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High temperature annealing of bent multicrystalline silicon rods
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Acta Materialia 60 (2012) 6762–6769 www.elsevier.com/locate/actamat
High temperature annealing of bent multicrystalline silicon rods Torunn Ervik a,⇑, Maulid Kivambe a, Gaute Stokkan a, Birgit Ryningen b, Otto Lohne a a
Department of Materials Science and Engineering, NTNU, Trondheim N-7491, Norway b SINTEF Materials and Chemistry, Trondheim, Norway Available online 1 October 2012
Abstract Dislocation etch-pit structures on multicrystalline silicon rods deformed at 900 °C in four-point bending were studied prior to and after a high-temperature annealing. After deformation, the majority of the dislocation etch-pits were aligned along traces of {1 1 1} planes. Certain localized areas revealed network structures, where etch-pit arrays deviated in the range of 2-10° from the {1 1 1} plane traces. After annealing at 1350 °C for 12 h, a marked change in dislocation density and structure which varied from grain to grain was observed. Some grains showed incomplete polygonized structures, with notable irregularities and Y-junctions. The results were compared with observations on as-cast industrial multicrystalline silicon wafers for solar cells, where similar incomplete polygonized structures can be found. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Dislocation structures; Plastic deformation; Recovery; Silicon; Solar cells
1. Introduction Inspired by experiments on single crystals of silicon performed by Patel in 1958 [1], which showed that dislocations originally forming slip lines on deformed samples will rearrange into polygon walls normal to the active slip plane upon annealing, this study aims to investigate bent multicrystalline silicon (mc-Si) rods. Multicrystalline silicon is produced by the directional solidification method, which is a less energy intensive technique than the Czochralski method used for single crystalline growth. The drawback of this process is that the material has a lower quality than silicon single crystals due to defects such as grain boundaries and dislocations, and to a higher impurity content. By comparing the microstructure on deformed and annealed rods with mc-Si wafers, this study investigates further how and when dislocation etch-pit structures such as reported and explored by Ryningen et al. [2] and Kivambe et al. [3] are formed during crystallization and cooling of the mc-Si ingot.
⇑ Corresponding author. Tel.: +47 95 130 890; fax: +47 73 550 203.
E-mail address: [email protected] (T. Ervik).
When a specimen is deformed, much more dislocations are produced than are necessary for plastic deformation. This excess production of dislocations can be attributed to the discontinuous burst of dislocations by activated dislocation sources [4]. Upon annealing, the excess dislocations will annihilate, and the remaining necessary dislocations will tend to reduce the internal energy by rearranging into low-energy configurations. Polygonization is such a recovery process, where dislocations align in polygonal walls in positions where they relieve each other’s elastic strain field [5]. The result is that the stored energy of the bent crystal is relieved and the crystal softens. In silicon, high temperatures are needed to initiate recovery processes since both glide and climb must occur [6]. A subsequent annealing will therefore require high temperatures for a long period [1]. There are several important differences between plastic deformation in single crystals and in multicrystalline materials [7]. In a multicrystalline material, the applied resolved shear stress varies from grain to grain, and grain boundaries act as strong barriers to dislocation motion. Stresses created by dislocations piled up at a boundary may cause dislocation sources in neighbouring grains to operate. Incompatibility stresses introduced at grain boundaries due to the elastic anisotropy in silicon will
1359-6454/$36.00 Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2012.08.049
T. Ervik et al. / Acta Materialia 60 (2012) 6762–6769
2. Experimental In a previous work by Cochard [10], mc-Si rods with a cross-section of 3 4 mm2 and 50 mm long were deformed in four-point bending at 900 °C in an argon atmosphere. In this work, these specimens are investigated with regard to the dislocation structure before and after a high-temperature annealing. Samples were deformed up to fracture or when the tensile apparatus reached its maximum allowed cell displacement. Fig. 1 shows a sketch of a rod deformed by four-point bending. The areas investigated in this work are located on the horizontal part between the two loading points A1 and A2, indicated with an arrow in Fig. 1. The material is solar-grade silicon (SoG-Si) multicrystals. In Fig. 2, the stress–strain curve for the two rods, Si78 and Si79, is shown. The two rods are deformed at different strain rates, Si78 at 105 s1 and Si79 at 5 106 s1. The mechanical behaviour of the material is found to correlate well to that of single crystals [10]. The upper yield point is followed by a sharp yield drop, after which a hardening rate sets in. A decrease in strain rate decreases the upper yield point, which is the same effect as in single crystals. Multicrystalline SoG-Si material will contain defects such as grain boundaries and a relatively high content of impurities compared to electronic-grade silicon. The wavy appearance of the hardening stage for sample Si78 might therefore be related to impurities diffusing to the dislocations or to grain boundaries impeding dislocation motion. After deformation, the rods were cooled to and stored at room temperature. They were then annealed at 1350 °C in an inert atmosphere in an alumina tube furnace for 12 h. The samples were heated and cooled at a rate of 600 K h1. Table 1 shows an overview of the experimental procedure of the two samples, Si78 and Si79. In all the steps which include polishing and etching, the samples were slightly mechanically polished and defect etched for 20 s using a
Fig. 1. Sketch of four-point bending. The dotted line shows the sample before the deformation has started. The two supporting beams, B1 and B2, have a constant position during the deformation. The investigated part is the horizontal region, shown by the arrow, between the two loading points, A1 and A2.
60 50
Stress σ (MPa)
introduce slip near the boundaries [8]. Since dislocations interfere with each other and prevent glide and climb, the occurrence of polygonization in different grains will vary [9], and different grains may therefore require different temperatures in order for polygonization to occur.
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40 30 20 Si78
10 0 0.0
Si79 0.5
1.0
1.5
2.0
Strain ε (%) Fig. 2. Stress–strain curves for the two deformed rods, Si78 and Si79.
