- Email: [email protected]
Systematic characterization of multi-crystalline silicon String Ribbon wafer
Journal of Crystal Growth 361 (2012) 38–43
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Systematic characterization of multi-crystalline silicon String Ribbon wafer a d ¨ ¨ C. Reimann a,b,n, G. Muller , J. Friedrich a,b, K. Lauer c, A. Simonis d, H. Watzig , d d d S. Krehan , R. Hartmann , A. Kruse a
Department of Crystal Growth, Fraunhofer IISB, Schottkystr.10, 91058 Erlangen, Germany Fraunhofer THM, Am St.-Niclas-Schacht 13, 09599 Freiberg, Germany c CiS Forschungsinstitut f¨ ur Mikrosensorik und Photovoltaik GmbH, Konrad-Zuse-Straße 14, 99099 Erfurt, Germany d Sovello AG, OT Thalheim, Sonnenallee 14-30, 06766 Bitterfeld-Wolfen, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 February 2012 Received in revised form 23 July 2012 Accepted 13 August 2012 Communicated by M.S. Goorsky Available online 4 September 2012
The String Ribbon technology provides multi-crystalline silicon wafers for low cost and high efficiency solar cells (16% efficiency on untextured cells). This paper deals with the systematic characterization of standard String Ribbon wafer material produced by the Sovello AG. The investigations of the grain structure, the dislocation density (EPD), the minority carrier lifetime t and the interstitial iron content Fei show a clear correlation between lifetime, EPD, and interstitial iron concentration. High lifetime areas consist mainly of S3 twinned grains with low EPD and low interstitial iron content in the range of 4 1011 atoms/cm3. The dislocation and interstitial iron distribution is non-uniform over the ribbon width. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A1. Defects A1. Impurities A2. String ribbon B2. Multi-crystalline silicon ribbon
1. Introduction Silicon ribbon crystal growth technologies, like Edge-defined Film-Fed growth (EFG), String-Ribbon growth (SRG), or Dendritic Web growth (DWG) are promising methods to achieve wafers at low cost (high ratio of watt-peak per gram (Wp/g) silicon) for high efficiency solar cells. All mentioned technologies avoid the kerf loss associated with mechanical sawing and damage etching. However, low-cost multi-crystalline silicon material can contain higher concentrations of impurities and crystal defects, like dislocations, which degrade the minority carrier lifetime. A well developed technology is the String Ribbon growth method [1–4]. It is based on the phenomenon of surface tension. Two thin carbon based strings are pulled upward through holes in a flat crucible with molten silicon by forming a silicon ribbon less than 200 mm thick and 80 mm wide between the two strings. Solar cells (untextured surface) produced at Sovello AG on 8 15 cm2 standard String Ribbon wafer have cell efficiencies of more than 16%. The goal is to improve this value by further improvements of the material quality. This can only be achieved if material properties and hence
n Corresponding author at: Department of Crystal Growth, Fraunhofer IISB, Schottkystr.10, 91058 Erlangen, Germany. Tel.: þ 49 9131 761272; fax: þ 49 9131 761280. E-mail address: [email protected] (C. Reimann).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.08.022
solar cell properties can be correlated to specific crystal defects. This paper is focused on grain structure, dislocation density and iron contamination. Some former characterization of String Ribbon wafers was done by Buonasissi et al. [5,6] and Geiger et al. [7] using material grown by Evergreen. The aim of this paper is to generate some deeper insight in performance limiting defects of typical String Ribbon wafer material. It will be shown that there is a strong correlation between dislocation density, iron concentration and minority charge carrier lifetime.
2. Characterization All investigations were performed on standard wafer material grown at Sovello AG. The boron doping was in the range of 5 1015–2 1016 atoms/cm3. The typical growth rate was around 10–20 mm/min with a slightly concave interface shape at the edges, as shown in Fig. 2. 2.1. Grain structure The grain orientations and grain boundary configurations were investigated by Electron backscatter diffraction (EBSD) measurements. For these purpose local measurements of the grain orientation as well as orientation mappings were performed. Therefore a
C. Reimann et al. / Journal of Crystal Growth 361 (2012) 38–43
39
2.3. Lifetime and interstitial iron concentration The minority charge carrier lifetime for passivated and unpassivated wafer surface was studied by the mPCD method [10]. Furthermore, the interstitial iron concentration was measured via the Fe–B dissociation method. The measurements were performed on SiNxpassivated wafer material at CIS Erfurt. The results were obtained on 8 2 cm2 samples according to the method of Lauer et al. [10].
3. Results 3.1. Grain structure
Fig. 2. Typical grain and dislocation distribution in a String Ribbon wafer revealed after Secco etching.
