A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.

We repon time-resolved photoluminescence spearoscopy oi highly porous silicon. Our results show that lhe luminescence i s due to localized quantumanfined cxcilons in undulating ayslalline silicon wires. The resonantly excited photoluminescence spec" exhibits sateUite suuclure due to momentumsonsewing phonons of aystalline silicon. l3i.s p r w i d s a desr signature of the crystalline-silimn elecuonic band Smcture. ?he spin s l a t s of Le localized exciton are split @ lhe elclron-hole exchange interaction. lhis splitting is manifested both in the strong dependence of the luminescence lifetime 011 temperatuq and as an energy gap m B e resonantly exated photoluminescence specuum. The aperimental splitting i in good agreement with the value calculated for a localiied m i t o n in aystalline silicon.The demonstration [ 11 of efficient visible luminescence from highly porous Si has stimulated a great deal of interest The attribution of this luminescence to quantumconfined camers in crystalline Si wires [l] has been supported by transmission electron microscopy [2] (EM) which shows the structure to consist of undulating columns of crystalline Si. Fit-principles calculations 131 for wires of the dimensions observed in EM account for both the quantum upshift of the luminescence energy and the radiative lifetimes. We report time-resolved photoluminescence (PL) spectroscopy which confirms the model of quantum confinement in crystalline S i . Our experiments show that the radiative states, even at room temperature, are localized excitons with a large exchange splitting. Our observation of momentumanserving phonon satellites

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