Parag Doshi,
Gerald E. Jellison, Jr.,
and Ajeet Rohatgi
P. Doshi and A. Rohatgi are with University Center of Excellence for Photovoltaics Research and Education, Department of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332.
G. E. Jellison is with Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831.
Parag Doshi, Gerald E. Jellison, and Ajeet Rohatgi, "Characterization and optimization of absorbing plasma-enhanced chemical vapor deposited antireflection coatings for silicon photovoltaics," Appl. Opt. 36, 7826-7837 (1997)
We have optimized plasma-enhanced chemical vapor deposition (PECVD) of
SiN-based antireflection (AR) coatings with special consideration for the
short-wavelength (<600 nm) parasitic absorption in SiN. Spectroscopic
ellipsometry was used to measure the dispersion relation for both the
refractive index n and the extinction coefficient
k, allowing a precise analysis of the trade-off between
reflection and absorption in SiN-based AR coatings. Although we focus on
photovoltaic applications, this study may be useful for photodetectors, IR
optics, and any device for which it is essential to maximize the transmission
of light into silicon. We designed and optimized various AR coatings for
minimal average (spectrally) weighted reflectance (〈Rw〉)
and average weighted absorptance (〈Aw〉), using the air mass 1.5
global solar spectrum. In most situations 〈Rw〉 decreased with higher
n, but 〈Aw〉 increased because k
increased with n. For the practical case of a
single-layer AR coating for silicon under glass, an optimum refractive index
of ∼2.23 (at 632.8 nm) was determined. Further simulations revealed that a
double-layer SiN stack with an n = 2.42 film
underneath an n = 2.03 film gives the minimum
total photocurrent loss. Similar optimization of double-layer
SiN/SiO2 coatings for silicon in air revealed an
optimum of n = 2.28 for SiN. To determine the
allowable tolerance in index and film thickness, we generated isotransmittance
plots, which revealed more leeway for n values below the
optimum than above because absorption begins to reduce photocurrent for high
n values.
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Values represent optimum SiN single-layer
coating under glass.
This “unrealistic” coating
represents a constant 2.40 index to show the ideal film if absorption is
neglected.
Values are for double-layer
antireflection case.
Values represent optimum single-layer SiN
coatings in air.
This “unrealistic” coating
represents a constant 2.0 index to show the ideal film if absorption is
neglected.
Values represent optimum double-layer
SiN/SiO2 coatings in air.
This “unrealistic” coating
represents a constant 2.55 index to show the ideal index if absorption is
neglected.
Values represent optimum SiN single-layer
coating under glass.
This “unrealistic” coating
represents a constant 2.40 index to show the ideal film if absorption is
neglected.
Values are for double-layer
antireflection case.
Values represent optimum single-layer SiN
coatings in air.
This “unrealistic” coating
represents a constant 2.0 index to show the ideal film if absorption is
neglected.
Values represent optimum double-layer
SiN/SiO2 coatings in air.
This “unrealistic” coating
represents a constant 2.55 index to show the ideal index if absorption is
neglected.