Author
Oliver Ambacher
Other affiliations: Osram, Siemens, Cornell University ...read more
Bio: Oliver Ambacher is an academic researcher from Fraunhofer Society. The author has contributed to research in topics: Amplifier & High-electron-mobility transistor. The author has an hindex of 64, co-authored 848 publications receiving 26256 citations. Previous affiliations of Oliver Ambacher include Osram & Siemens.
Papers published on a yearly basis
Papers
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TL;DR: In this paper, an enhancement of the quality factor by increasing the resonant frequency using strained resonator structures is proposed, which is the result of the thermal mismatch between heteroepitaxial SiC or AlN layers and the silicon substrates.
Abstract: A pulsed mode magnetomotive operation of micro- and nanoelectromechanical devices in air is demonstrated, where viscous damping determines the quality factor of the device. An enhancement of the quality factor by increasing the resonant frequency using strained resonator structures is proposed. Internal strain is the result of the thermal mismatch between heteroepitaxial SiC or AlN layers and the silicon substrates. Comparing unstrained and strained resonators, an increase of the quality factor by one order of magnitude from about 30 to 300 was achieved. This increase will improve the sensing performance of such resonant structures for an operation in ambient environment.
43 citations
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TL;DR: In this article, photothermal deflection spectroscopy (PDS) was used to study the absorption of GaN/InGaN/GaN double heterostructures in the energy range from 0.6 to 3.8 eV.
Abstract: Photothermal deflection spectroscopy (PDS) is used to study the absorption of GaN/InGaN/GaN double heterostructures in the energy range from 0.6 to 3.8 eV. The heterostructures containing 1–77-nm-thick InGaN single quantum wells were deposited by chemical vapour deposition from organometallic precursors. They are measured to investigate the absorption coefficient, bandgap, indium concentration and fluctuation of the quantum wells. A bandgap increase of hexagonal InxGa1-xN (x≈0.14) of 60 meV is observed with decreasing well thicknesses from 15 to 1 nm. The distribution of In-concentration of the InxGa1-xN layers was estimated from the slope of the absorption coefficient versus photon energy for energies below the bandgap and found to be Gaussian with a full width of half maximum of Δx=0.03.
42 citations
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TL;DR: In this paper, the energy shift of the In MNN peak was estimated during the depth profiling in order to obtain information about the position of the Fermi level, while the surface is clearly n-type.
Abstract: Non intentionally doped and Mg-doped InN layers were analyzed by sputter depth profiling in an Auger electron spectroscopy (AES) equipment and by Ultra-violet photoelectron spectroscopy (UPS). On the surface of both types of layers a high concentration of oxygen and a strong accumulation of electrons was observed, however, in contrast to the undoped layers the conductivity profile of Mg doped InN shows a strong discontinuity close to the surface. The depth of this discontinuity strongly depends on the oxygen concentration. The energy shift of the In MNN peak was estimated during the depth profiling in order to obtain information about the position of the Fermi level. In the bulk of the Mg-doped InN the In MNN peak shift of about 0.15 eV demonstrates the influence of the Mg on the Fermi level, while the surface is clearly n-type. A strong shift of the Fermi level close to the surface was observed, which might be attributed to the formation of In 2 O 3 . By the combination of AES and UPS a model for the band bending is proposed, which demonstrates that Mg doping indeed can compensate the n-type conductivity in the bulk and is therefore a prospective candidate to achieve p-type doping in InN.
42 citations
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TL;DR: In this article, the authors report on resonant MEMS and NEMS devices with functional layers of SiC, AlN and AlGaN/GaN heterostructures on different substrates, which have been investigated and analyzed in the course of an interdisciplinary research focus program of the German Research Foundation (DFG).
Abstract: Wide-bandgap semiconductors represent an attractive option to meet the increasing demands of micro- and nano-electromechanical systems (MEMS/NEMS) by offering new functionalities, high stability, biocompatibility and the potential for miniaturization and integration. Here, we report on resonant MEMS and NEMS devices with functional layers of SiC, AlN and AlGaN/GaN heterostructures on different substrates, which have been investigated and analysed in the course of an interdisciplinary research focus programme of the German Research Foundation (DFG). The specific deposition and etching technologies necessary for the three-dimensional micro-structuring are explained. Further, the implementation of appropriate electromechanical transduction schemes is discussed. In case of SiC and AlN resonators, actuation and sensing was achieved by a magnetomotive scheme. A piezoelectric coupling scheme where the counter electrode is formed by the two-dimensional electron gas at the interface of the III/V heterostructure was realized for the AlGaN/GaN resonators.
