High-Temperature Annealing of AlGaN

In the past few years, high-temperature annealing of AlN has become a proven method for providing AlN layers with low dislocation densities. Herein, the example of Al0.77Ga0.23N is used to investigate whether annealing can also improve the material quality of the ternary alloy. A detailed analysis of the influence of annealing temperature on structural and optical material properties is presented. It is found that with increasing annealing temperature, the threading dislocation density can be lowered from an initial value of 6.0 × 109 down to 2.6 × 109 cm−2. Ga depletion at the AlGaN surface and Ga diffusion into the AlN buffer layer are observed. After annealing, the defect luminescence between 3 and 4 eV is increased, accompanied by an increase in the oxygen concentration by about two orders of magnitude. Furthermore, due to annealing optical absorption at 325 nm (3.8 eV) occurs, which increases with increasing annealing temperature. It is assumed that the reason for this decrease in ultraviolet (UV) transmittance is the increasing number of vacancies caused by the removal of group-III and N atoms from the AlGaN lattice during annealing

High‐Temperature Annealing and Patterned AlN/Sapphire Interfaces

Using the example of epitaxial lateral overgrowth of AlN on trench-patterned AlN/sapphire templates, the impact of introducing a high-temperature annealing step into the process chain is investigated. Covering the open surfaces of sapphire trench sidewalls with a thin layer of AlN is found to be necessary to preserve the trench shape during annealing. Both the influence of annealing temperature and annealing duration are investigated. To avoid the deformation of the AlN/sapphire interface during annealing, the annealing duration or annealing temperature must be low enough. Annealing for 1 h at 1730 °C is found to allow for the lowest threading dislocation density of 3.5 × 108 cm−2 in the subsequently grown AlN, while maintaining an uncracked smooth surface over the entire 2 in. wafer. Transmission electron microscopy study confirms the defect reduction by high-temperature annealing and reveals an additional strain relaxation mechanism by accumulation of horizontal dislocation lines at the interface between annealed and nonannealed AlN. By applying a second annealing step, the dislocation density can be further reduced to 2.5 × 108 cm−2

Assessment of a Novel Process to Enable Roll-to-roll Production of Catalyst Coated Membranes

Hydrogen is becoming an increasingly important energy carrier within the next few years for many different applications within different industries, such as chemical industry, steel production or mobility. Furthermore, it can be used to store excess energy from renewable energy plants. Within this context, proton exchange membrane-electrolyzer and -fuel cells represent integral parts of this value chain, as they are responsible for hydrogen production and its reconversion to electricity. Both technologies have in common that they need a catalyst coated membrane (CCM) to enable the electrochemical conversion. Since nowadays electrolyzer and fuel cell production is still characterized by small-scale production processes, suitable large-scale production lines will be necessary for the market ramp-up. To address these challenges, a novel coating process for the CCMs is proposed by using a re-coatable transfer belt at which the catalyst ink is coated and dried first. Afterwards, the catalyst ink is transferred onto the membrane by applying a hot-pressing process. Within the presented research, the hot press process is focussed and assessed for the proposed concept. Therefore, the upstream production processes, such as catalyst ink production, coating and drying are described. A design of experiments is then conducted to investigate the applied process parameters within the hot-pressing process and optimized parameters are analysed. Afterwards, re-coating the transfer belt is tested, and the long-term usability of the employed belt is assessed by focussing structural changes

The Impact of AlN Templates on Strain Relaxation Mechanisms during the MOVPE Growth of UVB-LED Structures

Strain relaxation mechanisms in AlGaN based light emitting diodes emitting in the ultraviolet B spectral range (UVB-LEDs) grown on different AlN/sapphire templates are analyzed by combining in situ reflectivity and curvature data with transmission electron microscopy. In particular, the impact of dislocation density, surface morphology, and lattice constant of the AlN/sapphire templates is studied. For nonannealed AlN/templates with threading dislocation densities (TDDs) of 4 × 109 and 3 × 109 cm−2 and different surface morphologies strain relaxation takes place mostly by conventional ways, such as inclination of threading dislocation lines and formation of horizontal dislocation bands. In contrast, a TDD reduction down to 1 × 109 cm−2 as well as a reduction of the lattice constant of high temperature annealed AlN template leads to drastic changes in the structure of subsequently grown AlGaN layers, e.g., to transformation to helical dislocations and enhanced surface enlargement by formation of macrofacets. For the growth of strongly compressively strained AlGaN layers for UVB-LEDs the relaxation mechanism is strongly influenced by the absolute values of TDD and the lattice constant of the AlN templates and is less influenced by their surface morphology

