Electrophoretic Deposited Quartz Powder-Assisted Growth of Multicrystalline Silicon

Abstract

Ingot multicrystalline silicon (Mc-Si) needs to be improved in quality and reduced in cost compared with Czochralski monocrystalline silicon. A uniform and dense quartz nucleation layer was obtained by the electrophoretic deposition of quartz powder on the surface of the silicon wafer. The deposited silicon wafer was annealed at 600 °C for 1 h, and one side of the silicon wafer with the quartz layer was glued to the crucible. During the growth of Mc-Si crystal, the dense quartz powder can play a nucleation role. The results show that the average lifetime of the minority carriers a of quartz-assisted silicon ingot is 7.4 μs. The overall dislocation density of an electrophoretic deposition quartz-assisted silica ingot is low, and the defect density in the middle of the silica ingot is 1.5%, which is significantly lower than that of spray quartz (3.1%) and silicon particle (4.2%). Moreover, electrophoretic deposited quartz-assisted mc-Si can obtain oriented grains, which offers a potential to apply alkaline texturing on mc-Si wafers.

1. Introduction

Directionally solidified multicrystalline silicon (Mc-Si) is one of the main materials for industrial solar cells. In the past decade, the seed-assisted growth of Mc-Si has become very important. Due to the improvement of Mc-Si crystal quality and solar cell efficiency, the corresponding Mc-Si is called high performance Mc-Si. The early seed-assisted nucleation method was to introduce the nucleating agent based on solid silicon, for example, lay silicon particles on the inner bottom of the crucible [1,2]. These Si particles are partially melted in the melting stage, and the unmelted part provides a large number of nucleation sites for the homogeneous nucleation of solidified crystals. Therefore, small and homogeneous grains can be obtained [3,4]. Although the conventional homogeneous nucleation method can obtain a good crystal utilization rate, it requires a higher degree of supercooling, which also results in a large number of structural defects. In contrast, seed-assisted Mc-Si has a large number of small initial grains. Considering each grain, the stress between grains can be better released, and the defects can be confined to the small initial grains, However, there are also some drawbacks that cannot be ignored in partially melted seed-assisted Mc-Si growth [5,6,7]. First, the gap between seeds may lead to heterogeneous nucleation, which will cause local stress and form defects. At the same time, the unmelted silicon particles on the inner bottom of the crucible can change the temperature field, disturb the smoothness of the solid–liquid interface at the bottom of the crucible, and are prone to thermal stress and dislocation at the early stage of growth [8,9,10,11]. Secondly, during the whole growth process, the unmelted seed layer remains at high temperatures and is affected by the diffusion of impurities from the bottom layer, which contaminates the longer seed layer and cannot be used for the manufacture of solar cells. The former was solved by using silicon-based nucleating materials with high density and improving the thermal field [12,13,14]. The latter solution is to choose Mc-Si nucleation materials with a high melting point. Trempa et al. chose quartz seed material with a high melting point as the nucleation layer for the growth of Mc-Si, which improved the minority carrier life of Mc-Si crystal [15,16].

Hexagonal β -quartz is an ideal seed material for the growth of Mc-Si. Its lattice constant of 0.548 nm is close to that of Silicon (0.543 nm), and their lattice mismatch ratio is 0.9%. It is an ideal Mc-Si heterogeneous nucleation-assisted growth material. So far, the quartz nucleation layer is usually sprayed or brushed on the inner bottom of the crucible. The surface of the quartz layer is rough, which is not conducive to the synchronous nucleation of polysilicon on the quartz surface. We propose a method to enhance the growth of assisted polysilicon with smaller bottom grain size and lower defect density. In this paper, an effective method to assist the growth of Mc-Si is introduced. A layer of quartz powder is deposited by electrophoretic deposition on the surface of the silicon wafer to obtain a uniform quartz nucleation layer and annealed at 600 °C for 1 h under vacuum conditions. The results show that quartz-assisted polysilicon can form uniform initial grains. Photoluminescence (PL) and minority carrier lifetime images show that the defect density of Mc-Si is reduced by the quartz powder-assisted growth method. Trempa et al. found that different dense nucleation layers can be obtained by spraying SiO₂ with different particle sizes at the bottom of the crucible, and dense SiO₂ nucleation layers can improve the minority carrier life of a Mc-Si ingot. In this paper, a denser nucleation layer can be obtained by electrophoretic deposition quartz powder. The results show that the minority carrier lifetime of a Mc-Si ingot based on the deposition quartz (7.4 μs) method is higher than that of the typical partial melting (Si particle, 7.2 μs) and full melting (spray quartz, 6.9 μs) seed-assisted methods. Moreover, the results show that the quartz-assisted wafers have oriented grains, and a pyramid structure can be obtained after alkaline texturing.

