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

We report laser‐induced transient grating experiments performed at room temperature on self‐supporting p‐type porous silicon films with different porosities. With this technique the diffusion of the photocarriers can be studied with a time resolution of some tens of picoseconds. The gratings were created by two interfering pulses of the second‐harmonic 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 fundamental YAG laser frequency (1.15 eV). The observed grating decay time is very fast (hundreds of picoseconds) and shortens with decreasing porosity. Diffusion constants D=45, 24, and 5 cm2 s−1 have been found for the porosities of 64%, 68%, and 73%, respectively. To explain these high values of D we consider a simple kinetic model which takes into account two different types of carriers, delocalized and trapped ones.