(i)._Introduction to the resistivity surveying method. The resistivity of
Modelling of contact resistivity of fire-through Ag-Al ...
Transcript of Modelling of contact resistivity of fire-through Ag-Al ...
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SERIS is a research institute at the National University of Singapore (NUS). SERIS is supported by
NUS, the National Research Foundation Singapore (NRF), the Energy Market Authority of Singapore
(EMA) and the Singapore Economic Development Board (EDB).
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Modelling of contact resistivity of fire-through Ag-Al contacts to B-doped
polysilicon layers
Pradeep PADHAMNATH, Nitin NAMPALLI, Jeremie WERNER, Armin G. ABERLE, Shubham DUTTAGUPTA
10th Metallization & Interconnection Workshop, Genk, Belgium
15 Nov, 2021
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SERISSolar Energy Research Institute of Singapore
National Lab founded at NUS in 2008;
supported by NUS, NRF, EMA & EDB
Focuses on applied solar energy research
(solar cells, PV modules, PV systems)
> 120 staff & PhD students; state-of-the-art labs, ISO
certified (9001, 17025)
Close collaborations with industry & government agencies
Strategic priorities: To develop & commercialize solar
technologies suited for urban and tropical applications,
and support industry development and the energy
transformation towards higher solar adoption
SERIS
Solar cells
电池
PV modules
组件
Solar PV systems
PV系统
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SERIS’ monoPolyTM celliOx + Poly-Si deposited using PECVD tool (no wrap-around)
PECVD poly-Si
Best group Voc (mV) Jsc (mA/cm2) FF (%) η (%)
Batch median 701 41.04 82.5 23.7
Best cell 703 41.17 82.5 23.9
SDE + Texture
In-situ oxide + n+:a-Si (PECVD)
Annealing
Chemical edge isolation
Screen-printing + Firing
B-diffusion
Rear side etch
8 S
TE
PS
ARC + passivation
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p+ c-Si
n+ Poly-Si
SiNx
AlOx
SiNx
n-type c-Si
Front metal contact
iOx
Rear metal contact
n+ c-Si
p+ Poly-Si
SiNx
SiOx
SiNx
n-type c-Si
Front metal contact
iOx
Rear metal contact
monoPolyTM cellPossible configurations
Front Emitter n-type monPolyTM cell Rear Emitter n-type monPolyTM cell
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Metallization of n+ poly-Si has rapidly progressed.
Possible to contact 25nm poly-Si layers with
decent contact properties
Passivation properties largely independent of the
thickness of poly-Si layer
All experimental data shown here achieved on
planar surface with large area (156x156 mm2)
wafers
Contacts to n+ poly-SiWith screen-printed fire-through Ag metallization
Padhamnath et al., SolMat, 2019
25 50 100 150 200 2500.0
0.5
1.0
1.5
2.0
2.5
3.0
Thickness of poly Si [nm]
rc [mW
cm
2]
0 50 100 150 200 250251
10
100
1000 J0,metal
J0,surf
J0 [fA
/cm
2]
Thickness [nm]
J0,metal for Ag contacts on n+ c-Si
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Contacts to p+ poly-SiWith screen-printed fire-through Ag-Al metallization
Padhamnath et al., Solar Energy, 2021
P1 P2 P34
6
8
10
12
14
16
18
20
22
24
rc [mW
-cm
2]
FT Metal pastes50 100 150 200 250
0
10
20
30
40
50
60
rc [mW
-cm
2]
Poly-Si thickness [nm]
50 100 150 200 2500
5
10
15
J0,p
ass [fA/c
m2]
Thickness of poly-Si layer [nm]
50 100 150 200 250
100
1000
J0
,me
tal [
fA/c
m2]
poly-Si thickness [nm]
J0,metal on p+ c-Si Contacts to p+ poly-Si with FT Ag-Al pastes
not mature enough.
All experimental data shown here achieved on
planar surface with large area (156x156 mm2)
wafers
150nm poly-Si
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Contacts to p+ poly-SiWith screen-printed fire-through Ag-Al metallization
Padhamnath et al., Solar Energy, 2021
P1 P2 P34
6
8
10
12
14
16
18
20
22
24
rc [mW
-cm
2]
FT Metal pastes
Contact resistivity shows
dependency on glass layer
distribution.
Contact resistivity also
dependent on:
metal crystallites size
distribution of metal
crystallites
Surface texture
P1 P2 P32
4
6
8
10
12
14
16
18
20
% a
rea
co
ve
rag
e [
%]
FT Metal pastes
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Contacts to p+ poly-SiWith screen-printed fire-through Ag-Al metallization
Padhamnath et al., Solar Energy, 2021
Contact resistivity shows
dependency on glass layer
distribution.
Contact resistivity also
dependent on:
metal crystallites size
distribution of metal
crystallites
Surface texture
Plan Tex0
2
4
6
8
10
rc [
mW
-cm
2]
Data shown for 150nm poly-Si
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Contacts to p+ poly-SiWith screen-printed fire-through Ag-Al metallization
Padhamnath et al., Solar Energy, 2021
Ag-Al crystallites formed during high temp firing vary
greatly in size
For contacts to c-Si deeper junctions result in lower ρc
and J0,metal.
