Iron in crystalline silicon solar cells: fundamental properties, detection techniques,

and gettering

Daniel Macdonald, AnYao Liu, and Sieu Pheng Phang

Research School of Engineering The Australian National University, Canberra

2

• Origins of Fe in multicrystalline Si ingots

• Chemical states and recombination activity of Fe in silicon

• Measuring [Fei] by QSSPC and PL imaging

• Gettering of Fe during ingot growth and cell fabrication:

– Internal gettering – at GBs, dislocations and within grains

– External gettering by P, Al and B diffusions

Outline

3

• One of the most common metal contaminants

• Total Fe concentration, measured by NAA before and after P gettering

• Comes from the crucible, not the feedstock

• Typically between 1012 -1015 cm-3

Origins of Fe in multicrystalline Si ingots

Macdonald et al. 29th IEEE PVSC New Orleans (2002)

4

• Concentration increases towards top - segregation

• Also increases at bottom – solid-state diffusion from crucible

• Only a small fraction is interstitial - around 1%

• Remainder is precipitated (or substitutional)

• The dissolved fraction has a much larger impact on lifetime

Origins of Fe in multicrystalline Si ingots

0.1 1

1011

1012

1013

1014

1015

1016

1017

keff = 0.65B

keff < 0.05

keff < 0.05

interstitial Fe

Impu

rity

conc

entra

tion

(cm

-3)

Fraction from top of ingot

total Fe

Macdonald et al. J. Appl. Phys. 97 033523 (2005)

5

Recombination activity of interstitial Fe in silicon

conduction band

valence band

FeB0/+ (EV+0.1 eV)

FeB0/- (EC-0.26 eV)

Fei0/+ (EV+0.38 eV)

• Interstitial Fe (Fei) introduces a deep donor level

• Positively charged in p-type Si - mobile at RT - forms pairs with ionised acceptors.

• Two FeB levels – acceptor and donor

• Pairs break under illumination – only Fei present in working cells Courtesy of J. Schmidt, ISFH

6

Recombination activity of Fe in p-type silicon • Different energy levels and capture cross sections - different lifetime

curves • SRH modelling on left, QSSPC data on right (trapping restricts range)

1012 1013 1014 1015 1016 1017

1

10

100

crossoverpoint

1 Ω.cm p-type Si

FeB lifetime

Carri

er li

fetim

e (µs

)

Excess carrier density ∆n (cm-3)

Fei lifetime

Macdonald et al. J. Appl. Phys. 95 1021 (2004)

7

Detecting Fei using FeB pairing • Manipulating the SRH equations

shows that:

• Zoth and Bergholz developed a famous method based on SPV

• Extended to other methods (uW-PCD and QSSPC)

• Very sensitive (similar to DLTS) • Only works in p-Si • Must avoid the crossover point

1011 1012 1013 1014 1015 1016 1017

10

100

1000

crossover pointQSSPC

SPV

Fei lifetime FeB lifetime Auger lifetime

µW-PCD

Carri

er li

fetim

e (µs

)

Excess carrier concentration (cm-3)

p-Si, NA=1016cm-3, [Fei]=1012cm-3

−×=

FeBFei

i

CFeττ

11][

8

Detecting Fei using FeB pairing

• Pre-factor C is a function of doping and excess carrier density.

• In true low injection, C becomes constant – ideal measurement region.

• Not accessible to QSSPC or uW-PCD (trapping effects).

109 1011 1013 1015 1017-1x10-13

0

1x10-13

2x10-13

3x10-13

4x10-13

5x10-13

6x10-13

NA= 3x1016cm-3

1x1016

3x1015

1x1015

3x1014

1x1014

1/C

(µs

cm-3

)

Excess carrier density ∆n (cm-3)

−×=

FeBFei

i

CFeττ

11][

Macdonald et al. J. Appl. Phys. 95 1021 (2004)

9

Iron imaging with photoluminescence (PL)

• Band-to-band PL imaging - rapid and highly-resolved method for low-injection lifetime imaging – no trapping effects.

