A INTRODUCTION

In bulk crystalline semiconductors the use of the term bandgap is unambiguous: it refers to the energy gap between the maximum of the valence band and the minimum of the conduction band. The magnitude of this bandgap can be obtained simply by, for example, optical transmission measurements. The inhomogeneity of porous silicon means that assigning a value for the bandgap is no longer a simple matter. The dependence of the energies of the electronic states on dimensions that gives inter-sample variation in bandgap (see TABLE 1) also produces intra-sample variation. Consequently, a spread of bandgaps exists in every porous silicon sample, as is clearly demonstrated by the large inhomogeneous broadening of the photoluminescence (PL) from this material [1]. Quoting a single bandgap for a porous silicon sample is therefore an oversimplification; measurements, in general, yield averaged values that may additionally be weighted towards one end of the size distribution. The intra-sample variation in bandgap is, however, generally smaller than the inter-sample variation, and consequently meaningful comparisons can, with care, be made between the average bandgaps of different porous silicon samples.