To date, gallium-doped Czochralski (Cz) silicon has constituted a solar cell bulk material free of light-induced degradation. However, we measure light-induced degradation in gallium-doped Cz silicon in the presence of copper impurities. The measured degradation depends on the copper concentration and the material resistivity. Gallium-doped Cz silicon is found to be less sensitive to copper impurities than borondoped Cz silicon, emphasizing the role of boron in the formation of copper-related light-induced degradation.Peer reviewe

Haarahiltunen, Antti

Lindroos, Jeanette

Savin, Hele

Schubert, Martin C.

Yli-Koski, Marko

physica status solidi (RRL) - Rapid Research Letters

Jeanette Lindroos

Marko Yli-Koski

Antti Haarahiltunen

Martin C. Schubert

Hele Savin

Crossref

Light-induced degradation in copper-contaminated gallium-doped silicon

Aaltodoc

This is an electronic reprint of the original article.
This reprint may differ from the original in pagination and typographic detail.
Author(s): Lindroos, Jeanette & Yli-Koski, Marko & Haarahiltunen, Antti &
Schubert, Martin C. & Savin, Hele
Title: Light-induced degradation in copper-contaminated gallium-doped
silicon
Year: 2013
Version: Post print
Please cite the original version:
Lindroos, Jeanette & Yli-Koski, Marko & Haarahiltunen, Antti & Schubert, Martin C. &
Savin, Hele. 2013. Light-induced degradation in copper-contaminated gallium-doped
silicon. Physica Status Solidi RRL. Volume 7, Issue 4. 262-264. DOI:
10.1002/pssr.201307011.
Rights: © 2013 Wiley-Blackwell. This is the post print version of the following article: Lindroos, Jeanette & Yli-Koski,
Marko & Haarahiltunen, Antti & Schubert, Martin C. & Savin, Hele. 2013. Light-induced degradation in
copper-contaminated gallium-doped silicon. Physica Status Solidi RRL. Volume 7, Issue 4. 262-264. DOI:
10.1002/pssr.201307011, which has been published in final form at
http://onlinelibrary.wiley.com/doi/10.1002/pssr.201307011/abstract.
All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, and
duplication or sale of all or part of any of the repository collections is not permitted, except that material may
be duplicated by you for your research use or educational purposes in electronic or print form. You must
obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or
otherwise to anyone who is not an authorised user.
Powered by TCPDF (www.tcpdf.org)
Light-induced degradation in copper- 
contaminated gallium-doped silicon 
Jeanette Lindroos*, 1, Marko Yli-Koski1, Antti Haarahiltunen1, Martin C. Schubert2,  
and Hele Savin1 
1 Department of Micro and Nanosciences, Aalto University, Tietotie 3, 02150 Espoo, Finland  
2 Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstraße 2, 79110 Freiburg, Germany 
Received 10 January 2013, revised 20 February 2013, accepted 20 February 2013 
Published online 25 February 2013 
Keywords degradation, lifetime, silicon, gallium, copper 
* Corresponding author: e-mail jeanette.lindroos@aalto.fi, Phone: +358 9 470 22322, Fax: +358 9 470 25008
1 Introduction Light-induced degradation (LID) is a 
deleterious effect in crystalline silicon causing a significant 
solar cell efficiency loss during one day of illumination. 
LID has been found to increase in Czochralski (Cz) silicon 
with either increasing boron or oxygen concentration, at-
tributing LID to a boron–oxygen defect complex [1–4]. 
Therefore, traditional methods of minimizing LID consist 
of choosing either a high-resistivity boron-doped Cz bulk 
material or oxygen-free silicon, such as Magnetic Czo-
chralski (MCz) and Float Zone (FZ) silicon. Degradation-
free solar cells have also been manufactured by replacing 
boron doping with gallium or by switching to n-type sili-
con [2–4]. However, all these alternative materials come 
