55 results
Search Results
Now showing 1 - 10 of 55
- ItemSimulation of microwave circuits and laser structures including PML by means of FIT(München : European Geopyhsical Union, 2004) Hebermehl, G.; Schefter, J.; Schlundt, R.; Tischler, Th.; Zscheile, H.; Heinrich, W.Field-oriented methods which describe the physical properties of microwave circuits and optical structures are an indispensable tool to avoid costly and time-consuming redesign cycles. Commonly the electromagnetic characteristics of the structures are described by the scattering matrix which is extracted from the orthogonal decomposition of the electric field. The electric field is the solution of an eigenvalue and a boundary value problem for Maxwell’s equations in the frequency domain. We discretize the equations with staggered orthogonal grids using the Finite Integration Technique (FIT). Maxwellian grid equations are formulated for staggered nonequidistant rectangular grids and for tetrahedral nets with corresponding dual Voronoi cells. The interesting modes of smallest attenuation are found solving a sequence of eigenvalue problems of modified matrices. To reduce the execution time for high-dimensional problems a coarse and a fine grid is used. The calculations are carried out, using two levels of parallelization. The discretized boundary value problem, a large-scale system of linear algebraic equations with different right-hand sides, is solved by a block Krylov subspace method with various preconditioning techniques. Special attention is paid to the Perfectly Matched Layer boundary condition (PML) which causes non physical modes and a significantly increased number of iterations in the iterative methods.
- ItemEffect of electron blocking layer doping and composition on the performance of 310 nm light emitting diodes(Basel : MDPI, 2017) Kolbe, Tim; Knauer, Arne; Rass, Jens; Cho, Hyun Kyong; Hagedorn, Sylvia; Einfeldt, Sven; Kneissl, Michael; Weyers, MarkusThe effects of composition and p-doping profile of the AlGaN:Mg electron blocking layer (EBL) in 310 nm ultraviolet B (UV-B) light emitting diodes (LEDs) have been investigated. The carrier injection and internal quantum efficiency of the LEDs were simulated and compared to electroluminescence measurements. The light output power depends strongly on the temporal biscyclopentadienylmagnesium (Cp 2 Mg) carrier gas flow profile during growth as well as on the aluminum profile of the AlGaN:Mg EBL. The highest emission power has been found for an EBL with the highest Cp 2 Mg carrier gas flow and a gradually decreasing aluminum content in direction to the p-side of the LED. This effect is attributed to an improved carrier injection and confinement that prevents electron leakage into the p-doped region of the LED with a simultaneously enhanced carrier injection into the active region.
- ItemDiode laser based light sources for biomedical applications(Hoboken, NJ : Wiley, 2012) Müller, André; Marschall, Sebastian; Jensen, Ole Bjarlin; Fricke, Jörg; Wenzel, Hans; Sumpf, Bernd; Andersen, Peter E.Diode lasers are by far the most efficient lasers currently available. With the ever-continuing improvement in diode laser technology, this type of laser has become increasingly attractive for a wide range of biomedical applications. Compared to the characteristics of competing laser systems, diode lasers simultaneously offer tunability, high-power emission and compact size at fairly low cost. Therefore, diode lasers are increasingly preferred in important applications, such as photocoagulation, optical coherence tomography, diffuse optical imaging, fluorescence lifetime imaging, and terahertz imaging. This review provides an overview of the latest development of diode laser technology and systems and their use within selected biomedical applications.
- ItemCathodoluminescence and TEM investigations of structural and optical properties of AlGaN on epitaxial laterally overgrown AlN/sapphire templates(Milton Park : Taylor & Francis, 2013) Zeimer, U.; Mogilatenko, A.; Kueller, V.; Knauer, A.; Weyers, M.Surface steps as high as 15 nm on up to 10 μm thick AlN layers grown on patterned AlN/sapphire templates play a major role for the structural and optical properties of AlxGa1−xN layers with x ≥ 0.5 grown subsequently by metalorganic vapour phase epitaxy. The higher the Ga content in these layers is, the stronger is the influence of the surface morphology on their properties. For x = 0.5 not only periodic inhomogeneities in the Al content due to growth of Ga-rich facets are observed by cathodoluminescence, but these facets give rise to additional dislocation formation as discovered by annular dark-field scanning transmission electron microscopy. For AlxGa1−xN layers with x = 0.8 the difference in Al content between facets and surrounding material is much smaller. Therefore, the threading dislocation density (TDD) is only defined by the TDD in the underlying epitaxially laterally overgrown (ELO) AlN layer. This way high quality Al0.8Ga0.2N with a thickness up to 1.5 μm and a TDD ≤ 5x108 cm−2 was obtained.
