Function Of Impurity Concentration Research Articles

Anisotropy effects in tantalum, niobium, and vanadium down to the millikelvin temperature range

Experiments on the transition fields to the normal conducting state were made on single crystals of Ta, Nb, and V in the temperature range between 40 mK and the transition temperature. In tantalum results are presented on the thermodynamic quantitiesT c, μ0 H c(T),D(t), and γ in the clean limit and as a function of impurity concentration at low impurity levels (<0.15 at %). Special attention is paid to the phase transition between type I and type II superconductivity, which occurs at a certain conversion temperatureT*<T c in materials with κ(T c)<1/√2. This transition is found to be strongly anisotropic because of theH c2 anisotropy effect appearing in the type II superconducting state. It is shown that atT=0 no signs of type II superconductivity will appear for κ(T c)⩽0.44, whereas type II superconductivity will be found in every crystal direction for κ(T c)⩾0.50. The analysis ofH c2 anisotropy in Ta in terms of cubic harmonic functions demonstrates that the first anisotropic expansion coefficienta 4 remains finite atT*, whereas the second,a 6, vanishes when type II superconductivity disappears. No significant values of any higher order coefficient could be detected in Ta. For Nb and V the temperature dependence of the anisotropy coefficientsa 4,a 6,a 8, anda 10 was established in the entire temperature range. The diversity of results clearly indicates that different microscopic mechanisms contribute to the observedH c2 anisotropy effect in these materials.

Quantum spin glasses with randomly mixed uniaxial anisotropies

The authors study a long-range quantum spin glass model with randomly mixed local uniaxial anisotropies. Two types of magnetic impurities are considered, and the case of competing anisotropies will be discussed in particular. They calculate the phase diagrams of these systems as a function of impurity concentrations and strengths of the anisotropies. Depending on the parameters of the system, interesting transitions from longitudinal to transverse freezing and/or condensation in a non-magnetic state are observed.

Absence of a Coulomb gap in a two-dimensional impurity band.

The temperature dependence (30K&gt;T&gt;400 mK) and magnetic field dependence (H&lt;50 kG) of hopping conduction have been measured as a function of impurity concentration and surface electric field in a quasi-two-dimensional impurity band formed in the inversion layer of a sodium-doped Si metal-oxide-semiconductor field-effect transistor. We find that our observations can be accommodated by noninteracting, single-particle hopping models based on percolation theory in which the effect of Coulomb interactions between electrons on different sites is ignored. Our observations are not consistent with the existence of a Coulomb gap in the single-particle excitation spectrum, although the gap was expected to determine the conductivity under the conditions examined in these experiments.

Dimer formation at the earlier stage of dipole clustering in manganese-doped NaCl and KCl.

The aggregation kinetics of nearest-neighbor and next-nearest-neighbor ${\mathrm{Mn}}^{2+}$-cation--vacancy dipoles in manganese-doped NaCl and KCl has been analyzed at 333 and 343 K as a function of impurity concentration. To do this, we measured a large number of dipole- decay data points at short annealing times with use of the electron-paramagnetic-resonance technique. With this technique we achieved a good knowledge of the dipole-decay curve when only a small amount of dipoles had been clustered to form aggregates into the alkali halide matrix. The analyses of the data taken at the earlier stage of aggregation, without our assuming any reaction order a priori, indicates unambiguously that this dependence is the one expected for a pure second-order kinetics equation. From this result and those reported in our previous paper showing conclusively that dimers are the complexes which are formed during the very initial stage of the clustering process, it appears that the long-existing controversy of dimer versus trimer formation may be definitively solved.

Influence of radiation intensity and lead concentration in the room-temperature coloring of KBr.

The first stage of F-center coloring in quenched samples of lead-doped KBr has been investigated at room temperature as a function of impurity concentration and radiation intensity using optical absorption, photoluminescence, and ionic-thermocurrent techniques. For the range of lead concentration employed in this work (7 up to 250 ppm), the coloring increases monotonically with concentration when the samples are subjected to a radiation intensity of 10 R/min. Moreover, the amount of first-stage coloration was found to be proportional to the square root of impurity concentration, in agreement with the theoretical model recently proposed to account for F-center production in alkali halides doped with divalent cation impurities that change their valence state by irradiation. A quite different result was found to occur when the samples were irradiated at a higher radiation intensity (50 R/min). In this case, the coloring increases with lead concentration only at the very beginning of irradiation. As the irradiation time increases, however, the coloring curve for a heavily doped crystal crosses that for a slightly doped crystal in such a way that more coloring is produced during stage II of the F-center growth curve for the lower concentration. Moreover, as lead concentration increases, the coloring saturates more efficiently the higher the concentration. It was ascertained that the exposure of the crystals to the highest radiation intensity employed in this work also induces lead precipitation in the irradiated specimens as revealed by analyses of the emission and excitation spectra. This effect is produced by the ionizing radiation and not by the room-temperature annealing of the samples during the time in which the irradiations were performed. The influence of impurity precipitation occurring during stage I of the F-center growth curve on the kinetics of color-center formation is analyzed. It is proposed that the phenomenon of impurity precipitation induced by irradiation may be responsible for the experimental observation that more coloring is sometimes produced in slightly doped samples than in heavily doped samples.

