Contrary to standard silicon feedstock purified via the (more or less) standardized Sie-mens process, the modern refinement routes for upgraded metallurgical grade silicon yield material of highly differing specifications. The UMG-Si properties depend strongly on the raw silica and the purification recipe, which are mostly proprietary processes devel-oped in parallel by many silicon producers (section 2.1.2).
Since the silicon refining in the UMG-Si route is much less elaborate than the Siemens process, the impurity concentration can exceed standard silicon contamination by several orders of magnitude. Two groups of impurities are critical: transition metals, decreasing the minority carrier lifetime, and dopants, because industrial standard processes are in general optimized for a relatively narrow range of base resistivities.
Due to the silicon shortage around the year 2006, the PV industry as well as research institutes around the world have put a lot of effort into the determination of UMG-Si re-lated material and solar cell properties.
A large number of publications have covered the solar cell performance mainly of mul-ticrystalline UMG-Si, the influence of adapted processes – mainly the high-temperature fabrication steps like P-gettering – and UMG-Si related issues like the carrier mobility.
The mc-Si solar cell efficiencies of UMG material reported by various research institutes using industrial-like processing range between 15 - 16%, with best solar cells around 16.3%, which was only about 0.2-0.3% abs. below the reference values for this solar cell technology [41-44]. In a recent study by Junge et al., high-efficiency processing even yielded a performance of 18.4% on the best solar cell made from UMG mc-silicon pro-vided by CaliSolar [45], also being close to the standard reference value of the respective solar cell process. While these results were obtained in small-scale processing, Hoffmann et al. presented a long-term comparison between standard and UMG mc-silicon solar cells (Elkem feedstock) processed in an industrial facility in mass-production with a sta-tistical basis of in total 150,000 solar cells in 2008 [46]. He reported UMG-Si solar cell efficiencies of 15.4%, only about 0.1% abs. lower than the reference. This performance could be raised by 0.4% abs. to an average value of 15.8% in 2010 by optimizing the process in the same industrial process line, with best UMG-Si solar cells having an effi-ciency of 16.4% [47].
Note that the wafers used in these studies had been fabricated from feedstock resulting from different purification techniques of various silicon feedstock producers. Details on the differences between these materials and standard silicon wafers are not known. How-ever, it is evident that some examples for UMG silicon already compete with traditional Siemens-process based refinements on the solar cell performance level.
Besides the cell performance, the research has focused on the two critical UMG-material issues, namely the impact of transition metals and their behavior during high tempera-ture processing steps and the influence of the high dopant concentrations on the solar cell properties.
It was propagated mainly by Buonassisi and co-workers that metal-rich silicon wafers could benefit from “internal gettering” techniques, thus reducing the total interface be-tween silicon and metal atoms, which would minimize the recombination activity [6].
They suggested that internal gettering could be enhanced by the presence of e.g. Cu be-ing known to readily form co-precipitates with other transition metals [48]. Some impli-cations of this approach will be the topic of the next chapter 5.
The second obvious issue is the relatively high concentration of both dopants, boron and phosphorus. Since boron atoms act as acceptors while phosphorus atoms are donors, one dopant type compensates partly for the electrical influence of the other on the resulting free carrier density. This has consequences for the base resistivity distribution [49], the Cz-related boron-oxygen defect which causes light-induced degradation (LID) [50-52]
and for the charge carrier mobility [53].
This chapter summarizes the work which has been performed within this thesis to deter-mine the influences of high dopant concentrations on the silicon wafer properties. These studies have been done in collaboration with Juliane Geilker in the frame of her diploma thesis [54], which was supervised by the author of this thesis. In addition, several meas-urement results by Florian Schindler have been used, whose diploma thesis is currently being supervised by the author.
Cz-grown as well as block-cast upgraded metallurgical grade silicon of three different producers have been investigated. The first Cz-crystal is made of 100% UMG-Si feed-stock produced around the year 2008, called “UMG Cz 1”. For comparison, a Cz crystal containing a blend of 50% UMG-Si feedstock with virgin grade feedstock (“UMG Cz 2”) as well as a 0% reference were pulled, “UMG Cz 3”. In order to be able to distinguish the influence of dopant compensation on the properties of UMG-Si from all the other possible material-related impacts (e.g. background contamination), in the frame of the project SolarFocus, two Cz-crystals were grown from virgin grade silicon. Two different concen-trations of boron and phosphorus were added to the melt, thus intentionally compensat-ing the dopant influences: “Comp Cz 1” was weakly compensated by addcompensat-ing [B]=[P]=3x1016 at/cm3; “Comp Cz 2” was heavily compensated by doping with [B]=6x1016 at/cm3 and [P]=9x1016 at/cm3.
