Increasing Donor Concentration Research Articles

Effect of concentration and degree of saturation of topical fluocinonide formulations on in vitro membrane transport and in vivo availability on human skin.

The thermodynamic activity of drugs in topical vehicles is considered to significantly influence topical delivery. In vitro diffusion across a synthetic membrane was shown to be correlated to the degree of saturation of the drug in the applied vehicle and therefore offers a potential for increased topical drug delivery. Fluocinonide a topical corticosteroid, was chosen as a model compound to investigate in vitro and in vivo availability from formulations with different degrees of saturation. Sub-, as well as, supersaturated drug solutions were prepared using PVP as an antinucleant agent. In vitro membrane diffusion experiments across silicone membrane and in vivo pharmacodynamic activity assessments, using the human skin blanching assay, were carried out. Over the concentration range studied, the in vitro membrane transport of fluocinonide was proportional to the degree of saturation of the respective formulations. The in vivo pharmacodynamic response in the human skin blanching assay was related to the concentration of the drug in the vehicle irrespective of the degree of saturation. From the membrane permeation experiment it can be concluded, that the drug flux might be increased supra-proportionally with increasing donor concentration, drug (super-)saturation (proportional), beyond what would be anticipated based on ideal donor concentration and partition coefficient considerations only. These findings could not be confirmed in the in vivo investigation, probably due to additional vehicle effects (e.g., enhancement, irritation, drug binding) which have to be expected and could have altered the integrity of the stratum corneum and therewith topical bioavailability of the drug.

Slow photoconductivity decay in 3C-SiC on Si substrates

N-type 3C-SiC layers grown on p-type (001) Si substrates were characterized by the conventional photoconductivity decay method. A N2 laser (337 nm wavelength) was used as the excitation source. A very slow component with a time constant larger than 1 ms was observed in the photoconductivity decay curves. A numerical simulation considering a trap with a very small capture cross section for electrons (<1×10−21 cm2) was able to reproduce main qualitative features of the experimental results. From comparison of the experimental decay curves with the theoretical ones, the following conclusions were drawn about the trap in 3C-SiC. (1) The trap level Et is close to the conduction band edge Ec(Ec−Et=0.1–0.15 eV). (2) The concentration is considered to decrease with increasing donor concentration.

Transparent ZnO:Al films prepared by co-sputtering of ZnO:Al with either a Zn or an Al target

A ZnO:Al target was sputtered concurrently with additional Zn or Al, by co-sputtering it with either a Zn or an Al target. The influence of the deposition of these additional atoms on the ZnO:Al film properties was investigated. When co-sputtering the Zn target, the film resistivity shows a minimum for an adequate supply of Zn. With this supply of Zn, both the carrier concentration and the Hall mobility show maxima, and the optical transmittance and absorption edge are improved. When co-sputtering with the Al target, the carrier concentration alone is increased. These results indicate that there is a considerable Zn deficiency in the ZnO:Al films prepared in pure Ar gas, and additional Zn or Al atoms during film deposition can compensate for these Zn defects, in addition to the increased donor concentration in the film.

Background Si-doping effects on Zn diffusion-induced disordering in GaAs/AlGaAs multiple-quantum-well structures

Identical GaAs/Al0.2Ga0.8As multiple-quantum-well (MQW) structures uniformly doped with Si at various concentrations ranging from 1×1017 to 1×1019 cm−3 are grown by molecular-beam epitaxy to study the effects of the background Si-doping level on the Zn diffusion-induced disordering process. After Zn diffusions at 575 °C for 4 and 16 h, the structures are investigated by secondary-ion-mass spectrometry, and by transmission electron microscopy on cleaved wedges of the sample. The results show that the totally and partially disordered regions are always behind the Zn diffusion front. A dependence of the effective Zn diffusivity and of the disordering rate of the structures on the background Si-doping level is observed. The effective Zn diffusivity and the disordering rate are significantly reduced with increasing background Si concentration. Before Zn diffusion, photoluminescence spectra of the Si-doped MQW structures exhibit an increase in intensity of the Si donor–column-III vacancy complex emission band with increasing Si-doping level. This indicates that the concentration of column-III vacancies in the MQW structures increases as the background Si concentration increases. After Zn diffusion, an important decrease in intensity of the column-III vacancy related emission band is observed on the photoluminescence spectra taken in the Zn-diffused regions. The systematical analysis of the photoluminescence spectra of the Zn-diffused MQW structures as a function of diffusion time and as a function of etching depth below the sample surface makes it possible to describe the physical processes occurring during Zn diffusion. A model based on the ‘‘kick-out’’ mechanism of Zn diffusion is proposed to explain the effect of the background Si-doping level on the effective Zn diffusivity. The model shows that the effective Zn diffusivity is controlled by the concentration of column-III interstitials behind the Zn diffusion front and by the donor concentration in the sample. During the incorporation of Zn into the crystal lattice, column-III interstitials are generated. The supersaturation of these interstitials behind the Zn diffusion front is responsible for the enhancement of Al–Ga interdiffusion. Since column-III interstitials and column-III vacancies can mutually annihilate, the concentration of column-III interstitial and column-III vacancy in the Zn-diffused region is reduced with increasing Si-doping level, leading to a retardation of Zn diffusion into the MQW structure. On the other hand, a decrease of the effective Zn diffusivity caused by an increase in donor concentration in the samples is also demonstrated. Our results give evidence for the Fermi-level effect and the interactions between different point defects during Zn diffusion-induced disordering of GaAs/AlGaAs multilayered structures.

