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For future parabolic trough plants direct steam generation in the receiver pipes is a promising option for reducing the costs of solar thermal energy [1]. These new solar thermal power plants require innovative storage concepts, where the two phase heat transfer fluid poses a major challenge [2]. For the regions where the heat transfer fluid is in a single phase (water or steam), sensible heat storage using molten salt [3] or concrete [4] as storage material can be applied. However, efficient energy storage in the two-phase evaporation/condensation region requires heat storage operation within a narrow temperature range. For this two phase region, a high performance PCM storage technology was developed and demonstrated by DLR. A test module using 14 tons of PCM with 700 kWh capacity was built in 2009 and commissioned in 2010 in a direct steam test loop, set up at the power plant Litoral of Endesa in Carboneras, Spain [5]. The PCM-storage uses Sodium nitrate as phase change material with a melting temperature of 305 °C. Cycle testing has started end of 2010. Cycling tests have proven the expected discharge capacity of approx. 700 kWh for the PCM-storage module. System operation in constant pressure mode and sliding pressure mode has been conducted for the PCM-storage. While in the constant pressure mode a peak performance of the storage of more than 700 kW could be demonstrated, in the sliding pressure mode a constant power output over almost the whole charge and discharge period could be provided. The paper discusses the test results and evaluation for different operation modes for the phase change storage for discharge operation. Charging of the phase change storage is always in a once-through mode. However, for discharge, the steam can be generated either in forced or natural circulation mode or in once-through mode, leading to very different effects for the two-phase flow and filling level inside the heat exchanger pipes in the storage. The effects for forced and natural circulation discharge will be analysed and described in the paper.

Aberration corrected scanning transmission electron microscopy (STEM) with high angle annular dark field (HAADF) imaging and the newly developed annular bright field (ABF) imaging are used to define a new guideline for the polarity determination of semiconductor nanowires (NWs) from binary compounds in two extreme cases: (i) when the dumbbell is formed with atoms of similar mass (GaAs) and (ii) in the case where one of the atoms is extremely light (N or O: ZnO and GaN/AlN). The theoretical fundaments of these procedures allow us to overcome the main challenge in the identification of dumbbell polarity. It resides in the separation and identification of the constituent atoms in the dumbbells. The proposed experimental via opens new routes for the fine characterization of nanostructures, e.g., in electronic and optoelectronic fields, where the polarity is crucial for the understanding of their physical properties (optical and electronic) as well as their growth mechanisms.

We report on the electrochemical characteristics of GaN nanowire (NW) ensembles grown by plasma-assisted molecular beam epitaxy on Si111 substrates and on the influence of Si and Mg doping. The NW electrochemical properties in terms of surface capacitance (C(S)), surface resistance (R(S)) are extracted from electrochemical impedance spectra. While Mg doping of GaN NWs does not cause a significant variation of these quantities, an increase of the Si concentration leads to an increase of C(S) and a simultaneous decrease of R(S), indicating the presence of charge carriers in the NWs. According to the extracted values for R(S) and C(S) the NWs are classified into resistive and conductive. For conductive NWs charge transfer to a ferricyanide redox couple in the electrolyte is demonstrated and the ensemble average of the flatband voltage was determined. Variation of the lateral surface potential due to application of an external bias via the electrolyte is demonstrated.

The origin of exciton-phonon interaction in polar semiconductors is investigated. The relative contributions of Fr\"ohlich coupling and deformation potential scattering are identified by analyzing experimentally measured phonon-assisted luminescence using a rigorous many-body approach. Our experiment-theory comparison demonstrates that phonon scattering is significantly influenced by many-body interactions. Fr\"ohlich interaction can be strongly suppressed for excitons even when it dominates electronic single particle scattering. The results show that deformation potential scattering dominates the exciton-phonon interaction in ZnO, whereas Fr\"ohlich interaction prevails in CdS, and both coupling mechanisms yield almost equal contributions in ZnS.

We report on InGaN/GaN nanowires (NWs) as opto-chemical transducers for the detection of H2 and hydrocarbons. The NWs exhibit a strong photoluminescence (PL) which persists up to temperatures of 150°C and above. The GaN/InGaN NWs were grown by plasma-assisted molecular beam epitaxy on low-resistivity n-type Si (111) substrates and covered with a semitransparent catalytic Pt-coating after deposition. With the thin Pt coating on top, the PL intensity is dependent on the concentration of hydrogen or hydrocarbons in the surrounding atmosphere. Contrary to commercially available hydrogen sensors e.g. Pd or Pt gated high electron mobility transistors or catalytically activated metaloxide-silicon-carbide FETs our all-optical sensor system allows detection of ppm concentrations of H2 at low operating temperatures (30°C-120°C).