LEO 1530 Gemini from Zeiss with a Nordlys II EBSD-Detector with the Channel 5 software from Oxford was used. 2.2. Dislocation density Defect etching and subsequent optical microscopy was used to obtain the dislocation density in single grains. First the optimum defect etching parameters for String Ribbon material had to be found by a series of etching experiments as it was not published in literature to our best knowledge. Secco etch [8] was used at room temperature to reveal dislocations on as-grown ribbon material because it was known from literature [8] that dislocations will be etched independently from the grain orientation and will lead to circular or oval shaped etch pits. A Wright etch could have also been used but will lead to more crystallographic shaped etch pits [9]. The etching time was varied (1, 3, 4, 6 and 9 min) to obtain the maximum countable etch pit density (EPD) for different wafer positions. Fig. 1 shows the results. A maximum EPD for four different samples was achieved for 3 min of Secco etching. The resulting etch pits for this etching time were circular with diameters of 4 mm and show no significant overlap. The etch pit density decreases with longer etching time due to the overlap of the single etch pits, which could afterwards not be separated into single ones.
Fig. 2 shows an example of the distribution of grains and dislocations for String Ribbon wafers. The typical grain structure of as-grown samples (8 2 cm2) after Secco etching can be seen in Fig. 2a. Furthermore the pulling and growth direction is marked by black arrows; the shape of the solid/liquid interface revealed from reflected light microscopy is highlighted in white. Fig. 2b–f show in more detail distribution of grains and dislocations. The grains are more or less oriented parallel to the ribbon edges. A slight tilting between the growth direction and the grains can be observed in the middle of the ribbon (compare Fig. 2a). At the edges, a fine structure with small grains is found. At this position the grains are growing preferentially from the outer ribbon side to the inner (compare Fig. 2b). The dislocation density is inhomogeneously distributed over the ribbon width (compare Fig. 2c–e). Furthermore, regular linear alignments of the dislocations can be observed in some grains (compare Fig. 2f). A systematic investigation of the twinned areas, like the one shown in Fig. 2d was carried out for several samples coming from 20 different wafers. No dependence on the lateral wafer position could be observed. Obviously twinned regions are present at all wafer positions, which means that the used standard crystal growth conditions do not lead to twines at special lateral ribbon positions. The twined crystal growth is not controlled for standard crystal growth conditions. 3.2. Etch pit density (EPD) Fig. 3 shows the measured EPD for a typical String Ribbon sample across the ribbon width (80 mm) for each grain. The blue squares represent the position of the grain boundaries. The blue 1E+07
etch pit density [ 1/cm² ]
Fig. 1. Etch pit density (EPD) for different etching times revealed with a room temperature Secco etch.
1E+06 1E+05 1E+04 1E+03 1E+02
0
10
20
30
40
50
60
70
80
wafer width [mm] Fig. 3. Measured etch pit density (EPD) for each grain across the ribbon width. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
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C. Reimann et al. / Journal of Crystal Growth 361 (2012) 38–43
lines in between represent the measured mean values of the EPD in the investigated grain. The EPD varies over the ribbon width in the range of 2 102–5 106 1/cm2. A systematic investigation of the dislocation density of more than 20 String Ribbon wafers showed no significant dependence of the EPD on the lateral position on the wafer, which could mean that no systematic stress peak and therefore plastic deformation occur over the lateral ribbon position. Grains with higher dislocation density are neighbouring to grains with lower EPD. Often a difference of 2 orders of magnitude between adjacent grains is observed (compare Fig. 3). 3.3. Grain orientation Fig. 4 shows the etch pit density and the unpassivated lifetime values for several grains with different surface orientations measured on one wafer. The main surface orientations were (110), (331) and (210). It can be seen, that the EPD is independent from the grain orientations in this sample. That means that a grain with a specific surface orientation can have high or low EPD. The value of the measured unpassivated minority charge carrier lifetime correlates quite well with the EPD for the investigated grains. High lifetime values are linked in most cases to low dislocation densities and vice versa. Only two measuring points (marked with an arrow) show a high dislocation density and a higher lifetime value or a low dislocation density and a lower lifetime value, which could mean that the interstitial iron content and also the occurring iron precipitation are different in this regions. Fig. 5 shows a local EBSD orientation mapping of a highly twinned area of a typical String Ribbon sample. It can be seen that the grain surface orientation is in the area of the (101) corner of the EBSD pole
ehct pit density [1/cm²]
1.0E+07 1.0E+06 1.0E+05 (110) (331) (210)
1.0E+04 1.0E+03 1.0E+02
0
1 2 3 minority carrier lifetime [ µs ]
4
Fig. 4. Etch pit density and unpassivated lifetime mean values for several grains with different surface orientations measured at one sample.
