A INTRODUCTION

Porosity in solids can arise via several different routes. In this book we focus entirely on the 'subtractive' route whereby part of the semiconductor is selectively removed via an electrochemical etch (see Datareviews 1.1 1.3). This is because, to date, there has been much less interest in porous Si created by alternative techniques. Nevertheless, it is worth mentioning that the agglomeration or partial consolidation of Si 'nanocrystals' or 'ultrafine' Si powder prepared via a variety of techniques [1 7] can also generate highly porous Si. The porosity in this instance depends on the size, shape and manner of packing of the constituent Si particles. Alternative dry etching techniques for creating porous structures from bulk Si are also reported [8, 9].

The complexity and variety inherent in porous materials has led to a number of useful recommendations for categorising and characterising their structure and properties [10]. We describe some of these IUPAC (International Union of Pure and Applied Chemistry) guidelines here and their relevance to electrochemically prepared porous Si.

B PORE TYPE

FIGURE 1 shows schematically several types of pore that can be created in a Si wafer or film via electrochemical etching. What distinguishes a rough surface from a porous one? In the most general sense a 'pore' is an etch pit whose depth, d, exceeds its width, w (FIGURE 1(a)) [10]. Since at the ran scale, some increase in surface roughness...