Dopant Combinations Research Articles

Energy transfer and triplet exciton confinement in polymeric electrophosphorescent devices

AbstractEnergy transfer and triplet exciton confinement in polymer/phosphorescent dopant systems have been investigated. Various combinations of host‐guest systems have been studied, consisting of two host polymers, poly(vinylcarbazole) (PVK) and poly[9,9‐bis(octyl)‐fluorene‐2,7‐diyl] (PF), blended with five different phosphorescent iridium complexes with different triplet energy levels. These combinations of hosts and dopants provide an ideal situation for studying the movement of triplet excitons between the host polymers and dopants. The excitons either can be confined at the dopant sites or can flow to the host polymers, subject to the relative position of the triplet energy levels of the material. For PF, because of its low triplet energy level, the exciton can flow back from the dopants to PF when the dopant has a higher triplet energy and subsequently quench the device efficiency. In contrast, efficient electrophosphorescence has been observed in doped PVK films because of the high triplet energy level of PVK. Better energy transfer from PVK to the dopants, as well as triplet exciton confinement on the dopants, leads to higher device performance than found in PF devices. Efficiencies as high as 16, 8.0, and 2.6 cd/A for green, yellow, and red emissions, respectively, can be achieved when PVK is selected as the host polymer. The results in this study show that the energy transfer and triplet exciton confinement have a pronounced influence on the device performance. In addition, this study also provides material design and selection rules for the efficient phosphorescent polymer light‐emitting diodes. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2681–2690, 2003

On the Effects of Simultaneous Substitution of Al3 + and B 3 + in LiNiO2 Cathodes

A series of solid solutions of the formula with has been synthesized by adopting urea-nitrate solution combustion route. Two diamagnetic cations from the same non-transition metal group viz., and have been substituted simultaneously in to the crystal lattice of with a view to realize the combined advantages such as over-charge protection and longer cycle life respectively. The synthesized cathode materials were characterized in the light of diffraction (XRD), spectroscopic (Fourier transform infrared, FTIR) and electrochemical (CV and charge/discharge) analyses. It is manifested from XRD that the observed correlation between the amount of boron substituted and the requirement of heat-treating conditions such as heating time and temperature implied that the dopant plays a dual role, both as a dopant and as a flux. The dwelling time and the treating temperature required for the formation of phase pure compounds, either with a comparatively higher amount of boron with respect to Al or with the presence of maximum amount of B to the extent of 30% without Al were found to decrease as a function of boron content. The results of the XRD and FTIR studies established the complete mixing of Al and B with Ni, especially at the various levels and combinations of dopants attempted in the present study. The stabilization of Ni in the state and the existence of enhanced two-dimensional structure without any cation mixing have been achieved through this novel attempt of simultaneous substitution of diamagnetic cations viz., and in the layered matrix, as evidenced from the CV studies. The effect and the extent of substitution of Ni with Al and B on the structural and electrochemical performance of are discussed elaborately in this communication. © 2003 The Electrochemical Society. All rights reserved.

Electrical Conductivity in Rare-Earth Cation Co-Doped Ceria Solid-Solution Systems.

It is known that Sm-doped ceria shows the highest electrical conductivity among the ceria solid-solution systems with various rare-earth cation dopants. In order to clarify the ionic size effect on the oxide-ion conductivity, in the rare-earth co-doped ceria system, Ce0.8(Ln″1-xLn′x)0.2O1.8 (Ln′=Y, Gd and Ln″=Nd, La), the compositional fraction (x) of the co-doped cations was determined so that the average ionic size agreed with that of Sm3+. The lattice constants of the four ceria-based compounds, which were prepared with various combinations of rare-earth dopants, agreed well with the lattice constant of the Sm-doped sample. However, the electrical conductivity decreased abruptly with increasing difference of the two cationic radii. The electrical conductivity of these co-doped materials was discussed on the basis of the local ionic arrangement in the fluorite-type structure.