Fig. 5. EBSD orientation mapping of a highly twinned region with low EPD.
figure. The measured grain boundary configurations in between the grains are S3. The observed EPD for this highly S3-twinned region is nearly zero (compare upper left image of the etched region in Fig. 5). 3.4. Lifetime and interstitial iron content The unpassivated and passivated minority carrier lifetime was measured on more than 20 different wafers. Fig. 6 shows the passivated results for two typical samples (8 2 cm2) grown with the same crystallization process. The results show an inhomogeneous lifetime distribution over the ribbon width for both samples. High lifetime grains are adjacent to low lifetime ones. The different lifetime areas on a specific sample can be correlated nearly exactly to the grain structure. The passivated lifetime has locally values up to 30 ms in special grains (compare Fig. 6). A systematic investigation of several wafers shows no dependence of the lifetime on the ribbon width. The only common feature is a region at the edges of the ribbons with significantly lower lifetime. The measured interstitial iron content is also distributed inhomogeneously over the ribbon width. Grains with low iron contamination are neighbouring to grains with high iron contamination. The measured interstitial iron concentration is in the range of 4 1011–6.4 1012 atoms/cm3. In most cases values of high minority carrier lifetime correlate to a lower content of interstitial iron and vice versa (compare Fig. 6).
4. Discussion 4.1. Grain structure and dislocation distribution The investigated String Ribbon samples show a grain structure (compare Fig. 2) which is typical for String Ribbon wafer material [5–7]. The grains are more or less elongated parallel to the growth direction and therefore the grain boundaries are more or less perpendicular to the solid–liquid interface. The small grains occurring at the ribbon edges show a growth direction towards the inner region of the ribbon. This is a clear result of the concave interface shape in this region in combination with a heterogeneous nucleation of silicon at the strings. Former work by Ciszek et al. [11] showed promising results on improving these features by using alternative string materials with different wetting behaviour. Unfortunately these alternative string materials are causing other problems, e.g. contamination. The EPD is very inhomogeneous over the ribbon width (compare Fig. 3). Grains with high EPD are adjacent to grains with low EPD. The independence of the EPD from the grain orientation, as shown in Fig. 4, might give a hint, that global and/or local stress states are responsible for the dislocation formation due to plastic deformation during crystal growth and crystal cooling. For the continuous string-ribbon process a steep temperature gradient at the interface is necessary to conduct the latent heat as well as the heat flux from the meniscus into the crystal. After passing the thermal insulation ( 20 cm after the solid–liquid interface position) the heat flux is reduced more and more by radiation losses to the environment. This results in a very nonlinear temperature profile, causing thermal stresses, and partially relaxing by inplane plastic deformation increasing the dislocation density and partly by out of plane deformation, so called buckling. Behnken et al. [13] showed that the occurring stress along the ribbon is in the range of up to 15 MPa and therefore plastic deformation can lead to an increased dislocation density. This is supported by the observed linear alignments of the dislocations in several grains (compare Fig. 2). These linear alignments are possibly activated glide planes due to occurring stress.
C. Reimann et al. / Journal of Crystal Growth 361 (2012) 38–43
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Fig. 6. Measured passivated minority carrier lifetime t and interstitial iron concentration Fei along the ribbon width (8 2 cm2) for two typical samples.
Comparing the result in Fig. 4, grains with high EPD have a low minority charge carrier lifetime and vice versa. This results of spatially resolved EPD measurements for several specific grains correlate well with Buonassisi’s results [6]. Furthermore some regions which occur as highly twinned S3 region show very low EPD (compare Fig. 5) and high minority charge carrier lifetime. Similar observations were made by Bounassisi et al. [5] for Evergreens String Ribbon material. The results from several authors [12] provide a strong evidence that S3 {111} boundaries are the least energetically favourable type of a grain boundary for metal segregation and precipitation, thus they have the lowest recombination activity.
For a further investigation on the occurrence of the favourable
S3 twinned regions more data is needed in order to improve the statistics. Furthermore, the process conditions will be adjusted in order to grow material with a higher amount of S3 twinned regions. It should also be analysed whether lower dislocation densities in S3 twinned regions influence the residual stress in the wafer itself. 4.2. Lifetime and interstitial iron content The investigations of lifetime and interstitial iron content show an inhomogeneous distribution over the ribbon width (compare
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C. Reimann et al. / Journal of Crystal Growth 361 (2012) 38–43
Fig. 7. Correlation of the unpassivated lifetime t the interstitial iron content Fei, and the EPD over the width of a String Ribbon wafer for the same wafer area.
