Porous Silicon - Electrochemistry of Silicon and Its Oxide - page 408

8.6. FORMATION MECHANISMS
8.6.1. Historical Development
Since the discovery of PS in the late 1050s, numerous theories have been proposed
regarding the mechanisms of PS formation and the morphological features. Figures 8.58
and 8.59 detail the progress of research on PS over the last 50 years with respect to the
discovery of major PS features and the theories proposed for various mechanistic
aspects involved in PS formation and morphology. The parallel developments in the
understanding of the fundamental electrochemical reactions of a silicon electrode in HF
solutions are shown in Fig. 8.60.
Discovery of PS and the Initial Model. The formation of a solid surface layer
(porous silicon) on a silicon electrode during dissolution in aqueous HF solution was
first reported by Uhlir and Turner in the late 1950s. 33,957 In his study of electropolish-
ing of silicon in HF solutions, Turner found that the solid surface film forms only below
a critical current, J 1 (see Fig. 5.2). The effective dissolution valence in this current range
was found to be about 2. At potentials more positive than the critical current, elec-
tropolishing occurs. Because the film (formed on p -Si) was found to be amorphous and
contained fluoride, Turner suggested that the film could be a subfluoride (SiF 2 ) x grown
on the surface during the anodic dissolution of silicon. In the mid-1960s, this film was
found to consist mainly of polymerized silicon hydrides. 1134
Based on a systematic study of the charge transfer kinetics of the silicon/elec-
trolyte in HF solutions and the understanding that the anodic film was of amorphous