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Author: Boschker, Jos E. × DDC: 530 ×

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Now showing 1 - 4 of 4
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    Growth of crystalline phase change materials by physical deposition methods
    (Abingdon : Taylor & Francis Group, 2017) Boschker, Jos E.; Calarco, Raffaella
    Phase change materials are a technologically important materials class and are used for data storage in rewritable DVDs and in phase change random access memory. Furthermore, new applications for phase change materials are emerging. Phase change materials with a high structural quality, such as offered by epitaxial films, are needed in order to study the fundamental properties of phase change materials and to improve our understanding of this materials class. Here, we review the progress made in the growth of crystalline phase change materials by physical methods, such as molecular beam epitaxy, sputtering, and pulsed laser deposition. First, we discuss the difference and similarities between these physical deposition methods and the crystal structures of Ge2Sb2Te5, the prototype phase change material. Next, we focus on the growth of epitiaxial GST films on (0 0 1)- and (1 1 1)-oriented substrates, leading to the conclusion that (1 1 1)-oriented substrates are preferred for the growth of phase change materials. Finally, the growth of GeTe/Sb2Te3 superlattices on amorphous and single crystalline substrates is discussed.
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    Textured Sb2Te3 films and GeTe/Sb2Te3 superlattices grown on amorphous substrates by molecular beam epitaxy
    (New York : American Institute of Physics, 2017) Boschker, Jos E.; Tisbi, E.; Placidi, E.; Momand, Jamo; Redaelli, Andrea; Kooi, Bart J.; Arciprete, Fabrizio; Calarco, Raffaella
    The realization of textured films of 2-dimensionally (2D) bonded materials on amorphous substrates is important for the integration of this material class with silicon based technology. Here, we demonstrate the successful growth by molecular beam epitaxy of textured Sb2Te3 films and GeTe/Sb2Te3 superlattices on two types of amorphous substrates: carbon and SiO2. X-ray diffraction measurements reveal that the out-of-plane alignment of grains in the layers has a mosaic spread with a full width half maximum of 2.8°. We show that a good texture on SiO2 is only obtained for an appropriate surface preparation, which can be performed by ex situ exposure to Ar+ ions or by in situ exposure to an electron beam. X-ray photoelectron spectroscopy reveals that this surface preparation procedure results in reduced oxygen content. Finally, it is observed that film delamination can occur when a capping layer is deposited on top of a superlattice with a good texture. This is attributed to the stress in the capping layer and can be prevented by using optimized deposition conditions of the capping layer. The obtained results are also relevant to the growth of other 2D materials on amorphous substrates.
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    Modulation of van der Waals and classical epitaxy induced by strain at the Si step edges in GeSbTe alloys
    (London : Nature Publishing Group, 2017) Zallo, Eugenio; Cecchi, Stefano; Boschker, Jos E.; Mio, Antonio M.; Arciprete, Fabrizio; Privitera, Stefania; Calarco, Raffaella
    The present work displays a route to design strain gradients at the interface between substrate and van der Waals bonded materials. The latter are expected to grow decoupled from the substrates and fully relaxed and thus, by definition, incompatible with conventional strain engineering. By the usage of passivated vicinal surfaces we are able to insert strain at step edges of layered chalcogenides, as demonstrated by the tilt of the epilayer in the growth direction with respect of the substrate orientation. The interplay between classical and van der Waals epitaxy can be modulated with an accurate choice of the substrate miscut. High quality crystalline GexSb2Te3+x with almost Ge1Sb2Te4 composition and improved degree of ordering of the vacancy layers is thus obtained by epitaxial growth of layers on 3–4° stepped Si substrates. These results highlight that it is possible to build and control strain in van der Waals systems, therefore opening up new prospects for the functionalization of epilayers by directly employing vicinal substrates.
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    Coincident-site lattice matching during van der Waals epitaxy
    (London : Nature Publishing Group, 2015) Boschker, Jos E.; Galves, Lauren A.; Flissikowski, Timur; Lopes, Joao Marcelo J.; Kiemer, Alexandra K.; Riechert, Henning; Calarco, Raffaella
    Van der Waals (vdW) epitaxy is an attractive method for the fabrication of vdW heterostructures. Here Sb2Te3 films grown on three different kind of graphene substrates (monolayer epitaxial graphene, quasi freestanding bilayer graphene and the SiC (6√3 × 6√3)R30° buffer layer) are used to study the vdW epitaxy between two 2-dimensionally (2D) bonded materials. It is shown that the Sb2Te3 /graphene interface is stable and that coincidence lattices are formed between the epilayers and substrate that depend on the size of the surface unit cell. This demonstrates that there is a significant, although relatively weak, interfacial interaction between the two materials. Lattice matching is thus relevant for vdW epitaxy with two 2D bonded materials and a fundamental design parameter for vdW heterostructures.