Anti-Reflecting Nanostructures - Biomimetic Architectures by Plasma Processing - page 65

concluded that the replacement of discontinuities/steps in the RI
profile, observed for ARS I and II, with a continuous and smoother
profile for ARS III (except at the substrate interface) could suppress
reflectance, as demonstrated experimentally [73]. In fact, another
theoretical study also showed enhanced solar energy trapping with
modulated-diameter Si NWs photonic crystals compared with fixed-
diameter NRs [85].
The improved flatness of the RI profile near the air-silicon
interface [21] is the key for the AR performance. As observed in
Fig. 2.16, longer apexes (longer region I) (at the solid-ambient
interface) (Fig. 2.16c,d) actually results in a better AR performance at
all AOI (Fig. 2.16e) and all

s (Fig. 2.16f) [73,86]. Grating structures,
with groove depths exceeding 30

l

m, conventionally used in the THz
regime [87,88], indicates that longer NTs may perform better in the
THz regime since they would have a finer RI gradient at the air-solid
interface. However, the AR performance does not deteriorate at the
short wavelengths due to increasing lengths of SiNTs (Figs. 2.14c
and 2.16c-f).
Importance of the solid-ambient interface layer is also supported
by Lee et al. [89], who reported ZnO NRs, with tapered ends
(Fig. 2.18a), having much better AR properties (black line,
Fig. 2.18c) if they marginally differ in length (region I in Fig. 2.18b),
resulting in a rough profile of the apexes compared with an array of
identical length NRs resulting in a planar apex profile. A rough apex
profile is another way of decreasing the volume fraction (

µ

f ) and the

resultant . From the results of this work, it appears that arrays

of long cylindrical structures with tapered apexes and preferably of
unequal lengths is the near-ideal AR design to work over broadband
and larger AOIs [21,37].
In an earlier report, Lee at al. [90] also demonstrated SiNTs
arrays produced through deep reactive ion-etching (DRIE) process.
These DRIE produced SiNTs, although not categorised as biomimetic
by the authors, are larger (20 nm apex) than the single-step SMDE-
produced biomimetic SiNTs (5 nm apex) [73] and have a corkscrew-
like surface feature (Fig. 2.18d). Nevertheless, these NTs also showed
impressive AR characteristics, with minimum reflectance reached
at

n

eff

∼

8% (

l

= 1

µ

m) achieved by the longer NTs (higher RIE time)

t

(Fig. 2.18e) [90].