with tradeoffs either in cell efficiency or cost [5]. 
Recently, Savin et al. [6] have observed light-induced 
degradation similar to boron–oxygen LID in both boron- 
and phosphorous-doped FZ silicon by intentionally con-
taminating the material with low concentrations of copper. 
Copper concentrations of 1014 cm–3 did not to affect the 
initial FZ minority carrier lifetime, but degradation was 
measured during sample illumination. Consequently, light 
activation was proposed to enable the formation of highly 
recombination-active copper precipitates [7]. LID was 
more pronounced in copper-contaminated Cz silicon than 
in FZ silicon, suggesting that oxygen and its related defects 
provide nucleation sites for copper precipitation during  
illumination.    
Unintentional copper contamination can easily occur 
during solar cell processing, due to the high diffusivity and 
solubility of copper [8]. As expensive screen-printed silver 
front contacts are replaced with electroplated copper [9], 
copper contamination will increase in the production line, 
making copper-related LID a central issue in silicon solar 
cells. In order to understand the role of the dopant in the 
copper-related degradation process, we intentionally con-
taminate Ga-doped Cz silicon with copper and subject the 
material to illumination. We study the degradation behav-
ior as a function of copper and doping concentrations, 
comparing the results to boron-doped Cz silicon.  
2 Experimental details In this experiment, light-
induced degradation was studied in 6-inch wide, 300 μm 
thick gallium-doped Cz silicon and on 4-inch wide, 
525 μm thick boron-doped Cz silicon. Table 1 presents the 
wafer resistivities and oxygen concentrations. The results 
were also compared to previously published data on simi-
larly processed 17 Ω cm B-doped Cz silicon [6].  
All wafers were first subjected to RCA cleanings fol-
lowed by dry thermal oxide passivation. The boron-doped 
sample was oxidized for 40 min at 900 °C and annealed in 
a nitrogen atmosphere for 40 min at 950 °C, producing 
a 15 nm layer of oxide. A 26 nm passivating thermal oxide 
was grown on the Ga-doped wafers at 1000 °C. Then, 
the  wafers  were  intentionally  contaminated with copper  
To date, gallium-doped Czochralski (Cz) silicon has consti-
tuted a solar cell bulk material free of light-induced degrada-
tion.  However, we measure light-induced degradation in gal-
lium-doped Cz silicon in the presence of copper impurities.
The measured degradation depends on the copper concentra-
 tion and the material resistivity. Gallium-doped Cz silicon is 
found to be less sensitive to copper impurities than boron-
doped Cz silicon, emphasizing the role of boron in the forma-
tion of copper-related light-induced degradation. 
Table 1 Resistivity and oxygen concentration of the studied Cz 
silicon wafers. 
dopant resistivity (Ω cm) oxygen concentration (ppma) 
Ga  0.41 16.1 
Ga 
B 
B* 
10.1 
18–24 
17 
17.7 
11–13 
14.5 
* Ref. [6].
through the thermal oxide. The lower copper contamina-
tion levels in Figs. 1 and 2 were induced through droplets 
of 1, 5 and 20 ppb% (w/v) copper solution [10]. The higher 
surface contamination levels were achieved by spin coating 
the front surface with 8 and 100 ppm% (w/v) of copper so-
lution [11]. The wafers were subsequently annealed for 
20 min at 800 °C in a nitrogen atmosphere in order to dif-
fuse the copper contamination into the wafer bulk. Finally, 
a positive corona charge was deposited on the wafers, in 
order to keep copper in the wafer bulk [12]. 
The initial minority carrier lifetime of the wafers was 
first measured at medium injection level (~1014 cm–3) with 
Microwave Photoconductance Decay (μ-PCD), after which 
the wafers were subjected to illumination. On the spin-
contaminated wafers, small-area illumination (1 mm2) was 