Fig. 6). High lifetime correlates with low interstitial iron content and vice versa for most investigated samples. These observations are in contrast to Bounassisi et al. [6], who observed an anti-correlation between Fei–Bs state and passivated lifetime in String Ribbon wafer material which was grown from intentionally Fe contaminated silicon feedstock. The maximum observed interstitial iron level in [6] was 1.5 1012 atoms/cm3.The measured maximum interstitial iron concentration in this work is 6.4 1012 atoms/cm3 and is therefore higher than in [6]. The iron contamination can have different origins like feedstock material, graphite parts, gas atmosphere etc. Not yet understood is the reason which leads to the strongly inhomogeneous distribution of interstitial iron in the investigated wafers (compare Fig. 6). It is obvious that the cooling rates during ribbon growth are of magnitude higher than in conventional ingot processes [14]. Therefore, one would expect that the total iron content should be more uniformly distributed after crystal growth within the ribbon. The observed inhomogeneous interstitial iron distribution seems to be a result of diffusion and precipitation processes within shorter time frame in this material. Dissolved impurities can segregate to structural defects even at elevated temperatures, making them recombination-active after sample cooling. The degree to which a given structural defect will serve as a sink for metals depends on the energetic situation of its microstructure. Furthermore a high dislocation density could enhance the incorporation of interstitial iron. The authors do not believe that effects occurring during the crystal growth process, like inhomogeneous melt convection, lead to the observed iron distribution. This statement is supported by the fact, that the iron distribution is not the same for different ribbons grown by the same process conditions. Furthermore the orientation-dependent segregation effect could also not explain the results due to the fact that some neighbouring grains with the same grain orientation have different interstitial iron content. The total iron content, i.e. interstitial iron plus iron precipitates, was not investigated in this work. Former work [5,6] shows that the total iron content for String Ribbon material is in the order of 1014–1015 atoms/cm3. But local total iron content measurements are still challenging and therefore often missing in published literature. Detailed iron analysis at different axial ribbon positions, for different ribbons in one growth process, with varied feedstock and crucible materials is under work. These results should help to answer the open questions concerning the role of iron.
4.3. Lifetime limiting defects: a correlation between lifetime, interstitial iron content, and EPD Fig. 7 shows the unpassivated lifetime t, the interstitial iron content Fei and the EPD over the width of a String Ribbon wafer for the same wafer area. It can be seen, that in most of the wafer regions high minority carrier lifetime correlates with low interstitial iron content and low EPD and vice versa. Fig. 7 suggests that for this particular wafer the performance in the underperforming regions is limited primarily by dislocation-impurity (e.g. Fe) complexes.
5. Conclusion This contribution shows that: – String Ribbon wafer have an inhomogeneous dislocation distribution. – The EPD in String Ribbon wafer seems to be independent from crystal grain orientation. This could be due to global/local stress induced plastic deformation combined with dislocation formation. – String Ribbon wafer have an inhomogeneous distribution of interstitial Fei. This is possibly caused by local diffusion and precipitation processes and will be clarified in forthcoming work – The minority charge carrier lifetime is influenced by high EPD and high Fei content. – Twins with low EPD and low metal content are favourable for high lifetime. S3{111} boundaries are the least energetically favourable type of grain boundary for metal segregation and precipitation, thus they have the lowest recombination activity. The String ribbon process can efficiently be used to grow wafer with favourable S3{111} boundaries.
Acknowledgements The authors would like to thank the student co-workers B. Weisenseel and C. Strauchmann (IISB) for generating of the extensive dislocation data. Also thanks to S. Martin from IWW TUBA Freiberg for supporting the EBSD measurements.
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References [1] E.M. Sachs, D. Ely, J. Derby, Journal of Crystal Growth 82 (1987) 117–121. [2] R.L. Wallace, J.I. Honaka, A. Rohatgi, G. Crotty, Solar Energy Materials and Solar Cells 48 (1997) 179–186. [3] J.I. Hanoka, Solar Energy Materials & Solar Cells 65 (2001) 231–237. [4] J.P. Kalejs, Solar Energy Materials & Solar Cells 72 (2002) 139–153. [5] T. Bounassisi, G. Hahn, A.M. Gabor, J. Schischka, A.A. Istratov, M.D. Pickett, E.R. Weber, Proceedings of the 20th EUPVSEC, Barcelona, 2005. [6] T. Bounassisi, A.M. Lorenz, G.T. Tarnowski, Proceedings of the 21st EUPVSEC, Dresden, 2006. [7] P. Geiger, G. Kragler, G. Hahn, P. Fath, Solar Energy Materials & Solar Cells 85 (2005) 559–572.
