Anti-Reflecting Nanostructures - Biomimetic Architectures by Plasma Processing - page 67

2.4.4 Non-Silicon AR Nanostructures

In comparison to silicon-like high-RI material, logically, the option
of using starting materials with lower RI can be an alternative.
Interestingly, the published reports can broadly be classified into
following major groups: (i) SiO based, such as glass (RI

~

.52-1.80),

2

quartz (

~

1.54), and fused silica (

~

1.459), (ii) polymers (

~

1.46-

1.55), (iii) metals, such as gold ( 1.35), (iv) diamond

or diamond-like hard coatings, and (v) wide bandgap Group III-V
semiconductors such as GaAs, GaN, GaSb or InP.
Conical structures were fabricated on fused silica, quartz and glass
substrates using the lithographic principles involving mostly dry
etching technique (fluorocarbon or SF

~

0.47) or Ag (

~

plasma) [91,92]. Minimum
reflectance values of 0.5-1% and maximum 99% transmittance
could be achieved in VIS regime for glass and fused silica structures.
Wavelength-dependent reflectance profile was simulated by RCWA
model assuming the conical structures to be composed of a number
of square gratings having certain thickness, period and filling factor.
Optimised double-side SW grating structures on quartz achieved a
reflectance value of only 0.035% in the 450-525 nm range under
normal incidence compared with 3.5% in planar fused quartz [93].
This result is significant in the sense that it is damage resistant and is
transparent birefringent in the UV-IR region. Recently, a combination
of a bottom-up fabrication (GLAD) with a top-down engineering (ion
milling) approaches was employed to fabricate biomimetic ARCs
[94]. Here, GLAD technique was first utilised to produce nanopillar
arrays (NPAs) made of Si, SiO

6

, with broadened structures
(Fig. 2.19a), which were subsequently transformed into biomimetic
tapered geometries by means of post-deposition ion milling (Fig.
2.19b). This structure transformation remarkably decreases
reflection over a wide wavelength range (300-1700 nm) and field of
view (AOI < 60

and TiO

x 2

) (Fig. 2.19c) [94].
Compared with inorganic materials, polymer materials have
great practical value as flexible substrate with the ability of
controllable morphology and porosity ratio, and ease of large-
area processing [14]. In addition to the polymer nanostructures
described before, there are reports on pure or metal-coated
polymer structures demonstrating AR behaviour. The 'moth-eye'
structures have been fabricated using plasma (Ar

°

+

) treatment on