Besides silicon carbide, group-III nitrides are also suitable large-band-gap semiconductor materials for high-temperature gas sensor devices. Exposing GaN-based Schottky diodes with catalytically active platinum electrodes to hydrogen, we observed a decrease of the rectifying characteristics which we attribute to a decrease in Schottky barrier height. Current–voltage and elastic recoil detection measurements were used to investigate the H-sensing behavior of such devices. Our results indicate an interfacial effect as the origin of the sensor response to hydrogen.

For future parabolic trough plants direct steam generation in the absorber pipes is a promising option for reducing the costs of solar thermal energy. These new solar thermal power plants require innovative storage concepts, where the two phase heat transfer fluid poses a major challenge. A three-part storage system is proposed for the two phase fluid water/steam. Concrete storage is used for the process steps involving transfer of sensible heat — i.e. preheating of water and superheating of steam — while for the two-phase evaporation a phase change material (PCM) storage will be deployed. This technology is currently developed by DLR and Ed. Zublin AG within the project ITES, funded partly by the German Ministry for the Environment, Nature Conservation and Nuclear Safety. A combined storage solution with a 22 m3 concrete storage test module for superheating of steam and a 8.5 m3 PCM-storage for evaporation of water was build in 2009 in a direct steam test loop, set up at the power plant Litoral of Endesa in Carboneras, Spain. This high temperature storage system has a total capacity of approx. 1000 kWh and it will be the first demonstration of such a combined storage system for the two phase heat transfer fluid water/steam. Commissioning was completed in 2010, implying first heating-up of the concrete storage to expel the excess water in the concrete, first heating-up of the PCM storage including final filling of the storage with salt. Cycling tests for each storage unit separately are in progress. Combined testing will start in 2011. Results on the commissioning and testing will be reported in the paper.Copyright © 2011 by ASME

For future parabolic trough plants direct steam generation in the absorber pipes is a promising option for reducing the costs of solar thermal energy. These new solar thermal power plants require innovative storage concepts, where the two phase heat transfer fluid poses a major challenge. A three-part storage system is proposed for the two phase fluid water/steam. Concrete storage is used for the process steps involving transfer of sensible heat – i.e. preheating of water and superheating of steam - while for the two-phase evaporation a phase change material (PCM) storage will be deployed. This technology is currently being tested by DLR and Ed. Zublin AG within the project ITES, funded partly by the German Ministry for the Environment, Nature Conservation and Nuclear Safety. A combined storage solution with a 22 m³ concrete storage test module for superheating of steam and a 8.5 m³ PCM-storage for evaporation of water was build in 2009 in a direct steam test loop, set up at the power plant Litoral of Endesa in Carboneras, Spain. This high temperature storage system has a total capacity of ca. 1000 kWh and is the first demonstration of such a combined storage system for the two phase heat transfer fluid water/steam, details were reported in [Laing et al., 2009]. Cycle testing has started end of 2010. First, cycling tests for each storage unit separately have been performed successfully, showing the expected discharge capacities of ca. 250 kWh for the concrete unit and ca. 720 kWh for the PCM-storage module. System operation in constant pressure mode and sliding pressure mode has been conducted for the PCM-storage. While in the constant pressure mode the peak power of the storage of more than 700 kW could be demonstrated, in the sliding pressure mode a constant power output over almost the whole charge and discharge period could be provided. Combined system operation was also proven in constant pressure and sliding pressure mode, showing good system operability. The paper will describe the storage system and present the test results and evaluation of the single module testing as well as on the combined storage system testing in different operation modes.

The optical and structural properties of wurtzite GaN nanowires containing zinc-blende GaN inclusions of different thicknesses are investigated. Micro-photoluminescence spectra of single nanowires exhibit a series of narrow emission peaks with linewidth as low as 0.8 meV in the interval 3.1-3.42 eV. The peak energy blue-shifts with increasing excitation power following a similar to I(1/3) law due to the progressive band filling and to the screening of the internal field. The quantum confinement in these type-II crystal phase heterostructures was simulated in the framework of a one-dimensional effective mass model, accounting for the internal electrical polarization of the wurtzite GaN. The predicted transition energies are in good agreement with the energy statistics realized on more than 30 single nanowire emission spectra.

Individual GaN nanowires containing ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}/\text{GaN}$ quantum discs (QDiscs) with Al content $x\ensuremath{\le}16\mathrm{%}$ have been investigated by microphotoluminescence, transmission electron microscopy, and theoretical modeling. Single quantum discs show narrow emission lines with a linewidth as low as 3 meV at energies above the GaN band gap while the emission of nanowires containing multiple quantum discs shows multiple peaks with total spectral broadening that depends on the Al content in the barrier. As assessed by simulations of the quantum confinement based on a three-dimensional effective-mass model, the main factors influencing the spectral dispersion are: (i) strain relaxation in the QDiscs, strongly affected by the presence of a lateral AlGaN shell with a progressively changing thickness formed during the barrier growth; (ii) monolayer fluctuations in the QDisc thickness.