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[8] F. SeccodAragona, Journal Electrochemical Society: Solid-State Science and Technology (1972) 948. [9] M. Wright Jenkins, Journal Electrochemical Society: Solid-State Science and Technology 124 (5) (1977) 757. ¨ [10] K. Lauer, A. Laades, H. Ubensee, H. Metzner, A. Lawerenz, Journal of Applied Physics 104 (2008) 104503. [11] T.F. Ciszek, J.L. Hurd, M. Schietzelt, Journal Electrochemical Society. SolidState Science and Technology (1982) 2838–2843. [12] J. Chen, T. Sekiguchi, D. Yang, F. Yin, K. Kido, S. Tsurekawa, Journal of Applied Physics 96 (2004) 5490–5495. [13] H. Behnken, R. Hartmann, A. Aderhold, Proceedings of the 26th EUPVSEC, Hamburg, 2011. ¨ [14] G. Muller, J. Friedrich, 14th International Summer School on Crystal Growth. AIP Conference Proceedings, vol. 1270, 2010, pp. 255–281.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Systematic characterization of multi-crystalline silicon String Ribbon wafer a d ¨ ¨ C. Reimann a,b,n, G. Muller , J. Friedrich a,b, K. Lauer c, A. Simonis d, H. Watzig , d d d S. Krehan , R. Hartmann , A. Kruse a
Department of Crystal Growth, Fraunhofer IISB, Schottkystr.10, 91058 Erlangen, Germany Fraunhofer THM, Am St.-Niclas-Schacht 13, 09599 Freiberg, Germany c CiS Forschungsinstitut f¨ ur Mikrosensorik und Photovoltaik GmbH, Konrad-Zuse-Straße 14, 99099 Erfurt, Germany d Sovello AG, OT Thalheim, Sonnenallee 14-30, 06766 Bitterfeld-Wolfen, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 February 2012 Received in revised form 23 July 2012 Accepted 13 August 2012 Communicated by M.S. Goorsky Available online 4 September 2012
The String Ribbon technology provides multi-crystalline silicon wafers for low cost and high efficiency solar cells (16% efficiency on untextured cells). This paper deals with the systematic characterization of standard String Ribbon wafer material produced by the Sovello AG. The investigations of the grain structure, the dislocation density (EPD), the minority carrier lifetime t and the interstitial iron content Fei show a clear correlation between lifetime, EPD, and interstitial iron concentration. High lifetime areas consist mainly of S3 twinned grains with low EPD and low interstitial iron content in the range of 4 1011 atoms/cm3. The dislocation and interstitial iron distribution is non-uniform over the ribbon width. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A1. Defects A1. Impurities A2. String ribbon B2. Multi-crystalline silicon ribbon
1. Introduction Silicon ribbon crystal growth technologies, like Edge-defined Film-Fed growth (EFG), String-Ribbon growth (SRG), or Dendritic Web growth (DWG) are promising methods to achieve wafers at low cost (high ratio of watt-peak per gram (Wp/g) silicon) for high efficiency solar cells. All mentioned technologies avoid the kerf loss associated with mechanical sawing and damage etching. However, low-cost multi-crystalline silicon material can contain higher concentrations of impurities and crystal defects, like dislocations, which degrade the minority carrier lifetime. A well developed technology is the String Ribbon growth method [1–4]. It is based on the phenomenon of surface tension. Two thin carbon based strings are pulled upward through holes in a flat crucible with molten silicon by forming a silicon ribbon less than 200 mm thick and 80 mm wide between the two strings. Solar cells (untextured surface) produced at Sovello AG on 8 15 cm2 standard String Ribbon wafer have cell efficiencies of more than 16%. The goal is to improve this value by further improvements of the material quality. This can only be achieved if material properties and hence
n Corresponding author at: Department of Crystal Growth, Fraunhofer IISB, Schottkystr.10, 91058 Erlangen, Germany. Tel.: þ 49 9131 761272; fax: þ 49 9131 761280. E-mail address: [email protected] (C. Reimann).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.08.022
solar cell properties can be correlated to specific crystal defects. This paper is focused on grain structure, dislocation density and iron contamination. Some former characterization of String Ribbon wafers was done by Buonasissi et al. [5,6] and Geiger et al. [7] using material grown by Evergreen. The aim of this paper is to generate some deeper insight in performance limiting defects of typical String Ribbon wafer material. It will be shown that there is a strong correlation between dislocation density, iron concentration and minority charge carrier lifetime.