• Allows low-injection Fe imaging - similar to original SPV technique • Two PL images required, before and after breaking FeB pairs.

1011 1012 1013 1014 1015 1016 10171

10

100

1000 100% Fei : 0% FeB 75% : 25% 50% : 50% 25% : 75% 0% : 100%

Augerlifetime

crossover point

QSSPC

SPV PL imaging

µW-PCD

Carri

er lif

etim

e (µ

s)

Excess carrier concentration ∆n (cm-3)

p-Si, NA=1016cm-3, [Fei]+[FeB]=1012cm-3

Macdonald et al. J. Appl. Phys. 103, 073710 (2008)

10

Recombination activity of Fe in n-type silicon • Neutral charge state in n-type – less attractive for minority carriers

compared to p-type. • Higher lifetime in n-type in low- to mid-injection. • Possible incentive for using n-type substrates…

1012 1013 1014 1015 1016 10170.1

1

10

100

1000

10000 n-type FZ SiND=2.4x1015cm-3

[Fei]=3.8x1012cm-3

p-type FZ SiNA=2.8x1015cm-3

[Fei]=3.4x1012cm-3

Reco

mbi

natio

n life

time

(µs)

Excess carrier concentration (cm-3) 1010 1011 1012 1013 10140.01

0.1

1

10

100

1000

10000

n-type

p-type

Effe

ctive

lifet

ime τ Fe

+int

rinsic

(µs)

Interstitial Fe concentration [Fei ] (cm -3)

NA/D = 1016 cm -3, 0.1 suns illumination

Macdonald and Geerligs, Appl. Phys. Lett. 85 4061 (2004)

11

Fe images on mc-Si

• Wafer 20% from bottom of ingot

• High [Fei] (1013 cm-3)

• Internal gettering of Fe during ingot cooling at GBs, dislocation clusters

Liu et al. Progress in PV, 19 649 (2011)

12

Fe images on mc-Si • Wafer from near very

bottom of ingot

• Moderate [Fei] (1012 cm-3)

• Small grains

• Fewer dislocation clusters

• Lower [Fei] within grains – presence of precipitation sites

Liu et al. Progress in PV, 19 649 (2011)

13

Fe images on mc-Si • Wafer from middle of

ingot

• Low [Fei] (1011 cm-3)

• Reduced gettering at GBs (due to precipitation starting at lower T)

Liu et al. Progress in PV, 19 649 (2011)

14

Internal gettering of Fe at GBs during ingot cooling • Line-scans of PL images with resolution of 25 microns • 1D diffusion/capture model – 2 free parameters – diffusion length of Fei LD(Fei) and

precipitation velocity P of the GB • Have to take care of PL artifacts!

– Image smearing in the CCD camera - use point-spread function de-convolution – Carrier spreading in the sample – use low-lifetime – i.e. high [Fei] samples)

Liu and Macdonald, IEEE JPV 2, 479, (2012)

15

Internal gettering of Fe at GBs during ingot cooling • Same GB on different wafers reveals diffusion length of Fei LD(Fei) depends on initial [Fei] • Lower [Fei] means that precipitation begins later during cooling • Modelling ingot cooling time reveals data can only be explained if precipitation at GBs

commences after super-saturation ratio of about 50 is reached!

Liu and Macdonald, IEEE JPV 2, 479, (2012)

16

Internal gettering of Fe at GBs during annealing • Annealing at low temps can drive further precipitation at GBs, and within

grains. • Higher temperatures tend to homogenise the dissolved Fe

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• At 600 ºC, [Fei] is far above solubility limit: • Strong super-saturation • Drives precipitation at GBs,

and within grains

• At 800 ºC, [Fei] is approx equal to the solubility limit: • No precipitation

• At 900 ºC and above, [Fei] is

below solubility limit: • No precipitation • Homogenization by diffusion • Dissolution of precipitates

also possible

Internal gettering of Fe at GBs during annealing

18

Internal gettering of Fe at GBs during annealing

• 500 C annealing for various times

• 1D diffusion/capture model: • Widening denuded zone • Reduced intra-grain [Fei]

19

• At fixed temp annealing, diffusion length of Fe can be calculated from literature values.

