Solar Energy Materials & Solar Cells ] (]]]]) ]]]–]]]
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Solar Energy Materials & Solar Cells
journal homepage: www.elsevier.com/locate/solmat
Reverse saturation current density imaging of highly doped regions
in silicon: A photoluminescence approach
Jens Müller a,n, Karsten Bothe a, Sandra Herlufsen a, Helge Hannebauer a, Rafel Ferré a, Rolf Brendel a,b
a
b
Institute for Solar Energy Research Hamelin (ISFH), Am Ohrberg 1, 31860 Emmerthal, Germany
Institute of Solid-State Physics, University of Hannover, Appelstraße 2, 30167 Hannover, Germany
a r t i c l e i n f o
Keywords:
Photolumincescence
Charge carrier lifetime
Reverse saturation current density
Diffusion
abstract
We present a camera-based technique for the local determination of reverse saturation current
densities J0 of highly doped regions in silicon wafers utilizing photoconductance calibrated photoluminescence imaging (PC-PLI). We apply this approach to 12.5 12.5 cm2 float zone silicon samples
with textured surfaces and a homogeneous phosphorous diffusion with sheet resistances between 24
and 230 O/&. We find enhanced photoluminescence emission at metallized regions of a sample due to
reflection of long-wavelength light at the rear side of the sample. Our measurement setup comprises an
optical short pass filter in front of the camera effectively blocking wavelengths above 970 nm and
therefore ensuring a correct calibration of the PL signal in terms of excess charge carrier density Dn. We
analyze two sets of samples comprising metal contacts to highly doped regions prepared by Laser
Transfer Doping (LTD) as well as standard tube furnace phosphorus diffusion. We find a considerably
smaller J0 value of 370 fA/cm2 for the LTD approach compared to a standard diffusion process resulting
in J0 ¼ 570 fA/cm2. On the basis of these results we demonstrate that J0 imaging is a powerful analysis
technique for process optimization.
& 2012 Elsevier B.V. All rights reserved.
1. Introduction
In silicon solar cells, the recombination in highly doped regions
is characterized by the reverse saturation current density J0. For
process optimization J0 is usually investigated on test structures by
using photoconductance measurements [1]. However, this technique suffers from low spatial resolution and requires removing the
metal layer prior to measurements. Recently microwave-detected
photoconductance decay (MW-PCD) measurements [2] have been
demonstrated to overcome these limitations. However, MW-PCD
measurements suffer from long measurement times, since it is a
scanning technique. In addition to that, the metal layer acts as
a microwave reflector, drastically reducing the sensitivity of the
MW-PCD measurement [3].
In this contribution we demonstrate the application of photoconductance calibrated photoluminescence imaging (PC-PLI) [4]
to a spatially resolved analysis of the reverse saturation current
density J0 of highly doped layers. Our approach utilizes the
method of Kane and Swanson [1] to determine J0 images from
local injection-dependent carrier lifetime data. In contrast to
existing measurement techniques aiming at a determination of
n
Corresponding author. Tel.: þ49 5151999414; fax: þ49 5151999400.
E-mail address: j.mueller@isfh.de (J. Müller).
J0 of the solar cell base [5], our approach is employed to
determine J0 of a highly doped surface layer. Modifying the
measurement setup by an additional short pass filter in front of
the camera as described in Ref. [6] enables us to apply this
technique to partially metallized samples. Thus, with the presented measurement method we are able to quantify the recombination at metal contacts to highly doped silicon.
