Dominant Loss Mechanism Research Articles

Microwave losses in MgO, LaAlO3, and (La0.3Sr0.7)(Al0.65Ta0.35)O3 dielectrics at low power and in the millikelvin temperature range

We have investigated both the temperature and the power dependence of microwave losses for various dielectrics commonly used as substrates for the growth of high critical temperature superconductor thin films. We present measurement of niobium superconducting λ∕2 coplanar waveguide resonators, fabricated on MgO, LaAlO3, and (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT), at the millikelvin temperature range and at low input power. By comparing our results with the two-level system model, we have discriminated among different dominant loss mechanisms. LSAT has shown the best results as regards the dielectric losses in the investigated regimes.

Seismicity within a propagating ice shelf rift: The relationship between icequake locations and ice shelf structure

AbstractIceberg calving is a dominant mass loss mechanism for Antarctic ice shelves, second only to basal melting. An important process involved in calving is the initiation and propagation of through‐penetrating fractures called rifts; however, the mechanisms controlling rift propagation remain poorly understood. To investigate the mechanics of ice shelf rifting, we analyzed seismicity associated with a propagating rift tip on the Amery Ice Shelf, using data collected during the austral summers of 2004–2007. We apply a suite of passive seismological techniques including icequake locations, back projection, and moment tensor inversion. We confirm previous results that show ice shelf rifting is characterized by periods of relative quiescence punctuated by swarms of intense seismicity of 1 to 3 h. Even during periods of quiescence, we find significant deformation around the rift tip. Moment tensors, calculated for a subset of the largest icequakes (Mw > −2.0) located near the rift tip, show steeply dipping fault planes, horizontal or shallowly plunging stress orientations, and often have a significant volumetric component. They also reveal that much of the observed seismicity is limited to the upper 50 m of the ice shelf. This suggests a complex system of deformation that involves the propagating rift, the region behind the rift tip, and a system of rift‐transverse crevasses. Small‐scale variations in the mechanical structure of the ice shelf, especially rift‐transverse crevasses and accreted marine ice, play an important role in modulating the rate and location of seismicity associated with the propagating ice shelf rifts.

Numerical analysis of dominant loss parameters of ferrite thin films for conduction noise absorption in microstrip lines

The dominant power loss mechanism from conduction noise in a microstrip line was verified by investigating the effects of electrical conductivity and magnetic loss on noise reflection and transmission, and of power absorption in thin film absorbers (Ni-Zn ferrite thin film with high electrical resistivity, and electrically conductive ITO and Fe3O4 films). Using a simulation model of a microstrip line attached by thin films, the S parameters and power absorption were calculated for the frequency range from 0.05 to 3 GHz. Even if the film has a large value of magnetic loss due to ferromagnetic resonance, it is predicted that power dissipation by magnetic loss would be quite small, as was predicted for the Ni-Zn ferrite films. For the conductive and magnetic Fe3O4 thin film with similar magnetic loss dispersion, the contribution of magnetic loss to total power loss is also quite small. The values of S11, S21, and power absorption were almost the same as that for conductive ITO thin film of which the electrical resistivity was on the same order as that of the Fe3O4 thin film. The simulation of power absorption by the hybrid structure of ITO/Ni-Zn ferrite and ITO/Fe3O4 also supports this conclusion.

Limitation of Industrial Phosphorus-diffused Emitters by SRH Recombination

It is commonly assumed in the PV community that Auger recombination is the dominant loss mechanism in heavily phosphorus-doped emitters of industrial Si solar cells. Contrary to this assumption, we show in this work that most of the losses are caused by Shockley-Read-Hall (SRH) recombination via defect states introduced by inactive phosphorus. Using numerical device simulations, we successfully reproduce all the measured J0e values of a series of profiles (with various concentrations of inactive phosphorus) with an effective SRH capture crosses-section for holes, σp = 7.5×10-18 cm2. The composition of inactive phosphorus (e.g. in form of clusters of interstitial phosphorus, fine and rod-shaped precipitates) may vary under different fabrication conditions, hence we do not claim that this σp value is globally valid. However, because the series of profiles is representative for many emitters in mass production, our result implies generally higher SRH than Auger recombination rates and has decisive implications on the improvement of such industrial emitters: many emitters have not yet reached the Auger-limit and they may still be significantly improved by reducing their density of inactive phosphorus.