The polarization anisotropy of single GaN nanowires containing Al(x)Ga(1-x)N/GaN multiquantum disc (MQDisc) structures is characterized by polarization-resolved microphotoluminescence (mu PL). Single nanowires exhibit at T=4.2 K two main luminescence contributions: one is peaked at E=3.45-3.48 eV related to near-band-edge GaN bulk excitonic transitions and is polarized parallel to the nanowire axis (pi polarization) at moderate excitation-power density; the other, lying at higher energy, is related to excitonic transitions confined in the MQDisc and is polarized perpendicularly to the nanowire axis (sigma polarization). The results are interpreted in terms of the selection rules for excitonic transitions in wurtzite semiconductor crystals and of the polarization anisotropy arising from the elongated nanowire shape. Finally, the analysis of photoluminescence at T=300 K shows that the thermal population of light-hole states in the MQDisc produces a blueshift of the PL peak when polarization is rotated from sigma to pi.

We report a systematic study of the luminescence properties of AlxGa1-xN/GaN single and multi quantum disc structures in single nanowires with Al fraction xAl varying from 0.05 to 1. These quantum structures are situated on the top of GaN nanowires grown in the polar [0001] direction. Nanowires are synthesized by Plasma Assisted Molecular Beam Epitaxy under nitrogen rich growth conditions at a substrate temperature of T = 780 °C on Si (111) substrates. The PL energy of single and multi-quantum disc systems is explained in terms of Al content in the barriers, band bending at the upper polar surface, and strain relaxation in the heterostructure region (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the deposition of planar lipid bilayers (supported membranes) on gallium nitride (GaN) electrodes for potential applications as membrane-based biosensors. The kinetics of the lipid membrane formation upon vesicle fusion were monitored by simultaneous measurements of resistance and capacitance of the membrane using AC impedance spectroscopy in the frequency range between 50 mHz and 50 kHz. We could identify a two-step process of membrane spreading and self-healing. Despite its relatively low resistance, the membrane can be modeled by a parallel combination of an ideal resistor and capacitor, indicating that the membrane efficiently blocks the diffusion of ions.

III? nitride based Quantum Wells (QWs) have been extensively studied as active buildings blocks for different electronic and optoelectronic devices. Several samples of GaN/AIN Axial Multi Quantum Well nanowires (MQW NWs) have been synthesized, leading to NWs with a MQW structure with GaN wells of different widths along the growth axis, 1.5, 2,3 and 4 nm. In this work structural and optical properties are studied by means of high resolution transmission electron microscopy (HRTEM) and photoluminescence (PL) analysis respectively.

This paper reports about a piezoresistive β-SiC-on-silicon on insulator (SOI) pressure sensor with an on chip polycrystalline SiC thermistor for high operating temperatures. The β-SiC film was characterized by TEM-analysis, X-ray diffraction and Hall measurements. The investigations show a good single crystal quality of the β-SiC film and a reliable electrical isolation by the buried oxide layer from the substrate at temperatures up to 673 K. The fabricated pressure sensor chip was tested in the temperature range between room temperature and 573 K. The sensitivity at room temperature is S =2.0 mV V −1 bar −1 . The temperature coefficient of the sensitivity (TCS) between room temperature and 573 K is TCS=−0.16 %K −1 . The temperature coefficient of the resistivity (TCR) of the polycrystalline SiC thermistor is TCR=−0.17 %K −1 .

The selectivity of a LPCVD growth process for cubic silicon carbide with respect to the underlying substrate was investigated, and a highly selective growth process on structured Si/SiO 2 substrates was found at temperatures above 1150°C. The temperature dependence of this effect was systematically studied. In the temperature range between 1075–1150°C, polycrystalline SiC was grown on SiO 2 surfaces. At temperatures above 1150°C, no growth on SiO 2 occurred, whereas high-quality 3C-SiC layers were deposited onto the opened SiO 2 mask areas. This was shown by REM pictures and by removal of the SiO 2 layers in wet HF-etchant after the deposition. The selectively deposited 3C-SiC-structures were grown at a rate of 4.4 μm/h.

The self-assembled growth of GaN nanorods on Si (111) substrates by plasma-assisted molecular beam epitaxy under nitrogen-rich conditions is investigated. An amorphous silicon nitride layer is formed in the initial stage of growth that prevents the formation of a GaN wetting layer. The nucleation time was found to be strongly influenced by the substrate temperature and was more than 30 min for the applied growth conditions. The observed tapering and reduced length of silicon-doped nanorods is explained by enhanced nucleation on nonpolar facets and proves Ga-adatom diffusion on nanorod sidewalls as one contribution to the axial growth. The presence of Mg leads to an increased radial growth rate with a simultaneous decrease of the nanorod length and reduces the nucleation time for high Mg concentrations.

