Ab-initio Molecular Dynamis Simulation of Metastalde Defect by Electronic Excitation

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The nature of the isolated dangling-bonds (D) in amorphous silicon (a-Si), hydrogenated amorphous silicon (a-Si : H) and crystalline silicon (c-Si) are investigated using ab-initio molecular-dynamics simulation. We find that the bistability of dangling-bonds at sp^3- and sp^2-like configuration which exhibit a large lattice-relaxation in all cases. Neutral dangling-bond at sp^3-like configuration shows negative effective-U nature with large lattice-relaxation. Therefore the neutral dangling-bond (D^0) is unstable at the thermal equilibrium and disproportionate into negatively- and positively-charged dangling-bond ; 2D^0 --> D^- + D^+. On the other hand, neutral dangling-bond at sp^2-like configuration shows positive effective-U nature, and positively-, neutrally-, and negatively-charged dangling bonds appear with increasing the chemical potential.These bistabilities in dangling-bonds with the positive and negative effective-U nature are related to the microscopic mechanism of the X-ray-irradiation-enhanced crystallization in amorphous silicon in which the vacancy-interstitial-pair formation by X-ray irradiation reduces the formation energy of the vacancy and enhances the crystallization. We also find that the negative effective-U nature of the dangling-bond at sp^3-like configuration is related to the origin of Staebler-Wronski effect in which the neutral dangling-bonds of positive effective-U at sp^2-like configuration are created by the recombination-enhanced defect-reaction with the exposure to the visible light. Our prediction can explain all available experimental data very reasonably.