Sopori etchant [11]. The rods were imaged in a Zeiss Supra, 55VP field emission scanning electron microscope, equipped with an electron backscatter diffraction (EBSD) detector. The EBSD measurements were recorded using TSL OIMe data collection, and analysed by TSL OIMe data analysis version 6. 3. Results 3.1. After deformation, before annealing Fig. 3(a) shows a micrograph from a deformed rod, Si79, which was polished before deformation at 900 °C. Slip lines are therefore readily detected after deformation. The orientation map attached to the slip line picture, found from EBSD, shows four twin boundaries. These twins change the direction of the slip lines, as seen in the light microscope image. Fig. 3(b) shows an SEM image of the area marked with a square in Fig. 3(a) after polishing and etching. Etch-pit arrays are found to be parallel to the macroscopic slip lines. Traces of the {1 1 1} planes are found from EBSD and included in the image. After deformation, the stressed part between A1 and A2, in Fig. 1, of the rod was filled with overlapping dislocation etch-pits. 3.2. After annealing at 1350 °C for 12 h Etch-pit structures observed after annealing showed marked variation from grain to grain. Many grains revealed a polygonized structure, where etch-pits were aligned perpendicular to the traces of {1 1 1} planes. Fig. 4(a) shows an SEM image of a typical etch-pit structure found after annealing. The dislocation density is non-uniform across the grain and dislocation etch-pits are clustered in scattered or dense bundles. At some places, dislocation etch-pits are arranged in arrays perpendicular to a stacking fault. Fig. 4(b) is a sketch showing the investigated grain in Fig. 4(a). From EBSD analysis it is found that the surface plane of the grain is close to (0 1 1). In the sketch, {1 1 1} plane traces and the two h1 1 1i directions (normals) lying in the (0 1 1) surface plane are shown in the figure, and are indicated with black and white lines, respectively. Dislocation etch-pit
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Table 1 Experimental procedure. Sample
Si78
Si79
1. Polishing and Sopori etching 2. Deformation Deformation temperature Strain rate (_e) 3. Polishing and Sopori etching 4. Polishing 5. Annealing in alumina tube furnace Annealing temperature Annealing time 6. Polishing and Sopori etching
x x 900 °C 105 s1 x x x 1350 °C 12 h x
only polished x 900 °C 5 106 s1 x x x 1350 °C 12 h x
bundles are drawn as grey areas. The etched dislocation arrays lie in a [1 1 1] direction, this direction being marked with a black arrow in the sketch. The [1 1 1] direction lies on the (0 1 1) surface plane, and is perpendicular to a (1 1 1) plane trace. Fig. 5(a) shows the square marked A in Fig. 4(a) at higher magnification. The structure seen in the image may be an illustration of the first stages of polygonization. At first sight, the dislocation distribution looks random; however, small groups of dislocation etch-pits are found to form arrays perpendicular to the stacking fault (SF) and hence perpendicular to the traces of the {1 1 1} planes. Two such groups are indicated by white arrows in the
figure. It is a vague recovered structure, and many dislocation etch-pits are still arranged parallel to traces of the {1 1 1} planes. To the right in Fig. 4(a), the pattern is sharper; however, the etch-pits arrays still appear irregular and fragmented. A magnification of this area is shown in Fig. 5(b) (square marked B in Fig. 4(a)). The [1 1 1] direction is indicated by a white arrow. A closer look at the boundaries in Figs. 4 and 5 reveals that several of the dislocation etch-pit arrays are arranged in characteristic Y-shaped junctions. However, it is clear that the etch-pits are too wide for the structure to be seen properly. Most of the dislocation etch-pits shown in Figs. 4 and 5 are approximately 5 lm wide, and will therefore overlap as they come close to each other. The distance between each dislocation etch-pit in the array is of the order of 1 lm and less. The region seen in Fig. 6(a) represents the same structures seen in Fig. 5(b), though with much smaller etch-pits. This region was less etched than the other parts of the rod since it was covered by Teflon tape during etching. Four Y-junctions are clearly seen and pointed out in the image. The rod was polished and etched a second time, and Fig. 6(b) shows the exact same area after a normal etching procedure. Characteristic network structures are shown in Fig. 7(a) and (b). It should be noted that these structures were also found after deformation; however, the arrays appear
Fig. 3. (a) Light microscope image of a polished silicon rod (Si79) deformed at 900 °C. Slip lines are clearly seen here. An orientation map of this area with the {1 1 1} plane traces found from EBSD is included on the right. Four twin boundaries can be seen running through the orientation map and these are parallel to {1 1 1} traces. (b) SEM image of the marked square in (a) after Sopori etching. {1 1 1} slip lines and dislocation etch-pit arrays, marked with black lines, are parallel.
T. Ervik et al. / Acta Materialia 60 (2012) 6762–6769
Fig. 4. SEM image of a Sopori-etched rod (Si78) after annealing. (a) A typical dislocation etch-pit structure found after annealing. Dislocation etch-pits are arranged perpendicular to a stacking fault, and hence perpendicular to a {1 1 1} plane trace (b) Sketch showing the investigated grain. The surface plane is close to (0 1 1). Etch-pit bundles (drawn as grey areas) are aligned in a [1 1 1] direction perpendicular to a (1 1 1) plane trace (which also indicates the stacking fault (SF) running through the grain).
sharper after the high-temperature annealing and the SEM images shown in Fig. 7 are from after annealing. The two images show two different grains displaying a cell structure, with dislocation etch-pits arranged in arrays in two different directions. The surface plane in Fig. 7(a) is close to {1 1 1} and, from EBSD analysis, dislocation etch-pits were found to align in arrays deviating 5-10° from a {1 1 1} plane trace, which are shown as white lines in the figure. In Fig. 7(b), the surface plane is close to {1 0 0}, and here, too, the etch-pit arrays are not exactly parallel to a {1 1 1} plane trace. The deviation, of the order of 2-5°, is smaller than for the etch-pit arrays in Fig. 7(a). It can also be seen that the structure appears sharper in Fig. 7(b). For both grains, the experimentally determined cell side length is approximately 20 lm.
