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L. Wittmer

Bio: L. Wittmer is an academic researcher from Technische Universität München. The author has contributed to research in topics: Wurtzite crystal structure & Molecular beam epitaxy. The author has an hindex of 3, co-authored 3 publications receiving 2442 citations.

Papers
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Journal ArticleDOI
TL;DR: In this article, the authors investigated the role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces.
Abstract: Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlxGa1−xN/GaN/AlxGa1−xN and N-face GaN/AlxGa1−xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15

2,581 citations

Journal ArticleDOI
TL;DR: In this paper, a GaN/AlxGa1-xN/GaN high electron mobility transistors were grown by plasma-induced molecular beam epitaxy in order to study the formation and electric transport properties of two-dimensional electron gases.
Abstract: Nominally undoped GaN/AlxGa1-xN/GaN high electron mobility transistors were grown by plasma-induced molecular beam epitaxy in order to study the formation and electric transport properties of two-dimensional electron gases. By depositing an AlN nucleation layer on sapphire substrates before the growth of the GaN buffer layer, we were able to change the polarity of the wurtzite films from N- to Ga-face. The change in polarity causes a change in the sign of the spontaneous and piezoelectric polarization directed along the c-axis of the strained AlGaN barrier. The sign and the gradient in polarization at one of the GaN/AlGaN interfaces is mainly responsible for the generation and confinement of the two-dimensional electron gas. Ga- and N-face heterostructures with mobilities up to 1050 and 1200 cm2/Vs, respectively, and sheet carrier concentrations of up to 1.2×1013 cm-2 at room temperature were realized. Transistors processed from heterostructures with both polarities show maximum source-drain currents between 800 and 850 mA/mm and a transconductance of up to 250 mS/mm.

35 citations


Cited by
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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: In this article, the authors investigated the role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces.
Abstract: Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlxGa1−xN/GaN/AlxGa1−xN and N-face GaN/AlxGa1−xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15

2,581 citations

Journal ArticleDOI
07 Nov 2002
TL;DR: This paper attempts to present the status of the technology and the market with a view of highlighting both the progress and the remaining problems of the AlGaN/GaN high-electron mobility transistor.
Abstract: Wide bandgap semiconductors are extremely attractive for the gamut of power electronics applications from power conditioning to microwave transmitters for communications and radar. Of the various materials and device technologies, the AlGaN/GaN high-electron mobility transistor seems the most promising. This paper attempts to present the status of the technology and the market with a view of highlighting both the progress and the remaining problems.

1,849 citations

Journal ArticleDOI
TL;DR: This review explores different material classes for plasmonic and metamaterial applications, such as conventional semiconductors, transparent conducting oxides, perovskiteOxides, metal nitrides, silicides, germanides, and 2D materials such as graphene.
Abstract: Materials research plays a vital role in transforming breakthrough scientific ideas into next-generation technology. Similar to the way silicon revolutionized the microelectronics industry, the proper materials can greatly impact the field of plasmonics and metamaterials. Currently, research in plasmonics and metamaterials lacks good material building blocks in order to realize useful devices. Such devices suffer from many drawbacks arising from the undesirable properties of their material building blocks, especially metals. There are many materials, other than conventional metallic components such as gold and silver, that exhibit metallic properties and provide advantages in device performance, design flexibility, fabrication, integration, and tunability. This review explores different material classes for plasmonic and metamaterial applications, such as conventional semiconductors, transparent conducting oxides, perovskite oxides, metal nitrides, silicides, germanides, and 2D materials such as graphene. This review provides a summary of the recent developments in the search for better plasmonic materials and an outlook of further research directions.

1,836 citations

Journal ArticleDOI
TL;DR: In this paper, a combination of high resolution x-ray diffraction, atomic force microscopy, Hall effect, and capacitance-voltage profiling measurements is used to calculate the polarization induced sheet charge bound at the AlGaN/GaN interfaces.
Abstract: Two dimensional electron gases in Al x Ga 12x N/GaN based heterostructures, suitable for high electron mobility transistors, are induced by strong polarization effects. The sheet carrier concentration and the confinement of the two dimensional electron gases located close to the AlGaN/GaN interface are sensitive to a large number of different physical properties such as polarity, alloy composition, strain, thickness, and doping of the AlGaN barrier. We have investigated these physical properties for undoped and silicon doped transistor structures by a combination of high resolution x-ray diffraction, atomic force microscopy, Hall effect, and capacitance‐voltage profiling measurements. The polarization induced sheet charge bound at the AlGaN/GaN interfaces was calculated from different sets of piezoelectric constants available in the literature. The sheet carrier concentration induced by polarization charges was determined

1,439 citations