Observation of metastable self-organised structure during porous silicon formation

Abstract

The initial processes leading from an atomically flat Si surface to porous silicon formation are not well known and detailed surface analytical investigations on the dissolution steps are still lacking. By performing electrochemical experiments in dilute ammonium fluoride solutions at moderate pH values, we are able to investigate the process of dissolution from first pit formation to the formation of transitory mesa-type terraces on atomically flat n -Si(111) by atomic force microscopy. Mesa-type structures with flat extended terraces surrounded by strongly corrugated areas are observed after different conditioning procedures. The mesas exhibit terrace edges aligned with respect to preferred surface orientations, which results in pentagonal, hexagonal or higher composed structures. The findings can be interpreted in terms of the formation of sidewalls exhibiting only a limited number of crystal orientations: {111}, {110} and {113}. Because these faces can be electrochemically H terminated in a 1×1 manner, we suggest that their stability accounts for the variety of observed structures.