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Fig. 5. (a) A magnification of the square marked A in Fig. 4(a). Small groups of dislocation etch-pits are found to make arrays perpendicular to traces of slip planes (pointed out with white arrows). (b) Higher magnification image of the square marked B in Fig. 4(a). The etch-pit pattern is sharper. The white arrow indicates the [1 1 1] direction perpendicular to the stacking fault (SF). The stacking fault is parallel to a (1 1 1) plane trace.
{1 1 1} planes, found from EBSD analysis, are indicated with black lines in the images. Fig. 8(a) shows a grain where the surface plane is close to {1 1 0}. This is the same situation as for the grain seen in Figs. 4 and 5, and dislocation etch-pits in a polygonized structure will therefore be aligned in a h1 1 1i direction on the surface. In Fig. 8(b) the surface plane is close to {1 1 2}. Traces of the {1 1 1} planes are indicated with black lines and dislocation etchpits are seen to be aligned perpendicular to a {1 1 1} plane trace. Polygonized structures are seen to be coarser in Fig. 8(a), where the spacing between the polygon walls are approximately 50 lm, which is wider than for the dislocation etch-pit arrays seen in Fig. 8(b). In Fig. 8(b) the polygon wall spacing is of the order 20-40 lm. Y-junctions can still be seen in both images and are indicated by white arrows. 4. Discussion
3.3. Dislocation etch-pit structures observed on mc-Si wafers for solar cells Fig. 8 shows SEM images of a Sopori-etched as-cast mcSi wafer. Dislocation etch-pits are aligned perpendicular to stacking faults, marked SF in the image. Traces of the
The pictures in Figs. 4–7 show that the dislocation etchpit structures found after annealing of a deformed mc-Si rod vary from grain to grain. After annealing, recovery was observed in the form of polygonization and a decrease in the overall dislocation density. Since polygonization
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Fig. 6. SEM images of a Sopori-etched mc-Si rod (Si78) (a) Y-junctions are seen in a part of the annealed rod that was covered by Teflon tape during etching and was therefore only very gently etched. Dislocation etch-pits are of the order of 1 lm wide, and the structures become visible. Four Y-junctions are indicated with white arrows. (b) The same structure (marked square) after a normal etching procedure.
Fig. 7. SEM images of a Sopori-etched mc-Si rod (Si78) after deformation and heat treatment. (a) Dislocation etch-pit arrays constitute a network in a near {1 1 1} surface. The majority of the arrays deviate 5-10° from a {1 1 1} plane trace. (b) Dislocation etch-pit arrays constitute a network in a grain with surface plane close to {1 0 0}. The majority of the arrays deviate approximately 2-5° from a {1 1 1} plane trace.
involves dislocation glide as well as climb, high temperatures are required. Patel [1] annealed deformed single crystals at 1300 °C for 48 h before straight polygon walls were observed. Since the present samples are multicrystalline, other factors also affect the process of polygonization. In regions with multiple slip, dislocations can interfere with each other and prevent glide by the formations of barriers, such as the Lomer–Cottrell lock [9,12,13]. Grain boundaries are also obstacles to dislocation movement, and will lead to pile-ups of dislocations. In addition, impurities present in the specimen can lock dislocations and thereby play a role when it comes to dislocation mobility and therefore the alignment [14,15].
complete. Longer annealing times are necessary to bring about a uniform polygonization structure. To the right of Fig. 4(a), the pattern is sharper, and a magnified image is shown in Fig. 5(b). Here, the dislocation etch-pit arrays are lying in a [1 1 1] direction. This direction lies on the surface plane and is perpendicular to a (1 1 1) plane trace. It is also parallel to a (1 0 1) plane trace in the (0 1 1) surface. Hibbard and Dunn [16] divided the polygonization process into two stages, whereby dislocations first form shortrange boundary segments by a combination of glide and climb. The structure seen in Fig. 5(a) illustrates the first stages of polygonization well. For edge dislocations with the same sign lying on adjacent slip planes, forces will be a minimum when they lie vertically above one another. Thus, a vertical array of edge dislocations of the same sign is stable [17]. In the second stage the segments merge together by the motion of Y-junctions (see Fig. 6(a)). The movement of Y-junctions leads to polygon coarsening, namely an increase in polygon wall spacing. Polygon coarsening is expected to occur as long as a reduction in energy is possible; however, the kinetics involved have been found to indicate a relatively small driving energy for coarsening [18], and the coarsening process is indeed a slow process. The new boundary formed by two low-angle boundaries will have twice the angle but less energy than the sum of
4.1. Polygonized dislocation structures Dislocation structures where polygonization was observed were often heterogeneous, and both scattered and dense etch-pit arrays were observed in the grains. The etch-pit patterns are irregular and fragmented, as can be seen in Fig. 4. In this figure, several dislocation etch-pit arrays are aligned in the same direction. To the left of the image the etch-pit pattern is very vague, and dislocation etch-pits still lying along traces of slip planes are observed. It is clear that polygonization is far from
T. Ervik et al. / Acta Materialia 60 (2012) 6762–6769
Fig. 8. SEM images of a Sopori-etched as-cast mc-Si wafer. Dislocation etch-pits are aligned perpendicular to stacking faults, marked SF in the images. The white arrows indicate dislocation etch-pits arranged in Yjunctions. (a) The surface plane is close to {1 1 0}, and the distance between the dislocation etch-pit arrays is approximately 50 lm. (b) The surface plane is close to {1 1 2}, and dislocation etch-pits are aligned perpendicular to a {1 1 1} plane trace. The polygon wall spacing is smaller in this grain and is in the range of 20-40 lm.
the energies of the original boundaries [19]. The coarsening can be impeded by dislocations which are immobilized, e.g. by impurities [16,20]. Fig. 9 shows one Y-junction where it is clearly seen that the etch-pit distance decreases as the two etch-pit arrays merge together.