Deep levels in semiconductors

The role of deep-lying trapping centers in semi-insulating GaAs, polysilicon and polycrystalline tin oxide transparent electrode has been systematically investigated. It was demonstrated that some of the peculiar transport properties of these semiconductors can be elucidated by deep level compensation. A multilevel model is presented to determine the position of the Fermi level as a function of impurity concentrations. These include, quantitatively, the deep-lying levels which have been introduced by doping in the case of GaAs and by grain boundaries in the case of polycrystalline films. In the latter cases the dangling bonds, associated to lattice defects, are characterized by energy levels which are localized in the energy gap. These dangling bonds can act as electron traps when empty and hole traps when occupied. These are the deep levels.In each of the investigated three cases, this concept permitted the elucidation of some of the transport properties of these semiconductors.

Quasilocal impurity states in Pb1-xSnxTe and PbSe0.08Te0.92 liquid-phase epitaxial layers doped with group-III elements.

The doping properties of Ga, In, and Tl-doped PbTe, ${\mathrm{Pb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sn}}_{\mathrm{x}}$Te, and ${\mathrm{PbSe}}_{0.08}$${\mathrm{Te}}_{0.92}$ liquid-phase epitaxial layers were systematically studied as a function of impurity concentration, tin mole fraction, and temperature. The dependence of carrier concentration on impurity concentration was fitted to a model which assumes two doubly degenerate states per impurity atom, using the band parameters of the host material calculated according to the six-band model. From this fit the impurity-state energy and its dependence on temperature and x were obtained. The Ga and In impurity states lie deep within the conduction band, while that of Tl lies deep within the valence band, which explains the strong donor behavior of Ga and In and strong acceptor behavior of Tl. The impurity-state energy decreases linearly with increasing x. The temperature dependence of the In state could explain the anomalous temperature dependence of ${R}_{H}$ observed for ${\mathrm{Pb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sn}}_{\mathrm{x}}$Te heavily doped with In.

Spectral investigation of absorbing cholesteric liquid crystals

The investigation is made of the electron absorption spectra of pure and impure crystals of cholesteryl benzoate at 4, 2K and their Bragg reflection spectra as a function of impurity concentration (0,1 to 10%) and mesomorphic range temperature. The discrete structure of the electron absorption spectra is revealed and explained. The regularities of the mesophase order degree variation with temperature are repoted. The anomalous transmission of light by the cholesteryl benzoate mesophase (the Borrman effect analog) is measured as a function of temperature and impurity concentration.

Proximity effect sandwiches containing nonmagnetic localized states

The effects of resonant scattering by nonmagnetic localized states in the normal layer on the critical temperatures of proximity effect sandwiches are studied through the renormalization of the electron propagators in the McMillan proximity effect model. By treating both the resonant scattering of the normal layer electrons and the Coulomb correlation between electrons of opposite spins located on the same impurity site self-consistently, an expression for the decrease in the critical temperature that shows explicitly the dependences on the parameters that describe the impurity atoms is obtained. The decrease in the critical temperatures as a function of impurity concentration and of normal layer thickness is shown.

Two-dimensional density of states for electrons bound to impurities inside inversion layers at the semiconductor-insulator interface

The density of states for electrons bound to Na + impurities inside the inversion layer at the semiconductor-insulator interface of an MOS structure is calculated as a function of impurity concentration. The impurity potential is considered unscreened and the electrical quantum limit is assumed. A simple one-electron Hamiltonian is used and the disorder is treated through a cluster calculation. It is shown that the impurity band has a considerable bandwidth for impurity concentrations in a range of the experimental regime (this result agrees with the experimental findings of Hartstein and Fowler), and that the upper Hubbard band stands well above the lower band at very low concentration (in rough agreement with recent calculation done by Phelps and Bajaj on D − state).

