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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 morphology and surface electronic structure of indium nitride films grown by plasma-induced molecular beam epitaxy have been studied using atomic force microscopy as well as X-ray and ultraviolet photoelectron spectroscopy.

38 citations

Proceedings Article
22 May 2011
TL;DR: In this paper, a metamorphic HEMTMMIC with 20 nm gate length was developed for the fabrication of terahertz-wave monolithic integrated circuits (TMICs) with operational frequencies beyond 500 GHz.
Abstract: A metamorphic HEMT MMIC technology with 20 nm gate length is presented, developed for the fabrication of terahertz-wave monolithic integrated circuits (TMICs) with operational frequencies beyond 500 GHz. The MBE grown transistor heterostructure comprises a strained In 0.8 Ga 0.2 As channel with high electron mobility and high electron density for proper device scaling. The realized mHEMTs achieve a source resistance R S of 0.1 Ωmm which is required to minimize resistive losses in combination with an extrinsic maximum transconductance g m_max of 2500 mS/mm. The output characteristics of the 20 nm devices show no short channel effects and demonstrate sufficient pinch-off behavior for analog applications. For a transistor with 2 × 10 µm gate width a cut-off frequency ƒ T of 660 GHz was extrapolated which is to our knowledge the highest published ƒ T for any HEMT device. The presented 20 nm mHEMT technology was employed for the design of a compact four stage lownoise amplifier (LNA). The total small signal gain of the LNA exceeds 20 dB from 115 – 175 GHz.

38 citations

Journal ArticleDOI
TL;DR: In this article, the authors investigate Al0.22Ga0.78N/GaN high electron mobility transistors with GaN cap layer thicknesses of 0, 1, and 3 nm.
Abstract: We systematically investigate Al0.22Ga0.78N/GaN high electron mobility transistors with GaN cap layer thicknesses of 0, 1, and 3 nm. All samples have electron mobilities around 1700 cm2/Vs and sheet carrier concentrations around 8×1012 cm−2 as determined by Hall effect measurements. From photoreflectance measurements we conclude that the electric field strength within the AlGaN barrier increases with GaN cap layer thickness leading to a broadening of the transition peaks as determined by spectroscopic ellipsometry. The surface potential as determined by photoreflectance varies in the range between 0.585 and 0.249 eV dependent on the thickness of the GaN cap. Device results show a significant decrease in Ohmic contact resistance, an increase in ideality factor, a decrease in gate and drain leakage currents, an increase in gain, and an increase in power added efficiency with increasing cap layer thickness. Finally, devices with GaN cap show an improved direct current reliability compared to their counterpar...

37 citations

Journal ArticleDOI
TL;DR: In this paper, the intermediate indium oxide layer presents a body centered cubic (bcc) structure with bcc-In2O3(001)‖Al2O 3(0001) plane relationship, which offers a promising expectation to obtain high quality cubic InN layers superior to other highly mismatched cubic substrates used previously.
Abstract: Cubic InN layers were grown by molecular beam epitaxy on buffer layers of indium oxide prepared onto sapphire (0001) substrates. The structure was analyzed by means of electron diffraction and transmission electron microscopy. The intermediate indium oxide layer presents a body centered cubic (bcc) structure, with bcc-In2O3(001)‖Al2O3(0001) plane relationship. Thereupon, a zinc-blende phase of InN (001) was grown with a reticular misfit of 1.6% and a significant reduction of mismatch-related defects. This good coherence offers a promising expectation to obtain high quality cubic InN layers superior to other highly mismatched cubic substrates used previously.

37 citations

Journal ArticleDOI
TL;DR: In this paper, single crystalline and textured In2O3 thin films with (1 − 1 − 1) surface orientation were used as a model system to study the role of bulk and surface electron accumulation layer conductance for ozone sensing.
Abstract: Single crystalline and textured In2O3 thin films with (1 1 1) surface orientation, grown by plasma-assisted molecular beam epitaxy, were used as a model system to study the role of bulk and surface electron accumulation layer conductance for ozone sensing. Both conductance contributions, which add to the total film conductance, were systematically varied. The resulting ozone sensitivity was determined by total conductance measurements in synthetic air with defined ozone concentration using UV irradiation instead of heating to regenerate the In2O3 surface. Depletion of the surface electron accumulation by an oxygen plasma treatment, confirmed by X-ray photoelectron spectroscopy, rendered the films ozone insensitive. The ozone response of films with an accumulation layer was increased by thickness reduction or by designing the bulk of the film semi-insulating using deep acceptor doping by Mg. Our results of using electron accumulation layers for gas sensing and bulk doping by deep acceptors to increase sensitivity can be generalized to other gas sensing materials. The use of single crystalline films allows selecting the most sensitive crystallographic surface orientation and may have further advantages over polycrystalline films, such as increased stability and sensing speed.

37 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