Biomimetic subwavelength antireflective gratings
Chih-Hung Sun,1Brian J. Ho,1Bin Jiang,2and Peng Jiang1,*
1Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, USA
2Department of Mathematics and Statistics, Portland State University, Portland, Oregon 97201, USA
* Corresponding author: firstname.lastname@example.org
Received July 29, 2008; accepted August 15, 2008;
posted August 29, 2008 (Doc. ID 99567); published September 29, 2008
We have developed a simple and scalable bottom-up approach for fabricating moth-eye antireflective coat-
ings on GaAs substrates. Monolayer, non-close-packed silica colloidal crystals are created on crystalline
GaAs wafers by a spin-coating-based single-layer reduction technique. These colloidal monolayers can be
used as etching masks during a BCl3dry-etch process to generate subwavelength-structured antireflective
gratings directly on GaAs substrates. The gratings exhibit excellent broadband antireflective properties, and
the specular reflection matches with the theoretical prediction using a rigorous coupled-wave analysis
model. These bioinspired antireflection coatings have important technological applications ranging from ef-
ficient solar cells to IR detectors. © 2008 Optical Society of America
OCIS codes: 050.2770, 220.4241, 310.1210, 310.6628.
Gallium arsenide (GaAs) is a technologically impor-
tant semiconductor that has been widely used in op-
toelectronics, such as vertical cavity surface-emitting
lasers , near-IR photodetectors , and highly effi-
cient concentrator solar cells [3–5]. However, owing
to the high refractive index (RI) of GaAs (nGaAs?3.6
for visible wavelengths), more than 30% of incident
light is reflected back from the substrate surface.
This greatly reduces the efficiency of GaAs-based op-
toelectronic devices. To suppress the unwanted re-
flective losses, vacuum-deposited multilayer dielec-
tric (e.g., MgF2/ZnS? antireflection coatings (ARCs)
have been developed [6,7]. Unfortunately, these
multilayer ARCs are expensive to fabricate owing to
the stringent requirement of high vacuum, material
selection, and layer thickness control. Additionally,
thermal-mismatch-induced lamination and material
diffusion of the multilayer ARCs limit the device per-
formance at high power densities .
Inspired by the broadband antireflection of micro-
structured corneas of moths, which consist of non-
close-packed arrays of sub-300 nm nipples , sub-
extensively exploited [9–17]. These structures can re-
duce reflection over a wider range of wavelengths
and exhibit much improved thermal stability than
conventional multilayer ARCs. However, scalable
production of subwavelength ARCs is not a trivial
task for the current top-down nanolithography tech-
nologies (e.g., electron-beam lithography and inter-
ference lithography) [11,13]. Bottom-up colloidal li-
crystals as deposition or etching masks to pattern pe-
riodic nanostructures [18,19], provides a much sim-
pler and inexpensive alternative to nanolithography
in creating subwavelength gratings [16,20]. Unfortu-
nately, traditional colloidal assemblies suffer from
low throughput, small areas, incompatibility with
standard microfabrication, and limited close-packed
We have recently developed an inexpensive and
scalable colloidal lithography technology for creating
moth-eye antireflective gratings by using monolayer,
non-close-packed silica colloidal crystals prepared by
a simple spin-coating technique as etching masks
[21,22]. However, significant obstacles have been en-
countered when we employ this nonlithographic tech-
nique to GaAs substrates. Only disordered colloidal
monolayers can be assembled on GaAs by spin coat-
ing. This greatly reduces the uniformity and repro-
ducibility of the antireflective performance of tem-
plated ARCs. Here we report a generalized single-
reproducible production of monolayer colloidal crys-
tals on a variety of substrates that otherwise lead to
the formation of disordered crystals by the tradi-
tional spin-coating approach [23,24]. We also demon-
strate the scalable production of broadband ARCs on
crystalline GaAs substrates, and their antireflective
properties are characterized by both experimental
measurements and theoretical simulation.