Electrical characterization of micromachined AlN resonators at various back pressures

In this work, the fabrication process of piezoelectric cantilevers based on aluminum nitride films is presented. The cantilevers were electrically characterized in a vacuum chamber offering the possibility to close-loop control the back pressure from atmospheric conditions down to 10−2 mbar. The quality factor is an important figure of merit to evaluate the performance of micro resonators. In particular, three different modes were detected and analyzed: the first flexural mode, the second flexural mode and the first torsional mode. Among these three investigated modes, the first torsional mode has a higher quality factor compared to the others, which makes this mode more suitable for mass change detection applications as higher sensitivities are feasible. With the first torsional mode, quality factors of up to 4,557 are achievable at low chamber pressure, where the intrinsic damping is the dominating loss mechanism. The quality factor decreased to 1,050 at atmospheric pressure and is limited by viscous losses.

High efficiency back-contact back-junction thin-film monocrystalline silicon solar cells from the porous silicon process

This work demonstrates the fabrication of a 45 μm thick back-contact back-junction thin-film monocrystalline silicon solar cell from the porous silicon process with an energy conversion efficiency of 18.9%. We demonstrate an efficiency improvement of 5.4% absolute compared to our prior record of 13.5% for back-contact back-junction thin-film monocrystalline silicon solar cells. This increase in efficiency is achieved by reducing the recombination at the base contact using a back surface field and by increasing the generation with a front texture. We investigate the loss mechanisms in the cell using finite element simulations. A free energy loss analysis based on experiments and simulations determines the dominating loss mechanisms. The efficiency loss by base recombination is 0.8% absolute and the loss by base contact recombination is 0.5% absolute in the 18.9% efficiency cell.

Photocurrent extraction efficiency in colloidal quantum dot photovoltaics

The efficiency of photocurrent extraction was studied directly inside operating Colloidal Quantum Dot (CQD) photovoltaic devices. A model was derived from first principles for a thin film p-n junction with a linearly spatially dependent electric field. Using this model, we were able to clarify the origins of recent improvement in CQD solar cell performance. From current-voltage diode characteristics under 1 sun conditions, we extracted transport lengths ranging from 39 nm to 86 nm for these materials. Characterization of the intensity dependence of photocurrent extraction revealed that the dominant loss mechanism limiting the transport length is trap-mediated recombination.

Laboratory measurements of shear wave properties in marine sediments using bender element transducers

In shallow water, acoustic propagation is often controlled by the properties of the seabed. In environments where the shear speed in the sediment approaches the sound speed in the water column, wave conversion at the bottom has been identified as a dominant loss mechanism. The ultimate goal of this work is to develop a device for measuring both compressional and shear wave properties in situ in marine sediments. This work presents laboratory measurements using bender element transducers to measure shear wave properties in marine sediments. The transducer consists of two long, thin piezoceramic plates rigidly bonded along their lengths driven 180 degrees out of phase in the length extensional mode so that the differential change in length of each plate causes the composite element to bend [J. Acoust. Soc. Am 63, 1643–1645 (1978)]. When the transducer is embedded in a medium, mechanical motion is transferred from the bender to the particles in the medium in a manner such that particle motion is perpendicular to the length dimension of the element. Laboratory measurements demonstrate bender sensitivity to low amplitude shear waves in sandy and muddy sediments. [Work supported by ARL:UT IR&D.]

Correlations between the in situ acoustic properties and geotechnical parameters of sediments in the Yellow Sea, China

Knowledge about the marine sediment acoustic properties is a key to understanding wave propagation in sediments and is very important for military oceanography and ocean engineering. We developed a hydraulic-drived self-contained in situ sediment acoustic measurement system, and measured for the first time the in situ acoustic properties of sediments on 78 stations in the Yellow Sea, China, by employing this system. The relationships between the in situ measured acoustic properties and the onboard or laboratory determined geotechnical parameters were analyzed. Porosity was found to be the dominant factor in reducing velocity in a quadratic fashion; velocity showed an increment with bulk density and a decrement with mean grain size and clay content both with a nonlinear dependence; acoustic attenuation showed a bell-shaped correlation with porosity and mean grain size but reduced with clay content of the sediments. The attenuation results indicate that intergrain friction rather than viscous interactions between pore fluid and solid grains is the dominant loss mechanism in our marine sediments. The relationships established would be used to predict the geotechnical parameters from in situ measured acoustic properties and vice versa, as well as being an indicator of the seafloor processes, potential gas bubbles hazard and gas hydrates resources or other suitable targets of acoustic surveys.

