The document reports on an experiment measuring the interstitial iron concentration in multicrystalline silicon wafers using photoconductance and photoluminescence techniques. Samples underwent chemical treatment and passivation but yielded unexpectedly low minority carrier lifetimes (<20 μs) preventing analysis. Possible reasons for this include contamination during passivation from an unclean substrate holder or imperfections introduced during chemical treatment. Further experiments with a cleaned chamber could provide different results.

1. Internship Report
13/03/2015
Nanoscale Engineering Master
MEASURING INTERSTITIAL IRON CONCENTRATION USING
PHOTOCONDUCTANCE AND PHOTOLUMINESCENCE MINORITY CARRIER
LIFETIME
Chiras Dimitrios, Fourmond Erwann
CHIRAS Dimitrios
Institut des Nanotechnologies de Lyon (INL), at INSA University
Lyon, France
chirasphysics@yahoo.gr
The concentration of interstitial iron in compensated Si multicrystalline wafers was
studied, using photoconductance and photoluminescence contactless methods. The
wafers underwent chemical treatment and were passivated using PECVD. After
passivation each wafer was measured using PC and PL. Compairing the resulting
PC and PL graphs provides information about the concentration of interstitial
iron. The PC results of each wafer were compared for both the case of unpaired
interstitial iron and FeB pairs. The lifetime values of the wafers were lower than
20 μs, preventing any further research. Reasons behind this unexpected behavior
are discussed.
Keywords: Interstitial iron; Photoconductance; Photoluminescence; Minority carrier
lifetime; Piranha etching; Injection rate.
1. Introduction
Crystalline solids are known to have a
plethora of different properties even amongst
samples of same origins, due to the crystallographic
defects present in their structure. These defects are
interruptions of the regular pattern of the crystal
structure in crystalline solids, altering their
properties. The defects are characterized as
interstitials when the abnormality in the structure
consists of atoms occupying an otherwise
unoccupied site in the crystalline structure or two or
more atoms sharing one or more lattice sites.
In the case where the interstitials consist of
atoms which are not the same asthose in the original
lattice, the interstitials are called impurities.
Iron is an impurity known to cause a
reduction in the wafer minority-carrier diffusion
length, which is associated with both the lifetime of
the carriers as well as their mobility. This can
partially explain the solar-cell performance
degradation; ingots with high concentrations of iron
have beenreported to have a poorer crystallographic
structure [1] in comparison to uncontaminated ones.
Such defects are known to negatively influence the
gettering effectiveness [1] and therefore decrease
the solar-cell performance.

2. However, studies have shown that high
solar cell conversion efficiencies are possible even
in the case of high iron contamination. [2]. The
concentration of interstitial iron can also provide
information about the quality of the starting
material, the effectiveness of bulk hydrogenation
and may also allow to track any process-related
contaminations. Moreover, in compensated
materials such as compensated Si, interstitial iron
can bind with all the boron atoms available on the
material. These Fe-B pairs are responsible for
unexpected behaviors, such as the concentration of
the boron-oxygen defects not scaling with the boron
concentration [3].
A contactless and fast method to measure
the concentration of interstitial iron on compensated
silicon wafer is through combined
photoconductance (PC) and photoluminescence
(PL) measurements. Based on an eddy current
measurement [4,5], signals from both methods are
collected and converted into excess carrier
concentration, Δn.
However, simply performing
measurements on the samples can yield misleading
results which are due to the surfaces not being
smooth. Rough surfaces allow higher
recombination rates that results in very short
lifetimes, yielding results originating seldom from
the surface,while the main interest lies in measuring
the recombination of the whole bulk. Thus, in order
to decrease the recombination of carriers on the
surface, a chemical polish process is applied,
including chemical polish with HF and HNO3,
piranha etching and oxidization with HF just before
the deposition process.
Plasma-enhancedchemical vapor deposited
SiN (PECVD) at 400 𝑜 C was also used, in order to
passivate the surface and achieve measurable
maximum lifetime values.
2. Method
The samples used were cleaved sections of
5x5 𝑐𝑚2 CALISOLAR multicrystalline Si wafers,
co-doped with B-P-Ga, with the doping depending
on the position of the wafer in the same ingot. In
order to smoothen their rough surface, the wafers
underwent chemical treatment. In the following
formula. The process consisted of three steps:
In the first step, HF wasused to remove any
oxide, producing a quite clean and hydrophobic
surface.
