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 of non-polar a-plane and cubic InN on r-plane sapphire by molecular beam epitaxy

Abstract: Growth of non-polar III-nitrides has been an important subject recently due to its potential improvement on the efficiency of III-nitride-based opto-electronic devices. Despite study of non-polar GaN and GaN-based heterostructures, there are few reports on epitaxial growth of non-polar InN, which is also an important component of the III-nitride system. In this study, we report heteroepitaxial growth of non-polar InN on r -plane sapphire substrates using plasma-assisted molecular beam epitaxy. It is found that when a GaN buffer is used, the following InN film appears to be non-polar (11 2 0) a -plane which follows the a -plane GaN buffer. The room temperature Hall mobility of undoped a -plane InN is around 250 cm 2 /Vs with a carrier concentration around 6×10 18 cm -3 . Meanwhile, if InN film is directly deposited on r -plane sapphire without any buffer, the InN layer is found to consist of a predominant zincblende (cubic) structure along with a fraction of the wurtzite (hexagonal) phase with increasing content with proceeding growth.

Auger electron spectroscopy of silicon carbide

Abstract: The paper is devoted to evaluating the concentration of silicon on the surface of SiC exposed to bombardment by Ar + ions. For quantitative interpretation of the measured Auger spectra, the method was used in which the Auger current is determined as the area below the Auger peak in the direct N(E) spectrum after subtracting the background of inelastically scattered Auger electrons of the given peak and of the so-called true secondary electrons. We have found that bombardment of SiC by Ar + ions with energy 1 keV at an angle of 68 ◦ with respect to the surface normal and with the sample rotating at 6 rpm leads to Si depletion of SiC in comparison with its stoichiometric composition. The retrieved concentration of Si was 35.4at/nm 3 in the case of using the Si LVV spectra, and 39.7at/nm 3 if Si KLL spectra were used. K e y w o r d s: AES, quantitative analysis, sputtering, SiC, AES depth profiling

Poly-silicon CMOS compatible gate module for AlGaN/GaN-on-silicon MIS-HEMTs for power electronics applications

Abstract: In this paper we present a new poly-silicon gate process for AlGaN/GaN MIS-HEMT power transistors. Using a complete metal-free front-end processing of the gate module the process is fully CMOS compatible. Additionally, the gate reliability can be significantly increased. We used a three-step LPCVD SiN passivation fully enclosing the gate electrode made of polycrystalline silicon. As gate dielectrics LPCVD deposited SiN are used with a thickness of 20 nm and 120 nm. We compared these devices with MIS-HEMTs using Al as gate electrode. Constant current measurements have been performed that show with QBD, poiy, 20 nm = 714 C/cm2 and a MTTF0.5A/cm2 = 1293s significant higher charge pumping capability through the gate for the poly-Si gated devices compared to conventional metal gates.

Spurious Mode Suppression in the Design of GCPW Submillimeter-wave Power Amplifiers

Abstract: This work investigates the problem of spurious mode propagation in a grounded coplanar waveguide (GCPW) environment at frequencies between 200 GHz and 335 GHz. Design strategies focused on minimizing undesired effects are explored through full-wave electromagnetic (EM) analysis and experimental results from different test structures. It is shown that a $\lambda/13$ distance between via-holes should be chosen to avoid unwanted resonances at these high frequencies. The critical role of via-holes is also demonstrated through the experimental comparison of two power splitters. Finally, the need of closed RF pads to avoid any propagation of parasitic modes is experimentally shown in an application example of a power amplifier (PA) cell based on a 35 nm GaAs metamorphic high electron mobility transistor (mHEMT) technology. The PA cell with closed pads achieves a 0.5 dB bandwidth of 28 % with small-signal gain levels larger than 5.2 dB.

Excitonic Transitions in Homoepitaxial GaN

Abstract: The photoluminescence spectrum of a high quality homoepitaxial GaN film has been measured as a function of temperature. As temperature increases the recombination of free excitons dominates the spectra. Their energy shift has successfully fitted in that temperature range by means of the Bose-Einstein expression instead of Varshni's relationship. Values for the parameters of both semi-empirical relations describing the energy shift are reported and compared with the literature.

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.