Response of global soil consumption of atmospheric methane to changes in atmospheric climate and nitrogen deposition

AbstractSoil consumption of atmospheric methane plays an important secondary role in regulating the atmospheric CH4 budget, next to the dominant loss mechanism involving reaction with the hydroxyl radical (OH). Here we used a process‐based biogeochemistry model to quantify soil consumption during the 20th and 21st centuries. We estimated that global soils consumed 32–36 Tg CH4 yr−1 during the 1990s. Natural ecosystems accounted for 84% of the total consumption, and agricultural ecosystems only consumed 5 Tg CH4 yr−1 in our estimations. During the twentieth century, the consumption rates increased at 0.03–0.20 Tg CH4 yr−2 with seasonal amplitudes increasing from 1.44 to 3.13 Tg CH4 month−1. Deserts, shrublands, and xeric woodlands were the largest sinks. Atmospheric CH4 concentrations and soil moisture exerted significant effects on the soil consumption while nitrogen deposition had a moderate effect. During the 21st century, the consumption is predicted to increase at 0.05‐1.0 Tg CH4 yr−2, and total consumption will reach 45–140 Tg CH4 yr−1 at the end of the 2090s, varying under different future climate scenarios. Dry areas will persist as sinks, boreal ecosystems will become stronger sinks, mainly due to increasing soil temperatures. Nitrogen deposition will modestly reduce the future sink strength at the global scale. When we incorporated the estimated global soil consumption into our chemical transport model simulations, we found that nitrogen deposition suppressed the total methane sink by 26 Tg during the period 1998–2004, resulting in 6.6 ppb higher atmospheric CH4 mixing ratios compared to without considering nitrogen deposition effects. On average, a cumulative increase of every 1 Tg soil CH4 consumption decreased atmospheric CH4 mixing ratios by 0.26 ppb during the period 1998–2004.

Investigation of carrier spill‐over in InGaN‐based light‐emitting diodes by temperature dependences of resonant photoluminescence and open‐circuit voltage

AbstractIn InGaN‐based light‐emitting diodes (LEDs), carrier leakage to the outside of the active quantum wells (QWs) has been considered as one of the dominant carrier loss mechanisms at high current densities. In order to verify the carrier spill‐over phenomenon, we compared the electroluminescence (EL) efficiency, the resonant photoluminescence (PL) efficiency, and the open‐circuit voltage as functions of current, temperature, and excitation laser power. Very similar tendencies were experimentally observed among the three physical quantities, which implied that carriers were spilt over out of the QWs even in the resonant PL experiment.

Measuring TE1 mode Losses in Terahertz Parallel-Plate Waveguides

The parallel-plate waveguide (PPWG) has drawn considerable research interest [1–12], since it was first reported to support the undistorted propagation of broadband terahertz (THz) pulses. From that time, the transverse-electromagnetic (TEM) mode of the PPWG has been the popular choice for measurements due to its low loss, ease of quasi-optic coupling, and negligible dispersion as a result of the lack of a cutoff. Unlike the TEM mode, the lowest order transverse-electric (TE1) mode has a cutoff frequency, and thus this mode was largely unexplored in the THz regime until recently [13, 14]. This recent work showed that the cutoff frequency could be moved to lower frequencies to reduce the dispersion by increasing the plate separation, while matching the input beam size to the plate separation to realize dominantly single-mode propagation. This same work predicted the possibility of realizing ultra-low ohmic losses in the dB/km range by again utilizing the TE1 mode of this waveguide. These low ohmic losses could permit long-distance transport of THz radiation. However, with such long propagation distances a new concern arises: energy leakage out of the unconfined sides of the PPWG due to diffraction. In fact, this would be the dominant loss mechanism in the case under consideration, where the ohmic losses are virtually negligible. We addressed this diffraction problem in a recent article [15], where we showed that it is possible to inhibit diffraction losses for the TE1 mode by using a waveguide with slightly concave plates. Via a simple “bouncing plane wave” analysis, we demonstrated how to determine an ideal radius of curvature for the inner surfaces, for a waveguide operating at a given THz frequency. This is governed by a confocal condition, R 1⁄4 2bv .