The positive temperature coefficient of resistivity in barium titanate

Positive temperature coefficient of resistivity (PTCR) materials have become very important components, and among these materials barium titanate compounds make up the most important group. When properly processed these compounds show a high PTCR at the Curie temperature (the transition temperature from the ferroelectric tetragonal phase to the paraelectric cube phase). In the first half of this paper literature related to the resistivity-temperature behaviour is discussed. As explained by the well established Heywang model, the PTCR effect is caused by trapped electrons at the grain boundaries. From reviewing experimental results in the literature it is clear that the PTCR effect can not be explained by assuming only one kind of electron trap. It is concluded that as well as barium vacancies, adsorbed oxygen as 3d-elements can act as electron traps. In the second half of this paper, the influence of the processing parameters on the PTCR related properties is discussed. Special emphasis is placed on the phenomenon that the conductivity and grain size decrease abruptly with increasing donor concentration above ∼ 0.3 at%. Several models explaining this phenomenon are discussed and apparent discrepancies in experimental data are explained.

A model for the effects of primary substrates on the kinetics of reductive dehalogenation

A kinetic model that describes substrate interactions during reductive dehalogenation reactions is developed. This model describes how the concentrations of primary electron-donor and -acceptor substrates affect the rates of reductive dehalogenation reactions. A basic model, which considers only exogenous electron-donor and -acceptor substrates, illustrates the fundamental interactions that affect reductive dehalogenation reaction kinetics. Because this basic model cannot accurately describe important phenomena, such as reductive dehalogenation that occurs in the absence of exogenous electron donors, it is expanded to include an endogenous electron donor and additional electron acceptor reactions. This general model more accurately reflects the behavior that has been observed for reductive dehalogenation reactions. Under most conditions, primary electron-donor substrates stimulate the reductive dehalogenation rate, while primary electron acceptors reduce the reaction rate. The effects of primary substrates are incorporated into the kinetic parameters for a Monod-like rate expression. The apparent maximum rate of reductive dehalogenation (qm, ap) and the apparent half-saturation concentration (Kap) increase as the electron donor concentration increases. The electron-acceptor concentration does not affect qm, ap, but Kap is directly proportional to its concentration.

Shallow Donor Clusters in InP

Electronic states of a shallow donor in InP are investigated as functions of donor concentration (N D–N A) from 4.0×1015 to 5.5×1016/cm3 by electron spin resonance (ESR) at low temperatures. The Overhauser shift, nuclear spin-lattice relaxation time T 1N, and g-value are found to strongly depend on N D–N A; i.e., T 1N has a maximum at around 7×1015/cm3, while the g-value increases with increasing donor concentration, ranging from 1.2024 to 1.2143. Sweeping the magnetic field at appropriate speeds yields significantly inhomogeneous broadening of the ESR line, indicating that the ESR comes from bound donor electrons at least below N D–N A of 1.5×1016/cm3. A model of donor clusters is proposed to interpret the obtained results. The donor-concentration dependence of the g-values indicates that the donor states are significantly changed with the average donor-cluster size.

