Solid State Phenomena Vols. 205-206 (2014) pp 118-127
© (2014) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/SSP.205-206.118
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Photoluminescence imaging techniques have recently been extended to silicon bricks for
early production quality control and electronic characterisation in photovoltaics and
microelectronics. This contribution reviews the state of the art of this new method which is
fundamentally based on spectral luminescence analyses. We present highly resolved bulk lifetime
images that can be rapidly extracted from the side faces of directionally solidified or Czochralski
grown silicon bricks. It is discussed how detailed physical modelling and experimental verification
give good confidence of the best practice measurement errors. It is also demonstrated that bulk
lifetime imaging can further be used for doping and interstitial iron concentration imaging.
Additionally, we show that full spectrum measurements allow verification of the luminescence
modelling and are, when fitted to the theory, another accurate method of extracting the absolute
bulk lifetime.
Crystalline Si (c-Si) is and has been the work horse of the photovoltaic (PV) industry for many
years. In 2012, almost 90% of the fabricated solar cells worldwide were made from crystalline
silicon [1]. The crystalline silicon technology has been able to reduce the production costs
dramatically while slowly but constantly improving the efficiency of the devices. Thus, it remained
the most competitive PV technology with state of the art module conversion efficiencies of 16-22%.
The device structure for the most common industrial solar cells has remained largely the same and
is based on a structure first published in the 1970s. The industry tends to behave conservatively by
only advancing the technology incrementally to ensure the product quality is maintained at the
highest level to fulfil warranty requirements of 25 years or more.
A large proportion of the cost reduction that has been achieved is due to improved Si purification
and crystallization techniques. It seems that, in particular, increased throughputs and energy
efficiency measures have allowed this to occur [2]. Seeded and grain size controlled crystal growths
allow higher efficiencies to be achieved on directionally solidified multicrystalline Si (mc-Si). Ingot
sizes of typically 450 kg and in tests up to 1000 kg are currently used leading to large ingots that are
cut into 6x6 bricks, each six-inch squared in area and 25-26 cm in height, prior to wafer slicing.
High efficiency cell concepts can be expected to gain a larger market share in the midterm future
due to the rising proportion of the balance of system (BOS) cost in PV systems. Device
architectures beyond 20% efficiency are likely to be based on monocrystalline wafers and n-doped
bulk material is expected to gain market share. Oxygen complexes and interstitial iron do not limit
the performance of phosphorus doped silicon to the same extent as they do in boron doped
Czochralski-grown material [3,4]. However, the lower segregation coefficient of phosphorous
Solid State Phenomena Vols. 205-206
119
typically yields greater resistivity variation throughout the ingot, thus increases the need for
electrical characterisation and cell architectures that are more tolerant to variations in bulk doping.
It becomes evident that quality electrical characterisation of photovoltaic silicon bricks after
crystallization is of high and rising importance as it can increase not only the productivity of the
directional solidification but also permits selective cutting and specialised processing. In an
industrial environment, the desired characterisation method would allow non-destructive
measurements (i) with high spatial resolution, (ii) at inline speed, (iii) with high bulk lifetime
sensitivity, and (iv) with reliable accuracy.
A number of techniques have been used for the electrical characterisation of silicon bricks prior
to wafer slicing, such as microwave detected photoconductance decay (µ-PCD) [5,6] and
inductively coupled quasi-steady-state photoconductance (QSSPC) [7,8]. More recently steady state
PCD measurements were introduced, enabled by improved laser technology [9,10]. All of these
techniques are based on photoconductance measurement principles and generate spatially resolved
data by raster scanning the brick surface.
With the introduction of luminescence imaging [11,12] a purely optical technique is now
available for spatially resolved electrical characterisation. Numerous qualitative and quantitative
methods based on photo- and electroluminescence imaging have recently been developed. The
initial measurement principle was a simple qualitative steady-state measurement, but later also
spectral [13–16], time dependent [17,18] and polarization [19] dependent measurements were
introduced, all of which are adaptations of the basic PL imaging principle and setup [20]. In the last
few years the use of photoluminescence (PL) imaging has been further extended to allow
quantitative electrical characterisation of silicon bricks. This work will be reviewed in this article.