2. Measurement principle
Considering a p-type Si sample of thickness W and dopant
density NA, the inverse effective lifetime
1
teff
¼
1
tAuger
þ
1
trad
þ
1
tSRH
þðJ 0,front þJ 0,rear Þ
NA þ Dn
qn2i W
ð1Þ
depends on the Auger lifetime tAuger, the lifetime due to radiative
recombination trad, the Shockley–Read–Hall (SRH) lifetime tSRH,
the reverse saturation current densities of the front J0,front and
rear surface J0,rear, the excess carrier concentration Dn, the
elementary charge q and the intrinsic carrier concentration ni
[1,7]. From the parameterization of Kerr and Cuevas [8] tAuger and
trad are known and tSRH is assumed to be injection independent
under high injection conditions. Thus, we determine J0,front þJ0,rear
under high injection conditions (DnbNA) from the slope of
0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.solmat.2012.05.026
Please cite this article as: J. Müller, et al., Reverse saturation current density imaging of highly doped regions in silicon: A
photoluminescence approach, Solar Energy Materials and Solar Cells (2012), http://dx.doi.org/10.1016/j.solmat.2012.05.026
2
J. Müller et al. / Solar Energy Materials & Solar Cells ] (]]]]) ]]]–]]]
1/teff 1/tAuger 1/trad as a function of the excess charge carrier
density Dn.
In order to obtain local injection dependent carrier lifetime
data we acquire photoconductance calibrated photoluminescence
(PL) images at different high injection levels. Excess charge
carriers are excited in the sample under test by monochromatic
light at a wavelength of lexc ¼808 nm employing eight different
illumination intensities between 0.1 and 0.7 sun, which is sufficient to obtain high injection conditions in lowly doped Si. The
luminescence photons emitted from the sample due to radiative
recombination are detected by a Si-CCD camera. An optical long
pass filter in front of the camera blocks laser light reflected from
the sample surface. A sketch of the setup is shown in Fig. 1. For a
more detailed description of the setup we refer to Ref. [9].
According to Ref. [4] a quadratic dependence on the excess charge
carrier density Dn is expected for the PL signal
IPL ¼ aðDn2 þ NA DnÞ
ð2Þ
Here a denotes a calibration constant, which is determined
from calibrated conductivity measurements carried out in the
same setup for a specific sample area. Applying this calibration to
the whole PL image we obtain images of the local excess charge
carrier density Dnx,y. In a second step, these images are transferred into spatially resolved effective charge carrier lifetime
images according to
teff;x,y ¼ Dnx,y
W
Fð1Rfront Þ
ð3Þ
The incident photon flux F is measured using a Si reference
solar cell with known spectral response. The front reflectance
Rfront at the excitation wavelength of 808 nm follows from
reflectance measurements. Finally images of J0,rear þJ0,front are
determined from the local effective lifetime and excess charge
carrier density as described above.
For an experimental verification of our approach we use
12.5 12.5 cm2 sized and 275 mm thick p-type float-zone Si
samples with a doping concentration of NA ¼7 1013 cm 3. These
samples feature a double-sided phosphorous diffused highly
doped area electronically passivated by SiNx on textured surfaces
with sheet resistances ranging from 24 to 230 O/&. Since both
wafer surfaces are identical, we assume J0,rear and J0,front to be
Fig. 2. (a) J0 images of 12.5 12.5 cm2 sized samples comprising textured surfaces
and double sided phosphorous diffused highly doped regions with sheet resistances between 24 and 230 O/& passivated by SiNx (ni ¼8.6 109 cm 3).
(b) Averaged J0 values of the samples in (a) (red circles) as a function of sheet
resistance in comparison with data from Ref. [11] (blue triangles). The lines are
guides to the eyes. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
equal in (1). Fig. 2a shows the J0-images obtained from our
method. In Fig. 2a the lateral variation in J0 is below 10% for all
analyzed sheet resistances. This indicates a homogeneous surface
passivation due to the phosphorous diffusion in combination with
the SiNx layer. The averaged J0 values in Fig. 2a decrease with
increasing sheet resistance Rsh. This might be explained by a
decreasing bulk recombination within the highly doped layer due
to a decrease in both the surface dopant concentration as well as
the thickness of the diffused layer with increasing Rsh [10]. As
shown in Fig. 2b we measure an approximately six times higher J0
compared to data published by Cuevas et al. [11]. The reason
might be a lower surface dopant concentration of the highly
doped layers in Ref. [11] obtained by a thermal oxidation step. As
a result the bulk recombination is further reduced compared to a
standard non-oxidized phosphorous diffusion, which was carried
out for the samples shown in Fig. 2a.
