Homogeneous Doping Research Articles

Investigation of the electronic structure of polyaniline by electron energy-loss spectroscopy

The electronic structure of polyaniline as a function of protonation has been studied by electron energy-loss spectroscopy. Momentum dependent measurements indicate a homogeneous doping and do not support the granular metal model. The spectra are consistent with a polaron metal. Core-level spectroscopy on carbon and nitrogen indicates changes of the chemical bonding when going from the metallic to the semiconducting state.

Photoelectron spectroscopy study of FeCl 3 doped polyphenylacetylene

The XPS study of undoped polyphenylacetylene (PPA), shows that the feature of the C 1s signal is indicative of localized π electrons. Upon homogeneous doping of PPA with FeCl 3 in THF (tetrahydrofuran), the C 1s signal changes giving the evidence of a charge transfer reaction and of enhancement of the π→π ∗ shake-up satellite.

The influence of the ligands on the catalytic activity of a series of RhI complexes in reactions with phenylacetylene: Synthesis of stereoregular poly(phenyl) acetylene

AbstractThe catalytic activity of a series of [Rh L‐L chel]X complexes, in which we have varied the unsaturated ligand [L‐L = cis, cis‐cycloocta 1,5‐diene(cod) or 2,5‐norbornadiene(nbd) the nitrogen chelating ligand [chel = 2,2′‐bipyridine(bipy), 2,2′‐dipyridylamine(dipyam), 2,2′‐bipyrazine (bipz), 4,4′‐dimethyl‐2,2′‐bipyridine (4,4′‐Me2bipy)] and the counter ion [X = PF6, ClO4, BPh4], has been examined in reactions with phyenylacetylene (PA). The catalytic behaviour of the [Rh(cod)Cl2],tmeda (tmeda = N,N,N′,N′tetramethylethylendiamine), [Rh(cod)Cl2],teda] (teda = triethylendiamine), of the dimer [Rh(cod)Cl]2, and the use of NaOH as cocatalyst in different reaction conditions was also examined. The influence of the ligands on the catalytic activity of these RhI complexes is discussed. 1H and 13C NMR spectra have shown that highly stereoregular polyphenylacetilene can be obtained. Conditions for homogeneous doping of PPA, to obtain materials whose conductivity varies over 10–11 magnitude orders, are proposed. The stability of the doped polymers is also discussed.

Scattering time and single-particle relaxation time in a disordered two-dimensional electron gas.

We calculate the scattering time (which determines the conductivity) and the single-particle relaxation time (which determines the density of states) for a disordered two-dimensional electron gas in lowest order of the electron-impurity interaction. Analytical results and numerical results for remote impurity doping, homogeneous background doping, interface roughness scattering, and alloy disorder scattering in ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ga}}_{\mathrm{x}}$As quantum wells are presented. Self-consistency effects are also included and provide a theoretical frame for estimating the validity of the results calculated in lowest order of the electron-impurity interaction. A logarithmic singularity is found for the single-particle relaxation time in the case of homogeneous background doping.

On the electronic conductivity of composite electrolytes

The general problem of the distribution of minority charge carriers (here electrons) in space charge regions is discussed. It is shown that their profile is, boundary equilibrium supposed, determined by the behavior of the majority charge carriers (here ions). This result also has important implications for semiconductors. The contribution of electrons to the excess conductivity of space charge regions is calculated for one interface as well as for a dispersion. The Wagner-Hebb technique is extended to the problem of a non-uniform defect concentration in the quasi-parallel switching approximation. Wagner-Hebb experiments are performed on AgCl:γ-Al 2 O 3 composites. An increase of both n- and p-type conductivity is found with respect to pure AgCl. Applying the space charge model, the result can be explained by a double effect. Impurities are introduced (homogeneous doping) and enhance the hole concentration in the bulk (detected by the high- T behavior); space charge effects (heterogeneous doping) due to the interaction of γ-Al 2 O 3 surface with the silver cations (indicated by the influence on the ionic conductivity) enhance the n-conductivity in the space charge regions. The homogeneous effect is presumably due to O doping and is consistent with literature results on AgI-, LiI- and CuCl-alumina composites.

