Microacoustic evaluation of elastic parameters of highly porous silicon layers

Abstract

Non-destructive scanning acoustic microscopy investigations of elastic properties of porous silicon films, limited to low and medium porosities, are extended in this work to higher porosities (80 %) corresponding to the appearance of room temperature electroluminescence phenomena. Acoustic materials signatures were measured at various operating frequencies, f, and film thickness, d. It was shown that as these parameters increase the oscillatory V(z) behaviour disappears progressively to become completely attenuated, for f = 565 MHz and d = 6 µm, due to wave absorption, with α x = 0.2 µm −1. Moreover, from high frequency microechography, it was possible to determine longitudinal velocity, V L = 1680 m/s. To evaluate Young's modulus, E, and shear modulus, G, we used two different approaches to derive simple expressions for these constants in terms of just V L. The validity of such expressions was successfully put into evidence then applied to the highly porous silicon layers for which it was found that E = 1.29 GPa and G = 0.63 GPa.