OPTICAL STUDY OF PHOTON-TRAPPED POROUS SILICON LAYER

Porous-Si samples were optically studied by using the photoluminescenc e, Raman scattering, the absolute reflectance and ellipsometry methods . Results show that the porous Si has low optical constants, and can t rap more than 95% of the visible photons, but give no evidence of a st rong interband transition existing in the visible region, especially a t 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 recog nized in the spectra, but it was not seen in the experiments. Therefor e, a contradiction exists in the PL and optical absorption experiments . Except for other mechanisms, the calculations show that the layer di spersion 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 o ften seen with consistency in other material structures, strongly indi cates the same origin of visible luminescence as those suggested in th e literature. A possible mechanism for the luminescence and Raman enha ncement as well as the photon trap phenomenon was discussed, and was a ttributed mainly to random multiple micro-reflections occurring in the porous-Si layer that has extremely large internal micro surfaces.