Optical Study of Photon-Trapped Porous Silicon Layer

Abstract

Porous-Si samples were optically studied by using the photoluminescence, Raman scattering, the absolute reflectance and ellipsometry methods. Results show that the porous Si has low optical constants, and can trap more than 95% of the visible photons, but give no evidence of a strong interband transition existing in the visible region, especially at the 1.8-eV PL peak position, as suggested by the quantum size effect. The Lorentz oscillator and Bruggeman effective medium approximation (EMA) models were used in data analyses. Calculations indicate that if strong interband transition occurs, an optical structure can be recognized in the spectra, but it was not seen in the experiments. Therefore, a contradiction exists in the PL and optical absorption experiments. Except for other mechanisms, the calculations show that the layer dispersion effect may result in a shift of the luminescence peak for the porous Si. The 1.8-eV PL peak, not always shifted significantly but often seen with consistency in other material structures, strongly indicates the same origin of visible luminescence as those suggested in the literature. A possible mechanism for the luminescence and Raman enhancement as well as the photon trap phenomenon was discussed, and was attributed mainly to random multiple micro-reflections occurring in the porous-Si layer that has extremely large internal micro surfaces.