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formation of barriers in regions with more than one active slip system. The network structures often appear close to grain boundaries, while further away from the grain boundary the structure becomes more diffuse. This is a consequence of the incompatibility stresses at grain boundaries which give rise to secondary slip sources, and is an indication of the cell structure being a result of multiple slip, which is also a necessity when the dislocation motion is restricted to glide [25,26]. From the stress–strain curve, shown in Fig. 2, the hardening rate is seen to be low, suggesting that the majority of the stressed volume deforms in single glide [10]. However, this does not eliminate local deformation in multiple slip, particularly close to the grain boundaries, as is also observed. Several authors have discussed the importance of climb when it comes to dislocation cell formation. Patterning has been found to be depressed when the intrinsic point defect concentration is low [27]. In the case of the cell structure seen in this experiment, climb probably did not take place to a large degree during deformation since the deformation temperature was too low (0.7Tm). Aseev et al. [6] found that, for deformation temperatures higher than 0.8Tm (>1070 °C), both glide and climb occur at the same time. During annealing of the samples, climb takes place and modifications of the structure are possible; however, when comparing as-deformed and as-annealed, the structure do not show large changes except from appearing sharper. Thus the cell pattern seen is expected mainly to be a result of short-range interactions of dislocations belonging to different slip systems. These interactions are responsible for causing locks and junctions, whereas long-range interactions cause dislocations to move and align due to their mutual elastic strain fields acting on each other. In the latter case, climb is necessary to form network structures. 4.3. Comparison with dislocation etch-pit structures observed on mc-Si wafers for solar cells
4.2. Network structures
When comparing Figs. 6 and 8, it is observed that both the direction and the structure of the etch-pit arrays on an
Fig. 7(a) and (b) shows characteristic network structures where the dislocation etch-pit arrays form cells. For metals, cell structures are readily seen after deformation, and the evolution of such structures is reported to depend on the amount of strain and on the annealing time and temperature [21]. In addition, cell patterns are seen in compound semiconductors such as GaAs after crystal growth [22]. The deviation from {1 1 1} plane traces varied from grain to grain, and was in the range of 2-10°. Similar values have been reported for deformed copper and aluminium specimens [23,24], and the deviation was found to increase with the number of active slip systems. Structures such as the ones seen in Fig. 7(a) and (b) were also observed after deformation, indicating that these are deformation structures. It is expected that polygonization during the high-temperature annealing is difficult owing to the
Fig. 9. SEM image of an etched Y-junction; dislocation etch-pits in the two arrays combine to form a single array, and the etch-pit distance decreases as the two sub-boundaries merge together.
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mc-Si wafer are similar to those of the deformed and annealed mc-Si rods. Etch-pits are seen to be lying in Y-shaped junctions, as is also seen on the annealed rods, demonstrating an unfinished coarsening process. One important point is that the solid–liquid interface which dislocations intersect during solidification will be different from the free surface acting on the dislocations in an annealing experiment. A growing interface will therefore be a special situation with regards to recovery and polygonization. As a consequence of the recovery process being slow, temperatures during the growth of silicon must be close to the melting temperature for polygonization to occur. In a multicrystalline ingot, solid silicon will be at temperatures above 1300 °C for a period of the order of 2 h [28]. Dislocations in mc-Si ingots are thus assumed to be formed [29,30] near to the melt interface, followed by an immediate transfer to low-energy positions. Since dislocations in these positions are already in low-energy configurations, hightemperature annealing of multicrystalline silicon for solar cells is not expected to result in a lower density of dislocations in the bulk. It will lead to a reduction of dislocations only near the surface, as was found in etch-pit studies where the reductions were caused by image forces acting on dislocations lying close to the surface [31]. A coarsening of the existing polygon walls is, however, expected to be seen if annealing times are long enough. Additional factors such as stress have been found to enhance the dislocation density reduction when applied at high temperatures [32]. Cell structures such as those seen in Fig. 7(a) and (b) are not found in the clusters of as-grown mc-Si, and dislocation etch-pits lie in characteristic structures where etch-pits are aligned perpendicular to glide planes. 5. Conclusions Dislocation etch-pit structures on multicrystalline silicon rods deformed at 900 °C in four-point bending were studied prior to and after a high-temperature annealing. The main observations can be summarized as follows: 1. After deformation at 900 °C, the majority of the dislocation etch-pits were aligned in traces of {1 1 1} planes. In certain localized areas displaying network structures, a deviation from {1 1 1} plane traces was observed. The deviation seems to be correlated to the number of active slip systems, and was in the range of 2-10°. The network structures are believed to be deformation structures and a result of multiple slip and short-range interaction between dislocations. The structures appeared sharper after the high-temperature annealing. 2. After annealing at 1350° for 12 h, a significant change in etch-pit structure, which varied from grain to grain, was observed. In addition to a marked reduction in dislocation density, some grains showed polygonized structures where etch-pits were aligned in arrays perpendicular to {1 1 1} plane traces. The polygonization process was incomplete, and the dislocation etch-pit arrays appeared
irregular and were arranged in Y-junctions. This, in turn, implies a further possibility for polygon coarsening. 3. When the polygonized structures found after annealing are compared with observations of as-cast mc-Si wafers, it is clear that the structures appear similar. Dislocation etch-pits are aligned perpendicular to {1 1 1} planes, and are arranged in Y-junctions in a similar manner. The observations show that the results obtained by Patel [1] on single crystalline silicon rods can also be found on multicrystalline silicon rods. An increased understanding of how dislocation structures in mc-Si wafers are formed and developed during crystallization and cooling of the ingot has been gained by comparing the obtained results with industrial as-cast mc-Si wafers. Acknowledgements The present work was financially supported by NTNU and by the Norwegian Research Council, Elkem Solar and REC through the KMB-project “Defect Engineering in Crystalline Silicon Solar Cells”. The authors are grateful for the help of Julien Cochard in deforming samples and for his expertise on the mechanical behaviour of mc-Si crystals. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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Acta Materialia 60 (2012) 6762–6769 www.elsevier.com/locate/actamat
