Solar Energy Materials
and Solar Cells
ELSEVIER
Solar Energy Materials and Solar Cells 48 (1997) 145 150
Effect of boron gettering on minority-carrier
quality for FZ and CZ Si substrates
N. Ohe a'*, K. Tsutsui b, T. Warabisako b, T. Saitoh a
a Tokyo A&T University, Koganei, Tokyo 184, Japan
b Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185, Japan
Abstract
The gettering effect on boron-diffused FZ and CZ Si substrates was investigated by effective
lifetime measurement with a chemical passivation technique. After removal of the borondiffused layers, the effective lifetime increased about two times higher than initial values for
p-type FZ Si substrates. However, thermal processes in 0 2 and N 2 after boron diffusion
degraded the effective lifetime drastically for both the substrates. These results suggest that
lifetime killer impurities were gettered at defects in the P+-layers heavily doped by boron
diffusion and diffused out into bulk regions because of lowering surface B concentration by the
thermal processes. Heavy B concentration is needed to getter lifetime killer impurities.
Keywords." Boron gettering; Minority-carrier quality; Si substrates
1. Introduction
High-efficiency crystalline silicon solar cells include a back surface field (BSF)
structure fabricated using b o r o n (B) diffusion to reduce recombination at the rear
electrode [1-3]. However, B diffusion is a delicate process because sometimes B diffusion degraded cell performance by diffusing lifetime killer impurities or defects at high
temperature. This is one of the reasons why a theoretical efficiency of single crystalline
Si solar cell is difficult to be achieved. Furthermore, one of the ideas to reduce the cost
* Corresponding author.
0927-0248/97/$17.00 @ 1997 Elsevier Science B.V. All rights reserved
PII S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 0 8 6 - X
146
N. Ohe el al. S o l a r Em~tXr Malerials' atM Solar (_'ells' 48 (1997) 145 150
of solar cells is to use thinner Si substrates. In the thinner cell structure, the BSF
structure has to be fabricated hopefully by an aluminum alloying. However, the
alloying process is difficult to apply for thin substrates due to warping. To avoid the
problem, gas-phase, B diffusion should be employed to prepare p+-layers without
warping.
In this research, a detailed study on effective lifetime {rm-} variation during cell
processes related to B diffusion has been carried out by making the B-diffused
p+ layers for FZ and CZ single crystalline Si substrates. The effect of "de-gettering"
was also evaluated by measuring ze,f for Si substrates after thermal processes in 02 or
N2 ambient.
2. Experimental
Two different Si substrates of p-type, FZ (2 f~ cm) and CZ (10 f~ cm) were used in
this experiment. B was diffused at 1000'C for 60 min into both sides of the substrates
to fabricate p +-layers with a sheet resistance of 20 fl/[] using a pyrolytic boron nitride
solid source. To investigate the effect of thermal processes after B diffusion on bulk
lifetimes, two types of samples were prepared. One was oxidized at 1000°C for 60 min
to make a surface passivation layer and the other was annealed in N2 ambient at the
same condition. Furthermore, to investigate gettering and de-gettering effects in
detail, p-type CZ (10 ~ cm} substrates were processed to fabricate B-diffused samples
using consecutive B diffusion and oxidation. First, the B-diffused layers of three
samples were removed in an H F : HNO3 (1 : 20) solution. Then, the first sample was
oxidized at 1000C for 60 min, the second sample was diffused by B again and the
third sample was oxidized at the same condition after the second B diffusion.
A microwave detected photoconductivity decay method using a pulse laser of
904 nm was used to measure r~,-. To examine the effect of B gettering, bulk lifetimes
before and after B diffusion were measured using a chemical passivation ICP) technique. The CP technique using an ethanol solution of iodine was applied for reducing
surface recombination velocity at wafer surfaces [4]. To obtain bulk lifetimes, before
reff measurements, the B-diffused layers were removed from each side of the wafer in
an H F : HNO3(1 : 20t solution and then native oxide was removed in an H F : H 2 0
(1 : 10) solution.
3. Results and discussion
The effect of B diffusion process on r~,-ffor FZ and CZ wafers was investigated after
removal of the B-diffused layers. As shown in Fig. 1, the initial reff values for FZ-p and
CZ-p wafers were about 240 and 600 gs, respectively. After B diffusion, retf for the
FZ-p Si wafer increased about two times higher than the initial value although that for
the CZ wafer did not vary substantially. This result showed that reef increased due to
B gettering.
147
N. Ohe et aL/Solar Energy Materials and Solar Cells 48 (1997) 145 150
(f~.cm)
Before B diffusion ( FZ-p 2
CZ-p 10
\
FZ-p2
After B diffusion
CZ-p 10
0
100
200 300 400 500
Effective lifetime (ps)
600
Fig. 1. Effectof boron gettering on effectivelifetimefor p-type FZ and CZ wafers. The sample surfaces were
treated by a chemical passivation after removal of the B-diffused layers.
(f2-cm)
FZ-p 2
B diffusion
CZ-p 10
FZ-p 2
Oxidation
CZ-p 10
Annealing
in N2 ( FZ-p 2
\
CZ-p 10
1
10
100
1000
Effective lifetime (/Js)
Fig. 2. Effect of thermal processes in 02 o r N 2 ambient on effectivelifetime for p-type FZ and CZ wafers
after B diffusion.The sample surfaces were treated by a chemical passivation after removal of the B-diffnsed
layers.
