Seed manipulation for artificially controlled defect technique in new

Applied Physics Express 8, 105501 (2015) http://dx.doi.org/10.7567/APEX.8.105501

Seed manipulation for artificially controlled defect technique in new growth method for quasi-monocrystalline Si ingot based on casting Isao Takahashi, Supawan Joonwichien, Taisho Iwata, and Noritaka Usami Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan Received August 24, 2015; accepted August 31, 2015; published online September 18, 2015 We propose a new growth method for quasi-monocrystalline Si that achieves high-quality ingots and a high yield ratio. This method induces defect regions with compose dislocations surrounded by grain boundaries. These functional defects benefit the crystalline quality through impurity gettering, dislocation-propagation blocking, and stress relaxation with plastic deformation. Functional defect regions were grown from designed seeds and introduced to ingot edges where crystalline Si was disposed because of contamination. Preliminary experimentation demonstrated that functional defects could effectively form from the manipulated seeds. We call this method the Seed Manipulation for Artificially Controlled Defect Technique. © 2015 The Japan Society of Applied Physics

rystalline Si (c-Si) accounts for 90% of the starting materials for solar cells. However, cost reduction and a high crystalline quality are required to achieve grid parity in the photovoltaic industry. Quasi-mono c-Si is epitaxially grown from mono c-Si within a large crucible and has drawn interest in recent years1–3) because the crystalline quality can equal that of mono c-Si grown by the Czochralski (CZ) method. Furthermore, the low cost and high throughput enable large-scale ingot growth. Despite the great potential of quasi-mono c-Si, several problems exist, including the generation of dislocations from seeds, multicrystallization on the crucible walls, and impurity diffusion from the crucible.4–7) The problem of multicrystallization due to inhomogeneous nucleation on the crucible walls has been overcome by defect engineering. Kutsukake et al. demonstrated the suppression of multicrystalline grains with functional grain boundaries (GBs) artificially formed by multi-seed crystals.8,9) This demonstrates effective defect engineering in crystals, whereas defects typically have negative effects on the solar-cell performance. However, problems remain in the quasi-mono c-Si growth. The dislocation generation is the most detrimental problem because dislocations limit the solar-cell performance.10,11) Because CZ mono c-Si is utilized as the seed, several pieces of the arranged seed are required on the bottom of the crucible according to the seed size limitations. Therefore, junctions between multiple seeds cannot be avoided in industrial-scale ingots. Typically, dislocations are generated at the junction between neighboring seeds owing to the

C

impurity segregation, thermal shock, and angular differences in the crystallographic orientation of the seeds.12–17) To solve the problems of dislocation generation and impurity diffusion, we propose a new growth method combining defect engineering and the manipulation of defect distributions. The idea of the functional GB is expanded to functional defects comprising a high density of dislocations and GBs that provide the additional functions of trapping impurities and stress relaxation. These functions reduce the dislocation density and impurity concentration in the quasi-mono c-Si region. The defect distributions are precisely manipulated by arranging artificially designed seeds to form the functional defects. This growth method is called the Seed Manipulation for Artificially Controlled Defect Technique (SMART). In this study, we explain the SMART in detail and demonstrate its advantages. Figure 1 illustrates the SMART. A functional defect is defined as a central high-dislocation-density area surrounded by several GBs, as shown in Fig. 1(a). The GB structures were controlled by arranging the seed orientation to produce the functional defect structure. The GBs that generated dislocations during the crystal growth were utilized for the high-dislocation-density region. Functional defects were arranged along the inner wall of a quartz crucible where crystalline Si was disposed because of heavy contamination. Over 156 × 156 mm2 single crystalline Si blocks applied to solar cells were placed inside the functional defects. This area was defined as the quality-controlled region aided by functional defects. To suppress the dislocation generation

(a)

(b)

Fig. 1. Concept of the SMART. (a) Structure of the functional defects and (b) arrangement of seeds and grown crystals in a crucible.

