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.

Growth mechanism and electronic properties of epitaxial In2O3 films on sapphire

Abstract: In this work, we report on the epitaxial growth of high-quality cubic indium oxide thick films on c-plane sapphire substrates using a two-step growth process. The epitaxial relationship of In2O3 on (0001) Al2O3 has been investigated. The (222) plane spacing and lattice parameter of a most strain-relaxed high-quality In2O3 film have been determined to be 292.58 pm and 1013.53 pm, respectively. The electronic properties in dependence of the film thickness are interpreted using a three-region model. The density at the surface and interface totals (3.3±1.5)×1013cm-2, while the background electron density in the bulk was determined to be (2.4±0.5)×1018cm-3. Furthermore, post treatments such as irradiation via ultraviolet light and ozone oxidation have been found to influence only the surface layer, while the bulk electronic properties remain unchanged.

Surface properties of stoichiometric and defect-rich indium oxide films grown by MOCVD

Abstract: The influence of metalorganic chemical vapor deposition growth conditions on the indium oxide surface properties is investigated using photoelectron spectroscopy. Particular attention is paid to nanocrystalline samples grown at fairly low temperatures which are known for their high sensitivity to ozone. The results are compared to measurements on In2O3 films in cubic and rhombohedral crystal structure. It is shown that the growth conditions have a strong impact on the physical properties and that samples grown at 200 °C or below are highly oxygen-deficient and rich in defects, influencing the surface chemical and electronic properties and resulting in high ozone sensitivity.

Development of a high transconductance GaN MMIC technology for millimeter wave applications

Abstract: In this paper we report on the development of high transconductance GaN-based high electron mobility transistors (HEMTs) to improve the performance at W-Band frequencies. At first, the influence of the barrier thickness on the maximum transconductance (gm,max) was investigated by using two different technologies: growth of thin barrier layers and deep gate recess. Second, the effect of a gate length reduction down to 100 nm on gm,max was examined. The reduction of the barrier thickness results in a strong increase of the extrinsic transconductance up to 600 mS/mm. The technology was then used to fabricate HEMTs, with a cut-off frequency of 110 GHz, which are compatible to a MMIC technology (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AlGaN-based ultraviolet light detectors with integrated optical filters

Abstract: AlGaN-based ultraviolet light detectors with high responsivities in a narrow range of photon energy (≈0.3 eV) were grown by plasma induced molecular beam epitaxy on sapphire substrates. A thin Al0.4Ga0.6N film acting as an optical filter blocking high energy light was separated by an insulating AlGaN barrier from a third light sensitive Al0.3Ga0.7N layer. By optimizing the alloy compositions in the heterostructure and thickness of the filter layer, a peak responsivity of up to 35 A/W was achieved over a narrow range of wavelength with a peak position at 305 nm. The rejection of visible light response with respect to the peak responsivity was 5 orders of magnitude for a photoconductor device with a filter thickness of 1.4 μm. Decay times of the photoresponse after excimer laser exposure were determined to be between 40 and 330 ns.

Observation of ion-induced changes in the channel current of high electron mobility AlGaN/GaN transistors (HEMT)

Abstract: We report on the observation of channel current modulation by exposing AlGaN/GaN high electron mobility transistors (HEMT) to fluxes of ions of different signs generated by an ion spray technique. In these experiments, the gate was directly exposed to the ion flux without intermediate insulating or metallic layers. We were able to vary the channel current over several orders of magnitude in a reversible manner. The effect is likely to be caused by the compensation of bound ions at the GaN surface. Using this effect, we were able to realize a miniaturized charge-amplifying device sensitive to the sign and quantity of ion fluxes, with an amplification factor of about 1000.

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.