Quantum size effect in δ-doped AlGaAs heterostructures

We report a new structure formed by δ doping the barrier of an AlGaAs heterostructure. Our structure contains a pair of electronically coupled quantum systems that develop from the quantum size effect occurring in the barrier and at the heterointerface. Compared to conventional homogeneous doping, we find enhancements in interface density resulting in δ-doped structures for all spacer thicknesses. A mobility of 1.9×106 cm2/V s is observed at a spacer layer thickness of 360 Å. Systematic dependences of interface mobility and density on spacer thickness are deduced from the quantum Hall effect and variable temperature Hall measurements.

Combined homo-hetero doping for enhancement of ionic conductivity

Electrical conductivity of Na 0.005Ca 0.995F 1.995 and Y 0.008Ca 0.992F 2.008 samples containing 2 mol% CeO 2− x as the dispersed phase has been measured in the range 630 to 1030 K using an ac bridge at 1 kHz. The presence of the dispersed phase enhanced the conductivity of CaF 2 homogeneously doped with NaF. However, heterogeneous doping with CeO 2− x decreased the conductivity of YF 3-doped CaF 2. This suggests that preferential adsorption of F − ions to the CeO 2− x interface and the consequent creation of a space charge region with increased fluorine vacancies near the interface is the primary mechanism responsible for the enhancement of the conductivity of CaF 2 heterogeneously doped with CeO 2− x . By the synergetic effect of homogeneous and heterogeneous doping, the conductivity of polycrystalline CaF 2 can be enhanced by 4.5 orders of magnitude at 630 K.

Composite electrolytes

Thermodynamics of boundary regions manifested in the existence of space charge regions adjacent to each interface are able to explain many anomalous conductivity effects occurring in multiphase systems, if surface-surface interactions are introduced. The model of ‘heterogeneous doping’ is reviewed and, in particular, compared with classical homogeneous doping. Different kinds of interfaces (contact ion conductor/insulator; contact of two ion conductors; grain boundary) in macro-sized and micro-sized systems are discussed.

Kinetics of chemical n-doping of polyacetylene

Polyacetylene (PA) films were doped with tetrahydrofuran and benzene solutions of organo potassium and lithium compounds. The kinetics for doping were determined from the variation of the dopant concentration with time, as a function of film thickness, dopant concentration and doping temperature. The experimental doping rates were compared to computed values deduced from a doping model taking into account the interfibrillar diffusion, separately measured with an inert hydrocarbon. This comparison allows a minimum value (10 −15 cm 2/s) to be determined for the intrafibrillar diffusion coefficient D f of doped sites and cations. With such a value no homogeneous doping can be carried out at intermediate doping levels, for films thicker than about 50 microns, by a simple control of doping time or dopant concentration. This may, however, be achieved by a thermodynamic control using dopants of suitable redox potential.

Metal insulator transition due to surface roughness scattering in a quantum well

We calculate the critical width a c for a quantum well structure as function of the two-dimensional electron density. For a > a c the electron gas has a finite dc conductivity at temperature zero, and for a < a c the dc conductivity is zero. Homogeneous background doping, remote doping, and surface roughness scattering are considered. Due to surface roughness we find a strong increase of a c at high electron concentration, in novel contrast to heterostructures. Explicit results are presented for GaAs and InAs quantum wells and compared with experimental results. Experiments are suggested to test the predictions of the localization theory.