The investigated multicrystalline UMG silicon blocks were both cast from commercially available UMG-Si feedstock of different origin. One, named “UMG mc 1”, was crystallized in the experimental block-casting facility IS30 at SolarWorld in the frame of the joint pro-ject SolarFocus in the year 2009. For comparison, in the same facility a reference block –
“Ref mc 1” - was cast from high-purity feedstock. The second UMG-Si containing mc-Si ingot “UMG mc 2” was cast in the frame of the project ALBA2 also in the year 2009.
This chapter starts with the determination of the transition metal and the dopant concen-trations and focuses on the carrier mobility reduction due to the increased number of scattering centers in UMG-Si. In the second section, the limitations of both material classes are assessed, which covers in particular the boron-related defect in Cz-crystals made from compensated silicon feedstock. At last, the solar cell parameters of exemplary process runs of both material classes are compared to standard feedstock silicon.
4.1 Transition metal concentration
Although already developed 60 years ago, Neutron Activation Analysis (NAA) [55] re-mains one of the most sensitive techniques for the detection of trace impurities in silicon until now. It is based on the artificial activation of radioactive isotopes of most elements.
For this technique, a small sample – small pieces of silicon or crunched wafers – is placed in the range of a low energy neutron source (thermal neutrons), provided for example by nuclear reactors. When the irradiated neutrons collide with the sample atoms, instable radioactive isotopes form, which decay in average after the element-characteristic half-life in the order of minutes to several hours. In decomposing, the elements emit X-rays of characteristic wavelengths which are detected in usual scintillation detectors and used for the determination of the type and concentration of each element. The detection limit depends mainly on the sample volume and the radiative (wavelength-dependent) back-ground and is in the order of 1x1010 – 1x1012 at/cm3 for most transition metals in silicon.
Note that the NAA technique measures the total impurity content; it is not capable of distinguishing between the dissolved atoms, which are more harmful due to their large effective surface, and the precipitated fractions of the impurity content.
The ingot height-dependent concentration of the transition metals Cu, Fe, Ni, Cr and Co in the Cz-grown (“UMG Cz 1”) and block-cast mc (“UMG mc 1”) upgraded metallurgical grade silicon crystals are displayed in Figure 4.1. For comparison, the detected metal content of the reference mc-Si block is also plotted2. Besides the five transition metals, only spurious amounts of Au (in the order of 1x109 at/cm3) were measured. In addition, the UMG-silicon crystals contain a higher concentration of Ge, which is in the same ele-mental group as Si, and As, which acts as a donor. However, both impurity densities are not expected to affect the UMG silicon properties.
The NAA reveals that the UMG Cz-crystal mainly contains Cu (~1x1012-1x1013 at/cm3), Cr (~1x1012-1x1013 at/cm3) and Co (~1x1011-1x1012 at/cm3). Fe and Ni are not detected.
These values lie in the typical range of Cz-grown crystals made from standard silicon feedstock and generally below the metal profiles of both mc-Si blocks.
Both multicrystalline blocks contain a concentration of around 1x1013-1x1015 at/cm3 of Fe. The Cu contamination of both blocks is also comparable (~1x1013-5x1014 at/cm3) as is the Co content (~1x1011-1x1012 at/cm3). While the reference block holds in addition a low concentration of Cr (~1x1011 at/cm3), the UMG-Si block is contaminated with a rela-tively high concentration of Ni (~1x1014-1x1015 at/cm3).
The first conclusion is that the transition metal content of silicon wafers made from up-graded metallurgical grade feedstock material does not differ significantly from current standard mc-Si wafers, keeping in mind that variations due to different production condi-tions exist for every feedstock material. Referring to literature, a compilation of mc-Si impurity contents of different silicon wafer suppliers are given for example by Istratov et al. [56] and Macdonald et al. [26]. According to these publications, standard mc-Si wa-fers often contain Fe, Cr and Ni concentrations in the order of 1x1014-1x1015 at/cm3, re-sembling the results obtained on our UMG-Si material.
2 All NAA measurements were performed at Elemental Analysis, Inc. (EAI), USA, commissioned by I. Reis.
Figure 4.1: Transition metal concentration versus the ingot position (1 = launch of crys-tallization, 5 = end of crystallization) of Cz-grown (left) [57] and block-cast (middle) UMG silicon, compared to the concentration measured on a block-cast reference ingot (right)2.