Dielectric spectroscopy of conductive polyaniline salt films

AbstractDielectric responses of polyaniline (PANI) salt films, made from two different methods, were compared to investigate the effect of a dopant and film‐formation methods on the dielectric properties. One of the film‐formation methods for emeraldine salt was the film‐doping method, and the other, the solution‐doping method. The conductivity relaxation and dielectric properties were measured by a microdielectrometer at 293 K in the frequency range of 1–100,000 Hz. Dielectric spectra were a function of the dopant in the case of the film‐doping method, whereas it was a function of solvent–dopant combinations as well as the dopants in the solution‐doping method. The dielectric responses of film‐doped samples had similar patterns irrespective of the dopants, while those of solution‐doped samples were varied, probably due to different conjugation conditions or/and the conduction mechanism. In the case of film‐doped samples, dielectric loss and permittivity values were increased with decreasing dopant sizes. The dielectric relaxation times were also varied depending on the kind of dopants. The different solvent–dopant combinations led to variation of the dielectric responses in the solution‐doped PANI due to an altered chain structure. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2760–2769, 2001

Heteropolyacid/ m-cresol system as a primary/secondary dopant combination for polyaniline

The method for the preparation of soluble polyaniline PAn doped with heteropolyacid (HPA), using 12-phosphomolybdic acid (PMA) as an example, was proposed. It is established that new spectral and electrophysical properties of the PAn·PMA are the consequence of secondary doping of PAn with m-cresol. The solubility of the PAn·PMA enables the preparation of conducting composite films with dielectric polymers that allows the variation in conductivity value and the improvement of the mechanical properties of the conducting material.

The effect of co‐dopants on the processability of intrinsically conducting polymers

AbstractUsing a combination of functionalized dopants was found to be a simple method to improve the processability of intrinsically conducting polymers while retaining relatively high levels of conductivity. Two intrinsically conducting polymers, poly(3‐octylthiophene) and polyaniline were co‐doped with various combinations of dopants. In both systems, solubility was improved and coherent films were formed directly from common organic solvents without the need for a post‐processing doping step. Co‐doped intrinsically conducting polymer films exhibited conductivities up to 10−2 S/cm. Additionally, certain polyaniline complexes were capable of being melt processed without loss of conductivity.

Influence of dissolved gas and temperature on the light yield of new liquid scintillators

Sixteen new liquid scintillators, emitting green light, were studied. They are based on four solvents combined with four dopants. The influence of different gas atmospheres was studied. In particular, it was shown that by keeping these liquid scintillators in vacuum or in a neutral gas, the light yield increases up to 32% at 20°C and for the best solvent–dopant combinations. The dependence of the light yield on temperature was also studied for these scintillators. In the 20–60°C interval, some exhibit a light yield variation of ∼ 3% which is smaller than that of the NE 102A plastic scintillator.

Influence of Zn/In codoping on the optical properties of lithium niobate

The influence of a combined doping with Zn and In on the optical properties of lithium niobate is studied. While each of these dopants is known to improve the material’s nonlinear optical properties, it is an open question whether further improvements, especially of the nonlinear optical features, can be achieved by a suitable combination of dopants. Our measurements show that lithium niobate imposes certain limits which cannot be overcome. This can be described by a simple model which is based on the defect structure of lithium niobate.

Microstructure and flux pinning properties of melt textured grown doped YBa 2Cu 3O 7−δ

The physical properties and the microstructure of highly textured YBa 2Cu 3O 7/Y 2BaCuO 5 ceramics, doped with platinum and/or cerium have been investigated by high resolution electron microscopy (HREM), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The Ce, Pt and Ce + Pt doped samples show different J c behaviours as function of the applied field and a systematic comparison of the microstructure at different scales has been performed. The combination of both dopants leads to a drastic improvement of the critical current density either under zero field or with an external magnetic field (fishtail effect). A critical current density up to 56 000 A/cm 2 has been observed, which remains 43 000 A/cm 2 under 1 T in a sample doped with 2 wt% cerium oxide and 0.5 wt% platinum oxide. We suggest that a slight introduction of Ce in the YBa 2Cu 3O 7−δ matrix locally changes the superconducting properties, and enhances the flux pinning properties. Micron size BaCeO 3 particles, which appear with a preferential orientation in the YBa 2Cu 3O 7−δ matrix, are also potential candidates for strong pinning. Other secondary phases, formed during the growth and the presence of a nanocrystalline phase in the plate like gap of the Ce + Pt doped samples are other admixtures. Finally a high density of twins is observed in all samples with some differences between the cerium and the platinum doped samples. They most probably contribute to pinning under intermediate field.

