ETCHING OF SILICON IN NAOH SOLUTIONS .2. ELECTROCHEMICAL STUDIES OF N-SI(111) AND N-SI(100) AND MECHANISM OF THE DISSOLUTION

In Part I of this work, the bias dependence of the etching of silicon (111) has been investigated by means of in situ scanning tunneling mic roscopy observations. In this second part, current-voltage curves and etch rate results derived from the loss of material and performed with n-type Si samples of various orientations, show that electrochemical and chemical reactions coexist in the oxidation of Si. A model is pres ented for the oxidation of a Si atom in a kink site in different situa tions of polarization. The key feature of the description is the under standing of the persistent hydrogen termination of the surface in spit e of the continuous oxidative removal of Si atoms from the surface. Th e model includes the hydrolytic splitting of Si-H and Si-Si bonds as t he important chemical contributions to the etching process. At the res t potential, the chemical component is dominant. The sequence of react ions leaves the surface in the (1 x 1)-H terminated state. The anodic current is due to the injection of electrons which are produced during the substitution of Si-H by Si-OH bonds. This results above a critica l electrode potential in passivation. In this respect, (111) and (100) faces present quite different behaviors. At cathodic bias where the h ydrogen evolution becomes fast, due to the accumulation of electrons a t the surface, not only the anodic component of the etching reaction v anishes but also the chemical component decreases in rate and is event ually stopped.