Observation of a transitory structure during porous silicon formation stability of Si 1x1 H terminated surfaces and facets

The divalent dissolution of float zone FZ Si 111 is followed by contact mode atomic force microscopy AFM in dilute ammonium fluoride solutions at pH 4. Under 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 witdh surrounded by strongly corrugated areas are observed. Image analysis reveals that most border lines of the mesas can be obtained by drawing lines along angles of 0o, 30o and 30o and perpendicular to them with respect to a 0o reference line. Accordingly, the shape of the mesas can be constructed from a reduced number of crystallographic orientations, i. e. from 111 , 110 and 113 faces despite its complexity. Since the 111 , 110 and 113 faces can all be electrochemically hydrogen terminated in a 1x1 manner, the stability of 1x1 H surfaces is thought responsible for inducing the structure