The effective electron minority-carrier diffusion length, L_(n,eff), for 2.0 µm diameter Si wires that were synthesized by Cu-catalyzed vapor-liquid-solid growth was measured by scanning photocurrent microscopy. In dark, ambient conditions, L_(n,eff) was limited by surface recombination to a value of ≤ 0.7 µm. However, a value of L_(n,eff) = 10.5±1 µm was measured under broad-area illumination in low-level injection. The relatively long minority-carrier diffusion length observed under illumination is consistent with an increased surface passivation resulting from filling of the surface states of the Si wires by photogenerated carriers. These relatively large L_(n,eff) values have important implications for the design of high-efficiency, radial-junction photovoltaic cells from arrays of Si wires synthesized by metal-catalyzed growth processes

Atwater, Harry A.

Boettcher, Shannon W.

Kelzenberg, Michael D.

Lewis, Nathan S.

Putnam, Morgan C.

Turner-Evans, Daniel B.

English

Caltech Authors - Main

10 m minority-carrier diffusion lengths in Si wires synthesized
by Cu-catalyzed vapor-liquid-solid growth
Morgan C. Putnam,a Daniel B. Turner-Evans, Michael D. Kelzenberg,
Shannon W. Boettcher, Nathan S. Lewis, and Harry A. Atwater
California Institute of Technology, 1200 E. California Blvd., Pasadena, California 91125, USA
Received 5 August 2009; accepted 18 September 2009; published online 23 October 2009
The effective electron minority-carrier diffusion length, Ln,eff, for 2.0 m diameter Si wires that
were synthesized by Cu-catalyzed vapor-liquid-solid growth was measured by scanning
photocurrent microscopy. In dark, ambient conditions, Ln,eff was limited by surface recombination to
a value of 0.7 m. However, a value of Ln,eff=10.51 m was measured under broad-area
illumination in low-level injection. The relatively long minority-carrier diffusion length observed
under illumination is consistent with an increased surface passivation resulting from filling of the
surface states of the Si wires by photogenerated carriers. These relatively large Ln,eff values have
important implications for the design of high-efficiency, radial-junction photovoltaic cells from
arrays of Si wires synthesized by metal-catalyzed growth processes. © 2009 American Institute of
Physics. doi:10.1063/1.3247969
Photovoltaic cells based on Si wire arrays offer a number
of features that are compatible with obtaining high device
performance through the use of low-cost fabrication steps.
Some of these features include the opportunity to optimize
carrier collection through formation of either radial or axial
p-n junctions, the ability to directly synthesize large-area ar-
rays of Si wires through the use of chlorosilane precursors,
and the ability to embed the resulting Si wire arrays in flex-
ible polymeric sheets; thus enabling the facile removal of the
Si wire arrays and concomitant reuse of the templating
growth substrate.1–4
The vapor-liquid-solid VLS growth mechanism pro-
vides a demonstrated pathway to the fabrication of arrays of
one-dimensional Si wires.1 However, VLS growth processes
that employ metallic precursors unavoidably result in metal
incorporation into the Si wires. The presence of metallic im-
purities has the potential to degrade the minority-carrier life-
time and diffusion length in the Si wires. Hence, measure-
ments of the minority-carrier diffusion length are critical for
assessing the potential of VLS-grown Si wire arrays as com-
ponents in the fabrication of efficient photovoltaics.
Minority-carrier diffusion lengths up to 4 m have been
reported for Au-catalyzed, VLS-grown Si wires.3,5 These val-
ues are considerably smaller than the diffusion length in
high-purity bulk Si, but are in good agreement with the value
expected based on the measured concentration of the known
deep trap, Au, in the Si wires.6 Cu-catalyzed, VLS-grown Si
wires are expected to have longer diffusion lengths than
wires grown from Au catalysts, because Cu is not as detri-
mental as Au to the minority-carrier lifetime of Si.7,8 We
report herein, through use of scanning photocurrent micros-
copy SPCM, an effective electron minority-carrier diffu-
sion length, Ln,eff, of 10.51 m for Cu-catalyzed VLS-
grown Si wires.
Cu-catalyzed Si wires were grown at 1000 °C and 1 atm
from a 500:10 standard cubic centimeters per minute
SCCM gaseous H2:SiCl4 stream, in a process similar to
that reported previously1 see supporting information9. The
as-grown undoped wires were 2.0 m in diameter and
100 m in length. These wires were highly resistive, so in
subsequent growths 0.06 SCCM BCl3 5% in H2 was added
to the reactant stream. Four-point probe measurements of
wires that were removed from the Si growth substrate indi-
cated that the wires were p-type with a resistivity of
0.190.02  cm. This value corresponds to an acceptor
concentration NA of 1.050.151017 cm−3, assuming a
bulk hole mobility 3.1102 cm2 V−1 s−1 for Si. Prior to
SPCM characterization, a 5:1:1 by volume mixture of
de-ionized H2O 18 M cm resistivity:NH4OH29% :
H2O230% RCA1 was used to remove any organic con-
tamination, and a 6:1:1 by volume mixture of de-ionized
H2O:HCl37% :H2O2 RCA2 was used to remove residual
Cu. The chemical/native oxide was then removed by etching
the wires for 15 s in buffer HF Improved Transene, Inc..
The wires were then exposed to air for 3.5 days to grow a
