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© IMEC 2011
PASSIVATION OF A METAL CONTACT WITH A TUNNEL LAYER
X. LOOZEN, J. B. LARSEN, F. DROSS, M.
ALEMAN, T. BEARDA, B. J. O’SULLIVAN,
I. GORDON AND J. POORTMANS
© IMEC 2011
Literature overview
Highest efficiency cell on Si in 1984 with front contacts
passivated by thin SiO2 (full Al-BSF cell, 18.4%)
Tunnel SiO2 under front Al contacts on a n-type emitter
(PERC cell) ISFH
Voc gain of ~35mV by use of passivated contacts on IBC
cells (best cell 721mV, 24.2%)
Voc gain of 12mV by using an ALD Al2O3 tunnel layer
(best cell 673mV, 21.7%)
2
M.A. Green et al, IEEE TED (1984)
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN
J. Schmidt et al, Prog. Photovolt.: Res. Appl. (2008)
K. Jaeger-Hezel et al, EU PVSEC (1995)
P.J. Cousins et al, IEEE PVSC (2010)
D. Zielke et al, Phys. Status Solidi RRL (2011)
- High potential for contact passivation
- Need for characterization
© IMEC 2011
Outline
Literature overview
AlOx candidate for contact passivation?
AlOx contact passivation seen by
photoluminescence
Contact resistance
Conclusion
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 3
© IMEC 2011
Outline
Literature overview
AlOx candidate for contact passivation?
AlOx contact passivation seen by
photoluminescence
Contact resistance
Conclusion
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 4
© IMEC 2011
Use of AlOx for surface passivation
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 5
Effective passivation of n- & p-Si by
thick AlOx
Lifetime and Seff on bulk p- and n-Si passivated by
10nm AlOx + FGA 350ºC
2 Ωcm p-Si 2.4 Ωcm n-Si 2 Ωcm p-Si 2.4 Ωcm n-Si
© IMEC 2011
Can we use AlOx for contact passivation?
Target: Reduce recombination underneath
metal contacts via a tunneling layer
Question: Does an ultra-thin (~nm) AlOx layer
provide good surface passivation?
1) On base material?
2) On highly doped n- and p-type regions?
- Emitter
- FSF/BSF
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 6
© IMEC 2011
Can we use AlOx for contact passivation?
Target: Reduce recombination underneath
metal contacts via a tunneling layer
Question: Does an ultra-thin (~nm) AlOx layer
provide good surface passivation?
1) On base material?
2) On highly doped n- and p-type regions?
- Emitter
- FSF/BSF
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 7
© IMEC 2011
Passivation of base material by thin AlOx
Very low Seff already with 2nm thermal ALD AlOx
- 8 cm/s on n-Si (polished FZ)
- 20 cm/s on p-Si (polished FZ)
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 8
# ALD cycles ~ # Å of AlOx
Effective passivation of n- & p-Si with
very thin AlOx
Lifetime versus AlOx thickness after FGA 350ºC
2nm
© IMEC 2011
Can we use AlOx for contact passivation?
Target: Reduce recombination underneath
metal contacts via a tunneling layer
Question: Does an ultra-thin (~nm) AlOx layer
provide good surface passivation?
1) On base material?
2) On highly doped n- and p-type regions?
- Emitter
- FSF/BSF
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 9
© IMEC 2011
AlOx passivation on n+ diffused surfaces
Symmetrical deposition of thick 30nm AlOx on POCl3
diffused surfaces
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 10
Lifetime of AlOx passivated n+ surfaces versus Rsheet after FGA 350ºC
As diffused
After oxidation at 1000ºC
eff limited to ~100µs for 100 Ω/ doped n+ surfaces
with AlOx passivation. Why?
© IMEC 2011
Why poorer passivation with higher doping?
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 11
Base n-Si:
strong induced inversion
Medium n+ doping:
weak induced inversion
High n++ doping:
very weak or no induced
inversion
Base Si
- - - - + + +
- - - - - - - - - - - - + + + + + + + + +
+ + + + + + + + + n+
Base Si
- - - - - - - - - - - - - - - -
+ + + + + + + + +
n++
Base Si
- - - - - - - - - - - - - - - -
+ + + +
Floating junction passivation works well for base n-Si, but
not for highly doped n+ surfaces
AlOx with negative charges
© IMEC 2011
Outline
Literature overview
AlOx candidate for contact passivation?