Development Of Scalable Production Concepts For The Cost-Efficient Assembly Of PEM Fuel Cell Systems For Mobile Applications

Polymer-Electrolyte-Membrane (PEM) fuel cell systems will contribute to enable climate-neutral mobility through the chemical reaction of hydrogen and oxygen. PEM fuel cells address applications which are hardly decarbonized by HV batteries. But apart from its advantages, such as short refueling times and higher energy densities related to batteries or locally emission-free operation compared to conventional drivetrains, the fuel cell technology still faces challenges that inhibit its wide market penetration. Especially the low production volumes result in costly manufacturing processes. The assembly of the fuel cell stack and balance-of-plant components to a system is predominantly of manufactory character. There is a consensus in the literature that scaling up the production is associated with cost reduction effects. But in order to increase the demand that justifies a growth in unit numbers, the costs per system have to be reduced. With regard to this so-called "hen-and-egg problem", a reduction of production costs for small output numbers is necessary, while already considering the future necessity to scale the production. This paper discusses the development of scalable production concepts for PEM fuel cell system assemblies. In addition to a modular production concept, the associated production scenarios are also considered. For a generic fuel cell system, a possible assembly sequence and assembly tasks are derived from the bill of materials. The assembly durations for the individual steps are then determined according to the Methods-Time-Measurement (MTM) methodology. This methodological approach is intended to provide an estimate for each process step in the assembly and can be transferred to other fuel cell systems. The paper shows how a bill of materials can be used to estimate the cycle time for a system, but also the cycle time for defined stations. In addition, by considering different scaling mechanisms, further improvements in the assembly process are shown, based on the results from the MTM analysis

Synergy Analysis Methodology For Decreasing Fuel Cell Production Costs

For meeting CO2 emission targets in the mobility sector, decarbonization efforts of referring applications are necessary. Fuel cell electric vehicles powered by hydrogen demonstrate a viable option to achieve those targets, especially taking the targets of heavy-duty applications into consideration. Higher ranges, short fueling durations and locally emission-free transport represent advantages offered by fuel cells in comparison to internal combustion engines or battery-electric powertrains. However, production costs of fuel cells are still a drawback. Latest analyses show that the utilization of scale effects even in early technology adaption phases can heavily decrease production costs. As the cell structure of fuel cells and electrolyzers show many similarities, the assumption of production synergies is made. Taking advantage of referring synergies, increased production volumes and thus decreased production cost are assumed for fuel cells. This paper introduces a methodology to identify synergies between fuel cell and electrolyzer production. The methodology is used to evaluate a company's production process portfolio on the example of the three alternative coating processes, based on an initial evaluation of the processes and the use of the Analytic Network Process. The application of the methodology results in synergy coefficients for production processes, using the examples of slot die, gravure and spray coating. The coefficients are transferred into an overall benefit of a production process portfolio. Finally, the effect of the considered synergies between fuel cell and electrolyzer production on the overall benefit of a company's production process portfolio is visualized. This paper is concluded with a critical review of the methodology and a summary of further research

