Oliver Ambacher

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.

Tunable refractive beam steering using aluminum nitride thermal actuators

Abstract: Step-structured thermo-mechanical actuators based on aluminum nitride (AlN) thin films and their application in refractive beam steering are investigated. The actuators will tilt a suspended plate and deform a liquid surface to realize a micro-prism. Arrays of tunable micro-prisms will increase the resolution of compound eye systems. A numerical actuator description is presented and the beam geometry is investigated, considering achievable tilt angles and actuator linearity. For an accurate design, the coefficient of thermal expansion (CTE) of AlN is determined, while measuring the bow of a coated silicon substrate at different temperatures. For a temperature difference of 300 K, the results show a maximum tilt angle of 7.1 °, which is independent of actuator length. Furthermore, the fabrication process is introduced and the nano-crystalline structure of AlN at facets, which are caused by pre-structured substrates, is investigated.

High-Gain AlGaN/GaN HEMT Single Chip E-Band Power Amplifier MMIC with 30 dBm Output Power

Abstract: This paper reports on the design, fabrication and measured performance of an E-band power amplifier MMIC delivering >1 W of output power. AlGaN/GaN based high-electron-mobility transistors (HEMTs) with 100 nm of gate length have been processed in a grounded coplanar transmission line technology in order to realize the four-stage high power amplifier chip. For the designed prototype, measurement results yielded small-signal gain of 16-18 dB in the frequency range from 71 to 76 GHz (low E-band). A saturated output power of 30.8 dBm (1.2 W) has been reported under continuous wave (CW) operation with more than 11 dB of power gain, corresponding to a power density of around 1 W/mm at the output of the final stage. This demonstrates superior performance of GaN technology in the millimeter-wave design over other competing technologies.

Evaluation of AlN material properties through vibration analysis of thin membranes

Abstract: In this work, the mechanical properties of thin piezoelectric AlN films along with the methods and instruments to obtain this information with sufficient accuracy via dynamic (vibration) and static analyses of thin membranes are reported In addition, the impact of different damping mechanisms on the amplitude of forced oscillations have been considered in order to obtain the analytical expression relating the resonant amplitude of membrane to the ambient gas pressure (© 2012 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim)

3C-SiC:Ge alloys grown on Si (111) substrates by SSMBE

Abstract: In this work for the first time a comprehensive research of (Si1-xC1-y)Gex+y thin films epitaxially grown on Si (111) substrates by solid source molecular beam epitaxy are presented. The layers were grown at substrate temperatures ranging from 900 °C to 1040 °C with a growth rate of 0.6 nm/min. They were analysed by atomic force microscopy, scanning electron microscopy, X-ray diffraction, Auger electron spectroscopy, secondary ion mass spectroscopy and transmission electron microscopy. The structural analysis revealed that the grown epitaxial layer consists of the cubic polytype. A maximum Ge incorporation of 0.16% was achieved in epitaxial layers grown at 900 °C. It was obtained that the Ge concentration decreases with increasing growth temperature. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of GaN‐based lateral polarity heterostructures

Abstract: GaN-based lateral polarity heterostructures (LPH) were grown on sapphire substrates using molecular beam epitaxy. The LPH samples consist of periodical N-face (000-1) (grown directly on Al 2 O 3 ) and Ga-lace (0001) (grown on a AW bufferlayer) GaN stripes. The GaN-based LPH have been characterized using surface sensitive techniques. The GaN-based LPH consist of smooth Ga-face (0.4 nm RMS) and N-face regions of higher roughness (7 nm RMS) with small (<70 nm as measured by atomic force microscopy) and well-defined inversion domain boundary regions (IDBR). Further investigations have been performed by photoelectron emission microscopy (PEEM), photoelectron spectroscopy and cathodoluminescence to gain information about the surface composition including surface adsorbates as well as electronical and optical properties. PEEM reveals a "perfect" striped pattern with alternating Ga-and N-face regions with the desired periodicity over the entire field of view and, after thermal desorption of adsorbates well-defined IDBRs. In addition we found evidence for an increased chemical reactivity of the IDBs with oxygen.

I and i

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 …

Principles of Neural Science

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

Phonons and related crystal properties from density-functional perturbation theory

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.

Band parameters for III–V compound semiconductors and their alloys

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.

Conjugated polymer-based organic 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.