Spectroscopic investigation of light-emitting porous silicon photoetched in aqueous HF/I2 solution

The optical properties of porous silicon (PSi) photoetched in aqueous HF/I-2 solution are investigated using spectroellipsomety (SE), electroreflectance (ER), photovoltage (PV), photoconductivity (PC), photoluminescence (PL), and Fourier transform infrared (FTIR) spectroscopy. The PSi layers were formed in a HF/I-2 solution on n-Si substrates under Xe lamp illumination. The SE epsilon(E) and related data show an interference oscillation in the region below E similar to 3 eV, where the PSi material is nearly transparent. The PV and PC spectra reveal three individual peaks A, B, and C at similar to 1.2, similar to 1.7, and similar to 2.5 eV, respectively, arising from the PSi layer itself. Peak C is also observed in the ER spectrum, together with a broadened E-1 peak at similar to 3.4 eV. Change in the fundamental-absorption-edge nature (E-g(X)) from the indirect gap in crystalline silicon to the quasidirect gap in PSi is found in the PV and PC spectra. The PL spectrum shows a broad peak at similar to 2.0 eV(B). Peaks A, B, and C observed in the PSi layer may originate from the nondirect optical transitions at and above the lowest absorption edges E-g(X) (A and B) and E-g(L) (C). The quantum-mechanical size effect, i.e., a relaxation of the momentum conservation, makes possible the nondirect or quasidirect transitions at and above E-g(X) and E-g(L) in porous materials. The FTIR data support that the PL emission is due to the surface-sensitive quantum confinement effect. (C) 2007 American Institute of Physics.