Grundlinien der Wirtschaftsentwicklung 2010/2011

Das DIW Berlin rechnet für 2010 und 2011 mit einem Wirtschaftswachstum von jeweils rund zwei Prozent. Maßgebliche Triebkräfte kommen von der Binnennachfrage, die - mit Ausnahme der Unternehmensinvestitionen - in großem Umfang durch staatliche Stabilisierungsprogramme sowie durch die automatischen Stabilisatoren gestützt wird. Die wichtigste Säule bildet der private Verbrauch, der von beträchtlichen Kaufkraftzuwächsen der privaten Haushalte profitiert. Für die Exporte ist zunächst noch mit keiner kräftigen Erholung zu rechnen. Die deutschen Ausfuhren dürften aufgrund der Spezialisierung auf Investitionsgüter und des noch relativ geringen Marktanteils in den Wachstumszentren der Weltwirtschaft nur mit Verzögerung - und damit erst im nächsten Jahr - deutlicher am weltwirtschaftlichen Aufschwung teilhaben. Die Zahl der Arbeitslosen wird im kommenden Jahr zwar die Vier-Millionen-Marke übersteigen, angesichts der vorausgegangenen Produktionseinbrüche fällt der Beschäftigungsrückgang jedoch vergleichsweise schwach aus. Ermöglicht wird dies durch eine schwache Produktivitätsentwicklung und eine nur allmähliche Normalisierung der geleisteten Arbeitszeit. Gleichzeitig bleiben die Preise mit einer Inflationsrate um ein Prozent weitgehend stabil. Voraussetzung hierfür ist jedoch eine Beruhigung auf den Rohstoffmärkten, die in der Prognose unterstellt ist. Insgesamt sind die Rückschläge durch die schwere Wirtschaftskrise indes noch nicht überwunden: Erst gegen Ende 2011 dürfte die Wirtschaftskraft Deutschlands wieder an den Wert von Mitte 2008 und damit an das Niveau vor den dramatischen Produktionseinbrüchen heranreichen. Das entspricht rein rechnerisch mehr als drei Jahren mit Nullwachstum. In der Geldpolitik stellt sich die Frage nach dem richtigen Zeitpunkt für einen Ausstieg aus dem expansiven Kurs. Angesichts der noch bestehenden Unsicherheiten bezüglich der weiteren konjunkturellen Erholung und der Nachhaltigkeit der Finanzmarktstabilisierung ist eine nur allmähliche Rückführung der übermäßigen Liquiditätsversorgung empfehlenswert - zumal das Preisstabilitätsziel derzeit nicht gefährdet ist. Die Haushalts- und Finanzpolitik der Bundesregierung ist kritisch zu bewerten: Die Vorhaben der Bundesregierung - Abgabensenkung, Steuerreform, Gesundheitsreform und Einhaltung der Schuldenbremse ab 2016 - mögen für sich genommen jeweils eine gewisse Begründung haben, als Ganzes betrachtet sind diese Maßnahmen jedoch nicht gleichzeitig realisierbar. Diese Widersprüchlichkeit in der Wirtschaftspolitik kann erheblich zur Verunsicherung der privaten Haushalte und der Unternehmen beitragen. Hier wären eine stärkere Prioritätensetzung und eine klarere Gesamtkonzeption dringend geboten.Economic outlook, Business cycle forecast

Status and Prospects of AlN Templates on Sapphire for Ultraviolet Light-Emitting Diodes

Herein, the scope is to provide an overview on the current status of AlN/sapphire templates for ultraviolet B (UVB) and ultraviolet C (UVC) light-emitting diodes (LEDs) with focus on the work done previously. Furthermore, approaches to improve the properties of such AlN/sapphire templates by the combination of high-temperature annealing (HTA) and patterned AlN/sapphire interfaces are discussed. While the beneficial effect of HTA is demonstrated for UVC LEDs, the growth of relaxed AlGaN buffer layers on HTA AlN is a challenge. To achieve relaxed AlGaN with a low dislocation density, the applicability of HTA for AlGaN is investigated. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Improving AlN Crystal Quality and Strain Management on Nanopatterned Sapphire Substrates by High-Temperature Annealing for UVC Light-Emitting Diodes

Herein, AlN growth by metalorganic vapor-phase epitaxy on hole-type nanopatterned sapphire substrates is investigated. Cracking occurs for an unexpectedly thin-layer thickness, which is associated to altered nucleation conditions caused by the sapphire pattern. To overcome the obstacle of cracking and at the same time to decrease the threading dislocation density by an order of magnitude, high-temperature annealing (HTA) of a 300 nm-thick AlN starting layer is successfully introduced. By this method, 800 nm-thick, fully coalesced and crack-free AlN is grown on 2 in. nanopatterned sapphire wafers. The usability of such templates as basis for UVC light-emitting diodes (LEDs) is furthermore proved by subsequent growth of an UVC-LED heterostructure with single peak emission at 265 nm. Prerequisites for the enhancement of the light extraction efficiency by hole-type nanopatterned sapphire substrates are discussed. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei