Anodization of Layered Semiconductors: A Representation of the Layer Periodicity

Anodization of layered semiconductors, especially and , has been done to have a new representation of the layer periodicity which the layered semiconductors possess as their peculiar property. Constant current anodization (c.c.a.) in a solution of sodium borate in ethylene glycol (GSE) at pH 7 has been mainly adopted since the periodic surface condition for the successive monolayer film formation can be maintained by c.c.a. and GSE has been found to be an excellent electrolyte for the representation of the layer periodicity in these semiconductors. The layer periodicity has been represented by the periodically varying structures observed in the cell voltage vs.time and its second derivative curves for ,, , and , and in the in situ ellipsometric off‐null signal vs.time curves for in c.c.a. Unit line shapes of the and the surface coverage vs. time curves derived from curves have been investigated to understand the anodization process for the monolayer film formation in where the two‐dimensional nucleation and growth, which may result from a suppressed growth in the perpendicular direction to the cleavage surface by the potential barrier at the layer boundary, are taking place with the slow step of competing rate‐determining processes with comparable rates. The competing rate‐determining processes have been also recognized by the current transients in the potentiostatic anodization of in GSE on the analogy of the electrocrystallization study on nickel reported by Fleischmann et al. It has turned out in c.c.a. of in GSE that insoluble nonpassive films grow two dimensionally in the initial stage of anodization and transform into passive films. Another interesting observation has been made in the transient behaviors of characteristics. They have been classified into five types, depending on the kind of electrolytes and semiconductor samples used, or on being related to the current efficiency, the rate‐determining process and/or transformation from three‐dimensional to two‐dimensional growth of passive films.