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In 2009, the application of PL imaging on silicon brick side faces was explored firstly by Trupke et
al. [21]. It was shown that PL images can be measured on the side faces of cast boron doped mc-Si
bricks with high spatial resolution and short measurement times. Fig. 1a illustrates such a raw PL
image measured with a resolution of 1.5 megapixel and a 10 s exposure time.
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Fig. 1: (a) Qualitative raw PL image of a directionally solidified mc-Si brick. The image contrast is
given in digital count rates. (b) Calculated relative PL intensity as a function of bulk lifetime for
bare silicon brick at 880 nm monochromatic illumination
120
Gettering and Defect Engineering in Semiconductor Technology XV
The luminescence emitted by the brick is determined by the spontaneous emission inside the
sample resulting from the radiative recombination of excess minority electrons and holes. PL
images are typically taken at a photon flux equivalent to one sun (0.1 Wcm-1), and thus provide the
electronic properties that are representative for the operation conditions of typical industrial solar
cells. The minority carrier densities of a bare silicon brick remain in low injection (∆n«N) during a
steady-state one sun illumination. For this case, the spontaneous emission is defined simply by the
product of the excess minority carrier density ∆n and the background doping density N, i.e.
rsp ∝ ∆nN. The image contrast of Fig. 1 therefore represents the convoluted effect of variations in
both effective minority carrier lifetime and background doping.
The excess carrier density on the illuminated side of the silicon brick, in difference to a standard
silicon wafer, is not influenced by the rear surface. This enables an analytical solution of the carrier
density with dependence on depth, diffusion length and front surface recombination velocity,
following Bowden and Sinton [22]. Since the diffusion length of both directionally solidified and
Czochralski grown ingots are small compared to the sample thickness there is no saturation of the
measured effective lifetime with increasing bulk lifetime. Fig. 1b plots the PL intensity as a
function of bulk lifetime on double logarithmic scale using the analytical solution of the excess
carrier density with assuming a constant doping density, a Si CCD detector camera and no
additional filtering. The graph reveals that the bulk lifetime contrast in the PL signal decreases
towards higher lifetimes, but does not saturate.
A calibration constant and the local doping need to be measured separately to extract absolute
bulk lifetime information from a single PL image, such as the one shown in Fig. 1a. This can for
example be realised via an additional QSSPC measurement as specified in [21] and [15].
The analysis of just the luminescence intensity from a single image, as discussed above, is
somewhat limited in terms of converting the PL intensity into quantitative information.
Fundamentally this is related to difficulties in accurately measuring absolute photon fluxes in PL
measurements. However, quantitative data extraction is facilitated if additional information is
available, e.g. spectral information or time dependent data.
In the last few years the use of spectral luminescence intensity ratio imaging was initially
introduced to cell [13] and wafer measurements [14] and has more recently been extended to silicon
bricks. These are well suited samples for spectral photoluminescence analyses, given that
measurements can be performed on polished surfaces. This allows for a one-dimensional analytical
analysis. Thus, it has been possible to demonstrate the spectral photoluminescence intensity ratio
(PLIR) analysis as an independent quantitative measure for minority carrier bulk lifetimes on bare
bricks [15,23].
A spectral PLIR analysis eliminates the need for absolute luminescence intensities to be known,
since pure spectral variations define the bulk lifetime signature of the luminescence response.
Spectral changes occur due to luminescence reabsorption on the optical path from the origin to the
surface of the brick. Given that the rate of spontaneous emission rsp at low injection is defined by
the excess carrier density ∆n(x,τb), we calculate the bulk lifetime (τb) dependent PL intensity
following
d λ
PL(τ b ) ∝ ∫ ∫ rsp ,0 (λ )∆n( x, τ b )e −α (λ ) x Θ(λ )dλdx
(1)
0 λ0
where rsp,0 is the spontaneous emission rate at equilibrium, θ(λ) the spectral sensitivity of the
experimental apparatus and α(λ) the absorption coefficient of crystalline silicon. The analytical
solution of this integral is given by Green [24].
