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

Abstract

The optical properties of porous silicon (PSi) photoetched in aqueous HF∕I2 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∕I2 solution on n-Si substrates under Xe lamp illumination. The SE ε(E) and related data show an interference oscillation in the region below E∼3 eV, where the PSi material is nearly transparent. The PV and PC spectra reveal three individual peaks A, B, and C at ∼1.2, ∼1.7, and ∼2.5 eV, respectively, arising from the PSi layer itself. Peak C is also observed in the ER spectrum, together with a broadened E1 peak at ∼3.4 eV. Change in the fundamental-absorption-edge nature (EgX) 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 ∼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 EgX (A and B) and EgL(C). The quantum-mechanical size effect, i.e., a relaxation of the momentum conservation, makes possible the nondirect or quasidirect transitions at and above EgX and EgL in porous materials. The FTIR data support that the PL emission is due to the surface-sensitive quantum confinement effect.