High temperature annealing of bent multicrystalline silicon rods Torunn Ervik a,⇑, Maulid Kivambe a, Gaute Stokkan a, Birgit Ryningen b, Otto Lohne a a
Department of Materials Science and Engineering, NTNU, Trondheim N-7491, Norway b SINTEF Materials and Chemistry, Trondheim, Norway Available online 1 October 2012
Abstract Dislocation etch-pit structures on multicrystalline silicon rods deformed at 900 °C in four-point bending were studied prior to and after a high-temperature annealing. After deformation, the majority of the dislocation etch-pits were aligned along traces of {1 1 1} planes. Certain localized areas revealed network structures, where etch-pit arrays deviated in the range of 2-10° from the {1 1 1} plane traces. After annealing at 1350 °C for 12 h, a marked change in dislocation density and structure which varied from grain to grain was observed. Some grains showed incomplete polygonized structures, with notable irregularities and Y-junctions. The results were compared with observations on as-cast industrial multicrystalline silicon wafers for solar cells, where similar incomplete polygonized structures can be found. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Dislocation structures; Plastic deformation; Recovery; Silicon; Solar cells
1. Introduction Inspired by experiments on single crystals of silicon performed by Patel in 1958 [1], which showed that dislocations originally forming slip lines on deformed samples will rearrange into polygon walls normal to the active slip plane upon annealing, this study aims to investigate bent multicrystalline silicon (mc-Si) rods. Multicrystalline silicon is produced by the directional solidification method, which is a less energy intensive technique than the Czochralski method used for single crystalline growth. The drawback of this process is that the material has a lower quality than silicon single crystals due to defects such as grain boundaries and dislocations, and to a higher impurity content. By comparing the microstructure on deformed and annealed rods with mc-Si wafers, this study investigates further how and when dislocation etch-pit structures such as reported and explored by Ryningen et al. [2] and Kivambe et al. [3] are formed during crystallization and cooling of the mc-Si ingot.
⇑ Corresponding author. Tel.: +47 95 130 890; fax: +47 73 550 203.
E-mail address: [email protected] (T. Ervik).
When a specimen is deformed, much more dislocations are produced than are necessary for plastic deformation. This excess production of dislocations can be attributed to the discontinuous burst of dislocations by activated dislocation sources [4]. Upon annealing, the excess dislocations will annihilate, and the remaining necessary dislocations will tend to reduce the internal energy by rearranging into low-energy configurations. Polygonization is such a recovery process, where dislocations align in polygonal walls in positions where they relieve each other’s elastic strain field [5]. The result is that the stored energy of the bent crystal is relieved and the crystal softens. In silicon, high temperatures are needed to initiate recovery processes since both glide and climb must occur [6]. A subsequent annealing will therefore require high temperatures for a long period [1]. There are several important differences between plastic deformation in single crystals and in multicrystalline materials [7]. In a multicrystalline material, the applied resolved shear stress varies from grain to grain, and grain boundaries act as strong barriers to dislocation motion. Stresses created by dislocations piled up at a boundary may cause dislocation sources in neighbouring grains to operate. Incompatibility stresses introduced at grain boundaries due to the elastic anisotropy in silicon will
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T. Ervik et al. / Acta Materialia 60 (2012) 6762–6769
2. Experimental In a previous work by Cochard [10], mc-Si rods with a cross-section of 3 4 mm2 and 50 mm long were deformed in four-point bending at 900 °C in an argon atmosphere. In this work, these specimens are investigated with regard to the dislocation structure before and after a high-temperature annealing. Samples were deformed up to fracture or when the tensile apparatus reached its maximum allowed cell displacement. Fig. 1 shows a sketch of a rod deformed by four-point bending. The areas investigated in this work are located on the horizontal part between the two loading points A1 and A2, indicated with an arrow in Fig. 1. The material is solar-grade silicon (SoG-Si) multicrystals. In Fig. 2, the stress–strain curve for the two rods, Si78 and Si79, is shown. The two rods are deformed at different strain rates, Si78 at 105 s1 and Si79 at 5 106 s1. The mechanical behaviour of the material is found to correlate well to that of single crystals [10]. The upper yield point is followed by a sharp yield drop, after which a hardening rate sets in. A decrease in strain rate decreases the upper yield point, which is the same effect as in single crystals. Multicrystalline SoG-Si material will contain defects such as grain boundaries and a relatively high content of impurities compared to electronic-grade silicon. The wavy appearance of the hardening stage for sample Si78 might therefore be related to impurities diffusing to the dislocations or to grain boundaries impeding dislocation motion. After deformation, the rods were cooled to and stored at room temperature. They were then annealed at 1350 °C in an inert atmosphere in an alumina tube furnace for 12 h. The samples were heated and cooled at a rate of 600 K h1. Table 1 shows an overview of the experimental procedure of the two samples, Si78 and Si79. In all the steps which include polishing and etching, the samples were slightly mechanically polished and defect etched for 20 s using a
Fig. 1. Sketch of four-point bending. The dotted line shows the sample before the deformation has started. The two supporting beams, B1 and B2, have a constant position during the deformation. The investigated part is the horizontal region, shown by the arrow, between the two loading points, A1 and A2.
60 50
Stress σ (MPa)
introduce slip near the boundaries [8]. Since dislocations interfere with each other and prevent glide and climb, the occurrence of polygonization in different grains will vary [9], and different grains may therefore require different temperatures in order for polygonization to occur.
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40 30 20 Si78
10 0 0.0
Si79 0.5
1.0
1.5
2.0
Strain ε (%) Fig. 2. Stress–strain curves for the two deformed rods, Si78 and Si79.
