High Donor Concentrations Research Articles

Anomalous electron spin splitting in InAs pumped by injection

The electroluminescence and photoluminescence of bulk n-InAs with a high concentration of donors (N d ≈ 5 × 1016 cm−3) is studied experimentally in magnetic field in the Faraday layout of the experiment. Under the conditions of electrical injection, the photon energy corresponding to the electroluminescence peak exceeds the energy band gap E g. When a magnetic field is applied, the energy of the peak becomes lower than E g, and the peak splits into two circular-polarized components. The splitting depends on the injection current. In moderate magnetic fields (about 2 T), the splitting can be much more profound than the calculated splitting corresponding to the well-known g factor of electrons in InAs. The effect is attributed to different degrees of magnetic freezing-out of electrons with different spin orientation. The maximum in the dependence of the degree of polarization of photoluminescence on the magnetic field and the behavior of the photoluminescence line width support the model suggested.

Effect of donor-acceptor interaction strength on excitation energy migration and diffusion at high donor concentrations

The migration and diffusion modulated excitation energy transfer has been studied in a new dye pair 7-diethylamino-4-methylcoumarin (donor) to 3,3'-dimethyloxacarbocyanine iodide (acceptor) by steady-state and picosecond time-resolved spectroscopy. To reduce the artifact of self-absorption, at high donor concentrations, the time-resolved studies have been carried out in thin films of polyvinyl alcohol (solid matrix) and in methanol (liquid phase) at front-face geometry of excitation. The Forster-type (nonradiative) energy transfer [Discuss. Faraday Soc. 27, 7 (1959)] takes place directly from donor to acceptor in case of solid matrix, while Yokota-Tanimoto model [J. Phys. Soc. Jpn. 22, 779 (1967)] for diffusion has been found to be operating in the liquid phase. It has been found here that the high interaction strength between donor and acceptor molecules as compared to that among donors masks the effect of energy migration and diffusion at high donor concentrations. The rate and efficiency of energy transfer increase with increasing the acceptor concentration. This has been confirmed by the study of acceptor kinetics.

Purification and characterization of a thermostable intracellular β-xylosidase from the thermophilic fungus Sporotrichum thermophile

An intracellular β-xylosidase from the thermophilic fungus Sporotricum thermophile strain ATCC 34628 was purified to homogeneity by Q-Sepharose and Mono-Q column chromatographies. The protein properties correspond to molecular mass and p I values of 45 kDa and 4.2, respectively. The enzyme is optimally active at pH 7.0 and 50 °C. The purified β-xylosidase is fully stable at pH 6.0–8.0 and temperatures up to 50 °C and retained over 58% of its activity after 1 h at 60 °C. The enzyme hydrolyzes β-1,4-linked xylo-oligosaccharides with chain lengths from 2 to 6, releasing xylose from the non-reducing end, but is inactive against xylan substrates. The apparent K m and V max values from p-nitrophenyl β- d-xylopyranoside are 1.1 mM and 114 μmol p-nitrophenol min −1 mg −1, respectively. Alcohols inactivate the enzyme, ethanol at 10% (v/v) yields a 30% decrease of its activity. The enzyme is irreversibly inhibited by 2,3-epoxypropyl β- d-xylobioside while alkyl epoxides derived from d-xylose were not inhibitors of the enzyme. The enzyme catalyses the condensation reaction using high donor concentration, up to 60% (w/v) xylose.

Electrical conductivity mechanisms in zinc oxide thin films deposited by pulsed laser deposition using different growth environments

The mechanisms of electrical conduction in zinc oxide thin films, grown by pulsed laser deposition, have been investigated as a function of preparation conditions. The films were deposited on glass and silicon nitride coated silicon, using oxygen rich, oxygen deficient, or nitrogen atmospheres. The substrates were held at 473 K during deposition, and subsequently cooled down to room temperature in oxygen rich atmosphere of 4 Pa, or oxygen deficient atmosphere of 2×10−3 Pa. Films deposited and cooled in an oxygen deficient atmosphere exhibited very high donor concentration, originated in intrinsic defects, and an impurity band related mechanism of conduction. Films deposited under relatively high oxygen pressure were highly resistive and showed, upon ultraviolet light irradiation, grain boundary controlled electrical transport. An enhancement of the conductivity was observed when using a nitrogen atmosphere during the deposition, and oxygen atmosphere during cooling. In this case, the dependence of the conductivity with temperature followed Motts' Law of variable range hopping, characteristic of a material with localized states randomly located in space. Since the density of hopping centers appears to be much larger than the density of nitrogen incorporated in this sample, it is suggested that the nitrogen induces defects in the zinc oxide lattice that behave as localized hopping centers, as well as carrier suppliers, giving rise to the observed conductivity.

