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Intrinsic Stability of HD vs CD Si Pristine and Hydrogen Terminated Nanowires As A Function of Their Diameters and the H Chemical Potential

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Document pages: 16 pages

Abstract: Hexagonal diamond (HD or 2H) silicon nanowires (NWs) are promising for photovoltaic applications but are difficultto grow. In the present work, surfaces of HD Si are first studied via ab initio calculations, without and with hydrogen, including possible relaxations, and compared to Si cubic diamond (CD) surfaces, with a globally favorable energy ratio for HD surfaces. The energies of several plausible HD and CD Si NWs of different thicknesses are then calculated and compared to estimate their relative phase stabilities, again in favour of HD NWs, without and with hydrogen at surfaces. Analytically extrapolating the ab initio results as functions of bulk, surface and edge energy contributions, the main result is that HD Si NWs are intrinsically stable with respect to CD Si NWs for effective diameters up to about 15 nm, for pure Si NWs as well as for surface hydrogenated NWs. Thicker HD NWs can thus only be metastable. The diameter size limit for germanium, silicon s big brother, is three times smaller, making HD Ge NWs much less likely, in agreement with recent experimental attempts to grow Ge NWs.

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