A INTRODUCTION

There have been very few investigations of the elastic properties of porous silicon (PS), which are, however, expected to differ drastically from those of bulk silicon. Moreover, the use of PS for some applications is limited by the mechanical instability of this fragile material (upon drying for instance).

Young's moduli of different PS samples were measured by means of four different techniques: they appear to be drastically dependent on the porosity and on the doping level (p or p ⁺-type). For instance, highly porous silicon layers exhibit very low values of Young's modulus. The analysis of the data relating to the elastic properties of PS is reported in this paper, including a short discussion about the specific problem of the elastic properties of porous materials.

B X-RAY DIFFRACTION STUDY

The first investigation of the elastic properties of PS was performed by Barla et al [1] on p ⁺-type PS material by using X-ray diffraction. This paper was the first to show that PS material behaves as a nearly perfect single crystal. Indeed, they showed that when using a high resolution X-ray diffraction experiment, the rocking curve is composed of two well defined Bragg peaks, related to the silicon substrate and to the PS layer. Therefore, they concluded that porosity affects the PS lattice by producing a slight expansion. The angular distance between the two peaks is directly related to the lattice mismatch parameter ?a/a. The...