Growth and characterization of Sn doped ZnO thin films by pulsed laser deposition

AbstractSn:ZnO thin films with different Sn concentrations were grown by pulsed laser deposition (PLD) onto single‐crystal Si(001) substrates at an oxygen pressure of 2 × 10–2 mbar and substrate temperature of 600 °C. The targets used were high density Sn:ZnO pellets with different Sn concentrations produced by mixing ZnO and SnO2 by conventional ceramic routes. A deep structural and electrical characterization was carried out in order to determine the role of an increasing Sn nominal concentration on the ZnO film transport properties. Only films with a nominal 0.1 at% Sn show an improvement of the transport properties, lower resistivity and higher donor concentration, with respect to pure ZnO thin films. For films with larger Sn nominal concentrations segregated SnZnO phases appear that lead to larger film resistivities and no increase in donor concentration. The 0.1 at% Sn film is accordingly a good candidate to study the possible room temperature ferromagnetism when co doping with Mn. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Semiconductive properties of titanium anodic oxide films in McIlvaine buffer solution

The semiconductive properties of anodic oxide films grown on titanium surface at different formation potentials: 1.0, 2.0, 3.0 and 4.0 V were investigated by means of electrochemical impedance spectroscopy and cyclic voltammetry in McIlvaine buffers at pH 2, 4 and 5. The passive films show a Mott–Schottky behavior typical for an n-type semiconductor at the studied potential range with a high concentration of donors. On increasing the film formation potential, the flat band potential and the donor density decrease. Further, lower flat band potentials were obtained for higher pH buffer. These experimental results were related to the film thickness and composition. The influence of film thickness on the oxidation reactions taking place at the titanium electrodes covered by oxide film was evaluated.

Effect of external electric field on the microstructural evolution of La2O3-doped BaTiO3ceramics

Abstract The grain growth behavior of La-doped BaTiO3 specimens was investigated at 1350 °C under DC external electric field. In air atmosphere, grain growth was enhanced at the positive-biased region for the 0.15 mol% La-doped and 0.3 mol% La-doped specimens, while it was enhanced at the negative-biased region for the 1.0 mol% La-doped specimen. For the 0.6 mol% La-doped specimen, on the other hand, both positive- and negative-biased regions showed the same grain size. The demarcation donor composition (0.6 mol% La) shifted to a higher donor concentration when the specimen was heat-treated in nitrogen. The results were explained in terms of the grain boundary potential and its variation with donor concentration, oxygen partial pressure, and defect polarization.

Use of high acyl donor concentrations leads to penicillin acylase inactivation in the course of peptide synthesis

Enzyme inactivation has been observed in the course of penicillin acylase-catalyzed hydrolysis and aminolysis of d-phenylglycine amide. Inactivation was very sensitive to the d-phenylglycine amide concentration: at pH 9.5, 25 °C and 400 mM substrate, penicillin acylase lost more than 90% of its initial catalytic activity in half an hour, in the presence of 100 mM substrate, 50% of the initial activity in two hours, whereas in the absence of substrate, no significant enzyme inactivation was observed in three hours. Observed enzyme inactivation limits use of high acyl donor concentrations at penicillin acylase-catalyzed peptide synthesis.

