New models of clean and hydrogenated amorphous silicon surfaces

K Nepal and A Gautam and C Ugwumadu and DA Drabold, JOURNAL OF NON- CRYSTALLINE SOLIDS, 660, 123517 (2025).

DOI: 10.1016/j.jnoncrysol.2025.123517

We present new atomistic models of amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H) surfaces. The a-Si model included 4096 atoms and was obtained using local orbital density functional theory. By analyzing a slab model (periodic in two dimensions with a slab about 44 & Aring; thick), we observed strong correlation between surface structure and surface charge density, which might be compared to STM experiments. Hydrogen atoms added near the under- coordinated surface atoms passivate dangling bonds induce structural rearrangements. We analyze the electronic structure, including the localization of the states, and note resonant mixing between bulk and surface defect structures. We also compute the classical normal modes of the hydrogenated a-Si and compare them to experiments where possible. Our work is a step towards understanding the meaning of "surface reconstruction" for a noncrystalline material.

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