InGaN light-emitting diodes: Efficiency-limiting processes at high injection

The authors discuss a relatively comprehensive theoretical and experimental study aimed on unveiling the dominant efficiency loss mechanism at high injection levels in InGaN light-emitting diodes(LEDs), which still limits their application for general lighting despite the breathtaking performance demonstration. A large body of theoretical and experimental data ascribes the observed efficiency loss to overflow of hot electrons aggravated by nonuniform distribution of carriers in the active region as the primary origin of the efficiency droop-phenomenon, but Auger recombination has also been invoked as the genesis of the efficiency loss. The electron overflowand the associated efficiency loss can be reduced substantially by inserting, in the n-side of the InGaN active region, an InGaN stair-case electron injector (SEI) with a step-like increased indium composition to operate as an “electron cooler.” In contrast to electron-blocking layer usually employed to prevent the electron leakage from the active region, the SEI does not impede hole injection due to the absence of valence band offset with p-GaN. Moreover, SEI does not generate piezoelectric polarizationfield in addition to differential spontaneous polarization field that pulls down the conduction band at the AlGaN/GaN interface aggravating the electron rollover. In terms of the active region design, owing to their high three-dimensional density of states, it is argued that double heterostructures (DHs) are more attractive for general-lighting LEDs than necessarily quantum wells. The authors demonstrate that DH-based LED active regions,particularly wide ones and those composed of multiple DHs separated by thin (3 nm)In0.06Ga0.94N barriers of reduced barrier height, meant to allow efficient hole transport across the active regions, naturally act as an electron cooler, thus considerably reducing the electron overflow at high injection. However, a wide separation of electron and hole distribution functions in DHs wider than 6 nm substantially reduces the radiative recombination efficiency at injection current densities below∼200 A/cm2. Consequently, the LEDs with dual 6 nm and quad (4×) 3 nm DHs separated by 3-nm In0.06Ga0.94N barriers exhibit the highest external quantum efficiency with substantially reduced efficiency degradation at injection current densities of special interest for low-voltage general-lighting applications. The authors conclude that, for achieving the highest possible LED efficiency, it is imperative that optimum the SEI and the active region should be designed to operate in unison.

Observations of peroxyacetyl nitrate (PAN) in the upper troposphere by the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS)

Abstract. Peroxyacetyl nitrate (CH3CO·O2NO2, abbreviated as PAN) is a trace molecular species present in the troposphere and lower stratosphere due primarily to pollution from fuel combustion and the pyrogenic outflows from biomass burning. In the lower troposphere, PAN has a relatively short lifetime and is principally destroyed within a few hours through thermolysis, but it can act as a reservoir and carrier of NOx in the colder temperatures of the upper troposphere, where UV photolysis becomes the dominant loss mechanism. Pyroconvective updrafts from large biomass burning events can inject PAN into the upper troposphere and lower stratosphere (UTLS), providing a means for the long-range transport of NOx. Given the extended lifetimes at these higher altitudes, PAN is readily detectable via satellite remote sensing. A new PAN data product is now available for the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) version 3.0 data set. We report observations of PAN in boreal biomass burning plumes recorded during the BORTAS (quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites) campaign (12 July to 3 August 2011). The retrieval method employed by incorporating laboratory-recorded absorption cross sections into version 3.0 of the ACE-FTS forward model and retrieval software is described in full detail. The estimated detection limit for ACE-FTS PAN is 5 pptv, and the total systematic error contribution to the ACE-FTS PAN retrieval is ~ 16%. The retrieved volume mixing ratio (VMR) profiles are compared to coincident measurements made by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument on the European Space Agency (ESA) Environmental Satellite (ENVISAT). The MIPAS measurements demonstrated good agreement with the ACE-FTS VMR profiles for PAN, where the measured VMR values are well within the associated measurement errors for both instruments and comparative measurements differ no more than 70 pptv. The ACE-FTS PAN data set is used to obtain zonal mean distributions of seasonal averages from ~ 5–20 km. A strong seasonality is clearly observed for PAN concentrations in the global UTLS. Since the principal source of PAN in the UTLS is due to lofted biomass burning emissions from the pyroconvective updrafts created by large fires, the observed seasonality in enhanced PAN coincides with fire activity in different geographical regions throughout the year.