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© 2015 The Japan Society of Applied Physics

Appl. Phys. Express 8, 105501 (2015)

I. Takahashi et al. Table I. Grain orientations of prepared seeds in exp. I.

Growth orientation GB plane Sample surface

Seed 1

Seed 2

Seed 3

Seed 4

Seed 5

Seed 6

Seed 7

Seed 8

Seed 9

 (011)

 (011)

 (011)

(001)

 (001)

 (001)

 (011)

 (011)

 (011)

(111)  (211)

(111)  (211)

(111)  (21 1)

(130)  (310)

(120)  (210)

(130)  (310)

(111)  (211)

(111)  (211)

 (211)

GB structure

SA

Σ3

R

Tilt

Tilt

R

SA

(111) SA

Table II. Grain orientations of prepared seeds in exp. II. Seed 1

Seed 2

Seed 3

Seed 4

Seed 5

Seed 6

Seed 7

Growth orientation

(001)

(001)

 (001)

(001)

(001)

(001)

(001)

(001)

GB plane

(120)  (210)

(120)  (210)

(120)  (210)

(130)  (310)

(120)  (210)

(130)  (310)

(130)  (310)

(130)  (310)

Sample surface GB structure

SA

Σ5

Tilt

between these blocks, additional thin functional defects or GBs were inserted in the structure to suppress the dislocation generation. If the size of these functional defects was less than the kerf loss, the region was removed during the cutting process. A key issue in manipulating defect distributions is the generation and suppression of dislocations from the arranged seeds. This knowledge is based on our previous results, including a generation mechanism for dislocations from GBs or multiple seeds.18–22) Several advantages of this method were expected. First, the dislocation propagation in the functional defects can relax the shear stress that induces dislocation generation in the qualitycontrolled region. Propagated dislocations are effectively blocked by the surrounding GBs. Second, impurities diffused from the crucible and coating materials are trapped in functional defects during the crystal growth and the subsequent cooling process in the furnace. This mechanism is similar to the external gettering process in solar-cell fabrication. Third, the multicrystallization is blocked by the GBs of the functional defects. This effect was demonstrated by Kutsukake et al.8,9) These interesting aspects of functional defects provide a quality-controlled area with fewer dislocations and impurities and no multicrystallization. Furthermore, the yield ratio for solar-cell production increases if the functional defect thickness is less than the heavily contaminated area thickness disposed after the crystal growth. This condition can be achieved by optimizing the GB structure, dislocation density, and functional defect size. The SMART is quite different from the growth techniques proposed thus far. Initially, dislocations and GBs are intentionally induced in disposed area. In contrast, other studies only considered how to suppress the defect generation and density. Second, the intentionally induced defects are effectively used to improve the crystalline quality in other areas. Two sets of experiments were conducted to demonstrate the SMART. The first experiment investigated the relationship between the GB structure and the dislocation generation to reveal an appropriate structure for the functional defects. The second experiment was designed to prove the concept of the SMART. The first experiment (exp. I) involved four types of GBs fabricated into one ingot: small angle (SA), tilt (T), random (R), and Σ3. Table I presents the orientation of the prepared seeds and fabricated GB structures. CZ mono c-Si was used

Tilt

Tilt

SA

Seed 8

SA

for the seeds. The prepared seeds and feedstock of electricalgrade polycrystalline Si were placed in a 50-mm square within a silica crucible. The seeds were partially melted in a temperature gradient before the crystal growth. Crystals were then epitaxially grown by pulling the crucible down at 0.3 mm=min. The orientation of the prepared seeds in the second experiment (exp. II) is shown in Table II. SA and tilt GBs were chosen for dislocation generation and suppression, respectively. The seed orientations in the growth direction of the ingot were designed to align with only the (001) plane, because c-Si solar cells are fabricated from only (001)oriented wafers. Seeds 5 and 20 mm thick were used for the functional-defect and quality-controlled regions, respectively. The remaining growth conditions of the feedstock weight, crucible size, cooling rate, etc. were the same as those in exp. I. Grown ingots were cut along the growth direction perpendicular to the GB planes. Etch pits were identified using a Sopori solution.23) Etch-pit images were taken with an optical scanner. The crystallographic orientation and GB structure were characterized by electron backscattered diffraction. Figure 2 shows (a) orientation maps of the growth direction, (b) the GB plane, and (c) an etch-pit image of the sample used in exp. I. As shown in Fig. 2(c), dislocation clusters were generated along the SA and Σ3 GBs at the interface between the unmelted seeds and grown crystals — indicated by a dashed line — whereas no dislocation clusters were generated at the random GBs. Furthermore, the dislocation clusters began to generate in the middle of the tilt GBs. The dislocations generated from the SA and Σ3 GBs were blocked by the random GBs. The capability to generate dislocations is summarized as SA = Σ3 > tilt > random GBs. These results are attributed to the GB energy and the misorientation of the stable GB structure due to the limited accuracy of arranging or cutting the seeds. For example, the GB energy of Σ3 GB is quite low but rapidly increases with the misorientation relative to the ideal structure.24,25) Therefore, the Σ3 GB with a small misorientation angle can decrease the GB energy by introducing a subGB, which is defined as aligned dislocations. The experimental results of Σ3 GB clearly show that the gain in the GB energy from the misoriented structure to the stable structure should be larger than the elastic energy of the generated dislocations. Because the GB energy is lower in the lower Σ