A simple rating of the spectral dependence can be realised by a two-filter approach in which two
measurements of the luminescence signal are performed with different spectral filters. It is crucial
in this context to choose the band pass filters in such a way that the impact of bulk lifetime on the
Solid State Phenomena Vols. 205-206
121
PL intensity ratio is maximised (see Fig. 2). Furthermore, experimental conditions need to be
selected to most optimse for a maximal bulk lifetime signal. One-dimensional modelling of the
luminescence response of the specific experimental setup allows for an optimisation and definition
of a PLIR to bulk lifetime transfer function as illustrated in Fig. 2b. Note that the background
doping does not affect the spectral dependence of the luminescence response and consequently
cancels out in the PL ratio used for the bulk lifetime assessment.
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Fig. 2: (a) Dependence of the short (1000 nm, blue) and long (>1050 nm, red) pass filtered PL
intensities on bulk lifetime shown as a function of wavelength; (b) Calculated PL intensity ratio to
bulk lifetime transfer function for different camera and filter combinations.
Silicon CCD camera are widely used for PL imaging. Their strengths include extremely low read
and dark noise [23]. The relatively low price of scientific CCD chips and cameras allows for
industrial applications of PL (and EL) imaging.
The use of silicon CCDs for PLIR measurements needs to be considered with care since the low
quantum efficiency in the wavelength range of the measured luminescence emission causes a
spreading of luminescence light within the CCD, thus resulting in significant errors if not corrected
[23,25]. Two strategies were successfully demonstrated to largely overcome this limitation. One
consists of image deconvolution using an experimentally determined point spread function, which is
measured specifically for the experimental set up and separately for each spectral filter. The other
approach is to employ an InGaAs camera, which does not suffer of light spreading and also
improves the absolute accuracy for medium to high bulk lifetimes (see Fig. 2b) due to its fairly
constant quantum efficiency throughout the Si luminescence spectrum. However, the InGaAs
camera cannot deliver the same pixel resolution and accuracy for low (1-100 µs) bulk lifetimes with
currently available camera technology.
Fig. 3 presents PLIR detected bulk lifetime images using both camera types in direct
comparison. The signal to noise ratio (SNR) of the Si CCD detected images is superior. However
the InGaAs camera resolves smaller grain structures with better contrast. In the InGaAs image the
bulk lifetime contrast is almost entirely limited by lateral minority carrier diffusion into the grain
boundaries and dislocations and not by optical image blur, which dominates the Si CCD detected
image [26].
The uncertainty of the PLIR detected bulk lifetime has been estimated to be between 20-30%
relative, when measured with the current Si CCD based technology, a laser illumination at 880 nm
and using a deconvolution correction [23]. In principle, an InGaAs camera provides better accuracy,
the relatively high dark and read noise of current InGaAs cameras limits the applicability in
practice. It has been found necessary to apply a short pass filter with a band pass at ≥ 1075 nm to
achieve sufficient SNR in the short pass filtered image. Thus, only limited low bulk lifetime
sensitivity can be achieved since a wide gap between the short and long pass filtered images
122
Gettering and Defect Engineering in Semiconductor Technology XV
maximises the bulk lifetime sensitivity of the two filter method. Nonetheless, we believe that bulk
lifetimes exceeding 100 µs can be detected with an uncertainty of below 15% [23]. A round robin
experiment comparing PLIR with other measurement techniques such as QSSPC, MDP and
RCPCD [27] could provide more information about systematic errors of brick measurement
techniques.
(a)
(b)
Fig. 3: PLIR detected bulk lifetime image of a directionally solidified mc-Si brick employing (a) a
Si CCD camera with subsequent deconvolution and (b) an InGaAs camera. The scale gives bulk
lifetimes values in µs. The very top and bottom of the brick, which appears white in (b) could not be
evaluated due to a too low SNR of the InGaAs camera in this region.
A single PL image contains both bulk lifetime and doping information. A doping image can also be
extracted with the calculation of the PLIR bulk lifetime image (Fig. 3) since the bulk lifetime can be
used to calculate a normalised PL image, PLnorm. The latter represents the PL image that would be
expected for constant background doping concentration. A simple ratio of this PLnorm image and the
measured PL image thus provides a background doping density image. An example is shown in Fig.