native oxide on the wires.
To obtain diffusion lengths from the SPCM method, both
an Ohmic and a rectifying contact were required on the Si
wires.3,5 The Si wires were first removed from the growth
substrate and dispersed onto a Si3N4-coated Si100 wafer.
Photolithography was then used to pattern the contacts onto
individual Si wires. Following a 15 s etch in Buffer HF Im-
proved to remove the native oxide, Ohmic contacts were
formed by sputtering Al with 1% Si onto the p-Si wires.
Rectifying contacts were then formed by sputtering Al onto
the native oxide of the p-Si wires. This metal-insulator-
semiconductor MIS contact between Al and p-Si produces
a rectifying contact.10 After deposition of metal, a contact
anneal was performed at 300 °C for 10 min in forming gas
5% H2 in N2.
Figure 1 illustrates the observed rectifying behavior for a
device with an ideality factor of 1.8 and an effective barrier
height of 0.6 eV. Ideality factors ranged from 1.4 to 3.1, and
the effective barrier height ranged from 0.4 to 0.7 eV. The
inset of Fig. 1 displays an optical microscope image of the
device. The bright spot near the center of the wire corre-aElectronic mail: putnam@caltech.edu.
APPLIED PHYSICS LETTERS 95, 163116 2009
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sponds to the area illuminated by the laser, while the other
two spots are artifacts that arose from reflections in the mi-
croscope optics.
SPCM measurements were made using a WiTec scan-
ning near-field optical microscope in confocal mode. Local
illumination was provided by a 650 nm laser, chopped at
30 Hz, with a diffraction limited spot size of 0.4 m.
Broad-area illumination was provided by the microscope
light color temperature=3200 K. Using a photodiode to
measure the illumination power density, the optical carrier
generation density for both sources was calculated to be a
factor of 40 less than the equilibrium hole concentration of
11017 cm−3, thus satisfying the requirement for low-level-
injection illumination conditions see supporting informa-
tion. The photocurrent from the MIS rectifying contact was
detected by a preamplifier connected to a lock-in amplifier.3
For measurements at an applied bias, the Ohmic contact was
connected to a dc voltage source.
In SCPM, a two-dimensional map of the photocurrent is
generated by recording the photocurrent while scanning the
sample underneath a local illumination source the 650 nm
laser. Figure 2 depicts the zero-bias SPCM images of a Cu-
catalyzed Si wire measured in the dark Fig. 2b and in the
presence of broad-area illumination Fig. 2c. In the dark, a
small photocurrent note respective scale bars was observed
along the wire, as well as on the MIS rectifying contact in
regions immediately above and below the wire. The photo-
current above and below the wire on the MIS contact is
believed to arise from an optically thin coating of Al that
formed, as a result of the directional nature of sputtering and
the large ratio of the wire diameter to the thickness of the
contact, along the sidewall of the wire. Under broad-area
illumination, a much larger signal, which extended greater
than half the length of the device, was observed. Figure 2a
depicts a scanning electron microscope SEM image to fa-
cilitate correlation of the observed photocurrent response
with the geometry of the device.
The larger amplitude of the photocurrent and the in-
creased distance over which photocurrent was observed are
indicative of a larger effective electron minority-carrier dif-
fusion length, Ln,eff, under broad-area illumination than in the
dark. Because of the observed long time decay in Ln,eff time
scale of minutes after removal of the broad-area illumina-
tion, as shown by the dark photocurrent response after expo-
sure to broad-area illumination Fig. 2d, Fig. S19, we as-
sume that an improvement in surface passivation produces
the increase in Ln,eff with broad-area illumination. An in-
crease in Ln,eff, as a result of an increase in the bulk lifetime,
would be expected to decay on the time scale of the
minority-carrier lifetime 10–100 ns.
The photoinjection of electrons into the oxide oxide
trapped charge11 could produce a decrease in the surface
recombination velocity S and is consistent with the long time
decay in Ln,eff. Figure 2e depicts the expected band struc-
ture for p-Si coated with a native oxide that contains positive
fixed oxide charge. The presence of positive fixed oxide
charge, which is well known to exist in SiO2,12,13 will intro-
duce negative surface band bending. This band bending will
result in a large minority-carrier surface concentration and
hence produce a high surface-recombination velocity. How-
ever, the photoinjection of electrons into the oxide could bal-
ance the positive fixed oxide charge, thereby reducing the
negative surface band bending and decreasing S Fig. 2f.
A long time scale time scale of minutes for electrons to
tunnel out of an oxide14 is consistent with the observed long
time decay in Ln,eff under the proposed surface passivation
mechanism.
Figure 3 displays photocurrent cross sections along the
length of the wire, taken from SPCM images, as a function
of the bias voltage applied to the Ohmic contact. These data
allowed calculation of values of Ln,eff based upon the ex-
FIG. 1. Color online Current-voltage sweep of a 2.0 m diameter Si wire
