One of the key steps in nanotechnology is our ability to engineer and fabricate low-dimensional nano-objects, such as quantum dots, nanowires, two-dimensional atomic layers or three-dimensional nano-porous systems. Silicon carbide (SiC), a wide band-gap semiconductor, offers fascinating structural, thermomechanical, electronic, and chemical properties with a vast range of advanced applications including high-power, high-frequency, and high-temperature electronic devices and sensors. SiC has also a remarkable biocompatibility making it useful for biomedical applications. It is a substrate especially suitable for the growth of epitaxial graphene, with subsequent very promising potential applications in electronics and spintronics. Strain/stress interplay is the dominant driving force in SiC surface ordering, leading to more than 10 different surface reconstructions ranging from Si-rich to C-rich cubic SiC surfaces, and to the self-formation of highly stable massively parallel passive or active atomic lines and nanowires at the surface. Most interestingly, the interaction of H/D atoms with the Si-rich 3C-SiC(001)-3x2 surface reconstruction leads to surface metallization, which is the first example of H/D-induced metallization of a semiconductor surface [1]. Although there are some examples of voids or nano-cavities generated at a surface or below, most are in the µm or sub-µm scales, and none exhibit spatial ordering.
Here [2] we report the 1st evidence of nanotunnel opening taking place within the subsurface region of a semiconductor, SiC, as depicted in Figure 1. Such an effect is induced by selective hydrogen/deuterium interaction at the surface, which possesses intrinsic compressive stress. This finding is established with a combination of ab-initio and vibrational computations using VASP and MedeA, and vibrational spectroscopy & synchrotron-radiation-based photoemission experiments.
Figure 1: 3D view of a Nanotunnel. The nanotunnel opening induced by the interaction of H-atoms with the 3C-SiC(100)-3x2 surface is represented for the 8 H metallic structure.
Hydrogen/deuterium-induced puckering of the subsurface Si atoms marks the critical step in this nanotunnel opening. Depending on hydrogen/deuterium coverage, the nanotunnels are either metallic or semiconducting as identified by density of states (DOS) calculations and photoemission experiments performed on the TEMPO beamline at the 3rd generation SOLEIL synchrotron. The experimental system is optimized to monitor live the electronic properties during hydrogen exposure, thereby leading to identify the semiconducting/metal/semiconducting transition as a function of hydrogen exposures. Calculated and measured DOS are in excellent agreement. The vibrational frequencies for the clean and hydrogen-covered 3C-SiC(001)-3x2 surface, measured by high-resolution electron energy loss spectroscopy (HREELS) at the IMEM-CNR and Università di Genova joint laboratory, are also in excellent agreement with the computed ones.
Dangling bonds generated inside the nanotunnel offer a promising template to capture atoms or molecules. These features open nano-tailoring capabilities towards advanced applications in electronics, chemistry, storage, sensors or biotechnology. Understanding and controlling such a mechanism open routes towards selective surface/interface functionalization.
P. Soukiassian, E. Wimmer, E. Celasco, C. Giallombardo, S. Bonanni, L. Vattuone, L. Savio,
A. Tejeda, M. Silly, M. D'angelo, F. Sirotti, M. Rocca,
Nature Communications 4, 2800 doi: 10.1038/ncomms3800 (2013)
1 - V. Derycke, P. Soukiassian, F. Amy, Y.J. Chabal, M. D'angelo, H. Enriquez, M. Silly
Nanochemistry at the atomic scale revealed in H-induced semiconductor surface metallization
Nature Materials 2, 253 (2003)
2 - P. Soukiassian, E. Wimmer, E. Celasco, C. Giallombardo, S. Bonanni, L. Vattuone, L. Savio, A. Tejeda, M. Silly, M. D'angelo, F. Sirotti, M. Rocca, Hydrogen-induced nanotunnel opening within semiconductor subsurface, Nature Communications 4, 2800 (2013), doi: 10.1038/ncomms3800.
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