Figure 1(a) shows a typical scanning electron mi-
croscope (SEM) image of a colloidal monolayer con-
formed on GaAs by traditional spin coating. Inset, Fourier
transform of the image. (b) Double-layer colloidal crystals
formed on GaAs. (c) Monolayer colloidal crystal formed by
SLD. Inset, photograph of a monolayer crystal formed on a
2 in. GaAs wafer illuminated with white light. (d) Pair cor-
relation function calculated from a low-magnification SEM
image. For comparison, the PCF for an ideal hexagonal lat-
tice with?2D inter particle distance is also shown (black
(Color online) (a) Disordered colloidal monolayer
OPTICS LETTERS / Vol. 33, No. 19 / October 1, 2008
0146-9592/08/192224-3/$15.00© 2008 Optical Society of America
sisting of 320 nm silica spheres prepared on a crys-
talline GaAs substrate [N-type, (100), University
Wafer] by the standard spin-coating process [23,24].
The particles are completely disordered as confirmed
by the rings in the Fourier transform of the SEM im-
age. Contrary to the direct formation of colloidal
monolayers by spin coating, where the substrate
plays a crucial role in determining the resulting crys-
talline quality [23,24], double-layer colloidal crystals
are firstly assembled in SLD. Figure 1(b) shows a
double-layer colloidal crystal prepared by spin-
coating colloidal silica suspensions (20 vol. % in non-
volatile ethoxylated trimethylolpropane triacrylate
monomer) at 8000 rpm for 90 s. The non-close-
packed hexagonal arrangement for both the top- and
bottom-layer particles is clearly evident. After remov-
ing the top-layer particles by sweeping using a clean-
room Q-tip under flowing water, crystalline colloidal
monolayers with non-close-packed structure are
formed as shown in Fig. 1(c). The distinctive six-arm
Bragg diffraction star formed on the 2 in. ?5.08 cm?
GaAs wafer [inset of Fig. 1(c)] indicates the hexago-
nal ordering is over the whole wafer surface [24,25].
The interparticle distance of the non-close-packed
colloidal crystal is determined to be 1.40D, where D
is the diameter of silica spheres, by the first peak of
the pair-correlation function [PCF, Fig. 1(d)], g?r?,
which is calculated as g?r?=1/???dn?r,r+dr?/da?r,r
+dr?, where dn counts the number of spheres that lie
within a spherical shell, dr, of radius r from an arbi-
trary origin; da=2?rdr for a particular radial dis-
tance r; and ??? is the average particle number den-
sity. Figure 1(d) also shows the positions of the
oscillating PCF peaks agree well with those obtained
from a perfect hexagonal lattice with ?2D interpar-
ticle distance. Preliminary results show the SLD
technology can be easily and reproducibly applied to
other important substrates, such as Ni, Cr, glass, and
poly(methyl methacrylate). Only disordered mono-
layers are formed on these substrates by the tradi-
tional spin-coating technique .
The periodically arranged silica particles can then
be used as etching masks during a BCl3reactive ion
etch (RIE) process operating at 7.5 mTorr chamber
pressure, 20 SCCM (SCCM denotes cubic centime-
ters per minute at standard temperature and pres-
sure) BCl3, RIE=100 W, and inductively coupled
plasma ?ICP?=300 W on a Unaxis Shuttlelock reac-
tive ion etcher to generate arrays of cone shaped
nanopillars. Figures 2(a) and 2(b) show the tem-
plated GaAs structures after 9.5 min RIE and remov-
ing templating silica particles by a 2% hydrofluoric
acid wash. Interestingly, we observe the formation of
mushroom-like microstructures. The “stems” of the
mushrooms are cone-shaped and are caused by the
isotropic etching of GaAs by reactive chlorine ions.
The polymer wetting layer between the templating
silica spheres and the substrate , which protects
the GaAs surface immediately under the particles
from being etched, causes the formation of the flat
“caps” of the mushrooms. This unusual microstruc-
ture has been used to develop superoleophobic sur-
face , though it is not easily available by conven-
tional top-down and bottom-up techniques. Longer
dry etching leads to sharper conical stems and
smaller caps as revealed by the SEM image of a
12 min etched sample in Fig. 2(c). The residual caps
can finally be removed by a brief ultrasonication in
acetone, resulting in the formation of conical GaAs
gratings as shown in Fig. 2(d). The preservation of
the long-range hexagonal ordering and the interpar-
ticle distance of the original silica colloidal monolayer
throughout the templating process are clearly evi-
dent by comparing the SEM images of Figs. 1 and 2.