The second step consists of piranha cleaning
(a mixture of sulfuric acid H2SO4 and hydrogen
peroxide H2O2) in order to create a homogenous
layer of oxide. During this step, the samples are
securedwith a barover their positions on the handle.
That is necessary due to piranha cleaning’s rather
violent processes,during which gas bubbles emerge
through the boiling surface of the mixture. The bar
secures that the thin and fragile samples will not be
carried away by the gas bubbles. The samples, after
the piranha etching, are very hydrophobic and also
require great caution when handled, because they
might stick with each other.
The last step is the most important one,
during which HF was used to remove the layer we
created,along with any impurities. This step has to
be performed as soon as possible, before the
PECVD, so that the surface is smooth.
Following the chemical treatment is the
plasma-enhanced chemical-vapor deposited
(PECVD) SiN at 400 𝑜 C. The goal is to effectively
passivate the surface so that the lifetime of the
carrier is increased. During this process, a pump
creates a vacuum within the chamber. The
temperature of the chamber is then increased until it
reachesthe desired level. PECVDis used as a means
of depositing thin films from a gas state (vapor) to a
solid state in a substrate.

3. This process includes chemical reactions,
which take place after plasma is created from the
reacting gases. The plasma is created by
radiofrequency (AC) or DC discharge between two
electrodes, the space between which is filled with
the reacting gases. Plasma-deposited silicon nitride
can be formed either from silane and ammonia or
nitrogen [6].
A large amount of hydrogen can be found,
which can be bonded to silicon (Si-H) or nitrogen
(Si-NH). Infrared and ultraviolet absorption as well
as stability, mechanical stress and electrical
conductivity are heavily influenced by this
hydrogen.
After the aforementioned processes, the
samples are taken to a Sinton Instruments WCT-120
system, in order to perform the PC and PL
measurements.
Since these two forms of interstitial iron
(isolated or paired) have different recombination
properties (i.e. energy levels and capture cross
sections), they give rise to different carrier lifetimes.
The relationship between the recombination
properties and the carrier lifetime is as follows [7,8]
[ 𝑭𝒆𝒊] = 𝑪(
𝟏
𝝉 𝒍𝒊𝒈𝒉𝒕
−
𝟏
𝝉 𝒅𝒂𝒓𝒌
)
Here, 𝜏𝑙𝑖𝑔ℎ𝑡 a𝑛𝑑 𝜏 𝑑𝑎𝑟𝑘 are the carrier lifetimes for
which 𝐹𝑒𝑖 and FeB dominate, respectively.
It is known that 𝐹𝑒𝑖 under strong light is
present in its isolated form. However, leaving the
samples in the dark for a long period allows the FeB
pairs to re-form. The recombination parameters for
𝐹𝑒𝑖 and FeB, as well as the dopant concentration,
the injection level, and the temperature affect
directly the constant C which is different for p-type
silicon materials using dopants other than boron.
That means that the lifetime on a sample is
measured twice – firstly with the 𝐹𝑒𝑖 in isolated
form (after exposure to light), and again with it
paired with the acceptor atoms (after it spent
sufficient time for the 𝐹𝑒𝑖 to recombine with B in
the dark).
The figure 2.1 shows a typical pair of
lifetime measurements on a multicrystalline silicon
sample, before and after FeB pair dissociation. The
point where the curves cross over each other is
characteristic of such measurements involving FeB
pairs. Identifying correctly the crossoverpoint helps
validate that the lifetime changes observed are
indeed caused by FeB pair dissociation.
Figure 2.1: Lifetime measurements before and
after illumination
Each sample is initially exposed to
homogenous light ray with an intensity of 1 sun for
an amount of time sufficient to disassociate any FeB
pairs. Then PC and PL measurements are taken. It
is important to be noted that in order to guarantee
that the interstitial iron is not found in FeB pairs,
eachsample was exposed to light in betweenPCand
PL measurements.
Following the PC and PL measurements,
eachsample is setin the dark for a sufficient amount
of time in order to make sure that Fe has formed
again pairs with B and cannot be found in its free
form. Then the PC and PL measurements are
repeated.
PC and PL signals are collected at the same
region under identical illumination by a
photographic flash. Each signal is then converted
into excess carrier concentration, Δn.

