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Start date: 2020 × DDC: 530 × Publisher: Weinheim : Wiley-VCH × Subject: molecular beam epitaxy × WGL Institute: PDI ×

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    Self-Assembly of Well-Separated AlN Nanowires Directly on Sputtered Metallic TiN Films
    (Weinheim : Wiley-VCH, 2020) Azadmand, Mani; Auzelle, Thomas; Lähnemann, Jonas; Gao, Guanhui; Nicolai, Lars; Ramsteiner, Manfred; Trampert, Achim; Sanguinetti, Stefano; Brandt, Oliver; Geelhaar, Lutz
    Herein, the self-assembled formation of AlN nanowires (NWs) by molecular beam epitaxy on sputtered TiN films on sapphire is demonstrated. This choice of substrate allows growth at an exceptionally high temperature of 1180 °C. In contrast to previous reports, the NWs are well separated and do not suffer from pronounced coalescence. This achievement is explained by sufficient Al adatom diffusion on the substrate and the NW sidewalls. The high crystalline quality of the NWs is evidenced by the observation of near-band-edge emission in the cathodoluminescence spectrum. The key factor for the low NW coalescence is the TiN film, which spectroscopic ellipsometry and Raman spectroscopy indicate to be stoichiometric. Its metallic nature will be beneficial for optoelectronic devices using these NWs as the basis for (Al,Ga)N/AlN heterostructures emitting in the deep ultraviolet spectral range.
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    Evolution of Low-Frequency Vibrational Modes in Ultrathin GeSbTe Films
    (Weinheim : Wiley-VCH, 2021) Zallo, Eugenio; Dragoni, Daniele; Zaytseva, Yuliya; Cecchi, Stefano; Borgardt, Nikolai I.; Bernasconi, Marco; Calarco, Raffaella
    GeSbTe (GST) phase-change alloys feature layered crystalline structures made of lamellae separated by van der Waals (vdW) gaps. This work sheds light on the dependence of interlamellae interactions at the vdW gap on film thickness of GST alloys as probed by vibrational spectroscopy. Molecular beam epitaxy is used for designing GST layers down to a single lamella. By combining density-functional theory and Raman spectroscopy, a direct and simple method is demonstrated to identify the thickness of the GST film. The shift of the vibrational modes is studied as a function of the layer size, and the low-frequency range opens up a new route to probe the number of lamellae for different GST compositions. Comparison between experimental and theoretical Raman spectra highlights the precision growth control obtained by the epitaxial technique.