2. Experimental

A quartz layer was deposited on the surface of the silicon wafer by electrophoresis. The substrate material was the N-type silicon (1.2 Ω·cm, 156 × 156 mm²). A turbid solution was formed by mixing β -quartz powder (NCST, Shijiazhuang, China), aluminum nitrate, and water (mass ratio 20:1:1000) in a container for 1 h. The silicon wafer was then subjected to electrophoresis for 20 min in a mixture of β -quartz powder (10,000 mesh), aluminum nitrate, and water (mass ratio 20:1:10,000). During electrophoresis, the silicon substrate was connected to a negative electrode and the graphite plate to a positive electrode. The silicon wafers after quartz deposition were dried and annealed for 1 h under vacuum conditions of 600 °C to enhance the strength of the quartz powder. As shown in Figure 1, firstly, the quartz powder was deposited on the surface of the silicon wafer by the electrophoretic method. After deposition, the sample was vacuum annealed at 600 °C for 1 h, and one side of the deposited quartz powder was pasted on the bottom of the crucible. Silicon wafers were covered with silicon particles 2 cm thick to prevent damage to the silicon wafers at the bottom of the crucible during polysilicon loading.

Because the melting point of quartz is higher than that of silicon, the electrophoretic deposited quartz powder-assisted growth of Mc-Si can increase the melting temperature and eliminate the silicon particles at the bottom of the crucible. In this paper, three kinds of Mc-Si nucleation materials are studied, which are the electrophoretic deposited quartz powder, spray quartz powder, and silicon particles. The optical and electrical properties of Mc-Si crystals grown with the help of three nucleating materials are compared. A multicrystalline silicon ingot was prepared by directional solidification (GCL, Xuzhou, China). The vertical carrier lifetime distribution of the bricks was measured using microwave photoconductance decay (WT-2000, Semilab, Budapest, Hungary). The photoluminescence mapping of the wafers was measured (OPT-A101, OPT, Shandong, China) to analyze the defect distribution in the wafers.

3. Results and Discussions

Figure 2 shows the optical and scanning electron microscope (EVO-18, ZEISS, Jena, Germany) images of the electrophoretic deposited quartz powder, spray quartz powder, and silicon particle-assisted layer, respectively. In Figure 2c, the size of the silicon particles is approximately 5 mm and the distance between the particles is the largest compared with the other two nucleation layers. SEM images show that the surface of the electrophoretic deposited quartz powder is smoother than that of the spraying quartz powder, and annealing can also make the quartz layer stronger after electrophoretic deposition than that of the spray quartz, which contributes to the uniform nucleation of Mc-Si.

The lifetime of minority carriers was measured by microwave photoelectricity. The test instrument was the Semilab WT-2000PV. The minority carrier lifetime distribution of electrophoretic deposited quartz powder, spray quartz powder, and silicon particle-assisted crystal silicon is shown in Figure 3. The height of the low-life area (red area) at the bottom of Figure 3c is higher than that of Figure 3a,b, because the bottom of Figure 3c is an unmelted silicon particle layer. In the process of polycrystalline silicon melting, the silicon solution infiltrates the unmelted silicon particle layer and solidifies quickly, forming a low-life area. The same quartz powder nucleation material, but the length of the red zone at the bottom of Figure 3a, is lower than that in Figure 3b. There are two reasons, first of all, spraying at the bottom of the quartz crucible of the gap between particles is too big and easily spreads from the bottom of the crucible through clearance metal impurity to lowering the minority carrier lifetime of the Mc-Si ingot. This is followed by spraying quartz powder by van der Waals force between the adsorption and bonding strength is not high. In polycrystalline silicon, during the melting stage, liquid convection can make quartz powder fall off and diffuse into the solution to nucleate the low-life impurity region. Figure 3a shows that, concerning electrophoretic deposition quartz powder, the density of the nucleation layer can effectively prevent the metal impurity of the bottom of the crucible from diffusing into the inside of the crucible, which is associated with the chemical adsorption between electrophoretic deposition quartz powder and the high bonding strength between particles. The annealing process can further strengthen the bond strength between particles and can effectively reduce the result of the phenomenon in which the silicon solution convection nucleation layer material falls off. In summary, the average lifetime of minority carriers in Figure 3a is 7.4 μs higher than that in Figure 3b (6.9 μs) and Figure 3c (7.2 μs).

In order to better compare the effects of different nucleation layers on the homogeneousness of initial grain size of the Mc-Si ingot, the three samples were subjected to mixed acid (HNO₃/HF) corrosion treatment, as shown in Figure 4. From the comparison of photos after acid treatment, it can be seen that the initial grain size of Mc-Si nucleated by silicon particles is large but the homogeneousness is poor. The initial grain size of Mc-Si nucleated by electrophoretic deposited quartz powder is the most homogeneous. The reason is that the fine and uniform quartz particles in the electrophoretic deposition quartz layer form a large number of Mc-Si nucleation points at the bottom of the crucible, and a large number of initial grains grow synchronously and inhibit each other, which can reduce the extrusion stress between grains.

