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.

Method for separating two material layers

Abstract: The invention relates to a method for separating two material layers in such a way that both separated material layers remain essentially intact. According to the invention, an electromagnetic beam is radiated on an interface separating said layers and marking the place where the material layers are to be separated or in an area close to this interface through one of the two material layers. The electromagnetic beam is absorbed in the interface or in the area close to the interface. The absorption induces a separation of the materials on the interface.

Non-Polar a-plane AlScN($11\overline{2}0$) Thin Film Based SAW Resonators with Significantly Improved Electromechanical Coupling

Abstract: $1\ \mu\mathrm{m}$ thick non-polar a-plane AlScN( $11\overline{2}0$ ) thin films were prepared by magnetron sputter epitaxy on r-plane Al2O3( $1\overline{2}02$ ) substrates. X-ray diffraction (XRD) measurements confirmed it as a single-phase wurtzite a-plane AlScN( $11\overline{2}0$ ). Surface acoustic wave (SAW) resonators with wavelengths $\lambda=\{2, 2.5,\ 3,\ 4,\ 5,\ 6,\ 7,\ 8, 10\}\ \mu\mathrm{m}$ were fabricated on a-plane $\mathrm{Al}_{0.77}\mathrm{Sc}_{0.23}\mathrm{N}(11\overline{2}0)$ , a-plane $\mathrm{Al}_{0.68}\mathrm{Sc}_{0.32}\mathrm{N}(11\overline{2}0)$ , and c-plane $\mathrm{Al}_{0.77}\mathrm{Sc}_{0.23}\mathrm{N}(0001)$ thin films. Compared to c-plane $\mathrm{Al}_{0.77}\mathrm{Sc}_{0.23}\mathrm{N}$ -based resonators, 1.85–10 times higher effective electromechanical coupling $k_{\mathrm{eff}}^{2}$ was observed in the whole investigated resonator wavelength range. 50% higher figure of merit was obtained from a-plane Al0.77Sc0.23N-based highest frequency (1.71 GHz) SAW resonators and as high as 4.1% coupling was achieved for a-plane Al0.68Sc0.32N-based resonators (1.39 GHz). Moreover, SAW propagation properties of $\mathrm{AlScN}(11\overline{2}0)/\mathrm{Al}_{2}\mathrm{O}_{3}(1\overline{1}02)$ and AlScN(0001)/ Al2O3(0001) were analyzed using finite element method (FEM) simulations, the results were compared and they confirmed the experimentally observed trends.

Structural and photoconducting properties of MBE and MOCVD grown III-nitride double-heterostructures

Abstract: Investigations on standing wave (SW) interferometry come in focus of interest in the course of ongoing miniaturization of high precision length measurement systems. A key problem within these efforts is the development of a transparent ultra-thin photodetector for sampling the intensity profile of the generated SW. Group III-materials are promising candidates to ensure a good photodetector performance combined with the required optical transparency. In this work, we report on the interrelation of strain and dislocation density along with the influence of the structural properties on the sensitivity of double-heterostructure III-nitride photodetectors grown by molecular beam and metal organic vapour phase epitaxy.

Broadband absorption and emission millimeter-wave spectroscopy between 220 and 325 GHz

Abstract: A millimeter-wave spectroscope for the detection of triatomic gases has been constructed and characterized for frequencies between 230 and 325 GHz (H-band). The achieved results demonstrate a high sensitivity and low threshold detection. A circular lensed horn antenna transmits millimeter- waves into a gas-filled vacuum tube and excites triatomic gas molecules to a higher energy level, if the rotational resonance frequency of the molecule matches with the excitation frequency. At the other end of the tube a second lensed horn antenna receives the propagated electromagnetic wave and the millimeter-wave power is measured by a heterodyne receiver. By sweeping the radiated transmit frequency, the molecules' specific absorption can be detected. The measured absorption results are superimposed by standing wave effects within the tube. To eliminate the standing wave effects, spectroscopy on the basis of rotational spontaneous millimeter-wave emission was examined. This kind of spectroscopy decouples the transmitted from the received signal, whereby independent excitation and detection of the molecules are realized. The use of additional absorbers at the end of the gas tube decreases the decay time of the radiated wave inside the gas cell. In this paper, the detection of spontaneous emission of triatomic gas molecules with the use of a pulse-controlled transmitter and receiver is shown. Optimizations improved the stability and reproducibility of the measurements, and the detection threshold of nitrous oxide could be decreased to a ratio of 1/400. Furthermore, the implementation of a differential measurement method reduces the measurement time by a factor of 150 and simultaneously decouples of environmental influences.

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.