Morphological changes in porous silicon nanostructures: non-conventional photoluminescence shifts and correlation with optical absorption

In this paper, we show that the photoluminescence (PL) shifts of p-type por ous silicon (PS) are mainly attributed to some morphological changes relate d to anodisation conditions. We discuss how differences in the stirring and nature of the electrolytic solution can lead to morphological changes of t he PS layers. It has been found that when PS is formed in pure aqueous HF s olution, it can exhibit a non-conventional and reproducible "porosity - PL peak relationship". By correlating the PL spectral behaviour and PS morphol ogy throughout a quantum-confinement model, we explain both conventional an d non-conventional PL shifts. Correlation of PL and optical absorption (OA) shows that the PL peak energy and the optical absorption edge of PS exhibi t the same trend with size effect. The spectral behaviour of OA with regard to that of PL is well analysed within the quantum-confinement model throug hout the sizes and shapes of the nanocrystallites forming PS. The value of the effective band gap energy determined from the calculated lowest PL ener gy almost corresponds to that estimated from the optical absorption coeffic ient. These results suggest that the lowest radiative transition between th e valence band and the conduction band corresponds to the largest luminesce nt wires, and that the radiative recombination process leading to the PL em ission occurs in the c-Si crystallite core. (C) 2000 Elsevier Science B.V. All rights reserved.