Figure 5 shows PL imaging photos of the bottom wafer (the same height) assisted by electrophoretic deposited quartz powder, spray quartz powder, and silicon particles, respectively. PL mapping was used to characterize the distribution of macrostructural defects. The contrast of the PL image corresponded to the composite activity at this position, and the bottom wafer was selected to show the formation of defects during initial growth. It should be noted that in Figure 4c, a large area of the black pattern (red marked area) appears in the wafer, which indicates that a large number of small particles of unmelted silicon easily accumulates in the solid–liquid interface area at the beginning of growth. Silicon particles-assisted growth easily creates microcrystalline and forms structural defects in the early stage of growth. In Figure 4b, there is a large difference in grain size between different regions (red marked area). This indicates that the heterogeneity of quartz grain size (Figure 2b) in the nucleating layer affects the consistency of Mc-Si grain size. For electrophoretic quartz-assisted wafers, Figure 5a shows that the whole wafer has almost no obvious black pattern. This shows that the homogeneous nucleation layer (Figure 2a) leads to homogeneous grain distribution and reduces the internal stress. It should be noted that the actual growth height in Figure 5a is longer than that in Figure 5c because the starting position of the assisted growth of electrophoretic quartz is lower than that of partially molten silicon particles. Therefore, the nucleation quality of electrophoretic deposited quartz powder is obviously better than that of spray quartz powder and silicon particles. It shows that a homogeneous initial grain structure can maximize the release of concentrated stress and reduce the clusters of defects. The particle size distribution of Figure 5a is smaller and more homogeneous than that of Figure 5c, which was caused by the size of the nucleating powder shown in Figure 2.

In order to further analyze the effects of the three nucleation layers on the nucleation of Mc-Si, Ipp (Image pro plus) software was used for the statistical analysis of Figure 5. The statistical results showed that the crystal defects in Figure 6a,b were mainly concentrated in the grain boundary region of Mc-Si crystal. The defects in Figure 6c, not only existed at the grain boundary, but also had a large defect group inside the crystal, which was mainly caused by the low temperature of silicon seed crystals during the melting process and incomplete melting. According to IPP statistics, the defect area in Figure 6a accounted for 1.5% less than that in Figure 6b (3.1%). Due to the existence of large defect clusters, the defect area in Figure 6c accounted for 4.2%.

In order to more accurately understand the dislocation density distribution of the Mc-Si ingot with different nucleation materials assisted growth at different positions. Every 3 cm, we took a piece 6 cm from the bottom of the crucible and calculated the dislocation density at different positions through polishing and dislocation corrosion treatment. The dislocation distribution of the three nucleating materials is shown in Figure 7. It can be seen in the Figure 7, the electrophoretic quartz nucleation layer and silicon particle nucleation layer in the range from 5–10 cm play a good assisting role in the growth of Mc-Si, and the dislocation density of the silicon wafer is much lower than that of the silicon ingot assisted by spraying quartz. The nucleation effect of quartz electrophoretic deposition is slightly better than that of silicon particles. In the growth of polysilicon assisted by silicon particles, some silicon particles diffuse into the microcrystalline region formed in the solid–liquid interface region, which has a certain influence on the overall crystal quality of Mc-Si.

Another feature of the electrophoretic deposition method is that it can control the same orientation of quartz grains, and annealing can further adjust and enhance the adhesion between quartz grains and the monocrystalline silicon substrate. Figure 8 is the SEM image of alkaline etch based on the electrophoretic deposition method and conventional method. For the other two polycrystalline silicon ingot casting methods, the orientation of initial grains in the nucleation layer is random, and the orientation of Mc-Si grains is also randomly distributed. Therefore, after alkali texture treatment, a large area of smooth surface will appear in some areas (Figure 8b,c). For the sedimentary quartz method, it can be seen from Figure 8a that the grains have the same vertical orientation. The crystal structure of β -quartz is similar to that of silicon. The lattice constant of β -quartz is 0.548 nm, and the lattice constant of silicon is 0.543 nm. The lattice mismatch between β -quartz and silicon is 0.9%. Quartz is indeed an ideal material for the growth of Mc-Si.

4. Conclusions

The effect of electrophoretic deposition of the quartz layer after annealing on the quality of Mc-Si crystals was studied. Statistical results show that, compared with the other two assisted materials, the overall dislocation density of the electrophoretic quartz-assisted silicon ingot is lower, and the defect density of the electrophoretic quartz-assisted (1.5%) silicon ingot in the middle of the silicon ingot is significantly lower than that of spray quartz (3.1%) and silicon particle (4.2%). Compared with the results of Trempa et al., the initial particle size distribution of quartz-assisted casting by electrophoretic deposition is more uniform and smaller than that by the spraying quartz-assisted ingot. The minority carrier life of the quartz-assisted (7.4 μs) Mc-Si ingot by electrophoretic deposition is higher than that of Trempa et al.’s spraying quartz-assisted (7.0 μs) Mc-Si ingot. The electrophoretic deposition of quartz assisted Mc-Si wafer growth to obtain a good alkali texture surface, providing more effective methods for Mc-Si surface texture.