Sopori etchant [11]. The rods were imaged in a Zeiss Supra, 55VP field emission scanning electron microscope, equipped with an electron backscatter diffraction (EBSD) detector. The EBSD measurements were recorded using TSL OIMe data collection, and analysed by TSL OIMe data analysis version 6. 3. Results 3.1. After deformation, before annealing Fig. 3(a) shows a micrograph from a deformed rod, Si79, which was polished before deformation at 900 °C. Slip lines are therefore readily detected after deformation. The orientation map attached to the slip line picture, found from EBSD, shows four twin boundaries. These twins change the direction of the slip lines, as seen in the light microscope image. Fig. 3(b) shows an SEM image of the area marked with a square in Fig. 3(a) after polishing and etching. Etch-pit arrays are found to be parallel to the macroscopic slip lines. Traces of the {1 1 1} planes are found from EBSD and included in the image. After deformation, the stressed part between A1 and A2, in Fig. 1, of the rod was filled with overlapping dislocation etch-pits. 3.2. After annealing at 1350 °C for 12 h Etch-pit structures observed after annealing showed marked variation from grain to grain. Many grains revealed a polygonized structure, where etch-pits were aligned perpendicular to the traces of {1 1 1} planes. Fig. 4(a) shows an SEM image of a typical etch-pit structure found after annealing. The dislocation density is non-uniform across the grain and dislocation etch-pits are clustered in scattered or dense bundles. At some places, dislocation etch-pits are arranged in arrays perpendicular to a stacking fault. Fig. 4(b) is a sketch showing the investigated grain in Fig. 4(a). From EBSD analysis it is found that the surface plane of the grain is close to (0 1 1). In the sketch, {1 1 1} plane traces and the two h1 1 1i directions (normals) lying in the (0 1 1) surface plane are shown in the figure, and are indicated with black and white lines, respectively. Dislocation etch-pit
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Table 1 Experimental procedure. Sample
Si78
Si79
1. Polishing and Sopori etching 2. Deformation Deformation temperature Strain rate (_e) 3. Polishing and Sopori etching 4. Polishing 5. Annealing in alumina tube furnace Annealing temperature Annealing time 6. Polishing and Sopori etching
x x 900 °C 105 s1 x x x 1350 °C 12 h x
only polished x 900 °C 5 106 s1 x x x 1350 °C 12 h x
bundles are drawn as grey areas. The etched dislocation arrays lie in a [1 1 1] direction, this direction being marked with a black arrow in the sketch. The [1 1 1] direction lies on the (0 1 1) surface plane, and is perpendicular to a (1 1 1) plane trace. Fig. 5(a) shows the square marked A in Fig. 4(a) at higher magnification. The structure seen in the image may be an illustration of the first stages of polygonization. At first sight, the dislocation distribution looks random; however, small groups of dislocation etch-pits are found to form arrays perpendicular to the stacking fault (SF) and hence perpendicular to the traces of the {1 1 1} planes. Two such groups are indicated by white arrows in the
figure. It is a vague recovered structure, and many dislocation etch-pits are still arranged parallel to traces of the {1 1 1} planes. To the right in Fig. 4(a), the pattern is sharper; however, the etch-pits arrays still appear irregular and fragmented. A magnification of this area is shown in Fig. 5(b) (square marked B in Fig. 4(a)). The [1 1 1] direction is indicated by a white arrow. A closer look at the boundaries in Figs. 4 and 5 reveals that several of the dislocation etch-pit arrays are arranged in characteristic Y-shaped junctions. However, it is clear that the etch-pits are too wide for the structure to be seen properly. Most of the dislocation etch-pits shown in Figs. 4 and 5 are approximately 5 lm wide, and will therefore overlap as they come close to each other. The distance between each dislocation etch-pit in the array is of the order of 1 lm and less. The region seen in Fig. 6(a) represents the same structures seen in Fig. 5(b), though with much smaller etch-pits. This region was less etched than the other parts of the rod since it was covered by Teflon tape during etching. Four Y-junctions are clearly seen and pointed out in the image. The rod was polished and etched a second time, and Fig. 6(b) shows the exact same area after a normal etching procedure. Characteristic network structures are shown in Fig. 7(a) and (b). It should be noted that these structures were also found after deformation; however, the arrays appear
Fig. 3. (a) Light microscope image of a polished silicon rod (Si79) deformed at 900 °C. Slip lines are clearly seen here. An orientation map of this area with the {1 1 1} plane traces found from EBSD is included on the right. Four twin boundaries can be seen running through the orientation map and these are parallel to {1 1 1} traces. (b) SEM image of the marked square in (a) after Sopori etching. {1 1 1} slip lines and dislocation etch-pit arrays, marked with black lines, are parallel.
T. Ervik et al. / Acta Materialia 60 (2012) 6762–6769
Fig. 4. SEM image of a Sopori-etched rod (Si78) after annealing. (a) A typical dislocation etch-pit structure found after annealing. Dislocation etch-pits are arranged perpendicular to a stacking fault, and hence perpendicular to a {1 1 1} plane trace (b) Sketch showing the investigated grain. The surface plane is close to (0 1 1). Etch-pit bundles (drawn as grey areas) are aligned in a [1 1 1] direction perpendicular to a (1 1 1) plane trace (which also indicates the stacking fault (SF) running through the grain).
sharper after the high-temperature annealing and the SEM images shown in Fig. 7 are from after annealing. The two images show two different grains displaying a cell structure, with dislocation etch-pits arranged in arrays in two different directions. The surface plane in Fig. 7(a) is close to {1 1 1} and, from EBSD analysis, dislocation etch-pits were found to align in arrays deviating 5-10° from a {1 1 1} plane trace, which are shown as white lines in the figure. In Fig. 7(b), the surface plane is close to {1 0 0}, and here, too, the etch-pit arrays are not exactly parallel to a {1 1 1} plane trace. The deviation, of the order of 2-5°, is smaller than for the etch-pit arrays in Fig. 7(a). It can also be seen that the structure appears sharper in Fig. 7(b). For both grains, the experimentally determined cell side length is approximately 20 lm.
