11 results
Search Results
Now showing 1 - 10 of 11
- ItemImpact of the capture time on the series resistance of quantum-well diode lasers(Bristol : IOP Publ., 2020) Boni, A.; Wünsche, H.J.; Wenzel, H.; Crump, P.Electrons and holes injected into a semiconductor heterostructure containing quantum wells are captured with a finite time. We show theoretically that this very fact can cause a considerable excess contribution to the series resistivity and this is one of the main limiting factors to higher efficiency for GaAs based high-power lasers. The theory combines a standard microscopic-based model for the capture-escape processes in the quantum well with a drift-diffusion description of current flow outside the quantum well. Simulations of five GaAs-based devices differing in their Al-content reveal the root-cause of the unexpected and until now unexplained increase of the series resistance with decreasing heat sink temperature measured recently. The finite capture time results in resistances in excess of the bulk layer resistances (decreasing with increasing temperature) from 1 mΩ up to 30 mΩ in good agreement with the experiment. © 2020 The Author(s). Published by IOP Publishing Ltd.
- ItemFirst process-based simulations of climate change impacts on global tea production indicate large effects in the World’s major producer countries(Bristol : IOP Publ., 2020) Beringer, Tim; Kulak, Michal; Müller, Christoph; Schaphoff, Sibyll; Jans, YvonneModeling of climate change impacts have mainly been focused on a small number of annual staple crops that provide most of the world's calories. Crop models typically do not represent perennial crops despite their high economic, nutritional, or cultural value. Here we assess climate change impacts on global tea production, chosen because of its high importance in culture and livelihoods of people around the world. We extended the dynamic global vegetation model with managed land, LPJmL4, global crop model to simulate the cultivation of tea plants. Simulated tea yields were validated and found in good agreement with historical observations as well as experiments on the effects of increasing CO2 concentrations. We then projected yields into the future under a range of climate scenarios from the Inter-Sectoral Impact Model Intercomparison Project. Under current irrigation levels and lowest climate change scenarios, tea yields are expected to decrease in major producing countries. In most climate scenarios, we project that tea yields are set to increase in China, India, and Vietnam. However, yield losses are expected to affect Kenya, Indonesia, and Sri Lanka. If abundant water supply and full irrigation is assumed for all tea cultivation areas, yields are projected to increase in all regions.
- ItemStructural and luminescence imaging and characterisation of semiconductors in the scanning electron microscope(Bristol : IOP Publ., 2020) Trager-Cowan, C.; Alasmari, A.; Avis, W.; Bruckbauer, J.; Edwards, P.R.; Ferenczi, G.; Hourahine, B.; Kotzai, A.; Kraeusel, S.; Kusch, G.; Martin, R.W.; McDermott, R.; Naresh-Kumar, G.; Nouf-Allehiani, M.; Pascal, E.; Thomson, D.; Vespucci, S.; Smith, M.D.; Parbrook, P.J.; Enslin, J.; Mehnke, F.; Kuhn, C.; Wernicke, T.; Kneissl, M.; Hagedorn, S.; Knauer, A.; Walde, S.; Weyers, M.; Coulon, P.-M.; Shields, P.A.; Bai, J.; Gong, Y.; Jiu, L.; Zhang, Y.; Smith, R.M.; Wang, T.; Winkelmann, A.The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and cathodoluminescence (CL) hyperspectral imaging provide complementary information on the structural and luminescence properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres. EBSD provides crystal orientation, crystal phase and strain analysis, whilst ECCI is used to determine the planar distribution of extended defects over a large area of a given sample. CL reveals the influence of crystal structure, composition and strain on intrinsic luminescence and/or reveals defect-related luminescence. Dark features are also observed in CL images where carrier recombination at defects is non-radiative. The combination of these techniques is a powerful approach to clarifying the role of crystallography and extended defects on a material's light emission properties. Here we describe the EBSD, ECCI and CL techniques and illustrate their use for investigating the structural and light emitting properties of UV-emitting nitride semiconductor structures. We discuss our investigations of the type, density and distribution of defects in GaN, AlN and AlGaN thin films and also discuss the determination of the polarity of GaN nanowires. © 2020 The Author(s). Published by IOP Publishing Ltd.
