Deep Centers Research Articles

Structure and electrical properties of zinc oxide base iron doped ceramics

The search for new economically advantageous technologies of new zinc oxide based composite ceramic materials and the study of their structure and properties attract special attention today. These ceramics have a number of advantages as compared with materials prepared by more expensive technologies, due to the possibility to fabricate items having different shapes and sizes and particularly to vary their morphology, structure and phase composition. This allows controlling their functional properties by varying the powder particle size in charge, the temperatures, durations and atmospheres of synthesis and heat treatment, and the types of doping impurities in the ceramics. The structure and electrical properties of (FexOy)10(ZnO)90 ceramics (0 ≤ x ≤ 3; 1 ≤ y ≤ 4) synthesized in air using single- and two-stage synthesis methods have been studied. FeO, α-Fe2O3 and Fe3O4 powders or (α-Fe2O3 + FeO) mixture have been used for ZnO doping. X-ray diffraction, gamma-ray resonance spectroscopy and Raman spectroscopy data suggest that at average iron concentrations of 1–3 at.% the ceramic specimens contain at least three phases: the Zn1-δFeδO solid solution with a wurtzite structure, the ZnFe2O4 ferrite phase with a spinel structure and FexOy residual iron oxides which were used as doping impurities. Scanning electron microscopy and energy dispersion X-ray analysis have shown that the wurtzite phase grain size in the ceramic specimens decreases from several decades of microns for single-stage synthesis to submicron sizes for two- stage synthesis. We show that iron addition to ZnO induces a compression of the wurtzite phase crystal lattice, the compression of lattice magnitude being proportional to the oxygen content in the FexOy iron oxide doping agent. The temperature dependences of the electrical resistivity suggest that deep donor centers with an activation energy of about 0.37 eV are formed in the Zn1-δFeδO wurtzite phase. The temperature dependences of the electrical resistivity of electrons for undoped ZnO in the 6–300 K range and for doped (FeO)10(ZnO)90 ceramic synthesized in one stage exhibit a variable activation energy below 50 K which indicates a heavily disordered structure.

Environmental effects on zooplankton abundance on a sub-Arctic shelf off northern Norway

AB Aquatic Biology Contact the journal Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AB 27:75-86 (2018) - DOI: https://doi.org/10.3354/ab00697 Environmental effects on zooplankton abundance on a sub-Arctic shelf off northern Norway K. Eiane1,*, M. Espinasse1, B. Espinasse2 1Nord University, PO Box 1490, 8049 Bodø, Norway 2Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, BC, V6T1Z4, Canada *Corresponding author: ketil.eiane@nord.no ABSTRACT: We studied the abundance of common zooplankton on 8 dates and at 5 locations on the shelf off the Lofoten Islands, northern Norway, between September 2013 and August 2014. We tested for relationships between environmental variability and abundances. We found that, of 17 zooplankton taxa or groups for which we recorded abundances, 4 omnivorous or carnivorous groups persistently differed in abundance between stations. This difference probably reflected relatively deep centres of distribution in the water column, and abundances of these species were positively associated with bottom depth. In 10 taxa or groups, abundance correlated with sampling date or temperature, salinity, or fluorescence, but generally not with bottom depth. Most of these taxa were consumers on low trophic levels, with a vertical distribution often associated with near-surface waters. In the remaining taxa or groups, no association between abundance and the environment or with time of sampling was established. Our results suggest that on-shelf abundances of zooplankton with intermediate-to-deep vertical distribution patterns are mainly limited by bathymetry, while abundance variations in zooplankton with a predominately near-surface distribution are driven by intra-annual environmental variability. KEY WORDS: Plankton ecology · Sub-polar · Hydrography · Seasonality Full text in pdf format PreviousNextCite this article as: Eiane K, Espinasse M, Espinasse B (2018) Environmental effects on zooplankton abundance on a sub-Arctic shelf off northern Norway. Aquat Biol 27:75-86. https://doi.org/10.3354/ab00697 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AB Vol. 27. Online publication date: August 06, 2018 Print ISSN: 1864-7782; Online ISSN: 1864-7790 Copyright © 2018 Inter-Research.

Numerical Investigation on Flow Making System in Multi-purpose Basin

In this paper, numerical investigation on flow field of flow making system is carried out in the 1:10 scaled model of the multi-purpose basin for the deep sea engineering research center of Dalian University of Technology. According to the requirement of obtaining specified flow velocity in the working area, two kinds of numerical methods simulating pump are used. The calculation results show that the pump simulating method with controlling boundary conditions at inlet and outlet is not valid in the presence of free water surface. The problem is solved by the second pump simulating method in which the pump rotation is simulated in the multiple frame of reference (MRF) system. It is believed that the establishment of current model will provide an effective calculation tool for the simulation of flow characteristics of the deep water flow system.