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Fig. 5. (a) A magnification of the square marked A in Fig. 4(a). Small groups of dislocation etch-pits are found to make arrays perpendicular to traces of slip planes (pointed out with white arrows). (b) Higher magnification image of the square marked B in Fig. 4(a). The etch-pit pattern is sharper. The white arrow indicates the [1 1 1] direction perpendicular to the stacking fault (SF). The stacking fault is parallel to a (1 1 1) plane trace.
{1 1 1} planes, found from EBSD analysis, are indicated with black lines in the images. Fig. 8(a) shows a grain where the surface plane is close to {1 1 0}. This is the same situation as for the grain seen in Figs. 4 and 5, and dislocation etch-pits in a polygonized structure will therefore be aligned in a h1 1 1i direction on the surface. In Fig. 8(b) the surface plane is close to {1 1 2}. Traces of the {1 1 1} planes are indicated with black lines and dislocation etchpits are seen to be aligned perpendicular to a {1 1 1} plane trace. Polygonized structures are seen to be coarser in Fig. 8(a), where the spacing between the polygon walls are approximately 50 lm, which is wider than for the dislocation etch-pit arrays seen in Fig. 8(b). In Fig. 8(b) the polygon wall spacing is of the order 20-40 lm. Y-junctions can still be seen in both images and are indicated by white arrows. 4. Discussion
3.3. Dislocation etch-pit structures observed on mc-Si wafers for solar cells Fig. 8 shows SEM images of a Sopori-etched as-cast mcSi wafer. Dislocation etch-pits are aligned perpendicular to stacking faults, marked SF in the image. Traces of the
The pictures in Figs. 4–7 show that the dislocation etchpit structures found after annealing of a deformed mc-Si rod vary from grain to grain. After annealing, recovery was observed in the form of polygonization and a decrease in the overall dislocation density. Since polygonization
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Fig. 6. SEM images of a Sopori-etched mc-Si rod (Si78) (a) Y-junctions are seen in a part of the annealed rod that was covered by Teflon tape during etching and was therefore only very gently etched. Dislocation etch-pits are of the order of 1 lm wide, and the structures become visible. Four Y-junctions are indicated with white arrows. (b) The same structure (marked square) after a normal etching procedure.
Fig. 7. SEM images of a Sopori-etched mc-Si rod (Si78) after deformation and heat treatment. (a) Dislocation etch-pit arrays constitute a network in a near {1 1 1} surface. The majority of the arrays deviate 5-10° from a {1 1 1} plane trace. (b) Dislocation etch-pit arrays constitute a network in a grain with surface plane close to {1 0 0}. The majority of the arrays deviate approximately 2-5° from a {1 1 1} plane trace.
involves dislocation glide as well as climb, high temperatures are required. Patel [1] annealed deformed single crystals at 1300 °C for 48 h before straight polygon walls were observed. Since the present samples are multicrystalline, other factors also affect the process of polygonization. In regions with multiple slip, dislocations can interfere with each other and prevent glide by the formations of barriers, such as the Lomer–Cottrell lock [9,12,13]. Grain boundaries are also obstacles to dislocation movement, and will lead to pile-ups of dislocations. In addition, impurities present in the specimen can lock dislocations and thereby play a role when it comes to dislocation mobility and therefore the alignment [14,15].
complete. Longer annealing times are necessary to bring about a uniform polygonization structure. To the right of Fig. 4(a), the pattern is sharper, and a magnified image is shown in Fig. 5(b). Here, the dislocation etch-pit arrays are lying in a [1 1 1] direction. This direction lies on the surface plane and is perpendicular to a (1 1 1) plane trace. It is also parallel to a (1 0 1) plane trace in the (0 1 1) surface. Hibbard and Dunn [16] divided the polygonization process into two stages, whereby dislocations first form shortrange boundary segments by a combination of glide and climb. The structure seen in Fig. 5(a) illustrates the first stages of polygonization well. For edge dislocations with the same sign lying on adjacent slip planes, forces will be a minimum when they lie vertically above one another. Thus, a vertical array of edge dislocations of the same sign is stable [17]. In the second stage the segments merge together by the motion of Y-junctions (see Fig. 6(a)). The movement of Y-junctions leads to polygon coarsening, namely an increase in polygon wall spacing. Polygon coarsening is expected to occur as long as a reduction in energy is possible; however, the kinetics involved have been found to indicate a relatively small driving energy for coarsening [18], and the coarsening process is indeed a slow process. The new boundary formed by two low-angle boundaries will have twice the angle but less energy than the sum of
4.1. Polygonized dislocation structures Dislocation structures where polygonization was observed were often heterogeneous, and both scattered and dense etch-pit arrays were observed in the grains. The etch-pit patterns are irregular and fragmented, as can be seen in Fig. 4. In this figure, several dislocation etch-pit arrays are aligned in the same direction. To the left of the image the etch-pit pattern is very vague, and dislocation etch-pits still lying along traces of slip planes are observed. It is clear that polygonization is far from
T. Ervik et al. / Acta Materialia 60 (2012) 6762–6769
Fig. 8. SEM images of a Sopori-etched as-cast mc-Si wafer. Dislocation etch-pits are aligned perpendicular to stacking faults, marked SF in the images. The white arrows indicate dislocation etch-pits arranged in Yjunctions. (a) The surface plane is close to {1 1 0}, and the distance between the dislocation etch-pit arrays is approximately 50 lm. (b) The surface plane is close to {1 1 2}, and dislocation etch-pits are aligned perpendicular to a {1 1 1} plane trace. The polygon wall spacing is smaller in this grain and is in the range of 20-40 lm.
the energies of the original boundaries [19]. The coarsening can be impeded by dislocations which are immobilized, e.g. by impurities [16,20]. Fig. 9 shows one Y-junction where it is clearly seen that the etch-pit distance decreases as the two etch-pit arrays merge together.