- ItemThe decarbonisation of Europe powered by lifestyle changes(Bristol : IOP Publ., 2021) Costa, Luis; Moreau, Vincent; Thurm, Boris; Yu, Wusheng; Clora, Francesco; Baudry, Gino; Warmuth, Hannes; Hezel, Bernd; Seydewitz, Tobias; Rankovic, Ana; Kelly, Garret; Kropp, Jürgen P.Decision makers increasingly recognise the importance of lifestyle changes in reaching low emission targets. How the mitigation potential of changes in mobility, dietary, housing or consumption behaviour compare to those of ambitious technological changes in terms of decarbonisation remains a key question. To evaluate the interplay of behaviour and technological changes, we make use of the European Calculator model and show that changes in behaviour may contribute more than 20% of the overall greenhouse gas (GHG) emission reductions required for net-zero by 2050. Behaviour and technology-oriented scenarios are tested individually and in combination for the EU plus the UK and Switzerland. The impacts of behavioural change vary across sectors, with significant GHG emission reduction potential and broader benefits. Changes in travel behaviour limit the rising demand for electricity, natural resources and infrastructure costs from the electrification of passenger transport. Adopting a healthy diet reduces emissions substantially compared to intensifying agricultural practices, while at the same time making cropland available for conservation or bioenergy crops. The trade-offs between energy and food may be substantially alleviated when deploying technological and behavioural changes simultaneously. The results suggest that without behavioural change, the dependency of Europe on carbon removal technologies for its net-zero ambitions increases. Structural changes will be necessary to achieve full decarbonisation by 2050, yet changes in lifestyles are crucial, contributing to achieving climate targets sooner.
- ItemTheoretical investigation of a miniature microwave driven plasma jet(Bristol : IOP Publ., 2020) Klute, Michael; Porteanu, Horia-Eugen; Stefanovic, Ilija; Heinrich, Wolfgang; Awakowicz, Peter; Brinkmann, Ralf PeterRadio frequency driven plasma jets are compact plasma sources which are used in many advanced fields such as surface engineering or biomedicine. The MMWICP (miniature micro wave ICP) is a particular variant of that device class. Unlike other plasma jets which employ capacitive coupling, the MMWICP uses the induction principle. The jet is integrated into a miniature cavity structure which realizes an LC-resonator with a high quality factor. When excited at its resonance frequency, the resonator develops a high internal current which—transferred to the plasma via induction—provides an efficient source of RF power. This work presents a theoretical model of the MMWICP. The possible operation points of the device are analyzed. Two different regimes can be identified, the capacitive E-mode with a plasma density of ne ≈ 5 × 1017 m−3, and the inductive H-mode with densities of ne ⩾ 1019 m−3. The E to H transition shows a pronounced hysteresis behavior.
- ItemLow-index quantum-barrier single-pass tapered semiconductor optical amplifiers for efficient coherent beam combining(Bristol : IOP Publ., 2020) Albrodt, P.; Niemeyer, M.; Elattar, M.; Hamperl, J.; Blume, G.; Ginolas, A.; Fricke, J.; Maaßdorf, A.; Georges, P.; Lucas-Leclin, G.; Paschke, K.; Crump, P.The requirements for coherent combination of high power GaAs-based single-pass tapered amplifiers are studied. Changes to the epitaxial layer structure are shown to bring higher beam quality and hence improved combining efficiency for one fixed device geometry. Specifically, structures with large vertical near field and low wave-guiding from the active region show 10% higher beam quality and coherent combining efficiency than reference devices. As a result, coherent combining efficiency is shown to be limited by beam quality, being directly proportional to the power content in the central lobe across a wide range of devices with different construction. In contrast, changes to the in-plane structure did not improve beam quality or combining efficiency. Although poor beam quality does correlate with increased optical intensities near the input aperture, locating monolithically-integrated absorption regions in these areas did not lead to any performance improvement. However, large area devices with subsequently improved cooling do achieve higher output powers. Phase noise can limit coherent combining, but this is shown to be small and independent of device design. Overall, tapered amplifiers are well suited for high power coherent combining applications. © 2020 The Author(s). Published by IOP Publishing Ltd.
- ItemHigh-brightness broad-area diode lasers with enhanced self-aligned lateral structure(Bristol : IOP Publ., 2020) Elatta, M.; Brox, O.; Della Casa, P.; Maaßdorf, A.; Martin, D.; Wenzel, H.; Knigge, A.; Crump, P.Broad-area diode lasers with increased brightness and efficiency are presented, which are fabricated using an enhanced self-aligned lateral structure by means of a two-step epitaxial growth process with an intermediate etching step. In this structure, current-blocking layers in the device edges ensure current confinement under the central stripe, which can limit the detrimental effects of current spreading and lateral carrier accumulation on beam quality. It also minimizes losses at stripe edges, thus lowering the lasing threshold and increasing conversion efficiency, while maintaining high polarization purity. In the first realization of this structure, the current block is integrated within an extreme-triple-asymmetric epitaxial design with a thin p-doped side, meaning that the distance between the current block and the active zone can be minimized without added process complexity. Using this configuration, enhanced self-aligned structure devices with 90 µm stripe width and 4 mm resonator length show up to 20% lower threshold current, 21% narrower beam waist, and slightly higher (1.03 ) peak efficiency in comparison to reference devices with the same dimensions, while slope, divergence angle and polarization purity remain almost unchanged. These results correspond to an increase in brightness by up to 25%, and measurement results of devices with varying stripe widths follow the same trend. © 2020 The Author(s). Published by IOP Publishing Ltd.