performed by bias light (>25 W/cm2, 973.5 nm) at room 
temperature for up to 117 h [13]. During illumination, the 
bias light intensity was decreased momentarily to measure 
the lifetime as a function of illumination time at a high 
injection level (~1016 cm–3) with a pulsed 904 nm laser 
(200 ns). The wafers with droplets of lower copper solu-
tion concentration (ppb) were illuminated entirely with a 
halogen lamp (200 mW/cm2) for 20 min at a temperature 
below 50 °C. After illumination, the final lifetime was 
again measured at medium injection with μ-PCD.  
3 Results and discussion Figure 1 shows the life-
time map of half a high-resistivity Ga-doped Cz silicon 
wafer with three circular copper solution contamination ar-
eas (1, 5 and 20 ppb) measured after 20 min of halogen  
illumination. Figure 2 presents the average lifetime values 
measured before (τinitial) and after (τfinal) halogen illumina-
tion in the three contamination spots in the same wafer  
(orange bars). Figure 2 also depicts the average lifetime 
values in correspondingly copper-contaminated low-
resistivity Ga-doped Cz silicon (gray bars) and high-
resistivity B-doped Cz silicon (green bars, values from [6]). 
Assuming identical copper diffusion conditions in Ga-
doped Cz silicon as in B-doped Cz silicon at 800 °C  
[13, 14], the copper solution concentrations in Figs. 1 and 2 
are estimated as 1.6 × 1010 cm–2 in the 1 ppb spot, 
4.2 × 1010 cm–2 in the 5 ppb spot and 6.3 × 1010 cm–2 in the 
20 ppb contamination area [6]. 
In Fig. 2, the contamination level 0 ppb refers to the 
sample reference area located above the contamination 
spots in Fig. 1. Although this reference area has no inten- 
Figure 1 Lifetime map of half a 10.1 Ω cm Ga-doped Cz silicon 
wafer with three contamination spots (1, 5 and 20 ppb of copper 
solution) measured after 20 min of halogen illumination. 
tional contamination, it may still contain some copper as a 
result of unintentional contamination from the furnace dur-
ing the 800 °C anneal. Therefore, the small degradation 
measured in the B-doped Si reference area is most likely 
due to unintentional contamination.  
In the two Ga-doped samples, no lifetime change is ob-
served in the reference area or in the 1 and 5 ppb spots of 
copper solution. These results are in agreement with the 
previously reported LID-free behaviour of Ga-doped Cz 
silicon [2–4]. However, significant degradation is meas-
ured in the 20 ppb copper solution spot in the high-
resistivity Ga-doped sample in both Figs. 1 and 2. This is 
an interesting result, as copper contamination appears to 
cause light-induced degradation also in Ga-doped Cz sili-
con. 
In the high-resistivity B-doped sample, LID is detected 
in all three contamination spots as discussed in Ref. [6]. As 
LID is observed only in the 20 ppb spot in the high-
resistivity Ga-doped Cz wafer, the dopant has a clear effect 
on copper-related LID in Cz silicon. Gallium doping ap-
pears to make Cz silicon less sensitive to copper contami-
nation than boron doping.  
Figure 2 Average lifetime measured before (τintial) and after 
(τfinal) illumination in a reference area and in three contamination 
spots (1, 5 and 20 ppb of copper solution) in Ga-doped Cz and 
B-doped Cz silicon. 
Figure 3 High-injection lifetime as a function of illumination 
time in Ga-doped and B-doped Cz silicon with varying copper so-
lution concentrations. 
As the low-resistivity Ga-doped Cz silicon wafer does 
not show any degradation in Fig. 2, the copper concentra-
tions were increased in the second experiment. Figure 3 
presents the time dependence of the degradation process at 
these higher copper concentrations. As expected, no degra-
dation is observed in the clean Ga-doped wafers, but clear 