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formation of barriers in regions with more than one active slip system. The network structures often appear close to grain boundaries, while further away from the grain boundary the structure becomes more diffuse. This is a consequence of the incompatibility stresses at grain boundaries which give rise to secondary slip sources, and is an indication of the cell structure being a result of multiple slip, which is also a necessity when the dislocation motion is restricted to glide [25,26]. From the stress–strain curve, shown in Fig. 2, the hardening rate is seen to be low, suggesting that the majority of the stressed volume deforms in single glide [10]. However, this does not eliminate local deformation in multiple slip, particularly close to the grain boundaries, as is also observed. Several authors have discussed the importance of climb when it comes to dislocation cell formation. Patterning has been found to be depressed when the intrinsic point defect concentration is low [27]. In the case of the cell structure seen in this experiment, climb probably did not take place to a large degree during deformation since the deformation temperature was too low (0.7Tm). Aseev et al. [6] found that, for deformation temperatures higher than 0.8Tm (>1070 °C), both glide and climb occur at the same time. During annealing of the samples, climb takes place and modifications of the structure are possible; however, when comparing as-deformed and as-annealed, the structure do not show large changes except from appearing sharper. Thus the cell pattern seen is expected mainly to be a result of short-range interactions of dislocations belonging to different slip systems. These interactions are responsible for causing locks and junctions, whereas long-range interactions cause dislocations to move and align due to their mutual elastic strain fields acting on each other. In the latter case, climb is necessary to form network structures. 4.3. Comparison with dislocation etch-pit structures observed on mc-Si wafers for solar cells
4.2. Network structures
When comparing Figs. 6 and 8, it is observed that both the direction and the structure of the etch-pit arrays on an
Fig. 7(a) and (b) shows characteristic network structures where the dislocation etch-pit arrays form cells. For metals, cell structures are readily seen after deformation, and the evolution of such structures is reported to depend on the amount of strain and on the annealing time and temperature [21]. In addition, cell patterns are seen in compound semiconductors such as GaAs after crystal growth [22]. The deviation from {1 1 1} plane traces varied from grain to grain, and was in the range of 2-10°. Similar values have been reported for deformed copper and aluminium specimens [23,24], and the deviation was found to increase with the number of active slip systems. Structures such as the ones seen in Fig. 7(a) and (b) were also observed after deformation, indicating that these are deformation structures. It is expected that polygonization during the high-temperature annealing is difficult owing to the
Fig. 9. SEM image of an etched Y-junction; dislocation etch-pits in the two arrays combine to form a single array, and the etch-pit distance decreases as the two sub-boundaries merge together.
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mc-Si wafer are similar to those of the deformed and annealed mc-Si rods. Etch-pits are seen to be lying in Y-shaped junctions, as is also seen on the annealed rods, demonstrating an unfinished coarsening process. One important point is that the solid–liquid interface which dislocations intersect during solidification will be different from the free surface acting on the dislocations in an annealing experiment. A growing interface will therefore be a special situation with regards to recovery and polygonization. As a consequence of the recovery process being slow, temperatures during the growth of silicon must be close to the melting temperature for polygonization to occur. In a multicrystalline ingot, solid silicon will be at temperatures above 1300 °C for a period of the order of 2 h [28]. Dislocations in mc-Si ingots are thus assumed to be formed [29,30] near to the melt interface, followed by an immediate transfer to low-energy positions. Since dislocations in these positions are already in low-energy configurations, hightemperature annealing of multicrystalline silicon for solar cells is not expected to result in a lower density of dislocations in the bulk. It will lead to a reduction of dislocations only near the surface, as was found in etch-pit studies where the reductions were caused by image forces acting on dislocations lying close to the surface [31]. A coarsening of the existing polygon walls is, however, expected to be seen if annealing times are long enough. Additional factors such as stress have been found to enhance the dislocation density reduction when applied at high temperatures [32]. Cell structures such as those seen in Fig. 7(a) and (b) are not found in the clusters of as-grown mc-Si, and dislocation etch-pits lie in characteristic structures where etch-pits are aligned perpendicular to glide planes. 5. Conclusions Dislocation etch-pit structures on multicrystalline silicon rods deformed at 900 °C in four-point bending were studied prior to and after a high-temperature annealing. The main observations can be summarized as follows: 1. After deformation at 900 °C, the majority of the dislocation etch-pits were aligned in traces of {1 1 1} planes. In certain localized areas displaying network structures, a deviation from {1 1 1} plane traces was observed. The deviation seems to be correlated to the number of active slip systems, and was in the range of 2-10°. The network structures are believed to be deformation structures and a result of multiple slip and short-range interaction between dislocations. The structures appeared sharper after the high-temperature annealing. 2. After annealing at 1350° for 12 h, a significant change in etch-pit structure, which varied from grain to grain, was observed. In addition to a marked reduction in dislocation density, some grains showed polygonized structures where etch-pits were aligned in arrays perpendicular to {1 1 1} plane traces. The polygonization process was incomplete, and the dislocation etch-pit arrays appeared
irregular and were arranged in Y-junctions. This, in turn, implies a further possibility for polygon coarsening. 3. When the polygonized structures found after annealing are compared with observations of as-cast mc-Si wafers, it is clear that the structures appear similar. Dislocation etch-pits are aligned perpendicular to {1 1 1} planes, and are arranged in Y-junctions in a similar manner. The observations show that the results obtained by Patel [1] on single crystalline silicon rods can also be found on multicrystalline silicon rods. An increased understanding of how dislocation structures in mc-Si wafers are formed and developed during crystallization and cooling of the ingot has been gained by comparing the obtained results with industrial as-cast mc-Si wafers. Acknowledgements The present work was financially supported by NTNU and by the Norwegian Research Council, Elkem Solar and REC through the KMB-project “Defect Engineering in Crystalline Silicon Solar Cells”. The authors are grateful for the help of Julien Cochard in deforming samples and for his expertise on the mechanical behaviour of mc-Si crystals. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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