Concentration-dependent photoinduced electron-transfer reactions: 1:1 and 1:2 radical-ion complexes

For some exciplexes of 9, 10-dicyanoanthracene (DCA) with naphthalenes and phenanthrenes as donors, the fluorescence quantum yields and lifetimes decrease with increasing donor concentration. The rate constants for these self-quenching reactions increase with decreasing redox energy of the pair and with increasing solvent polarity. Both parameters increase the charge-transfer (CT) character of the exciplex. When the CT character is low, the quenching rate constants are too small to measure (<10 7 M -1 s -1 ), but with increasing CT character, to the limit that the exciplex becomes essentially a contact radical-ion pair, the interception rate constant approaches the diffusion-controlled limit. The product of the self-quenching reactions is a 1:2 radical-ion complex

The effects of dopant concentration on the properties of single-heterostructure AlGaAs LEDs with diffused and grown junctions

The emission spectra, frequency characteristics and output power of single-heterostructure AlGaAs LEDs are studied experimentally and theoretically. The peak wavelength of spectra increases with increasing donor concentration, which is explained as being due to gradual changes of emission regions from the n to p region, based on the calculations of emission spectra and injection ratios of carriers. The cut-off frequency increases with increasing donor concentration but shows little change with acceptor concentration. To understand the phenomena, frequency response is derived taking injection of both types of carriers into account. The calculated cut-off frequency agrees with that of experiments on both donor and acceptor dependences. Considerable change in the ratio of emission from the two regions is shown to be the origin of the phenomena. The output power decreases with the increase of donor concentration and shows a mild trade-off against the cut-off frequency. These characteristics are explained provided that quantum efficiencies in the Te-doped n region decrease with increasing donor concentration, though ratios of the emission from the n region decrease concurrently.

Characterization of in Vitro Transdermal Iontophoretic Delivery of Insulin

In-vitro studies were conducted to characterize the transdermal iontophoretic delivery of insulin, thus avoiding potential complications from various biological variations, which may be encountered during in-vivo studies. The proteolytic degradation of insulin in skin homogenates and degradation under the experimental conditions used was investigated. Appropriate adjuvants were incorporated to minimize the potential degradation problems of insulin. 125I-Insulin was observed to penetrate into and accumulate in the skin during the iontophoresis period. It was then released gradually from this depot, as a mixture of intact and 125-I labelled fragments, into the receptor medium. Drug desorption studies supported the theory of skin depot or reservoir formation. It was found that an electric field could be used to facilitate the desorption of drug from the depot. The post-application flux of insulin (or its fragments) from the skin depot formed during iontophoresis was monitored to study the factors affecting the iontophoretic delivery of insulin. Stripping and delipidization of the skin were noted to increase the skin permeation rate of insulin. The cumulative radioactivity permeated and accumulated in the skin was higher at pH 3.6 than at pH 7.4. The iontophoresis-facilitated transdermal delivery was observed to increase with increasing duration of current application and increasing donor concentration of insulin. Modulation of drug delivery by multiple applications was also found to be feasible.

Photoelectrochemical effects of BaTiO3 thin film electrodes prepared by the sol-gel method

The photoelectrochemical properties of La3+-doped BaTiO3 thin film electrodes prepared by the sol-gel method have been investigated as a function of the amount of La3+ content, film thickness, heating atmosphere (air/H2), and optical absorption. In the air-heated electrodes doped with 0.1, 0.2, and 0.3 mol % La3+, the maximum quantum efficiencies have been obtained when the film thicknesses were 10 000, 8000, and 3000 Å, respectively. When the film thicknesses are over certain absorption depths, whose calculated values are over 10 000, 8000, and 6500 Å, respectively, their quantum efficiencies decrease. In the hydrogen-heated electrodes, the depletion layer decreases as the donor concentration increases, and the optical absorption edge moves to longer wavelength by 0.07 eV. Also, the quantum efficiency decreases as the film thickness increases due to the bulk recombination.

Excitonic optical bistability in n-type-doped semiconductors

A resonant monochromatic pump laser generates coherent excitons in an n-type-doped semiconductor. Both exciton–exciton and exciton–donor interactions come into play. The former interaction can give rise to the appearance of optical bistability, which is heavily influenced by the latter one. When optical bistability occurs at a fixed laser frequency both its holding intensity and hysteresis-loop size are shown to decrease with increasing donor concentration. Two possibilities are suggested for experimentally determinng one of the two parameters of the system, the exciton–donor coupling constant or the donor concentration if the other is known.