4.2 Dopant concentration
The high concentration of both dopant types, boron and phosphorus, has implications on the base resistivity distribution, on the net doping concentration and the charge carrier mobility. The latter decreases the carrier diffusion length of holes and electrons, which affects the solar cell parameters because the pn-junction collects less charge carriers.
In standard, uncompensated p-type material, the resistivity ρ correlates directly with the acceptor concentration via
( )
NA = qµC( )
N1A NAρ (4-1),
where µC(NA) stands for the (known) doping-dependent majority carrier mobility. The determination of many other measurands like the minority carrier lifetime is based on the three variables ρ, µC(NA) and NA. However, in compensated material these derivations are more complex. The interdependencies are visualized in Figure 4.2.
Instead of the acceptor concentration, the electrically active fraction of dopants, the net doping concentration p0, has to be used:
D
A N
N
p0 = − (4-2).
Although the electrically active fraction is lower than NA and ND, both dopants take effect as scattering centers for free holes and electrons. Hence, both the majority and the mi-nority carrier mobility are lower than in uncompensated material of same net doping.
Therefore, the mobility model which is used for the evaluation of µC(NA) in equation (4-1) is not valid any more in compensated material and cannot be used for the determination of the net doping concentration. A different majority carrier mobility model has to be im-plemented, depending on NA and ND [50]:
Due to the lack of data on compensated silicon material in literature, the carrier mobili-ties of compensated Si wafers were investigated and compared to existing mobility mod-els in order to assess their quality.
To do this, measurements of the base resistivity and at least two of the three meas-urands p0, NA and ND are necessary:
A) The retrieval of the base resistivity can be done e.g. by the four-point-probe tech-nique or via inductive sheet resistance measurement methods [58].
B) The net doping concentration can be measured directly via the CV-method or with the help of a technique which has been newly developed in the frame of this thesis in col-laboration with S. Rein and J. Geilker [54, 57]. The so-called FCA-FTIR method is ex-plained in detail in section 4.2.1.
C) For the detection of NA or ND, the limits of existing techniques such as NAA, induc-tively coupled plasma mass spectrometry (ICP-MS), ICP optical emission spectros-copy (ICP-OES), ICP atomic absorption spectrosspectros-copy (ICP-AAS), glow-discharge mass spectrometry (GDMS) and secondary ion mass spectrometry (SIMS) are often in the order of 1016 at/cm3, which is also the expected dopant content. Another, more sensi-tive method would therefore be desirable. Macdonald showed a way to independently determine the boron concentration with the help of the iron-boron pairing time con-stant τassocFe−B [59] with the detection limit estimated to be reliably below 1x1016 at/cm3 [54]:
Here, the constant M=5.0x105 s/Kcm3 and T is the sample temperature. Macdonald proved that the iron-boron pairing in p-type compensated material was not altered by the high concentration of phosphorus [59], equation (4-4) therefore remaining valid.
The acceptor concentration can thus be measured by the simple means of time and temperature-dependent carrier lifetime techniques such as quasi-steady state photo-conductance (QSSPC). Note that the absolute values of the carrier lifetime are not important; therefore the measurement does not have to be corrected for the different mobility in compensated material (section 4.3.1). Neither does the iron concentration matter. The only constraint on the Fe-content is that the FeB-pairs (and the interstitial species Fei) must have a significant impact on the minority carrier lifetime, which is true in many cases. Otherwise, the wafers can be intentionally doped with Fe, e.g. by
scratching iron onto the wafer surface and driving the contaminant into the wafer bulk during a high-temperature step.
The feasibility of this approach is shown in the next section with the help of intention-ally Fe-contaminated Fz-samples.
Note, however, that this method does not work on n-type compensated silicon as no FeB-pairs are formed.
D) When the net doping concentration p0 and the acceptor concentration are known, the donor concentration ND can be calculated with the help of eq. (4-2).
Figure 4.2: Interdependencies between the measurands relevant for the characterization of UMG silicon wafers. The encircled measurands can be directly analyzed with the help of existing techniques. The dotted rectangles identify measurands which have to be de-termined by a combination of several other values.
By another approach with the same number of measurements, the conductivity mobility can also be determined by Hall mobility measurements µH when the Hall factor r(NA,ND) is known:
(
AH D)
C r N N
µ µ
= , (4-5).
Conductivity and Hall mobilities of UMG silicon and intentionally compensated Cz-wafers and the deduced Hall factors are presented in section 4.2.4.