Shallow donor induced n-type conductivity in deuterated boron-doped diamond

We show that deuteration of a series of boron-doped (100) diamond epitaxial layers can lead to a p-type to n-type conversion. The n-type diamond epilayers have electrical conductivities of a few S/cm at 300 K. These values are a factor 103–105 higher than the n-type conductivities achieved with phosphorus doping. This is a consequence of the shallow donor character of the deuterium-related donors inducing the conversion (ionization energy of 0.34 eV) and of the high donor concentrations. We also show that these donors are formed in a two-step process. First, all the acceptors form (B, D) complexes with one deuterium per boron atom. Then, the presence of excess and more mobile deuterium triggers the formation of the shallow donors. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Atomic structure of B-related defects and B diffusion in Si predoped with P impurities

We perform first-principles pseudopotential calculations to investigate the atomic structure and energetics of various defects consisting of B and P impurities and the effect of P on B diffusion in Si. In the equilibrium case, we find that a B-P pair is energetically most stable when P is positioned at a second-neighbor site of B. When excess Si interstitials are generated by implantation, B and P impurities tend to form I s -B-P complexes with self-interstitials (I s ), where P still prefers to a second-neighbor site of B, particularly for high donor concentrations. Using the nudged elastic band method, we examine the diffusion of B and its energy barrier in the presence of P. We find that the diffusion pathways of B are similar to those reported for the I s -B pair without P; however, the migration energy for B diffusion from the I s -B-P complex increases by about 0.2 eV. The increase of the activation energy and the formation of the B-P pair acting as a trap for B diffusion provide a clue for understanding the suppression of B diffusion observed in Si predoped heavily with donor impurities.

Pitting initiation and propagation of hypoeutectoid iron-based alloy with inclusions of martensite in chloride-containing nitrite solutions

The pitting initiation and the pitting propagation of a hypoeutectoid iron-based alloy with inclusions of martensite in nitrite solutions containing chloride ions were studied by various electrochemical techniques. It was shown, by scanning reference electrode technique (SRET), that pitting occurred only on the martensite phase rather than hypoeutectoid areas. Anodic polarization behaviors of martensite electrode in nitrite solution with and without chloride ions are similar to those of hypoeutectoid electrode: both of martensite and hypoeutectoid electrodes were passive and the passive films could be broken by chloride ion. However, the open circuit potential of martensite electrode is more negative than that of hypoeutectoid electrode, and the passive current density of martensite electrode in the solution containing chloride ions was larger than that of hypoeutectod electrode. It was shown, by Mott-Schottky analysis, that the passive film of martensite electrode in the solution containing chloride ions had a higher donor concentration than that of hypoeutectoid electrode. There was a galvanic interaction between martensite and hypoeutectoid electrodes and it was this interaction that induced and accelerated the pitting corrosion on martensite area. Current and potential fluctuations, which reflected pitting initiation, were observed when martensite and hypoeutectoid electrodes were coupled together.

Optical and Microstructural Properties of N- and Ga-Polarity GaN

ABSTRACTThe effect of polarity on the optical and microstructural properties of GaN is presented. A sample with adjacent domains of Ga- and N-polarity material was grown by varying the nucleation layer. This allows a unique opportunity to study the two different polarities under controlled conditions. We found that the N-polarity material has a much lower dislocation density than the Ga-polarity material. The N-polarity material contains voids that are not present in the Ga-polarity region. The surface roughness of the N-polarity material appears to be caused by Ga-polarity inversion domains which lead the growth. A cathodoluminescence study showed that the N-polarity material is much brighter than the Ga-polarity material, suggesting a higher donor concentration, probably due to increased impurity incorporation in the [0001] growth direction or possibly due to an increase in intrinsic point defects. There is also evidence that the N-polarity region contains two types of material, one is flat and can be etched, the other has an inclined facet and does not etch.

Phenomenological Analysis of Codoping Role of Statistical Fluctuations

The effect of codoping in semiconductors takes on the appearance of reduced impurity ionisation and enhanced solubility of the majority impurity in presence of the compensating one. These two issues were addressed by considering the changes in the impurity ionisation process due to a high concentration of donors and acceptors. Specifically potential fluctuations associated with random distributions of impurities are dealt with in a simple but tractable model which allows one to define the optimum conditions of codoping. The required compensation ratio turns out to be around 0.5. Low activation energy is also predicted. Potential fluctuations are shown to affect as well impurity incorporation from an external phase in codoping experiments. A pending question is the actual carrier mobility in such disordered material.