• Very good agreement with fitted values (500 ºC)

• A method to measure diffusivity?

Internal gettering of Fe at GBs during annealing

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• After homogenization on left, after 14 hour anneal at 500 °C on right. • Precipitation rate varies from grain to grain.

Internal gettering of Fe within the grains

21

• Precipitation rate varies despite initial Fe concentrations being similar

Internal gettering of Fe within the grains

22

Final [Fei] with respect to intra grain dislocation density, after annealing at 500oC for 14.5 hours

Microscope image of a defect etched mc-Si wafer

Internal gettering of Fe within the grains

• Less Fe precipitation in grains of low dislocation density • Average distance between dislocations is less than Fe diffusion length • Dislocations act as nucleation sites for Fe precipitation within grains

23

External gettering of Fei by P, Al and B diffusions • Fe-implanted, annealed, mono FZ p-Si, detected by QSSPC.

implant Fe anneal P, B or Al diffusion

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P, 8

50 C

P, 8

00 C

P, 7

50 C

P, 8

50 +

650

C -- -- -- -- -- --

0.01

0.1

1

10

100

Per

cent

age

of F

e re

mai

ning

(%)

Phosphorus gettering of Fei • P gettering removes between 90-99% of Fe – better at lower temp • Adding a post-getter anneal improves gettering further – segregation ratio

improves

Phang and Macdonald, JAP 109, 073521 2011

25

Phosphorus gettering of Fei • Driving in P diffusion reduces gettering effectiveness

• Very heavily doped region (>1020 cm-3) required for best gettering

Phang and Macdonald, IEEE JPV accepted

26

Aluminium gettering of Fei • Al gettering removes more than 99.9%! • However, typical BSF firing is too short for Al gettering to rear side…

P, 8

50 C

P, 8

00 C

P, 7

50 C

P, 8

50 +

650

CAl

, 850

C, 5

5 m

inAl

, 750

C, 5

5 m

inAl

, 750

C, 1

5 se

c -- -- --

0.01

0.1

1

10

100

Per

cent

age

of F

e re

mai

ning

(%)

Phang and Macdonald, JAP 109, 073521 2011

27

Boron gettering of Fei • B gettering very effective if Boron Rich Layer (BRL) is present. • However, BRL is oxidised in-situ to allow low Joe - re-injects Fe into base. • Even a low temp anneal does not help much…

P, 8

50 C

P, 8

00 C

P, 7

50 C

P, 8

50 +

650

CAl

, 850

C, 5

5 m

inAl

, 750

C, 5

5 m

inAl

, 750

C, 1

5 se

cB,

950

C +

BRL

B, 9

50 C

no

BRL

B, 9

50+6

50 n

o BR

L

0.01

0.1

1

10

100

Per

cent

age

of F

e re

mai

ning

(%)

Phang et al., IEEE JPV 3, 261, 2013

28

Boron gettering of Fei • One solution is to remove the BRL at low temp by etch-back

• Preserves gettering • Allows lower Joe

Phang et al., IEEE JPV 3, 261, 2013

29

Boron gettering of Fei • Another idea is to overlay a light phosphorus diffusion – ‘buried emitter’. • Gives very good gettering

Phang and Macdonald, IEEE JPV accepted

30

Conclusions • Fe is common in mc-Si wafers, both dissolved and precipitated • Dissolved iron more active in p-type than n-type due to charge state • QSSPC allows very sensitive [Fei] measurements • PL allows spatially resolved Fe imaging • Gettering of dissolved Fe is critical for mc-Si cells

• Internal gettering at GBs, dislocations and in intra-grain regions • Strong super-saturation required

• External gettering via P, B or Al diffusions can be very effective • Al and B more effective than P • However, B diffusions require the presence of a BRL

Acknowledgements: this work has been supported by the Australian Research Council (ARC).