Effects of Anodization Current Density on Photoluminescence Properties of Porous Silicon

Photoluminescence (PL) mechanisms of porous Si (PS) have been investigated by comparing Fourier transform infrared (FTIR) absorption structures with PL properties obtained using a He–Cd laser as a photoexcitation source. PS samples investigated were prepared by anodization at different anodization current densities (i a) or by different total electric charge densities (q T). The densities of Si–H and Si–H2 species and the amount of oxygen are constant independent of both i a and q T, and the oxygen is inferred to exist on the topmost surface of the PS layer. On the other hand, the PL intensity depends only on i a and increases drastically with increasing i a. Therefore, there is no relationship between the PL intensity and the hydride densities or the amount of oxygen, indicating that newly formed surface complexes, related to hydrogen or oxygen, including hydrogenated amorphous Si, are not essential to the PL origin, and that the Si atomic configuration, at least within the ~ 100 A detection depth in the PL experiment, varies with the change of i a. Macroscopic structures such as porosity and specific surface area were evaluated to be unchanged among the samples. The FTIR and PL results together with this evaluation suggest that photoluminescent elements, which do not affect the macroscopic structures fundamentally, exist in the region of the internal surface of PS pores and the number of elements increases with increasing i a. A quantum-sized crystallite is a most promising photoluminescent element for which these results are well explained. The PL peak energies were almost constant among the samples, which suggests that the minimum size of the quantum-sized crystallites, during anodization, is limited.