2. Characterization All investigations were performed on standard wafer material grown at Sovello AG. The boron doping was in the range of 5 1015–2 1016 atoms/cm3. The typical growth rate was around 10–20 mm/min with a slightly concave interface shape at the edges, as shown in Fig. 2. 2.1. Grain structure The grain orientations and grain boundary configurations were investigated by Electron backscatter diffraction (EBSD) measurements. For these purpose local measurements of the grain orientation as well as orientation mappings were performed. Therefore a
C. Reimann et al. / Journal of Crystal Growth 361 (2012) 38–43
39
2.3. Lifetime and interstitial iron concentration The minority charge carrier lifetime for passivated and unpassivated wafer surface was studied by the mPCD method [10]. Furthermore, the interstitial iron concentration was measured via the Fe–B dissociation method. The measurements were performed on SiNxpassivated wafer material at CIS Erfurt. The results were obtained on 8 2 cm2 samples according to the method of Lauer et al. [10].
3. Results 3.1. Grain structure
Fig. 2. Typical grain and dislocation distribution in a String Ribbon wafer revealed after Secco etching.
LEO 1530 Gemini from Zeiss with a Nordlys II EBSD-Detector with the Channel 5 software from Oxford was used. 2.2. Dislocation density Defect etching and subsequent optical microscopy was used to obtain the dislocation density in single grains. First the optimum defect etching parameters for String Ribbon material had to be found by a series of etching experiments as it was not published in literature to our best knowledge. Secco etch [8] was used at room temperature to reveal dislocations on as-grown ribbon material because it was known from literature [8] that dislocations will be etched independently from the grain orientation and will lead to circular or oval shaped etch pits. A Wright etch could have also been used but will lead to more crystallographic shaped etch pits [9]. The etching time was varied (1, 3, 4, 6 and 9 min) to obtain the maximum countable etch pit density (EPD) for different wafer positions. Fig. 1 shows the results. A maximum EPD for four different samples was achieved for 3 min of Secco etching. The resulting etch pits for this etching time were circular with diameters of 4 mm and show no significant overlap. The etch pit density decreases with longer etching time due to the overlap of the single etch pits, which could afterwards not be separated into single ones.
Fig. 2 shows an example of the distribution of grains and dislocations for String Ribbon wafers. The typical grain structure of as-grown samples (8 2 cm2) after Secco etching can be seen in Fig. 2a. Furthermore the pulling and growth direction is marked by black arrows; the shape of the solid/liquid interface revealed from reflected light microscopy is highlighted in white. Fig. 2b–f show in more detail distribution of grains and dislocations. The grains are more or less oriented parallel to the ribbon edges. A slight tilting between the growth direction and the grains can be observed in the middle of the ribbon (compare Fig. 2a). At the edges, a fine structure with small grains is found. At this position the grains are growing preferentially from the outer ribbon side to the inner (compare Fig. 2b). The dislocation density is inhomogeneously distributed over the ribbon width (compare Fig. 2c–e). Furthermore, regular linear alignments of the dislocations can be observed in some grains (compare Fig. 2f). A systematic investigation of the twinned areas, like the one shown in Fig. 2d was carried out for several samples coming from 20 different wafers. No dependence on the lateral wafer position could be observed. Obviously twinned regions are present at all wafer positions, which means that the used standard crystal growth conditions do not lead to twines at special lateral ribbon positions. The twined crystal growth is not controlled for standard crystal growth conditions. 3.2. Etch pit density (EPD) Fig. 3 shows the measured EPD for a typical String Ribbon sample across the ribbon width (80 mm) for each grain. The blue squares represent the position of the grain boundaries. The blue 1E+07
etch pit density [ 1/cm² ]
Fig. 1. Etch pit density (EPD) for different etching times revealed with a room temperature Secco etch.