The growth and properties of In-doped metalorganic vapor phase epitaxy interdiffused multilayer process (HgCd)Te

Indium doping of Hg1−xCdxTe layers grown by metalorganic vapor phase epitaxy using the interdiffused multilayer process has been investigated using trimethyl indium as the dopant source. The epitaxial growth was performed onto 2° off 〈100〉 CdTeSe substrates at ∼350 °C using dimethyl cadmium (DMCd) and di-isopropyl telluride as the alkyl sources. Doping in the range 1017–2×1018 cm−3 was achieved. By comparing Hall effect measurements of carrier concentration with secondary ion mass spectrometry analyses of the In concentrations in the layers, it was found that the In was only 30% active in the as-grown layers but ∼100% active after Hg-rich isothermal annealing at 250 °C for 48 h. At In concentrations greater than 2×1018 cm−3, the carrier concentration levels off, probably due to the solubility limit of In being reached. The annealed doped layers show slightly higher carrier mobilities than as-grown layers for the same carrier concentration. An apparent shift of the absorption edge to shorter wavelength with increasing donor concentration is thought to be due to the Moss–Burstein effect rather than a change in alloy composition x. A significant ‘‘memory’’ effect has been found with trimethyl In which persists for several runs and is probably due to strong adsorption onto the stainless steel surfaces in the growth system. This can be overcome by vacuum baking of the pipework. The growth of a heterostructure using In doping has shown that there is no serious diffusion problem with In under the growth conditions used.

Dependence of the grain boundary potential barrier height of BaTiO3 ceramics on donor dopant concentration

Barium titanate ceramics exhibiting the positive temperature coefficient of resistance (PTCR) effect were prepared with donor concentrations ranging from 0.05-1.8 at.%. The resistance of the various samples (normalised to give the specific resistance of a unit area of grain boundary rho L in order to eliminate the effect of the observed reduction in grain size with increased donor concentration) was measured in the vicinity of the ferroelectric transition temperature Tc. Arrhenius plots of rho L against (T- theta )/T, where T is the absolute temperature and theta is the Curie point, were then obtained for T>Tc. The plots were found to be linear up to a few degrees below the temperature corresponding to the maximum value of rho L, in agreement with the well known Heywang model. The potential barrier heights, calculated from the slopes of the Arrhenius plots, were found to be similar for samples containing 0.1-1.4 at.% ( approximately 0.2 eV at 150 degrees C) and greater in more heavily doped specimens (0.4 eV at the same temperature). However, the Arrhenius plots belonging to samples containing 0.05 at.% were linear for a small temperature range since the PTCR jump was very small and consequently it was not possible to calculate the potential barrier height with any accuracy.

Density-functional approach to doped magnetic semiconductors: Evolution of bound states of electrons as the donor concentration increases.

We present a simple method for calculating dense bound states of electrons coupled with localized spins in doped magnetic semiconductors within the density-functional formalism. This paper deals with the case in which the defect in antiferromagnetic semiconductors behaves as a trapped center with an effective charge +\ensuremath{\Vert}e\ensuremath{\Vert}. We apply our method to clarify the evolution of the magnetic impurity state (MIS) and the bound magnetic polaron (BMP) as the donor concentration increases on a pseudocrystal approximation for defect sites, using the physical parameters relevant to an antiferromagnetic semiconductor EuTe. In the single MIS the localized spins around the defect are canted in the antiferromagnetic matrix by the exchange interaction between impurity electrons and localized spins, while in the single BMP, the localized spins around the defect are aligned completely ferromagnetically in the matrix. The evolution obtained for the MIS is substantially different from that for the BMP in both the electronic state of impurity electrons and the magnetic ordering of localized spins.In the MIS the evolution in the electronic state is qualitatively similar to that in doped nonmagnetic semiconductors, and a nonmetal-metal transition occurs at the donor concentration where the ferromagnetic moment of localized spins, induced by MIS, starts to decrease. The induced ferromagnetic moment, thus, seems like an obstruction for the metallic state. On the other hand, in the BMP, the strong exchange interaction favors the completely ferromagnetic polarization of electron spins; so the transition to a metallic state occurs when the ferromagnetic ordering of localized spins, produced by BMP, is established over a large part of the crystal. The comparison with the experiment strongly suggests that the doped EuTe, such as ${\mathrm{Eu}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Gd}}_{\mathrm{x}}$Te and ${\mathrm{EuTe}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{I}}_{\mathrm{x}}$, is a typical material for the BMP, but not for the MIS. The effect of the random distribution of defect sites on the magnetic ordering is also studied in a conventional way, which implies that the heavily doped EuTe shows a complex magnetic structure involving the randomly distributed large ferromagnetic clusters and the remainder of the material more or less antiferromagnetic.