We investigate the origin of the surface conductivity of H-terminated diamond films immersed in aqueous electrolyte. We demonstrate that in contrast to the in air situation, charge transfer across the diamond interface does not govern the surface conductivity in aqueous electrolyte when a gate electrode controls the diamond/electrolyte interfacial potential. Instead, this almost ideally polarizable interface allows the capacitive charging of the surface. This description resolves the observed disagreement of the pH sensitivity of the diamond surface conductivity in air and in aqueous electrolyte.

Direct steam generation (DSG) represents a promising option to improve today’s parabolic trough technology for solar thermal power generation. The European DISS and INDITEP projects have proven the feasibility of the DSG process under real solar conditions at the DISS test facility at the Plataforma Solar de Almería (PSA) (Zarza, E., Valenzuela, L., Léon, J., Hennecke, K., Eck, M., Weyers, H.-D., Eickhoff, M., 2004, “Direct Steam Generation in Parabolic Troughs Final Results and Conslusions of the DISS Project,” Energy, 29, pp. 635–644). These projects have also shown that the recirculation mode is the preferred operation mode for DSG collector fields. This concept requires water-steam separators at the end of the evaporation section of the collector loop. Both compact water-steam separators for every single row or huge separation drums for the whole collector field are considered. Small compact water-steam separators show a lower inertia, reducing the time for start-up. Within INDITEP and the German R&D project SOLDI compact water-steam separators have been developed, manufactured, and tested by DLR and Siemens, with its subcontractor Framatome ANP. Prototypes of a cyclone and a baffle separator have been implemented into the DISS test facility. More than 200 tests have been performed to investigate the separation efficiency, the pressure loss, and the performance under transient conditions. This paper focuses on the steady-state tests.

Quantum dots (QDs) are of special interest for both fundamental physics and optoelectronic applications. Controlled growth of ensembles of self assembled GaN/AIN QDs showing efficient room-temperature luminescence has been demonstrated recently. On the other hand, GaN-based devices have been shown useful to detect chemically induced changes of the surface potential, thereby acting as chemical sensors in liquid or gaseous environments. We demonstrate the applicability of GaN QDs embedded in an A1N matrix and covered with a catalytic Pt layer as chemical sensors with contactless optical readout, benefiting from the optical transparency of both the sapphire substrate and the A1N matrix. The optical response of such a system in terms of a change in luminescence characteristics upon exposure to hydrogen is discussed.

MEMS micro heater devices capable of long-term operation at temperatures up to 1000 degrees C are presented. The enhanced long-term stability has been achieved by employing antimony-doped tin oxide (SnO2:Sb) as a substitute for the conventionally used noble metal heater resistors. A detailed investigation of its high-temperature stability reveals that degradation is caused by out-diffusion of Sb impurities from the SnO2 film. (c) 2007 Elsevier B.V. All rights reserved.

The self-assembled formation of ordered, vertically stacked rocksalt/wurtzite Mg x Zn 1−xO heterostructures by planar phase separation is shown. These heterostructures form quasi “natural” two-dimensional hetero-interfaces between the different phases upon annealing of MgO-oversaturated wurtzite Mg x Zn 1−xO layers grown by plasma-assisted molecular beam epitaxy on c-plane sapphire substrates. The optical absorption spectra show a red shift simultaneous with the appearance of a cubic phase upon annealing at temperatures between 900 °C and 1000 °C. Transmission electron microscopy reveals that these effects are caused by phase separation leading to the formation of a vertically ordered rock salt/wurtzite heterostructures. To explain these observations, we suggest a phase separation epitaxy model that considers this process being initiated by the formation of a cubic (Mg,Zn)Al2O4 spinel layer at the interface to the sapphire substrate, acting as a planar seed for the epitaxial precipitation of rock salt Mg x Zn 1−xO. The equilibrium fraction x of magnesium in the resulting wurtzite (rock salt) layers is approximately 0.15 (0.85), independent of the MgO content of the as-grown layer and determined by the annealing temperature. This model is confirmed by photoluminescence analysis of the resulting layer systems after different annealing temperatures. In addition, we show that the thermal annealing process results in a significant reduction in the density of edge- and screw-type dislocations, providing the possibility to fabricate high quality templates for quasi-homoepitaxial growth.

