In order to reveal the mechanism of the electrochemical etching process in ultrapure water, first-principles molecular -dynamics simulations for etching process of the hydrogen-terminated Si(001) surfaces by hydroxyl ion in ultrapure water were carried out on the basis of the Kohn-Sham local-density-functional formalism. A plane-wave basis set and a norm-conserving pseudopotential were used. The standard molecular-dynamics method for the optimization of the ionic system and the steepest-descents (SD) method for the quenching procedure of the electronic degrees of freedom were adopted. It was confirmed that the chemisorbed four hydroxyl ions on the hydrogen-terminated Si(001) step edge decrease the covalent bond strength of Si-Si back-bond and initiate the etching process.