Effect of impurities on the grain growth of chemical vapor deposited polycrystalline silicon films

In this paper the effect of the most commonly used n-type dopants (phosphorus and arsenic) and of some contaminants (oxygen and chlorine) on the polysilicon grain growth during high temperature annealing is presented. The impurities are ion implanted into the polysilicon films after the chemical vapor deposition. The average grain size (as determined by TEM observations or x-ray analysis) of 0.44 um-thick polysilicon layers contamined with As, P or oxygen is studied as a function of impurity concentrations. It is shown that grain growth is reduced by the presence of oxygen or chlorine contaminants: for oxygen this effect appears for concentrations higher than about 10 20 cm −3. As far as dopant impurities, P dopant monotonically enhances grain growth (for concentrations higher than about 3 × 10 20 cm −3), while the grain growth enhancement effect disappears for As-doped films with concentrations higher than the solid solubility limit. In the case of P-doped poly Si films, the influence of the annealing temperature on grain size is also presented and discussed.

Electron and hole carrier mobilities for liquid phase epitaxially grown GaP in the temperature range 200–550 K

Mobility data for GaP in the temperature range 200–550 K as a function of impurity concentration are presented. The data were determined from sheet Hall coefficient and resistivity measurements obtained using the van der Pauw method. The results are given for a wide range of concentrations ranging from unintentionally doped to degenerately doped conditions. The maximum values of the electron and hole mobilities at room temperature were found to be 160 and 135 cm2/Vs, respectively. The observed average temperature dependence of the mobility is T−1.7 for electrons and T−2.3 for holes.

Photoluminescence and impurity concentration in GaxIn1−xAsyP1−y alloys lattice-matched to InP

The photoluminescence spectra of GaInAsP alloys are examined as a function of impurity concentration at several different temperatures and alloy compositions. The photoluminescence linewidth depends on both the impurity concentration and temperature, and our experimental results at 295 °K are described by the empirical relation: ND(ΔE) =9.1×1011 exp(ΔE/4.3 ×10−3) cm−3. It is proposed that photoluminescence linewidth can be used for a simple and rapid determination of the impurity concentration for n- and p-type samples whose total impurity concentration lies in the range 1016 cm−3 &amp;lt;ND +NA &amp;lt;1019 cm−3.

M-center production in europium-doped NaCl and KCl

Abstract The first stage of the M-center coloring has been investigated in europium-doped NaCl and KCl at room temperature, as a function of impurity concentration in the range 30–800 ppm. The results indicate that the M-center production can be described by a statistical process in which the relationship M=K1F2 is obeyed. It was also ascertained that the value for the constant K1 is dependent on the impurity concentration and not very sensitive to the type of the alkali halide matrix.

Ionic thermocurrent studies in lithium ammonium sulfate

Four distinct peaks denoted as $A$, $B$, $C$, and $D$ are found to occur at temperatures -25, 10, 186, and 235\ifmmode^\circ\else\textdegree\fi{}C in the ionic-thermocurrent (ITC) spectrum measured from -196 to 250\ifmmode^\circ\else\textdegree\fi{}C along the ferroelectric $a$ axis in LiN${\mathrm{H}}_{4}$S${\mathrm{O}}_{4}$ single crystal. All the four peaks, with slight changes in temperature for current maxima for peaks $A$ and $D$, appear in the ITC spectra of samples doped with specific amounts of ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Zn}}^{2+}$ as impurities. Detailed investigations of the spectrum as a function of impurity concentration, poling temperature, poling time, and polarizing field have been carried out. The experimental results show that the reorientation of the impurity-vacancy complexes identified as (${\mathrm{Cu}}^{2+}{V}_{p}^{\ensuremath{-}}$) and (${\mathrm{Zn}}^{2+}{V}_{p}^{\ensuremath{-}}$) are responsible for peak $A$ in doped samples. $B$ does not exhibit characteristic features of an ITC peak. Peak $C$ is found to be due to the protons released during the phase transition occurring in the crystal at this temperature (186\ifmmode^\circ\else\textdegree\fi{}C) and hence is an example of "protonic thermocurrent" which is reported here for the first time. Peak $D$ is accounted for as due to the dissociation of the impurity-vacancy ($I\ensuremath{-}V$) complexes. The numerical values of the following parameters have been calculated from the experimental results: (a) the activation energies in the various phases of the crystal, (b) the reorientation energy, (c) the association energy, and (d) the frequency factor for the relaxation time of the $I\ensuremath{-}V$ complexes. There is complete agreement between the present results and the data obtained from earlier electrical conductivity studies.