Fast ion confinement and stability in a neutral beam injected reversed field pinch

The behavior of energetic ions is fundamentally important in the study of fusion plasmas. While well-studied in tokamak, spherical torus, and stellarator plasmas, relatively little is known in reversed field pinch plasmas about the dynamics of fast ions and the effects they cause as a large population. These studies are now underway in the Madison Symmetric Torus with an intense 25 keV, 1 MW hydrogen neutral beam injector (NBI). Measurements of the time-resolved fast ion distribution via a high energy neutral particle analyzer, as well as beam-target neutron flux (when NBI fuel is doped with 3–5% D2) both demonstrate that at low concentration the fast ion population is consistent with classical slowing of the fast ions, negligible cross-field transport, and charge exchange as the dominant ion loss mechanism. A significant population of fast ions develops; simulations predict a super-Alfvénic ion density of up to 25% of the electron density with both a significant velocity space gradient and a sharp radial density gradient. There are several effects on the background plasma including enhanced toroidal rotation, electron heating, and an altered current density profile. The abundant fast particles affect the plasma stability. Fast ions at the island of the core-most resonant tearing mode have a stabilizing effect, and up to 60% reduction in the magnetic fluctuation amplitude is observed during NBI. The sharp reduction in amplitude, however, has little effect on the underlying magnetic island structure. Simultaneously, beam driven instabilities are observed as repetitive ∼50 μs bursts which coincide with fast particle redistribution; data indicate a saturated core fast ion density well below purely classical predictions.

Design of GaAs Solar Cells Operating Close to the Shockley–Queisser Limit

With recent advances in device design, single-junction GaAs solar cells are approaching their theoretical efficiency limits. Accurate numerical simulation may offer insights that can help close the remaining gap between the practical and theoretical limits. Significant care must be taken, however, to ensure that the simulation is self-consistent and properly comprehends thermodynamic limits. In this paper, we use rigorous photon recycling simulation coupled with carrier transport simulation to identify the dominant loss mechanisms that limit the performance of thin-film GaAs solar cells.

Quantum effects in silicon for photovoltaic applications

AbstractQuantum confinement effects in silicon might help to overcome the theoretical efficiency limit of ∼33% of silicon cells [Shockley and Queisser, J. Appl. Phys. 32, 510 (1961)]. The dominant loss mechanism in solar cells is the thermalization of photo‐excited carriers to the band gap. Engineering of the band gap makes it in principle possible to obtain a conversion efficiency of ∼44% [Pavesi and Turan (eds.), Silicon Nanocrystals, Fundamentals Synthesis and Application (Wiley‐VCH, Berlin, 2010)]. Band gap engineering is achieved by controlling the silicon‐nanocrystal (Si‐NC) dimension. We currently use Si‐NCs in solar cells to modify the solar spectrum in a way that the incoming light is more efficiently converted into electricity and also, to realize multiple junction cells to minimize thermalization losses. The Si‐NC energy of the band gap and the Si‐NC optical properties depend strongly on the NC size, which can be controlled by the processing parameters. Therefore, light management is possible by varying the refractive index of the Si‐NCs to improve the antireflective coating (ARC) and by conversion of absorbed high energy photons re‐emitted in the red spectral region of light. Thus, in this approach Si‐NCs are used as spectral down‐shifters and for ARC optimization. Further, NCs allow the fabrication of higher band gap solar cells, to be used on top of standard silicon cells as tandem cell elements. Due to the tunability of the band gap, it is possible to construct all silicon multilayer or tandem cells. The concepts and research results for both ideas will be presented.