Transmission of Light by Crystals with Deep Impurities Affected by a Contribution of Two-Center Mechanism of Nonlinear Photoexcitation

It is demonstrated that transmission of laser light sharply decreases on a time scale of several nanoseconds with increase in light intensity in the range of ~105–106 W/cm2 when a two-center mechanism of nonlinear photoexcitation of a transparent crystal with high concentration of deep impurity centers is taken into consideration.

Static and Dynamic Disorder in Triple-Cation Hybrid Perovskites

A detailed understanding of the carrier dynamics and emission characteristics of organic-inorganic lead halide perovskites is critical for their optoelectronic and energy harvesting applications. In this work, we reveal the impact of the crystal lattice disorder on the photo-generated electron-hole pairs through low-temperature photoluminescence measurements. We provide strong evidence that the intrinsic disorder forms a sub-bandgap tail density of states, which determines the emission properties at low temperature. The PL spectra indicate that the disorder evolves with increasing temperature, changing its character from static to dynamic. This change is accompanied by a rapid drop of the PL efficiency, originating from the increased mobility of excitons/polarons, which enables them to reach deep non-radiative recombination centers more easily.

New Route for "Cold-Passivation" of Defects in Tin-Based Oxides.

Transparent conductive oxides (TCOs) are essential in technologies coupling light and electricity. For Sn-based TCOs, oxygen deficiencies and undercoordinated Sn atoms result in an extended density of states below the conduction band edge. Although shallow states provide free carriers necessary for electrical conductivity, deeper states inside the band gap are detrimental to transparency. In zinc tin oxide (ZTO), the overall optoelectronic properties can be improved by defect passivation via annealing at high temperatures. Yet, the high thermal budget associated with such treatment is incompatible with many applications. Here, we demonstrate an alternative, low-temperature passivation method, which relies on cosputtering Sn-based TCOs with silicon dioxide (SiO2). Using amorphous ZTO and amorphous/polycrystalline tin dioxide (SnO2) as representative cases, we demonstrate through optoelectronic characterization and density functional theory simulations that the SiO2 contribution is twofold. First, oxygen from SiO2 passivates the oxygen deficiencies that form deep defects in SnO2 and ZTO. Second, the ionization energy of the remaining deep defect centers is lowered by the presence of silicon atoms. Remarkably, we find that these ionized states do not contribute to sub-gap absorptance. This simple passivation scheme significantly improves the optical properties without affecting the electrical conductivity, hence overcoming the known transparency–conductivity trade-off in Sn-based TCOs.

Effect of Deep Centers on the Statistical Delay of Microplasma Breakdown in Gallium-Arsenide Light-Emitting Diodes

A statistical study of the microplasma-breakdown delay in gallium-arsenide light-emitting diodes is performed. The significant effect of deep centers on the microplasma breakdown of gallium arsenide p–n junctions is detected. It is shown that the statistical delay of the microplasma breakdown makes it possible to estimate the energy spectrum of deep levels in the microplasma channel when varying the charge state of deep centers by decreasing the reverse voltage applied to the p–n junction. In the temperature range of 250–350 K, the effect of three deep levels is detected and their parameters are determined.

Effect of Deep Centers on Charge-Carrier Confinement in InGaN/GaN Quantum Wells and on LED Efficiency

The deep-center-assisted tunneling of carriers in p–n structures of light-emitting diodes (LEDs) with InGaN/GaN quantum wells (QWs) makes smaller the effective height of the injection barrier, but leads to a dependence of the radiation efficiency on the density and energy spectrum of defects in GaN. In the case of hopping conduction across the space charge region, the forward voltage mainly drops near the QW boundary, where the density of deep states at the quasi Fermi-level is the lowest. As a result, band bending at the boundary decreases, and, with increasing current, the direction of the electric field also changes, which leads to a weaker confinement of holes, to their non-radiative recombination in the n barrier, and to an efficiency droop. The low efficiency of green GaN LEDs is associated with the dominance of deep centers and insufficient density of shallow centers in the energy spectrum of defects in barrier layers near the boundaries with the QW. The proposed model is confirmed by the stepwise experimental dependences of the current, capacitance and efficiency of green and blue LEDs in the case of forward bias, which reflect the contribution of color centers responsible for the defect photoluminescence bands in GaN.