In the present study, the effects of various types of strain and indium concentration on the total energy and optoelectronic properties of GaN nanowires (NWs) with embedded InxGa1−xN nanodisks (NDs) are examined. In particular, the bi-axial, hydrostatic, and uniaxial strain states of the embedded InxGa1−xN NDs are investigated for multiple In concentrations. Density functional theory is employed to calculate the band structure of the NWs. The theoretical analysis finds that the supercell-size-dependent characteristics calculated for our 972-atom NW models are very close to the infinite supercell-size limit. It is established that the embedded InxGa1−xN NDs do not induce deep states in the band gap of the NWs. A bowing parameter of 1.82 eV is derived from our analysis in the quadratic Vegard's formula for the band gaps at the various In concentrations of the investigated InxGa1−xN NDs in GaN NW structures. It is concluded that up to ∼10% of In, the hydrostatic strain state is competitive with the bi-axial ...

The covalent functionalization of GaN and AlN surfaces with organosilanes is demonstrated. Both octadecyltrimethoxysilane and aminopropyltriethoxysilane form self-assembled monolayers on hydroxylated GaN and AlN surfaces, confirmed by x-ray photoelectron spectroscopy and atomic force microscopy. The monolayer thickness on GaN was determined to 2.5±0.2nm by x-ray reflectivity. Temperature-programmed desorption measurements reveal a desorption enthalpy of 240kJ∕mol. The realization of micropatterned self-assembled monolayers and the hybridization of deoxyribonucleic acid molecules on biofunctionalized GaN surfaces are shown.

This paper presents a summary of the main results and conclusions achieved in the DISS (Direct Solar Steam) project. The test facility implemented at the Plataforma Solar de Almeria (PSA) in 1997–8, the so-called PSA DISS test facility, was operated for more than 3000 h in 1999–2000 and 2001 to investigate the Direct Steam Generation (DSG) process under real solar conditions. The feasibility of the DSG process in horizontal parabolic trough collectors has been proven and an important know how has been acquired by the project partners regarding the thermo-hydraulic parameters of the water/steam flow in DSG solar fields.

For the realization of good quality 3C-SiC films, epitaxially grown on Si, the perfection of the film during the early stage of growth is substantial. In this work the beneficial role of Flash Lamp Process (FLP) for defect elimination and strain reduction in the 3C-SiC films is discussed. FLP is a highly transient process with a flash duration of a few milliseconds (ms), preferentially occurring at the SiC/Si interface where the irradiated flux has its maximum absorption. If the energy density is sufficiently high, the temperature at the interface increases above the melting point of silicon. As a result, Si at the interface melts and subsequently dissolves the 3C-SiC in the vicinity of the interface, while the non-melted part of the film is annealed. During the re-solidification phase separation occurs and that part of the SiC film which is already dissolved into silicon is epitaxially deposited on the annealed uppermost part, which acts as the seed layer. The process resembles liquid phase epitaxy (LPE) resulting in substantial improvement of the SiC film. This process also eliminates the cavities and the stress at the 3C-SiC/Si interface. The irradiated 3C-SiC films were characterized by Transmission Electron Microscopy.

Using thermally induced hydrosilylation, organic molecules were covalently bonded to H-terminated crystalline silicon (111) and hydrogenated amorphous silicon (a-Si:H) surfaces. The resulting chemical surface structure was analyzed by X-ray photoelectron spectroscopy (XPS) and compared to that of silicon surfaces covered by a native oxide or terminated with hydrogen. For both kinds of substrates, the presence of oxygen on the surface is found to hinder the hydrosilylation reaction. Stable H-termination as a starting point for a successful hydrosilylation can be obtained on a-Si:H surfaces with much less technological effort than on crystalline silicon surfaces. Photoconductivity measurements of the different a-Si:H surfaces at low intensity of illumination (monomolecular recombination regime) indicate that the hydrosilylated surface has less defects than the H-terminated surfaces or surfaces covered with native oxide. Spin-dependent photoconductivity measurements identify the dominant paramagnetic defect at the hydrosilylated a-Si:H surface to be the silicon dangling bond.

For measuring the cylinder pressure in combustion engines a high temperature pressure sensor has been developed. The sensor is made of a piezoresistive /spl beta/-SiC-on-SOI (SiCOI) sensor chip and a specially designed housing. The SiCOI sensor was characterized under static pressure up to 20 MPa. The sensitivity of the sensor at room temperature is approximately 1.9 mV/MPa and decreases to about 1.2 mV/MPa at 300/spl deg/C. For monitoring the dynamic cylinder pressure in the combustion chamber the SiCOI sensor was placed into the cylinder head of a gasoline engine. The measurements were performed at 1500 rpm under different loads, and for comparison a quartz pressure transducer from Kistier was used as a reference. The maximum pressure at partial load operation amounts to about 2.0 MPa. The difference between the calibrated SiCOI sensor and the reference sensor is significantly less than 0.1 MPa during the whole operation.

