Porous silicon: a quantum sponge structure for silicon based optoelectronics

The striking photoluminescence properties of porous silicon have attracted considerable research interest since their discovery in 1990. Luminescence is due to excitonic recombination quantum confined in Si nanocrystals which remain after the partial electrochemical dissolution of silicon. Porous si licon is constituted by a nanocrystalline skeleton (quantum sponge) immerse d in a network of pores. As a result, porous silicon is characterized by a very large internal surface area (of the order of 500 m(2)/cm(3)). This int ernal surface is passivated but remains highly chemically reactive which is one of the essential features of this new and complex material. We present an overview of the experimental characterization and theoretical modeling of porous silicon, from the preparation up to various applications. Emphasi s is devoted to the optical properties of porous silicon which are closely related to the quantum nature of the Si nanostructures. The characteristics of the various luminescence bands are analyzed and the underlying basic me chanisms are presented. In the quest of an efficient electroluminescent dev ice, we survey the results for several porous silicon contacts, with partic ular attention to the interface properties, to the stability requirement an d to the carrier injection mechanisms. Other device applications are discus sed as well. (C) 2000 Elsevier Science B.V. All rights reserved.