Porous Silicon Electroluminescence Mechanisms and Defect Analysis

The purpose of this research was to determine the mechanisms contributing to the electroluminescence (EL) of porous silicon and to identify and analyze the effects of porous silicon defects on the luminescence. Time dependence of the EL, voltage excitation, and electrical current were compared. Luminescence spectra and temperature dependence were measured. Two components of the EL were observed with significantly different EL decay times. They appear to result from recombination in regions of the porous silicon having different structure sizes and different electrical connectivity. The larger structures can support extrinsic conduction, giving rise to electrical excitation of EL. The smaller particles are more isolated electrically and in addition behave more like insulators, by virtue of their increased binding energy of substitutional impurities. These considerations are manifested in significant differences in the temperature dependence of the EL. Defects in the porous silicon were examined using EPR. A Pb center defect has been unambiguously identified with a silicon dangling orbital. The disposition of these defects confirms that the nanocrystallites of silicon within the porous silicon skeletal structure retain their original crystal lattice and orientation. Correlation of defect density and photoluminescence (PL) under different conditions of anneal demonstrates that the Pb defect is responsible for some of the quenching of the PL, but that there is also another quenching mechanism.