Ionic Transport in Heterogeneously and Homogeneously Doped Thallium (I)‐Chloride

AbstractThe effect of alumina admixtures on the ionic conductivity of TlCl is studied. In agreement with the model of “Heterogeneous Doping” a change of the intrinsic anionic conduction (Cl ‐vacancies) into a Tl+‐conduction (Tl+‐vacancies) is induced at lower temperatures (≦ 300 K). The experimental conductivity data as a function of temperature, concentration and grain‐size can be quantitatively described with an interface defect concentration considerably lower than for AgCl and AgBr. Likewise, for pure polycrystalline TlCl space charge regions enriched in cation vacancies around grain boundaries are assumed to provide an increase of the conductivity. Similarities and differences between heterogeneous and the classical homogeneous doping are clearly reflected by experiments using PbCl2 as a dopant for both TlCl and TlCl: Al2O3

Homogeneous doping of silica by uranyl ions using a chemical mixing procedure

Spectral data showed that in silica doped by uranyl ions using a chemical mixing procedure the uranyl ions are homogeneously dispersed in a solution-like environment.

Submicron Highly Doped Silicon Epitaxial Layers Grown by Lpvpe

AbstractBy low pressure vapour phase epitaxy (LPVPE) epitaxial growth down to 760ºC has been achieved in the SiC12H2/H2 system, as revealed by RBS and channeling measurements ( min ∼4.0%). High and relative homogeneous doping was obtained with boron resp. phosphorus in the range 1x1018 -5x1019 cm-3 for T: 800º - 900ºC. Sharp transitions in the boron doping profile to the substrate, of the order of 15mn, result only for temperatures much lower than 850ºC. Schottky barrier enhanced diodes with promising performances were formed with Ti evaporated on p+n layers grown selectively on n+ substrates.

Hydrogenic impurities in superlattices with parabolic quantum well potentials

Parabolic quantum-wells (PQWs) in compositional GaAs/Al x Ga 1−x As and homogeneous doping GaAs superlattices are being built using the technique of molecular beam epitaxy. The binding energy (Eb) and wave function of a hydrogenic impurity in an effective PQW are calculated variationally. Identifications of the parabolic coefficient K allows us to predict physical ranges of Eb. For typical compositional-PQWs, Eb depends logarithmically on the fraction of the conduction band discontinuity, Qe, hence appropriate experiments may be designed to confirm one of its two values (0.85 and 0.57) proposed elsewhere. Comparison with works of other authors indicates that 1) compositional-PQWs of typical widths (≳300Å) yield larger values of Eb compared to those of finite or even infinite square wells of same width, and 2) doping-superlattices with linear effective potentials produce physical ranges for |Eb| considerably larger than those obtained by doping-PQWs.

Homogeneous doping and semiconductor-to-"metal" transition in polyacetylene

The conductivity of iodine-doped cis- and trans-polyacetylenes was measured as a function of dopant concentration per C-H unit, $y$, from parts-per-million to the metallic regime. The sharp semiconductor-to-"metal" ($S\ensuremath{-}M$) transitions span over only 2 orders of magnitude of $y$ and the $log\ensuremath{\sigma}\ensuremath{-}\mathrm{vs}\ensuremath{-}logy$ relationships for the two isomers are identical over a large portion of the transitional region. The change in thermoelectric power $S$ occurs even more abruptly over only twofold increase in dopant concentration. The midpoints of the transition for $\ensuremath{\sigma}$ vs $y$ and $S$ vs $y$ are nearly the same at $y\ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. The number of Curie spins decreases rapidly during the transition; the EPR linewidth and line shape of cis-polyacetylene were not affected by doping. These behaviors are suggested as indicative of uniform doping. Polyacetylenes, both cis and trans, doped with As${\mathrm{F}}_{5}$ by a "slow" method yielded inhomogeneous materials devoid of a distinct $S\ensuremath{-}M$ transition. Using an improved "cyclic" doping procedure, we obtained homogeneously doped trans-${[\mathrm{CH}{(\mathrm{As}{\mathrm{F}}_{5})}_{y}]}_{x}$ which exhibits $\ensuremath{\sigma}\ensuremath{-}\mathrm{vs}\ensuremath{-}y$ and $\ensuremath{\sigma}\ensuremath{-}\mathrm{vs}\ensuremath{-}S$ dependence nearly the same as iodine-doped polymers. The results are interpreted in terms of a phase transition. In lightly doped materials the charged soliton and polaron defects are pinned by the potential of one dopant ion, and these materials are said to be in a glassy state. Conversion to a liquid state takes place when the average separation of uniformly distributed dopant ions is less than the defect domain widths so that a defect interacts with two or more dopant ions. Conduction is probably via electron hopping between the charged soliton and polaron states in the transitional regime. This solition melting transition occurs at about 100-fold lower in dopant concentration than that for the onset of Pauli susceptibility. We were unable to achieve uniform doping of cis-polyacetylene with As${\mathrm{F}}_{5}$.

