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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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TL;DR: In this article, photoluminescence measurements under intense magnetic fields were performed in indium nitride samples with carrier concentration ranging from about $7.5 to $5.5.
Abstract: We present photoluminescence measurements under intense magnetic fields ($B$ up to 30 T) in $n$-doped indium nitride samples with carrier concentration ranging from about $7.5\ifmmode\times\else\texttimes\fi{}{10}^{17}\text{ }{\text{cm}}^{\ensuremath{-}3}$ to $5\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }{\text{cm}}^{\ensuremath{-}3}$. The observation of transitions involving several Landau levels permits to determine the carrier-reduced mass $\ensuremath{\mu}$ around the $\ensuremath{\Gamma}$ point. Depending on the carrier concentration, we find $\ensuremath{\mu}$ ranging between $0.093{m}_{0}$ and $0.107{m}_{0}$ (${m}_{0}$ is the electron mass in vacuum). This finding poses a lower limit to the electron effective mass, whose unexpectedly large value $({m}_{e}\ensuremath{\ge}0.093{m}_{0})$ indicates that the sources of $n$ doping in InN perturb strongly the crystal conduction band near its minimum.

25 citations

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TL;DR: In this article, a systematic study of epitaxial growth, processing technology, device performance and reliability of GaN HEMTs and MMICs manufactured on 3-inch SiC substrates is presented.
Abstract: We present a systematic study of epitaxial growth, processing technology, device performance and reliability of our GaN HEMTs and MMICs manufactured on 3 inch SiC substrates. Epitaxy and processing are optimized for both performance and reliability. The deposition of the AlGaN/GaN HEMT epitaxial structures is designed for low background carrier concentration and a low trap density in order to simultaneously achieve a high buffer isolation and low DC to RF dispersion. Device fabrication is performed using standard processing techniques involving both electron-beam and stepper lithography. Gate lengths of 250 nm and 500 nm are employed for 10 GHz and 2 GHz applications, respectively. The developed HEMTs demonstrate excellent high-voltage stability, high power performance and large power added efficiencies. Devices exhibit two-terminal gate―drain breakdown voltages in excess of 160 V (current criterion 1 mA/mm) across the entire 3 inch wafer with parasitic gate and drain currents well below 1 mA/mm when biased up to 80 V drain bias under pinch-off conditions. Load-Pull measurements at 2 GHz on 800 μm gate width devices return a well-behaved relationship between bias-voltage and output-power as well as power-added- efficiencies beyond 60% up to U DS = 100 V. For a drain bias of 100 V an output-power-density around 22 W/mm with 26 dB linear gain is obtained. On large devices (32 mm gate width packaged in industry-standard ceramic packages) an output power beyond 100 W is achieved with a PAE above 50% and a linear gain around 15 dB. Dual-stage MMICs in microstrip transmission line technology yield a power added efficiency of 40% at 8.56 GHz for a power level of 11 W. A single-stage MMIC yields a PAE of 46% with 7 W of output power at V DS = 28 V. Reliability is tested on HEMT devices having a gate periphery of 8 x 60 μm at an operating bias of 50 V under both DC and RF conditions. About 10% drain-current change under DC-stress (50 mA/mm) is observed af- ter more than 1000 h of operation with an extrapolated drain- current degradation below 20% after 200000 h (more than 20 years) of operation. Under RF stress (2 GHz, 1 dB compression) the observed change in output power density is be- low 0.2 dB after more than 1000 h.

25 citations

Journal ArticleDOI
TL;DR: In this article, the effect of oxygen and methane reactive ion etching plasma on the electrical characteristics of nominally undoped GaN/Al 0.25 Ga 0.75 N/GaN high mobility heterostructures on sapphire substrates grown by plasma-induced molecular beam epitaxy and metalorganic chemical vapor deposition (MOCVD).
Abstract: Plasma-induced damage often reduces the electrical and optical performance and the lifetime of compound semiconductor devices. We have investigated the effect of oxygen and methane reactive ion etching plasma on the electrical characteristics of nominally undoped GaN/Al 0.25 Ga 0.75 N/GaN high mobility heterostructures on sapphire substrates grown by plasma-induced molecular beam epitaxy and metalorganic chemical vapor deposition (MOCVD). The electrical transport properties of the two-dimensional electron gas in the AlGaN/GaN heterostructures were investigated by a combination of capacitance–voltage profiling and Hall effect measurements. The performance degradation of the heterostructures is attributed to the reduction of the carrier density and mobility due to ion bombardment, which is causing a creation of surface and deep acceptor states. After rapid thermal annealing (RTA) at temperatures between 400°C and 800°C, the electrical properties of the heterostructures exposed at moderated plasma power density and bias were mostly recovered. However, samples exposed at high power density lost the significant part of the electron sheet carrier concentration unrecoverable even after RTA at 800°C.

25 citations

Journal ArticleDOI
TL;DR: In this paper, compositional fluctuations in GaN/GaInN/GAN double heterostructures investigated by photoluminescence excitation and resonant Raman spectroscopy are reported.
Abstract: We report on compositional fluctuations in GaN/GaInN/GaN double heterostructures investigated by photoluminescence excitation and resonant Raman spectroscopy. The energy and line-shape of both luminescence and longitudinal optical (LO) phonon Raman peaks strongly depend on excitation energy due to selective excitation of regions with different In content. For a given excitation energy, luminescence from In-rich regions takes place and in addition, with increasing sample temperature, resonantly excited luminescence from regions of lower In content is superposed. Thus, the luminescence strongly broadens and on average shifts to higher energies with increasing temperature. The spectral variation of the photoluminescence and Raman cross sections is determined and correlated with the GaInN absorbance as measured by photothermal deflection spectroscopy.

25 citations

Journal ArticleDOI
TL;DR: In this paper, GaN substrates of up to 2-mm diameter were grown on the Ga-face and N-face of these substrates with a low-pressure MOCVD system and PL spectra resolved the bound and free exciton peaks from the overgrown GaN.
Abstract: Free-standing GaN substrates of up to 2″ diameter can be produced by delamination of thick HVPE (hydride vapour phase epitaxy) films from sapphire with a laser-induced liftoff process. Homoepitaxial films of up to 2 μm thickness were grown on the Ga-face and N-face of these substrates with a low-pressure MOCVD system. The pre-overgrowth polishing of the substrates influences the final surface morphology and growth modus, which also depend on the crystal polarity. PL spectra resolve the bound and free exciton peaks from the overgrown GaN.

25 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