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 microscopy 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 presented for the oxidation of a Si atom in a kink site in different situations of polarization. The key feature of the description is the understanding of the persistent hydrogen termination of the surface in spite of the continuous oxidative removal of Si atoms from the surface. The model includes the hydrolytic splitting of Si-H and Si-Si bonds as the important chemical contributions to the etching process. At the rest potential, the chemical component is dominant. The sequence of reactions 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 critical electrode potential in passivation. In this respect, (111) and (100) faces present quite different behaviors. At cathodic bias where the hydrogen evolution becomes fast, due to the accumulation of electrons at the surface, not only the anodic component of the etching reaction vanishes but also the chemical component decreases in rate and is eventually stopped.