degradation is measured in all copper-contaminated wafers, 
including the low-resistivity Ga-doped sample.  
Even though the degradation is visible in all contami-
nated samples, the degradation time constants tCu vary 
notably [13]. At the lower contamination solution level 
(8 ppm), the lifetime decreases rapidly in the high-
resistivity Ga-doped wafer. In the low-resistivity Ga-doped 
wafer, the same copper solution concentration does 
not cause degradation until 100 min in illumination, result-
ing in a degradation time constant of 3710 min. The slower 
degradation rate in the low-resistivity material may be 
caused by the higher gallium concentration, due to the 
resistivity dependency of the minority carrier injection 
level at constant bias light intensity. The slightly lower 
oxygen concentration might also reduce the degradation 
rate. 
Although the degradation rate is much slower in the 
low-resistivity material, both the degradation rate and the 
relative degradation increased after increasing the copper 
solution concentration to 100 ppm. Thus, LID appears to 
increase in both low- and high-resistivity Ga-doped Cz sil-
icon with increasing copper concentration. However, com-
pared to boron-doped silicon, Ga-doped Cz silicon requires 
higher copper concentrations and longer illumination times 
for copper-related LID to occur.   
4 Conclusion Gallium-doped Cz silicon wafers were 
intentionally contaminated with copper and subjected to 
illumination. Light-induced degradation was clearly ob-
served in the contaminated material and found to increase 
with increasing copper concentration or with increasing re-
sistivity. Higher copper concentrations and longer illumi-
nation times were required to induce LID in Ga-doped Cz 
silicon than in B-doped Cz silicon. Therefore, boron dop-
ing renders Cz silicon more sensitive to copper contamina-
tion. Previously, oxygen has also been shown to increase 
copper-related LID [6]. Hence, silicon with both high bo-
ron and high oxygen concentration appears to have the 
lowest tolerance to copper contamination.  
Acknowledgements This research was undertaken at the 
Micronova Nanofabrication Centre of Aalto University. The au-
thors acknowledge the financial support of the Academy of Fin-
land, Okmetic Oyj and the Finnish Funding Agency for Technol-
ogy and Innovation. 
References 
  [1] J. Schmidt, A. G. Aberle, and R. Hezel, in: Proceedings 
26th IEEE PVSC, Anaheim, CA, USA (IEEE, New York, 
1997), p. 15.  
 [2] S. W. Glunz, S. Rein, W. Warta, J. Knobloch, and W. Wett-
ling, Sol. Energy Mater. Sol. Cells 65, 223 (2001).  
  [3] J. Schmidt and K. Bothe, Phys. Rev. B 69, 024107 (2004).  
  [4] S. W. Glunz, S. Rein, J. Knobloch, W. Wettling, and T. Abe, 
Prog. Photovolt. 7, 466 (1999).  
  [5] A. Goetzberger, C. Hebling, and H. W. Schock, Mater. Sci. 
Eng. R 40, 1 (2003). 
  [6] H. Savin, M. Yli-Koski, and A. Haarahiltunen, Appl. Phys. 
Lett. 95, 152111 (2009). 
  [7] R. Sachdeva, A. A. Istratov, and E. R. Weber, Appl. Phys. 
Lett. 79, 2937 (2001). 
  [8] A. A. Istratov, C. Flink, H. Heislmair, E. R. Weber, and 
T. Heiser, Mater. Sci. Eng. B 72, 99 (2000). 
  [9] J. Bartsch, A. Mondon, K. Bayer, C. Schetter, M. Hörteis, 
and S. W. Glunz, J. Electrochem. Soc. 157(10), H942 
(2010). 
[10] W. B. Henley, D. A. Ramappa, and L. Jastrezbski, Appl. 
Phys. Lett. 74, 278 (1999). 
[11] M. Hourai, T. Naridomi, Y. Oka, K. Murakami, S. Sumita, 
N. Fujino, and T. Shiraiwa, Jpn. J. Appl. Phys. 27, L2361 
(1988). 
[12] J. Lindroos, M. Yli-Koski, A. Haarahiltunen, and H. Savin, 
Appl. Phys. Lett. 101, 232108 (2012). 
[13] H. Väinölä, M. Yli-Koski, A. Haarahiltunen, and J. Sink-
konen, J. Electrochem. Soc. 150(12), G790 (2003). 
[14] A. A. Istratov, C. Flink, H. Heislmair, E. R. Weber, and 
T. Heiser, Phys. Rev. Lett. 81(6), 1244 (1998). 


