supplementary information for: surface passivated gaasp ... · by adjusting the growth parameters...
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Supplementary information for:
Surface passivated GaAsP single-nanowire solar cells exceeding 10% efficiency grown on silicon
Jeppe V. Holm1†, Henrik I. Jørgensen1†, Peter Krogstrup2, Jesper Nygård2,4, Huiyun Liu3 and Martin
Aagesen1†∗
1SunFlake A/S, Universitetsparken 5, DK-2100 Copenhagen, Denmark, 2Nano-Science Center, Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark, 3Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, UK, 4Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.
†These authors contributed equally to this work
∗e-mail: [email protected]
The supplement includes more details on:
1. Nanowire core growth
2. Ideality factor, series and shunt resistance
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Supplementary Figures
Supplementary Figure S1. TEM image showing the full nanowire from Figure 1b and higher resolution
images of sections of the same wire. The crystal structure is almost phase perfect except from a few single
twins along the nanowire which are indicated by black arrows. At the bottom and top part of the nanowire
larger sections with multiple twins are present, as indicated by white arrows. The almost perfect middle
section shows that it is possible, by adjusting the growth parameters, to essentially eliminate
twinning in the GaAsP nanowires.
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Supplementary Figure S2. A TEM image of a bottom nanowire section. During the first part of the
nanowire growth, large sections of single twins or würtzite crystal segments form. Since the nanowire is
tilted slightly the hexagonal shape of the nanowire can be seen in the twin segments. The twinnings are not
caused by strain from the difference in lattice constants between the GaAsP and the Si, but are due to the
different growth conditions during this part of the nanowire growth. By adjusting the growth parameters
correctly it should hence be possible to eliminate the twinning entirely.
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Supplementary Figure S3. Diffraction image from core only GaAsP. The diffraction spots are a signature of
the face centred cubic crystal structure when viewed in the [011] zone axis. Since there are two atoms in
the brillouin zone (Ga and As/P) it is a zinc-blende signature. The image is obtained when looking at the
middle section of a nanowire. At the top and bottom of the nanowires, some of the spots may have
additional mirror spots as a signature of one or more twins. At areas with multiple twins close to each other
streaks between the main spots begin to develop. No other type of diffraction pattern has been found,
indicating that the wires have 6 equivalent [011] side facets.
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Supplementary Figure S4. EDX of core only GaAs1-xPx nanowires. a, b and c, Phosphorus content [x] of two
nanowires from three different nanowire growths. The sudden change in P content at the highest point is
measured at the very top of the nanowire. It should be noted that the content of group V in the liquid is
generally very low (see Supplementary Figure S4d,e,f), which implies a high uncertainty in the Group
V mole fractions. a, A growth where the P content was within the intended range x ~0.25. b, A growth
where the P content was lower than required x ~0.5-0.10. c, A growth where the P content was deliberately
changed from low (0.15) to high (0.70) during the growth. d, e and f, The atomic content of one of the
wires from respectively the a, b and c panels. The top data point shows that the catalyst particle is mostly
gallium. Since these are raw data the gallium content is not displayed as exactly 50%.
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Supplementary Figure S5. The emitted wavelength from room temperature photo luminescence
measurements of core only GaAs1-xPx nanowires standing on half 3” growth substrates. a, same growth as
Supplementary Figure S4a. b, same growth as Figure 1c in the main paper c, same growth as
Supplementary Figure S4b. Every pixel represents a 0.5 x 0.5 mm square of substrate and thousands of
nanowires. The axes indicate scan distance [mm]. Note that the wavelength scale is different in each panel.
Below each plot is written the approximate phosphor content which was extracted from the EDX
measurements, and the calculated bandgap and equivalent wavelength25. Comparing the measured
wavelengths to the EDX derived ones, we observe that the EDX and PL qualitatively agree, but that
wavelengths calculated from the EDX are slightly below the wavelength of the emitted light. A small
variation in bandgap is observed across the substrate in Supplementary Figure S5c. This is the result of a
small temperature gradient across the substrate during nanowire growth and growth initiation. The
temperature gradient results in a variation in nanowire density and because the P and As diffusion lengths
are different this leads to the observed variation in bandgap. Using positioned growth and improved
substrate temperature calibration this bandgap variation should be removed.
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Supplementary Discussion
Nanowire core growth
For vapor-liquid-solid (VLS) growth of Ga-assisted nanowires the morphology of the liquid-solid growth
region plays an important role on the relative crystal structure formation probabilities26. Thus, having a
correct V/III ratio is crucial for obtaining perfect zinc-blende nanowire crystals. Since the effective V/III ratio
at the growth region changes throughout the nanowire growth7 it is important to counteract this by being
able to adjust the external group V and III fluxes accordingly. The Ga-assisted axial nanowire core growth
can be divided into 3 growth stages, 1) The initial growth stage: formation of a stable liquid-solid growth
mode takes typically a few hundred nm of growth; 2) quasi steady-state growth of the main part of the
nanowire; and 3) the final growth stage where the droplet is consumed by nucleation from the
supersaturated Ga droplet either a) during cooling after nanowire growth or b) when growing without a Ga
flux. By adjusting the As and P fluxes during part 2 we have been able to obtain essentially a perfect single
crystal zinc-blende structure. Twinnings at the lower section of the nanowire, away from the main photon
absorption areas, and at the top of the nanowire have not yet been attempted removed. The crystal
quality, material composition and bandgap of the nanowires grown, were probed using transmission
electron microscopy (TEM), room temperature photo-luminescence (PL) and energy dispersive x-ray
spectroscopy (EDX). Selected data from different core nanowire growths are shown in Supplementary
Figures S1, S2, S3, S4 and S5.
Ideality factor, series and shunt resistance
We extract the ideality factor, series resistance and shunt resistance by fitting the current-voltage curves to
a simple model schematically shown below. It consists of a diode with ideality factor η, photo current Iph
and a series and shunt resistor (RSe, and RSh). The source-drain current (ISd) versus source-drain voltage (VSd)
can be found via a parameterization of the diode voltage (Vd) using the following three equations:
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0( ) exp 1
( ) ( )
( ) ( )
dd d ph
B
dsd d d d
Sh
dsd d Se d d d
Sh
eVI V I Ik T
VI V I VR
VV V R I V VR
η
= − −
= +
= + +
Figure 3a, b shows fits (solid lines) to the measured IV curves in the dark (blue circles), and illuminated by
global AM1.5 light (red circles). We hereby extract for the passivated (unpassivated) device an ideality
factor of about 2.0 (2.2), and series resistances of 70MΩ (10MΩ). The shunt resistance depends on the light
intensity; under dark conditions it is >100GΩ for both devices.
Supplementary References
25. Vurgaftman, I., Meyer, J. R. & Ram-Mohan, L. R. Band parameters for III–V compound
semiconductors and their alloys. Journal of Applied Physics 89, 5815–5875 (2001).
26. Krogstrup, P. et al. Impact of the Liquid Phase Shape on the Structure of III-V Nanowires.
Phys. Rev. Lett. 106, 125505 (2011).