The specular reflectivity of the templated subwave-
length gratings are evaluated using visible- and
near-IR reflectivity measurement at normal inci-
dence by using an Ocean Optics HR4000 spectrom-
eter. The solid curves in Fig. 3 show the measured re-
flection from a bare GaAs wafer and the nanocone
grating as shown in Fig. 2(d). The wafer exhibits
?30% reflectivity, while much reduced reflection
??3%? is obtained for the templated subwavelength
grating. Importantly, the resulting moth-eye ARCs on
GaAs are broadband, exhibiting consistent low reflec-
tion over both visible and near-IR wavelengths.
The optical measurement is complemented by the-
oretical simulation using a rigorous coupled-wave
analysis (RCWA) model . A conical profile is used
to simulate the templated nipples as shown in Fig.
2(d). We divide the cone array into 100 horizontal cir-
cular layers. The intercone distance is defined as?2D
by the PCF calculation shown in Fig. 1(d). We can
tures after 9.5 min BCl3RIE. (b) Higher magnification im-
age of (a). (c) Conical nipples formed after 12 min BCl3
RIE. (d) Same sample as (c) after a brief ultrasonication in
(a) Templated mushroom-like GaAs microstruc-
specular reflection at normal incidence from a bare GaAs
wafer and the conical grating as shown in Fig. 2(d).
(Color online) Experimental and RCWA-simulated
October 1, 2008 / Vol. 33, No. 19 / OPTICS LETTERS
calculate the fraction of GaAs in each layer as f?z*? Download full-text
=??r*?2/?3D2, where z* and r* are the z coordinate
and the bottom radius of the cone, respectively. Based
on the effective medium theory, the effective RI n?z*?
of the layer at height z* can be approximated by
[8,29]. We can then calculate the total reflectance of
the layered system by solving Maxwell’s equation to
express the electromagnetic (EM) field in each layer
and matching EM boundary conditions between
neighboring layers. The complex RI of GaAs is used
to calculate the resulting reflectance .
The curves with circles in Fig. 3 show the simu-
lated reflection from a flat GaAs wafer and an array
of conical nipples templated from 320 nm silica
spheres. The calculated spectrum for the featureless
substrate has the same shape as the experimental re-
sult. The simulated reflection from the subwave-
length nanocone array also agrees reasonably well
with the experimental spectrum. The excellent
broadband antireflection properties of the templated
conical gratings can be understood by mapping the
calculated effective RI at ?=700 nm across the height
of 320 nm conical nipples (Fig. 4). For bare wafer, the
RI changes sharply from air ?RI=1.0? to bulk GaAs
?RI=3.772?, while for templated nipples, the RI
changes gradually from 1.0 to 3.41 and then to 3.772.
As the reflection ?R? from an interface between two
materials with RI of n1 and n2 is governed by
Fresnel’s equation R=?n1−n2/n1+n2?2, the RI
gradient exhibited by the conical nipples thus leads
to very low reflection over a wide range of wave-
In summary, we have developed a nonlithographic
technology for creating moth-eye antireflection coat-
ings on GaAs substrates. The technique is scalable
and compatiblewith standard
promising for developing more economic, efficient,
and reliable GaAs-based optoelectronic devices.
This work was supported by the National Science
Foundation (NSF) under grants CBET-0651780 and
CBET-0744879, ACS Petroleum Research Fund, and
the University of Florida Research Opportunity In-
centive Seed Fund.
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at ?=700 nm from the wafer surface ?depth=0? to the bot-
tom of templated GaAs nanocones with 320 nm height. The
dotted line indicates the refractive index of bulk GaAs at
the same wavelength.
Change of the calculated effective refractive index
OPTICS LETTERS / Vol. 33, No. 19 / October 1, 2008