The gauge factor of /spl beta/-SiC was measured for dependence on the crystal quality, the doping type and the doping level between room temperature and 400/spl deg/C. A new measurement method using micromachined SiC bridge structures together with a piezoelectric micromanipulator set-up was applied to make non-isolated SiC films on silicon accessable to measurements. The most intriguing result is that the gauge factor of n-doped mono-crystalline /spl beta/-SiC is negative, whereas that of poly-crystalline material is positive. Considering the dependence of the gauge factor on the temperature, the crystal orientation and the doping type this could be explained by the greater influence of grain boundaries in the case of polycrystalline wide bandgap materials compared to silicon.

This paper reports about the first piezoresistive pressure sensor for high operating temperatures using single crystalline, n-type /spl beta/-SiC piezoresistors on Silicon On Insulator (SOI) substrates. The new Silicon Carbide On Insulator (SiCOI) layer structure prevents a leakage current flow through the substrate at high temperatures up to 723 K. The sensor was tested in the temperature range between room temperature and 573 K. The sensitivity of the device at room temperature is approximately 20.2 /spl mu/V/VkPa. This corresponds to a longitudinal gauge factor of -32 in the [100]-direction. The Temperature Coefficient of Sensitivity (TCS) is -0.16 %K/sup -1/ at 573 K.

International audience; In this work we analyse the performance of Pt- and Ni-based Schottky metallizations on AlxGa1−xN (x = 0, 0.31). An intermediate thin Ti layer is shown to enhance the thermal stability of Pt/Au, and leads to an increase of the Schottky barrier height. Pt/Ti/Au contacts on GaN provide a barrier height of 1.18 ± 0.07 eV, increasing up to 2.0 ± 0.1 eV on Al0.31Ga0.69N. Further improvement of Schottky contacts is achieved by surface passivation with plasma-enhanced chemical vapour deposited SiO2 or SixNy, which reduces the leakage current by two orders of magnitude and structural modifications in the metal due to thermal ageing.

We report on electrostatic screening of polarization-induced internal electric fields in AlN/GaN nanowire heterostructures with Germanium-doped GaN nanodiscs embedded between AlN barriers. The incorporation of Germanium at concentrations above $10^{20}\,\text{cm}^{-3}$ shifts the photoluminescence emission energy of GaN nanodiscs to higher energies accompanied by a decrease of the photoluminescence decay time. At the same time, the thickness-dependent shift in emission energy is significantly reduced. In spite of the high donor concentration a degradation of the photoluminescence properties is not observed., Comment: Manuscript including Supplemental material (15 pages, 5 figures)

The present paper gives an overview over different sensor applications of GaN thin films and AlGaN/GaN heterostructures. The response of Pt-GaN Schottky diodes towards hydrogen and hydrogen containing gases is analysed and their gas sensitivity is characterized from room temperature up to 600°C. In addition, the sheet carrier density of a two dimensional electron gas confined at the heterointerface in AlGaN/GaN high electron mobility transistors (HEMTs) is shown to be significantly influenced by changes in the electronic properties of the device surface. This effect is successfully exploited for the realization of ion detectors and sensors for fluid monitoring based on AlGaN/GaN HEMTs with non-metalized gate areas. Promising possibilities of fabricating monolithically integrated sensor devices for wireless signal transmission are demonstrated by the realization of SAW devices on epitaxial AlN-films.

In order to fulfill future emissions and OBD regulations, whilst meeting increasing demands for driveability and refinement, new technologies for SI engines have to be found in terms of sensors and algorithms for engine control units. One promising way, explored in the AENEAS collaborative project between DaimlerChrysler, Kistler, Ricardo and the European Commission, is to optimize the behavior of the system by using in-cylinder measurements and analysing them with modern control algorithms. In this paper a new engine management system based on combustion pressure sensing is presented. The pressure sensor is designed to give a reliable and accurate signal of the full pressure trace during a working cycle. With the application of new technologies low cost manufacturing appears to be achievable, so that an application in mass production can be considered. Furthermore, model-based algorithms were developed to allow optimal control of the engine based on the in-cylinder measurements. The algorithms incorporate physical principles to improve efficiency, emissions and to reduce the parameterisation effort. In the paper, applications of the combustion pressure signal for air mass estimation, knock detection, ignition control cam phase detection and diagnosis are discussed. (author)

Conventional Engine Management Systems (EMS) are primarily parameter based systems in which there is no underlying model to describe the behaviour of the engine. Such systems therefore require many parameters to control the engine under steady-state, transient and varying ambient conditions. The introduction of Model Based Control in order to overcome this problem has been hindered mainly by the lack of inexpensive sensing devices which can closely monitor the engine’s performance. In particular, the measurement of cylinder pressure which would give valuable information on the engine’s performance, has been very expensive due to the harsh environment of the combustion chamber. In recent years, attention has focused upon the development of inexpensive cylinder pressure sensors in order to realise a system more appropriate for the application of Model Based control. This paper describes the application of new robust pressure sensing technology coupled with Model Based control and diagnostics algorithms.