Contact resistivity also dependent on metal crystallites
size and distribution
Ideal contact – crystallites stay within the poly-Si layer
Modeling of Ag-Al contacts to p+ poly-Si less explored
0.0 0.2 0.4 0.6 0.8 1.01015
1016
1017
1018
1019
1020
B (
cm
-3)
depth [mm]
c-Si
poly-Si
Al crystallites
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Modeling of Contacts to p+ poly-SiModeling approach
Padhamnath et al., Solar Energy, 2021
Huge difference exist between theoretical and
experimentally achievable ρc .
Glass layer could impede the crystallites formation
and charge flow, especially on planar surfaces.
Glass layer could lead to artificially increased effective
Schottky barrier height (Φb) 1.00 1.05 1.10 1.15-20
-15
-10
-5
0
Ln(r
c)
1/ÖNpoly (x10-10)
Current FT paste
Ideal FT paste
Ideal contact
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Modeling of Contacts to p+ poly-SiModeling approach
Crystallites divided into strips of δx (one order of
magnitude smaller than estimated depth)
ρc calculated for each individual crystal by parallel
summation of individual strips.
Dopant change considered as per the dopant profile
Fermi-Dirac statistics used for charge carrier
distribution including BGN effect for [B]>1x1019 cm-3
Crystallites within the poly-Si layer – larger influence
on ρc
δx
Assumption
Constant dopant
concentration over the
width of single strip
For detailed modeling approach and theory : E. Lohmüller, et al. "Impact of boron doping profiles on the specific contact resistance of screen printed Ag–Al contacts on
silicon," Solar Energy Materials and Solar Cells, vol. 142, pp. 2-11, 2015
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Modeling of Contacts to p+ poly-SiModeling approach
Current transport mechanism Specific contact resistance Eq
Thermionic Emission (TE)
(N < 1018 cm-3) [1]𝜌𝑐(𝑇𝐸) =
𝑘𝐵𝑞𝐴∗𝑇
𝑒𝑥𝑝𝑞∅𝑏
′
𝑘𝐵𝑇(1)
Thermionic Field Emission (TFE)
(1018 cm-3 < N < 1020 cm-3 ) [2,3]
𝜌𝑐(𝑇𝐹𝐸) = 𝐶𝑇𝐹𝐸𝑘𝐵𝑞𝐴∗𝑇
𝑒𝑥𝑝𝑞∅𝑏
′
𝐸0(2)
𝐶𝑇𝐹𝐸 =𝑘𝐵𝑇 𝑐𝑜𝑠ℎ 𝐺 𝑐𝑜𝑡ℎ 𝐺
𝜋 𝑞∅𝑏′ +𝐸𝐹 𝐸00
𝑒𝑥𝑝𝐸𝐹
𝐸0−
𝐸𝐹
𝑘𝐵𝑇, 𝐺 =
𝐸00
𝑘𝐵𝑇
(3)
Field Emission (FE) (N > 1020 cm-3) [2,3]
𝜌𝑐(𝐹𝐸) = 𝐶𝐹𝐸𝑘𝐵𝑞𝐴∗𝑇
𝑒𝑥𝑝𝑞∅𝑏
′
𝐸00(4)
𝐶𝐹𝐸 = ቈ
𝜋
𝑠𝑖𝑛𝜋𝑘𝐵𝑇
2𝐸00𝑙𝑛 𝑀
−
2𝐸00𝑘𝐵𝑇 𝑙𝑛 𝑀
𝑒𝑥𝑝 −𝐸𝐹 𝑙𝑛 𝑀
2𝐸00𝑀 = 𝑙𝑛
4𝑞∅𝑏′
𝐸𝐹
(5)
𝜌𝑐 Τ𝑇𝐹𝐸 𝐹𝐸 [2] 1/1
𝜌𝑐 𝑇𝐹𝐸+
1
𝜌𝑐 𝐹𝐸(6)
𝜌𝑐 Τ𝑇𝐸 𝑇𝐹𝐸 [4] 1/1
𝜌𝑐 𝑇𝐸+
1
𝜌𝑐 𝑇𝐹𝐸(7)
Characteristic Energy, E0 [5] 𝐸0 = 𝐸00 𝑐𝑜𝑡ℎ 𝐺 (8)
Characteristic Energy, E00 [5]𝐸00 =
𝑞ℎ
4𝜋
𝑁
𝑚𝑡∗𝜀𝑟𝜀0
(9)
[1] S. Sze and K. Ng, Physics Of Semiconductor Devices. John Wiley& Sons. Inc.,
2007.
[2] K. Varahramyan and E. Verret, "A model for specific contact resistance applicable
for titanium silicide-silicon contacts," Solid-State Electronics, vol. 39, no. 11, pp.