A calculation of the magnetisation of alloys of Ni with Rh, Ru, Ir and Os

A theoretical discussion of the magnetisation as a function of impurity concentration for the alloys NiRh, NiRu, NiIr and NiOs is presented. The various features of these curves are attributed to the interplay of several influences including the larger band width of the impurities relative to that of Ni, the relatively more repulsive potentials on Ru and Os than Rh and Ir, and the greater spin-orbit coupling parameters of Ir and Os. The agreement between theory and experiment is satisfactory in all cases of reasonable values of the parameters. The resultant value of one of these parameters, the energy differences between the Fermi energy and the top of the majority spin-band in pure Ni is 46 MeV, in excellent agreement with experimental photoemission estimates.

RECIPE—A two-dimensional VLSI process modeling program

REnsselaer Computer integrated Circuits Process Engineering (RECIPE) is a two-dimensional (2-D) integrated circuit process modeling program developed for use in VLSI applications. The program incorporates a 2-D diffusion model which includes the concentration dependence of the diffusion coefficients. An incremental solution method is used to compute the appropriate diffusion coefficients as a function of impurity concentration throughout space. RECIPE also incorporates a 2-D ion-implantation model. While intended as a general-purpose modeling program, RECIPE has been used to study channel-length decrease of short-channel MOSFET's during high-temperature processing. A typical phosphorus-implanted (150 keV, 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) 1-µm gate transistor had no channel after processing for 60 min at 1000°C, while an arsenic-implanted device had an effective channel length of ∼ 0.1 µm after similar processing.

Kondo effect and spin-glass freezing of the magnetic impurities Cr, Mn, and Fe in superconducting palladium hydride

Through low-field ac susceptibility measurements we have determined the depression of the superconducting transition temperature T/sub c/ in palladium hydride (T/sub c/0 = 9.3 K) as a function of impurity concentration x for Cr, Mn, and Fe. For Cr and Fe similar values for the initial T/sub c/ depression were found, i.e., -150 K/at. % Cr and -145 K/at. % Fe. From resistivity experiments we are able to estimate the Kondo temperatures T/sub K/, i.e., T/sub K/approx. =10 K for Cr and T/sub K/approx. =5 K for Fe. Since i.e., T/sub K/approx. =10 K for Cr and T/sub K/approx. =5 K for Fe. Since systems exhibits an enhanced pair breaking as described by the theory of Mueller-Hartmann and Zittartz. In contrast, for Mn the initial T/sub c/ depression is -21 K/at. % and T/sub K/<

Band-gap narrowing in heavily doped many-valley semiconductors

Band-gap shrinkage in impure $n$-Si and Ge as a function of impurity concentration is studied theoretically at $T=0$ K. Above the Mott critical density it is assumed that electrons occupy the host conduction band in the form of an electron gas. Interactions among the free carriers and with the ionized point impurities give rise to a downward shift of the conduction band. Interband matrix elements are explicitly included for the valence bands. As a consequence, the two valence bands at the center of the zone are shifted upwards by the same amount. Correlation and impurity scattering both give corrections to the band gap, which are of the same order of magnitude. In particular, one finds that the band gap depends rather sensitively on the arrangement of donor ions. The choice of dielectric screening is investigated, and comparison with previous calculations is made. A comparison between present theoretical results and experimental estimates of the band-gap narrowing is also indicated. One notes an order-of-magnitude agreement, although there is considerable scatter in the experimental data. Shortcomings of the present theory are briefly discussed.

Critical behavior of the Baxter-Wu model with quenched impurities

We have used an importance-sampling Monte Carlo technique to study the Baxter-Wu model with various fractions of the lattice sites occupied by random, quenched, nonmagnetic, site impurities. We found the system had long-time fluctuations which were caused by the creation and motion of domain boundaries. For this reason our study required the use of a large number of Monte Carlo steps per spin. The data were analyzed using finite-size scaling to extract the infinite-lattice critical exponents and critical amplitudes from the Monte Carlo simulation of finite lattices. This analysis of the data for the pure Baxter-Wu model yielded results which agreed well with exact results and series-expansion predictions. The addition of quenched site impurities caused a dramatic change in the critical behavior. The pure-lattice critical exponents ($\ensuremath{\nu}=\frac{2}{3}$, $\ensuremath{\alpha}=\frac{2}{3}$, ${\ensuremath{\gamma}}_{m}\ensuremath{\simeq}1.17$) change upon the addition of only a few percent impurities to $\ensuremath{\nu}=1.00\ifmmode\pm\else\textpm\fi{}0.07$, $\ensuremath{\alpha}\ensuremath{\lesssim}0.0$, and ${\ensuremath{\gamma}}_{m}=1.95\ifmmode\pm\else\textpm\fi{}0.08$. The phase diagram as a function of impurity concentration and an estimate for the infinite-lattice percolation limit are also given.