4. The PC signal is converted through the
relationship connecting conductivity (σ) and Δnpc.
[9]:
𝜟𝒏 𝒑𝒄 = 𝝈/𝒒𝑾𝝁 𝒕
in which q is the elementary electron charge,
W corresponds to the sample thickness, and 𝜇 𝑡
corresponds to the sum of electron and hole
mobilities.
PL signal is converted into Δnpl using:
𝑷𝑳 = 𝑨𝒊 𝑩𝜟𝒏 𝒑𝒍(𝜟𝒏 𝒑𝒍 + 𝑵 𝑨
𝑫⁄ )
with Ai representing the scaling factor while
considering that in general the PL signal is
measured only in relative units [9] and B
representing the radiative recombination
coefficient. This value is directly connected to the
total carrier density within the wafer.
3. Results and discussion
A typical EXCEL spreadsheet displaying
all the data acquired from PC and PL measurements
can be seen in FIGURE 3.1 and 3.2 respectively.
Figure 3.1:Results measuring a wafer using
PC mode
Figure 3.2: Results of measuring a
wafer using PL mode
On a generalized mode photoconductance
measurement, the sample name as well as the
optical constant and the measured resistivity can be
found under their respective labels. The actualvalue
of resistivity that is taken into account for the
creation of the various graphs, must be modified by
the user. So, in order to acquire better fitting in the
resulting graphs, we adjust the said value according
to the one measured for each sample. The lower
right graph displays the relationship between the
lifetime of the minority carriers (without the Auger
electron correction) and its density.
3 different batches of 6 samples each were
processed throughout this work. The first two
batches were chemically treated and passivated,
then measured accordingly. The third batch was
chemically treated but due to time restrictions did
not reach the passivation stage. In each batch there
existed 5 regular samples and 1 used as a reference,
lacking doping. In the second batch, a sample from
a different origin was used.
In PC mode, there are four different modes of
taking measurements available to the user; QSS,
Transient and Generalized 1/1 and 1/64. The QSS
mode requires knowing the optical constant of the
sample. In Transient mode the first measurement
should be taken after the flash has stopped and all
the carriers have been created.

5. That is why an option to adjust the zoom of
the flash is available, allowing the user to move the
point of the first measurement further to the right.
This method has the advantage that does not require
preexisting knowledge of the amount of light, the
reflectivity of the wafer or the absorption in it,
because in the case of Transient mode the first
measurement is taken after the flash has ended.
However, if the lifetime of the carriers is too small
(less than about 100μs, the transient mode is
ineffective, mainly due to the fact that the flash
shines for a very small time period.
Both of the aforementioned modes though
are based on approximations on the formula
𝝉 =
𝜟𝒏
𝑮 𝒅𝜟𝒏
𝒅𝒕⁄
considering either G (rate of generation of electron-
hole pairs) in the case of Transient or dΔn/dt in the
case of QSS as negligible. However, this mode
requires knowledge of the optical constant.
Each sample was measured in both
Generalized 1/1 and Transient mode during PC
measurements. The results for all samples apart
from the reference show that the lifetime of the
carriers was too small (less than 20μs in all cases
and in some samples less than 10μs) to yield any
actualdata. The flash outlived the carriers in almost
all cases,and in the rare cases (2 samples) that the
carriers’ lifetime was borderline traceable (at
around 45μs), it was not possible to conduct
measurements in Transient mode.
In PL measurements, the transient mode
can determine the injection level dependent
effective excess carrier lifetime 𝜏 𝑒𝑓𝑓 𝛥( 𝑛). PL is a
sensitive lifetime technique with measurements not
significantly affected by excess carriers
accumulated in space charge regions in contrast to
its counterpart, PC. However,with lifetimes smaller
than 20μs in all cases,the results of the PL method
were also unavailable to yield proper graphs.
Not being able to get the desired graphs in
at leastone type of measurementin eachcase (either
PCor PL),there wasno feasible way to compare the
two methods’ graphs and track the crossover point,
at least not in a region free of noise. One possible
cause for the small carrier lifetimes could be a
complication during the chemical treatment. All the
processes included (chemical polishing, piranha
cleaning, oxidation) were required to be performed
timely and homogenously for each batch so that
there are no abnormalities on the surface. Also,
PECVD has to start immediately after the oxidation
step of the chemical treatment, meaning that the
morphology of the surface could have changed in
the time it takes for the plasma chamber to reach the
desired state (temperature, pressure).