Effect of dopants on the magnetic properties of MnZn ferrites for high frequency power supplies

The effect of additions on the magnetic properties of MnZn ferrites for high frequency power supplies was studied. The investigation revealed that various combinations of dopants added to the ferrite modify the grain boundary resistivity and the magnetic permeability spectra at constant microstructure. Particularly, the combined effect of Sn/sup 4+/, Ti/sup 4+/ and Ta/sup 5+/ was found to increase the grain boundary resistivity in the ferrites studied and remarkably decrease the power loss at higher frequencies.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Residual defect control when doping thin layers in diamond

Doping of a diamond layer by boron ion implantation has been conclusively demonstrated and reported in the literature. To reach this objective, implantation is done at low target temperatures (typically liquid nitrogen) followed by rapid annealing; the so-called CIRA method. The object of the exercise is to activate the implanted atoms by dopant atom-vacancy combinations, and to decrease the residual damage by means of self-interstitial-vacancy recombinations. This paper is part of an ongoing study on the effects that the implantation-annealing parameters have on the doping process. Is is known that in order to improve the activation ratio, the initial implanted radiation damage has to be increased. However, this causes a higher residual damage after annealing. It is therefore proposed and theoretical arguments as well as preliminary experimental data are advanced to show that another route to control the CIRA doping process may be achieved by multiple, low-dose, implantation-annealing sequences.

Understanding doped V2O3 as a functional positive temperature coefficient material

Doped V2O3 is examined to understand its useful properties as a positive temperature coefficient (PTC) material. The effects of different dopant species and combinations of dopant species and the effects of microstructure are examined in terms of the properties of transition temperature, transition width, resistivity ratio, and transition hysteresis size. In addition to the previously used dopants of Cr and Al, the dopants In, Al+Cr, Al+In, and Al+Ga are also found to be effective at producing a PTC phase transition. The combination of Al+In produced the best resistivity ratio for its transition temperature. Higher sintering temperatures are found to produce larger grains and a sharper transition. The sharper transition is interpreted as the combined effect of heterogeneous nucleation and self-catalysation within a single grain. The hysteresis decreases from over 100 K between the first cooling and the following heating to around 50 K for subsequent thermal cycles. This is most likely a result of the formation of microcracks which are observed in the ceramic. An additional five cycles, though, shows no measurable decrease in the hysteresis size indicating no further growth of the microcracks.

Synthesis of Chemically Stabilized Cristobalite

The synthesis of chemically stabilized cristobalite (CSC) by a wet‐chemical route is described. CSC is a form of silica (containing small amounts of other oxides) which is similar in many ways to β‐cristobalite, but does not invert to α‐cristobalite upon cooling. The effects of changing the dopant levels and various synthetic parameters on the phases formed in the CaO‐Al2O3‐SiO2 system have been investigated. Stabilization of cristobalite with a number of different dopant combinations has been attempted, with mixed results. The use of wet‐chemical techniques has led to the synthesis of apparently phase‐pure CSC in a composition region where traditional solid‐state synthetic techniques yield a mixture of other phases.

Mechanisms of Photochromic and Photorefractive Effects in Doubly Doped Lithium Niobate Crystals

The influence of various dopants and their combinations on the photorefractive effect in lithium niobate crystals is investigated and the principal opportunity for controlling the photorefractive parameters by use of combinations of dopants is shown. It is found, that some combinations of impurities cause a photoinduced inverse change of the absorption coefficient (photochromic effect) in lithium niobate crystals. A model of the photochromic effect, based on charge transfer between impurity centres, is suggested. [Russian Text Ignored]

Interstitial oxygen determination near epitaxial silicon and Czochralski silicon interface

The oxygen content near epitaxial layer-substrate silicon interface was investigated using a micro Fourier transform infrared technique. Systematic measurements, performed in a transversal wafer cross-section configuration, clearly indicated the presence of interstitial oxygen in the epitaxial layer when it was grown on n-type substrates, while no evidence of oxygen in it was obtained (in concentration detectable by the infrared technique) when grown on p-type substrates. Interstitial oxygen profiles, near the epitaxial layer-substrate interface, obtained by analyzing the optical data, are reported and discussed considering different combinations of dopants in the substrate and in the epilayer.