Correlation of Absorption Profile and Fill Factor in Organic Solar Cells: The Role of Mobility Imbalance

AbstractWe investigate the role of the spatial absorption profile within bulk heterojunction small molecule solar cells comprising a 50 nm ZnPc:C60 active layer. Exploiting interference effects the absorption profile is varied by both the illumination wavelength and the thickness of an optical spacer layer adjacent to the reflecting electrode. The fill factor under 1 sun illumination is observed to change from 43 to 49% depending on the absorption profile which approximately equals the charge‐carrier generation profile. It is shown by varying the mixing ratio between ZnPc and C60 that the importance of the generation profile is correlated with the imbalance of mobilities. Therefore, it is concluded that non‐geminate recombination is the dominating loss mechanism in these devices. Numerical drift‐diffusion simulations reproduce the experimental observations showing that charge carrier extraction is more efficient if charge carriers are generated close to the contact collecting the less mobile charge carrier type. Furthermore, this effect can explain the dependence of the internal quantum efficiency measured at short circuit on wavelength and implies that the spectral mismatch for a given solar simulator and device depends on the applied voltage.

Evaluating the fate and behaviour of cyclic volatile methyl siloxanes in two contrasting North American lakes using a multi-media model

The behaviour of cyclic volatile methyl siloxanes (cVMS) in lakes was explored using a fugacity-based steady-state non-equilibrium multimedia fate and transport model (a modified version of QWASI). Three substances were investigated: Octamethylcyclotetrasiloxane (D4), Decamethylcyclopentasiloxane (D5) and Dodecamethylcyclohexasiloxane (D6) in two contrasting North American lakes: Lake Ontario and Lake Pepin (a natural shallow lake on the Mississippi river between Minnesota and Wisconsin). Values for the principal partition coefficients and the hydrolysis rate constant were adjusted for the mean annual temperatures of each lake. Hydrolysis rate constants were also adjusted for the fraction of chemical calculated to be in the freely dissolved phase. Half-lives in sediment were calculated from partitioning theory, assuming hydrolysis can occur only in the dissolved phase. Best estimates of substance-specific emissions were obtained by combining current per capita approximations of usage and fraction lost to domestic waste water, the population of the lake catchment and cVMS removal in waste water treatment. Concentrations were generally lower and chemical residence times longer in Lake Ontario than in Lake Pepin, owing to greater depth, a higher hydraulic retention time and a higher degree of dilution. Overall persistence in Lake Pepin is significantly influenced by the high rate of sediment burial assumed in the model, as well as by a relatively high rate of water discharge. Despite the many similarities of the compounds considered, the dominant loss mechanisms vary significantly and are not the same in each lake system. This highlights the pitfalls of subjective evaluation of chemical fate and illustrates the important role which models have to play in providing a quantitative framework for assessing chemical behaviour objectively under the influence of a complex and interacting set of factors.

CZTS nanocrystals: a promising approach for next generation thin film photovoltaics

Cu2ZnSn(S,Se)4 (CZTSSe) has received considerable attention as a material capable of driving the development of low-cost and high performance photovoltaics. Its high absorption coefficient, optimal band gap, and non-toxic, naturally abundant elemental constituents give it a number of advantages over most thin film absorber materials. In this manuscript, we discuss the current status of CZTSSe photovoltaics, and provide a comprehensive review of Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) nanocrystal (NCs)-based fabrication methods and solar cell characteristics. The focus will be on the relevant synthetic chemistry, film deposition, and the production of high efficiency photovoltaic devices. Various colloidal synthesis routes are currently used to form the highest quality CZTSSe film from the nanocrystals with controllable phase, size, shape, composition, and surface ligands. A variety of recipes are summarized for producing nanocrystal inks that are appropriate for forming CZTSSe absorber materials with a wide range of controllable optoelectronic properties. Deposition and post-processing, such as annealing and selenization treatments, play an important role in defining the phase and structure of the resulting material. Various film treatment strategies are outlined here, and their resulting material quality, device performance, and dominant photovoltaic loss mechanisms are discussed. Suggestions regarding needed improvements and future research directions are provided based on the current field of available literature.