Another possible cause is the substrate holder
in the plasma chamber was not properly cleaned.
This could have been the cause of contaminations
during the deposition which could have led in a
decrease in the passivation of the layer, eventually
resulting in a decrease in the lifetime of the carriers.
Samples of different origin were also
measured in order to verify whether the issue lied
with the samples of the specific ingot. The results
ruled out this possible explanation, as the
aforementioned samples showed the expected
behavior.
4. Summary and conclusion
CALISOLAR multicrystalline Si wafer
samples (co-doped with B-P-Ga) were chemically
treated in order to acquire a smooth surface,
underwent PECVD to further passivate said surface
and had the lifetime of the minority carriersmeasured
via PC and PL. The goal was to compare the results
of each method for every sample and acquire
information about the concentration of interstitial iron
in the bulk of the wafer. However,the lifetime of the
carriers was so small in all cases, that the software
was unable to plot graphs, especially in PC mode.

6. This abnormal behavior can be caused by a
plethora of reasons; the passivation process might
have been contaminated due to the substrate holder
not being clean from previous depositions; or a not
properly timed step during the chemical treatment
of the waferscould have resulted in faulty polishing,
thus significantly reducing the lifetime of the
carriers.
Also, the process of piranha is quite violent,
with the whole mixture boiling when the samples
are submerged into it, possibly causing
unhomogenous polishing. The latest batch
underwent the chemical treatment, however did not
get passivated through PECVD due to lack of time.
At the time the whole chamber has been thoroughly
cleaned, thus possible future experiments could
yield different results.
5. Acknowledgements
This research is supported by the Universities of
Claude Bernard Lyon 1, École Centrale de Lyon,
INSA and Institut des Nanotechnologies de Lyon.
6. References
1. Pizzini S. Advanced Silicon Materials
for Photovoltaic Applications. Hoboken, NJ: John Wiley
& Sons, 2012. Print.
2. D. H. Macdonald, L. J. Geerligs and A.
Azzizi, “ Iron detection in crystalline silicon by carrier
lifetime measurements for arbitrary injection and doping
”.J. Appl. Phys. 95 , (2004) pp. 1021-1028
3. Macdonald, Daniel, and An Liu.
"Recombination Activity of Iron-boron Pairs in
Compensated P-type Silicon." Physica Status Solidi (b)
247.9 (2010): 2218-221.
4. Stevenson, Donald T., and Robert J.
Keyes. "Measurement of Carrier Lifetimes in
Germanium and Silicon." Journal of Applied Physics
26.2 (1955): 190.
5. Sinton RA, Cuevas A, Stuckings M.
Quasi-steady-state photoconductance, a new method for
solar cell material and device characterization. 25th
IEEE
Photovoltaic Specialists Conference, 1996: 457-460.
6. Pecora, A., L. Maiolo, G. Fortunato,and
C. Caligiore. "A Comparative Analysis of Silicon
Dioxide Films Deposited by ECR-PECVD, TEOS-
PECVD and Vapox-APCVD." Journal of Non-
Crystalline Solids 352.9-20 (2006): 1430-433.
7. Z. Hameiri, T. Trupke, N. Gao, R. A.
Sinton, and J. W. Weber, “Effective bulk doping
concentration of diffused and undiffused silicon wafers
obtained from combined photoconductance and
photoluminescence measurements,” Prog. Photovoltaics
Res. Appl., pp. 942–949, Mar. 2012.
8. G. Zoth and W. Bergholz, J. Appl.
Phys. 67, 1990, pp. 6764-6771
9. Schroder, Dieter K., and Lawrence G.
Rubin. "Semiconductor Material and Device
Characterization." New Jersey: Willey-IEEE Press, 2006
10. Giesecke, J. A., M. C. Schubert, D.
Walter, and W. Warta. "Minority Carrier Lifetime in
Silicon Wafers from Quasi-steady-state
Photoluminescence." Applied Physics Letters 97.9
(2010): 092109.
11. D. Macdonald, T. Roth, P. N. K.
Deenapanray, T. Trupke and R. A. Bardos, Appl. Phys.
Lett. 89, 2006, pp. 142107.