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

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The optical properties of porous silicon (PSi) photoetched in aqueous HF/I2 solution areinvestigated 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 lampillumination. The SE ε(E) and related data show an interference oscillation in the region belowE～3 eV, where the PSi material is nearly transparent. The PV and PC spectra reveal threeindividual peaks A, B, and C at ～1.2, ～1.7, and ～2.5 eV, respectively, arising from the PSi layeritself. 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 siliconto the quasidirect gap in PSi is found in the PV and PC spectra. The PL spectrum shows a broadpeak at ～2.0 eV(B). Peaks A, B, and C observed in the PSi layer may originate from the nondirectoptical transitions at and above the lowest absorption edges EgX (A and B) and EgL (C). Thequantum-mechanical size effect, i.e., a relaxation of the momentum conservation, makes possiblethe nondirect or quasidirect transitions at and above EgX and EgL in porous materials. The FTIR datasupport that the PL emission is due to the surface-sensitive quantum confinement effect.
2007-09-15