English

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This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Lindroos, Jeanette & Yli-Koski, Marko & Haarahiltunen, Antti &Schubert, Martin C. & Savin, HeleTitle: Light-induced degradation in copper-contaminated gallium-dopedsiliconYear: 2013Version: Post printPlease cite the original version:Lindroos, Jeanette & Yli-Koski, Marko & Haarahiltunen, Antti & Schubert, Martin C. &Savin, Hele. 2013. Light-induced degradation in copper-contaminated gallium-dopedsilicon. Physica Status Solidi RRL. Volume 7, Issue 4. 262-264. DOI:10.1002/pssr.201307011.Rights: © 2013 Wiley-Blackwell. This is the post print version of the following article: Lindroos, Jeanette & Yli-Koski,Marko & Haarahiltunen, Antti & Schubert, Martin C. & Savin, Hele. 2013. Light-induced degradation incopper-contaminated gallium-doped silicon. Physica Status Solidi RRL. Volume 7, Issue 4. 262-264. DOI:10.1002/pssr.201307011, which has been published in final form athttp://onlinelibrary.wiley.com/doi/10.1002/pssr.201307011/abstract.All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Light-induced degradation in copper- contaminated gallium-doped silicon Jeanette Lindroos*, 1, Marko Yli-Koski1, Antti Haarahiltunen1, Martin C. Schubert2,  and Hele Savin1 1 Department of Micro and Nanosciences, Aalto University, Tietotie 3, 02150 Espoo, Finland  2 Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstraße 2, 79110 Freiburg, Germany Received 10 January 2013, revised 20 February 2013, accepted 20 February 2013 Published online 25 February 2013 Keywords degradation, lifetime, silicon, gallium, copper * Corresponding author: e-mail jeanette.lindroos@aalto.fi, Phone: +358 9 470 22322, Fax: +358 9 470 250081 Introduction Light-induced degradation (LID) is a deleterious effect in crystalline silicon causing a significant solar cell efficiency loss during one day of illumination. LID has been found to increase in Czochralski (Cz) silicon with either increasing boron or oxygen concentration, at-tributing LID to a boron–oxygen defect complex [1–4]. Therefore, traditional methods of minimizing LID consist of choosing either a high-resistivity boron-doped Cz bulk material or oxygen-free silicon, such as Magnetic Czo-chralski (MCz) and Float Zone (FZ) silicon. Degradation-free solar cells have also been manufactured by replacing boron doping with gallium or by switching to n-type sili-con [2–4]. However, all these alternative materials come with tradeoffs either in cell efficiency or cost [5]. Recently, Savin et al. [6] have observed light-induced degradation similar to boron–oxygen LID in both boron- and phosphorous-doped FZ silicon by intentionally con-taminating the material with low concentrations of copper. Copper concentrations of 1014 cm–3 did not to affect the initial FZ minority carrier lifetime, but degradation was measured during sample illumination. Consequently, light activation was proposed to enable the formation of highly recombination-active copper precipitates [7]. LID was more pronounced in copper-contaminated Cz silicon than in FZ silicon, suggesting that oxygen and its related defects provide nucleation sites for copper precipitation during  illumination.    Unintentional copper contamination can easily occur during solar cell processing, due to the high diffusivity and solubility of copper [8]. As expensive screen-printed silver front contacts are replaced with electroplated copper [9], copper contamination will increase in the production line, making copper-related LID a central issue in silicon solar cells. In order to understand the role of the dopant in the copper-related degradation process, we intentionally con-taminate Ga-doped Cz silicon with copper and subject the material to illumination. We study the degradation behav-ior as a function of copper and doping concentrations, comparing the results to boron-doped Cz silicon.  2 Experimental details In this experiment, light-induced degradation was studied in 6-inch wide, 300 μm thick gallium-doped Cz silicon and on 4-inch wide, 525 μm thick boron-doped Cz silicon. Table 1 presents the wafer resistivities and oxygen concentrations. The results were also compared to previously published data on simi-larly processed 17 Ω cm B-doped Cz silicon [6].  