Observation of a transitory structure during porous silicon formation: Stability of Si(1 × 1)-H-terminated surfaces and facets

The divalent dissolution of float zone (FZ) Si(1 1 1) in dilute ammonium fluoride solutions at pH 4 is followed by contact mode atomic force microscopy (AFM). Using a variety of photoelectrochemical preparation conditions rather similar structures are found. The dissolution charges range between 9 and 22 bilayer equivalents in potentiostatic and mixed potentiostatic and chronoamperometric experiments. Mesa-type structures with flat extended terraces (∼200 nm width) surrounded by strongly corrugated areas are observed. Image analysis reveals that most border lines of the mesas can be obtained by drawing lines parallel or perpendicular to a reference border line or by lines being rotated by ±30° with respect to these lines. Accordingly, the shape of the mesas can be constructed from a reduced number of crystallographic orientations, i.e., from {1 1 1}, {1 1 0} and {1 1 3} faces despite its complexity. Since the {1 1 1}, {1 1 0} and {1 1 3} faces can all be electrochemically hydrogen terminated in a (1 × 1) manner, the stability of (1 × 1)-H surfaces is thought to be responsible for inducing the structures.