In-plane control of morphology and tunable photoluminescence in porous silicon produced by metal-assisted electroless chemical etching

Photoluminescent porous silicon (PSi) was produced by Pt-assisted electroless etching of p ^--Si (100) in a 1:2:1 solution of HF, H ₂O ₂, and methanol. The peak emission wavelength of the PSi could be tuned in the range 500 nm≤λ≤600 nm simply by changing the time of etching. The luminescence is sufficiently intense at all wavelengths to be visible by eye. Furthermore, by patterning the metal areas on the surface prior to etching, the luminescence can be controlled spatially. To investigate the relationship among processing variables - principally etch time and spatial proximity to Pt - and morphology, scanning electron microscopy (SEM), true color fluorescence microscopy, and spatially resolved phonon line shape studies were undertaken. SEM images show nanocrystalline features in the region where the luminescence originates, a region which shifts spatially as a function of etch time, as indicated by fluorescence microscopy. Raman scattering measurements of the shift and broadening of the longitudinal optical phonon band interpreted in the context of the phonon confinement model were used to estimate crystallite sizes. As with the luminescence, the crystallite sizes were found to vary as a function of distance from the Pt patterned area and etch time. These results are interpreted in light of an etching mechanism in which H ₂O ₂ reduction results in hole injection deep into the valence band, which then drifts spatially and plays a critical role in determining the rate at which Si is removed from the surface.