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Supporting Information for:
PROFILING PHOTOINDUCED CARRIER GENERATION IN
SEMICONDUCTOR MICROWIRE ARRAYS VIA
PHOTOELECTROCHEMICAL METAL DEPOSITION
M
ITA
D
ASOG
1,†
,
A
ZHAR
I.
C
ARIM
1,†
,
S
ISIR
Y
ALAMANCHILI
2
,
H
ARRY
A.
A
TWATER
2
AND
N
ATHAN
S.
L
EWIS
1,3,4*
1
Division of Chemistry a
nd Chemical Engineering
2
Division of Engineering and Applied Sciences
3
Kavli Nanoscience Institute
4
Beckman Institute
California Institute of Technology
Pasadena, CA 91125
These authors contributed equally
*Corresponding Author:
nslewis@caltech.edu
S1.
Contents
This document contains a descr
iption of the experimental and co
mputational methods
utilized in this work (Secti
ons S2 and S3) and voltammetry data
(Section S4).
S2.
Experimental Methods
Materials and Chemicals
.
HAuCl
4
·3H
2
O (99.999%, Sigma-Aldrich), KCl
(99%, Sigma-Aldrich),
buffered HF (Transene Inc), In (99.999 %, Alfa Aesar), Ga (99.9
99 %, Alfa Aesar), and CH
3
OH
(ACS grade, BDH chemicals) were used as received. H
2
O with a resistivity ≥ 18.2 MΩ cm
(Barnstead Nanopure System) was used throughout. Microwire arra
ys were constructed by etching
either p-Si(100) wafer sections (1.0 – 10.0 Ω cm, B-doped, 525
± 25 μm thick, single-side polished,
Addison Engineering) or n-Si (100) wafer sections (1.0 – 10.0 Ω
cm, P-doped, 525 ± 25 μm thick,
single-side polished, A
ddison Engineering).
Fabrication of Microwire Arrays
.
S1813 positive photoresist was spin coated at 4000 rpm onto
the top of Si wafers. The wafers were then heated on a hotplate
at 115 ºC for 1 min. The photoresist
was then exposed using UV illumination through a square lattice
mask consisting of holes with a
3 μm diameter and a 7 μm pitch. The photoresist was developed u
sing MF-319 developer and the
wafers were baked for 10 m
in at 115 ºC. A 200 nm thick Al
2
O
3
mask was then deposited onto the
patterned wafer using electron-b
eam evaporation. PG-remover was
used to liftoff the photoresist.
After the liftoff, Fomblin oil was applied to the back of the w
afers to act as a thermal contact
material, and the wafers were loaded into the etching chamber.
Wires were etched using a
cryogenic inductively coupled plasma reactive ion etching proce
ss (ICP-RIE) with an Oxford
DRIE 100 ICP-RIE system. Etching was performed at low capacitiv
e coupled power of 5 W to
reduce damage due to the momentum of the ions. A high inductive
ly coupled power of 900 W was
used to increase the number of ions in the plasma to achieve hi
gh rates of chemical etching. The
chamber was maintained at -120 ºC and a pressure of 10 mTorr du
ring the etching process. Etching
was performed using SF
6
: O
2
ratios ranging from 70 sccm : 5.5 sccm to 70 sccm : 8 sccm for
30
min. Variation of the O
2
concentration in the plasma enabled control over the wire tape
r. After
etching, the wafers were dippe
d in buffered HF to chemically re
move the Al
2
O
3
mask.
Photoelectrochemical Deposition
.
Si microwire arrays were cut into ~ 1 cm x 1 cm sections.