All wafers were first subjected to RCA cleanings fol-lowed by dry thermal oxide passivation. The boron-doped sample was oxidized for 40 min at 900 °C and annealed in a nitrogen atmosphere for 40 min at 950 °C, producing a 15 nm layer of oxide. A 26 nm passivating thermal oxide was grown on the Ga-doped wafers at 1000 °C. Then, the  wafers  were  intentionally  contaminated with copper  To date, gallium-doped Czochralski (Cz) silicon has consti-tuted a solar cell bulk material free of light-induced degrada-tion.  However, we measure light-induced degradation in gal-lium-doped Cz silicon in the presence of copper impurities.The measured degradation depends on the copper concentra- tion and the material resistivity. Gallium-doped Cz silicon is found to be less sensitive to copper impurities than boron-doped Cz silicon, emphasizing the role of boron in the forma-tion of copper-related light-induced degradation. Table 1 Resistivity and oxygen concentration of the studied Cz silicon wafers. dopant resistivity (Ω cm) oxygen concentration (ppma) Ga  0.41 16.1 Ga B B* 10.1 18–24 17 17.7 11–13 14.5 * Ref. [6].through the thermal oxide. The lower copper contamina-tion levels in Figs. 1 and 2 were induced through droplets of 1, 5 and 20 ppb% (w/v) copper solution [10]. The higher surface contamination levels were achieved by spin coating the front surface with 8 and 100 ppm% (w/v) of copper so-lution [11]. The wafers were subsequently annealed for 20 min at 800 °C in a nitrogen atmosphere in order to dif-fuse the copper contamination into the wafer bulk. Finally, a positive corona charge was deposited on the wafers, in order to keep copper in the wafer bulk [12]. The initial minority carrier lifetime of the wafers was first measured at medium injection level (~1014 cm–3) with Microwave Photoconductance Decay (μ-PCD), after which the wafers were subjected to illumination. On the spin-contaminated wafers, small-area illumination (1 mm2) was performed by bias light (>25 W/cm2, 973.5 nm) at room temperature for up to 117 h [13]. During illumination, the bias light intensity was decreased momentarily to measure the lifetime as a function of illumination time at a high injection level (~1016 cm–3) with a pulsed 904 nm laser (200 ns). The wafers with droplets of lower copper solu-tion concentration (ppb) were illuminated entirely with a halogen lamp (200 mW/cm2) for 20 min at a temperature below 50 °C. After illumination, the final lifetime was again measured at medium injection with μ-PCD.  3 Results and discussion Figure 1 shows the life-time map of half a high-resistivity Ga-doped Cz silicon wafer with three circular copper solution contamination ar-eas (1, 5 and 20 ppb) measured after 20 min of halogen  illumination. Figure 2 presents the average lifetime values measured before (τinitial) and after (τfinal) halogen illumina-tion in the three contamination spots in the same wafer  (orange bars). Figure 2 also depicts the average lifetime values in correspondingly copper-contaminated low-resistivity Ga-doped Cz silicon (gray bars) and high-resistivity B-doped Cz silicon (green bars, values from [6]). Assuming identical copper diffusion conditions in Ga-doped Cz silicon as in B-doped Cz silicon at 800 °C  [13, 14], the copper solution concentrations in Figs. 1 and 2 are estimated as 1.6 × 1010 cm–2 in the 1 ppb spot, 4.2 × 1010 cm–2 in the 5 ppb spot and 6.3 × 1010 cm–2 in the 20 ppb contamination area [6]. In Fig. 2, the contamination level 0 ppb refers to the sample reference area located above the contamination spots in Fig. 1. Although this reference area has no inten- Figure 1 Lifetime map of half a 10.1 Ω cm Ga-doped Cz silicon wafer with three contamination spots (1, 5 and 20 ppb of copper solution) measured after 20 min of halogen illumination. tional contamination, it may still contain some copper as a result of unintentional contamination from the furnace dur-ing the 800 °C anneal. Therefore, the small degradation measured in the B-doped Si reference area is most likely due to unintentional contamination.  In the two Ga-doped samples, no lifetime change is ob-served in the reference area or in the 1 and 5 ppb spots of copper solution. These results are in agreement with the previously reported LID-free behaviour of Ga-doped Cz silicon [2–4]. However, significant degradation is meas-ured in the 20 ppb copper solution spot in the high-resistivity Ga-doped sample in both Figs. 1 and 2. This is an interesting result, as copper contamination appears to cause light-induced degradation also in Ga-doped Cz sili-con. In the high-resistivity B-doped sample, LID is detected in all three contamination spots as discussed in Ref. [6]. As LID is observed only in the 20 ppb spot in the high-resistivity Ga-doped Cz wafer, the dopant has a clear effect on copper-related LID in Cz silicon. Gallium doping ap-pears to make Cz silicon less sensitive to copper contami-nation than boron doping.  