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.

Effect of dislocations on electrical and electron transport properties of InN thin films. II. Density and mobility of the carriers

Abstract: The influence of dislocations on electron transport properties of undoped InN thin films grown by molecular-beam epitaxy on AlN(0001) pseudosubstrates is reported. The microstructure and the electron transport in InN(0001) films of varying thickness were analyzed by transmission electron microscopy and variable temperature Hall-effect measurements. It was found that crystal defects have strong effects on the electron concentration and mobility of the carriers in the films. In particular, the combined analysis of microscopy and Hall data showed a direct dependence between free carrier and dislocation densities in InN. It was demonstrated that threading dislocations are active suppliers of the electrons and an exponential decay of their density with the thickness implies the corresponding decay in the carrier density. The analysis of the electron transport yields also a temperature-independent carrier concentration, which indicates degenerate donor levels in the narrow band-gap InN material. The relative in...

Evidence of electron accumulation at nonpolar surfaces of InN nanocolumns

Abstract: High-quality InN nanocolumns grown by molecular beam epitaxy on n-type Si(111) have been electrically characterized by atomic force microscopy. Current-voltage characteristics were measured on InN nanocolumns with similar heights but different diameters. The conductivity scales the nanocolumns reciprocal diameter, pointing to the nanocolumn lateral surface as the main conduction path. These results, opposing those found in undoped GaN nanocolumns where the conductivity is rather independent of the diameter (conduction through the volume), agree well with a model that predicts electron accumulation by Fermi level pinning within the conduction band on nonpolar (m plane) InN surfaces reconstructed under In-rich conditions.

AlGaN/GaN biosensor—effect of device processing steps on the surface properties and biocompatibility

Abstract: The impact of typical device processing steps (KOH, HCl, HF wet chemical etching, SF 6 and Cl plasma etching) on the surface properties (roughness, chemical composition, contact angle to water) of group III-nitride based chemical sensors is investigated with emphasis on the electrical performance of the sensor and the biocompatibility. An AlGaN/GaN high electron mobility transistor serves as basic sensing device. For our studies, the widely distributed mammalian cell cultures HEK 293FT and CHO-K1 are used as biological model systems. The processing of the devices has only little influence on the cell growth onto the sensor, which is in all cases superior to that on silicon surfaces. Fluorine dry etching leads to oxidation and smoothing of the surface, thus, improving the electrical properties of the AlGaN/GaN sensor. In contrast, autoclave treatment enhances the carbon contamination with negative impact on the sensor properties and increased the contact angle to water, which can be used as indicator for the state of the sensor surface. For all other treatments the contact angle recaptures a stable value of about 50 ± 5° after exposure to air or water droplets for some hours due to contamination by hydrocarbons.

Defect-related optical transitions in GaN.

Abstract: Sub-band-gap absorption of GaN grown by metal-organic chemical vapor deposition on sapphire was investigated by photothermal deflection spectroscopy (PDS), transmission measurements, and the constant photocurrent method (CPM). We determine acceptor binding energies in undoped GaN at 220 and about 720 meV. A comparison between absorption and CPM spectra yields the dependence of the quantum efficiency-mobility-lifetime-product ($\ensuremath{\eta}\ensuremath{\mu}\ensuremath{\tau}$) versus energy and gives relevant information about the excitation mechanisms. CPM spectra show a significantly smaller absorption (up to a factor of $\frac{1}{10}$) in the range between 3.0 and 3.3 eV as compared to PDS. This indicates that the majority of carriers excited with these photon energies have a relatively small $\ensuremath{\eta}\ensuremath{\mu}\ensuremath{\tau}$ product and thus do not contribute to the externally detected photocurrent. We propose that in this energy range the spectrum is dominated by interband absorption in isolated cubic-phase crystallites in the hexagonal matrix and by excitation of electrons from occupied acceptors into the conduction band of the main hexagonal crystal modification ($h$-GaN). Temperature-dependent photoluminescence measurements, excited with energies below and above the direct band gap of hexagonal GaN, confirm this interpretation and can be correlated with the subgap absorption detected by PDS. Transient photocurrent measurements show a persistent photoconductivity, which can also be explained by the existence of isolated cubic-phase inclusions.

Integration of In2O3 nanoparticle based ozone sensors with GaInN∕GaN light emitting diodes

Abstract: There is a high demand for compact low-cost ozone sensors. It has been shown recently that In2O3 nanolayers can act as ozone sensitive films activated at room temperature by ultraviolet light. In the present work, the authors integrate ultrathin layers of In2O3 nanoparticles and a GaInN∕GaN based blue light emitting diode (LED) on a single sensor chip. The integrated sensor was found to be sensitive to O3 concentrations as low as ∼40ppb. These results demonstrate that by integrating GaInN∕GaN based blue LEDs and metal oxide sensing layers back to back on a single chip, compact and robust gas sensors can be realized.

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.