device. Inset: optical microscope image of the measured device; the Ohmic
contact is on the left and the MIS contact is on the right. The laser illumi-
nation spot can be seen in the center of the wire.
FIG. 2. Color online a SEM image of the device of Fig. 1. b SPCM
image of the device measured in the dark. c SPCM image of the device
measured under low-level-injection, broad-area illumination. d SPCM im-
age of the device measured in the dark for a measurement started within 5 s
after exposure to broad-area illumination. All SPCM images are at zero bias
and took 4 min and 14 s to acquire. e Schematic illustration of the pro-
posed band diagram in the dark. f Schematic diagram of the proposed band
diagram under broad-area illumination.
163116-2 Putnam et al. Appl. Phys. Lett. 95, 163116 2009
Downloaded 13 Nov 2009 to 131.215.193.211. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
pected relationship for diffusion-limited minority-carrier
transport in the quasineutral region: Jphexp−xMIS
−x /Ln,eff, where xMIS−x is the distance between the MIS
rectifying contact and the laser illumination. Under broad-
area illumination, the best-performing individual device ex-
hibited a value of Ln,eff=9.50.2 m for photocurrent cross
sections along the center of the wire and a value of Ln,eff
=11.50.2 m for photocurrent cross sections along the
sidewall of the wire Fig. 2c: see supporting information,
producing an average Ln,eff of 10.51 m for this sample.
Measurements in the dark yielded Ln,eff0.7 m. The value
of Ln,eff measured in the dark is an upper bound for the actual
value of Ln,eff under these conditions, because the photocur-
rent variation produced by the Gaussian profile of the laser
beam is significant for such small diffusion lengths. Four
other devices yielded Ln,eff values under illumination of 4, 5,
6, and 7 m, with the latter value limited by the contact-
to-contact spacing in the device under study. The observed
variation of Ln,eff is not surprising based on the proposed
surface passivation mechanism. The highest measured value
of 10.5 m can thus be taken as a lower bound on the true
bulk minority-carrier diffusion length of such wires. Mea-
surements made at both forward and reverse bias, as well as
zero-bias photovoltaic collection conditions, confirmed that
the measured effective minority-carrier diffusion lengths
were independent of bias, and thus rule out significant con-
tributions from drift current to the Ln,eff measurements.
The bulk minority-carrier electron diffusion length is
2101 m for p-Si that contains the thermodynamic equi-
librium concentration 1017 atoms /cm3 of Cu in Si at
1000 °C.8 Hence, the Ln,eff measured for these Cu-catalyzed
Si wires may still be limited by surface recombination and/or
by the presence of other impurities in the bulk of the Si
wires.
Bounds of S9102 cm s−1 under broad-area illumi-
nation and S3105 cm s−1 in the dark can be estimated
from the measured values of Ln,eff for the best-performing
device, assuming a bulk diffusion length of 2101 m see
supporting information.5,15 From the value of S measured in
the dark, the Si–SiO2 interface trap density can be estimated
to exceed 31013 cm−2, assuming reasonable values of the
thermal velocity 	th=107 cm s−1 and of the trap capture
cross section 
=10−15 cm2 Dit=1 / S •
 •	th. This is a
large interface trap density, but is comparable with previous
results5 and is much smaller than the surface density of Si
atoms. Achievement of Ln,eff=10.5 m in Si microwires
grown from a low-cost Si precursor is a significant result for
Si wire array photovoltaics. Semiconductor device transport
models suggest that wire array cells with a Si wire diameter
of 3.0 m, a wire length of 25 m, and a 1018 cm−3 base
doping concentration should be able to produce air mass 1.5,
1 sun, efficiencies of =14.5%.16
The authors thank BP, the Department of Energy Office
of Basic Energy Sciences, and the Caltech Center for Sus-
tainable Energy Research for support, as well as the Kavli
Nanoscience Institute at Caltech, the Molecular Materials
Research Center at Caltech, and the Center for the Science of
Materials and Engineering NSF MRSEC: DMR 0520565
for use of facilities, and thank Emily Warren, Josh Spurgeon,
Brendan Kayes, and Michael Filler for their contributions.
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H. A. Atwater, Proceedings of the 34th IEEE PVSC, 2009 unpublished.
FIG. 3. Color online Photocurrent cross sections along the length of the
wire, as a function of the bias voltage applied to the Ohmic contact. a
Measured in the dark b and measured under low-level-injection, broad-
area illumination. In both a and b the zero-bias fit is shown as a dashed
black line and is used to calculate effective electron minority-carrier diffu-
sion lengths Ln,eff of 0.7 and 9.7 m, respectively.
163116-3 Putnam et al. Appl. Phys. Lett. 95, 163116 2009
Downloaded 13 Nov 2009 to 131.215.193.211. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp


10 µm minority-carrier diffusion lengths in Si wires synthesized by Cu-catalyzed vapor-liquid-solid growth

http://authors.library.caltech.edu/16767/1/Putnam2009p6371Appl_Phys_Lett.pdf

10 µm minority-carrier diffusion lengths in Si wires synthesized by Cu-catalyzed vapor-liquid-solid growth

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