- ItemThe 2020 UV emitter roadmap(Bristol : IOP Publ., 2020) Amano, Hiroshi; Collazo, Ramón; De Santi, Carlo; Einfeldt, Sven; Funato, Mitsuru; Glaab, Johannes; Hagedorn, Sylvia; Hirano, Akira; Hirayama, Hideki; Ishii, Ryota; Kashima, Yukio; Kawakami, Yoichi; Kirste, Ronny; Kneissl, Michael; Martin, Robert; Mehnke, Frank; Meneghini, Matteo; Ougazzaden, Abdallah; Parbrook, Peter J.; Rajan, Siddharth; Reddy, Pramod; Römer, Friedhard; Friedhard, Jan; Sarkar, Biplab; Scholz, Ferdinand; Schowalter, Leo J; Shields, Philip; Sitar, Zlatko; Sulmoni, Luca; Wang, Tao; Wernicke, Tim; Weyers, Markus; Witzigmann, Bernd; Wu, Yuh-Renn; Wunderer, Thomas; Zhang, YueweiSolid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm - due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments. © 2020 IOP Publishing Ltd.
- ItemWavelength stabilized high pulse power 48 emitter laser bars for automotive light detection and ranging application(Bristol : IOP Publ., 2020) Klehr, Andreas; Liero, Armin; Christopher, Heike; Wenzel, Hans; Maaßdorf, Andre; Della Casa, Pietro; Fricke, Jörg; Ginolas, Arnim; Knigge, AndreaDiode lasers generating optical pulses with high peak power and lengths in the nanosecond range are key components for light detection and ranging systems, e.g. for autonomous driving and object detection. We present here an internally wavelength stabilized distributed Bragg reflector broad area laser bar with 48 emitters. The vertical structure based on AlGaAs (confinement and cladding layers) and InGaAs (active quantum well) is specifically optimized for wavelength-stabilized pulsed operation, applying a surface Bragg grating with high reflectivity. The bar is electrically driven by a new in-house developed high-speed driver based on GaN transistors providing current pulses with amplitudes of up to 1000 A and a repetition frequency of 10 kHz. The generated 4 ns to 10 ns long optical pulses are nearly rectangular shaped and reach a pulse peak power in excess of 600 Watts at 25 °C. The optical spectrum with a centre wavelength of about 900 nm has a width of 0.15 nm (FWHM) with a side mode suppression ratio > 30 dB. © 2020 IOP Publishing Ltd.
- ItemExploring uncertainties in global crop yield projections in a large ensemble of crop models and CMIP5 and CMIP6 climate scenarios(Bristol : IOP Publ., 2021) Mueller, Christoph; Franke, James; Jaegermeyr, Jonas; Ruane, Alex C.; Elliott, Joshua; Moyer, Elisabeth; Heinke, Jens; Falloon, Pete D.; Folberth, Christian; Francois, LouisConcerns over climate change are motivated in large part because of their impact on human society. Assessing the effect of that uncertainty on specific potential impacts is demanding, since it requires a systematic survey over both climate and impacts models. We provide a comprehensive evaluation of uncertainty in projected crop yields for maize, spring and winter wheat, rice, and soybean, using a suite of nine crop models and up to 45 CMIP5 and 34 CMIP6 climate projections for three different forcing scenarios. To make this task computationally tractable, we use a new set of statistical crop model emulators. We find that climate and crop models contribute about equally to overall uncertainty. While the ranges of yield uncertainties under CMIP5 and CMIP6 projections are similar, median impact in aggregate total caloric production is typically more negative for the CMIP6 projections (+1% to −19%) than for CMIP5 (+5% to −13%). In the first half of the 21st century and for individual crops is the spread across crop models typically wider than that across climate models, but we find distinct differences between crops: globally, wheat and maize uncertainties are dominated by the crop models, but soybean and rice are more sensitive to the climate projections. Climate models with very similar global mean warming can lead to very different aggregate impacts so that climate model uncertainties remain a significant contributor to agricultural impacts uncertainty. These results show the utility of large-ensemble methods that allow comprehensively evaluating factors affecting crop yields or other impacts under climate change. The crop model ensemble used here is unbalanced and pulls the assumption that all projections are equally plausible into question. Better methods for consistent model testing, also at the level of individual processes, will have to be developed and applied by the crop modeling community.