Spectral characteristics of visible light emission from porous Si: Quantum confinement or impurity effect?

Emission and excitation spectra of visibly luminescing porous Si structures subjected to a long-term air impregnation are analyzed. A red shift of emission spectrum with increasing initial porosity is reported. A correlation of excitation spectra with the energy of Γ′25-Γ15 transition in Si monocrystal is found in the samples differing in the initial porosity. The results seem to be hard to explain in terms of the quantum-confinement models in their present state. An alternative model based on band-gap widening in heavily doped silicon structures is considered as well.