Persistence of negative vacancy and self-interstitial formation energies in atomistic models of amorphous silicon

Abstract

We use classical potential atomistic modeling to demonstrate that negative vacancy and self-interstitial formation energies are inherently present in amorphous silicon ($a$-Si). Our conclusions are based on comprehensive calculations of vacancy and self-interstitial formation energies in nine different models, ranging in size from 1000 to 2744 atoms and formed by quenching liquid Si at rates between ${10}^{10}$ and ${10}^{12}\phantom{\rule{0.16em}{0ex}}\mathrm{K}/\mathrm{s}$. We account for a range of possible reference energies corresponding to different choices of reservoirs for atoms inserted into or removed from our models. Moreover, we show that successive addition of either vacancies or self-interstitials does not deplete the number of negative formation energy sites. Instead, it further lowers defect formation energies. These findings lead us to conclude that $a$-Si is not a structure at or near a local enthalpy minimum nor can it be brought to a local enthalpy minimum through the introduction of constitutional defects.