1E+06 1E+05 1E+04 1E+03 1E+02
0
10
20
30
40
50
60
70
80
wafer width [mm] Fig. 3. Measured etch pit density (EPD) for each grain across the ribbon width. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
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C. Reimann et al. / Journal of Crystal Growth 361 (2012) 38–43
lines in between represent the measured mean values of the EPD in the investigated grain. The EPD varies over the ribbon width in the range of 2 102–5 106 1/cm2. A systematic investigation of the dislocation density of more than 20 String Ribbon wafers showed no significant dependence of the EPD on the lateral position on the wafer, which could mean that no systematic stress peak and therefore plastic deformation occur over the lateral ribbon position. Grains with higher dislocation density are neighbouring to grains with lower EPD. Often a difference of 2 orders of magnitude between adjacent grains is observed (compare Fig. 3). 3.3. Grain orientation Fig. 4 shows the etch pit density and the unpassivated lifetime values for several grains with different surface orientations measured on one wafer. The main surface orientations were (110), (331) and (210). It can be seen, that the EPD is independent from the grain orientations in this sample. That means that a grain with a specific surface orientation can have high or low EPD. The value of the measured unpassivated minority charge carrier lifetime correlates quite well with the EPD for the investigated grains. High lifetime values are linked in most cases to low dislocation densities and vice versa. Only two measuring points (marked with an arrow) show a high dislocation density and a higher lifetime value or a low dislocation density and a lower lifetime value, which could mean that the interstitial iron content and also the occurring iron precipitation are different in this regions. Fig. 5 shows a local EBSD orientation mapping of a highly twinned area of a typical String Ribbon sample. It can be seen that the grain surface orientation is in the area of the (101) corner of the EBSD pole
ehct pit density [1/cm²]
1.0E+07 1.0E+06 1.0E+05 (110) (331) (210)
1.0E+04 1.0E+03 1.0E+02
0
1 2 3 minority carrier lifetime [ µs ]
4
Fig. 4. Etch pit density and unpassivated lifetime mean values for several grains with different surface orientations measured at one sample.
Fig. 5. EBSD orientation mapping of a highly twinned region with low EPD.
figure. The measured grain boundary configurations in between the grains are S3. The observed EPD for this highly S3-twinned region is nearly zero (compare upper left image of the etched region in Fig. 5). 3.4. Lifetime and interstitial iron content The unpassivated and passivated minority carrier lifetime was measured on more than 20 different wafers. Fig. 6 shows the passivated results for two typical samples (8 2 cm2) grown with the same crystallization process. The results show an inhomogeneous lifetime distribution over the ribbon width for both samples. High lifetime grains are adjacent to low lifetime ones. The different lifetime areas on a specific sample can be correlated nearly exactly to the grain structure. The passivated lifetime has locally values up to 30 ms in special grains (compare Fig. 6). A systematic investigation of several wafers shows no dependence of the lifetime on the ribbon width. The only common feature is a region at the edges of the ribbons with significantly lower lifetime. The measured interstitial iron content is also distributed inhomogeneously over the ribbon width. Grains with low iron contamination are neighbouring to grains with high iron contamination. The measured interstitial iron concentration is in the range of 4 1011–6.4 1012 atoms/cm3. In most cases values of high minority carrier lifetime correlate to a lower content of interstitial iron and vice versa (compare Fig. 6).
4. Discussion 4.1. Grain structure and dislocation distribution The investigated String Ribbon samples show a grain structure (compare Fig. 2) which is typical for String Ribbon wafer material [5–7]. The grains are more or less elongated parallel to the growth direction and therefore the grain boundaries are more or less perpendicular to the solid–liquid interface. The small grains occurring at the ribbon edges show a growth direction towards the inner region of the ribbon. This is a clear result of the concave interface shape in this region in combination with a heterogeneous nucleation of silicon at the strings. Former work by Ciszek et al. [11] showed promising results on improving these features by using alternative string materials with different wetting behaviour. Unfortunately these alternative string materials are causing other problems, e.g. contamination. The EPD is very inhomogeneous over the ribbon width (compare Fig. 3). Grains with high EPD are adjacent to grains with low EPD. The independence of the EPD from the grain orientation, as shown in Fig. 4, might give a hint, that global and/or local stress states are responsible for the dislocation formation due to plastic deformation during crystal growth and crystal cooling. For the continuous string-ribbon process a steep temperature gradient at the interface is necessary to conduct the latent heat as well as the heat flux from the meniscus into the crystal. After passing the thermal insulation ( 20 cm after the solid–liquid interface position) the heat flux is reduced more and more by radiation losses to the environment. This results in a very nonlinear temperature profile, causing thermal stresses, and partially relaxing by inplane plastic deformation increasing the dislocation density and partly by out of plane deformation, so called buckling. Behnken et al. [13] showed that the occurring stress along the ribbon is in the range of up to 15 MPa and therefore plastic deformation can lead to an increased dislocation density. This is supported by the observed linear alignments of the dislocations in several grains (compare Fig. 2). These linear alignments are possibly activated glide planes due to occurring stress.
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Fig. 6. Measured passivated minority carrier lifetime t and interstitial iron concentration Fei along the ribbon width (8 2 cm2) for two typical samples.
