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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
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Journal ArticleDOI
TL;DR: In this article, the authors observed exciton dead layers in Pt/GaN Schottky diodes with N and Ga-face polarity and showed that these layers determine the shape of electroreflectance and photocurrent spectra close to the band gap in dependence on the bias voltage.
Abstract: For the first time, we observed exciton dead layers in Pt/GaN Schottky diodes with N and Ga-face polarity. We show that these layers determine the shape of electroreflectance and photocurrent spectra close to the band gap in dependence on the bias voltage. Analysis of experimental data using a spatially dependent dielectric function of GaN due to an inhomogeneous electric field in the depletion region yields the ionised impurity concentration and the surface band bending. In addition, exciton and broadening energies for the field-free limit are obtained.

2 citations

Journal ArticleDOI
TL;DR: In this paper, a 3D strain relaxation mechanism was proposed to yield unstrained, high quality epitaxial GaN layers on nanocrystalline AlN thin film by means of a three-dimensional strain relaxation.
Abstract: Usually, the fabrication of microelectromechanical systems (MEMS) requires unstrained or tensile strained active layers on a selectively removable sacrificial layer. Compressive strain would lead to instabilities due to buckling effects. For group III-nitride based MEMS, AlN is a promising material for sacrificial layers since it can be epitaxially overgrown and etched selectively to GaN. However, due to the larger lattice constants GaN is growing compressively strained on AlN. Nanoheteroepitaxy opens a way to yield unstrained, high quality epitaxial GaN layers on nanocrystalline AlN thin film by means of a 3D strain relaxation mechanism. For this purpose sputtered nanocrystalline AlN films were overgrown with single crystalline GaN and AlGaN/GaN layers by metalorganic chemical vapor deposition. The high quality of the layers is proven by an atomically flat surface and a 2D electron gas at the interface of the AlGaN/GaN heterostructure.

2 citations

Proceedings ArticleDOI
16 Jun 2014
TL;DR: In this article, a portable, low-cost focusing system based on compact dielectric lenses and its suitability for the use in a wide band 94 GHz FMCW radar for civil safety applications is discussed.
Abstract: In this paper we discuss the design of a portable, low-cost focusing system based on compact dielectric lenses and its suitability for the use in a wide band 94 GHz FMCW Radar for civil safety applications. A Vivaldi antenna transmits the Radar signal directly into a dielectric cone in order to control the aperture angle. By using an additional lens system, the signal can be focused at targets in the range of several meters. Dielectric properties and dispersion of different materials have been investigated at W-band frequencies to optimize the lenses within the systems based on quasi-optical ray tracing models.

2 citations

Proceedings ArticleDOI
01 Oct 2016
TL;DR: In this paper, a scalable matrix shell model for microwave dual-gate and cascode HEMTs is presented, which is based on 2.5D EM simulation of the passive metallization structures.
Abstract: A new versatile parasitic network extraction method for microwave dual-gate and cascode HEMTs is presented which is based on 2.5D EM simulation of the passive metallization structures. After port count reduction and offset capacitance correction at the internal FET reference planes, a scalable matrix shell model is obtained. The intrinsic FET model is extracted from a separate characterization of a common-source FET. The matrix shell model is compared to a fully distributed model and to measured S-parameters for AlGaN/GaN HEMTs with a gate length of 0.1 μm.

2 citations

Journal ArticleDOI
TL;DR: In this paper, a technological process flow enabling fabrication of transversal nanofeatures in nanocrystalline diamond-based MEMS is presented, which can be used for fabrication of nanostructured templates, projection exposure lithography and bi-layer electron beam lithography.

2 citations


Cited by
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[...]

08 Dec 2001-BMJ
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

Journal ArticleDOI
06 Jun 1986-JAMA
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

Journal ArticleDOI
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

Journal ArticleDOI
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

Journal ArticleDOI
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