Optical and Microstructural Investigation of Heavy B-Doping Effects in Sublimation-Grown 3C-SiC

In this work, a complementary microstructural and optical approach is used to define processing conditions favorable for the formation of deep boron-related acceptor centers that may provide a pathway for achieving an intermediate band behavior in highly B-doped 3C-SiC. The crystallinity, boron solubility and precipitation mechanisms in sublimation-grown 3C-SiC crystals implanted to 1-3 at.% B concentrations were investigated by STEM. The revealed defect formation and boron precipitation trends upon thermal treatment in the range 1100-2000°C have been cross-correlated with the optical characterization results provided by imaging PL spectroscopy. We discuss optical activity of the implanted B ions in terms of both shallow acceptors and deep D-centers, a complex formed by a boron atom and a carbon vacancy, and associate the observed spectral developments upon annealing with the strong temperature dependence of the D-center formation efficiency, which is further enhanced by the presence of implantation-induced defects.

Development of efficient scintillator materials based on LiLaP4O12 doped with rare earth ions

Microstructural and luminescent properties of lithium lanthanum phosphate LiLaP4O12 (LiLaPO), undoped and doped with trivalent rare earth (RE3+, RE = Ce, Eu, and Tb) ions, were studied. LiLaPO powders were synthesized by sol-gel method assisted by polyvinyl alcohol (PVA). The structural phase analysis was confirmed by X-ray powder diffraction (XRD). Scintillator properties were investigated to evaluate the luminescent response of the material under excitation with X-rays. The scintillation efficiency of the doped samples were compared to the one measured for a known BGO scintillator and it was shown that Tb-doped LiLaPO showed efficiency 30% higher than the BGO one. The chromaticity diagram was obtained from radioluminescent (RL) measurements. The stability of the RL response was investigated during long term X-ray exposure. The results showed that the RL efficiency of the LiLaPO increased almost by 40%, for the Ce-doped sample, and almost 25%, for Eu and Tb-doped samples, as the exposure time increased. This behavior was attributed to the presence of the deep trapping centers that trap charge carriers during the first stage of the irradiation, with a subsequent release of the trapped charges increasing the probability of the electron-hole pair recombination as a function of the time. Time-resolved RL was carried out to investigate the luminescent decay time for doped samples and, for Ce-doped samples, a fast decay component with luminescent decay time of 17 ns dominates. Typical millisecond decay components with decay time constants of 3.12 ms and 5.17 ms were observed for Tb3+ and Eu3+-doped LiLaPO, respectively. Additionally, a slow component with decay time constant of 31.3 ms was also found for Eu3+-doped LiLaPO and this component was attributed to the existence very shallow traps in the material.

Generation Kinetics of Nonequilibrium Charge Carriers in Crystals with Deep Impurities Involving Two-Center Transitions between Band and Impurity States

A new and efficient mechanism of nonlinear photoexcitation of a transparent crystal with deep impurity centers is proposed. It is hypothesized that the energy of a quantum of light is smaller than the energy gap between the bottom of the conduction band and the impurity level, but is larger than the gap between the impurity level and the top of the valence band. The kinetics of nonlinear cascade generation of nonequilibrium charge carriers is considered taking into account two-center processes in which the energy transfer and the photon absorption occur in one and the same elementary event. The dependences of the quasi-equilibrium concentrations of nonequilibrium charge carriers in the bands and of the occupancy of impurity states on the laser-radiation intensity are obtained. It is shown that the generation process of nonequilibrium electron–hole pairs is of a threshold nature. Depending on the concentration of impurity centers, the threshold intensities can be ~105–107 W/cm2, while the setting time of the quasi-equilibrium occupancies of electronic states is ~10–0.1 ns.

Influence of dislocation density and carbon impurities in i-GaN layer on the performance of Schottky barrier ultraviolet photodetectors

The influences of dislocation density and carbon impurities in i-GaN layer on the performance of Schottky barrier ultraviolet photodetectors are investigated. It is found that an increase in dislocation density leads to a decrease in the responsivity, mainly because that the dislocation defects act as charge traps in GaN which can increase the recombination probability of photo-generated electron-hole pairs. On the other hand, it is shown that high carbon impurity content in the i-GaN layer also causes a decrease of the responsivity, mainly because the fact that the photo-generated carriers can’t be effectively collected due to the effect of C-induced deep level centers related to carbon impurities.To fabricate high performance GaN based ultraviolet photodetectors, it is important to reduce dislocation density and keep a relatively low concentration of residual carbon during the material growth.