Requirements on future vehicles are reduction of energy consumption and emissions. To fulfil these, an optimization of the combustion process including all relevant parameters is necessary and therefore parameter field based control systems have to be replaced by closed loop regulating circuits. To realize the latter in series applications, new types of sensors designed for high temperatures and harsh environments have to be developed. The present paper describes the development of a combustion pressure sensor based on cubic Silicon Carbide (β-SiC) layers on Silicon. Results of in-cylinder measurements will be given in comparison to a Kistler reference sensor. SiC technology based on the hexagonal 4H and 6H polytypes we are developing for integrated signal electronics. The latter is needed for smart sensors and independently operating modules, applied in harsh environments. JFET transistors, which are the basis for monolithic circuit design, will be described and I-V-characteristics measured at elevated temperatures up to 400°C will be given.

Die Auflosung der Unternehmungsgrenzen stellt einen aktuellen „Megatrend” der betriebswirtschaftlichen Diskussion dar (vgl. hierzu Reis 1996 a, S. 195). Die hohe Dynamik und Komplexitat der Wettbewerbsumfelder stellen pull-Faktoren dar, die die Leistungsfahigkeit herkommlicher organisatorischer Losungen in Frage stellen und die Suche nach neuen organisatorischen Konzepten fur die zukunftsfahige Unternehmung auslosen. Vor diesem Hintergrund werden durch die rasanten Fortschritte der modernen Informationstechnologien Moglichkeiten erschlossen, die Auflosung der Unternehmungsgrenzen in die Realitat umzusetzen. Die Potentiale der Informationstechnologien wirken somit als push-Faktoren.

The structural properties and the strain state of InGaN/GaN superlattices embedded in GaN nanowires were analyzed as a function of superlattice growth temperature, using complementary transmission electron microscopy techniques supplemented by optical analysis using photoluminescence and spatially resolved microphotoluminescence spectroscopy. A truncated pyramidal shape was observed for the 4 nm thick InGaN inclusions, where their (0001¯) central facet was delimited by six-fold {101¯l} facets towards the m-plane sidewalls of the nanowires. The defect content of the nanowires comprised multiple basal stacking faults localized at the GaN base/superlattice interface, causing the formation of zinc-blende cubic regions, and often single stacking faults at the GaN/InGaN bilayer interfaces. No misfit dislocations or cracks were detected in the heterostructure, implying a fully strained configuration. Geometrical phase analysis showed a rather uniform radial distribution of elastic strain in the (0001¯) facet of the InGaN inclusions. Depending on the superlattice growth temperature, the elastic strain energy is partitioned among the successive InGaN/GaN layers in the case of low-temperature growth, while at higher superlattice growth temperature the in-plane tensile misfit strain of the GaN barriers is accommodated through restrained diffusion of indium from the preceding InGaN layers. The corresponding In contents of the central facet were estimated at 0.42 and 0.25, respectively. However, in the latter case, successful reproduction of the experimental electron microscopy images by image simulations was only feasible, allowing for a much higher occupancy of indium adatoms at lattice sites of the semipolar facets, compared to the invariable 25% assigned to the polar facet. Thus, a high complexity in indium incorporation and strain allocation between the different crystallographic facets of the InGaN inclusions is anticipated and supported by the results of photoluminescence and spatially resolved microphotoluminescence spectroscopy.

We investigated the electric-field dependence of the photoluminescence-emission properties of InGaN/GaN quantum dot multilayers in contact with an electrolyte. Controlled variations of the surface potential were achieved by the application of external electric fields using the electrolytic Schottky contact and by variation of the solution's pH value. Prior to characterization, a selective electrochemical passivation process was required to suppress leakage currents. The quantum dot luminescence is strongly affected by surface potential variations, i.e., it increases exponentially with cathodic bias and acidic pH values. The results cannot be explained by a modification of intra-dot polarization induced electric fields via the quantum confined Stark effect but are attributed to the suppression/enhancement of non-radiative recombination processes, i.e., mainly hole transfer into the electrolyte. The results establish a link between the photoluminescence intensity and the magnitude of electric fields at the semiconductor/electrolyte interface.

