Hexagonal Boron Nitride Nanowalls Synthesized by Unbalanced RF Magnetron Sputtering
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Hexagonal Boron Nitride Nanowalls Synthesized by Unbalanced RF Magnetron Sputtering Boumédiène BenMoussa1, Jan D’Haen1,2, Christian Borschel3, Marc Saitner1, Ali Soltani4, Vincent Mortet1,2, Carsten Ronning3, Marc D’Olieslaeger1,2, Hans-Gerd Boyen1, and Ken Haenen1,2 1 Hasselt University, Institute for Materials Research (IMO), Diepenbeek, Belgium 2 IMEC vzw, Division IMOMEC, Diepenbeek, Belgium 3 Friedrich-Schiller-Universität Jena, Institut für Festkörperphysik, Jena, Germany 4 Institut d'Electronique de Microélectronique et de Nanotechnologie, Villeneuve d'Ascq, France ABSTRACT A recurrent problem in the synthesis of hexagonal boron nitride (h-BN) is contamination with oxygen and carbon, leading to possible detrimental effects on optical and electronic properties. Here it is shown that the addition of H2 to the N2/Ar mixture used during the deposition process, clearly suppresses the incorporation of these elements, reducing their combined level below 5 %. The surface morphology, assessed with scanning electron microscopy (SEM), revealed the presence of h-BN nanowalls, i.e. vertically positioned 2D structures consisting out of several h-BN sheets. While Fourier transform infrared (FTIR) spectroscopy revealed the sp2 nature of the bonds, confirming the hexagonal nature of the nanowalls, the quasi-perfect stoichiometry of the material was evidenced by combining energy dispersive X-ray analysis (EDX) and Rutherford backscattering spectroscopy (RBS). The dimensions and density of these walls are clearly film thickness dependent and cross-sectional TEM images confirmed the increasing level of porosity with film thickness. A dense layer of material is present at the substrate-film interface, which gradually evolves into the 2D nanowall structures. INTRODUCTION Since the recent work by Watanabe and co-workers [1-3] showing remarkable optoelectronic properties of hexagonal boron nitride (h-BN), this material is considered as a promising candidate for a wide range of advanced applications based on its luminescent properties. To fulfill these potential applications, it is necessary to deposit BN thin films on a variety of substrates. While the high pressure, high temperature (HPHT) technique produces high quality crystals, the technique itself is rather cumbersome, leading to small, irregular crystals that are difficult to handle due to their fragile nature. Similar to carbon, BN has many allotropes. It exists in the forms of sp2-bonded rhombohedral (r-BN), turbostratic (t-BN), and hexagonal BN (h-BN), or sp3-bonded wurtzite (wBN) and cubic BN (c-BN). Being a layered structure, h-BN has many excellent physical properties analogous to graphene, being mechanically strong and showing high chemical and thermal stability [4-7]. It has a wide band gap of up to 5.9 eV [8], which has important applications for its possible use, as a deep-ultraviolet-light emitter and detector [1,9]. Because of the structural similarity to graphite, h-BN has even recently shown to be a competitive and alternative substrate candidate for graphene dev
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