Quantum confinement contribution to porous silicon photoluminescence spectra

Photoluminescence (PL) spectra of freshly etched and ambient-aged porous silicon specimens have been measured and analyzed to extract spectral contributions due to quantum confinement (QC) and non-QC effects. It is shown that all spectra can be deconvolved into five Gaussian bands with typical peak energies 1.59, 1.76, 1.84, 1.94, and 2.07 eV. Ambient aging induces ∼0.1 eV blueshift in each of the three highest energy peaks, which is attributed to QC effects. In contrast, the two lowest energy peaks remain unshifted as expected for non-QC effects. Because size of the porous silicon nanoparticles is deduced from the magnitude of blueshift, it is imperative to correctly identify the spectral components associated solely with QC. The three closely spaced Gaussian bands are summed and the resulting single Gaussian band is analyzed with the model of Islam and Kumar [J. Appl. Phys. 93, 1753 (2003)]. Results show that peak energy of the freshly etched sample is 1.86 eV with average nanoparticle size L0=3.90 nm and dispersion σ=0.065 nm. Ambient aging for 40 days causes this peak to blueshift to 2.07 eV and reduces the nanoparticle size to L0=3.37 nm with dispersion σ=0.088 nm. The reduction in particle size (ΔL=0.53 nm) is attributed to oxidation of the nanocrystalline surface, and corresponds to approximately 2 monolayers of oxide growth.