Figure 2 Average lifetime measured before (τintial) and after (τfinal) illumination in a reference area and in three contamination spots (1, 5 and 20 ppb of copper solution) in Ga-doped Cz and B-doped Cz silicon. Figure 3 High-injection lifetime as a function of illumination time in Ga-doped and B-doped Cz silicon with varying copper so-lution concentrations. As the low-resistivity Ga-doped Cz silicon wafer does not show any degradation in Fig. 2, the copper concentra-tions were increased in the second experiment. Figure 3 presents the time dependence of the degradation process at these higher copper concentrations. As expected, no degra-dation is observed in the clean Ga-doped wafers, but clear degradation is measured in all copper-contaminated wafers, including the low-resistivity Ga-doped sample.  Even though the degradation is visible in all contami-nated samples, the degradation time constants tCu vary notably [13]. At the lower contamination solution level (8 ppm), the lifetime decreases rapidly in the high-resistivity Ga-doped wafer. In the low-resistivity Ga-doped wafer, the same copper solution concentration does not cause degradation until 100 min in illumination, result-ing in a degradation time constant of 3710 min. The slower degradation rate in the low-resistivity material may be caused by the higher gallium concentration, due to the resistivity dependency of the minority carrier injection level at constant bias light intensity. The slightly lower oxygen concentration might also reduce the degradation rate. Although the degradation rate is much slower in the low-resistivity material, both the degradation rate and the relative degradation increased after increasing the copper solution concentration to 100 ppm. Thus, LID appears to increase in both low- and high-resistivity Ga-doped Cz sil-icon with increasing copper concentration. However, com-pared to boron-doped silicon, Ga-doped Cz silicon requires higher copper concentrations and longer illumination times for copper-related LID to occur.   4 Conclusion Gallium-doped Cz silicon wafers were intentionally contaminated with copper and subjected to illumination. Light-induced degradation was clearly ob-served in the contaminated material and found to increase with increasing copper concentration or with increasing re-sistivity. Higher copper concentrations and longer illumi-nation times were required to induce LID in Ga-doped Cz silicon than in B-doped Cz silicon. Therefore, boron dop-ing renders Cz silicon more sensitive to copper contamina-tion. Previously, oxygen has also been shown to increase copper-related LID [6]. Hence, silicon with both high bo-ron and high oxygen concentration appears to have the lowest tolerance to copper contamination.  Acknowledgements This research was undertaken at the Micronova Nanofabrication Centre of Aalto University. The au-thors acknowledge the financial support of the Academy of Fin-land, Okmetic Oyj and the Finnish Funding Agency for Technol-ogy and Innovation. References   [1] J. Schmidt, A. G. Aberle, and R. Hezel, in: Proceedings 26th IEEE PVSC, Anaheim, CA, USA (IEEE, New York, 1997), p. 15.   [2] S. W. Glunz, S. Rein, W. Warta, J. Knobloch, and W. Wett-ling, Sol. Energy Mater. Sol. Cells 65, 223 (2001).    [3] J. Schmidt and K. Bothe, Phys. Rev. B 69, 024107 (2004).    [4] S. W. Glunz, S. Rein, J. Knobloch, W. Wettling, and T. Abe, Prog. Photovolt. 7, 466 (1999).    [5] A. Goetzberger, C. Hebling, and H. W. Schock, Mater. Sci. Eng. R 40, 1 (2003).   [6] H. Savin, M. Yli-Koski, and A. Haarahiltunen, Appl. Phys. Lett. 95, 152111 (2009).   [7] R. Sachdeva, A. A. Istratov, and E. R. Weber, Appl. Phys. Lett. 79, 2937 (2001).   [8] A. A. Istratov, C. Flink, H. Heislmair, E. R. Weber, and T. Heiser, Mater. Sci. Eng. B 72, 99 (2000).   [9] J. Bartsch, A. Mondon, K. Bayer, C. Schetter, M. 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Light-induced degradation in copper-contaminated gallium-doped silicon

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