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Appl. Phys. Express 8, 105501 (2015)

I. Takahashi et al.

(a)

(a)

(b)

(b)

(c)

(d)

(c)

Fig. 2. Exp. I results. (a) Orientation maps of growth direction, (b) the GB plane, and (c) an etch pit image. In the orientation maps, the GBs — except for the SA GB — are illustrated with lines. The dashed lines show the interface between the unmelted seeds and grown crystals.

GB, lower Σ GBs with misorientation easily generate dislocations from the seed junction, reducing the GB energy. The ideal structure for SA GB corresponds to Σ1 GB, i.e., is single crystalline; hence, an artificially arranged SA GB actively generates dislocations, as shown in Fig. 2(c). Considering these results, we chose an SA GB for the dislocation generation and a tilt GB for the blocking to manipulate the distribution of the dislocations. Figure 3 shows (a) orientation maps of the growth direction, (b) the GB plane, (c) an etch-pit image, and (d) magnified etch pit images for the sample used in exp. II. The functional defect region was fabricated with SA, Σ5, and tilt GBs on both sides of the ingot. The orientations in the growth direction of the ingot were controlled to align with only the (001) plane for application to solar-cell products. Crystalline Si solar cells are fabricated by using (001)-oriented wafers because the texture structure is easily formed with alkali solutions. In the center of the ingot, i.e., the quality-controlled region, seeds larger than those in the functional defects were prepared. As shown in Figs. 3(c) and 3(d), the SA GBs generated dislocations that were blocked by the tilt GBs, which was expected. The tilt GB located in the center did not generate dislocations, resulting in a low dislocation density in the quality-controlled area. Although the further study of the effects of the stress relaxation and impurity trapping in the functional defects should be conducted, the aforementioned results clearly indicate that defect distributions can be manipulated effectively and easily by arranging the seeds. We proposed and demonstrated a new growth method called the SMART for quasi-mono c-Si. It comprises two techniques: defect manipulation with artificially arranged seeds and defect engineering aided by defect functions. In the proposed structure, the functional defects comprise high-

Fig. 3. Exp. II results. (a) Orientation maps of growth direction, (b) the GB plane, (c) an etch pit image, and (d) magnified images taken by an optical microscope. In the orientation maps, the GBs — except for the SA GB — are separated by lines. The number in (d) corresponds to the observed area in (c). The dashed lines show the interface between the unmelted seeds and grown crystals.

dislocation-density areas, and dislocation-blocking GBs are placed in the area surrounding ingots where crystals cannot be used for solar cells because of the heavy contamination. In the center of the ingot, the dislocation density and impurity concentration are expected to be low because the functional defects act as stress-relaxation and impurity-trapping sites. A preliminary experiment was conducted and indicated that defect distributions can successfully be manipulated by designing the seed orientation, size, and arrangement. The SMART is very easy to implement and may overcome the problems of the dislocation generation and impurity contamination for quasi-mono c-Si. Therefore, our results contribute to the production of high-quality and high-yieldratio c-Si ingots based on the casting method. Acknowledgments The authors thank Kentaro Kutsukake at Tohoku University for advice about seed preparation and fruitful discussions.

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