AlOx contact passivation seen by
photoluminescence
Contact resistance
Conclusion
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 12
© IMEC 2011
AlOx passivation seen by photoluminescence
Target: passivation of front contact in i-PERC cells
For PL measurement
- Suppress rear metallization
- Use tunnel candidate on front
- Compare passivated/non-passivated areas
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 13
PL illumination
CZ p-Si
SiOx/SiNx passivation stack
100 Ω/ n+ emitter
SiNx ARC
Al BSF
CZ p-Si
SiOx/SiNx passivation stack
n+
SiNx ARC
© IMEC 2011
AlOx passivation seen by photoluminescence
Target: passivation of front contact in i-PERC cells
For PL measurement
- Suppress rear metallization
- Use tunnel candidate on front
- Compare passivated/non-passivated areas
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 14
PL illumination
CZ p-Si
SiOx/SiNx passivation stack
100 Ω/ n+ emitter
SiNx ARC
Al BSF
CZ p-Si
SiOx/SiNx passivation stack
n+
Tunnel AlOx
© IMEC 2011
AlOx passivation seen by photoluminescence
Target: passivation of front contact in i-PERC cells
For PL measurement
- Suppress rear metallization
- Use tunnel candidate on front
- Compare passivated/non-passivated areas
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 15
PL illumination
CZ p-Si
SiOx/SiNx passivation stack
100 Ω/ n+ emitter
SiNx ARC
Al BSF
CZ p-Si
SiOx/SiNx passivation stack
n+
Tunnel AlOx
© IMEC 2011
No coating
(native oxide) Ti/Pd/Ag
Tunnel
AlOx
+
Ti/Pd/Ag
Tunnel
AlOx
AlOx passivation seen by photoluminescence
Comparison passivated/non-
passivated areas
Area for QSSPC calibration
Reflection on metal no direct
comparison
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 16
CZ p-Si
SiOx/SiNx passivation stack
100 Ω/ n+ emitter
SiNx ARC
Al BSF
CZ p-Si
SiOx/SiNx passivation stack
n+
Tunnel AlOx
© IMEC 2011
AlOx passivation seen by photoluminescence
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 17
No coating
(native oxide) Ti/Pd/Ag
1.5 nm
Al2O3
+
Ti/Pd/Ag
1.5nm Al2O3
Lifeti
me (
µs)
Dn=5.1014cm-3 on non-metalized area
and lower on metalized areas
CZ 180µm p-Si, with 100Ω / POCl3 emitter
Low AlOx reference lifetime - Limited AlOx passivation on n+ Si
30
10
15
20
25
0
5 17.4µs
6.4µs
25µs
23µs
Significant increase of lifetime under metal with 15 AlOx
ALD cycles (~1.5nm)
© IMEC 2011 CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 18
Lifetime versus AlOx thickness
Ultra-thin AlOx under contact
improves passivation
Average lifetime on the Si areas covered by AlOx and/or metal
1.5-2nm
© IMEC 2011
Outline
Literature overview
AlOx candidate for contact passivation?
AlOx contact passivation seen by
photoluminescence
Contact resistance
Conclusion
CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 19
© IMEC 2011 CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN
p-Si
n+, 120 Ω/
500 µm
Ti/Pd/Ag dots
V AlOx
20
Flat emitter
Departure from ohmic contact from 20 AlOx ALD cycles (~2 nm)
Textured emitter
Large current and ohmic contact through all AlOx layers, even through
20nm AlOx !!!
Influence of thickness on contact resistance
© IMEC 2011 CONTACT PASSIVATION – 3RD METALLIZATION WORKSHOP, CHARLEROI 2011 – X. LOOZEN 21
Reduced recombination under Ti/Pd/Ag contacts
with AlOx tunnel layers is observed with PL
Ohmic behavior observed on flat emitters with up
to 1.5 nm AlOx (= optimal thickness for passivation)
Contact resistance on textured emitters not clear,
but no show-stopper indication
Conclusion
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© IMEC 2011
Thank you!