On Homogeneous Phosphorus Doping of Silicon Using Ionising Radiation

The doping inhomogeneities in phosphorus doped (n-type) silicon crystals, prepared by the conventional zone- melting techniques, can be reduced drastically by accomplishing doping through nuclear transmutation of silicon. Details of the method are given for obtaining homogeneous and controlled phosphorus concentrations in Si crystals used especially for manufacturing high-power devices like thyristors and rectifiers. Factors influencing the final resistivity distribution in irradiated crystals have been discussed.

The Influence of the Growth Process on Doping Profile and Mobility Profile during LPE of the Pseudobinary Sn ‐ GaAs System

With heavily Sn‐doped layers grown from liquid phase epitaxy (LPE) it is demonstrated that the profiles of the carrier concentration and the total concentration reflect the respective growth processes. This is derived from the temporally varying growth rate and from an experimentally verified relation between the average growth rate and the average doping concentration. The mobility profile is correspondingly influenced. Initial supersaturation of the melt and a fast cooling rate of the process are best for obtaining a homogeneous doping profile.

“Head-to-Head” and “Tail-to-Tail” Domains in barium titanate crystals

The influence of the nature of doping distribution, its average concentration and crystal thickness on the distribution of spontaneous induction, the distortion of the energy bonds and the behaviour of free energy of barium titanate crystals have been considered.It has been shown that during homogeneous parabolic and Gaussian doping distributions through the crystal thickness, there exist stability intervals of a monodomain state and “head-to-head and tail-to-tail” domains. The nature of the doping distribution also influences the width and energy of the transient regions.

Application of thermal neutron irradiation for large scale production of homogeneous phosphorus doping of floatzone silicon

The limitations of conventional melt doping of phosphorus in silicon are discussed in relation to the obtainable homogeneity. Due to "theoretical-design" possibilities and increased manufacturing yield for power components based on n-type silicon, the method of thermal neutron irradiation doping has been developed for large scale production of floatzone silicon of homogeneous resistivity. It is shown that radioactivity problems do not interfere for resistivities above approximately 5Ω- cm and that lattice radiation defects can be annealed out to the extent that they appear harmless for all major applications. The doping homogeneity is discussed in view of the influencing nuclear reactor characteristics and the choice of starting material. Doping variations less than 1 percent across slices of up to 80-mm diameter are demonstrated to be obtainable with production results typically 3-10 percent. The minority carrier lifetime of neutron-doped silicon is shown to lie in the range of 100-1000 µs.

High-voltage thyristors and diodes made of neutron-irradiated silicon

Neutron irradiation is a way to produce homogeneous and well-defined phosphorus doping in large diameter silicon crystals. One major application of such silicon is in the field of high-voltage power devices. The relation between resistivity and breakdown voltage determined for neutron-irradiated silicon is in good agreement with the theory of Sze and Gibbons. It is shown that by changing from conventionally doped silicon to neutron-irradiated silicon noticeable increases in the blocking capability of thyristors can be achieved without an increase of the n-base width. Production of power thyristors with blocking voltages of 3 to 5 kV has been successful, indicating that applications of neutron-irradiated silicon have already left the laboratory stage.