Abstract
The enhanced surface area of silicon nanotexture is an important metric for solar cell integration as it affects multiple properties including optical reflectance, dopant diffusion, and surface recombination. Silicon nanotexture is typically characterized by its surface-area-to-projected-area ratio or enhanced area factor (EAF). However, traditional approaches for measuring EAF provide limited statistics, making correlation studies difficult. In this article, silicon's dominant ultraviolet reflectance peak, R(E2), which is very sensitive to surface etching, is applied to EAF spatial mapping. A clear decay correlation between R(E2) and EAF is shown for multiple textures created using reactive ion etching and metal catalyzed chemical etching. This correlation is applied to R(280 nm) reflectance mapping to yield accurate, high-resolution full-wafer EAF spatial mapping of silicon nanotextures. R(280 nm) mapping is also shown to be sensitive enough to correlate the impact of nanotexture spatial variation on post-diffusion sheet resistance. Finite-difference time-domain simulations of several nanoscale pyramid textures confirm a decay band for R(E2) versus EAF, consistent with our measurements. We suggest that R(E2) mapping may prove useful for other silicon nanotexture properties and applications where EAF is important.
Highlights
T HE enhanced surface area of nanotextured silicon and “black silicon” (B-Si) are of great interest for various applications including lithium battery anodes [1], [2], photocathodes for photoelectrochemical hydrogen production [3], [4], and ultralow reflectance for photovoltaics [5], [6]
We demonstrate a clear correlation between R(E2) and enhanced area factor (EAF) using a wide range of silicon nanotexture conditions prepared using reactive ion etching (RIE) and industrial metal catalyzed chemical etching (MCCE)
We presented a clear empirical decay correlation between R(E2) and EAF for multiple silicon nanotexture conditions prepared with MCCE and RIE
Summary
T HE enhanced surface area of nanotextured silicon and “black silicon” (B-Si) are of great interest for various applications including lithium battery anodes [1], [2], photocathodes for photoelectrochemical hydrogen production [3], [4], and ultralow reflectance for photovoltaics [5], [6]. The peak in the imaginary part of the pseudodielectric function, measured with spectroscopic ellipsometry, has been shown to be sensitive to microscopic surface roughness, described as a “form of density deficit” [30]. Such results indicate that E2 optical spectra peaks like R(E2) are very sensitive to the amount of etched silicon, which is related to a texture’s void volume (i.e., “density deficit”). As the etched amount (or etched thickness) is related to texture surface area, we propose that R(E2) should be well suited for monitoring spatial variation in silicon nanotextures. The R(E2) versus EAF relation is studied using finite-difference time-domain (FDTD) simulations of nanoscale pyramid textures with a wide range of void volumes
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