3. Application to partially metallized samples
Fig. 1. Sketch of the measurement setup comprising an optical excitation of
excess charge carriers in a Si sample placed on a photoconductance calibration
unit. The reference cell measures the incident photon flux. Photoluminescence
(PL) photons emitted by the sample are detected with a Si-CCD camera. An optical
short pass filter effectively absorbs PL reflected from the sample rear.
In the non-metallized region of partially metallized samples
we are able to relate the PL signal to the excess charge carrier
density Dn. However, applying this calibration to the metallized
Please cite this article as: J. Müller, et al., Reverse saturation current density imaging of highly doped regions in silicon: A
photoluminescence approach, Solar Energy Materials and Solar Cells (2012), http://dx.doi.org/10.1016/j.solmat.2012.05.026
J. Müller et al. / Solar Energy Materials & Solar Cells ] (]]]]) ]]]–]]]
areas would result in an overestimation of Dn. The reason is the
enhanced PL signal at metallized sample surfaces, due to the
detection of high wavelength light reflected at the metallized
sample rear. For this reason we modify our setup by integrating
an additional optical short pass filter in front of the Si CCD
camera. This filter effectively blocks wavelengths above 970 nm
corresponding to an absorption length of 88 mm in Si. Hence, we
detect less than 10% of the PL intensity reflected at the rear of
samples thicker than 140 mm, which enables us to use the same
calibration for the metallized and non-metallized areas.
In order to experimentally verify these considerations, we
examine the impact of the additional SP filter on the measured
PL intensity on a 275 mm thick planar sample comprising a
dielectric layer on both sides. On the rear side of this sample
3
aluminum layers are deposited in different areas by evaporation
and screen printing. Fig. 3a shows the PL lifetime image acquired
without the SP filter. Metallized areas are clearly visible and show
artificially enhanced lifetime values. Fig. 3b shows the PL lifetime
image of the same sample but acquired with the SP filter in front
of the camera. In this case the metallized regions are no longer
visible and the lifetime values correspond to the actual effective
lifetime. Thus, Fig. 3b demonstrates the effective blocking of long
wavelength light reflected at the sample rear by using the optical
short pass in front of the camera.
The modified setup is used to determine J0 of metal contacts. In
this contribution we analyze the recombination of contacts to a
standard phosphorous diffusion as well as to a phosphorous diffusion process combined with the recently introduced Laser Transfer
Doping (LTD) technique [12]. For LTD a pulsed laser is employed to
transfer a layer of amorphous Si doped with phosphorous from a
glass substrate to a silicon surface. Fig.4 shows a schematic of the
process sequence, where after the LTD a standard phosphorous
diffusion is carried out. This results in a low sheet resistance in the
area processed with LTD. We use a standard Ag screen printing
process and contact firing to analyze the applicability of this process
as selective emitter for industrial type solar cells.
