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.

Polarization induced 2D hole gas in GaN/AlGaN heterostructures

Abstract: The generation of high density 2D hole gases is crucial for further progress in the field of electronic and optoelectronic nitride devices. In this paper, we present results of C-V profiling measurements for N-face GaN/AlGaN heterostructures and systematic theoretical studies of Mg- doped GaN/AlGaN gated heterostructures and superlattices. Our calculations are based on a self- consistent solution of the multiband k.p Schrodinger and Poisson equation and reveal that the hole 2D sheet density is mainly determined by the polarization induced interface charges. For an Aluminium concentration of 30%, the induced hole density in the heterostructure can reach values up to 1.5×1013 cm−2. In the GaN/AlGaN superlattices, the hole sheet density increases with the superlattice period and saturates for a period of 40 nm at a value of 1.5×1013 cm−2.

In-Plane Oriented Stacks of c-AlScN/Mo (110) for BAW Resonators Grown by Magnetron Sputter Epitaxy

Abstract: Magnetron sputter epitaxy was used to grow AlScN/Mo/AlN stacks on Si(111) substrates. The influence of temperature on the in-plane orientation of various layers was studied using XRD pole figures and ϕ-scans. As an alternative route to improve the quality of AlScN grown on Mo, Mo layers were annealed at various temperatures prior to overgrowth and the quality of the layers was evaluated using AFM, XRD, XRR and non-contact sheet resistance measurements.

Atomic layer deposited P-type nickel oxide and cobalt oxide for ethanol gas sensing

Abstract: We present a comprehensive study of atomic layer deposited materials for ethanol gas sensing with large-scale fabrication techniques. Ultra-thin nickel oxide and cobalt oxide films show very promising performance in ethanol sensing. The films were measured at different temperatures and humidity levels. Unlike most semiconducting metal oxides used for gas sensing, nickel and cobalt oxide are p-type materials. The reactivity and sensitivity of p-type metal oxides offer their full potential only in thin compact form, where the highly resistive bulk does not dominate the current. Atomic layer deposition is an excellent method of fabrication for ultra-thin films and offers a versatile palette of potential semiconducting metal oxides materials for more target gases.

A GaN-on-Si-Based Logic, Driver and DC-DC Converter Circuit with Closed-Loop Peak Current-Mode Control

Abstract: This work presents an entirely GaN-implemented DC-DC converter circuit with closed-loop peak currentmode control. The circuit combines logic inverter, level-shifter, differential amplifier, comparator, NOR-gate, RS flip-flop, power transistor with integrated gate driver, and power diode, all based on a d-mode GaN-on-Si technology. A prototype converter (200-to-100 V, 500 kHz, 100 W, 97% efficiency) was designed and characterized. The GaN-based control circuit requires only 0.5% active chip area compared to the GaN-based driver and power devices. Thus, the work demonstrates that the addition of basic logic circuits in GaN power ICs enables a high gain in functionality at negligible additional area requirement.

Localized donor state above the conduction band minimum in InN revealed by high pressure and temperature transport experiments

Abstract: We have studied the pressure and temperature dependences of Hall electron concentration and mobility in degenerated, not intentionally doped InN samples. The results obtained for a whole set of samples with electron concentrations from 5.4 x 10 17 cm -3 to 3.3 x 10 18 cm -3 consistently revealed the existence of a localized donor type state resonant with the conduction band. The concentration of this state is of the same order in all studied samples and not exceeds 10 18 cm -3 . Population of this state determines electrical properties of InN samples with low carrier concentration. This donor state is not the main source of conducting electrons in these not intentionally doped samples and can be entirely populated and hidden in samples with high electron concentration exceeding about 3 x 10 18 cm -3 .

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.