Energy Levels of Defects Created in Silicon Supersaturated with Transition Metals

Intermediate-band semiconductors have attracted much attention for use in silicon-based solar cells and infrared detectors. In this work, n-Si substrates have been implanted with very high doses (1013 cm−2 and 1014 cm−2) of vanadium, which gives rise to a supersaturated layer inside the semiconductor. However, the Mott limit was not exceeded. The energy levels created in the supersaturated silicon were studied in detail by means of thermal admittance spectroscopy. We found a single deep center at energy near EC − 200 meV. This value agrees with one of the levels found for vanadium in silicon. The capture cross-section values of the deep levels were also calculated, and we found a relationship between the capture cross-section and the energy position of the deep levels which follows the Meyer–Neldel rule. This process usually appears in processes involving multiple excitations. The Meyer–Neldel energy values agree with those previously obtained for silicon supersaturated with titanium and for silicon contaminated with iron.

Compensation and persistent photocapacitance in homoepitaxial Sn-doped β-Ga2O3

The electrical properties of epitaxial β-Ga2O3 doped with Sn (1016–9 × 1018 cm−3) and grown by metalorganic chemical vapor deposition on semi-insulating β-Ga2O3 substrates are reported. Shallow donors attributable to Sn were observed only in a narrow region near the film/substrate interface and with a much lower concentration than the total Sn density. For heavily Sn doped films (Sn concentration, 9 × 1018 cm−3), the electrical properties in the top portion of the layer were determined by deep centers with a level at Ec-0.21 eV not described previously. In more lightly doped layers, the Ec-0.21 eV centers and deeper traps at Ec-0.8 eV were present, with the latter pinning the Fermi level. Low temperature photocapacitance and capacitance voltage measurements of illuminated samples indicated the presence of high densities (1017–1018 cm−3) of deep acceptors with an optical ionization threshold of 2.3 eV. Optical deep level transient spectroscopy (ODLTS) and photoinduced current transient spectroscopy (PICTS) detected electron traps at Ec-0.8 eV and Ec-1.1 eV. For lightly doped layers, the compensation of film conductivity was mostly provided by the Ec-2.3 eV acceptors. For heavily Sn doped films, deep acceptor centers possibly related to Ga vacancies were significant. The photocapacitance and the photocurrent caused by illumination at low temperatures were persistent, with an optical threshold of 1.9 eV and vanished only at temperatures of ∼400 K. The capture barrier for electrons causing the persistent photocapacitance effect was estimated from ODLTS and PICTS to be 0.25–0.35 eV.

Temperature-dependent OSL properties of nano-phosphors LiAlO2:C and α-Al2O3:C

The present study focuses on the synthesis and characterization of carbon doped nano-phosphors, LiAlO2 and α-Al2O3 and their temperature-dependent optically stimulated luminescence (TA-OSL) characteristics in the temperature ranges of 25–350 °C. These nano-phosphors with the carbon concentration of 0.01 mol% exhibits high luminescent intensity for LiAlO2:C in the low dose range of 1 mGy-7 Gy and for α-Al2O3:C in the range of 100 mGy-1 kGy. Both these nano-phosphors are of polycrystalline in nature, having grain size 15–50 nm as confirmed by the X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM), respectively. The maximum TA-OSL intensities are observed at 125 °C for LiAlO2:C and 200 °C for Al2O3:C, and reveal the presence of deep defect centres. The Arrhenius analysis shows the activation energies Ea = 0.06 ± 0.02 eV for LiAlO2:C and Ea = 0.04 ± 0.01 eV, & Eb = 0.48 ± 0.07 eV for Al2O3:C. The TA-OSL and OSL characteristics are discussed with special reference to the medical and high radiation dosimetry. These compounds, LiAlO2:C and α-Al2O3:C, are non-toxic, robust and are potential candidates for reusable dosimetry.

Quantum efficiency of femtosecond-laser sulfur hyperdoped silicon solar cells after different annealing regimes

Annealing dependent absorption and quantum efficiency spectra are determined on femtosecond-laser, sulfur hyperdoped silicon solar cells in a spectral range from 300 to 2500 nm. Although the samples, which are rapidly quenched after annealing at T = 1250 °C, do not feature the highest sub-bandgap absorption, the highest sub-bandgap quantum efficiency is measured on the same level as on non-annealed samples, featuring the highest sub-bandgap absorption. Findings on annealing dependent absorption are carried over, in order to explain the measured quantum efficiency spectra. In the sub-bandgap spectral range conversion is more efficient, when deep sulfur centers are obtained from annealing at higher temperatures at preferably rapid quenching. For those annealing regimes the above-bandgap quantum efficiency is decreased, which is ascribed to less shallow sulfur donors. Lower temperatures or slow cooling rates results in shallow donors, featuring a more efficient conversion in the above-bandgap spectral range. This is on the expense of deep sulfur centers and the corresponding sub-bandgap conversion ability. Therewith it is concluded, that the main absorption occurs only in the laser-induced sulfur emitter layer. Furthermore, to some extent a prediction is proposed on sulfur hyperdoped silicon prepared by ion implantation and subsequent pulsed laser melting, featuring rapid cooling from the melt.

