Porous silicon photoluminescence: Implications from in situ studies

Abstract

Photoluminescence and Raman measurements have been performed on an anodized silicon surface in an HF/ethanol anodization solution and after replacement of this solution with water. Immediately after anodization and while resident in HF/ethanol, the porous silicon produced does not exhibit intense photoluminescence. Intense photoluminescence develops spontaneously in HF/ethanol after 18–24 h or with replacement of the HF/ethanol with water. The results are consistent with a quantum confinement mechanism in which electron-hole pair migration to traps and nonradiative recombination dominates the de-excitation pathways until silicon nanocrystals are physically separated and energetically decoupled by hydrofluoric acid etching or surface oxidation. Raman spectra show the development of nanometer-size silicon crystals concurrent with intense photoluminescence. Illumination of the silicon surface during anodization tends to inhibit the formation of nanocrystalline silicon, but even very thin layers (tens of nanometers or less) exhibit photoluminescence. As produced by anodization, the porous silicon surface is apparently inhomogeneous on the millimeter scale, showing large variations in photoluminescence intensity and peak wavelength.