46 research outputs found

    Epitaxial pulsed laser crystallization of amorphous germanium on GaAs

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    We have investigated the crystallization of amorphous germanium films on GaAs crystals using nanosecond laser pulses. The structure and composition of the crystallized layers is dominated by nonequilibrium effects induced by the fast cooling process following laser irradiation. Perfect epitaxial films are obtained for fluencies that completely melt the Ge film, but not the substrate. For higher fluencies, partial melting of the substrate leads to the formation of a (GaAs)(1-x)Ge-2x epitaxial alloy with a graded composition profile at the interface with the substrate. Since Ge and GaAs are thermodynamically immiscible in the solid phase, the formation of the alloy is attributed to the suppression of phase separation during the fast cooling process. Lower laser fluencies lead to polycrystalline layers with a patterned surface structure. The latter is attributed to the freeze-in of instabilities in the melt during the fast solidification process. (C) 2001 American Institute of Physics.9052575258

    Temperature and Magnetic Field Effects on the Transport Controlled Charge State of a Single Quantum Dot

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    Individual InAs/GaAs quantum dots are studied by micro-photoluminescence. By varying the strength of an applied external magnetic field and/or the temperature, it is demonstrated that the charge state of a single quantum dot can be tuned. This tuning effect is shown to be related to the in-plane electron and hole transport, prior to capture into the quantum dot, since the photo-excited carriers are primarily generated in the barrier

    Competing magnetic interactions in MnAs studied via thin film domain pattern analysis

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    Manganese arsenide is one of the few ferromagnetic metals that can be grown on semiconductor substrates as a thin film with high structural perfection. The coupled magnetic and structural phase transition around 40°C leads to a variety of different phenomena such as the self-organized stripe formation on GaAs(001) substrates or the anisotropic lattice shrinkage. By investigating the domain pattern in the phase coexistence region we provide experimental evidence that the magnetic order is due to competing ferromagnetic double-exchange and antiferromagnetic direct exchange interactions. This scenario corroborates recent theoretical calculations and may explain the frequently observed angle of 38° in the domain pattern of epitaxial MnAs films. © 2005 The American Physical Society

    Semiautomatic wet chemical etching of an array of MnAs nanodots and their magnetic properties

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    MnAs layers grown on GaAs substrates are processed to arrays of submicron-scale islands in a semiautomatic manner by wet chemical etching. Magnetic-force microscopy reveals the importance of the nonuniform material characteristics of MnAs layers originating from the stress for the island formation. The fabrication technique relies on the cracks generated in the MnAs layers by the etching. The stress due to the volume change associated with the phase transition between the α and β phases of MnAs is indicated to be responsible for the crack generation. © 2004 Elsevier B.V. All rights reserved

    Semiautomatic wet chemical etching of an array of MnAs nanodots and their magnetic properties

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    MnAs layers grown on GaAs substrates are processed to arrays of submicron-scale islands in a semiautomatic manner by wet chemical etching. Magnetic-force microscopy reveals the importance of the nonuniform material characteristics of MnAs layers originating from the stress for the island formation. The fabrication technique relies on the cracks generated in the MnAs layers by the etching. The stress due to the volume change associated with the phase transition between the α and β phases of MnAs is indicated to be responsible for the crack generation. © 2004 Elsevier B.V. All rights reserved

    Surface-acoustic-wave transducers for the extremely-high-frequency range using AlN/SiC(0001)

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    The fast sound propagation in AlN layers is demonstrated to enable generation of surface acoustic waves (SAWs) by interdigital transducers at a frequency beyond 30 GHz. While scaling down the wavelength of the transducers fabricated on AlN/SiC structures and the thickness of the AlN top layer to raise the operation frequency, the excitation of SAWs becomes intricate due to the weak electromechanical coupling in SiC. We examine the dependence of the feasibility of SAW generation on the AlN layer thickness

    High-aspect ratio patterning of MnAs films

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    We report the high-aspect ratio patterning of epitaxial MnAs-on-GaAs(0 0 1) films. The control of strain is key since MnAs-on-GaAs(0 0 1) exhibits a strain-stabilized coexistence of two chemically, elastically and magnetically distinct phases forming a self-organized stripe structure over a temperature range of 10-40 °C. Anisotropic plasma etching allows for high-aspect ratios and good reproducibility. Using Ti films as an etch mask, arbitrarily oriented structures can be transferred into films of up to 300 nm thickness. The removal of the masking material is challenging as MnAs reacts with all common acids, alkalis and even water. Optimum results are obtained by etching the Ti mask in hydrofluoric acid at elevated temperatures (>50 °C), where MnAs is entirely in its β-phase. © 2006 IOP Publishing Ltd

    Micromagnetic properties of MnAs-on-GaAs(001) films

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    Strained MnAs films on GaAs(001) grown by molecular beam epitaxy exhibit unique micro-magnetic properties due to the strain-mediated coexistence of hexagonal ferromagnetic a-MnAs and orthorhombic paramagnetic β-MnAs arranged in selforganized periodic stripe patterns. To explore the internal structure of the magnetization, which is not accessible by the magnetic imaging techniques AFM and XMCDPEEM, detailed micromagnetic simulations are needed. Otherwise, physically unreasonable models would be developed. © 2006 WILEY-VCH Verlag GmbH and Co. KGaA

    Micromagnetic properties of epitaxial MnAs films on GaAs surfaces

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    We present a systematic study of the micromagnetic properties of MnAs deposited by molecular-beam epitaxy on GaAs(001) and GaAs(111)B surfaces. In epitaxial MnAs films, the strain state in MnAs-on-GaAs(001) (anisotropic) and MnAs-on-GaAs(111)B (isotropic) has a strong influence on the magneto-structural phase transition and thus the micromagnetic properties. The ferromagnetic α and the β phase coexist over a wide temperature range exhibiting self-organized, magnetically coupled nanostructures. Independent of the substrate orientation, magnetic flux-closure domain patterns are formed in the basal plane of MnAs. The spatial distribution of the phases in equilibrium (stripes and quasi-hexagonal islands, respectively) stabilizes various magnetic states, which were found experimentally and confirmed by micromagnetic simulations. © 2007 WILEY-VCH Verlag GmbH and Co. KGaA

    Selective etching of epitaxial MnAs films on GaAs(001): Influence of structure and strain

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    Strain in epitaxial MnAs thin films on GaAs(001) substrates plays an important role in the coupled magnetostructural phase transition. As a result of strain, the phase transition from the ferromagnetic α phase to the paramagnetic Β phase proceeds over a wide temperature range and the coexisting phases form a periodic stripe array. Employing suitable wet chemical etchants, the two MnAs phases can be etched selectively. Perpendicular to the α-Β -stripe structure, the built-up strain relaxes in the course of the etching process by the formation of cracks. The combination of both strain relaxation mechanisms allows for the defined patterning of two-dimensional arrays of nanomagnets. Through micromagnetic investigations, it is possible to identify the location of α - and Β-MnAs which helps to clarify two major aspects of the etching process. First, it is possible to determine the etch rates of α - and Β-MnAs and follow the complex interplay of strain and phase composition during the etching process. Second, as strain reflects itself in a shifted phase-transition temperature, temperature-dependent micromagnetic studies allow to determine the strain environment of the cracks. © 2005 American Institute of Physics
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