losses scale with the effective charge, Z, of the plasma. Radiative losses due to bremsstrahlung are proportional to Z2; those due to recombination are proportional to Z4. Figure 1 shows the impurity concentration above which ignition cannot be achieved as a function of impurity atomic number for an experimental power reactor. The advantage of using a low-Z material as the first wall of a fusion machine (i.e., as the plasma-facing material) is evident.The major plasma impurities observed in most experiments are oxygen, carbon, and first-wall material. These impurities are introduced into the plasma by many mechanisms, including outgassing, desorption, chemical interaction, sputtering, and evaporation. Chemical and physical interactions between the first wall and the energetic hydrogenic ions produced in the plasma are important factors in impurity production. (Here “hydrogen” refers to hydrogen and its isotopes, deuterium and tritium.)The energy contained in the plasma can be quite large (many megajoules) and can interact with the first wall on a short rime-scale (e.g., a millisecond). It is possible that no material will prove suitable for all operational scenarios. Therefore, the burden of reducing the plasma/first-wall interaction to an acceptable level rests on both plasma and materials scientists.

Stationary Study of the Boundry Plasma of Large Tokamaks with Divertor

AbstractThe hydrogen plasma and impurity flows parallel to the magnetic field lines in a collissional tokamak scrape‐off layer are numerically investigated. The physical model is developed on the basis of electron‐ion fluid equations for hydrogen plasma and rate equations for impurities. A complex model valid for arbitrary concentration of impurity ions based on 21‐moment Grad approximation is used to describe transport coefficients and thermal forces. Plasma interactions with neutral particles recycling at the divertor plate are taken into account. Steady state spatial profiles of the plasma and impurity (0, C) parameters are computed using the time relaxation method Possibility of helium exhaust is investigated.

Anomalous Solid Phase Epitaxy Near the Compensation Point in Amorphous Silicon with Boron and Phosphorus Impurity Profiles

AbstractWe made high precision cw laser interferometric measurements of the variations of the rate of solid phase epitaxial regrowth (SPER) of amorphous layers on (100) silicon implanted with both boron and phosphorus. Depth profiles of SPER were correlated with the implanted boron and phosphorus distributions measured by secondary ion mass spectroscopy (SIMS). The results showed that: (1) the minimum (SPER) rate did not occur at the depth where the implanted impurity concentrations were equal; (2) the maximum activation energy for SPER (∼2.9 eV; ≈0.18 eV greater than for SPER in intrinsic Si) occurred at the depth where the regrowth rate was a minimum; (3) the regrowth rates in the dual implanted sample were different from those of the samples doped only with phosphorus at the same net phosphorusc concentration; and (4) the rate at the depth where the impurity concentrations were equal was different from the intrinsic rate. Further interpretation of the results suggests that the SPER rate in the dual implanted samples is equal to the value for intrinsic silicon at a depth where the net ionized impurity concentration is compensated. The SPER rate was a minimum at a depth where the net ionized impurity concentration was slightly p type.

Electrical characterization of gallium planar-doped ZnSe grown by molecular-beam epitaxy

We report electrical characterization of a series of ZnSe samples which are planar doped by a new approach to doping involving periodic deposition of sheets of Ga on Zn- or Se-rich surfaces. For samples planar doped on Zn-rich surfaces, the mobility could be described by ionized-impurity scattering and polar optical-phonon scattering mechanisms, while planar-doped samples on Se-rich surfaces and uniformly doped samples require additional scattering mechanisms to describe their mobilities. The latter two cases give higher mobilities than the first, a result which is in conflict with the fact that the latter have higher total ionized-impurity concentrations and compensation. These results of higher mobilities along with higher total ionized-impurity concentrations are interpreted as evidence of donor-acceptor pair formation.

Ion concentrations in a binary liquid mixture: Implications for wetting behavior.

The concentration of impurity ions in the two phases of a binary liquid mixture can affect long-range electrostatic forces responsible for wetting layers at a charged substrate. For the particular system ${\mathrm{CS}}_{2}$+${\mathrm{CH}}_{3}$${\mathrm{NO}}_{2}$, we estimate from conductivity measurements the concentration of impurity ions for various temperatures near the critical point. The estimated ion concentrations are consistent with values previously obtained from fitting to theory reflectivity data for wetting on a glass substrate [D. Ripple, X.-l. Wu, and C. Franck, Phys. Rev. B 38, 9054 (1988)].

Alkaline Earth Impurity Segregation at the Basal {0001} and Prism {1010} Surfaces of α-Al2O3

Abstract Atomistic computer simulation methods have been used to model calcium and magnesium segregation at the basal {0001} and prism {1010} surfaces of α-Al2O3 and their interaction with charge compensating defects. The results demonstrate that segregation is surface specific, varying with temperature, impurity ion size and concentration. The variation of defect energy as a function of distance from the basal surface is also given and the results are discussed in the light of the sintering behaviour of α-Al2O3 doped with calcium and magnesium.

Electron-beam-induced current and photoetching investigations of dislocations and impurity atmospheres in n-type liquid-encapsulated Czochralski GaAs

Typical defects in n-type liquid-encapsulated Czochralski GaAs, i.e., grown-in dislocations and grown-in and then stress-induced glide (GS) dislocations have been studied by electron-beam-induced current (EBIC) and diluted Sirtl-like etching with light (DSL photoetching). The study of GS dislocations was possible because both EBIC and DSL can reveal the recombinative traces left behind the moving dislocations. Grown-in dislocations and the ending points of GS dislocations exhibit a central feature more recombinative than the external atmosphere. No such central feature has been detected at the starting points of the GS dislocations. A model for the formation of the impurity atmospheres in the GS defects has been put forward. By quantitative EBIC measurements performed right in the impurity atmospheres (∼5–20 μm in size) around dislocations, the local space-charge region width of the EBIC Schottky diode was determined. Information on the net ionized impurity concentration in the atmospheres could thus be obtained. It has been established that silicon (dopant) autocompensation along with the gettering of other unknown impurities and point defect reactions probably involving arsenic interstitials are important features of the atmospheres.

Recent developments for plasma edge diagnostics using atomic beams

Li-atom beams with velocities of 1.5 × 10 5 cm/s and 1× 10 6 cm/s have been used to measure n e profiles in the density range 10 11–10 13 cm −3 from the Li line emission with a spatial resolution of about 1 mm. Injecting in addition a different type of atoms, the ionization rate of which shows a strong dependance on the electron temperature in the interesting range of 5–100 eV (e.g. carbon and helium), also radial T e profiles are obtained. A high intensity 30 keV Li beam is employed to measure radial profiles of impurity ion concentrations (e.g. He, C, 0) by charge exchange recombination spectroscopy. For the different purposes several types of injectors have been developed: thermal beams using ovens, suprathermal beams using laser ablation techniques, and a high energy beam using a plasma ion source in combination with a charge exchange cell. They are located at different poloidal and toroidal positions. The spatial line emission profiles of the beams are recorded either by Si diode array cameras or by a photomultiplier in combination with a scanning mirror so that a spatial resolution better than 2 mm is achieved. The combination of all these systems delivers information about the structure of the TEXTOR boundary layer in poloidal and toroidal direction.