Radiation hard detectors from silicon enriched with both oxygen and thermal donors: improvements in donor removal and long-term stability with regard to neutron irradiation

Detectors made on the silicon wafers with high concentration of thermal donors (TD), which were introduced during the high temperature long time (HTLT) oxygenation procedure, have been investigated in the study of radiation hardness with regard to neutron irradiation and donor removal problems in irradiated high resistivity Si detectors. Two facts have been established as the evidence of radiation hardness improvement of HTLT(TD) Si detectors irradiated below ∼10 14 n/cm 2 compared to detectors made on standard silicon wafers: the increase of space charge sign inversion fluence (of 1 MeV neutrons) due to lower initial Si resistivity dominated by TD, and the gain in the reverse annealing time constant τ favourable for this material. Coupled with extremely high radiation tolerance to protons observed earlier (“beta zero” behaviour in a wide range of fluence), detectors from HTLT(TD) Si may be unique for application in the experiments with multiple radiations. The changes in the effective space charge density ( N eff) in as-irradiated detectors as a function of neutron fluence have been fitted using three different models. It has been shown that a new model with a universal donor removal rate for both materials, and considering the contribution of non-removable TD to the N eff provides good fit to the experimental data. A defect level related to TD has been observed in DLTS spectra of HTLT Si(TD) near T∼75 K. The physics of donor removal in irradiated silicon detectors is discussed.

Uniformity of passive films formed on ferrite and martensite by different inorganic inhibitors

A comparison of uniformity of passive films formed on ferrite and martensite by three kinds of inorganic inhibitors, chromate, bicarbonate and nitrite, has been made by anodic polarization curve and AC impedance measurements. It was found, by anodic polarization curve analyses, that there was different pitting susceptibility of passive films formed on ferrite and martensite by chromate or bicarbonate. In the solution containing chloride ions, the broken potential of the passive film formed on ferrite by chromate or bicarbonate was more negative than that on martensite. However, the passive films formed on both ferrite and martensite by nitrite had similar pitting susceptibility. Their broken potentials were almost the same. The difference in pitting susceptibility of the passive films was explained by Mott–Schottky analyses. It was found that the passive film formed on ferrite by chromate or bicarbonate had a higher donor concentration than the passive film on martensite. The passive films with higher donor concentrations were more sensitive to chloride ions. However, the passive films formed on both ferrite and martensite by nitrite had the same donor concentration. A uniform passive film can be formed on a martensite mild steel with ferrite bands by nitrite.

Temperature dependence of electrical properties of gallium-nitride bulk single crystals doped with Mg and their evolution with annealing

Comprehensive studies of the electrical properties of Mg-doped bulk GaN crystals, grown by high-pressure synthesis, were performed as a function of temperature up to 750 °C. Annealing of the samples in nitrogen ambient modifies qualitatively their resistivity values ρ and the ρ(T) variation. It was found that our material is characterized by a high concentration of oxygen-related donors and that the charge transport in the studied samples is determined by two types of states, one of shallow character (Mg-related state, EA≈0.15 eV), and the second one much more deep, E2≈0.95 eV (above the valence band). Depending on the effective concentration of either states, different resistivities ρ can be observed: lower resistivity (ρ<104 Ω cm at ambient temperature) in samples with dominant EA states and very high resistivity (ρ>106 Ω cm at ambient temperature) in samples with dominant E2 states. For the first type of samples, annealing at Tann<500 °C leads to a decrease of their resistivity and is associated with an increase of the effective concentration of the shallow Mg acceptors. Annealing of both types of samples at temperatures between 600 and 750 °C leads to an increase of the deep state concentration. The presence of hydrogen ambient during annealing of the low-resistivity samples strongly influences their properties. The increase of the sample resistivity and an appearance of a local vibrational mode of hydrogen at 3125 cm−1 were observed. These effects can be removed by annealing in hydrogen-free ambient.

Direct Measurements of the Charge-Recombination Dynamics of Geminate Ion Pairs Formed upon Electron-Transfer Quenching at High Donor Concentration

The rate constants of back-electron-transfer (BET) reaction within geminate ion pairs generated upon static ET quenching of cyano-substituted anthracenes by aromatic amines and methoxy-substituted benzenes (MSB) at high concentration in acetonitrile have been measured directly using ultrafast multiplex transient grating spectroscopy. The free energy of BET, ΔGBET, was varied between −3.0 and −0.6 eV, a range corresponding, in principle, to the inverted, barrierless, and normal regimes. When plotted vs ΔGBET, the measured rate constants, kBET, exhibit a large scattering. Good fits of the semiclassical expression for nonadiabatic ET are obtained if the rate constants are sorted according to the electron donor. The resulting electronic coupling matrix elements V are larger and the solvent reorganization energies smaller than those reported for BET within solvent-separated ion pairs, suggesting that BET takes place between ions in contact. However, in the low exergonicity region, the observed BET rate constan...

