Furrey Undergraduate Research Symposium v3 Public

Al-Induced Crystallization:
Cheaper, More Efficient
Solar Cells
PATRICK FURREY, ELISABETH MCCLURE, SETH HUBBARD
NANOPOWER RESEARCH LABS, ROCHESTER NY

What is a single junction solar cell?
Undergraduate Research Symposium 2015
August 7, 2015 Rochester, NY
¹ http://pveducation.org/pvcdrom/solar-cell-operation/solar-cell-structure ² William Shockley and Hans J. Queisser, "Detailed Balance
Limit of Efficiency of p-n Junction Solar Cells", Journal of Applied Physics, Volume 32 (March 1961), pp. 510-519; doi:10.1063/1.1736034
2
• Generation of light-generated carriers
• Collection light-generated carriers to
generate current
• Generation of large voltage across
solar cell
• Dissipation of power in load and
parasitic resistances
• Shockley-Queisser limit maximizes
single p-n junction with 1.34eV
bandgap at AM1.5 to be 33.7% ²
¹

What is a multi junction solar cell?
• III-V solar cells are direct bandgap
semiconductors, as opposed to Si and is are
great absorbers
• Multijunction cells can account for otherwise
lost photons
• Grown by MOCVD, GaInP/GaInAs/Ge cells can
reach 31.3% efficiency at AM1.5 and 41.6%
under concentration
3

History of AIC and its relation to our Goal
• AIC uses inexpensive techniques, such as
sputtering or thermal evaporation, to work
towards these kinds of efficiencies; however,
much work is still needed to make it
commercially viable.
• Polycrystalline materials have grain
boundaries which act as defects, whereas
small grains can lead to increased
nonradiative recombination that impedes
current generation.
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Highly (111)-oriented Ge thin films on insulators formed by Al-induced crystallizationToko, K. and Kurosawa, M. and Saitoh, N. and Yoshizawa,
N. and Usami, N. and Miyao, M. and Suemasu, T., Applied Physics Letters, 101, 072106 (2012), DOI:http://dx.doi.org/10.1063/1.4744962
Our goal is to pick up whereToko, et al, left off
in exploring different variables for increasing
grains size and preferential orientation.

Our Process and Tools
• Started with 1 1 1 Si wafers
• RCA cleaned them to proceed to grow a 1-micron oxide layer
• Sputtered/thermally evaporated 50nm of Al
• Broke vacuum for 30 mins/remained under vacuum
• Sputtered/thermally evaporated 50nm of Ge
• Annealed for 10/20 hour
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CVC 601 Sputter System Kurt J. Lesker PVD 75c Lindberg BlueTube Furnace
Si
SiO2
Al
Ge
Al2O3
Al2O3
Ge

Variable Time
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Constants:
• 260 C anneal
• No Al2O3
• Sputtered
Pre anneal shows no Ge peak
meaning the Ge is amorphous
Both anneals showed Ge
peaks, meaning crystals
formed
20h Ge peak had a higher
intensity than the 10h,
showing a greater number of
crystals

Variable Time- SEM
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Constants:
• 260 C anneal
• No Al2O3
• Sputtered
Pre anneal shows no Ge peak
meaning the Ge is amorphous
Both anneals showed Ge
peaks, meaning crystals
formed
20h Ge peak had a higher
intensity than the 10h,
showing a greater number of
crystals
20 hour anneal
10 hour anneal

Scherrer Equation
A formula that relates X-ray diffraction readings to grain sizes.
𝜏 =
𝐾𝜆
𝛽𝑐𝑜𝑠𝜃
K is a dimensionless shape factor that has a typical value of 0.9, varying with crystalline structure.
𝜆 is the X-ray wavelength.
The Scherrer Equation is applicable up to 0.1 to 0.2um.
A K of 0.9 and the 𝜆 of Cu, 1.541838 Å, was used.
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Sample FWHM (𝛽) Bragg Angle (𝜃) Mean Size (or less)
10h, no Al2O3 0.230 27.2º 6 Å
20h, no Al2O3 0.226 27.1º 6 Å
Patterson, A. (1939). "The Scherrer Formula for X-Ray Particle Size Determination". Phys. Rev. 56 (10): 978–982. Bibcode:1939PhRv...56..978P.doi:10.1103/PhysRev.56.978.

With or Without Al2O3
9
Constants:
• 10h period
• 260 C anneal
• Sputtered
SinceAl2O3 acts as a
barrier, layer transfer
cannot occur at such a
low temperature and
the Ge remains
amorphous
As the Al tries to
diffuse, it is impeded
by the Al2O3 and
forms dendritic grains

With or Without Al2O3
– Nomarski Microscopy
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Constants:
• 10h period
• 260 C anneal
• Sputtered
SinceAl2O3 acts as a
barrier, layer transfer
cannot occur at such a
low temperature and
the Ge remains
amorphous
The Al peak shift is
either an Al alloy
created or Al2O3
Without Al2O3
With Al2O3

Different Deposition Methods
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Constants:
• 20h period
• 260 C anneal
• With Al2O3
Neither method
shows Ge crystals
formed
Sputtering shows
larger peaks
meaning bigger
crystals or more
uniform crystal
orientation

Different Deposition
Methods - SEM
12
Constants:
• 20h period
• 260 C anneal
Neither method
shows Ge crystals
formed
Sputtering shows
larger peaks
meaning bigger
crystals or more
uniform crystal
orientation
Thermal Evaporation with Al2O3
Sputter without Al2O3

Variable Temperature
13
Constants:
• 20h period
• With Al2O3
• Thermally Evaporated
The Al2O3 barrier is
overcome and layer
transfer can occur at 325
C.This results in 111 Ge
grains.

Variable Temperature –
Nomarski Microscopy
14
Constants:
• 20h period
• With Al2O3
• Thermally Evaporated
The Al2O3 barrier is
overcome and layer
transfer can occur at 325
C.This results in 111 Ge
grains.
325 C Anneal
260 C Anneal

Conclusions and Further Work
• THE THERMAL EVAPORATED SAMPLE AT 325 C SHOWED THE
BEST IMAGES THROUGH MICROSCOPY.
• THE 20H ANNEAL WITH NO VACUUM BREAK SHOWED THE
STRONGEST XRD PEAKS FOR 1 1 1 GERMANIUM.
• HYPOTHESIS IS THAT THE ALUMINUM GROWS DENDRITICALLY
AND THE GERMANIUM GROWS IN A MORE ROUNDED SHAPE.
• Changing the thicknesses ofAl and Ge
• More higher temperature anneals, >260 C, needed to find the “sweet
spot”
• Explore 20h anneal sputtered with Al2O3 layer.
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Acknowledgements
• GEORGE NELSON
• MICHAEL SLOCUM, NATHAN COX, JASON STAUB
• ZAC BITTNER
• THE OFFICE OF NAVAL RESEARCH
Questions?
N00173-14-1-G011
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