Stress and temperature coupling effects on dislocation
density reduction in multicrystalline silicon
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Citation
Castellanos, Sergio et al. “Stress and Temperature Coupling
Effects on Dislocation Density Reduction in Multicrystalline
Silicon.” 2010 35th IEEE Photovoltaic Specialists Conference
(PVSC), 20-25 June 2010, Honolulu, HI, IEEE (2010).
000357–000358. CrossRef. Web. ©2010 IEEE.
As Published
http://dx.doi.org/10.1109/PVSC.2010.5616893
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Institute of Electrical and Electronics Engineers
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Final published version
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http://hdl.handle.net/1721.1/78276
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Detailed Terms
STRESS AND TEMPERATURE COUPLING EFFECTS ON DISLOCATION DENSITY
REDUCTION IN MULTICRYSTALLINE SILICON
Sergio Castellanos, Mariana I. Bertoni, Michelle Vogl, Alexandria Fecych, Tonio Buonassisi
Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
ABSTRACT
In multicrystalline silicon (mc-Si), the presence of
dislocation-rich areas limits solar cell conversion
efficiencies [1-2]. Previous studies have demonstrated that
6
-2
dislocation densities higher than 10 cm can dramatically
decrease the minority carrier lifetime [3]. High dislocation
densities, and their decoration with impurities, can limit
minority carrier lifetime even after phosphorous diffusion
or hydrogen passivation [4-5].
We previously proposed a method to remove dislocations
from mc-Si by high-temperature annealing, demonstrating
dislocation density reductions of 95% approximately [6].
We demonstrated that the dependence of dislocation
density reduction on annealing temperature is much more
pronounced that the dependence on annealing time [7]. In
this contribution, we propose stress as an additional
mechanism to enhance dislocation density reduction. We
discuss the relationship between temperature, stresses
and dislocation density in string ribbon.
NON-DESTRUCTIVE IMAGING OF DISLOCATION
BANDS VIA INFRARED BIREFRINGENCE
Based on previous work by Buonassisi et al. [8] and
Ganapati et al. [9] we probe local stresses surrounding
dislocations using infrared birefringence imaging (IBI) [10].
Stress, either externally applied or internal (residual
stress, defect-related stress), can induce birefringence in
photoelastic cubic solids such as silicon. By measuring the
birefringence of transmitted infrared light, Ganapati et al.
[9] demonstrated that dislocation bands can be detected in
mc-Si. IBI measurements in Figure 1 were taken on the
same sample before and after annealing at 1300 °C for 6
hours.
Before Annealing
bands visible
treatment.
before
annealing
disappear
after
The highlighted region demonstrates high stresses around
45 degree angle lines, which chemical etching revealed to
be dislocation bands. When comparing the two images, it
is evident that annealing results in a considerable
reduction of these diagonal dislocation bands.
DISLOCATION DENSITY REDUCTION VIA RELIEF OF
RESIDUAL STRESS
During mc-Si growth, the crystal is subject to internal
stresses due to thermal gradients and geometric
constraints. Upon cooling, these stresses are fixed in
place, resulting in residual stresses. When submitted to
high temperatures, the mc-Si crystal can experience a
considerable stress relief, resulting in dislocation density
reduction. We demonstrated that samples with higher
initial residual stresses present higher dislocation
reductions upon annealing; detailed results will be
published in an upcoming manuscript [11].
We further investigate the effect of externally applied loads
at high temperatures. An experimental setup consisting of
a three-point bending device was engineered. A String
Ribbon sample was loaded as shown in Figure 2, to apply
a tensile and compressive load simultaneously while
annealing at high temperatures.
After Annealing
Highly dislocated region
Figure 1 Non-destructive imaging of dislocation
2
bands: Infrared birefringence images of 2.5x8 cm
String Ribbon wafers before and after hightemperature annealing. Bright yellow/orange and light
blue regions indicate higher stresses; dark red and
blue regions indicate lower stresses. Dislocation
978-1-4244-5892-9/10/$26.00 ©2010 IEEE
Figure 2 Stress Distribution Representation: Threepoint-bending fixture showing highly localized
stresses at the contact points and a gradual stress
distribution: compression at the top part, and tension
at the bottom end.
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The sample under bending demonstrates a dislocation
density gradient throughout its face, with some areas
resulting in significant dislocation density reduction. While
detailed results will be published in a peer-reviewed
journal article [11], our preliminary results suggest that
tailoring stress distributions in ingots, bricks, or wafers
may present a promising path towards reducing
dislocation densities.
REFERENCES
[1] G. Stokkan et al., J. Appl. Phys. 101, 053515 (2007).
[2] S. Pizzini et al., J. Electrochem. Soc. 133, 2363 (1986).
[3] C. Donolato, J. Appl. Phys. 84, 2656 (1998).
[4] A. Bentzen et al., J. Appl. Phys. 99, 093509 (2006).
[5] O. Schultz et al., Prog. Photovolt. 14, 711 (2006).
[6] K. Hartman et al., Appl. Phys. Lett. 93, 122108 (2008).
[7] M.I. Bertoni, C. Colin, and T. Buonassisi, Physica
Status Solidi 156-158, 11-18 (2010).
[8] T. Buonassisi, S. Reitsma, R. Sweeney, M.D. Pickett,
W. Huang, J. Lesniak, and M.L. Spencer, Proc. 22nd
European Photovoltaic Solar Energy Conference, Milan,
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[9] V. Ganapati, S. Schoenfelder, S. Castellanos, S.
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Morhenn, G. Hahn, J. Bagdahn, and T. Buonassisi,, J.
Appl. Phys. (2010) (in press).
[10] J.W. Dally and W.F. Riley, Experimental Stress
Analysis, McGraw-Hill, New York, 1991
[11] M.I. Bertoni et al., Appl. Phys. Lett., submitted (2010)
978-1-4244-5892-9/10/$26.00 ©2010 IEEE
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