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InxGa1-xAs Nanowires on Silicon:

1D Heterogeneous Epitaxy, Bandgap Engineering, and Photovoltaics

Jae Cheol Shin1, Kyou Hyun Kim2, Ki Jun Yu1, Hefei Hu2, Leijun Yin3, Cun-Zheng Ning3, John

A. Rogers2,1

, Jian-Min Zuo2, and Xiuling Li

1,2*

*xiuling@illinois.edu

1Department of Electrical and Computer Engineering,

2Department of Materials Science and

Engineering, University of Illinois, Urbana, IL 61801,3Department of Electrical Engineering,

Arizona State University, Tempe, AZ 85287

The InxGa1-xAs NWs do not grow vertically in the 1D fashion indefinitely. The average

height of InxGa1-xAs NWs first scales linearly with growth time, then becomes nearly saturated

after a certain height, as shown in Fig. S1. The saturation point (height) increases as In%

increases.

Figure S1. SEM images of In0.3Ga0.7As NWs grown under the same growth condition but different

growth time (7, 14, and 20 mins, as labeled). The average height of NWs increases linearly with growth

time then becomes saturated at around 5 m. The amount of bending in the NW enhances with growth

time indicating increased composition inhomogeneity. After height saturation, the average diameter of the

NWs increases as a result of sidewall growth. The scale bars represent 1 m.

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The energy-dispersive X-ray analysis (EDX) shows no systematic composition variation

along the axial and radial direction of an In0.85Ga0.15As nanowire as seen in Fig. S2. No tapering

or enhanced growth at the base has been observed at all. This indicates that the growth mode is

different from the metal-catalyzed InGaAs NWs (ref. Nano Lett. 2006, 6, 599) where surface

diffusion dominates.

Figure S2. a, b Low-resolution TEM images of In0.85Ga0.15As NWs indicating the locations where an

energy-dispersive X-ray analysis (EDX) have been measured. c, d are the atomic fraction of the specific

point where is corresponding the marks in a and b, respectively.

The InGaAs NW array naturally increases a light trapping on Si substrate thus enhancing

the absorption rate of solar cells. The Fig. S3 shows the absorption spectra in the wavelength

range of 0.5-1 m measured from planar Si (300 m thick) without and with NW array. The

absorption rate of planar Si was increased due to the anti-reflection coating (ARC) on top and

further increased by having InxGa1-xAs NW array on the surface. 80 nm of SiNx is used for ARC.

Normal incidence light was shined, transmitted onto the samples, and then collected by detector

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positioned at ~ 0.5 cm. The absorption rate measured is probably slightly overestimated due to

the fact that the light cannot be entirely collected into the detector as there is a distance between

the sample and the detector.

Figure S3. Absorption rate as a function of wavelength for the planar Si (black and red color) and

InGaAs NW array grown on planar Si (blue color). The absorption rate of planar Si in the wavelength

range of 0.5-1 m increases with ARC on top and further increases with InGaAs NW array due to the

light trapping effect.

InxGa1-xAs NWs have been passivated to reduce surface recombination and phase

separation by a GaAs layer in situ or monolithically. The uniform GaAs sidewall passivation was

confirmed by dipping the NWs in a selective InGaAs etchant (citric acid:H2O2), which etched the

unpassivated InGaAs NWs but left the GaAs passivated InGaAs NWs intact, as shown in Fig. S4.

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Figure S4. SEM images show InGaAs NWs (a) without and (b) with GaAs passivation shell, after

dipping in InGaAs selective etchant (i.e., Citric: H2O2) for 25 sec. The NWs in a is attacked by the etchant

while those in b appear to be intact, confirming the existence and uniform passivation using a GaAs layer.

The scale bars represent 300 nm.

Note that although the GaAs passivation layer was uniformly grown on the surface of

InGaAs NWs as confirmed in Fig. S4, only small and sparse islands were formed instead of

continuous 2-D growth of GaAs on Si substrate, as can be seen in Fig. S5. SU-8 filling further

isolates these islands from contacting the nanowire solar cell in the solar cell devices.

Figure S5. SEM image for the In0.85Ga0.15As NWs with GaAs passivation layer grown in situ. The GaAs

passivation layer was grown at 620oC in situ for 2 min. The scale bar represents 1 m.

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The fabrication procedure of the photovoltaic device consisting of InxGa1-xAs NW array

is detailed in Fig. S6.

Figure S6. SEM images for the photovoltaic device processing. a, n-type In0.3Ga0.7As NW array grown

on p-type Si (111) substrate. b, SU8 filled and RIE etched to expose the tips of NWs. c, ITO is depositedon top of NW array. d, Au finger pattern deposited on top of ITO. e, fully fabricated photovoltaic device.

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The illuminated area (3 x 1 mm2) of the InGaAs NW photovoltaic device was defined

using black tapes as seen in the figure S7.

Figure S7. Top-viewSEM images for the photovoltaic device. The illuminated area was defined using

black tapes. The scale bar represents 1mm.

Table S1 shows detailed growth conditions for different composition InxGa1-xAs NWs on Si

(111) substrate, including growth temperature, group III and V precursor molar flows, and

calculated V/III ratio.

Table S1. Detailed growth condition for InxGa1-xAs NWs on Si (111).

Growth

Temp. (oC)

TMIn

(mol/min)

TMGa

(mol/min)In/(In+Ga)

AsH3(mol/min)

V/III

InAs 540 6.70E-06 1.00 4.91E-04 73

In0.85Ga0.15As 570 1.30E-05 7.60E-06 0.63 7.10E-04 34

In0.30Ga0.70As 590 4.50E-06 8.70E-06 0.34 5.80E-04 44

In0.20Ga0.80As 590 4.50E-06 1.80E-05 0.20 5.80E-04 26