Linewidth Study of Electric Dipole Induced Spin Resonance in Uniaxially Stressed n-InSb in Far Infrared Regions: Experimental

We have carried out the linewidth measurements of the electric dipole induced spin resonance (EDSR) of conduction electrons in uniaxially stressed n-InSb in the far-infrared (FIR) regions. Our employed samples of n-InSb are highly doped with donors (6.0×10 15 cm -3 and 8.5×10 15 cm -3 ) and thus have metallic properties due to degenerate electrons. The linewidths of the EDSR have been obtained as the functions of the magnetic field (2.10∼6.39 T), the temperature (4.2∼70 K) and the donor concentration. The main features of the experimental results are the followings: (1) It is found that the minimum of the linewidth appears on the magnetic field dependence at 4.2 K. (2) In the lower resonance magnetic field (2.10 T and 2.74 T) and lower temperature regions ( T<∼20 K), we have recognized a slight reduction of the linewidth with the increasing of temperature. (3) For moderately high temperatures ( T>∼20 K), the increment of the linewidth is found on our employed experimental conditions for all samples. (4) For the sample with higher donor concentration, the enhancement of the linewidth is also found in comparison with the sample with lower donor concentration. In this paper, the linewidth of the EDSR spectrum is mainly discussed in respect of the experimental results. The detailed theoretical discussion will be presented in the following paper by Sugihara et al.

Characterization of the film formed on iron in borate solution by electrochemical impedance spectroscopy

The passive film anodically formed on iron in 0.1 mol l −1 sodium borate solution has been evaluated by electrochemical impedance spectroscopy. An equivalent circuit is assumed in order to characterize the film capacitance, the charge transfer resistance at the interfaces and the ionic flux across the film. The Mott–Schottky representation was used plotting the capacitance as a function of the applied potential. The film can be described as an n-type semiconductor with a high concentration of donors and very low ionic conduction.

Proportion of geometrical hydroperoxide isomers generated by radical oxidation of methyl linoleate in homogeneous solution and in aqueous emulsion

The proportion of geometrical hydroperoxide isomers generated by aerobic oxidation of methyl linoleate (18:2 Me) in either aqueous emulsion consisting of Tris-HCl buffer (pH 7.4) or in a homogeneous dichloromethane solution was determined to understand the mechanism of lipid oxidation in different reaction systems. Four geometrical isomers were generated after oxidation of 18:2 Me in dichloromethane: methyl 13-hydroperoxy-cis-9, trans-11-octadecadienoate, methyl 13-hydroperoxy-trans-9,trans-11-octadecadienoate, methyl 9-hydroperoxy-trans-10,cis-12-octadecadienoate, and methyl 9-hydroperoxy-trans-10, trans-12-octadecadienoate in the ratios of 1:4:1:4, respectively. The ratios between each isomer did not change until the peroxide value (PV) increased to 58 meq/kg. Oxidation of 18:2 Me in aqueous emulsion yielded the same geometrical isomers of hydroperoxide. However, the ratios were different: 3:2:3:2 until the PV increased to 110 meq/kg. Predominant (60%) formation of trans,trans hydroperoxide isomers was obtained in the oxidation of a mixture of 18:2 Me and methyl laurate (12:0 Me). These results are interpreted to reflect the importance of the concentration of hydrogen atom-donating equivalents to the kinetic preference for different products. The high effective concentration of hydrogen donors in the oxidation of 18:2 Me in emulsions favored the formation of the less stable cis,trans isomers. The lower concentration of hydrogen donor in the dichloromethane solution effectively slowed hydrogen donation and led to the strong preference for the more stable trans,trans isomers. This interpretation was further tested by preparing emulsions of 18:2 Me and 12:0 Me to dilute concentration of hydrogen-donating species using the nonhydrogen-donating 12:0 Me. Consistent with the proposed hypothesis, the proportion of trans,trans isomers increased as a result of 12:0 Me addition.