ELECTROCHEMISTRY OF SEMICONDUCTORS - THE RETURN OF SILICON

Semiconductor electrochemistry has known an important development in the eighties, especially aimed to the photoelectrochemical conversion of solar energy. More fundamental work is now being addressed toward the most simple and well-known semiconductors, especially silicon. In spite of its propensity toward oxidation, silicon has demonstrated its suitability for realizing photoelectrochemical cells in non-aqueous solvents, with good stability and performance comparable to those of dry solid-state cells. Upon rinsing in fluoride medium, the silicon surface comes out covered with Si-H bonds. In contrast to common expectation, this coating is stable on an hour time scale even in the presence of water. In a solvent such as methanol, the formation of SiOCH3 surface groups still reinforces surface stability. Chemical dissolution of silicon in alkaline media has long been known and used for its anisotropic properties. Recent studies have shown that, here again, the surface during the dissolution appears essentially hydrogenated, and the mechanism responsible for the anisotropy is being clarified. On the opposite, surface oxidation may be looked for, for example in view of the manufacturing of MOS devices or for the passivation of integrated circuits. Anodic oxidation may indeed provide oxides with a good electronic quality and improved thickness control. In fluoride media, significant work has been directed toward understanding the electrochemical dissolution processes of silicon. At moderate potentials, anodic dissolution may lead to the formation of porous silicon. This material, whose formation mechanism is still imperfectly understood, exhibits the characteristics of an original semiconductor. Especially, it may exhibit an intense luminescence in the visible range. This luminescence is presently arousing many excitement in view of its possible applications, but its origin is a matter of controversy. For more positive potentials, the anodic dissolution of silicon in fluoride media exhibits a polishing regime. For the most positive potentials, an original resonant electrical behavior of the interface has been observed. This behavior arises from an oscillating behavior on the microscopic scale. The origin of this oscillation, associated to a variation of the interfacial oxide thickness, is still to be elucidated.