4. A single calibration point is sufficient to transfer the resulting relative data into an absolute
doping image. This can be achieved for example by an eddy current point measurement.
Solid State Phenomena Vols. 205-206
123
Fig. 4: Relative doping image of the same directionally solidified mc-Si brick derived from PLIR
detected bulk lifetime image and a short pass filtered image. Influence of the surface polish is
visible, which indicates a current measurement artefact. Preliminary data is shown and uncertainties
prevail in the top and bottom due to residual impact of image blur.
A recent extension to the analysis has enabled the extraction of interstitial iron concentration images
by employing the reversibility between the paired iron-boron and the dissociated interstitial state of
iron in boron doped silicon [28–30]. We show preliminary quantitative data for PL based images of
the Fei concentration (see Fig. 5) that might extend the application of the PLIR and which are
expected to be relevant for predictive models that correlate material properties with final device
performance [31].
Fig. 5 shows that central areas of the brick contain the lowest interstitial iron concentration,
which is known to be one of the most detrimental impurities in silicon solar cells. A slowly
increasing Fei concentration level towards the top and sharp edges towards the bottom and top are
observed. This information is obtained from a procedure that takes less than 5 min and includes the
use of a strong xenon flash lamp for dissociation of the FeB pairs. A complete analysis and detailed
description of PL based iron imaging of silicon bricks will be presented in an upcoming publication.
124
Gettering and Defect Engineering in Semiconductor Technology XV
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Fig. 5: (a) PLIR detected interstitial iron concentration image of a directional solidified mc-Si brick
shown as log([Fei]). (b) Corresponding cross-sectional average concentration as a function of brick
height. No reliable data could be extracted in the bottom 10% and top 5% of the brick. The central
dashed curve is given as a guide to the eye. Note that values are absolute but as yet not calibrated.
The analysis of the full PL spectrum can add to the PLIR analysis and extend its capabilities, while
providing a different seemingly more direct access to bulk lifetime information.
We have been able to experimentally verify the physical modelling of the luminescence emission
on silicon bricks [32]. The theoretical description of luminescence emission from an indirect
semiconductor such as silicon was provided by Würfel et al. [33]. Spectral PL data on silicon
samples have been used in a number of applications, for example as a sensitive measure to
determine the silicon absorption coefficient at very low values and for different types of doping
concentrations as shown by Daub et al. [34,35].
Green demonstrated that the full PL emission of a monochromatically illuminated silicon brick
can be described analytically [24]. We have been able to show that PL spectra measured on silicon
bricks can be fitted by the formula and bulk lifetime information can be extracted, if measured with
a calibrated detector [32]. Fig. 6 displays the measured spectra of four measurement spots taken on
a directionally solidified (spot 4 and 5) and on a Cz-grown brick (spot 2 and 3) and their best curve
fits according to the expression given by Green. We show that absolute bulk lifetime can be
assessed with a statistical error of less than 14% with a confidence of 99% (11% with confidence of
95%) using the full spectral PL description by Green (see Fig. 6b). Note that the doping
concentration does not have to be known for this procedure. However, the fitting procedure is found
to be sensitive to the source of absorption coefficient data (see [32]).
Solid State Phenomena Vols. 205-206
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Fig. 6: (a) Non-linear full spectrum fits of experimental spectra covering a wide lifetime range, with
theoretical local bulk lifetime values listed; (b) Sum of the least square residuals for spot 3 with
minimum and 95 and 99% confidence intervals
A PL imaging based calibrated lifetime measurement for silicon bricks has been developed that is
based on the PL intensity ratio method. It can be applied to silicon bricks directly after solidification
and polishing, i.e. prior to wafer slicing and enables important early stage electronic
characterisation of silicon bricks. It combines the strengths of photoluminescence imaging,
specifically the short measurement times and high spatial resolution with excellent bulk lifetime
sensitivity and accuracy. Proof of concept data for Fe images were shown in this paper.
This research has been supported by the Australian Government through the Australian Renewable
Energy Agency (ARENA). The authors thank J. Nyhus from Norner AS for the supply of silicon
bricks.
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