FREE CARRIER RECOMBINATION LIFETIME CALCULATION FROM PHOTOCONDUCTIVITY DECAY MEASUREMENT IN NON–PASSIVATED SILICON

In indirect band gap semiconductors, for example, in silicon, the free carrier recombination lifetime is determined by recombination through deep level centers and inversely proportional to their concentration. This parameter is of the utmost importance for characterizing the quality of the material. Contactless methods of free carrier recombination lifetime measurements by protoconductivity decay analysis are most widely used. The measurement results are largely affected by surface recombination. The calculation of the lifetime in the bulk of a sample from the characteristic time of photoconductivity decay remains relevant since there is no ambiguous analytical solution of the continuity equation for this case. In this paper, an analysis of the relaxation of photoconductivity in single−crystal silicon wafers with non−passivated surfaces was carried out with numerical methods. The applicability of the well–known formulas for estimating the contribution of surface recombination to the effective photoconductivity decay time was discussed. We show that the time in which the «fast» exponents disappear depends on the relative thickness of the sample. It is only this part of the relaxation curve that the effective decay time is determined by the maximum value of the surface component of the relaxation time and is described by the well−known formulas. The saturation of the effective relaxation time at the point when the signal intensity reaches 45 % of the peak one (the onset point of effective decay time counting pursuant to the SEMI MF 1535 standard recommendation) only occurs in samples with thicknesses less than 3—5 diffusion lengths. For thick samples the contribution of the «fast» exponentials to the effective photoconductivity relaxation time is observed up to 5 % of the peak signal (i.e., until the noise level of the measured signal is reached). Use of the recommended formulas, including for the «infinite recombination rate» case at which the maximum surface lifetime is d2/π2D, leads to a sufficiently large (up to 20 %) error in free carrier recombination lifetime calculation.

The suppression of Cu-related charge localized defects in Cu2ZnSnS4 thin film solar cells

Cu2ZnSnS4 (CZTS) is a promising low cost thin-film solar cell material. However, the charge localized defects greatly hinder the improvement of the solar cell efficiency, thus the identification and knowledge of the possible charge localized defects in it are extremely important. Using hybrid functional calculation, we find that CuSn and CuZn are the main charge localized defects in CZTS. In detail, our results show that CuSn is a deep level recombination center. Moreover, the growth condition of Sn determines the population of CuSn because the stable chemical potential region of ΔμSn ([− 1.74, 0] eV) is larger than that of ΔμCu ([− 0.77, 0] eV). Thus Sn-rich growth condition is proposed to suppress the CuSn. As for CuZn antisites, part is deep acceptors which will be beneficial for the efficiency of solar cell, while part forms donor-acceptor pairs with ZnCu. The CuZn+ZnCu donor-acceptor pairs will lead to large potential fluctuation in CZTS, which is a disadvantageous factor. Fortunately, such pairs can be greatly suppressed by Cd doping due to two reasons: one is that the Cd atoms prefer to substitute the Zn atomic sites leading to the reduction of the CuZn concentration, and the other is that the Cd dopant in the CZTS makes it difficult for its neighboring Zn atom be substituted by Cu atom.

Deep Radiation-Induced Defect Centers Created by a Fast Neutron Flux in CdZnTe Single Crystals

The effect of a fast neutron flux (Φ = 1014–1015 cm–2) on the electrical and photoluminescence properties of p-CdZnTe single crystals is studied. Isothermal annealing is performed (T = 400–500 K), and the activation energy of the dissociation of radiation-induced defects is determined at ED ≈ 0.75 eV.

Восстановление временной зависимости интенсивности света, частота модуляции которой больше граничной частоты фотоприемника, изготовленного на основе кремния с глубокими рекомбинационными центрами.

Восстановление временной зависимости интенсивности света, частота модуляции которой больше граничной частоты фотоприемника, изготовленного на основе кремния с глубокими рекомбинационными центрами.