Surface contributions to the effective optical properties of porous silicon

Porous silicon (PS) presents efficient photoluminescence and electroluminescence with potential applications in the optoelectronic industry, in particular in photovoltaic devices. It is now generally accepted that the interesting optical properties of PS are due to two combined aspects: on the one hand, the quantum confinement and on the other, the surface states. Although there has been a great effort to study PS, its transport properties are still not well understood. Due to the complex structure of PS an averaging theory to describe its effective properties is justified. In this work the effective dielectric function, effective absorption coefficient and effective refractive index are calculated using the volume averaging method for a model of periodic columns with different surface coatings simulating porous silicon. This approach allows analytical results within certain approximations and the analysis of surface contributions. The method uses parameters to characterize the bulk and the surface. We choose for the bulk c-Si, and cover it with three different possible surfaces: siloxane, a-Si:H and SiO2. The results are compared with experimental data and other theoretical approaches for silicon wires. We obtain good agreement with some experimental results showing the important role of the surface in the effective response of porous silicon. (C) 1998 Elsevier Science B.V. All rights reserved.