Chemistry Reference
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GaAs-based sub-wavelength gratings (SWG) have also been
developed by employing colloidal lithography with a dry etching
process patterned on GaAs substrates. By using non-close-packed
silica colloidal monolayer and etching masks during a BCl
dry-etch
process, the cone shaped SWG has been directly generated on GaAs
substrates, which exhibited reduction in reflectivity loss from 30%
to 3% in 0.4-1.0
3
µ
m range [108]. Similar conical structures were
also reported using a self-assembled monolayer of PS spheres as
dry etching mask (under plasma of Ar, Cl
mixture) [109].
Average reflectance of a bare GaAs wafer was reduced from 35.0%
to 2.3% across a spectral range of 300-1200 nm. The broadband AR
properties for both the structures match well with the theoretical
prediction using RCWA model [108,109].
GaAs nanograsses (NGs) has been fabricated using a simple, one-
step, maskless (H
and O
2
2
) plasma etching in an inductively coupled plasma
chemical vapour-deposition technique [110]. The NGs (diameter
<
2
∼
200 nm) (Fig. 2.20e, f) exhibited remarkable
broadband AR properties, with less sensitivity to TE- (Fig. 2.20g)
and TM-polarised light (Fig. 2.20h) over a wide range of AOI
compared with the bare GaAs substrate. The GaAs NGs showed an
enhanced absorption of 72-98% when the incident energy is less
than the GaAs bandgap (
20 nm, length
= 873-2400 nm), and the absorption was
enhanced by 98-100% above the bandgap [110].
CVD-fabricated GaP NRs were shown to increase transmittance
by
l
13%, compared with the planar GaP, at below-bandgap range.
Such NRs AR structures have potential in LED application for
efficient light extraction [111]. Similarly, GaSb SWG with 'moth-eye'-
like feature suppresses the reflectance of planar GaSb by
∼
≥
80% in
µ
the 0.5-2.0
m spectral region [112]. The conventional ARCs of ZnS
and MgF
degrade with heating, limiting the solar cells performance,
especially in thermophotovoltaic cells, while GaSb SWG did show
very stable performance. In another work, the 2D submicron
periodic structures made of InP, achieved using a two time laser (He-
Cd) holography followed by dry and wet etching, showed reasonable
reduction in reflectance in the 0.4-1.2
2
m region [113].
Interestingly, the non-silicon AR structures did not perform
inferior to their silicon counterparts, although the advanced
knowledge of silicon processing results in better control of the latter
structures. Compared with crystalline materials, there is scope for
further development in amorphous starting materials, such as glass
and polymers.
µ
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