  Recent progress achieved in metal-assisted chemical etching (MACE) has
enabled the production of high-quality micropillar arrays for various
optoelectronic applications. Si micropillars produced by MACE often show a
porous Si/SiOx shell on crystalline pillar cores introduced by local
electrochemical reactions. In this paper, we report the distinct optoelectronic
characteristics of the porous Si/SiOx shell correlated to their chemical
compositions. Local photoluminescent (PL) images obtained with an immersion oil
objective lens in confocal microscopy show a red emission peak (about 650 nm)
along the perimeter of the pillars that is threefold stronger compared to their
center. On the basis of our analysis, we find an unexpected PL increase (about
540 nm) at the oil/shell interface. We suggest that both PL enhancements are
mainly attributed to the porous structures, a similar behavior observed in
previous MACE studies. Surface potential maps simultaneously recorded with
topography reveal a significantly high surface potential on the sidewalls of
MACE-synthesized pillars (+0.5 V), which is restored to the level of planar Si
control (-0.5 V) after removing SiOx in hydrofluoric acid. These distinct
optoelectronic characteristics of the Si/SiOx shell can be beneficial for
various sensor architectures.
