ABSTRACT
Porous silicon (PSi) films are typically prepared by electrochemical etching of crystalline Si (c-Si) in HF-based solutions. This process is compatible with the fabrication of standard silicon-based microelectronic devices. The pore morphology, diameter, and pore direction can all be controlled by the c-Si wafer surface orientation, doping level and type, the etching solution, and the current density. Under these preparation conditions, the freshly prepared PSi surface is covered with silicon-hydrogen (Si-Hx) bonds. The hydrogen-terminated porous silicon (PSi-H) film is of good electronic quality, but the Si-Hx bonds formed on its surface do not protect against photoluminescence (PL) quenching, leading to slow degradation of PL upon exposure to air and concomitant degradation of the electronic properties of the material. This limitation restricts the use of PSi in the fabrication of commercial devices. Moreover, the poor aqueous stability of PSi remains a major challenge in its medical and biological applications. Indeed, the PSi-H surface reacts in ambient air to form an oxide sub-monolayer. Although, the Si-O bond is relatively stable in air, noncontrolled oxidation of the PSi surface introduces surface defects, responsible for the PL quenching. Furthermore, the Si-O bond slowly hydrolyzes and dissolves in water; this phenomenon is accelerated in alkaline media. These chemical transformations, occurring upon exposure of PSi surface to ambient air and/or aqueous-based solutions, affect the optical/electronic but also the mechanical integrity of the porous matrix.