LPE Growth of Doped Sige Layers Using Multicomponent Phase Diagrams Calculations

ABSTRACTHeavy doping of n-type Si-Ge alloys is necessary for improving their high temperature thermoelectric properties. Because of the limited solid solubility of the best dopant available, phosphorus, simultaneous additions of gallium were started several years ago. The substantially higher carrier concentration values obtained in such super-saturated, hot-pressed SiGe/GaP materials point to significant changes in dopant solid solubilities. To better understand the behavior of these alloys, investigation of homogeneous single crystalline materials were needed. As a near-thermodynamic equilibrium technique with processing temperatures well below the melting point of these materials, liquid-phase epitaxy (LPE) was particularly suited to the study of the mechanisms of multidoping. Based on successful crystal growth of Si1-xGex thin films using metals such as Ga, In, Sn and Bi for solvents, several experiments were designed to grow multi-doped SiGe layers with III-V dopant combinations. Knowledge of ternary and quaternary phase diagrams is essential to develop the LPE process. Ternary Si-Ge-M systems computations in good agreement with experimental determinations were used to calculate some of the necessary multicomponent phase diagrams and assess the strength of the various III–V interactions.

Doped calcium sulphide as a thermoluminescence dosimetry material

Methods for the production of doped CaS phosphors and the analysis of thermoluminescence following excitation by UV, X- and gamma rays are presented. The thermoluminescence emission intensity affords a method of determining the dose of excitation radiation received; a number of dopant combinations were examined in an attempt to produce more efficient materials for this purpose and the results are given. One material, CaS:Cd, Na, was found to match closely the stated ideal criteria and in particular to exhibit a linear emission vs dose characteristic over a wide range of UV and gamma doses; in addition, with gamma excitation a single emission peak was found, though a more detailed analysis indicates a double trap structure.

Process and film characterization of low pressure tetraethylorthosilicate–borophosphosilicate glass

The pyrolytic decomposition of tetraethylorthosilicate (TEOS) has been used to deposit highly doped borophosphosilicate glass (BPSG) films. The experiments were carried out in a modified low pressure chemical vapor deposition furnace in the 620–680 °C range at a deposition pressure of 0.5 Torr. Trimethylborate and phosphine together with oxygen were found to be the best dopant source combination. The deposited layers adhere strongly to the quartzware and low particle counts of typically less than 0.2 per cm2 were measured. The thickness uniformity was below ±6%, the dopant uniformity was ±0.1 wt. % over a wafer. If compared to low pressure SiH4–BPSG films, the TEOS–BPSG shows superior step coverage and much higher chemical stability up to dopant levels of about 5 wt. % B, 5 wt. % P. The etch rate behavior in buffered HF corresponds to the trends reported for SiH4–BPSG. The electrical characterization of thin TEOS–BPSG and TEOS–SiO2 layers yielded an electrical breakdown strength in the 7–8 MV/cm range. The glass flow properties (wet and dry ambient) were investigated and found to be very good in the 800–850 °C region. By using a rapid optical anneal system, the required step tapering could be achieved at time/temperature combinations of 40 s/900 °C or 10 s/1000 °C. Detailed infrared studies showed strong interaction among the different oxide absorption bands of the TEOS–BPSG layers.

Modeling of dopant diffusion during rapid thermal annealing

Observations of enhanced dopant diffusion during rapid thermal annealing (RTA) have been made and correlated with the existence of the following phenomena: (1) amorphization of the ion implanted layer; (2) damage-induced dislocation formation; (3) damage annealing; (4) self-interstitial trapping; or (5) dopant solubility enhancement. Thus, modeling the diffusion of As, B, P, and Sb during RTA presents a unique challenge, since point-defect generation, damage annealing, and solid solubility levels are occurring or changing during the short diffusion intervals. The base line modeling environment for studying transient diffusion must include normal, thermally assisted diffusion processes such as electric field effects, concentration-dependent diffusion, clustering or precipitation, dopant misfit strain, etc. In addition, transient phenomena must be introduced. We have handled transient modeling by overlaying the time-dependent effects on steady-state thermally assisted diffusion calculations. Temperature and point-defect transients are calculated by performing calculations over small time intervals in which average parameter values are determined. Calculations and measurements have been performed for As, P, B, and BF2 implants into crystalline and preamorphized Si. Atomic level models are proposed and activation energies have been determined for each substrate–dopant combination.