In order to reveal the mechanism of the electrochemical etching process in ultrapure water, first-principles molecular-dynamics simulations of Si(001) surfaces interacting with OH molecules were carried out on the basis of the Kohn-Sham local-density-functional formalism. A plane-wave basis set was used, and the cut-off energy is 327eV(24Ry). A norm-conserving pseudopotential was also used. We adopt the standard molecular-dynamics method for the optimization of the ionic system and the preconditioned conjugate-gradient (CG) method for the quenching procedure of the electronic degrees of freedom. We determined the optimized ionic configurations and electronic distributions for OH chemisorbed Si(001) surfaces and clarify the etching mechanism. It was confirmed that three OH molecules can chemisorb on the Si surface atom but it cannot be etched and four OH molecules cannot chemisorb on. We concluded that the Si surface atom cannot be etched as Si(OH)4 molecule and hydrogen-termination is necessary to etch the Si surface atom.