Enhanced percutaneous absorption of formoterol furmarate via pulsed iontophoresis. I. Effect of constant current and constant voltage

The effect of iontophoresis on percutaneous absorption of formoterol fumarate (FF) was investigated in vitro with abdominal skin excised from guinea pig. By passive diffusion, the flux at steady state for stripped skin was about 230 times greater than that for intact skin. Therefore, it is suggested that stratum corneum is a barrier for the percutaneous absorption of FF. Penetrated amount of FF increased significantly with an increase in donor concentration from 0.035% to 0.07%. Under the constant current iontophoresis the large flux accompanied with the high applied voltage was shown at the beginning of iontophoresis. Thereafter the flux decreased with a decrease in applied voltage. Under the constant voltage iontophoresis a linear relationship between penetrated amount of FF and time was observed.

Modification of oxide films by ion implantation: TiO 2-films modified by Ti + and O + as example

Oxide films can be modified by ion implantation. Changes in the electrochemical properties of the films are due to the deposition profile of the implanted ion, ie doping and stoichiometric changes, as well as to the radiation damage. The latter is due to the formation of Frenkel defects and at high concentrations to a complete amorphization of the oxide film. TiO x-films with 1 < x < 2.5 were produced by implantation of Ti + - and O + -ions into anodic oxide films on titanium. The electrode capacity shows always the behaviour of an n-type semiconductor with an almost constant flatband potential but a strong maximum donor concentration at about 3% Ti + concentration. Oxygen implantation, on the other hand, causes a small increase of donor concentration only at high concentration of O +, Electron transfer reactions show strong modification of the electronic behaviour of the oxide film with a maximum again at 3% titanium. Photocurrent spectra prove the increasing amorphization and show interband states 2.6 eV above the VB or below the CB. During repassivation measurements at various potentials different defects formed by Ti + - and O + -implantation become mobile. A tentative model of the band structure is constructed which takes into account the interband states due to localised Ti + -and O + -ions. The modification of ion implanted oxide films is compared with the effects of other preparation techniques.

Shallow donor-like impurity states in magnetic field in n-type InP

We present the evidence of hydrogenic behavior of shallow donor-like impurity states in n-type InP (8.5 × 10 15 cm −3 ≲ N d−N a ≲ 6.2 × 10 16 cm −3) from magnetic freeze out experiments at magnetic fields up to 18 T. This occurs at T ≳ 10 K and at a magnetic field sufficiently high to induce a metal-insulator transition. At high fields, the reduction of the binding energy with respect to the hydrogenic model of LARSEN can be well accounted for by the increasing overlap of the impurity wave functions as the donor concentration increases.

Electron hopping and magnetic field dependent spin dynamics of radical ions in solution

The role of hopping on the geminate recombination of radical ions (N,N-dimethylaniline cation and anthracene anion) in acetonitrile is studied via the nanosecond time-resolved magnetic field effect on the triplet yield and the influence of donor concentration thereon. Increasing donor concentration leads to lifetime broadening of the magnetic field dependence of the triplet yield. Responsible for this effect is the perturbation of the coherent spin motion caused by hopping of the electron spin between donor sites of different nuclear spin configuration. Comparison of experimental results with calculations based on the semiclassical theory of spin motion yields an estimate of the hopping rates. Deuteration of both radicals influences the halfwidth of the magnetic field effect: at long probing times and low donor concentrations the halfwidth measured for protonated radical ions exceeds the one for the deuterated species: at short delay times and large donor concentrations, i.e. high hopping rates, this isotopic effect is reversed.

Bound Magnetic Polaron in Europium Chalcogenides. Magnetic Properties

AbstractThe effect of shallow‐donor impurities and the bound magnetic polaron (BMP) formation on the magnetic properties of some Eu chalcogenides is considered. The contribution of the inhomogeneous spin polarization around the donors to the magnetic susceptibility and the total magnetization is calculated. It is shown that due to the donors a maximum of the zero‐field differential susceptibility shifts to a higher temperature as the donor concentration increases. Also the magnetization is non‐zero far above the Curie temperature when the donors are present. The results agree with the experimental data obtained in low‐conducting EuSe samples with various donor concentrations. Moreover, the calculated results indicate that the donors must form clusters with large unperturbed parts of the crystal between the clusters in order to explain the details in the experimental susceptibility.