The sample under test is a 275 mm thick p-type float-zone Si
wafer with NA ¼7 1013 cm 3. The planar surface oriented to the
front during the measurement is electronically passivated by a stack
of Al2O3 and SiNx resulting in a negligible J0,front in Eq. (1). On the
textured side oriented to the rear for the measurement four different
areas are prepared: in areas (I) and (II) a highly doped region is
prepared by a Laser Transfer Doping (LTD) process resulting in a sheet
resistance of 45 O/& of the highly doped region. Subsequently a
100 O/& tube furnace P-diffusion is applied to the whole sample rear
with a passivating SiNx layer on top, which will be further analyzed in
areas (III) and (IV). In addition silver paste is screen printed on areas
(II) and (IV). Full area contacts are then formed by rapid thermal
annealing. As can be seen from Fig. 5a, the J0-image shows a higher J0
for the LTD process J0,I ¼280 fA/cm2 compared to the P-diffusion
J0,III ¼140 fA/cm2 when no metal contacts are applied. However, in
the case of metal contacts we obtain significantly lower contact
recombination of J0,cont,II ¼370 fA/cm2 using the LTD process compared to the light P-diffusion J0,cont,IV ¼570 fA/cm2. The latter finding
is explained by the lower sheet resistance of area (II) compared to
(IV). The high recombination at the contacted surface is in this case
shielded by the thick highly doped region in area (II), which is not
the case for the thinner highly doped area in area (IV). In Fig. 5b the
curves 1/teff (Dn) of the marked pixels in Fig. 5a demonstrate the
applicability of the proposed method for individual pixels.
4. Summary
Fig. 3. Lifetime images acquired using (a) photoconductance calibrated photoluminescence imaging (PC-PLI) and (b) PC-PLI with additional SP filter of a sample
with passivated surfaces and aluminum deposited in the marked areas (dashed
lines) by evaporation and screen-printing.
Camera-based photo-conductance calibrated photoluminescence measurements are shown to allow for a local measurement
of the reverse saturation current density J0 of highly doped
Fig. 4. Combination of Laser Transfer Doping (LTD) with a standard phosphorous diffusion results in a low sheet resistance layer in the area processed with LTD (not to
scale).
Please cite this article as: J. Müller, et al., Reverse saturation current density imaging of highly doped regions in silicon: A
photoluminescence approach, Solar Energy Materials and Solar Cells (2012), http://dx.doi.org/10.1016/j.solmat.2012.05.026
4
J. Müller et al. / Solar Energy Materials & Solar Cells ] (]]]]) ]]]–]]]
10%. At metallized sample rear surfaces the PL signal is enhanced
due to reflection of high wavelength light at the sample rear.
Thus, an additional short pass filter effectively blocking wavelengths above 970 nm is installed in front of the camera. Using
this configuration the calibration function relating the camera PL
signal to the excess carrier density Dn can be transferred from a
non-metallized to a metallized region.
We combine a standard phosphorous diffusion with the Laser
Transfer Doping (LTD) technique, to achieve local low sheet
resistance areas. Analyzing samples comprising metal contacts
to highly doped regions prepared by LTD as well as standard tube
furnace phosphorus diffusion, we find a considerably smaller J0
value of 370 fA/cm2 for the LTD approach compared to standard
diffusion process resulting in J0 ¼570 fA/cm2.
Acknowledgments
The authors thank H. Kohlenberg, M. Wolter, C. Marquardt and
R. Bock for their help with sample processing. This work was
financially supported by the State of Lower Saxony, Germany.
References
Fig. 5. (a) J0-image of a sample with well passivated front and P-diffused rear side
comprising four areas I–IV described in the body of the text. The high J0 values at
the edges of the four areas are artifacts resulting from sample cutting after
processing. (ni ¼ 8.6 109 cm 3). (b) Inverse effective lifetime as a function of the
excess charge carrier density of the marked pixels (white circles) in (a).
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charge carrier density Dn for individual pixels.
We apply the proposed method to 12.5 12.5 cm2 float zone
silicon samples with textured surfaces and a homogeneous
phosphorous diffusion with sheet resistances between 20 and
200 O/& in the highly doped layer. We find a decrease in J0 with
increasing sheet resistance in accordance to previous results.
The lateral variation of J0 across the wafer is found to be below
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Please cite this article as: J. Müller, et al., Reverse saturation current density imaging of highly doped regions in silicon: A
photoluminescence approach, Solar Energy Materials and Solar Cells (2012), http://dx.doi.org/10.1016/j.solmat.2012.05.026