  Recent advances in fabrication have enabled radial-junction architectures for
cost-effective and high-performance optoelectronic devices. Unlike a planar PN
junction, a radial-junction geometry maximizes the optical interaction in the
three-dimensional (3D) structures, while effectively extracting the generated
carriers via the conformal PN junction. In this paper, we report
characterizations of radial PN junctions that consist of p-type Si micropillars
created by deep reactive-ion etching (DRIE) and an n-type layer formed by
phosphorus gas diffusion. We use electron-beam induced current (EBIC)
microscopy to access the 3D junction profile from the sidewall of the pillars.
Our EBIC images reveal uniform PN junctions conformally constructed on the 3D
pillar array. Based on Monte-Carlo simulations and EBIC modeling, we estimate
local carrier separation/collection efficiency that reflects the quality of the
PN junction. We find the EBIC efficiency of the pillar array increases with the
incident electron beam energy, consistent with the EBIC behaviors observed in a
high-quality planar PN junction. The magnitude of the EBIC efficiency of our
pillar array is about 70 % at 10 kV, slightly lower than that of the planar
device (about 81 %). We suggest that this reduction could be attributed to the
unpassivated pillar surface and the unintended recombination centers in the
pillar cores introduced during the DRIE processes. Our results support that the
depth-dependent EBIC approach is ideally suitable for evaluating PN junctions
formed on micro/nanostructured semiconductors with various geometry.