Thus, flexural and longitudinal vibration modes were excited and characterized using electrical and optical techniques. The measured key parameters of resonant frequency and quality factor are related to geometry, material and environmental parameters using analytical and finite element (FE) models. Finally, potential sensor applications are experimentally investigated.
42 citations
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TL;DR: In this article, the x-ray standing wave technique was used to determine the polarity of a 1 μm thick GaN film grown by molecular beam epitaxy on an α-Al2O3(0001) single crystal.
Abstract: The x-ray standing wave technique was used to determine the polarity of a 1 μm thick GaN film grown by molecular beam epitaxy on an α-Al2O3(0001) single crystal. The standing wave was generated by x-ray diffraction from the GaN film. The Ga Kα fluorescence yield was recorded as a function of incidence angle within the range of the GaN(0002) reflection. Analysis of the data reveals that the film has grown with N polarity, i.e., the nitrogen atoms occupy the top half of the wurtzite (0001) bilayers.
42 citations
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TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.
Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …
33,785 citations
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TL;DR: The editors have done a masterful job of weaving together the biologic, the behavioral, and the clinical sciences into a single tapestry in which everyone from the molecular biologist to the practicing psychiatrist can find and appreciate his or her own research.
Abstract: I have developed "tennis elbow" from lugging this book around the past four weeks, but it is worth the pain, the effort, and the aspirin. It is also worth the (relatively speaking) bargain price. Including appendixes, this book contains 894 pages of text. The entire panorama of the neural sciences is surveyed and examined, and it is comprehensive in its scope, from genomes to social behaviors. The editors explicitly state that the book is designed as "an introductory text for students of biology, behavior, and medicine," but it is hard to imagine any audience, interested in any fragment of neuroscience at any level of sophistication, that would not enjoy this book. The editors have done a masterful job of weaving together the biologic, the behavioral, and the clinical sciences into a single tapestry in which everyone from the molecular biologist to the practicing psychiatrist can find and appreciate his or
7,563 citations
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TL;DR: In this paper, the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method, is reviewed.
Abstract: This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long-wavelength vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.
6,917 citations
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TL;DR: In this article, the authors present a comprehensive, up-to-date compilation of band parameters for the technologically important III-V zinc blende and wurtzite compound semiconductors.
Abstract: We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.
6,349 citations
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TL;DR: This review gives a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells, and discusses the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells.
Abstract: The need to develop inexpensive renewable energy sources stimulates scientific research for efficient, low-cost photovoltaic devices.1 The organic, polymer-based photovoltaic elements have introduced at least the potential of obtaining cheap and easy methods to produce energy from light.2 The possibility of chemically manipulating the material properties of polymers (plastics) combined with a variety of easy and cheap processing techniques has made polymer-based materials present in almost every aspect of modern society.3 Organic semiconductors have several advantages: (a) lowcost synthesis, and (b) easy manufacture of thin film devices by vacuum evaporation/sublimation or solution cast or printing technologies. Furthermore, organic semiconductor thin films may show high absorption coefficients4 exceeding 105 cm-1, which makes them good chromophores for optoelectronic applications. The electronic band gap of organic semiconductors can be engineered by chemical synthesis for simple color changing of light emitting diodes (LEDs).5 Charge carrier mobilities as high as 10 cm2/V‚s6 made them competitive with amorphous silicon.7 This review is organized as follows. In the first part, we will give a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells. In the second part, we will focus on conjugated polymer/fullerene bulk heterojunction solar cells, mainly on polyphenylenevinylene (PPV) derivatives/(1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61) (PCBM) fullerene derivatives and poly(3-hexylthiophene) (P3HT)/PCBM systems. In the third part, we will discuss the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells. In the fourth part, we will suggest possible routes for further improvements and finish with some conclusions. The different papers mentioned in the text have been chosen for didactical purposes and cannot reflect the chronology of the research field nor have a claim of completeness. The further interested reader is referred to the vast amount of quality papers published in this field during the past decade.
6,059 citations