A INTRODUCTION

The internal surface of porous silicon (PS) layers varies from about 200 to 600 m ²/cm ³ depending on the dopant level of the substrate from which they are formed [1].

Such an inner surface contains a high density of sites for original impurities such as hydrogen [2] and fluorine [3, 4] coming from the electrolyte used for the anodic dissolution. After the formation, the layers are generally kept in ambient air for a few hours and even a few days. During this period, natural oxidation occurs and also contamination by chemical species from the atmosphere.

The optical and electrical properties depend on these impurities. The luminescence properties for example depend on the presence of hydrogen which passivates the silicon dangling bonds [5], and also on the presence of oxides which change the crystallite size and introduce surface states which may be non-radiative (Si dangling bonds) or radiative recombination centres [6]. Moreover, luminescence in the blue spectral range [7] appears from aged PS which has been attributed to the presence of oxide. PS is also known to be sensitive to moisture [8]. The conductivity for example is increased by the presence of acetone vapour or ammonia, so sensors can be realised [9].

It is hence indispensable to know the chemical composition of PS layers when studying their physical properties. In this Datareview we discuss the main results concerning the chemical composition of...