Comparing the result in Fig. 4, grains with high EPD have a low minority charge carrier lifetime and vice versa. This results of spatially resolved EPD measurements for several specific grains correlate well with Buonassisi’s results [6]. Furthermore some regions which occur as highly twinned S3 region show very low EPD (compare Fig. 5) and high minority charge carrier lifetime. Similar observations were made by Bounassisi et al. [5] for Evergreens String Ribbon material. The results from several authors [12] provide a strong evidence that S3 {111} boundaries are the least energetically favourable type of a grain boundary for metal segregation and precipitation, thus they have the lowest recombination activity.
For a further investigation on the occurrence of the favourable
S3 twinned regions more data is needed in order to improve the statistics. Furthermore, the process conditions will be adjusted in order to grow material with a higher amount of S3 twinned regions. It should also be analysed whether lower dislocation densities in S3 twinned regions influence the residual stress in the wafer itself. 4.2. Lifetime and interstitial iron content The investigations of lifetime and interstitial iron content show an inhomogeneous distribution over the ribbon width (compare
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Fig. 7. Correlation of the unpassivated lifetime t the interstitial iron content Fei, and the EPD over the width of a String Ribbon wafer for the same wafer area.
Fig. 6). High lifetime correlates with low interstitial iron content and vice versa for most investigated samples. These observations are in contrast to Bounassisi et al. [6], who observed an anti-correlation between Fei–Bs state and passivated lifetime in String Ribbon wafer material which was grown from intentionally Fe contaminated silicon feedstock. The maximum observed interstitial iron level in [6] was 1.5 1012 atoms/cm3.The measured maximum interstitial iron concentration in this work is 6.4 1012 atoms/cm3 and is therefore higher than in [6]. The iron contamination can have different origins like feedstock material, graphite parts, gas atmosphere etc. Not yet understood is the reason which leads to the strongly inhomogeneous distribution of interstitial iron in the investigated wafers (compare Fig. 6). It is obvious that the cooling rates during ribbon growth are of magnitude higher than in conventional ingot processes [14]. Therefore, one would expect that the total iron content should be more uniformly distributed after crystal growth within the ribbon. The observed inhomogeneous interstitial iron distribution seems to be a result of diffusion and precipitation processes within shorter time frame in this material. Dissolved impurities can segregate to structural defects even at elevated temperatures, making them recombination-active after sample cooling. The degree to which a given structural defect will serve as a sink for metals depends on the energetic situation of its microstructure. Furthermore a high dislocation density could enhance the incorporation of interstitial iron. The authors do not believe that effects occurring during the crystal growth process, like inhomogeneous melt convection, lead to the observed iron distribution. This statement is supported by the fact, that the iron distribution is not the same for different ribbons grown by the same process conditions. Furthermore the orientation-dependent segregation effect could also not explain the results due to the fact that some neighbouring grains with the same grain orientation have different interstitial iron content. The total iron content, i.e. interstitial iron plus iron precipitates, was not investigated in this work. Former work [5,6] shows that the total iron content for String Ribbon material is in the order of 1014–1015 atoms/cm3. But local total iron content measurements are still challenging and therefore often missing in published literature. Detailed iron analysis at different axial ribbon positions, for different ribbons in one growth process, with varied feedstock and crucible materials is under work. These results should help to answer the open questions concerning the role of iron.
4.3. Lifetime limiting defects: a correlation between lifetime, interstitial iron content, and EPD Fig. 7 shows the unpassivated lifetime t, the interstitial iron content Fei and the EPD over the width of a String Ribbon wafer for the same wafer area. It can be seen, that in most of the wafer regions high minority carrier lifetime correlates with low interstitial iron content and low EPD and vice versa. Fig. 7 suggests that for this particular wafer the performance in the underperforming regions is limited primarily by dislocation-impurity (e.g. Fe) complexes.
5. Conclusion This contribution shows that: – String Ribbon wafer have an inhomogeneous dislocation distribution. – The EPD in String Ribbon wafer seems to be independent from crystal grain orientation. This could be due to global/local stress induced plastic deformation combined with dislocation formation. – String Ribbon wafer have an inhomogeneous distribution of interstitial Fei. This is possibly caused by local diffusion and precipitation processes and will be clarified in forthcoming work – The minority charge carrier lifetime is influenced by high EPD and high Fei content. – Twins with low EPD and low metal content are favourable for high lifetime. S3{111} boundaries are the least energetically favourable type of grain boundary for metal segregation and precipitation, thus they have the lowest recombination activity. The String ribbon process can efficiently be used to grow wafer with favourable S3{111} boundaries.
Acknowledgements The authors would like to thank the student co-workers B. Weisenseel and C. Strauchmann (IISB) for generating of the extensive dislocation data. Also thanks to S. Martin from IWW TUBA Freiberg for supporting the EBSD measurements.
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