Václav Holý

High-resolution x-ray scattering from thin films and multilayers

Part 1 Experimental realization: basic elements of an equipment resolution elements diffractometers and reflectometers. Part 2 The theory of X-ray diffraction and its realization by the experiment: kinematical X-ray scattering from ideal crystals kinematical X-ray diffraction from deformed thin layers kinematical X-ray diffraction from randomly disturbed layers dynamical X-ray diffraction in perfect layers dynamical X-ray diffraction in slightly deformed layers optical reflection of X-rays from ideal layers optical reflection of X-rays from layers with rough interfaces dynamical X-ray diffraction in strongly asymmetric cases grazing incidence diffraction (GID). Appendices: elements of the formal theory of scattering structure factors, dispersion corrections and extinction length.

High-Resolution X-Ray Scattering

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Tetragonal phase of epitaxial room-temperature antiferromagnet CuMnAs

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Strain and composition distribution in uncapped SiGe islands from x-ray diffraction

Recent studies have demonstrated the potential of antiferromagnets as the active component in spintronic devices. This is in contrast to their current passive role as pinning layers in hard disk read heads and magnetic memories. Here we report the epitaxial growth of a new high-temperature antiferromagnetic material, tetragonal CuMnAs, which exhibits excellent crystal quality, chemical order and compatibility with existing semiconductor technologies. We demonstrate its growth on the III-V semiconductors GaAs and GaP, and show that the structure is also lattice matched to Si. Neutron diffraction shows collinear antiferromagnetic order with a high Néel temperature. Combined with our demonstration of room-temperature-exchange coupling in a CuMnAs/Fe bilayer, we conclude that tetragonal CuMnAs films are suitable candidate materials for antiferromagnetic spintronics.

X-ray Nanodiffraction on a Single SiGe Quantum Dot inside a Functioning Field-Effect Transistor

We have investigated the strain and composition distribution in uncapped SiGe islands grown on Si (001) by x-ray diffraction. In order to be sensitive to the dot layer on the sample surface, and at the same time being able to measure in-plane strain and strain in growth direction, we utilized a scattering geometry at grazing incidence angles, but with high exit angles. The measured intensity distribution is compared to simulations based on the strain distribution calculated by a finite element method. Although pure Ge has been deposited during island growth by molecular beam epitaxy, the Ge composition varies from 0.5 at the island base to 1.0 at the top of the islands. Even at this top, the elastic relaxation reaches only about 50%.

Magnetostrictive thin films for microwave spintronics

For advanced electronic, optoelectronic, or mechanical nanoscale devices a detailed understanding of their structural properties and in particular the strain state within their active region is of utmost importance. We demonstrate that X-ray nanodiffraction represents an excellent tool to investigate the internal structure of such devices in a nondestructive way by using a focused synchotron X-ray beam with a diameter of 400 nm. We show results on the strain fields in and around a single SiGe island, which serves as stressor for the Si-channel in a fully functioning Si–metal–oxide semiconductor field-effect transistor.

Self-assembled carbon-induced germanium quantum dots studied by grazing-incidence small-angle x-ray scattering

Multiferroic composite materials, consisting of coupled ferromagnetic and piezoelectric phases, are of great importance in the drive towards creating faster, smaller and more energy efficient devices for information and communications technologies. Such devices require thin ferromagnetic films with large magnetostriction and narrow microwave resonance linewidths. Both properties are often degraded, compared to bulk materials, due to structural imperfections and interface effects in the thin films. We report the development of epitaxial thin films of Galfenol (Fe81Ga19) with magnetostriction as large as the best reported values for bulk material. This allows the magnetic anisotropy and microwave resonant frequency to be tuned by voltage-induced strain, with a larger magnetoelectric response and a narrower linewidth than any previously reported Galfenol thin films. The combination of these properties make epitaxial thin films excellent candidates for developing tunable devices for magnetic information storage, processing and microwave communications.

Multiple-stable anisotropic magnetoresistance memory in antiferromagnetic MnTe.

We present a structural investigation of buried C-induced Ge quantum dot multilayers grown on (001) Si by molecular-beam epitaxy. Using grazing-incidence small angle x-ray scattering, we determine the shape, the mean radius, height, and dot distance. The dot distribution is isotropic within the (001) interfaces, and no correlation of the dot positions along growth direction was found.

Structural Investigations of Core-shell Nanowires Using Grazing Incidence X-ray Diffraction.

Commercial magnetic memories rely on the bistability of ordered spins in ferromagnetic materials. Recently, experimental bistable memories have been realized using fully compensated antiferromagnetic metals. Here we demonstrate a multiple-stable memory device in epitaxial MnTe, an antiferromagnetic counterpart of common II–VI semiconductors. Favourable micromagnetic characteristics of MnTe allow us to demonstrate a smoothly varying zero-field antiferromagnetic anisotropic magnetoresistance (AMR) with a harmonic angular dependence on the writing magnetic field angle, analogous to ferromagnets. The continuously varying AMR provides means for the electrical read-out of multiple-stable antiferromagnetic memory states, which we set by heat-assisted magneto-recording and by changing the writing field direction. The multiple stability in our memory is ascribed to different distributions of domains with the Néel vector aligned along one of the three magnetic easy axes. The robustness against strong magnetic field perturbations combined with the multiple stability of the magnetic memory states are unique properties of antiferromagnets.

Diffuse x‐ray scattering from misfit dislocations in SiGe epitaxial layers with graded Ge content

The fabrication of core-shell structures is crucial for many nanowire device concepts. For the proper tailoring of their electronic properties, control of structural parameters such as shape, size, diameter of core and shell, their chemical composition, and information on their strain fields is mandatory. Using synchrotron X-ray diffraction studies and finite element simulations, we determined the chemical composition, dimensions, and strain distribution for series of InAs/InAsP core-shell wires grown on Si(111) with systematically varied growth parameters. In particular we detect initiation of plastic relaxation of these structures with increasing shell thickness and/or increasing phosphorus content. We establish a phase diagram, defining the region of parameters leading to pseudomorphic nanowire growth. This is important to avoid extended defects which are detrimental for their electronic properties.