- ItemNormally-off GaN transistors for power applications(Milton Park : Taylor & Francis, 2014) Hilt, O.; Bahat-Treidel, E.; Brunner, F.; Knauer, A.; Zhytnytska, R.; Kotara, P.; Wuerfl, J.Normally-off high voltage GaN-HFETs for switching applications are presented. Normally-off operation with threshold voltages of 1 V and more and with 5 V gate swing has been obtained by using p-type GaN as gate. Different GaN-based buffer types using doping and backside potential barriers have been used to obtain blocking strengths up to 1000 V. The increase of the dynamic on-state resistance is analyzed for the different buffer types. The best trade-off between low dispersion and high blocking strength was obtained for a modified carbon-doped GaN-buffer that showed a 2.6x increase of the dynamic on-state resistance for 500 V switching as compared to switching from 20 V off-state drain bias. Device operation up to 200 °C ambient temperature without any threshold voltage shift is demonstrated.
- ItemOrigin of a-plane (Al,Ga)N formation on patterned c-plane AIN/sapphire templates(Milton Park : Taylor & Francis, 2013) Mogilatenko, A.; Kirmse, H.; Hagedorn, S.; Richter, E.; Zeimer, U.; Weyers, M.; Tränkle, G.a-plane (Al,Ga)N layers can be grown on patterned c-plane AlN/sapphire templates with a ridge direction along [1bar 100]Al2O3. Scanning nanobeam diffraction reveals that the formation of a-plane layers can be explained by nucleation of c-plane (Al,Ga)N with [11bar 20](Al,Ga)N
- ItemDynamics of micro-integrated external-cavity diode lasers: Simulations, analysis and experiments(Berlin : Weierstraß-Institut für Angewandte Analysis und Stochastik, 2014) Radziunas, Mindaugas; Tronciu, Vasile Z.; Luvsandamdin, Erdenetsetseg; Kürbis, Christian; Wicht, Andreas; Wenzel, HansThis paper reports the results of numerical and experimental investigations of the dynamics of an external cavity diode laser device composed of a semiconductor laser and a distant Bragg grating, which provides an optical feedback. Due to the influence of the feedback, this system can operate at different dynamic regimes. The traveling wave model is used for simulations and analysis of the nonlinear dynamics in the considered laser device. Based on this model, a detailed analysis of the optical modes is performed, and the stability of the stationary states is discussed. It is shown, that the results obtained from the simulation and analysis of the device are in good agreement with experimental findings.
- ItemKohärente Strahlquelle für die optische Freiraumkommunikation auf der Basis von hybrid integrierten Diodenlasern und Halbleiterlaser-Verstärkern : Abschlussbericht(Hannover : Technische Informationsbibliothek (TIB), 2012) Tränkle, G.; Erbert, G.; Wicht, A.; Spiessberger, S.[no abstract available]
- ItemEpitaxie, Herstellung, und Charakterisierung von III-N basierenden Deep-UV LEDs mit Schwerpunkt auf dem UV-B bis UV-C, Teilvorhaben : Projekt-Abschlussbericht ; im Verbundvorhaben: Deep-UV LEDs auf der Basis von (AlGaln)N/GaN Quantenfilmen für den UV-A, UV-B und UV-C Wellenlängenbereich(Hannover : Technische Informationsbibliothek (TIB), 2011) Weyers, M.; Knauer, A.; Einfeldt, Sven; Rodriguez, Hernan; Kneissl, M.; Knüller, V.; Tessaro, T.; Petzke, T[no abstract available]
- ItemHochbitratige Schaltungen mit InP-HBTs : Schlussbericht(Hannover : Technische Informationsbibliothek (TIB), 2005) Heinrich, W.; Brunner, F.; Kurpas, P.; Meliani, C.; Rudolph, M.; Sidorov, V.; Würfl, J.[no abstract available]