Immediately before deposition, each section was rinsed with CH
3
OH, followed by H
2
O, and then
immersed in buffered HF(aq) for ~ 60 s to remove any surficial
SiO
x
from the Si. The sample was
then rinsed with H
2
O and dried with a stream of N
2
(g). A Ga/In eutectic was scratched into the
back side of each section using
a carbide scribe, and the secti
on was mounted on
a ~ 2 cm x 2 cm
Ti plate using Cu tape. This assembly was then sealed with a Ti
backplate into a single
compartment O-ring compression th
at confined the contact region
between the solution and the Si
microwire array section t
o a circular area of 0.1 cm
2
. The cell was equippe
d with a pyrex window
that enabled illumination during deposition. A three-electrode
configuration was utilized with a
graphite-rod counter electrode (
99.999 %, Sigma-Aldrich) and a
Ag/AgCl reference electrode (3
M KCl, Bioanalytical Systems). Au was deposited from an aqueous
solution of 0.010 M HAuCl
4
and 0.100 M KCl. All depositions were performed using a Princet
on Applied Research Model 273
potentiostat. Depositions were carried out at room temperature
by biasing the illuminated
microwire array potentiostatica
lly at -1.25 V vs. Ag/AgCl until
an integrated charge density of
0.10 C cm
-2
had been passed.
Electrode Illumination
.
Illumination for the photoelectrochemical depositions was prov
ided by
narrowband diode (LED) sources (Thorlabs) with respective inten
sity-weighted λ
avg
values and
spectral bandwidths (FWHM) of 461 and 29 nm (M470L2), 516 and 3
0 nm (M505L3), 630 and
18 nm (M625L3), and 940 and 30 nm (M940L2), respectively. The o
utput of each diode source
was collected and collimated with an aspheric condenser lens (Ø
30 mm, f = 26.5 mm). The light
intensity incident on the electrode was measured by placing a c
alibrated Si photodiode (Thorlabs
FDS10X10), in the photoelectrochem
ical cell with the electrolyt
e, at the location of the electrode,
and measuring the steady-state
current response of the Si photo
diode. Depositions that utilized the
diode with λ
avg
= 461 nm as the illumination source were performed with a ligh
t intensity of 77
mW cm
-2
at the electrode. Deposit
ions using the diodes with λ
avg
= 516, 630, and 940 nm, were
performed with intensities of 64, 50, and 33 mW cm
-2
, respectively.
Microscopy
.
Scanning-electron micrographs (SEMs) were obtained with a FEI
Nova NanoSEM
450 at an accelerating voltage of 5.00 kV and working distance
of 5.00 mm using an Everhart-
Thornley secondary e
lectron detector.
S3.
Computational Methods
Generation Profile Simulation
.
Photocarrier generation profile
s in Si wire arrays under steady
-
state illumination with unpolarized light were simulated using
three-dimensional full wave
electromagnetic simulations via a finite-difference time domain
(FDTD) method. Modeling was
performed using the FDTD Solutions software package (Lumerical)
.
Si microwire arrays were
simulated using a single three
-dimensional unit cell with perio
dic boundary conditions along the
x and y axes to depict a 7 μm s
quare lattice similar to the fab
ricated arrays, in
conjunction with
infinite boundary conditions rende
red as perfectly matched laye
rs (PML) along the z axis.
Idealized microwire arrays corr
esponding to the four different
fabricated shapes were constructed
in the 3D simulation region by approximating the tapered microw
ires as truncated cones or
multiple sections of truncated cones. The absorption was simula
ted under polarized illumination
and then the CW-generation rate analysis group in the FDTD Solu
tions software was used to derive
the generation rate per unit vo
lume. The generation rate under
unpolarized illumination was
obtained by averaging the genera
tion rate data with itself by i
nterchanging the x- and y-axes as
permitted by the square symmetry of the wire lattice.
S4.
Voltammetry
Figure S1.
Linear sweep voltammogram (at a scan rate of 50 mV s
-1
) obtained with a p-Si microwire array with a ~ 3
μm wire diameter, 7 μm wire pitch, and 30 μm wire height in an
aqueous solution of 0.010 M HAuCl
4
and 0.100 M
KCl under chopped illumination with a LED source having λ
avg
= 461 nm and an intensity of 77 mW cm
-2
.