Zn1–xMgxO epitaxial films with Mg concentrations 0≤x≤0.3 were grown by plasma-assisted molecular beam epitaxy on a-plane sapphire substrates. Precise determination of the Mg concentration x was performed by elastic recoil detection analysis. The bandgap energy was extracted from absorption measurements with high accuracy taking electron-hole interaction and exciton-phonon complexes into account. From these results a linear relationship between bandgap energy and Mg concentration is established for x≤0.3. Due to alloy disorder, the increase of the photoluminescence emission energy with Mg concentration is less pronounced. An analysis of the lattice parameters reveals that the epitaxial films grow biaxially strained on a-plane sapphire.

We review different strategies to achieve a three-dimensional energy bandgap modulation in a nanowire (NW) by the introduction of self-assembled 0D, 1D and 2D quantum structures, quantum dots (QDs), quantum wires (QWRs) and quantum wells (QWs). Starting with the well-known axial, radial (coaxial/prismatic) or polytypic quantum wells in GaN/AlN, GaAs/AlAs or wurtzite/zinc-blende systems, respectively, we move to more sophisticated structures by lowering their dimensionality. New recent approaches developed for the self-assembly of GaN quantum wires and InAs or AlGaAs quantum dots on single nanowire templates are reported and discussed. Aberration corrected scanning transmission electron microcopy is presented as a powerful tool to determine the structure and morphology at the atomic scale allowing for the creation of 3D atomic models that can help us to understand the enhanced optical properties of these advanced quantum structures.

Ausgehend von der Beobachtung, das die Absatzmarkte immer mehr zum Engpas der unternehmerischen Tatigkeit werden, wird die Einfuhrung eines Strategischen Marketing gefordert. Das Strategische Marketing wird dabei als eine Fuhrungskonzeption verstanden, die sich durch eine besondere Kundenorientierung auszeichnet. In dieser Sichtweise fallt der strategischen Marketingplanung die Aufgabe zu, strategische Innovationen zu entwickeln, die die Antwort auf die vielfaltigen Herausforderungen der Unternehmungsumwelt darstellen (vgl. Raffee (1989), S. 4 ff.). Das Strategische Marketing-Controlling unterstutzt die Strategische Unternehmungsfuhrung durch den Aufbau eines strategischen Marketing-Planungs- und Kontrollsystems sowie eines entsprechenden Informationsversorgungssystems. In der strategischen Marketingplanung kommen im wesentlichen die bereits vorgestellten Instrumente zum Einsatz.

In den letzten Jahren ist im Bereich der Unternehmungsfuhrung eine Ablosung der klasssischen Sichtweise, die durch ein analytisches und isolierendes Denken gepragt ist, zu beobachten. Dieser Wandel zielt auf eine ganzheitliche Sichtweise der Unternehmungsfuhrung. Im Kern dieses integrierenden und zusammenfugenden Ansatzes steht die Uberlegung, ein isolierendes Vorgehen aufzugeben, um den Blick auf die Unternehmung als Ganzes zu richten. Nur ein solches Verstandnis der Unternehmungsfuhrung scheint geeignet, die aus zunehmender Komplexitat und Dynamik der Umwelt resultierenden Probleme zu handhaben. Im Mittelpunkt der Uberlegungen stehen die Systemvorstellung (die Unternehmung als soziales System) und die Kybernetik, worunter das Denken in Regelkreisen, Prozessen und Wechselwirkungen zu verstehen ist (vgl. Ulrich (1970), S. 105 ff.). Das systemorientiert-ganz-heitliche Management dient somit der Gestaltung, Lenkung und Entwicklung der Unternehmung. Mit dieser ganzheitlichen Orientierung ist eine erhebliche Erweiterung der Umweltperspektive verbunden, was neben okonomischen auch zur Berucksichtigung politischer, gesellschaftlicher und okologischer Aspekte fuhrt.

Die Entwicklung in der Unternehmungsumwelt ist in der jungeren Vergangenheit mit einer zunehmenden Dynamik und Komplexitat verbunden. Kennzeichnend ist dabei, das die Umweltanderungen relativ uberraschend erfolgen.

Die Ableitung von Controlling-Leitsatzen hat sich gleichermasen an den Funktionen des Strategischen Controlling und an der spezifischen Unternehmungs- und Umweltsituation zu orientieren (vgl. Abbildung 38).

Die Strategische Planung stellt eine Antwort auf die Umweltentwicklungen dar, wie sie unter Abschnitt 1.1 beschrieben wurden. Unter Planung wird allgemein ein “...systematisches, zukunftsbezogenes Durchdenken von Zielen, Masnahmen, Mitteln und Wegen zur Zielerreichung...” (Wild (1982), S. 13) verstanden. Strategische Planung ist als Prozes aufzufassen, in dem auf der Basis rationaler Analysen der Unternehmungssituation sowie von Chancen und Risiken Ziele festgelegt werden (vgl. Abbildung 5).