Supporting Information for:
Enhanced Absorption and < 1% Spectrum-and-Angle-Ave
raged
Reflection in Tapered Microwire Arrays
Sisir Yalamanchili
1,3†
,
Hal S. Emmer
1,†
, Katherine T. Fountaine
2,6
, Christopher T. Chen
1
,
Nathan S. Lewis
2-5*
, and Harry A. Atwater
1,3,4
1
Division of Engineering and Applied Sciences, Calif
ornia Institute of Technology, Pasadena, CA 91125
2
Division of Chemistry and Chemical Engineering, Cal
ifornia Institute of Technology, Pasadena, CA
91125
3
The Joint Center for Artificial Photosynthesis, Cal
ifornia Institute of Technology, Pasadena, CA 91125
4
Kavli Nanoscience Institute, California Institute o
f Technology, Pasadena, CA 91125
5
Beckman Institute, California Institute of Technolo
gy, Pasadena, CA 91125
6
NGNext, Northrop Grumman Aerospace Systems, 1 Space
Park Dr., Redondo Beach, CA 90278
†
These authors contributed equally
*Corresponding Author:
haa@caltech.edu
This is a 7 page document with 3 sections ( S1, S2,
and S3), and 5 figures (S1-S5)
S1. Transmission Electron Microscopy (TEM)
Figure S1:
Transmission electron microscopy (TEM) study of a s
ingle etched microwire
immediately after ICPRIE with the Al2O3 mask intact
on top and with no cleaning except a
solvent rinse is aligned to the 001 zone axis. A la
rge area corresponding to the tip of a wire was
studied (far left). Bright field images (BF) are de
void of contrast within the etched wire other
than bend contours introduced during sample prepara
tion, and selected area diffraction (SAD)
shows a clean pattern. Higher resolution images at
the top of the wire (center) shows no evidence
of damage to the wire tip which was protected by th
e hard mask during growth. However, a high
resolution imaging reveals a thin (~ 2-3 nm) region
on the sidewalls (far right) which could
correspond to damage introduced during the etching
process. These data were collected at 300
kV in an FEI Technai TF30 TEM.
Sample preparation for transmission electron micros
copy:
Etched microwires were removed
from the host Si wafer by mechanical cleaving with
a razor blade before being dispersed in
isopropyl alcohol. The resulting solution was spun
onto a Si wafer for 30 s at 2000 rpm. The
wire-coated wafer was then coated with ~ 20 nm of Al
2
O
3
with atomic layer deposition. Axial
cross sections were made in a FEI Versa dual-beam f
ocused ion beam instrument, extracted with
an FEI EasyLift NanoManipulator and welded to an Om
niprobe Cu liftout TEM grid before
thinning to electron transparency.
S2. Microwave photoconductivity decay lifetime meas
urements
Schematic of the custom built microwave photoconduc
tivity decay (MW-PCD) lifetime
measurement setup is shown in Fig. S2. This techniq
ue is similar to the widely used quasi-
steady-state photoconductance technique (QSSPC), ex
cept that the photoconductivity decay is
obtained by measuring the reflection of a microwave
at the sample following a light pulse (1064
nm for 5ns in our case) instead of the permeability
change of the sample measured via coupling
the sample by a coil to a radio-frequency bridge
1
.
Figure S2:
Schematic of home built microwave detected photocon
ductivity decay setup
Figure S3
: Microwave reflectivity of ALD Al
2
O
3
passivated tapered microwire arrays with
in
situ
back surface passivation using 5.8M HCl measured u
sing the microwave photoconductivity
setup with 0.75 μ s carrier lifetime
2.00E-06
3.00E-06
4.00E-06
5.00E-06
6.00E-06
Microwave Reflectivity (a.u.)
Time (s)
S3. Additional details regarding tapered microwire
samples
Tapered microwires embedded PDMS films:
Total thickness of the film is 120 μ m with 75 μ m
tall tapered microwires embedded in them. The shape
of the microwires is very close to a cone
75 μ m tall, 25 nm top radius, and 3.5 μ m bottom rad
ius. The fill fraction of Si in the film is
plotted in Fig. S4 below.
Figure S4
: (a) Si fill fraction variation from the top of th
e peeled off PDMS films with tapered
microwires embedded in it. (b) SEM cross section im
age of the films showing the tapered
microwires popping out of PDMS at the bottom of the
film.
Figure S5
: 2D plot of angular averaged reflection from Si su
bstrate with tapered microwires
etched into it (in red), and angular averaged absor
ption in tapered microwire arrays embedded in
PDMS (in blue) plotted in 400 nm – 1100 nm waveleng
th range.
(
1) Rein, S., Lifetime measurement techniques. In
Lifetime Spectroscopy - A method of defect
characterization in silicon for photovoltaic applic
ations
, Springer: Germany, 2005; pp 59-68.