O n the contrary, as indicated in Fig. 2, m e a s u r e d "L'ef for both F Z a n d C Z wafers
oxidized after B diffusion decreased drastically to as low as 3 ~ts. The values were two
orders of m a g n i t u d e lower t h a n those for the B gettered samples. F u r t h e r m o r e , similar
zeff decreases to a b o u t 10-20 Its were o b t a i n e d for B-diffused wafers a n n e a l e d in
N2 ambient. This m e a n s that the drastic zeff decrease was caused by the t h e r m a l
processes rather t h a n the a m b i e n t effect.
148
N. Ohe el al./Solur Energy Materials and Solar Cells 48 (1997) 145 150
The effect of consecutive thermal processes on zefrwas examined for CZ-p 10 ~ cm
Si wafers after removing the first B-diffused layers. After the second B diffusion, as
shown in Fig. 3, r~rt- was about 500 gs as high as that for the first B-diffused sample,
but rerf after subsequent oxidation decreased to about 15 ItS. On the other hand, in the
oxidation after removal of B-diffused layers, refr of about 150 ps was not so low as
compared with the B-gettered samples and higher than samples oxidized after the first
and second B diffusions. This result suggests that the twice B gettering is more
effective than the once gettering and "de-gettering" occurs during thermal processes
for samples with p+-layers.
To investigate gettering and "de-gettering" mechanisms, carrier profiles before and
after oxidation for the FZ-p wafers were measured by the C V method with an
electrochemical etching. As shown in Fig. 4, surface carrier concentration before
oxidation was about 5 x 101'~ cm -~, which means B was heavily doped at the surface.
Gettering sites were generated in the heavily B-diffused layers due to the atomic size
difference between Si and B atoms. As a result, lifetime killer impurities, probably F e ,
were diffused and gettered at the sites. On the other hand, surface B concentration
after oxidation decreased to l x 101~ cm ~. Therefore, the gettering sites in the
B-diffused layers became lower after oxidation. As a result, lifetime killer impurities
could not be trapped at the defects and were diffused out from the B-diffused layers
into bulk regions.
In order to estimate the gettering mechanism, the dependence of wafer thickness on
re~.r was investigated by repetitive etching of oxidized, B-diffused the CZ-p Si wafers.
As indicated in Fig. 5, the lifetimes degraded for whole regions of the wafers. This
First B diffusion
Oxidation
! iiii ii
Removed
B-diffused layers , ~
l
Oxidation
Second B diffusion
Oxidation
I
I0
I O0
1000
Effective lifetime (us)
Fig. 3. Effectof consecutive B diffusion and oxidation processeson effectivelifetimefor p-type CZ 10 ~ cm
wafers.
N. Ohe et al./Solar Energy. Materials and Solar Cells 48 (1997) 145-150
149
1020
E
tO
1019 ~ - . . ~
-.-..:. . . . . . ,
.,.,j After
1018
¢.3
tO
r~
1017
'E
cO
o
-1I °xidati°n
......................
i .............
......................
i
"i
2
2.5
. . . . .
1016
0
0.5
1
1.5
3
Depth ~ m )
Fig. 4. Carrier profiles before and after oxidation for B-diffused p-type FZ 2 ~2cm wafers as determined by
the C V method using an electrochemical etching.
1000
J
O~
lO0
Wafer
E
Q)
._>
"6
Q)
After B diffusion
/
Removal of
10
B-diffusec ~ayers
U::
LIJ
1
100
ii
i
i
I
i
150
i
i
i
f
200
,
,
,
,
I
250
,
,
,
,
300
Wafer thickness ~ m )
Fig. 5. Dependence of wafer thickness on effective lifetime for an oxidized and B-diffused p-type CZ wafer.
The samples were etched repeatedly in an HF : HNO3 (1 : 20) solution and treated by the CP.
result i n d i c a t e d that lifetime killer i m p u r i t i e s diffused into a l m o s t the center of the
wafer. U s i n g an e q u a t i o n of x / D " t where D is the diffusion coefficient a n d t the
o x i d a t i o n time, a diffusion d i s t a n c e of F e is e s t i m a t e d to be 3 m m using the D of a b o u t
3 x 10 . 5 cm2/s at 1000°C I-5]. This suggests t h a t a fast diffuser like F e m i g h t be related
to the gettering a n d " d e - g e t t e r i n g " processes p r o b a b l y by d e c o m p o s i n g a n d r e c o m bining a F e - B c o m p l e x [6].
150
N. ()he et al. 'Solar Em'r~v Materials and Solar Cells 48/1997) 145 150
4. Conclusion
After B diffusion, bulk lifetimes increased about two times higher than initial value
for p-type FZ Si substrate. However, thermal processes in O~ or N2 after boron
diffusion affected the drastic degradation of bulk lifetimes. These results suggest that
lifetime killer impurities, probably Ve, were gettered at defects in the heavily B-doped
p+-layers, whereas diffused out into bulk regions because of lowering surface B concentration by thermal processes.
Acknowledgements
This work was supported by the New Energy and Industrial Technology Development Organization as a part of the New Sunshine Program under the Ministry of
International Trade and Industry, Japan.
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