CARRIER DIFFUSION IN POROUS SILICON STUDIED BY TRANSIENT LASER-INDUCED GRATING SPECTROSCOPY

We report laser-induced transient grating experiments performed at roo m temperature on self-supporting p-type porous silicon films with diff erent porosities. With this technique the diffusion of the photocarrie rs can be studied with a time resolution of some tens of picoseconds. The gratings were created by two interfering pulses of the second-harm onic radiation from an Nd3+:yttrium-aluminum-garnet YAG laser (2.3 eV) and their decay in time was studied by a time-delayed pulse of the fu ndamental YAG laser frequency (1.15 eV). The observed grating decay ti me is very fast (hundreds of picoseconds) and shortens with decreasing porosity, Diffusion constants D=45, 24, and 5 cm(2) s(-1) have been f ound for the porosities of 64%, 68%, and 73%, respectively. To explain these high values of D we consider a simpie kinetic model which takes into account two different types of carriers, delocalized and trapped ones. (C) 1996 American Institute of Physics.