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longer wavelengths (IR and above), the reflectance increases. Again,
most AR structures fail when it goes to grazing AOI. In fact, this
poses a challenge for most optical devices, such as solar cells, which
utilise sunlight incident at over ~180
solid angle. Research efforts
are ongoing for a structure that would provide lowest possible total-
or hemispherical-reflection values over broad wavelength bands
and large AOI, leading to broadband and omni-directional anti-
reflectivity.
°
2.4.3 Silicon Nanotips for Broadband, Omnidirectional
AR
Since the industrial advancement in silicon technology, silicon isit
has become the most-explored material for AR coatings/surfaces. As
observed in previous sections, the development of AR structures, from
porous silicon- to moth eye-inspired biomimetic nanostructures,
has mainly grown with Si [29,34,52,53,55-57,59,63].
Interestingly, crystalline silicon in general demonstrates high
reflectivity (
35% in VIS range) due to its high RI (3.8 at 600 nm).
Hence, the RI has to be graded in the range of 3.8 (in silicon) to 1.0
(in air) for the AR applications. A recent report has successfully
demonstrated a broadband and omnidirectional AR phenomenon
in silicon nanotip (SiNTs) array, having apex diameter of
~
∼
3-5 nm,
9
-2
base diameter of
∼
200 nm, and packing density of
∼
6
×
10
cm
[73],
outperforming the existing Si-based AR microstructures.
These SiNTs (Fig. 2.14a-b) fabricated by using self-masked dry
etching (SMDE) technique (involving a plasma of silane, methane,
hydrogen and argon), monolithically on planar silicon wafers [74-
77], evidently, did show a remarkable suppression in reflectance,
especially for the long NTs (16
m), over a broadband spanning
UV-VIS-NIR, mid-IR, far-IR, and terahertz (THz) (Fig. 2.14c-f,
respectively) [73].
In the UV-VIS-NIR region, the 16
µ
m long SiNTs showed highest
reflectance of only 0.7% (Fig. 2.14c), which, however expectedly,
increased with
µ
. Even in the THz regime, the 16 mm long SiNTs
showed quite an appreciable reflectance loss than in planar wafers
(Fig. 2.14f). The strong dependence of specular reflectance with
AOI, observed in polished silicon, could be nearly removed upon
nanostructuring the surface for both the
l
s
- and
p
-polarised light (Fig.
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