1601-1607, 1996
[3] A. Y. C. Yu, "Electron tunneling and contact resistance of metal-silicon contact
barriers," Solid-State Electronics, vol. 13, no. 2, pp. 239-247, 1970/02/01/ 1970
[4] E. Lohmüller, et al. "Impact of boron doping profiles on the specific contact
resistance of screen printed Ag–Al contacts on silicon," Solar Energy Materials and
Solar Cells, vol. 142, pp. 2-11, 2015
[5] F. Padovani and R. Stratton, "Field and thermionic-field emission in Schottky
barriers," Solid-State Electronics, vol. 9, no. 7, pp. 695-707, 1966.
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Modeling of Contacts to p+ poly-SiModeling approach
Symbol Definition Value Units
kB
Boltzmann
constant1.38x10-23 m2 kg s-2 K-1
T Temperature 300 K
q Electronic charge 1.6x10-19 C
hPlanck’s
constant6.62 x10-34 m2 kg/s
me Mass of electron 9.31 x10-31 kg
m*t/me [1]
Ratio of
tunnelling mass
to electronic
mass
0.5 No unit
ε0
Permittivity of
free space8.85 x10-12 C2 (Nm2)-1
εSi
Relative
permittivity of Si11.7 No unit
A*
Effective
Richardson
constant [12]
32 (Acm-2K-2)
Assumptions
No illumination
No voltage bias
If dopant density is known
then, Φb can be calculated
𝑉𝑏𝑖 = ∅𝑏 −1
𝑞𝐸𝐹 − 𝐸𝑉 = 0
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Modeling of Contacts to p+ poly-SiModeling approach
A) SEM micrograph of etch pits for a 250-nm p+ poly-Si layer
deposited on a planar sample metallized with a FT Ag-Al
paste. B) Image converted to 8-bit. C) Processed image
showing the area of the etch pits analyzed. To reveal the etch
pits, the bulk metal and the metal crystallites were etched
away
50 100 150 200 250 300 350 400 4500
50
100
150
200
250
Co
un
t
Area of the etch pits [nm2]
etch pits area fraction - 9.5%
3 6 9 12 15 18 21 24 270
20
40
60
80
100
120
140
160
180
200
220
27%
Count
Edge length of etch pits [nm]
59%
9% 4%
Ignored
(< 0.2%)
Histogram of the edge lengths of the pits. The bin width is
taken as 5 nm. 0.2% of the etch pits were ignored (shown
by the shaded region).
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Modeling of Contacts to p+ poly-SiModeling results
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.23.95
3.96
3.97
3.98
3.99
4.00
rc [
mW
-cm
2]
fB [eV]
TETFE TE/TFE
TFE/FE
rc,exp
rc,theo
Model ϕB [eV] ρc,exp [mΩ-cm2] ρc,theo [mΩ-cm2]
TE 0.51568 ± 0.00111 3.98 ± 0.45335 3.98213 ± 0.00278
TFE 0.95697 ± 0.00228 3.98 ± 0.45335 3.98287 ± 0.00167
TE/TFE 0.94697 ± 0.00236 3.98 ± 0.45335 3.98231 ± 0.00 135
TFE/FE 1.16548 ± 0.00872 3.98 ± 0.45335 3.98133 ± 0.00025
Ideal ϕB for contact to p-type silicon - 0.53 - 0.55 eV for Ag contacts and 0.57 -
0.58 eV for Al contacts
Ideal model for the dopant range : TFE / or TE/TFE
ϕB obtained for TE model agrees with literature
For TFE model, ϕB almost 2x the expected value
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Modeling of Contacts to p+ poly-SiDiscussion
Ideal ϕB range - 0.53 - 0.55 eV for Ag contacts and 0.57 - 0.58 eV for Al contacts
Ideal ϕB obtained for TE model applicable for dopant conc <1018
ϕB higher for TFE or TE/TFE models – charge transport hindered
Increased ρc due to the increased ϕB
Thick and uniform glass layer on planar samples → increased ϕB→ Increased ρc
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Summary
Industrial metallization solutions needed for commercial success of p+ poly-Si.
FT Ag-Al or Al pastes ideal candidate for commercial screen-printed
metallization.
Hypothesis – thick glass layers impede the charge carriers – high ρc
0 50 100 150 200 250 3000
20
40
60
80
100
Mod. %
cry
sta
llite
s [%
]
Distance from the surface [nm]
Tex
Planideal FT paste
(front application)
Ideal FT paste
(rear application,
crytallites limited to
150 nm poly-Si)
Probable values
J0,metal <50fA/cm2
rc <0.5 mW-cm2
Probable values
J0,metal <200fA/cm2
rc <0.5 mW-cm2
Methods to improve ρc
Limiting the penetration depth of Al
crystallites by alloying with Si
Achieving thinner glass layer
Doping the glass layer
Developing FT Al pastes
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Acknowledgment 致谢
SERIS is a research institute at the National University of Singapore (NUS). SERIS is
supported by NUS, the National Research Foundation Singapore (NRF), the Energy Market
Authority of Singapore (EMA) and the Singapore Economic Development Board (EDB).
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Thank you for your attention!
Contact:
Pradeep Padhamanth ([email protected])
Head, Solar Cell Metallization Group
Next-Generation Industrial Solar Cells & Modules
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